Origin of the variation of exciton binding energy in semiconductors

Engineering Trap Distribution by Doping Rare Earth Ion for Mechanoluminescence Enhancement

Mechanoluminescence materials exhibit fascinating optical properties due to their energy harvesting and controllable release capabilities. SrAl2O4:Eu2+ (SAOE) has been extensively studied as a traditional mechanoluminescence material, however, the luminescence intensity enhancement and the luminescence mechanism of its mechanoluminescence remain an unresolved issue, which hinders the development and widespread application of excellent phosphors. Herein, a promising rare earth (Re3+ = Sm3+, Dy3+, Er3+, and Tm3+) doping strategy was proposed to achieve intense mechanoluminescence of SAOE. By introducing different Re3+ ions to manipulate the energy level positions in SAOE phosphors, the depth and density of electron and hole traps can be tuned, resulting in the maximum mechanoluminescence intensity of SrAl2O4:Eu2+, Tm3+ is about 11-fold higher than that of SAOE. The mechanism governing trap distribution has been unveiled through thermoluminescence glow curve analysis and density functional theory calculations. Our research provides valuable guidance for designing high-performance phosphors and opens up new opportunities for multifunctional applications.

Revealing Intrinsic Free Charge Generation: Promoting the Construction of Over 19% Efficient Planar p-n Heterojunction Organic Solar Cells

Due to high binding energy and extremely short diffusion distance of Frenkel excitons in common organic semiconductors at early stage, mechanism of interface charge transfer-mediated free carrier generation has dominated the development of bulk heterojunction (BHJ) organic solar cells (OSCs). However, considering the advancements in materials and device performance, it is necessary to reexamine the photoelectric conversion in current-stage efficient OSCs. Here, we propose that the conjugated materials with specific three-dimensional donor-acceptor conjugated packing potentially exhibit distinctive charge photogeneration mechanism, which spontaneously split Wannier-Mott excitons to free carriers in pure phases. Subsequently, the pure planar p-n heterojunction (PHJ) OSCs based on green orthogonal solvents were prepared and exhibited comparable even greater performance to that of BHJ OSCs. More interestingly, by introducing PVDF-TrFE as intrinsic region to regulate built-in electric field of the device, the planar p-i-n PHJ OSCs achieved much higher efficiency (>18%) and stability. Moreover, a prominent efficiency of over 19% has been obtained via ternary optimization, which is the new efficiency record for PHJ OSCs up to date. This study points towards the distinguishing intrinsic free charge generation mechanism, opens up a new avenue for OSCs to collectively realize high-efficiency, long-term duration, and simplified device engineering for future commercialization.

Large exciton binding energy in a bulk van der Waals magnet from quasi-1D electronic localization

Excitons, bound electron-hole pairs, influence the optical properties in strongly interacting solid-state systems and are typically most stable and pronounced in monolayer materials. Bulk systems with large exciton binding energies, on the other hand, are rare and the mechanisms driving their stability are still relatively unexplored. Here, we report an exceptionally large exciton binding energy in single crystals of the bulk van der Waals antiferromagnet CrSBr. Utilizing state-of-the-art angle-resolved photoemission spectroscopy and self-consistent ab-initio GW calculations, we present direct spectroscopic evidence supporting electronic localization and weak dielectric screening as mechanisms contributing to the amplified exciton binding energy. Furthermore, we report that surface doping enables broad tunability of the band gap offering promise for engineering of the optical and electronic properties. Our results indicate that CrSBr is a promising material for the study of the role of anisotropy in strongly interacting bulk systems and for the development of exciton-based optoelectronics.

Boosting exciton dissociation in anion and cation co-doped polymeric semiconductor for selective oxidation reaction

The inherently low dielectric properties and weak shielding effect of polymeric semiconductors cause excitons to dominate their photoexcitation process, which greatly restricts the photocatalytic performances mediated by charge carriers. Here, an anion and cation co-doping strategy was proposed to weaken the binding energy of excitons by forming distinct positive and negative charge regions, where the charge asymmetry produced an external potential to drive exciton dissociation. Using polymeric carbon nitride as a typical model framework, we show that the incorporation of anions (Cl-, Br-, I-) and cations (Na+, K+) could create a significant spatial separation of electrons and holes, thereby promoting exciton dissociation. Specifically, K+ and Cl- co-doped polymeric carbon nitride could effectively promote the dissociation of excitons into hot carriers, contributing to the outstanding efficiency in hot-electron-involved photocatalytic processes, such as the generation of superoxide radicals (O2˙-) and the oxidation of phenylboric acid under visible light. This work presents a practical approach for promoting excitons dissociation through the introduction of charge asymmetry.

Large-Area Epitaxial Growth of Transition Metal Dichalcogenides

Over the past decade, research on atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) has expanded rapidly due to their unique properties such as high carrier mobility, significant excitonic effects, and strong spin-orbit couplings. Considerable attention from both scientific and industrial communities has fully fueled the exploration of TMDs toward practical applications. Proposed scenarios, such as ultrascaled transistors, on-chip photonics, flexible optoelectronics, and efficient electrocatalysis, critically depend on the scalable production of large-area TMD films. Correspondingly, substantial efforts have been devoted to refining the synthesizing methodology of 2D TMDs, which brought the field to a stage that necessitates a comprehensive summary. In this Review, we give a systematic overview of the basic designs and significant advancements in large-area epitaxial growth of TMDs. We first sketch out their fundamental structures and diverse properties. Subsequent discussion encompasses the state-of-the-art wafer-scale production designs, single-crystal epitaxial strategies, and techniques for structure modification and postprocessing. Additionally, we highlight the future directions for application-driven material fabrication and persistent challenges, aiming to inspire ongoing exploration along a revolution in the modern semiconductor industry.

