Chapter 4 One-Dimensional Nanostructures : Nanotube, Nanowires and Nanorods

Synthesis Methods Bottom up:

Spontaneous Growth A growth driven by reduction of Gibbs free energy or chemical potential. This can be from either recrystallization or a decrease in supersaturation. Growth along a certain orientation faster than other direction – anisotropic growth. For nanowire, growth occurs only along one direction, but no growth along other directions.

Evaporation (Dissolution) – Condensation Growth

Rate Limiting Steps Step 2 can be rate limiting if the supersaturation or concentration of growth species is low. When a sufficient supersaturation or a high concentration of growth species is present, Step 4 will be the rate-limiting process. Either adsorption-desorption of growth species on the growth surface (step 2) or surface growth (step 4) can be rate limiting process.

Step 2 is rate limiting process The growth rate is determined by condensation rate, J (atoms/cm2sec). J = ασPo/(2πmkT)2, Where α is the accommodation coefficient, and σ= (P – Po)/Po and is the supersaturation of the growth species in the vapor in which Po is the equilibrium vapor pressure (or concentration of the growth species in the vapor) of the crystal at temperature T, m is the atomic weight of the growth species, k is the Boltzmann constant, αis the fraction of impinging growth species that becomes accommodated on the growing surface, and is a surface specific property. A significant difference in α in different facets would result in anisotropic growth.

The impinging growth species onto the surface is function of residence time and /or diffusion distance before escapting back to the vapor phase.

Step Growth or KSV Theory 4. Kink site (4 CB), 5. ledge-kink site (3 CB), 7. adatom (1 CB, unstable), 8. ledge site (2 CB)

BCF Theory – presence of screw dislocation ensures continuous growth and enhances the growth rate

Mechanism leading to formation of nanowire The growth rate of a facet increases with an increased density of screw dislocations parallel to the growth direction. It is known that different facets can have a significantly different ability to accommodate dislocations. The presence of dislocations on a certain facet can result in anisotropic growth, leading to the formation of nanowire or nanorods.

PBC Theory (100), F-face, unstable (110), S-face (111), K-face

Mechanism leading to Anisotropic Growth Therefore, α = 1 for {111} and {110}, α< 1 for {100}. This leads to that growth rate for {111}, {110} is greater than that for {100}. For both {111} and {110}, the growth process is always adsorption limited. Facets with high growth rate (high surface energy) disappears while facets with low growth rate (low surface energy) survives. This leads to anisotropic growth and results in nanowires. In addition, defects-induced growth and impurity-inhibited growth are the possible mechanisms for growth along axis of nanowires. A low supersaturation is required for anisotropic growth. A higher supersaturation supports bulk crystal growth or homogeneous nucleation leading to formation of polycrystalline or powder.

Growth of Single Crystal Nanobelts of Semiconducting or metal oxides Evaporating the metal oxides (ZnO, SnO2, In2O3, CdO) at high temperatures under a vacuum of 300 torr and condensing on an alumina substrate, placed inside the same alumina tube furnace, at relatively low temperature. Or heating the metal oxide or metal nanoparticles at T=780 - 820oC in air, Nanorods can be obtained depending upon annealing T and time. Nanowires such as ZnO, Ga2O3, MgO, CuO or Si3N4 and SiC can be made by this method.

By controlling growth kinetics, a consequence of minimizing the total energy attributed by spontaneous polarization and elasticity, left-handed helical nanostructures and nano-rings can be formed.

Dissolution and Condensation Growth The growth species first dissolve into a solvent or a solution, and then diffuse through the solvent and deposit onto the surface resulting growth of nanowires.

Growth of Se Nanowires

Growth of SeTe Nanowires

Growth of Ag Nanowire Using Pt Nanoparticles as Growth Seeds Precursor: AgNO3 Reduction agent: ethylene glycol Surfactant: polyvinyl pyrrolidone (PVP) The surfactant absorbed on some growth surfaces and blocks the growth, resulting in the formation of uniform crystalline silver nanowires.

