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Creators/Authors contains: "Liu, Chi"

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  1. Abstract

    Translation of chirality and asymmetry across structural motifs and length scales plays a fundamental role in nature, enabling unique functionalities in contexts ranging from biological systems to synthetic materials. Here, we introduce a structural chirality transfer across the organic–inorganic interface in two-dimensional hybrid perovskites using appropriate chiral organic cations. The preferred molecular configuration of the chiral spacer cations,R-(+)- orS-(−)-1-(1-naphthyl)ethylammonium and their asymmetric hydrogen-bonding interactions with lead bromide-based layers cause symmetry-breaking helical distortions in the inorganic layers, otherwise absent when employing a racemic mixture of organic spacers. First-principles modeling predicts a substantial bulk Rashba-Dresselhaus spin-splitting in the inorganic-derived conduction band with opposite spin textures betweenR- andS-hybrids due to the broken inversion symmetry and strong spin-orbit coupling. The ability to break symmetry using chirality transfer from one structural unit to another provides a synthetic design paradigm for emergent properties, including Rashba-Dresselhaus spin-polarization for hybrid perovskite spintronics and related applications.

  2. Hybrid perovskites incorporating conjugated organic cations enable unusual charge carrier interactions among organic and inorganic structural components, but are difficult to prepare as films due to disparate component chemical/physical characteristics ( e.g. , solubility, thermal stability). Here we demonstrate that resonant infrared matrix-assisted pulsed laser evaporation (RIR-MAPLE) mitigates these challenges, enabling facile deposition of lead-halide-based perovskite films incorporating variable-length oligothiophene cations. Density functional theory (DFT) predicts suitable organic and inorganic moieties that form quantum-well-like structures with targeted luminescence or exciton separation/quenching. RIR-MAPLE-deposited films enable confirmation of these predictions by optical measurements, which further display excited state behavior transcending traditional quantum-well models— i.e. , with appropriate selection of specially synthesized organic/inorganic moieties, intercomponent carrier transfer efficiently converts excitons from singlet to triplet states in organics for which intersystem crossing cannot ordinarily compete with recombination. These observations demonstrate the uniquely versatile excited-state behavior in hybrid perovskite quantum wells, and the value of integrating DFT, organic synthesis, RIR-MAPLE and spectroscopy for screening/preparing rationally devised complex structures.
  3. Abstract

    The island of Sardinia has been of particular interest to geneticists for decades. The current model for Sardinia’s genetic history describes the island as harboring a founder population that was established largely from the Neolithic peoples of southern Europe and remained isolated from later Bronze Age expansions on the mainland. To evaluate this model, we generate genome-wide ancient DNA data for 70 individuals from 21 Sardinian archaeological sites spanning the Middle Neolithic through the Medieval period. The earliest individuals show a strong affinity to western Mediterranean Neolithic populations, followed by an extended period of genetic continuity on the island through the Nuragic period (second millennium BCE). Beginning with individuals from Phoenician/Punic sites (first millennium BCE), we observe spatially-varying signals of admixture with sources principally from the eastern and northern Mediterranean. Overall, our analysis sheds light on the genetic history of Sardinia, revealing how relationships to mainland populations shifted over time.