Native Vacancies Research Articles

Realizing high thermoelectric performance and thermal stability in CuInTe2 through heavy dose Mg doping

Cu based ternary compounds have received intensive attentions as thermoelectric materials but their carrier mobility and thermal stability are subject to native Cu vacancy. In this work, the synergistic improvement in thermoelectric performance and stability in CuInTe2 is presented, facilitated by local chemical bond enhancement. Heavy dose Mg doping on In site can successfully suppress the formation of Cu vacancy in CuInTe2 and its thermal stability is significantly improved. As a result, In comparison to alternative dopants, Mg doping demonstrates a notable capacity for reinforcing the lattice structure through the inhibition of copper vacancies, thereby yielding a considerable enhancement in mobility by 20 %∼50 %, and the zT value of CuIn0.94Mg0.06Te2 exceeds 1.2 at 873 K. By further alloying with Ga on In site, the thermal conductivity is greatly reduced and more surprisingly the thermal stability is continuously enhanced. Notably, in Cu(In0.4Ga0.6)0.94Mg0.06Te2, a peak zT value of 1.72 at 973 K is achieved, while in Cu(In0.6Ga0.4)0.94Mg0.06Te2, an average zT value of 0.82 is attained. This study offers valuable insights into optimizing the thermoelectric performance and thermal stability of Cu-based ternary compounds through effective doping and defect regulation, providing guidance for future research in this field.

Symmetric bipolar resistive switching in copper oxide nanostructure/ITO lateral device under exposure to atmospheric oxygen and application in artificial synaptic devices

Capacitively coupled resistive switching behaviour was observed in a planar-geometry two-terminal device based on nanostructures of copper oxide as active material and indium tin oxide (ITO) as electrodes. When tested in ambient atmosphere, current-voltage I−V characteristic was found to be symmetric with pronounced hysteresis loop signifying two different states of resistance, namely the low resistive state (LRS) and high resistive state (HRS). Capacitive effect was apparent in the low voltage regime due to the presence of offset voltage at zero current and offset current at zero bias. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of native oxygen vacancies in copper oxide. Additionally, exposure to atmospheric oxygen played the dominant role in creation of excess vacancy sites by electrochemical reaction near positive electrode. Creation of excess density of oxygen vacancy near the electrodes in conjunction with trapping and detrapping of electrons from these defect sites is perceived as the prime mechanism for the observed switching behaviour. The device ON/OFF ratio, ION/IOFF∼1.5 was found to be stable over at least 100 continuous cycles. When tested under vacuum, the device I−V characteristics was linear in shape without any hysteresis signifying ohmic transport. Analysis of ultraviolet photoelectron spectroscopy (UPS) showed the Fermi energy level of copper oxide is at ∼ 4.69 eV, which is in a close match with the work function of ITO-electrode. The essential synaptic characteristics such as learning and forgetting curves and paired pulse facilitation (PPF) are established. The nonlinearity factors (NLF) are very low indicating the copper oxide-based devices are promising candidates for neuromorphic applications.

Native Pb vacancy defects induced p-type characteristic in epitaxial monolayer PbSe

PbSe, a predicted two-dimensional (2D) topological crystalline insulator (TCI) in the monolayer limit, possess excellent thermoelectric and infrared optical properties. Native defects in PbSe take a crucial role for the applications. However, little attention has been paid to the defect induced doping characteristics. Here, we provide an experimental and theoretical investigation of defect induced p-type characteristic on epitaxial monolayer PbSe on Au(111). Scanning tunneling microscopy (STM) measurements demonstrate an epitaxial PbSe monolayer with a fourfold symmetric lattice. Combined scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations reveal a quasi-particle bandgap of 0.8 eV of PbSe. STM results unveil that there are two types of defects on the surface, one is related the vacancies of Pb atoms and the other is the replacement of the absent Se atoms by Pb. Corresponding theoretical optimization confirms the structures of the defects. More importantly, both STS measurements and DFT calculations give evidence that the Pb vacancies move the Fermi energy inside the valence band and produce extra holes, leading to p-type characteristics of PbSe. Our work provides effective information for the future research of device performance based on PbSe films.

