Band Bending Research Articles

Synergistic defect passivation and strain compensation toward efficient and stable perovskite solar cells

Rational interface engineering is essential for minimizing interfacial nonradiative recombination losses and enhancing device performance. Herein, we report the use of bidentate diphenoxybenzene (DPOB) isomers as surface modifiers for perovskite films. The DPOB molecules, which contain two oxygen (O) atoms, chemically bond with undercoordinated Pb2+ on the surface of perovskite films, resulting in compression of the perovskite lattice. This chemical interaction, along with physical regulations, leads to the formation of high-quality perovskite films with compressive strain and fewer defects. This compressive strain-induced band bending promotes hole extraction and transport, while inhibiting charge recombination at the interfaces. Furthermore, the addition of DPOB will reduce the zero-dimensional (0D) Cs4PbBr6 phase and produce the two-dimensional (2D) CsPb2Br5 phase, which is also conducive to the improvement of device performance. Ultimately, the resulting perovskite films, which are strain-released and defect-passivated, exhibit exceptional device efficiency, reaching 10.87% for carbon-based CsPbBr3 device, 14.86% for carbon-based CsPbI2Br device, 22.02% for FA0.97Cs0.03PbI3 device, respectively. Moreover, the unencapsulated CsPbBr3 PSC exhibits excellent stability under persistent exposure to humidity (80%) and heat (80 °C) for over 50 days.

S-scheme homojunction of 0D cubic/2D hexagonal ZnIn2S4 for efficient photocatalytic reduction of nitroarenes

The targeted conversion of toxic nitroarenes to corresponding aminoarenes presents significant promise in simultaneously addressing environmental pollution concerns and producing value-added fine chemicals. In this study, we synthesize a 0D/2D ZnIn2S4 homojunction (CH-ZnIn2S4) by in situ growth of cubic ZnIn2S4 (C-ZnIn2S4) quantum dots onto the surface of ultrathin hexagonal ZnIn2S4 (H-ZnIn2S4) nanosheets for photocatalytic reduction of nitroarenes to aminoarenes using water as a hydrogen donor. The optimal performance of photocatalytic nitro reduction over the 0D/2D CH-ZnIn2S4 homojunction reaches 96.1% within 20 min of visible light irradiation, which is 2.45 and 1.52 times than that of C-ZnIn2S4 (39.3%) and H-ZnIn2S4 (63.3%), respectively. The improved photocatalytic performance can be attributed to the formation of a step-type S-scheme homojunction, characterized by identity chemical composition and natural lattice matching. The configuration enables continuous band bending and a low energy barrier of charge transportation, benefiting the charge transfer across the interface while maximizing their redox capabilities. Furthermore, the 2D structure of H-ZnIn2S4 nanosheets offers abundant surface sites to immobilize the 0D C-ZnIn2S4 that provides ample exposed active sites with low overpotential for HER, thereby ensuring high hydrogenation reduction activity of nitroarenes. The study is expected to inspire further interest in the reasonable design of homojunction structures for efficient and sustainable photocatalytic redox reactions.

Eliminating Hole Extraction Barrier in 1D/3D Perovskite Heterojunction for Efficient and Stable Carbon-Based CsPbI3 Solar Cells with a Record Efficiency.

Carbon-based perovskite solar cells (C-PSCs) have the advantages of low-cost and high-stability, but their photovoltaic performance is limited by severe defect-induced recombination and low hole extraction efficiency. 1D perovskite is proven to effectively passivate the defects on the perovskite surface, therefore reducing non-radiative recombination loss. However, the unsuitable energy level of most 1D perovskite renders an undesired downward band bending for 3D perovskite, resulting in a high hole extraction barrier and reduced hole extraction efficiency. Therefore, rational design and selection of 1D perovskites as the modifiers are essential in balancing defect passivation and hole extraction. In this work, based on simulation calculations, thiocholine iodide (TchI) is selected to prepare 1D perovskite with high work function and then constructs TchPbI3/CsPbI3 1D/3D perovskite heterojunction. Experimental results show that this strategy eliminates the hole extraction barrier at the perovskite/carbon interface, which improves the hole extraction efficiency of corresponding devices. Meanwhile, the strong interaction between the thiol group and Pb suppresses defect-induced recombination effectively and improves the stability of CsPbI3. The assembled C-PSCs exhibit a champion efficiency of 19.08% and a certified efficiency of 18.7%. To the best of the knowledge, this is a new efficiency record for inorganic C-PSCs.

