PhD Defense: Jon Gustafson
Location
Physics : 401
Date & Time
November 19, 2021, 11:30 am – 2:00 pm
Description
ADVISOR: L. Michael Hayden
TITLE: Ultrafast Carrier Dynamics in Monolayer Transition Metal Dichalcogenides via Time-Resolved Terahertz Spectroscopy
ABSTRACT: Atomically thin transition metal dichalcogenides (TMDs) are a class of two-dimensional materials that have attracted much attention in the past decade because of their unique properties. One such property is the existence of tightly bound excitons at room temperature. These excitons are a consequence of quantum confinement and the reduced dielectric environment enhancing the Coulombic interactions between electrons and holes. The strong Coulombic interactions in TMDs are sufficient to support charged excitons (trions) as well. As people look to fabricate devices out of TMDs, a fundamental understanding of how excitons, trions, electrons, holes, and other quasiparticles behave in these two-dimensional systems is essential.
In this work, we present three studies in which we characterize the behavior of charge carriers in monolayer TMDs utilizing time-resolved terahertz spectroscopy (TRTS). TRTS is a pump-probe technique in which a terahertz pulse probes the photogenerated excited state of a material. By measuring this interaction between the terahertz pulse and the excited-state carriers, we can monitor the frequency-dependent photoconductivity dynamics of a material with sub-picosecond resolution.
In the first study, we investigate the carrier dynamics of monolayer WS2 in vacuum. Here, we observe a photoinduced increase in conductivity due to defect-mediated positive trion formation. Furthermore, we find that trions in monolayer WS2 contribute to the conductivity in three ways: a Drude response, a broad resonance response, and a dissociation response at the trion binding energy.
We then consider the role the environment plays in modulating the photoconductivity response of monolayer TMDs. In the second study, we investigate the effect of molecular oxygen on the photoconductivity of monolayer MoS2. Here, we find that the photoconductivity shifts from negative to positive as the MoS2 environment changes from vacuum to atmospheric pressure. This transition results from physically adsorbed oxygen depleting electrons from the n-type MoS2.
In the third study, we investigate the effect of the dielectric environment on the photoconductivity of monolayer WS2. Here, we find that the broad resonance response and the dissociation response at the trion binding energy shift to lower frequencies as the dielectric constant of the environment increases. This shift is due to increased electrostatic screening. These results should prove useful to those who study the ultrafast carrier dynamics in two-dimensional materials, as well as to those who look to make devices out of these nanomaterials.
TITLE: Ultrafast Carrier Dynamics in Monolayer Transition Metal Dichalcogenides via Time-Resolved Terahertz Spectroscopy
ABSTRACT: Atomically thin transition metal dichalcogenides (TMDs) are a class of two-dimensional materials that have attracted much attention in the past decade because of their unique properties. One such property is the existence of tightly bound excitons at room temperature. These excitons are a consequence of quantum confinement and the reduced dielectric environment enhancing the Coulombic interactions between electrons and holes. The strong Coulombic interactions in TMDs are sufficient to support charged excitons (trions) as well. As people look to fabricate devices out of TMDs, a fundamental understanding of how excitons, trions, electrons, holes, and other quasiparticles behave in these two-dimensional systems is essential.
In this work, we present three studies in which we characterize the behavior of charge carriers in monolayer TMDs utilizing time-resolved terahertz spectroscopy (TRTS). TRTS is a pump-probe technique in which a terahertz pulse probes the photogenerated excited state of a material. By measuring this interaction between the terahertz pulse and the excited-state carriers, we can monitor the frequency-dependent photoconductivity dynamics of a material with sub-picosecond resolution.
In the first study, we investigate the carrier dynamics of monolayer WS2 in vacuum. Here, we observe a photoinduced increase in conductivity due to defect-mediated positive trion formation. Furthermore, we find that trions in monolayer WS2 contribute to the conductivity in three ways: a Drude response, a broad resonance response, and a dissociation response at the trion binding energy.
We then consider the role the environment plays in modulating the photoconductivity response of monolayer TMDs. In the second study, we investigate the effect of molecular oxygen on the photoconductivity of monolayer MoS2. Here, we find that the photoconductivity shifts from negative to positive as the MoS2 environment changes from vacuum to atmospheric pressure. This transition results from physically adsorbed oxygen depleting electrons from the n-type MoS2.
In the third study, we investigate the effect of the dielectric environment on the photoconductivity of monolayer WS2. Here, we find that the broad resonance response and the dissociation response at the trion binding energy shift to lower frequencies as the dielectric constant of the environment increases. This shift is due to increased electrostatic screening. These results should prove useful to those who study the ultrafast carrier dynamics in two-dimensional materials, as well as to those who look to make devices out of these nanomaterials.