Electronically defective tellurium-doped TiO2 catalysts for enhanced photoelectrochemical water splitting

Density functional theory (DFT) was employed to explore the effect of Tellurium substitutional (TeTi) doping in titanium dioxide (TiO2). The incorporation of Te was found to be favorable under O-rich conditions. Te doping resulted in band gap narrowing with the formation of mid-gap states, maintaining high dielectric constant, and reducing the birefringence of TiO2 in the UV region. The combination of TeTi with native oxygen vacancies (Ov) and hydrogen interstitials (Hi) was also studied. Band-gap narrowing was found to be prevalent in all structures, revealing their potential as good catalysts for photoelectrochemical water-splitting. The finite difference time domain (FDTD) analysis of 150-nm particle showed increased absorption cross-section over a wide band (up to ∼700 nm), better than that of Silicon at most wavelengths, and surpassing the performance of Hi without TeTi. Finally, the catalytic activity of the materials was investigated by studying the water-splitting intermediate reactions. In case of oxygen evolution reaction (OER), the TeTi + Ov material showed the best catalytic activity followed by the TeTi material. Whereas for the hydrogen evolution reaction (HER), the TeTi + Hi material was the best performing followed by the TeTi material, where the ΔGH was found to be −0.04 eV, close to the ideal 0 value and also has the closest d band center position to fermi level.

Pressure Effect on Diffusion of Native Defects and Mg Impurity in Mg‐Ion‐Implanted GaN during Ultrahigh‐Pressure Annealing

Herein, transmission electron microscopy direct observations are used to examine the time evolution of dislocation loops in Mg‐ion‐implanted GaN during annealing in N2 atmospheres of 2.0 and 0.3 GPa. It is indicated in the results that the diffusion of the native defects, for both interstitials and vacancies, is retarded during annealing at the higher pressure. Furthermore, secondary ion mass spectrometry shows that annealing at the higher pressure retards the migration of Mg and increases Mg‐acceptor concentration in the ion‐implanted region. These findings provide a design principle of the postimplantation annealing process to activate ion‐implanted Mg in GaN.

Highly Reversible Local Structural Transformation Enabled by Native Vacancies in O2-Type Li-Rich Layered Oxides with Anion Redox Activity.

A novel O2-phase Li1.033Ni0.2[□0.1Mn0.5]O2 cathode with native vacancies (denoted as "□") was delicately designed. By a combination of noninvasive 7Li pj-MATPASS NMR and electron paramagnetic resonance measurements, it is unequivocally shown that the reservation of native vacancies enables the fully reversible local structural transformation without the formation of Li in the Li layer (Litet) in Li1.033Ni0.2[□0.1Mn0.5]O2 during the initial and subsequent cycling. In addition, the pernicious in-plane Mn migration that would result in the generation of trapped molecular O2 is effectively mitigated in Li1.033Ni0.2[□0.1Mn0.5]O2. As a result, the cycle stability of Li1.033Ni0.2[□0.1Mn0.5]O2 is significantly enhanced compared to that of the vacancy-free Li1.033Ni0.2Mn0.6O2, showing an extraordinary capacity retention of 102.31% after 50 cycles at a rate of 0.1C (1C = 100 mA g-1). This study defines an efficacious strategy for upgrading the structural stability of O2-type Li-rich layered oxide cathodes with reversible high-voltage anion redox activity.

Effect of Mn+2 Doping and Vacancy on the Ferromagnetic Cubic 3C-SiC Structure Using First Principles Calculations

