COMPLETE SPECIFICATION

Novel catalyst-free method for the production of Si-Ge and C-Si-Ge nanotubes by CVD method Field of Invention:

The present invention relates to the field of nanotechnology and material science wherein the novel catalyst-free Silicon-Germanium nanotubes and Carbon-Silicon-Germanium nanotubes are produced by two zone CVD furnace.

Background of the Invention:

One-dimensional (ID) nanostructures such as nanotubes, nanowires and nanobelts play a crucial role as both interconnects and functional components in future electronic and optical devices basically associated with their low dimensionality and the quantum confinement effect. Among the elements in group 14, carbon (C), silicon (Si) and germanium (Ge) have been mostly used for the development of advanced nanoelectronic devices. C, Si and Ge nanostructures exhibit significant differences in physical and chemical properties from their bulk, which have been exploited to serve as a building block for advanced logic devices and as sensors to detect biological and chemical reagents. C, Si and Ge nanowires (NWs) have been synthesized by various techniques such as: chemical vapor deposition (CVD), laser ablation, molecular beam epitaxy, solution growth and catalytic etching. Si NWs have been reported to grown via vapor-liquid-solid (VLS) using liquid alloy catalysts (Au, Ag, Ga, Au-Ga, and In) and via the vapour-solid-solid (VSS) process using solid alloy catalysts (Ti, Al, Ni, Cu, and Pd).

The major drawbacks of the existing state-of-art are ustilization of catalysts like gold for the synthesis of Si,Ge and Si-Ge nanowires, wherein these catalysts acts as an impurities and also creates tapering effect at the tip of Si-Ge hetrostructure. This approach limits the growth of long wires with urn length, hinders the performance of the material in electronic applications. Another disadvantage is that Ge and Si nanowires are reported to get easily oxidized upon exposure to ambient conditions which subsequently affects the properties of the materials. The present invention reports metal-free synthesis of high-density single-crystal Si-Ge and C-Si-Ge nanotubes with tubular morphology by CVD. This approach avoids the tapering effect by catalyst free synthesis method. Si-Ge and C-Si-Ge nanotubes were synthesized by nucleation on nanocrystalline Ge seeds and subsequent one-dimensional anisotropic growth without using external catalyst. Systematic control of the diameters with tight distribution and tunable doping concentration were realized by adjusting the growth conditions, such as growth temperature and ratio of precursor partial pressures. This growth approach offers a method to eliminate potential metal catalyst contamination and open new avenues for the application of Si-Ge and C-Si-Ge nanotubes in electronic and photonic devices.

Brief description of the prior art:

No prior art such as closely related to the present one is already available. U.S. Patent No 20110262341 Al describes about the catalyst free production of carbon nanotubes by the discharge method, whereas the present invention relates to the catalyst free production of Carbon-Silicon-Germanium and Silicon -Germanium nanotubes. CN101270470B relates to the synthesizing of non metal catalyst self organizing growth carbon nano tube with CVD wherein Carbon Nanotubes are grown whereas in the present invention Silicon-Germanium and Carbon-Silicon-Germanium nanotubes are synthesized without using any metal catalyst in CVD U.S. Patent No 6843850B2 relates to the growth of single walled carbon nanotubes using SiC substrates at high temperature whereas C-Si-Ge and Si-Ge are grown without any metal catalyst. Phenyltrimethylgermane and Trimethylphenylsilane are used as the precursor for Germanium, Silicon and carbon. U.S. Patent No 6958253B2 describes about process of deposition of Si-Ge films using silanes germanium precursor whereas in the present invention Si-Ge and C- Si-Ge nanotubes are grown using Phenyltrimethylgermane and Trimethylphenylsilaneinvention as the precursors.

Objective of the Invention:

1) The main objective of present invention is the single step method for the production of Si-Ge and C-Si-Ge nanotubes using Chemical vapour deposition method.

