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
2Polyacetylene (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 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.
4Bulk graphite loosely bound layers of carbon atomsGraphite flakes in pencil marks:Including flakes only one atom thick!Discovered by Andre Geim and his group, 2004
5Resistance per square charge neutrality pointof 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)
6Resistance of graphene flakeViera 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 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 temperatureResistance (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)
8Methods 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 films4) Chemical vapour deposition on thin Ni layers (Kim et al. 2009) - large-scale patterned graphene films - stretchable, highly-conducting transparent electrodes5) Graphene Nanoflakes ( ~ 30 nm) with edges decorated with carboxylic acid groups (Green et al. 2009)
9 Reduced graphene oxideCristina 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 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
11Conclusions on conduction mechanisms in reduced grapheneoxide: 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.
12Applications 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 film4) Support for samples in Transmission Electron Microscope5) Ultra-sensitive chemical sensors (single molecules)6) Nano-electro-mechanical systems (NEMS): light, stiff and strong7) Graphene powder: Field emission (Geim and Novoselov, Nature Mater. 2007; Geim, Science 2009)
13Towards 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 gap3) Transistor circuitry could be created in a graphene sheet:molecular electronicsbut with top-down gateapproach: drain source
14 Conduction in thick and thin SWCNT networks Measurements by Viera Skákalová, Max-Planck-Institut, Stuttgart thick network Fluctuation-assisted (SWNT paper) tunnelling betweenapprox 50 mm thick: 1 mm metallic regions Variable-range thin network: hopping between localized states 2 mm AFM trace: 50 nm 50 nm
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
16Thin 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
17Rolled-up Graphene: Single-Wall Carbon Nanotube thin networksEnhancement of transmittance and conductance ofby removal of volatile solvent (butylamine): annealed unannealed Butylamine removed S. Ravi, A.B. Kaiser and C.L. Bumby, Chem. Phys Lett. (2010)
18Conductance 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)