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In halide perovskite solar cells the formation of secondary-phase excess lead iodide (PbI 2 ) has some positive effects on power conversion efficiency (PCE) but can be detrimental to device stability and lead to large hysteresis effects in voltage sweeps. We converted PbI 2 into an inactive (PbI 2 ) 2 RbCl compound by RbCl doping, which effectively stabilizes the perovskite phase. We obtained a certified PCE of 25.6% for FAPbI 3 (FA, formamidinium) perovskite solar cells on the basis of this strategy. Devices retained 96% of their original PCE values after 1000 hours of shelf storage and 80% after 500 hours of thermal stability testing at 85°C.

Quantum emitters are needed for a myriad of applications ranging from quantum sensing to quantum computing. Hexagonal boron nitride (hBN) quantum emitters are one of the most promising solid-state platforms to date due to their high brightness and stability and the possibility of a spin-photon interface. However, the understanding of the physical origins of the single-photon emitters (SPEs) is still limited. Here we report dense SPEs in hBN across the entire visible spectrum and present evidence that most of these SPEs can be well explained by donor-acceptor pairs (DAPs). On the basis of the DAP transition generation mechanism, we calculated their wavelength fingerprint, matching well with the experimentally observed photoluminescence spectrum. Our work serves as a step forward for the physical understanding of SPEs in hBN and their applications in quantum technologies. Ministry of Education (MOE) National Research Foundation (NRF) Submitted/Accepted version J.Z. acknowledges support from the National Basic Research Program of China (grant no. 2017YFA0303401, 2016YFA0301200), Beijing Natural Science Foundation (JQ18014), and Strategic Priority Research Program of Chinese Academy of Sciences (grant no. XDB28000000) and CAS Interdisciplinary Innovation team. W.G. acknowledges the Singapore National Research Foundation and DSO National Laboratories under the QEP grants NRF2021- QEP2-03-P01, 2019-0643 (QEP-P2), and 2019-1321 (QEPP3), CRP award nos. NRF-CRP21-2018-0007, NRF-CRP22- 2019-0004, and NRF-CRP23-2019-0002, and the Singapore Ministry of Education (MOE2016-T3-1-006 (S)). I.A. acknowledges the financial support from the Australian Research Council (via CE200100010).

Multiferroic materials have great potential in non-volatile devices for low-power and ultra-high density information storage, owing to their unique characteristic of coexisting ferroelectric and ferromagnetic orders. The effective manipulation of their intrinsic anisotropy makes it promising to control the multiple degrees of freedom of the storage "medium". Here, we have discovered intriguing electrical and magnetic anisotropies within the intralayer of CuCrP2S6, a promising van der Waals multiferroic material. The in-plane uniaxial anisotropies of the current rectifications, magnetic properties and magnon modes are demonstrated and manipulated by electric direction/polarity, temperature variation and magnetic field. More important, we have observed spin-flop transition corresponding to specific magnon modes, and it is well supported by theoretical calculations. Our work provides the first observation of electrical and magnetic anisotropies with same easy axis in van der Waals multiferroics, which will stimulate novel device applications of artificial bionic synapses, multi-terminal spintronic chips and magnetoelectric devices.

The intentionally designed band alignment of heterostructures and doping engineering are keys to implement device structure design and device performance optimization. According to the theoretical prediction of several typical materials among the transition metal dichalcogenides (TMDs) and group-IV metal chalcogenides, MoS

In halide perovskite solar cells the formation of secondary-phase excess lead iodide (PbI

Tetragonal ZrO2 high-k material as the dielectric layer of dynamic random access memory (DRAM) capacitors faces bulk defect related leakage current, which is one of the main obstacles to the down-scaling of DRAM devices. Boron and hydrogen impurities are known to be responsible for leakage current degradation and are hard to be removed in DRAM capacitors. However, the defect origins of boron and hydrogen leakage current are still puzzling, and corresponding suppression methods are urged. In this work, the properties of boron and hydrogen impurities in tetragonal ZrO2 are investigated using first-principles calculations, and defect types such as boron and hydrogen interstitials are discovered to have detrimental defect levels related to leakage current. Based on the discovery, a chlorine co-doping approach that can passivate detrimental defects by forming defect complexes is further proposed. By introducing level repulsion due to coupling between defect states, defect levels of passivated defect complexes are moved out of the region of leakage current contribution. Thus, bulk defect related leakage current in tetragonal ZrO2 based DRAM capacitors can be effectively suppressed without device structure modification, and a broad vista is opened for next-generation DRAM devices.

