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Because of the excellent electrical properties, III–V semiconductor nanowires are promising building blocks for next-generation electronics; however, their rich surface states inevitably contribute large amounts of charge traps, leading to gate bias stress instability and hysteresis characteristics in nanowire field-effect transistors (FETs). Here, we investigated thoroughly the gate bias stress and hysteresis effects in InAs nanowire FETs. It is observed that the output current decreases together with the threshold voltage shifting to the positive direction when a positive gate bias stress is applied, and vice versa for the negative gate bias stress. For double-sweep transfer characteristics, the significant hysteresis behavior is observed, depending heavily on the sweeping rate and range. On the basis of complementary investigations of these devices, charge traps are confirmed to be the dominant factor for these instability effects. Importantly, the hysteresis can be simulated well by utilizing a combination of the rate equation for electron density and the empirical model for electron mobility. This provides an accurate evaluation of carrier mobility, which is in distinct contrast to the overestimation of mobility when using the transconductance for calculation. All these findings are important for understanding the charge trap dynamics to further enhance the device performance of nanowire FETs.

Spinel oxides (AB2O4) with unique crystal structures have been widely explored as promising alternative catalysts for efficient oxygen evolution reactions; however, developing novel methods to fabricate robust, cost-effective, and high-performance spinel oxide based electrocatalysts is still a great challenge. Here, utilizing a complementary experimental and theoretical approach, pentavalent vanadium doping in the spinel oxides (i.e., Co3O4and NiFe2O4) has been thoroughly investigated to engineer their surface structures for the enhanced electrocatalytic oxygen evolution reaction. Specifically, when the optimal concentration of vanadium (ca. 7.7 at. %) is incorporated into Co3O4, the required overpotential to reach a certain jGEOMand jECSAdecreases dramatically for oxygen evolution reactions in alkaline media. Even after 30 h of chronopotentiometry, the required potential for V-doped Co3O4just increases by 16.3 mV, being much lower than that of the undoped one. It is observed that the pentavalent vanadium doping introduces lattice distortions and defects on the surface, which in turn exposes more active sites for reactions. DFT calculations further reveal the rate-determining step changing from the step of *-O to *-OOH to the step of *-OH to *-O, while the corresponding energy barriers decrease from 1.73 to 1.57 eV accordingly after high-valent V doping. Moreover, the oxygen intermediate probing method using methanol as a probing reagent also demonstrates a stronger OH* adsorption on the surface after V doping. When vanadium doping is performed in the inverse spinel matrix of NiFe2O4, impressive performance enhancement in the oxygen evolution reaction is as well witnessed. All these results clearly illustrate that the V doping process can not only efficiently improve the electrochemical properties of spinel transition metal oxides but also provide new insights into the design of high-performance water oxidation electrocatalysts.

Due to the efficient photocarrier separation and collection coming from their distinctive band structures, superlattice nanowires (NWs) have great potential as active materials for high-performance optoelectronic devices. In this work, InGaZnO NWs with superlattice structure and controllable stoichiometry are obtained by ambient-pressure chemical vapor deposition. Along the NW axial direction, perfect alternately stacking of InGaO(ZnO)4+blocks and InO2–layers is observed to form a periodic layered structure. Strikingly, when configured into individual NW photodetectors, the Ga concentration is found to significantly influence the amount of oxygen vacancies and oxygen molecules adsorbed on the NW surface, which dictate the photoconducting properties of the NW channels. Based on the optimized Ga concentration (i.e., In1.8Ga1.8Zn2.4O7), the individual NW device exhibits an excellent responsivity of 1.95 × 105A/W and external quantum efficiency of as high as 9.28 × 107% together with a rise time of 0.93 s and a decay time of 0.2 s for the ultraviolet (UV) photodetection. Besides, the obtained NWs can be fabricated into large-scale parallel arrays on glass substrates as well to achieve fully transparent UV photodetectors, where the performance is on the same level or even better than many transparent photodetectors with high performance. All the results discussed above demonstrate the great potential of InGaZnO superlattice NWs for next-generation advanced optoelectronic devices.

