PhD defence of Ayse Melis Aygar – High density, on-chip, alkali doping of graphene and the characterization of the electronic structure of franckeite

Abstract

This thesis presents contributions to the understanding of the fundamental properties of van der Waals materials, motivated by the long-term goal of developing methods to tune and control the physical properties of van der Waals materials. In the first part of this thesis, a method to alkali dope graphene on-chip in an inert glovebox environment is presented, enabling an electron density of 4 x 10¹⁴ cm^-2 to be achieved. The method is suitable for charge transport and optical spectroscopy measurements at ultra-high charge carrier density. In our experiments, cesium vapour inside a cavity promotes cesium atom adsorption and ionization at the graphene surface, doping the graphene to ultra-high density levels. At the electron density of 4 x 10¹⁴ cm^-2, a drop in room temperature mobility to 140 cm²/Vs is recorded, in accord with the effective mass increase. Once doped, graphene can be hermetically sealed to protect against oxidation from the ambient environment, enabling experimental manipulation outside the glovebox environment. This method is substantively more facile than state-of-the-art ultra-high vacuum doping methods. In heavily doped graphene, a large magnetoresistance at low temperature, T = 1.3K, is attributed to electron density fluctuations. Temperature dependent weak localization reveals the prevalence of trigonal warping, in accord with high electron density. Non-resonant Raman scattering at a 785 nm pump wavelength independently confirms the high electron density achieved via the dynamic contribution to the Raman G-band shift.

The second part of the thesis presents the experimental characterization of several electronic properties of franckeite, a naturally occurring sulfosalt mineral with a van der Waals superlattice structure composed of alternating incommensurate two-dimensional layers: pseudo-tetragonal PbS and hexagonal SnS₂ layers. Experimental observations of the franckeite atomic structure, using state-of-the-art high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and atom probe tomography (APT) are presented. Angle-resolved photoemission spectroscopy (ARPES) measurements of the band structure reveal an anisotropic two-valley valence band with in-plane effective mass values, 19.1 and 1.6 m0. Optical reflection hyperspectra of exfoliated franckeite is used to determine the direct optical band-gap at Eg = 1.9 - 2.0 eV. Thermoelectric measurements of exfoliated franckeite flakes reveal a Seebeck coefficient of S= +45 uV/K and verify the intrinsic carriers to be p-type.