Wide bandgap semiconductors doped with transition metals are attracting significant attention in the fabrication of dilute magnetic semiconductor devices (DMSs). The working principle of DMSs is based on the manipulation of the electron spin, which is useful for magnetic memory devices and spintronic applications. Using the density functional theory (DFT) calculation with the GGA+U approximation, we investigated the effect of native defects on the magnetic and electronic structure of Mn+2-doped 3C-SiC structure. Three structures were selected with variations in the distance between two impurities of (Mn+2)-doped 3C-SiC, which are 4.364 Å, 5.345Å, and 6.171 Å, respectively. We found ferromagnetic coupling for single and double Mn+2 dopant atoms in the 3C-SiC structure with magnetic moments of 3 μB and 6 μB respectively. This is due to the double exchange because of p-d orbital hybridization. The p-orbitals of C atoms play important roles in the stability of the ferromagnetic configuration. The impact of Si-vacancy (nearby, far) and C-vacancy (near) of (Mn+2)-doped 3C-SiC plays an important role in the stabilization of AFM due to super-exchange coupling, while the C-vacancy (far) model is stable in FM. All electronic structures of Mn+2-doped 3C-SiC reveal a half-metallic behavior, except for the Si-vacancy and C-vacancy of (nearby), which shows a semiconductor with bandgap of 0.317 and 0.828 eV, respectively. The Curie temperature of (Mn+2)-doped 3C-SiC are all above room temperature. The study shows that native vacancies play a role in tuning the structure from (FM) to (AFM), and this finding is consistent with experiments reported in the literature.

Effect of native carbon vacancies on evolution of defects in ZrC1- under He ion irradiation and annealing

The dynamic study of radiation-induced defects with annealing is critical for the material design for next-generation nuclear energy systems. The native vacancy could affect the development of defects, which lacks study. In the present work, the as-hot pressed ZrC1-x (x=0, 0.15, 0.3) ceramics which comprised crystallites of a few microns in size with different amounts of carbon vacancies were irradiated by 540 keV He2+ ions at room temperature with a fluence of 1 × 1017/cm2. The radiation-induced lattice expansion was found to be a common phenomenon in a sequence of ZrC0.85≥ZrC1.0>ZrC0.7. Both X-ray and electron diffractions confirmed maintenance of structural integrity without amorphization after irradiation. Inside the irradiated region, only “black-dot” type defects, i.e., clusters of point defects were observed while no helium-induced cavities, cracks, or extended dislocations were detected. The as-irradiated ZrC1-x were then annealed at different high temperatures. Upon annealing at 800 °C, very tiny helium-induced cavities were found to be generated and the crystal lattice recovered to a great extent, especially for the sub-stoichiometric samples. While annealed at 1500 °C, all the samples almost fully recovered the crystal lattices close to those of as-hot pressed ones. Meanwhile, large cavities and extended dislocations were generated. With increasing amount of native carbon vacancies, the size of cavities increased while the length and density of extended dislocations decreased. Inverse changes of lattice parameters during irradiation and annealing processes have been interpreted by the kinetics of defects. Finally, the correlation between native vacancies and damage behavior is discussed.

Raman spectroscopy study of damage in swift heavy ion‐irradiated ceramics

AbstractRaman scattering is applied to probe the radiation damage in swift heavy ion‐irradiated ceramics, namely zirconium nitride (ZrN), ceria (CeO2), and yttria‐stabilized zirconia (ZrO2: Y, or YSZ) for about the same high electronic stopping power of heavy ions. Raman spectra show that these ceramics are radiation‐resistant materials that are not amorphized by such irradiations even for large ion track overlap at high fluences. However, for ZrN, the increase of the TA/LA and TO/LO band intensities versus fluence is evidenced after 100‐MeV Xe ion irradiation up to a saturation for the fluence of 3 × 1012 cm−2. The band growths are ascribed to the increase of the concentration of Zr and N vacancies induced by electronic excitations inside tracks. However, the diffuse reflectance spectra do not exhibit any clear modifications of the electronic structure. For ceria, the decrease and broadening of the main F2g peak of the fluorite‐like structure and the growth of a broad defect band assigned to oxygen vacancy formation is observed versus fluence up to 1014 cm−2 for 200‐MeV Xe ion irradiation. For YSZ, Raman spectra mainly give evidence of the intrinsic lattice disorder due to the native oxygen vacancies even up to high fluences of about 3 × 1013 cm−2 for 200‐MeV I and 200‐MeV Au ion irradiation. Results are discussed on the basis of the interplay between the native structural disorder and the radiation‐induced disorder by electronic excitations in these three materials.

