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Nanoceria prepared with different Ce3+/Ce4+ ratios show different oxidase mimetic activities. The activity is enhanced selectively in presence of fluoride ions and used for glutathione detection.

Deoxyribonucleic acid (DNA) has been hypothesized to act as a molecular wire due to the presence of an extended π-stack between base pairs, but the factors that are detrimental in the mechanism of charge transport (CT) across tunnel junctions with DNA are still unclear. Here we systematically investigate CT across dense DNA monolayers in large-area biomolecular tunnel junctions to determine when intrachain or interchain CT dominates and under which conditions the mechanism of CT becomes thermally activated. In our junctions, double-stranded DNA (dsDNA) is 30-fold more conductive than single-stranded DNA (ssDNA). The main reason for this large change in conductivity is that dsDNA forms ordered monolayers where intrachain tunneling dominates, resulting in high CT rates. By varying the temperature T and the length of the DNA fragments in the junctions, which determines the tunneling distance, we reveal a complex interplay between T, the length of DNA, and structural order on the mechanism of charge transport. Both the increase in the tunneling distance and the decrease in structural order result in a change in the mechanism of CT from coherent tunneling to incoherent tunneling (hopping). Our results highlight the importance of the interplay between structural order, tunneling distance, and temperature on the CT mechanism across DNA in molecular junctions.

Non-noble-metal-based electrocatalysts with superior oxygen reduction reaction (ORR) activity to platinum (Pt) are highly desirable but their fabrications are challenging and thus impeding their applications in metal-air batteries and fuel cells. Here, we report a facile molten salt assisted two-step pyrolysis strategy to construct carbon nanosheets matrix with uniformly dispersed Fe3N/Fe nanoparticles and abundant nitrogen-coordinated Fe single atom moieties (Fe@FeSA-N-C). Thermal exfoliation and etching effect of molten salt contribute to the formation of carbon nanosheets with high porosity, large surface area and abundant uniformly immobilized active sites. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image, X-ray absorption fine spectroscopy, and X-ray photoelectron spectroscopy indicate the generation of Fe (mainly Fe3N/Fe) and FeSA-N-C moieties, which account for the catalytic activity for ORR. Further study on modulating the crystal structure and composition of Fe3N/Fe nanoparticles reveals that proper chemical environment of Fe in Fe3N/Fe notably optimizes the ORR activity. Consequently, the presence of abundant FeSA-N-C moieties, and potential synergies of Fe3N/Fe nanoparticles and carbon shells, markedly promote the reaction kinetics. The as-developed Fe@FeSA-N-C-900 electrocatalyst displays superior ORR performance with a half-wave potential (E1/2) of 0.83 V versus reversible hydrogen electrode (RHE) and a diffusion limited current density of 5.6 mA cm−2. In addition, a rechargeable Zn-air battery device assembled by the Fe@FeSA-N-C-900 possesses remarkably stable performance with a small voltage gap without obvious voltage loss after 500 h of operation. The facile synthesis strategy for the high-performance composites represents another viable avenue to stable and low-cost electrocatalysts for ORR catalysis.

Layered sodium titanates (NTO), one of the most promising anode materials for advanced sodium-ion batteries (SIBs), feature high theoretical capacity and no serious safety concerns. The pristine NTO electrode, however, has unfavorable Na+ transport kinetics, due to the dominant two-dimensional (2D) Na-ion transport channels within the crystal along the low energy barrier octahedron layers, which impedes the practical application of this class of potential materials. Herein, an interesting concept of opening three-dimensional (3D) fast ion transport channels within the intrinsic NTO frameworks is proposed to enhance the electrochemical performance through a combination of oxygen vacancy generation and cation substitution strategies, by which the interlayer spacing of the NTO frameworks is expanded for fast 3D Na-ion transport. It is evidenced that the oxygen-deficient and bismuth-substituted HBNTO (BixNa2-xTi3Oy, 0 < x < 2, 0 < y < 7, HBNTO) exhibits obvious enhancements on the reversible capacity (∼145% enhancement at 20 mAh g-1 compared with NTO), the rate capability (∼200% enhancement at 500 mAh g-1 compared with NTO), and the cycling stability (∼210% enhancement of retention capacity after 150 cycles at 20 mAh g-1 compared with NTO). The molecular dynamic simulations and theoretical calculations demonstrate that the enhanced performance of HBNTO is contributed by the multiplied sodium diffusion pathways and the increased ion migration rates with the successful opening of 3D internal ion transport channels. This work demonstrates the effectiveness of the strategies in opening the 3D intercrystal ion transport channels for boosting the electrochemical performance of SIBs.

Sodium-ion batteries (SIBs) are emerging as an inexpensive and more sustainable alternative to lithium-ion batteries in the energy storage market. To advance their commercialization, one major scientific undertaking is to develop low-cost, reliable anode materials from abundant resources, like the success of graphite in the lithium-ion batteries. However, graphite is chemically inactive in storing sodium ions and, to render it viable in sodium-ion batteries, additional modification of graphite is required. Herein, we demonstrate a green and facile method to prepare cost-effective and stable graphitic SIB anodes. The modification process started with the electrochemical oxidation of expanded graphite to widen the interlayer and functionalize graphite layers, followed by a fast (20 min) thermal treatment at 150 °C to achieve controlled deoxygenation. The thermally processed electrochemical graphite oxide could provide a high reversible capacity of 268 mAh g−1 at 100 mA g−1 and 163 mAh g−1 at 500 mA g−1 as well as low fading in capacity (in average 0.0198% loss per cycle) over 2000 cycles. The electrochemical route eliminates the need for the harsh chemical oxidation of graphite, offering a promising approach for industrial production of low-cost anodes for sodium-ion batteries.