2) The secondary objective the present invention is to synthesize the Si-Ge and C-Si-Ge nanotubes without utilizing any metal catalysts.

3) The third objective of the present invention is to produce Si-Ge and C-Si-Ge nanotubes with crystalline and tubular structure.

4) The fourth objective of the present invention is use of precursor phenyltrimethylgermane (PTMG): trimethylphenylsilane (TMPS) in the ratio 1:1 is used for growing the Si-Ge nanotubes.

5) The final objective of the present invention is use of precursor phenyltrimethylgermane (PTMG): trimethylphenylsilane (TMPS) in the ratio 1:4 is used for growing the C-Si-Ge nanotubes.

Summary of the invention:

The present invention is about a preparation of Si-Ge and C-Si-Ge nanotubes which is achieved by CVD method without employing any metal catalysts. Silicon-Germanium Nanotube The process temperature of 800 °C and feed ratio of phenyltrimethylgermane (PTMG) and trimethylphenylsilane (TMPS) (1:1) was indicated as critical parameters for successful growth of Si-Ge nanotubes . The Si-Ge nanotubes with tailored dimensions of 45 nm (outer diameter), 12 nm (inner diameter) and several um lengths were produced. The peaks present in the PL spectra confirm the photon energy band of nanotubes at 2.9 eV. Carbon-Silicon-Germanium Nanotube The process temperature of 800 °C and feed ratio of PTMG and TMPS (1:4) was indicated as critical parameters for successful growth of C-Si-Ge nanotubes. The C-Si-Ge nanotubes exhibited tube dimension of 45 nm (outer diameter), 10 nm (inner diameter) and several \im lengths. The peaks present in the PL spectra confirm the photon energy band of nanotubes in 3.2 eV. These results may offer new possibilities for the application of Si-Ge nanotube and C-Si-Ge nanotubes in electronic and photonic devices. Brief Description of the Drawings:

1. X-Ray diffraction pattern of Si-Ge nanotubes
2. SEM images of Si-Ge nanotubes.
3. EDAX spectrum of Si-Ge nanotubes
4. (a) and (b) SEM images of C-Si-Ge nanotubes using PTMG and TMPS
5. EDAX spectrum of C-Si-Ge nanotubes.
6. TEM images of Si-Ge nanotubes.
7. HRTEM images of Si-Ge nanotubes with tubular morphology.
8. TEM images of C-Si-Ge nanotubes.
9. (a) & (b) HRTEM images of C-Si-Ge nanotubes with tubular morphology.
10. Raman spectrum of Si-Ge nanotubes (a) with optical-phonon modes associated with local Ge-Ge, Ge-Si, Si-Si vibrations and (b) with peaks confirming the tubular structure.
11. Raman spectrum of C-Si-Ge nanotubes.
12. Photo Luminescence (PL) spectrum (a) Si-Ge nanotube and (b) C- Si-Ge nanotubes

Detailed description of the invention:

The single step method for the production of Si-Ge and C-Si-Ge nanotubes can be broadly defined with following steps:
1. Performing chemical vapour deposition (CVD) using precursors which comprises of C, Si and Ge.
2. Production of Si-Ge and C-Si-Ge nanotubes without the aid of any metal catalyst.
3. Production of Si-Ge and C-Si-Ge nanotubes with crystalline and tubular structure.

The Si, Ge and C precursor has a general formula Ge R(4-X)LX / Si R<4-x)Lx where x=l,2,or 3; R may consists of alkyl, cycloalkyl or aryl group and L=Hydrogen , halide or alkoxide. In a particular useful embodiment the general formula for combined silicon-carbon and germanium-carbon source is SiR4 and GeR4 respectively. Where R = alkyl, cycloalkyl or aryl group. In the present invention the combined sources utilized for silicon-carbon and germanium-carbon consists of Trimethylphenylsilane and Phenyltrimethylgermane respectively. The method adopted for the production of Si-Ge nanotube and C-Si-Ge nanotubes specifically involves the following steps :

1. the dissolving of combined sources of Si, Ge and C without using any solvents/catalysts to form the feed solution

2. dispersing the feed solution in a stream of carrier gases

3. introducing the feed solution along with the carrier gas at the inlet of the reactor

4. decomposition of the precursor into Si, Ge and C at their volatization temperature in the lower temperature zone of the reactor

5. growth of Si-Ge nanotube and C-Si-Ge nanotubes in the high temperature zone of the reactor.

