Electronic Band Structure and Optical Properties of HgPS3 Crystal and Layers.

Transition metal thiophosphates (MPS3) are of great interest due to their layered structure and magnetic properties. Although HgPS3 may not exhibit magnetic properties, its uniqueness lies in its triclinic crystal structure and in the substantial mass of mercury, rendering it a compelling subject for exploration in terms of fundamental properties. In this work, we present comprehensive experimental and theoretical studies of the electronic band structure and optical properties for the HgPS3 crystal and mechanically exfoliated layers from a solid crystal. Based on absorption, reflectance and photoluminescence measurements supported by theoretical calculations, it is shown that the HgPS3 crystal has an indirect gap of 2.68 eV at room temperature. The direct gap is identified at the Γ point of the Brillouin zone (BZ) ≈ 50 meV above the indirect gap. The optical transition at the Γ point is forbidden due to selection rules, but the oscillator strength near the Γ point increases rapidly and therefore the direct optical transitions are visible in the reflectance spectra approximately at 60-120 meV above the absorption edge, across the temperature range of 40 to 300 K. The indirect nature of the bandgap and the selection rules for Γ point contribute to the absence of near-bandgap emission in HgPS3. Consequently, the photoluminescence spectrum is primarily governed by defect-related emission. The electronic band structure of HgPS3 undergoes significant changes when the crystal thickness is reduced to tri- and bilayers, resulting in a direct bandgap. Interestingly, in the monolayer regime, the fundamental transition is again indirect. The layered structure of the HgPS3 crystal was confirmed by scanning electron microscopy (SEM) and by mechanical exfoliation.

The Defects Genome of Janus Transition Metal Dichalcogenides.

2D Janus Transition Metal Dichalcogenides (TMDs) have attracted much interest due to their exciting quantum properties arising from their unique two-faced structure, broken-mirror symmetry, and consequent colossal polarization field within the monolayer. While efforts are made to achieve high-quality Janus monolayers, the existing methods rely on highly energetic processes that introduce unwanted grain-boundary and point defects with still unexplored effects on the material's structural and excitonic properties Through high-resolution scanning transmission electron microscopy (HRSTEM), density functional theory (DFT), and optical spectroscopy measurements; this work introduces the most encountered and energetically stable point defects. It establishes their impact on the material's optical properties. HRSTEM studies show that the most energetically stable point defects are single (VS and VSe) and double chalcogen vacancy (VS -VSe), interstitial defects (Mi), and metal impurities (MW) and establish their structural characteristics. DFT further establishes their formation energies and related localized bands within the forbidden band. Cryogenic excitonic studies on h-BN-encapsulated Janus monolayers offer a clear correlation between these structural defects and observed emission features, which closely align with the results of the theory. The overall results introduce the defect genome of Janus TMDs as an essential guideline for assessing their structural quality and device properties.

Waveguide-Coupled Light Photodetector Based on Two-Dimensional Molybdenum Disulfide.

The integration of transition metal dichalcogenides with photonic structures such as sol-gel SiOx:TiOy optical waveguides (WGs) makes possible the fabrication of photonic devices with the desired characteristics in the visible spectral range. In this study, we propose and experimentally demonstrate a MoS2-based photodetector integrated with a sol-gel SiOx:TiOy WG. Based on the spectroscopic measurements performed for our device, we concluded that the light entering the WG is almost completely channeled out from the WG and absorbed by the MoS2 flake, which is deposited on the WG. Therefore, this device works as a photodetector. The light coupling into the MoS2 region in this device construction is due to the high contrast of refractive index between the van der Waals crystal and the sol-gel WG, which is ∼4 and ∼1.8, respectively. The obtained MoS2-based photodetectors exhibit a photoresponsivity of 0.3 A W-1 (n-type MoS2) and 7.53 mA W-1 (p-type MoS2) at a bias voltage of 2 V. These results reveal great potential in the integration of sol-gel WGs with van der Waals crystals in optoelectronic applications.

