Unveiling defect-mediated carrier dynamics in monolayer semiconductors by spatiotemporal microwave imaging.

The optoelectronic properties of atomically thin transition-metal dichalcogenides are strongly correlated with the presence of defects in the materials, which are not necessarily detrimental for certain applications. For instance, defects can lead to an enhanced photoconduction, a complicated process involving charge generation and recombination in the time domain and carrier transport in the spatial domain. Here, we report the simultaneous spatial and temporal photoconductivity imaging in two types of WS2 monolayers by laser-illuminated microwave impedance microscopy. The diffusion length and carrier lifetime were directly extracted from the spatial profile and temporal relaxation of microwave signals, respectively. Time-resolved experiments indicate that the critical process for photoexcited carriers is the escape of holes from trap states, which prolongs the apparent lifetime of mobile electrons in the conduction band. As a result, counterintuitively, the long-lived photoconductivity signal is higher in chemical-vapor deposited (CVD) samples than exfoliated monolayers due to the presence of traps that inhibits recombination. Our work reveals the intrinsic time and length scales of electrical response to photoexcitation in van der Waals materials, which is essential for their applications in optoelectronic devices.

Construction of Co4 Atomic Clusters to Enable Fe-N4 Motifs with Highly Active and Durable Oxygen Reduction Performance.

Fe-N-C catalysts with single-atom Fe-N4 configurations are highly needed owing to the high activity for oxygen reduction reaction (ORR). However, the limited intrinsic activity and dissatisfactory durability have significantly restrained the practical application of proton-exchange membrane fuel cells (PEMFCs). Here, we demonstrate that constructing adjacent metal atomic clusters (ACs) is effective in boosting the ORR performance and stability of Fe-N4 catalysts. The integration of Fe-N4 configurations with highly uniform Co4 ACs on the N-doped carbon substrate (Co4 @/Fe1 @NC) is realized through a "pre-constrained" strategy using Co4 molecular clusters and Fe(acac)3 implanted carbon precursors. The as-developed Co4 @/Fe1 @NC catalyst exhibits excellent ORR activity with a half-wave potential (E1/2 ) of 0.835 V vs. RHE in acidic media and a high peak power density of 840 mW cm-2 in a H2 -O2 fuel cell test. First-principles calculations further clarify the ORR catalytic mechanism on the identified Fe-N4 that modified with Co4 ACs. This work provides a viable strategy for precisely establishing atomically dispersed polymetallic centers catalysts for efficient energy-related catalysis.

Carbon Nitride Photocatalysts with Integrated Oxidation and Reduction Atomic Active Centers for Improved CO2 Conversion.

Single-atom active-site catalysts have attracted significant attention in the field of photocatalytic CO2 conversion. However, designing active sites for CO2 reduction and H2 O oxidation simultaneously on a photocatalyst and combining the corresponding half-reaction in a photocatalytic system is still difficult. Here, we synthesized a bimetallic single-atom active-site photocatalyst with two compatible active centers of Mn and Co on carbon nitride (Mn1 Co1 /CN). Our experimental results and density functional theory calculations showed that the active center of Mn promotes H2 O oxidation by accumulating photogenerated holes. In addition, the active center of Co promotes CO2 activation by increasing the bond length and bond angle of CO2 molecules. Benefiting from the synergistic effect of the atomic active centers, the synthesized Mn1 Co1 /CN exhibited a CO production rate of 47 µmol g-1 h-1 , which is significantly higher than that of the corresponding single-metal active-site photocatalyst.

Carbon-Supported Single-Atom Catalysts for Formic Acid Oxidation and Oxygen Reduction Reactions.

