Selenium Vacancies Research Articles

Anomalous Temperature and Polarization Dependences of Photoluminescence of Metal‐Organic Chemical Vapor Deposition‐Grown GeSe2

AbstractGermanium diselenide (GeSe2) is a 2D semiconductor with air stability, a wide bandgap, and anisotropic optical properties. The absorption and photoluminescence (PL) of single‐crystalline 2D GeSe2 grown by metal‐organic chemical vapor deposition and their dependence on temperature and polarization are studied. The PL spectra exhibit peaks at 2.5 eV (peak A) and 1.8 eV (peak B); peak A displays a strongly polarized emission along the short axis of the crystal, and peak B displays a weak polarization perpendicular to that of peak A. With increasing temperature, peak B shows anomalous behaviors, i.e., an increasing PL energy and intensity. The excitation energy‐dependent PL, time‐resolved PL, and density functional theory calculations suggest that peak A corresponds to the band‐edge transition, whereas peak B originates from the inter‐band mid‐gap states caused by selenium vacancies passivated by oxygen atoms. The comprehensive study on the PL of single‐crystalline GeSe2 sheds light on the origins of light emission in terms of the band structure of anisotropic GeSe2, making it beneficial for the corresponding optoelectronic applications.

Electron density control in WSe2 monolayers via photochlorination

Modulation of the Fermi level using an ultraviolet (UV)-assisted photochemical method is demonstrated in tungsten diselenide monolayers. Systematic shifts and relative intensities between charged and neutral exciton species indicate a progressive and controllable decrease of the electron density and switch tungsten diselenide from n-type to a p-type semiconductor. The presence of chlorine in the 2D crystal shifts the Fermi level closer to the valence band while the effect can be only partially reversible via continuous wave laser rastering process. Chlorine species in the lattice are validated by x-ray photoelectron spectroscopy, while density functional theory calculations predict that adsorption of chlorine on the selenium vacancy sites leads to p-type doping. The results of our study indicate that photochemical techniques have the potential to enhance the performance of various 2D materials, making them suitable for integrated optoelectronics such as lateral nanopatterned p–n junctions.

Plasma-optimized contact for high-performance PdSe2 nanoflake-based field-effect transistors

Low-resistance contact has long been pursued in the two-dimensional (2D) electronic/optoelectronic device community. Still, an economy-efficient method highly compatible with the conventional 2D device fabrication process in laboratory remains to be explored. Herein, we report a plasma-optimized contact strategy for high-performance PdSe2 nanoflake-based field-effect transistors (FETs). Selenium vacancies created by air plasma can introduce p-type doping in the contact area, thus optimizing the device performance. The effect of plasma treatment on PdSe2 nanoflake is corroborated by high-resolution transmission electron microscopy, energy-dispersive x-ray spectroscopy spectrum, atomic force microscopy, and Kelvin probe force microscopy. The PdSe2 FET with plasma-optimized contact exhibits significantly improved field-effect carrier mobilities, current on/off ratios, and reduced contact resistance than that without plasma treatment fabricated from the same PdSe2 nanoflake. Moreover, this strategy has also been proven effective to prepare high-performance FETs based on 2D WSe2 and MoSe2 nanoflakes, further demonstrating its application prospect.

Armchair Janus WSSe Nanotube Designed with Selenium Vacancy as a Promising Photocatalyst for CO2 Reduction.

Photocatalytic conversion of carbon dioxide into chemical fuels offers a promising way to not only settle growing environmental problems but also provide a renewable energy source. In this study, through first-principles calculation, we found that the Se vacancy introduction can lead to the transition of physical-to-chemical CO2 adsorption on Janus WSSe nanotube. Se vacancies work at the adsorption site, which significantly improves the amount of transferred electrons at the interface, resulting in the enhanced electron orbital hybridization between adsorbents and substrates, and promising the high activity and selectivity for carbon dioxide reduction reaction (CO2RR). Under the condition of illumination, due to the adequate driving forces of photoexcited holes and electrons, oxygen generation reaction (OER) and CO2RR can occur spontaneously on the S and Se sides of the defective WSSe nanotube, respectively. The CO2 could be reduced into CH4, meanwhile, the O2 is produced by the water oxidation, which also provides the hydrogen and electron source for the CO2RR. Our finding reveals a candidate photocatalyst for obtaining efficient photocatalytic CO2 conversion.

