arXiv subjects

Stefan Hecht

Publications and source records attributed to Stefan Hecht.

5 recordsLinked to original sources

arXiv2024

An atomic-scale perspective on individual thiol-terminated molecules anchored to single S vacancies in MoS$_2$

Sulphur vacancies in MoS$_2$ on Au(111) have been shown to be negatively charged as reflected by a Kondo resonance. Here, we use scanning tunneling microscopy to show that these vacancies serve as anchoring sites for thiol-based molecules (CF$_3$-3P-SH) with two distinct reaction products, one of them showing a Kondo resonance. Based on comparisons with density-functional theory (DFT) calculations, including a random structure search and computation of energies and electronic properties at a hybrid exchange-correlation functional level, we conclude that both anchored molecules are charge neutral. One of them is an anchored intact CF$_3$-3P-SH molecule while the other one is the result of catalytically activated dehydrogenation to CF$_3$-3P-S with subsequent anchoring. Our investigations highlight a perspective of functionalizing defects with thiol-terminated molecules that can be equipped with additional functional groups, such as charge donor- or acceptor-moieties, switching units or magnetic centers.

cond-mat.mtrl-sci↗

arXiv2023

Bond dissociation dynamics of single molecules on Ag(111)

The breaking of a chemical bond is fundamental in most chemical reactions. To understand chemical processes in heterogeneous catalysis or on-surface polymerization the study of bond dissociation in molecules adsorbed on crystalline surfaces is advantageous. Single molecule studies of bond breaking can give details of the dissociation dynamics, which are challenging to obtain in mole-scale ensemble experiments. Bond breaking in single adsorbed molecules can be triggered using the energy of the tunnelling electrons in a scanning tunnelling microscope (STM) at selected positions to investigate the dissociation dynamics. Single bond dissociation dynamics has been deeply investigated only in small molecules, but not in larger molecules that exhibit distinct rotational degrees of freedom. Here, we use low temperature (7 K) STM to dissociate a single bromine atom from an elongated molecule (dibromo-terfluorene) adsorbed on a Ag(111) surface. This rod-like molecule allows to clearly identify not only displacement of the reaction fragments, but also their rotation. The results show that the molecular fragment binds to the nearest silver atom and only further rotation is allowed. Moreover, the excitation responsible for the bond breaking can propagate through the molecular backbone to dissociate a bromine atom that is not located at the pulse position. These results show the important role of the metal substrate in conditioning the bond dissociation dynamics. Our results might allow to improve the control of the synthesis of 2D materials and targeted engineering of molecular architectures.

physics.chem-ph↗

arXiv2023

Right On Time: Ultrafast Charge Separation Before Hybrid Exciton Formation

Organic/inorganic hybrid systems offer great potential for novel solar cell design combining the tunability of organic chromophore absorption properties with high charge carrier mobilities of inorganic semiconductors. However, often such material combinations do not show the expected performance: while ZnO, for example, basically exhibits all necessary properties for a successful application in light-harvesting, it was clearly outpaced by TiO$_2$ in terms of charge separation efficiency. The origin of this deficiency has long been debated. This study employs femtosecond time-resolved photoelectron spectroscopy and many-body ab initio calculations to identify and quantify all elementary steps leading to the suppression of charge separation at an exemplary organic/ZnO interface. We demonstrate that charge separation indeed occurs efficiently on ultrafast (350 fs) timescales, but that electrons are recaptured at the interface on a 100 ps timescale and subsequently trapped in a strongly bound (0.7 eV) hybrid exciton state with a lifetime exceeding 5 $\mu$s. Thus, initially successful charge separation is followed by delayed electron capture at the interface, leading to apparently low charge separation efficiencies. This finding provides a sufficiently large timeframe for counter-measures in device design to successfully implement specifically ZnO and, moreover, invites material scientists to revisit charge separation in various kinds of previously discarded hybrid systems.

cond-mat.mtrl-sci↗

arXiv2018

Dynamically switching the surface electronic and electrostatic properties of indium tin oxide electrodes with photochromic monolayers

The chemical modification of electrodes with organic materials is a common approach to tune the electronic and electrostatic landscape between interlayers in optoelectronic devices, thus facilitating charge injection at the electrode/semiconductor interfaces and improving their performance. The use of photochromic molecules for the surface modification allows dynamic control of the electronic and electrostatic properties of the electrode and thereby enables additional functionalities in such devices. Here, we show that the electronic properties of a transparent indium tin oxide (ITO) electrode are reversibly and dynamically modified by depositing organic photochromic switches (diarylethenes) in the form of self-assembled monolayers (SAMs). By combining a range of surface characterization and density functional theory calculations, we present a detailed picture of the SAM binding onto ITO, the packing density of molecules, their orientation, as well as the work function modification of the ITO surface due to the SAM deposition. Upon illumination with ultraviolet and green light, we observe a reversible shift of the frontier occupied levels by 0.7 eV, and concomitantly a reversible work function change of ca. 60 meV. Our results prove the viability of dynamic switching of the electronic properties of the electrode with external light stimuli, which could be used to fabricate ITO-based photo-switchable optoelectronic devices.

physics.chem-ph↗

arXiv2015

Zinc Oxide Modified with Benzylphosphonic Acids as Transparent Electrodes in Regular and Inverted Organic Solar Cell Structures

An approach is presented to modify the WF of solution-processed sol-gel derived ZnOover an exceptionally wide range of more than 2.3 eV. This approach relies on the formation of dense and homogeneous self-assembled monolayers based on phosphonic acids with different dipole moments. This allows us to apply ZnO as charge selective bottom electrodes in either regular or inverted solar cell structures, using P3HT:PCBM as the active layer. These devices compete with or even exceed the performance of the reference cell on ITO/PEDOT:PSS. Our finding challenges the current view that bottom electrodes in inverted solar cells need to be electron-blocking for good device performance.

cond-mat.mtrl-sci↗