Time-Resolved Terahertz Spectroscopy of Semiconductor Nanomaterials
Creators: Jensen, Charles E
Publisher: name: University of Alberta Library
Year: 2024

<p>We investigate the microscopic nature of charge-carrier conduction in nanoscale semiconducting materials using time-resolved terahertz spectroscopy (TRTS). TRTS uses picosecond pulses of terahertz radiation (0.5-2.5 THz) to explore photoexcited charge-carrier lifetimes and dynamics with picosecond resolution. Owing to the small length scales probed by THz radiation (∼ 10 nm), TRTS provides a valuable, non-contact approach to characterize charge-carrier transport in the novel nanomaterials studied herein for the first time.</p>

<p>Photoexcited charge-carriers are known to catalyze chemical reactions in CdS nanowires, however, the underlying ultrafast photoconductivity dynamics remain relatively unexplored. Here, we combine TRTS and time-resolved photoluminescence (TRPL) to form a new, self-consistent understanding of ultrafast transport in CdS nanowires wrapped in C3N5 nanosheets. We find that charge carriers exhibit Drude-Smith conductivity spectrum, with bulk-like mobilities (∼ 400 cm2/Vs) on short length scales (&lt; 25 nm). We construct a theoretical model of band-edge photoconductivity and photoluminescence lifetimes in CdS nanowires that encompasses charge-carriers undergo subpicosecond hot-carrier cooling, bimolecular recombination, and surface recombination for the first time. By simultaneously fitting photoconductivity and photoluminescence lifetimes we reveal that charge-separation (∼ 150 ps) plays a significant role in the relaxation dynamics of CdS nanowires. </p>

<p>Atomically thin materials are a rapidly growing field of study, and there is a growing number of studies conducted in the THz frequency band. Here, we explore the temperature and fluence-dependent charge carrier dynamics in a WSe2 monolayer. We find that ultrafast exciton dissociation yields localized charge-carriers that dominate the photoconductivity in the THz frequency range. Furthermore, we find that the charge carrier mobility does not significantly change with fluence (50 - 500 µJ/cm2) or temperature (80 - 295 K). High mobility semiconductors are desirable in battery applications, but the price is prohibitive in large-scale manufacturing. Inverse-opal nanostructures can simultaneously reduce<br>
costs and enhance device performance, however, it is unknown how this affects charge-carrier participation in battery chemistry. Here, we use TRTS to investigate charge-carrier dynamics in SixGe17−x inverse opal films. Our Drude-Smith fits indicate long-range (&gt; 30 nm) mobilities of ∼ 3 − 19 cm2/Vs. The photoconductivity lifetimes indicate significant charge-carrier trapping in inverse-opal films, with activation energies of ∼ 40 − 140 meV.</p>

<p>Silicene is a candidate for next-generation optoelectronics and photovoltaics because it<br>
may be easier to integrate into existing technologies than graphene, however, silicene quickly oxidizes when exposed to air. Encapsulation techniques prevent oxygen from reaching the nanosheet, but it is unknown how C12H25 (dodecene) encapsulation impacts the photoconductivity of a silicon nanosheet (SiNS). Here, we use TRTS to explore ultrafast photoconductivity in SiNSs for the first time. Dodecene encapsulation results in a short-lived (∼ 1 ps) photoconductivity, which is an improvement over the nonexistent photoconductivity we observe in hydrogen terminated SiNSs. Our Drude-Smith fits indicate strong localization in the dodecene-encapsulated SiNSs (c = 0.99 ± 0.06) that nearly halts long range (&gt; 30 nm) transport.</p>

<p>Gallium nitride nanowalls permit control over the facets exposed to erosion during photocatalysis, which can enhance the lifetime of the nanowalls. How this morphology impacts<br>
the ultrafast photoconductivity is unknown. Here, we use TRTS to study charge-carrier dynamics in GaN nanowalls for the first time. We find that the photoconductivity lifetimes are described by a bi-exponential model, with recombination lifetimes of 5.45 ± 0.08 ps and 67 ± 3 ps. We also find a long-lived recombination mechanism that lasts longer than 125 ps.</p>