David A. Strubbe's research while affiliated with University of California and other places

Course-based Undergraduate Research Experiences (CUREs) bring the excitement of research into the classroom to improve learning and the sense of belonging in the field. They can reach more students, earlier in their studies, than typical undergraduate research. Key aspects are: students learn and use research methods, give input into the project, generate new research data, and analyze it to draw conclusions that are not known beforehand. CUREs are common in other fields but have been rare in materials science and engineering. I propose a paradigm for computational material science CUREs, enabled by web-based simulation tools from nanoHUB.org that require minimal computational skills. After preparatory exercises, students each calculate part of a set of closely related materials, following a defined protocol to contribute to a novel class dataset which they analyze, and also calculate an additional property of their choice. This approach has been used successfully in several class projects.

Spin-flip (SF) methods applied to excited-state approaches like the Bethe–Salpeter equation allow access to the excitation energies of open-shell systems, such as molecules and defects in solids. The eigenstates of these solutions, however, are generally not eigenstates of the spin operator S^2 . Even for simple cases where the excitation vector is expected to be, for example, a triplet state, the value of ⟨S^2⟩ may be found to differ from 2.00; this difference is called ‘spin contamination’. The expectation values ⟨S^2⟩ must be computed for each excitation vector, to assist with the characterization of the particular excitation and to determine the amount of spin contamination of the state. Our aim is to provide for the first time in the SF methods literature a comprehensive resource on the derivation of the formulas for ⟨S^2⟩ as well as its computational implementation. After a brief discussion of the theory of the SF Bethe–Salpeter equation (BSE) and some examples further illustrating the need for calculating ⟨S^2⟩ , we present the derivation for the general equation for computing ⟨S^2⟩ with the eigenvectors from an SF-BSE calculation, how it is implemented in a Python script, and timing information on how this calculation scales with the size of the SF-BSE Hamiltonian.

Point defects in two-dimensional materials are of key interest for quantum information science. However, the parameter space of possible defects is immense, making the identification of high-performance quantum defects very challenging. Here, we perform high-throughput (HT) first-principles computational screening to search for promising quantum defects within WS2, which present localized levels in the band gap that can lead to bright optical transitions in the visible or telecom regime. Our computed database spans more than 700 charged defects formed through substitution on the tungsten or sulfur site. We found that sulfur substitutions enable the most promising quantum defects. We computationally identify the neutral cobalt substitution to sulfur (CoS0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{{{{{{{{\rm{S}}}}}}}}}^{0}$$\end{document}) and fabricate it with scanning tunneling microscopy (STM). The CoS0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{{{{{{{{\rm{S}}}}}}}}}^{0}$$\end{document} electronic structure measured by STM agrees with first principles and showcases an attractive quantum defect. Our work shows how HT computational screening and nanoscale synthesis routes can be combined to design promising quantum defects.

Thermoelectric materials with high electrical conductivity and low thermal conductivity (e.g., Bi2Te3) can efficiently convert waste heat into electricity; however, in spite of favorable theoretical predictions, individual Bi2Te3 nanostructures tend to perform less efficiently than bulk Bi2Te3. We report a greater-than-order-of-magnitude enhancement in the thermoelectric properties of suspended Bi2Te3 nanoribbons, coated in situ to form a Bi2Te3/F4-TCNQ core–shell nanoribbon without oxidizing the core–shell interface. The shell serves as an oxidation barrier but also directly functions as a strong electron acceptor and p-type carrier donor, switching the majority carriers from a dominant n-type carrier concentration (∼10²¹ cm–3) to a dominant p-type carrier concentration (∼10²⁰ cm–3). Compared to uncoated Bi2Te3 nanoribbons, our Bi2Te3/F4-TCNQ core–shell nanoribbon demonstrates an effective chemical potential dramatically shifted toward the valence band (by 300–640 meV), robustly increased Seebeck coefficient (∼6× at 250 K), and improved thermoelectric performance (10–20× at 250 K).

Point defects in two-dimensional materials are of key interest for quantum information science. However, the space of possible defects is immense, making the identification of high-performance quantum defects extremely challenging. Here, we perform high-throughput (HT) first-principles computational screening to search for promising quantum defects within WS 2 , which present localized levels in the band gap that can lead to bright optical transitions in the visible or telecom regime. Our computed database spans more than 700 charged defects formed through substitution on the tungsten or sulfur site. We found that sulfur substitutions enable the most promising quantum defects. We computationally identify the neutral cobalt substitution to sulfur (Co$_{\rm S}^{0}$) as very promising and fabricate it with scanning tunneling microscopy (STM). The Co$_{\rm S}^{0}$ electronic structure measured by STM agrees with first principles and showcases an attractive new quantum defect. Our work shows how HT computational screening and novel defect synthesis routes can be combined to design new quantum defects.

