VLS is a method for growth of One-Dimensional Structure like Nanowires from Chemical Vapour Deposition.

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)

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