Microscopic conductivity and ultrafast photocurrents in chalcogenides from 2D to 3D and beyond

Kushnir, Kateryna

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Microscopic conductivity and ultrafast photocurrents in chalcogenides from 2D to 3D and beyond

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The subject of this thesis is the dynamics of charge carriers and optical excitations in semiconducting chalcogenides, specifically compounds that contain sulfur or selenium. Chalcogenides are fascinating, as they display a wide array of properties that are attractive for both solar energy conversion and optoelectronics. With band gaps in the visible and near infrared, they strongly absorb light in the visible range, resulting in the excitation of charge carriers with long lifetimes and high mobility. Some of these materials exhibit room temperature ferroelectricity and ferroelasticity, pronounced nonlinear optical effects, and topologically protected surface states. Their properties can be further tailored by their structure and dimensionality, from bulk, three-dimensional (3D) materials to nanocrystals and inherently two-dimensional (2D) van der Waals materials. 2D chalcogenides are particularly appealing, as they feature enhanced light-matter interactions due to reduced dielectric screening, confinements of charge carriers in individual layers, and no dangling bonds at their surfaces. They are flexible and can be deposited on a variety of substrates. This thesis discusses photoexcited carrier dynamics in a number of 2D and 3D chalcogenides: \ce{ GeS }, \ce{ GeSe }, \ce{SnSe}, \ce{SnS2}, \ce{PbS}, \ce{Bi2S3}, and \ce{(Bi_{1-x}In_x)_2Se_3}. Of them, 2D \ce{ GeSe } and \ce{ GeS } have strongly anisotropic electronic and optical properties, owing to robust room temperature ferroelectric polarization in the layers. 2D \ce{(Bi_{1-x}In_x)_2Se_3} undergoes a transition from a topological to trivial band insulator behavior with the indium content increasing from zero to a few percent, and exhibits pronounced changes in its carrier density, mobility, and response to photoexcitation. Another 2D chalcogenide, \ce{ SnS2 }, was investigated as a possible photoanode material due to its high optical absorption and mobility. Additionally, we discovered that it also exhibits intriguing nonlinear effects upon photoexcitation that result in the emission of terahertz (THz) radiation. Finally, the polycrystalline (quasi-3D) chalcogenides \ce{ PbS } and \ce{ Bi2S3 } were investigated as photovoltaic and photodetector materials. Polycrystalline films are easy to deposit and more cost effective for large scale manufacturing, but, unlike bulk single crystals, can suffer from deleterious effects of grain or domain boundary defects. For all of these materials, their potential applications in photonic and optoelectronic devices require a detailed understanding of the optical excitation, microscopic photoconductivity, dynamics of optically injected charge carriers and photoexcitations, and their dependence on chemical structure and morphology. Relevant photophysical processes, such as free carrier absorption, carrier scattering, trapping, recombination, and interactions between the optically injected carriers and lattice vibrational modes, occur over sub-picosecond to nanoseconds time scales, and often have characteristic energies in the THz range (0.1 - 30 THz or 4 - 120 meV). The all-optical techniques in the THz spectroscopy toolbox use broadband, picosecond-duration, phase-stable THz pulses to probe the dynamics of low-energy excitations in materials and microscopic photoconductivity without a reliance on electrical contacts, and are therefore ideally suited for investigating photophysics in bulk as well as nanoscale systems. In this thesis we have used three experimental THz spectroscopic approaches: 1) time domain THz spectroscopy, which probes intrinsic free carriers, low energy phonons, and other low energy excitations, 2) time-resolved THz spectroscopy, which measures microscopic transient photoconductivity following optical excitation, and 3) THz emission spectroscopy, which provides a window into nonlinear optical properties and ultrafast photocurrents. With these techniques, we have demonstrated generation of ultrafast shift currents, or bias-free bulk photovoltaic effects, in 2D \ce{ GeSe } and \ce{ GeS }. We have observed the scattering of optically excited carriers from bulk conduction band states into two distinct sets of high mobility topological surface states in 2D \ce{Bi2Se3}, and showed that the dynamics of the photoexcited free carriers are affected by these twin domain boundaries and are sensitive to the disorder introduced by indium substitution in 2D \ce{(Bi_{0.75}In_{0.25})_2Se_3} and \ce{(Bi_{0.50}In_{0.50})_2Se_3}. We have also characterized the lifetimes of free photoexcited carriers, their mobility, and their interactions with interfaces and boundaries in 2D \ce{ SnS2 } and \ce{GeS}, and in polycrystalline \ce{ Bi2S3 } and \ce{ PbS }. Our experiments uncovered the dynamics of photoexcitation over sub-picosecond to nanosecond time scales, and revealed the relationship between structure and optoelectronic properties in studied 2D and 3D chalcogenide materials. We have set the stage for their applications in efficient photovoltaic, photoelectrochemical and optoelectronic devices.

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