my group's research
My group's research focuses on the development, implementation and application of new simulation methods to study quantum dynamics of electrons (and) nuclei under the influence of electromagnetic radiation outside the linear response regime. We deal with all the process that goes from pen & paper design of methods, the implementation of these methods in high performance computing platforms and working together with experimental groups on the application of these methods to provide insights on processes that happen on the fs time-scale. Our methods are based on the approximate but very accurate DFTB Hamiltonian, the combination of this theoretical framework together with highly efficient GPU based computational techniques allows us to study the quantum dynamics of electrons well out of equilibrium for systems of thousands of atoms. Here I show some examples of our current research projects.
dynamics of energy transfer
Efficient light harvesting starts by capturing energy from the sun and channeling it into places where actual chemistry can be done. Our methods allow to study the dynamic of excitation flow in real time in complex molecular systems. As an example of the different systems we deal with is the case of photosynthetic antenna complexes. We have studied the excitation flow in the Fenna-Mathews-Olson complex from Prosthecochloris aestuarii and determined the influence of the protein environment on the regulation of energy flow. According to our calculations the presence of the protein backbone modifies the dynamics of energy flow in order to make energy transfer more efficient from the chlorosome to the reaction centre. At present we are studying other systems for which highly directional energy transfer occurs in order to unravel the dynamical reasons behind this directionality and how it can be used for the design of new and efficient light harvesting supramolecular structures.
photoinduced charge separation
Dye semiconductor complexes are the core of Dye Sensitized Solar Cells. Photo-excitation of the dye causes an ultra-fast charge transfer process to the SC nanoparticle. The dye is regenerated from a redox couple present in solution and electrons are harvested by a conducting electrode on which the complex is supported. We have used our techniques to study the dynamics of photo-induced electron (or hole) injection into the semiconductor conduction band upon resonant dye illumination. Our results allow us to distinguish between Type I and Type II dyes and predict charge transfer efficiencies. We predicted that for some dyes, illumination at certain wave-lengths woud induce holes to be injected into the valence band of the SC, a fact that was later confirmed by experiment. At present we are using massively parallel simulations at a range of wavelengths in order to asses overall efficiencies under broadband illumination from black-body radiation sources more akin to realistic working conditions.
fluorescent Ag-DNA complexes
DNA-Ag complexes represent new biocompatible and highly efficient flourophores. These emmiters posses a very distinctive characteristic s in their optical properties. Independent of the strand sequence the fluorophore always posses two intense absoprtion bands. One of these bands, in the visible range, is highly tunable by changing the strand sequence, the number of Ag atoms in the complex and their state of charge. The other, in the UV range is common to all emitters independently of the position of the visible band. It has been determined that the UV band corresponds to the excitation of the strand bases. The characteristic that makes these systems interesting is the fact that the emission occurs in always in the visible range for excitation of any of the two absorption bands. Up to very recently the mechanism by which energy was transfered from the UV to the visible band occurred was unknown. Our work has shown that upon excitation of the UV band an ultra fast electron transfer occurs to empty excited states of the embedded silver wire. These states are the same as those excited when the visible band is illuminated. Despite that we are (as yet) unable to predcit fluorescence spectra, upon studying the evolution of electron-hole wavepackets we see that thy remain well separated by a "gap" in the electronic spectrum. Further evidence comes from the evolution of e-h wavepackets in oxidized complexes which are know not to fluoresce in which we see ultrafast e-h annihilation.
relaxation of plasmonic excitations
We have been simulating the relaxation of surface (or localized) plasmon resonances (LSPR) in metallic nanoparticles since the early stages of formation of my group using a variety of methods. Our most recent work regards the damping of these excitations by adsorbed molecules. This phenomenon is known as Chemical Interface Damping and has a long history in the field. The usual model due to Pearson implies that excited electrons, by spending time on adsorbate states loose coherence with their companions therefore reducing the lifetime of the collective excitation. Our work has provided a deeper understanding of the mechanisms behind CID. From studying the dynamics of relaxation of LSPR in Ag nanoparticles covered with adsorbates that bind with different strengths we have observed that electrons rarely "jump" into adsorbates. Hoever, we propose that the broadening is caused by mixing of adsorbate states with nanoparticle states. This mixing blurs the energy of particle states over a broader energy range enhancing the natural Landau Damping mechanism. At present we are focused on describing hot-electron generation in Ag and Au nanoparticles and the injection of these into other materials (such as titania), and studying the reactions these hot-electrons cause by means of non-adiabatic simulations that include nuclear motion.