Bandgap Engineering in Wurtzite GaAs Nanowires by Hydrostatic Pressure

Band structure of wurtzite （WZ） GaAs nanowires （NWs） is investigated by using photoluminescenee measurements under hydrostatic pressure at 6 K. We demonstrate that WZ GaAs NWs have a direct bandgap transition with an emission energy of 1.53eV, corresponding to the optical transition between conduction band Г7c and valence band Г9v in WZ GaAs. The direct-to-pseudodirect bandgap transition can be observed by applying a pressure approximately above 2.5 GPa.

Regulating the key performance parameters for Hg-based IR NLO chalcogenides via bandgap engineering strategy

Recently,non-centrosymmetric(NCS)Hg-based chalcogenides have garnered significant interest due to their strong second-harmonic-generation intensities(deff),making them attractive candidates for infrared nonlinear optical(IR-NLO)application.However,achieving both wide band gaps(Eg)and large phasematched deffsimultaneously in these materials remains a challenge due to their inherent constraints on each other.In this research,we have successfully obtained two quaternary NCS Hg-based chalcogenides,Rb2HgGe_(3)S_(8)and Cs_(2)HgGe_(3)S_(8),by implementing a bandgap engineering strategy that involves alkali metal introduction and Hg/Ge ratio regulation.Both compounds consist of 2D[Hg Ge_(3)S_(8)]_(2)–anionic layers made of 1D[HgGeS_(6)]^(6–)chains and dimeric[Ge_(2)S_(6)]_(4–)polyhedra arranged alternately,and the charge-balanced Rb+/Cs+cations located between these layers.Remarkably,Rb_(2)HgGe_(3)S_(8)and Cs_(2)HgGe_(3)S_(8)exhibit overall properties required for promising IR-NLO materials,including sufficient PM deff(0.55–0.70×AgGaS_(2)@20_(5)0 nm),large Eg(3.27–3.41 e V),giant laser-induced damage thresholds(17.4–19.7×AgGaS_(2)@1064 nm),broad optical transmission intervals(0.32–17.5μm),and suitable theoretical birefringence(0.069–0.086@2050 nm).Furthermore,in-depth theoretical analysis reveals that the exceptional IRNLO performance is attributed to the synergy effects of distorted[HgS_(4)]and[GeS_(4)]tetrahedra.Our study provides a useful strategy for enhancing the Eg and advancing Hg-based IR-NLO materials,which is expected to extended and implemented in other chalcogenide systems.

Bandgap engineering of high mobility two-dimensional semiconductors toward optoelectronic devices

Over the last few years,great advances have been achieved in exploration of high-mobility two-dimensional(2D)semiconductors such as metal chalcogenide InSe and noble-transition-metal dichal-cogenide PdSe_(2).These materials are competitive candidates for constructing next-generation optoelec-tronic devices owing to their unique crystalline and electronic structures.Moreover,the optical and electronic properties of 2D materials can be efficiently modified via precisely engineering their band structures,which is critical for widening specific applications ranging from high-performance opto-electronics to catalysis and energy harvesting.In this review,we focus on the progress in bandgaps engineering of newly emerging high-mobility 2D semiconductors and their applications in optoelec-tronic devices,incorporating our recent study in the InSe and PdSe_(2)systems.First of all,we discuss the structure-property relationship of typical high-mobility 2D semiconductors(InSe and PdSe 2).Next,we analyze several viable strategies for bandgap engineering,including thickness,strain or pressure,alloying,heterostructure,surface modification,intercalation,and so on.Furthermore,we summarize the optoelectronic devices fabricated with such high-mobility 2D semiconductors.The conclusion and outlook in this topic are finally presented.This review aims to provide valuable insights in bandgap engineering of newly emerging 2D semiconductors and explore their potential in future optoelectronic applications.

