We investigate monolayer MoS_2xSe_2(1-x) alloys across the full composition range using optical spectroscopy. We demonstrate continuous tuning of the optical gap over ∼0.35 eV, accompanied by a systematic reduction of the B–A exciton splitting, in agreement with density functional theory calculations. Temperature-dependent measurements reveal a progressive increase of the average phonon energy from Se-rich to S-rich alloys that follows a simple reduced-mass scaling model. Polarization-resolved spectroscopy further shows a monotonic increase of the circular polarization from nearly zero in MoSe_2 to ∼15% in MoS_2 at 78 K. The observed evolution of the polarization is attributed to alloy-induced modifications of the electronic structure that modify bright–dark exciton mixing and the associated valley depolarization. These findings establish alloy engineering as an effective strategy for controlling excitonic properties in TMD monolayers.

The stability of all-inorganic metal halide perovskite anodes is critically dependent on the electrode fabrication method, a factor often overlooked. This work compares the structural and electrochemical performance of slurry-cast versus solvent-free dry-processed anodes using CsPbBr3 (lead-based) and Cs2AgBiBr6 (lead-free) perovskites. We find that the conventional slurry process induces a catastrophic degradation of the CsPbBr3 structure into its constituent products before cycling, whereas the dry-processing method successfully preserves the pristine perovskite phase. Consequently, the dry-fabricated CsPbBr3 electrode exhibits excellent cycling stability driven by a reversible Li-Pb alloying mechanism, significantly outperforming the rapidly fading Cs2AgBiBr6 anode. These findings demonstrate that for this class of materials, optimizing the fabrication process to prevent chemical degradation is a primary and critical step toward achieving stable electrochemical performance.

Semiconductor-based electrocatalysts with tunable electronic structures and efficient interfacial charge transfer hold great promise for sustainable energy conversion. In this study, we present a novel nanostructured electrocatalyst composed of (3-Co(OH)2 nanolayers uniformly anchored on CdIn2S4 (CIS) thiospinel nanoparticles, synthesized via a combined hydrothermal and photochemical deposition strategy. Comprehensive structural and spectroscopic analyses, supported by theoretical calculations, demonstrate the formation of a well-defined p-n heterojunction at the (3-Co(OH)2/CIS interface, which promotes directional charge transport and reduces interfacial resistance, collectively accelerating oxygen evolution reaction (OER) kinetics. The improved electrocatalytic activity arises from the synergistic effects of interfacial electric field modulation, topotactic electrochemical transformation of (3-Co(OH)2 into catalytically active (3-CoOOH, and stabilization of high-valent Co4+ species under anodic polarization. The optimized 10 wt% Co@CIS catalyst achieves a low overpotential of 294 mV at 10 mA cm- 2, a small Tafel slope of 61.2 mV dec- 1, and excellent operation stability over 100 h in alkaline electrolyte. When integrated into a two-electrode alkaline water electrolyzer, it enables overall water splitting at a cell voltage of only 1.60 V at 10 mA cm- 2. These results highlight the critical importance of interfacial engineering in designing high-performance, durable and cost-effective electrocatalysts for renewable energy conversion technologies.

The influence of a photochemical doping method on the spin-valley polarization degree (Pc) of excitons in WSe2 monolayers is reported. By varying the carrier density and transitioning from an excess of electrons (n-type) to an excess of holes (p-type), a non-monotonic dependence of Pc on the doping level is observed. Using controlled, single-shot photochlorination steps, this non-monotonic behavior is unveiled, with Pc reaching a minimum value of less than 10% at 78 K near the charge neutrality point, while increasing by a factor of three at a hole density of 5 x 1011 cm-2. The impact of the doping on Pc is explained using a phenomenological model that accounts for various mechanisms influencing exciton polarization dynamics, including exciton-carrier scattering processes and exciton-to-trion conversion rates. Among these, exciton-carrier collisions emerge as the dominant mechanism driving the observed variations in Pc, while the exciton effective lifetime remains nearly independent of doping. These findings highlight the potential of photochemical methods for investigating valley physics and for effectively tuning the exciton polarization degree in transition metal dichalcogenide monolayers.

Silicon-based dielectric nanoantennas provide an effective platform for engineering light-matter interactions in van der Waals semiconductors. Here, we demonstrate near-field coupling between monolayer MoS2 and silicon nanoantennas arranged in hexagonal lattices with tunable geometric parameters, leading to a three-fold enhancement in photoluminescence and an excitation-wavelength-dependent emission that aligns with Mie-resonant modes. Raman spectroscopy reveals an up to 8-fold enhancement in the vibrational modes of MoS2, while second-harmonic generation exhibits a 20 to 30-fold increase in efficiency, closely correlating with the presence of the underlying nanoantennas. Our experiments and simulations quantify the tunable benefits of the near-field interactions, taking into account thin-film interference and strain-induced effects. Our findings present dielectric nanoantennas as a promising platform for tailoring linear and nonlinear optical properties in 2D materials, with potential applications in nanophotonic devices and integrated photonics.