An intense femtosecond-laser excitation of a solid induces highly nonthermal conditions. In materials like silicon, laser-induced bond-softening leads to a highly incoherent ionic motion and eventually nonthermal melting. But is this outcome an inevitable consequence, or can it be controlled? Here, we performed ab initio molecular dynamics simulations of crystalline silicon after multiple femtosecond-laser pulse excitations with total energy above the nonthermal melting threshold. Our results demonstrate an excitation mechanism, which can be generalized to other materials, that pauses nonthermal melting and creates a metastable state instead, which has an electronic structure similar to the ground state. Our findings open the way for a search for metastable structural and/or electronic transitions in the high-excitation regime. In addition, our approach could be used to switch off nonthermal contributions in experiments, which could be used to obtain reliable electron-phonon coupling constants more easily.

The properties of $\mathrm{MoS_2}$ can be tuned or optimized through doping. In particular, Ni doping has been shown to improve the performance of $\mathrm{MoS_2}$ for various applications, including catalysis and tribology. To enable investigation of Ni-doped $\mathrm{MoS_2}$ with reactive molecular dynamics simulations, we developed a new ReaxFF force field to describe this material. The force field parameters were optimized to match a large set of density-functional theory (DFT) calculations of 2H-$\mathrm{MoS_2}$ doped with Ni, at four different sites (Mo-substituted, S-substituted, octahedral intercalation, and tetrahedral intercalation), under uniaxial, biaxial, triaxial, and shear strain. The force field was evaluated by comparing ReaxFF- and DFT-relaxed structural parameters and the tetrahedral/octahedral energy difference in doped 2H, energies of doped 1H and 1T monolayers, and doped 2H structures with vacancies. We demonstrated the force field with reactive simulations of sputtering deposition and annealing of Ni-doped MoS$_2$ films. Results show that the developed force field can accurately model the phase transition of Ni-doped $\mathrm{MoS_2}$ from amorphous to crystalline. The newly developed force field can be used in subsequent investigations to study the properties and behavior of Ni-doped $\mathrm{MoS_2}$ using reactive molecular dynamics simulations.

The assignment of an exact space group to the tetragonal CH$_3$NH$_3$PbI$_3$ perovskite structure is experimentally challenging and controversial in the literature. Average orientation of the methylammonium ion that gives symmetry to the experimental measurement is not captured in a static density functional theory calculation, although the quasi-I4cm and quasi-I4/mcm structures are commonly used in calculations. In this work we have developed a methodology to quantify the hidden symmetry of these structures using group theory, to enable use of symmetries in understanding spectroscopy and other properties. We study the approximate symmetry of vibrational modes, including analysis of degenerate representations, as well as the dielectric, elastic, electro-optic, Born effective charge, and Raman tensors and the dynamical matrix. Comparing to each subgroup of the full tetragonal D$_{4h}$, our results show that the quasi-I4cm is best described by the expected corresponding point group C$_{4v}$, whereas the quasi-I4/mcm (despite corresponding to point group D$_{4h}$) is best described by the lower symmetry C$_{2v}$. Our methodology can be useful generally for analysis of other soft hybrid materials or any approximately symmetric material.

One-dimensional (1D) organic metal halide hybrids exhibit strongly anisotropic optical properties, highly efficient light emission, and large Stokes shift, holding promises for novel photodetection and lighting applications. However, the fundamental mechanisms governing their unique optical properties and in particular the impacts of surface effects are not understood. Here, we investigate 1D C4N2H14PbBr4 by polarization-dependent time-averaged and time-resolved photoluminescence (TRPL) spectroscopy, as a function of photoexcitation energy. Surprisingly, we find that the emission under photoexcitation polarized parallel to the 1D metal halide chains can be either stronger or weaker than that under perpendicular polarization, depending on the excitation energy. We attribute the excitation-energy-dependent anisotropic emission to fast surface recombination, supported by first-principles calculations of optical absorption in this material. The fast surface recombination is directly confirmed by TRPL measurements, when the excitation is polarized parallel to the chains. Our comprehensive studies provide a more complete picture for a deeper understanding of the optical anisotropy in 1D organic metal halide hybrids.

Organic metal halide hybrids with low-dimensional structures at the molecular level have received great attention recently for their exceptional structural tunability and unique photophysical properties. Here we report for the first time the synthesis and characterization of a one-dimensional (1D) organic metal halide hybrid material, which contains metal halide nanoribbons with a width of three octahedral units. It is found that this material with a chemical formula C$_8$H$_{28}$N$_5$Pb$_3$Cl$_{11}$ shows a dual emission with a photoluminescence quantum efficiency (PLQE) of around 25% under ultraviolet (UV) light irradiation. Photophysical studies and density functional theory (DFT) calculations suggest the coexisting of delocalized free excitons and localized self-trapped excitons in metal halide nanoribbons leading to the dual emission. This work shows once again the exceptional tunability of organic metal halide hybrids that bridge between molecular systems with localized states and crystalline ones with electronic bands.