Unlocking the Interfacial Adsorption‐Intercalation Pseudocapacitive Storage Limit to Enabling All‐Climate, High Energy/Power Density and Durable Zn‐Ion Batteries

AbstractSluggish storage kinetics and insufficient performance are the major challenges that restrict the transition metal dichalcogenides (TMDs) applied for zinc ion storage, especially at the extreme temperature conditions. Herein, a multiscale interface structure‐integrated modulation concept was presented, to unlock the omnidirectional storage kinetics‐enhanced porous VSe2−x⋅n H2O host. Theory research indicated that the co‐modulation of H2O intercalation and selenium vacancy enables enhancing the interfacial zinc ion capture ability and decreasing the zinc ion diffusion barrier. Moreover, an interfacial adsorption‐intercalation pseudocapacitive storage mechanism was uncovered. Such cathode displayed remarkable storage performance at the wide temperature range (−40–60 °C) in aqueous and solid electrolytes. In particular, it can retain a high specific capacity of 173 mAh g−1 after 5000 cycles at 10 A g−1, as well as a high energy density of 290 Wh kg−1 and a power density of 15.8 kW kg−1 at room temperature. Unexpectedly, a remarkably energy density of 465 Wh kg−1 and power density of 21.26 kW kg−1 at 60 °C also can be achieved, as well as 258 Wh kg−1 and 10.8 kW kg−1 at −20 °C. This work realizes a conceptual breakthrough for extending the interfacial storage limit of layered TMDs to construct all‐climate high‐performance Zn‐ion batteries.

Unlocking the Interfacial Adsorption-Intercalation Pseudocapacitive Storage Limit to Enabling All-Climate, High Energy/Power Density and Durable Zn-Ion Batteries.

Sluggish storage kinetics and insufficient performance are the major challenges that restrict the transition metal dichalcogenides (TMDs) applied for zinc ion storage, especially at the extreme temperature conditions. Herein, a multiscale interface structure-integrated modulation concept was presented, to unlock the omnidirectional storage kinetics-enhanced porous VSe2-x ⋅n H2 O host. Theory research indicated that the co-modulation of H2 O intercalation and selenium vacancy enables enhancing the interfacial zinc ion capture ability and decreasing the zinc ion diffusion barrier. Moreover, an interfacial adsorption-intercalation pseudocapacitive storage mechanism was uncovered. Such cathode displayed remarkable storage performance at the wide temperature range (-40-60 °C) in aqueous and solid electrolytes. In particular, it can retain a high specific capacity of 173 mAh g-1 after 5000 cycles at 10 A g-1 , as well as a high energy density of 290 Wh kg-1 and a power density of 15.8 kW kg-1 at room temperature. Unexpectedly, a remarkably energy density of 465 Wh kg-1 and power density of 21.26 kW kg-1 at 60 °C also can be achieved, as well as 258 Wh kg-1 and 10.8 kW kg-1 at -20 °C. This work realizes a conceptual breakthrough for extending the interfacial storage limit of layered TMDs to construct all-climate high-performance Zn-ion batteries.

Selenium vacancies regulate d-band centers in Ni3Se4 toward paired electrolysis in anion-exchange membrane electrolyzers for upgrading N-containing compounds

Selenium vacancies regulate d-band centers in Ni3Se4 toward paired electrolysis in anion-exchange membrane electrolyzers for upgrading N-containing compounds

Thermoelectric Composite of (Bi0.98In0.02)2Te2.7Se0.3/Bi2Se3 with Enhanced Thermopower and Reduced Electrical Resistivity