Revisiting a (001)-oriented layered lead chloride templated by 1,2,4-triazolium: structural phase transitions, lattice dynamics and broadband photoluminescence.

This study revisits a (001)-oriented layered lead chloride templated by 1,2,4-triazolium, Tz2PbCl4, which recently has been an object of intense research but still suffers from gaps in characterization. Indeed, the divergent reports on the crystal structures of Tz2PbCl4 at various temperatures, devoid of independent verification of chiral phases through second harmonic generation (SHG), have led to an unresolved debate regarding the existence of a low-temperature phase transition (PT) and the noncentrosymmetric nature of the low-temperature phase. Now, by combining differential scanning calorimetry, single-crystal X-ray diffraction, dielectric, as well as linear and nonlinear optical spectroscopies on Tz2PbCl4, we reveal a sequence of reversible PTs at T1 = 361 K (phase I-II), T2 = 339 K (phase II-III), and T3 = 280 K (phase III-IV). No SHG activity could be registered for any of the four crystal phases, as checked by wide-temperature range SHG screening, supporting their centrosymmetry. The dipole relaxation processes indicate a decrease in activation energy with increasing temperature, from 0.60, 0.38, to 0.24 eV observed for phase IV (space group P21/c), phase III (Pnma), and phase II (Cmcm), respectively. This change is interpreted as a result of the diminishing strength of H-bonds as the system transforms from phase IV to III and subsequently to II. The weaker H-bonds facilitate the reorientation of Tz+ cations in the presence of an external electric field. The photoluminescence spectra of Tz2PbCl4 reveal an intriguing interplay of narrow and broadband emission, linked respectively to free excitons and excitons trapped on defects. Notably, as the temperature decreases from 300 K to 16 K, both the emission bands exhibit distinctive blue and red shifts, indicative of increased in-plane octahedral distortion. This dynamic behaviour transforms the photoluminescence of Tz2PbCl4 from greenish-blue at 300 K to yellowish-green at 13 K, enriching our understanding of 2D lead halide perovskites and highlighting the optoelectronic potential of Tz2PbCl4.

Large-Area, Flexible, Lead-Free Sn-Perovskite Solar Modules.

For the first time, large-area, flexible organic-inorganic tin perovskite solar modules are fabricated by means of an industry-compatible and scalable blade-coating technique. An 8-cell interconnected mini module with dimensions of 25 cm2 (active area = 8 × 1.5 cm2) reached 5.7% power conversion efficiency under 1000 W/m2 (AM 1.5G) and 9.4% under 2000 lx (white-LED).

Stability of mechanically exfoliated layered monochalcogenides under ambient conditions.

Monochalcogenides of groups III (GaS, GaSe) and VI (GeS, GeSe, SnS, and SnSe) are materials with interesting thickness-dependent characteristics, which have been applied in many areas. However, the stability of layered monochalcogenides (LMs) is a real problem in semiconductor devices that contain these materials. Therefore, it is an important issue that needs to be explored. This article presents a comprehensive study of the degradation mechanism in mechanically exfoliated monochalcogenides in ambient conditions using Raman and photoluminescence spectroscopy supported by structural methods. A higher stability (up to three weeks) was observed for GaS. The most reactive were Se-containing monochalcogenides. Surface protrusions appeared after the ambient exposure of GeSe was detected by scanning electron microscopy. In addition, the degradation of GeS and GeSe flakes was observed in the operando experiment in transmission electron microscopy. Additionally, the amorphization of the material progressed from the flake edges. The reported results and conclusions on the degradation of LMs are useful to understand surface oxidation, air stability, and to fabricate stable devices with monochalcogenides. The results indicate that LMs are more challenging for exfoliation and optical studies than transition metal dichalcogenides such as MoS2, MoSe2, WS2, or WSe2.

Surface Engineering of Methylammonium Lead Bromide Perovskite Crystals for Enhanced X-ray Detection.