Nanoscale-engineered surfaces induce regulated strain in atomic layers of 2D materials that could be useful for unprecedented photonics applications and for storing and processing quantum information. Nevertheless, these strained structures need to be investigated extensively. Here, we present texture-induced strain distribution in single-layer WS2 (1L-WS2) transferred over Si/SiO2 (285 nm) substrate. The detailed nanoscale landscapes and their optical detection are carried out through Atomic Force Microscopy, Scanning Electron Microscopy, and optical spectroscopy. Remarkable differences have been observed in the WS2 sheet localized in the confined well and at the periphery of the cylindrical geometry of the capped engineered surface. Raman spectroscopy independently maps the whole landscape of the samples, and temperature-dependent helicity-resolved photoluminescence (PL) experiments (off-resonance excitation) show that suspended areas sustain circular polarization from 150 K up to 300 K, in contrast to supported (on un-patterned area of Si/SiO2) and strained 1L-WS2. Our study highlights the impact of the dielectric environment on the optical properties of two-dimensional (2D) materials, providing valuable insights into the selection of appropriate substrates for implementing atomically thin materials in advanced optoelectronic devices.

The in-plane anisotropic properties of two-dimensional (2D) group IV monochalcogenides provide an additional degree of freedom which can be used in future optoelectronic devices. Here, it is shown that the third harmonic generation (THG) signal produced by ultrathin tin (II) sulfide (SnS) is in-plane anisotropic with respect to the incident linear polarization of the laser field. We fit the experimental polarization-resolved THG (P-THG) measurements with a nonlinear optics model, which accounts for the orthorhombic crystal structure of 2D SnS. We calculate the relative magnitudes of the \{chi}^(3) tensor components by recording and simultaneously fitting both orthogonal components of the P-THG intensity. Furthermore, we introduce a THG anisotropy ratio, whose calculated values compare the total THG intensity when the excitation linear polarization is along the armchair crystallographic direction with the case when it is along the zigzag direction. Our results provide quantitative information on the anisotropic nature of the THG process in SnS, paving the way to a better understanding of anisotropic nonlinear light-matter interactions, and the development of polarization-sensitive nonlinear optical devices.

Two-dimensional (2D) graphene and graphene-related materials (GRMs) show great promise for future electronic devices. GRMs exhibit distinct properties under the influence of the substrate that serves as support through uneven compression/ elongation of GRMs surface atoms. Strain in GRM monolayers is the most common feature that alters the interatomic distances and band structure, providing a new degree of freedom that allows regulation of their electronic properties and introducing the field of straintronics. Having an all-optical and minimally invasive detection tool that rapidly probes strain in large areas of GRM monolayers, would be of great importance in the research and development of novel 2D devices. Here, we use Polarization-resolved Second Harmonic Generation (P-SHG) optical imaging to identify strain distribution, induced in a single layer of WS2 placed on a pre-patterned Si/SiO2 substrate with cylindrical wells. By fitting the P-SHG data pixel-by-pixel, we produce spatially resolved images of the crystal armchair direction. In regions where the WS2 monolayer conforms to the pattern topography, a distinct cross-shaped pattern is evident in the armchair image owing to strain. The presence of strain in these regions is independently confirmed using a combination of atomic force microscopy and Raman mapping.

We perform micro-photoluminescence and Raman experiments to examine the impact of biaxial tensile strain on the optical properties of WS2 monolayers. A strong shift on the order of −130 meV per % of strain is observed in the neutral exciton emission at room temperature. Under near-resonant excitation, we measure a monotonic decrease in the circular polarization degree under the applied strain. We experimentally separate the effect of the strain-induced energy detuning and evaluate the pure effect coming from the biaxial strain. The analysis shows that the suppression of the circular polarization degree under the biaxial strain is related to an interplay of energy and polarization relaxation channels as well as to variations in the exciton oscillator strength affecting the long-range exchange interaction.

