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Research and software development projects

Photo-induced real time spin dynamics in 2D magnets

Laser-induced switching of spins in materials is of great interest to revolutionize future magnetic storage technology and spintronics, which is generally realized in multicomponent ferrimagnetic (FiM) compounds but rare in 2D magnets. Using density functional theory (DFT) calculations, we show that 2D MXenes of type Cr2VC2F2  have unusual FiM order. Interestingly, our real-time time-dependent DFT simulations demonstrate that laser pulses can directly induce ultrafast spinselective charge transfer between magnetic sublattices in a few femtoseconds and further generate dramatic changes in the magnetic structure of these MXenes, including a transition from FiM to transient ferromagnetism (FM). The microscopic mechanism behind this ultrafast switching of spin is governed by the optically induced intersite spin transfer (OISTR) effect, which theoretically enables the ultrafast optical manipulation of the magnetic state in MXenes.

Ultrafast energy transfer through an intermolecular CoIn

Conical intersections (CoIns) of multidimensional potential energy surfaces are ubiquitous in nature and control pathways and yields of many photo-initiated intramolecular processes. Such topologies can be potentially involved in the energy transport in aggregated molecules or polymers but are yet to be uncovered. Here, using ultrafast two-dimensional electronic spectroscopy (2DES), we reveal the existence of intermolecular CoIns in molecular aggregates relevant for photovoltaics. Ultrafast, sub-10-fs 2DES tracks the coherent motion of a vibrational wave packet on an optically bright state and its abrupt transition into a dark state via a CoIn after only 40 fs. Non-adiabatic dynamics simulations identify an intermolecular CoIn as the source of these unusual dynamics. Our results indicate that intermolecular CoIns may effectively steer energy pathways in functional nanostructures for optoelectronics.

TD-DFT/B real-time charge dynamics at hybrid interfaces

In a recent work, we have studied the photodynamic process in a supramolecular arrangement composed of a hydrogenated nanodiamond (C190H110) interacting with a 3,4,9,10-perylenetetra-carboxylic acid diimide (PDI) molecule as acceptor. After pulse irradiation in tune with the photoexcitation of the acceptor, the system shows an ultrafast charge transfer process reaching a stable steady state in a few tens of femtoseconds. We proposed a purely electronic reordering of the system after charge separation as the reason for the irreversibility of the process.

Also van der Waals heterostructures from layered molecular stacks might offer means to efficient optoelectronics, due to adjustable photoactive, charge separation and collection layers. TD-DFTB simulations reveals nonlinear interlayer exciton dissociation without recombination in perylene acceptors and cove graphene nanoribbon donors on top of armchair graphene platforms.

Coherent real-time charge transport across a donor−acceptor interface

We theoretically have studied the ultrafast charge transport in thiophene:fullerene stacks using time-dependent density functional tight-binding theory combined with Ehrenfest molecular dynamics for open systems. Our results reveal coherent oscillations of the charge density between neighboring donor sites, persisting for ∼200 fs and promoting charge transport within the polymer stacks. At the donor−acceptor interface, vibronic wave packets are launched, propagating coherently over distances of more than 3 nm into the acceptor region. This supports previous experimental observations of long-range ballistic charge-carrier motion in organic photovoltaic systems and highlights the importance of vibronic coupling engineering as a concept for tailoring the functionality of hybrid organic devices.

Photocatalysis on metal oxide surfaces

Femtosecond X-ray laser pulses synchronized with an optical laser were employed to investigate the dynamics of the photocatalytic oxidation of CO on the anatase TiO2(101) surface in real-time. Our results provide evidence of ultrafast timescales for the photooxidation of CO to CO2 and clarify the initial mechanism of oxygen activation that is crucial to unraveling the underlying processes for a range of photocatalytic reactions relevant to air purification and self-cleaning surfaces. The photocatalytic oxidation of CO takes place between 1.2 - 2.8 ( 0.2) ps after irradiation with an ultrashort laser pulse leading to the formation of CO2, prior to which no intermediate species were observed. We found that the presence of intragap unoccupied O2 levels leads to the formation of a charge transfer complex that can be activated by visible light at a photon energy of 1.6 eV, in line with theoretical calculations. This allows the reaction to be initiated via a newly proposed mechanism involving the direct transfer of electrons from TiO2 to physisorbed O2.

DFTB+ Software development

DFTB+ is a versatile community developed open source software package offering fast and efficient methods for carrying out atomistic quantum mechanical simulations. By implementing various methods approximating density functional theory (DFT), such as the density functional based tight binding (DFTB) and the extended tight binding method, it enables simulations of large systems and long timescales with reasonable accuracy while being considerably faster for typical simulations than the respective ab initio methods. Based on the DFTB framework, it additionally offers approximated versions of various DFT extensions including hybrid functionals, time dependent formalism for treating excited systems, electron transport using non-equilibrium Green’s functions, and many more. DFTB+ can be used as a userfriendly standalone application in addition to being embedded into other software packages as a library or acting as a calculation-server accessed by socket  communication. We give an overview of the recently developed capabilities of the DFTB+ code, demonstrating with a few use case examples, discuss the strengths and weaknesses of the various features, and also discuss on-going developments and possible future perspectives.

Former projects

Information about some former projects of us can be found here.