The surface quality of lead halide perovskite crystals can extremely influence their optoelectronic properties and device performance. Here, we report a surface engineering crystallization technique in which we in situ grow a polycrystalline methylammonium lead tribromide (MAPbBr3) film on top of bulk mm-sized single crystals. Such MAPbBr3 crystals with a MAPbBr3 passivating film display intense green emission under UV light. X-ray photoelectron spectroscopy demonstrates that these crystals with emissive surfaces are compositionally different from typical MAPbBr3 crystals that show no emission under UV light. Time-resolved photoluminescence and electrical measurements indicate that the MAPbBr3 film/MAPbBr3 crystals possess less surface defects compared to the bare MAPbBr3 crystals. Therefore, X-ray detectors fabricated using the surface-engineered MAPbBr3 crystals provide an almost 5 times improved sensitivity to X-rays and a more stable baseline drift with respect to the typical MAPbBr3 crystals.

Bandgap Pressure Coefficient of a CH3NH3PbI3 Thin Film Perovskite.

Recent scientific interest in examining the bandgap evolution of a MAPbI3 hybrid perovskite by applying hydrostatic pressure has mostly focused on a room-temperature tetragonal phase. In contrast, the pressure response of a low-temperature orthorhombic phase (OP) of MAPbI3 has not been explored and understood. In this research, we investigate for the first time how hydrostatic pressure alters the electronic landscape of the OP of MAPbI3. Pressure studies using photoluminescence combined with calculations within density functional theory at zero temperature allowed us to identify the main physical factors affecting the bandgap evolution of the OP of MAPbI3. The negative bandgap pressure coefficient was found to be strongly dependent on the temperature (α120K = -13.3 ± 0.1 meV/GPa, α80K = -29.8 ± 0.1 meV/GPa, and α40K = -36.3 ± 0.1 meV/GPa). Such dependence is related to the changes in the Pb-I bond length and geometry in the unit cell as the atomic configuration approaches the phase transition as well as the increasing phonon contribution to octahedral tilting as the temperature increases.

The influence of Fermi level position at the GaN surface on carrier transfer across the MAPbI3/GaN interface.

Both gallium nitride (GaN) and hybrid organic-inorganic perovskites such as methylammonium lead iodide (MAPbI3) have significantly influenced modern optoelectronics. Both marked a new beginning in the development of important branches in the semiconductor industry. For GaN, it is solid-state lighting and high-power electronics, and for MAPbI3, it is photovoltaics. Today, both are widely incorporated as building blocks in solar cells, LEDs and photodetectors. Regarding multilayers, and thus multi-interfacial construction of such devices, an understanding of the physical phenomena governing electronic transport at the interfaces is relevant. In this study, we present the spectroscopic investigation of carrier transfer across the MAPbI3/GaN interface by contactless electroreflectance (CER) for n-type and p-type GaN. The effect of MAPbI3 on the Fermi level position at the GaN surface was determined which allowed us to draw conclusions about the electronic phenomena at the interface. Our results show that MAPbI3 shifts the surface Fermi level deeper inside the GaN bandgap. Regarding different surface Fermi level positions for n-type and p-type GaN, we explain this as carrier transfer from GaN to MAPbI3 for n-type GaN and in the opposite direction for p-type GaN. We extend our outcomes with a demonstration of a broadband and self-powered MAPbI3/GaN photodetector.

Crack-Templated Wire-Like Semitransparent Electrodes with Unique Irregular Patterns.

The development of novel methods of producing transparent electrodes is important because of their ever-evolving applications and thus the additional parameters they must meet. In this work, we present a method of manufacturing semitransparent silver electrodes. This technique involves cracking the polyvinylpyrrolidone layer in the presence of a colloidal nanodispersion of zinc oxide. The resulting cracked polymer layer serves as the disposable mask for metal deposition. The whole procedure is valuable due to the fast and easy step of cracks formation caused by the elevated temperature and reduced pressure. The obtained electrodes have high transparency (82.4%) in a wide spectral range, which is only limited by the transparency of the applied substrate, and low resistivity (27.3 × 10-7 Ωm). The presence of unique patterns suggests new ideas for the applications of such electrodes, such as coding, security, and antiplagiarism protection.

