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Synthesis and characterization of silicon and germanium nanowires, silica nanotubes, and germanium telluride/tellurium nanostructures

Dissertation
Author: Hsing-Yu Tuan
Abstract:
A supercritical fluid-liquid solid (SFLS) nanowire growth process using alkanethiol-coated Au nanoparticles to seed silicon nanowires was developed for synthesizing silicon nanowires in solution. The organic solvent was found to significantly influence the silicon precursor decomposition in solution. 46.8 mg of silicon nanowires with 63% yield of silicon nanowire synthesis were achieved while using benzene as a solvent. The most widely used metal for seeding Si and Ge nanowires is Au. However, Au forms deep trap in both Si and Ge and alternative metal seeds are more desirable for electronic applications. Different metal nanocrystals were studied for Si and Ge nanowire synthesis, including Co, Ni, CuS, Mn, Ir, MnPt 3 , Fe2 O3 , and FePt. All eight metals have eutectic temperatures with Si and Ge that are well above the nanowire growth temperature. Unlike Au nanocrystals, which seed nanowire growth through the formation of a liquid Au:Si (Au:Ge) alloy, these other metals seed nanowires by forming solid silicide alloys, a process we have called "supercritical fluid-solid-solid" (SFSS) growth. Moreover, Co and Ni nanoparticles were found to catalyze the decomposition of various silane reactants that do not work well to make Si nanowires using Au seeds. In addition to seeding solid nanowires, CuS nanoparticles were found to seed silica nanotubes via a SFSS like mechanism. 5% of synthesized silica nanotubes were coiled. Heterostructured nanomaterials are interesting since they merge the properties of the individual materials and can be used in diverse applications. GeTe/Te heterostructures were synthesized by reacting diphenylgermane (DPG) and TOP-Te in the presence of organic surfactants. Aligned Te nanorods were grown on the surface facets of micrometer-size germanium telluride particles.

Table of Contents List of Tables........................................................................................................xii List of Figures......................................................................................................xiii Chapter 1: Introduction............................................................................................1 1.1 One-Dimensional Nanostructures ...........................................................1 1.2 Nanowire Synthesis ................................................................................2 1.3 Supercritical Fluids .................................................................................5 1.4 Surfactant-Mediated Colloidal Nanocrystal Synthesis ...........................8 1.5 Dissertation Overview ..........................................................................11 1.6 References .............................................................................................13 Chapter 2: High Yield Si Nanowire Synthesis in Supercritical Benzene..............17 2.1 Introduction ...........................................................................................18 2.2 Supercritical Fluid Continuous Flow Reaction .....................................18 2.2.1 Si Nanowire Synthesis ..............................................................18 2.2.2 Si Nanowire Characterization ...................................................20 2.3 History of Silicon Nanowire Synthesis in Organic Solvent ..................21 2.4 Monophenylsilane Disproportionation .................................................22 2.5 Organic Solvent Selection ....................................................................23 2.6 Results and Discussion .........................................................................23 2.7 Conclusions ...........................................................................................29 2.8 References .............................................................................................30 Chapter 3: Nanocrystal-Mediated Crystallization of Silicon and Germanium Nanowires in Supercritical Fluid: The Role of Catalysis and Solid-Phase Seeding..........................................................................................................32 3.1 Introduction.............................................................................................32 3.2 Experimental...........................................................................................33 3.2.1 Different Metal Nanoparticles Preparation.................................33

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3.2.2 Semi-Batch Nanowire Synthesis................................................36 3.2.3 Silicon Precursor Solutions.........................................................37 3.3 Results and Discussion...........................................................................39 3.2.1 Effect of Metal nanoparticles on Si and Ge Nanowire Growth..39 3.2.2 Catalytic Heterogeneous Decomposition of Silicon Precursors by Metal Nanoparticles...............................................................47 3.2.2.1 The Role of Au Nanoparticles in Si Precursor Decomposition...................................................................47 3.2.2.2 The Role of Ni Nanoparticles in Si Precursor Decomposition...................................................................48 3.2.2.3 The Role of Co Nanoparticles in Si Precursor Decomposition...................................................................51 3.4 Conclusions.............................................................................................56 3.5 References...............................................................................................56 Chapter 4: Germanium Nanowire Synthesis: An Example of Solid- Phase Seeded Growth with Nickel Nanocrystals....................................................61 4.1 Introduction.............................................................................................61 4.2 Experimental...........................................................................................63 4.3 Results.....................................................................................................64 4.4 Discussion...............................................................................................70 4.4.1 Melting Point Depression of Ni Nanocrystals............................70 4.4.2 The NiGe x Seed Particle Shape..................................................72 4.4.3 Diameter Distribution of SFSS-grown Ge Nanowires................75 4.4.4 Diffusion-Limited Growth..........................................................79 4.5 Conclusions.............................................................................................81 4.6 References...............................................................................................82 Chapter 5: Silicon Nanowires and Silica Nanotbues Seeded by Copper Sulfide Nanocrystals..................................................................................................85 5.1 Introduction.............................................................................................85 5.2 Experimental...........................................................................................88 5.2.1 Reaction Chemicals....................................................................88

