Band-edge Light Research Articles

Direct Z-scheme GaTe/SnS2 van der Waals heterojunction with tunable electronic properties: A promising highly efficient photocatalyst

The production of hydrogen through photocatalytic water splitting offers a secure and eco-friendly approach. In this study, the application of GaTe/SnS2 heterojunction in the field of photocatalysis was studied in depth by using the first-principles calculation method. Based on single-layer GaTe and SnS2, a novel two-dimensional GaTe/SnS2 van der Waals heterojunction (vdwH) was constructed. Through analytical calculations, it is obvious that the GaTe/SnS2 heterojunction has an indirect bandgap of 0.85eV. Furthermore, its type-II band alignment facilitates efficient separation of photogenerated electrons and holes across different layers. The application of Bader charge analysis revealed an intriguing observation: the GaTe layer transferred a charge of 0.058e to the SnS2 layer. This charge transfer led to the creation of a robust built-in electric field, which played a pivotal role in efficiently inhibiting the recombination of photogenerated electrons and holes. Under the condition of pH = 0, GaTe/SnS2 heterojunction can promote redox reaction and realize water splitting. In addition, when the biaxial strain of -3%–3 % is applied to the GaTe/SnS2 heterojunction, the band edge position and light absorption properties are effectively changed, and more photons participate in the water splitting process. More importantly, the solar-to-hydrogen (STH) efficiency of GaTe/SnS2 heterojunction reaches 56.6 %, and when ε = 3 %, ηSTH increases to 58.21 %. Hence, our research showcases the GaTe/SnS2 heterojunction's potential as a highly efficient Z-scheme photocatalyst for water splitting, offering promising prospects for hydrogen production.

One-Year Water-Stable and Porous Bi(III) Halide Semiconductor with Broad-Spectrum Antibacterial Performance.

Hybrid metal halide semiconductors are a unique family of materials with immense potential for numerous applications. For this to materialize, environmental stability and toxicity deficiencies must be simultaneously addressed. We report here a porous, visible light semiconductor, namely, (DHS)Bi2I8 (DHS = [2.2.2] cryptand), which consists of nontoxic, earth-abundant elements, and is water-stable for more than a year. Gas- and vapor-sorption studies revealed that it can selectively and reversibly adsorb H2O and D2O at room temperature (RT) while remaining impervious to N2 and CO2. Solid-state NMR measurements and density functional theory (DFT) calculations verified the incorporation of H2O and D2O in the molecular cages, validating the porous nature. In addition to porosity, the material exhibits broad band-edge light emission centered at 600 nm with a full width at half-maximum (fwhm) of 99 nm, which is maintained after 6 months of immersion in H2O. Moreover, (DHS)Bi2I8 exhibits bacteriocidal action against three Gram-positive and three Gram-negative bacteria, including antibiotic-resistant strains. This performance, coupled with the recorded water stability and porous nature, renders it suitable for a plethora of applications, from solid-state batteries to water purification and disinfection.

The Fine-Structure Constant as a Ruler for the Band-Edge Light Absorption Strength of Bulk and Quantum-Confined Semiconductors.

Low-dimensional semiconductors have found numerous applications in optoelectronics. However, a quantitative comparison of the absorption strength of low-dimensional versus bulk semiconductors has remained elusive. Here, we report generality in the band-edge light absorptance of semiconductors, independent of their dimensions. First, we provide atomistic tight-binding calculations that show that the absorptance of semiconductor quantum wells equals mπα (m = 1 or 2 with α as the fine-structure constant), in agreement with reported experimental results. Then, we show experimentally that a monolayer (superlattice) of quantum dots has similar absorptance, suggesting an absorptance quantum of mπα per (confined) exciton diameter. Extending this idea to bulk semiconductors, we experimentally demonstrate that an absorptance quantum equal to mπα per exciton Bohr diameter explains their widely varying absorption coefficients. We thus provided compelling evidence that the absorptance quantum πα per exciton diameter rules the band-edge absorption of all direct semiconductors, regardless of their dimension.

