Formation Of Intrinsic Defects Research Articles

Equilibrium densities of intrinsic defects in transition metal diselenides of molybdenum and tungsten.

Point defects are thermodynamically stabilized in all crystalline materials, with increased densities negatively impacting the properties and performance of transition metal dichalcogenides (TMDs). While recent point defect reduction methods have led to considerable improvements in the optoelectronic properties of TMDs, there is a clear need for theoretical work to establish the lower limit of defect densities, as represented by thermal equilibrium. To that end, an abinitio and thermodynamic analysis of the equilibrium densities of intrinsic point defects in MoSe2 and WSe2 is presented. The intrinsic defect formation energies at the limits of the selenium and metal-rich regimes are determined by density functional theory (DFT) and then augmented with elemental chemical potential functions to determine temperature- and pressure-dependent formation energies. Equilibrium defect densities are determined for MSe, SeM, vM, and vSe, where M and v, respectively, represent the metal and the vacancy, as a function of synthesis temperature and pressure. The effects of vibrational free energy contributions and treatment of the DFT exchange-correlation potential are found to be non-negligible. Calculated equilibrium densities are several orders of magnitude below reported defect densities in TMDs made by chemical vapor deposition, chemical vapor transport, and flux methods, thereby establishing that current synthesis methods are either kinetically limited or impurity dominated.

The A-Ni chemical bond in AIIINiSb (AIII=Sc, Y, Er) half-Heusler materials triggers the formation of anomalous vacancy defects

Defects exert a profound influence on thermoelectric materials by altering electronic band structures and significantly impacting their performance. Despite being a potentially promising thermoelectric material, the mechanism behind defect formation in half-Heusler (HH) compounds ABX remains unclear, impeding the enhancement of their thermoelectric properties. In this study, we investigated the intrinsic defect formation energies for AⅢNiSb (AⅢ = Sc, Y, Er) and other 9 HH compounds, namely AⅣNiSn (AⅣ = Ti, Zr, Hf), AⅤFeSb (AⅤ = V, Nb, Ta), and AⅣCoSb (AⅣ = Ti, Zr, Hf), using first-principles calculations and thermodynamics. The results reveal that AⅢNiSb (AⅢ = Sc, Y, Er) exhibits anomalous B (Ni) vacancy defects, with their formation energies being significantly lower than those of the corresponding B vacancy defects in the other 9 HH compounds. This anomaly can be attributed to the strength of the A-B bonds, where a decrease in bond strength leads to a decrease in the formation energy of B vacancies. This approach of exploring the influence of interatomic bond strength on defect formation is not only insightful for HH compounds but also holds potential applications in defect studies across various materials, offering a broader perspective on the fundamental mechanisms governing defect formation and stability.

Synergistic Tailoring of Electronic and Thermal Transports in Thermoelectric Se-Free n-Type (Bi,Sb)2Te3.

Se-free n-type (Bi,Sb)2Te3 thermoelectric materials, outperforming traditional n-type Bi2(Te,Se)3, emerge as a compelling candidate for practical applications of recovering low-grade waste heat. A 100% improvement in the maximum ZT of n-type Bi1.7Sb0.3Te3 is demonstrated by using melt-spinning and excess Te-assisted transient liquid phase sintering (LPS). Te-rich sintering promotes the formation of intrinsic defects (TeBi), elevating the carrier concentration and enhancing the electrical conductivity. Melt-spinning with excess Te fine-tunes the electronic band, resulting in a high power-factor of 0.35 × 10-3 W·m-1 K-2 at 300 K. Rapid volume change during sintering induces the formation of dislocation networks, significantly suppressing the lattice thermal conductivity (0.4 W·m-1 K-1). The developed n-type legs achieve a high maximum ZT of 1.0 at 450 K resulting in a 70% improvement in the output power of the thermoelectric device (7.7 W at a temperature difference of 250 K). This work highlights the synergy between melt-spinning and transient LPS, advancing the tailored control of both electronic and thermal properties in thermoelectric technology.

