Browsing by Subject "Ultrafast spectroscopy"

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    Hot electron dynamics at semiconductor surfaces: implications for quantum dot photovoltaics.
    (2010-07) Tisdale, William A.
    Finding a viable supply of clean, renewable energy is one of the most daunting challenges facing the world today. Solar cells have had limited impact in meeting this challenge because of their high cost and low power conversion efficiencies. Semiconductor nanocrystals, or quantum dots, are promising materials for use in novel solar cells because they can be processed with potentially inexpensive solution-based techniques and because they are predicted to have novel optoelectronic properties that could enable the realization of ultra-efficient solar power converters. However, there is a lack of fundamental understanding regarding the behavior of highly-excited, or "hot," charge carriers near quantum-dot and semiconductor interfaces, which is of paramount importance to the rational design of high-efficiency devices. The elucidation of these ultrafast hot electron dynamics is the central aim of this Dissertation. I present a theoretical framework for treating the electronic interactions between quantum dots and bulk semiconductor surfaces and propose a novel experimental technique, time-resolved surface second harmonic generation (TR-SHG), for probing these interactions. I then describe a series of experimental investigations into hot electron dynamics in specific quantum-dot/semiconductor systems. A two-photon photoelectron spectroscopy (2PPE) study of the technologically-relevant ZnO(10-10) surface reveals ultrafast (sub-30fs) cooling of hot electrons in the bulk conduction band, which is due to strong electron-phonon coupling in this highly polar material. The presence of a continuum of defect states near the conduction band edge results in Fermi-level pinning and upward (n-type) band-bending at the (10-10) surface and provides an alternate route for electronic relaxation. In monolayer films of colloidal PbSe quantum dots, chemical treatment with either hydrazine or 1,2-ethanedithiol results in strong and tunable electronic coupling between neighboring quantum dots. A TR-SHG study of these electronically-coupled quantum-dot films reveals temperature-activated cooling of hot charge carriers and coherent excitation of a previously-unidentified surface optical phonon. Finally, I report the first experimental observation of ultrafast electron transfer from the higher excited states of a colloidal quantum dot (PbSe) to delocalized conduction band states of a widely-used electron acceptor (TiO2). The electric field resulting from ultrafast (<50fs) separation of charge carriers across the PbSe/TiO2(110) interface excites coherent vibration of the TiO2 surface atoms, whose collective motions can be followed in real time.
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    Structural Explorations into Singlet Fission Chromophores for Solar Photochemistry with Femtosecond Stimulated Raman Spectroscopy
    (2020-10) Bera, Kajari
    Understanding physical, chemical or biological changes require monitoring the molecular vibrations aka nuclear coordinates on the timescale of their movements, which is hundreds of femtoseconds. Thus, ultrafast spectroscopic techniques capable of providing molecular conformational changes on the femtosecond timescale are desirable to provide in-depth knowledge of any photophysical process. This thesis discusses the strength of using a structurally sensitive ultrafast spectroscopic technique, femtosecond stimulated Raman spectroscopy (FSRS) to understand the photophysics in a chemical process called singlet fission. Thin films and single crystals which may undergo singlet fission are promising for solar energy conversion strategies as they can generate two charge carriers by the absorption of only one photon. A major bottleneck in the singlet fission field is the challenge of designing or discovering new molecules with enhanced photophysical properties, while not altering other parameters such as crystal packing or solubility which can hinder overall device performance. Chemical modifications to acenes may lead to singlet fission-based photovoltaics with improved solar energy conversion efficiencies, but identifying how molecular structural changes impact the rate and yield of fission is challenging to model and predict. Traditionally, spectroscopic measurements have not provided much insight for rational design of new chromophores, but rather have been used to understand why an existing system works well. To this end, I have used FSRS to examine the excited state structural dynamics in organic chromophores undergoing singlet fission and have provided a predictive model for molecular designing guidelines to obtain efficient singlet fission systems for their use in solar energy based devices. Specifically, I have used ultrafast FSRS to monitor the changes in the molecular structure in rubrene during singlet fission and observed that singlet fission in crystalline rubrene is associated with a loss of electron density in the tetracene backbone. Armed with this knowledge and a hypothesis to improve singlet fission rate by reducing electron density in the tetracene core, I screened rubrene derivatives to study their excited state dynamics using FSRS. From a series of derivatives, I screened two new rubrene derivatives with electron withdrawing substituents, FM-rubrene & F-rubrene, to prime the system for singlet fission, without impacting intermolecular interactions. Using these rationally designed rubrene derivatives I found that both the rate and yield of singlet fission are significantly improved, proving that spectroscopic insight is crucial to successfully designing new chromophores. This shows that the long-held promise of spectroscopy-informed small molecule design for organic optoelectronic materials can be realized in rubrene-based singlet fission materials. As the first graduate student to use FSRS in a microscope to look at crystals, I realized that distinguishing Raman from non-Raman features can be complicated in molecular crystals, where the narrowband transient vibronic couplings due to less heterogeneous broadening can overwhelm the excited state Raman bands. To overcome this, we redesigned the ultrafast FSRS experimental setup to help ease the data extraction and interpretation associated with this technique. By adding a pair of mirrors and a slit to the existing grating filter setup, we generated an additional Raman excitation pulse that identifies Raman peaks from a pool of non-Raman peaks. In this thesis, I present the successful implementation of FSRS to provide unprecedented structural dynamics in organic semiconducting materials with the aim to achieve better performing singlet fission based-photovoltaic devices and the experimental modifications made to support FSRS as a more user-friendly technique for its widespread adoption to understand chemical reactions.