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17 October 2011 NZ Institute of Physics Conference VICTORIA UNIVERSITY OF WELLINGTON Te Whare Wānanga o te Ūpoko o te Ika a Māui Alan B. Kaiser Shrividya Ravi and Chris Bumby * MacDiarmid Institute for Advanced Materials and Nanotechnology, Victoria University of Wellington * Now at Industrial Research Ltd, Gracefield

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Research 4: A Kaiser

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Page 1: 15.30 o5 a kaiser

17 October 2011

NZ Institute of Physics Conference

VICTORIA UNIVERSITY OF WELLINGTON

Te Whare Wānanga o te Ūpoko o te Ika a Māui

Alan B. Kaiser

Shrividya Ravi and Chris Bumby *

MacDiarmid Institute for Advanced Materials and Nanotechnology,

Victoria University of Wellington

* Now at Industrial Research Ltd, Gracefield

Page 2: 15.30 o5 a kaiser

polyacetylene

(CH)n

intrinsic conductivity

similar to metals

carbon-based

electronics

typical nanofibre

diameter 20 ~ 40 nm

electrode separation

~ 150 nm

Polyacetylene (conducting polymer) nanofibre

Yung Woo Park et al.

2

Page 3: 15.30 o5 a kaiser

Nobel prize for Physics 2010

Andre Geim and Kostya Novoselov

Awarded 2010 Nobel Prize for Physics for their ground- breaking

experiments on the two-dimensional material graphene

- Demonstrated novel physics of electrons in graphene owing to

unusual band structure around Fermi level.

3

Page 4: 15.30 o5 a kaiser

Bulk graphite 4

loosely bound layers

of carbon atoms

Graphite flakes in pencil marks:

Including flakes only one atom

thick!

Discovered by Andre Geim and

his group, 2004

Page 5: 15.30 o5 a kaiser

Gate voltage Vg shifts Fermi

energy up (or down)

Resistance per square

of graphene:

electrons conduct

5

holes conduct

Mobility can extremely high - up to 120,000 cm2/Vs at 240 K

in suspended graphene

(Andrei et al. 2008, Bolotin, Kim et al. 2008, Geim, Novoselov et al. 2008)

- higher than any semiconductor (mean free path up to 1 mm)

Re

sis

tan

ce

(kW

)

charge neutrality point

Page 6: 15.30 o5 a kaiser

6

Resistance of graphene flake

-20 -15 -10 -5 0 5 10 15 20

1

2

3

4

5

before T-cycle

after T-cycle

R (

kW

)

Gate Voltage (V)

Viera Skákalová, Max Planck Institute, Stuttgart

charge neutrality point

Mesoscopic “Universal

Conductance

Fluctuations” very

persistent in graphene

- up to > 50 K

Page 7: 15.30 o5 a kaiser

0 50 100 150 200 2500.6

0.8

1.0

1.2

1.4

high

energy

phonons fluctuations

acoustic phonons

residual resistance

Resis

tan

ce (

kW

)

Temperature (K)

low temperature

anomaly

- monotonic but

can be up or down

7

Graphene: temperature dependence of resistance

R(T) above 50K

consistent with

scattering by

acoustic and high-

energy phonons

(as shown by Chen

et al., Morosov et al.

2008)

Skakalova, Kaiser et al. Phys. Rev. B (2009)

Page 8: 15.30 o5 a kaiser

1) Flakes from graphite crystal: lift off with sticky tape, or rub

graphite crystallite on Si/SiO2 substrate (Geim, Novoselov 2004)

2) Epitaxial films from SiC: heat to remove Si at surface, leaving C

layer (Berger, de Heer 2006)

3) Chemically-derived by forming graphene oxide sheets (which

disperse in water), depositing them and then removing oxygen by

chemical reduction (Burghard, Kaner 2007)

– can deposit as macroscopic graphene films

4) Chemical vapour deposition on thin Ni layers (Kim et al. 2009)

- large-scale patterned graphene films

- stretchable, highly-conducting transparent electrodes

5) Graphene Nanoflakes ( ~ 30 nm) with edges decorated with

carboxylic acid groups (Green et al. 2009)

8

Methods of making graphene sheets:

Page 9: 15.30 o5 a kaiser

only parts of sample are oxidized in separation of graphene oxide sheets

- remain disordered after oxygen removed by reduction

9

Reduced graphene oxide

well-ordered crystalline regions in regions not oxidized

STM image:

Cristina Gómez-Navarro, Marko Burghard et al., Max Planck Institute, Stuttgart

Page 10: 15.30 o5 a kaiser

10

Conductance of reduced graphene oxide:

temperature-independent conductance

at low T, higher electric field

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

-26

-24

-22

-20

-18

-16

-14

-12

Vg=-20V

Vg=-15V

Vg=-10V

Vg=-5V

Vg=0

Vg=10V

Vg=20V

(c)

