Siloxene -- Articles & patents

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Siloxene

https://pubs.rsc.org/en/content/articlelanding/2018/ee/c8ee00160j#!divAbstract

Two-dimensional siloxene nanosheets: novel high-performance supercapacitor electrode materials
Karthikeyan Krishnamoorthy, et al.

Abstract
Silicon-based materials have attracted considerable interest for the development of energy storage devices because of their ease of integration with the existing silicon semiconductor technology. Herein, we have prepared siloxene sheets—a two-dimensional (2D) silicon material—and investigated their energy storage properties via fabrication of a symmetric supercapacitor (SSC) device containing 0.5 M tetraethylammonium tetrafluoroborate as the electrolyte. The formation of 2D siloxene sheets functionalized with oxygen, hydrogen, and hydroxyl groups was confirmed through X-ray diffraction, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and laser Raman mapping analyses. Cyclic voltammetric studies of the siloxene SSC device revealed the presence of pseudocapacitance in the siloxene sheets that arose from an intercalation/deintercalation phenomenon. The galvanostatic charge–discharge profiles of the device displayed sloped symmetric triangular curves with a maximum specific capacitance of 2.18 mF cm−2, high energy density of 9.82 mJ cm−2, good rate capability, and excellent cycling stability of 98% capacitance retention after 10 000 cycles. The siloxene SSC device delivered a maximum power density of 272.5 mW cm−2, which is higher than those of other silicon- and carbon-based SSCs, highlighting their potential for application in energy storage.

https://www.cambridge.org/core/journals/journal-of-materials-research/article/local-and-electronic-structure-of-siloxene/F4AB681764E0682EBD74EFF6D7FC9636

Local and Electronic Structure of Siloxene
    Ernst Z. Kurmaev, et al.

Abstract
Silicon L2,3 x-ray emission spectra (XES) of siloxene powder samples prepared according to Wöohler and Kautsky (Wöhler and Kautsky siloxene) are presented. The results are compared with the Si L2,3 spectra of the reference compounds a-Si, c-Si, SiO2, and SiOx. A close similarity of the electronic structure of Wöhler siloxene to that of a-SiO0.43: H and of Kautsky siloxene to that of a-SiO0.87: H is found. We determine the number of oxygen atoms per Si atom at ~0.5 in Wöhler siloxene and ~0.8 in Kautsky siloxene. The relative concentrations are in good agreement with the results of infrared absorption measurements on the same samples.

http://statistics.roma2.infn.it/~marsili/Grafene/PhysRevB.56.13132.pdf

Structure and optical properties of the planar silicon compounds polysilane and Wo ̈ler siloxene
U. Dettlaff-Weglikowska, et al.

Structure and optical properties of the planar silicon compounds polysilane and Wo ̈hler siloxeneU. Dettlaff-Weglikowska, W. Ho ̈nle, A. Molassioti-Dohms, S. Finkbeiner,*and J. WeberMax-Planck-Institut fu ̈r Festko ̈rperforschung Postfach 80 06 65, D-70506 Stuttgart, Germany~Received 9 June 1997!The two-dimensional silicon backbone structure of planar polysilane and Wo ̈hler siloxene is responsible fortheir exciting luminescing properties. We have prepared single crystals of siloxene by a topotactic reactionfrom crystalline CaSi2. The chemical composition was determined as@Si6H3~OH!3#n. The x-ray crystalstructure   analysis   identifies   the   so-called   Wo ̈hler   siloxene   as   2D-poly@1,3,5-trihydroxocyclohexasilane#.Polysilane exhibits the same structural properties but with a chemical composition@Si6H6#n. The opticalproperties~infrared transmission, photoluminescence, excitation spectroscopy!of these well-defined materialsare presented. A heat treatment above 350 ° C in vacuum of Wo ̈hler siloxene results in a destruction of theplanar`2@Si2#structure by internal rearrangements, which is evidenced by the x-ray-diffraction pattern andcharacteristic changes in the optical spectra. The involvement of Wo ̈hler siloxene in the optical properties ofporous Si is critically reviewed.

https://www.worldscientific.com/doi/abs/10.1142/9789812812995_0020
https://doi.org/10.1142/9789812812995_0020
Porous Silicon, pp. 417-441 (1994)

SPIN RESONANCE INVESTIGATIONS OF POROUS SILICON AND SILOXENE
MARTIN S. BRANDT and MARTIN STUTZMANN

Abstract
We present an overview of the current work on electron spin resonance (ESR) and related techniques (light-induced electron spin resonance (LESR), spin-dependent photoconductivity (SDPC), and optically detected magnetic resonance (ODMR)) in light emitting porous silicon and siloxene. In particular, we discuss the ESR signatures of the dominant defect in these materials, the silicon dangling bond. In ODMR, the microscopic structure of the excited state is identified as a triplet exciton with a dipolar diameter of about 4Å. This excited state is also observed in other silicon-based systems emitting light in the red (annealed siloxene, hydrogenated silicon-oxygen alloys).

http://zfn.mpdl.mpg.de/data/Reihe_B/35/ZNB-1980-35b-0025.pdf

The Topochemical Reaction of CaSi2 to a Two-Dimensional Subsiliceous Acid Si6H3(OH)3 (= Kautskys' Siloxene)
Armin Weiss

[ PDF ]

https://onlinelibrary.wiley.com/doi/abs/10.1002/9783527610761.ch16
https://www.researchgate.net/publication/230154389_Kautsky-Siloxene_Analogous_Monomers_and_Oligomers
DOI: 10.1002/9783527610761.ch16

Kautsky‐Siloxene Analogous Monomers and Oligomers
In book: Silicon Chemistry: From the Atom to Extended Systems, pp.214 - 225, Chapter · May 2007
Harald Stueger

Abstract
Because of its indirect band structure, crystalline silicon does not show visible light emission at room temperature. Low-dimensional silicon polymers such as polysilanes, or silicon polymers with sheet-like structures, however, have a direct band structure. They attract considerable attention in solid-state physics mainly because of their outstanding luminescence properties. Especially the two-dimensional sheet polymers such as siloxene (Si6O3H6)n or polysilyne (SiH)n are promising candidates for technological application because of their higher mechanical stability and higher conductivity. Since (SiH)n, accessible from (SiBr)n and LiAlH4 or, more recently, from CaSi2 and HCl, is highly unstable and shows luminescence only in the UV part of the spectrum, siloxene with its strong room temperature photoluminescence in the green or yellow has been most thoroughly investigated. Research on siloxene has been further intensified by the recent suggestion that the efficient visible luminescence observed in porous silicon, a phenomenon of considerable current interest, is mainly due to the presence of siloxene species on the surface of the porous silicon particles.

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.832.4509&rep=rep1&type=pdf

Photo- and Chemiluminescence from Wöhler Siloxenes
A. Molassioti-Dohm, et al.

ABSTRACT
The photo- and chemiluminescence of Wohler siloxenes, Si,H3(OH)3+, in the two oxidation stages with x =0:yel-low sioxene and x =1:red sioxene were studied. During slow oxidation both compounds exhibit a shift of the photolu-minescence spectra at room temperature, but the position of the chernilurninescence is not affected. Our results supportthe buckled-layer structure of Wöhler sioxenes with the Si6-ring in chair conformation as the building block. The Si-Hbonds on the rmg can be successively oxidized (0 x3),leading to a shift in the photoluminescence spectrum. Totaloxidation of all three Si-H bonds results in an unstable modification, which transforms rapidly to a new substance withdistinctively different optical properties. The results on siloxene have common features with measurements on porous sil-icon. However, the quantitative differences in the spectra indicate that in porous silicon, the emission properties are notdirectly associated with Wohler siloxenes.

https://pubs.rsc.org/en/content/articlelanding/2015/ta/c5ta00321k#!divAbstract
https://pdfs.semanticscholar.org/6751/101e0818a5e8549f3d3b4b313d1f775709df.pdf

Synthesis of a calcium-bridged siloxene by a solid state reaction for optical and electrochemical properties
Haruo Imagawa, et al.

Abstract
A Ca-bridged siloxene (Ca-siloxene) resulting from the deintercalation of Ca from CaSi2 was synthesized via a solid state reaction with ethanol wash using TaCl5. The presence of fragmented, two-dimensional siloxene planes with Ca bridging was confirmed. The Ca-siloxene exhibited tunable optical properties, as well as stable Li storage performance by Ca bridging.

https://pdfs.semanticscholar.org/259d/088203a82cab26339aafc6f7e753837d28bb.pdf

The observation of silicon nanocrystals in siloxene
by RF Pinizzotto

Oxygen Adsorption Properties of Microporous Silica Derived from Layered Siloxene by Oxidation
S. Yamanaka

Abstract
Layer structured siloxene prepared by the reaction of CaSi2 with hydrochloric acid was further oxidized by a treatment with water. During this treatment, the Si-Si and the Si-H bonds of the siloxene were oxidized to Si-O-Si and Si-OH bonds, respectively, and the interlayers were cross-linked by the condensation of the Si-OH groups. The resulting compounds have high surface areas (400–600 m2/g) with pores in the micropore to mesopore ranges. The compounds calcined in a vacuum up to 1000°C keep the high surface area and show peculiar electron spin resonance (ESR) centers on adsorption of oxygen, which are attributed to the formation of silicon dangling bonds. The ESR centers disappear reversibly on desorption of oxygen.

https://www.jstage.jst.go.jp/article/jcersj2/122/1428/122_JCSJ-P14038/_pdf

Formation of Si-based nanosheets by extractionof Ca from CaSi2layers on Si substrates
Xiang MENG, et al.

It was demonstrated thatflake-like structures containing Si-based nanosheets were successfully synthesized on Si(111) substrates.By exposure of CaSi2/Si substrates to CrCl2vapor, Ca atoms were extracted from CaSi2, then, the Si-based nanosheets wereformed. The structural properties of the Si-based nanosheets formed at the edge of theflakes were examined. It is noted that theflake-like structures were rooted to the substrates, and that the Si-based nanosheets were easily exfoliated from theflake-likestructures, to expose the surfaces corresponding to the Si{111}planes of the nanosheet, originated from the corrugated Si(111)layers linked by Ca in CaSi2

https://iopscience.iop.org/article/10.1088/0031-8949/1992/T45/067

Visible luminescence from porous silicon and siloxene
H D Fuchs, et al.

https://www.degruyter.com/downloadpdf/j/znb.1980.35.issue-1/znb-1980-0108/znb-1980-0108.pdf

The Topochemical Reaction of CaSi2 to a Two-Dimensional Subsiliceous Acid Si6H3(OH)3 (= Kautskys' Siloxene)

https://europepmc.org/articles/pmc6565710

Nickel@Siloxene catalytic nanosheets for high-performance CO2 methanation
Xiaoliang Yan, et al.

Abstract
Two-dimensional (2D) materials are of considerable interest for catalyzing the heterogeneous conversion of CO2 to synthetic fuels. In this regard, 2D siloxene nanosheets, have escaped thorough exploration, despite being composed of earth-abundant elements. Herein we demonstrate the remarkable catalytic activity, selectivity, and stability of a nickel@siloxene nanocomposite; it is found that this promising catalytic performance is highly sensitive to the location of the nickel component, being on either the interior or the exterior of adjacent siloxene nanosheets. Control over the location of nickel is achieved by employing the terminal groups of siloxene and varying the solvent used during its nucleation and growth, which ultimately determines the distinct reaction intermediates and pathways for the catalytic CO2 methanation. Significantly, a CO2 methanation rate of 100 mmol gNi−1 h−1 is achieved with over 90% selectivity when nickel resides specifically between the sheets of siloxene.

https://pubs.acs.org/doi/full/10.1021/acsami.8b15323?src=recsys

Understanding the Thermal Treatment Effect of Two-Dimensional Siloxene Sheets and the Origin of Superior Electrochemical Energy Storage Performances
Parthiban Pazhamalai, et al.

Abstract
Two-dimensional siloxene sheets are an emerging class of materials with an eclectic range of potential applications including electrochemical energy conversion and storage sectors. Here, we demonstrated the dehydrogenation/dehydroxylation of siloxene sheets by thermal annealing at high temperature (HT) and investigated their supercapacitive performances using ionic liquid electrolyte. The X-ray diffraction analysis, spectroscopic (Fourier transform infrared, laser Raman, and X-ray photoelectron spectroscopy) studies, and morphological analysis of HT-siloxene revealed the removal of functional groups at the edges/basal planes of siloxene, and preservation of oxygen-interconnected Si6 rings with sheet-like structures. The HT-siloxene symmetric supercapacitor (SSC) operates over a wide potential window (0–3.0 V), delivers a high specific capacitance (3.45 mF cm–2), high energy density of about 15.53 mJ cm–2 (almost 2-fold higher than that of the as-prepared siloxene SSC), and low equivalent series resistance (compared to reported silicon-based SSCs) with excellent rate capability and long cycle life over 10 000 cycles.