Bandgap engineering of tetragonal phase CuFeS_(2)quantum dots via mixed-valence single-atomic Ag decoration for synergistic Cr(VI)reduction and RhB degradation

Bandgap engineering through single-atom site binding on semiconducting photocatalyst can boost the intrinsic activity,selectivity,carrier separation,and electron transport.Here,we report a mixed-valence Ag(0)and Ag(I)single atoms co-decorated semiconducting chalcopyrite quantum dots(Ag/CuFeS_(2)QDs)photocatalyst.It demonstrates efficient photocatalytic performances for specific organic dye(rhodamine B,denoted as RhB)as well as inorganic dye(Cr(VI))removal in water under natural sunlight irradiation.The RhB degradation and Cr(VI)removal efficiencies by Ag/CuFeS_(2)QDs were 3.55 and 6.75 times higher than those of the naked CuFeS_(2)QDs at their optimal pH conditions,respectively.Besides,in a mixture of RhB and Cr(VI)solution under neutral condition,the removal ratio has been elevated from 30.2%to 79.4%for Cr(VI),and from 95.2%to 97.3%for RhB degradation by using Ag/CuFeS_(2)QDs after 2 h sunlight illumination.The intrinsic mechanism for the photocatalytic performance improvement is attributed to the narrow bandgap of the single-atomic Ag(I)anchored CuFeS_(2)QDs,which engineers the electronic structure as well as expands the optical light response range.Significantly,the highly active Ag(0)/CuFeS_(2)and Ag(I)/CuFeS_(2)effectively improve the separation efficiency of the carriers,thus enhancing the photocatalytic performances.This work presents a highly efficient single atom/QDs photocatalyst,constructed through bandgap engineering via mixed-valence single noble metal atoms binding on semiconducting QDs.It paves the way for developing high-efficiency single-atom photocatalysts for complex pollutions removal in dyeing wastewater environment.

Application of machine learning in perovskite materials and devices:A review

Metal-halide hybrid perovskite materials are excellent candidates for solar cells and photoelectric devices.In recent years,machine learning(ML)techniques have developed rapidly in many fields and provided ideas for material discovery and design.ML can be applied to discover new materials quickly and effectively,with significant savings in resources and time compared with traditional experiments and density functional theory(DFT)calculations.In this review,we present the application of ML in per-ovskites and briefly review the recent works in the field of ML-assisted perovskite design.Firstly,the advantages of perovskites in solar cells and the merits of ML applied to perovskites are discussed.Secondly,the workflow of ML in perovskite design and some basic ML algorithms are introduced.Thirdly,the applications of ML in predicting various properties of perovskite materials and devices are reviewed.Finally,we propose some prospects for the future development of this field.The rapid devel-opment of ML technology will largely promote the process of materials science,and ML will become an increasingly popular method for predicting the target properties of materials and devices.

Effect of Lewis acid-base additive on lead-free Cs_(2)SnI_(6) thin film prepared by direct solution coating process

Inorganic Cs_(2)SnI_(6) perovskite has exhibited substantial potential for light harvesting due to its exceptional optoelectronic properties and remarkable stability in ambient conditions.The charge transport characteristics within perovskite films are subject to modulation by various factors,including crystalline orientation,morphology,and crystalline quality.Achieving preferred crystalline orientation and film morphology via a solution-based process is challenging for Cs_(2)SnI_(6) films.In this work,we employed thiourea as an additive to optimize crystal orientation,enhance film morphology,promote crystallization,and achieve phase purity.Thiourea lowers the surface energy of the(222)plane along the(111)direction,confirmed by x-ray diffraction,x-ray photoelectron spectroscopy,ultraviolet photoelectron spectroscopy studies,and density functional theory calculations.Varying thiourea concentration enables a bandgap tuning of Cs_(2)SnI_(6) from 1.52 eV to1.07 eV.This approach provides a novel method for utilizing Cs_(2)SnI_(6) films in high-performance optoelectronic devices.

