Bianco, Elisabeth F
Ajayan, Pulickel M; Ringe, Emilie
Doctor of Philosophy
Two-dimensional (2D) materials, which are only one or a few atoms thick, can possess exciting and tunable optical/electronic properties unique from their 3D counterparts, providing a platform to tailor physics through chemistry and engineering. Structure-property relationships in 2D materials, including the room-temperature quantum Hall effect in graphene, have inspired the search for new 2D materials, amongst which are those from non-layered parent crystals, such as germanene. In this research, I have designed novel synthesis routes aimed to exploit attractive properties in 2D and ultrathin (<10 nm) chalcogens Se and Te, whose non-layered, anisotropic crystal structures provoke axis-dependent optical and electronic properties. Ultrathin Te films with controllable thickness, 2.5-10 nm, were grown by two methods: pulsed laser deposition and controllably-unbalanced magnetron sputtering. These films are the largest area ultrathin chalcogens reported (cm2), robust toward oxidation for several days, and exhibit the anisotropic P3121 Te structure. Furthermore, the crystallographic orientation of sputtered ultrathin Te is found to be controllable by the growth substrate; the anisotropic Te<0001> lies in the plane of the substrate on highly-oriented pyrolytic graphite (HOPG) but aligns orthogonally to MgO(100) substrates. Complementary high-resolution transmission electron microscopy (HRTEM), polarized Raman spectroscopy, and Hall effect measurements unravel a correlation between this tunable orientation and optical/electrical anisotropy in ultrathin Te, providing both a rational handle to access desired properties and a simple platform for device fabrication. Also, Raman signals are acutely dependent on film thickness from 20-2.5 nm displaying dramatic blue shifts of both basal plane and axial modes—a phenomenon not observed in layered 2D materials. The relative shifts between modes are orientation-dependent, suggesting that optical anisotropy persists and is even enhanced at the ultrathin limit. Lastly, vapor transport deposition is demonstrated for 0.85-3.0 nm-thick Se and Te, and TEM/scanning transmission electron microscopy (STEM) reveal a novel 2D α-phase in the case of three-atom-thick Te. This research has resulted in the first ultrathin chalcogens grown by a truly scalable technique with rational control of orientation and large-area uniformity, pushing these materials toward practical utility. Furthermore, evidence of thickness-dependent optical properties and 2D α-phase reconstruction reveals the quasi-2D nature of ultrathin Te.