2D heterostructures provide another exciting opportunity for extending the application of 2D materials in energy conversion and storage devices, due to their flexibility in electronic structure modulations and surface chemistry regulations. Herein, by coupling liquid-exfoliated and mildly oxidized black phosphorus nanosheets (BP-NSs) with wet-chemically synthesized 2Dnickel Ni(OH)2 nanosheets (NH-NSs), 2D/2D heterostructured nanosheets (BNHNSs) are rationally constructed with a favorable transition of electron structure and desired intermediate adsorptions for alkaline oxygen evolution reaction (OER) catalysis. When used as an OER catalyst, to reach a current density of 10mAcm−2, the overpotential of 2D/2D BNHNSs is only 297mV, corresponding to a considerable decrease of 22% and 34% compared with the individual 2D NH-NSs and 2D BP-NSs, respectively. The structural tracking at the initial reconstruction stage via time-dependent Raman spectra confirms that the phosphorus oxidization into the P–OH and the phase transformation into oxyhydroxide (NiOOH) significantly promote the electron transfer and electrocatalytic efficiency and thus endow the 2D/2D BNHNSs with much enhanced OER catalytic activity. This work offers new insights on the electron structure modulation of 2D-based heterostructures and opens new avenues for regulating the adsorption of emerging phosphorene-based materials for electrocatalysis.

The surface of hydrogen-terminated diamond (H-terminated diamond) supports a p-type surface conductivity when interfaced with high electron-affinity surface acceptors through the surface transfer doping process. High electron-affinity transition metal oxides (TMOs), such as MoO3, have been regarded as superior candidates in surface transfer doping of diamond, holding great promise for enabling practical diamond electronic devices. In this work, a simple, solution-processed method is demonstrated to deposit a molybdenum trioxide (MoO3) layer on H-terminated diamond surface to achieve surface transfer doping of diamond. The surface of diamond following the deposition of solution-processed MoO3 (sp-MoO3) experienced significant reduction in sheet resistivity, corresponding to an increase in hole density, compared with the pristine air-doped diamond. This hole accumulation layer induced by sp-MoO3 is thermally stable up to 450 °C, and demonstrates metallic conductivity down to 250 mK with a strong spin-orbit interaction (SOI) in the induced two-dimensional (2D) surface conducting layer. Our study demonstrates that sp-MoO3 is comparable with thermally-evaporated MoO3 in doping performance but with great advantage as a simple, inexpensive and highly flexible fabrication method for developing diamond electronics.

Delafossite CuFeO2 is a p-type oxide semiconductor with a band gap of ∼1.5 eV, which has attracted great interests for applications in solar energy harvesting and oxide electronics. However, there are still some discrepancies in the literature regarding its fundamental electronic structure and transport properties. In this paper, we use a synergistic combination of resonant photoemission spectroscopy and X-ray absorption spectroscopy to directly study the electronic structure of well-defined CuFeO2 epitaxial thin films. Our detailed study reveals that CuFeO2 has an indirect and d-d forbidden band gap of 1.5 eV. The top of the valence band (VB) of CuFeO2 mainly consists of occupied Fe 3d states hybridized with Cu 3d and O 2p, and the bottom of the conduction band (CB) is primarily made up of unoccupied Fe 3d states. The localized nature of the Fe 3d states at both CB and VB edges would limit the carrier mobility and the dynamics of photoexcited carriers. In addition, Mg doping at Fe sites in CuFeO2 increases the hole carrier concentration and leads to a gradual shift of the Fermi level toward the VB. These insights into its electronic structure are of fundamental importance for rational designing and improving the performance of CuFeO2 as photocatalysts.

Double perovskite oxides are one of the most promising bifunctional electrocatalysts for efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) due to their adjustable electronic structures via doping with different metal cations or engineering of defects. Herein, we report a systematic study on the tuning of the electronic structure of La2-xSrxNiMnO6 with 0 ≤ x ≤ 1.0 to promote the bifunctional OER/ORR activity. The bifunctional index (ΔE) is substantially reduced with increasing of Sr contents and achieves an optimal value of 0.85 V for La1.4Sr0.6NiMnO6, exceeding that of widely studied LaNiO3. Our study on electronic structures reveals that the enhancement of the ORR and OER activities strongly correlates with the appearance of Ni3+ oxidation states and the upshift of the O 2p-band center promoted by Sr doping. Furthermore, an increase of hole states, derived from Ni3+ states, reduces the energy barrier for the electron transfer from 0.44 to 0.12 eV and hence improves the intrinsic OER activities. The tuning of the electronic structure that leads to higher OER and ORR activities in La2-xSrxNiMnO6 can be extended to other materials for the design of active bifunctional electrocatalysts.

Periodic nanostructures, a group of nanomaterials consisting of single or multiple nano units/components periodically arranged into ordered patterns (e.g., vertical and lateral superlattices), have attracted tremendous attention in recent years due to their extraordinary physical and chemical properties that offer a huge potential for a multitude of applications in energy conversion, electronic and optoelectronic applications. Recent advances in the preparation strategies of periodic nanostructures, including self-assembly, epitaxy, and exfoliation, have paved the way to rationally modulate their ferroelectricity, superconductivity, band gap and many other physical and chemical properties. For example, the recent discovery of superconductivity observed in "magic-angle" graphene superlattices has sparked intensive studies in new ways, creating superlattices in twisted 2D materials. Recent development in the various state-of-the-art preparations of periodic nanostructures has created many new ideas and findings, warranting a timely review. In this review, we discuss the current advances of periodic nanostructures, including their preparation strategies, property modulations and various applications.