Density functional characterization of Bi-based photocatalysts: BiTaO4, Bi4Ta2O11 and Bi7Ta3O18

Using density-functional theory (DFT) calculations, we make a comparative study for the structural, electronic, and optical properties and photocatalysis of triclinic BiTaO4, orthorhombic BiTaO4, triclinic Bi4Ta2O11, and triclinic Bi7Ta3O18. We predict the photocatalytic capacity by analyzing the band edge position and light absorption. Besides, we calculate the effective masses of holes and electrons to compare the carriers’ mobility of the four bismuth tantalates. The optimized lattice parameters are obtained and crystal structures are analyzed. Electronic properties are studied by analyzing the band structures, partial density of state (PDOS), charge density, Mulliken population, and the hybridization intensity of Bi 6s-O 2p in the valence band maximum (VBM). Optical properties are investigated by calculating the optical absorption coefficient, optical refractive index, optical extinction coefficient, and dielectric function. By comparing the relative ratio of effective mass, we found that triclinic BiTaO4 and triclinic Bi7Ta3O18 not only have excellent mobility of carriers but also have excellent separation of photoexcited electron-hole pairs. Because of desirable mobility and separation of carriers, some absorption in visible light, and suitable redox potential, the four bismuth tantalates are predicted as promising catalyst candidates.

Ti-Alloying of BaZrS3 Chalcogenide Perovskite for Photovoltaics.

BaZrS3, a prototypical chalcogenide perovskite, has been shown to possess a direct band gap, an exceptionally strong near band edge light absorption, and good carrier transport. Coupled with its great stability, nontoxicity with earth-abundant elements, it is thus a promising candidate for thin film solar cells. However, its reported band gap in the range of 1.7–1.8 eV is larger than the optimal value required to reach the Shockley–Queisser limit of a single-junction solar cell. Here, we report the synthesis of Ba(Zr1–xTix)S3 perovskite compounds with a reduced band gap. It is found that Ti-alloying is extremely effective in band gap reduction of BaZrS3: a mere 4 atom % alloying decreases the band gap from 1.78 to 1.51 eV, resulting in a theoretical maximum power conversion efficiency of 32%. Higher Ti-alloying concentration is found to destabilize the distorted chalcogenide perovskite phase.

Slow light with interleaved p-n junction to enhance performance of integrated Mach-Zehnder silicon modulators

Abstract Slow light is a very important concept in nanophotonics, especially in the context of photonic crystals. In this work, we apply our previous design of band-edge slow light in silicon waveguide gratings [M. Passoni et al, Opt. Express 26, 8470 (2018)] to Mach-Zehnder modulators based on the plasma dispersion effect. The key idea is to employ an interleaved p-n junction with the same periodicity as the grating, in order to achieve optimal matching between the electromagnetic field profile and the depletion regions of the p-n junction. The resulting modulation efficiency is strongly improved as compared to common modulators based on normal rib waveguides, even in a bandwidth of 20–30 nm near the band edge, while the total insertion loss due to free carriers is not increased. The present concept is promising in view of realizing slow-light modulators for silicon photonics with reduced energy dissipation.

Growth of BiVO4 nanoparticles on a WO3 porous scaffold: improved water-splitting by high band-edge light harvesting

We demonstrate a heterojunction photoanode of WO3 coated with BiVO4 nanoparticles with high band-edge light absorption.

Optoelectronic properties of single-crystalline GaInAsSb quaternary alloy nanowires**Project supported by the National Natural Science Foundation of China (Grant Nos. 51525202, 61505051, 1137049, 61474040, and 61635001), the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province, China, and the Fundamental Research Funds for the Central Universities, China.

Bandgap engineering of semiconductor nanomaterials is critical for their applications in nanoelectronics, optoelectronics, and photonics. Here we report, for the first time, the growth of single-crystalline quaternary alloyed Ga0.75In0.25As0.49Sb0.51 nanowires via a chemical-vapor-deposition method. The synthesized nanowires have a uniform composition distribution along the growth direction, with a zinc-blende structure. In the photoluminescence investigation, these quaternary alloyed semiconductor nanowires show a strong band edge light emission at 1950 nm (0.636 eV). Photodetectors based on these alloy nanowires show a strong light response in the near-infrared region (980 nm) with the external quantum efficiency of 2.0 × 104% and the responsivity of 158 A/W. These novel near-infrared photodetectors may find promising applications in integrated infrared photodetection, information communication, and processing.

Optimizing band-edge slow light in silicon-on-insulator waveguide gratings.