Two-dimensional Be2P4 as a promising thermoelectric material and anode for Na/K-ion batteries

Incredibly effective and flexible energy conversion and storage systems hold great promise for portable self-powered electronic devices. Owing to their large surface area, exceptional atomic structures, superior electrical conductivity and good mechanical flexibility, two-dimensional (2D) materials are recognized as an attractive option for energy conversion and storage application. In this work, we examined the stability, electronic, thermoelectric and electrochemical aspects of a novel 2D Be2P4 monolayer by adopting density functional theory (DFT). The Be2P4 monolayer exhibits a direct semiconductor gap of 0.9 eV (HSE06), large Young's modulus (∼198 GPa), high carrier mobility (∼104 cm2 V-1 s-1) and a low excitonic binding energy of 0.11 eV. Our calculated findings suggest that Be2P4 shows a lattice thermal conductivity of 1.02 W m K-1 at 700 K, resulting in moderate thermoelectric performance (ZT ∼ 0.7), encouraging its use in thermoelectric materials. In addition, a higher adsorption energy of -2.28 eV (-2.52 eV) and less diffusion barrier of 0.22 eV (0.17 eV) for Na(K)-ion batteries promote fast ion transport in the Be2P4 monolayer. This material also shows a high specific capacity and superior energy density of 8460 W h kg-1 (8883 W h kg-1) for Na(K)-ion batteries. Thus, our results offer insightful information for investigating potential thermoelectric and flexible anode materials based on the Be2P4 monolayer.

Exploration of the synergistic effect of chrysene-based core and benzothiophene acceptors on photovoltaic properties of organic solar cells

To improve the efficacy of organic solar cells (OSCs), novel small acceptor molecules (CTD1-CTD7) were designed by modification at the terminal acceptors of reference compound CTR. The optoelectronic properties of the investigated compounds (CTD1-CTD7) were accomplished by employing density functional theory (DFT) in combination with time-dependent density functional theory (TD-DFT). The M06 functional along with a 6-311G(d,p) basis set was utilized for calculating various parameters such as: frontier molecular orbitals (FMO), absorption maxima (λmax), binding energy (Eb), transition density matrix (TDM), density of states (DOS), and open circuit voltage (Voc) of entitled chromophores. A red shift in the absorption spectra of all designed chromophores (CTD1-CTD7) was observed as compared to CTR, accompanied by low excitation energy. Particularly, CTD4 was characterized by the highest λmax value of 685.791 nm and the lowest transition energy value of 1.801 eV which might be ascribed to the robust electron-withdrawing end-capped acceptor group. The observed reduced binding energy (Eb) was linked to an elevated rate of exciton dissociation and substantial charge transfer from central core in HOMO towards terminal acceptors in LUMO. These results were further supported by the outcomes from TDM and DOS analyses. Among all entitled chromophores, CTD4 exhibited bathochromic shift (685.791 nm), minimum HOMO/LUMO band gap of 2.347 eV with greater CT. Thus, it can be concluded that by employing molecular engineering with efficient acceptor moieties, the efficiency of photovoltaic materials could be improved.

Atomic-scale visualization of the interlayer Rydberg exciton complex in moiré heterostructures

Excitonic systems, facilitated by optical pumping, electrostatic gating or magnetic field, sustain composite particles with fascinating physics. Although various intriguing excitonic phases have been revealed via global measurements, the atomic-scale accessibility towards excitons has yet to be established. Here, we realize the ground-state interlayer exciton complexes through the intrinsic charge transfer in monolayer YbCl3/graphite heterostructure. Combining scanning tunneling microscope and theoretical calculations, the excitonic in-gap states are directly profiled. The out-of-plane excitonic charge clouds exhibit oscillating Rydberg nodal structure, while their in-plane arrangements are determined by moiré periodicity. Exploiting the tunneling probe to reflect the shape of charge clouds, we reveal the principal quantum number hierarchy of Rydberg series, which points to an excitonic energy-level configuration with unusually large binding energy. Our results demonstrate the feasibility of mapping out the charge clouds of excitons microscopically and pave a brand-new way to directly investigate the nanoscale order of exotic correlated phases.

Nanostructured ZnO with Tunable Morphology from Double-Salt Ionic Liquids as Soft Template

ZnO nanostructures with tunable morphology were synthesized by the hydrothermal method from two ionic liquids (ILs), 1-ethyl-3-methylimidazolium acetate [C2mim]CH3CO2 and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [C2mim](CF3SO2)2N and their corresponding double-salt ILs (DSILs). ILs served as soft templates. DSILs were noted for the production of smaller particle size along with uniformity compared to their pure IL counterparts. A changeover of the shape of ZnO from nano-prism to a hexagonal disk-like structure was observed with the addition of [C2mim]CH3CO2 in the medium during synthesis while nano-dice- and rod-shaped particles were obtained from [C2mim](CF3SO2)2N. The effect of concentration of both ILs was explored for the variations of size and shape, and at high concentrations, the morphology was distinct and sharp with uniform size in each case. The synthesized products exhibited excellent phase (wurtzite) purity and polycrystalline nature. The smallest crystallite size was acquired from DSILs, indicating the advantageous effect of the dual anions. The selective adsorption effect of [C2mim]CH3CO2 on certain facets promoted the growth of ZnO clusters along the [1010] direction, while [C2mim](CF3SO2)2N favored the growth along the [0001] direction. Consequently, DSILs rendered interpenetrating hexagonal disks due to the combined action of the anions for controlling the shape. The band gap energies of the nanoparticles (NPs) were consistent with the distribution of size. Extremely strong red emission and negligible UV emission for the synthesized ZnO NPs demonstrate their potential in the advancement of optoelectronic devices.