The oxygen evolution reaction (OER) plays a crucial role in many electrochemical energy technologies, and creating multiple beneficial factors for OER catalysis is desirable for achieving high catalytic efficiency. Here, we highlight a new halogen-chlorine (Cl)-anion doping strategy to boost the OER activity of perovskite oxides. As a proof-of-concept, proper Cl doping at the oxygen site of LaFeO3 (LFO) perovskite can induce multiple favorable characteristics for catalyzing the OER, including rich oxygen vacancies, increased electrical conductivity and enhanced Fe-O covalency. Benefiting from these factors, the LaFeO2.9-δCl0.1 (LFOCl) perovskite displays significant intrinsic activity enhancement by a factor of around three relative to its parent LFO. This work uncovers the effect of Cl-anion doping in perovskites on promoting OER performance and paves a new way to design highly efficient electrocatalysts.

Bismuth-based materials (e.g., metallic, oxides and subcarbonate) are emerged as promising electrocatalysts for converting CO2 to formate. However, Bio-based electrocatalysts possess high overpotentials, while bismuth oxides and subcarbonate encounter stability issues. This work is designated to exemplify that the operando synthesis can be an effective means to enhance the stability of electrocatalysts under operando CO2RR conditions. A synthetic approach is developed to electrochemically convert BiOCl into Cl-containing subcarbonate (Bi2O2(CO3)xCly) under operando CO2RR conditions. The systematic operando spectroscopic studies depict that BiOCl is converted to Bi2O2(CO3)xCly via a cathodic potential-promoted anion-exchange process. The operando synthesized Bi2O2(CO3)xCly can tolerate − 1.0 V versus RHE, while for the wet-chemistry synthesized pure Bi2O2CO3, the formation of metallic Bio occurs at − 0.6 V versus RHE. At − 0.8 V versus RHE, Bi2O2(CO3)xCly can readily attain a FEHCOO- of 97.9%, much higher than that of the pure Bi2O2CO3 (81.3%). DFT calculations indicate that differing from the pure Bi2O2CO3-catalyzed CO2RR, where formate is formed via a *OCHO intermediate step that requires a high energy input energy of 2.69 eV to proceed, the formation of HCOO− over Bi2O2(CO3)xCly has proceeded via a *COOH intermediate step that only requires low energy input of 2.56 eV.

Porous metal-organic frameworks (MOFs) have become one of the fastest growing fields with broad applications due to the presence of various active sites (e.g., cavities, metal nodes, and organic linkers). Notably, an on-surface synthesis represents a facile method for the fabrication of two-dimensional (2D) MOFs, where the substrate not only supports and dictates the symmetry of the surface nanostructures but also provides the metal atoms to form the metal nodes. However, the realization of on-surface synthesized 2D metal-inorganic porous networks is rarely explored, except for a recently reported gold-phosphorus network. Here, we integrate one more dimension into the growth process (activated gas phase atoms) and demonstrate the on-surface synthesis of ternary metal-inorganic gold-phosphorus-nitrogen porous networks on Au(111). The atomic structure is precisely identified through a combination of low-temperature scanning tunneling microscopy, density functional theory calculations, and X-ray photoelectron spectroscopy. Using the thermal energy from the heated Au(111) substrate, the phosphorus precursor can simultaneously grab gold atoms from the surface and nitrogen atoms from activated nitrogen gas to build up a long-range ordered 2D surface porous structure. Our study opens a completely new platform for the on-surface synthesis of tailored metal-inorganic frameworks.

Hydrogen-terminated diamond possesses an intriguing p-type surface conductivity which is induced via thermodynamically driven electron transfer from the diamond surface into surface acceptors such as atmospheric adsorbates, a process called surface transfer doping. High electron affinity transition metal oxides (TMOs) including MoO3 and V2O5 have been shown to be highly effective solid-state surface acceptors for diamond, giving rise to a sub-surface two-dimensional (2D) hole layer with metallic conduction. In this work, low temperature magnetotransport is used as a tool to show the presence of a Rashba-type spin-orbit interaction with a high spin-orbit coupling of 19.9 meV for MoO3 doping and 22.9 meV for V2O5 doping, respectively, through the observation of a transition in the phase-coherent backscattering transport from weak localization to weak antilocalization at low temperature. Surface transfer doping of diamond with TMOs provides a 2D hole system with spin-orbit coupling that is over two times larger than that reported for diamond surfaces with atmospheric acceptors, opening up possibilities to study and engineer spin transport in a carbon material system.

Hydrogen-rich organic molecules such as alcohols are widely used as hydrogen donors in transfer hydrogenation. Nevertheless, water as a more abundant and ecofriendly hydrogen source has hardly been used due to the high difficulty in splitting water molecules. Herein, we designed a photocatalytic water-donating transfer hydrogenation (PWDTH) technique, in which hydrogen was extracted from water under light illumination and then in situ added to different unsaturated bonds (C-C, C-O, N-O) for chemical synthesis. Platinum-loaded carbon nitride (Pt/CN) was used as the model catalyst for this cascade reaction, which is beyond its normal applications for water splitting. This approach was highly accessible to efficiency optimization, either by modifying CN for extended light absorption and enhanced charge transfer or by alloying Pt with another metal for better catalytic activities. Remarkably, a quantum efficiency of up to 21.8% was achieved for nitrobenzene hydrogenation under 380 nm irradiation, which is 3 times higher than that obtained in the single water-splitting reaction, indicating that the PWDTH can be more rewarding than hydrogen evolution for solar energy harvesting. Deep insights into the underlying mechanism were provided by detailed measurements and interpretations of femtosecond transient absorption spectra, action spectra (quantum efficiency as a function of excitation wavelength), and reaction kinetic profiles under varied conditions including the variation of light intensities, temperatures, and water isotopes. The mild reaction conditions, simple processing, and broad substituent group tolerance endow this approach with a high potential toward a general solar-to-chemical conversion technique.

