arXiv subjects

Torsten Fritz

Publications and source records attributed to Torsten Fritz.

9 recordsLinked to original sources

arXiv2026

Tuning of Photoexcited Electron Dynamics at Monolayer h-BN/Metal Interfaces by Corrugation

Atomic-scale corrugation in two-dimensional materials can modify interfacial electronic coupling, yet its influence on ultrafast carrier relaxation remains poorly established. Here, we compare image potential states (IPS) at monolayer h-BN/Ir(111) and h-BN/Pt(111) interfaces using structural characterization and time-resolved two-photon photoemission spectroscopy. Consistent with literature, h-BN is strongly corrugated on Ir(111) but comparatively flat on Pt(111). The first (n = 1) and second (n = 2) IPS appears at similar energies on both substrates, whereas their relaxation dynamics differ markedly. On h-BN/Ir(111), the IPS decay is response-limited (<20 fs), while h-BN/Pt(111) exhibits lifetimes of 56 fs (n = 1) and 75 fs (n = 2). The lifetime contrast is most consistently explained by corrugation-enhanced overlap of the IPS wavefunction with the metal substrate, which accelerates electron decay. These results indicate that atomic-scale corrugation is an effective physical parameter for tuning ultrafast electron dynamics at two-dimensional material and metal interfaces.

cond-mat.mtrl-sci↗

arXiv2026

Bottom-up realization of a type-II organic-TMD heterointerface: Pentacene on monolayer WS2

Stacked van der Waals heterostructures based on transition metal dichalcogenides (TMDs) exhibit a rich variety of exotic interfacial phenomena. Substituting one component with an organic semiconductor (OSC) enables the design of hybrid heterostructures with tunable functionalities for optoelectronic, photovoltaic, and spintronic applications. In this work, exploiting scanning tunneling spectroscopy (STS), photoemission orbital tomography (POT) and G0W0 electronic structure calculations, we experimentally and theoretically demonstrate the self-assembly of an ordered single layer of pentacene (5A) above monolayer WS2, exhibiting a type-II (staggered) band alignment in the hybrid 5A/WS2 interface. Central to this result is the synthesis of extended, atomically flat WS2 - an essential prerequisite for a highly ordered and electronically homogeneous OSC/TMD interface - which can only be reliably achieved via bottom-up growth, most notably molecular beam epitaxy (MBE). We realize this by leveraging Au(111) as an atomically clean and conductive sample for epitaxial growth - a necessary requirement for reliable and comparable STS/POT characterizations. The high quality of the synthesized heterostructure, together with its type-II band alignment, establishes pentacene/WS2 as a model system for orbital-resolved studies of charge transfer, energy-level renormalization, and non-equilibrium interfacial processes in hybrid organic-inorganic-2D heterostructures.

cond-mat.mtrl-sci↗

arXiv2025

Dial It Down: The Effect of Strongly Interacting Adsorbates on the BiAg2 Rashba Surface State

Organic semiconductors interfaced with spin-orbit coupled materials offer a rich playground for fundamental studies of controlling spin dynamics in spintronic devices. The adsorbate-surface interactions at such interfaces play a key role in determining the valence electronic and spin structure and consequently, the device physics as well. Here we present the adsorption and electronic structure of the strong organic electron acceptor 2,7-dinitropyrene-4,5,9,10-tetrone (NO2-PyT, C16H4N2O8) on the Rashba spin-orbit coupled surface alloy BiAg2/Ag(111). We show that the strong adsorbate-surface alloy interaction leads to weakening of the electronic coupling between the surface alloy atoms and quench the spin-orbit coupled surface state in BiAg2/Ag(111). Our findings demonstrate an important challenge associated with using molecular adsorbates to tailor the spin polarization in BiAg2/Ag(111), and our work provides guidelines to consider while designing interfacial systems to engineer the spin polarization in Rashba surface alloys.

cond-mat.mtrl-sci↗

arXiv2023

Lifshitz Transition and Band Structure Evolution in Alkali Metal Intercalated 1Tprime-MoTe2

In van der Waals materials, coupling between adjacent layers is weak, and consequently interlayer interactions are weakly screened. This opens the possibility to profoundly modify the electronic structure, e.g., by applying electric fields or with adsorbates. Here, we show for the case of the topologically trivial semimetal 1Tprime-MoTe2 that potassium dosing at room temperature significantly transforms its band structure. With a combination of angle-resolved photoemission spectroscopy, scanning tunneling microscopy, x-ray photoemission spectroscopy, and density functional theory we show that i) for small concentrations of K, 1Tprime-MoTe2 undergoes a Lifshitz transition with the electronic structure shifting rigidly, and ii) for larger K concentrations 1Tprime-MoTe2 undergoes significant band structure transformation. Our results demonstrate that the origin of this electronic structure change stems from alkali metal intercalation.

cond-mat.mtrl-sci↗

arXiv2022

Heterostructure films of SiO$_2$ and HfO$_2$ for high power laser optics prepared by plasma-enhanced atomic layer deposition

Absorption losses and laser-induced damage threshold (LIDT) are considered as the major constraint for the development of optical coatings for high-power laser optics. Such coatings require paramount properties like low losses due to optical absorption, high mechanical stability, and enhanced damage resistance to withstand high-intensity laser pulses. In this work, heterostructure films were developed by the intermixing of SiO$_2$ and HfO$_2$ using plasma-enhanced atomic layer deposition (PEALD) technique. Thin film characterization techniques such as spectroscopic ellipsometry, spectrophotometry, substrate curvature measurements, x-ray reflectivity, and Fourier transform infrared spectroscopy were employed for extracting optical constants, spectral inter-pretation, residual stress, layer formation, and functional groups present in the heterostructures, respectively. These heterostructures demonstrate tunable refractive index, bandgap, and improved optical losses and LIDT properties. The films were incorporated into antireflection coatings (multilayer stacks and graded index coatings) and the LIDT was determined at 355 nm wavelength by the R-on-1 method. Optical absorptions at the reported wavelengths were characterized using photothermal common-path interferometry and laser-induced deflection techniques.