The commercialization of fuel cells, especially for direct formic acid fuel cells (DFAFCs) and proton-exchange membrane fuel cells (PEMFCs), is significantly restrained by the high cost, poor stability, and sluggish kinetics of platinum group metals (PGM) catalysts for both the anodic formic acid oxidation reaction (FAOR) and the cathodic oxygen reduction reaction (ORR). Currently, it has confronted with challenges, including exploring highly active, cost-effective, and stable catalysts to replace PGM for DFAFCs and PEMFCs. Recently, the increasing investigation has been focused on the single-atom catalysts (SACs) to enhance the catalytic performance owing to the maximum atom utilization and highly exposed active sites. The aim of this review is to present the recent research activities on carbon supported SACs. At the beginning of the review, metal-based SACs supported on different carbon supports, and the typical characterization methods are introduced. Subsequently, recent advances in metal-based SACs for FAOR and ORR catalysis are scientifically summarized. Particularly, some representative metal-based SACs for ORR activity are further exemplified with a deeper understanding of structure-activity relationships. Finally, the challenges and opportunities of SACs are prospected, such as the mechanism understanding and commercial applications.

An Adjacent Atomic Platinum Site Enables Single-Atom Iron with High Oxygen Reduction Reaction Performance.

The modulation effect has been widely investigated to tune the electronic state of single-atomic M-N-C catalysts to enhance the activity of oxygen reduction reaction (ORR). However, the in-depth study of modulation effect is rarely reported for the isolated dual-atomic metal sites. Now, the catalytic activities of Fe-N4 moiety can be enhanced by the adjacent Pt-N4 moiety through the modulation effect, in which the Pt-N4 acts as the modulator to tune the 3d electronic orbitals of Fe-N4 active site and optimize ORR activity. Inspired by this principle, we design and synthesize the electrocatalyst that comprises isolated Fe-N4 /Pt-N4 moieties dispersed in the nitrogen-doped carbon matrix (Fe-N4 /Pt-N4 @NC) and exhibits a half-wave potential of 0.93 V vs. RHE and negligible activity degradation (ΔE1/2 =8 mV) after 10000 cycles in 0.1 M KOH. We also demonstrate that the modulation effect is not effective for optimizing the ORR performances of Co-N4 /Pt-N4 and Mn-N4 /Pt-N4 systems.

Energy-Resolved Photoconductivity Mapping in a Monolayer-Bilayer WSe2 Lateral Heterostructure.

Vertical and lateral heterostructures of van der Waals materials provide tremendous flexibility for band-structure engineering. Because electronic bands are sensitively affected by defects, strain, and interlayer coupling, the edge and heterojunction of these two-dimensional (2D) systems may exhibit novel physical properties, which can be fully revealed only by spatially resolved probes. Here, we report the spatial mapping of photoconductivity in a monolayer-bilayer WSe2 lateral heterostructure under multiple excitation lasers. As the photon energy increases, the light-induced conductivity detected by microwave impedance microscopy first appears along the heterointerface and bilayer edge, then along the monolayer edge, inside the bilayer area, and finally in the interior of the monolayer region. The sequential emergence of mobile carriers in different sections of the sample is consistent with the theoretical calculation of local energy gaps. Quantitative analysis of the microscopy and transport data also reveals the linear dependence of photoconductivity on the laser intensity and the influence of interlayer coupling on carrier recombination. Combining theoretical modeling, atomic-scale imaging, mesoscale impedance microscopy, and device-level characterization, our work suggests an exciting perspective for controlling the intrinsic band gap variation in 2D heterostructures down to a regime of a few nanometers.

Reversible mechanochromic luminescence at room temperature in cationic platinum(II) terpyridyl complexes.

Reversible mechanochromic luminescence in cationic platinum(II) terpyridyl complexes is described. The complexes [Pt(Nttpy)Cl]X2 (Nttpy = 4'-(p-nicotinamide-N-methylphenyl)-2,2':6',2″-terpyridine, X = PF6 (1), SbF6 (2), Cl (3), ClO4 (4), OTf (5), BF4 (6)) exhibit different colors under ambient light in the solid state, going from red to orange to yellow. All of these complexes are brightly luminescent at both room temperature and 77 K. Upon gentle grinding, the yellow complexes (4-6) turn orange and exhibit bright red luminescence. The red luminescence can be changed back to yellow by the addition of a few drops of acetonitrile to the sample. Crystallographic studies of the yellow and red forms of complex 5 suggest that the mechanochromic response is likely the result of a change in intermolecular Pt···Pt distances upon grinding.

