Low-Dimensional Materials at the Nanoscale: Transition Metal Chalcogenides, Carbon Nanomaterials and Organic Semiconductors
The overall theme of this dissertation is the electronic transport and electromechanical study of low dimensional materials at the nanoscale. The dissertation is divided into three parts based on the class of materials: I. collective ground states in ultrathin materials, II. carbon nanomaterials based nanomechanical resonators and III. organic semiconductors.
In part I, the superconductivity and charge density waves in transition metal chalcogenides are introduced. Crystal synthesis of transition metal chalcogenides by chemical vapor transport is presented. The materials have quasi-low dimensional crystal structure: either quasi-two dimensional (e.g. NbSe2, TaS2, WTe2, FeSe) or quasi-one dimensional (e.g. NbSe3, TaS3, (NbSe4)3I). Monolayer NbSe2, grown by molecular beam epitaxy, shows a superconducting transition at Tc=2K and is studied down to 50mK with magnetic fields. The sliding charge density waves in NbSe3 nanoribbons are studied with narrowband noise, which directly probes the order parameter. A proposal to scale down the contactless conductivity measurement technique for nanoscale samples with lithographically fabricated planar coils is presented.
In part II, microstructures of suspended carbon nanotubes and graphene are studied as nanomechanical resonators. Carbon nanotubes are clamped on one end and the other end is free to enable field emission. The field emission provides a means of electrical readout. Fabrication of carbon nanotube field emitting mechanical resonators on an integrated platform are explored. The platform is designed to allow the study of the nanomechanical motion across multiple characterization techniques. Graphene nanomechanical resonators are studied as a first step in the development of a microactuator-based platform to control strain fields in graphene. In particular, non-uniaxial strains for large pseudo-magnetic field effects are intended.
In part III, organic nanowire formation with DPP-TPA molecules for use in photovoltaics is explored. The nanowire’s charge carrier mobility is characterized in a field effect transistor. In addition, the use of rubrene single crystals for the study of photophysics at the interface with novel acceptor molecules is explored.