Nickel-hexagonal phase induced in Ni3Si-monoclinic intermetallic phase by ion beam irradiation

A hypereutectic Ni alloy with 22 at. % Si was irradiated with a high-energy nickel ion beam and high ion fluence at 650°C to induce the formation of intrinsic and extrinsic point defects. Under these irradiation conditions, high concentrations of point defects were formed in the irradiated region. The recovery process initiated due to the damage induced by irradiation led to the formation of a nanoparticle population in the irradiated region. The irradiated region was characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The experimental evidence obtained permitted the establishment of an event sequence that culminated in nanoparticle population formation. The event sequence had the following stages: (a) Ni atom population implantation within a specific zone of the irradiated region; (b) nucleation and growth of a Nickel-hexagonal phase due to ordering of the implanted nickel atoms; and (c) Ni-hexagonal phase nanoparticle growth within a specific region of the irradiated region. The HRTEM analysis yielded crucial insights into the mechanisms underlying nanoparticle formation. The two most important aspects are as follows: firstly, the nucleation of Nickel-hexagonal phase into an amorphous region, and secondly, the non-equilibrium nature of the Nickel-hexagonal phase of the Nickel metallic system. These experimental findings are important for implantation technology; implanting atomic species into certain materials to create composite materials can lead to the formation of non-equilibrium phases and amorphous regions. Both of these outcomes have the potential to adversely affect the properties of the composite material.

Effective nitrogen doping of TiO2 polymorphs at mild temperatures for visible-light-responsive hydrogen evolution

Herein, a mild-temperature nitrogen doping route with the urea-derived gaseous species as the active doping agent is proposed to realize visible-light-responsive photocatalytic hydrogen evolution both for the anatase and rutile TiO2. DFT simulations reveal that the cyanic acid (HOCN), derived from the decomposition of urea, plays a curial role in the effective doping of nitrogen in TiO2 at mild temperatures. Photocatalytic performance demonstrates that both the anatase and rutile TiO2 doped at mild temperatures exhibit the highest hydrogen evolution rates, although the ones prepared at high temperatures possess higher absorbance in the visible range. Steady-state and transient surface photovoltage characterizations of these doped TiO2 polymorphs prepared at different temperatures reveal that harsh conditions (high temperature reaction) typically result in the formation of intrinsic defects that are detrimental to the transport of the low-energy visible-light-induced electrons, while the mild-temperature nitrogen-doping could flatten the pristine upward band bending without triggering the formation of Ti3+, thus achieving enhanced visible-light-responsive hydrogen evolution rates. We anticipate that our findings will provide inspiring information for shrinking the gap between the visible-light-absorbance and the visible-light-responsiveness in the band engineering of wide-bandgap metal-oxide photocatalysts.

Formation of Grown-In Nitrogen Vacancies and Interstitials in Highly Mg-Doped Ammonothermal GaN.

The formation of intrinsic point defects in the N-sublattice of semi-insulating Mg-doped GaN crystals grown by the ammonothermal method (SI AT GaN:Mg) was investigated for the first time. The grown-in defects produced by the displacement of nitrogen atoms were experimentally observed as deep traps revealed by the Laplace transform photoinduced transient spectroscopy in the compensated p-type crystals with the Mg concentrations of 6 × 1018 and 2 × 1019 cm-3 and resistivities of ~1011 Ωcm and ~106 Ωcm, respectively. In both kinds of materials, three closely located traps with activation energies of 430, 450, and 460 meV were revealed. The traps, whose concentrations in the stronger-doped material were found to be significantly higher, are assigned to the (3+/+) and (2+/+) transition levels of nitrogen vacancies as well as to the (2+/+) level of nitrogen split interstitials, respectively. In the material with the lower Mg concentration, a middle-gap trap with the activation energy of 1870 meV was found to be predominant. The results are confirmed and quantitatively described by temperature-dependent Hall effect measurements. The mechanism of nitrogen atom displacement due to the local strain field arising in SI AT GaN:Mg is proposed and the effect of the Mg concentration on the charge compensation is discussed.