Mixology of MA1-x EA x PbI3 Hybrid Perovskites: Phase Transitions, Cation Dynamics, and Photoluminescence.

Mixing molecular cations in hybrid lead halide perovskites is a highly effective approach to enhance the stability and performance of optoelectronic devices based on these compounds. In this work, we prepare and study novel mixed 3D methylammonium (MA)-ethylammonium (EA) MA1-x EA x PbI3 (x < 0.4) hybrid perovskites. We use a suite of different techniques to determine the structural phase diagram, cation dynamics, and photoluminescence properties of these compounds. Upon introduction of EA, we observe a gradual lowering of the phase-transition temperatures, indicating stabilization of the cubic phase. For mixing levels higher than 30%, we obtain a complete suppression of the low-temperature phase transition and formation of a new tetragonal phase with a different symmetry. We use broad-band dielectric spectroscopy to study the dielectric response of the mixed compounds in an extensive frequency range, which allows us to distinguish and characterize three distinct dipolar relaxation processes related to the molecular cation dynamics. We observe that mixing increases the rotation barrier of the MA cations and tunes the dielectric permittivity values. For the highest mixing levels, we observe the signatures of the dipolar glass phase formation. Our findings are supported by density functional theory calculations. Our photoluminescence measurements reveal a small change of the band gap upon mixing, indicating the suitability of these compounds for optoelectronic applications.

Stress-Tuned Optical Transitions in Layered 1T-MX2 (M=Hf, Zr, Sn; X=S, Se) Crystals.

Optical measurements under externally applied stresses allow us to study the materials' electronic structure by comparing the pressure evolution of optical peaks obtained from experiments and theoretical calculations. We examine the stress-induced changes in electronic structure for the thermodynamically stable 1T polytype of selected MX2 compounds (M=Hf, Zr, Sn; X=S, Se), using the density functional theory. We demonstrate that considered 1T-MX2 materials are semiconducting with indirect character of the band gap, irrespective to the employed pressure as predicted using modified Becke-Johnson potential. We determine energies of direct interband transitions between bands extrema and in band-nesting regions close to Fermi level. Generally, the studied transitions are optically active, exhibiting in-plane polarization of light. Finally, we quantify their energy trends under external hydrostatic, uniaxial, and biaxial stresses by determining the linear pressure coefficients. Generally, negative pressure coefficients are obtained implying the narrowing of the band gap. The semiconducting-to-metal transition are predicted under hydrostatic pressure. We discuss these trends in terms of orbital composition of involved electronic bands. In addition, we demonstrate that the measured pressure coefficients of HfS2 and HfSe2 absorption edges are in perfect agreement with our predictions. Comprehensive and easy-to-interpret tables containing the optical features are provided to form the basis for assignation of optical peaks in future measurements.

Role of Temperature in Arsenic-Induced Antisurfactant Growth of GaN Microrods.

Due to the antisurfactant properties of arsenic atoms, the self-induced dodecagonal GaN microrods can be grown by molecular beam epitaxy (MBE) in Ga-rich conditions. Since temperature is a key parameter in MBE growth, the role of temperature in the growth of GaN microrods is investigated. The optimal growth temperature window for the formation of GaN microrods is observed to be between 760 and 800 °C. Lowering the temperature to 720 °C did not change the growth mechanism, but the population of irregular and amorphous microrods increased. On the other hand, increasing the growth temperature up to 880 °C interrupts the growth of GaN microrods, due to the re-evaporation of the gallium from the surface. The incorporation of As in GaN microrods is negligible, which is confirmed by X-ray diffraction and transmission electron microscopy. Moreover, the photoluminescence and cathodoluminescence characteristics typical for GaN are observed for individual GaN microrods, which additionally confirms that arsenic is not incorporated inside microrods. When the growth temperature is increased, the emission related to the band gap decreases in favor of the defect-related emission. This is typical for bulk GaN and attributed to an increase in the point defect concentration for GaN microrods grown at lower temperatures.

Strong Substrate Strain Effects in Multilayered WS2 Revealed by High-Pressure Optical Measurements.