Tunable Interlayer Delocalization of Excitons in Layered Organic-Inorganic Halide Perovskites

Layered organic-inorganic halide perovskites exhibit remarkable structural and chemical diversity and hold great promise for optoelectronic devices. In these materials, excitons are thought to be strongly confined within the inorganic metal halide layers with interlayer coupling generally suppressed by the organic cations. Here, we present an in-depth study of the energy and spatial distribution of the lowest-energy excitons in layered organic-inorganic halide perovskites from first-principles many-body perturbation theory, within the GW approximation and the Bethe-Salpeter equation. We find that the quasiparticle band structures, linear absorption spectra, and exciton binding energies depend strongly on the distance and the alignment of adjacent metal halide perovskite layers. Furthermore, we show that exciton delocalization can be modulated by tuning the interlayer distance and alignment, both parameters determined by the chemical composition and size of the organic cations. Our calculations establish the general intuition needed to engineer excitonic properties in novel halide perovskite nanostructures.

Stabilization of Guanidinate Anions [CN3 ]5- in Calcite-Type SbCN3

The stabilization of nitrogen-rich phases presents a significant chemical challenge due to the inherent stability of the dinitrogen molecule. This stabilization can be achieved by utilizing strong covalent bonds in complex anions with carbon, such as cyanide CN- and NCN2- carbodiimide, while more nitrogen-rich carbonitrides are hitherto unknown. Following a rational chemical design approach, we synthesized antimony guanidinate SbCN3 at pressures of 32-38 GPa using various synthetic routes in laser-heated diamond anvil cells. SbCN3 , which is isostructural to calcite CaCO3 , can be recovered under ambient conditions. Its structure contains the previously elusive guanidinate anion [CN3 ]5- , marking a fundamental milestone in carbonitride chemistry. The crystal structure of SbCN3 was solved and refined from synchrotron single-crystal X-ray diffraction data and was fully corroborated by theoretical calculations, which also predict that SbCN3 has a direct band gap with the value of 2.20 eV. This study opens a straightforward route to the entire new family of inorganic nitridocarbonates.

Emerging quasi-one-dimensional material NbS4 with high carrier mobility and good visible-light adsorption performance for nanoscale applications

Due to their unique structure, abundant properties and potential applications, low-dimensional materials with covalently bonded building blocks through van der Waals (vdW) interactions have sparked widespread interest. Recently, the bulk phase NbS4 consisting of one-dimensional (1D) chains has been synthesized successfully, adding a new member to the group V metallic polychalcogenide family. In the present study, based on density functional theory calculations, we obtained a better understanding of the stability, mechanical properties, electronic structures, transport properties and optical performances of the bulk phase NbS4. Furthermore, the possibility of exfoliating 1D single-chain nanowires from the bulk phase was uncovered. Both bulk phase and 1D nanowires show dynamic, thermal, and mechanical stabilities. The bulk phase possesses an indirect band gap of 1.39 eV with high anisotropic carrier mobilities of 471.814 cm2 s-1 v-1 for electrons (along the b axis direction) and 546.92 cm2 s-1 v-1 for holes (along the a axis direction). The single-chain nanowire exhibits remarkable flexibility and can resist 24% tensile strain along the chain direction. The decreased dimension from the bulk phase to the individual 1D chain not only makes the band gap increase to 1.81 eV but also results in an indirect-to-direct band gap transition, indicating a strong quantum confinement effect. The 1D single-chain nanowire also shows high carrier mobilities of 111.91 cm2 s-1 v-1 for electrons and 316.63 cm2 s-1 v-1 for holes along the chain direction. In addition, both bulk phase and 1D nanowire display excellent visible light absorption performance along the chain direction and the absorption coefficients reach the order of 106 and 105 cm-1. These promising properties render quasi 1D NbS4 as candidate materials for nanoscale applications in high-performance optoelectronic and nanoelectronic devices. The predicted unconventional properties of NbS4 not only provide a meaningful complement to the fascinating quasi 1D material family, but also will attract extensive interest from a wide audience to explore unanticipated properties and design new nanoscale devices based on NbS4.

Regulating excitonic effects in non-oxide based XPSe3 (X = Cd, Zn) monolayers towards enhanced photocatalysis for overall water splitting

The non-oxide 2D materials have garnered considerable interest due to their potential utilization as photocatalysts, which offer a superior substitute to metal-oxide-based photocatalysts. This study investigates the impact of the dielectric environment on the size and binding energy of excitons in atomically thin, experimentally synthesized semiconducting monolayers [XPSe₃, X = (Cd, Zn)] to address the critical problem of electron-hole recombination, which significantly hinders the efficiency of most photocatalysts. We employ a precise non-hydrogenic model surpassing the hydrogenic-based Mott-Wannier model. Our findings are among the first few demonstrations of an increase in exciton size (and decrease in exciton binding energy) as environmental screening increases. These findings have implications for photocatalytic water splitting and are not limited to metal phosphorus trichalcogenides, but can be applied to other classes of 2D materials as well. This work also compares metal-oxide photocatalysts, which have been the focus of much research over the past five decades, to non-oxide-based metal phosphorus trichalcogenide photocatalysts, which offer a superior alternative due to their ability to address issues such as light-harvesting inability in the visible spectrum and unwanted charge recombination centres. Furthermore, the implications of this study extend beyond photocatalysts and are significant for the design and development of next-generation optoelectronic devices that incorporate excitonic processes, such as solar cells, photodetectors, LEDs, etc.

