The zero-phonon lines, phonon sidebands and the dynamics of optically induced spin polarization of the neutral divacancies (VSiVC)degrees in 4H-SiC have been investigated by EPR spectroscopy. Via in-situ resonant optical excitation at T = 4 K the zero phonon lines (ZPL) between the 3A2 groundstate and the 3E excited states and their absorption phonon side bands (PSB) have been accurately determined for each of the four VV degrees centers (hh, kk, kh, hk) from the spectral dependence of the EPR line intensity. Their values, respectively 1130.7 nm (hh), 1129 nm (kk), 1107.5 nm (kh) and 1077.3 nm (hk) are in good agreement with previous PL studies and with theoretical predictions. Decomposition of the phonon sidebands of the basal VV centers reveals a coupling with phonons of 24, 36, and 47.5 meV (Vkh) and 11.5, and 18.6 meV (Vhk). The low-temperature optical excitation of the VV degrees centers gives rise to a 100% ground-state spin polarization which persists at T = 4 K for hundreds of seconds, when the excitation is switched off. Contrary to the case of VV centers in n-type nitrogen doped 4H-SiC, the VV centers in p-type doped 4H-SiC do not show any charge-state instability under optical excitation.

A comprehensive study of second harmonic generation on thermally oxidized MoS 2 flakes with the thickness ranging from monolayer up to seven layers is presented. Observing the fundamental nonlinear behavior for nontreated and oxidized MoS 2 reveals that oxidation causes significant changes in the second harmonic response for all investigated structures. Excitation-power-dependent measurements to analyze the nonlinear behavior with respect to the oxidation time show progressive oxidation within the maximum oxidation time of 6 h under the considered oxidation conditions. Here, polarization-dependent measurements reveal the structural changes due to oxidation. Additionally, it is found that the oxidation depth is restricted to the topmost S layer and the oxidation behavior exhibits a layer dependency. These findings are supported by theoretical band structure calculations. The results demonstrate that the thermal oxidation progress of two-dimensional MoS 2 can be monitored with nonresonant and noninvasive SH microscopy by following the distinct fingerprints of structural modification in the nonlinear response.

A comprehensive study of second harmonic generation on thermally oxidized MoS2 flakes with thickness ranging from monolayer up to seven layers is presented. Observing the fundamental nonlinear behavior for non-treated and oxidized MoS2 reveals that oxidation causes significant changes in the second harmonic (SH) response for all investigated structures. Excitation power dependent measurements to analyze the nonlinear behavior with respect to the oxidation time show progressive oxidation within the maximum oxidation time of six hours, under the considered oxidation conditions. Here, polarization dependent measurements reveal the structural changes due to oxidation. Additionally, it is found that the oxidation depth is restricted to the top most layer and the oxidation behavior exhibits a layer dependency. These findings are supported by theoretical band structure calculations. The results demonstrate that the thermal oxidation progress of two dimensional MoS2 can be monitored with non-resonant and non-invasive SH microscopy, by following distinct fingerprints of structural modification in the nonlinear response.

We investigate the role of topological defects in exciton behavior in (6,5) semiconducting single-walled carbon nanotubes using density functional theory. Our study identifies the helical Stone-Wales defect as a prominent trap for triplet excitons, characterized by a large zero-field splitting consistent with experimental data and a small singlet-triplet gap. The weak electron-phonon coupling, as evidenced by a Huang-Rhys factor of 0.74, renders it a promising single-photon emitter, with the zero-phonon line predicted at 1.6 μm, within the telecom range. These insights into defect-engineered electronic structure and exciton dynamics offer promising opportunities for improving the performance of carbon nanotube-based quantum light sources and optoelectronic devices.

