Effects of isotope doping on the phonon modes in graphene

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 41-46).Carbon related systems have attracted a large amount of attention of the science and technology community during the last few decades. In particular, graphene and carbon nanotubes have remarkable properties that have inspired applications in several fields of science and engineering. Despite these properties, creating structurally perfect samples is a difficult objective to achieve. Defects are usually seen as imperfections that degrade the properties of materials. However, defects can also be exploited to create novel materials and devices. The main topic of this thesis is studying the effect of isotope doping on the phonon properties of graphene. The advantage of the isotope enrichment technique is that only phonon frequencies or thermal properties can be modified without changing the electrical or chemical properties. We calculated the values of the phonon lifetimes due to isotope impurity scattering for all values of isotopic fractions, isotopic masses and for all wave-vectors using second order perturbation theory. We found that for natural concentrations of 13C, the contribution of isotopic scattering of optical modes is negligible when compared to the contribution from the electron-phonon interaction. Nevertheless, for atomic concentrations of 13C as high as [rho] = 0.5 both the isotopic and electron-phonon contributions become comparable. Our results are compared with recent experimental results and we find good agreement both in the 13C atomic density dependence of the lifetime as well as in the calculated spectral width of the G-band. Due to phonon scattering by 13C isotopes, some graphene phonon wave-functions become localized in real space. Numerical calculations show that phonon localized states exist in the high-energy optical phonon modes and in regions of flat phonon dispersion. In particular, for the case of in-plane optical phonon modes, a typical localization length is on the order of 3 nm for 13C atomic concentrations of [rho] ~~ 0.5. Optical excitation of phonon modes may provide a way to experimentally observe localization effects for phonons in graphene.by Joaquin F. Rodriguez-Nieva.S.M

Novel electronic behaviors in graphene nanostructures

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Physics, 2016.Cataloged from PDF version of thesis.Includes bibliographical references (pages 167-185).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.by Joaquin F. Rodriguez-Nieva.Ph. D