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

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

Aims

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

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

A New CNT Growth Method using Germanium

New Germanium based route to SWNT Start with either SiGe (30% Ge) layer grown on silicon Or Stranski-Krastanow Ge (d = nm) dots on silicon

Pre-Treat Substrates Carbon ion implant – 3  cm -2 30keV Strip native oxide with HF vapour Chemically oxidise with H 2 0 2

Chemical Vapour Deposition Substrate Gas outlet Quartz tube Ceramic boat Oven Gas inlet (CH 4, Ar, H 2 ) Two Step Process Anneal – 1000 o C, H sccm, Ar 1000 sccm, 10min Growth – 850 o C, CH sccm, Ar 300 sccm, 10 min Temperature (C) Time (min) RT

Initial results

SEM Post Growth: SiGe Samples 500 nm (a) 500 nm (a) 500 nm (b) 500 nm (b) As-grown After HF Treatment 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

Raman Spectra CNTs Thick fibers excitation = 633nm Clear G-band signal No D- band observed Radial breathing modes indicate SWNT with diameters in range nm Thick fibres give broad peak at 1400 cm-1 similar to ones reported for amorphous carbon.

Ge dot samples Raman spectra of Ge dot sampleCNTs on Ge dots

TEM Images A bundle of SWNTs (b) (a) 10 nm 50 nm (b) (a) 10 nm 50 nm Oxide nanofibers

Possible Mechanisms

Analysis of Experimental Data SampleImplantPre-treatmentGrowth GasCNT growth 1C ionH2O2H2O2 CH 4, H 2 CNTs 2C ion-CH 4, H 2 Lower CNT density 3C ionH2O2H2O2 Ar, H 2 No CNTs, oxide fibres 4NoH2O2H2O2 CH 4, H 2 No CNTs 5No-CH 4, H 2 No CNTs

Further Analysis Evidence of C diffusion to surface C expected to aid nanotube growth Ge nanoparticles formed during pre-anneal After implantAfter pre-heating SEM AFM

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

Refinement of CNT Process

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

Ge Nanoparticle Fabrication

Recent Results

Ge Nanoparticles 3E16cm -2 Ge implant No C implant 600C anneal 3E16cm -2 Ge implant No C implant 1000C anneal

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

After Nanotube Growth Carbon nanotubes formedNo carbon nanotubes formed 3E16cm -2 Ge implant No C implant 600C anneal 3E16cm -2 Ge implant No C implant 1000C anneal New process allows nanotube growth without C implant

Analysis of Experimental Data Carbon implant widens process window for nanotube growth

Intensity (arb. units) Wave number (cm -1 ) G D #2 no C+ implant Si RBM Si RBM Si #2 C+ implant G M Wave number (cm -1 ) Intensity (arb. units) Effect of Carbon Implant No C implantC implant No D band for C implanted samples Carbon implant improves nanotube “quality”

Conclusions Developed a new route to SWNT growth Evidence shows Ge nanoparticles key to growth SWNTs produced have diameter range nm 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”