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

Nanotubes

P. Ashburn, T. Uchino, C.H. de GrootSchool of Electronics and Computer Science

D.C. Smith, G. N Ayre, K.N. BourdakosSchool of Physics and Astronomy

A.L. Hector, B. MazumderSchool 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 = 20-250nm) dots on silicon

Pre-Treat Substrates

• Carbon ion implant – 31016 cm-2 30keV

• Strip native oxide with HF vapour

• Chemically oxidise with H202

Chemical Vapour Deposition

Substrate

Gas outlet

Quartz tube

Ceramic boat

Oven

Gas inlet(CH4, Ar, H2)

Two Step Process

Anneal – 1000oC, H2 300 sccm, Ar 1000 sccm, 10min

Growth – 850oC, CH4 1000 sccm, Ar 300 sccm, 10 min

Tem

per

atu

re (

C)

1000

850

Time (min)30 60 90

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

CNTs

Thick fibers

Raman shift (cm-1)

1200 1400 1600 1800

G band (a)

Inte

ns

ity

(arb

. u

nit

s)

-400-300-200-100

SiRBM (b)

1200 1400 1600 1800

(c)

Raman shift (cm-1)

1200 1400 1600 1800

G band (a)

1200 1400 1600 1800

G band (a)

Inte

ns

ity

(arb

. u

nit

s)

-400-300-200-100

SiRBM (b)

-400-300-200-100

SiRBM (b)

1200 1400 1600 1800

(c)

1200 1400 1600 1800

(c)

• excitation = 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.

Ge dot samples

Raman spectra of Ge dot sample

CNTs on Ge dots

Raman shift (cm-1)In

ten

sit

y (a

rb.

un

its

)

1200 1400 1600 1800

-250 -200 -150 -100

Raman shift (cm-1)

Raman shift (cm-1)In

ten

sit

y (a

rb.

un

its

)

1200 1400 1600 1800

-250 -200 -150 -100

Raman shift (cm-1)

-250 -200 -150 -100

Raman shift (cm-1)

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

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

Further Analysis• Evidence of C diffusion to surface

• C expected to aid nanotube growth

•Ge nanoparticles formed during pre-anneal

Depth (nm)

1017

1018

1019

1020

1021

1022

1023

0 40 8010-4

10-3

10-2

10-1

100

101

102

0 40 80

C,

O C

on

ce

ntr

ati

on

(c

m-3

)

Ge

co

nte

nt (%

)

(a) (b)Ge

C

O

C

Ge

O

Depth (nm)

1017

1018

1019

1020

1021

1022

1023

0 40 8010-4

10-3

10-2

10-1

100

101

102

0 40 80

C,

O C

on

ce

ntr

ati

on

(c

m-3

)

Ge

co

nte

nt (%

)

(a) (b)Ge

C

O

C

Ge

O

1 µm1 µm

After implant After 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

SiO2

Si

Ge implant

SiO2

Si

Anneal

Si

Oxide etchGe nanoparticles

Recent Results

Ge Nanoparticles

3E16cm-2 Ge implantNo C implant600C anneal

3E16cm-2 Ge implantNo C implant1000C anneal

Ge Nanoparticle Sizes

3E16cm-2 Ge implantNo C implant600C anneal

3E16cm-2 Ge implantC implant600C anneal

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

After Nanotube Growth

Carbon nanotubes formed No carbon nanotubes formed

3E16cm-2 Ge implantNo C implant600C anneal

3E16cm-2 Ge implantNo C implant1000C anneal

•New process allows nanotube growth without C implant

Temperature Time Temperature Time without C+ C+

1100 ℃ 5 min 850 ℃ 20 min No CNTs No CNTs

1050 ℃ 5 min 850 ℃ 20 min No CNTs 0.6 ± 0.1

1000 ℃ 5 min 850 ℃ 20 min No CNTs 2.7 ± 0.8

950 ℃ 10 min 850 ℃ 20 min 0.6 ± 0.2 2.0 ± 0.6

900 ℃ 10 min 850 ℃ 20 min 3.5 ± 1.0 4.1 ± 1.2

850 ℃ 10 min 850 ℃ 20 min 1.3 ± 0.1 1.2 ± 0.3

N/A N/A 1000 ℃ 20 min No CNTs No CNTs

N/A N/A 950 ℃ 20 min 1.6 ± 0.5 3.2 ± 2.6

N/A N/A 900 ℃ 20 min No CNTs 0.4 ± 0.1

N/A N/A 850 ℃ 20 min No CNTs No CNTs

Pre-anneal CNT growth CNT area density (μm in length / μm2)

Analysis of Experimental Data

•Carbon implant widens process window for nanotube growth

Inte

ns

ity

(arb

. u

nit

s)

1200 1400 1600 1800 2000

Wave number (cm-1)

G

D

#2 no C+ implant

100 200 300 400 500

Si

RBM

RBM

Si

100 200 300 400 500

RBM

SiSi

#2 C+ implant

1200 1400 1600 1800 2000

G

M

Wave number (cm-1)

Inte

ns

ity

(arb

. u

nit

s)

Effect of Carbon Implant

No C implant C 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 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”