Manipulation of triplet states lies at the origin of the emerging applications in quantum sensing and spin-based optoelectronics. In this work, we employ optically detected magnetic resonance (ODMR) spectroscopy to investigate how sp3 functionalization of (6,5) single-walled carbon nanotubes (SWCNTs) influences triplet exciton (TE) behavior. Functionalization with closed-shell 4-nitrophenyl groups at varying defect densities reveals that similar to singlet excitons, the TEs localize at the defect sites, leading to reduced zero-field splitting (ZFS) parameters and a distortion from the axial symmetry typically observed for pristine tubes. ODMR contrast is highest at low defect densities, suggesting that interdefect interactions significantly affect TE generation and spin polarization. Density functional theory (DFT) confirms the experimental observations that a reduced ZFS is observed for the sp3-functionalized SWCNTs. Open-shell (radical) functionalization introduces strong exchange interactions between the radical's unpaired electron and the TEs, resulting in an effective S = 3/2 system with enhanced ODMR contrast. These findings highlight how tuning the nature and spatial arrangement of sp3 defects offers a powerful strategy to control TE dynamics in SWCNTs, toward their integration into advanced quantum materials and devices.

A novel self-assembly strategy fabricates low-density dimer gold nanoparticles in a nanoparticle-on-metallic-mirror (NPoM) platform with ~3 nm gaps, utilizing M13 bacteriophage as a template, enabling dynamic and reversible tunability for versatile plasmonic applications.

First-principles calculations reveal how topological defects in semiconducting carbon nanotubes trap triplet excitons and enable single-photon emission at telecom wavelengths, offering new insights into their potential for photonic devices.

Paramagnetic point defects in silicon provide qubits that could open up pathways towards silicon-technology based, low-cost, room-temperature (RT) quantum sensing. The silicon dangling bond (db) is a natural candidate, given its sub-nanometer localization and direct involvement in spin-dependent charge-carrier recombination, allowing for electrical spin readout. In crystalline silicon, however, rapid loss of db spin-coherence at RT due to free-electron trapping, strongly limits quantum applications. In this work, by combining density-functional theory and multifrequency (100 MHz-263 GHz) pulsed electrically detected magnetic resonance spectroscopy, we show that upon electron capture, dbs in a hydrogenated amorphous silicon matrix form metastable spin pairs in a well-defined quasi two-dimensional (2D) configuration. Although highly localized, these entangled spin pairs exhibit nearly vanishing intrinsic dipolar and exchange coupling. The formation of this magic-angle-like configuration involves a >0.3 eV energy relaxation of a trapped electron, stabilizing the pair. This extends RT spin coherence times into the microsecond range in silicon required for a future spin-based quantum sensing technology.

Batteries based on heavier alkali ions are considered promising candidates to substitute for current Li-based technologies. In this theoretical study, we characterize the structural properties of a novel material, i.e., F-doped RbTiOPO4 (RbTiPO4F, RTP:F), and discuss aspects of its electrochemical performance in Rb-ion batteries (RIBs) using density functional theory (DFT). According to our calculations, RTP:F is expected to retain the so-called KTiOPO4 (KTP)-type structure, with lattice parameters of 13.236 Å, 6.616 Å, and 10.945 Å. Due to the doping with F, the crystal features eight extra electrons per unit cell, whereby each of these electrons is trapped by one of the surrounding Ti atoms in the cell. Notably, the ground state of the system corresponds to a ferromagnetic spin configuration (i.e., S=4). The deintercalation of Rb leads to the oxidation of the Ti atoms in the cell (i.e., from Ti3+ to Ti4+) and to reduced magnetic moments. The material promises interesting electrochemical properties for the cathode: rather high average voltages above 2.8 V and modest volume shrinkages below 13% even in the fully deintercalated case are predicted.

Polarons influence decisively the performance of lithium niobate for optical applications. In this project, the formation of (defect) bound polarons in lithium niobate is studied by ab-initio molecular dynamics. The calculations show a broad scatter of polaron formation times. Rising temperature increases the share of trajectories with long formation times, which leads to an overall increase of the average formation time with temperature. However, even at elevated temperatures, the average formation time does not exceed the value of 100 femtoseconds, i.e., a value close to the time measured for free, i.e., self-trapped polarons. Analyzing individual trajectories, it is found that the time required for the structural relaxation of the polarons depends sensitively on the excitation of the lithium niobate high-frequency phonon modes and their phase relation.