Crystal symmetry enables high thermoelectric performance of rhombohedral GeSe(MnCdTe2)x

High symmetry favors high power factor by virtue of the balanced Seebeck coefficient and carrier mobility, nonetheless, the role of crystal symmetry in enhancing the material’s thermoelectric performance is abstruse. Here, we employ the interplay between crystal symmetry and native point defects towards high zT of IV-VI semiconductor GeSe, which is scarce in thermoelectric study. Pristine orthorhombic GeSe has a low zT ~ 0.05 due to the high formation energy of Ge vacancy and thus the low carrier concentration (~ 1016 cm−3). Alloying GeSe with MnCdTe2 stabilizes higher-symmetry rhombohedral structure at ambient conditions, thereby effectively lowering the formation energy of Ge vacancy and raising the carrier concentration by four orders of magnitude. Meanwhile, compared to orthorhombic GeSe, the rhombohedral Ge1-yBiySe(MnCdTe2)x own higher valley degeneracy and smaller band effective mass, rendering the decent Seebeck coefficient and larger carrier mobility, respectively. Moreover, the generated multiscale microstructures in rhombohedral Ge0.96Bi0.04Se(MnCdTe2)0.10, including atomic-scale native Ge vacancies and substitution point defects, nanoscale domain structures, and micron-sized secondary phases effectively depress the lattice thermal conductivity. As a result, a state-of-the-art zT ~ 1.0 at 723 K is achieved in Ge0.96Bi0.04Se(MnCdTe2)0.10. These results attest to the efficacy of the interplay between crystal symmetry and native point defects towards high performance GeSe-based and other thermoelectric materials.

Native Vacancy Defects in MXenes at Etching Conditions

Two-dimensional MXenes have recently received increased attention due to their facile synthesis process and extraordinary properties suitable for many different applications. During the wet etching synthesis of MXenes, native defects, such as metal and carbon or nitrogen vacancies, are produced, but the underlying defect formation processes are poorly understood. Here, we employ first-principles calculations to evaluate formation energies of Ti, C, and N vacancies in Ti3C2 and Ti2N MXenes under etching conditions. We carefully account for the mixed functionalization of the surfaces as well as the chemical environment in the solution (pH and electrode potential). We observe that the formation energies of the metal vacancies differ significantly for different types of surface functionalization as well as for different local and global environments. We attribute these differences to electrostatic interactions between vacancies and the surrounding functional groups. We predict that Ti vacancies will be prevalent on bare or OH-functionalized surfaces but not on O-functionalized ones. In contrast, C and N vacancies are more prevalent in O-functionalized surfaces. In addition, our results suggest that the pH value of the etching solution and the electrode potential strongly affect vacancy formation. In particular, the predicted conditions at which abundant vacancy formation is expected are compared to experiments and found to coincide with conditions at which MXenes oxidize readily. This suggests that Ti vacancy formation is a crucial step in initiating the oxidation process.

Stabilization of Cu2O through Site-Selective Formation of a Co1Cu Hybrid Single-Atom Catalyst

Single-atom catalysts (SACs) consist of a low coverage of isolated metal atoms dispersed on a metal substrate, called single-atom alloys (SAAs), or alternatively single metal atoms coordinated to oxygen atoms on an oxide support. We present the synthesis of a new type of Co1Cu SAC centers on a Cu2O(111) support by means of a site-selective atomic layer deposition technique. Isolated metallic Co atoms selectively coordinate to the native oxygen vacancy sites (Cu sites) of the reconstructed Cu2O(111) surface, forming a Co1Cu SAA with no direct Co–Ox bonds. The centers, here referred to as Co1Cu hybrid SACs, are found to stabilize the active Cu+ sites of the low-cost Cu2O catalyst that otherwise is prone to deactivation under reaction conditions. The stability of the Cu2O(111) surface was investigated by synchrotron radiation-based ambient-pressure X-ray photoelectron spectroscopy under reducing CO environment. The structure and reduction reaction are modeled by density functional theory calculations, in good agreement with experimental results.

