Novel electronic behaviors in graphene nanostructures

Abstract

Recently, it has been shown that graphene can be combined with a variety of nanoscale systems, such as other two-dimensional crystals, to form novel electronic nanostructures. These systems inherit the unique characteristics of graphene, such as high mobility, Berry phase, photoresponse mediated by hot carriers, and at the same time acquire new features due to nanoscale heterogenities. In this thesis, I explore the novel electronic behaviors which emerge in this fashion. I focus on two types of systems: (i) vertically-stacked structures in which graphene layers are interspaced with insulating materials and (ii) in-plane structures formed by spatially-varying electrostatic potentials in graphene. The outline of this thesis is as follows: first, I show that the vertical structures grant access to distinct transport behaviors and new kinds of photoresponse. Those include, in particular, photo-induced negative differential resistance, bistability, and hysteretic I-V characteristics. This wide variety of behaviors is enabled by a number of interesting physical phenomena which can be accessed in these structures, such as resonant tunneling, thermionic emission and field emission. I explore the different knobs which are available to control these phenomena and new ways to employ them to design the I-V response. Second, I study in-plane nanostructures such as pn junction rings induced by local charges, and show that they enable confinement of electronic states in graphene. Confined states in these graphene quantum dots arise due to constructive interference of electronic waves scattered at the pn junction and inward-reflected from the ring by the so-called Klein scattering process. Key fingerprints of confined states are resonances appearing periodically in scanning tunneling spectroscopy maps. Besides the novel mechanism for confinement, I also demonstrate that graphene quantum dots can be exploited for accessing exotic and potentially useful behavior which is not available in conventional quantum dots. An example of such behavior is a giant non-reciprocal effect of quantum dot resonances which is induced by the Berry phase. Third, I study manifestations of defects in the Raman spectral maps of disordered graphene systems. Two salient Raman features, namely the D and D' bands, provide useful information about the nature of defects. I perform a detailed analysis of the origin of the Raman scattering cross section which is routinely measured in experiments and discuss how it can be used to obtain information about defects. Overall, this thesis demonstrates the versatility of graphene nanostructures. This is manifested in numerous phenomena which have implications both in basic science, e.g. Berry phase effects, as well as in applied research, e.g. photodetection in graphene Schottky junctions. Furthermore, several of the ideas discussed here can be extended to achieve other interesting and potentially useful effects, such as localized valley-polarized states in graphene quantum dots and exciton confinement.