Gas adsorption on activated AlGaN photocathode: A first-principle study on charge redistribution, work function variation and emission performance attenuation

AlGaN photocathode has important applications in photoelectronic imaging and vacuum electronic devices. However, The short lifetime of the photocathode results in higher costs of usage. This can be attributed to the decrease in vacuum degree and the adsorption of residual gases. In this study, first-principle study and transmission matrix are utilized to investigate five gases (H2O, CO, CO2, H2, CH4) adsorbed on Cs-activated AlGaN (100) surface. Results suggest that all these five gases are prior to deposit on the surface at low coverage of 0.25 ML with adsorption energy of -2 eV. CO has the highest adsorption possibility with its adsorption energy lower than 0 eV. Significant charge transfer occurs at Cs-gas interface, particularly near the oxygen atoms of oxygen-containing molecules. CO2 and CO exhibit a noticeable increase in work function. The increase in the work function of the H2O adsorption is primarily due to its longer dipole length rather than dipole charge by analysing the surface dipole moment. CH4 and H2 have negligible impact on surface properties. Upon gas adsorption, the electrons on the AlGaN surface flow back to the adatoms, causing the CBM shift towards higher energy and upwards band bending. Combined with the surface electron affinity, the iteration of band structures further explains the increase of work function. The open energy of free electron escaping into the vacuum after CO2 adsorption increases to the level of inactivated surface of 5.55 eV from PBE-D2. This study reveals the mechanism of performance decay of the photocathode and is expected to guide the optimization of packaging technology.

In2O3/ZnO heterojunction thin film transistor for high recognition accuracy neuromorphic computing and optoelectronic artificial synapses

Solid electrolyte-gated transistors exhibit improved chemical stability and can fulfill the requirements of microelectronic packaging. Typically, metal oxide semiconductors are employed as channel materials. However, the extrinsic electron transport properties of these oxides, which are often prone to defects, pose limitations on the overall electrical performance. Achieving excellent repeatability and stability of transistors through the solution process remains a challenging task. In this study, we propose the utilization of a solution-based method to fabricate an In2O3/ZnO heterojunction structure, enabling the development of efficient multifunctional optoelectronic devices. The heterojunction’s upper and lower interfaces induce energy band bending, resulting in the accumulation of a large number of electrons and a significant enhancement in transistor mobility. To mimic synaptic plasticity responses to electrical and optical stimuli, we utilize Li+-doped high-k ZrO x thin films as a solid electrolyte in the device. Notably, the heterojunction transistor-based convolutional neural network achieves a high accuracy rate of 93% in recognizing handwritten digits. Moreover, our research involves the simulation of a typical sensory neuron, specifically a nociceptor, within our synaptic transistor. This research offers a novel avenue for the advancement of cost-effective three-terminal thin-film transistors tailored for neuromorphic applications.

Exploring the Structural Stability of 1T-PdO2 and the Interface Properties of the 1T-PdO2/Graphene Heterojunction.