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5.2.2 Reactor Setup and Procedure ......................................................88 5.2.3 Silicon Nanowire Synthesis........................................................89 5.2.4 Silica Nanotube Synthesis..........................................................90 5.2.5 Materials Characterization..........................................................90 5.3 Results and Discussion...........................................................................91 5.3.1 Silicon Nanowire Synthesis with CuS Nanocrystals..................91 5.3.2 Silica Nanotube Synthesis..........................................................94 5.3.3 Silica Nanotubes and Nanofibers Seeded with Au Nanocrystals97 5.3.4 Helical Silica Nanotubes Seeded by CuS Nanocrystals.............98 5.3.5 Silica/Metal Interface Morphology...........................................101 5.3.6 Sulfur Remaining in the CuS Seed Particle after Si Nanowire Growth......................................................................................103 5.3.7 A Small Proportion of Crystalline Si nanowires were Observed in the reactions with Trace Water and Oxygen.........................106 5.4 Conclusions...........................................................................................108 5.5 References.............................................................................................109 Chapter 6: Synthesis of Bipyramidal Germanium Telluride (GeTe) Particles and GeTe/Te Heterostructures...........................................................................119 6.1 Introduction .........................................................................................119 6.2 Experimental .......................................................................................120 6.3 Results and Discussion .......................................................................121 6.3.1 Bipyramidal germanium telluride (GeTe) nanoparticle ..........121 6.3.2 Heterostructured GeTe/Te Nanomaterials................................122 6.3.3 The Role of Octanol on GeTe/Te Heterostructure Synthesis .127 6.3.4 Surfactant Effect On GeTe/Te Heterostructure Synthesis ......128 6.4 Conclusions .........................................................................................129 6.5 References ...........................................................................................130 Chapter 7: Conclusions and Recommendations..................................................133 7.1 Conclusions...........................................................................................133

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7.1.1 High Yield Silicon Nanowire Synthesis in Supercritical Benzene ...................................................................................133 7.1.2 Nanocrystal-Mediated Crystallization of Silicon and Germanium Nanowires in Supercritical Fluid: The Role of Catalysis and Solid-Phase Seeding...........................................133 7.1.3 Germanium Nanowire Synthesis: An Example of Solid- Phase Seeded Growth with Nickel Nanocrystals ..............................135 7.1.4 Silicon Nanowires and Silica nanotbues Seeded by Copper Sulfide Nanocrystals ...............................................................136 7.1.5 Synthesis of Bipyramidal Germanium Telluride (GeTe) Particles and GeTe/Te Heterostructures .................................138 7.2 Recommendations.................................................................................139 7.2.1. Gram-Scale Silicon Nanowire Synthesis.................................139 7.2.2 Chemical Surface Passivation of Silicon Nanowires................140 7.2.3 Germanium Telluride Nanowire Synthesis...............................141 Bibliography (Heading 2,h2 style: TOC 2).........................................................142 Vita 156

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List of Tables Table 3.1: Summary of seed nanocrystal composition and selected properties and reaction conditions.....................................................................46 Table 4.1: Parameters Used to Calculate the Diameter Dependence of the Melting Temperature of Ni Using Eq1.............................................71

