Growth and transmission electron microscopy studies of nanomaterials 5 7

Chapter Growth of silicon carbide nanocones Chapter Growth of silicon carbide nanocones In this chapter, we continue to use the VLS method to grow silicon carbide nanocones 5.1 Introduction SiC is an important group IV semiconductor material with a bandgap of 2.3 eV It has received much attention for its applications in high frequency, high power and high temperature devices due to its high breakdown electric field, electron mobility and good thermal conductivity [1-5] Theoretical calculations and experimental results show that the electricity and strength of SiC nanorods are better than those of large whiskers and bulk crystalline SiC, [6-7] the elastic constant can reach the theoretical value of 600 Gpa for (111) oriented SiC [6] 5.2 Motivation Controlling nanostructure formation is one of the key steps in fabricating nanomaterials and has been the focus of intensive research efforts For example, creating architectured assembly of dimensional nanostructures is essential for device integration in field emission arrays, sensor arrays, multi-tip arrays for dip-pen lithography, photonic waveguides etc [8-10] There have been extensive research efforts on the synthesis and assembly of nanorods and nanotubes for such 128 Chapter Growth of silicon carbide nanocones mechanical or electronic applications However there have been relatively few systematic studies on ways to precisely engineer the shape and form of the components for functional nanoscale mechanical devices For example, a conical structure offers substantially higher mechanical and thermal stability than a narrow cylinder Structurally, nanocones can have nanometer-sized tips and micrometer sized bases, rendering their manipulation easier than nanotubes Nanocones are also mechanically stiffer and less prone to bending and thermal shock, making them ideal candidates for scanning probe tips, or as nano-syringes for quantum dot injection into biological cells Zhang [11] reported the chemical vapor deposition (CVD) synthesis of tubular graphite cones consisting of annular rings of graphene planes concentric with a hollow interior The conical structure is due to the progressively shorter terrace edges going from the inside to outside, arising from the sequential growth of shorter secondary layers on the inner layers as the central tube grows upward Khrisnan [12] used arc discharge methods to synthesize geometrically precise, hollow nanocones that consisted of folded conical graphene planes Vladmir [13] balanced the growth and etching effects of the acetylene-ammonia mixtures to synthesize carbon nanocones made of cylindrical carbon nanofibres surrounded by carbon precipitates on the outside All the nanocone structures reported thus far are straight-growing, crystalline cones made of carbon 129 Chapter Growth of silicon carbide nanocones Many researchers have been trying various methods to synthesize one-dimensional SiC rods The detailed methods are listed in table 5.1 Table 5.1 Methods for synthesizing SiC nanorods and nanowires Researchers Shape of SiC Synthesis methods Year Dai et al SiC nanorods nanotube template and SiO or Si+I2 1995[14] Meng et al SiC nanorod within carbothermal reduction of silica 1999[15] SiO2 nanorod xerogels at 1650 ˚C SiC-SiOx Heating SiC-Fe-Co mixtures under CO heterojunction nanowires atmosphere at 1500 ˚C SiC nanorods hot filament CVD on Si substrate using Zhu et al Lai et al 1999[16] 2000[17] mixture of Si, SiO2 and carbon above 2300 ˚C Hu et al SiC nanorods high pressure and low temperature using CCl4, Si and Na 2000[18] Li et al SiC SiC rod as the anode to arc discharge 2001[19] Needle-Like SiC carbothermal reduction (CNTs) of SiO 2001[20] Nanorods at 1410 ˚C, Fe SiC–C coaxial silicon substrates and carbon deposition nanocables using pyrolysis of CH4 at 1100 ˚C, Fe SiC nanowire carbothermal reduction of Si and SiO2 networks under 1250 ˚C nanorod/nanorod Gao et al Kim et al Li et al 2003[21] 2003[22] Although various methods have been applied for synthesizing