VICTORIA UNIVERSITY OF WELLINGTON Te Whare Wānanga o te Ūpoko o te Ika a Māui Siegmar Roth Super-Jubilee 26 June 2015 Alan B. Kaiser MacDiarmid Institute for Advanced Materials and Nanotechnology, SCPS, Victoria University of Wellington in collaboration with Viera Skákalová (University of Vienna) Corey Hewitt and David Carroll (Wake Forest University, North Carolina, USA) Cristina Gómez-Navarro (Universidad Autonoma de Madrid,Spain) Siegmar Roth, Yung Woo Park and others

2 Ehinger and Roth found no evidence for conduction by solitons in conducting polymers: Non-solitonic conductivity in polyacetylene, Phil. Mag. (1986) At low dopant concentrations carrier hopping between localized defect states dominates, while at high doping levels only the inter-fibrillar contact resistances are important. In an intermediate doping region the prevailing contribution to the conductivity is carrier tunnelling between highly conducting parts of the sample separated by less-conducting zones. There seems to be no doping regime in which carrier transport via mobile conjugational defects (solitons) plays a detectable role.

3 Discussing the Heterogeneous Model….? - some remarkable similarities for the three materials - but also key differences … on Mt Gwanak, Seoul (~1995)

4 Variable-Range Hopping (VRH) conduction : Siegmar Roth, One-Dimensional Metals, 1st ed. (VCH Weinheim) Hops between available electron states near the Fermi level at randomly located positions require different energies.

5 … on Mt Gwanak, Seoul (~1995) Only parts of sample are oxidized in preparation of graphene oxide sheets - remain disordered after oxygen removed by reduction Reduced graphene oxide We found free-electron conduction in well-ordered crystalline regions not oxidized in series with VRH between localized states in disordered regions Cristina G ó mez-Navarro, Ravi Sundaram and Marko Burghard et al., Max Planck Institute, Stuttgart

3D carbon networks: Conductivity vs Electric Field agrees with VRH models for field-assisted thermally driven VRH, and our extension for larger fields: SEM image Data from Govor et al., Phys. Rev. B 62 (2000) Cheah, Gómez-Navarro, Jaurigue & Kaiser, J.Phys-Condens.Mat. 25, (2013) Pollak-Riess VRH Shklovskii VRH 6

7 Review for Chemical Society Reviews: - some remarkable similarities for the three materials - but also key differences

J. G. Checkelsky and N. P. Ong, Phys. Rev. B 80, 8 (2009), Large Thermopower S (of different sign) in graphene: Near charge neutrality point: 190  V/K thermopower difference for electron and hole graphene at room temperature – so could provide large ZT ? 8