To avoid crosstalk and suppress leakage currents in resistive random access memories (RRAMs), a resistive switch and a current rectifier (diode) are usually combined in series in a one diode–one resistor (1D–1R) RRAM. However, this complicates the design of next-generation RRAM, increases the footprint of devices and increases the operating voltage as the potential drops over two consecutive junctions1. Here, we report a molecular tunnel junction based on molecules that provide an unprecedented dual functionality of diode and variable resistor, resulting in a molecular-scale 1D–1R RRAM with a current rectification ratio of 2.5 × 104 and resistive on/off ratio of 6.7 × 103, and a low drive voltage of 0.89 V. The switching relies on dimerization of redox units, resulting in hybridization of molecular orbitals accompanied by directional ion migration. This electric-field-driven molecular switch operating in the tunnelling regime enables a class of molecular devices where multiple electronic functions are preprogrammed inside a single molecular layer with a thickness of only 2 nm. A multifunctional molecule acting both as diode and variable resistor is used to fabricate compact molecular switches with a thickness of 2 nm, good current rectification and resistive on/off ratio, and requiring a drive voltage as low as 0.89 V.

Hematite (Fe2O3) is a well-known oxide semiconductor suitable for photoelectrochemical (PEC) water splitting and industry gas sensing. It is widely known that Sn doping of Fe2O3 can enhance the device performance, yet the underlying mechanism remains elusive. In this work, we determine the relationship between electronic structure, optical properties, and PEC activity of Sn-doped Fe2O3 by studying highly crystalline, well-controlled thin films prepared by pulsed laser deposition (PLD). We show that Sn doping substantially increases the n-type conductivity of Fe2O3, and the conduction mechanism is better described by a small-polaron hopping (SPH) model. Only 0.2% Sn doping significantly reduces the activation energy barrier for SPH conduction from at least 0.5 eV for undoped Fe2O3 to 0.14 eV for doped ones. A combination of X-ray photoemission, X-ray absorption spectroscopy, and DFT calculations reveals that the Fermi level gradually shifts toward the conduction band minimum with Sn doping. A localized Fe2+-like gap state is observed at the top of the valence band, accounting for the SPH conduction. Interestingly, different from the literature, only 0.2% Sn doping in Fe2O3 significantly improves the PEC activity, while more Sn decreases it. The improved PEC activity is partially attributed to an increased band bending potential which facilitates the charge separation at the space charge region. The reduced activation energy barrier for SPH will facilitate the transport of photoexcited carriers for the enhanced PEC, which is of interest for further carrier dynamics study.

The single layer diamond – diamane, a two-dimensional (2D) form of diamond with a bilayer sp3 carbon nanostructure, has been initially predicted in 2009, while its experimental synthesis has only been reported very recently. This work carries out a comprehensive study on the vibrational properties of diamane nanoribbon (DNR) targeting the ultra-sensitive sensing applications. Based on in silico studies, it is found that the DNR resonator possesses a higher natural frequency and a large quality factor (Q-factor) on the order of 105 higher than those of a bilayer nanoribbon resonator. Under pre-tensile strain, the natural frequency of the DNR resonator receives a remarkable increase and its Q-factor maintains a high magnitude yielding to an extremely high figure of merit on the order of 1015. It is further found that the randomly distributed surface hydrogenation exerts negligible influence on the vibrational properties of the DNR resonator. However, an unevenly distributed hydrogenation results in out-of-plane deformation and significantly changes its vibrational properties. It is additionally found that the stacking configuration of the diamane leads to negligible influence on its vibrational properties. This study reveals that the DNR resonator has excellent vibrational properties, which are promising for the construction of ultra-sensitive resonator-based sensors.

Perovskite transition metal oxides are promising non-precious metal electrocatalysts for promoting the oxygen evolution reaction (OER) in many electrochemical energy conversion devices. This work reports a systematic study of the relation between the electronic structure and OER performance of perovskite La1−xSrxFeO3 (0 ≤ x ≤ 1). The partial substitution of La for Sr in LaFeO3 results in the oxidation of Fe3+ to Fe4+ and substantially enhances OER activity. A comprehensive X-ray spectroscopic study reveals a strong correlation of the enhanced OER activity with these Fe4+ states. The presence of Fe4+ leads to a stronger Fe–O bond due to stronger Fe 3d–O 2p orbital hybridization and shifts the energy position of the valence band (VB) to EF. Such an electronic modulation optimizes the surface adsorption energetics of *OH intermediates, contributing to faster OER kinetics. Furthermore, a new unoccupied state is created at ∼0.9 eV below the conduction band. This hole state reduces the energy barrier for electron transfer from 1.34 eV to 0.44 eV, thereby facilitating charge transfer at the interface. The Fe4+-induced electronic states responsible for higher OER activity have implications for the understanding of structure–activity relationships in other Fe-based electrocatalysts such as highly active Fe-doped NiOOH.

