ABSTRACT
“Ultrafast Terahertz Spectroscopy of Charge Transport and Photoexcited Carrier Dynamics in Two-Dimensional MXene Thin-Films”
Ultrafast terahertz (THz) spectroscopy provides a powerful, non-contact approach for probing charge transport and photoexcited carrier dynamics in complex materials. In this dissertation, THz time-domain spectroscopy (THz-TDS) and optical pump–THz probe (OPTP) spectroscopy techniques are used in combination
with four-point probe electrical measurements to investigate the intrinsic and photoinduced electronic properties of a recently-established class of two-dimensional (2D) transition metal carbide/nitride thin-films. Known as MXenes, these materials are comprised of many 2D nanoflakes. THz spectroscopy and four-point probe measurements enable direct access to both short-range, intraflake conductivity and long-range, interflake conductivity, in addition to ultrafast photoexcited carrier dynamics in this class of materials.
By applying these measurement techniques across a range of MXene systems, including Ti3C2Tx, Nb2CTx, Ti2CTx, and Ti3CNTx, a more comprehensive understanding of how MXene chemical composition influences its electronic properties and structure is revealed. Overall, it is found that the choice of transition metal sets the density of states at the Fermi level, determining general electronic properties including conductivity and charge carrier density, while the choice of carbon or nitrogen as the material base, along with the number of layers in the system, can allow for significant "tuning" of these electronic properties. The choice of surface termination can also modify the electronic properties and structure of MXenes, but more experimental data is needed to understand how.
Furthermore, it is hypothesized that ultrafast photoconductivity is governed by the relative magnitude of intrinsic and photoexcited carrier populations. Specifically, when the intrinsic carrier density exceeds the photoinjected carrier density (N >> ΔN), photothermal effects dominate, leading to a transient reduction in conductivity after photoexcitation. In contrast, when the intrinsic carrier density is comparable to the injected carrier density (N ~ ΔN), photoexcitation results in a transient conductivity enhancement as the increase in carrier mobility by photoexcited carriers outweighs photothermal effects.
Finally, the use of other, non-MXene materials to further engineer the electronic properties and structure of MXenes is examined. In particular, it is found that adsorbing sodium tripolyphosphate (TPP) at the MXene flake edges or encapsulating MXene flakes with silk fibroin can improve the environmental stability of MXenes, preserving their useful electronic properties against degradation, while simultaneously allowing control over their photothermal relaxation dynamics.
Together, these results deepen our understanding of charge transport and photoexcited carrier dynamics in MXenes, and provide a means of engineering their properties for applications in THz devices, sensing, and photothermal technologies.
Advisor: Professor Lyubov Titova
Committee Members: Professor Doug Petkie, Professor Aaron Deskins and Professor Jeannine Coburn