Motivated by a recent study on the air stability of PdSe2, which also reports the metastability of the PdO2 monolayer [Hoffman A. N.. npj 2D Mater. Appl.2019, 3( (1), ), 50.], in this work, we use density functional theory (DFT) to further explore the thermal, dynamic, and mechanical stability of monolayer PdO2 and study its structural and electronic properties. We further studied its vertical heterojunction composed of 1T-PdO2 and graphene monolayers. We show that both the monolayer and the heterojunction are energetically and dynamically stable with no negative frequencies in the phonon spectrum and belong to the vdW-type. 1T-PdO2 is an indirect-band-gap semiconductor with band-gap values of 0.5 eV (GGA) and 1.54 eV (HSE06). The interface properties of the heterojunction show that the n-type Schottky barrier height (SBH) becomes negative at the vertical interface in the PdO2/graphene contact, forming an Ohmic contact and mainly suggesting the potential of graphene for efficient electrical contact with the PdO2 monolayer. However, at the same time, a negative band bending occurs at the lateral interface based on the current-in-plane model. Moreover, the optical absorption of the PdO2/graphene heterojunction under visible-light irradiation is significantly enhanced compared to the situation in their free-standing monolayers.

Simulation of electrical rectification effect in two-dimensional MoSe2/WSe2 lateral heterostructures

Two-dimensional (2D) transition metal dichalcogenides lateral heterostructures exhibit excellent performance in electrics and optics. The electron transport of the heterostructures can be effectively regulated by ingenious design. In this study, we construct a monolayer MoSe2/WSe2 lateral heterostructure, covalently connecting monolayer MoSe2 and monolayer WSe2. Using the Extended Huckel Theory method, we explored current-voltage characteristics under varied conditions, including altering carrier density, atomic replacement and interface angles. Calculations demonstrate a significant electrical rectification ratio (ERR) ranging from 200 to 800. Additionally, Employing Density Functional Theory with non-equilibrium Green’s function method, we investigated electronic properties, attributing the rectification effect to electronic state distribution differences, asymmetric transmission coefficients and band bending of projected local density of states. The expandability of the interfacial energy barrier enhances the rectification effect through adjustments in carrier concentration, atomic replacements and interface size. However, these enhancements introduce challenges such as increased electron-boundary scattering and reduced ambipolarity, resulting in a lower ERR. This study provides valuable theoretical insights for optimizing 2D electronic diode devices, offering avenues for precise control of the rectification effect.

The role of interface energetics in Sb2Se3 thin film solar cells

We comprehensively simulated the interface energetics at the Sb2Se3/CdS interfaces and showed its impact on device performance. The interface discontinuity, band bending at interface and energy level alignment generates interfaces issues and must be optimized for an optimal device performance. The design parameters for controlling interface. Metal contact work function preferably higher than electron affinity (EA) and Fermi level (EF) combined (EA + EF), should result in near Ohmic behaviour of contact. Secondly electron affinity of buffer could be tuned to achieve small positive conduction bandoffset (spike barrier) at absorber/buffer interface which lowers the chances of recombination through interface states. A pn + configuration with highly doped buffer layer, as compared to p-absorber, is favourable as it will extend depletion in absorber, providing additional drift to photo-generated carriers. Lastly, acceptor defect at Sb2Se3-CdS interface generate surface inversion and detrimental to performance. Donor defects occupying interface states are preferred condition for optimal device performance. We have compiled the optimal ranges for these controlling parameters, to achieve theoretically ideal values of energy level alignment and energetics, leading to optimal performance.

Effects of the Addition of Tin Powder to Perovskite Precursor Solutions on Band Bending at PEDOT:PSS/Perovskite Interfaces in Mixed-Cation Mixed-Halide Tin Perovskite Solar Cells.

Using electron spin resonance (ESR) spectroscopy, we investigated the effects of the addition of tin (Sn) powder to perovskite layers on band bending at the perovskite surface near poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) hole-transport layers in perovskite solar cells (PSCs) involving formamidinium (FA)-methylammonium (MA)-mixed-cation I-Br-mixed-halide tin perovskites. We performed dark ESR spectroscopy measurements of a PEDOT:PSS/FA0.75MA0.25Sn(I0.75Br0.25)3 stack and of a PEDOT:PSS/Sn-powder-added FA0.75MA0.25Sn(I0.75Br0.25)3 stack. The results indicate that FA0.75MA0.25Sn(I0.75Br0.25)3 layers have significant downward band bending near PEDOT:PSS layers. Such downward band bending is unfavorable for hole selectivity and surface passivation at the interface. However, the addition of Sn powder to the tin perovskite precursor solution was found to significantly prevent the downward band bending and rather cause upward band bending, which can improve the hole selectivity and field-effect passivation quality. This can be due to prevented oxidation of perovskite layers by Sn powder addition. These findings are crucial for developing highly efficient and stable tin perovskite solar cells.

