Graphene is perhaps the most studied material around the globe in recent years. It has served as a classic example of 2D materials not just because of the historical reasons, but importantly, due to distinctly observable dimensional crossover in it, from 2D to 3D, via Bernal stacked (AB) bilayer to multilayer finally culminating in graphite. The interlayer interactions that are thus responsible, however, tend to differ vastly in presence of defects or disorders. Of particular interest is the angular disorder causing the layers to stack in a manner away from the conventional AB packing. The new class of graphene systems involving a twist among otherwise highly crystalline 2D layers, is often termed as twisted graphene. The twist as a new degree of freedom induces several angle dependent properties, from visible absorption to superconductivity, unheard of in the case of graphene itself. In some instances, the layers may become highly decoupled such that a layer under twist may behave as though suspended, being free from any substrate influence. This is when the extraordinary properties of graphene also are truly observable. The work in the laboratory relates to developing a new synthetic process to produce good amounts of twisted graphene stacks. It is a modified chemical vapour deposition process involving joule heating of Ni catalyst foil coated with an aromatic hydrocarbon. The method has also been employed to improve the quality of graphene grown by other methods.
Graphene in its purest form is expected to exhibit a semiconducting to metallic transition in its temperature dependent conductivity with a transition temperature, Tc, depending sensitively on the disorder or defects present in it. Even for good quality graphene, these disorder are non-negligible and the transition temperature appears above the ambient, practically rendering it to be only semiconducting over a wide range of temperature. The transport behaviour of twisted multilayer graphene grown in the laboratory, the Tc value was found to be well below the ambient temperature. The graphene layers in the stacks are thus highly decoupled due to the angular rotation among them, as a result, they exhibit very high Raman I2D/IG values (up to 12) with narrow 2D width (16 – 24 cm-1). The photoresponse behaviour also corroborates well with the transition in transport behaviour. The studies have resulted in many international publications and a patent. Collaborative project activities with an Industry were also undertaken. This is yet another unique activity from the laboratory.
(a) Properties of twisted graphene.
(b) Raman and transport data of the twisted graphene stack revealing intrinsic nature of the graphene layers.
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