Se Vacancies Research Articles

Unlocking hidden spins in centrosymmetric 1T transition metal dichalcogenides by vacancy-controlled spin-orbit scattering

Spin current generation and manipulation remain the key challenge of spintronics, in which relativistic spin-orbit coupling (SOC) plays a ubiquitous role. In this paper, we demonstrate that hidden Rashba spins in the nonmagnetic, centrosymmetric lattice of multilayer ${\mathrm{SnSe}}_{2}$ can be efficiently activated by spin-orbit scattering introduced by Se vacancies. Via vacancy scattering, conduction electrons with hidden spin-momentum locked polarizations acquire out-of-plane magnetization components, which effectively break the chiral symmetry between the two Se sublattices of an ${\mathrm{SnSe}}_{2}$ monolayer when electron spins start precession in the strong built-in Rashba SOC field. The resulting spin separations are manifested in quantum transport as vacancy concentration- and temperature-dependent crossovers from weak antilocalization to weak localization, with the distinctive spin relaxation mechanism of the D'yakonov-Perel' type. In a nonlocal geometry, the generated spins exhibit a long diffusion length exceeding 5 $\textmu{}\mathrm{m}$, when fast momentum scattering protects the effective spin polarizations by driving the random-walk evolution of spin precessions subject to rapidly changing Rashba SOC field. Our study shows the great potential of two-dimensional systems with hidden-spin textures for spintronics.

Calculation of defect formation energy of point defects in CdGa2Se4

The presented main studies of point defects in CdGa2Se4 chalcopyrite are considered to quantify the formation energies of three possible models of Cd, Ga and Se vacancy defects using various local density approximation (LDA) and general gradient approximation (GGA) approximations. Using supercells with 56 atoms, interatomic distances were calculated both for an ideal cell and a supercell containing vacancies. It was found that interatomic distance does not change only in a supercell containing Ga vacancy. Investigation shows that the Ga vacancy has a high defect formation energy (DFE) among other intrinsic vacancies and is therefore unstable. We found that for all vacancies, [Formula: see text] charge states are favorable.

Facile microwave plasma driven 3D-WSe2 2H-1T phase modulation for improving NO2 gas sensing performance

In recent years, transition metal dichalcogenides (TMDs) have been widely used for gas sensors. Here, three-dimensional (3D) WSe2 nanosheet arrays were surface treated by microwave plasma. Based on the original 3D structure, a 1T/2H hybrid phase structure was constructed by phase modulation, and Se vacancies were introduced to effectively improve its gas sensing performance. After only 60 s of treatment, the response (52.24 %), response/recovery time of the sample for 1 ppm NO2 were significantly improved with excellent stability and selectivity at room temperature. The intrinsic mechanism of its performance enhancement was elicited through various characterizations and molecular model construction. It is demonstrated that microwave plasma is a promising treatment method to improve the gas-sensitive performance of TMDs.

Magnetic and transport properties of two-dimensional ferromagnet VSe2 with Se vacancies

As a room-temperature van der Waals intrinsic ferromagnetic semiconductor, monolayer VSe2 was studied extensively for both fundamental research and potential low-dimensional semiconductor spintronic applications. Defects are inevitably encountered and can affect the electronic, magnetic, and transport properties during the experimental synthesis of VSe2 monolayer. However, the study of defects in VSe2 monolayer is still lacking at the atomic level. Here, the electronic, magnetic, and transport properties of VSe2 monolayer with Se vacancies are investigated by first-principles density-functional method. With the direction of easy magnetization axes unchanged, introducing Se vacancies results in the decrease of magnetic anisotropy energy and Curie temperature, which can be further increased by applying in-plane biaxial strain. After Se vacancies are introduced, the conductivity of the system decreases, while the larger doping range is obtained for pure spin-polarized current. The transition from semiconductor to half-metal with 100% spin polarization can be realized by introducing Se bivacancy in VSe2 monolayer, which is highly promising for next-generation spin filter, spin injection and spin transport nanodevices.PACS: 61.72.jd; 73.20.At; 75.50.Pp; 75.70.Ak

Improved Carrier Lifetimes of CdSe Thin Film via Te Doping for Photovoltaic Application.