SiC nanorods and nanowires, the nanorods show no ordered arrangement on the specific substrates, and are randomly deposited on the wall of the chamber, in the crucible or on the cathode Such fabrication methods are unsuitable for the manufacturing of nanodevices Moreover, since Si or SiO2 were commonly used as the source, 130 Chapter Growth of silicon carbide nanocones temperatures of more than 1000 ˚C were needed for these reactions In this study, we report for the first time the synthesis of silicon oxide-ensheathed silicon carbide nancones using tetramethylsilane as the single source precursor at temperatures under 900 ˚C It is discovered that the nanocones show a propensity to undergo bending at various angles and we will discuss the growth mechanism 5.3 Growth conditions for SiC nanocones Silicon wafers coated with 20 nm of nickel film were introduced into the MW-PECVD system The nickel-coated Si(100) substrate was pretreated at 800 ˚C using a pure hydrogen plasma in order to remove the impurities and oxide film from the substrate prior to growth A single source precursor, tetramethylsilane, was used for the deposition of the SiC nanocones The details of sample pretreatment and the growth conditions used in the MP-PECVD system are listed as follows: H2 pre-treatment Growth condition Temperature (°C) 800 900 Flow rate (sccm) 100 H2 Bias (V) 10 Si(CH3)4 100 H2 -150 -20 Pressure (torr) 23 23 Time (minutes) 20 20 131 Chapter Growth of silicon carbide nanocones 5.4 Growth and phase determination of SiC-SiO2 nanocones Figure 5.1 High Resolution SEM image of dense, uniform SiC nanocones At the end of the deposition, a deep blue film could be seen on the Ni -coated Si substrate Visualization of the surface deposits by SEM reveals the growth of both straight and bent conical fibres with an overall length of several micrometers, as shown in figure 5.1 Beneath the nanocones, the substrate is covered by ball-like deposits about 0.5 m in diameter The base of the cone is micrometer-sized and is anchored to the micron-sized ball whilst the tip is nanometer-sized 132 Chapter Growth of silicon carbide nanocones 65 60 a Transmittance 55 b 50 45 40 Si-O 35 Si-C Si-Si 30 25 Si-O-Si 20 500 1000 1500 2000 2500 -1 wavenumber (cm ) Figure 5.2 FTIR transmission spectra of the Si substrate (a) before deposition; (b) after deposition with nanocones The composition of the film was investigated by FTIR transmission spectroscopy Figure 5.2 shows IR spectra for the substrate before and after deposition with tetramethylsilane The band at 606 cm−1 corresponds to the phonon mode of the Si substrate We also can see three transverse optic phonon bands of SiO2 in both spectra: the lowest frequency band at 458 cm−1 corresponds to the Si–O rocking vibration, the peaks at 792 cm−1 and 1107 cm−1 correspond to the symmetric and asymmetric stretching vibrations of the Si–O–Si The broad band of asymmetric stretching vibrations at 1107 cm-1 consists of two bands with peaks at 1088 cm−1 and 1200 cm−1 respectively [23,24] The second band is originally IR-inactive but can be 133 Chapter Growth of silicon carbide nanocones activated by disorder-induced mode coupling The intensity of SiO2 peaks after deposition becomes stronger compared to the peaks due to native oxide before deposition, which indicates the film consists of large quantity of amorphous SiO2 The additional peak at 820 cm-1 was observed after deposition which is the characteristic stretching vibration of crystalline SiC [25] (a) Figure 5.3 (a) TEM image of the tip of the cone The size of the tip is around 20 nm, one straight rod with diameter 10 nm is at the center of the cone; (b) High resolution TEM image which shows that the inner nanorod is crystalline cubic SiC growing along the direction, the resolved {111} lattice planes are separated by 0.25 nm (b) The internal microstructure of the nanocone was studied using TEM Figure 5.3(a) shows a low magnification TEM image of a nanocone The inset shows the whole morphology of this nanocone The peripheral wall of the cone is smooth and