By using the solid-state reaction approach, composite polycrystalline samples of (Bi0.98In0.02)2Te2.7Se0.3/x%Bi2Se3 were created with varying amounts of Bi2Se3, (x = 5%, 10%, 15%, and 20%). The hexagonal crystal structure of the composite was revealed by x-ray diffraction (XRD) with a space group of Roverline{3 }m. The surface of the samples was seen to have secondary particles using a field emission scanning electronic microscope. Every sample displayed the typical semi-conducting behaviour across the entire temperature range. In the complex (Bi0.98In0.02)2Te2.7Se0.3, it was found that bismuth was coordinated with six selenium atoms and there were significant selenium vacancies. With an increase in bismuth selenide concentration, the dissolution pattern shifted to a substitutional pattern. A two fold decrease in electrical resistivity for (Bi0.98In0.02)2Te2.7Se0.3/20%Bi2Se3 composition was seen compared to (Bi0.98In0.02)2Te2.7Se0.3/5%Bi2Se3. The granular material was produced by sintering and scattering of potential barrier, a thermal process that increases the Seebeck coefficient. A 200% increase was observed in thermopower for (Bi0.98In0.02)2Te2.7Se0.3/20%Bi2Se3 compared to (Bi0.98In0.02)2Te2.7Se0.3/5%Bi2Se3 compound.Graphical

Synergized selenium-vacancy heterogeneous interface and carbon nanotubes for insight into efficient oxidation of pollutants via photocatalytic peroxymonosulfate activation

Advanced strategies for enhancing catalytic performance of catalysts include multi-metal active site exposure, vacancy growth and electronic structure optimization. Here, carbon nanotube-loaded CoSe2/FeSe2 catalysts (Co/Fe-CNT) with multiphase boundaries and embedded selenium vacancies (VSe) were prepared using a one-step hydrothermal method. The catalysts feature unique electronic structures and abundant active sites, and were found to exhibit unexpectedly high activity and stability in photocatalytic mediated sulphate radical (SR-photo) system, with impressive oxidation capacity for complex matrix polluted water. The spontaneously formed internal electric field strongly promoted charge transport and significantly enhanced the photocatalytic property. The VSe formed at the heterogeneous interface could modulate the surface electronic structure and accelerate the redox cycling of transition metals, thereby accelerating the production of reactive oxygen species and boosting the efficiency of contaminant degradation. This work provides guidance for the design of high-performance catalysts for VSe engineering and expands their potential for application in wastewater purification.

Selenium vacancies induced surface oxygen adsorption and sensitization mechanism of PbSe film: Experimental and computational

Lead selenide (PbSe) is widely considered to be an ideal material for room temperature mid-wave infrared (MWIR) detection. The MWIR response of PbSe leaps obviously after sensitization in a particular oxidizing process. However, the mechanism of such characteristics has not been specifically understood. Here, the oxygen (O) sensitization reaction process is affirmed by experiment and computation. In the process of film preparation, Se atoms fractionate from the (200) plane of PbSe lattice and leave Se vacancies (VSe). Then, O2 molecules adsorbed at VSe and subsequently disintegrate into O atoms occupying VSe. The O indrawing forms new energy levels and reduces the band gap (Eg), realizes the conductivity transformation from n- to p-type, decreases carrier concentration, and enhances carrier mobility. The variation of conductivity inhibits the dark current of PbSe detectors and increases the mobility of photon-generated carriers at the same time, resulting in the improvement of the IR detection performance.

Plasma Etching of Pyrite-Type Nickel Diselenide Nanosheets to Create Selenium Vacancies for Applications as Electrocatalysts for Hydrogen Evolution

Boosting the hydrogen evolution reaction (HER) process with highly efficient non-noble metal catalysts is crucial for hydrogen-powered fuel cells. Pyrite-type NiSe2 is one of the most promising HER catalysts due to its inexpensive cost and good conductivity, although its intrinsic activity is moderate. Using a plasma etching method on a carbon cloth substrate material, we created a large number of selenium vacancies on NiSe2 nanosheets (NiSe2-VSe/CC). With a low overpotential of 91 mV and a Tafel slope of 91 mV dec–1 at 10 mA cm–2, as well as a durability of 20 h at high current (100 mA cm–2), the NiSe2-VSe/CC demonstrates superior HER performance for its catalytic activity and long-term stability. Cross-validated by measurement analyses and density functional theory calculations, the introduction of vacancies reduces the resistivity of NiSe2 and optimizes its hydrogen adsorption energy (GH*) and water adsorption energy (GH2O*). Based on defect engineering, this study proposes an approach to designing improved electrocatalysts for alkaline HER.