Exciton transfers are ubiquitous and extremely important processes, but often poorly understood. A recent example is the triplet exciton transfer in tetracene sensitized silicon solar cells exploited for harvesting high-energy photons. The present ab initio molecular dynamics calculations for tetracene-Si(111):H interfaces show that Si dangling bonds, intuitively expected to hinder the exciton transfer, actually foster it. This suggests that defects and structural imperfections at interfaces may be exploited for excitation transfer.

The triplet exciton transfer from tetracene to silicon has recently found interest in the context of efficiently harvesting high-energy photons with sensitized silicon solar cells. Density-functional calculations presented here show that at covalently bonded, atomically perfect interfaces, no exciton transfer can be expected. The situation changes, however, as soon as realistic interface models are taken into account. Unsaturated Si dangling bonds on n-type silicon assist the exciton transfer. The calculations show furthermore that incompletely bonded interfaces as well as horizontally attached seed molecules foster excitation transfer. Backfilling the interface, on the other hand, is detrimental to the interface's transport properties.

Most properties of solid materials are defined by their internal electric field and charge density distributions which so far are difficult to measure with high spatial resolution. Especially for 2D materials, the atomic electric fields influence the optoelectronic properties. In this study, the atomic-scale electric field and charge density distribution of WSe2 bi- and trilayers are revealed using an emerging microscopy technique, differential phase contrast (DPC) imaging in scanning transmission electron microscopy (STEM). For pristine material, a higher positive charge density located at the selenium atomic columns compared to the tungsten atomic columns is obtained and tentatively explained by a coherent scattering effect. Furthermore, the change in the electric field distribution induced by a missing selenium atomic column is investigated. A characteristic electric field distribution in the vicinity of the defect with locally reduced magnitudes compared to the pristine lattice is observed. This effect is accompanied by a considerable inward relaxation of the surrounding lattice, which according to first principles DFT calculation is fully compatible with a missing column of Se atoms. This shows that DPC imaging, as an electric field sensitive technique, provides additional and remarkable information to the otherwise only structural analysis obtained with conventional STEM imaging.

Hyperfine splittings play an important role in quantum information and spintronics applications. They allow for the readout of the spin qubits, while at the same time providing the dominant mechanism for the detrimental spin decoherence. Their exact knowledge is thus of prior relevance. In this work, we analytically investigate the relativistic effects on the hyperfine splittings of hydrogen-like atoms, including finite-size effects of the nucleis’ structure. We start from exact solutions of Dirac’s equation using different nuclear models, where the nucleus is approximated by (i) a point charge (Coulomb potential), (ii) a homogeneously charged full sphere, and (iii) a homogeneously charged spherical shell. Equivalent modelling has been done for the distribution of the nuclear magnetic moment. For the 1s ground state and 2s excited state of the one-electron systems H1, H2, H3, and He+3, the calculated finite-size related hyperfine shifts are quite similar for the different structure models and in excellent agreement with those estimated by comparing QED and experiment. This holds also in a simplified approach where relativistic wave functions from a Coulomb potential combined with spherical-shell distributed nuclear magnetic moments promises an improved treatment without the need for an explicit solution of Dirac’s equation within the nuclear core. Larger differences between different nuclear structure models are found in the case of the anisotropic 2p3/2 orbitals of hydrogen, rendering these excited states as promising reference systems for exploring the proton structure.

Realizing plasmonic nanogaps with a refractive index (n = 1) environment in metallic nanoparticle (NP) structures is highly attractive for a wide range of applications. So far in self-assembly-based approaches, without surface functionalization of metallic NPs, achieving such extremely small nanogaps is challenging. Surface functionalization introduces changes in the refractive index at nanogaps, which in turn deteriorates the desired plasmonic properties. In addition, fabrication of low-density dimer NP designs with smaller nanogaps poses a big challenge. Here, we introduce a simple and straightforward self-assembly-based strategy for the fabrication of low-density, isolated dimer gold nanoparticles in a nano-particle-on-metallic-mirror (NPoM) platform. A minimum interparticle gap distance between NPs of similar to 3 nm is achieved without surface functionalization. This is possible by utilizing the M13 bacteriophage as the spacer layer instead of SiO2 in NPoM. Density functional theory calculations on Au atom adsorption on SiO2 and M13 bacteriophage surface constituents trace the NP assembly on the latter to a comparatively weak interaction with the substrate. Our study offers an attractive route for fabricating low density plasmonic dimer structures featuring small nanogaps and will enrich structure specific/isolated studies benefitting a variety of optical, actuator, and sensing applications.

