Omer Yaffe, Yinsheng Guo, Trevor Hull, Costas Stoumpos, Liang Tan, David Egger, Fan Zheng, Guilherme Szpak, Octavi Semonin, Alexander Beecher, Tony Heinz, Leeor Kronik, Andrew Rappe, Mercouri Kanatzidis, Jonathan Owen, Marcos Pimenta, Louis Brus
We combine low frequency Raman scattering measurements with first-principles molecular dynamics (MD) to study the nature of dynamic disorder in hybrid lead-halide perovskite crystals. We conduct a comparative study between a hybrid (CH3NH3PbBr3) and an all-inorganic lead-halide perovskite (CsPbBr3). Both are of the general ABX3 perovskite formula, and have a similar band gap and structural phase sequence, orthorhombic at low temperature, changing first to tetragonal and then to cubic symmetry as temperature increases. In the high temperature phases, we find that both compounds show a pronounced Raman quasi-elastic central peak, indicating that both are dynamically disordered.
The study of the adsorption and growth of metals on ceramics is a
rapidly growing area, as these interactions are key to understanding
many materials and processes used in modern technology. In particular,
oxide-supported catalysts have been extensively studied, due to their
widespread industrial applications. Knowledge of the role played by
the underlying metal oxide in the reactivity of the metal catalyst can
give insights into the design of more effective catalysts. Here, we
use density functional theory (DFT) to investigate the adsorption of
CO onto 1) bulk Pt, 2) Pt thin layers supported on an alpha-alumina
surface and 3) Pt nanoparticles on alpha-alumina. Our results
show strong binding for CO molecules on the surfaces of both the thin
Pt layers and the 3-atom nanoparticles supported on alumina
substrates. This enhanced binding can possibly lead to more reactive
catalysts. Further calculations on reaction products are needed to
determine the effectiveness of these new systems.
Using density functional theory, we have performed structural relaxations of Rh(111) and Cu(100). To obtain accurate results, these calculations must be converged with respect to all computational approximations. In particular, it is vital to treat Brillouin zone integration with care, taking into account the effect of finite k-point sampling on surface and bulk structural properties. A new method is described and demonstrated for minimizing the error of finite k-point sampling in predicting surface relaxations accurately.
Density functional theory calculations are performed to determine the nature of vibrational modes associated with carbon monoxide chemisorbed to the copper (100) surface. The electronic states and charge density are determined using a plane-wave pseudopotential method within the local density approximation. The surface is modeled using a periodic slab geometry, and the force constant matrix is computed from first principles by displacing each atom in turn and determining the resulting forces on it and all other atoms. This matrix is then diagonalized to yield normal-mode polarization vectors and frequencies. The eigenvectors provide information about the strength of coupling between copper atom motions and CO vibrations. The computed vibrational spectrum is used to predict the transient response of the system to nonequilibrium heating, and the results are compared with recent pulse-laser experiments on this system.
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