Intrinsic Nature of Graphene Revealed in Temperature-Dependent Transport of Twisted Multilayer Graphene

Graphene in its purest form is expected to exhibit a semiconducting to metallic transition in its temperature-dependent conductivity as a result of the interplay between Coulomb disorder and phonon scattering, the transition temperature, <i>T</i>_c, depending sensitively on the disorder induced carrier density (<i>n</i>_c). Even for good quality graphene, the <i>n</i>_c can be quite high (∼10¹² cm^–2) and the transition temperature may be placed well above the ambient, practically rendering it to be only semiconducting over a wide range of temperature. Here we report an experimental study on the transport behavior of twisted multilayer graphene (tMLG) exhibiting <i>T</i>_c well below the ambient temperature. The graphene layers in these tMLG are highly decoupled with one another due to the angular rotation among them; as a result, they exhibit very high Raman I_2D/I_G values (up to 12) with narrow 2D width (16–24 cm^–1). The observed <i>T</i>_c values seem to go hand in hand with the Raman I_2D/I_G values; a multilayer with mean I_2D/I_G value of 4.6 showed a <i>T</i>_c of 180 K, while that with mean I_2D/I_G of 4.9 showed lower a <i>T</i>_c of 160 K. Further, another multilayer with even higher mean I_2D/I_G value of 6.9 was metallic down to 5 K, indicating a very low disorder. The photoresponse behavior also corroborates well with the transition in transport behavior