a nanorod with diameter of 10 nm is concentric to the cone, tipped by a catalyst particle A high magnification view reveals that the center of the cone has a coaxial crystalline rod of about 10 nm diameter The HRTEM image of this coaxial rod in 134 Chapter Growth of silicon carbide nanocones figure 5.3(b) shows lattice fringe separations of 0.25 nm consistent with the cubic β-SiC {111} interplanar separation, whilst the outer coat is amorphous TEM observation shows that the inner SiC nanorod grows preferentially along the direction Figure 5.4 Energy filtered maps of silicon carbide/silicon dioxide nanocones The maps show that the center rod is crystalline silicon carbide and the outer layer is silicon dioxide The elemental distribution was verified by EELS mapping in figure 5.4 The elemental maps were obtained from the L23 edge of Si, and the K edge of C and O Silicon is found in both the body and the tip of the cone although the Si signal at the tip arises from the overlap of the Ni M and Si L edge Carbon existing at the center of the cone and this indicates that the central rod is silicon carbide, whilst the outer 135 Chapter Growth of silicon carbide nanocones conical coat is silicon oxide Since the atomic concentration of silicon in SiO2 is lower than that in SiC, the brightness indicates that the density of silicon atoms in the outer sheath is lower than that of the center The metal particle at the tip is nickel Elemental mapping clearly proves that center core of the nanocone is cubic SiC, while the amorphous outer sheath layer is SiO2 14 Intensity(x10 ) 12 Si-L2L3 10 C-K O-K B A 100 200 300 400 500 eV Figure 5.5 Electron energy loss spectra (a) collected at region ‘A’, focusing on the rod; (b) collected at region ‘B’, focusing on the side of a cone More information on the SiC-SiO2 nanocones is obtained by interpreting the energy loss near edge structure (ELNES) of the Si, C and O edges The near edge energy loss structure arises from the excitation from core shell electrons to the vacant levels above the Fermi level The fine structure and position of the Si-L2,3 edge depends on the ionicity of the bond with the ligand atoms We can identify and differentiate the SiX2 (X=O, C, P, N) compound using ELNES as a fingerprint 136 Chapter Growth of silicon carbide nanocones method [26] Background substracted EEL spectra taken from side and tip of the cone are shown in figure 5.5 Focusing the electron beam on the narrower region ‘A’ of the cone where the rod is more prominent produces a spectrum with C and Si edges characteristic of SiC Focusing on the thicker region ‘B’ on the side produces an EELS spectrum with Si and O edges, which are characteristic of the SiO4 tetrahedral cluster The Si L-edge consists of two sharp peaks at 107.3 and 113.9 eV, and a third broad peak at 129.4 eV which is separated by about 22 eV from the first peak Such ELNES structures of SiC and SiO2 had also been previously recorded by L.A Garvie [27] The small C-K edge signal may come from the supporting holey carbon film The composition of the SiC-SiO2 cone was determined by the quantification method which is described in chapter The atomic ratio of the inner SiC rod and side SiO2 wall of the cone is Si/C=1.0:(0.9±0.12) and Si/O=1.0:(2.12±0.25) respectively All these results indicate the SiC-SiO2 nanocone consists of the nearly stoichiometric SiC nanorods covered by an amorphous SiO2 layer The presence of nickel catalyst at the tip of the SiC rod suggests tip-catalyzed growth following the classic vapor-liquid-solid mechanism [28] The single source precursor tetramethylsilane decomposed into SiC vapor and diffused into the Ni catalyst particles Since the {111} plane of β-SiC has the lowest surface free energy, estimated to be about 2830 erg/cm2, which is much lower than those of other planes 137 Chapter Novel heterogeneous reaction route to Cu chalcopyrite thin films excess In2S3 existed mainly as an amorphous phase under 300 ˚C At 400 ˚C, the indium-rich film changes to a nearly stoichiometric composition With a further increase in