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Open Access Publications from the University of California

Scholarly Works (4 results)

  • Article
  • Peer Reviewed
UC Berkeley Previously Published Works (2016)
Acene molecular crystals are of current interest in organic optoelectronics, both as active materials and for exploring and understanding new phenomena. Phonon scattering can be an important facilitator and dissipation mechanism in charge separation and carrier transport processes. Here, we carry out density functional theory (DFT) calculations of the structure and the full phonon dispersion of crystalline naphthalene, a well-characterized acene crystal for which detailed neutron-diffraction measurements, as well as infrared and Raman spectroscopy, are available. We evaluate the performance, relative to experiments, of DFT within the local density approximation (LDA); the generalized gradient approximation of Perdew, Burke, and Ernzerhof (PBE); and a recent van der Waals-corrected nonlocal correlation (vdW-DF-cx) functional. We find that the vdW-DF-cx functional accurately predicts lattice parameters of naphthalene within 1%. Intermolecular and intramolecular phonon frequencies across the Brillouin zone are reproduced within 7.8% and 1%, respectively. As expected, LDA (PBE) underestimates (overestimates) the lattice parameters and overestimates (underestimates) phonon frequencies, demonstrating their shortcomings for predictive calculations of weakly bound materials. If the unit cell is fixed to the experimental lattice parameters, PBE is shown to lead to improved phonon frequencies. Our study provides a detailed understanding of the phonon spectrum of naphthalene, and highlights the importance of including van der Waals dispersion interactions in predictive calculations of lattice parameters and phonon frequencies of molecular crystals and related organic materials.
    • Thesis
    • Peer Reviewed
    UC Berkeley Electronic Theses and Dissertations (2019)

    In this dissertation we present first principles electronic structure calculations of acene solids, achieving better understanding of their electronic and vibrational properties relevant to their optoelectronic function. The family of acene molecular crystals serves as a testbed for broader classes of organic crystalline semiconductors, which have come under increasing focus for their many favorable optoelectronic properties. Among them are relative ease of processing, strong and tunable absorption, and charge carrier mobilities sufficiently high for applications. Despite numerous computational and experimental efforts, the details of the underlying mechanisms of these optoelectronic processes are still actively disputed, especially concerning the role of electron-phonon coupling and its impact on the acene electric structure at finite temperatures. To further improve the efficiency of these systems, and to develop new materials that can overcome existing challenges, better understanding of the underlying principles and structure-function relationships that determine acene properties is thus needed.

    Here, we calculate the charged and neutral electronic excitations of the acene crystal series within many-body perturbation theory (MBPT), based on van der Waals (vdW)–corrected density functional theory (DFT). We compare the performance of various functionals and vdW corrections in predicting the experimental lattice parameters and investigate the sensitivity of excited states to these structural parameters. Generally, low-lying charged and neutral excitations are well described by the MBPT methods used here, provided that optimized geometries close to experiment are used. The inclusion of vdW interactions to account for the weak intermolecular interactions in molecular crystals is thus found to be a prerequisite for the predictive and accurate calculation of excited state energies in these systems.

    To investigate the effect of vibrational coupling at zero and finite temperatures in organic crystals, we calculate the phonon band structure and electron-phonon contributions to the electron self-energy for the case of the naphthalene crystal. We first provide a comprehensive analysis of the computed phonon band structure, comparing to neutron diffraction data. Again, vdW corrections are necessary to obtain phonon frequencies from DFT calculations that are in good agreement with experiment. Based on these results, we compute the electron-phonon self-energy in naphthalene using vdW-corrected DFT and MBPT to lowest order. This self-energy provides the contribution of phonons to the renormalization of band structure energies and to the scattering lifetimes of electronic states. The resulting renormalized band gap at room temperature, and the temperature-dependent mobilities of electron and hole charge carriers are in good agreement with experimental values. Finally, we explore an eigenvalue–self-consistent computational scheme for the electron-phonon self-energy that leads to the prediction of strong satellite bands in the quasiparticle band structure.

    The methods presented in this dissertation are general and our conclusions are applicable to other molecular crystals, thus providing a template for future predictive calculations of optoelectronic properties of acenes and related systems, in which both structures and excited states are calculated entirely from first principles.

      Cover page: Electronic excitations, phonons, and electron-phonon coupling in acenes
      • Article
      • Peer Reviewed
      UC Berkeley Previously Published Works (2020)
      Organic molecular crystals are expected to feature appreciable electron-phonon interactions that influence their electronic properties at zero and finite temperature. In this work, we report first-principles calculations and an analysis of the electron-phonon self-energy in naphthalene crystals. We compute the zero-point renormalization and temperature dependence of the fundamental band gap, and the resulting scattering lifetimes of electronic states near the valence- and conduction-band edges employing density functional theory. Further, our calculated phonon renormalization of the GW-corrected quasiparticle band structure predicts a fundamental band gap of 5 eV for naphthalene at room temperature, in good agreement with experiments. From our calculated phonon-induced electron lifetimes, we obtain the temperature-dependent mobilities of electrons and holes in good agreement with experimental measurements at room temperature. Finally, we show that an approximate energy self-consistent computational scheme for the electron-phonon self-energy leads to the prediction of strong satellite bands in the electronic band structure. We find that a single calculation of the self-energy can reproduce the self-consistent results of the band gap renormalization and electrical mobilities for naphthalene, provided that the on-the-mass-shell approximation is used, i.e., if the self-energy is evaluated at the bare eigenvalues.
        Cover page: Band gap renormalization, carrier mobilities, and the electron-phonon self-energy in crystalline naphthalene
        • Article
        • Peer Reviewed
        UC Berkeley Previously Published Works (2022)
        Diamine-appended Mg2(dobpdc) (dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) metal-organic frameworks are a promising class of CO2 adsorbents, although their stability to SO2─a trace component of industrially relevant exhaust streams─remains largely untested. Here, we investigate the impact of SO2 on the stability and CO2 capture performance of dmpn-Mg2(dobpdc) (dmpn = 2,2-dimethyl-1,3-propanediamine), a candidate material for carbon capture from coal flue gas. Using SO2 breakthrough experiments and CO2 isobar measurements, we find that the material retains 91% of its CO2 capacity after saturation with a wet simulated flue gas containing representative levels of CO2 and SO2, highlighting the robustness of this framework to SO2 under realistic CO2 capture conditions. Initial SO2 cycling experiments suggest dmpn-Mg2(dobpdc) may achieve a stable operating capacity in the presence of SO2 after initial passivation. Evaluation of several other diamine-Mg2(dobpdc) variants reveals that those with primary,primary (1°,1°) diamines, including dmpn-Mg2(dobpdc), are more robust to humid SO2 than those featuring primary,secondary (1°,2°) or primary,tertiary (1°,3°) diamines. Based on the solid-state 15N NMR spectra and density functional theory calculations, we find that under humid conditions, SO2 reacts with the metal-bound primary amine in 1°,2° and 1°,3° diamine-appended Mg2(dobpdc) to form a metal-bound bisulfite species that is charge balanced by a primary ammonium cation, thereby facilitating material degradation. In contrast, humid SO2 reacts with the free end of 1°,1° diamines to form ammonium bisulfite, leaving the metal-diamine bond intact. This structure-property relationship can be used to guide further optimization of these materials for CO2 capture applications.
          Cover page: Evaluation of the Stability of Diamine-Appended Mg2(dobpdc) Frameworks to Sulfur Dioxide
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