Patents

CN106058232
Siloxene material, preparation method of silicon-based oxide and cathode material

The invention provides a siloxene material. A chemical formula I of the siloxene material is shown as Si6OxHy(OH)6-y, wherein each of x and y is greater than or equal to 1 and less than or equal to 6. The siloxene material has Si-O-Si bond, Si-H bond and Si-OH bond. The siloxene material is simple in preparation process and easy in large-scale preparation. A cathode material prepared by using the siloxene material has high circulating stability, high initial Coulomb efficiency and high rate performance. Experiment results show that under a condition of 150mA/g, initial lithium-removing capacity is 845-1329mAh/g, initial Coulomb efficiency is 65.2-84.1%, and capacity retention ratio after circulating for 200 circles is 75.0-85.3%; under a condition of 1.5A/g, capacity retention ratio is 71.1-86.0%. The invention further provides a preparation method of silicon-based oxide and the cathode material.

JP2012001604
SILOXENE COMPOSITE MATERIAL, SILOXENE DERIVATIVE, AND METHOD FOR PRODUCING THESE

PROBLEM TO BE SOLVED: To provide a method for producing a siloxene composite material, which can optionally select a functional group incorporated in siloxene.SOLUTION: The method for producing a siloxene composite material represented by the composition formula (1): SiH(O-E-CONRR)(wherein Rand Rare the same as those in the following formula (3); and E and a are the same as those in the following formula (2)) comprises reacting a siloxene derivative represented by formula (2): SiH(O-E-COOR)(wherein Ris a 1-10C hydrocarbon group; E is a 1-30C hydrocarbon group; and 1≤a≤5) with a nitrogen-containing compound represented by formula (3): HNRR(wherein Rand Rdenote a hydrogen atom, a hydrocarbon group or a hetero group, and may be bonded to each other in the NRRto form a nitrogen-containing hetero ring), in an organic solvent.

JP2009184839
SILICA NANOSHEET, METHOD FOR PRODUCING THE SAME AND DISPERSION OF SILICA NANOSHEET IN ORGANIC SOLVENT

PROBLEM TO BE SOLVED: To provide a method for producing a silica nanosheet by which a silica nanosheet dispersible in an organic solvent can be obtained. ;SOLUTION: The method for producing a silica nanosheet includes a step of mixing monocalcium disilicide and concentrated hydrochloric acid to form layered siloxene, and a step of mixing and stirring the layered siloxene and an organic solvent to form a silica nanosheet in the organic solvent.

US5578379
Device comprising a luminescent material

Siloxene and siloxene derivatives are compatible with silicon and may be generated as epitaxial layer on a silicon monocrystal. This permits the production of novel and advantageous electroluminescent devices, such as displays, image converters, optoelectric integrated circuits. Siloxene and siloxene derivatives may also be advantageously employed in lasers as laser-active material and in fluorescent lamps or tubes as luminescent material...

Siloxene Si6 O3 H6 and Si6 O3+n H6-m and its derivatives are known, cf. for example Gmelin, Silicon, volume B, p. 591-596. It forms a highly polymerized solid insoluble substance which contains as essential constituent silicon six-membered rings or silicon planes.

Three different structural modifications of siloxene are illustrated in FIGS. 1 to 3, each of which shows part of the [111]-plane of a siloxene crystal. The large striped circles denote Si atoms, the large blank circles O atoms and the small striped circles H atoms. The most stable structure is that according to FIG. 1, which contains pronounced six-membered rings linked together by oxygen atoms.

It is also known that siloxene exhibits photoluminescence and chemoluminescence and that the luminescence color can be varied by substitution of the H atoms of the siloxene. However, a fact hitherto not recognized was that siloxene, in particular on suitable contacting, can also exhibit electroluminescence, and nor had it been recognized that due to its constitution and the production methods available siloxene is excellently suited to uses in optical and electrooptical devices, in particular in conjunction with silicon and silicon semiconductor devices.

A particular advantage is that the lattice constants of siloxene are practically identical to those of silicon. Consequently, a siloxene layer can be applied epitaxially to a silicon monocrystal, in particular to a [111]-surface of the silicon. It is further possible to chemically convert a surface layer of a monocrystalline or polycrystalline silicon body to siloxene without substantial modification of the structure of said layer. The same applies to sapphire substrates and so-called SOS structures ("Silicon-on-Sapphire"). A further important advantage is that siloxene layers can also be applied at relatively low temperatures, in particular.ltoreq.800 DEG C., and also to semiconductor substrates, in particular silicon substrates, which contain an integrated circuit so that by using conventional semiconductor fabrication techniques, such as photomasking, a desired configuration of electroluminescent devices on the integrated circuit can be produced. This gives an integrated electronic-electrooptical circuit. Siloxene and its derivatives can also advantageously be used on other substrates, such as glass, quartz, organic polymers (plastics), for example PMMA.

Siloxene for use as luminescent material can be made in various ways, for example by a known wet chemical method according to Kautsky (see for example Z. anorg. Chemie 117, 209-242 (1921) or W ohler (see for example Liebig's Annals 127,257, 1863), and further by deposition from the gas phase on a suitable substrate, in particular, as mentioned above, monocrystalline silicon, for example using a mixture of SiH4, H2 O and/or H2, O2, and a dilution gas such as argon or another noble gas or nitrogen, and the reaction can be activated by light, in particular laser radiation, a plasma discharge, a high-frequency or microwave discharge, cyclotron resonance, heat, for example heating the substrate. Further possibilities are deposition from the liquid phase, for example liquid phase epitaxy, spin-coating by means of a suspension, and also molecular beam deposition, for example molecular beam epitaxy, silicon being evaporated from an effusion cell and hydrogen and oxygen being employed as ion beams. Electrochemical methods for producing siloxene are also known.

A further wet chemical method for producing siloxene resides in subjecting calcium disilicide CaSi2 (or another suitable metal silicide of similar structure, for example silicides of other alkaline earth elements, such as magnesium, strontium and barium) to the action of fuming hydrochloric acid (38% HCl) for several hours and washing the reaction product with ether or water to remove the chlorine. In another method of this type the metal silicide is treated for several hours with an ice-cooled mixture of alcohol, water or HCl and thereafter washed with ether. In this case, a mixture of 60 vol. parts ethanol, 11 vol. parts H2 O and 2 vol. parts conc. HCl has proved suitable.

The reaction and the subsequent washing should be carried out away from oxygen in the dark.

To produce a homogeneous epitaxial siloxene layer on a monocrystalline silicon substrate, metal silicide, in particular CaSi2, can advantageously be formed epitaxially on a [111] surface of a silicon body, as is known from publications by J. F. Morar et al. in Physical Review B, vol. 37, no. 5, Feb. 15, 1988, p. 2618-2621 and J. Vac. Sci. Technol. A 6(3), May/June 1988, p. 1340-1342. In this method, on a surface of a silicon substrate, preferably on a polished [111] surface of an Si monocrystal, a layer of calcium metal is vapour deposited with a desired thickness, for example 100 to 500 nm, and alloyed in by heating to about 800 DEG C. This layer is then converted wet chemically to siloxene in the manner described above by the action of HCl. In this manner a dense homogeneous epitaxial siloxene layer is obtained on a monocrystalline Si substrate which can be structured by photoetching, corpuscular beam etching or any other known semiconductor structuring method.

It is also possible to apply siloxene suspended in a suitable solvent (e.g. ethanol) as a uniform thin layer to a substrate, for example by spin coating...

In an electroluminescent electronic device the substrate of the possibly structured siloxene layer has a corresponding electrode pattern and a corresponding transparent counter electrode array, for example of a thin transparent layer of gold or tin oxide, is applied to the free surface of the siloxene.

A particularly advantageous configuration of an electrooptical device consists of a layer structure including in order a substrate of p or n-conductive doped monocrystalline silicon, a thin transparent dielectric layer of for example SiO2 or Si3 N4, a layer of siloxene or siloxene derivative and a transparent counter electrode, for example of gold, tin or indium oxide. The transparent dielectric intermediate layer has the important function of being able to generate highly excited charge carriers by a high electric field (injection through tunnelling). The structure described can be protected from harmful environmental influences by a thin transparent layer.

The siloxene should also be protected in other cases by a protective layer, for example of a transparent material, such as SiO2, which protects the siloxene from the action of oxygen, moisture and other harmful environmental influences but does not impair the optical function.

A particular advantage of the use of siloxene as luminescent material resides in that the luminescence color can be varied by heating and/or substitution, also in situ and locally selectively. This is known in the case of photoluminescence of siloxene and for adjusting the luminescence color the known substitution methods and substituents may be employed.

The hydrogen atoms of the siloxene may be successively substituted, for example by halogens, in particular Cl, by OH groups, by alcohol, a bond of a C atom of the alcohol replacing for example the hydrogen of the siloxene, or by other monovalent radicals having an electron negativity different from hydrogen. The substitution results in a widening to the crystal lattice of the siloxene in the [111]-direction. Furthermore, a shift of the fluorescence towards longer wavelengths takes place. For the substitution known method may be employed, cf. for example E. Hengge, Advances of Chemical Research 9, 145 (1967) and H. Ubara et. al., J. Non-Cryst. Solids 59&60, 641 (1983).

Since the substitution can take place after the formation and structuring of a siloxene layer, the substitution can be carried out locally differently for example by covering parts of the siloxene layer in which no or a different substitution is to take place during the substitution, for example photolithographically. In this manner siloxene regions may be provided on an integrated circuit or other electrooptical device which on electrical or optical stimulation emit different colors.

The electrical properties of the siloxene or the siloxene derivatives can be modified specifically by heating and/or doping. The electrical conductivity of the siloxene may be influenced by substitution of a silicon atom of the plane or by interstitial doping, in particular with alkali metals, for example Li or Cs. Furthermore, the conductivity of the plane can be modified by intercollation of suitable molecules between every two planes. Finally, since undoped siloxene is an insulator, the conductivity of the silicon planes in a field-effect configuration can also be varied by charge transfer from a conductive substrate into the silicon planes.

One property of luminescence of siloxene and its derivatives is a wide spread of excitation lifetimes which may be undesirable for uses in optoelectronic communications technology (data transmission). It may be necessary here to control the excitation lifetimes specifically by incorporating recombination centres, in particular in the form of crystal defects. This may for example be done by treating with ionizing radiation, such as ion beams, or by oxidation, and the incorporation of recombination centres may be carried out in locally selective manner in order to obtain a local structuring of the emission properties of a siloxene layer.

Due to their structural properties and the optical properties, in particular the efficient photoluminescence, it is also advantageous to use siloxene and siloxene derivatives in other optical devices which operate with luminescence but not with electrical stimulation of the siloxene. It is possible with the production methods referred to above to deposit siloxene epitaxial layers or thin films with spatially constant or controlled variable redractive index on substrates such as crystalline silicon. Examples of such applications are the use of thin siloxene layers on silicon as sensor elements or optical waveguides. A concrete example is a sensor or- oxidizing media. The oxidation of the siloxene generates a chemoluminescence which is detected electrically in the silicon.

Also advantageous is the deposition of siloxene or siloxene derivative on existing photosensitive structures for spectral matching (solar cells, image converters, in particular for converting corpuscular radiation, X-ray radiation or ultraviolet light to visible radiation, for improving the sensitivity). This makes it possible in particular to compensate at least partially the pronounced drop of the quantum efficiency of photodiodes on the basis of crystalline silicon in the wavelength range above 400 nm. It is possible to increase the efficiency of Si-solar cells by a thin layer of siloxene which acts as antireflex layer and converts short-wave light, in particular UV light, to light of longer wavelengths for which the Si-sdar cell has a greater sensitivity.

Since siloxene does not exhibit inversion symmetry (the crystalline structure cannot be transferred to itself by reflecting at a point) it can be employed for optical frequency doubling.

Siloxene layers of adequate thickness are suitable as scintillator material for detecting ionizing radiation and may be employed with a material combination (siloxene on silicon) which is completely compatible with silicon technology. The advantage here resides in particular in the fact that no impurities are introduced and in the deposition of siloxene, for example from a plasma, large surfaces can also be formed. This use is expedient for example in combination with large-area amorphous silicon substrates.

A further field of use is integrated optics on the basis of silicon. Here, at present materials such as SiO2 or oxinitrides are considered which however are not optically active in the visible range. Siloxene is compatible with silicon technology similarly to the aforementioned dielectrics, has a smaller refractive index than silicon and moreover due to its high luminescence yield has properties which can be exploited in the production of optically active components in integrated optics without requiring for this purpose substances foreign to silicon technology. For example, siloxene or siloxene derivative can be employed in such applications as optically nonlinear material for optical modulators, for multiplexers and lasers. In crystalline form or in a suspension, siloxene may also be employed directly as active laser material.