Transition metal dichalcogenide (TMD) monolayers (1L) in the 2H-phase are two-dimensional semiconductors with two valleys in their band structure that can be selectively populated using circularly polarized light. The choice of the substrate for monolayer TMDs is an essential factor for the optoelectronic properties and for achieving a high degree of valley polarization at room temperature (RT). In this work, we investigate the RT valley polarization of monolayer WS 2 on different substrates. A degree of polarization of photoluminescence (PL) in excess of 27% is found from neutral excitons in 1L-WS 2 on graphite at RT, under resonant excitation. Using chemical doping through photochlorination we modulate the polarization of the neutral exciton emission from 27% to 38% for 1L-WS 2 /graphite. We show that the valley polarization strongly depends on the interplay between doping and the choice of the supporting layer of TMDs. Time-resolved PL measurements, corroborated by a rate equation model accounting for the bright exciton population in the presence of a dark exciton reservoir support our findings. These results suggest a pathway towards engineering valley polarization and exciton lifetimes in TMDs, by controlling the carrier density and/or the dielectric environment at ambient conditions.

Modulation of the Fermi level using an ultraviolet (UV)-assisted photochemical method is demonstrated in tungsten diselenide monolayers. Systematic shifts and relative intensities between charged and neutral exciton species indicate a progressive and controllable decrease of the electron density and switch tungsten diselenide from n-type to a p-type semiconductor. The presence of chlorine in the 2D crystal shifts the Fermi level closer to the valence band while the effect can be only partially reversible via continuous wave laser rastering process. The presence of chlorine species in the lattice is validated by X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations predict that adsorption of chlorine on the selenium vacancy sites leads to p-type doping. The results of our study indicate that photochemical techniques have the potential to enhance the performance of various 2D materials, making them suitable for potential applications in optoelectronics.

Modulation of the Fermi level using an ultraviolet (UV)-assisted photochemical method is demonstrated in tungsten diselenide monolayers. Systematic shifts and relative intensities between charged and neutral exciton species indicate a progressive and controllable decrease of the electron density and switch tungsten diselenide from n-type to a p-type semiconductor. The presence of chlorine in the 2D crystal shifts the Fermi level closer to the valence band while the effect can be only partially reversible via continuous wave laser rastering process. Chlorine species in the lattice are validated by x-ray photoelectron spectroscopy, while density functional theory calculations predict that adsorption of chlorine on the selenium vacancy sites leads to p-type doping. The results of our study indicate that photochemical techniques have the potential to enhance the performance of various 2D materials, making them suitable for integrated optoelectronics such as lateral nanopatterned p-n junctions.

Two-dimensional (2D) tin(II) sulfide (SnS) crystals belong to a class of orthorhombic semiconducting materials that are lately attracting significant interest, given their remarkable properties, such as in-plane anisotropic optical and electronic response, multiferroic nature and lack of inversion symmetry. The 2D SnS crystals exhibit anisotropic response along the in-plane armchair (AC) and zigzag (ZZ) crystallographic directions, offering an additional degree of freedom in manipulating their behavior. Therefore, calculating the AC/ZZ directions is important in characterizing the 2D SnS crystals. In this work, we take advantage of the lack of inversion symmetry of the 2D SnS crystal, that produces second harmonic generation (SHG), to perform polarization-resolved SHG (P-SHG) nonlinear imaging of the in-plane anisotropy. We fit the P-SHG experimental data with a nonlinear optics model, that allows us to calculate the AC/ZZ orientation from every point of the 2D crystal and to map with high-resolution the AC/ZZ direction of several 2D SnS flakes belonging in the same field of view. It is found that the P-SHG intensity polar patterns are associated with the crystallographic axes of the flakes and with the relative strength of the second order nonlinear susceptibility tensor in different directions. Therefore, our method provides quantitative information of the optical in-plane anisotropy of orthorhombic 2D crystals, offering great promise for performance characterization during device operation in the emerging optoelectronic applications of such crystals.

Transition Metal Dichalcogenides (TMDs) of the form MX 2 (where M=Mo or W and X=S, Se, Te), are a special class of 2D-layered materials [1]. Unlike their 3D-counterparts that are indirect gap semiconductors, single layers of MX 2 have a direct-gap, with tremendous consequences in the PL quantum yield [2]. In addition, TMDs are characterized by valley dependent optical selection rules that make them ideal candidates to store and process quantum information (valleytronics), due to broken inversion symmetry in combination with time reversal symmetry [3,4]. In this work, the effect of doping (i.e., the electron/hole density) on the optoelectronic properties of single layers of WSe 2 is investigated. Photochemical doping is realized by intense UV laser pulses in a gas environment that provides the dopant atoms or molecules. By controlling systematically, the irradiation parameters, it is possible to control the doping level and consequently the electronic band structure of the monolayer crystal [5]. In the case of WSe 2 , photochlorination induces n-type doping that is a partially reversible process achieved by a raster scanning procedure using a continuous wavelength laser. In addition, a significant increase in the spin valley polarization was achieved in accordance with an increase of the electron density after the photochemical doping [6,7].