Finding sustainable and renewable energy to replace traditional fossil fuel is critical for reducing greenhouse gas emission and avoiding environment pollution. Solar cells that convert energy of sunlight into electricity offer a viable route for solving this issue. At present, halide perovskites are the most potential candidate materials for solar cell with considerable power conversion efficiency, whereas their stability remains a challenge. In this work, we summarize four different key factors that influence the stability of halide perovskites: (a) effect of environmental moisture on the degradation of halide perovskites. The performance of halide perovskite solar cells is reduced due to hydrated crystal hinders the diffusion of photo‐generated carriers, which can be solved by materials encapsulation technique; (b) photo‐induced instability. Through uncovering the underlying physical mechanism, we note that materials engineering or novel device structure can extend the working life of halide perovskites under continuous light exposure; (c) thermal stability. Halide perovskites are rapidly degraded into PbI2 and volatile substances as heating due to lower formation energy, whereas hybrid perovskite is little changed; (d) electric field effect in the degradation of halide perovskites. The electric field impacts significantly on the carrier separation, changes direction of photo‐induced currents and generates switchable photovoltaic effect. For each key factor, we have shown in detail the underlying physical mechanisms and discussed the strategies to overcome this stability difficulty. We expect this review from both theoretical and experimental points of view can be beneficial for development of perovskite solar cell materials and promotes practical applications.

Two-dimensional (2D) Janus materials with large exciton binding energies have attracted enormous attention for their novel photoelectric properties. Here, the quasiparticle (QP) band structures and...

Perovskite light-emitting diodes (PeLEDs) have showed significant progress in recent years; the external quantum efficiency (EQE) of electroluminescence in green and red regions has exceeded 20%, but the efficiency in blue lags far behind. Here, a large cation CH3CH2NH2+ is added in PEA2(CsPbBr3)2PbBr4 perovskite to decrease the Pb–Br orbit coupling and increase the bandgap for blue emission. X-ray diffraction and nuclear magnetic resonance results confirmed that the EA has successfully replaced Cs+ cations to form PEA2(Cs1-xEAxPbBr3)2PbBr4. This method modulates the photoluminescence from the green region (508 nm) into blue (466 nm), and over 70% photoluminescence quantum yield in blue is obtained. In addition, the emission spectra is stable under light and thermal stress. With configuration of PeLEDs with 60% EABr, as high as 12.1% EQE of sky-blue electroluminescence located at 488 nm has been demonstrated, which will pave the way for the full color display for the PeLEDs., Blue light-emitting diodes (LEDs) are critical for displays. Employing a large organic cation into a quasi-two dimensional perovskite with green emission, Chu et al. achieve LEDs exhibiting a high external quantum efficiency of 12.1% and stable spectra in the sky-blue region.

