Probing the electronic properties of atomically thin graphitic layers with optical spectroscopy.

摘要

Since its discovery, graphene (a mono-layer of graphitic layer) has attracted tremendous attention from the physics community. Being one-atom thick, Single Layer Graphene (SLG) is an ideal model for fundamental studies of 2-Dimensional (2D) systems. One of the most interesting aspects of SLG is the resemblance of its electronic structure to an important class of particles in high energy physics, the Dirac massless Fermions, i.e., massless Fermions with 4 flavors. Such an electronic structure leads to many peculiar phenomena in fundamental physics and many desirable properties for technological applications of this new material. Therefore, a basic understanding of the electronic properties of SLG is of the greatest importance in this field.;In this thesis, the electronic structure of single- and few-layer graphene is investigated with optical spectroscopic techniques. In the first part, the optical absorption spectrum of SLG is measured. In the mid/near infrared range, the absorbance is a universal constant equal to pialpha = 2.29%, a being the fine structure constant, independent of photon energies, the particular sample characteristics and all parameters describing the band structure of SLG. This reflects a fundamental fact that the charge carriers in SLG indeed behave as 2D Dirac massless Fermions at low energies. Since each Fermionic species absorbs by pialpha/4, the 4 spin-nodal flavors of Fermions in SLG gives an absorbance of pialpha. On the other hand, strong modifications of optical absorption in the visible/UV range due to many body interactions are observed. Such effects give rise to a breakdown of the universal absorbance and experimental signatures of saddle point excitons.;In the second part, the electronic structure of Bi-Layer Graphene (BLG) is studied in the presence of strong applied electric fields. In contrast to SLG, the low-energy electrons in BLG are described as chiral massive Fermions. The linearly dispersing bands in SLG are replaced by split hyperbolic bands due to inter-layer interactions. Therefore, the existence of van Hove singularities in the Joint Density of States (JDOS) gives rise to optical resonance features that are tunable by an applied electric field. This interesting behavior is investigated in a BLG field effect transistor (FET). One striking observation at high gate bias fields is that a band gap is opened and its size is tunable with the electric field strength. This is a direct consequence of field-induced inversion symmetry breaking in BLG, which lifts the degeneracy at the K-point of the Brillouin zone. Band gaps as large as 200 meV can be achieved, which implies potential applications in room temperature FETs.;In the third part of the thesis, the electronic structure of Few-Layer Graphene (FLG) and its relationship to bulk graphite is studied. Compared to SLG, the electronic structure of FLG is much richer and more flexible for engineering purposes. Two different families of crystallographic orientations are found in FLG, the Bernel (ABABAB...) and rhombohedral (ABCABC...) stacking. Dramatic differences of the electronic properties between the two cases are observed. For Bernel FLG, the electronic structure is derived by zone folding of the band structure of bulk graphite and can be decomposed into independent massless and massive components with different band gaps. For rhombohedral FLG, due to its peculiar electronic structure, the JDOS has divergent (1-Dimensional (1D) like) singularities, as revealed in the measured absorption spectra. Furthermore, the presence of surface conducting states and its relationship to semimetallic rhombohedral bulk graphite are discussed.;Finally, results concerning the electronic properties of other atomically thin layered materials are presented. Particular attention is given to a particular semiconducting transition-metal dichalcogenide material, MoS2. We find that a dramatic crossover from indirect to direct gap of the material occurs in the monolayer limit. This result suggests interesting opportunities for other layered materials in the limit of few atomic layers.