The electroluminescence may be stimulated both with direct current and with alternating current.

With reference to FIGS. 4 to 8, some specific examples of embodiments of the invention will now be explained hereinafter.

FIG. 4 shows an electroluminescent device including a substrate 10 of doped monocrystalline silicon. In this example the substrate is highly n-conductive and has a specific resistance of the order of magnitude of a few ohm-cm. Disposed on the silicon monocrystal forming the substrate 10 is an epitaxial siloxene layer 12 and on the latter a transparent electrode 14 of SnO2 or other known transparent conductive material compatible with siloxene is disposed. On the side of the substrate 10 remote from the siloxene layer 12 a rear electrode 16 in the form of a vapour-deposited aluminium layer or the like is applied.

If a voltage is applied to the electrodes 14, 16 electrons are injected from the n-conductive substrate 10 and holes from the transparent counter electrode 14 into the siloxene layer 12 so that an electroluminescence occurs in the latter.

Instead of an n-conductive substrate, a p-conductive substrate may also be employed, the aforementioned signs of the charge carriers then being reversed.

The embodiment according to FIG. 5 corresponds to that according to FIG. 4 except that between the substrate 10 and the siloxene layer 14 a thin insulating layer 18 is arranged. In a practical example of embodiment the insulating layer 18 consists of SiO2 and has a thickness of about 5 nm to 100 nm. For this purpose, possibly a commercially available Si substrate material with buried oxide layer may be employed. The insulating layer 18 is so thin that the charge carriers, i.e. in this case the electrons, tunnel therethrough when a voltage of adequate magnitude negative with respect to the electrode 14 is applied to the electrode 16. In this arrangement, the charge carriers injected into the siloxene layer 14 have a higher energy as regards the conduction band edge of siloxene than in the device according to FIG. 4 and consequently a more efficient injection and thus higher luminescence yield are obtained.

The device according to FIG. 6 contains as substrate 10a a pn or pin photodiode of conventional design which has a rear electrode 16 and a transparent electrode 14 on its light entrance side. Disposed on the transparent electrode 14a is a siloxene layer 12 which in turn is covered by a transparent protective layer 20, for example of silicon dioxide. Electromagnetic radiation, in particular shortwave radiation such as UV or X-ray radiation, as well as corpuscular radiation, which enters the siloxene layer 12 through the protective layer 20 is converted therein to radiation of longer wavelength which in turn is converted by the photodiode 10a, 14a, 16 with high efficiency to electrical signal.

In a modification of the device according to FIG. 6, which can be used as sensor device, the protective layer 20 is not present or is permeable or perforated so that a substance to be detected can generate in the siloxene layer chemoluminescence which is detected by the photodiode 10a.

FIG. 7 shows a sensor device which includes a substrate 10 of doped silicon, a real electrode 16 attached to the rear side thereof and a siloxene layer 12 disposed on the upper side. On the siloxene layer there is an electrode structure 22 which ill this case is formed in comb-like manner and leaves free portions 12a of the surface of the siloxene layer so that aid portions are exposed to the atmosphere. Under the action of radicals, such as oxidizing radicals, halogens, etc., the conductivity of the diode-like structure formed by the silicon substrate 10 and the siloxene layer 12 varies. A corresponding signal then occurs in a circuit 24 which contains this structure and is only schematically illustrated.

FIG. 8 shows a particularly simple example of an embodiment according to the present invention. The device according to FIG. 8 includes a thin siloxene layer 12 which is provided on the one side with a rear electrode 16 and on the other side with a transparent electrode 14. By applying an ac voltage to the electrodes 14, 16 the siloxene is stimulated to electroluminescence. The rear electrode can be formed as a support structure and consists then for example of a metal plate and/or forms an injecting contact, for example a Schottky contact. Between the siloxene layer 12 and the transparent electrode 14, in this case a thin transparent dielectric layer 26 of SiO2 or the like is also disposed and permits generation of a higher field strength in the siloxene. Such a dielectric layer may also be provided in the devices according to FIGS. 4, 5 and 7.

Siloxene may also be advantageously employed as luminescent material in fluorescent lamps or tubes, in particular in combination with other luminescent materials. Here as well the siloxene is stimulated both by the UV radiation generated in the discharged and by electron bombardment.

US2008050573
Silicon Nanosheet , Nanosheet Solution and Process for Producing the Same, Nanosheet -Containing Composite, and Nanosheet Aggregate

Inventor(s):    NAKANO HIDEYUKI et al.

A silicon nanosheet comprising a silicon atom layer formed by bonding two-dimensionally and periodically arranged silicon atoms to each other through an Si-Si bond. A nanosheet solution prepared by dispersing or suspending the silicon nanosheets in a solvent. A nanosheet-containing composite having the silicon nanosheets on the surface and/or in the interior of a substrate. A nanosheet aggregate formed by aggregating the silicon nanosheets. A process for producing a nanosheet solution comprising: an acid treatment step of bringing a layered silicon compound into contact with an aqueous acid solution to derive a siloxene compound; and an exfoliation step of adding the siloxene compound into a solvent containing a surfactant, shaking the mixture, and peeling off the siloxene compound.

FIELD OF THE INVENTION

[0001] The present invention relates to: a silicon nanosheet comprising a single silicon atom layer formed by arranging silicon atoms two-dimensionally and periodically; a silicon nanosheet formed by aggregating plural such silicon atom layers; a nanosheet solution containing the silicon nanosheet and a process for producing the nanosheet solution; a nanosheet aggregate; and a nanosheet-containing composite.

BACKGROUND OF THE INVENTION

[0002] Silicon has heretofore been widely used as an important electronic material for a semiconductor integrated circuit, a thin-film transistor, and the like. Silicon has excellent characteristics as a material for an electronic device and is used also as an electronic data processing device such as a DRAM, an LSI, or the like. Meanwhile, a hydrogenated/hydroxylated compound, such as siloxene (Si6O3H6) or the like, which is a compound having a layered silicon skeleton, porous silicon, hydrogenated amorphous silicon, and others are known as luminous materials.

[0003] Recently it has been found that porous silicon formed by electrochemically etching a silicon substrate has the capability of visible light emitting (refer to L. T. Canham, "Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers," Applied Physics Letters, AMERICAN INST. OF PHYSICS, U.S.A., Sep. 3, 1990, p. 1046 to 1048). The light-emitting mechanism of porous silicon has not been clarified yet but it is estimated that the light emission is caused by a quantum effect, a surface structure, or a surface oxide film.

[0004] Meanwhile in recent years, research on a nanosheet comprising a semiconductor material such as TiO2, MnO2, Ca2Nb3O10, or the like has actively been worked on. TiO2 can be obtained from Cs0.7Ti1.825O4 as lepidocrocite type layered titanate (refer to T. Sasaki and M. Watanabe, Journal of the American Chemical Society, U.S.A., 1998, Vol. 120, p. 4682 to 4689), MnO2 can be obtained from K0.45MnO2 as layered manganese oxide of an [alpha]-NaFeO2 type associated structure (refer to JP-A No. 335522/2003 and Y. Omomo, T. Sasaki, L. Z. Wang and M. Watanabe, Journal of the American Chemical Society, U.S.A., 2003, Vol. 12, p. 3568 to 3575), and Ca2Nb3O10 can be obtained from KCa2Nb3O10 of a layered perovskite structure (refer to Y. Ebina, T. Sasaki and M. Watanabe, Solid State Ionics, U.S.A., 2002, Vol. 151, p. 177 to 182). In any of the cases, a nanosheet is produced by: exchanging ions between layers for hydrogen ions; thereafter intercalating quaternary ammonium ions (particularly tetrabutyl ammonium) between the layers; expanding the gap between the layers by hydration swelling and violently shaking it; and thereby peeling off a layered compound.

[0005] With regard to silicon materials, nanocrystal silicon is cited as a nanoscale material. Nanocrystal silicon: shows quantum effect-which a conventional silicon material does not show; and hence attracts attention as a novel electronic material and the like. It is known that the nanocrystal silicon can generally be produced by, for example, a process of: simultaneously sputtering silicon and quartz glass; thereby forming an amorphous film wherein Si is excessively contained in SiO2 on another silicon substrate; and applying heat treatment at 900[deg.] C. to 1,100[deg.] C. (refer to JP-A No. 040348/2001).

[0006] With such background, the development of porous silicon and nanocrystal silicon as electronic materials for the next generation has been worked on increasingly actively in recent years. The development of a useful silicon material having a novel structure has been desired with the aim of using it as a more novel electronic material or a more novel luminous material.

[0007] In the case of nanocrystal silicon, however, the nanocrystal silicon is an indirect transition semiconductor and it requires the change of momentum when an excited carrier falls into a ground state. As a result, the nanocrystal silicon has a low luminous efficiency and has not been suitable for a luminous material for a luminous element and the like.

[0008] Further, nanocrystal silicon has very fewer variations in the synthesis process thereof in spite of the fact that it is very highly appreciated as an electronic material and the like in practical use. As a process of synthesizing nanocrystal silicon, the process of forming an SiO2 film containing an excessive amount of Si under a reduced pressure and annealing the film at a high temperature of the vicinity of 1,000[deg.] C. has mainly been used.

[0009] In contrast, in the case of a nanosheet comprising silicon, quantum effect and luminousness can be expected and hence the nanosheet is expected as a novel electronic material and a novel luminous material. However, all of the materials which have heretofore been successful in forming a nanosheet have been transition metal oxide and there has not been a case where a nanosheet comprising non-oxide silicon is produced.

SUMMARY OF THE INVENTION

[0010] An object of the present invention is to provide: a novel silicon nanosheet wherein silicon atoms are arranged two-dimensionally and periodically; a nanosheet solution wherein the silicon nanosheets disperse and suspend and a process for producing the nanosheet solution; a nanosheet aggregate formed by aggregating silicon nanosheets; and a nanosheet-containing composite which contains silicon nanosheets.

[0011] Further, another object of the present invention is to provide a silicon nanosheet having a high luminous efficiency and being useful as an electronic material.

[0012] A silicon nanosheet according to the present invention which attains the above objects is a silicon nanosheet comprising a silicon atom layer formed by bonding two-dimensionally and periodically arranged silicon atoms to each other through an Si-Si bond.

[0013] Further, a nanosheet solution according to the present invention is a nanosheet solution prepared by dispersing or suspending the silicon nanosheets according to the present invention in a solvent.

[0014] Furthermore, a nanosheet-containing composite according to the present invention is a nanosheet-containing composite having the silicon nanosheets according to the present invention on the surface and/or in the interior of a substrate.

[0015] Yet further, a nanosheet aggregate according to the present invention is a nanosheet aggregate formed by aggregating the silicon nanosheets according to the present invention.

[0016] Still further, a process for producing a nanosheet solution according to the present invention comprises: an acid treatment step of bringing a layered silicon compound into contact with an aqueous acid solution to derive a siloxene compound; and an exfoliation step of adding the siloxene compound into a solvent containing a surfactant, shaking the mixture, and peeling off the siloxene compound.

[0017] In addition, another process for producing a nanosheet solution according to the present invention comprises: a hydrothermal treatment step of dispersing a layered silicon compound in a solvent comprising a mixture of an amine having 3 or more carbon atoms and water and applying hydrothermal treatment; and a separation step of separating unreacted materials.

[0018] When a layered silicon compound reacts with acid, the interlayer atoms of the layered silicon compound are replaced with acid molecules and a siloxene compound is formed. Successively, when a surfactant is added to a solution containing the siloxene compound and the mixture is shaken, the surfactant intrudes between layers and the siloxene compound swells infinitely. Otherwise, when a kind of amine is added to a layered silicon compound and the mixture is subjected to hydrothermal treatment, bulky amines are substituted for the interlayer atoms of the layered silicon compound and the layered silicon compound swells infinitely. As a result, a silicon nanosheet containing a single silicon atom layer or plural silicon atom layers is obtained.

[0019] A thus obtained silicon nanosheet is substantially composed of Si atoms, has two-dimensional anisotropy, and hence shows such unprecedentedly excellent characteristics as stated below. That is, a thus obtained silicon nanosheet:
(1) emits fluorescence of specific wavelengths with a specific excitation wavelength;
(2) has a remarkably large bandgap in comparison with bulk silicon;
(3) is excellent in traceability of a shape when a substrate surface is coated with the nanosheet;
(4) has an extremely high surface to volume ratio;
(5) exhibits high catalytic performance in the event of chemical reaction; and
(6) has a high thermal conductivity.

[0026] A silicon nanosheet according to the present invention can be used for: an electronic material constituting a semiconductor integrated circuit, a thin-film transistor, and others; a luminous element; an electronic element; a chemical catalyst; a catalyst carrier; a negative electrode active material for a lithium secondary battery; and others.

