Márton Vörös

Optically controlled switching of the charge state of a single nitrogen-vacancy center in diamond at cryogenic temperatures.

P. Siyushev, ∗ H.Pinto, A.Gali, 3 F. Jelezko, and J. Wrachtrup 5 3.Physikalisches Institut and Stuttgart Research Center of Photonic Engineering (SCoPE), Universität Stuttgart, Pfaffenwaldring 57, Stuttgart, D-70569, Germany Institute for Solid State Physics and Optics, Wigner Research Center for Physics, Hungarian Academy of Sciences, Budapest, POB 49, H-1525, Hungary Department of Atomic Physics, Budapest University of Technology and Economics, Budafoki ut 8, H-1111, Budapest, Hungary Institut für Quantenoptik, Universität Ulm, D-89081 Ulm, Germany Max Planck Institute for Solid State Research, Heisenbergstraße 1, D-70569 Stuttgart, Germany

The absorption of oxygenated silicon carbide nanoparticles

We have investigated the absorption of 0.9-1.4 nm silicon carbide nanoparticles (SiC NPs) by time-dependent density functional calculations, focusing on the effect of various oxygen adsorbates of the surface. We have found that Si-O and C-O single bonds result in relatively large optical gaps in the ultraviolet region while Si=O and C=O double bonds will dramatically lower the optical gap into the visible blue and red regions, respectively. Our findings can help interpret recent experiments on colloidal SiC NPs and their utilization in biological applications.

The absorption spectrum of hydrogenated silicon carbide nanocrystals from ab initio calculations

The electronic structure and absorption spectrum of hydrogenated silicon carbide nanocrystals (SiC NCs) have been determined by first principles calculations. We show that the reconstructed surface can significantly change not just the onset of absorption but the shape of the spectrum at higher energies. We compare our results with two recent experiments on ultrasmall SiC NCs.

High-Energy Excitations in Silicon Nanoparticles

We have investigated high energy excitations in approximately 1-2 nm Si nanoparticles (NPs) by ab initio time-dependent density functional calculations, focusing on the influence on excitation spectra, of surface reconstruction, surface passivation by alkyl groups, and the interaction between NPs. We have found that surface reconstruction may change excitation spectra dramatically at both low and high energies above the gap; absorption may be enhanced nonlinearly by the presence of alkyl groups, compared to that of unreconstructed, hydrogenated Si NPs, and by the interaction between NPs. Our findings can help interpret the recent experiments on multielectron generation in colloidal semiconductor NPs as well as help optimize photovoltaic applications of NPs.

Novel silicon phases and nanostructures for solar energy conversion

Silicon exhibits a large variety of different bulk phases, allotropes, and composite structures, such as, e.g., clathrates or nanostructures, at both higher and lower densities compared with diamond-like Si-I. New Si structures continue to be discovered. These novel forms of Si offer exciting prospects to create Si based materials, which are non-toxic and earth-abundant, with properties tailored precisely towards specific applications. We illustrate how such novel Si based materials either in the bulk or as nanostructures may be used to significantly improve the efficiency of solar energy conversion devices.

Tuning colloidal quantum dot band edge positions through solution-phase surface chemistry modification

Band edge positions of semiconductors determine their functionality in many optoelectronic applications such as photovoltaics, photoelectrochemical cells and light emitting diodes. Here we show that band edge positions of lead sulfide (PbS) colloidal semiconductor nanocrystals, specifically quantum dots (QDs), can be tuned over 2.0 eV through surface chemistry modification. We achieved this remarkable control through the development of simple, robust and scalable solution-phase ligand exchange methods, which completely replace native ligands with functionalized cinnamate ligands, allowing for well-defined, highly tunable chemical systems. By combining experiments and ab initio simulations, we establish clear relationships between QD surface chemistry and the band edge positions of ligand/QD hybrid systems. We find that in addition to ligand dipole, inter-QD ligand shell inter-digitization contributes to the band edge shifts. We expect that our established relationships and principles can help guide future optimization of functional organic/inorganic hybrid nanostructures for diverse optoelectronic applications.

Characterization of NiFe oxyhydroxide electrocatalysts by integrated electronic structure calculations and spectroelectrochemistry