Electrochemical, spectroscopic and theoretical studies of a simple bifunctional cobalt corrole catalyst for oxygen evolution and hydrogen production.

Six cobalt and manganese corrole complexes were synthesized and examined as single-site catalysts for water splitting. The simple cobalt corrole [Co(tpfc)(py)2] (1, tpfc = 5,10,15-tris(pentafluorophenyl)corrole, py = pyridine) catalyzed both water oxidation and proton reduction efficiently. By coating complex 1 onto indium tin oxide (ITO) electrodes, the turnover frequency for electrocatalytic water oxidation was 0.20 s(−1) at 1.4 V (vs. Ag/AgCl, pH = 7), and it was 1010 s(−1) for proton reduction at −1.0 V (vs. Ag/AgCl, pH = 0.5). The stability of 1 for catalytic oxygen evolution and hydrogen production was evaluated by electrochemical, UV-vis and mass measurements, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX), which confirmed that 1 was the real molecular catalyst. Titration and UV-vis experiments showed that the pyridine group on Co dissociated at the beginning of catalysis, which was critical to subsequent activation of water. A proton-coupled electron transfer process was involved based on the pH dependence of the water oxidation reaction catalyzed by 1. As for manganese corroles 2­6, although their oxidizing powers were comparable to that of 1, they were not as stable as 1 and underwent decomposition at the electrode. Density functional theory (DFT) calculations indicated that water oxidation by 1 was feasible through a proposed catalytic cycle. The formation of an O­O bond was suggested to be the rate-determining step, and the calculated activation barrier of 18.1 kcal mol(−1) was in good agreement with that obtained from experiments.

Cobalt porphyrin electrode films for electrocatalytic water oxidation.

Catalysts play very important roles in artificial photosynthesis for solar energy conversion. In this present study, two water-insoluble cobalt porphyrin complexes, cobalt(II) meso-tetraphenylporphyrin (CoP-1) and cobalt(II) 5,10,15,20-tetrakis-(4-bromophenyl)porphyrin (CoP-2), were synthesized and coated as thin films on the FTO working electrode. The films showed good activities for electrocatalytic water oxidation in aqueous solutions at pH 9.2. The Faradaic efficiencies of both films approached to ~100%, measured using a fluorescence-based oxygen sensor. The turnover frequencies were close to 0.50 s(−1) and 0.40 s(−1) for CoP-1 and CoP-2, respectively, under an applied anodic potential of 1.3 V (vs. Ag/AgCl) at pH 9.2. Importantly, no cobalt oxide particles were observed on the working electrode after catalysis. The stability of the catalyst films was further evaluated by UV-vis spectroscopy, inhibition measurements, mass spectrometry, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The pH dependence of water oxidation on CoP-1 and CoP-2 suggested a proton-coupled electron transfer (PCET) mechanism. The catalyst films could be recycled and showed almost unchanged catalytic activities when they were reused in new electrocatalytic studies of water oxidation.

Facile deposition of nanostructured cobalt oxide catalysts from molecular cobaloximes for efficient water oxidation.