Molecule-like electrocatalysts with FeN4motifs have been demonstrated to be excellent candidates for various renewable energy conversions. The ability to further tune the electronic properties of molecular FeN4motifs and integrate them onto conductive supports represents a key step towards the synthesis of highly robust and efficient single-atom catalysts (SACs) for practical applications. Here, we developed a new route for the synthesis of a well-defined single-atom FeN4electrocatalystvia in situpolymerization of four amino groups functionalized iron phthalocyanine (NH2-FePc) molecules on conductive carbon nanotubes. The intermolecular oxidative dimerization between the amino groups of NH2-FePc creates the desired electron-withdrawing pyrazine linker between FeN4motifs, which can significantly optimize their electrocatalytic performances. As a result, the FeN4-SAC exhibits both outstanding ORR activity (a half-wave potential of 0.88 Vvs.RHE) and excellent performance in Zn-oxygen batteries, outperforming the commercial Pt/C and pristine iron phthalocyanine (FePc) catalysts. Our theoretical calculations reveal that the presence of electron-withdrawing linkers shifts the occupied antibonding states towards lower energies and thus weakens the Fe-O bond, which is primarily responsible for the enhancement of ORR activity.

To realise molecular scale electrical operations beyond the von Neumann bottleneck, new types of multi-functional switches are needed that mimic self-learning or neuromorphic computing by dynamically toggling between multiple operations that depend on their past. Here we report a molecule that switches from high to low conductance states with massive negative memristive behaviour that depends on the drive speed and number of past switching events, with all measurements fully modelled using atomistic and analytical models. This dynamic molecular switch (DMS) emulates synaptic behaviour and Pavlovian learning, all within a 2.4 nm thick layer that is three orders of magnitude thinner than a neuronal synapse. The DMS provides all fundamental logic gates necessary for deep learning because of its time-domain and voltage-dependent plasticity. The synapse-mimicking multi-functional DMS represents adaptable molecular scale hardware operable in solid-state devices opening a pathway to simplify dynamic complex electrical operations encoded within a single ultra-compact component.

Rechargeable aqueous zinc-ion batteries (ZIBs) are promising in stationary grid energy storage due to their advantages in safety and cost-effectiveness, and the search for competent cathode materials is one core task in the development of ZIBs. Herein, the authors design a 2D heterostructure combining amorphous vanadium pentoxide and electrochemically produced graphene oxide (EGO) using a fast and scalable spray drying technique. The unique 2D heterostructured xerogel is achieved by controlling the concentration of EGO in the precursor solution. Driven by the improved electrochemical kinetics, the resultant xerogel can deliver an excellent rate capability (334 mAh g

In this paper, we report insights into the local atomic and electronic structure of ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ epitaxial thin films and its correlation with electrical, optical, and magnetic properties. We grew structurally well-defined ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ epitaxial thin films with controlled properties on $\mathrm{Mg}{\mathrm{Al}}_{2}{\mathrm{O}}_{4}(001)$ substrates using pulsed laser deposition. Films grown at low temperatures ($l400{\phantom{\rule{0.28em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$) exhibit a ferrimagnetic and metallic behavior, while those grown at high temperatures are nonmagnetic semiconductors. The electronic structure and cation local atomic coordination of the respective films were investigated using a combination of resonant photoemission spectroscopy, x-ray absorption spectroscopy, and ab initio calculations. Our results unambiguously reveal that the ${\mathrm{Ni}}^{3+}$ valence state promoted at low growth temperature introduces delocalized $\mathrm{Ni}\phantom{\rule{0.28em}{0ex}}3d$-derived states at the Fermi level (${E}_{F}$), responsible for the metallic state in ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$, while the $\mathrm{Co}\phantom{\rule{0.28em}{0ex}}3d$-related state is more localized at higher binding energy. In the semiconducting films, the valence state of Ni is lowered and $\ensuremath{\sim}+2$. Further structural and defect chemistry studies indicate that the formation of oxygen vacancies and secondary CoO phases at high growth temperature are responsible for the ${\mathrm{Ni}}^{2+}$ valence state in ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$. The $\mathrm{Ni}\phantom{\rule{0.28em}{0ex}}3d$-related state becomes localized away from ${E}_{F}$, opening a band gap for a semiconducting state. The band gap of the semiconducting ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ is estimated to be $l0.8\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, which is much smaller than the quoted values in the literature ranging from 1.1 to 2.58 eV. Despite the small band gap, its optical transition is $d\text{\ensuremath{-}}d$ dipole forbidden, and therefore, the semiconducting ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ still shows reasonable transparency in the infrared-visible light region. The present insights into the role of ${\mathrm{Ni}}^{3+}$ in determining the electronic structure and defect chemistry of ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ provide important guidance for use of ${\mathrm{NiCo}}_{2}{\mathrm{O}}_{4}$ in electrocatalysis and opto-electronics.

2D black phosphorus (BP) is one promising electrocatalyst toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) catalysis. The too strong adsorption of oxygen intermediates during OER, while the too weak adsorption of hydrogen intermediate during HER, however, greatly compromise its practical water splitting applications with overpotentials as high as 450 mV for OER and 420 mV for HER to achieve 10 mA cm-2 under alkaline conditions. Herein, by rationally introducing the nanosized iridium (Ir) modifier together with optimized exposing surface toward electrolytes, an efficient Ir-modified BP electrocatalyst with much favorable adsorption energies toward catalytic intermediates possesses an outstanding pH-universal water splitting performance, surpassing the nearly all reported BP-based catalysts and the commercial noble-metal catalysts. The Ir-modified BP catalyst with the optimized exposed surfaces only requires an overall cell voltage of 1.54 and 1.57 V to achieve 10 mA cm-2 in acidic and alkaline electrolysers, respectively. This design uncovers the potential applications of 2D BP in practical electrocatalysis fields via decreasing reaction intermediate adsorption energy barriers and promoting the interfacial electron coupling for heterostructured catalysts, and offers new insights into the surface-dependent activity enhancement mechanism.

