Transitions Metal Dichalcogenides: Growth, Fermiology Studies, and Few-Layered Transport Properties
Rhodes, Daniel Adam (author)
Balicas, Luis (professor co-directing dissertation)
Bonesteel, N. E. (professor co-directing dissertation)
Riley, Mark A. (committee member)
Chiorescu, Irinel (committee member)
Florida State University (degree granting institution)
College of Arts and Sciences (degree granting college)
Department of Physics (degree granting department)
Transition metal dichalcogenides (TMDs or TMDCs) have garnered much interest recently due to their weakly layered structures, allowing for mechanical exfoliation down to a single atomic layer. As such, it is pertinent to re-examine the bulk properties of these materials in order to completely understand and predict what is happening in the few-layered limit. A large majority of these systems were first investigated in the 1950s and 1960s. As such, many of the current growth methods rely on these reports, making new growth techniques for lowering defects of importance as well. In this thesis, both topics are taken into consideration and discussed, though the latter remains to be investigated in much more detail and should be the work of future research efforts. Orthorhombic MoTe₂ and its isostructural compound WTe₂ were recently claimed to belong to a new class (type II) of Weyl semimetals characterized by a linear touching between hole and electron Fermi surfaces in addition to nodal lines. These compounds have recently been shown to display very large non-saturating magnetoresistances which have been attributed to nearly perfectly compensated densities of electrons and holes. Here, we present a detailed study on the temperature and angular dependence of the Shubnikov-de-Haas (SdH) effect in the semi-metal WTe₂ and MoTe₂. In WTe₂, we observe four fundamental SdH frequencies and attribute them to spin-orbit split, electron- and hole-like, Fermi surface (FS) cross-sectional areas. Their angular dependence seems consistent with ellipsoidal FSs with volumes suggesting a modest excess in the density of electrons with respect to that of the holes. We show that density functional theory (DFT) calculations fail to correctly describe the FSs of WTe₂. When their cross-sectional areas are adjusted to reflect the experimental data, the resulting volumes of the electron/hole FSs obtained from the DFT calculations would imply a pronounced imbalance between the densities of electrons and holes. We find evidence for field-dependent Fermi surface cross-sectional areas by fitting the oscillatory component superimposed onto the magnetoresistivity signal to several Lifshitz-Kosevich components. We also observe a pronounced field-induced renormalization of the effective masses. Taken together, our observations suggest that the electronic structure of WTe₂ evolves with the magnetic field due to the Zeeman splitting. This evolution is likely to contribute to its pronounced magnetoresistivity. For β-MoTe2, high quality single-crystals were synthesized by flux in excess tellurium. We find that its superconducting transition temperature depends on disorder as quantified by the ratio between the room- and low-temperature resistivities, (residual resistivity ratio, RRR. Similar to WTe₂, its magnetoresistivity does not saturate at high magnetic fields and can easily surpass 10₂⁶%, with the superimposed Shubnikov de Haas oscillations revealing a non-trivial Berry phase of ≃pi. The geometry of the Fermi surface, as extracted from the quantum oscillations, is markedly distinct from the calculated one. A broad anomaly seen in the heat capacity and in the Hall-effect indicates that the crystallographic and the electronic structures evolve upon cooling below 100 K, likely explaining the discrepancy between these recent predictions and our experimental observations. In α-MoTe₂, grown at lower temperatures in vapor transport, we focus on few-layered crystals mechanically exfoliated onto a 270 nm thick SiO₂ layer. Previous reports found that the field-effect mobility of transition metal dichalcogenides TMDs tends to increase, reaching a maximum value when crystals are composed of approximately 10 atomic layers. We show that the overall performance of a MoTe₂ based field-effect transistor is comparable to similar devices based on MoS₂ or MoSe₂. But with an optical gap quite close in value to the one of Si and an enhanced spin-orbit interaction (since Te is a 5p element) suggests that this compound might be particularly suitable for optoelectronic applications in a complimentary range of wavelengths. In the case of MoSe₂, we show that multi-layered (~ 10 atomic layers) field-effect transistors can display ambipolar behavior at room temperature when using a standard combination of metals, Au on Ti, for all the electrical contacts. The 4.33 eV work function of Ti is closely matched by the electron affinity of bulk MoSe₂, 4.45 ± 0.11 eV. This implies that the Fermi level of Ti is very close to the bottom of the conduction band of MoSe₂, and therefore that one should expect a rather small Schottky barrier for electron conduction through the Ti:Au contacts. One extracts through Hall effect measurements, Hall mobilities in excess of 250 cm₂²/(V s) for both holes and electrons at room temperature. These values are remarkable, since they are comparable or higher than most values reported so far for transition metal dichalcogenides at room temperature, but are obtained without the use of high k-dielectrics such as HfO₂, doping, or of a particular combination of metals for the electrical contacts. Our results suggest that improvements in fabrication, and on the quality of the starting material (with a lower amount of defects) could make field-effect transistors based on few atomic layers of synthetic MoSe₂ excellent candidates for complementary logic electronics. Finally, we report on alloys of MoTe₂ and WTe₂, Mo₁₋xWxTe₂, grown by a chemical vapor transport process with the goal of obtaining a phase diagram with respect to doping and temperature. These crystals have been analyzed for composition via EDS and investigated through high resolution transmission electron microscopy, scanning tunneling microscopy (STM) and ARPES. Transmission electron microscopy images clearly show that through W substitution we are able to synthesize both 2H, trigonal prismatic, and Td, orthorhombic structures. As opposed to other reports, we find a phase transition from the 2H- to the Td-phase at 10 % W doping, much lower than expected. This structure is characterized by a linear arrangement of atoms, as opposed to a hexagonal pattern. In addition, through examination of monolayers via TEM, we find that the W disperses randomly amongst the crystal as opposed to forming grain boundaries. Given the crystallinity and quality of the material, mapping the phase diagram should be of relative ease, however this work is still ongoing. As such, the phase diagram has not yet been completed and remains unreported in this manuscript.
Fermi Surface, Magnetoresistance, MoTe2, Synthesis, TMDs, WTe2
July 13, 2016.
A Dissertation submitted to the Department of Physics in partial fulfillment of the requirements for the degree of Doctor of Philosophy.
Includes bibliographical references.
Luis Balicas, Professor Co-Directing Dissertation; Nicholas Bonesteel, Professor Co-Directing Dissertation; Theo Siegriest, University Representative; Mark Riley, Committee Member; Irinel Chiorescu, Committee Member.
Florida State University
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