Disadvantages of Evaporation – Condensation Deposition Nanowire grown by EC most likely have faceted morphology and are generally short in length with relatively small aspect ratios, particular when grown in liquid medium. However, anisotropic growth induced by axial imperfections, such as screw dislocation, microtwins and stacking faults, or by impurity poisoning, can result in the growth of nanowires with large aspect ratios.

Vapor (or solution)-Liquid-solid (VLS) Growth It is noted that the surface of liquid has a large accommodation coefficient, and is therefore a preferred site for deposition.

Wagner summarized the requirements for VLS growth over 30 years ago.

VLS Growth Process

The growth rate for VLS is much faster The liquid surface can be considered as a rough surface. Rough surface is composed of only ledge, ledge-kink, or kink sites. That is, every site over the entire surface is to trap the impinging growth species. The accommodation coefficient is unity. It is reported that the growth rate of silicon nanowire using a liquid Pt-Si alloy is about 60 times higher than directly on the silicon substrate at 900oC. The liquid acts as a sink for the growth species in the vapor phase, it also act as a catalyst for the heterogeneous reaction or deposition.

Compound Semiconductor Nanowires Nanowires of binary group III-V materials (GaAs, GaP, InAs, and InP), ternary III-V materials (GaAs/P, InAs/P), binary II-VI compounds (ZnS, ZnSe, CdS, and CdSe), and binary IV-IV SiGe alloys have been made in bulk quantities as high purity (>90%) single crystals.

Table 1. Summary of single crystal nanowires synthesized Table 1. Summary of single crystal nanowires synthesized. The growth temperatures correspond to ranges explored in these studies. The minimum and average nanowire diameters were determined from TEM and FESEM images. Structures were determined using electron diffraction and lattice resolved TEM imaging: ZB, zinc blende; W, wurtzite; and D, diamond structure types. Compositions were determined from EDX measurements made on individual nanowires. All of the nanowires were synthesized using Au as the catalyst, except GaAs, for which Ag and Cu were also used. The GaAs nanowires obtained with Ag and Cu catalysts have the same size distribution, structure, and composition as those obtained with the Au catalyst.

Choice of Catalyst The catalysts for VLS growth can be chosen in the absence of detailed phase diagrams by identifying metals in which the nanowire component elements are soluble in the liquid phase but that do not form solid compounds more stable than the desired nanowire phase; i.e., the ideal metal catalyst should be physically active but chemically stable. From this perspective the noble metal Au should represent a good starting point for many materials. This noble metal also has been used in the past for the VLS growth of surface supported nanowires by metal-organic chemical vapor deposition (MOCVD).

In general, the nanowires grown by VLS have a cylindrical morphology, i.e. without facets on the side surface and having a uniform diameter. This is attributed to the growth at a temperature greater than the roughening temperature (surface undergoing a transition from faceted (smooth) to rough surface).

VLS Growth of Nanowires Precursors: Compound gas (e.g. SiCl4), evaporation of solids, Laser ablation of solid targets.

Methods for Growth of CNTs Furnace at 1200 C Ar gas Graphite target Laser Water-cooled copper collector Nanotube growing along tip of collector Laser Ablation Process Formation of nanotubes Note: The target may be made by pressing Si powder mixed with 0.5% iron. Arc-Discharge System Water in out graphite, cathode graphite anode Water in He pump Water out Power Supply mass flow controller auto pressure controller

Advantages of Oxide Assisted Growth (OAG) Si Nanowires pp. 635-640.

SLS Growth of InP Nanowires

Growth of Silicon Nanowires by SLS method

Template-Based Synthesis-Electrochemical Deposition

Electroless Electrolysis

Using polycarbonate membrane as template – Electrochemical Deposition (for conducting polymer) or electroless electrolysis (for polymer)

Electrophoretic Deposition Stabilization of colloids is generally achieved by electrostatic double layer mechanism.

Using polycarbonate membrance as template

Template Filling

Template Filling

Incomplete Filling of the Template

Template Filling Assisted with Centrifugation Force Centrifugation Force which must be greater than the repulsion force between particles

Converting Through Chemical Reactions

Electrospining – using electric forces (overcome surface tension) to porduce polymer fibers