The role of selenium vacancies functionalized mediator of bimetal (Co, Fe) selenide for high-energy–density lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries are currently only in the basic research stage and have not been commercialized, which is mainly affected by the poor conductivity of sulfur/lithium sulfide (S/Li2S), volume expansion effect of sulfur and the shuttle effect of lithium polysulfides (LiPSs). Herein, a three dimensional (3D) carbon nanotubes (CNTs) decorated cubic Co9Se8−x/FeSe2−y (0 ＜ x ＜ 8, 0 ＜ y ＜ 2) composite (Co9Se8−x/FeSe2−y@CNTs) is developed, and used as the functionalized mediator on polypropylene (PP) in Li-S batteries. Benefiting from the good electrical conductivity, large number of Se vacancies and the triple block/adsorption/catalytic effects of Co9Se8−x/FeSe2−y@CNTs, the cell with Co9Se8−x/FeSe2−y@CNTs//PP modified separator delivers a high reversible capacity (1103.5 mA h g−1) at 1C after three cycles activation at 0.5C and remains 446 mA g h−1 after 750 cycles with a 0.08% capacity decay rate each cycle. Moreover, at 0.2C, a high areal capacity of 3.63 mA h cm−2 after 100 cycles with a high sulfur loading of 4.1 mg cm−2 is obtained. The in-situ XRD tests revealing the transition path of α-S8 → Li2S → β-S8 during the first charge–discharge process, then β-S8 → Li2S → β-S8 conversion reaction in the next cycles, and firstly determine the sulfur-selenide active intermediates (Se1.1S6.9) during cycles. The work provides a new insight into the development of bimetallic selenide composites by defect engineering with highly adsorptive and catalytic properties for Li-S batteries.

Engineering Selenium Vacancies on CoSe2 Nanoarrays for Alkaline Hydrogen Evolution

The development of highly efficient nonnoble metals toward hydrogen evolution reaction (HER) electrocatalysts is significant for producing hydrogen via water electrolysis. Selenides are becoming increasingly attractive owing to their excellent HER activity. Herein, CoSe2 nanorods were successfully manufactured on a carbon cloth (CC) and selenium vacancies were obtained using a plasma cleaner (CoSe2-VSe/CC) to enhance their intrinsic catalytic activity. The synthetic catalyst exhibited a lower overpotential of 88 mV at 10 mA cm–2 in 1 M KOH and high durability over 100 h at 100 mA cm–2, which can be ascribed to the increased electrochemical surface area and rapid electron transfer. In addition, density functional theory (DFT) calculations showed that introducing Se vacancies optimized the hydrogen adsorption energy on the Se site and reduced the transition-state energy barrier of water dissociation. This study laid a firm foundation for preparing high-performance and stable electrocatalysts via anion defect engineering.

Catalytic Activity of Defect-Engineered Transition Me tal Dichalcogenides Mapped with Atomic-Scale Precision by Electrochemical Scanning Tunneling Microscopy.

Unraveling structure-activity relationships is a key objective of catalysis. Unfortunately, the intrinsic complexity and structural heterogeneity of materials stand in the way of this goal, mainly because the activity measurements are area-averaged and therefore contain information coming from different surface sites. This limitation can be surpassed by the analysis of the noise in the current of electrochemical scanning tunneling microscopy (EC-STM). Herein, we apply this strategy to investigate the catalytic activity toward the hydrogen evolution reaction of monolayer films of MoSe2. Thanks to atomically resolved potentiodynamic experiments, we can evaluate individually the catalytic activity of the MoSe2 basal plane, selenium vacancies, and different point defects produced by the intersections of metallic twin boundaries. The activity trend deduced by EC-STM is independently confirmed by density functional theory calculations, which also indicate that, on the metallic twin boundary crossings, the hydrogen adsorption energy is almost thermoneutral. The micro- and macroscopic measurements are combined to extract the turnover frequency of different sites, obtaining for the most active ones a value of 30 s-1 at -136 mV vs RHE.