Charge transfer (CT) dynamics across metal-molecule interfaces has important implications for performance and function of molecular electronic devices. CT times, on the order of femtoseconds, can be precisely measured using synchrotron-based core-hole clock (CHC) spectroscopy, but little is known about the impact on CT times of the metal work function and the bond dipole created by metals and the anchoring group. To address this, here we measure CT dynamics across self-assembled monolayers bound by thiolate anchoring groups to Ag, Au, and Pt. The molecules have a terminal ferrocene (Fc) group connected by varying numbers of methylene units to a diphenylacetylene (DPA) wire. CT times measured using CHC with resonant photoemission spectroscopy (RPES) show that conjugated DPA wires conduct electricity faster than aliphatic carbon wires of a similar length. Shorter methylene connectors exhibit increased conjugation between Fc and DPA, facilitating CT by providing greater orbital mixing. We find nearly 10-fold increase in the CT time on Pt compared to Ag due to a larger bond dipole generated by partial electron transfer from the metal-sulfur bond to the carbon-sulfur bond, which creates an electrostatic field that impedes CT from the molecules. By fitting the RPES signal, we distinguish electrons coming from the Fe center and from cyclopentadienyl (Cp) rings. The latter shows faster CT rates because of the delocalized Cp orbitals. Our study demonstrates the fine tuning of CT rates across junctions by careful engineering of several parts of the molecule and the molecule-metal interface.

This paper describes the transition from the normal to inverted Marcus region in solid-state tunnel junctions consisting of self-assembled monolayers of benzotetrathiafulvalene (BTTF), and how this transition determines the performance of a molecular diode. Temperature-dependent normalized differential conductance analyses indicate the participation of the HOMO (highest occupied molecular orbital) at large negative bias, which follows typical thermally activated hopping behavior associated with the normal Marcus regime. In contrast, hopping involving the HOMO dominates the mechanism of charge transport at positive bias, yet it is nearly activationless indicating the junction operates in the inverted Marcus region. Thus, within the same junction it is possible to switch between Marcus and inverted Marcus regimes by changing the bias polarity. Consequently, the current only decreases with decreasing temperature at negative bias when hopping is “frozen out,” but not at positive bias resulting in a 30-fold increase in the molecular rectification efficiency. These results indicate that the charge transport in the inverted Marcus region is readily accessible in junctions with redox molecules in the weak coupling regime and control over different hopping regimes can be used to improve junction performance., Y.H., C.N., M.S.M., and S.K.K. contributed equally to this work. The authors acknowledge the Ministry of Education (MOE) for supporting this research under award no. MOE2019-T2-1-137. Prime Minister's Office, Singapore under its Medium sized centre program is also acknowledged for supporting this research. The authors also gratefully acknowledge the Australian Synchrotron (ANSTO) soft X-ray spectroscopy beamline where the SAM characterization was conducted. C.N. and E.d.B. acknowledge support from the US National Science Foundation (grant no. ECCS#1916874). D.Q. acknowledges the support of the Australian Research Council (Grant No. FT160100207). Authors also acknowledge the Marie-Sklodowska-Curie project ITN iSwitch (GA-642196), Spanish Ministry of Economy and Competitiveness (project FANCY CTQ2016-80030-R, GENESIS PID2019-111682RB-I00 and Severo Ochoa programme for Centers of excellence in R&D (SEV-2015-0496)). J.C. is a research director of the Belgian National Fund for Scientific Research (FNRS).

Being a metallic transition-metal dichalcogenide, monolayer vanadium diselenide (VSe2) exhibits many novel properties, such as charge density waves and magnetism. Its interfaces with other materials can potentially be used in device applications as well as for manipulating its intrinsic properties. Here, we present a scanning tunneling microscopy and synchrotron-based X-ray photoemission spectroscopy study of the surface charge-transfer doping using efficient electron-withdrawing and electron-donating materials, that is, molybdenum trioxide (MoO3) and potassium (K), on the molecular beam epitaxy-grown monolayer VSe2 on highly oriented pyrolytic graphite (HOPG). We demonstrate that monolayer VSe2 is immune to MoO3- and K-doping effects. However, at the monolayer edges where the local chemical reactivity is higher because of Se deficiency, MoO3 is seen to react with VSe2 to form molybdenum dioxide (MoO2) and vanadium dioxide (VO2). Compared to the obvious charge-transfer doping effects of MoO3 and K on HOPG, the electronic structure of monolayer VSe2 is barely perturbed. This is attributed to the large density of states at the Fermi level of monolayer VSe2 carrying the metallic character. This work provides new insights into the chemical and electronic properties of monolayer VSe2, important for future VSe2-based electronic device design.

Practical applications of two-dimensional (2D) black phosphorus (BP) are limited by its fast degradation under ambient conditions, for which many different mechanisms have been proposed; however, an atomic level understanding of the degradation process is still hindered by the absence of bottom-up methods for the growth of large-scale few-layer black phosphorus. Recent experimental success in the fabrication of single-layer blue phosphorus provides a model system to probe the oxidation mechanism of two-dimensional (2D) phosphorene down to single-layer thicknesses. Here, we report an atomic-scale investigation of the interaction between molecular oxygen and blue phosphorus. The atomic structure of blue phosphorus and the local binding sites of oxygen have been precisely identified using qPlus-based noncontact atomic force microscopy. A combination of low-temperature scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements reveal a thermally reversible oxidation process of blue phosphorus in a pure oxygen atmosphere. Our study clearly demonstrates the essential role of oxygen in the initial oxidation process, and it sheds further light on the fundamental pathways of the degradation mechanism.