First-principles study of intrinsic defects and helium in tungsten trioxide

Understanding the behavior of intrinsic defects and helium (He) in tungsten oxides is useful for the application of tungsten (W) in a fusion environment because of the oxidation of W surfaces. The formation and diffusion energies of intrinsic defects and He in monoclinic γ-WO3 have been investigated using first-principles density functional theory calculations. The formation energy and diffusion activation energy of O defects are lower than W defects. O vacancy prefers to diffuse along the ⟨001⟩ direction, then followed by ⟨010⟩ and ⟨100⟩ directions; however, the W vacancy is immobile at temperatures lower than 2000 K. The stability of Schottky defects (SDs) is sensitive to their geometry and orientation. W interstitials prefer to move along the [100] direction, while O interstitials jump around W atoms rather than through the W quasi-cubic centers. He interstitial atoms are predicted to have a high solubility and an anisotropic diffusion mechanism in γ-WO3. In addition, the effect of biaxial strain on the solubility and diffusivity of He interstitials was investigated. He interstitials prefer to reside at individual sites rather than clusters. He atoms are weakly trapped by single vacancies or SDs. Vacancies assist the local migration of nearby He. Correspondingly, He self-clustering and bubble formation are less likely to form in γ-WO3 relative to bcc W. The energetics obtained in this work can be used to predict the microstructure evolution of the WO3 layer on a W substrate exposed to He plasmas at different temperatures.

Dual-Functionalized Ca Enables High Sodiation Kinetics for Hard Carbon in Sodium-Ion Batteries.

Hard carbons (HCs), while a leading candidate for sodium-ion battery (SIB) anode materials, face challenges in their unfavorable sodiation kinetics since the intricate microstructure of HCs complicates the Na+ diffusion channel. Herein, a Hovenia dulcis-derived HC realizes a markedly enhanced high-rate performance in virtue of dual-functionalized Ca. The interlayer doped Ca2+ effectively enlarges the interlayer spacing, while the in situ-formed CaSe templates induce the formation of hierarchical pore structures and intrinsic defects, significantly providing fast Na+ diffusion channels and abundant active sites and thus enhancing the sodium storage kinetics. Achieved by the synergistic effect of regulation of intrinsic microcrystalline and pore structures, the optimized HC shows remarkable performance enhancements, including a high reversible capacity of 350.3 mA h g-1 after 50 cycles at 50 mA g-1, a high-capacity retention rate of 95.3% after 1000 cycles, and excellent rate performance (108.4 mA h g-1 at 2 A g-1). This work sheds light on valuable insight into the structural adjustment of high-rate HCs, facilitating the widespread utilization of SIBs.

A first principles investigation of defect energetics and diffusion in actinide dioxides

In this work, we have studied the formation and migration of various neutral intrinsic defects in AnO2. The non-stoichiometric defects such as vacancy defect, interstitial defect, and antisite defect as well as stoichiometric defects such as Schottky defect and Frenkel pairs has been considered for investigation. The formation energies of point defects are much higher in ThO2 as compared to UO2 and PuO2. The binding energy of defect clusters indicate that formation of Frenkel pair and Schottky defect is favorable over individual defects in ThO2. In case of UO2 and PuO2, hyper-stoichiometric defects are more stable than hypo-stoichiometric defects. The results obtained in this work are in good agreement with experimental results.

First-principles calculation of intrinsic point defects and doping performance of MoSi<sub>2</sub>N<sub>4</sub>

MoSi<sub>2</sub>N<sub>4</sub> is an emergent two-dimensional (2D) material, which has received much attention because of its excellent performance over semiconductors, including excellent environmental stability and high carrier mobility. However, the formation of intrinsic defects in semiconductors is often inevitable and can significantly affect device performance. By using density functional theory (DFT), we analyze the properties and effects of intrinsic point defects in MoSi<sub>2</sub>N<sub>4</sub>. We first confirm the consistency of our results with current experimental data. After that, the formation energy values of twelve native defects reveal that the antisite defect of molybdenum substituting for silicon (Mo<sub>Si</sub>) defect dominates in all intrinsic defects. Under the constraint of overall charge neutrality, self-consistent Fermi level calculations reveal that MoSi<sub>2</sub>N<sub>4</sub> with only intrinsic defects exhibits intrinsic characteristics, highlighting its potential as a semiconductor device material. However, this intrinsic nature contradicts the p-type characteristics observed in two-dimensional MoSi<sub>2</sub>N<sub>4</sub>. In the subsequent defect concentrations, we find that both n-type and p-type behavior can be easily realized by doping appropriate impurities without being compensated by native defects. This suggests that the p-type characteristics of MoSi<sub>2</sub>N<sub>4</sub> during growth may result from p-type impurities introduced under non-equilibrium growth conditions or silicon vacancy defects. Our findings not only demonstrate the potential applications of MoSi<sub>2</sub>N<sub>4</sub> in semiconductor devices but also provide valuable guidance for future studying the defect mechanisms of this material.