The optical properties of two-dimensional materials can be effectively tuned by strain induced from a deformable substrate. In the present work we combine first-principles calculations based on density functional theory and the effective Bethe-Salpeter equation with high-pressure optical measurements to thoroughly describe the effect of strain and dielectric environment onto the electronic band structure and optical properties of a few-layered transition-metal dichalcogenide. Our results show that WS2 remains fully adhered to the substrate at least up to a -0.6% in-plane compressive strain for a wide range of substrate materials. We provide a useful model to describe effect of strain on the optical gap energy. The corresponding experimentally determined out-of-plane and in-plane stress gauge factors for WS2 monolayers are -8 and 24 meV/GPa, respectively. The exceptionally large in-plane gauge factor confirms transition metal dichalcogenides as very promising candidates for flexible functionalities. Finally, we discuss the pressure evolution of an optical transition closely lying to the A exciton for bulk WS2 as well as the direct-to-indirect transition of the monolayer upon compression.

Toward h-BN/GaN Schottky Diodes: Spectroscopic Study on the Electronic Phenomena at the Interface.

Hexagonal boron nitride (h-BN), together with other members of the van der Waals crystal family, has been studied for over a decade, both in terms of fundamental and applied research. Up to now, the spectrum of h-BN-based devices has broadened significantly, and systems containing the h-BN/III-V junctions have gained substantial interest as building blocks in, inter alia, light emitters, photodetectors, or transistor structures. Therefore, the understanding of electronic phenomena at the h-BN/III-V interfaces becomes a question of high importance regarding device engineering. In this study, we present the investigation of electronic phenomena at the h-BN/GaN interface by means of contactless electroreflectance (CER) spectroscopy. This nondestructive method enables precise determination of the Fermi level position at the h-BN/GaN interface and the investigation of carrier transport across the interface. CER results showed that h-BN induces an enlargement of the surface barrier height at the GaN surface. Such an effect translates to Fermi level pinning deeper inside the GaN band gap. As an explanation, we propose a mechanism based on electron transfer from GaN surface states to the native acceptor states in h-BN. We reinforced our findings by thorough structural characterization and demonstration of the h-BN/GaN Schottky diode. The surface barriers obtained from CER (0.60 ± 0.09 eV for GaN and 0.91 ± 0.12 eV for h-BN/GaN) and electrical measurements are consistent within the experimental accuracy, proving that CER is an excellent tool for interfacial studies of 2D/III-V hybrids.

Experimental and Theoretical Studies of the Electronic Band Structure of Bulk and Atomically Thin Mo1-x W x Se2 Alloys.

We present studies focused on the evolution of the electronic band structure of the Mo1-x W x Se2 alloy with the tungsten content, which was conducted by combining experimental and theoretical methods. Employed spectroscopic techniques, namely, photoreflectance, photoacoustic spectroscopy, and photoluminescence, allowed observing indirect and direct transitions at high and beyond high-symmetry points of the Brillouin zone (BZ). Two excitons (A and B) associated with the K point of the BZ were observed together with other optical transitions (C and D) related to band nesting. Moreover, we have also identified the indirect transition for the studied crystals. Obtained energies for all transitions were tracked with a tungsten content and compared with results of calculations performed within density functional theory. Furthermore, based on the mentioned comparison, optical transitions were assigned to specific regions of the BZ. Finally, we have obtained bowing parameters for experimentally observed features, for, i.e., thin-film samples: b(A) = 0.13 ± 0.03 eV, b(B) = 0.14 ± 0.03 eV, b(C) = 0.044 ± 0.008 eV, and b(D) = 0.010 ± 0.003 eV.

Kinetics of the radiative and nonradiative recombination in polar and semipolar InGaN quantum wells.