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List of Figures Figure 1.1: Phase diagram of Au:Si and a schematic illustration of a VLS type Au seeded silicon nanowire growth which involves (I) alloying, (II) liquidation, (III) supersaturation, and (IV) nanowire crystallization..3 Figure 1.2: Schematic pressure-temperature equilibrium phase diagram showing the triple point, the critical point and the supercritical region..................................................................................................6 Figure 1.3: Equilibrium phase diagram of n-hexane showing density as a function of temperature and pressure..................................................7 Figure 1.4: Shape control of colloidal nanocrystals via kinetic control of anisotropic crystal growth or selective adhesion of organic surfactants. a. The high-energy facets grow faster than low energy facets. b. kinetic shape control by selective adhesion of organic surfactants.(images taken from ref 34)...............................................9 Figure 1.5: Hybrid nanocrystal heterostructures starting from rod-like seeds: a second material nucleates at polar nanorod ends which have higher reactivity. (images taken from ref 35)..............................................11 Figure 2.1: Schematic of the high pressure reactor system used for nanowire synthesis in a supercritical fluid........................................................20 Figure 2.2: Formation of four center activated complex and the molecular phenyl bond rearrangement...............................................................23 Figure 2.3: Au-seeded silicon nanowires via SFLS using (a) hexane, (c) toluene and (e) hexane and the corresponding synthesis results (b-f)...........24

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Figure 2.4: XRD pattern of Si nanowire sample.................................................25 Figure 2.5: Silicon nanowire weight and yield of reactions carried out in different solvents...............................................................................26 Figure 2.6: Photograph of Si nanowires produced in a single reaction in supercritical benzene.........................................................................27 Figure 2.7: HRSEM images of silicon nanowires obtained from monophenysilane in the presence of Au nanoparticles in supercritical benzene with a Au/Si molar ratio of 1:1000................28 Figure 2.8: High-resolution TEM (HRTEM) images of a Si nanoiwre produced by Au-SFLS process in supercritical benzene..................................29 Figure 3.1: TEM images of metal nanocrystals studied as Si and Ge nanowire seeds: (a) Co, (b) Ni, (c) CuS (d) Mn, (e) Ir, (f) MnPt 3 , (g) Fe 2 O 3

and (h) FePt. Inset scale bars are 2 nm.............................................34 Figure 3.2: A semi-batch supercritical fluid experimental setup for silicon and germanium nanowire synthesis using different metal nanoparticles in supercritical fluid..........................................................................37 Figure 3.3: Molecular structures of selected organosilane Si precursors for investigation of catalytic properties of metal nanoparticle...............38 Figure 3.4: SEM images of (a-h) Si and (i-p) Ge nanowires synthesized in supercritical toluene from MPS (150 mM, 500°C, 10.3 MPa) and DPG (80 mM, 460°C, 10.3 MPa), respectively.............................40

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Figure 3.5: HRTEM images of Si (a-c) and Ge (d-f) nanowires seeded by (a-c) Co, (d-e) Fe 2 O 3 and (f) CuS showing <110>, <111> and <112> growth directions. Co, Fe 2 O 3 and CuS seeding gave Si and Ge nanowires with equal proportions of <111> and <110> oriented nanowires, with ~5% of the sample containing <112> oriented nanowires, usually with longitudinal {111} twins, as in (c). The 0.326 nm lattice spacing agrees with the (111) d-spacing for bulk Ge (0.327nm)....................................................................................41 Figure 3.6: TEM images of particles located at the tips of Si nanowires seeded with (a) Co and(c) CuS nanocrystals. Energy dispersive X-ray spectra (EDS) taken at the particle tips shows thecomposition to be (b) Co-Si and (d) Cu-Si alloys. In (b), the Cu signal is from the copper TEM grid and the Ni signal in (d) is from the nickel TEM grid. Note that in (d), no S signal was detected in the tip.................44 Figure 3.7: TEM images of particles observed at the tips of Ge nanowires seeded with (a) Co, (c) Fe 2 O 3 , and (e,f) FePt nanocrystals. The associated EDS data (b,d,g) was obtained by focusing the electron beam on the tip to reveal its composition. The Cu signal is from the TEM grid. Note the nanowire in (f), which shows the appearance of an interesting <113>-oriented twin at 29° withrespect to <111> growth axis........................................................................................45

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Figure 3.8: Si synthesized with octylsilane in toluene at 17.9 MPa and 460°C: SEM images of product obtained using (a) Au and (b) Ni nanocrystals ([Si]/[Ni]=100) and (c,d) TEM images of the Si nanowires synthesized by Ni-seeded SFSS from octylsilane. In (c) and (d), note the characteristic amorphous shell that coats the crystalline core that results from sidewall deposition of octylsilane.49 Figure 3.9: Si produced from trisilane in hexane at 14.3 MPa and 450 o C: SEM images of product obtained using (a) Au nanocrystals ([Si]/[Au]=5), (b) Ni nanocrystals ([Si]/[Ni]=10), and (c) Ni nanocrystals ([Si]/[Ni]=5). (d,e) TEM images of of nanowires obtained from trisilane in the presence of Ni nanocrystals. In contrast to nanowires grown from MPS and octylsilane, the nanowires shown here have grown in the <111> direction. Even in the case of the kinked wire in (D), the growth direction remains <111>................................................................................................50