(a)

Vds= 0.1V

ln I (

A)

T1/3

(K-1/3

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

-26

-24

-22

-20

-18

-16

-14

-12

(b)

Vds= 0.5 V

ln I (

A)

T-1/3

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

-18

-16

-14

-12

(a)

Vds= 2 V

ln I (

A)

T -1/3

(K -1/3

)

ln(

I )

(A

) ln(

I )

(A

) 1I T 1/3) (K-1/3)

2D variable-range hopping at high T

for different gate voltages

1I T 1/3) (K-1/3) ln

( I )

(A

)

Vds = 0.1 V

Vds = 2.0 V

Vds = 0.5 V

1 01/3( ) exp

BG T G G

T

Kaiser, Gómez-Navarro, Burghard et al., Nano Lett. (2009)

Page 11: 15.30 o5 a kaiser

11

Conclusions on conduction mechanisms in reduced graphene

oxide:

Conduction is highly heterogeneous:

1) relatively high metallic conductivity in the crystalline regions

with delocalized carrier density showing the usual

dependence on gate voltage;

2) thermally-driven variable-range hopping in disordered barrier

regions that dominates the resistance above 40 K;

3) purely field-driven T-independent tunnelling conduction at

larger fields and low temperature: tunnelling between

localized states in barrier regions, and through barrier regions

at their thinnest points between delocalized states in metallic

regions. The lowest barrier energies are inferred to be of

order of 40 meV.

These oxide-related barriers, if made in a controlled fashion,

could define conducting channels on graphene sheets.

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Applications of graphene:

1) Conducting composites with filling factors < 1%

2) Highly stretchable (up to 20% - more than any other crystal)

3) As membranes: gases cannot pass through monolayer

graphene film

4) Support for samples in Transmission Electron Microscope

5) Ultra-sensitive chemical sensors (single molecules)

6) Nano-electro-mechanical systems (NEMS): light, stiff and strong

7) Graphene powder: Field emission

(Geim and Novoselov, Nature Mater. 2007; Geim, Science 2009)

12

Page 13: 15.30 o5 a kaiser

Towards Carbon-based Electronics:

1) Graphene with ballistic conduction at 300 K as very fast field-

effect transistor (FET) (Avouris et al.)

2) Graphene nanoribbon transistors with band gap

3) Transistor circuitry could be created in a graphene sheet:

13

drain

source

gate

molecular electronics

but with top-down

approach:

Page 14: 15.30 o5 a kaiser

Conduction in thick and thin SWCNT networks

thin network:

2 mm

50 nm

1 mm

50 nm

thick network

(SWNT paper)

approx 50 mm thick:

AFM trace:

14

Measurements by Viera Skákalová, Max-Planck-Institut, Stuttgart

Fluctuation-assisted

tunnelling between

metallic regions

Variable-range

hopping between

localized states

Page 15: 15.30 o5 a kaiser

0 20 40 60 80 100

10-3

10-2

10-1

100

101

Buckypaper

Net 4

Net 1

Net 2

S

qu

are

Co

nd

ucta

nce

(S)

Transmittance (%)

Net 3

Transparency of thin SWCNT networks

Thick free-standing

SWCNT network

SWCNT networks

become thinner

Net 4 made with 4 return

air-brush strokes

Net 1 made with 1 return

air-brush stroke

Co

nd

ucta

nce

pe

r sq

ua

re (

S)

15

Measurements by Viera Skákalová, MPI Stuttgart

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Thin transparent single-wall carbon nanotube films:

drop casting with SWCNTs

in solvent on square glass

cover slip:

very thin SWCNT film with

metal contacts

thicker film

Shrividya Ravi and Dr Chris Bumby (Victoria University of Wellington)

16

Page 17: 15.30 o5 a kaiser

Rolled-up Graphene: Single-Wall Carbon Nanotube thin networks

Enhancement of transmittance and conductance of

by removal of volatile solvent (butylamine):

S. Ravi, A.B. Kaiser and C.L. Bumby, Chem. Phys Lett. (2010)

annealed

unannealed

Butylamine removed

17

Page 18: 15.30 o5 a kaiser

Conductance of single-wall carbon nanotube network (log scale)

s

found „metallic‟

behaviour below 3 K

1/T1/4

variable-range hopping conduction

A few percolating metallic paths with thin tunnelling barriers -

some similarity to chemically-derived graphene !

S. Ravi, A.B. Kaiser and C.L. Bumby, Chem. Phys Lett. (2010)

18