Although graphene oxide (GO) has shown enduring popularity in the research community, its synthesis remains cost prohibitive for many of its demonstrated applications. While significant progress ha...

This work reports a fundamental study on the relationship of the electronic structure, catalytic activity and surface reconstruction process of Fe doped NiS2 (FexNi1-xS2) for the oxygen evolution reaction (OER). A combined photoemission and X-ray absorption spectroscopic study reveals that Fe doping introduces more occupied Fe 3d6 states at the top of the valence band and thereby induces a metallic phase. Meanwhile, Fe doping also significantly increases the OER activity and results in much better stability with the optimum found for Fe0.1Ni0.9S2. More importantly, we performed detailed characterization to track the evolution of the structure and composition of the catalysts after different cycles of OER testing. Our results further confirmed that the catalysts gradually transform into amorphous (oxy)hydroxides which are the actual active species for the OER. However, a fast phase transformation in NiS2 is accompanied by a decrease of OER activity, because of the formation of a thick insulating NiOOH layer limiting electron transfer. On the other hand, Fe doping retards the process of transformation, because of a shorter Fe-S bond length (2.259 Å) than Ni-S (2.400 Å), explaining the better electrochemical stability of Fe0.1Ni0.9S2. These results suggest that the formation of a thin surface layer of NiFe (oxy)hydroxide as an active OER catalyst and the remaining Fe0.1Ni0.9S2 as a conductive core for fast electron transfer is the base for the high OER activity of FexNi1-xS2. Our work provides important insight and design principle for metal chalcogenides as highly active OER catalysts.

Transition metal oxides are being actively pursued as low-cost electrocatalysts for the oxygen evolution reaction (OER) in many electrochemical energy devices. A fundamental understanding of the oxide electronic structures, along with the ability to rationally tune them, is a key step toward designing of highly active catalysts. Here, we report the tuning of the electronic structure of NiO via Li doping (LixNi1–xO) to enhance the OER activities. We identified that Li0.5Ni0.5O (LiNiO2) has the highest OER activity, comparable to or exceeding that of the benchmark perovskite Ba0.5Sr0.5Co0.8Fe0.2O3−δ and LaNiO3. More importantly, a synergistic combination of synchrotron-based photoemission spectroscopy, X-ray absorption spectroscopy, and density functional theory was used to unravel the electronic structure of LixNi1–xO with unprecedented accuracy, thus providing deep insight into the origin of the enhanced catalytic activity. The results unambiguously reveal the creation of a new hole state at 1.1 eV above the Fermi level and an enhanced degree of O 2p–Ni 3d hybridization induced by Li doping optimize the adsorption energetics of OH intermediates and thereby facilitate the fast kinetics for the OER. The LixNi1–xO would serve as a new platform to study the relationship of composition–electronic structure–activity for OER electrocatalysts, beyond the extensively studied Co-based perovskites.

Despite being a bona-fide bulk insulator, diamond develops an intriguing two-dimensional (2D) p-type surface conductivity when its surface is terminated by hydrogen and exposed to appropriate surface adsorbate layer as a result of the surface transfer doping process. Consequently, the surface of diamond presents a versatile platform for exploiting some of the extraordinary physical and chemical properties of diamond, leading to applications such as chemical/biological sensing and the development of high-power and high-frequency field-effect transistors (FETs). In this talk, I will describe our recent work on the surface transfer doping of diamond by transition metal oxides (TMOs), which give rise to an underlying two-dimensional (2D) hole conducting layer on diamond that can be harnessed for building devices and the exploration of quantum transport properties.

Herein, we report the first room temperature switchable Fe(iii) molecular spin crossover (SCO) tunnel junction. The junction is constructed from [FeIII(qsal-I)2]NTf2 (qsal-I = 4-iodo-2-[(8-quinolylimino)methyl]phenolate) molecules self-assembled on graphene surfaces with conductance switching of one order of magnitude associated with the high and low spin states of the SCO complex. Normalized conductance analysis of the current–voltage characteristics as a function of temperature reveals that charge transport across the SCO molecule is dominated by coherent tunnelling. Temperature-dependent X-ray absorption spectroscopy and density functional theory confirm the SCO complex retains its SCO functionality on the surface implying that van der Waals molecule—electrode interfaces provide a good trade-off between junction stability while retaining SCO switching capability. These results provide new insights and may aid in the design of other types of molecular devices based on SCO compounds., Herein, we report the first room temperature switchable Fe(iii) molecular spin crossover (SCO) tunnel junction.