The most preferably quantity of feed stock solution for the growth of Si-Ge nanotube comprises of combined sources of germanium to silicon in the ratio of 1:1. The same feed stock solution for the growth of C-Si-Ge nanotube comprises of combined sources of germanium-carbon to silicon-carbon in the ratio of 1:4. The carrier gas may be selected from the group of inert gases (nitrogen, argon, helium), hydrogen and mixtures thereof. Among the total mixture of gases, hydrogen with 5-30% and rest argon is desirable. The temperature in the preheat zone of the precursor is maintained at 200 to 300 °C and the reaction temperature preferred is 700 to 900 °C.

The said Silicon, Germanium and Carbon precursor has a general formula Ge R<4-X)W Si R(4-X)LX where x = 1 ,2, or 3; R may be of alkyl, cycloalkyl or aryl group and L=Hydrogen ,halide or alkoxide. For the purpose a particular useful personification in the present invention, radical R is selected from the following radicals: phenol, methyl, ethyl, vinyl, n-propyl, iso-propyl, allyl, n-butyl, terta- butyl, cyclopentadienyl and mixtures thereof. The product obtained from the reaction has various characteristics. The Si-Ge nanotubes have specific characteristics, that it consists of 20.04% Si and 79.96% Ge (atom %). The C-Si-Ge nanotubes have specific characteristics, that it consists of 14.80% Si, 27.98% Ge and rest carbon (atom %). The products existed 100% crystalline without any traces of amorphous elements of the constituents. The morphology of obtained product with tubular structure and micrometer length are particularly desirable. The nanotube has diameter in the range of 20 to 100 nm and length several urn. The purity of the product was obtained without performing any purification steps. These crystalline nanotubes find application in the field of nanpelectronics but not limited to nanophotonics, sensors, optics, optoelectronics and batteries. The following example is provided to demonstrate the observations in the invention.

Example

Preparation of Si-Ge nanotubes and C- Si-Ge nanotubes The optimized growth of Si-Ge nanotubes and C-Si-Ge nanotubes was performed in a two-zone CVD furnace. The reactor consisted of quartz tube of 100 mm diameter and a quartz boat was inserted at the reaction zone for additional deposition surface. Neat Phenyltrimethylgermane (PTMG) and Trimethylphenylsilane (TMPS) with volume ratio 0:1,1:0,1:1,1:2,1:3,1:4,1:5 were injected into the preheat zone of the reactor at a rate of 1 ml/h. After volatilization in the preheat zone (~250°C), the precursors was carried into the reaction zone of the furnace, maintained at 700-900 °C, by a 10% I-b/Ar sweep gas (750 cm3/min flow rate). The CVD runs were carried out for 2 hours duration, after which the material deposited on the quartz boat and walls of quartz tube was collected for analysis.

Characterizations

The X-ray diffractograms (XRD) of the Si-Ge nanotubes (1) were obtained from a PANalytical XTert using Ni-filtered Cu Ka radiation equipped with liquid nitrogen cooled germanium solid-state detector. The diffractogram of the catalyst was recorded in the 29 range of 20-80°, respectively, and at the scanning rate of 0.02° with the counting time of 5 seconds at each point. The crystalline nature of the Si-Ge nanotubes and C-Si-Ge were evaluated by Raman spectroscopy (LabRam HR800 system). The morphology of the materials was studied using scanning electron microscopy (VEGA 3 SBH - TESCAN USA, SEM) and transmission electron microscopy (JEOL 2100, 200KV, TEM). The optical characterization of the material was done with photoluminescence (PL) studies. For this purpose laser Argon ion laser (Horiba Jobin) with 488 nm excitation wavelength at room temperature was used. Powder X-ray diffraction was performed for the identification of the crystalline phases present in the as-synthesized material.