A systematic analysis of photonic bands and group index in silicon grating waveguides is performed, in order to optimize band-edge slow-light behavior in integrated structures with low losses. A combination of numerical methods and perturbation theory is adopted. It is shown that a substantial increase of slow light bandwidth is achieved when decreasing the internal width of the waveguide and the silicon thickness in the cladding region. It is also observed that a reduction of the internal width does not undermine the performance of an adiabatic taper.

Photoinduced Charge Transfer in Poly(3-hexylthiophene)/TiO2 Hybrid Inverse Opals: Photonic vs Interfacial Effects

We study the effects of tailored light–matter interactions on charge transfer in conjugated polymer films. We use inverse opal structures of titania as the electron acceptor and the model polymer poly(3-hexylthiophene) (P3HT) as the electron donor. We systematically tune the periodicity of the inverse opal to study how the photophysical properties of the conformally coated P3HT are affected by the photonic stop band and band-edge light localization to observe both suppression and enhancement of absorption. Surprisingly, we observe changes in the vibronic coupling in the P3HT absorption spectra in the inverse opal structures as compared to control films. We determine that the polymer in the inverse opals shows more J-aggregate-like behavior with exciton bandwidths of 18 meV, compared to that of 124 meV for P3HT on a planar mesoporous TiO2 film. We also study charge transfer at the polymer/inorganic interface by photoinduced absorption spectroscopy. We find that the polaron signal depends on the excitation wavelength, the periodicity of the inverse opal, and the interfacial area. The inverse opal structures exhibit significantly increased charge generation compared to the control films, and we determine that photonic effects of the lattice, while observable, play a secondary role in this enhancement relative to the increased surface area.

Water Splitting Progress in Tandem Devices: Moving Photolysis beyond Electrolysis

Water photolysis is a sustainable technology to convert natural solar energy and water into chemical fuels and is thus considered a thorough solution to the forthcoming energy crises. Unassisted water splitting that could directly harvest solar light and subsequently split water in a single device has become an important research theme. Three types of tandem devices including photoelectrochemical (PEC), photovoltaic (PV) cell/PEC and PV/electrolyser tandem cells are proposed to realize water photolysis at different levels of integration and component. Recent progress in tandem water splitting devices is summarized, and crucial issues on device optimization from the perspective of each photo‐absorber functionalities in band edge potential, light absorptivity and transmittance are discussed. By increasing the performances of stand‐alone PEC or PV devices, a 20% solar to hydrogen efficiency is predicted that is a significant value towards further application in practice. Accordingly, the challenges for materials development and configuration optimization are further outlined.

Identification of photo-induced spin-triplet recombination centers situated at Si surfaces and Si/SiO2 interfaces

A recombination center at Si surfaces and Si/SiO2 interfaces was identified using highly sensitive spin-dependent recombination (SDR) detection of electron paramagnetic resonance (EPR). The defect (Si-KU1) has a spin-triplet (S = 1) state owing to excitation by band-edge light employed in measurement. The Si-KU1 SDR-EPR spectrum exhibits orthorhombic symmetry along two equivalent ⟨110⟩ axes running parallel to the interface, which disappears upon mechanical lapping of the surface oxide or heat treatment above 400 °C. However, removal of surface oxide with diluted HF solution sharpens the Si-KU1 SDR-EPR lines. A plausible structural model of the center responsible for Si-KU1 is presented.

Band Edge Modulation and Light Emission in InGaN Nanowires Due to the Surface State and Microscopic Indium Distribution

InGaN nanowires (NWs) show exceptional optical properties and have huge potential in applications such as light-emitting diodes, laser diodes, and solar cells. Although lots of work has focused on improving their optical performance, little is known about the influence of the In distribution and surface states on the microscopic light emission mechanism. In order to give an atomic-level understanding, we investigate the electronic structures of the wurtzite Ga-rich InGaN NWs with different In distributions using first-principles calculations, in which both the unsaturated and saturated NWs are considered. We find that In atoms are apt to distribute on the surface of the NWs and that the short surface In–N chains can be easily formed. For the unsaturated NWs, several new bands are induced by the surface states, which can be modified by the surface In microstructures. The randomly formed surface In–N chains can highly localize the electrons/holes at the band edges and dominate the interband optical transiti...