NH3 plasma-etching-derived porous N-doped carbon nanotubes with FeP nanoparticles and intrinsic carbon defects for boosting oxygen reduction in rechargeable Zn–air batteries

The formation of intrinsic carbon defects and phase engineered FeP nanoparticles regulated by NH3 plasma etching significantly enhanced the oxygen reduction reaction performance of N-doped carbon nanotubes.

Formation of intrinsic point defects in AlN: a study of donor and acceptor characteristics using hybrid QM/MM techniques

The wide-gap material aluminium nitride (AlN) is gaining increasing attention for its applications in optoelectronics, energy, and quantum computing, making the investigation of its defect properties crucial for effective use in these fields.

Multiscale Probing and Redesign of the Solid-State Synthesis Toward Defect-Free Ni-Rich Layered Cathodes

Nickel-rich lithium layered oxides (LiNixCoyMnzO2, NCM, x+y+z=1, x≥0.6)) have received tremendous attention as the most promising cathode materials for the practical development of high-energy lithium-ion batteries. However, they commonly suffer from rapid capacity decay along with structural and morphological degradation. Previous studies have unraveled that the general failure mechanism involves the mechanical deterioration of electrode particles, originating from micro-cracking in particles due to the build-up of significant anisotropic strains during phase transition and the chemical parasitic reactions with electrolytes through the crevice created. Such chemo-mechanical failures are triggered and aggravated by various structural defects, including internal void spaces, surface reconstruction layers, and intragranular nanopores, typical features of the conventionally synthesized nickel-rich layered oxide materials. It remains elusive how these defects are formed regardless of the use of single-crystalline or polycrystalline particles. A fundamental understanding of the general defect formations is thus indispensable to clarify how the synthesis process induces the formation of intrinsic defects from the precursor to the final product stage and how these defects affect the electrochemical degradation at the primary- and secondary-particle levels. Critical questions that require in-depth study include how the spatially inhomogeneous solid-state reaction begins at the interface of reacting precursors, propagates during the synthesis, and triggers the generation of defects. In this presentation, I will show the hidden synthetic mechanism of nickel-rich layered oxide materials using a combination of high-end and multi-length-scale analysis methods, including aberration-corrected TEM, in situ heating TEM with gas control, and in situ XRD. The kinetic interplay in synthesis of nickel-rich lithium layered oxide and its implications will be discussed in detail. Furthermore, the microstructural pore defects of NCN9235 during the heating process were quantitatively characterized. Based on our mechanistic understanding, we propose a redesign of solid-state synthesis pathway to achieve structurally integrated nickel-rich layered oxide cathodes.

Intrinsic defects generated by iodine during TiO2 crystallization and its relationship with electrical conductivity and photoactivity

AbstractDefect formation during synthesis is one of the strategies used to improve the photoactivity of polycrystalline semiconductors such as titanium dioxide (TiO2). Defects can modify the electronic structure of TiO2 and change the surface of the interaction between the photocatalyst and the reactants. In this study, TiO2 relationship between processing in the presence of iodine and the consequent formation of intrinsic defects were explored. TiO2 nanoparticles were synthesized using the polymeric precursor method and exposed to iodine ions at concentrations up to 5 mol%. After calcination at 350°C, detailed chemical analyses revealed that iodine was absent in the samples. However, the TiO2 properties, such as specific surface area, crystallite sizes, and specific grain boundary area, were affected. Further experiments, such as electron paramagnetic resonance, diffuse reflectance, optical measurements, and electrochemical impedance spectroscopy indicated the presence of defects in the iodine‐processed samples. These defects directly influenced the electrical properties of the material, which affected the photoactivity, measured by the degradation of acetaminophen.

A computational study of CaWO4: Raman spectrum, intrinsic defects, and excited state properties

In this study, we present an ab initio investigation of the optical properties of CaWO4. The crystal structure of the bulk material along with its vibrational (in the ground state) and optical properties (in both ground and excited states) were analyzed. Our simulations accurately identified the observed Raman signature of the material. Additionally, we simulated the intrinsic luminescence properties of CaWO4 and identified the charge transfer that occurs during the de-excitation process. Finally, through the use of ab initio techniques, the intrinsic defect formation energies and their corresponding concentrations were determined providing the first instance of such findings for this material.

Geometric, electronic and transport properties of bulged graphene: A theoretical study.