The crystal family of potassium titanyl phosphate (KTiOPO4) is a promising material group for applications in quantum and nonlinear optics. The fabrication of low-loss optical waveguides, as well as high-grade periodically poled ferroelectric domain structures, requires a profound understanding of the material properties and crystal structure. In this regard, Raman spectroscopy offers the possibility to study and visualize domain structures, strain, defects, and the local stoichiometry, which are all factors impacting device performance. However, the accurate interpretation of Raman spectra and their changes with respect to extrinsic and intrinsic defects requires a thorough assignment of the Raman modes to their respective crystal features, which to date is only partly conducted based on phenomenological modelling. To address this issue, we calculated the phonon spectra of potassium titanyl phosphate and the related compounds rubidium titanyl phosphate (RbTiOPO4) and potassium titanyl arsenate (KTiOAsO4) based on density functional theory and compared them with experimental data. Overall, this allows us to assign various spectral features to eigenmodes of lattice substructures with improved detail compared to previous assignments. Nevertheless, the analysis also shows that not all features of the spectra can unambigiously be explained yet. A possible explanation might be that defects or long range fields not included in the modeling play a crucial rule for the resulting Raman spectrum. In conclusion, this work provides an improved foundation into the vibrational properties in the KTiOPO4 material family.

As a benchmark, the structural, electronic and optical properties of the three main phases of TiO 2 crystals have been calculated using Hubbard U correction and hybrid functional methods in density-functional theory. These calculations are compared concerning the available experimental observations on pristine TiO 2 crystals. Modified hybrid functionals, particularly the PBE0 functional with 11.4% fraction of exact exchange, are shown to provide highly accurate atomic structures and also accurate electronic structure data, including optical excitation energies. With DFT + U, accurate optical spectra are also possible, but only if the Hubbard U is applied on the O 2p electrons exclusively. Furthermore, both methods, the 11.4%-PBE0 hybrid functional and the DFT + U p scheme have been used to study TiO 2 amorphous ultra-thin films, confirming the agreement of the two methods even with respect to small details of the optical spectra. Our results show that the proposed DFT + U p methodology is computationally efficient, but still accurate. It can be applied to well-ordered TiO 2 polymorphs as well as to amorphous TiO 2 and will allow for the calculations of complex titania-based structures.

The nitrogen-vacancy (NV) centers (NCVSi)− in 4H silicon carbide (SiC) constitute an ensemble of spin S = 1 solid state qubits interacting with the surrounding 14N and 29Si nuclei. As quantum applications based on a polarization transfer from the electron spin to the nuclei require the knowledge of the electron–nuclear interaction parameters, we have used high-frequency (94 GHz) electron–nuclear double resonance spectroscopy combined with first-principles density functional theory to investigate the hyperfine and nuclear quadrupole interactions of the basal and axial NV centers. We observed that the four inequivalent NV configurations (hk, kh, hh, and kk) exhibit different electron–nuclear interaction parameters, suggesting that each NV center may act as a separate optically addressable qubit. Finally, we rationalized the observed differences in terms of distinctions in the local atomic structures of the NV configurations. Thus, our results provide the basic knowledge for an extension of quantum protocols involving the 14N nuclear spin.

The third‐order susceptibility of lithium niobate (LiNbO 3 ) is calculated within a Berry‐phase formulation of the dynamical polarization based on the electronic structure obtained within density‐functional theory (DFT). Maximum values of the order of m V are calculated for photon energies between 1.2 and 2 eV, i.e., in the lower half of the optical bandgap of lithium niobate. Both free and bound electron (bi)polarons are found to lead to a remarkable enhancement of the third‐order susceptibility for photon energies below 1 eV.