Cubic gallium nitride (c-GaN) is an attractive material system for various electronic and optoelectronic device applications, as it lacks internal piezoelectric polarization fields unlike its hexagonal counterpart. Yet, the fabrication of high-performing device structures depends on the growth of high-quality c-GaN thin films, which remains a challenging task. Here, we present a systematic characterization of the electrical properties of cubic GaN/AlN heterostructures with varying thicknesses grown by molecular beam epitaxy on 3C-SiC/Si templates. Impedance spectroscopy is used as a powerful tool for characterizing transport through the sample structure as well as the defect density at the existing interfaces. While the bare template exhibits a single feature in the impedance spectrum associated with the 3C-SiC/Si interface, a second low-frequency process arises under forward bias in samples with cubic GaN/AlN layers on top. Quantitative analysis of the impedance response reveals a reduction in the impurity density with increasing c-GaN film thickness. This improvement in structural quality with extended growth time is accompanied by a decrease in defect density at the interfaces. These results establish a direct link between optimized growth conditions and structural quality in cubic nitride heterostructures, providing a pathway toward the reliable design of next-generation electronic and optoelectronic devices.

Photoelectrochemical water splitting is a promising route to sustainable hydrogen production, but it requires semiconductor electrodes with optimal bandgap, proper band-edge alignment to the water redox potentials, and high corrosion resistance. Cubic silicon carbide (3C-SiC) is a compelling candidate due to its near-ideal bandgap energy and excellent chemical stability. Here, we systematically characterize SiC photoelectrodes comprising of n-type and p-type 3C-SiC thin films grown on Si substrates of matching dopant type. Linear-sweep voltammetry and electrochemical impedance spectroscopy yield the key photoelectrochemical parameters including the flat-band potential and open-circuit potential. Ultraviolet photoelectron spectroscopy and low-energy inverse photoelectron spectroscopy provide the valence-band maximum, conduction-band minimum, Fermi level positions, and bandgap energies. Together, these results elucidate the detailed energy band landscapes for both n- and p-3C-SiC/electrolyte interfaces. The energy diagrams explain the observed behavior with and without illumination, confirming that n-doped 3C-SiC functions as efficient photoanode for oxygen evolution while p-doped 3C-SiC acts as photocathode for hydrogen evolution in neutral aqueous electrolyte. Establishing these quantitative band-edge alignments provides a blueprint for designing durable, bias-free tandem PEC architectures. Given the scalability and stability of SiC, these insights advance pathways toward cost-effective, large-scale green-hydrogen production with a reduced environmental footprint.

2D indium selenide (InSe) is a layered semiconductor with high electron mobility and a tunable band gap ranging from 1.25 eV in the bulk to 2.8 eV in the monolayer limit. However, growing phase-pure InSe remains challenging due to the complex indium-selenium (In-Se) phase diagram. This complexity and the sensitivity of chemical precursors to growth conditions make it difficult to control which In-Se phase forms during synthesis during, e.g., metal-organic chemical vapor deposition (MOCVD). MOCVD is considered the most promising approach for growing InSe, as it enables wafer-scale, uniform, and controllable deposition-key requirements for device integration. We present a systematic investigation of InSe synthesis on c-plane sapphire substrates at low temperatures. By varying Se/In precursor ratio and growth temperature, we create a phase diagram that covers In-rich, equal stoichiometric, and Se-rich InxSey phases. Raman spectroscopy and atomic force microscopy, supported by energy dispersive X-ray spectroscopy and scanning transmission electron microscopy, reveal formation conditions of 2D InSe. The epitaxial alignment is verified by in-plane X-ray diffraction. Samples grown under optimized conditions exhibit a strong optical absorption in the visible range and especially a comparably high electron mobility, underlining the potential of the MOCVD-grown material for future applications.