temperature, the percentage of In decreases significantly and the segregation of the copper sulfide phase occurs Thus, a deposition temperature of 400 ˚C was found to be optimal for the growth of CuInS2 films on copper substrates using From the XRD and SEM results, high-quality CuInS2 film can be achieved at a deposition temperature of 400 ˚C More information of the film is given by side view SEM image and RBS The thickness of the CuInS2 film is estimated at ca µm from side view SEM image (figure 6.9) Figure 6.9 Side view SEM image of the CuInS2 film grown at 400 °C 165 Chapter Novel heterogeneous reaction route to Cu chalcopyrite thin films 4000 3500 3000 Counts 2500 B simulated O Si S Cu In 2000 1500 1000 500 1200 1400 1600 1800 2000 Energy (keV) Figure 6.10 RBS spectrum from the CuInS2 film grown at 400 °C The original Rutherford backscattering measurements and simulated spectrum are shown in figure 6.10 The calculated elemental composition from the simulated spectrum is Cu:In:S = 0.24:0.26:0.50, which suggests the composition of the film is homogeneous within the whole film and stoichiometric CuInS2 The presence of oxygen not only has been identified in this film, but also detected in many chemically deposited chalcopyrite films in other works Therefore the result clearly indicates that the heterogeneous reaction of with copper affords a more efficient route for the growth of stoichiometric, highly ordered CuInS2 films The formation of ternary CuInS2 films may involve the formation of binary sulphide through the thermal decomposition of followed by a solid state reaction to form the ternary compound [36] Therefore, it is likely that Cu2S has been formed by a reaction between sulfur contained in the precursor vapor or its decomposed products such as (PhCO)2S [30] and the copper substrate in our system In the mean 166 Chapter Novel heterogeneous reaction route to Cu chalcopyrite thin films time, In2S3 is deposited on the surface of the substrate A solid-state reaction between copper sulfide and indium sulfide results in the formation of stoichiometric CuInS2 At higher temperatures, the CuInS2 phase decomposes due to its thermal instability Therefore at 600 ˚C, only Cu2S and Cu1.96S are obtained and all the other chalcopyrite CuInS2 peaks disappear 20 30 40 50 * (215) CuGaS2 (312) * Cu1.96S Cu1.96S-CuGaS2 Ga2S3(311) * (200) * (201) CuGaS2 (204) Ga2S3(220) * (202) * (115) Ni0.96S (110) * (212) Ni0.96S (002) Ni0.96S (101) Ni0.96S (102) * (104) * (102) CuGaS2 (112) * (110) * (103) Ga2S3(111), Ni0.96S (100) * (101) Intensity (a.u.) 6.4.4 Growth of GaCuS2 from the precursor [Et3NH][Ga(SC{O}Ph)4]·H2O [2] Ga2S3-Ni0.96S 60 70 theta (degree) Figure 6.11 X-Ray diffractograms of the films deposited on a Cu coated Si substrate (top) and a Ni coated Si substrate (bottom) using precursor We attempted to synthesize Ga2S3 and CuGaS2 thin films using the precursor [Et3NH][Ga(SC{O}Ph)4]·H2O (2) γ-Ga2S3 (JCPDS No 43-916) films were grown on nickel-coated silicon substrates using the precursor at 400 ˚C However, XRD of the film as shown in figure 6.11 indicated the formation of Ni0.96S along with Ga2S3 A GaCuS2 and Cu1.96S mixture was achieved when precursor was used for 167 Chapter Novel heterogeneous reaction route to Cu chalcopyrite thin films deposition on a Cu-Si substrate Although precursor has similar properties to 1, it appears that the CuGaS2 decomposition temperature is lower than the evaporation temperature, which resulted in the formation of a nonstoichiometic film and a mixture, even though the experimental conditions were changed during the reaction On the other hand, the diffusion rate of GaS species is lower than that of the corresponding InS species, which also lead to a mixture of two products forming in the CuGaS2 films 6.5 Growth of CuInE2 from precursor In(EPh)3 (E= S, Se) Since high-quality CuInS2 thin films have been successfully grown on Cu coated Si substrates by heterogeneous reaction, CuInSe2, which resembles CuInS2 and has the highest light conversion efficiency achieved in lab, is expected to have a