Semiconductor materials play a central role in the integrated circuits, photovoltaics, information and communication, microelectronic devices, lighting, and so on. However, the performance of semiconductor optoelectronic devices depends critically on the dopability and defect engineering of semiconductor materials. In this paper, firstly, we introduce the doping properties in semiconductors and the theoretical progress of charged defect calculations over the past decades, including the traditional jellium model. To overcome the disadvantages and limitations of the traditional jellium model, recently, we propose a straightforward and universal theory, i.e., transfer real state model (TRSM), which can calculate directly the charged defect properties in both bulk and low-dimensional semiconductors. In the jellium model for a finite supercell size calculation, it suffers a serious problem to determine the defect properties in low-dimensional semiconductors due to charge distribution in the vacuum region. However, our TRSM method by putting the ionized electrons or holes on a real host band edge states naturally keeps the supercell neutral and provides clear physical meaning. For three-dimensional bulk materials, the defect formation energy and transition energy level calculated by our TRSM method are almost the same as the results obtained by using the traditional jellium model. For low dimensional semiconductors, however, the TRSM method cures the divergence issue that occurred in the jellium model due to long-range electrostatic Coulomb interactions. Secondly, we discuss the wide band gap semiconductors. By analyzing the doping limit law, we elucidate the effective methods for defect engineering in oxide wide band gap semiconductors, and then how their p-type conductivity can be improved. Those methods are based on two rules: (1) Reducing the ionization energy of acceptors; (2) suppressing the formation of compensating donors. Further, the physical mechanism of the difference in conductivity between ionic and covalent compounds in the amorphous wide band gap semiconductors is introduced. A band coupling model is employed to clarify the difference between pseudo-hydrogen passivated and real-hydrogen passivated low dimensional wide band gap semiconductors. Thirdly, we discuss the semiconductor alloys with high doping concentration. In the nonisovalent semiconductor alloys, due to strong wave function localization of the band edge states, the physical properties of nonisovalent alloys do not meet the statistical average law, distinct from the isovalent alloys. The band crossing phenomenon in large mismatched isovalent semiconductor alloys is often misinterpreted by a phenomenological two-level band anti-crossing model. Fortunately, this phenomenon has now been properly explained by the band broadening picture. When doping small concentration of Bi or N impurities into GaAs, the defect levels are localized due to the weak interactions between impurities. The band edges of GaAs1− x Bi x and GaAs1− x N x consist of the host atoms. The impurity level gradually broadens out with the increase of the impurity concentration, and thus the band edges of defective GaAs are dominated by impurities. Finally, we focus on the diffusion of metal impurities in semiconductors. The fundamental reason for the differences in the diffusion of Ag and Cu atoms between ionic and covalent semiconductors is clarified. The s-d coupling between the d orbitals of the diffusors and the s orbitals of the host materials are responsible for the behavior of diffusive atoms. The deep understanding of metal impurities in semiconductor materials offers effective theoretical guidance for controlling the diffusion properties of impurities in different types of semiconductor materials.

Recent years have seen swift increase in the power conversion efficiency of perovskite solar cells (PSCs). Interface engineering is a promising route for further improving the performance of PSCs. Here we perform first-principles calculations to explore the effect of four candidate buffer materials (MACl, MAI, PbCl2 and PbI2) on the electronic structures of the interface between MAPbI3 absorber and TiO2. We find that MAX (X = Cl, I) as buffer layers will introduce a high electron barrier and enhance the electron-hole recombination. Additionally, MAX does not passivate the surface states well. The conduction band minimum of PbI2 is much lower than that of MAPbI3 absorber, which significantly limits the band bending of the absorber and open-circuit voltage of solar cells. On the other side, suitable bandedge energy level positions, small lattice mismatch with TiO2 surfaces, and excellent surface passivation make PbCl2 a promising buffer material for absorber/electron-transport-layer interface engineering in PSCs. Our results in this work thus provide deep understanding on the effects of interface engineering with a buffer layer, which shall be useful for improving the performance of PSCs and related optoelectronics.

Ge has the potential to replace Si as the future field-effect transistors channel material due to its superior hole mobility, and cubic zirconia with high dielectric constant and small lattice mismatch can be selected as its oxide layer. At present, the mechanism of charge trapping caused by defects in the Ge oxide layer interface and bulk of such device has not been accurately analyzed. In our work, we have constructed the cubic Ge/ZrO2 interface, and studied the electronic structure and hole trapping characteristics of the interface structure by first-principles hybrid-functional calculations with Marcus theory. According to research oxygen vacancies with the different distances (abbr. d O-int) away from the Ge substrate, we confirm that the oxygen vacancy can act as a fast trap center to capture the hole of the valence band maximum (VBM) from Ge, resulting in the ultrafast or fast transient charge trapping in the high-k gate dielectric. We found that, when a given range of applied electric field, the hole trap is ultrafast with capture time of 10−6–10−5 μ s when d O-int is within the range of 2–7 A ∘ , and there is a 2–3 order of magnitude increases in capture time as d O-int exceeds 7 A ∘ with the maximum capture cross section reducing substantially. Here, our work provides a clear and reasonable description of the distance-dependent hole trapping process at the Ge/high-k dielectrics metal-oxide-semiconductor (MOS) devices and provides significant support for solving the reliability problem of microelectronic devices.