Oriented adsorption and efficient sensing of NO2 on MoSe2 monolayer: a comparative study with WSe2 monolayer

Oriented adsorption is very rare on two-dimensional inorganic thin layers. Based on preliminary study, density functional theory (DFT) was used to explore the adsorption and sensing behavior of NO2 on MoSe2 perfect and defective monolayers including selenium vacancy (VSe), molybdenum vacancy (VMo), and antisite defect of selenium on molybdenum site (SeMo) by large-scale sampling. The adsorption energy can reach 0.29 eV with 0.182 e charge transfer on the perfect monolayer. The adsorption ability is improved largely on VSe, VMo, and SeMo monolayers (3.27, 0.93, and 1.20 eV) with considerable charge transfer (0.583, 0.152, and 0.139 e, respectively), demonstrating the potential of MoSe2 monolayers as NO2 sensing materials. Meanwhile, similar to WSe2 monolayer, oriented adsorption of NO2 is disclosed. NO2 molecule extends mainly along the Se–Mo–Se trough and the Se–Mo bond on all the four MoSe2 monolayers. Whereas, the oriented adsorption ability is weaker than the WSe2 monolayer, which can be confirmed by the relatively small proportion of oriented adsorption conformations. This kind of double oriented adsorption behavior (vertical adsorption and adsorption along specific direction of the two-dimensional material) was considered to be due to the good polarity match between molecule and monolayer. Molecular dipole moment and material bond population are believed to be two important parameters that determine the oriented adsorption capacity of molecular-material systems. The successive discovery of the unique oriented adsorption and the deepening of understanding provide chance for molecular design of oriented adsorption of inorganic small molecules on two-dimensional layered materials. The strong dissociation ability of the anionic vacancy monolayer to NO2 molecule indicates that MoSe2 monolayer has a very high adsorption, capture, and activation potential for nitrogen oxides. Comparative study proved that the Mo site activity was less than the W site activity on the perfect monolayer, and defects could help the Mo site activity overtake. These findings provide pivotal guidance for creating advanced surface regulated electronic systems, such as sensors.

Effect of Te doping on oxidation resistance and electronic structure of two-dimensional InSe

InSe is a typical two-dimensional (2D) layered semiconductor material, which has excellent electrical properties and moderate adjustable band gap. It is found that InSe has an attractive application prospect in optoelectronic devices. However, some studies have shown that InSe in a single selenium vacancy (Vse) system is easily degraded when exposed to the environment of O<sub>2</sub> molecule, which seriously affects the application of InSe in the field of electronic devices. In order to improve the environmental stability of the material, the substitution doping method of Te is proposed in this work. Density functional theory (DFT) is used to analyze the electronic structure, adsorption energy, Bader charge and energy reaction paths of the different systems. It is found that Te substitution doping can significantly improve the stability of InSe. At the same time, the defect state produced by Vse can be eliminated. The specific research results are as follows. First, the dissociation barrier of O<sub>2</sub> molecule on Te doped InSe surface (InSe—Te) is as high as 2.67 eV, indicating that Te-doped InSe has a strong antioxidant capacity. Second, the distance between O<sub>2</sub> molecule and the surface of InSe—Te is 3.87 Å, and the adsorption energy is only –0.03 eV, indicating that O<sub>2</sub> molecules are physically adsorbed on the monolayer surface. Third, Te doping not only improves the antioxidant capacity of the InSe, but also eliminates the defect state produced by Vse. Fourth, the Te-doping obviously eliminates the original Vse defect state or impurity band. The density of states and band structure of Te-doped InSe are almost the same as those of perfect InSe, which can maintain the stability of InSe structure and effectively reduce the damage of oxidation environment to defective InSe monolayer. The results of this study will be helpful in improving the environmental stability of InSe 2D material devices and promoting the research and development of InSe 2D devices.

Deep defects limiting the conversion efficiency of Sb2Se3 thin-film solar cells.

Quasi-one-dimensional (Q1D) semiconductor antimony selenide (Sb2Se3) shows great potential in the photovoltaic field, but the photoelectric conversion efficiency (PCE) of Sb2Se3-based solar cells has shown no obvious breakthrough during the past several years, of which the intrinsic reasons are pending experimentally. Here, we prepare high-quality Q1D Sb2Se3 thin films via the vapor transport deposition technique. By investigating the bandedge electronic level structure and carrier relaxation/recombination dynamics, we find that (i) the optimized Se-rich growth conditions can highly improve the crystal quality of the Q1D Sb2Se3 thin films, the carrier lifetime of which is substantially increased up to ∼8.3 μs; (ii) the Se-rich growth conditions have advantages to annihilate the deep selenium vacancies VSei (i = 1 and 3 for non-equivalent Se atomic sites) but is not effective for the deep donor VSe2, which locates at ∼0.3 eV (300 K) below the conduction band and intrinsically limits the PCE value of devices below ∼7.63%. This work suggests that further optimizing the Se-rich conditions to technically eliminate this kind of deep defect is still essential for preparing high-performance Sb2Se3 film solar cells.