physics.optics↗

arXiv2022

Atomically Thin Metal-Dielectric Heterostructures by Atomic Layer Deposition

Heterostructures increasingly attracted attention over the past several years to enable various optoelectronic and photonic applications. In this work, atomically thin interfaces of Ir/Al2O3 heterostructures compatible with micro-optoelectronic technologies are reported. Their structural and optical properties were determined by spectroscopic and microscopic techniques (XRR, XPS, HRTEM, spectroscopic ellipsometry, and UV/VIS/NIR spectrophotometry). The XRR and HRTEM analyses reveal a layer-by-layer growth mechanism of Ir in atomic scale heterostructures, which is different from the typical island-type growth of metals on dielectrics. Alongside, XPS investigations imply the formation of Ir-O-Al bonding at the interfaces for lower Ir concentrations, in contrast to the nanoparticle core-shell structure formation. Precisely tuning the ratio of the constituents ensures the control of the dispersion profile along with a transition from effective dielectric to metallic heterostructures. The Ir coating thickness was varied ranging from a few {\AA} to films of about 7 nm in the heterostructures. The transition has been observed in the structures containing individual Ir coating thicknesses of about 2-4 nm. Following this, show epsilon-near-zero metamaterials with tunable dielectric constants by precisely varying the composition of such heterostructures. Overall, a comprehensive study on structural and optical properties of the metal-dielectric interface of Ir/Al2O3 heterostructures was addressed indicating an extension of the material portfolio available for optical system design.

physics.app-ph↗

arXiv2022

Frontier Orbital Degeneracy: A new Concept for Tailoring the Magnetic State in Organic Semiconductor Adsorbates

Kondo resonances in molecular adsorbates are an important building block for applications in the field of molecular spintronics. Here, we introduce the novel concept of using frontier orbital degeneracy for tailoring the magnetic state, which is demonstrated for the case of the organic semiconductor 1,4,5,8,9,11-Hexaazatriphenylenehexacarbonitrile (HATCN, C18N12) on Ag(111). Low-temperature scanning tunneling microscopy/spectroscopy (LT-STM/STS) measurements reveal the existence of two types of adsorbed HATCN molecules with distinctly different appearances and magnetic states, as evident from the presence or absence of an Abrikosov-Suhl-Kondo resonance. Our DFT results show that HATCN on Ag(111) supports two almost isoenergetic states, both with one excess electron transferred from the Ag surface, but with magnetic moments of either 0 or 0.65 uB. Therefore, even though all molecules undergo charge transfer of one electron from the Ag substrate, they exist in two different molecular magnetic states that resemble a free doublet or an entangled spin state. We explain how the origin of this behavior lies in the twofold degeneracy of the lowest unoccupied molecular orbitals of gas phase HATCN, lifted upon adsorption and charge-transfer from Ag(111). Our combined STM and DFT study introduces a new pathway to tailoring the magnetic state of molecular adsorbates on surfaces, with significant potential for spintronics and quantum information science.

cond-mat.mtrl-sci↗

arXiv2021

Tuning exciton recombination rates in doped transition metaldichalcogenides

Monolayer transition metal dichalcogenides (TMDs) are direct gap semiconductors that hold great promise for advanced applications in photonics and optoelectronics. Understanding the interplay between their radiative and non-radiative recombination pathways is thus of crucial importance not only for fundamental studies but also for the design of future nanoscale on-chip devices. Here, we investigate the interplay between doping and exciton-exciton annihilation (EEA) and their impact on the photoluminescence quantum yield in different TMD samples and related heterostructures. We demonstrate that the EEA threshold increases in highly doped samples, where the radiative and non-radiative recombination of trions dominates.

cond-mat.mes-hall↗

arXiv2020

Surface Self-Assembly of Functionalized Molecules on Ag(111): More Than Just Chemical Intuition

The fabrication of nanomaterials involves self-ordering processes of functional molecules on inorganic surfaces. To obtain specific molecular arrangements, a common strategy is to equip molecules with functional groups. However, focusing on the functional groups alone does not provide a comprehensive picture. Especially at interfaces, processes that govern self-ordering are complex and involve various physical and chemical effects, often leading to structures that defy chemical intuition, as we showcase here on the example of a homologous series of quinones on Ag(111). From chemical intuition one could expect that such quinones, which all bear the same functionalization, form similar motifs. In salient contrast, our joint theoretical and experimental study shows that profoundly different structures are formed. Using a machine-learning-based structure search algorithm, we find that this is due to a shift of the balance of three antagonizing driving forces: adsorbate-substrate interactions governing adsorption sites, adsorbate-adsorbate interactions favoring close packing, and steric hindrance inhibiting certain otherwise energetically beneficial molecular arrangements. The theoretical structures show excellent agreement with our experimental characterizations of the organic/inorganic interfaces, both for the unit cell sizes and the orientations of the molecules within. With a detailed examination of all driving forces, we are further able to devise a design principle for self-assembly of functionalized molecules. The non-intuitive interplay of similarly strong interaction mechanisms will continue to be a challenging aspect for the design of functional interfaces. Our agreement between theory and experiment combined with the new physical insights indicates that these methods have now reached the necessary accuracy to do so.

cond-mat.mes-hall↗