REVISTA DE BIOLOGIA E CIÊNCIAS DA TERRA ISSN 1519-5228 - Artigo_Bioterra_V24_...
Vapour Solid Liquid Growth
1.
2. Introdution
Process
Requirements
Various Nanowires
Control of Size
Precursors and catalysts
3. VLS- Vapour Solid Liquid Growth
VLS is a method for growth of One-Dimensional Structure like Nanowires
from Chemical Vapour Deposition.
The name VLS mechanism reflects the pathway of Si, which coming from
the vapour phase diffuses through the liquid droplet and ends up as a
solid Si wire.
4. o Take Silicon Wafer
o Deposit a thin Au film(~1-10 nm) which act as catalyst
Via Sputter deposition or thermal evaporation
o Silicon + Gold 385 C Liquid Mixture(Droplet)
o Silicon Species evaporate
o Diffusion (Liquid-Vapour Interface )
o Condense Liquid Droplet
o Supersaturation
o Precipitate (Solid-Liquid Interface)
o Nucleation
o Crystal Growth in unidirection ( Solid-Liquid Interface)
5.
6. Must form Liquid solution
Distribution coefficient must be less than unity
Equilibrium vapour Pressure over liquid droplet
must be small
Physically active but Chemically inert
Solid solubility must be low
7. Solid-Liquid interface must be well defined
crystallographically
General Supersaturation should kept relatively low
For Cylindrical shape:
Lateral Growth rate < Longitudinal Growth rate
8. For Si Nanowires:
Catalyst-Mixture of silicon powder with 5 wt% Fe
Temp-1200 C
Nanowire Diameter ~15nm
Nanowire Length – few 10 to several 100 micrometers
Amorphose oxide layer of ~2nm over coated the
outside of silicon nanowires
For GaAs Nanowires:
Catalyst- Gold, Silver & Copper
9. Size of Nanowire ~ Liquid Catalyst Droplet ~ Layer thickness of Catalyst
For 10nm Au size~ Germanium nanowire 150nm
For 5nm Au size~ Germanium nanowire 80nm
Further Reduction ~ No Change
Use of Monosized catalyst colloids:
Size of Nanowire ~ Catalyst nanoclusters
For 28.2 nm diameters Gold colloids ~ 30.2 nm GaP nanowires
For 18.5 nm diameters Gold colloids ~ 20.0 nm GaP nanowires
For 8.4 nm diameters Gold colloids ~ 11.4 nm GaP nanowires
Further Reduction ~ Small Droplet ~ High Solubility ~ High Supersaturation
Results in Lateral Growth on side surface
Nanowires like conical structure
10. For Silicon Nanowires:
Gaseous Precursors: SiCl4
For Ge Nanowires:
Precursors: Ge(s) + GeI4(g) 2GeI2(g)
In Liquid Catalyst: 2GeI2 Ge(l) + GeI4(g)
ZnO Nanowires:
Catalyst: Au coated ~2-50nm
1:1 mixture of ZnO & Graphite powder
Temp:900-925 C
Constant flow of argon ~ 5-30min
ZnO + C Zn + CO
Au Coating of 50 nm ~ 80-120 nm Nanowires diameter~ 10-20 μm length
Au Coating of 3 nm ~ 40-70 nm Nanowires diameter ~ 5-10 μm length