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Title:Transient extreme ultraviolet spectroscopy of semiconductors
Author(s):Verkamp, Max Andrew
Director of Research:Vura-Weis, Josh
Doctoral Committee Chair(s):Vura-Weis, Josh
Doctoral Committee Member(s):Dlott, Dana D; Schleife, André; Abbamonte, Peter
Department / Program:Chemistry
Discipline:Chemical Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):extreme ultraviolet spectroscopy
carrier dynamics
Abstract:Transient extreme ultraviolet (XUV) spectroscopy is used to investigate ultrafast photophysics in lead iodide (PbI2) and methylammonium lead iodide perovskite (MAPbI3). Sub-30 fs pulses of XUV light are produced to use as a probe in a tabletop instrument using high-harmonic generation. PbI2 and MAPbI3 both absorb XUV radiation at the iodine N4,5 edge, which arises from transitions from the core I 4d orbitals to the valence and conduction bands of the semiconductor materials. Static measurements at this edge probe the iodine partial density of states of the conduction band, showing good agreement with spectra predicted using density functional theory (DFT). Excitation in the visible promotes electrons from the valence bands to the conduction bands, resulting in photogenerated charge carriers (holes and electrons). The XUV valence band region shows new transitions from the core states into the unoccupied holes, providing a tool for understanding hole dynamics in semiconductor materials. The transient signals in the XUV conduction band region result from a combination of state-blocking (band-filling) and band-gap renormalization. This can be disentangled but will require further theoretical modeling to fully extract electron dynamics. XUV and optical transient absorption (OTA) together reveal unequal cooling in MAPbI3, with the initial excitation giving more excess energy to the hole distribution than the electron distribution. The distributions show rapid cooling by carrier-phonon coupling in the first few hundred fs followed by slow cooling due to the hot-phonon bottleneck effect for tens to hundreds of ps. The cooling dynamics differ, indicating that the electron-phonon and hole-phonon coupling pathways are independent. Additional modeling and DFT prediction are used to better understand fitting OTA data to extract carrier distributions with unequal energy in each band. It is shown that OTA is more sensitive to the lower energy carrier, or to the lighter carrier for unequal effective masses.
Issue Date:2019-07-08
Rights Information:Copyright 2019 Max Verkamp
Date Available in IDEALS:2019-11-26
Date Deposited:2019-08

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