Two-dimensional (2D) materials have been attracted highly interest in recent years due to their low structural symmetry, excellent photoresponse and high air stability. However, most 2D materials can only response to specific light, which limits the development of wide spectrum photodetectors. Proper band gap and the regulation of Fermi level are foundations for realizing electronic multi-channel transitions, which is an effective method to achieve wide spectral response. Herein, a noble 2D material, Palladium phosphide sulfide (PdPS), is designed and synthesized. The band gap of PdPS is around 2.1 eV and the formation of S vacancies, interstitial Pd and P atoms promote the Fermi level very close to the conduction band. Therefore, PdPS-based photodetector shows impressive wide spectral response from solar blind ultraviolet (SBUV) to near infrared (NIR) based on the multi-channel tranisiton. It also exhibits superior optoelectrical properties with photoresponsivity (R) of 1×103 A/W and detectivity (D*) of 4×1011 Jones at 532 nm. Moreover, PdPS exhibits good performance of polarization detection with dichroic ratio of ∼3.7 at 808 nm. Significantly, it achieves polarimetric imaging and hidden target detection in complex environments through active detection. This article is protected by copyright. All rights reserved.

The application of germanium (Ge)-based transistors has long been restricted by the poor reliability of the gate dielectrics. One solution proposed in the experiment is capping the GeO[Formula: see text] layer with high-k dielectrics and further doping the dielectric with yttrium (Y) atoms. However, the strategy only works at a very small doping concentration window, and the underlying mechanism remains unclear. Here, we carry out first-principles calculations on a concrete Ge/GeO[Formula: see text]/ZrO[Formula: see text] stack to study the structural and electronic properties of various defects before and after Y-doping and further calculate their exact charge-trapping rates by the Marcus charge transfer theory. We show that the Y atoms can effectively weaken the charge-trapping capability of vacancy defects in the ZrO[Formula: see text] layer, but on the other hand, they can induce some new types of active defects if the density is high. In addition, it is found that the Y atoms can have a very different effect even when doped to the same material. These results indicate that a precise control of the doping position and doping concentration is necessary to promote the reliability of Ge transistors.

Thermoelectric materials which enable heat-to-electricity conversion are fundamentally important for heat management in semiconductor devices. Achieving high thermoelectric performance requires blocking the thermal transport and maintaining the high electronic transport, but it is a challenge to satisfy both criteria simultaneously. We propose that tuning the interlayer distance can effectively modulate the electrical and thermal conductivities. We find group IV-VI and V semiconductors with a moderate interlayer distance can exhibit high thermoelectric performance. Taking SnSe as an example, we reveal that in the out-of-plane direction the delocalized pz orbitals combined with the relatively small interlayer distance lead to overlapping of the antibonding state wave functions, which is beneficial for high electronic transport. However, because of the breakdown of the chemical bond, the out-of-plane thermal conductivity is small. This study provides a strategy to enhance electrical conductivity without increasing thermal conductivity and thus sheds light on the design of thermoelectric devices.

Carbon vacancy ${\mathrm{V}}_{\mathrm{C}}$ is the dominant detrimental defect in SiC, and hydrogen passivation of ${\mathrm{V}}_{\mathrm{C}}$ is often used to facilitate its application in electronic devices. However, the exact nature of hydrogen passivation of ${\mathrm{V}}_{\mathrm{C}}$ in $4H$-SiC remains inconclusive in view of the available divergent experiment and theoretical findings. Here, using the Heyd-Scuseria-Ernzerhof screened hybrid density functional calculations, we demonstrate that the ${\mathrm{V}}_{\mathrm{C}}$ defect can capture up to four hydrogens, and the electrically active levels within the band gap can be entirely passivated, in line with recent reported experimental observations. This paper, thus, casts light on the role of hydrogen passivation in SiC.

