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
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
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
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
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
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
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)
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:
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
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)
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.
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
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:
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
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
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
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
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