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Figure 3.10: Co nanocrystal-seeded Si nanowires. HRSEM images of Si nanowires synthesized in supercritical toluene (10.3 MPa) from (a) MPS (500°C), (b) trisilane (400°C), and (c) octylsilane (500°C). HRSEM images of Si nanowires synthesized in supercritical toluene (10.3 MPa) from trisilane at (d) 350°C, (e) 400°C, and (f) 450°C. (g-i) TEM images of Si nanowires synthesized in supercritical toluene (10.3 MPa) with (d) trisilane (400°C), (e) trisilane (450°C) and (f) octylsilane (500°C). The nanowires in (h) and (i) are coated with an amorphous shell, as shown more clearly in the low resolution TEM images in the insets in (e) and (f). The shell material in (h) is amorphous Si and in (i) it is amorphous 3:1 C:Si (by EDS)..........................................................53 Figure 3.11: Si nanowire growth via SFSS. (a) Homogeneous MPS decomposition occurring by disproportionation versus heterogeneous MPS, octylsilane, and trisilane decomposition catalyzed by the Ni and Co surface. (b) Si atoms diffuse into the Ni and Co nanocrystal until reaching saturation. (c) Silicon nanowire nucleates and crystallizes from the Ni:Si or Co:Si alloy interface, growing to produce the high aspect ratio nanowire illustrated in (d).................................................................................55

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Figure 4.1: SEM images of Ni nanocrystal-seeded Ge nanowires obtained using 80 mM DPG fed into toluene at (a) 460°C and (b) 410ºC at 23.4MPa. (c) The pseudo Ni-Ge phase diagram shows the Ge nanowire synthesis temperature (“Genws”; dashed line) at 352ºC below the lowest eutectic temperature..............................................65 Figure 4.2: TEM images of Ni nanocrystal-seeded Ge nanowires revealing both (a-c) <110> and (d-f) <111> growth directions that are independent of diameter........................................................................................65 Figure 4.3: Histogram showing the relative occurrence of <110> and <111>- oriented Ge nanowires as a function of diameter..............................67 Figure 4.4: (a) TEM image of a Ni:Ge alloy seed particle at the end of a 14.5 nm Ge nanowire. Nanometer-scale EDS reveals (b) only Ge in the core of the wire and (c) Ge and Ni in the particle at the nanowire tip. The Cu signal is from the copper TEM grid............................68 Figure 4.5: HRTEM of two NiGe 2 seeds at the ends of (a) <111> and (b) <110> oriented Ge nanowires. (Insets) Fast Fourier transform (FFTs) of the HRTEM images. The FFTs index to orthorhombic NiGe 2 and the visible lattice spacings of (a) 0.25 nm and (b) 0.271 nm also match the NiGe2 (112) and (400) d-spacings, respectively.69 Figure 4.6: Size-dependent melting temperatures (T m ) of Ni and Au nanocrystals normalized by the bulk melting temperature (T o ) calculated using the modified Pawlow theory (Eqn 1).Note : The Au parameters were taken from Ref. 28...........................................71

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Figure 4.7: (a) TEM image of a Ni nanocrystal. (b,d,f) TEM images showinig the NiGe x seed particles at the tips of Ge nanowires with increasing diameters: 15 nm, 32 nm, 67 nm. (h) TEM image of Au seeds at the ends of 22 nm and 98 nm diameter Ge nanowires grown by SFLS. (c,e,g) Illustrations of seed particle aggregation that occurs during nanowire growth....................................................................74 Figure 4.8: Diameter distributions of Ge nanowires synthesized in toluene at 460°C, 23.4MPa, and a Ge:metal mole ratio of 100:1 using (a) 5.6 nm diameter Ni nanocrystals and (b) 2 nm diameter Au nanocrystals as seeds........................................................................76 Figure 4.9: TEM images of Ge nanowires seeded by (a-c) Ni and (d) Au nanocrystals......................................................................................78 Figure 4.10: Schematic of two possible SFSS growth mechanisms: (b-d) surface-enhanced solid state diffusion process; (e-h) solid-state volume counter-diffusion process.....................................................81