Germanium is crystalized in the cubic diamond structure, but its high energy hexagonal Ge (lonsdaleite) phase has many novel properties such as direct band gap. Using first-principles calculations, we show that the hexagonal lonsdaleite phase of Ge can be stabilized by introducing carriers, either electrons or holes, because Ge in the cubic and hexagonal phases form a type-I band alignment with both electrons and holes localized at the hexagonal site. This result is distinct from that in zinc-blende compounds such as ZnSe, because due to the lack of inversion symmetry, the crystal-field splitting, zone folding, and symmetry-controlled level repulsion between valence and conduction band states lead to a type-II band alignment between its cubic and hexagonal phases, so the hexagonal (wurtzite) phase of ZnSe can only be stabilized, in principle, by holes. This distinction reveals that, due to the symmetry differences, the well-investigated understanding of band structure differences between zinc-blende and wurtzite phases should not be simply extended to that of diamond and lonsdaleite phases despite the remarkable structure resemblance between the two cases.

Nonradiative-recombination-related defects are significant for optoelectronic semiconductor devices. Here, we analyze nonradiative-recombination processes in ionic semiconductors using first-principles density-functional theory. In ionic group II-VI semiconductors, we find that large lattice relaxations of anion vacancies caused by strong Coulomb interactions between different charged defect states can significantly enhance recombination processes through a two-level recombination mechanism. Specifically, we show that the defect level of the 2+ charged anion vacancy $\mathrm{(}{V}_{\mathrm{Se}}^{2+})$ in group II-VI $\mathrm{Zn}\mathrm{Se}$ is close to the conduction-band minimum and easily captures an electron to form a metastable 1+ charged state $\mathrm{(}{V}_{\mathrm{Se}}^{+})$; then, the large lattice relaxation, on account of the change in Coulomb interactions locally in the different charged states, rapidly changes this metastable state to a stable one and simultaneously move the defect level of ${V}_{\mathrm{Se}}^{+}$ closer to that valence-band maximum, and thus, increases the hole-capture rate. Compared with the Shockley-Read-Hall nonradiative-recombination theory based on a single defect level, this two-level recombination mechanism involving anion vacancies can greatly increase the nonradiative-recombination rate in ionic group II-VI semiconductors. This understanding is expected to be useful for the study of the nonradiative-recombination process in ionic semiconductors for applications in the field of optoelectronic devices.

Traditional methods of discovering new materials, such as the empirical trial and error method and the density functional theory (DFT)‐based method, are unable to keep pace with the development of materials science today due to their long development cycles, low efficiency, and high costs. Accordingly, due to its low computational cost and short development cycle, machine learning is coupled with powerful data processing and high prediction performance and is being widely used in material detection, material analysis, and material design. In this article, we discuss the basic operational procedures in analyzing material properties via machine learning, summarize recent applications of machine learning algorithms to several mature fields in materials science, and discuss the improvements that are required for wide‐ranging application.

Transition metal dichalcogenides (TMDCs) with different thickness can greatly influence the performance of photonic devices. However, how to accurately control the layers of TMDCs and realize the a...

Defect states and exciton of two-dimensional semiconductors play an important role in fundamental research and device applications. Here, we reported the defect emissions and exciton–lattice intera...

We investigated the diffusion properties of metal atoms in van der Waals layered materials using first-principles calculations combined with group theory analysis. We found that there is an abnormal diffusion behavior of Cu in MoS2, which originated from the competition of the electronic and strain energies. Although the atom diffusing in bulk MoS2 constantly changes the symmetry of the system with reduced electronic energy due to p–d coupling between occupied Cu d and unoccupied anion p states, the strain energy is dominant. As a result, the energy barrier of metal atoms is mainly determined by their size, making the diffusion rate of Cu faster than those of other metal atoms. Nevertheless, in the monolayer MoS2, the strain energy is negligible and the electronic coupling is significant, so that the strong d–d coupling between occupied Cu d and unoccupied cation d states leads to the highest diffusion barrier of Cu atoms.