similar reaction route The precursor In(EPh)3 was selected as the method preparation of this precursor only involves a one-step addition of reactants and high yields can be achieved [37] However the crystal structure of this precursor had never been determined before as no single crystal suitable for single crystal X-ray diffraction experiment had been obtained TGA was used to investigate the decomposition of In(EPh)3 on heating Experiments were performed at ambient pressure, under N2 flow and a heating rate of 10 ˚C/min The weight loss was associated with decomposition of the precursor to In2S3 for In(SPh)3 as the residue weight was 35.8%, while for In(SePh)3, it gave a residue weight of 41.3% (theoretical weight residue for In2S3 is 36.9% and that of In2Se3 is 40.0%) The results show that the precursors are suitable for deposition of the CuInE2 films because the final stable product In2E3 contained only In and E, elements that are both needed for the formation of the CuInE2 films 168 Chapter Novel heterogeneous reaction route to Cu chalcopyrite thin films Table 6.4 EI-MS peaks of In(EPh)3 In(SPh)3 In(SePh)3 m/e ion intensity m/e ion intensity 77 Ph ·+ strong 77 Ph ·+ strong 110 SPh ·+ strong 154 SePh ·+ strong 185 SPh2 ·+ medium 234 SePh2 ·+ strong 218 S2Ph2 ·+ strong 314 Se2Ph2 ·+ strong 224 InSPh ·+ medium 348 InSe(SePh) ·+ medium 339 In2(SPh)·+ Medium 432 In(SePh)2·+ weak 442 In(SPh)3·+ medium 506 InSe(SePh)2·+ weak 666 In2(SPh) 4·+ weak 582 In(SePh) 3·+ weak 775 In2(SPh) 5·+ weak 697 In2(SePh) 3·+ weak Fragmentation patterns in vapor state of both precursors using electron impact mass spectroscopy were used to investigate the decomposition pathways of these precursors and thus a possible route to the formation of the ternary compounds Both characterizations were performed in vacuum and the temperature was 230 ˚C for In(SPh)3 and 200 ˚C for In(SePh)3 precursors Table 6.4 lists the peaks in Electron Ionization–Mass Spectrometer (EI-MS) for In(SPh)3 and In(SePh)3 Weak peaks at m/e = 775 and 442 corresponding to In2(SPh)5·+ and In(SPh)3·+ suggest that this precursor might volatilize as a dimer as well as a monomer Since the mass spectrum showed a possible dimer of In(SPh)3, it might have a polymeric structure like the In(SePh)3 precursor Structural characterization of In(SePh)3 by Parkin and others [38] had recently discovered that In(SePh)3 has a polymeric structure where the In centres are coordinated to four bridging SePh and 169 Chapter Novel heterogeneous reaction route to Cu chalcopyrite thin films one terminal SePh group instead of the previously reported six bridging SePh coordination [39] It could be probable that In(SPh)3 may have the same structure as In(SePh)3 because their EI-MS show similar fragmentation patterns and intensities EI-MS of In(SePh)3 also contains a weak ion peak of In(SePh)3·+, strong peaks of Ph2Se2·+ and SePh2·+, similar to In(SPh)3·+, Ph2S2·+ and SPh2·+ From the existence of the monomers In(SPh)3·+, In(SePh)3·+ and dimer peaks of In2(SPh)4·+, In2(SPh)5·+ and In2(SePh)3·+ in their spectrum, it is obvious that both solid precursors had volatized (116), (312) (204), (220) (204) (211) CuInSe2 (200) (116) (312) (224) Intensity (a.u.) (112) (112) into the vapor phase before they decomposed into fragments CuInS2 20 30 40 50 60 theta (degree) Figure 6.12 XRD pattern of CuInS2 and CuInSe2 films Previous work showed that the optimal growth condition for CuInS2 films was found to be at a substrate temperature of 400 ˚C Figure 6.12 shows the XRD pattern of CuInS2 and CuInSe2 film The XRD pattern reveals the tetragonal structure and crystalline nature of the deposited CuInS2 and CuInSe2 films The 170 Chapter Novel heterogeneous reaction route to Cu chalcopyrite thin films lattice spacing of CuInS2 has been determined to be a = 5.52 Å and c in the range of 11.05 - 11.07 Å and they are in good agreement with that of the JCPDS data (75 106) of a = 5.517 Å and c = 11.06 Å The lattice constants of CuInse2 were determined to be a = 5.76 Å and c = 11.53 Å All the peaks in the XRD indicate pure