Metal-free-catalyst for the growth of Single Walled Carbon Nanotubes

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Metal-free-catalyst for the growth of Single
Walled Carbon Nanotubes

• P. Ashburn, T. Uchino, C.H. de Groot
• School of Electronics and Computer Science
• D.C. Smith, G. N Ayre, K.N. Bourdakos
• School of Physics and Astronomy
• A.L. Hector, B. Mazumder
• School of Chemistry
• University of Southampton

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Contents

• Aims
• New CNT growth process using Ge
• Initial results
• Possible mechanisms
• Refinement of CNT process
• Recent results
• Conclusions

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Aims
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Standard Growth Methods

• Require
• Metal catalyst nanoparticles
• Metals include Fe, Ni, Co and others
• Particles need to be a few nanometers in diameter
• Carbon containing gas
• SWNT favoured by smaller non-conjugated molecules
• Energy to decompose the feedstock
• Thermal energy CVD
• RF Plasma enhanced CVD
• Often include
• Hydrogen
• encourages SWNT growth
• Oxidising agent
• appears to regenerate catalyst

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Non-metallic routes to SWNT

• But standard standard growth methods have
  disadvantage of using metal catalysts
• Metals are lifetime killers in silicon also
  degrade yield
• Non-metallic catalyst is desirable for silicon
  compatibility
• Aim of this research is to search for
  non-metallic routes to SWNT growth

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A New CNT Growth Method using Germanium
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New Germanium based route to SWNT

• Start with either
• SiGe (30 Ge) layer grown on silicon
• Or
• Stranski-Krastanow Ge (d 20-250nm) dots on
  silicon

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Pre-Treat Substrates

• Carbon ion implant 3?1016 cm-2 30keV
• Strip native oxide with HF vapour
• Chemically oxidise with H202

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Chemical Vapour Deposition
Gas inlet (CH4, Ar, H2)
Quartz tube
Gas outlet
Substrate
Ceramic boat
Oven
Two Step Process Anneal 1000oC, H2 300 sccm,
Ar 1000 sccm, 10min Growth 850oC, CH4 1000
sccm, Ar 300 sccm, 10 min
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Initial results
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SEM Post Growth SiGe Samples
As-grown

• Two types of fibre observed after growth
• Fat Curly Fibres
• Removed by HF vapour etch
• Oxide nanofibers
• Thin straighter fibres
• Remain after HF vapour etch
• Carbon nanotubes

After HF Treatment
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Raman Spectra

• lexcitation 633nm
• Clear G-band signal
• No D- band observed
• Radial breathing modes indicate SWNT with
  diameters in range 1.2-1.6nm
• Thick fibres give broad peak at 1400 cm-1 similar
  to ones reported for amorphous carbon.

CNTs
CNTs
Thick fibers
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Ge dot samples
Raman spectra of Ge dot sample
CNTs on Ge dots
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TEM Images
A bundle of SWNTs
Oxide nanofibers
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Possible Mechanisms
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Analysis of Experimental Data
Sample Implant Pre-treatment Growth Gas CNT growth
1 C ion H2O2 CH4, H2 CNTs
2 C ion - CH4, H2 Lower CNT density
3 C ion H2O2 Ar, H2 No CNTs, oxide fibres
4 No H2O2 CH4, H2 No CNTs
5 No - CH4, H2 No CNTs
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Further Analysis

• Evidence of C diffusion to surface
• C expected to aid nanotube growth
• Ge nanoparticles formed during pre-anneal

SEM
After implant
After pre-heating
AFM
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Possible Mechanisms

• Vapour-liquid-solid growth one possibility Ge
  nanoparticle would be seen at tip of nanotube
• Nanotube growth from root another possibility
• TEM shows no evidence on particle at tip

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Refinement of CNT Process
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Issues

• Ge nanoparticles responsible for growth
• But need to control nanoparticle size
• Ge implantation widely used to create Ge
  nanoparticles in oxide
• Has advantage that nanoparticle size controlled
  by implant dose and anneal conditions

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Ge Nanoparticle Fabrication
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Recent Results
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Ge Nanoparticles
3E16cm-2 Ge implant No C implant 600C anneal
3E16cm-2 Ge implant No C implant 1000C anneal
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Ge Nanoparticle Sizes
3E16cm-2 Ge implant No C implant 600C anneal
3E16cm-2 Ge implant C implant 600C anneal

• 600C anneal gives 2nm Ge nanoparticles
• C implant gives smaller Ge nanoparticles

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After Nanotube Growth
3E16cm-2 Ge implant No C implant 600C anneal
3E16cm-2 Ge implant No C implant 1000C anneal
Carbon nanotubes formed
No carbon nanotubes formed

• New process allows nanotube growth without C
  implant

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Analysis of Experimental Data

• Carbon implant widens process window for nanotube
  growth

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Effect of Carbon Implant
No C implant
C implant

• No D band for C implanted samples
• Carbon implant improves nanotube quality

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Conclusions

• Developed a new route to SWNT growth
• Evidence shows Ge nanoparticles key to growth
• SWNTs produced have diameter range 1.2 -1.6nm
• SWNTs are highly quality as measured by Raman
• Implanted Ge nanoparticles give more reproducible
  SWNT growth.
• C implant widens process window for SWNT growth
  improves nanotube quality