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Figure 5.1: Si nanowires synthesized in supercritical toluene at 10.3 MPa and 500°C using MPS as a reactant in the presence of CuS nanocrystals. (a) HRSEM image of Si nanowires. Inset: low-resolution TEM of three Si wires. (b-d) TEM images of Si nanowires with three different growth directions: <111>, <112>, and <110>. <111> is the predominant growth direction. (e) TEM image of a Cu-Si alloy particle at the end of a 19.3 nm diameter Si nanowire. (f) Nanobeam EDS data obtained from the metal seed at the tip of the nanowire, revealing the presence of Cu and Si. (The Ni signal originates from the Ni TEM grid.) (g) XRD peaks from the reaction product matches diamond cubic Si (PDF #27-1402)..........94 Figure 5.2: Silica nanotubes produced from MPS in supercritical toluene at 10.3 MPa at 500°C with trace water and oxygen in the presence of CuS nanocrystals. (a) HRSEM image of a field of silica nanotubes. (b-g) TEM images of silica nanotubes. The dark particles in the images are Cu. Note that the nanotubes in (e) and (f) have a bamboo morphology. (h) EDS linescans across the silica nanotube in the inset. Both oxygen and silicon are present and their concentration profiles mirror each other............................96 Figure 5.3: (a,c) STEM images and (b,d) EELS line scans of an (a,b) Si nanowire synthesized in supercritical toluene at 500 o C, 10.3 MPa and CuS nanocrystals under inert conditions with MPS and (c,d) an SiO 2 nanotube made under similar reaction conditions in the presence of water and oxygen...........................................................97

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Figure 5.4: Silica nanotubes and nanofibers formed in the presence of Au nanocrystals by decomposing 350 mM MPS in toluene at 10.3 MPa at 500°C with Si:Au=1000:1 and trace water and oxygen: (a) HRSEM of a field of silica nanofiber; (b-c) TEM images of silica nanofibers with Au nanoparticle at their ends; (d) TEM of a region with a mix of silica nanofibers and nanotubes and (e) a single silica nanotube seeded with Au. Nanotubes made up approximately 5% of the sample.....................................................................................98 Figure 5.5: Helical silica nanotubes seeded by CuS nanocrystals in toluene at 10.3 MPa and 500°C with 150 mM MPS and trace water and oxygen.............................................................................................100 Figure 5.6: TEM images of (a) Cu nanoparticles embedded in Si formed by MPS decomposition in toluene at 500 o C and 10.3 MPa under inert reaction conditions (i.e., no oxygen and water); (b) Cu nanoparticles embedded in SiO 2 formed when trace oxygen and water were added to the reactions; and (c) Au nanoaprticles embedded in SiO 2 formed when trace oxygen and water were added to the reactions................................................................................102

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Figure 5.7: (a-e) TEM images of silicon nanowires with amorphous Cu-Si-S “seed” particles. The TEM images in (f-h) are higher magnification images obtained at different positions of the nanowire in (e). The image of the nanowire shown in (f) reveals that the nanowire is crystalline. The images in (g) and (h) show that the seed is an amorphous cluster of particles. (i) EDS on the tip confirms the presence of Si, Cu and S.................................................................104 Figure 5.8: TEM images of an Si nanowire that was seeded by an amorphous Cu-Si-Si particle. Higher magnification images along the length of the nanowire (b-h) reveal many extended defects, including {111} twin planes...........................................................................105 Figure 5.9: TEM images of a crystalline Si nanowire where the seed particle ends up surrounded by an amorphous coating that physically separates the seed form the nanowire.............................................106 Figure 5.10: Very large diameter (>150 nm) crystalline Si nanowires with very rough surfaces were also found as a byproduct in the reactions carried out with trace oxygen in which silica nanotubes were the primary reaction product.................................................................107 Figure 6.1: SEM images of bipyramidal GeTe nanoparticles (a-c) and simulated octahedral structure........................................................122 Figure 6.2: GeTe-Te nanoparticle-nanorod heterostructure synthesis by adding 10% octanol in a reaction. The Te nanorods were grown epitaxially on some facet planes of GeTe particles..........................................123 Figure 6.3: HRTEM of GeTe/Te heterostructures............................................124