The dependence of photosensitivity on composition for thin films of Ge x As y Se1–x–y chalcogenide glasses

We have prepared twelve Ge–As–Se chalcogenide glass films with different chemical compositions and investigated their stability to exposure with near bandedge light. The evolution of two key parameters, the refractive index at 1550 nm and the bandgap with increasing fluence were fitted with stretched-exponential functions. While most of the films showed photo-bleaching (or photodarkening) behavior, we found that for films with a mean coordination number (MCN) around ≈2.45–2.50, neither the bandgap nor the refractive index changed upon irradiation, demonstrating that photostable glasses exist with a particular chemical composition corresponding to the strongest glass formers. Such photostable glasses are the best choice for applications in photonics.

Photo-induced and Thermal Annealing of Chalcogenide Films for Waveguide Fabrication

We describe the effect of light-induced annealing of As2S3 thin films and its impact on the performance of an optical waveguide. An as-deposited film was subjected to illumination with band-edge light before being used to fabricate sub- micron thick waveguides using photolithography and dry plasma etching. Studies of the film microstructure revealed a difference between the atomic bonds and linked phases between the as-deposited, the thermally-annealed, and the optically-annealed films. Although optical-annealing evolves the structure of the film in a similar way to thermal annealing (i.e., it causes polymerization of the glass network), it can drive the film much closer to bulk state in terms of refractive index and the numbers of wrong bonds. The waveguides showed, however, almost the same propagation losses to those from thermally-annealed material. Nonetheless our results indicate that optical-annealing provides some advantages over thermal annealing for waveguide fabrication.

First-Principles Study of 2.2 nm Silicon Nanocrystals Doped with Boron

First-principles study of boron (B)-doped silicon nanocrystals (Si NCs) at 0 K in the framework of density functional theory has been carried out. It is found that B prefers residing at the surface of Si NCs, similar to phosphorus (P). Different from P, B induces surface restructuring when B is one- or two-coordinated at the NC surface. B doping does not significantly change the bandgap of Si NCs, but in most cases B introduces deep energy levels in the bandgap of Si NCs. This explains the B-doping induced quenching of band-edge light emission usually observed in experiments. The negligible infrared absorption of B-doped Si NCs may result from the fact that only three-coordinated B is formed at the NC surface. The electronic transitions involving the energy levels induced by these three-coordinated B are not in the infrared range.

Effect of periodicity on optical forces between a one-dimensional periodic photonic crystal waveguide and an underlying substrate

We numerically investigate the attractive optical force between a suspended one-dimensional periodic photonic crystal waveguide and underlying substrate in a silicon-on-insulator platform. We show that the optical force is enhanced by designing the waveguide cross section to make the mode approach the band edge or substrate light line. We show that for periodic waveguides, the optical force is nonmonotonic with waveguide-substrate separation. This effect may enable the design of compact, integrated optical power limiters.

Microscopic origin of light emission fromAlyGa1−yN/GaNsuperlattices: Band profile and active site

We present first-principles calculations of AlGaN/GaN superlattice, clarifying the microscopic origin of the light emission and revealing the effect of local polarization within the quantum well. Profile of energy band and distributions of electrons and holes demonstrate the existence of a main active site in the well responsible for the main band-edge light emission. This site appears at the position where the local polarization becomes zero. With charge injection, the calculated optical spectra show that the broadening of the band gap at the active site leads to the blueshift of emission wavelength.

Efficient band-edge light injection in two-dimensional planar photonic crystals using a gradual interface

Experimental results about efficient band-edge light coupling into two-dimensional planar photonic crystals by the use of a gradual interface are reported. It is shown that a chirp of both the lattice period and the hole diameter on short lengths below 3 µm allows maintaining high optical transmission spectra close to the band-gap frequency where strongly dispersive phenomena can be exploited. If compared with nonoptimized structures, increased optical levels by 15 to 20 dB are obtained using gradual tapering stages.

Band-edge emission enhancement by longitudinal stress field in GaN

Bright ultraviolet luminescence in the wing region of epitaxial-lateral-overgrowth GaN was studied by cathodoluminescence. Analysis of dislocation bending and movement by transmission electron microscopy demonstrates the emergence of another longitudinal stress field, which effectively enhances the band-edge light emission. Ab initio calculations of interband transition probability provide a model showing that introduction of appropriate additional stress component in the longitudinal direction of GaN will improve the efficiency of band-edge emission.