Germanene nanomeshes: Cooperative effects of degenerate perturbation and uniaxial strain on tuning bandgap

Based on the detailed first-principles calculations, we have carefully investigated the defect induced band splitting and its combination with Dirac cone move in bandgap opening. The uniaxial strain can split the π -like bands into πa and πz bands with energy interval Estrain to shift the Dirac cone. Also, the inversion symmetry preserved antidot can split πa （πz） into πa1 and πa2 （πz1 and πz2） bands with energy interval Edefect to open bandgap in the nanomesh with Γ as four-fold degenerate Dirac point according to the band-folding analysis. Though the Edefect would keep almost unaffected, the Estrain would be increased by enhancing the uniaxial strain to continuously tune the gap width. Then the bandgap can be reversibly switched on/off. Our studies of the inversion symmetry preserved nanomesh show distinct difference in bandgap opening mechanism as compared to the one by breaking the sublattice equivalence in the （GaAs）6 nanoflake patterned nanomesh. Here, the π-band gap remains almost unchanged against strain enhancing.

Prediction of a monolayer spin-spiral semiconductor:CoO with a honeycomb lattice

The recent successful fabrication of two-dimensional(2D)CoO with nanometer-thickness motivates us to investigate monolayer CoO due to possible magnetic properties induced by Co atoms.Here,we employ first-principles calculations to show that monolayer CoO is a 2D spin-spiral semiconductor with a honeycomb lattice.The calculated phonon dispersion reveals the monolayer's dynamical stability.Monolayer CoO exhibits a type-I spin-spiral magnetic ground state.The spinspiral state and the direct bandgap character are both robust under biaxial compressive strain(-5%)to tensile strain(5%).The bandgap varies only slightly under either compressive or tensile strain up to 5%.These results suggest a potential for applications in spintronic devices and offer a new platform to explore magnetism in the 2D limit.

Controllable Vapor Growth of Large-Area Aligned CdS_xSe_(1-x) Nanowires for Visible Range Integratable Photodetectors

The controllable growth of large area band gap engineered-semiconductor nanowires(NWs) with precise orientation and position is of immense significance in the development of integrated optoelectronic devices. In this study, we have achieved large area in-plane-aligned CdS_xSe_(1-x) nanowires via chemical vapor deposition method. The orientation and position of the alloyed CdS_xSe_(1-x)NWs could be controlled well by the graphoepitaxial effect and the patterns of Au catalyst. Microstructure characterizations of these as-grown samples reveal that the aligned CdS_xSe_(1-x)NWs possess smooth surface and uniform diameter. The aligned CdS_xSe_(1-x)NWs have strong photoluminescence and high-quality optical waveguide emission covering almost the entire visible wavelength range. Furthermore, photodetectors were constructed based on individual alloyed CdS_xSe_(1-x)NWs. These devices exhibit high performance and fast response speed with photoresponsivity ～670 A W^(-1) and photoresponse time ～76 ms. Present work provides a straightforward way to realize in-plane aligned bandgap engineering in semiconductor NWs for the development of large area NW arrays,which exhibit promising applications in future optoelectronic integrated circuits.

Intrinsic photocatalytic water oxidation activity of Mn-doped ferroelectric BiFeO3

The development of stable and efficient visible light-absorbing oxide-based semiconductor photocatalysts is a desirable task for solar water splitting applications.Recently,we proposed that the low photocurrent density in film-based BiFeO_(3)(BFO)is due to charge recombination at the interface of the domain walls,which could be largely reduced in particulate photocatalyst systems.To demonstrate this hypothesis,in this work we synthesized particulate BFO and Mn-doped BiFeO_(3)(Mn-BFO)by the sol-gel method.Photocatalytic water oxidation tests showed that pure BFO had an intrinsic photocatalytic oxygen evolution reaction(OER)activity of 70μmol h^(-1) g^(-1),while BFO-2,with an optimum amount of Mn doping(0.05%),showed an OER activity of 255μmol h^(-1) g^(-1) under visible light(λ≥420 nm)irradiation.The bandgap of Mn-doped BFO could be reduced from 2.1 to 1.36 eV by varying the amount of Mn doping.Density functional theory(DFT)calculations suggested that surface Fe(rather than Mn)species serve as the active sites for water oxidation,because the overpotential for water oxidation on Fe species after Mn doping is 0.51 V,which is the lowest value measured for the different Fe and Mn species examined in this study.The improved photocatalytic water oxidation activity of Mn-BFO is ascribed to the synergistic effect of the bandgap narrowing,which increases the absorption of visible light,reduces the activation energy of water oxidation,and inhibits the recombination of photogenerated charges.This work demonstrates that Mn doping is an effective strategy to enhance the intrinsic photocatalytic water oxidation activity of particulate ferroelectric BFO photocatalysts.