Aging and Charge Compensation Effects of the Rechargeable Aqueous Zinc/Copper Hexacyanoferrate Battery Elucidated Using In Situ X-ray Techniques

The zinc/copper hexacyanoferrate (Zn/CuHCF) cell has gained attention as an aqueous rechargeable zinc-ion battery (ZIB) owing to its open framework, excellent rate capability, and high safety. However, both the Zn anode and the CuHCF cathode show unavoidable signs of aging during cycling, though the underlying mechanisms have remained somewhat ambiguous. Here, we present an in-depth study of the CuHCF cathode by employing various X-ray spectroscopic techniques. This allows us to distinguish between structure-related aging effects and charge compensation processes associated with electroactive metal centers upon Zn2+ ion insertion/deinsertion. By combining high-angle annular dark-field-scanning electron transmission microscopy, X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy, and elemental analysis, we reconstruct the picture of both the bulk and the surface. First, we identify a set of previously debated X-ray diffraction peaks appearing at early stages of cycling (below 200 cycles) in CuHCF. Our data suggest that these peaks are unrelated to hypothetical ZnxCu1–xHCF phases or to oxidic phases, but are caused by partial intercalation of ZnSO4 into graphitic carbon. We further conclude that Cu is the unstable species during aging, whose dissolution is significant at the surface of the CuHCF particles. This triggers Zn2+ ions to enter newly formed Cu vacancies, in addition to native Fe vacancies already present in the bulk, which causes a reduction of nearby metal sites. This is distinct from the charge compensation process where both the Cu2+/Cu+ and Fe3+/Fe2+ redox couples participate throughout the bulk. By tracking the K-edge fluorescence using operando XAS coupled with cyclic voltammetry, we successfully link the aging effect to the activation of the Fe3+/Fe2+ redox couple as a consequence of Cu dissolution. This explains the progressive increase in the voltage of the charge/discharge plateaus upon repeated cycling. We also find that SO42– anions reversibly insert into CuHCF during charge. Our work clarifies several intriguing structural and redox-mediated aging mechanisms in the CuHCF cathode and pinpoints parameters that correlate with the performance, which will hold importance for the development of future Prussian blue analogue-type cathodes for aqueous rechargeable ZIBs.

Realizing n-type gete through suppressing the formation of cation vacancies and bi-doping* *Supported by the National Science Fund for Distinguished Young Scholars (Grant No. 51725102), and the National Natural Science Foundation of China (Grant No. 51861145305).

It is known that p-type GeTe-based materials show excellent thermoelectric performance due to the favorable electronic band structure. However, n-type doping in GeTe is of challenge owing to the native Ge vacancies and high hole concentration of about 1021 cm−3. In the present work, the formation energy of cation vacancies of GeTe is increased through alloying PbSe, and further Bi-doping enables the change of carrier conduction from p-type to n-type. As a result, the n-type thermoelectric performance is obtained in GeTe-based materials. A peak zT of 0.34 at 525 K is obtained for (Ge0.6Pb0.4)0.88Bi0.12Te0.6Se0.4. These results highlight the realization of n-type doping in GeTe and pave the way for further optimization of the thermoelectric performance of n-type GeTe.

Transition‐Metal Vacancy Manufacturing and Sodium‐Site Doping Enable a High‐Performance Layered Oxide Cathode through Cationic and Anionic Redox Chemistry

AbstractTriggering the anionic redox chemistry in layered oxide cathodes has emerged as a paradigmatic approach to efficaciously boost the energy density of sodium‐ion batteries. However, their practical applications are still plagued by irreversible lattice oxygen release and deleterious structure distortion. Herein, a novel P2‐Na0.76Ca0.05[Ni0.23□0.08Mn0.69]O2 cathode material featuring joint cationic and anionic redox activities, where native vacancies are produced in the transition‐metal (TM) layers and Ca ions are riveted in the Na layers, is developed. Random vacancies in the TM sites induce the emergence of nonbonding O 2p orbitals to activate anionic redox, which is confirmed by systematic electrochemical measurements, ex situ X‐ray photoelectron spectroscopy, in situ X‐ray diffraction, and density functional theory computations. Benefiting from the pinned Ca ions in the Na sites, a robust layered structure with the suppressed P2‐O2 phase transition and enhanced anionic redox reversibility upon charge/discharge is achieved. Therefore, the electrode displays exceptional rate capability (153.9 mA h g−1 at 0.1 C with 74.6 mA h g−1 at 20 C) and improved cycling life (87.1% capacity retention at 0.1 C after 50 cycles). This study provides new opportunities for designing high‐energy‐density and high‐stability layered sodium oxide cathodes by tuning local chemical environments.