Hydrophilic ionic liquid assisted hydrothermal synthesis of ZnO nanostructures with controllable morphology

Nanostructured ZnO with controllable morphology was prepared by a hydrothermal method in the presence of three different hydrophilic ionic liquids (ILs), 1-ethyl-3-methylimidazolium methylsulfate, ([C2mim]CH3SO4), 1-butyl-3-methylimidazolium methylsulfate, ([C4mim]CH3SO4) and 1-ethyl-3-methylimidazolium ethylsulfate, ([C2mim]C2H5SO4) as soft templates. The formation of ZnO nanoparticles (NPs) with and without IL was verified using FT-IR and UV-visible spectroscopy. X-ray diffraction (XRD) and selected area electron diffraction (SAED) patterns indicated the formation of pure crystalline ZnO with a hexagonal wurtzite phase. Field emission scanning electron microscopic (FESEM) and high-resolution transmission electron microscopic (HRTEM) images confirmed the formation of rod-shaped ZnO nanostructures without using IL, whereas the morphology varied widely following addition of ILs. With increasing concentrations of [C2mim]CH3SO4, the rod-shaped ZnO nanostructures transformed into flower-shaped nanostructures whereas with rising concentrations of [C4mim]CH3SO4 and [C2mim]C2H5SO4 the morphology changed into petal- and flake-like nanostructures, respectively. The selective adsorption effect of the ILs could protect certain facets during the formation of ZnO rods and promote the growth in directions other than [0001] to yield petal- or flake-like architectures. The morphology of ZnO nanostructures was, therefore, tunable by the controlled addition of hydrophilic ILs of different structures. The size of the nanostructures was widely distributed and the Z-average diameter, evaluated from dynamic light scattering measurements, increased as the concentration of the IL increased and passed through a maximum before decreasing again. The optical band gap energy of the ZnO nanostructures decreased when IL was added during the synthesis which is consistent with the morphology of the ZnO nanostructures. Thus, the hydrophilic ILs serve as self-directing agents and soft templates for the synthesis of ZnO nanostructures and the morphology and optical properties of ZnO nanostructures are tunable by changing the structure of the ILs as well as systematic variation of the concentration of ILs during synthesis.

Big Data in a Nano World: A Review on Computational, Data-Driven Design of Nanomaterials Structures, Properties, and Synthesis

The recent rise of computational, data-driven research has significant potential to accelerate materials discovery. Automated workflows and materials databases are being rapidly developed, contributing to high-throughput data of bulk materials that are growing in quantity and complexity, allowing for correlation between structural-chemical features and functional properties. In contrast, computational data-driven approaches are still relatively rare for nanomaterials discovery due to the rapid scaling of computational cost for finite systems. However, the distinct behaviors at the nanoscale as compared to the parent bulk materials and the vast tunability space with respect to dimensionality and morphology motivate the development of data sets for nanometric materials. In this review, we discuss the recent progress in data-driven research in two aspects: functional materials design and guided synthesis, including commonly used metrics and approaches for designing materials properties and predicting synthesis routes. More importantly, we discuss the distinct behaviors of materials as a result of nanosizing and the implications for data-driven research. Finally, we share our perspectives on future directions for extending the current data-driven research into the nano realm.

Photoreflectance and Photoluminescence Study of Antimony Selenide Crystals

Among inorganic, Earth-abundant, and low-toxicity photovoltaic technologies, Sb₂Se₃ has emerged as a strong material contender reaching over 10% solar cell power conversion efficiency. Nevertheless, the bottleneck of this technology is the high deficit of open-circuit voltage (V _OC) as seen in many other emerging chalcogenide technologies. Commonly, the loss of V _OC is related to the nonradiative carrier recombination through defects, but other material characteristics can also limit the achievable V _OC. It has been reported that in isostructural compound Sb₂S₃, self-trapped excitons are readily formed leading to 0.6 eV Stokes redshift in photoluminescence (PL) and therefore significantly reducing the obtainable V _OC. However, whether Sb₂Se₃ has the same limitations has not yet been examined. In this work, we aim to identify main radiative carrier recombination mechanisms in Sb₂Se₃ single crystals and estimate if there is a fundamental limit for obtainable V _OC. Optical transitions in Sb₂Se₃ were studied by means of photoreflectance and PL spectroscopy. Temperature, excitation intensity, and polarization-dependent optical characteristics were measured and analyzed. We found that at low temperature, three distinct radiative recombination mechanisms were present and were strongly influenced by the impurities. The most intensive PL emissions were located near the band edge. In conclusion, no evidence of emission from self-trapped excitons or band-tails was observed, suggesting that there is no fundamental limitation to achieve high V _OC, which is very important for further development of Sb₂Se₃-based solar cells.

An extension of electron acceptor sites around Thiazolothiazole unit for evaluation of large power conversion efficiency: A theoretical insight

Small organic solar cells containing thiazolothiazole unit as an electron acceptor for solution processed bulk heterojunction (BHJ) small donor-acceptor-donor (D-A-D) type materials have been designed and studied theoretically with state-of-the-art density functional theory and time-dependent density functional theory (TD-DFT) for reliable estimation of their excited state and charge transfer photophysical characteristics for estimating their power conversion efficiencies. The suggested possible synthetic routes with complete reaction information have been also provided for synthesis. The electron acceptor sites around the thiazolothiazole unit have been enlarged by introducing different strong electron withdrawing groups and checked their effects on the voltages (V_OC) and fill factor (FF) which are the two main parameters directly influences on power conversion efficiencies. Out of five theoretically studied molecules, the experimental reported data of TT-TTPA (Thiazolothiazole-thiaophene triphenyl amine) has been compared with four designed molecules and concluded that extension of acceptor sites significantly contributed towards the better charge transport properties of electron and hole.

