Research 4: A Kaiser

1. 17 October 2011
NZ Institute of Physics Conference
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
VICTORIA UNIVERSITY OF WELLINGTON
Te Whare Wānanga o te Ūpoko o te Ika a Māui

2. 2
Polyacetylene (conducting polymer) nanofibre
polyacetylene
(CH)n
intrinsic conductivity
similar to metals
 carbon-based
electronics
typical nanofibre
diameter 20 ~ 40 nm
electrode separation
~ 150 nm
Yung Woo Park et al.

3. 3
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.

4. 4
Bulk graphite
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

5. 5
Resistance per square charge neutrality point
of graphene:
Resistance
(kW)
electrons
holes conduct
conduct
Gate voltage Vg shifts Fermi
energy up (or down)
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)

6. 6
Resistance of graphene flake
Viera Skákalová, Max Planck Institute, Stuttgart
charge neutrality point
5
4
before T-cycle
after T-cycle
R (kW)
3
Mesoscopic “Universal
2 Conductance
Fluctuations” very
1 persistent in graphene
- up to > 50 K
-20 -15 -10 -5 0 5 10 15 20
Gate Voltage (V)

7. 7
Graphene: temperature dependence of resistance
Skakalova, Kaiser et al. Phys. Rev. B (2009)
1.4
R(T) above 50K
consistent with
1.2 scattering by
low temperature
Resistance (kW)
acoustic and high-
anomaly energy phonons
- monotonic but high
1.0 can be up or down (as shown by Chen
energy et al., Morosov et al.
phonons
fluctuations 2008)
0.8 acoustic phonons
residual resistance
0.6
0 50 100 150 200 250
Temperature (K)

8. 8
Methods of making graphene sheets:
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)

9. 9
Reduced graphene oxide
Cristina Gómez-Navarro, Marko Burghard et al., Max Planck Institute, Stuttgart
STM image:
only parts of sample are oxidized
in separation of graphene oxide
sheets
- remain disordered after oxygen
removed by reduction
well-ordered crystalline regions in
regions not oxidized

10. 10
Conductance of reduced graphene oxide:
Kaiser, Gómez-Navarro, Burghard et al., Nano Lett. (2009)
2D variable-range hopping at high T -12
Vds = 0.5 V
for different gate voltages -14 Vds= 0.5 V (b)
(a)
-16
-12
Vds 0.1V0.1 V (c)
Vds= =
) (A)
-18
-14
ln( I ln I (A)
-20
-16
-22
-18
ln( I )ln I (A)
(A)
-24
-20 Vg=-20V
Vg=-15V
Vg=-10V -26
-22
Vg=-5V -12
0.1 0.2 0.3 0.4
-24
Vg=0
Vg=10V
V 0.6 2.0 V (a) 0.9
0.5
= 0.7 0.8
Vds= 2 V
ds
-1/3
ln( I ln)I (A)(A)
-14 T
Vg=20V
-26
-16
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1/3 -1/3 -18
T(K
1I T 1/3) (K-1/3) 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
1I T 1/3) (K (K-1/3)
-1/3 -1/3
T )
 B 
G(T )  G1 exp   1/ 3   G0 temperature-independent conductance
 T  at low T, higher electric field

11. 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.

12. 12
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)

13. 13
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:
molecular electronics
but with top-down gate
approach:
drain
source

14. 14
Conduction in thick and thin SWCNT networks
Measurements by Viera Skákalová, Max-Planck-Institut, Stuttgart
thick network Fluctuation-assisted
(SWNT paper) tunnelling between
approx 50 mm thick: 1 mm
metallic regions
Variable-range
thin network: hopping between
localized states
2 mm
AFM trace: 50 nm 
50 nm

15. 15
Transparency of thin SWCNT networks
Thick free-standing
SWCNT network
Conductance per squareS(S)
Square Conductance( ) 1
10 Buckypaper
0
10
SWCNT networks
-1
10 become thinner
-2
10 Net 4
Net 3
-3 Net 2
10
Net 1
0 20 40 60 80 100
Transmittance (%)
Net 4 made with 4 return Net 1 made with 1 return
air-brush strokes air-brush stroke
Measurements by Viera Skákalová, MPI Stuttgart

16. 16
Thin transparent single-wall carbon nanotube films:
Shrividya Ravi and Dr Chris Bumby (Victoria University of Wellington)
drop casting with SWCNTs

in solvent on square glass
cover slip:
very thin SWCNT film with
metal contacts
thicker film

17. 17
Rolled-up Graphene: Single-Wall Carbon Nanotube thin networks
Enhancement of transmittance and conductance of
by removal of volatile solvent (butylamine):
annealed
unannealed
Butylamine removed
S. Ravi, A.B. Kaiser and C.L. Bumby, Chem. Phys Lett. (2010)

18. 18
Conductance of single-wall carbon nanotube network (log scale)
variable-range hopping conduction
found „metallic‟
behaviour below 3 K
1/T1/4
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)