Tunnel junctions based on Fe storing globular proteins are an interesting class of biomolecular tunnel junctions due to their tunable Fe ion loading, symmetrical structure and thermal stability, and are therefore attractive to study the mechanisms of charge transport (CT) at the molecular level. This paper describes a temperature-induced change in the CT mechanism across junctions with large globular (similar to 25 nm in diameter) E2-proteins bioengineered with Fe-binding peptides from ferritin (E2-LFtn) to mineralise Fe ions in the form of iron oxide nanoparticles (NPs) inside the protein's cavity. The iron oxide NPs provide accessible energy states that support high CT rates and shallow activation barriers. Interestingly, the CT mechanism changes abruptly, but reversibly, from incoherent tunnelling (which is thermally activated) to coherent tunnelling (which is activationless) across the E2-LFtn-based tunnel junctions with the highest Fe ion loading at a temperature of 220-240 K. During this transition the current density across the junctions increases by a factor of 13 at an applied voltage of V = -0.8 V. X-ray absorption spectroscopy indicates that the iron oxide NPs inside the E2-LFtn cages undergo a reversible phase transition; this phase transition opens up new a tunnelling pathway changing the mechanism of CT from thermally activated to activationless tunnelling despite the large size of the E2-LFtn and associated distance for tunnelling. Ministry of Education (MOE) Published version D. Q. acknowledges the support of the Australian Research Council (Grant No. FT160100207). We acknowledge the Ministry of Education (MOE) for supporting this research under award no. MOE2019-T2-1-137. Prime Minister’s Office, Singapore, under its Medium-sized centre program is also acknowledged for supporting this research.

Ultrathin freestanding membranes with a pronounced metal–insulator transition (MIT) have huge potential for future flexible electronic applications as well as provide a unique aspect for the study of lattice–electron interplay. However, the reduction of the thickness to an ultrathin region (a few nm) is typically detrimental to the MIT in epitaxial films, and even catastrophic for their freestanding form. Here, we report an enhanced MIT in VO2-based freestanding membranes, with a lateral size up to millimeters and the VO2 thickness down to 5 nm. The VO2 membranes were detached by dissolving a Sr3Al2O6 sacrificial layer between the VO2 thin film and the c-Al2O3(0001) substrate, allowing the transfer onto arbitrary surfaces. Furthermore, the MIT in the VO2 membrane was greatly enhanced by inserting an intermediate Al2O3 buffer layer. In comparison with the best available ultrathin VO2 membranes, the enhancement of MIT is over 400% at a 5 nm VO2 thickness and more than 1 order of magnitude for VO2 above 10 nm. Our study widens the spectrum of functionality in ultrathin and large-scale membranes and enables the potential integration of MIT into flexible electronics and photonics. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version

Empowering conventional materials with unexpected magnetoelectric properties is appealing to the multi-functionalization of existing devices and the exploration of future electronics. Recently, owing to its unique effect in modulating a matter's properties, ultra-small dopants, for example, H, D, and Li, attract enormous attention in creating emergent functionalities, such as superconductivity, and metal–insulator transition. Here, an observation of bipolar conduction accompanied by a giant positive magnetoresistance in D-doped metallic Ti oxide (TiOxDy) films is reported. To overcome the challenges in intercalating the D into a crystalline oxide, a series of TiOxDy is formed by sequentially doping Ti with D and surface/interface oxidation. Intriguingly, while the electron mobility of the TiOxDy increases by an order of magnitude larger after doping, the emergent holes also exhibit high mobility. Moreover, the bipolar conduction induces a giant magnetoresistance up to 900% at 6 T, which is ≈6 times higher than its conventional phase. This study paves a way to empower conventional materials in existing electronics and induce novel electronic phases. Ministry of Education (MOE) National Research Foundation (NRF) Accepted version

Coordination crosslinking of oligoamide nanorods yields two dimensional metallosupramolecular framework.

Materials with flat bands are considered as ideal platforms to explore strongly correlated physics such as the fractional quantum hall effect, high-temperature superconductivity, and more. In theory, a Kagome lattice with only nearest-neighbor hopping can give rise to a flat band. However, the successful fabrication of Kagome lattices is still very limited. Here, we provide a new design principle to construct the Kagome lattice by trapping atoms into Kagome arrays of potential valleys, which can be realized on a potassium-decorated phosphorus-gold surface alloy. Theoretical calculations show that the flat band is less correlated with the neighboring trivial electronic bands, which can be further isolated and dominate around the Fermi energy with increased Kagome lattice parameters of potassium atoms. Our results provide a new strategy for constructing Kagome lattices, which serve as an ideal platform to study topological and more general flat band phenomena.

Artificial iconic memories, also called photomemories, are new types of nonvolatile memory that can simultaneously detect and store light information in a monolithic device. Several approaches have been proposed to construct artificial iconic memories, such as three-terminal field effect transistors, which can achieve an effective control of the gate voltage and external light terminals. The drawbacks in constructing these memories involve complicated fabrication processes, and the resulting performance of, for example, perovskite transistor-type photomemories is limited by the low carrier mobilities and poor ambient stabilities, whereas architectures based on floating gate modulations entail strict interface engineering and poor device reliability. In this paper, we propose a novel monolithic artificial iconic memory with a multilayer architecture of indium tin oxide/perovskite/gold/perovskite/silver, which combines the memory and photodetector functionalities of perovskites in an integrated device. The bottom perovskite layer plays the role of a photodetector, modulating the voltage bias on the top perovskite layer that serves as a resistive switching memory. This multilayer perovskite device can store photo-sensing data in its resistive states, with a memory retention of 5 × 103 s and ambient stability longer than sixty days. As a prototype demonstration, a 7 × 7 artificial iconic memory array is constructed to detect and store data on light intensity distribution, enabling a nonvolatile imaging functionality. Our work provides a new platform for designing perovskite-based architectures with simultaneous light detection and data storage capabilities. Published version This work was supported by the University of New South Wales SHARP Project. D.-C. Qi acknowledges the support of the Australian Research Council (Grant No. FT160100207) and the continued support from the Queensland University of Technology (QUT) through the Centre for Materials Science.