Cadmium selenide (CdSe) solar cells have proven to be a remarkable potential top cell for a silicon-based tandem application. However, the defects and short carrier lifetimes of CdSe thin films greatly limit the solar cell performance. In this work, a Te-doped strategy is proposed to passivate the Se vacancy defects and increase the carrier lifetime of the CdSe thin film. The theoretical calculation helps to reveal the mechanism of nonradiative recombination of the CdSe thin film in depth. After Te-doping, the calculated capture coefficient of CdSe can be reduced from 4.61 × 10-8 cm3 s-1 to 2.32 × 10-9 cm3 s-1. Meanwhile, the carrier lifetime of CdSe thin film is increased nearly 3-fold from 0.53 to 1.43 ns. Finally, the efficiency of the Cd(Se,Te) solar cell is improved to 4.11%, about a relative 36.5% improvement compared to the pure CdSe solar cell. Both theoretical calculations and experiments prove that Te can effectively passivate bulk defects and improve the carrier lifetime of CdSe thin films, deserving further exploration to improve solar cell performance.

Single Selenium Atomic Vacancy Enabled Efficient Visible-Light-Response Photocatalytic NO Reduction to NH3 on Janus WSSe Monolayer

The NO reduction reaction (NORR) toward NH3 is simultaneously emerging for both detrimental NO elimination and valuable NH3 synthesis. An efficient NORR generally requires a high degree of activation of the NO gas molecule from the catalyst, which calls for a powerful chemisorption. In this work, by means of first-principles calculations, we discovered that the NO gas molecule over the Janus WSSe monolayer might undergo a physical-to-chemical adsorption transition when Se vacancy is introduced. If the Se vacancy is able to work as the optimum adsorption site, then the interface's transferred electron amounts are considerably increased, resulting in a clear electronic orbital hybridization between the adsorbate and substrate, promising excellent activity and selectivity for NORR. Additionally, the NN bond coupling and *N diffusion of NO molecules can be effectively suppressed by the confined space of Se vacancy defects, which enables the active site to have the superior NORR selectivity in the NH3 synthesis. Moreover, the photocatalytic NO-to-NH3 reaction is able to occur spontaneously under the potentials solely supplied by the photo-generated electrons. Our findings uncover a promising approach to derive high-efficiency photocatalysts for NO-to-NH3 conversion.

First-principles study of the native defects with charge states in ZrSe[formula omitted

Study of the native defects in transition metal dichalcogenides (TMDCs) is of fundamental interest and potential technological importance. The atomic and electronic structures of native defects with charge states in ZrSe2 are systematically investigated for the first time based on first-principles calculations with the optB86R-vdW and HSE06 functionals. We identify the configurations of relatively stable defects including Zr interstitial (Zrint), Se vacancy (VSe) and Zr antisite (ZrSe). The positive charged Zr interstitial defect (Zrint2+) has the lowest formation energy and thus is most commonly seen in ZrSe2, which is in good agreement with experimental observation. The ZrSe defect is found to form a DX-like configuration. Moreover, the non-interacting Zr Frenkel pairs are the most favorable form of intrinsic defects. The electronic structure analyses of these defects reveal that the extra Zr atoms adjacent to the defects act as donors, which are responsible for the native n-type conductivity of as-grown ZrSe2. Our study can provide an insight into understanding the properties of native defects in ZrSe2.

Extended phase homogeneity and improved out-of-plane charge transfer in Sb and Te co-alloyed n-type BiSe layered compound with extraordinary thermoelectric performance

BiSe is a promising n-type layered thermoelectric compound near room temperature. However, its full potential is not realized because of its overall high electron density and inferior interlayer charge transport. In this study, we demonstrate that these drawbacks are well addressed by a feasible Sb and Te co-alloying approach. Sb, preferably substituting Bi atoms in the Bi2Se3 quintuplet layer, effectively neutralizing excess electrons of BiSe by suppressing positively charged Se vacancies. On the other hand, Te exclusively replaces Se, and promotes interlayer charge transfer by increasing chemical bond covalency. In addition, Te alloying significantly impedes phonon propagation of BiSe by introducing considerable size and mass disorders. More importantly, it is found that co-alloying expands the phase homogeneity of BiSe, which further intensifies the respective functions of Sb and Te alloying. As a consequence, a peak ZT value of 0.6 at 425 K is realized in the sample Bi0.6Sb0.4Se0.6Te0.4, which is 2.5 times that of pristine BiSe. Our work suggests that Sb and Te co-alloyed BiSe is a robust candidate for thermoelectric application near room temperature.