AlxGa1–xAs(100) (x ∼ 0.3) surfaces treated with aqueous sodium sulfide solution: Chemistry and electronic structure

Chemical composition and electronic structure of the native-oxide-covered AlxGa1–xAs(100) (x ∼ 0.3) surfaces were investigated by x-ray photoelectron spectroscopy and photoluminescence before and after treatment at room temperature with a concentrated aqueous solution of sodium sulfide in order to get inside into mechanism of sulfide solution interaction with aluminum-containing III–V alloys. Even short treatment of the n-AlGaAs(100) surface leads to the removal of the most of the native oxide layer so that the surface is covered with a thin layer of residual aluminum and gallium oxides with a thickness of approximately 1 ML, which can be formed during air exposure after termination of the chemical treatment. Longer treatment of the n-AlGaAs(100) surface does not further reduce the amount of residual aluminum and gallium oxides. The etching of native oxide layer on the p-AlGaAs(100) surface proceeds slower and the lowest amount of residual aluminum and gallium oxides is achieved after etching for 12 min. At the same time sulfur is hardly adsorbed at the AlGaAs(100) surfaces after interaction with the solution. It is found that the lower the amount of residual aluminum and gallium oxides on n- and p-AlGaAs(100) surfaces, the higher is the photoluminescence intensity. The band bending on the native-oxide-covered n- and p-AlGaAs(100) surfaces is about 0.85 and 0.5 eV, respectively, while the ionization energy is nearly the same for both surfaces. Treatment of n- and p-AlGaAs(100) surfaces with an aqueous sodium sulfide solution causes simultaneous decrease in their ionization energy.

Passivation of 1D@3D perovskite ferroelectric film with hydrophobic molecules for ferro-pyro-phototronic effect-based self-driven photodetector.

High-performance, self-driven photodetectors in commercial and public applications show promising prospects. The pyro-phototronic effect is a promising method for building these detectors, but limitations in interfacial contact conditions hinder the use of the ferro-pyro-phototronic effect. By modifying the surface of 1D@3D perovskite ferroelectric film with tetra-ethyl ammonium (TEAI) molecules, the interfacial defect density is reduced, resulting in a high-performance, stable photodetector. Moreover, the passivation can greatly enhance the ferro-pyro-phototronic effect, which can be explained by the increased band bending and decreased trap states at the SnS/perovskite interface resulting in less re-distribution of charge carriers directly across the interface. Our work offers a feasible and effective method for producing pyro-phototronic responses in perovskite films-based devices, and thus presents a feasible solution for high-performance, self-driven and flexible photodetection.

Bismuth as a buffer layer for metal contact with silicon carbide studied by In situ photoelectron spectroscopy

Silicon carbide (SiC) is a promising third-generation semiconductor due to its wide bandgap. However, the high Schottky barrier and metal-induced gap states (MIGS) at the metal/SiC interface present significant challenges for device fabrication, leading to high contact resistance and poor current delivery. This study proposes the use of bismuth (Bi), with its semimetallic properties and gap-state saturation effect, as a contact buffer layer to address these issues. We conducted a systematic investigation of the chemical and electronic characteristics of the Pt/Bi/4H-SiC(0001) system, fabricated via molecular beam epitaxy (MBE), using in situ X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Our findings reveal weak bonding between the Bi buffer layer and the 4H-SiC(0001) surface, resulting in a slight downward band bending effect and the formation of a substantial dipole across the Bi/4H-SiC(0001) interface. Moreover, UPS spectra indicate a reduction in the work function of Pt/Bi/4H-SiC(0001), suggesting the potential for achieving low contact resistance. Notably, the Pt/Bi/4H-SiC(0001) system remains stable when exposed to 1.6×109 Langmuir of oxygen at room temperature, while a bare Bi buffer layer undergoes partial oxidation. These results provide a comprehensive understanding of the Pt/Bi/4H-SiC(0001) interfaces and strategies for improving metal/SiC contacts.