Within the plethora of different vanadium oxide phases, vanadium dioxide (VO2) is a well-studied material for thermochromic applications such as smart windows. To obtain crystalline VO2 thin films, usually high in situ substrate temperatures are necessary. This, in turn, is detrimental on industrial scales, hence, alternatively postgrowth temperature treatment is a viable option. Even so, the surface morphology and roughness of the thin films exhibit strong temperature dependence. As these characteristics directly govern the physical, optical, and electronic responses, precise control of surface morphology-while maintaining phase stability-is vital. This is even more critical in multilayer architectures, where surface morphology influences the multilayer's interfaces. Here, we use ion-beam sputter deposition for the reproducible growth of vanadium oxide and assess a variety of annealing procedures to manipulate both the surface and phase of the material. We show that the surface morphology as well as the phase transition depend on the annealing parameters, such as atmosphere, pressure, temperature, and duration of the chosen treatment.

In the context of solid-state batteries (SSBs), minimizing the partial electronic conductivity (6el) of solid electrolytes is essential to minimize self-discharge and degradation processes, ensuring the viability of SSBs. Thus, a precise determination of 6el is of high interest as it varies significantly across the operational range of practical SSBs. Here, ion-blocking Hebb-Wagner polarization measurements are a well-established technique for assessing the partial conductivity of minority charge carriers in solid ion conductors (i.e., 6el). However, measuring 6el of lithium-ion conducting solid electrolytes often yields inaccurate results due to electrochemical reduction reactions at low (reference) potentials. This study provides a comprehensive framework encompassing fundamental derivations, accurate testing protocols, and comprehensive data analysis for determining 6el of lithium-ion conducting solid electrolytes. The partial ionic and electronic conductivities of lithium halides (LiX, with X = Cl, Br, or I)-components of typical interphases in SSBs-were systematically evaluated over a range of the lithium activity. Pristine Hebb-Wagner cells (i.e., reservoir-free SSBs) exhibit "self-polarization", which we discuss as part of the formal treatment, and cause the material to be probed in the extrinsic region. Our experimental findings offer systematic insight into the electronic properties of lithium halides, extrapolated to room temperature, revealing conductivities in the order of 10-9 S cm-1 or lower within the tested activity range. These findings deepen the understanding of the role of LiX in solid electrolyte interphase formation affecting the performance and stability of SSBs.

Accurate characterization of plasmas within the discharge chambers of gridded ion engines is essential for their advancement. This study showcases the effectivity of terahertz time-domain spectroscopy (THz-TDS) as a non-invasive technique for profiling low-pressure inductively coupled plasmas, mimicking the conditions of the discharge chamber in a radio-frequency ion thruster. Operating at pressures of 3 - 7· 10^-3 mbar inside the discharge chamber and a supply power of the radio frequency generator ranging from 10 - 80 W, THz-TDS reveals electron densities in the range of 2· 10^16 - 2· 10^17 m ^-3 for xenon, krypton, and argon plasmas. Our results show good agreement with Langmuir probe measurements and global plasma modeling, highlighting the accuracy and reliability of THz-TDS. This validation, conducted under conditions representative of gridded ion engines, demonstrates that THz-TDS is, in principle, suitable for characterizing the electron system of the plasma ignited in such thrusters in operation. This offers great potential of developing a promising additional tool for plasma diagnostics in electric propulsion systems.

Global energy consumption and the imperative to mitigate climate change have driven the exploration of innovative technologies to enhance energy efficiency in buildings. Among these, smart windows utilizing thermochromic vanadium dioxide-based materials as active materials have emerged as a promising avenue. These windows dynamically modulate their optical properties in response to environmental conditions, helping to reduce the energy consumption for heating/cooling of the building. However, the practical implementation of VO2-based smart windows faces challenges related to optimizing their performance and durability. In this context, this review advocates for further research into VO2-based smart windows incorporating titanium dioxide (TiO2) buffer layers and TiO2 antireflection (AR) coatings. Rutile TiO2 buffer layers are used to promote growth of the VO2 functional films. Additionally, TiO2 antireflection coatings can improve the optical performance of smart windows by minimizing reflections and maximizing light transmission. This work highlights how TiO2-based layers may be used to form a multilayer system surrounding the active VO2 layer in order to enhance the efficiency of VO2-based smart windows. The proposed application of rutile TiO2 buffer layers and anatase TiO2 AR layers holds promise for advancing the development of energy-efficient building technologies.