Because of their fascinating properties, two-dimensional (2D) nanomaterials have attracted a lot of attention for developing next-generation electronics and optoelectronics. However, there is still a lack of cost-effective, highly reproducible, and controllable synthesis methods for developing high-quality semiconducting 2D monolayers with a sufficiently large single-domain size. Here, utilizing a NaOH promoter and W foils as the W source, we have successfully achieved the fabrication of ultralarge single-domain monolayer WS2films via a modified chemical vapor deposition method. With the proper introduction of a NaOH promoter, the single-domain size of monolayer WS2can be increased to 550 μm, while the WS2flakes can be well controlled by simply varying the growth duration and oxygen concentration in the carrier gas. Importantly, when they are fabricated into global backgated transistors, WS2devices exhibit respectable peak electron mobility up to 1.21 cm2V–1s–1, which is comparable to those of many state-of-the-art WS2transistors. Photodetectors based on these single-domain WS2monolayers give an impressive photodetection performance with a maximum responsivity of 3.2 mA W–1. All these findings do not only provide a cost-effective platform for the synthesis of high-quality large single-domain 2D nanomaterials, but also facilitate their excellent intrinsic material properties for the next-generation electronic and optoelectronic devices.

Controlled synthesis of lead halide perovskite (LHP) nanostructures not only benefits fundamental research but also offers promise for applications. Among many synthesis techniques, although catalytic vapor–liquid–solid (VLS) growth is recognized as an effective route to achieve high-quality nanostructures, until now, there is no detailed report on VLS grown LHP nanomaterials due to the emerging challenges in perovskite synthesis. Here, we develop a direct VLS growth for single-crystalline all-inorganic lead halide perovskite (i.e., CsPbX3; X = Cl, Br, or I) nanowires (NWs). These NWs exhibit high-performance photodetection with the responsivity exceeding 4489 A/W and detectivity over 7.9 × 1012Jones toward the visible light regime. Field-effect transistors (FET) based on individual CsPbX3NWs are also fabricated, where they show the superior hole mobility of up to 3.05 cm2/(V s), higher than other all-inorganic LHP devices. This work provides important guidelines for the further improvement of these perovskite nanostructures for utilizations.

Due to the superior optoelectronic properties of halide perovskites, it has attracted wide attentions during the past decade, especially in the field of energy. When the dimension of halide perovskites reduces, novel properties will appear, such as large exciton binding energy, high photoluminescence quantum yield, and enlarged band gap. This way, low dimensional halide perovskites, especially two-dimensional (2D) halide perovskites, have attracted intensive attention in recent years and showed promising applications, including photodetectors, light emitting diodes (LEDs), and solar cells. Various methods were then developed for the synthesis of 2D halide perovskites and studying the exciting optoelectronic properties. The optoelectronic devices based on 2D halide perovskites showed excellent performance, particularly high sensitive photodetection, high electroluminescence with sharp emission peak, and environmental stable solar cells with decent power conversion efficiency. In this case, 2D halide perovskites become superstar materials in perovskites research area. In this review, we summarize the recent progresses in the synthesis and energy applications of 2D halide perovskites, and challenges and future directions in the field.

Recently, due to the excellent and tunable optoelectrical properties, quasi-two-dimensional (quasi-2D) layered perovskites have attracted tremendous attention for next-generation optoelectronic devices. However, fabricating high-quality 2D perovskite films, especially with low trap density, is still a challenge. Here, we successfully incorporate different concentrations of FA and Cs cations (i.e.mixed cations) into the quasi-2D perovskites of (iBA)2(MA)3Pb4I13(FA = formamidinium; iBA = iso-butylamine; MA = methylamine) to modulate their thin film qualities, improving their subsequently fabricated device performances. When configured into photodetectors on rigid substrates, the optimal mixed cation-incorporated (iBA)2(MA)3Pb4I13perovskites exhibit impressive photodetection properties, which are comparable or even better than those of other 2D perovskite-based photodetectors previously reported. Once fabricated as a flexible photodetector on polyimide, the quasi-2D perovskite device demonstrates further improved performances, yielding a good responsivity of 400 mA W−1, a high detectivity up to 1.68 × 1012Jones and fast response speeds (rise and decay time constants of 43 and 22 ms, respectively) under 532 nm illumination. Importantly, the obtained devices possess excellent mechanical flexibility and durability with photocurrent maintaining 82% of the initial value even after 9000 bending cycles. This work can provide valuable design guidelines of 2D perovskites to obtain high-performance flexible photodetectors for next-generation optoelectronics.

