Tunable Electronic and Optical Properties of Layered 2D Material

The field of 2D layered materials has been continuously growing and evolving for decades. The early works on graphene by Novosolev and Geim drove the field into the mainstream. Since then it has moved beyond carbon based systems to encompass a wide variety of materials with varied chemical compositions and properties. The increased research interest has been driven by the advances in synthesis techniques and the ease with which these properties can be modulated to access novel science and vast application possibilities. For example graphene has shown evidence of massless Dirac fermions and quantum hall effect and MoS2 has great potential for flexible/low power electronic applications and novel charge density wave states.;This dissertation presents work on various tunable optical and electronic properties of 2D layered materials focusing primarily on the transition metal dichalcogenide family. First, the modulation of the optical response of MoS 2 by coupling to a substrate is discussed. A classical multiple reflection model is developed describing resonant activity in the SiO2 layer beneath the MoS2. The model is compared to experimental data and discrepancies are discussed. The model fails to consider charge transfer effects between the MoS2 and Si layers at low SiO2 thicknesses. In addition the model fails to account for the excitonic origins and nature of the MoS 2 photoluminescence.;The second project details experimental observations of tunable exciton dynamics at the contact region of MoS2 based field effect devices under varying applied fields. A decomposition of the photoluminescence response by fitting to the sum of 3 lorentzian functions reveals a ~40 meV blue shift of all excitonic recombination at the contact when compared to the channel region. In addition the ratios of peak intensities associated with the different exciton recombinations vary with applied source-drain bias voltage. This effect is not associated with field induced dissociation due to the asymmetric band bending that comes about from applying a bias voltage.;Finally there is a discussion of work extending previous research on the use of charged gas molecules to alter the conductivity of 2D layered material electronic devices. MoS2 field effect devices when exposed to positive ions show an increase in negative carrier concentration consistent with a gating mechanism for carrier induction. Conversely negative ion exposure is shown to induce hole type carriers in the same type of device. A polymer capping mechanism is developed to ensure that the ion-gating effect persists over extended periods of time with minimal impact to the electronic properties of the device.