[0027] Further, a nanosheet solution according to the present invention can be used for: a liquid fluorescent agent; a raw material for the production of a nanosheet-containing composite and a nanosheet aggregate; and others.

[0028] Furthermore, a nanosheet-containing composite according to the present invention can be used for: various kinds of electronic elements; a heat radiating sheet; a raw material for a powder magnetic core; and others.

[0029] In addition, a nanosheet aggregate according to the present invention can be used for: various kinds of catalyst carriers; a negative electrode active material for a lithium secondary battery; an absorbent; and others.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG. 1 is a view showing the crystal structure of a silicon nanosheet according to a first embodiment of the present invention;
[0031] FIG. 2 is a view showing the crystal structure of CaSi2 which is a kind of layered silicon compound;
[0032] FIG. 3 is a view showing the crystal structure of a siloxene compound derived from CaSi2;
[0033] FIG. 4 is a view showing the crystal structure of a compound wherein acid molecules in the Si layered network structure of the siloxene compound shown in FIG. 3 are replaced with a surfactant;
[0034] FIG. 5 is an explanatory view showing a state wherein silicon atom layers peel off from the compound shown in FIG. 4;
[0035] FIG. 6 shows an X-ray diffraction pattern of a nanosheet solution obtained in Example 1;
[0036] FIG. 7 is a view showing the result of observing a silicon nanosheet obtained in Example 1 with an atomic force microscope (AFM);
[0037] FIG. 8 shows an electron diffraction pattern of a silicon nanosheet obtained in Example 1;
[0038] FIG. 9 shows a fluorescence spectrum of a nanosheet solution obtained in Example 1;
[0039] FIG. 10 is a perspective view showing the wholeness of a nanosheet-containing composite obtained in Example 3;
[0040] FIG. 11 is an enlarged partial view of a section of the nanosheet-containing composite shown in FIG. 10;
[0041] FIG. 12 shows a fluorescence spectrum of a nanosheet-containing composite obtained in Example 3;
[0042] FIG. 13 is a view showing the crystal structure of YbSi2 which is a kind of layered silicon compound;
[0043] FIG. 14 is a view showing the crystal structure of a silicon nanosheet according to a second embodiment of the present invention;
[0044] FIG. 15 shows an electron diffraction pattern of a silicon nanosheet obtained in Example 7;
[0045] FIG. 16 shows a TEM photograph of an end of a silicon nanosheet obtained in Example 7;
[0046] FIG. 17 shows a result of EDX analysis of a silicon nanosheet obtained in Example 7;
[0047] In FIG. 18, FIG. 18A is a view showing the crystal structure of a diamond and FIG. 18B is a view showing the crystal structure of a silicon nanosheet obtained in Example 7; and
[0048] FIG. 19 shows a result of UV-vis measurement of a silicon nanosheet obtained in Example 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0049] An embodiment according to the present invention is hereunder explained in detail.

[0050] A silicon nanosheet according to the present invention contains a silicon atom layer.

[0051] In the present invention, the term "a silicon atom layer" means a single atom layer formed by bonding two-dimensionally and periodically arranged silicon atoms to each other through an Si-Si bond.

[0052] Further, the term "two-dimensionally arranged silicon atoms" means that plural silicon atoms are arranged periodically in the directions of a and b axes (in the directions parallel with the layer plane) and substantially only one Si atom exists in the direction of c axis (in the direction perpendicular to the layer plane).

[0053] A silicon nanosheet according to the present invention comprises a single silicon atom layer or a layered product formed by laminating plural silicon atom layers.

[0054] In the case of a silicon nanosheet comprising a single silicon atom layer (hereunder referred to as "a single layered nanosheet"), the thickness thereof is about 0.5 to 1 nm. The thickness of a silicon atom layer somewhat varies in accordance with the structure thereof. For example, when silicon atoms are regularly arranged so as to form the same structure as a (111) plane in a diamond structure, the thickness of the single layered nanosheet is 0.6 to 0.8 nm.

[0055] A single layered nanosheet has a large size-anisotropy (the ratio of a width to a thickness). As a result, when a substrate or the like is coated with a silicon nanosheet for example, the shape of the surface of the substrate can be traced in a well reproducible manner. Further, since the area of the sheet is remarkably large in comparison with the thickness thereof, the surface of the substrate can efficiently be coated without largely changing the shape of the substrate surface.

[0056] In contrast, in the case of a silicon nanosheet comprising a layered product formed by laminating plural silicon atom layers (hereunder referred to as "a multilayered nanosheet"), the thickness thereof increases in proportion to the number of the silicon atom layers. By using a production process to be described later and optimizing the production conditions, it is possible to synthesize a silicon nanosheet about 10 to 20 nm in thickness.

[0057] Further, when a silicon nanosheet according to the present invention is used for a luminous element, an indicating element, or the like, a preferable thickness thereof is 10 nm or less. When the thickness of a silicon nanosheet exceeds 10 nm, the quantum size effect does not show up and hence it may undesirably be impossible to emit fluorescence with a specific excitation wavelength.

[0058] The width of a silicon nanosheet depends on the after-mentioned production conditions (for example, agitation strength at shaking in an exfoliation step, or the like). By using the after-mentioned process, a nanosheet 10 nm to 10 [mu]m in width can be obtained. In general, when the width of a silicon nanosheet is less than 10 nm, the nanosheet tends to reaggregate in a liquid. On the other hand, when the width of a silicon nanosheet exceeds 1 [mu]m, the nanosheet tends to precipitate. In order to suppress the reaggregation and precipitation of a nanosheet, a preferable width of the nanosheet is 50 to 500 nm.

[0059] A silicon nanosheet has various kinds of structures and compositions in accordance with the production conditions. Concretely there are following examples.

[0060] A first concrete example of a silicon nanosheet is a nanosheet comprising a silicon atom layer of "a diamond type."

[0061] Diamond has a structure wherein regular tetrahedrons each of which has carbon atoms in the center and at the apexes thereof are linked to each other in the manner of sharing the apexes (refer to FIG. 18A). The unit lattice of a diamond structure is configured by containing four such regular tetrahedrons in a cube and has eight atoms. The atoms are located at the positions of (a) 0, 0, 0 and face-centering translation and (b) 1/4, 1/4, 1/4 and face-centering translation.

[0062] When a (111) plane of such a diamond structure is observed from the <111> orientation, it looks like carbon six-membered rings are arranged periodically. Among the six atoms constituting a carbon six-membered ring, non-neighboring three atoms are located at the positions of 0, 0, 0 and face-centering translation (namely, on a (111) plane in the expression of Miller's indices). The other three atoms are located at the positions of 1/4, 1/4, 1/4 and face-centering translation (namely, on a (444) plane in the expression of Miller's indices). That is, a carbon six-membered ring in a diamond structure has a zigzag wavy structure.

[0063] The term "a silicon atom layer of a diamond type" means that the silicon atoms are regularly arranged so as to form the structure of a (111) plane of diamond. In other words, "a silicon atom layer of a diamond type" means that Si six-membered rings are arranged two-dimensionally and periodically and, among the six silicon atoms constituting each of the Si six-membered rings, the non-neighboring three silicon atoms are located on a plane corresponding to a (111) plane of Si having a diamond structure and the other three silicon atoms are located on a plane corresponding to a (444) plane of Si having a diamond structure.

[0064] A silicon atom layer of a diamond type has various kinds of compositions in accordance with the type of a starting material and production conditions.

[0065] For example, when CaSi2 is subjected to acid treatment with concentrated hydrochloric acid at room temperature and swelled infinitely with a surfactant, a nanosheet having a composition represented by the expression (a) below and containing a silicon atom layer of a diamond type can be obtained;
Si6H3-[delta](OH)3+[delta](0≤[delta]≤3) (a).

[0066] FIG. 1 shows the structure of a silicon nanosheet having a composition represented by the expression (a). In FIG. 1, a silicon nanosheet 1 is composed of a single silicon atom layer 15 of a diamond type. The silicon atom layer 15 has a zigzag wavy structure like As and P. Among the four couplings of a Si atom, three couplings are used for Si-Si bond and the remaining one coupling links to H or OH.

[0067] The surface of a silicon nanosheet having a composition represented by the expression (a) can be negatively electrified. By using the surface charge, it is possible to easily coat a substrate surface with the silicon nanosheet.

[0068] Otherwise for example, when CaSi2 is subjected to acid treatment with concentrated hydrochloric acid at around -30[deg.] C. and swelled infinitely with a surfactant, a nanosheet having a composition represented by the expression (b) below and containing a silicon atom layer of a diamond type can be obtained;
(SiH)n (b).

[0069] A silicon nanosheet having a composition represented by the expression (b) has the same structure as the silicon nanosheet 1 shown in FIG. 1 except that, in the case of the former silicon nanosheet, all of the couplings which do not contribute to the Si-Si bond link to H.

[0070] A second concrete example of a silicon nanosheet is a nanosheet comprising a silicon atom layer of "a graphite type."

[0071] Graphite has a structure wherein planes (the c planes) formed with carbon six-membered rings are laminated in the c axis direction. The term "a silicon atom layer of a graphite type" means that silicon atoms are regularly arranged so as to form a c plane structure of graphite or a structure similar thereto. In other words, "a-silicon atom layer of a graphite type" means that: Si six-membered rings are arranged two-dimensionally and periodically; and the distance in the c axis direction between the Si six-membered rings composed of the six silicon atoms (in the direction perpendicular to the layer plane of the silicon atom layer) is narrower than the distance between a (111) plane and a (444) plane of a diamond-structured Si. That is, "a silicon atom layer of a graphite type" means that the six silicon atoms constituting a Si six-membered ring: are located on an identical plane; or form a zigzag structure wherein the amplitude thereof in the c axis direction is smaller than that of a silicon atom layer of a diamond type.

[0072] By using the after-mentioned process, it is possible to obtain a nanosheet containing a silicon atom layer wherein the angle formed by the level plane and the direction of an adjacent Si atom is in the range of 0 to 100 when viewed from the direction of the a axis of the silicon atom layer (from the direction parallel with the layer plane).

[0073] A silicon atom layer of a graphite type has various kinds of compositions in accordance with the type of a starting material and production conditions.

[0074] For example, when YbSi2 is subjected to acid treatment under prescribed conditions and swelled infinitely with a surfactant, a nanosheet having a composition represented by the expression (c) below and containing a silicon atom layer of a graphite type can be obtained;
Si6H3-[delta](OH)3+[delta](0≤[delta]≤3) (c).

[0075] FIG. 14 shows the structure of a silicon nanosheet having a composition represented by the expression (c). In FIG. 14, a silicon nanosheet 7 is composed of a single silicon atom layer 75 of a graphite type. Among four couplings of a Si atom, three couplings are used for Si-Si bond and the remaining one coupling links to H or OH.

[0076] Meanwhile for example, when CaSi2 is subjected to hydrothermal treatment in an amine aqueous solution, a nanosheet having a composition represented by the expression (d) below and containing a silicon atom layer of a graphite type can be obtained;
SiOx (0≤x≤0.5) (d).

[0077] FIG. 18B shows the structure of a silicon nanosheet having a composition represented by the expression (d). Here, the structure of diamond type silicon is also shown in FIG. 18A. A silicon nanosheet having a composition represented by the expression (d) contains a planar silicon atom layer formed with Si six-membered rings. Further, oxygen is added to parts of the silicon atom layer.

[0078] A part of a silicon atom layer may be modified with an organic modified group. In each of such various kinds of silicon atom layers as stated above, a Si atom has a coupling which does not contribute to Si-Si bond. It is estimated that such a coupling links to H, OH, O, or the like. When all or some of such couplings are modified with an organic modified group (a substituent), it is possible to impart the functions of the organic modified group (for example, the functions as a chemical catalyst and the like) to a silicon nanosheet.

[0079] As organic modified groups for example, there are an alkyl group, an alkenyl group, an alkoxy group, a carboxyl group, an acyl group, a thiol group, a sulfo group, an amino group, and others.

[0080] The most remarkable point of a silicon nanosheet according to the present invention is that: the silicon nanosheet comprises a single silicon atom layer wherein silicon atoms are arranged in a nearly planar shape; or the silicon nanosheet is formed by aggregating plural silicon atom layers in each of which silicon atoms are arranged in a nearly planar shape. That is, a silicon nanosheet according to the present invention: is a novel one which comprises a silicon atom layer having two-dimensional anisotropy; and is different from a nanosheet comprising conventional transition metal oxide such as manganese oxide, titanium oxide, and the like. A silicon nanosheet is basically non-oxide, has two-dimensional anisotropy, and hence has such excellent characteristics as stated below.