Electrostatic and Environmental Control of the Trion Fine Structure in Transition Metal Dichalcogenide Monolayers

Charged excitons or trions are essential for optical spectra in low-dimensional doped monolayers (ML) of transitional metal dichalcogenides (TMDC). Using a direct diagonalization of the three-body Hamiltonian, we calculate the low-lying trion states in four types of TMDC MLs as a function of doping and dielectric environment. We show that the fine structure of the trion is the result of the interplay between the spin-valley fine structure of the single-particle bands and the exchange interaction. We demonstrate that by variations of the doping and dielectric environment, the fine structure of the trion energy can be tuned, leading to anticrossing of the bright and dark states, with substantial implications for the optical spectra of the TMDC ML.

Mechanism of the trivalent lanthanides' persistent luminescence in wide bandgap materials

The trivalent lanthanides have been broadly utilized as emitting centers in persistent luminescence (PersL) materials due to their wide emitting spectral range, which thus attract considerable attention over decades. However, the origin of the trivalent lanthanides' PersL is still an open question, hindering the development of excellent PersL phosphors and their broad applications. Here, the PersL of 12 kinds of the trivalent lanthanides with the exception of La³⁺, Lu³⁺, and Pm³⁺ is reported, and a mechanism of the PersL of the trivalent lanthanides in wide bandgap hosts is proposed. According to the mechanism, the excitons in wide bandgap materials transfer their recombination energy to the trivalent lanthanides that bind the excitons, followed by the generation of PersL. During the PersL process, the trivalent lanthanides as isoelectronic traps bind excitons, and the binding ability is not only related to the inherent arrangement of the 4f electrons of the trivalent lanthanides, but also to the extrinsic ligand field including anion coordination and cation substitution. Our work is believed to be a guidance for designing high-performance PersL phosphors.

Dynamics of photoconversion processes: the energetic cost of lifetime gain in photosynthetic and photovoltaic systems

The continued development of solar energy conversion technologies relies on an improved understanding of their limitations. In this review, we focus on a comparison of the charge carrier dynamics underlying the function of photovoltaic devices with those of both natural and artificial photosynthetic systems. The solar energy conversion efficiency is determined by the product of the rate of generation of high energy species (charges for solar cells, chemical fuels for photosynthesis) and the energy contained in these species. It is known that the underlying kinetics of the photophysical and charge transfer processes affect the production yield of high energy species. Comparatively little attention has been paid to how these kinetics are linked to the energy contained in the high energy species or the energy lost in driving the forward reactions. Here we review the operational parameters of both photovoltaic and photosynthetic systems to highlight the energy cost of extending the lifetime of charge carriers to levels that enable function. We show a strong correlation between the energy lost within the device and the necessary lifetime gain, even when considering natural photosynthesis alongside artificial systems. From consideration of experimental data across all these systems, the emprical energetic cost of each 10-fold increase in lifetime is 87 meV. This energetic cost of lifetime gain is approx. 50% greater than the 59 meV predicted from a simple kinetic model. Broadly speaking, photovoltaic devices show smaller energy losses compared to photosynthetic devices due to the smaller lifetime gains needed. This is because of faster charge extraction processes in photovoltaic devices compared to the complex multi-electron, multi-proton redox reactions that produce fuels in photosynthetic devices. The result is that in photosynthetic systems, larger energetic costs are paid to overcome unfavorable kinetic competition between the excited state lifetime and the rate of interfacial reactions. We apply this framework to leading examples of photovoltaic and photosynthetic devices to identify kinetic sources of energy loss and identify possible strategies to reduce this energy loss. The kinetic and energetic analyses undertaken are applicable to both photovoltaic and photosynthetic systems allowing for a holistic comparison of both types of solar energy conversion approaches.

Coupling between Emissive Defects on Carbon Nanotubes: Modeling Insights

Covalent functionalization of single-walled carbon nanotubes (SWCNTs) with organic molecules results in red-shifted emissive states associated with sp³-defects in the tube lattice, which facilitate their improved optical functionality, including single-photon emission. The energy of the defect-based electronic excitations (excitons) depends on the molecular adducts, the configuration of the defect, and concentration of defects. Here we model the interactions between two sp³-defects placed at various distances in the (6,5) SWCNT using time-dependent density functional theory. Calculations reveal that these interactions conform to the effective model of J-aggregates for well-spaced defects (>2 nm), leading to a red-shifted and optically allowed (bright) lowest energy exciton. H-aggregate behavior is not observed for any defect orientations, which is beneficial for emission. The splitting between the lowest energy bright and optically forbidden (dark) excitons and the pristine excitonic band are controlled by the single-defect configurations and their axial separation. These findings enable a synthetic design strategy for SWCNTs with tunable near-infrared emission.