The synthesis of MesDPM group 13 (B─In, MesDPM = 1,5,9-trimesityldipyrromethene) compounds with halide substituents (Cl─I) is described. All compounds were fully characterized including NMR and IR spectroscopy as well as mass spectrometry. In addition, the solid state molecular structures have been determined by X-ray diffraction (XRD) analysis. As higher representatives of BODIPY (boron difluoride dipyrromethene) dyes, some of these MesDPM triel dihalides also exhibit an intense green fluorescence when exposed to sunlight. In this regard, the optical properties were investigated by UV/Vis and photoluminescence spectroscopy giving absorption maxima around 520 nm and fluorescence emission in the range between 550 and 660 nm. Fluorescence quantum efficiencies up to 42% could be obtained from measurements in toluene solution. Further, reactivity studies were carried out which opened-up access to mixed substituted MesDPM triels with one alkyl and one halide substituent.

Herein, the simple synthesis of triel dihydride complexes with the sterically demanding 1,5,9-trimesityldipyrromethene (MesDPM) ligand is reported. With these compounds, reactivity studies are carried out, opening access to new classes of MesDPM triels. All compounds are fully characterized by standard analytic methods such as NMR, IR, and UV/vis spectroscopy as well as mass spectrometry. In addition, the molecular structures in solid state are determined by single-crystal X-ray diffraction analysis. In some instances, green fluorescence can also be observed upon solar irradiation, which prompted further investigations of the photoluminescence behavior.

Lead halide perovskites and related hybrid metal halides exhibit exceptional semiconductor properties, enabling diverse applications in photovoltaics, solid-state lighting, and photocatalysis. Multinary halido metalates, combining multiple metals, offer unique opportunities to tune the optical and electronic properties of these materials for specific applications. Here, we present the synthesis and characterization of (Hpiz)4BiCu4I11·2MeCN (piz = piperazine), the most copper-rich molecular iodido bismuthate reported to date, featuring a Cu/Bi ratio of 4:1. It extends the "all-in-one" design concept of halido cuprates with cationic ligands to multinary systems and exhibits a low optical band gap of 1.82 eV (681 nm) and broad red photoluminescence centered at 1.69 eV (735 nm), making it a promising candidate for light-harvesting and near-infrared emission applications. Quantum chemical analyses attribute the reduced band gap to strong electronic interactions between Cu-(I) and Bi-(III). Additionally, the monometallic analogs (H2piz)-CuI3 and (H2piz)-Bi2I8 reveal the role of heterometallic interactions in modulating the optical properties. This study provides valuable insights into the design of copper-bismuth iodide systems, enriching the library of hybrid materials with customized semiconductor characteristics.

Herein, the simple synthesis of triel dihydride complexes with the sterically demanding 1,5,9‐trimesityldipyrromethene ( Mes DPM) ligand is reported. With these compounds, reactivity studies are carried out, opening access to new classes of Mes DPM triels. All compounds are fully characterized by standard analytic methods such as NMR, IR, and UV/vis spectroscopy as well as mass spectrometry. In addition, the molecular structures in solid state are determined by single‐crystal X‐ray diffraction analysis. In some instances, green fluorescence can also be observed upon solar irradiation, which prompted further investigations of the photoluminescence behavior.

We present our investigation of cubic zinc blende InGaN thin films grown by plasma-assisted molecular beam epitaxy on c-GaN/c-AlN/3C-SiC/Si substrates oriented in the (001) orientation. Through spectroscopic ellipsometry analyses spanning the infrared and visible-ultraviolet spectral ranges, we extract the dielectric function, which contains contributions from phonons, plasmons, and interband transitions. A linear shift of TO-phonon frequency, ranging from 467cm−1 for InN to 555cm−1 for GaN, was observed. Moreover, we consider a non-parabolic conduction band, many-body effects such as bandgap renormalization, and Burstein–Moss shift, as well as strain-induced alterations in the absorption edge. From this, the fundamental bandgap energies at room temperature and a bowing parameter of bG=1.61eV can be determined.