The XRD pattern of the Si-Ge nanotubes obtained at 800 °C from PTMG and TMPS feed ratio 1:1. The pattern exhibit peaks related to a crystalline material. The peaks originating at 26 = 19.62, 24.74, 36.20, 39.86, 42.79, 44.49, 50.75, 53.21, 59.28, 68.39, 74.16, 75.16 and 78.04 corresponds to Ge (2) (101), (111), (211), (202), (212), (113), (222), (302), (223), (331), (314), (205) and (413) planes of tetragonal structure (Joint Committee on Powder Diffraction Standards (JCPDS PDF 72-1089)) and 20 =26.9, 38.55 and 55.66 corresponds to Si (3) (110), (201), (210) and (220) planes of hexagonal structure (JCPDS PDF#750841). Thus the Si-Ge nanotubes are crystalline in nature and consist of only silicon and germanium elements. The SEM images (4) of materials grown at 800 °C, by a 10% Fb/Ar sweep gas (750 cm3/min flow rate) using PTMG and TPS ratio 1:1. The materials exhibit the morphology of a tubular structure with high aspect ratio. Observations from the EDAX spectrum (5) indicate the presence of silicon and germanium. The Si-Ge nanotubes have specific characteristics, that it consists of 20.04% Si (6) and 79.96%Ge (7) (atom%).

Absence of carbon might be due to less availability of carbon in the mixture, which runs away as volatile product at the reaction zone. Formation of Si-Ge nanotubes at this reaction condition is worth investigation. The SEM images (8) of materials grown at 800 °C, by a 10% IVAr sweep gas (750 cm3/min flow rate) using PTMG and TMPS ratio (1:4). The materials clearly depict tubular morphology with diameter in the range of 30 to 100 nm. The EDAX spectrum (9) confirms the formation of carbon, silicon and germanium elements in the material. The carbon inherent in the precursors is considered responsible for the formation of graphitic carbon in C-Si-Ge nanotubes. Quantitative EDAX spectrum analysis of the tube indicates a composition of 27.98 % Ge (10), 14.80 % Si (11) and 57.22% C (12) (atom %). The nanotubes appear to be very smooth without the presence of any impurities and amorphous carbon. The tubes have high aspect ratio, with length in the order of urn. Thermal decomposition of PTMG and TMPS at lower temperature (250°C), generate large amount of carbon atoms compared to Si and Ge, thus diffusion of C and Si in Ge attains a super saturation limit at reaction zone (800°C). The reaction carried at 700°C and 900°C did not yield any nanotubes.

The incomplete decomposition of the precursor at 700°C is hinders the growth and produced nanotubes of shorter length. At 900°C, amorphous carbon was deposited over the tubes due to the sintering effect. For detailed microstructure investigation of the nanotubes, TEM analyses were also performed. The TEM images of Si-Ge nanotubes (13) grown at 800 °C, by a 10% H2/Ar sweep gas (750 cm3/min flow rate) using PTMG and TMPS ratio (1:1). Nanotube structures with hollow core can be observed from the images. The Si-Ge nanotubes have internal diameter of 12 nm and outer diameter 45nm with length several micrometers observed in HRTEM (14). The lattice spacing between in Si-Ge nanotube was calculated to be 3.3 A. The nanotubes are straight, bent or kinked. The TEM images of C-Si-Ge obtained (15) at 800 °C, by a 10% H2/Ar sweep gas (750 cm3/min flow rate) using PTMG and TMPS ratio (1:4). A tubular structure of the sheets stacked almost parallel to the tube axis and a hollow core with dimensions of 45 nm outer diameter, 10 nm inner diameter can also be noticed. The lattice spacing of C-Si-Ge was calculated to be 2.3 A using HRTEM observation (16)