Designing highly transparent cerium doped Y_(2)O_(3) ceramics with high mechanical and thermal properties for UV-shielding in extreme conditions

Ultraviolet(UV)radiation poses risks to both human health and organics.In response to the urgent demand for UV-shielding across various applications,extensive endeavors have been dedicated to developing UV-shielding materials spanning from wide-bandgap semiconductors to organo-inorganic composite films.However,existing UV shielding materials,though suitable for daily use,cannot meet the demands of extreme conditions.In this work,we incorporated CeO_(2)as a UV absorber into Y_(2)O_(3)transparent ceramics for UV-shielding.The effect of CeO_(2)concentration on the optical,mechanical,and thermal properties of Y_(2)O_(3)ceramics was systematically investigated.These findings indicate that CeO_(2)serves not only as a UV absorber but also as an effective sintering aid for Y_(2)O_(3)transparent ceramics.The 5 at%Ce-doped Y_(2)O_(3)transparent ceramics exhibit the optimal optical quality,with in-line transmittance of~77%at 800 nm.The introduction of Ce shifted the UV cutoff edge of Y_(2)O_(3)transparent ceramics from 250 to 375 nm,which was attributed to the visible band absorption of Ce^(4+).This shift grants UV shielding capabilities to Y_(2)O_(3)transparent ceramics,resulting in 100%shielding for ultraviolet C(UVC,100-280 nm)and ultraviolet B(UVB,280-320 nm)and~95%shielding for ultraviolet A(UVA,320-400 nm).The service stability(optical properties)under various corrosive conditions(acid,alkali,UV irradiation,and high temperature)was investigated,confirming the excellent stability of this transparent ceramic UV-shielding material.A comparison of the performance parameters of transparent ceramics with those of traditional UV shielding materials such as glasses,films,and coatings was conducted.Our work provides innovative design concepts and an effective solution for UVshielding materials for extreme conditions.

Bandgap tunable preparation of GaS nanosheets and their application in photoelectrochemical photodetectors

Bandgap engineering of two-dimensional(2D)materials is essential for the design of photoelectrochemical(PEC)devices.Gallium(II)sulfide(GaS),a layered semiconductor material with a direct bandgap of approximately 3.05 eV,has recently gained extensive attention owing to its unique photoresponse property.However,its bandgap tunability relative to the number of layers has not been experimentally confirmed;thus,the effect of bandgap on the photoresponse has not been explored yet.Herein,fewlayered GaS nanosheets(Ns)are prepared using a simple liquid-phase exfoliation(LPE)approach.After centrifuging at different speeds,GaS Ns with defined layers are obtained,which enable verification of the tunable bandgap from 2.02 to 3.15 eV.When applied as a PEC-type photodetector,the responsivity of the photodetector is 4.77 mA W^(−1)and 33.7μA W^(−1)under bias voltages of 0.6 and 0 V,respectively.Theoretical models of the electronic structure suggest that a reduction in the number of layers,leading to a decrease of the effective mass at the valence band maximum(VBM),can enhance the carrier mobility of GaS Ns.This results in high photocurrents and indicates that 2D GaS Ns are ideal materials for future high-performance optoelectronic systems.