[0081] Firstly, a silicon nanosheet shows a peak in the visible light region in the measurement of a fluorescence spectrum. When a silicon nanosheet is irradiated with excitation light having a specific excitation wavelength, it emits fluorescence having specific wavelengths. More specifically, a silicon nanosheet emits fluorescence having the wavelengths of 450 to 600 nm when an excitation wavelength in the range of 400 to 500 nm is applied. Further, by optimizing the thickness of a nanosheet, the nanosheet emits fluorescence composed of three kinds of light having the peak wavelengths of 465±5 nm, 505±5 nm, and 560±5 nm, respectively when an excitation wavelength of, for example, 400 nm is applied. Here, the term "peak" means an apex of a spectrum in fluorescence spectrum measurement.

[0082] All of those wavelengths are in the visible light region and can be observed as green fluorescence. By using this phenomenon, it is possible to use a silicon nanosheet or a nanosheet solution prepared by dispersing or suspending the silicon nanosheets in a solvent for a luminous element, an indicating material, or the like.

[0083] Secondly, in the case of a silicon nanosheet, the bandgap obtained through light absorption is 3.0 eV or more.

[0084] The bandgaps of major materials used as semiconductors are 1.1135 eV in the case of Si, 1.428 eV in the case of GaAs, 3.02 eV in the case of 4H-SiC, and 5.47 eV in the case of diamond. Bulk silicon shows only a three-dimensional structure of a diamond type at normal pressures and another kind of structure thereof has not been known. For that reason, the bandgap of conventional silicon has never exceeded the value of the bulk silicon (1.1135 eV).

[0085] In contrast, a silicon nanosheet shows a larger bandgap than bulk silicon. The bandgap tends to increase as the thickness of a nanosheet reduces. By using the after-mentioned process, a silicon nanosheet having a bandgap of 3.0 eV or more can be obtained. The detailed mechanism of obtaining a wide bandgap by forming a nanosheet is not obvious but it is estimated that the planar structure, oxygen contained in a small amount, and others may presumably influence the phenomenon.

[0086] When conventionally used silicon is replaced with a material having a large bandgap and a large dielectric breakdown electric field, it is possible to reduce the thickness of each layer of an element and also to apply high concentration doping. As a consequence, it is possible to make an element having a high withstand voltage and a small on resistance. That is, by using a silicon nanosheet according to the present invention, it is possible to get rid of the trade-off between withstand voltage and on resistance, and make a low-loss high-voltage power element.

[0087] Thirdly, a silicon nanosheet has large two-dimensional shape anisotropy. Thereby, it is possible to coat the outer and inner surfaces of various kinds of substrates with nanosheets. Further, by so doing, it is possible to impart the functions intrinsic to a nanosheet to a substrate.

[0088] Fourthly, a silicon nanosheet or an aggregate of the silicon nanosheets has an extremely high surface to volume ratio. Consequently, by making use of the high surface to volume ratio, it is possible to use a silicon nanosheet or an aggregate of the silicon nanosheets for various applications (for example, a photocatalyst, a solid lubricant, and the like).

[0089] Fifthly, a silicon nanosheet exhibits high activity in the event of chemical reaction. Thereby, it is possible to use a silicon nanosheet as, for example, a negative electrode active material of a lithium secondary battery. Further, the negative electrode active material comprising a silicon nanosheet can be a high capacity material, thereby can be an ultrathin membrane electrode with the use of a small amount, and hence has the characteristics of hardly causing the downward slip from the electrode and being excellent in cyclic performance.

[0090] Sixthly, a silicon nanosheet is substantially composed of Si and hence has a high thermal conductivity. Consequently, by complexing a silicon nanosheet with, for example, resin, it is possible to increase the heat dissipation of the resin.

[0091] A silicon nanosheet according to the present invention has such excellent characteristics as stated above and hence can be used as: an electronic material constituting a semiconductor integrated circuit, a thin-film transistor or the like; a luminous element; an indicating element; or the like.

[0092] Next, a nanosheet solution according to the present invention is explained. A nanosheet solution according to the present invention is prepared by dispersing or suspending silicon nanosheets according to the present invention in a solvent.

[0093] As the solvents in which silicon nanosheets are dispersed, polar solvents such as water, alcohol, glycol, ether and the like, and a solvent comprising the mixture of those can be used.

[0094] Further, in the silicon nanosheet solution, a surfactant or an amine caused by the after-mentioned production process is contained. A surfactant or an amine has the effects of: preventing silicon nanosheets dispersing in a solvent from aggregating; and maintaining a stable dispersed state.

[0095] The concentration of silicon nanosheets in a solution is not particularly limited and can arbitrarily be selected in accordance with the application of the solution. In general, when the concentration of silicon nanosheets is too low, the operation efficiency lowers in the use for various applications and the various functions of the nanosheets lower. On the other hand, when the concentration of silicon nanosheets is too high, the silicon nanosheets may undesirably aggregate in the solution. In order to stably disperse silicon nanosheets without the deterioration of operation efficiency and functions, the concentration of silicon nanosheets is preferably 0.1 to 10 wt %, and yet preferably 0.5 to 3.0 wt %.

[0096] In order to stably disperse silicon nanosheets in a solvent, it is preferable that the width of a silicon nanosheet is as small as possible. More specifically, when the width of a silicon nanosheet is 1 [mu]m or less, a stable colloidal solution is formed and so-called "a Tyndall phenomenon" shows up.

[0097] Next, a process for producing a nanosheet solution according to the present invention is explained.

[0098] A process for producing a nanosheet solution according to a first embodiment of the present invention includes an acid treatment step and an exfoliation step.

[0099] An acid treatment step is a step of bringing a layered silicon compound into contact with an aqueous acid solution to derive a siloxene compound.

[0100] Here, the term "a layered silicon compound" means one of a group of compounds represented by the chemical composition formula AxSi2 (A represents Ca and/or Yb, 0.8≤x≤1.2). A layered silicon compound has a structure wherein an A atom layer is sandwiched between layered network structures (silicon atom layers) of Si. The interlayer atom A may be either Ca or Yb, or both of Ca and Yb.

[0101] The term "a siloxene compound" means a compound obtained by bringing a layered silicon compound into contact with an aqueous acid solution, wherein acid molecules are substituted for all or some of the interlayer atoms A. Adjacent silicon atom layers are attracted to each other by the acid molecules interposed between the layers.

[0102] The term "an aqueous acid solution" means an aqueous solution containing acid such as hydrochloric acid or the like. As the solvent of an aqueous acid solution, besides water, a solvent comprising the mixture of water and alcohol such as ethanol, methanol, or the like can be used.

[0103] It is difficult to directly remove atoms (Ca or Yb) existing between layers of a layered network structure formed with Si. In the acid treatment step, therefore, acid molecules are substituted for interlayer atoms by subjecting a layered silicon compound to acid treatment and a siloxene compound is derived.

[0104] The type of acid used in the acid treatment step is not particularly limited but concentrated hydrochloric acid is particularly preferable. By using concentrated hydrochloric acid as an aqueous acid solution, it is possible to easily remove interlayer atoms without oxidizing the two-dimensional skeleton of Si.

[0105] Further, with regard to the amount of an aqueous acid solution, any amount is acceptable as long as the amount is not less than an amount which allows acid molecules to be substituted for interlayer atoms contained in a layered silicon compound. The optimum amount varies in accordance with the type of a layered silicon compound, the concentration of an aqueous acid solution, and others. Usually however, concentrated hydrochloric acid (12 N) is added by 100 ml per 1 g of a layered silicon compound.

[0106] When a layered silicon compound is subjected to acid treatment, the temperature of the aqueous acid solution influences the composition of a siloxene compound. For example, when CaSi2 which is a kind of layered silicon compound is processed with concentrated hydrochloric acid at around room temperature, a siloxene compound (Weiss type siloxene) represented by the chemical composition formula Si6H3-[delta](OH)3+[delta](0≤[delta]≤3) is obtained. In contrast for example, when CaSi2 is processed with concentrated hydrochloric acid at around -30[deg.] C., a siloxene compound (layered polysilane) represented by the chemical composition formula (SiH)n is obtained.

[0107] Further, in general, as the temperature of an aqueous acid solution rises, the two-dimensional skeleton of Si tends to be oxidized. In order to remove interlayer atoms with scarcely oxidizing the two-dimensional skeleton of Si constituting a layered silicon compound, it is preferable to lower the temperature of an aqueous acid solution as low as possible. More specifically, a desirable temperature of an aqueous acid solution is 0[deg.] C. or lower. In this case, it is possible to easily synthesize a siloxene compound scarcely containing impurities such as silica (SiO2) and the like.

[0108] With regard to the duration of acid treatment, any duration may be accepted as long as it is sufficiently long for substituting acid molecules for the interlayer atoms of a layered silicon compound. The optimum duration: varies somewhat in accordance with the concentration and temperature of an aqueous acid solution, the amount of a layered silicon compound added to the aqueous acid solution, and others; but usually is 1 to 3 days.

[0109] Further, in order to prevent the two-dimensional skeleton of Si from oxidizing, it is preferable to apply acid treatment in an inert atmosphere such as an atmosphere of argon, N2, or the like.

[0110] A siloxene compound obtained through the acid treatment step either may be subjected directly to the after-mentioned exfoliation step or may be washed further with an aqueous solution of acid such as hydrochloric acid or the like (a first washing step). When the siloxene compound is further washed with acid after the acid treatment, salt (for example, calcium salt) as a by-product can be removed and hence a nanosheet solution of a higher purity can be produced.

[0111] Further, the siloxene compound may be washed further with an organic solvent after washed with an aqueous acid solution (a second washing step). When the siloxene compound is further washed with an organic solvent, excessive acid such as hydrochloric acid used in the first washing step can be removed and hence a nanosheet solution of a far higher purity can be produced. As organic solvents used for washing, for example, there are acetone, ethanol, methanol, tetrahydrofuran, and others.

[0112] Further, in a siloxene compound obtained by subjecting a layered silicon compound to acid treatment, some of the Si atoms are hydrogenated and/or hydroxylated. When the siloxene compound is further dispersed in water and agitated at room temperature, it is possible to add oxygen to some of the Si atoms. Otherwise, when the siloxene compound is further refluxed in an aqueous solution containing iodide RI (R represents an alkyl group, an alkenyl group, an alkoxy group, a carboxyl group, an acyl group, a thiol group, a sulfo group, an amino group, or the like) including methyl iodide (CH3I) or the like, it is possible to add an organic modified group to some of the Si atoms. An oxidized, hydrogenated, or hydroxylated silicon nanosheet or a silicon nanosheet to which an organic modified group is added can be produced after the after-mentioned exfoliation step in accordance with the kinds of processes applied to a siloxene compound.

[0113] An exfoliation step is a step of adding a siloxene compound into a solvent containing a surfactant, shaking the mixture, and peeling off the siloxene compound. By so doing, the siloxene compound swells infinitely and a single layered or multilayered nanosheet wherein silicon atoms are arranged nearly planarly and periodically can be obtained.

[0114] As surfactants, there are an anionic surfactant, a cationic surfactant, a neutral surfactant, and others. Any of those surfactants may be used in the present invention.

[0115] As the anionic surfactants, there are, for example, sodium dodecyl sulfate (SDS), sodium perfluorooctanoate (SPFO), sodium alkylbenzenesulfonate such as sodium dodecylbenzenesulfonate or the like, sodium stearate, and others.

[0116] As the cationic surfactants, there are, for example, tetrabutyl ammonium hydroxide (TBAOH), tetramethylammonium ((CH3)4NOH), tetraethylammonium ((C2H5)4NOH), tetrapropyl ammonium ((C3H7)4NOH), n-ethylamine (C2H5NH2), n-propylamine (C3H7NH2), and others.

[0117] As the neutral surfactants, there are, for example, P-1,2,3 (block copolymer, HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH20)20H), and others.

[0118] With regard to a solvent to which a surfactant is added, any solvent may be used as long as it can dissolve the surfactant. As the concrete solvents, water, ethanol, ethylene glycol, and others can be used.

[0119] The concentration of a surfactant in a solution affects the thickness of a silicon nanosheet. A. surfactant has the functions of intruding between the layers of a siloxene compound, expanding the distance between the silicon atom layers, and facilitating the exfoliation. In general, as the concentration of a surfactant relatively increases, the surfactant tends to be substituted for all of the acid molecules and the constituent ions thereof intruding between the layers and hence it becomes easier to peel off the silicon atom layers and form single layers. On the other hand, when the concentration of a surfactant relatively decreases, a multilayered nanosheet tends to be obtained. Here, a multilayered nanosheet may sometimes be formed also by aggregating nanosheets which are once peeled off to form single layers in a solution.

[0120] The optimum concentration: varies in accordance with the structure and composition of a nanosheet to be produced, the type of the surfactant, and others; but is usually in the range of 0.01 to 1.0 mol/dm<3> .