Database of Wannier tight-binding Hamiltonians using high-throughput density functional theory

Wannier tight-binding Hamiltonians (WTBH) provide a computationally efficient way to predict electronic properties of materials. In this work, we develop a computational workflow for high-throughput Wannierization of density functional theory (DFT) based electronic band structure calculations. We apply this workflow to 1771 materials (1406 3D and 365 2D), and we create a database with the resulting WTBHs. We evaluate the accuracy of the WTBHs by comparing the Wannier band structures to directly calculated spin-orbit coupling DFT band structures. Our testing includes k-points outside the grid used in the Wannierization, providing an out-of-sample test of accuracy. We illustrate the use of WTBHs with a few example applications. We also develop a web-app that can be used to predict electronic properties on-the-fly using WTBH from our database. The tools to generate the Hamiltonian and the database of the WTB parameters are made publicly available through the websites https://github.com/usnistgov/jarvis and https://jarvis.nist.gov/jarviswtb .

Surface structure-dependent photocatalytic O2 activation for pollutant removal with bismuth oxyhalides

The purification of water and air by semiconductor photocatalysis is a rapidly growing area for academic research and industrial innovation, featured with ambient removal of organic or inorganic pollutants by using solar light as the energy source and atmospheric O₂ as the green oxidant. Both charge transfer and energy transfer from excited photocatalysts can overcome the spin-forbidden nature of O₂. Layered bismuth oxyhalides are a new group of two-dimensional photocatalysts with an appealing geometric and surface structure that allows the dynamic and selective tuning of O₂ activation at the surface molecular level. In this Feature Article, we specifically summarize our recent progress in selective O₂ activation by engineering surface structures of bismuth oxyhalides. Then, we demonstrate selective photocatalytic O₂ activation of bismuth oxyhalides for environmental control, including water decontamination, volatile organic compound oxidation and nitrogen oxide removal, as well as selective catalytic oxidations. Challenges and opportunities regarding the design of photocatalysts with satisfactory performance for potential environmental control applications are also presented.

Materials chemistry and engineering in metal halide perovskite lasers

The invention and development of the laser have revolutionized science, technology, and industry. Metal halide perovskites are an emerging class of semiconductors holding promising potential in further advancing the laser technology. In this Review, we provide a comprehensive overview of metal halide perovskite lasers from the viewpoint of materials chemistry and engineering. After an introduction to the materials chemistry and physics of metal halide perovskites, we present diverse optical cavities for perovskite lasers. We then comprehensively discuss various perovskite lasers with particular functionalities, including tunable lasers, multicolor lasers, continuous-wave lasers, single-mode lasers, subwavelength lasers, random lasers, polariton lasers, and laser arrays. Following this a description of the strategies for improving the stability and reducing the toxicity of metal halide perovskite lasers is provided. Finally, future research directions and challenges toward practical technology applications of perovskite lasers are provided to give an outlook on this emerging field.

C, N dual-doped ZnO nanofoams: a potential antimicrobial agent, an efficient visible light photocatalyst and SXAS studies

It is crucial to develop an environmentally friendly and low-cost method to treat industrial effluent that contains soluble dyes and microbes. Most of the photocatalysts have been studied using an external light source that increases the cost of the purification process of effluent. This study focuses on developing efficient solar photocatalytic nanofoams. The controlled growth of ZnO nanofoams (CNZ nanofoams) in a simple method of thermal oxidation using a soft template is reported. Prepared nanofoams are characterized using X-ray diffraction, scanning electon microscopy and synchrotron soft X-ray absorption spectroscopy. By photocatalysis studies under direct sunlight it was found that within 120 min CNZ nanofoams degraded 99% of the dye. In addition, antimicrobial studies of multi-drug-resistant E. Fergusonii isolated from wastewater was carried out. These antimicrobial results showed a good inhibition zone, indicating that prepared nanofoams are both an effective solar photocatalyst and an antimicrobial agent.

Variational Excitations in Real Solids: Optical Gaps and Insights into Many-Body Perturbation Theory

We present an approach to studying optical band gaps in real solids in which quantum Monte Carlo methods allow for the application of a rigorous variational principle to both ground and excited state wave functions. In tests that include small, medium, and large band gap materials, optical gaps are predicted with a mean absolute deviation of 3.5% against experiment, less than half the equivalent errors for typical many-body perturbation theories. The approach is designed to be insensitive to the choice of density functional, a property we exploit in order to provide insight into how far different functionals are from satisfying the assumptions of many-body perturbation theory. We explore this question most deeply in the challenging case of ZnO, where we show that, although many commonly used functionals have shortcomings, there does exist a one-particle basis in which perturbation theory's zeroth-order picture is sound. Insights of this nature should be useful in guiding the future application and improvement of these widely used techniques.

Hubbard excitons in two-dimensional nanomaterials

Excitons in two-dimensional nanomaterials are studied by solving the many-electron Hamiltonian with a configuration-interaction approach. It is shown that graphene or phosphorene nanoflakes can not accommodate any excitonic bound states if the long-range Coulomb interaction is suppressed when the systems are placed in a high-k dielectric environment or on a metal substrate. Hence it is revealed that an electron-hole pair created by an optical excitation does not always form an exciton even in a confined nanostructure. The negative exciton binding energy is found to exhibit distinct dependence on the strength of short-range Coulomb interaction as the system undergoes a phase transition from non-magnetic to anti-ferromagnetic. It is further shown that the electron-hole pair may form an exciton state only when the long-range Coulomb interaction is recovered in the nanoflakes.