Topological insulator Bi2Se3 for highly sensitive, selective and anti-humidity gas sensors

Chemiresistive gas sensors generally surfer from low selectivity, inferior anti-humidity, low response signal or signal-to-noise ratio, severely limiting the precise detection of chemical agents. Herein, we exploit high-performance gas sensors based on topological insulator Bi2Se3 that is distinguished from conventional materials by robust metallic surface states protected by time-reversal symmetry. In the presence of Se vacancies, Bi2Se3 nanosheets exhibit excellent gas sensing capability toward NO2, with a high response of 93% for 50ppm and an ultralow theoretical limit of detection concentration about 0.06ppb at room temperature. Remarkably, Bi2Se3 demonstrates ultrahigh anti-humidity interference characteristics, as the response with standard deviation of only 3.63% can be achieved in relative humidity range of 0-80%. These findings are supported by first-principles calculations, with analyses on adsorption energy and charge transfer directly revealing the anti-humidity and selectivity. This work may pave the way for implementation of exotic quantum states for intelligent applications.

Computational Analysis of Metal Contact on Bi2O2Se with Se Surface Vacancies

AbstractIn this work, a theoretical study on the electronic properties of the metal/Bi2O2Se interface is presented through the density functional theory calculation. Particularly, the effects of Cr, Pd, Pt, Au, and Bi on monolayer, bilayer, and trilayer Bi2O2Se are explored. Naturally created Se vacancies on the Bi2O2Se surface are also considered by constructing two interface structures: the metal/Bi2O2Se with the Se vacancies remaining or filled with metals. For the metal/monolayer Bi2O2Se, it is observed that the Bi2O2Se layer is fully metallized since Bi2O2Se strongly interacts with the metal. Meanwhile, for the metal/bilayer and trilayer Bi2O2Se, the Bi2O2Se layers remain semiconducting except for the layer right next to metal. Among the considered metals, regardless of whether the Se vacancies are filled with metal or not, the semiconducting layers in bilayer and trilayer Bi2O2Se form Ohmic contact with Bi. It is also found that filling the Se vacancies with the metals heavier than Se increases the interface distance between metal and Bi2O2Se, and hence results in the weak Fermi level pinning effect.

Efficient Electrocatalytic Reduction of CO2 to Ethanol Enhanced by Spacing Effect of CuCu in Cu2‐xSe Nanosheets

AbstractIt is highly desired yet challenging to strategically steer carbon dioxide (CO2) electroreduction reaction (CO2ER) toward ethanol (EtOH) with high activity, which provides a promising way for intermittent renewable energy reservation. Controlling spatial distance between the adjoining active centers and promoting the CC coupling progress are crucial to realize this purpose. Herein, ultrathin 2D Cu2‐xSe is prepared with abundant Se vacancies, where the spatial distance between the CuCu around the Se vacancies is effectively shortened because of the lattice stress. Besides, the moderate spatial distance induced by Se vacancies can significantly decrease the Gibbs free energy of asymmetric *CO*CHO coupling progress, effectively change the local charge distribution, decrease the valence state of Cu atoms and increase the electron‐donating capacity of the dual active sites. Combining experimental observations and density functional theory simulations, the CuCu dual sites with spatial distance of 2.51 Å in VSe‐Cu2‐xSe sample can catalyze CO2ER to EtOH with high selectivity in a potential range from −0.4 to −1.6 V, and reach the highest faradaic efficiency of 68.1% at −0.8 V. This work reveals the influence of spacing effect on ethanol selectivity, and provides a new idea for future design of catalysts with chain elongation reaction, which can bring extensive attention.

Comprehensively Understanding the Role of Anion Vacancies on K-Ion Storage: A Case Study of Se-Vacancy-Engineered VSe2.