As an important semiconductor nanomaterial, InP nanowires (NWs) grown with a typical vapor–liquid–solid mechanism are still restricted from their low electron mobility for practical applications. Here, nonpolar-oriented defect-free wurtzite InP NWs with electron mobility of as high as 2000 cm2V–1s–1can be successfully synthesized viaPd-catalyzed vapor–solid–solid growth. Specifically, PdIn catalyst particles are involved and found to expose their PdIn{210} planes at the InP nucleation frontier due to their minimal lattice mismatch with nonpolar InP{2̅110} and {1̅100} planes. This appropriate lattice registration would then minimize the overall free energy and enable the highly crystalline InP NW growth epitaxially along the nonpolar directions. Because of the minimized crystal defects, the record-high electron mobility of InP NWs (i.e., 2000 cm2V–1s–1at an electron concentration of 1017cm–3) results, being close to the theoretical limit of their bulk counterparts. Furthermore, once the top-gated device geometry is employed, the device subthreshold slopes can be impressively reduced down to 91 mV dec–1at room temperature. In addition, these NWs exhibit a high photoresponsivity of 104A W–1with fast rise and decay times of 0.89 and 0.82 s, respectively, in photodetection. All these results evidently demonstrate the promise of nonpolar-oriented InP NWs for next-generation electronics and optoelectronics.

Quasi two-dimensional (2D) layered organic–inorganic perovskite materials (e.g., (BA)2(MA)n−1PbnI3n+1; BA = butylamine; MA = methylamine) have recently attracted wide attention because of their superior moisture stability as compared with three-dimensional counterparts. Inevitably, hydrophobic yet insulating long-chained organic cations improve the stability at the cost of hindering charge transport, leading to the unsatisfied performance of subsequently fabricated devices. Here, we reported the synthesis of quasi-2D (iBA)2(MA)n−1PbnI3n+1perovskites, where the relatively pure-phase (iBA)2PbI4and (iBA)2MA3Pb4I13films can be obtained. Because of the shorter-branched chain of iBA as compared with that of its linear equivalent (n-butylamine, BA), the resulting (iBA)2(MA)n−1PbnI3n+1perovskites exhibit much enhanced photodetection properties without sacrificing their excellent stability. Through hot-casting, the optimized (iBA)2(MA)n−1PbnI3n+1perovskite films with n= 4 give the significantly improved crystallinity, demonstrating the high responsivity of 117.09 mA/W, large on–off ratio of 4.0 × 102, and fast response speed (rise and decay time of 16 and 15 ms, respectively). These figure-of-merits are comparable or even better than those of state-of-the-art quasi-2D perovskite-based photodetectors reported to date. Our work not only paves a practical way for future perovskite photodetector fabrication via modulation of their intrinsic material properties but also provides a direction for further performance enhancement of other perovskite optoelectronics.