The effective manipulation of excitons is important for the realization of exciton-based devices and circuits, and doping is considered a good strategy to achieve this. While studies have shown that 2D semiconductors are ideal for excitonic devices, preparation of homogenous substitutional foreign-atom-doped 2D crystals is still difficult. Here we report the preparation of homogenous monolayer Sb-doped MoS2 single crystals via a facile chemical vapor deposition method. A and B excitons are observed in the Sb-doped MoS2 monolayer by reflection magnetic circular dichroism spectrum measurements. More important, compared with monolayer MoS2, the peak positions of two excitons show obvious shifts. Meanwhile, the degeneration of A exciton is also observed in the monolayer Sb-doped MoS2 crystal using photoluminescence spectroscopy, which is ascribed to the impurity energy levels within the band-gap, confirmed by density function theory. Our study opens a door to developing the doping of 2D layered transition metal dichalcogenides with group-V dopants, which is helpful for the fundamental study of the physical and chemical properties of transition metal dichalcogenides. Excitons in two-dimensional transition metal dichalcogenides can be tuned by incorporation of group-V dopants. A team led by Zhongming Wei at the Institute of Semiconductors, Chinese Academy of Sciences and Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences used chemical vapour deposition to synthesize Sb-doped MoS2 monolayers, and investigated the spectral position of the A and B excitons. Microscopy and spectroscopy results indicated that Sb doping is substitutional and homogeneous, and stably replaces Mo atoms in the MoS2 lattice, forming Mo0.91Sb0.09S2 crystals. A combination of reflection magnetic circular dichroism spectroscopy and photoluminescence spectroscopy revealed that the A and B excitons exhibit a clear shift if compared to pristine MoS2. Density functional theory calculations showed that Sb doping gives rise to formation of impurity-related energy level within the band-gap of stoichiometrically pure MoS2.

In recent years, with the rapid development of transfer technologies related to graphene and other two-dimensional layered materials (2DLMs), graphene sandwiched 2DLMs have been confirmed to be outstanding tunneling and optoelectronic devices. Here, compared to the planar SnSe2-Au device, the SnSe2 device with different thicknesses (12-256 nm) is incorporated into graphene sandwiched structures for photodetection. The results indicate that the photoresponse properties are dependent on the thickness and gate voltage. In particular, under 532 nm illumination and at a Vg of +80 V, the SnSe2 device with a thickness of 96.5 nm shows an impressively high responsivity of 1.3 × 103 A W-1, an external quantum efficiency of 3 × 105%, and a detectivity of 1.2 × 1012 Jones. Besides, a high response speed (a rise time of 30.2 ms and a decay time of 27.2 ms) and flat photoswitching behavior are achieved without the gate voltage. In addition, the intrinsic mechanisms are further discussed through the relative spatial potential difference and the band alignment diagrams of the graphene-SnSe2-graphene and Au-SnSe2-Au structures. These findings indicate that SnSe2 has great potential for practical applications in next generation high performance optoelectronics.

Ultra-wide band-gap nitrides have huge potential in micro- and optoelectronics due to their tunable wide band-gap, high breakdown field and energy density, excellent chemical and thermal stability. However, their application has been severely hindered by the low p-doping efficiency, which is ascribed to the ultrahigh acceptor activation energy originated from the low valance band maximum. Here, a valance band modulation mode is proposed and a quantum engineering doping method is conducted to achieve high-efficient p-type ultra-wide band-gap nitrides, in which GaN quantum-dots are buried in nitride matrix to produce a new band edge and thus to tune the dopant activation energy. By non-equilibrium doping techniques, quantum engineering doped AlGaN:Mg with Al content of 60% is successfully fabricated. The Mg activation energy has been reduced to about 21 meV, and the hole concentration reaches higher than 1018 cm−3 at room temperature. Also, similar activation energies are obtained in AlGaN with other Al contents such as 50% and 70%, indicating the universality of the quantum engineering doping method. Moreover, deep-ultraviolet light-emission diodes are fabricated and the improved performance further demonstrates the validity and merit of the method. With the quantum material growth techniques developing, this method would be prevalently available and tremendously stimulate the promotion of ultra-wide band-gap semiconductor-based devices.