Phase junction CdS decorated by plasmonic Cu1.8S: Realizing directed carriers transfer path and broad solar light-response for efficient photocatalytic water splitting

Phase engineering is an emerging strategy for tuning the electronic structure by manipulating atomic configuration and accumulating the carrier transfer, making it important in the field of photocatalysis. In this paper, Cu1.8S@phase junction CdS is successfully synthesized. Phase junction CdS (HCC) exhibits band bending to optimize the separation and transfer of photogenerated carriers. The introduction of plasmonic Cu1.8S not only endows CdS with a broad photoresponse ability to the near-infrared (NIR) light region and a photothermal effect but also forms the p-n junction to further accelerate the carrier separation rate. The synergistic effect of phase junction and p-n junction realizes an excellent photocatalytic hydrogen activity in the full spectrum. Cu1.8S@HCC exhibits an H2 production rate of 3.7 mmol·g−1·h−1 and 374 μmol·g−1·h−1 under Visible (Vis)-light (780 ≥ λ ≥ 420 nm) and NIR light (λ ≥ 780 nm) illumination, which is 31 times and 9 times higher than the Vis-light driven photocatalytic H2 rate of cubic CdS and HCC. Importantly, through detailed characterization and density functional theory (DFT) calculations, the migration pathway of charge carriers and the reactive active site in multiple interfaces are determined. This work presents an exploitation of advanced photocatalysts in multiple phases for excellent solar energy conversion.

R‐NG●‐Regulated Electron‐Phonon Coupling to Enhance Hot‐Hole Injection for Inverted Perovskite Solar Cells

AbstractExtracting hot‐carriers before they relax back to the band edge will reduce thermal dissipation above the bandgap, which is one of the primary sources of efficiency loss in perovskite solar cells (PSCs). It requires slow cooling of hot‐carriers together with the efficient extraction of hot‐carriers. Here, a novel interface engineering is demonstrated by embedding radical‐containing nanographene (r‐NG●) between perovskite and poly[bis(4‐phenyl)(2,4‐dimethoxyphenyl)‐amine] (PTOAA) to harvest this excess energy. This strategy accelerates the extraction rate of the hot‐hole (3 times that of control at 400 K) by augmenting the relaxation channel, which is related to the “quasi‐SOMO” energy state of r‐NG●. Nonadiabatic molecular dynamics calculation further revealed that radical character promotes the rotation of the cation in perovskite, which contributes to generating strong nonadiabatic couplings and multiple phonon modes. In addition, the band bending effect of buried perovskite and improved conductivity of PTOAA also facilitate exciton dissociation and hole collection efficiency (97.5%). Consequently, the resultant r‐NG● decorated PSCs delivered an impressive efficiency of 24.20%, along with better thermal and humidity stability. Moreover, it can maintain 93% of its initial efficiency after 300 hours of illumination.

Modeling the electroluminescence of atomic wires from quantum dynamics simulations.

