Strong correlation between electrical and thermal transport properties in graphene/graphite films beyond the Wiedemann-Franz law

Due to the unique band structure of graphene, defects in graphene and graphene-based structures often lead to complicated electrical transport properties, even at a very low level. Herein, we found a novel phase transition from semiconducting to metallic behaviors (a negative temperature coefficient of resistance to positive one) along with temperature increase in commercially available graphite/graphene films. Comparison studies on samples from different sources confirm the commonly existed transition in such graphite/graphene-based structures, but with variable transition temperatures ranging from about 320 K to 750 K. Further studies reveal that samples with lower absolute electrical resistance often obtains lower transition temperature, which is related to the defects level that strongly influence the carrier mobilities or concentrations and determines the electrical transport properties, especially at low temperatures. Along with the electrical transport properties, the thermal transport properties are simultaneously characterized from beyond the transition temperatures down to around 90 K. For the first time, an unpredicted linear relation between the transition temperature and the residual thermal reffusivity was observed, which are both strongly correlated to the defects level. The current work shed light on the understanding of electron and phonon transport behaviors and their correlation in graphite/graphene-based structures.