Cobaloximes are usually used as molecular hydrogen evolution reaction (HER) catalysts. Herein we report for the first time the use of molecular cobaloximes as catalyst precursors for water oxidation when anodic potentials were applied. Highly active thin catalyst films were deposited at +1.5 V and +1.1 V (vs. Ag/AgCl) in 0.1 M borate buffer solution at pH = 9.2 containing 1 mM cobaloximes. Four catalyst films (CoOx-1-CoOx-4) were synthesized from four different cobaloximes. The current densities of CoOx-1 were up to ~5.5 mA cm(-2) and ~2.6 mA cm(-2) when the applied potentials were +1.5 V and +1.1 V, respectively, which were higher than the current densities of CoOx-2, CoOx-3 and CoOx-4 under the same conditions. Scanning electron microscopy (SEM) images reveal that the nanometer-sized particles of CoOx-1 possibly contribute to its high activity while the other three catalysts have micrometer-sized amorphous materials on the surface of FTO. X-ray photoelectron spectroscopy (XPS) data displayed the valence state of the cobalt element as Co(ii) or Co(iii) oxide species. The morphological stability of the CoOx-1 catalyst was further studied using SEM.

Rhodium and Carbon Sites with Strong d-p Orbital Interaction for Efficient Bifunctional Catalysis.

Efficient and stable catalysts are highly desired for the electrochemical conversion of hydrogen, oxygen, and water molecules, processes which are crucial for renewable energy conversion and storage technologies. Herein, we report the development of hollow nitrogenated carbon sphere (HNC) dispersed rhodium (Rh) single atoms (Rh1HNC) as an efficient catalyst for bifunctional catalysis. The Rh1HNC was achieved by anchoring Rh single atoms in the HNC matrix with an Rh-N3C1 configuration, via a combination of in situ polymerization and carbonization approach. Benefiting from the strong metal atom-support interaction (SMASI), the Rh and C atoms can collaborate to achieve robust electrochemical performance toward both the hydrogen evolution and oxygen reduction reactions in acidic media. This work not only provides an active site with favorable SMASI for bifunctional catalysis but also brings a strategy for the design and synthesis of efficient and stable bifunctional catalysts for diverse applications.

One-step synthesis of single-site vanadium substitution in 1T-WS2 monolayers for enhanced hydrogen evolution catalysis.

Metallic tungsten disulfide (WS2) monolayers have been demonstrated as promising electrocatalysts for hydrogen evolution reaction (HER) induced by the high intrinsic conductivity, however, the key challenges to maximize the catalytic activity are achieving the metallic WS2 with high concentration and increasing the density of the active sites. In this work, single-atom-V catalysts (V SACs) substitutions in 1T-WS2 monolayers (91% phase purity) are fabricated to significantly enhance the HER performance via a one-step chemical vapor deposition strategy. Atomic-resolution scanning transmission electron microscopy (STEM) imaging together with Raman spectroscopy confirm the atomic dispersion of V species on the 1T-WS2 monolayers instead of energetically favorable 2H-WS2 monolayers. The growth mechanism of V SACs@1T-WS2 monolayers is experimentally and theoretically demonstrated. Density functional theory (DFT) calculations demonstrate that the activated V-atom sites play vital important role in enhancing the HER activity. In this work, it opens a novel path to directly synthesize atomically dispersed single-metal catalysts on metastable materials as efficient and robust electrocatalysts.

Mobility-Fluctuation-Controlled Linear Positive Magnetoresistance in 2D Semiconductor Bi2O2Se Nanoplates.

Linear magnetoresistance is generally observed in polycrystalline zero-gap semimetals and polycrystalline Dirac semimetals with ultrahigh carrier mobility. We report the observation of positive and linear magnetoresistance in a single-crystalline semiconductor Bi2O2Se grown by chemical vapor deposition. Both Se-poor and Se-rich Bi2O2Se single-crystalline nanoplates display a linear magnetoresistance at high fields. The Se-poor Bi2O2Se exhibits a typical 2D conduction feature with a small effective mass of 0.032m0. The average transport Hall mobility, which is lower than 5500 cm2 V-1 s-1, is significantly reduced, compared with the ultrahigh quantum mobility as high as 16260 cm2 V-1 s-1. More interestingly, the pronounced Shubnikov-de Hass oscillations can be clearly observed from the very large and nearly linear magnetoresistance (>500% at 14 T and 2 K) in Se-poor Bi2O2Se. A close analysis of the results reveals that the large and linear magnetoresistance observed can be ascribed to the spatial mobility fluctuation, which is strongly supported by Fermi energy inhomogeneity in the nanoplate samples detected using an electrostatic force microscopy images and multiple frequencies in a Shubnikov-de Hass oscillation. On the contrary, the Se-rich Bi2O2Se exhibits a transport mobility (<300 cm2 V-1 s-1) much smaller than that observed in Se-poor samples and shows a much smaller linear magnetoresistance ratio (less than 150% at 14 T and 2 K). More strikingly, no Shubnikov-de Hass oscillations can be observed. Therefore, the linear magnetoresistance in Se-rich Bi2O2Se is governed by the average mobility rather than the mobility fluctuation.