Static and time-dependent quantum-mechanical approaches have been employed in the literature to characterize the physics of light-emitting molecules and nanostructures. However, the electromagnetic emission induced by an input current has remained beyond the realm of molecular simulations. This is the challenge addressed here with the help of an equation of motion for the density matrix coupled to a photon bath based on a Redfield formulation. This equation is evolved within the framework of the driven-Liouville von Neumann approach, which incorporates open boundaries by introducing an applied bias and a circulating current. The dissipated electromagnetic power can be computed in this context from the time derivative of the energy. This scheme is applied in combination with a self-consistent tight-binding Hamiltonian to investigate the effects of bias and molecular size on the electroluminescence of metallic and semiconducting chains. For the latter, a complex interplay between bias and molecular length is observed: there is an optimal number of atoms that maximizes the emitted power at high voltages but not at low ones. This unanticipated behavior can be understood in terms of the band bending produced along the semiconducting chain, a phenomenon that is captured by the self-consistency of the method. A simple analytical model is proposed that explains the main features revealed by the simulations. The methodology, applied here at a self-consistent tight-binding level but extendable to more sophisticated Hamiltonians such as density functional tight binding and time dependent density functional theory, promises to be helpful for quantifying the power and quantum efficiency of nanoscale electroluminescent devices.

Structural and electrical conductivity studies of Polyaniline - WO3 hybrid nanocomposites for gas sensing applications

Abstract Conducting polymer – metal oxide based hybrid nanocomposites are a fascinating class of materials for miniaturized and flexible gas sensor devices. They exhibit enhanced physiochemical properties such as sensitivity, selectivity towards various volatile and hazardous chemical and bioanalytes. Our study focuses on conducting polyaniline (PANI) and WO3 nanocomposites, where different weight percentages (wt.%) of WO3 nanoparticles are embedded within the conducting PANI matrix using an in-situ oxidation polymerization synthesis technique. The surface morphology analysis indicated that the WO3 nanoparticles with an average grain size of ~200 nm are homogeneously distributed within the PANI nanofibers. The Fourier Transform Infra-Red (FTIR) spectrum analysis showed that the absorption peaks at 1111,1291, 1385, 1474, and 1560 cm−1 are typical of the conducting PANI emeraldine phase. We attribute the additional broad peak ranging between 840 to 720 cm−1 in the spectrum to WO3 phase, wherein, the intensity of the peak increases with WO3 content in case of hybrid composites. Current-voltage (I-V) characteristics for all our samples showed linear behaviour up to 1.2 volts. Temperature-dependent DC electrical conductivity (σ) studies measured from room temperature to 120°C for pure PANI, and PANI-WO3 composites showed an enhanced electrical conductivity of values up to 0.12 S/cm for PANI as compared to WO3 with σ ~ 1.4 x 10−3 S/cm. Pure PANI exhibits semiconducting behavior with an increase in electrical conductivity with temperature due to the charge carrier delocalization within the dispersed PANI backbone. The addition of higher concentrations of WO3 in composites leads to a metallic-like behavior, characterized by a decrease in electrical conductivity with temperature. These observations are attributed to the field-assisted band bending effects at the interfaces of PANI and WO3. Our composites show desired electrical characteristics suitable for gas sensing applications.

Carbon coated iron pyrite (C–FeS2) photo-electrode for photo-electrochemical water splitting

In this paper, we present fabrication of ‘FeS2’ and carbon coated FeS2 (C–FeS2) photo-electrodes and demonstrated that coating of carbon on FeS2 significantly enhanced photo-electrochemical properties. The hydrothermal method has been adopted for the synthesis of FeS2 and carbon coating on FeS2 was achieved by CVD technique. The materials are subsequently characterized using X-ray diffraction, Raman spectroscopy and Field enhanced scanning electron microscope (FESEM) techniques for phase identification and morphological studies, respectively. The photo-electrochemical measurements revealed significant enhancement in the current density from ∼2 to 3.7 mA/cm2 (∼1.8 times higher) for C–FeS2 in comparison to the FeS2 photo-electrode under visible light. The enhancement in FeS2 after carbon coating has been attributed to reduction in recombination of photo generated charge carriers, enhancement in the band bending and lowering the charge transfer resistance at the semiconductor/electrolyte interface. To date, very few studies have been reported on FeS2 based composites for water splitting applications and use of carbon coating can be a novel approach for developing efficient solar to hydrogen conversion devices. The mechanism for the photocurrent enhancement was supported by Mott-Schottky and electrochemical impedance spectroscopy measurements.