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Figure 6.4: EDS of (a)GeTe particles and (b) Ge-doped Te nanorods..............125 Figure 6.5: X-ray diffraction pattern of as GeTe/Te heterostructures...............126 Figure 6.6: SEM images of synthesis result of GeTe/Te heterostructure by adding (a) 0% (b) 5% (c) 10%, and (d) 20% octanol......................128 Figure 6.7: The synthesis result of GeTe/Te heterostructures using (a) octanol, (b) oleic acid, (c) isoprene, and (d) 1-hexadecanethiol...................129 Figure 7.1: Figure 7.1. A 250 ml Parr reactor apparatus for scale-up supercritical fluid nanowire synthesis. (a) reactor design and (b) a 250 ml reactor cell..........................................................................140

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Chapter 1: Introduction 1.1 ONE - DIMENSIONAL N ANOSTRUCTURES

Intensive efforts of materials science focus on dealing with nanoscale materials, i.e., with characteristic dimensions between 1 and 100 nm. The advance of synthetic strategies and characterization techniques help researchers precisely control and utilize these tiny materials. When the size shrinks to the nanoscale, the large surface-to-volume ratio and quantum size effects of materials give rise to unique electrical, optical, magnetic, mechanical and chemical properties and can be used in diverse applications such as optoelectronics, sensing, catalysts, medical cures and have been proposed as the building blocks of future electronics. In addition to size, the dimensionality of the nanostructures has strong affects material properties. One-dimensional (1D) nanostructures, such as nanowires, nanorods, and nanotubes, have attracted attention because of their possible use as electronic channels, interconnects and functional building blocks in electronic, optoelectronic and nanofluidic devices. For example, semiconductor nanowires can be used to fabricate various prototype nanoscale electronic devices such as field effect transistors, photodetectors, and chemical sensors. 1-5 Hollow inorganic nanotubes might serve as nanoscale pipes to transport fluid and molecular species and have been proposed as building blocks for nanofluidic systems. 6-9

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Semiconductor nanorods have been explored for use in efficient solar cell devices. 10- 12 Group IV semiconductors such as silicon, germanium and their compounds, germanium telluride, and silica are interesting because of their importance in the semiconductor industry. Therefore, understanding, the effect of dimensionality and size control on nanostructures is of great interest and robust synthetic methods of these nanomaterials are desired. 1.2

N ANOWIRE SYNTHESIS

Vapor-liquid-solid (VLS) growth is an effect approach to synthesize group IV nanowires that are single crystalline, straight with a narrow size distribution, and free of defects. The VLS growth mechanism was first discovered by Wagner and Ellis in the 1960s. 13 In the first VLS experiments, silicon atoms from degradation of a silicon precursor from vapor phase by chemical vapor decomposition were deposited on an Au-film coated silicon substrate (CVD). Because the substrate temperature is above the Au:Si eutectic point, the silicon dissolves into the Au to form a liquid Au:Si eutectic droplets instead of depositing on the surface. C ontinuous feeding of silicon atoms into this liquid Au:Si eutectic droplets leads to supersaturation and nucleation of solid Si from the droplet and sequential wire growth. In conclusion, VLS nanowire growth involves four stages: (1) alloying, (2) liquidation, (3) supersaturation, and (4) nanowire nucleation

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Figure 1.1 illustrated the VLS growth mechanism of Au-seeded Si nanowires. Successful wire growth by VLS requires crystallization and a sufficient concentration of semiconductor to sustain growth. The growth temperature must exceed the metal/semiconductor eutectic temperature. 14 The concentration of the semiconductor must be large enough to sustain nanowire growth. In binary metal- semiconductor systems such as Au-Si, Fe-Si, Co-Si, and Ni-Si…etc., offering a reaction condition with high temperature over binary eutectic temperatures and high concentration of feeding semiconductor atoms can promote the whisker growth.

Figure 1.1: Phase diagram of Au:Si and a schematic illustration of a VLS type Au seeded silicon nanowire growth which involves (I) alloying, (II) liquidation, (III) supersaturation, and (IV) nanowire crystallization.

