Data_Sheet_1_Efficient Calculation of the Negative Thermal Expansion in ZrW2O8.ZIP

<p>We present a study of the origin of the negative thermal expansion (NTE) on ZrW₂O₈ by combining an efficient approach for computing the dynamical matrix with the Lanczos algorithm for generating the phonon density of states in the quasi-harmonic approximation. The simulations show that the NTE arises primarily from the motion of the O-sublattice, and in particular, from the transverse motion of the O atoms in the W–O and W–O–Zr bonds. In the low frequency range these combine to keep the WO₄ tetrahedra rigid and induce internal distortions in the ZrO₆ octahedra. The force constants associated with these distortions become stronger with expansion, resulting in negative Grüneisen parameters and NTE from the low frequency modes that dominate the positive contributions from the high frequency modes. This leads us to propose an anharmonic, two-frequency Einstein model that quantitatively captures the NTE behavior.</p

Anomalous Structural Disorder in Supported Pt Nanoparticles

Supported Pt nanocatalysts generally exhibit anomalous behavior, including negative thermal expansion and large structural disorder. Finite temperature DFT/MD simulations reproduce these properties, showing that they are largely explained by a combination of thermal vibrations and low-frequency disorder. We show here that a full interpretation is more complex and that the DFT/MD mean-square relative displacements (MSRD) can be further separated into vibrational disorder, “dynamic structural disorder” (DSD), and long-time equilibrium fluctuations of the structure dubbed “anomalous structural disorder” (ASD). We find that the vibrational and DSD components behave normally, increasing linearly with temperature while the ASD decreases, reflecting the evolution of mean nanoparticle geometry. As a consequence the usual procedure of fitting the MSRD to normal vibrations plus temperature-independent static disorder results in unphysical bond strengths and Grüneisen parameters

Operando Effects on the Structure and Dynamics of Pt<i>_n</i>Sn_<i>m</i>/γ-Al₂O₃ from Ab Initio Molecular Dynamics and X‑ray Absorption Spectra

Alumina-supported Pt–Sn nanocluster catalysts are widely used in reforming processes, yet a theoretical understanding of their structure and function is far from complete. In an attempt to elucidate their behavior under operando conditions, we have carried out a detailed investigation of nanoscale bimetallic clusters of Pt and Sn supported on γ-Al₂O₃ using a combination of finite temperature ab initio molecular dynamics and theoretical X-ray absorption spectroscopy (XAS). Our simulations reveal a rich nonequilibrium structure over several time scales, with vibrational and anomalous structural disorder and fluctuating charge transfer to the support. In contrast with bulk Pt–Sn materials, the clusters are found to be markedly inhomogeneous, with substantial differences between surface and internal structure. The Sn atoms are preferentially segregated to the surface and fluctuate between different Pt bonds over a picosecond time scale. Importantly, these properties show how an improved XAS analysis of these systems should take into account both their inhomogeneity and dynamic structural disorder. Although our study is limited to small nanoclusters due to the limitations of ab initio molecular dynamics, we argue that their unusual dynamical structure also has important implications for catalytic behavior of these systems, which is briefly illustrated by the adsorption and dissociation reactivity of H₂

Molecular Dynamics Simulations of Supported Pt Nanoparticles with a Hybrid Sutton–Chen Potential

Understanding the physical and chemical behavior of supported nanoscale catalysts is of fundamental and technological importance. However, their behavior remains poorly understood, in part due to their complex, dynamical structure and the nature of interactions at the nanoscale. We found previously that real-time ab initio finite temperature DFT simulations provide fundamental insights into the dynamic and electronic structure of nanoparticles. Unfortunately, such first-principles calculations are very computationally intensive. To make such simulations more feasible, we have developed a hybrid version of the classical Sutton–Chen model potential which is orders of magnitude more efficient. This potential is parametrized to previous DFT/MD simulations and accounts for many-body effects induced by the support. The model is applied to Pt_10,20 nanoparticles supported on a model γ-Al₂O₃ surface. In addition to the thermal variation of the internal structure, the model also predicts diffusion coefficients and bond-breaking rates. The simulations reveal size-dependent dynamical changes with increasing temperature, as the clusters go from a “frozen” state attached to the support, to a “liquid” state where they are free to diffuse. These changes provide a rationale for the observed negative thermal expansion. Implications for nanoscale catalysis are briefly discussed

