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Open Access Publications from the University of California

Scholarly Works (19 results)

  • Article
  • Peer Reviewed
UC Merced Previously Published Works (2013)
For linear electron-phonon coupling, the Huang-Rhys factor, S, gives the intensity ratio of the one-quantum vibronic transition (0 → 1) to the purely electronic origin transition (0 → 0) in a vibrationally resolved, zero-temperature absorption or emission spectrum. It is often assumed that the overtone to fundamental integrated intensity ratio in resonance Raman scattering of semiconductor nanocrystals is equal to or proportional to S, or that S may be determined from the overtone intensity in some other straightforward manner. In fact, this is not generally possible because of different excitation profiles for overtones and fundamentals, differential sensitivity of overtones and fundamentals to electronic dephasing, and interference effects from partially overlapping electronic transitions. Here we examine the relationship between the Huang-Rhys factor and the overtone to fundamental intensity ratio through spectroscopic simulations using parameters appropriate to II-VI semiconductor nanocrystals such as CdSe. A simple equation relating the overtone to fundamental Raman intensity ratio to the Huang-Rhys factor is obtained only in the case of a single resonant electronic state, excitation at the maximum of the inhomogeneously broadened absorption band, and a homogeneous line width small compared with the phonon frequency.
    Cover page: Resonance Raman Overtone Intensities and Electron–Phonon Coupling Strengths in Semiconductor Nanocrystals
    • Article
    • Peer Reviewed
    UC Merced Previously Published Works (2019)
    This perspective reviews the topic of exciton-phonon coupling (EPC) in II-VI semiconductor nanocrystals. First, EPC is defined and its relevance is discussed, both as it influences the properties of the materials relevant to applications and as a probe of electronic structure. Different experimental and theoretical methods for probing EPC are outlined. Results for several different classes of II-VI nanocrystals are summarized. Finally, possible future directions are outlined.
      Cover page: Exciton-optical phonon coupling in II-VI semiconductor nanocrystals
      • Article
      • Peer Reviewed
      UC Merced Previously Published Works (2020)
      Second order nonlinear responses such as sum frequency and second harmonic generation arise from the response of a material system to the second power of an incident electromagnetic field through the material's first hyperpolarizability or second-order optical susceptibility. These quantities are nonzero only for noncentrosymmetric systems, but different length scales of the noncentrosymmetry give rise to second harmonic or sum frequency radiation with different spatial and coherence characteristics. This perspective discusses the possible contributions to the second-order signal from films of small semiconductor quantum dots and addresses whether such experiments are expected to selectively enhance transitions to surface defects or trap states in such systems. It points out how "surface" and "bulk" contributions to the sum frequency or the second harmonic signal should be distinguishable through their angular dependence in a scattering geometry. It also explores possible mechanisms whereby second order spectroscopies might provide access to surface states that are very weak or absent in other forms of optical spectroscopy.
        Cover page: Can second order nonlinear spectroscopies selectively probe optically “dark” surface states in small semiconductor nanocrystals?
        • Article
        • Peer Reviewed
        UC Merced Previously Published Works (2016)
        Three empirical interatomic force fields are parametrized using structural, elastic, and phonon dispersion data for bulk CdSe and their predictions are then compared for the structures and phonons of CdSe quantum dots having average diameters of ˜2.8 and ˜5.2 nm (˜410 and ˜2630 atoms, respectively). The three force fields include one that contains only two-body interactions (Lennard-Jones plus Coulomb), a Tersoff-type force field that contains both two-body and three-body interactions but no Coulombic terms, and a Stillinger-Weber type force field that contains Coulombic interactions plus two-body and three-body terms. While all three force fields predict nearly identical peak frequencies for the strongly Raman-active "longitudinal optical" phonon in the quantum dots, the predictions for the width of the Raman peak, the peak frequency and width of the infrared absorption peak, and the degree of disorder in the structure are very different. The three force fields also give very different predictions for the variation in phonon frequency with radial position (core versus surface). The Stillinger-Weber plus Coulomb type force field gives the best overall agreement with available experimental data.
          Cover page: Comparison of three empirical force fields for phonon calculations in CdSe quantum dots
          • Thesis
          • Peer Reviewed
          UC Merced Electronic Theses and Dissertations (2011)

          Active pharmaceutical ingredients (APIs) are often chiral. In many cases, different stereoisomers have significantly different physiological effects. As a result, the FDA requires clinical testing of each stereoisomer in any stereoisomeric mixture submitted as a candidate therapeutic and testing of the proposed mixture. As a result, clinical testing of any new stereoisomeric mixture requires three separate clinical trials. This process is cost-prohibitive. As a result, chiral pharmaceuticals are almost exclusively produced as single stereoisomers. Racemic production of chiral APIs limits production to an absolute maximum of 50% yield. A more atom-economical approach is asymmetric synthesis, which makes use of reactions that produce predominately or even exclusively one enantiomer. In recent years, organocatalysis has attracted attention as a potential tool for drug development. Organocatalysts avoid the usage of hazardous metals that can persist as trace contaminants in pharmaceuticals. Many organocatalytic systems have been developed in the past years with outstanding yields and selectivities. However, the mechanistic details for most of those reactions are still not well understood. In this dissertation, the author will first discuss the mechanism of Hajos-Parrish reaction, a proline-catalyzed intramolecular aldol reaction by performing kinetic isotope effect (KIE) study and ab initio density functional theory calculation. Secondly, the author will present the mechanistic study of asymmetric borane reduction catalyzed by Corey-Bakshi-Shibata (CBS) catalyst, and demonstrate the significance of steric effects in asymmetric reactions by measuring steric isotope effects. Additionally, two related fundamental studies on the Alpine-borane reduction and Friedel-Crafts acylation reaction will be presented as they are united with the main subjects covered in this dissertation.

