arXiv subjects

Damien Riedel

Publications and source records attributed to Damien Riedel.

7 recordsLinked to original sources

arXiv2026

Quantifying Electronic and Vibronic Contributions to Charge Transfer at the Nanoscale

Charge transfer (CT) is governed by complex multiscale dynamics sensitive to environmental factors. In molecules, charge state and vibrational effects shape energy levels, charge distribution, and reactivity, thereby controlling CT efficiency. Quantifying these contributions in CT is experimentally challenging, as vibrational effects remain difficult to isolate due to limits in precision, sensitivity, and stability. In such context, the original Marcus theory, often lacks the refinement required to accurately capture CT rates in complex environments, necessitating new approaches that incorporate vibronic effects. Here, we examine a non-covalent H2TPP dyad on a semi-insulating CaF2/Si(100) surface. Using a low-temperature (9 K) scanning tunneling microscope (STM), we generate tunneling electrons that trigger CT events by creating locally cationic states while activating transient vibronic modes at specific molecular sites in one fragment. This initiates a meso N tautomerization serving as a CT signature in the second fragment. By tuning the tunnel electron energy, CT rates measurements reveal a periodic modulation. Quantitative analysis with the Marcus-Levich-Jortner model identifies key reorganization energies and resonant vibronic modes, while DFT provides complementary conformational and vibrational insights. Extending the Marcus-Antoniewicz models to explicit reaction coordinates reveals that CT is governed by the interplay between electronic and vibronic contributions, establishing surface-supported systems as a model framework.

physics.chem-ph↗

arXiv2026

Atomic-Scale Surface Imaging of bulk Epitaxial CsPbBr3 Perovskite Single Crystals on Mica using Light Assisted Scanning Tunneling Microscopy at Low-Temperature (80 K)

Epitaxial single-crystalline CsPbBr3 perovskite films on mica, prepared ex-situ, are explored using a low-temperature scanning tunneling microscope (STM) by probing the unoccupied electronic states of their surface in ultra-high vacuum (UHV) at 80 K. Light-assisted STM measurements under a broadband illumination with visible light were employed to enhance and stabilize surface conductivity. STM imaging across the surface of macroscopic bulk CsPbBr3 films reveals large flat terraces characterized by a specific type of surface reconstruction, consisting of parallel rows of U-shaped atomic nanostructures. These structures are spaced by 12 angstroms and exhibit an internal periodicity of 5.1 angstroms. Density functional theory (DFT) calculations reproduce the experimental observations and reveal a competition between different orthorhombic CsPbBr3(110) surface reconstructions: a Cs-rich structure, identified as the most energetically stable, and three alternative PbBr rich reconstructions, which are slightly higher in energy yet remain consistent with the STM data. Additional analyses that explicitly account for the mica substrate exclude the cubic CsPbBr3 phase and other orthorhombic surface orientations, while showing that variations in the mica surface termination do not alter the preferred CsPbBr3(110) reconstruction. This combined approach thereby confirms our assignment and resolves previous STM interpretations of CsPbBr3.

cond-mat.mtrl-sci↗

arXiv2023

Optoelectronic Readout of single Er Adatom's Electronic States Adsorbed on the Si(100) Surface at Low Temperature (9K)

Integrating nanoscale opto-electronic functions is vital for applications such as optical emitters, detectors, and quantum information. Lanthanide atoms show great potential in this endeavor due to their intrinsic transitions. Here, we investigate Er adatoms on Si(100)-2x1 at 9K using a scanning tunneling microscope (STM) coupled to a tunable laser. Er adatoms display two main adsorption configurations that are optically excited between 800 nm and 1200 nm while the STM reads the resulting photocurrents. Our spectroscopic method reveals that various photocurrent signals stem from the bare silicon surface or Er adatoms. Additional photocurrent peaks appear as the signature of the Er adatoms relaxation, triggering efficient dissociation of nearby trapped excitons. Calculations using the density functional theory with spin-orbit coupling correction highlight the origin of the observed photocurrent peaks as specific 4f->4f or 4f->5d transitions. This spectroscopic technique can pave the way to an optoelectronic analysis of atomic and molecular assemblies by offering unique insight into their intrinsic quantum properties.

physics.app-ph↗

arXiv2022

Periodically Spaced CaF$_2$ Semi-Insulating Thin Ribbons Growth Study on the Si(100) Surface