Nitride-based semiconductors are vital for efficient optoelectronic devices in the ultraviolet to green spectral range. However, producing red-emitting InGaN micro-LEDs is challenging due to lattice mismatch with traditional GaN substrates. This mismatch causes strain relaxation, compositional gradients, and defects in high-indium-content InGaN films. These issues severely limit device efficiency, and the potential of alternative substrates to address these challenges is not fully explored. Here, we show that Al1-xScxN pseudosubstrates with adjustable lattice parameters greatly improve lattice matching of InGaN. Using plasma-assisted molecular beam epitaxy, we grow 120 nm-thick, phase-pure Al1-xScxN layers (0.1 < xSc < 0.2). This enables high-quality deposition of In0.28Ga0.72N layers and a uniform indium distribution compared to growth directly on GaN. AlScN-supported films exhibit no compositional pulling effect common for conventional substrates. This uniformity is confirmed by room-temperature photoluminescence, showing a narrow emission at 538 nm. Our results demonstrate that AlScN pseudosubstrates are promising for future integrated red micro-LED devices.

Lead halide perovskites have catalyzed the rise of main-group metal halide materials as promising candidates for next-generation optoelectronics, including solar cells, light-emitting diodes, lasers, sensors, and photocatalysts. Among these, effi-cient light-emission arises from self-trapped excitons, wherein excited states induce transient lattice distortions that localize excitons. However, the complex interplay of factors, such as lattice distortions, lattice softness, and electron-phonon cou-pling dynamics, obscures the direct structure-property relationships complicating the targeted material design. In this study, we advance the understanding of self-trapped exciton (STE)-based emission in hybrid antimony and bismuth halides, em-phasizing the interplay of structural and electronic factors that enhance white-light emission. We systematically vary com-position, anion dimensionality, connectivity, and the organic cation and find that the presence of Bi/Sb and Cl in edge-sharing anion motifs promotes white-light emission and optimal electron-phonon coupling. Chlorides outperform bromides, and organic cations, such as CMA and BZA, only subtly influence optical behavior by altering lattice dynamics and rigidity, resulting in tunable emission characteristics without compromising STEs. This work deepens the understanding of the emis-sion mechanisms in hybrid halide perovskites and establishes guiding principles for tailoring optoelectronic properties, paving the way for advanced materials with enhanced white-light emission for next-generation optoelectronic applications.

Cubic III-nitrides are a promising alternative to conventional wurtzite-based InGaN systems for visible light emission, particularly in the red spectral region, due to the absence of internal polarization fields. We present a systematic study of cubic InGaN layers grown by plasma-assisted molecular beam epitaxy using two different growth schemes: conventional growth and metal-modulated epitaxy (MME). Three types of structures were investigated, namely, bulk layers, multiple quantum wells (MQWs), and single quantum wells (SQWs). MME-grown samples show improved surface morphology and enhanced phase purity, including a complete suppression of hexagonal inclusions. Photoluminescence (PL) measurements confirm red emission from all samples and reveal multiple emission peaks for the quantum well samples. Based on micro-cathodoluminescence mapping, the main emission peak is most likely attributed to carrier recombination in the QW or bulk regions, while the high-energy peak is associated with localized surface features, such as pits. Temperature-dependent PL measurements show different thermal quenching behavior for the two peaks. Power-dependent measurements confirm that all samples exhibit remarkable emission energy stability over nearly two orders of magnitude variation in excitation power. The spectrally integrated PL intensities recorded at various temperatures demonstrate the robust emission efficiency retaining up to 25% of their low-temperature PL intensity at room temperature. These results demonstrate the potential of MME-grown cubic InGaN for efficient red emission and underline its relevance for future micro-LED applications.

Readily implemented broadband THz sources extend the reach of THz in diagnostic and sensing applications. Coherently controlled photocurrents in pure, cost-effective bulk Germanium and Germanium quantum wells render such broadband and gapless terahertz emission, overcoming the bandwidth limitations of traditional III-V and II-VI semiconductor-based THz emitters. Precise phase control between the fundamental and its second harmonic governs the THz field dynamics. The presented scheme, which utilizes low-energy 100 fs optical pulses centered around 1550 nm, is readily accessible through affordable ultrafast fiber laser technology and frequency doubling.