[0121] With regard to the amount of a surfactant solution, any amount is acceptable as long as the amount is sufficient to efficiently insert the surfactant between the layers of a siloxene compound. The optimum amount varies in accordance with the concentration of a surfactant, the amount of the siloxene compound added to the solution, and others, but usually a surfactant solution containing a surfactant in the amount corresponding to the amount one to two times the siloxene compound by mol is added.

[0122] Further, it is preferable that a surfactant solution is acidic. In the case where a siloxene compound is added to a surfactant solution and shaken, when the pH of the solution increases, the two-dimensional skeleton of Si is oxidized and silica tends to form. In contrast, when the solution is acidic, the oxidation of the two-dimensional skeleton of Si can be suppressed. Specifically, the pH of a surfactant solution is preferably 5 or lower.

[0123] For example, when an anionic surfactant or a neutral surfactant is used as a surfactant, it is preferable to add acid such as hydrochloric acid, nitric acid, sulfuric acid, or acetic acid in a solution and adjust pH to 5 or lower. In particular, in the case of an anionic surfactant, a siloxene compound is easily peeled off and moreover silica is more easily and surely prevented from forming by adjusting the pH of a solution, and hence the anionic surfactant is particularly suitable for a surfactant used in the exfoliation step.

[0124] On the other hand, when a cationic surfactant is used as a surfactant, acid is formed during the reaction process and hence it sometimes happens that the exfoliation of silicon atom layers resultantly proceeds in an acidic region. In such a case, even when pH is not particularly adjusted in an acidic region, it is possible to peel off a siloxene compound without oxidizing the two-dimensional skeleton of Si.

[0125] After a siloxene compound is added to a surfactant solution, the solution is shaken mechanically or with supersonic waves. The strength and time of the shaking influence the thickness and width of a nanosheet. In general, as the strength of shaking increases and/or the time thereof also increases, the thickness of a nanosheet tends to reduce (namely, the probability of obtaining a single layered nanosheet increases) and/or the width of the nanosheet tends to reduce. The optimum shaking time varies in accordance with a shaking process, a shaking strength, and others, but usually is 3 to 7 days.

[0126] For example, in order to peel off a siloxene compound to a thickness of 10 nm or less, it is preferable to: add a surfactant solution of 100 to 1,000 mL having the surfactant concentration of 0.01 to 1.0 mol/dm<3 > to a siloxene compound of 1 g; and shake the solution for 3 to 7 days under the condition of 100 to 250 rpm.

[0127] Otherwise for example, in order to peel off a siloxene compound to form single layers, it is preferable to add a surfactant solution of 500 to 1,000 mL having the surfactant concentration of 0.5 to 0.8 mol/dm<3 > to a siloxene compound of 1 g; and shake the solution for 5 to 10 days under the condition of 100 to 250 rpm.

[0128] When a siloxene compound is added to a surfactant solution and the mixture is shaken under prescribed conditions, the siloxene compound peels off and a nanosheet solution containing silicon nanosheets is obtained. For example, when Weiss type siloxene Si6H3(OH)3 is added to a surfactant solution and the mixture is shaken, a suspended nanosheet solution of pale yellow-green color is obtained.

[0129] Here, it sometimes happens that solid materials (an unreacted siloxene compound, a coarse nanosheet, and others) may settle in a solution under some shaking conditions. In such a case, the solid materials are removed by centrifugal separation or the like. A nanosheet solution after solid materials have been removed is extremely stable and shows light scattering (a Tyndall phenomenon) intrinsic to colloid. Further, when a nanosheet solution is subjected to centrifugal separation, by optimizing the separation conditions, most of multilayered nanosheets are removed from the solution and a nanosheet solution having a high content of single layered nanosheets can be obtained.

[0130] The structure and composition of a silicon nanosheet contained in a thus obtained nanosheet solution vary in accordance with the kind of the layered silicon compound and the production conditions. For example, when CaSi2 is used as a layered silicon compound and subjected to acid treatment and exfoliation, a nanosheet containing a silicon atom layer of a diamond type is obtained.

[0131] A crystal structure of CaSi2 which is a kind of layered silicon compound is shown in FIG. 2. In FIG. 2, CaSi2 (a layered silicon compound 12) has a structure wherein a Ca atom layer 22 is sandwiched between layers of a layered network structure 21 of Si.

[0132] CaSi2 is one of typical Zintl phases and the formal charge thereof is represented by Ca<2+> (Si<-> )2. Here, Si<-> has an electron structure equivalent to phosphorus which is one of the fifth family elements in the periodic law of elements and forms a layered network structure similar to arsenic and black phosphorus. That is, CaSi2 is viewed as being a layered crystal wherein a Ca atom layer 22 is inserted between (111) planes of Si having a diamond structure (a layered network structure 21 of Si).

[0133] When such a layered silicon compound is processed with an aqueous acid solution, the interlayer atoms are removed and acid molecules intrude between the layers. The reaction caused when CaSi2 as a kind of layered silicon compound is subjected to acid treatment with an HCl aqueous solution is shown with the expression (1) below;
3CaSi2+7HCl->Si6H3(OH)3.HCl+3CaCl2+3H2 (1)

[0134] The crystal structure of a siloxene compound obtained through the reaction represented by the expression (1) is shown in FIG. 3. When CaSi2 is processed with an HCl aqueous solution, Ca between the layers is removed and Weiss type siloxene (Si6H3(OH)3.HCl) which is a kind of siloxene compound is derived. Weiss type siloxene (a siloxene compound 3) has a structure wherein HCl molecules are inserted between silicon atom layers 15 as shown in FIG. 3. The silicon atom layers 15 are attracted to each other through the HCl. The distance between the silicon atom layers 15 is about 6 Ȧ.

[0135] A siloxene compound obtained through acid treatment is: if needed, subjected to washing with an aqueous acid solution (a first washing step), washing with an organic solvent (a second washing step), oxidation, modification with an organic modified group, and others; and thereafter added to a surfactant solution of a prescribed concentration and a prescribed amount. When a siloxene compound is added to a surfactant solution, the surfactant intrudes between layers of the siloxene compound and the distance between the layers expands.

[0136] The crystal structure of a compound obtained by adding a siloxene compound 3 shown in FIG. 3 to a sodium dodecyl sulfate (SDS) solution is shown in FIG. 4. As shown in FIG. 4, when the siloxene compound 3 is added to a solution containing a surfactant (SDS) 4, HCl molecules or the constituent ions thereof inserted between silicon atom layers 15 are replaced with bulky guest molecules (SDS). As a result, the distance between the silicon atom layers 15 expands to about 100 Ȧ.

[0137] When the surfactant 4 is inserted between the silicon atom layers 15, the bonding force between the silicon atom layers 15 weakens. Thereby, when the solution is shaken, the constraining force between the silicon atom layers 15 is cut and the silicon atom layers 15 peel off as shown in FIG. 5. Then, by optimizing the conditions in the exfoliation step, it is possible to peel off the silicon atom layers 15 to form single layers. By further optimizing the conditions in the exfoliation step, it is possible to suppress the oxidation of a Si skeleton when the silicon atom layers 15 are peeled off. As a result, a silicon nanosheet substantially composed of Si is obtained.

[0138] The crystal structure of YbSi2 which is a kind of layered silicon compound is shown in FIG. 13. In FIG. 13, YbSi2 (a layered silicon compound 6) has a structure wherein a Yb atom layer 62 is sandwiched between layers of a layered network structure 61 of Si and forms an AlB2-type layered disilicide structure. In YbSi2, Si forms a flat layered network structure 61 similar to graphite.

[0139] When YbSi2 is used as a layered silicon compound too, a siloxene compound is derived through acid treatment likewise. Successively, when the obtained siloxene compound is added to a surfactant solution and the solution is shaken, a silicon nanosheet 7 containing a silicon atom layer 75 of a graphite type is obtained as shown in FIG. 14.

[0140] Silicon nanosheets according to the present invention are contained in a thus obtained nanosheet solution and hence, by making use of the characteristics, the nanosheet solution can be used for various kinds of applications as shown below.

[0141] (1) The silicon nanosheet, as stated above, emits fluorescence of specific wavelengths with a specific excitation wavelength. Hence, the nanosheet solution can be used as it is for a liquid fluorescent agent and the like.

[0142] (2) When a substrate surface is coated with the nanosheet solution or a substrate is dipped into the nanosheet solution, it is possible to coat the substrate surface with a nanosheet. Since a nanosheet solution in a colloidal state is excellent in film-formability and casting properties in particular, it is possible to uniformly coat a substrate surface with a nanosheet even when the substrate has a complicated shape.

[0143] (3) When a substrate surface coated with the nanosheet is heated under appropriate conditions, hydrogen, a hydroxyl and others are separated and a silicon crystal or a silicon membrane can be formed on the substrate surface. That is, by using the nanosheet solution, it is possible to obtain a silicon crystal or a silicon membrane with ease without employing a crushing means, a melting means, a sputtering means, a vapor deposition means, or the like. The obtained silicon crystal and membrane can be used for an electronic material for the production of: a semiconductor integrated circuit, a thin-film transistor, or the like; a luminous element; an indicating element; or the like.

[0144] (4) When a substrate surface is coated with the nanosheet and thereafter the coated substrate is heated in an oxidizing atmosphere, it is possible to uniformly coat the substrate surface with a very thin silica (SiO2) membrane.

[0145] (5) Whereas silicon obtained by a chemical vapor deposition process (a CVD process) or the like is generally spherical, a flaky silicon nanosheet is obtained by a production process according to the present invention. The nanosheet has a very high surface to volume ratio and exhibits high catalytic performance in the event of chemical reaction, and hence a nanosheet solution in which the nanosheets disperse can be used as various kinds of catalysts.

[0146] Next, a process for producing a nanosheet solution according to a second embodiment of the present invention is explained.

[0147] A process for producing a nanosheet solution according to the present embodiment includes a hydrothermal treatment step and a separation step.

[0148] The hydrothermal treatment step is a step of dispersing a layered silicon compound in a solvent comprising the mixture of an amine having 3 or more carbon atoms and water and applying hydrothermal treatment.

[0149] In the present invention, the term "an amine" means an organic compound having an amino group (-NH2) (a first amine). The amine may be an amine having one amino group (a univalent amine) or an amine having two or more amino groups (a multivalent amine). In particular, a univalent amine has the large effect of peeling off silicon atom layers to form single layers and hence is suitable for an amine used in a hydrothermal treatment step.

[0150] Further, it is preferable that an amine has 3 or more carbon atoms. Furthermore, it is preferable that an amine is a straight-chain type. When an amine of molecules having a relatively large number of carbon atoms and/or molecules of a straight-chain type (namely somewhat bulky molecules) is used, the amine is inserted between silicon atom layers, thereby the distance between the silicon atom layers expands, and the silicon atom layers is easily peeled off to form single layers.

[0151] As the concrete amines, there are propylamine (C3H7NH2), butylamine (C4H9NH2), pentylamine (C5H11NH2), hexylamine (C6H13NH2), peptylamine (C7H15NH2), octylamine (C8H17NH2), and others.

[0152] Here, with regard to a layered silicon compound, the production process thereof is the same as the production process according to the first embodiment and therefore the explanations are omitted.

[0153] With regard to the concentration of an amine contained in a mixed solvent, any concentration is acceptable as long as it is a concentration which allows the amine to be efficiently substituted for the interlayer atoms contained in a layered silicon compound. In general, as an amine concentration increases, the amine is more easily substituted for interlayer atoms. Meanwhile, when an amine concentration is too high, a lamella structure wherein a silicon layer is linked to the amine is undesirably formed.

[0154] In order to efficiently obtain a nanosheet, an amine concentration in a mixed solvent is preferably 10 to 30 vol %, and yet preferably 15 to 25 vol %.

[0155] With regard to the amount of a mixed solvent used in a hydrothermal treatment step, any amount is acceptable as long as it is an amount which allows the amine to be substituted for the interlayer atoms contained in a layered silicon compound. The optimum amount varies in accordance with the kind of a layered silicon compound, the concentration of a mixed solvent, and others, but is usually 50 to 200 mL per 1 g of a layered silicon compound.

[0156] A preferable hydrothermal treatment temperature is 120[deg.] C. to 180[deg.] C. When the hydrothermal treatment temperature is lower than 120[deg.] C., the exfoliation of silicon atom layers does not proceed within a practical time. On the other hand, when the hydrothermal treatment temperature exceeds 180[deg.] C., a sheet is likely to decompose.

[0157] With regard to hydrothermal treatment duration, the optimum duration is selected in accordance with a hydrothermal treatment temperature. In general, when hydrothermal treatment duration is short, the exfoliation of silicon atom layers is insufficient. On the other hand, superfluous hydrothermal treatment has no practical benefits. For example, when a hydrothermal treatment temperature is 120[deg.] C., a nanosheet can be obtained through hydrothermal treatment for 3 days or longer.