tetragonal CuInSe2 film (JCPDS card No 80-535) The strong (112) orientation of CuInSe2 gives a good lattice match with CdS in order to form high efficiency CuInSe2/ CdS/ ZnO solar cell devices [40] SEM images reveal that homogeneous and dense films were deposited for both CuInS2 and CuInSe2 The cross-sectional images obtained from SEM revealed that the film is approximately m thick for CuInS2 and 0.8 m thick for the CuInSe2 film It is probable that the mechanism of formation of CuInSe2 is also equally to that of CuInS2 For CuInSe2 formation from metallic precursors, it has been established generally that it involves Cu2Se and In2Se3 reaction to form the ternary compound [32, 41] Their field desorption mass spectra showed strong molecular peaks of Ph2Se2·+, SePh2·+ and weak peaks of In(SePh)4·+ and In2Ph(SePh)4·+ It can be deduced that In(SePh)3 after volatilization in CVD could also give In2Se3 on the Cu substrate in our heterogeneous reaction Therefore the reaction pathways for the heterogeneous reaction here may be suggested to be as follows: SePh + 2Cu Cu2Se SePh2 + 2Cu Cu2Se Se2Ph2 + 4Cu 2Cu2Se Cu2Se + In2Se3 2CuInSe2 Fragments such as SePh, SePh2, and Se2Ph2, formed from decomposition of the precursor, reached the Cu substrate to form Cu2Se The volatile by-products are 171 Chapter Novel heterogeneous reaction route to Cu chalcopyrite thin films then carried away into the vacuum The other fragments of In would then decompose and form In2Se3 on the substrate of higher temperature than its evaporation temperature In2Se3 could then further undergo reaction with Cu2Se to deposit the desired ternary semiconductor 6.6 Conclusion Thermogravimetric and pyrolysis experiments showed that these single precursors are good for the preparation of M2S3 bulk materials Thin films of crystalline M2S3 and CuMS2 (M = In, Ga) have been prepared using [Et3NH][M(SC{O}Ph)4]·H2O as single-source precursors While high quality In2S3 thin films could be prepared on Ni films using 1, only a mixture of γ-Ga2S3 and hexagonal Ni0.96S were obtained on Ni from On Cu films, a heterogeneous reaction route between the Indium sulfide precursor and copper resulted in the growth of a crystalline film of CuInS2 at 400 °C Growth at temperatures higher than 400 °C resulted in the segregation of the Cu1.96S phase Only a mixture of CuGaS2 and CuS2 was obtained from precursor High quality CuInS2 and CuInSe2 thin films have been deposited using precursors and in CVD As Cu is not incorporated into the precursor, the formation of CuInS2, CuInSe2 and CuGaS2 has to involve a reaction between the gaseous precursor vapors and the solid Cu on Si substrate Results have shown that the deposition of pure and single phase CuInS2 and CuInSe2 films could be achieved from the heterogeneous route This novel approach to CuInS2 and CuInSe2 films has never been documented before This new route may open the door to the deposition of thin films where suitable precursor material is not available 172 Chapter Novel heterogeneous reaction route to Cu chalcopyrite thin films References [1] Contreras, M.A.; Egaas, B.; Ramanathan, K Prog Photovolt Res Appl 1999, 7, 311 [2] Isomura, S.; Shirakata, S.; Abe, T Sol Energy Mater Sol Cells 1991, 22, 223 [3] Agnihotri, O P.; Ran, P R.; Thangraj, R.; Sharma, A K.; Raturi, A Thin Solid Films 1983, 102, 291 [4] Yamaguchi, T.; Matsufusa, J.; Yoshida, A Sol Energy Mater Sol Cells 1992, 27, 25 [5] Yuksel, O.F.; Basol, B.M.; Safak, H.; Karabiyik, H Appl Phys A: Mater Sci & Processing 2001, 73, 387 [6] Harris, J.D.; Hehemann, D.G.; Cowen, J.E Conference Record of the IEEE Photovolt Specialists Conference 2000, 28th, 563 [7] Meese, J.M.; Manthurtuthil, J.C.; Locker, D.R Bull Am Phys Soc 1975, 20, 696 [8] Moller, H J Semiconductors for Solar Cells Artech House, Boston 1993, 35 [9] Dzionk, C.; Metzher, H.; Hessler, S.; Mahnke, H E Thin Solid Films 1997, 299, 38 [10] Barron, A.R CVD of Compound Semiconductors, VCH, Weinheim 1997, Ch.5 [11] MacInnes, A.N.; Power, M.B.; Barron, A.R.; Jenkins, P.P.; Hepp, A.F Appl Phys Lett 1993, 62, 711 [12] Oishi, K.; Kobayashi, S.; Ohta, S.I.; Tsuboi, N.; Kaneko, F J Cryst Growth 1997, 177, 88 [13] Bochmann, M Chem Vap Deposition 1996, 2, 85 [14] O’Brien, P ; Nomura, R J Mater Chem 1995, 5, 1761 [15] Carmalt, C J.; Morrison, D E.; Parkin, I P J Mater Chem 1998, 8, 2209 173 Chapter Novel heterogeneous reaction route to Cu chalcopyrite thin films [16] Yasusada, U.; Takashi, S.; Youhei, H.; Hideki, M Denki Kagaku oyobi Kogyo Butsuri Kagaku 1993, 61, 1206 [17] Landry, C.C.; Barron, A.R Science 1993, 260, 1653 [18] Landry, C.C.; Lockwood, J.; Barron, A.R Chem Mater 1995, 7, 699 [19] Jiang, Y ; Wu, Y ; Mo, X ; Yu, W.; Xie, Y.; Qian, Y Inorg Chem 2000, 39, 2964 [20] Lu, Q.; Hu, J.; Tang, K.; Qian, Y.; Zhou, G.; Liu, X Inorg Chem 2000, 39, 1606 [21] Hollingsworth, J A.; Hepp, A F.; Buhro, W E Chem Vap Deposition 1995, 5, 105 [22] Jones C.; Brien, P.O CVD of compound semiconductors: precursor synthesis, development and applications, Weinheim, 1997 [23] Horley, G A.; Chunggaze, M.; O’Brien, P.; White, A J P.; Williams, D J J Chem Soc Dalton Trans 1998, 4205 [24] Hirpo, W.; Dhingra, S.; Sutorik, A.C.; Kanatzidis, M.G J Am Chem Soc.1993, 115, 1597 [25] Hollingsworth, J.A PhD Thesis, Washington University, St Louis, USA, 1999 [26] Nomura, R.; Seki, Y.; Matsuda, H J Mater Chem 1992, 2, 765 [27] Nomura, R.; Fujii, S.; Kanaya, K.; Matsuda, H Polyhedron 1990, 9, 361 [28] Nomura, R.; Seki, Y.; Matsuda, H Thin Solid Films 1992, 209, 145 [29] Banger, K.K.; Cowen, J.; Hepp, A.F Chem Mater 2001, 13, 3827 [30] Shang, G.; Kunze, K.; Hampden-Smith, M J.; Duesler, E N Chem Vap Deposition 1996, 2, 242 [31] Bini, S.; Bindu, K.; Lakshmi, M.; Kartha, C.S.; Vijayakumar, k.P.; Kashiwaba, Y.; Abe, T Renew Energ 2000, 20, 405 174 Chapter Novel heterogeneous reaction route to Cu chalcopyrite thin films [32] Domashevskaya, E.P.; Gorbachev, V.V.; Terekhov, V.A.; Kashkarov, V.M.; Panfilova, E.V.; Shchukarev, A.V J Electron Spectrosc 2001, 114, 901 [33] Rockett A.; Abou-Elfotouh, F.; Albin, D.; Bode, M.; Ermer, J.; Klenk, R.; Lommasson, T.; Russell, T.W.F.; Tomlinson, R.D.; Tuttle, J.; Stolt, L.; Walter, T.; Peterson, T.M Thin Solid Films, 1994, 237, [34] Domashevskaya, E.P.; Gorbachev, V.V.; Terekhov, V.A.; Kashkarov, V.M.; Panfilova, E.V.; Shchukarev, A.V J Electron Spectrosc 2001, 114, 901 [35] Barreau, N.; Marsillac, S.; Bernede, J.C Vacuum 2000, 56, 101 [36] Krunks, M.; Mikli, V.; Bijakina, O.; Mellikov, E Appl Surf Sci 1999, 142, 356 [37] Kumar, R.; Mabrouk, H E.; Tuck, D G Inorganic syntheses 1992, 29, 15 [38] Kuchta, M C.; Rheingold A L.; Barkin G New J Chem 1999, 23, 957 [39] Annan, T A.; Kumar, R.; Mabrouk, H E.; Tuck, D G Polyhedron 1989, 18, 865 [40] Gupta, A.; Isomura, S Sol Energy Mater Sol Cells 1998, 53, 385 [41] Guillen, C.; Herrero, J Vacuum 2002, 67, 659 [42] Gysling, H.J.; Wernberg, A.A Chem Mater 1992, 4, 900 175 Chapter Conclusion Chapter Conclusion 7.1 General conclusion I have presented several chapters on the synthesis of inorganic semiconductor nanomaterials using vapor-phase CVD methods in this thesis Insights into the internal structure, growth and phase transformation processes of the nanomaterials have been obtained Results from the TEM studies indicate that nucleation and growth of the BN nanocapsules, ZnS nanowires and SiC nanocones obtained in this thesis agreed with the vapor-liquid-solid (VLS) method Ternary phase compounds such as CuInS2 and CuInSe2 thin films were synthesized by the vapor-solid (VS) method In the VLS reaction, it involves the diffusion and dissolution of gaseous precursors, such as ammonia, ZnS vapor and tetramethylsilane, into the molten catalyst, followed by the subsequent recrystallization of BN, ZnS and SiC nanomaterials The thickness and diameter of the ultimate products are determined by the size of the catalysts, while the length of nanowires and the quantities of products obtained are controlled by the reaction time and the degree of supersaturation during the experiments In the VS reaction, the formation of CuInS2 and CuInSe2 ternary films resulted from a solid state reaction between the