We studied mechanisms of recombination in InGaN quantum wells in polar and semipolar structures. Photoluminescence measurements show that the optical emission linewidths for polar and semipolar structures are almost identical suggesting the same level of indium fluctuations in quanutm wells. Their "peak-energy-versus-temperature" relations demonstrate very pronounced "s-shape" effect. Emission linewidth measured by cathodoluminescence does not depend on area from which the light is collected meaning that the fluctuations are smaller that 100 nm. The time scale of recombination process are of the order of 80 ns for polar and 2 ns for semipolar. Energy dispersion of the recombination time is strong in polar structures and very weak in semipolar ones which can be interperted in terms of electric field influence on photoluminescence lifetime energy dispersion. At room temparture emmission is dominated by Schockley-Hall-Read recombination and does not show any dispersion. Rate equation analysis of photoluminescence transients show domination of excitonic recombination in the case of polar samples (low temperature) and bimolecular in the case of semipolar ones. Both types of quantum wells, polar and semipolar look similar from the point of view of localization but differ in their radiative recombination mechanisms.

Exfoliated CrPS4 with Promising Photoconductivity.

Layered semiconductors have attracted significant attention due to their diverse physical properties controlled by composition and the number of stacked layers. Herein, large crystals of the ternary layered semiconductor chromium thiophosphate (CrPS4 ) are prepared by a vapor transport synthesis. Optical properties are determined using photoconduction, absorption, photoreflectance, and photoacoustic spectroscopy exposing the semiconducting properties of the material. A simple, one-step protocol for mechanical exfoliation onto a transmission electron microscope grid is developed, and multiple layers are characterized by advanced electron microscopy methods, including atomic resolution elemental mapping confirming the structure by directly showing the positions of the columns of different elements' atoms. CrPS4 is also liquid exfoliated, and in combination with colloidal graphene, an ink-jet-printed photodetector is created. This all-printed graphene/CrPS4 /graphene heterostructure detector demonstrates a specific detectivity of 8.3 × 108 (D*). This study shows a potential application of both bulk crystal and individual flakes of CrPS4 as active components in light detection, when introduced as ink-printable moieties with a large benefit for manufacturing.

Exciton Binding Energy of Two-Dimensional Highly Luminescent Colloidal Nanostructures Determined from Combined Optical and Photoacoustic Spectroscopies.

Reduced dimensionality of structures such as 0D quantum dots, 1D nanorods, and 2D nanoplatelets is predicted to favor the creation of tightly bound excitons stable at room temperature, making experimental determination of the exciton binding energy ( R x) crucial for evaluating the performance of semiconductor nanoparticles. We propose a fully optical approach for R x determination based on a complementary combination of photoacoustic and transmission spectra, using 5.5, 4.5, and 3.5 ML CdSe nanoplatelets as a benchmark system. The absence of excitonic features in photoacoustic spectra allows for probing the band-to-band transition, leading to the band gap determination. Such an unusual effect is explained by efficient re-emission of the absorbed radiation typical for high quantum yield structures, keeping the crystal lattice from excess phonon generation. The determined exciton binding energy for CdSe nanoplatelets ranges from 130 to 230 meV, confirming the presence of robust excitons in highly confined 2D systems.

Optical Properties of In2 xGa2-2 xO3 Nanowires Revealed by Photoacoustic Spectroscopy.

Group III oxides, such as In2O3 and Ga2O3, have proved to be good candidates as active materials for novel electronic devices, including high-mobility transistors, gas sensors, and UV photodetectors. The ability to tune optical and electronic properties is provided by alloying In2 xGa2-2 xO3 (InGaO) in a broad compositional range. Further development of InGaO compounds in the form of nanowires (NWs) would overcome the technological limitations, such as the substrate crystal lattice mismatch and the inability to fabricate high quality structures above the critical thickness. In this work, optical properties of alloyed InGaO NWs in a wide compositional range are carefully assessed. Unlike classical optical characterization methods, photoacoustic spectroscopy reveals the fundamental absorption edge despite the strong light scattering in porous and randomly oriented nanowires structure. An unusual compositional band gap dependence is also observed, giving insight into the phase segregation effect and increased quality of mixed NWs. In addition, photoacoustic measurements disclose potential applications of InGaO NWs in remote, light-driven loudspeakers because of intense photoacoustic effect in nanowire ensembles in this material system.