[0158] The separation step is a step of separating unreacted materials from a mixed solvent after hydrothermal treatment. By separating unreacted materials from a mixed solvent, a nanosheet solution according to the present invention is obtained. A process for separating unreacted materials is not particularly limited but centrifugal separation is preferable.

[0159] By using a process according to the present embodiment, a nanosheet solution containing silicon nanosheets having various widths or a nanosheet solution wherein single-layered nanosheets and multilayered nanosheets coexist is obtained. When the obtained nanosheet solution is subjected to centrifugal separation under appropriate conditions, coarse nanosheets are separated and a nanosheet solution of a colloidal state is obtained. Also, by optimizing separation conditions, a nanosheet solution containing a larger amount of single-layered nanosheets is obtained.

[0160] The detailed mechanism wherein a silicon nanosheet is obtained by the process according to the present embodiment is not obvious but it is estimated that the reason why a silicon nanosheet is obtained is that bulky amine molecules are substituted for the interlayer atoms of a layered silicon compound through hydrothermal treatment and the silicon atom layers swell infinitely. Further, it is estimated that the reason why a multilayered nanosheet is obtained by the process according to the present embodiment is that the nanosheets which have once been peeled off to form single layers by hydrothermal treatment are reaggregated in a solution.

[0161] Furthermore, in the process according to the present embodiment, by changing the kind of a used layered silicon compound and conditions, silicon nanosheets having various structures are obtained. For example, when CaSi2 is used as a layered silicon compound and subjected to hydrothermal treatment in a mixed solvent containing a straight-chain univalent amine, a nanosheet containing a silicon atom layer of a graphite type can be synthesized.

[0162] Next, a nanosheet-containing composite according to the present invention and a process for producing the composite are explained.

[0163] A nanosheet-containing composite according to the present invention is configured by having a silicon nanosheet according to the present invention on the surface and/or in the interior of a substrate.

[0164] Here, the term "a substrate" means a material other than a silicon nanosheet, and the kind and the shape of the material make no matter. That is, the kind of the substrate may be any of glass, ceramics, metal, resin and others. Further, the shape of the substrate may be any of a plate, a bar, a pipe, a sheet, a porous body, powder, and others.

[0165] The term "having a silicon nanosheet on the surface of a substrate" means that all or a part of the surface (including the inner surface) of a substrate is coated with a silicon nanosheet. The substrate surface may be coated with either a single-layered silicon nanosheet or a two-or-more-layered silicon nanosheet.

[0166] The term "having a silicon nanosheet in the interior of a substrate" means that silicon nanosheets disperse in the interior of a substrate. The nanosheets may uniformly disperse in an overall substrate or the content of nanosheets may vary according to the location.

[0167] A nanosheet-containing composite according to the present invention can be produced by a variety of processes. Specifically, there are the following processes.

[0168] A first process is a process of coating a substrate surface directly with a nanosheet solution or dipping a substrate directly into a nanosheet solution.

[0169] A nanosheet is likely to be charged negatively in a solution. In such a case, it is preferable to use a material which is charged positively in a solution (for example, polydialyldimethylammonium (PDDA), polyethyleneimine (PEI), or the like) as the substrate. When a material which is likely to be charged positively is used as the substrate, it is possible to easily form a nanosheet membrane on the substrate surface due to the electrical interaction between the substrate and the nanosheet solution merely by coating the substrate with a nanosheet solution or dipping the substrate into a nanosheet solution.

[0170] A second process is a process of making a substrate surface alternately adsorb a nanosheet and a material having an electric charge of a polarity opposite that of the nanosheet (hereunder referred to as "a second material") in a solution.

[0171] When a substrate is a material which has no electric charge or has electric charge of the same polarity as that of a nanosheet (for example, poly(sodium4-styrenesulfonate) (-CH2CH(C6H4SO3Na)n-) or the like) in a solution, a uniform nanosheet membrane is hardly formed even when the substrate surface is directly coated with a nanosheet solution. In contrast, it is possible to easily form a nanosheet membrane on the substrate surface due to the electrical interaction between the second material and the nanosheet solution by coating a substrate surface with the second material beforehand.

[0172] As stated above, a nanosheet is likely to be charged negatively in a solution. In such a case, a substrate surface is coated beforehand with a material which is positively charged in a solution. As concrete materials which are positively charged, there are cationic resins such as polydiallyl dimethyl ammoniumchloride (PDADMAC), polyethyleneimine (PEI), polyallylamine hydrochloride (PAH), and others. A substrate surface may be coated with one layer of the second material and one layer of the noanosheet in this order or the second material and the nanosheet may be adsorbed alternately to form plural layers.

[0173] Further, when a substrate is a material having electric charge of a polarity opposite that of a nanosheet in a solution, as stated above, a nanosheet membrane can be formed merely by coating the substrate surface with a nanosheet solution. However, the thickness of a nanosheet membrane is almost determined by the thickness of a nanosheet in a solution and it is difficult to form a thick membrane. In contrast, it is possible to increase the thickness of a nanosheet membrane nearly in proportion to the repetition rate of alternate adsorption without regard to the existence or the polarity of electric charge on the substrate surface by making a substrate surface adsorb a nanosheet and the second material alternately.

[0174] A third process is a process of mixing a nanosheet solution or nanosheets with a solution containing a substrate or melt of a substrate; and solidifying the mixture.

[0175] The third process does not particularly limit the material kind of a substrate but is suitable particularly for a process of dispersing nanosheets in the interior of a resin material. By using the process, it is possible to produce, for example, a resin film wherein silicon nanosheets disperse. A silicon nanosheet is excellent in thermal conductivity and hence a resin film wherein silicon nanosheets disperse has both electrical insulating properties and thermal conductivity. Consequently, it can be used as, for example, an electrical insulating material and the like having high heat dissipation in an electronic part and the like.

[0176] A nanosheet-containing composite obtained by the aforementioned process can be used for various applications as it is. Further, it may be subjected to heat treatment under prescribed conditions if the material of a substrate allows the heat treatment.

[0177] For example, by applying a nanosheet on the surface or in the interior of a substrate and thereafter heating the substrate at 800[deg.] C. to 1,000[deg.] C. in an inert atmosphere, it is possible to remove hydrogen and a hydroxyl contained in the silicon nanosheet. As a result, it is possible to form a crystal or a membrane substantially composed of only Si atoms on the surface or in the interior of the substrate.

[0178] Further for example, it is possible to form a crystal or a membrane of silica (SiO2) on the surface or in the interior of the substrate by applying a nanosheet on the surface or in the interior of a substrate and thereafter heating the substrate at 500[deg.] C. to 1,000[deg.] C. in an oxidizing atmosphere.

[0179] Next, a nanosheet aggregate according to the present invention and a process for producing it are explained.

[0180] A nanosheet aggregate according to the present invention is obtained by aggregating nanosheets according to the present invention.

[0181] In a nanosheet solution, silicon nanosheets are in the state of dispersing or suspending in a liquid (a solvent). As processes for aggregating the nanosheets, there are the following processes;

[0182] (1) a process of adjusting the pH of a nanosheet solution to 7 to 5 and aggregating nanosheets (so called "a sol-gel process"),

[0183] (2) a process of controlling the concentration of an electrolyte in a nanosheet solution in the range of 1 to 10 moles and aggregating nanosheets,

[0184] (3) a process of heating or freeze-drying a nanosheet solution and thereby removing a solvent in the solution, and others.

[0185] Since a silicon nanosheet has a very large two-dimensional anisotropy, a nanosheet aggregate obtained by aggregating the silicon nanosheets comprises fine particles (0.1 to 3 [mu]m in average diameter) and has a high surface to volume ratio (50 to 200 m<2> /g). Consequently, a nanosheet aggregate can be used as various kinds of carriers, an absorbent, and others. Further, by using a nanosheet aggregate as a negative electrode active material of a lithium secondary battery, it is possible to configure the lithium secondary battery having a large capacity and being excellent in cyclic performance.

EXAMPLES

Example 1 1. Derivation of Weiss Type Siloxene

[0186] A nanosheet solution was produced by using CaSi2 (refer to FIG. 2) as a layered silicon compound. Firstly, CaSi2 powder was brought into contact with concentrated hydrochloric acid (12 N) at a rate of 100 cm<3 > of the latter to 1 g of the former and was reacted at 0[deg.] C. in an argon atmosphere (an acid treatment step). After the reaction for 8 hours, Weiss type siloxene (Si6H3(OH)3.HCl, refer to FIG. 3) of yellowish green was obtained.

[0187] Successively, the powder was filtered and thereafter washed with concentrated hydrochloric acid cooled to 0[deg.] C. in an argon atmosphere, and thus CaCl2 as a by-product was removed (a first washing step). Thereafter, the powder was further washed with acetone (a second washing step). 2. Exfoliation of Weiss Type Siloxene

[0188] Next, Weiss type siloxene of 0.2 g was added to a sodium dodecyl sulfate (SDS) aqueous solution having the concentration of 0.1 mol/dm<3 > and a pH of 5 or less which was adjusted with hydrochloric acid. By so doing, the HCl having intruded between silicon atom layers of the Weiss type siloxene was replaced with the SDS (the surfactant) and the distance between the silicon atom layers expanded from about 6 Ȧ to about 100 Ȧ (refer to FIGS. 3 and 4).

[0189] Successively, shaking was applied at the rate of about 100 rpm for 10 days with a shaker (made by As One Corporation) (an exfoliation step) and the Weiss type siloxene containing the SDS was peeled off to a thickness of 10 nm or less, namely a thickness nearly equal to the thickness of a single layered silicon atom layer (refer to FIG. 5). Thereby, a nanosheet solution wherein silicon nanosheets 1 having the structure shown in FIG. 1 dispersed was obtained.

[0190] In the present example, since SDS which was an anionic surfactant was used as a surfactant in particular, it was possible to replace Cl<-> ions among the HCl molecules which had intruded between silicon atom layers of a siloxene compound with the surfactant. Then, since the Cl<-> ions were replaced with the anionic surfactant and shaking was applied under strong acidic conditions of pH 5 or lower, it was possible to easily peel off the siloxene compound to a thickness of 10 nm or less. 3. Evaluation

[0191] It was observed that, when the nanosheet solution was put into a beaker and irradiated with a light from one side, a Tyndall phenomenon intrinsic to a colloidal solution appeared. Therefore, it was understood that the nanosheet solution obtained in the present example was a colloidal solution.

[0192] Further, the nanosheet solution was subjected to X ray diffractometry and the distance between silicon atom layers was measured. The result is shown in FIG. 6. From FIG. 6, it was understood that most of the silicon atom layers dispersed in the solvent with a space of 100 Ȧ or more, namely most of the silicon nanosheets were composed of single layered silicon atom layers.

[0193] Next, the nanosheet solution was dropped on a mica, dried, and thereafter observed with an atomic force microscope (AFM) (D3100, made by Veeco Instruments Inc.). The result is shown in FIG. 7. From FIG. 7, it was understood that the thickness of the nanosheet was 0.7 to 0.8 nm and the width thereof was around 100 nm.

[0194] Next, the nanosheet solution was dropped on a copper mesh for observation with a transmission electron microscope (TEM) (made by JEOL Ltd.), dried, and thereafter observed with the TEM. As a result, it was found that the concentration was uniform in a silicon nanosheet and the thickness of the silicon nanosheet was very uniform.

[0195] Next, an electron diffraction pattern of the silicon nanosheet was measured with a TEM apparatus (made by JEOL Ltd.). The result is shown in FIG. 8.

[0196] As shown in FIG. 8, it was confirmed that the silicon nanosheet showed a diffraction pattern which was formed when a silicon nanosheet was irradiated with a beam from the (111) direction of a crystal having a face centered cubic (FCC) lattice structure. That is, it was understood that the silicon nanosheet according to the present example was a nanosheet obtained by peeling off a layered silicon compound comprising CaSi2 which was used as the starting material to form single layers (monomolecular layers) in the state where the (111) plane of Si in the layered silicon compound was maintained as it was.

[0197] Here, in FIG. 8, the white dots show the (220) planes of a face centered cubic (FCC) lattice and, since the dots are hardly visible in the figure, the dots are indicated with the arrows for convenience. As shown in FIG. 8, the dots were observed at the positions of the apexes of a nearly regular hexagon.