Despite that organic-inorganic lead halide perovskites have attracted enormous scientific attention for energy conversion applications over the recent years, the influence of temperature and the type of the employed hole transport layer (HTL) on the charge carrier dynamics and recombination processes in perovskite photovoltaic devices is still largely unexplored. In particular, significant knowledge is missing on how these crucial parameters for radiative and non-radiative recombinations, as well as for efficient charge extraction vary among different perovskite crystalline phases that are induced by temperature variation. Herein, we perform micro photoluminescence (μPL) and ultrafast time resolved transient absorption spectroscopy (TAS) in Glass/Perovskite and two different Glass/ITO/HTL/Perovskite configurations at temperatures below room temperature, in order to probe the charge carrier dynamics of different perovskite crystalline phases, while considering also the effect of the employed HTL polymer. Namely, CH3NH3PbI3 films were deposited on Glass, PEDOT:PSS and PTAA polymers, and the developed Glass/CH3NH3PbI3 and Glass/ITO/HTL/CH3NH3PbI3 architectures were studied from 85 K up to 215 K in order to explore the charge extraction dynamics of the CH3NH3PbI3 orthorhombic and tetragonal crystalline phases. It is observed an unusual blueshift of the bandgap with temperature and the dual emission at temperature below of 100 K and also, that the charge carrier dynamics, as expressed by hole injection times and free carrier recombination rates, are strongly depended on the actual pervoskite crystal phase, as well as, from the selected hole transport material.

This cover illustration represents a laser field interacting with an ultrathin SnS crystal and the production of a second harmonic generation (SHG) field. Two representative polar diagrams of the polarization-resolved SHG intensity from different SnS crystals, and the fitting with the nonlinear optics model, are also shown, enabling the extraction of quantitative information on the in-plane anisotropy in SnS. For further details see article number 2102776 by Emmanuel Stratakis and co-workers.

The sufficient control of the carrier density of a single layer WS2 (1L-WS2) has been realized by the pulsed laser irradiation doping technique. Chlorine atoms are incorporated on the surface of the atomically thin lattice in a precursor gas atmosphere. In this work, we demonstrate spin-valley polarization tunability by more than 40% in 1L-WS2 on hBN via photochlorination. Polarization photoluminescence spectroscopy was performed in the temperature range from 4K to 300K. The decrease in circular polarization after the photochlorination treatment is attributed to the significant reduction of the active defect sites in 1L-WS2 and, consequently, to the increase in the non-radiative exciton lifetime. Ultrafast time-resolved transient absorption spectroscopy measurements support our findings. The above results indicate a useful approach of controlling the density of the active defect sites and the valley polarized light emission in doped monolayer crystal lattices.

Degenerate minima in momentum space-valleys-provide an additional degree of freedom that can be used for information transport and storage. Notably, such minima naturally exist in the band structure of transition metal dichalcogenides (TMDs). When these atomically thin crystals interact with intense laser light, the second harmonic generated (SHG) field inherits special characteristics that reflect not only the broken inversion symmetry in real space but also the valley anisotropy in reciprocal space. The latter is present whenever there exists a valley population imbalance (VPI) between the two valleys and affects the polarization state of the detected SHG. In this work, it is shown that the temperature-induced change of the SHG intensity dependence on the excitation field polarization is a fingerprint of VPI in TMDs. In particular, pixel-by-pixel VPI mapping based on polarization-resolved raster-scanning imaging microscopy was performed inside a cryostat to generate the SHG contrast in the presence of VPI from every point of a TMD flake. The generated contrast is marked by rotation of the SHG intensity polar diagrams at low temperatures and is attributed to the VPI-induced SHG.

Atomically thin two-dimensional (2D) materials can be vertically stacked with van der Waals bonds, which enable interlayer coupling. In the particular case of transition metal dichalcogenide (TMD) bilayers, the relative direction between the two monolayers, coined as twist-angle, modifies the crystal symmetry and creates a superlattice with exciting properties. Here, we demonstrate an all-optical method for pixel-by-pixel mapping of the twist-angle with a resolution of 0.55(°), via polarization-resolved second harmonic generation (P-SHG) microscopy and we compare it with four-dimensional scanning transmission electron microscopy (4D STEM). It is found that the twist-angle imaging of WS2 bilayers, using the P-SHG technique is in excellent agreement with that obtained using electron diffraction. The main advantages of the optical approach are that the characterization is performed on the same substrate that the device is created on and that it is three orders of magnitude faster than the 4D STEM. We envisage that the optical P-SHG imaging could become the gold standard for the quality examination of TMD superlattice-based devices.