Anion vacancy engineering (AVE) is widely used to improve the Li-ion and Na-ion storage of conversion-type anode materials. However, AVE is still an emerging strategy in K-ion batteries, which are promising for large-scale energy storage. In addition, the role of anion vacancies on ion storage is far from clear, despite several proposed explanations. Herein, by employing VSe2 as a model conversion-type anode material, Se vacancies are intentionally introduced (labeled as P-VSe2-x ) to investigate their effect on K+ storage. The P-VSe2-x shows excellent cyclability in half cells (143mAhg-1 at 3.0Ag-1 after 1000 cycles) and high energy density in coin-type full cells (206.8Whkg-1 ). By applying various electrochemical techniques, the effects of Se vacancies on the redox potentials of K-ion insertion/extraction and the K-ion diffusion in electrodes upon cycling are uncovered. In addition, the structural evolution of Se vacancies during potassiation/de-potassiation using various operando and ex characterizations is revealed. Moreover, it is demonstrated that Se vacancies can facilitate the breaking of VSe bonds upon the P-VSe2-x conversion using theoretical calculations. This work comprehensively explains the role of anion vacancies in ion storage for developing high-performance conversion-type anode materials.

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.

Defect-Engineering of 2D Dichalcogenide VSe2 to Enhance Ammonia Sensing: Acumens from DFT Calculations

Opportune sensing of ammonia (NH3) gas is industrially important for avoiding hazards. With the advent of nanostructured 2D materials, it is felt vital to miniaturize the detector architecture so as to attain more and more efficacy with simultaneous cost reduction. Adaptation of layered transition metal dichalcogenide as the host may be a potential answer to such challenges. The current study presents a theoretical in-depth analysis regarding improvement in efficient detection of NH3 using layered vanadium di-selenide (VSe2) with the introduction of point defects. The poor affinity between VSe2 and NH3 forbids the use of the former in the nano-sensing device's fabrications. The adsorption and electronic properties of VSe2 nanomaterials can be tuned with defect induction, which would modulate the sensing properties. The introduction of Se vacancy to pristine VSe2 was found to cause about an eight-fold increase (from -012 eV to -0.97 eV) in adsorption energy. A charge transfer from the N 2p orbital of NH3 to the V 3d orbital of VSe2 has been observed to cause appreciable NH3 detection by VSe2. In addition to that, the stability of the best-defected system has been confirmed through molecular dynamics simulation, and the possibility of repeated usability has been analyzed for calculating recovery time. Our theoretical results clearly indicate that Se-vacant layered VSe2 can be an efficient NH3 sensor if practically produced in the future. The presented results will thus potentially be useful for experimentalists in designing and developing VSe2-based NH3 sensors.

NO2 Physical-to-Chemical Adsorption Transition on Janus WSSe Monolayers Realized by Defect Introduction.

As is well known, NO2 adsorption plays an important role in gas sensing and treatment because it expands the residence time of compounds to be treated in plasma-catalyst combination. In this work, the adsorption behaviors and mechanism of NO2 over pristine and Se-vacancy defect-engineered WSSe monolayers have been systematically investigated using density functional theory (DFT). The adsorption energy calculation reveals that introducing Se vacancy acould result in a physical-to-chemical adsorption transition for the system. The Se vacancy, the most possible point defect, could work as the optimum adsorption site, and it dramatically raises the transferred-electron quantities at the interface, creating an obviously electronic orbital hybridization between the adsorbate and substrate and greatly improving the chemical activity and sensing sensitivity of the WSSe monolayer. The physical-to-chemical adsorption transition could meet different acquirements of gas collection and gas treatment. Our work broadens the application filed of the Janus WSSe as NO2-gas-sensitive materials. In addition, it is found that both keeping the S-rich synthetic environments and applying compression strain could make the introduction of Se vacancy easier, which provides a promising path for industrial synthesis of Janus WSSe monolayer with Se vacancy.