Solution-Processed Mixed-Dimensional Hybrid Perovskite/Carbon Nanotube Electronics.

Benefiting from their extraordinary physical properties, methylammonium lead halide perovskites (PVKs) have attracted significant attention in optoelectronics. However, the PVK-based devices suffer from low carrier mobility and high operation voltage. Here, we utilize sorted semiconducting single-walled carbon nanotubes (95% s-SWCNTs) to enhance the performance of thin-film transistors (TFTs) based on the mixed-cation perovskite (MA1-xFAx)Pb(I1-xBrx)3, enabling mixed-dimensional solution-processed electronics with high mobility (32.25 cm2/(V s)) and low voltage (∼3 V) operation. The resulting mixed-dimensional PVK/SWCNT TFTs possess ON/OFF ratios on the order of 107, enabling the fabrication of high-gain inverters.

Quasi-Two-Dimensional Se-Terminated Bismuth Oxychalcogenide (Bi2O2Se).

Since the discovery of graphene, van der Waals (vdW) two-dimensional (2D) materials have attracted considerable attention for various potential applications. Recently, a Se-terminated bismuth oxychalcogenide, Bi2O2Se, has been fabricated using the vapor deposition method. Bi2O2Se is not a vdW 2D material, but the as-grown substance shows 2D characteristics. For example, Bi2O2Se exhibits layer number-dependent absorption spectra in experiments, but until now, there has been no reasonable explanation as to why. Here, we propose a 50% Se-passivation surface model, which elucidates the production of such spectra. Our model is also consistent with recent observations using scanning tunneling microscopy. Moreover, high-resolution transmission electron microcopy observations show a broken zipper-like structure in Bi2O2Se. We ascribe Bi2O2Se as a zipper 2D material, and we summarize the characteristics of zipper 2D materials while proposing the development of others. Zipper 2D materials not only are an important subset of 2D materials but also bridge the gap between vdW 2D materials and traditional 3D materials. Because they are a big family, including insulators, semiconductors, and magnetic metals, zipper 2D materials lend themselves to a plethora of applications.

Gate-Tunable and Multidirection-Switchable Memristive Phenomena in a Van Der Waals Ferroelectric.

Memristive devices have been extensively demonstrated for applications in nonvolatile memory, computer logic, and biological synapses. Precise control of the conducting paths associated with the resistance switching in memristive devices is critical for optimizing their performances including ON/OFF ratios. Here, gate tunability and multidirectional switching can be implemented in memristors for modulating the conducting paths using hexagonal α-In2 Se3 , a semiconducting van der Waals ferroelectric material. The planar memristor based on in-plane (IP) polarization of α-In2 Se3 exhibits a pronounced switchable photocurrent, as well as gate tunability of the channel conductance, ferroelectric polarization, and resistance-switching ratio. The integration of vertical α-In2 Se3 memristors based on out-of-plane (OOP) polarization is demonstrated with a device density of 7.1 × 109 in.-2 and a resistance-switching ratio of well over 103 . A multidirectionally operated α-In2 Se3 memristor is also proposed, enabling the control of the OOP (or IP) resistance state directly by an IP (or OOP) programming pulse, which has not been achieved in other reported memristors. The remarkable behavior and diverse functionalities of these ferroelectric α-In2 Se3 memristors suggest opportunities for future logic circuits and complex neuromorphic computing.