Enhanced photocatalysis for water splitting in layered tin chalcogenides with high carrier mobility

Due to their proper band gaps (between 1.40 eV and 2.34 eV), newly fabricated tin monochalcogenides (SnX, X = S, Se) and dichalcogenides SnX2, whose monolayer formation energies are much smaller than MoS2, are promising materials for harvesting visible light. Moreover, the anisotropic carrier mobility is up to 2486.93 cm2 V-1 s-1 for SnSe and 2181.96 cm2 V-1 s-1 for SnS2. By applying low tensile strain, the band edge of SnX can be lowered to meet the criteria for water splitting. Meanwhile, the photo-generated exciton binding energies are pretty low, which indicates that the electron-hole can separate efficiently, and may lead to remarkable activity for photocatalysis. Promisingly, it is possible to stack SnS and SnS2 to fabricate a vertical heterostructure (VHT). According to band analysis, we found that the global valence and conduction bands are from SnX and SnX2, respectively. Due to the weak interaction between the two monolayers, the optical gaps can slightly decrease in the two monolayers compared to those in the corresponding isolated ones. Therefore, the VHT can meet the two primary conditions of a photocatalyst for water splitting to generate H2 in SnX and O2 in SnX2. The strong electronegativity difference between the two layers develops an effective potential gradient between the SnS and SnS2 layers, which evokes an effective electric field between them. Thus, it is of benefit for quick charge separation and inter-layer charge transfer. High efficiency of light harvesting can be realized, and improved photocatalytic efficiency.

Phonon coherences reveal the polaronic character of excitons in two-dimensional lead halide perovskites

Hybrid organic-inorganic semiconductors feature complex lattice dynamics due to the ionic character of the crystal and the softness arising from non-covalent bonds between molecular moieties and the inorganic network. Here we establish that such dynamic structural complexity in a prototypical two-dimensional lead iodide perovskite gives rise to the coexistence of diverse excitonic resonances, each with a distinct degree of polaronic character. By means of high-resolution resonant impulsive stimulated Raman spectroscopy, we identify vibrational wavepacket dynamics that evolve along different configurational coordinates for distinct excitons and photocarriers. Employing density functional theory calculations, we assign the observed coherent vibrational modes to various low-frequency (≲50 cm^-1) optical phonons involving motion in the lead iodide layers. We thus conclude that different excitons induce specific lattice reorganizations, which are signatures of polaronic binding. This insight into the energetic/configurational landscape involving globally neutral primary photoexcitations may be relevant to a broader class of emerging hybrid semiconductor materials.

Prediction of an extremely long exciton lifetime in a Janus-MoSTe monolayer

The electron-hole separation efficiency is a key factor that determines the performance of two-dimensional (2D) transition metal dichalcogenides (TMDs) and devices. Therefore, searching for novel 2D TMD materials with a long timescale of carrier lifetime has become one of the most important topics. Here, based on time-domain density functional theory (TD-DFT), we propose a brand new TMD material, namely Janus-MoSTe, which exhibits a strong built-in electric field. Our results show that in the Janus-MoSTe monolayer, the exciton consisting of an electron and hole has a relatively wide spatial extension and low binding energy. In addition, a slow electron-hole recombination process is observed, with a timescale on the order of 1.31 ns, which is even comparable to those of van der Waals (vdW) heterostructures. Further analysis reveals that the extremely long timescale for electron-hole recombination could be ascribed to the strong Coulomb screening effect as well as the small overlap of wavefunctions between electrons and holes. Our findings establish the built-in electric field as an effective factor to control the electron-hole recombination dynamics in TMD monolayers and facilitate their future applications in light detection and harvesting.

Observation of intrinsic dark exciton in Janus-MoSSe heterosturcture induced by intrinsic electric field

The nature of exciton in heterostructure exhibits distinct properties, which makes heterostructure a promising candidate for valleytronic and optoelectronic applications. Therefore, understanding of exciton behaviour in heterostructure is the key approach to design novel devices. Here, we investigate the electronic properties including quasiparticle-energy calculations (on the level of the GW approximation) and optical properties (on the level of the Bethe-Salpeter equation) of Janus-MoSSe based heterostructure. Our results show that the build-in electric field caused by spontaneous polarization of Janus-MoSSe monolayer can significantly affect the interlayer interactions within the heterostructure, giving rise to a bright-to-dark exciton transition. To shed light on this phenomenon, a theoretical model is developed, which illustrates that the dark exciton can be ascribed to a coherence cancellation at the band edge positions induced by the strong interlayer coupling. Our findings may provide a new way for modulating and developing of van der Waals heterostructure that have applications in valleytronic and optoelectronic devices.

Trions in bulk and monolayer materials: Faddeev equations and hyperspherical harmonics

The negatively T ^- and positively T ⁺ charged trions in bulk and monolayer semiconductors are studied in the effective mass approximation within the framework of a potential model. The binding energies of trions in various semiconductors are calculated by employing the Faddeev equation with the Coulomb potential in 3D configuration space. Results of calculations of the binding energies for T ^- are consistent with previous computational studies, while the T ⁺ is unbound for all considered cases. The binding energies of trions in monolayer semiconductors are calculated using the method of hyperspherical harmonics by employing the Keldysh potential. It is shown that 2D T ^- and T ⁺ trions are bound and the binding energy of the positive trion is always greater than for the negative trion due to the heavier effective mass of holes. Our calculations demonstrate that screening effects play an important role in the formation of bound states of trions in 2D semiconductors.

Exciton emission of quasi-2D InGaN in GaN matrix grown by molecular beam epitaxy

We investigate the emission from confined excitons in the structure of a single-monolayer-thick quasi-two-dimensional (quasi-2D) In_xGa_1−xN layer inserted in GaN matrix. This quasi-2D InGaN layer was successfully achieved by molecular beam epitaxy (MBE), and an excellent in-plane uniformity in this layer was confirmed by cathodoluminescence mapping study. The carrier dynamics have also been investigated by time-resolved and excitation-power-dependent photoluminescence, proving that the recombination occurs via confined excitons within the ultrathin quasi-2D InGaN layer even at high temperature up to ~220 K due to the enhanced exciton binding energy. This work indicates that such structure affords an interesting opportunity for developing high-performance photonic devices.

