STUDY OF ELECTRONIC STRUCTURES AND CORRELATIONS IN GRAPHENE USING OPTICAL SPECTROSCOPIES

Ph.DDOCTOR OF PHILOSOPH

Optical conductivity study of screening of many-body effects in graphene interfaces

Theoretical studies have shown that electron-electron (e-e) and electron-hole (e-h) interactions play important roles in many observed quantum properties of graphene making this an ideal system to study many body effects. In this report we show that spectroscopic ellipsometry can enable us to measure this interactions quantitatively. We present spectroscopic data in two extreme systems of graphene on quartz (GOQ), an insulator, and graphene on copper (GOC), a metal which show that for GOQ, both e-e and e-h interactions dominate while for GOC e-h interactions are screened. The data further enables the estimation of the strength of the many body interaction through the effective fine structure constant, $\alpha_{g}^{*}$. The $\alpha_{g}^{*}$ for GOQ indicates a strong correlation with an almost energy independent value of about 1.37. In contrast, $\alpha_{g}^{*}$ value of GOC is photon energy dependent, is almost two orders of magnitude lower at low energies indicating very weak correlation.Comment: Main Article (4 pages, 4 figures); Supporting Online Material (12 pages, 9 figures

Point Defects and Localized Excitons in 2D WSe2

Identifying the point defects in 2D materials is important for many applications. Recent studies have proposed that W vacancies are the predominant point defect in 2D WSe2, in contrast to theoretical studies, which predict that chalcogen vacancies are the most likely intrinsic point defects in transition metal dichalcogenide semiconductors. We show using first principles calculations, scanning tunneling microscopy (STM) and scanning transmission electron microscopy experiments, that W vacancies are not present in our CVD-grown 2D WSe2. We predict that O-passivated Se vacancies (O_Se) and O interstitials (Oins) are present in 2D WSe2, because of facile O2 dissociation at Se vacancies, or due to the presence of WO3 precursors in CVD growth. These defects give STM images in good agreement with experiment. The optical properties of point defects in 2D WSe2 are important because single photon emission (SPE) from 2D WSe2 has been observed experimentally. While strain gradients funnel the exciton in real space, point defects are necessary for the localization of the exciton at length scales that enable photons to be emitted one at a time. Using state-of-the-art GW-Bethe-Salpeter-equation calculations, we predict that only Oins defects give localized excitons within the energy range of SPE in previous experiments, making them a likely source of previously observed SPE. No other point defects (O_Se, Se vacancies, W vacancies and Se_W antisites) give localized excitons in the same energy range. Our predictions suggest ways to realize SPE in related 2D materials and point experimentalists toward other energy ranges for SPE in 2D WSe2

Electrochemically induced amorphization and unique lithium and sodium storage pathways in FeSbO4 nanocrystals

The increasing energy demands have prompted research on conversion and alloying materials, offering high lithium and sodium storage capacities. However, most of these materials suffer from huge volume expansion and degradation over the thousands of charging and discharging cycles required for commercial applications. In this study, we demonstrate a facile route to synthesize FeSbO4 nanocrystals that possess theoretical lithium and sodium storage capacity of 1220 mAh g–1. Operando X-ray diffraction studies reveal the electrochemically induced amorphization of the nanocrystals upon alkali-ion storage. We achieved specific storage capacities of ∼600 mAh g–1 for lithium and ∼300 mAh g–1 for sodium, respectively. The disparity in the lithium and sodium electrochemistry arises from the unique lithiation/sodiation pathways adopted by the nanocrystals. This study offers new insights into the chemistry and mechanism of conversion- and alloying-based energy storage materials that would greatly assist the development of next-generation active materials for energy storage.NRF (Natl Research Foundation, S’pore)MOE (Min. of Education, S’pore)Accepted versio