Evidence for intrinsic defects and nanopores as hotspots in 2D PdSe2 dendrites for plasmon-free SERS substrate with a high enhancement factor

Surface-enhanced Raman spectroscopy (SERS), a very powerful tool for the identification of molecular species, has relied mostly on noble metal-based substrates to obtain a high enhancement factor. In this work, we demonstrate that self-driven intrinsic defects in 2D palladium di-selenide (PdSe2) dendrites grown at low temperature (280 °C) act as hotspots for high SERS enhancement. We grow 2D dendritic PdSe2 with ample intrinsic defects to exploit it for SERS application. X-ray electron spectroscopy (XPS) analysis reveals 9.3% outer layer and 4.7% interior Se vacancies. A detailed examination of atomic-scale defects revealed Se vacancy (VSe) coupled with Se–Pd–Se vacancy (VSe-Pd-Se) in monolayer PdSe2, and an array of line defects (Se vacancies) and nanopores in bilayer PdSe2 dendrites. Interestingly, our studies reveal that Se vacancy-rich PdSe2 gives rise to line defects that act like hotspots for SERS enhancement. Remarkably, the vacancy-rich dendritic PdSe2 shows a SERS enhancement factor >105 and can detect RhB at a concentration down to 10−8 M. We speculate that the topological line defects and the edge construction in PdSe2 dendrites act as metallic wire or edge, which is partly responsible for the high enhancement in the SERS signal. The high SERS sensitivity is explained on the basis of multiple charge transfer processes combined with the predicted metal-like behavior of the defected 2D PdSe2. Our conclusions are fully supported by the density functional theory calculation of the electronic density of states of the defective bilayer (2L) PdSe2, which remarkably exhibits metallic character. Being a defect-enabled SERS substrate, dendritic 2D PdSe2 fills the gap between conventional plasmonic SERS substrate and plasmon-free SERS substrate.

Synergetic optimization of thermoelectric properties in SnSe film via manipulating Se vacancies

Thin film thermoelectric materials have attracted much attention due to the application prospect in nanoscale devices. However, the commonly used optimization strategy in bulk materials, such as defect engineering, is hard to be realized in films. In this work, we synthesized SnSe films by the magnetron sputtering method and manipulated the Se vacancy concentration simply by controlling the sputtering time. The carrier transport properties were systematically investigated by combining theoretical simulation and experimental measurement. In the 130 nm-thick ultrathin film, Se vacancies were formed due to the difference in atomic mass between Sn and Se. The electrical conductivity and carrier mobility are improved by the weak electron-phonon coupling strength and small indirect band gap. Meanwhile, the thermal conductivity of the vacancy structure has an average reduction of 33.8% compared with the pristine SnSe films due to the strong vacancy scattering. The Seebeck coefficient maintains above 300 μV K-1 due to the relatively large direct band gap. A high ZT of 0.6 was finally achieved at 700 K in the film sample with Sn:Se ratio of 1.08, 50% improvement over the pristine SnSe film. However, a transition to the metallic characteristic occurs when Se vacancy concentration further increases, resulting in deterioration of ZT at high temperature. The present work demonstrates a guiding principle of manipulating vacancy concentration for high thermoelectric performance and reveals how vacancies influence energy transfer.

Se vacancy modulation of centimeter-scale 2D MoSe2 continuous films via Se evaporating temperature

Vacancy defects, often generated during the in situ CVD synthesis, may lead to the deterioration of the physical and chemical properties of 2D atomically thin layers. Due to the weak chemical reactivity of Se, it is still a great challenge to achieve high-quality selenization of large-area 2D transition metal selenides without Se vacancies. In our work, Se evaporating temperature acts as a facile method for the Se-vacancy modulation of centimeter-scale 2D MoSe2 continuous films. It affects the supply of Se source in vapor phase and hence determines the selenization reaction of 2D MoSe2. Not only the Se-vacancy concentration but also layer-thickness distribution could be effectively controlled by changing Se evaporating temperature. The Se-vacancy concentration of the as-selenided MoSe2 continuous films approximately reduces from 5% at 300 ℃ to 0% at 500 ℃. Correspondingly, their layer thicknesses gradually decrease and tend to become an atomically thin layer with centimeter-scale surface coverage. It demonstrates that the sufficient Se supply is crucial for the complete selenization of 2D MoSe2 via elevating Se evaporating temperature. In addition, 2D MoSe2 continuous films with less Se vacancies exhibit better crystalline quality and higher layer-thickness uniformity. Therefore, Se evaporating temperature provides a feasible strategy for optimizing the selenization reaction and modulating the Se-vacancy concentration of 2D transition metal selenides.

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.