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11 Publications visible to you, out of a total of 11
Sensing and sensitivity: Computational chemistry of graphene‐based sensors

Abstract (Expand)

Highly efficient, tunable, biocompatible, and environmentally friendly electrochemical sensors featuring graphene‐based materials pose a formidable challenge for computational chemistry. In silico … rationalization, optimization and, ultimately, prediction of their performance requires exploring a vast structural space of potential surface‐analyte complexes, further complicated by the presence of various defects and functionalities within the infinite graphene lattice. This immense number of systems and their periodic nature greatly limit the choice of computational tools applicable at a reasonable cost. An alternative approach using finite nanoflake models opens the doors to many more advanced and accurate electronic structure methods, while sacrificing the realism of representation. Locating the surface‐analyte complex is followed by an in‐depth in silico analysis of its energetic and electronic properties using, for example, energy decomposition schemes, as well as simulation of the signal, for example, a zero‐bias transmission spectra or a current–voltage curve, by means of the nonequilibrium Green's function method. These and other properties are examined in the context of a sensor's selectivity, sensitivity, and limit of detection with an aim to establish design principles for future devices. Herein, we analyze the advantages and limitations of diverse computational chemistry methods used at each of these steps in simulating graphene‐based electrochemical sensors. We present outstanding challenges toward predictive models and sketch possible solutions involving such contemporary techniques as multiscale simulations and high‐throughput screening.

Authors: Anna Piras, Christopher Ehlert, Ganna Gryn'ova

Date Published: 3rd Mar 2021

Publication Type: Journal

Ultrafast dynamics of 2-thiouracil investigated by time-resolved Auger spectroscopy

Abstract (Expand)

We present time-resolved ultraviolet-pump x-ray probe Auger spectra of 2-thiouracil. An ultraviolet induced shift towards higher kinetic energies is observed in the sulfur 2p Auger decay. The difference … Auger spectra of pumped and unpumped molecules exhibit ultrafast dynamics in the shift amplitude, in which three phases can be recognized. In the first 100 fs, a shift towards higher kinetic energies is observed, followed by a 400 fs shift back to lower kinetic energies and a 1 ps shift again to higher kinetic energies. We use a simple Coulomb-model, aided by quantum chemical calculations of potential energy states, to deduce a C–S bond expansion within the first 100 fs. The bond elongation triggers internal conversion from the photoexcited S2 to the S1 state. Based on timescales, the subsequent dynamics can be interpreted in terms of S1 nuclear relaxation and S1-triplet internal conversion.

Authors: F Lever, D Mayer, D Picconi, J Metje, S Alisauskas, F Calegari, S Düsterer, C Ehlert, R Feifel, M Niebuhr, B Manschwetus, M Kuhlmann, T Mazza, M S Robinson, R J Squibb, A Trabattoni, M Wallner, P Saalfrank, T J A Wolf, M Gühr

Date Published: 17th Dec 2020

Publication Type: Journal

PSIXAS : A Psi4 plugin for efficient simulations of X‐ray absorption spectra based on the transition‐potential and Δ‐Kohn–Sham method

Abstract (Expand)

Near edge X‐ray absorption fine structure (NEXAFS) spectra and their pump‐probe extension (PP‐NEXAFS) offer insights into valence‐ and core‐excited states. We present PSIXAS, a recent implementation … for simulating NEXAFS and PP‐NEXAFS spectra by means of the transition‐potential and the Δ‐Kohn–Sham method. The approach is implemented in form of a software plugin for the Psi4 code, which provides access to a wide selection of basis sets as well as density functionals. We briefly outline the theoretical foundation and the key aspects of the plugin. Then, we use the plugin to simulate PP‐NEXAFS spectra of thymine, a system already investigated by others and us. It is found that larger, extended basis sets are needed to obtain more accurate absolute resonance positions. We further demonstrate that, in contrast to ordinary NEXAFS simulations, where the choice of the density functional plays a minor role for the shape of the spectrum, for PP‐NEXAFS simulations the choice of the density functional is important. Especially hybrid functionals (which could not be used straightforwardly before to simulate PP‐NEXAFS spectra) and their amount of “Hartree‐Fock like” exact exchange affects relative resonance positions in the spectrum.

Authors: Christopher Ehlert, Tillmann Klamroth

Date Published: 15th Jul 2020

Publication Type: Journal

Au 21 cage structures and their magic number tri‐cations

Abstract (Expand)

We examine the stability and properties of three Au21 cage structures, one with D3 symmetry and denoted as Au21 (D3), which is novel, and the other two with C2v symmetry. One, denoted as Au21 (C2v-1), … has been previously reported but the other, denoted as Au21 (C2v-2), is novel. As reference Au21 structures, we also examine a sheet isomer and a compact isomer, Au21 (Cs-Tetra), formed by adsorbing an Au atom on Au20 (Td). For all structures, we consider charge ranging from −1 to +4. For the Au21 cage structures, a primary property of interest is their spherical aromaticity, as measured by their nucleus independent chemical shift. Our focus is on charge +3 since each gold atom is assumed to contribute one (6s) valence electron, and 18 is a magic number for shell closing. We find that, although Au21 (D3)+3 has the largest aromaticity, it is not the most stable urn:x-wiley:00207608:media:qua26191:qua26191-math-0001 cage species. Surprisingly, Au21 (C2v-2), which is not even stable as a neutral cage species, is the most stable tri-cation cage species. We also examine the stability of (neutral) Au21Xn cage structures relative to Au21Xn structures derived from Au21 (Cs-Tetra) (with X = F, Cl, Br, I and n = 1, 3, 5). We find that, although Au21F3 derived from Au21 (D3) has the largest aromaticity, it is not the most stable Au21F3 cage structure. Nonetheless, all cage structures are stabilized relative to Au21 (Cs-Tetra) and, remarkably, for the trichlorinated, tribrominated, and triiodineated clusters, at least one cage structure is more stable than its Au21 (Cs-Tetra) counterpart.

Authors: Christopher Ehlert, Xiaojing J. Liu, Ian P. Hamilton

Date Published: 5th Jun 2020

Publication Type: Journal

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