Boosting the UV-vis-NIR Photodetection Performance of MoS2 through the Cavity Enhancement Effect and Bulk Heterojunction Strategy.

Navigating more effective methods to enhance the photon utilization of photodetectors poses a significant challenge. This study initially investigates the impact of morphological alterations in 2H-MoS2 on photodetector (PD) performance. The results reveal that compared to layered MoS2 (MoS2 NLs), MoS2 nanotubes (MoS2 NTs) impart a cavity enhancement effect through multiple light reflections. This structural feature significantly enhances the photodetection performance of the MoS2-based PDs. We further employ the heterojunction strategy to construct Y-TiOPc NPs:MoS2 NTs, utilizing Y-TiOPc NPs (Y-type titanylphthalocyanine) as the vis-NIR photosensitizer and MoS2 NTs as the photon absorption enhancer. This approach not only addresses the weak absorption of MoS2 NTs in the near-infrared region but also enhances carrier generation, separation, and transport efficiency. Additionally, the band bending phenomenon induced by trapped-electrons at the interface between ITO and the photoactive layer significantly enhances the hole tunneling injection capability from the external circuit. By leveraging the synergistic effects of the aforementioned strategies, the PD based on Y-TiOPc NPs:MoS2 NTs (Y:MT-PD) exhibits superior photodetection performance in the wavelength range of 365-940 nm compared to MoS2 NLs-based PD and MoS2 NTs-based PD. Particularly noteworthy are the peak values of key metrics for Y:MT-PD, such as EQE, R, and D* that are 4947.6%, 20588 mA/W, and 1.94 × 1012 Jones, respectively. The multiperiod time-resolved photocurrent response curves of Y:MT-PD also surpass those of the other two PDs, displaying rapid, stable, and reproducible responses across all wavelengths. This study provides valuable insights for the further development of photoactive materials with a high photon utilization efficiency.

Picosecond carrier dynamics in InAs and GaAs revealed by ultrafast electron microscopy.

Understanding the limits of spatiotemporal carrier dynamics, especially in III-V semiconductors, is key to designing ultrafast and ultrasmall optoelectronic components. However, identifying such limits and the properties controlling them has been elusive. Here, using scanning ultrafast electron microscopy, in bulk n-GaAs and p-InAs, we simultaneously measure picosecond carrier dynamics along with three related quantities: subsurface band bending, above-surface vacuum potentials, and surface trap densities. We make two unexpected observations. First, we uncover a negative-time contrast in secondary electrons resulting from an interplay among these quantities. Second, despite dopant concentrations and surface state densities differing by many orders of magnitude between the two materials, their carrier dynamics, measured by photoexcited band bending and filling of surface states, occur at a seemingly common timescale of about 100 ps. This observation may indicate fundamental kinetic limits tied to a multitude of material and surface properties of optoelectronic III-V semiconductors and highlights the need for techniques that simultaneously measure electro-optical kinetic properties.

Construction of a p-p heterojunction Co3O4/MoS2 electrocatalyst for boosting hydrogen evolution reaction

Building p-p heterojunctions with Fermi level difference, due to the motion of energy levels, band bending, and internal electric field generation, can effectively improve the electronic structure of the interface, enrich the active sites of heterojunctions, and promote the adsorption of reaction intermediates, which is an effective approach to constructing high-performance electrocatalysts. In this study, a Co3O4/MoS2 p-p heterojunction catalyst with nano-particle/nano-flower structure was designed, inducing charge transfer from Co3O4 to MoS2, thereby increasing the negative charge of MoS2, improving *H adsorption through electrostatic interaction, enhancing electrocatalytic kinetics, and reducing reaction overpotential. The catalytic performance of MoS2 for hydrogen evolution reaction was effectively enhanced. In 1 M KOH solution, the current density was 10 mA cm−2, and the overpotential was 156 mV, with good stability after 40 h of operation. This work provides a new direction for the design of p-p heterojunction electrocatalysts and offers more choices for promoting the development of high-performance non-noble metal catalysts.