Unintentional hydrogen doped impurity induced complex paramagnetic centers in ZnO nanoparticles

The complexity between hydrogen donor and VZn states of synthesized ZnO NPs is highly appreciated under environment imbalance. Single-electron coupling with native defects caused a compromised state (polaronic), and such state behavior is identified as using EPR spectroscopy. 1s e− with native vacancy complex order (VZn – Hi) resonance is found to have a great deal originating EPR resonance. The room temperature and low temperature (77 K) EPR measurements upon heating (800 °C), leading to new paramagnetic centers. Extremes g~1.9558/1.9557 is considered to be a function of thermal gradient and doubly ionized VZn(VZn−)−2(DIV) ~2.0055 sharing one hole is a matter of interest.1s e− in association with the intrinsic defects is deep level trapping, causing complex (VZn – Hi) order signal, encouraging green luminescence emissions. Activation of O−ion-related multi-EPR extremes are having g~2.0255, 2.0220 along with more signals have encountered during low-temperature measurement after annealing at 800 °C.

First-principles study of the vacancy defects in ZnIn2Te4 and CdIn2Te4

First-principles calculations were carried out to study the stability and electronic properties of native vacancy defects in the semiconducting ZnIn2Te4 (ZIT) and CdIn2Te4 (CIT). The Zn/Cd and In vacancies are acceptor defects, while the Te vacancy is donor defect. However, the In and Te vacancies dominate in the [Formula: see text]-type and [Formula: see text]-type semiconducting environments, respectively. The Te vacancy is not excited, so it could not compensate the majority of free carriers. The In vacancy prefers to be excited, which generates free hole carriers to compensate the majority of electron carriers. The Zn vacancy is rare in a typical semiconducting environment. Furthermore, all the vacancies induce localized defect states which may be trap centers for the free carriers. Accordingly, these native vacancy defects are destructive for the development of solar cells based on ZIT and CIT, so they should be avoided as much as possible during the growth process.

Realization low resistivity of high AlN mole fraction Si-doped AlGaN by suppressing the formation native vacancies

The influence of resistivity for n-AlGaN is investigated by changing Si concentration, growth temperature and growth rate. It is found that the resistivity strongly depended on the growth conditions and it reaches to a small value of 5 × 10−3 Ω cm at the growth temperature of 1040 °C. In addition, it is found that there is a strong correlation between the resistivity and the PL emission intensity of group-III-vacancy–Si (VIII-Si) complexes. It indicates that defect compensation can play a key role in high resistivity of n-AlGaN films.

Tuning of lattice oxygen reactivity and scaling relation to construct better oxygen evolution electrocatalyst

Developing efficient and low-cost electrocatalysts for oxygen evolution reaction is crucial in realizing practical energy systems for sustainable fuel production and energy storage from renewable energy sources. However, the inherent linear scaling relation for most catalytic materials imposes a theoretical overpotential ceiling, limiting the development of efficient electrocatalysts. Herein, using modeled NaxMn3O7 materials, we report an effective strategy to construct better oxygen evolution electrocatalyst through tuning both lattice oxygen reactivity and scaling relation via alkali metal ion mediation. Specifically, the number of Na+ is linked with lattice oxygen reactivity, which is determined by the number of oxygen hole in oxygen lone-pair states formed by native Mn vacancies, governing the barrier symmetry between O–H bond cleavage and O–O bond formation. On the other hand, the presence of Na+ could have specific noncovalent interaction with pendant oxygen in *OOH to overcome the limitation from linear scaling relation, reducing the overpotential ceiling. Combining in situ spectroscopy-based characterization with first-principles calculations, we demonstrate that an intermediate level of Na+ mediation (NaMn3O7) exhibits the optimum oxygen evolution activity. This work provides a new rational recipe to develop highly efficient catalyst towards water oxidation or other oxidative reactions through tuning lattice oxygen reactivity and scaling relation.

Interplay between invasive single atom Pt and native oxygen vacancy in rutile TiO2(110) surface: A theoretical study

Interplay between invasive single atom Pt and native oxygen vacancy in rutile TiO2(110) surface: A theoretical study