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.

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.

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.

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.

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.

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.