Multidirection Piezoelectricity in Mono- and Multilayered Hexagonal α-In2Se3.

Piezoelectric materials have been widely used for sensors, actuators, electronics, and energy conversion. Two-dimensional (2D) ultrathin semiconductors, such as monolayer h-BN and MoS2 with their atom-level geometry, are currently emerging as new and attractive members of the piezoelectric family. However, their piezoelectric polarization is commonly limited to the in-plane direction of odd-number ultrathin layers, largely restricting their application in integrated nanoelectromechanical systems. Recently, theoretical calculations have predicted the existence of out-of-plane and in-plane piezoelectricity in monolayer α-In2Se3. Here, we experimentally report the coexistence of out-of-plane and in-plane piezoelectricity in monolayer to bulk α-In2Se3, attributed to their noncentrosymmetry originating from the hexagonal stacking. Specifically, the corresponding d33 piezoelectric coefficient of α-In2Se3 increases from 0.34 pm/V (monolayer) to 5.6 pm/V (bulk) without any odd-even effect. In addition, we also demonstrate a type of α-In2Se3-based flexible piezoelectric nanogenerator as an energy-harvesting cell and electronic skin. The out-of-plane and in-plane piezoelectricity in α-In2Se3 flakes offers an opportunity to enable both directional and nondirectional piezoelectric devices to be applicable for self-powered systems and adaptive and strain-tunable electronics/optoelectronics.

Crystalline Copper Phosphide Nanosheets as an Efficient Janus Catalyst for Overall Water Splitting.

Hydrogen is essential to many industrial processes and could play an important role as an ideal clean energy carrier for future energy supply. Herein, we report for the first time the growth of crystalline Cu3P phosphide nanosheets on conductive nickel foam (Cu3P@NF) for electrocatalytic and visible light-driven overall water splitting. Our results show that the Cu3P@NF electrode can be used as an efficient Janus catalyst for both the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). For OER catalysis, a current density of 10 mA/cm2 requires an overpotential of only ∼320 mV and the slope of the Tafel plot is as low as 54 mV/dec in 1.0 M KOH. For HER catalysis, the overpotential is only ∼105 mV to achieve a catalytic current density of 10 mA cm-2. Moreover, overall water splitting can be achieved in a water electrolyzer based on the Cu3P@NF electrode, which showed a catalytic current density of 10 mA/cm2 under an applied voltage of ∼1.67 V. The same current density can also be obtained using a silicon solar cell under ∼1.70 V for both the HER and the OER. This new Janus Cu3P@NF electrode is made of inexpensive and nonprecious metal-based materials, which opens new possibilities based on copper to exploit overall water splitting for hydrogen production. To the best of our knowledge, such high performance of a copper-based water oxidation and overall water splitting catalyst has not been reported to date.

High catalytic activity for water oxidation based on nanostructured nickel phosphide precursors.

For the first time, noble-metal-free nickel phosphide (Ni2P) was used as an excellent catalyst precursor for water oxidation catalysis. The lowest onset potential was observed at ∼1.54 V (vs. RHE) and a Tafel slope of 60 mV dec(-1) was obtained in alkaline solution (pH = 13.6).

Synthesis and enhanced electrochemical catalytic performance of monolayer WS2(1-x) Se2x with a tunable band gap.

The first realization of a tunable band-gap in monolayer WS2(1-x) Se2x is demonstrated. The tuning of the bandgap exhibits a strong dependence of S and Se content, as proven by PL spectroscopy. Because of its remarkable electronic structure, monolayer WS2(1-x) Se2x exhibits novel electrochemical catalytic activity and offers long-term electrocatalytic stability for the hydrogen evolution reaction.