Alternating metal-modulated molecular beam epitaxy enables the growth of both self-assembled c-InGaN/GaN quantum wells and fully alloyed c-InGaN layers. In situ reflection high-energy electron diffraction (RHEED) analysis coupled with ex situ structural characterization investigates the growth mechanism and prerequisites for the self-assembled c-InGaN quantum well formation. The data reveal that indium accumulates without incorporating into the underlying c-GaN layer during an indium deposition step. However, the accumulated indium forms c-InGaN during a subsequent GaN growth step consistent with vertical cation segregation. Furthermore, X-ray diffraction, time-of-flight secondary ion mass spectrometry depth profiles, and scanning transmission electron microscopy imaging show homogeneous and well-defined c-InGaN layers. The presented growth mechanism requires high substrate temperatures and gallium fluxes. Still, limit testing suggests that indium contents of up to 37% are feasible. This encourages the implementation of metal-modulated grown c-InGaN in red light-emitting devices. Furthermore, combining RHEED operando diagnostics and a precise understanding of the growth mechanism is vital for progressing toward automated growth of complex heterostructures.

Cubic InGaN alloys are a promising candidate material for next-generation optoelectronic applications as they lack internal fields and promise to cover large parts of the electromagnetic spectrum from the deep UV toward the mid-infrared. This demands high-quality epitaxial growth of cubic InGaN/GaN quantum wells, especially for the red energy range. However, the growth of indium-bearing nitride quantum wells in the metastable cubic phase still poses many challenges. InGaN and GaN are typically grown at different temperatures and with different metal fluxes in molecular beam epitaxy, leading to either long waiting periods for temperature adjustment or growth under suboptimal conditions. Both degrade the crystal quality and optical properties. In this study, we apply a metal-modulated growth approach in molecular beam epitaxy that enables us to grow either self-assembled, phase pure, cubic InGaN/GaN multi quantum wells (MQWs) or homogeneous c-InGaN layers, only by adjusting the shutter duration times for Ga and In. We achieve smooth surfaces and sharp interfaces with a quantum well thickness tunable from 6 to 16 nm and a barrier thickness ranging from 4 to 10 nm. X-ray diffraction confirms >99% phase purity of our cubic layers, while time-of-flight secondary ion mass spectrometry, scanning transmission electron microscopy, and energy-dispersive X-ray spectroscopy provide detailed information on the quantum well composition and strain. Photoluminescence measurements at room temperature demonstrate the emission properties of the samples, with the emission wavelength ranging from 540 to 670 nm. Changing the barrier and QW thickness results in a shift of emission energy of up to 400 meV, which is explained by quantum confinement and strain. The high interface quality and excellent optical properties of the quantum wells without the need for multiple metal sources or long waiting times represent a significant advance in the development of next-generation optoelectronic devices.

Rabi splitting is a defining signature of strong light-matter interaction, emerging when a two-level system is resonantly driven by an optical field, resulting in a spectral doublet separated by the Rabi energy. In solid-state systems, Rabi splitting occurs at exciton resonances, where it is shaped by many-body interactions intrinsic to the material. Here, we investigate the Rabi splitting dynamics in two paradigmatic two-dimensional semiconductors: a hBN-encapsulated MoSe2 monolayer and a (Ga,In)As multiple quantum well structure. In MoSe2, strong Coulomb interactions dominate over light-matter coupling, while in the quantum wells, both interactions are of comparable strength. While both systems exhibit clear Rabi splitting under resonant excitation, their behavior diverges under increased excitation strength. MoSe2 displays sublinear Rabi splitting due to excitonic correlations, whereas (Ga,In)As quantum wells reveal additional spectral resonances and coherent optical gain, indicating a transition beyond the simple two-level regime. These contrasting behaviors are quantitatively captured by a unified microscopic many-body theory based on Heisenberg equations of motion and an exciton expansion. Our findings elucidate the impact of many-body interactions on coherent exciton dynamics and establish a framework for tailoring strong-field optical responses in two-dimensional materials.

Excitations in spatially indirect transitions feature such as excitons in type-II heterostructures or charge-transfer excitations in molecular crystals feature a permanent dipole moment which influences the transition dipole moments commonly probed by optical spectroscopy. Epitaxially grown III-V quantum heterostructures featuring suitable band alignments are ideal model systems to study the interplay between the two. Selected topics discussed include the AC Stark effect and signatures of spatially indirect coherent biexciton states as well as the potential for coherent optical current injection dynamics.