Significance The conversion of water to oxygen and hydrogen molecules is essential for a variety of renewable energy technologies. Nickel–iron (NiFe) oxyhydroxide is an important, earth-abundant electrocatalyst for the oxygen evolution reaction. A combined experimental and computational study of pure Ni oxyhydroxide and mixed NiFe oxyhydroxide thin films elucidates the chemistry governing their different electrochemical and optical properties. The Ni and Fe oxidation states in each system are assigned as a function of applied potential based on quantum-mechanical calculations, cyclic voltammetry, and UV-visible spectroscopy. In the more catalytically active NiFe system, oxidation to Fe4+ coincides with the onset of oxygen evolution. Synergy between experiment and theory provides a detailed, atomistic understanding of this robust catalyst. NiFe oxyhydroxide materials are highly active electrocatalysts for the oxygen evolution reaction (OER), an important process for carbon-neutral energy storage. Recent spectroscopic and computational studies increasingly support iron as the site of catalytic activity but differ with respect to the relevant iron redox state. A combination of hybrid periodic density functional theory calculations and spectroelectrochemical experiments elucidate the electronic structure and redox thermodynamics of Ni-only and mixed NiFe oxyhydroxide thin-film electrocatalysts. The UV/visible light absorbance of the Ni-only catalyst depends on the applied potential as metal ions in the film are oxidized before the onset of OER activity. In contrast, absorbance changes are negligible in a 25% Fe-doped catalyst up to the onset of OER activity. First-principles calculations of proton-coupled redox potentials and magnetizations reveal that the Ni-only system features oxidation of Ni2+ to Ni3+, followed by oxidation to a mixed Ni3+/4+ state at a potential coincident with the onset of OER activity. Calculations on the 25% Fe-doped system show the catalyst is redox inert before the onset of catalysis, which coincides with the formation of Fe4+ and mixed Ni oxidation states. The calculations indicate that introduction of Fe dopants changes the character of the conduction band minimum from Ni-oxide in the Ni-only to predominantly Fe-oxide in the NiFe electrocatalyst. These findings provide a unified experimental and theoretical description of the electrochemical and optical properties of Ni and NiFe oxyhydroxide electrocatalysts and serve as an important benchmark for computational characterization of mixed-metal oxidation states in heterogeneous catalysts.

Electronic and optical properties of pure and modified diamondoids studied by many-body perturbation theory and time-dependent density functional theory

Diamondoids are small diamond nanoparticles (NPs) that are built up from diamond cages. Unlike usual semiconductor NPs, their atomic structure is exactly known, thus they are ideal test-beds for benchmarking quantum chemical calculations. Their usage in spintronics and bioimaging applications requires a detailed knowledge of their electronic structure and optical properties. In this paper, we apply density functional theory (DFT) based methods to understand the electronic and optical properties of a few selected pure and modified diamondoids for which accurate experimental data exist. In particular, we use many-body perturbation theory methods, in the G0W0 and G0W0+BSE approximations, and time-dependent DFT in the adiabatic local density approximation. We find large quasiparticle gap corrections that can exceed thrice the DFT gap. The electron-hole binding energy can be as large as 4 eV but it is considerably smaller than the GW corrections and thus G0W0+BSE optical gaps are about 50% larger than the Kohn-Sham (KS) DFT gaps. We find significant differences between KS time-dependent DFT and GW+BSE optical spectra on the selected diamondoids. The calculated G0W0 quasiparticle levels agree well with the corresponding experimental vertical ionization energies. We show that nuclei dynamics in the ionization process can be significant and its contribution may reach about 0.5 eV in the adiabatic ionization energies.

Colloidal nanoparticles for Intermediate Band solar cells

The Intermediate Band (IB) solar cell concept is a promising idea to transcend the Shockley-Queisser limit. Using the results of first principles calculations, we propose that colloidal nanoparticles (CNPs) are a viable and efficient platform for the implementation of the IB solar cell concept. Focusing on CdSe CNPs, we show that (1) a well-defined intragap state arises inside the gap of the CdSe CNPs, and (2) the intra-gap states of the isolated CNPs with reconstructed surfaces combine to form an IB in arrays of CNPs. This IB is well separated from the valence and conduction band edges. We also show that in solution such IB may be electron doped using, e.g. decamethylcobaltocene, thus activating an IB-induced absorption process. Our results, together with the recent report of a nearly 9% efficient CNP solar cell indicate that colloidal nanoparticle intermediate band solar cells are a promising platform to overcome the Shockley-Queisser limit.

Ab initio optoelectronic properties of silicon nanoparticles: Excitation energies, sum rules, and Tamm-Dancoff approximation

We present an ab initio study of the excited state properties of silicon nanoparticles (NPs) with diameters of 1.2 and 1.6 nm. Quasiparticle corrections were computed within the G0W0 approximation. The absorption spectra were computed by time-dependent density functional theory (TDDFT) using the adiabatic PBE approximation, and by solving the Bethe-Salpeter equation (BSE). In our calculations, we used recently developed methods that avoid the explicit inversion of the dielectric matrix and summations over empty electronic states. We found that a scissor operator reliably describes quasiparticle corrections for states in the low energy part of the spectra. Our results also showed good agreement between the positions of the absorption peaks obtained using TDDFT and the BSE in the low part of the spectra, although the peak intensities differ. We discuss the effect of the Tamm-Dancoff approximation on the optical properties of the NPs and present a quantitative analysis in terms of sum rules. In the case of the BSE we found that, even in the absence of the Tamm-Dancoff approximation, the f-sum rule is not fully satisfied due to an inconsistency between the approximations used for the BSE kernel and for the quasiparticle Hamiltonian.