Two - Dimensional van der Waals Materials: Characterization and Electronic Device Applications
The successful exfoliation of graphene and studies into its unique electrical and thermal properties has motivated searches for other quasi two-dimensional (2D) materials with interesting properties that could be used for practical applications. In this dissertation, I describe my research of the properties of inorganic van der Waals materials – layered transition metal dichalcogenides and devices based on these materials. The first part of the dissertation deals with the selective gas sensing using MoS2 thin-film transistors. The sensing is enabled by the change in the channel conductance, characteristic transient time, and spectral density of the low-frequency current fluctuations. The back-gated MoS2 thin-film field-effect transistors were fabricated on Si/SiO2 substrates. The exposure to ethanol, acetonitrile, toluene, chloroform, and methanol vapors resulted in drastic changes in the source-drain current. It was established that the transient time of the current change and the normalized spectral density of the low-frequency current fluctuations can be used as additional sensing parameters for selective gas detection with thin-film MoS2 transistors. The second part of this dissertation involves the Raman study of 1T-TaSe2 thin-films. Bulk 1T-TaSe2 exhibits unusually high charge density wave (CDW) transition temperatures of 600 and 473 K below which the material exists in the incommensurate (I-CDW) and the commensurate (C-CDW) charge-density-wave phases, respectively. The C-CDW reconstruction of the lattice coincides with new Raman peaks resulting from zone-folding of phonon modes from middle regions of the original Brillouin zone back to Γ. The C-CDW transition temperatures as a function of film thickness were determined from the evolution of these new Raman peaks, and they are found to decrease from 473 to 413 K as the film thicknesses decrease from 150 to 35 nm. The results of the dissertation contribute to better understanding of properties of 2D materials and may lead to their practical applications in electronics.