The crystalline nature of the nanotubes was investigated using Raman spectroscopy. The spectrum of Si-Ge hetrostructure obtained with PTMG and TMPS ratio 1:1. The Raman spectrum (17) of Si-Ge hetrostructure with peaks near 280-290, 400 and 480-500 cm"1, corresponds to optical-phonon modes associated with local Ge-Ge (18), Ge-Si (19), Si-Si (20) vibrations respectively. In general Ge-Ge, Ge-Si, Si-Si bond length relative to c-Ge, Ge-Si, c- Si are different, hence structural disorder exists in Si-Ge hetrostructure. Thus the distribution of Ge-Ge, Ge-Si, Si-Si is average because of the disorder. This leads to inhomogenity in the three local mode frequencies and broadening in the three Raman peaks. The spectrum reflects the characteristic peaks for nanotubes with a G-band at about 1597 cm"1 (21) and D-band (at about 1338 cm-1 (22)). This observation was consistent with TEM results. Presence of high content of Ge than Si in 1:1 ratio, initiates diffusion of Si into Ge. Smaller size of the Si-atoms causes lower strain and so the activation energy should be lower. Consequently Ge act as self-seed for the Si-Ge nanotubes formation. Ultimately after attaining the supersaturating condition in diffusion, re-crystallization results in Si-Ge nanotubes.

Presumably the high miscibility of Si in Ge, makes carbon present in the feed solution escape as volatile product without participating in the reaction. The Raman spectrum of C-Si-Ge obtained with PTMG and TMPS ratio (1:4). The Raman spectrum of C-Si-Ge (23) hetrostructure with peaks near 292 (24), 400 (25) and 539 cm"1 (26), corresponds to optical-phonon modes associated with local Ge-Ge, Ge-Si, Si-Si vibrations respectively. The spectrum reflects the characteristic peaks for graphitic carbon with a G-band at about 1598 cm"1 (27) and D-band (at about 1333 cm"1 (28)). The G-band originates mainly from the graphite in plane E2g vibration mode of hexagonal graphite and is related to the vibration of sp -hybridized carbon atoms in the graphite layer. The D-band indicates the presence of structural disorder within the carbon sheet. This observation was consistent with TEM results. Larger atomic size of Ge compared to Si , promote Ge to act as seed for the C-Si-Ge nanotubes formation . The growth process involves diffusion of C along with Si and Ge into the Ge seeds. Ultimately after attaining the supersaturation condition, re-crystallization results in C-Si-Ge nanotubes.

The photoluminescence (PL) studies was measured using Argon ion laser with 488 nm excitation wavelength at room temperature. The PL spectrum corresponding to Si-Ge nanotubes are shown in figure 8a. The PL spectrum of exhibit strong emission peaks in the range of 350 to 450 nm (29). The peaks confirms the photon energy band of Si-Ge nanotubes at 2.9 eV. The nanosize of the Si-Ge hetrostructure enables the generated hot carriers to undergo ballistic transport. These carriers tend to reach the Si-Ge and are trapped by the luminescent centers present at the interface layers. The blue light emission at 3.0 eV is due to some intrinsic defects centers present in hetrostructure which act as recombination centers. The blue light emission at 3.4 eV was reported due to the recombination of luminescent centers present at the interface layers of Si-Ge hetrostructure . The PL spectrum corresponding to C-Si-Ge nanotubes are shown in figure 8b . The PL spectrum also exhibit strong emission peaks in the range of 350 to 450 nm (30). The peaks confirms the photon energy band of C-Si-Ge nanotubes at 3.2 eV. The blue light emission at 3.4 and 3.5 eV was reported due to the recombination of luminescent centers present at the interface layers of C-Si-Ge hetrostructure. Noteworthy the PL spectrum corresponding to silicon nanostructure in the range of 600-800 nm was absent in both the spectrum.