The use and the study of semi-insulating layers on metals and semiconductors surfaces have found continuous interest in the past decades. So far, the control of the sizes and growth location of the insulating islands on the substrate is either ill-defined or usually constrained to the use of evaporation masks which size can easily exceed tenth of nanometers. Here, we show that it is possible to grow self-organized periodically spaced thin ribbons of semi-insulating stripes on the bare Si(100) surface. The epitaxial growth of these structures is obtained by the evaporation of CaF$_2$ molecules on the silicon surface with a coverage of 1.2 monolayers. They are investigated via scanning tunneling techniques at low temperature (9K). The obtained ribbons exhibit a surface bandgap of ~3.2 eV as well as a resonant state at the central part of the ribbons at ~2.0 eV below the Fermi level energy. The use of the density functional theory allows suggesting a model structure of the observed ribbons and reproducing the experimental STM topographies. The formation of the thin ribbons is discussed and we point out the influence of the mechanical forces inside and between the structures that may influence their periodicity.

cond-mat.mtrl-sci↗

arXiv2020

Two Dimensional Functionalized Ultrathin Semi Insulating CaF2 Layer on the Si(100) Surface at a Low Temperature for Molecular Electronic Decoupling AH-2083 arXiv submit/3480305

The ability to precisely control the electronic coupling/decoupling of adsorbates from surfaces is an essential goal. It isimportant for fundamental studies not only in surface science but alsoin several applied domains including, for example, miniaturizedmolecular electronic or for the development of various devices suchas nanoscale biosensors or photovoltaic cells. Here, we provide atomic-scale experimental and theoretical investigations of a semi-insulatinglayer grown on a silicon surface via its epitaxy with CaF2. We showthat, following the formation of a wetting layer, the ensuing organizedunit cells are coupled to additional physisorbed CaF2molecules,periodically located in their surroundings. This configuration shapesthe formation of ribbons of stripes that functionalize the semi-conductor surface. The obtained assembly, having a monolayerthickness, reveals a surface gap energy of 3.2 eV. The adsorption of iron tetraphenylporphyrin molecules on the ribbons of stripes is used to estimate the electronic insulating properties of thisstructure via differential conductance measurements. Density functional theory (DFT) including several levels of complexity(annealing, DFT +U, and nonlocal van der Waals functionals) is employed to reproduce our experimental observations. Ourfindings offer a unique and robust template that brings an alternative solution to electronic semi-insulating layers on metal surfacessuch as NaCl. Hence, CaF2/Si(100) ribbon of stripe structures, whose lengths can reach more than 100 nm, can be used as aversatile surface platform for various atomic-scale studies of molecular devices.

cond-mat.mtrl-sci↗

arXiv2018

Probing Charge Transfer Dynamics in a Single Iron Tetraphenylporphyrin Dyad Adsorbed on an Insulating Surface

Although the dynamics of charge transfer (CT) processes can be probed with ultimate lifetime resolution, the helplessness to control CT at the nanoscale constitutes one of the most important road-blocks to revealing some of its deep fundamental aspects. In this work, we present an investigation of CT dynamics in a single iron tetraphenylporphyrin (Fe-TPP) donor/acceptor dyad adsorbed on a CaF2/Si(100) insulating surface. The tip of a scanning tunneling microscope (STM) is used to create local ionic states in one fragment of the dyad. The CT process is monitored by imaging subsequent changes in the neighbor acceptor molecule and its efficiency is mapped revealing the influence of the initial excited state in the donor molecule. In validation of the experiments, simulations based on density functional theory show that holes have a higher donor-acceptor CT rate compared to electrons and highlight a noticeable initial state dependence on the CT process. We leverage the unprecedented spatial resolution achieved in our experiments to show that the CT process in the dyad is governed via molecule-molecule coherent tunneling with negligible surface-mediated character.

physics.chem-ph↗

arXiv2017

Investigating charge transfer dynamics at the nanoscale

Acquiring quantitative information on charge transfer (CT) dynamics at the nanoscale remains an important scientific challenge. In particular, CT processes in single molecules at surfaces needs to be investigated to be properly controlled in various devices. To address this issue, the dynamics of switching molecules can be exploited. Here, a Nickel-tetraphenylporphyrin adsorbed on the Si(100) surface is used to study the CT process ruling the reversible activation of two chiral molecular conformations. Via the electrons of a scanning tunneling microscope (STM), a statistical study of the molecular switching reveals two specific locations of the molecule for which their efficiency is optimized. The CT mechanism is shown to propagate from two lateral aryls groups towards the porphyrin macrocycle inducing an intramolecular movement of two symmetric pyrroles. The measured switching efficiencies can thus be related with a Markus-Jordner model to estimate relevant parameters that describe the dynamics of the CT process. Numerical simulations provide a precise description of the molecular conformations and unveil the molecular energy levels that are involved in the CT process. This quantitative method opens a completely original approach to study CT at the nanoscale.

cond-mat.mes-hall↗