[0198] In the present example as stated above, a silicon nanosheet having an uniform thickness of 0.7 to 0.8 nm and comprising the same structure as the (111) plane of silicon, wherein the interior of the plane was composed of a monocrystal, could be obtained. In the obtained silicon nanosheet, as shown in FIG. 1, a part of the hydrogen of Weiss type siloxene (Si6H3(OH)3.HCl) was hydroxylated. Taking it into consideration that a small amount of gas was generated in the exfoliation step, it was estimated that the hydroxylation was caused by such reaction between the Weiss type siloxene and water in the exfoliation step as represented by, for example, the expression (2) below,
Si6H3(OH)3+[delta]H2O->Si6H3-[delta](OH)3+[delta]+[delta]H2 (0≤[delta]≤3) (2).

[0199] Next, the fluorescence spectrum of the nanosheet solution was measured with a fluorescence spectrophotometer (FP-6600, made by JASCO Corporation). A fluorescence spectrum when the excitation wavelength was set at 400 nm and the solid concentration (the concentration of silicon nanosheets) was set at 0.5% by weight is shown in FIG. 9. In FIG. 9, the horizontal axis represents a wavelength (nm) and the vertical axis represents a strength. Further, in FIG. 9, the measurement result of the fluorescence spectrum is shown with the solid line and the waveforms obtained by the waveform separation of the measurement result (the solid line) are shown with the dotted lines.

[0200] From FIG. 9, it was understood that:
(1) when a silicon nanosheet according to the present example was irradiated with light of the excitation wavelength of 400 nm, a fluorescence having the wavelengths of 450 to 600 nm was emitted; and
(2) the fluorescence spectrum could be separated into three types of waveforms having the peak wavelengths of 465±5 nm, 505±5 nm, and 560±5 nm, respectively.

[0203] Further, although it is not shown in the figure, when the concentration of the solid content in a nanosheet solution was diluted to 0.05% by weight, the strength of the peak of the waveform having the peak wavelength of 465±5 nm increased. The increase of the peak strength can be explained as the quantum size effect of the silicon nanosheet.

[0204] As stated above, a silicon nanosheet according to the present example emits a green fluorescence comprising three types of lights having peaks in a visible light region. By making use of the phenomenon, such a silicon nanosheet and a nanosheet solution wherein the silicon nanosheets are dispersed or suspended can be used for an indicating material and others.

Example 2

[0205] Following the same procedure as described in the item 1 of Example 1, Weiss type siloxene (Si6H3(OH)3.HCl) was produced. Successively, the Weiss type siloxene of 0.2 g was added to a tetrabutyl ammonium (TBAOH, a cationic surfactant) aqueous solution. In this case, the concentration of the tetrabutyl ammonium aqueous solution was set at the same mol concentration as the Cl in the Weiss type siloxene (Si6H3(OH)3.HCl), namely 0.1 mol/dm<3> . The solution was shaken for 10 days at the rate of about 100 rpm with a shaker (made by As One Corporation) and resultantly a nanosheet solution was obtained.

[0206] In the case of the nanosheet solution obtained in the present example too, in the same way as Example 1, it was confirmed that silicon nanosheets nearly comprising single layered silicon atom layers dispersed in a solvent and had a peak in the visible light region in the measurement of a fluorescence spectrum.

Example 3 1. Production of a Nanosheet-Containing Composite

[0207] Following the procedure below, as shown in FIGS. 10 and 11, a nanosheet-containing composite 5 wherein three resin layers 53 and three nanosheet layers 55 were laminated alternately on the surface of a substrate 51 was produced.

[0208] Firstly, a quartz glass substrate 20 mm*20 mm in area and 2 mm in thickness was prepared as the substrate. Further, a nanosheet solution prepared in Example 1 and a polymer solution containing PDADMAC and NaCl were prepared. In the polymer solution, the concentration of the PDADMAC was 1 mg/mL and the concentration of the NaCl was 0.5 M. Furthermore, water was prepared for washing.

[0209] Successively, the glass substrate was immersed in the polymer solution and dried, and thus a resin layer comprising the PDADMAC was formed on the surface of the glass substrate (a resin layer forming step). The glass substrate was washed with water and dried (a washing step). Thereafter, the glass substrate was immersed in the nanosheet solution and dried again, and thereby a nanosheet layer comprising a silicon nanosheet was formed on the resin layer (a nanosheet layer forming step). After the glass substrate was washed again with water, the resin layer forming step, the washing step, and the nanosheet forming step were further applied repeatedly. 2. Evaluation

[0210] The fluorescence spectrum of the obtained nanosheet-containing composite was measured with a fluorescence spectrophotometer (FP-6600, made by JASCO Corporation). The fluorescence spectrum obtained when the excitation wavelength was set at 450 nm is shown in FIG. 12. As shown in FIG. 12, in the case of the nanosheet-containing composite of the present example, the peak was observed at the wavelength of 540±5 nm. That is, it was found that the nanosheet-containing composite of the present example emitted fluorescence in the visible light range.

[0211] Further, when the nanosheet-containing composite was subjected to heat treatment at 800[deg.] C. to 1,000[deg.] C. in a vacuum (10<-5 > Pa or lower), hydrogen and a hydroxyl in the silicon nanosheets were removed and a silicon membrane could be formed on the glass substrate.

Example 4

[0212] A nanosheet solution produced in Example 1 was freeze-dried and thus a nanosheet aggregate was obtained. Specifically, a nanosheet solution adjusted to 0.5% by weight was frozen with liquid nitrogen and, by sucking the nanosheet solution with a rotary pump, the solvent was removed gradually. The nanosheet aggregate comprised fine particles (0.1 to 0.3 [mu]m) and had a high surface to volume ratio (50 to 200 m<2> /g).

Example 5

[0213] A nanosheet solution was produced following the same procedure as Example 1 except that YbSi2 (refer to FIG. 13) was used as the layered silicon compound.

[0214] It was confirmed that, in the obtained nanosheet solution, silicon nanosheets 7 each of which contained a silicon atom layer 75 having a nearly flat structure as shown in FIG. 14 dispersed in the solvent.

Example 6

[0215] Following the same procedure as described in the item 1 of Example 1, a nanosheet solution was produced.

[0216] Successively, a siloxene compound of 0.2 g was added to an ethanol solution of P-1,2,3 (block copolymer, HO(CH2CH2O)20(CH2CH(CH3)O)70(CH2CH2O)20H) the concentration of which was 0.01 mol/dm<3> , and shaken hard (an exfoliation step). As a result, a dark brown suspension wherein silicon nanosheets dispersed was obtained.

[0217] When the suspension was put in a beaker and irradiated with light from one side, a Tyndall phenomenon intrinsic to a colloidal solution appeared. More specifically, it was understood that the suspension was a colloidal solution wherein silicon nanosheets suspended.

[0218] As stated above, it was understood that, even when a neutral surfactant was used as a surfactant, it was possible to produce a colloidal solution wherein silicon nanosheets dispersed.

Example 7 1. Production of a Nanosheet Solution

[0219] CaSi2 (0.1 g) was dispersed in a solvent comprising the mixture of propylamine (2 mL) and distilled water (8 mL). The mixture was subjected to hydrothermal treatment at 120[deg.] C. for 3 days in a beaker made of tetrafluoroethylene. Thereafter, unreacted materials were centrifugally separated at the number of revolutions of 10,000 rpm and the supernatant liquid was fractionated. 2. Evaluation of a Structure

[0220] FIG. 15 shows an electron diffraction pattern on the planes of a silicon nanosheet contained in the obtained nanosheet solution. From FIG. 15, it was understood that the spots appeared in the form of a regular hexagon and had hexagonally rotational symmetry. The obtained spots appeared at the positions of 0.215 nm and 0.124 nm and were indexed as (220) and (440), respectively. Assuming that the diffraction spots were based on the ≤111> incidence, the spacing of lattice planes was about 10% larger in comparison with the values of bulk Si (0.192 nm in the case of Si (220) and 0.096 nm in the case of Si (440)).

[0221] Further, an end of a silicon nanosheet sometimes uprose and the shape of the lamination of the sheets was precisely observed from the end portion. A TEM photograph of an end of a silicon nanosheet is shown in FIG. 16. From FIG. 16, it was understood that the observed nanosheet had a layered structure comprising 20 to 30 layers. The distance between layers was 0.30 nm in the actual measurement.

[0222] A result of EDX analysis of a silicon nanosheet is shown in FIG. 17. In FIG. 17, the counts of C and Cu were caused by the grid during the TEM observation. From FIG. 17, it was understood that, besides Si, oxygen was included in the nanosheet.

[0223] From the above results, the silicon nanosheet obtained in the present example could be indexed on the basis of a hexagonal system, and the lattice constants were determined as a=0.43 nm and c=0.30 nm. On the other hand, CaSi2 as the starting material was indexed as a=0.38 nm and c=0.31 nm on the basis of the hexagonal system. This showed that the error was too large to regard the silicon nanosheet obtained in the present example as comprising conventional sp<3 > orbits. Then assuming that the silicon nanosheet comprised sp<2 > obits like graphite (a planar structure) and the distance of Si-Si bond was 0.25 nm, the lattice constants of the hexagonal system were a=0.43 nm and c=0.30 nm and the result well coincided with the actual measurement. Taking the result together with the results of FIGS. 15 and 16 into consideration, it was estimated that the silicon nanosheet obtained in the present example had a structure formed by laminating silicon atom layers each of which had a planar structure as shown in FIG. 18B. 3. Evaluation of Photophysical Properties

[0224] A nanosheet solution containing silicon nanosheets by 1% by weight was subjected to UV-vis measurement. The result is shown in FIG. 19. From FIG. 19, in the case of the silicon nanosheet obtained in the present example, the bandgap obtained from the absorption edge of the rising portion was 3.6 eV. The value meant the expression of quantum effect. More specifically, the value supported the fact that the nanosheets dispersing in the solution were nanosized silicon of 1 nm or less.

[0225] In the above explanations, the embodiments according to the present invention have been described in detail. However, the present invention is not limited to the above embodiments at all and various modifications are also included in the present invention in the range not deviating from the tenor of the present invention.

JPH0964417
MANUFACTURE OF SILOXENE COMPOUND THIN FILM

NISHIMURA KOUSUKE; NAGAO YASUYUKI +
Applicant(s):    KOKUSAI DENSHIN DENWA CO LTD +

PROBLEM TO BE SOLVED: To form a siloxene compound thin film that the composition distribution in a film thickness direction is uniform with the silicide of calcium used as a main material on a silicon substrate by using a depositing method using calcium disilicide and silicon as evaporation sources, a quick heating method, and a chemically treating method of hydrochloric acid. SOLUTION: One of calcium disilicide (CaSi2 ), calcium silicide (CaSi) or dicalcium silicide (Ca2 Si) as the silicide of calcium and Si are used as evaporation sources, two or more layers of CaSix (0<x<2) thin film and an Si thin film are alternately laminated in vacuum on a silicon (Si) substrate 111 by a depositing method. The CaSix /Si multilayer thin film deposited Si substrate is heated in vacuum or in inert atmosphere to form a CaSi2 single crystal thin film on the substrate 111. Then, the CaSi2 single crystal thin film laminated layer Si substrate is treated with hydrochloric acid to form a siloxene compound thin film on the substrate 111.

US2912308
Improvements in the preparation of mono-silane

Siloxene Si6 O3 H6 is prepared as a grey powder by reacting a metal disilicide with an aqueous-alcoholic solution of mineral acid. Silicides specified are calcium, barium, and strontium silicide. Acids specified are hydrochloric, sulphuric, and acetic acid. Alcohols specified are methyl, ethyl, n- or iso-propyl, or n-, iso- or tert-butyl alcohol. 90-97% of alcohol may be present in the reaction mixture. Acidic hydrogen to metal ratios of 2-18 to 1 are specified. The temperature of reaction may be 0-95 DEG C. but is usually room temperature. The reaction mixture may be allowed to stand for 17 hours. The siloxene may be separated by filtration, washed with ethanol, and vacuum dried at 60 DEG C. Monosilane is produced by reacting siloxene, preferably prepared as described above, with liquid or gaseous ammonia, in a non-oxidizing atmosphere and at a temperature of at least -33 DEG C. Argon or helium may be employed as a carrier for gaseous ammonia which may be at a temperature of 350 DEG C. The system may be evacuated, or an inert gas atmosphere employed, when using liquid ammonia. In order to use liquid ammonia, increased pressure may be employed and/or up to 30% of ammonium chloride, bromide, or iodide may be added to the reaction mixture. The monosilane which is evolved together with ammonia and hydrogen may be condensed together with the ammonia in a liquid
EXAMPLE I 125

Siloxene was prepared by adding 6 7 grams of calcium disilicide ( 0 05 mol) to a solution of 41.5 milliliters of concentrated hydrochloric acid ( 0 50 mol) in 1000 milliliters of ethanol.

This corresponds to an acidic hydrogen to 130 831,289 silane which comprises reacting siloxene with ammonia in a non-oxidising atmosphere at a 60 temperature of at least -33 C and recovering the resulting gaseous mono-silane from the reaction mixture.