Claims:

We Claim:

1. A process for preparation novel catalyst-free Si-Ge and C-Si-Ge nanotubes by CVD method comprises of two embodiment,

a) wherein, the first embodiment is synthesis of silicon-germanium (Si-Ge) nanotubes

b) wherein, the second embodiment is synthesis of carbon-silicon-germanium (C-Si-Ge) nanotubes

c) wherein , the said two embodiments comprises of carbon atom, silicon atom, germanium atom, trimethylphenylsilane precursor, phenyltrimethylgermane precursor, carrier gas, pre- heat zone temperature, reaction temperature and two zone chemical vapour deposition (CVD) furnace.

2. As claimed in the claim 1, wherein the said embodiments are single step procedure for production of Si-Ge and C-Si-Ge nanotubes without employing any metal catalyst.

3. As claimed in the claim 1,

a) wherein the said precursor used for the synthesis of Si-Ge and C-Si-Ge nanotubes comprises of phenyltrimethylgermane (PTMG) and trimethylphenylsilane (TMPS),

b) wherein, the said precursor has a general formula Ge R(4-X)LX/ Si R(4. X)LX where x comprises of 1,2 or 3; radical R comprises of alkyl, cycloalkyl or aryl group; L comprises of Hydrogen, halide or alkoxide,

4. As claimed in the first embodiment of the claim 1,

a) wherein the said Si-Ge nanotube material is grown at 800°C two zone CVD furnace

b) wherein the said precursors phenyltrimethylgermane (PTMG) : . trimethylphenylsilane (TMPS) comprises of the ratio 1:1 is used for growing the Si-Ge nanotubes

c) wherein the said carrier gas comprising of 10% K^/Ar gas with the flow rate of 750 cm3/min

d) wherein the said material exhibit the morphology of tubular structure with the said ratio

5. As claimed in the first embodiment of the claim 1,

a) wherein the Si-Ge nanotubes comprises of 20.04% of Silicon and 79.96% of Germanium (atom %)

b) wherein the carbon atom evades as a volatile product at the reaction zone thereby lessening the availability of the carbon in the mixture furthermore growing the tubular nanostructure eventually comprising Silicon-Germanium void of carbon atom as claimed in the first embodiment

6. As claimed in the second embodiment of the claim 1,

a) wherein the formation of said tubular nanostructure comprising of C-Si-Ge is due to the carbon inherent in the precursor

b) the PTMG and TMPS precursors in the ratio of 1:4 is used for growing the C-Si-Ge nanotube whereby forming lOOnm diameter range tubular nanostructure

c) wherein the said nanotubes comprises of 27.98% of Ge, 14.80% of Si and 57.22% of Carbon element,

7. As claimed in the second embodiment of the claim 1,

a) wherein the thermal decomposition of PTMG and TMPS at lower temperature at 250°C thereby generating large amount of carbon atoms compared to Si and Ge, wherein the diffusion of Carbon and Silicon in Ge attains a super saturation limit at reaction zone at 800°C

b) wherein the at 700°C the incomplete decomposition of precursors hinders the growth and at 900°C the amorphous carbon was deposited over the tubes due to the sintering effect

c) wherein the said carrier gas comprising of 10% H2/Ar gas with the flow rate of 750 cm3/min

8. As claimed in the claim 1,

a) wherein the optimized growth of pure, crystalline Si-Ge nanotubes and C-Si-Ge nanotubes are produced in the two zone CVD furnace without any transition metal catalyst

b) wherein the reactor comprises of quartz boat which is inserted at the reaction zone for additional deposition of surface

9. As claimed in the claim 1, wherein the production of Si-Ge and C-Si-Ge nanotubes is a simple, single step, expeditious, low temperature and cost effective procedure.