            Cover page: Mechanistic investigation of organocatalysis
            • Article
            • Peer Reviewed
            UC Merced Previously Published Works (2018)
            • 1 supplemental PDF
            Cover page: Weak Exciton–Phonon Coupling in CdSe Nanoplatelets from Quantitative Resonance Raman Intensity Analysis
            • Article
            • Peer Reviewed
            UC Merced Previously Published Works (2019)
            • 1 supplemental PDF
            Cover page: Resonance Hyper-Raman Scattering from CdSe and CdS Nanocrystals
            • Article
            • Peer Reviewed
            UC Merced Previously Published Works (2017)
            Zinc to cadmium cation exchange of ZnSe quantum dots has been used to produce a series of alloyed Zn1-xCdxSe quantum dots. As x increases and the lowest-energy exciton shifts to the red, the peak initially broadens and then sharpens as x approaches 1. Resonance Raman spectra obtained with excitation near the lowest excitonic absorption peak show a gradual shift of the longitudinal optical phonon peak from 251 cm-1 in pure ZnSe to 210 cm-1 in nearly pure CdSe with strong broadening at intermediate compositions. The LO overtone to fundamental intensity ratio, a rough gauge of exciton-phonon coupling strength, increases considerably for intermediate compositions compared with those of either pure ZnSe or pure CdSe. The results indicate that partial localization of the hole in locally Cd-rich regions of the alloyed particles increases the strengths of local internal electric fields, increasing the coupling between the exciton and polar optical phonons.
            • 1 supplemental PDF
            Cover page: Nonuniform Excitonic Charge Distribution Enhances Exciton–Phonon Coupling in ZnSe/CdSe Alloyed Quantum Dots
            • Article
            • Peer Reviewed
            UC Merced Previously Published Works (2014)
            The origin of the ubiquitous low-frequency shoulder on the longitudinal optical (LO) phonon fundamental in the Raman spectra of CdSe quantum dots is examined. This feature is usually assigned as a "surface optical" (SO) phonon, but it is only slightly affected by modifying the surface through exchanging ligands or adding a semiconductor shell. Here we present excitation profile data showing that the low-frequency shoulder loses intensity as the excitation is tuned to longer wavelengths, closer to resonance with the lowest-energy 1Se-1S3/2 excitonic transition. Calculations of the resonance Raman spectra are carried out using a fully atomistic model with an empirical force field to calculate the phonon modes and the standard effective mass approximation envelope function model to calculate the electron and hole wave functions. When a force field of the Tersoff type is used, the calculated spectra closely resemble the experimental ones in showing mainly the higher-frequency LO phonon with 1Se-1S3/2 resonance but showing intensity in lower-frequency features with 1Pe-1P3/2 resonance. These calculations indicate that the main LO phonon peak involves largely motion of the interior atoms, while the low-frequency shoulder is more equally distributed throughout the crystal but not surface-localized. Interestingly, very different results are obtained with the widely used Coulomb plus Lennard-Jones force field developed by Rabani, which predicts far more disordered structures and more localized phonon modes for the nanocrystals compared with the Tersoff-type potential.
            • 1 supplemental PDF
            Cover page: The “Surface Optical” Phonon in CdSe Nanocrystals
            • Article
            • Peer Reviewed
            UC Merced Previously Published Works (2015)
            Resonance Raman spectra and excitation profiles have been measured and semiquantitatively modeled for core/shell quantum dots consisting of 2.7 nm diameter zincblende CdSe cores and thin (0.5 nm) or thick (1.6 nm) CdS shells. The Raman spectra show previously reported trends of increased peak frequency for both the CdSe and the CdS longitudinal optical (LO) phonons with increasing shell thickness. We also find a strong dependence of the peak CdS frequency on excitation energy and a large discrepancy between the experimental frequency of the CdSe + CdS combination band and the sum of the corresponding fundamental frequencies. This suggests that the dominant transitions at high excitation energies are localized on either the CdSe core or the CdS shell and thereby cannot enhance combination band transitions between core and shell. The CdS to CdSe Raman intensity ratios at high excitation energies further support this picture. The electron-phonon coupling for the CdSe LO phonon in the lowest excitonic transition is slightly weaker in the core/shell structures than in pure CdSe quantum dots, contrary to expectations for the Fröhlich coupling mechanism. Possible explanations for this discrepancy are discussed.
            • 1 supplemental PDF
            Cover page: Electron–Phonon Coupling in CdSe/CdS Core/Shell Quantum Dots
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