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The use of nanosize metal particles to seed nanowire growth is critical to obtaining wires with nanometer scale distrbution. In 1995, Buhro et al. used a solution-liquid-solid (SLS) method to synthesize GaAs and GaP nanowires in organic solvent. In 1998, Morales and Lieber used a laser ablation method to produce nanoscale nanoparticles to seed Si and Ge nanowire with diameters less than 30 nm via VLS mechanism. 15 They later used this laser catalyzed growth (LCG) method to synthesize various semiconductors including, Si, Ge, GaAs, and GaP. Followed their work, chemical vapor depositon (CVD) and supercritical fluid synthesis were later developed to produce other semiconductor nanowires such as GaAs, InAs and GaP nanowires. 15-25

The problem with laser ablation is that a very broad size distribution of nanoparticles is generated, leading to a broad nanowire distribution. In 2000, Holmes et al., reported using dodecanethiol-coated monodisperse Au nanoparticles to seed nanowires with diameters smaller than 10 nm and relatively narrow size distributions by using a supercritical fluid at a high temperature and a high pressure. Lieber and co-workers later reported controlling the diameter Si and InP grown by VLS nanowires with a narrow distribution of Au nanoparticles by combing CVD using size-monodisperse Au particles. 26

Although CVD based nanowire synthesis allows high temperature reaction conditions and can be used to produce many different semiconductor nanowires, the throughput of one-batch nanowires is much less than the solution-based method. In a

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solution-based synthesis, a larger reaction volume and higher precursor concentration gives much larger quantities of nanowires. Buhro and co-workers have shown various III-V nanowires can be synthesized in high boiling point organic solvents; however, this method can not applied to synthesize Si and Ge nanowires because the boiling points of most organic solvents are lower than the eutectic temperatures of Si and Ge. 16 Organic solvents, however, can be pressuried and heated above their critical points to access very high temperatures (up to 600 ˚C), offering reaction conditions suitable for Si and Ge nanowire synthesis. Nanowire growth in supercirtical conditions using metal nanocrystals as seeds is called supercritical fluid-liquid-solid growth mechanism (SFLS). The SFLS method has been developed to produce crystalline Si and Ge nanowires with diameters less than 30 nm and length longer than 10µm in solution. Moreover, as shown in Chapter 2, the SFLS method is proven to produce silicon nanowires of ~50mg in a single reaction.

1.3

S UPERCRITICAL F LUIDS

A supercritical fluid (SCF) is a substance with a pressure and temperature above its critical point. Figure 1.2 shows the general pressure-temperature phase diagram and shows that a single phase fluid exists beyond the critical point where the vapor-liquid coexistence curve disappears. SCFs have the properties such as density and diffusivity intermediate between those of liquids and gas and can be

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modulated gas-like to liquid-like behavior by tiny changes in temperature and pressure. 27-28 As shown in Figure 1.3, the density of the solvent can be changed from 0.1g/m to 0.6 g/m by adjusting temperature and pressure, which causes big different statuses of the same solvent. High diffusivity and low viscosity of SCFS also provide an idea platform for transporting reactants which is usually limited in conventional liquid phase.

Figure 1.2: Schematic pressure-temperature equilibrium phase diagram showing the triple point, the critical point and the supercritical region.

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Figure 1.3: Equilibrium phase diagram of n-hexane showing density as a function of temperature and pressure Supercritical carbon dioxide and water are the most common supercritical media and are used in industrial processes such as extraction, chromatography, and cleaning. A few years ago, researchers start to apply SCFs as part of the synthetic strategies to produce different nanomaterials. For example, Cu and CuO nanoparticles were synthesized in supercritical water and semiconductor Si and Ge nanoparticles were synthesized in supercritical hexane. 29-32 Arrested precipitation methods using organic surfactants were used to stabilize the nanoparticles and control their sizes. In the Korgel group, one dimensional nanomaterials such as

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silicon and germanium nanowires and carbon nanotbues were also synthesized via a supercritical fluid-liquid-solid growth mechanism. In conclusion, supercritical fluid is considered as a useful platform for preparing one-dimensional nanomaterials, providing access to high reaction temperatures (up to 600 ˚C) for degradation of precursors and crystallization. 1.4 S URFACTANT - MEDIATED C OLLOIDAL NANOCRYSTAL S YNTHESIS

Colloidal nanocrystal synthesis in solution often involves the interactions between inorganic nanoparticles and organic surfactants to control the size and shape of nanoparticles. Surfactant molecules adhere to the surfaces of growing nanocrystals and act as stabilizing agents (also called capping ligands) to control the size and shape of nanocrystals. Murray, Norris and Bawendi in 1993 33 synthesized monodisperse CdSe nanoparticles using trioctylphosphine oxide (TOPO) as surfactants. These organic surfactants such as alkyl phosphine oxide, alky phosphonic acids, alkyl posphines, fatty acids and amines have a bonding head group and a hydrocarbon chain with hydrophobic character. In addition to nanocrystals with nearly spherical shapes, nanocrystals with anisotropic crystallographic characteristics have different energies in different surfaces. Since the growth rate of surfaces is exponentially proportional to the surface energy, highly anisotropic shapes of nanocrystals such as nanorods, nanodisks are obtained. The surface energy of the nanocrystals can be changed by introducing surfactants undergo selective adhesion on the nanocrystal’s surface. The

