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Open Access Publications from the University of California

Scholarly Works (6 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2023)

Lead halide perovskites are the subject of great interest owing to the unique photophysical properties of perovskites where elucidating the nature of those properties highly relies on the understanding the photophysics of semiconducting materials. Upon photoexcitation, excited electrons and holes are generated, where bound electron-hole pair is called an exciton, what decides the optoelectronic properties depends on how charges carriers are relaxed back to their ground state. Given that the charges carriers can be coupled to lattice vibrations, called phonons, while relaxing, taking a closer look at how the structural fluctuations produced by the lattice affects the behavior of charge carriers is crucial to elucidate the microscopic origin of the properties of semiconducting materials. However, including the dynamical effects of phonons is in general difficult. Although the study on how phonons affect the properties of free charges has a long history, there still remains active open area for studying how excitonic properties are altered by the interaction with phonons. Especially for perovskites, the complex structure and anharmonic nature makes theoretical study even more challenging.

Herein, in this thesis, we explore the properties of lead halide perovskites in various dimensions by using an atomistic molecular dynamics simulations, allowing us to capture all orders of anharmonicity. For the system of layered perovskites with bulky organic molecules, it is showed that how the anharmonicity of organic molecules affects the vibrational relaxation dynamics following photoexcitation. And for a system of single chain perovskite nanowire, we are able to analyze the structural dynamics. In terms of studying excitonic properties in bulk perovskites, with this explicit anharmonic perovskite lattice, we describe quantum particles with path integral framework and we have developed a Gaussian field theory to describe the effective interactions between electrons and holes as mediated by the perovskite lattice using path integral molecular dynamics, illustrating the method of use to study exceptional excitonic properties of perovskites. Lastly, we end this by briefly introducing the application of this framework for studying the multiparticle excitations in perovskite nanocrystals.

    Cover page: Interplay between structural dynamics and optoelectronic properties in lead halide perovskites
    • Article
    • Peer Reviewed
    UC Berkeley Previously Published Works (2022)
    We employ quasiparticle path integral molecular dynamics to study how the excitonic properties of model semiconductors are altered by electron-phonon coupling. We describe ways within a path integral representation of the system to evaluate the renormalized mass, binding energy, and radiative recombination rate of excitons in the presence of a fluctuating lattice. To illustrate this approach, we consider Fröhlich-type electron-phonon interactions and employ an imaginary time influence functional to incorporate phonon-induced effects nonperturbatively. The effective mass and binding energies are compared with perturbative and variational approaches, which provide qualitatively consistent trends. We evaluate electron-hole recombination rates as mediated through both trap-assisted and bimolecular processes, developing a consistent statistical mechanical approach valid in the reaction limited regime. These calculations demonstrate how phonons screen electron-hole interactions, generically reducing exciton binding energies and increasing their radiative lifetimes.
      Cover page: Renormalization of excitonic properties by polar phononsCreative Commons 'BY-NC' version 4.0 license
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2023)
      We study the energetics of quasiparticle excitations in CsPbBr3 perovskite nanocrystals using path integral molecular dynamics simulations. Employing detailed molecular models, we elucidate the interplay of anharmonic lattice degrees of freedom, dielectric confinement, and electronic correlation on exciton and biexciton binding energies of a range of nanocrystal sizes. We find generally good agreement with some experimental observations of binding energies and additionally explain the observed size-dependent Stokes shift. The explicit model calculations are compared with simplified approximations to rationalize the lattice contributions to binding. We find that polaron formation significantly reduces exciton binding energies, whereas these effects are negligible for biexciton interactions. While experimentally the binding energy of biexcitons is uncertain, based on our study, we conclude that biexcitons are bound in CsPbBr3.
        Cover page: Biexcitons are bound in CsPbBr3 perovskite nanocrystalsCreative Commons 'BY' version 4.0 license
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2023)
        The order of bright and dark excitonic states in lead-halide perovskite nanocrystals is debated. It has been proposed that the Rashba effect, driven by lattice-induced symmetry breaking, causes a bright excitonic ground state. Direct measurements of excitonic spectra, however, show the signatures of a dark ground state, bringing the role of the Rashba effect into question. We use an atomistic theory to model the exciton fine structure of perovskite nanocrystals, accounting for realistic lattice distortions. We calculate optical gaps and excitonic features that compare favorably with experimental works. The exciton fine structure splittings show a nonmonotonic size dependence due to a structural transition between cubic and orthorhombic phases. Additionally, the excitonic ground state is found to be dark with spin triplet character, exhibiting a small Rashba coupling. We additionally explore the effects of nanocrystal shape on the fine structure, clarifying observations on polydisperse nanocrystals.
          Cover page: Size-Dependent Lattice Symmetry Breaking Determines the Exciton Fine Structure of Perovskite NanocrystalsCreative Commons 'BY' version 4.0 license
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2021)
          Organic-inorganic layered perovskites, or Ruddlesden-Popper perovskites, are two-dimensional quantum wells with layers of lead-halide octahedra stacked between organic ligand barriers. The combination of their dielectric confinement and ionic sublattice results in excitonic excitations with substantial binding energies that are strongly coupled to the surrounding soft, polar lattice. However, the ligand environment in layered perovskites can significantly alter their optical properties due to the complex dynamic disorder of the soft perovskite lattice. Here, we infer dynamic disorder through phonon dephasing lifetimes initiated by resonant impulsive stimulated Raman photoexcitation followed by transient absorption probing for a variety of ligand substitutions. We demonstrate that vibrational relaxation in layered perovskite formed from flexible alkyl-amines as organic barriers is fast and relatively independent of the lattice temperature. Relaxation in layered perovskites spaced by aromatic amines is slower, although still fast relative to bulk inorganic lead bromide lattices, with a rate that is temperature dependent. Using molecular dynamics simulations, we explain the fast rates of relaxation by quantifying the large anharmonic coupling of the optical modes with the ligand layers and rationalize the temperature independence due to their amorphous packing. This work provides a molecular and time-domain depiction of the relaxation of nascent optical excitations and opens opportunities to understand how they couple to the complex layered perovskite lattice, elucidating design principles for optoelectronic devices.
            Cover page: Vibrational relaxation dynamics in layered perovskite quantum wells
            • Article
            • Peer Reviewed
            UC Berkeley Previously Published Works (2023)
            Halide perovskite is a unique dynamical system, whose structural and chemical processes happening across different timescales have significant impact on its physical properties and device-level performance. However, due to its intrinsic instability, real-time investigation of the structure dynamics of halide perovskite is challenging, which hinders the systematic understanding of the chemical processes in the synthesis, phase transition, and degradation of halide perovskite. Here, we show that atomically thin carbon materials can stabilize ultrathin halide perovskite nanostructures against otherwise detrimental conditions. Moreover, the protective carbon shells enable atomic-level visualization of the vibrational, rotational, and translational movement of halide perovskite unit cells. Albeit atomically thin, protected halide perovskite nanostructures can maintain their structural integrity up to an electron dose rate of 10,000 e-/Å2·s while exhibiting unusual dynamical behaviors pertaining to the lattice anharmonicity and nanoscale confinement. Our work demonstrates an effective method to protect beam-sensitive materials during in situ observation, unlocking new solutions to study new modes of structure dynamics of nanomaterials.
              Cover page: Direct Observation of Transient Structural Dynamics of Atomically Thin Halide Perovskite Nanowires
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