First-Principles Phonon Quasiparticle Theory Applied to a Strongly Anharmonic Halide Perovskite

Understanding and predicting lattice dynamics in strongly anharmonic crystals is one of the long-standing challenges in condensed matter physics. Here we propose a first-principles method that gives accurate quasiparticle (QP) peaks of the phonon spectrum with strong anharmonic broadening. On top of the conventional first-order self-consistent phonon (SC1) dynamical matrix, the proposed method incorporates frequency renormalization effects by the bubble self-energy within the QP approximation. We apply the developed methodology to the strongly anharmonic $\alpha$-CsPbBr$_3$ that displays phonon instability within the harmonic approximation in the whole Brillouin zone. While the SC1 theory significantly underestimates the cubic-to-tetragonal phase transition temperature (\tc) by more than 50\%, we show that our approach yields \tc = 404--423~K, in excellent agreement with the experimental value of 403~K. We also demonstrate that an accurate determination of QP peaks is paramount for quantitative prediction and elucidation of lattice thermal conductivity.Comment: 6 pages, 3 figure

Temperature Dependence of the Energy Levels of Methylammonium Lead Iodide Perovskite from First-Principles.

Environmental effects and intrinsic energy-loss processes lead to fluctuations in the operational temperature of solar cells, which can profoundly influence their power conversion efficiency. Here we determine from first-principles the effects of temperature on the band gap and band edges of the hybrid pervoskite CH3NH3PbI3 by accounting for electron-phonon coupling and thermal expansion. From 290 to 380 K, the computed band gap change of 40 meV coincides with the experimental change of 30-40 meV. The calculation of electron-phonon coupling in CH3NH3PbI3 is particularly intricate as the commonly used Allen-Heine-Cardona theory overestimates the band gap change with temperature, and excellent agreement with experiment is only obtained when including high-order terms in the electron-phonon interaction. We also find that spin-orbit coupling enhances the electron-phonon coupling strength but that the inclusion of nonlocal correlations using hybrid functionals has little effect. We reach similar conclusions in the metal-halide perovskite CsPbI3. Our results unambiguously confirm for the first time the importance of high-order terms in the electron-phonon coupling by direct comparison with experiment

Atomic and Electronic Structure of the BaTiO\u3csub\u3e3\u3c/sub\u3e(001) (√5×√5)\u3cem\u3eR\u3c/em\u3e26.6° Surface Reconstruction

This contribution presents a study of the atomic and electronic structure of the (√5×√5)R26.6° surface reconstruction on BaTiO3 (001) formed by annealing in ultrahigh vacuum at 1300 K. Through density functional theory calculations in concert with thermodynamic analysis, we assess the stability of several BaTiO3 surface reconstructions and construct a phase diagram as a function of the chemical potential of the constituent elements. Using both experimental scanning tunneling microscopy (STM) and scanning tunneling spectroscopy measurements, we were able to further narrow down the candidate structures, and conclude that the surface is either TiO2-Ti3/5, TiO2-Ti4/5, or some combination, where Ti adatoms occupy hollow sites of the TiO2 surface. Density functional theory indicates that the defect states close to the valence band are from Ti adatom 3d orbitals (≈1.4  eV below the conduction band edge) in agreement with scanning tunneling spectroscopy measurements showing defect states 1.56±0.11  eV below the conduction band minimum (1.03±0.09  eV below the Fermi level). STM measurements show electronic contrast between empty and filled states’ images. The calculated local density of states at the surface shows that Ti 3d states below and above the Fermi level explain the difference in electronic contrast in the experimental STM images by the presence of electronically distinctive arrangements of Ti adatoms. This work provides an interesting contrast with the related oxide SrTiO3, for which the (001) surface (√5×√5)R26.6° reconstruction is reported to be the TiO2 surface with Sr adatoms

Density Functional Theory Study of Nucleation and Growth of Pt Nanoparticles on MoS₂(001) Surface

The dispersion of Pt metallic nanoparticles on different supports is of high relevance for designing more efficient and less expensive catalysts. In order to understand the nucleation and epitaxial growth of Pt nanoparticles and thin films on MoS₂ monolayers, we have systematically analyzed, by first-principles density functional calculations, the evolution of morphology and atomic structure of supported (Pt)<i>_n</i> nanoparticles (NPs) on MoS₂(001) for <i>n</i> ≤ 12. We find that <i>n</i> = 5 is the cluster size where the growth of the NPs transforms from two- to three-dimensional (2D to 3D). Owing to the topography of MoS₂(001), the 2D NPs mostly attach to the support via direct bonding with Mo atoms that sit in the troughs of the surface, while the 3D NPs are bonded to the sulfur atoms that are more extended in the vacuum region. Furthermore, we find that Pt is sufficiently mobile on the surface where the number of hopping events per second is ≈10³ s^–1 along [101̅] and ≈10 s^–1 along [11̅0] at room temperature. The somewhat large mobility suggests that monomer diffusion is not likely to be the rate-limiting step for Oswald ripening and that Pt sputtering on MoS₂(001) will result in relatively large particles rather than a fine dispersion. The existence of a fast diffusion channel along [101̅] suggests that the morphology of the NPs is anisotropic

Van der Waals Epitaxial Growth of Transition Metal Dichalcogenides on Pristine and N‑Doped Graphene

The stability and the electronic structure of layered heterostructures MX₂ (M = Mo or W and X = S or Se) and graphene (GA) are systematically investigated using first-principles methods. The calculations cover pristine and defected GA systems with up to 12% nitrogen substitutional defects. It is found that the van der Waals (vdW) epitaxy of MX₂ on undoped GA substrate, whether pristine or defected, follows a Volmer–Weber growth-mode resulting in thick MX₂ films. On the other hand, nitrogen doping of pristine GA (N-GA) and also of GA with Stone–Wales (SW) defects increases the MX₂/GA heterostructure adhesion energy favoring the growth of ultrathin MX₂ layers. This growth-mode change in MoS₂ due to nitrogen doping is in agreement with recent experiments. Furthermore, our study demonstrates that the yield of ultrathin MX₂ films can be increased if the N-GA samples have a larger concentration of SW defects or nitrogen. The underpinnings of the extra stability of these N-GA substrates are due to charge-transfer effects that decrease the Pauli repulsion between the two layered systems

Trends in the Adsorption and Growth Morphology of Metals on the MoS₂(001) Surface

Contacts between metal surfaces and MoS₂ are crucial for the utilization of MoS₂ in different technologies. Here we systematically investigate using first-principles density functional theory the adsorption and diffusion on MoS₂(001) of a wide range of metals from Groups I–IV in addition to all of the 3d transition metals (TMs) and selected 4d and 5d TMs. The binding mechanisms as well as trends in the binding energies are elucidated by examining the electronic structure of the system, and in particular the interplay between Coulomb interactions, Pauli repulsion, and <i>nd</i>^<i>m</i>(<i>n</i> + 1)<i>s</i>^<i>x</i> → <i>nd</i>^<i>m</i>+1(<i>n</i> + 1)<i>s</i>^<i>x</i>–1 (<i>x</i> = 1, 2; <i>n</i> = 3, 4, 5) promotion energies. We show that the metal-induced workfunction reduction is correlated with the ionization potential of the isolated atom and is furthermore linearly dependent on the interfacial dipole moment with an offset term. Additionally, the growth morphologies of the metal nanoparticles on MoS₂ are predicted by analyzing the monomer adhesion energy and its mobility on the substrate. Our results are in line with recent experiments showing that Ag and Au follow a Volmer–Weber growth mode on MoS₂(001)