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surface energy of the particular facets selectively stabilized by organic surfactants is lower and grows slower than the other facets. Selective adhesion of surfactants can not only direct elongation along one axis but it can also compress the plane growth along other axes as shown in Figure 1.4b. 34 More complicated shapes of nanocrystals can be obtained by this selective adhesion mechanism of organic surfactants. . Organic surfactant-mediated synthesis of nanocrystals can be extended to hybrid nanocrystal synthesis. When some materials such as cadmium chalcogenide nanoparticles form in the wurtzite structure in the presence of some surfactants, the polar facets grow much faster than non-polar facets and form rods or tetrapods. The polar facets have higher reactivity and can allow a second material nucleate at the location, forming hybrid nanoparticles in some cases as shown in Figure 1.5. 35-36 As described above, by interaction with inorganic nanoparticle surface, organic surfactants have been proposed as useful tools to manipulate the growth of colloidal nanocrystals. Moreover, organic surfactants are expected to have more structural and compositional control over the colloidal nanocrystal synthesis. For example, Chapter 6 respresents GeTe/Te heterostructure synthesis as one illustration of organic surfact mediated growth of colloidal nanomaterial heterostructures

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Figure 1.4: Shape control of colloidal nanocrystals via kinetic control of anisotropic crystal growth or selective adhesion of organic surfactants. a. The high- energy facets grow faster than low energy facets. b. kinetic shape control by selective adhesion of organic surfactants.(images taken from ref 34)

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Figure 1.5: Hybrid nanocrystal heterostructures starting from rod-like seeds: a second material nucleates at polar nanorod ends which have higher reactivity. (images taken from ref 35) 1.5 D ISSERTATION OVERVIEW

Chapter 2 represents an example of scale-up synthesis of silicon nanowires using an Au-seeded SFLS growth mechanism. Silicon precursor decomposition mechanism in solution is crucial for nanowire synthesis and is discussed in depth. The influence of various solvents on the conversion of Si precursors for silicon nanowire growth is investigated. Optical photographs and the weight of collected Si nanowires from Au-seeded Si nanowire synthesis in different solvents are compared. Silicon and germanium nanowire seeded by different metal nanoparticles instead of Au in supercritical fluid are discussed in Chapter 3. The morphology, yield, crystallinity of silicon and germanium nanowires using eight different metal

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Abstract: A supercritical fluid-liquid solid (SFLS) nanowire growth process using alkanethiol-coated Au nanoparticles to seed silicon nanowires was developed for synthesizing silicon nanowires in solution. The organic solvent was found to significantly influence the silicon precursor decomposition in solution. 46.8 mg of silicon nanowires with 63% yield of silicon nanowire synthesis were achieved while using benzene as a solvent. The most widely used metal for seeding Si and Ge nanowires is Au. However, Au forms deep trap in both Si and Ge and alternative metal seeds are more desirable for electronic applications. Different metal nanocrystals were studied for Si and Ge nanowire synthesis, including Co, Ni, CuS, Mn, Ir, MnPt 3 , Fe2 O3 , and FePt. All eight metals have eutectic temperatures with Si and Ge that are well above the nanowire growth temperature. Unlike Au nanocrystals, which seed nanowire growth through the formation of a liquid Au:Si (Au:Ge) alloy, these other metals seed nanowires by forming solid silicide alloys, a process we have called "supercritical fluid-solid-solid" (SFSS) growth. Moreover, Co and Ni nanoparticles were found to catalyze the decomposition of various silane reactants that do not work well to make Si nanowires using Au seeds. In addition to seeding solid nanowires, CuS nanoparticles were found to seed silica nanotubes via a SFSS like mechanism. 5% of synthesized silica nanotubes were coiled. Heterostructured nanomaterials are interesting since they merge the properties of the individual materials and can be used in diverse applications. GeTe/Te heterostructures were synthesized by reacting diphenylgermane (DPG) and TOP-Te in the presence of organic surfactants. Aligned Te nanorods were grown on the surface facets of micrometer-size germanium telluride particles.