Because of sluggish kinetics of the oxygen evolution reaction (OER), designing low-cost, highly active, and stable electrocatalysts for OER is important for the development of sustainable electrochemical water splitting. Here, {112} high-index facet exposed porous Co3O4nanosheets with oxygen vacancies on the surface have been successfully synthesized via a simple hydrothermal method followed by NaBH4reduction. As compared with the pristine and other faceted porous Co3O4nanosheets (e.g., {110} and {111}), the as-prepared {112} faceted porous nanosheets exhibit a much lower overpotential of 318 mV at a current density of 10 mA cm–2. Importantly, these nanosheets also give excellent electrochemical stability, displaying an insignificant change in the required overpotential at a current density of 10 mA cm–2even after a 14 h long-term chronoamperometric test. All these superior OER activity and stability could be attributed to their unique hierarchical structures assembled by ultrathin porous nanosheets, {112} high-index exposed facets with higher ratio of Co2+/Co3+and oxygen vacancies on the surface, which can substantially enhance the charge transfer rate and increase the number of active sites. All these findings not only demonstrate the potency of our Co3O4nanosheets for efficient water oxidation but also provide further insights into developing cost-effective and high-performance catalysts for electrochemical applications.

Van der Waals heterostructures built from two-dimensional materials on a conventional semiconductor offer novel electronic and optoelectronic properties for next-generation information devices. Here we report that by simply stacking a vapor-phase-synthesized multilayer n-type WS2film onto a p-type Si substrate, a high-responsivity Zener photodiode can be achieved. We find that above a small reverse threshold voltage of 0.5 V, the fabricated heterojunction exhibits Zener tunneling behavior which was confirmed by its negative temperature coefficient of the breakdown voltage. The WS2/Si heterojunction working in the Zener breakdown regime shows a stable and linear photoresponse, a broadband photoresponse ranging from 340 to 1100 nm with a maximum photoresponsivity of 5.7 A/W at 660 nm and a fast response speed of 670 μs. Such high performance can be attributed to the ultrathin depletion layer involved in the WS2/Si p–n junction, on which a strong electric field can be created even with a small reverse voltage and thereby enabling an efficient separation of the photogenerated electron–hole pairs.

Functioning both as electrochromic (EC) and transparent-conductive (TC) coatings, WO3/Ag/WO3(WAW) trilayer film shows promising potential application for ITO-free electrochromic devices. Reports on thermal-evaporated WAW films revealed that these bifunctional WAW films have distinct EC characteristics; however, their poor adhesive property leads to rapid degradation of coloring-bleaching cycling. Here, we show that WAW film with improved EC durability can be prepared by reactive sputtering using metal targets. We find that, by introducing an ultrathin tungsten (W) sacrificial layer before the deposition of external WO3, the oxidation of silver, which leads to film insulation and apparent optical haze, can be effectively avoided. We also find that the luminous transmittance and sheet resistance were sensitive to the thicknesses of tungsten and silver layers. The optimized structure for TC coating was obtained to be WO3(45 nm)/Ag (10 nm)/W (2 nm)/WO3(45 nm) with a sheet resistance of 16.3 Ω/□ and a luminous transmittance of 73.7%. Such film exhibits compelling EC performance with decent luminous transmittance modulation ΔTlumof 29.5%, fast switching time (6.6 s for coloring and 15.9 s for bleaching time), and long-term cycling stability (2000 cycles) with an applied potential of ±1.2 V. Thicker external WO3layer (45/10/2/100 nm) leads to larger modulation with maximum ΔTlumof 46.4%, but at the cost of significantly increasing the sheet resistance. The strategy of introducing ultrathin metal sacrificial layer to avoid silver oxidation could be extended to fabricating other oxide–Ag–oxide transparent electrodes via low-cost reactive sputtering.

Initiated by graphene, two-dimensional (2D) layered materials have attracted much attention owing to their novel layer-number-dependent physical and chemical properties. To fully utilize those properties, a fast and accurate determination of their layer number is the priority. Compared with conventional structural characterization tools, including atomic force microscopy, scanning electron microscopy, and transmission electron microscopy, the optical characterization methods such as optical contrast, Raman spectroscopy, photoluminescence, multiphoton imaging, and hyperspectral imaging have the distinctive advantages of a high-throughput and nondestructive examination. Here, taking the most studied 2D materials like graphene, MoS _2, and black phosphorus as examples, we summarize the principles and applications of those optical characterization methods. The comparison of those methods may help us to select proper ones in a cost-effective way.