Cristián G. Sánchez

Dynamical simulation of inelastic quantum transport

A method for correlated quantum electron–ion dynamics is combined with a method for electronic open boundaries to simulate in real time the heating, and eventual equilibration at an elevated vibrational energy, of a quantum ion under current flow in an atomic wire, together with the response of the current to the ionic heating. The method can also be used to extract inelastic current–voltage corrections under steady-state conditions. However, in its present form the open-boundary method contains an approximation that limits the resolution of current–voltage features. The results of the simulations are tested against analytical results from scattering theory. Directions for the improvement of the method are summarized at the end.

The transfer of energy between electrons and ions in solids

In this review we consider those processes in condensed matter that involve the irreversible flow of energy between electrons and nuclei that follows from a system being taken out of equilibrium. We survey some of the more important experimental phenomena associated with these processes, followed by a number of theoretical techniques for studying them. The techniques considered are those that can be applied to systems containing many non-equivalent atoms. They include both perturbative approaches (Fermis Golden Rule and non-equilibrium Greens functions) and molecular dynamics based (the Ehrenfest approximation, surface hopping, semi-classical Gaussian wavefunction methods and correlated electron–ion dynamics). These methods are described and characterized, with indications of their relative merits.

Molecular conduction: Do time-dependent simulations tell you more than the Landauer approach?

A dynamical method for simulating steady-state conduction in atomic and molecular wires is presented which is both computationally and conceptually simple. The method is tested by calculating the current-voltage spectrum of a simple diatomic molecular junction, for which the static Landauer approach produces multiple steady-state solutions. The dynamical method quantitatively reproduces the static results and provides information on the stability of the different solutions.

An embedded atom approach to underpotential deposition phenomena

Abstract We have performed embedded atom calculations for a number of systems of electrochemical interest involving a metallic single-crystal substrate and a metallic adsorbate. Different thermodynamic contributions to the so-called underpotential shift are calculated and analyzed comparatively, drawing some general trends. The metal pairs considered involve silver, gold, platinum, palladium and copper. We consider the possibility of underpotential deposition through the excess of binding energy, arriving at two novel conclusions. First, for some systems consisting of metal M 1 and M 2 , underpotential deposition should be energetically possible in both cases, that is M 1 on M 2 and M 2 on M 1 . Second, anions may play a decisive role in changing the energetics of some systems. In particular cases like copper on Au(111) they may be responsible to a large extent for the existence of an underpotential deposition. Entropic contributions were neglected in the present analysis.

Photoelectrochemical Hole Injection Revealed in Polyoxotitanate Nanocrystals Functionalized with Organic Adsorbates

We find that crystallographically resolved Ti17O24(OPr(i))20 nanoparticles, functionalized by covalent attachment of 4-nitrophenyl-acetylacetonate or coumarin 343 adsorbates, exhibit hole injection into surface states when photoexcited with visible light (λ = 400-680 nm). Our findings are supported by photoelectrochemical measurements, EPR spectroscopy, and quantum dynamics simulations of interfacial charge transfer. The underlying mechanism is consistent with measurements of photocathodic currents generated with visible light for thin layers of functionalized polyoxotitanate nanocrystals deposited on FTO working electrodes. The reported experimental and theoretical analysis demonstrates for the first time the feasibility of p-type sensitization of TiO2 solely based on covalent binding of organic adsorbates.

A theoretical study of the optical properties of nanostructured TiO2.

Optical properties of TiO(2) nanoclusters (with more than 30 TiO(2) units) were calculated within a fully atomistic quantum dynamic framework. We use a time dependent tight-binding model to describe the electronic structure of TiO(2) nanoclusters in order to compute their optical properties. We present calculated absorption spectra for a series of nanospheres of different radii and crystal structures. Our results show that bare TiO(2) nanoclusters have the same adsorption edge for direct electronic transition independently of the crystal structure and the nanocluster size. We report values of the adsorption edge of around 3.0 eV for all structures analyzed. In the present work we demonstrate that, for small clusters, both the direct transition absorption edge and the blue shifting phenomena are masked by thermal disorder.

Cu underpotential deposition on Au(111) and Au(100). Can this be explained in terms of the energetics of the Cu/Au system?

In this work we undertake a first principles study of the deposition of Copper on Au(111) and Au(100) surfaces from a density functional point of view. The analysis of the different energetic contributions to the so-called underpotential shift indicates that the deposition of Cu on any of these surfaces should occur at overpotentials. We conclude that some other factors must be taken into account to explain the experimental results, such as anion coadsorption. We also analyze the work function and the electronic density in the system.

Underpotential versus overpotential deposition: a first-principles calculation

In this work we take the first steps towards a systematic theoretical study of underpotential deposition (upd) systems involving d metals and compare them with systems that present overpotential deposition (opd). We report preliminary results from first-principles calculations based on density functional theory for two typical systems: Ag on Au(111)(upd), where the lattice mismatch is practically negligible, and Cu on Ag(111)(opd). Comparative calculations were made for Au adsorption on Ag(111), with the prediction of upd for this system.

Quantum dynamical simulations of local field enhancement in metal nanoparticles

Field enhancements (Γ) around small Ag nanoparticles (NPs) are calculated using a quantum dynamical simulation formalism and the results are compared with electrodynamic simulations using the discrete dipole approximation (DDA) in order to address the important issue of the intrinsic atomistic structure of NPs. Quite remarkably, in both quantum and classical approaches the highest values of Γ are located in the same regions around single NPs. However, by introducing a complete atomistic description of the metallic NPs in optical simulations, a different pattern of the Γ distribution is obtained. Knowing the correct pattern of the Γ distribution around NPs is crucial for understanding the spectroscopic features of molecules inside hot spots. The enhancement produced by surface plasmon coupling is studied by using both approaches in NP dimers for different inter-particle distances. The results show that the trend of the variation of Γ versus inter-particle distance is different for classical and quantum simulations. This difference is explained in terms of a charge transfer mechanism that cannot be obtained with classical electrodynamics. Finally, time dependent distribution of the enhancement factor is simulated by introducing a time dependent field perturbation into the Hamiltonian, allowing an assessment of the localized surface plasmon resonance quantum dynamics.

Transition dipole moments of the Qy band in photosynthetic pigments.

From studying the time evolution of the single electron density matrix within a density functional tight-binding formalism we calculate the Q(y) transition dipole moments vector direction and strength for a series of important photosynthetic pigments. We obtain good agreement with first-principles and experimental results and provide insights into the detailed nature of these excitations from the time evolving populations of molecular orbitals involved as well as correlations between pigment chemistry and dipole strength.