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.
The impinging growth species onto the surface is function of residence time and /or diffusion distance before escapting back to the vapor phase.
4. Kink site (4 CB), 5. ledge-kink site (3 CB), 7. adatom (1 CB, unstable), 8. ledge site (2 CB)
(100), F-face, unstable
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.
Growth of Se Nanowires
Growth of SeTe Nanowires
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.
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. The growth temperatures correspond to ranges exploredin these studies. The minimum and average nanowire diameters were determined from TEM and FESEMimages. Structures were determined using electron diffraction and lattice resolved TEM imaging: ZB, zincblende; W, wurtzite; and D, diamond structure types. Compositions were determined from EDX measurementsmade on individual nanowires. All of the nanowires were synthesized using Au as the catalyst, except GaAs, forwhich Ag and Cu were also used. The GaAs nanowires obtained with Ag and Cu catalysts have the same sizedistribution, structure, and composition as those obtained with the Au 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).
Methods for Growth of CNTs
Water-cooled copper collector
Furnace at 1200 C
Laser Ablation Process
Nanotube growing along tip of collector
mass flow controller
auto pressure controller
Formation of nanotubes
Note: The target may be made by pressing Si powder mixed with 0.5% iron.
Advantages of Oxide Assisted Growth (OAG) Si Nanowires
Growth of Silicon Nanowires by SLS method
Using polycarbonate membrane as template – Electrochemical Deposition (for conducting polymer) or electroless electrolysis (for polymer)
Stabilization of colloids is generally achieved by electrostatic double layer mechanism.
Using polycarbonate membrance as template
Centrifugation Force which must be greater than the repulsion force between particles
Electrospining – using electric forces (overcome surface tension) to porduce polymer fibers