Fluorination vs. chlorination： a case study on high performance organic photovoltaic materials

Halogenation is a very efficient chemical modification method to tune the molecular energy levels, absorption spectra and molecular packing of organic semiconductors. Recently, in the field of organic solar cells(OSCs), both fluorine-and chlorinesubstituted photovoltaic materials, including donors and acceptors, demonstrated their great potentials in achieving high power conversion efficiencies(PCEs), raising a question that how to make a decision between fluorination and chlorination when designing materials. Herein, we systemically studied the impact of fluorination and chlorination on the properties of resulting donors(PBDB-T-2 F and PBDB-T-2 Cl) and acceptors(IT-4 F and IT-4 Cl). The results suggest that all the OSCs based on different donor and acceptor combinations can deliver good PCEs around 13%–14%. Chlorination is more effective than fluorination in downshifting the molecular energy levels and broadening the absorption spectra. The influence of chlorination and fluorination on the crystallinity of the resulting materials is dependent on their introduction positions. As chlorination has the advantage of easy synthesis, it is more attractive in designing low-cost photovoltaic materials and therefore may have more potential in largescale applications.

Superionic conductor-mediated growth of ternary ZnCdS nanorods over a wide composition range

Composition regulation of semiconductors can engineer their bandgaps and hence tune their properties. Herein, we report the first synthesis of ternary ZnxCd1-xS semiconductor nanorods by superionic conductor （AgRS）-mediated growth with [（C4H9）2NCS2]2M （M = Zn, ca） as single-source precursors. The compositions of the ZnKCd1-xS nanorods are conveniently tuned over a wide range by adjusting the molar ratio of the corresponding precursors, leading to tunable bandgaps and hence the progressive evolution of the light absorption and photoluminescence spectra. The nanorods present well-distributed size and length, which are controlled by the uniform Ag2S nanoparticles and the fixed amount of the precursors. The results suggest the great potential of superionic conductor-mediated growth in composition regulation and bandgap engineering of chalcogenide nanowires/nanorods.

Perovskite/Si tandem solar cells: Fundamentals, advances,challenges, and novel applications

The world record device efficiency of single-junction solar cells based on organic–inorganic hybrid perovskites has reached 25.5%.Further improvement in device power conversion efficiency(PCE)can be achieved by either optimizing perovskite films or designing novel device structures such as perovskite/Si tandem solar cells.With the marriage of perovskite and Si solar cells,a tandem device configuration is able to achieve a PCE exceeding the Shockley–Queisser limit of single-junction solar cells by enhancing the usage of solar spectrum.After several years of development,the highest PCE of the perovskite/Si tandem cell has reached 29.5%,which is higher than that of perovskite-and Si-based singlejunction cells.Here,in this review,we will(1)first discuss the device structure and fundamental working principle of both two-terminal(2T)and four-terminal(4T)perovskite/Si tandem solar cells;(2)second,provide a brief overview of the advances of perovskite/Si tandem solar cells regarding the development of interconnection layer,perovskite active layer,tandem device structure,and lightmanagement strategies;(3)third,discuss the challenges and opportunities for further developing perovskite/Si tandem solar cells.This review article,on the one hand,provides a comprehensive understanding to readers on the development of perovskite/Si tandems.On the other hand,it proposes various novel applications that may bring such tandems into the market in a near future.

Revealing the output power potential of bifacial monolithic all-perovskite tandem solar cells

Bifacial monolithic all-perovskite tandem solar cells have the promise of delivering higher output power density by inheriting the advantages of both tandem and bifacial architectures simultaneously.Herein,we demonstrate,for the first time,the bifacial monolithic all-perovskite tandem solar cells and reveal their output power potential.The bifacial tandems are realized by replacing the rear metal electrodes of monofacial tandems with transparent conduction oxide electrodes.Bandgap engineering is deployed to achieve current matching under various rear illumination conditions.The bifacial tandems show a high output power density of 28.51 mW cm−2 under a realistic rear illumination(30 mW cm−2).Further energy yield calculation shows substantial energy yield gain for bifacial tandems compared with the monofacial tandems under various ground albedo for different climatic conditions.This work provides a new device architecture for higher output power for all-perovskite tandem solar cells under real-world conditions.