Thermally Driven Point Defect Transformation in Antimony Selenosulfide Photovoltaic Materials.

Thermal treatment of inorganic thin films is a general and necessary step to facilitate crystallization and, in particular, to regulate the formation of point defects. Understanding the dependence of the defect formation mechanism on the annealing process is a critical challenge in terms of designing material synthesis approaches for obtaining desired optoelectronic properties. Herein, a mechanistic understanding of the evolution of defects in emerging Sb2 (S,Se)3 solar cell films is presented. A top-efficiency Sb2 (S,Se)3 solar-cell film is adopted in this study to consolidate this investigation. This study reveals that, under hydrothermal conditions, the as-deposited Sb2 (S,Se)3 film generates defects with a high formation energy, demonstrating kinetically favorable defect formation characteristics. Annealing at elevated temperatures leads to a two-step defect transformation process: 1) formation of sulfur and selenium vacancy defects, followed by 2) migration of antimony ions to fill the vacancy defects. This process finally results in the generation of cation-anion antisite defects, which exhibit low formation energy, suggesting a thermodynamically favorable defect formation feature. This study establishes a new strategy for the fundamental investigation of the evolution of deep-level defects in metal chalcogenide films and provides guidance for designing material synthesis strategies in terms of defect control.

Charge-Polarized Selenium Vacancy in Nickel Diselenide Enabling Efficient and Stable Electrocatalytic Conversion of Oxygen to Hydrogen Peroxide.

Vacancy engineering is deemed as one of the powerful protocols to tune the catalytic activity of electrocatalysts. Herein, Se-vacancy with charge polarization is created in the NiSe2 structure (NiSe2 -VSe ) via a sequential phase conversion strategy. By a combined analysis of the Rietveld method, transient photovoltage spectra (TPV), in situ Raman and density functional theory (DFT) calculation, it is unequivocally discovered that the presence of charge-polarized Se-vacancy is beneficial for stabilizing the structure, decreasing the electron transfer kinetics, as well as optimizing the free adsorption energy of reaction intermediate during two-electron oxygen reduction reaction (2e- ORR). Benefiting from these merits, the as-prepared NiSe2 -VSe delivered the highest selectivity of 96% toward H2 O2 in alkaline media, together with a selectivity higher than 90% over the wide potential range from 0.25 to 0.55V, ranking it in the top level among the previously reported transition metal-based electrocatalysts. Most notably, it also displayed admirable stability with only a slight selectivity decay after 5000 cycles of accelerated degradation test (ADT).

MoSe2 and NiCo2O4/NiO Based HybridNanostructure as Novel Electrocatalyst for High Performance Rechargeable Zinc-Air Battery

Efficient air electrocatalysts are needed to develop eco-friendly rechargeable Zinc-air (Zn-air) batteries for renewable energy storage. Here, we investigate the electrocatalytic activity of hydrothermally synthesized MoSe2, NiCo2O4/NiO and NiCo2O4/NiO-MoSe2 hybrid nanostructure by performing oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) studies. The NiCo2O4/NiO-MoSe2 hybrid nanostructure (Tafel slope∼ 54 mV decade−1) shows best electrocatalytic activity and stability for OER study among synthesized electrocatalysts. Further, the oxygen reduction behaviors of prepared catalysts have been observed in oxygen saturated electrolyte solution. The excellent electrocatalytic activity of hybrid nanostructure can be associated with its high electrochemical active surface area, number of active sites and possibly due to the presence of selenium vacancies and edges in MoSe2. Furthermore, we demonstrate Zn-air batteries with these electrocatalysts as air cathode, showing open circuit potential of 1.42 V, the specific capacity of 1023 mA h gZn−1 and energy density of 1195 W h kgZn−1, with high durability for hybrid nanostructure-based Zn-air battery. To the best of our knowledge, this is the first study demonstrating pristine MoSe2 and NiCo2O4/NiO-MoSe2 hybrid nanostructure-based cathodes for Zn-air batteries with excellent specific capacity and energy density.