Operando Effects on the Structure and Dynamics of Pt<i>_n</i>Sn_<i>m</i>/γ-Al₂O₃ from Ab Initio Molecular Dynamics and X‑ray Absorption Spectra

Alumina-supported Pt–Sn nanocluster catalysts are widely used in reforming processes, yet a theoretical understanding of their structure and function is far from complete. In an attempt to elucidate their behavior under operando conditions, we have carried out a detailed investigation of nanoscale bimetallic clusters of Pt and Sn supported on γ-Al₂O₃ using a combination of finite temperature ab initio molecular dynamics and theoretical X-ray absorption spectroscopy (XAS). Our simulations reveal a rich nonequilibrium structure over several time scales, with vibrational and anomalous structural disorder and fluctuating charge transfer to the support. In contrast with bulk Pt–Sn materials, the clusters are found to be markedly inhomogeneous, with substantial differences between surface and internal structure. The Sn atoms are preferentially segregated to the surface and fluctuate between different Pt bonds over a picosecond time scale. Importantly, these properties show how an improved XAS analysis of these systems should take into account both their inhomogeneity and dynamic structural disorder. Although our study is limited to small nanoclusters due to the limitations of ab initio molecular dynamics, we argue that their unusual dynamical structure also has important implications for catalytic behavior of these systems, which is briefly illustrated by the adsorption and dissociation reactivity of H₂

Electronic Fingerprints of DNA Bases on Graphene

We calculate the electronic local density of states (LDOS) of DNA nucleotide bases (A,C,G,T), deposited on graphene. We observe significant base-dependent features in the LDOS in an energy range within a few electronvolts of the Fermi level. These features can serve as electronic fingerprints for the identification of individual bases in scanning tunneling spectroscopy (STS) experiments that perform image and site dependent spectroscopy on biomolecules. Thus the fingerprints of DNA-graphene hybrid structures may provide an alternative route to DNA sequencing using STS

Electronic Fingerprints of DNA Bases on Graphene

We calculate the electronic local density of states (LDOS) of DNA nucleotide bases (A,C,G,T), deposited on graphene. We observe significant base-dependent features in the LDOS in an energy range within a few electronvolts of the Fermi level. These features can serve as electronic fingerprints for the identification of individual bases in scanning tunneling spectroscopy (STS) experiments that perform image and site dependent spectroscopy on biomolecules. Thus the fingerprints of DNA-graphene hybrid structures may provide an alternative route to DNA sequencing using STS

Electronic Fingerprints of DNA Bases on Graphene

We calculate the electronic local density of states (LDOS) of DNA nucleotide bases (A,C,G,T), deposited on graphene. We observe significant base-dependent features in the LDOS in an energy range within a few electronvolts of the Fermi level. These features can serve as electronic fingerprints for the identification of individual bases in scanning tunneling spectroscopy (STS) experiments that perform image and site dependent spectroscopy on biomolecules. Thus the fingerprints of DNA-graphene hybrid structures may provide an alternative route to DNA sequencing using STS

Polarization Dependent High Energy Resolution X‑ray Absorption Study of Dicesium Uranyl Tetrachloride

Dicesium uranyl tetrachloride (Cs₂UO₂Cl₄) has been a model compound for experimental and theoretical studies of electronic structure of U­(VI) in the form of UO₂²⁺ (uranyl ion) for decades. We have obtained angle-resolved electronic structure information for oriented Cs₂UO₂Cl₄ crystal, specifically relative energies of 5f and 6d valence orbitals probed with extraordinary energy resolution by polarization dependent high energy resolution X-ray absorption near edge structure (PD-HR-XANES) and compare these with predictions from quantum chemical Amsterdam density functional theory (ADF) and ab initio real space multiple-scattering Green’s function based FEFF codes. The obtained results have fundamental value but also demonstrate an experimental approach, which offers great potential to benchmark and drive improvement in theoretical calculations of electronic structures of actinide elements

Resonant Inelastic X‑ray Scattering of Molybdenum Oxides and Sulfides

Molybdenum oxides and sulfides were studied using resonant inelastic X-ray scattering (RIXS). The 2p_3/23d Mo RIXS planes show a rich structure with considerably more spectral information than in conventional X-ray absorption near edge structure (XANES) spectroscopy. The spectra were simulated using FEFF9 giving generally good agreement and detailed electronic information can be derived. The reference materials serve as a starting point for detailed electronic and geometric investigations of a broad range of compounds. In particular, this can provide insights in the properties and performance of unknown and changing materials like those in catalysis