Electronic and excitonic properties of two-dimensional and bulk InN crystals

Motivated by potential extensive applications in nanoelectronics devices, we calculate structural and optoelectronic properties of two-dimensional InN as well as its three-dimensional counterparts by using density functional theory.

Effect of Water Adsorption on the Photoluminescence of Silicon Quantum Dots

The optical properties of silicon quantum dots (Si QDs) are strongly influenced by circumjacent surface-adsorbed molecules, which would highly affect their applications; however, water, as the ubiquitous environment, has not received enough attention yet. We employed the time-dependent density functional calculations to investigate the water effect of photoluminescence (PL) spectra for Si QDs. In contrast with the absorption spectra, PL spectra exhibit distinct characteristics. For Si32H38, PL presents the single maximum in the dry and humid environment, while the emission spectrum displays a dual-band fluorescence spectroscopy in the low-humidity environment. This phenomenon is also observed in the larger Si QDs. The distinct character in spectroscopy is dominated by the stretching of the Si-Si bond, which could be explained by the self-trapped exciton model. Our results shed light on the Si-water interaction that is important for the development of optical devices based on Si-coated surfaces.

Enhanced Exciton Binding Energy of ZnO by Long-Distance Perturbation of Doped Be Atoms

The excitonic effect in semiconductors is sensitive to dopants. Origins of dopant-induced large variation in the exciton binding energy (E(b)) is not well understood and has never been systematically studied. We choose ZnO as a typical high-E(b) material, which is very promising in low-threshold lasing. To the best of our knowledge, its shortest wavelength electroluminescence lasing was realized by ZnO/BeZnO multiple quantum wells (MQWs). However, this exciting result is shadowed by a controversial E(b) enhancement claimed. In this Letter, we reveal that the claimed E(b) is sensible if we take Be-induced E(b) variation into account. Detailed first-principle investigation of the interaction between dopant atoms and the lattice shows that the enhancement mainly comes from the long-distance perturbation of doped Be atoms rather than the local effect of doping atoms. This is a joint work of experiment and calculation, which from the angle of methology paves the way for understanding and predicting the E(b) variation induced by doping.

Robust room temperature valley polarization in monolayer and bilayer WS2

We report robust room temperature valley polarization in chemical-vapor-deposition (CVD) grown monolayer and bilayer WS2via polarization-resolved photoluminescence measurements using excitation below the bandgap. We show that excitation with energy slightly below the bandgap of the multi-valleyed transition metal chalcogenides can effectively suppress the random redistribution of excited electrons and, thereby, greatly enhance the efficiency of valley polarization at room temperature. Compared to mechanically exfoliated WS2, our CVD grown WS2 films also show enhancement in the coupling of spin, layer and valley degree of freedom and, therefore, provide improved valley polarization. At room temperature, using below-bandgap excitation and CVD grown monolayer and bilayer WS2, we have reached a record-high valley polarization of 35% and 80%, respectively, exceeding the previously reported values of 10% and 65% for mechanically exfoliated WS2 layers using resonant excitation. This observation provides a new direction to enhance valley control at room temperature.

Charge-Carrier Dynamics in Organic-Inorganic Metal Halide Perovskites

Hybrid organic-inorganic metal halide perovskites have recently emerged as exciting new light-harvesting and charge-transporting materials for efficient photovoltaic devices. Yet knowledge of the nature of the photogenerated excitations and their subsequent dynamics is only just emerging. This article reviews the current state of the field, focusing first on a description of the crystal and electronic band structure that give rise to the strong optical transitions that enable light harvesting. An overview is presented of the numerous experimental approaches toward determining values for exciton binding energies, which appear to be small (a few milli-electron volts to a few tens of milli-electron volts) and depend significantly on temperature because of associated changes in the dielectric function. Experimental evidence for charge-carrier relaxation dynamics within the first few picoseconds after excitation is discussed in terms of thermalization, cooling, and many-body effects. Charge-carrier recombination mechanisms are reviewed, encompassing trap-assisted nonradiative recombination that is highly specific to processing conditions, radiative bimolecular (electron-hole) recombination, and nonradiative many-body (Auger) mechanisms.

Hydrophilic ionic liquid-assisted control of the size and morphology of ZnO nanoparticles prepared by a chemical precipitation method

A hydrophilic ionic liquid (IL), [EMIM][MeSO₄] served as a self directing template during synthesis of ZnO nanoparticles (NPs) by chemical precipitation and the size and morphology of ZnO NPs depended on the concentration of the IL.

The origin of enhanced optical absorption of the BiFeO3/ZnO heterojunction in the visible and terahertz regions

Optical absorption is improved for the BiFeO3/ZnO heterostructure prepared by a sol-gel process, especially, in the terahertz energy region. A dipole-corrected slab model is used to describe the bilayer film, and first-principles calculations agree with the experiments which present unambiguous explanation for the enhancement of the optical properties. Two-dimensional electrons in the ZnO side of the heterostructure are found to play an essential role in forming the photoinduced carriers and the enhancement of the absorption. The conducting layers tend to penetrate into the interface and decrease the band gap, leading to the transport of carriers through the interface to the BiFeO3 side. The photoinduced carriers can be separated by the ferroelectric domains in BiFeO3, and this mechanism makes the heterostructure an ideal candidate for BiFeO3-based ferroelectric photovoltaic cells.