A solar cell is a photovoltaic device that converts solar radiation energy to electrical energy, which plays a leading role in alleviating global energy shortages and decreasing air pollution levels typical of conventional fossil fuels. To render solar cells more efficient, high visible-light absorption rates and excellent carrier transport properties are required to generate high carrier levels and high output voltage. Hence, the core material, i.e., the absorption layer, should have an appropriate direct band gap and be effectively doped by both p- and n-types with minimal carrier traps and recombination centers. Consequently, defect properties of absorbers are critical in determining solar cell efficiency. In this work, we review recent first-principles studies of defect properties and engineering in four representative thin-film solar cells, namely CdTe, Cu(In,Ga)Se2, Cu2ZnSnS4, and halide perovskites. The focal points include basic electronic and defect properties, existing problems, and possible solutions in engineering defect properties of those materials to optimize solar cell efficiency.

Ultra-wide bandgap (UWBG) semiconductors have great potential for high-power electronics, radio frequency electronics, deep ultraviolet optoelectronic devices, and quantum information technology. Recently, the two-dimensional UWBG GaPS4 was first applied to the solar-blind photodetector in experiments, which was found to have remarkable performance, such as high responsivity, high quantum efficiency, etc., and promising applications in optoelectronic devices. However, the knowledge of monolayer (ML) GaPS4 for us is quite limited, which hinders its design and application in optoelectronic devices. Here, we focus on the properties of electronic structure and intrinsic defects in ML GaPS4 by first-principles calculations. We confirmed that the fundamental gap of ML GaPS4 is 3.87 [Formula: see text], while the optical gap is 4.22 [Formula: see text]. This discrepancy can be attributed to the inversion symmetry of its structure, which limits the dipole transitions from valence band edges to conduction band edges. Furthermore, we found that intrinsic defects are neither efficient p-type nor n-type dopants in ML GaPS4, which is consistent with experimental observations. Our results also show that if one expects to achieve p-type ML GaPS4 by selecting the appropriate dopant, P-rich conditions should be avoided for the growth process, while for achieving n-type doping, S-rich growth conditions are inappropriate. This is because due to the low strain energy, [Formula: see text] has very low formation energy, which leads to the Fermi levels ([Formula: see text]) pinning at 0.35 [Formula: see text] above the valence band maximum and is not beneficial to achieve p-type ML GaPS4 under the P-rich conditions; the large lattice relaxation largely lowers the formation energy of [Formula: see text], which causes the [Formula: see text] pinning at 0.72 [Formula: see text] below the conduction band minimum and severely prevents ML GaPS4 from being n-type doping under the S-rich conditions. Our studies of these fundamental physical properties will be useful for future applications of ML GaPS4 in optoelectronic devices.

The band offsets between semiconductors are significantly associated with the optoelectronic characteristics and devices design. Here, we investigate the band offset trends of few-layer and bulk IV–VI semiconductors MX and MX2 (M = Ge, Sn; X = S, Se, Te). For common-cation (anion) systems, as the atomic number increases, the valence band offset of MX decreases, while that of MX2 has no distinct change, and the physical origin can be interpreted using band coupling mechanism and atomic potential trend. The band edges of GeX2 system straddle redox potentials of water, making them competitive candidates for photocatalyst. Moreover, layer number modulation can induce the band offset of GeSe/SnS and GeSe2/GeS2 heterojunction undergoing a transition from type I to type II, which makes them suitable for optoelectronic applications.