deposited binary compound and the Cu substrate The growth rate is determined by the growth 176 Chapter Conclusion temperature, volatility of the inorganic precursors as well as the thickness of the pre-deposited Cu film Due to the ease of precursor as well as substrate preparation, coupled to the high purity of the nanomaterials obtained, the methods developed in this thesis may be useful for the bulk production of nanomaterials for technological applications 7.2 Future work Based on the research results in this thesis, further research in the following areas can be continued: It is timely to examine the applications of the nanomaterials prepared in this thesis For applications in catalysis, bulk synthesis of randomly oriented nanomaterials is sufficient to meet the general requirements However a high degree of control in the size uniformity and orientation of the nanoparticles will be needed for applications in photonics or magnetic recording media It has been suggested that the 2D organization of magnetic nanoparticles could be a critical step towards the realization of high-density recording media BN layers can passivate the metal nanoparticles, forming an insulator-metal structure Based on the results in chapter 3, the size of the catalysts will dictate the shape of the outer BN sheath and the performance of these magnetic particles One method to generate homogeneously-sized nanoparticle is to use the newly developed nanocluster beam deposition technique This technique can be used to deposit nanoparticles on a 177 Chapter Conclusion substrate with a very narrow dispersion in sizes BN sheets can be formed on the deposited metal particles by reacting the latter with borazine, which will result in the metal-encapsulated BN particles with uniform size These 2D magnetic nanoparticles passivated by the BN external sheath can be useful in high-density magnetic storage devices In addition, the nanocluster beam technique can also be used for the deposition of copper nanoclusters, which can be reacted with the single source precursor to form of Indium copper cluster via the solid state reaction presented in Chapter The probe tip is the key part of a scanning electron microscope High-aspect ratio and sharp tips are required for high resolution imaging Moreover, normal probe tips made of Si or SiN are not robust enough to survive mechanical abrasion during operation At present, carbon nanotubes are utilized as probe tips for probing high aspect-ratio structures Alternatively, SiC-SiOx nanocones, with strong micro-sized bases and sharp 5-10 nm sized tips, may offer applications as nanocantilevers Since SiC nanorods have been reported to have high elastic modules and toughness, the tips made of SiC-SiOx nanocones will be more sensitive and stronger than those tips made of other materials Considerable efforts are needed to optimize the fabrication processes of these nanocones before they can be commercialized as standard AFM tips In this regard, the presence of Ni particles at the tip due to catalyzed growth may be useful for magnetic imaging 178 Chapter Conclusion The CuInS2 and CuInSe2 films in chapter are potentially useful in solar panel applications as energy absorbing materials, therefore more work can be carried out in order to examine the conversion efficiency of solar energies to electrical energies The photovoltaic properties of grown film are the research focus in the future work 179 ... Product of decomposition 72 (-), 120(-), Anhydrous 151 (-), 220(-), In2S3a 368(+), 474 (+) 65( -), 1 15( -), 40- 86 97. 8 ( 97. 6) NA Anhydrous 155 (-), 2 25( -), 123 - 50 2 15. 2 (16.3) 17. 7 Ga2S3b 450 (+), 51 5(+)... lattice spacing of CuInS2 has been determined to be a = 5. 52 Å and c in the range of 11. 05 - 11. 07 Å and they are in good agreement with that of the JCPDS data ( 75 106) of a = 5. 5 17 Å and c = 11.06... (Å) 0 .71 073 0 .71 073 Crystal System Orthorhombic Orthorhombic Space Group P212121 a (Å) P212121 12.9124(6) 12 .7 250 (3) b (Å) 12.91 87( 7) 12. 8 57 3(1) c (Å) 21 .56 10(11) 21.6933(4) 359 6.6(3) 354 9.2(1)