Phosphorene, a two-dimensional material that can be exfoliated from black phosphorus, exhibits remarkable mechanical, thermal, electronic, and optical properties. In this work, we demonstrate that the unique structure of pristine phosphorene endows this material with exceptional quantum-mechanical performance by using first-principles calculations. Upon charge injection, the maximum actuation stress is 7.0 GPa, corresponding to the maximum actuation strain as high as 36.6% that is over seven times larger than that of graphene (4.7%) and comparable with natural muscle (20–40%). Meanwhile, the maximum volumetric work density of phosphorene (207.7 J/cm3) is about three orders of magnitude larger than natural muscle (0.008–0.04 J/cm3) and approximately six times larger than graphene (35.3 J/cm3). The underlying mechanism of this exceptional electromechanical performance in phosphorene is well revealed from the analysis of atomic structure and electronic structure. Finally, the influence of charge on the mechanical behaviors of phosphorene is examined by mechanical tests, indicating the sufficient structural integrity of phosphorene under the combined electromechanical loading. These findings shed light on phosphorene for promising applications in developing nanoelectromechanical actuators.

The exploration of promising nonlinear optical materials, which allows for the construction of high-performance optical devices in fundamental and industrial applications, has become one of the fastest-evolving research interests in recent decades and plays a key role in the development and innovation of optics in the future. Here, by utilizing the optical nonlinearity of a recently synthesized, two dimensional material AuTe2Se4/3 prepared by the self-flux method, a passively mode-locked fiber laser operating at 1557.53 nm is achieved with 147.7 fs pulse duration as well as impressive stability (up to 91 dB). The proposed mode-locked fiber laser reveals superior overall performance compared with previously reported lasers which are more widely studied in the same band. Our work not only investigates the optical nonlinearity of AuTe2Se4/3, but also demonstrates its ultrafast photonics application. These results may stimulate further innovation and advancement in the field of nonlinear optics and ultrafast photonics. Two dimensional materials can exhibit unique optical properties, making them interesting for new photonic devices and laser sources. Here, the strong optical nonlinearity of AuTe2Se4/3 is exploited to achieve a femtosecond infra-red laser with high stability.

Structural symmetry is a simple way to quantify the anisotropic properties of materials toward unique device applications including anisotropic transportation and polarization-sensitive photodetection. The enhancement of anisotropy can be achieved by artificial symmetry-reduction design. A core-shell SbI

Magnetic tunneling junctions (MTJs) have atomic thickness due to the use of two-dimensional (2D) materials. Combining density functional theory with non-equilibrium Green's function formalism, we systematically investigate the structural and magnetic properties of CrX3/h-BN/CrX3 (X = Br, I) MTJs, as well as their spin-dependent transport characteristics. Through calculation of the transmission spectrum, the large tunneling magnetoresistance (TMR) effect was observed in these MTJs. Moreover, their conductance based on two-dimensional materials was greatly improved over that of traditional MTJs. The transmission mechanism was analyzed using the symmetry of the orbit and the eigenstates of the transmitted electrons. We also discuss the problem of Schottky contact between different metal electrodes and devices. Our results suggest that MTJs based on two-dimensional ferromagnets are feasible.

Forming novel van der Waals (vdW) heterostructures by combining different two-dimensional (2D) materials is significant to achieve more desirable properties. Using first-principles calculations, we demonstrate the electronic and optical properties of the InSe/InTe van der Waals heterostructure. Our results suggest that this heterostructure has an intrinsic type-II band alignment with a direct band gap. The electrons and holes are respectively localized in the InSe and InTe layers. The spatial separation of the lowest energy electron–hole pairs can occur, implying that the InSe/InTe heterostructure is a good candidate for a high-efficiency solar cell. In addition, the optical absorption in heterostructures can be enhanced compared with both of the monolayers. Moreover, tuning of the values with a direct band gap can be induced by applying normal strain, and the band gap exhibits linear variation. Meanwhile, an intrinsic type-II band alignment can be tuned to become type-I, which is suitable for light emission applications. These results indicate that the flexible InSe/InTe vdW heterostructure can provide new ways to utilize two-dimensional materials in future optoelectronic devices.