Out-of-plane deformation in graphene is unavoidable during both synthesis and transfer procedures due to its special flexibility, which distorts the lattice and eventually imposes crucial effects on the physical features of graphene. Nowadays, however, little is known about this phenomenon, especially for zero-dimensional bulges formed in graphene. In this work, employing first-principles-based theoretical calculations, we systematically studied the bulge effect on the geometric, electronic, and transport properties of graphene. We demonstrate that the bulge formation can introduce mechanical strains (lower than 2%) to the graphene's lattice, which leads to a significant charge redistribution throughout the structure. More interestingly, a visible energy band splitting was observed with the occurrence of zero-dimensional bulges in graphene, which can be attributed to the interlayer coupling that stems from the bulged structure. In addition, it finds that the formed bulges in graphene increase the electron states near the Fermi level, which may account for the enhanced carrier concentration. However, the lowered carrier mobility and growing phonon scattering caused by the formed bulges diminish the transport of both electrons and heat in graphene. Finally, we indicate that bulges arising in graphene increase the possibility of intrinsic defect formation. Our work will evoke attention to the out-of-plane deformation in 2D materials and provide new light to tune their physical properties in the future.

Defect properties and solution energies of dopants in NASICON-type LiGe2(PO4)3 solid electrolyte: a first-principles study.

NASICON-type solid electrolytes are suitable choices for solid state batteries considering safer and more stable electrochemical performance compared to other potential solid electrolytes. The present study investigates intrinsic defects and dopant incorporation energetics in the LiGe2(PO4)3 (LGP) electrode material using density functional theory-based calculations. The formation energies of intrinsic defects (Frenkel, Schottky and anti-sites) indicate that Li Frenkel pair formation is the most energetically feasible process. With an aim to improve the lithium ion conductivity and chemical stability by suitable doping, solution energies are calculated for various trivalent (M3+ = B3+, Al3+, Ga3+, Sc3+, In3+, Y3+, Gd3+, La3+) and tetravalent (M4+ = Si4+, Ti4+, Sn4+ and Zr4+) ions substituted at the Ge4+ site. The most favourable trivalent and tetravalent dopants are Al3+ and Ti4+, respectively. The changes in lattice parameters with doping are correlated with channel/bottleneck size for Li+ migration. Alkali atom doping at the Li+ site is energetically favourable whereas alkali-earth doping at the Li+ site is not. Analysis based on Bader charges and density of states delineates changes in chemical interactions between the dopant atoms and the host LGP.

Probing site-selective doping and charge compensating defects in KMgF3: insights from a hybrid DFT study.

Design of optoelectronic materials with tunable properties using activators and defect clusters has become one of the prime interests of current research. In this study, detailed Density Functional Theory based calculations have been presented to investigate the geometries and electronic structures of various possible defect clusters using Eu-KMgF3 as a probe which has numerous technological and industrial applications. Using a more reliable hybrid density functional, we have calculated defect formation energies and thermodynamic transition levels to get knowledge about the site selectivity of Eu. It has been observed that the electronic structure of Eu-KMgF3 is not only dependent on the site of doping but also on the oxidation state of Eu (2+/3+). The present study also investigates the relative stability of different kinds of defects and defect clusters under various synthetic growth conditions. The ultimate aim is to find out the microscopic origin of the fundamental optical properties of Eu-KMgF3 and provide an unambiguous explanation of available experimental results. Thus, it has been revealed that doping with Eu results in the spontaneous formation of intrinsic defects, which contribute to the observed optical behaviour. We have also extended our study to investigate the role of codoping with Li in determining the geometry and electronic structure of Eu-KMgF3 aiming to explain its impact on the optical properties. Thus, a complete presentation of the influence of the activator in the absence and presence of lattice defects on the optical properties of KMgF3 has been accomplished in the current study. We strongly believe that the present study will be helpful in designing tunable phosphor materials by a defect-controlled synthesis strategy.

Synthetic Control of Intrinsic Defect Formation in Metal Oxide Nanocrystals Using Dissociated Spectator Metal Salts.

Crystallographic defects are essential to the functional properties of semiconductors, controlling everything from conductivity to optical properties and catalytic activity. In nanocrystals, too, defect engineering with extrinsic dopants has been fruitful. Although intrinsic defects like vacancies can be equally useful, synthetic strategies for controlling their generation are comparatively underdeveloped. Here, we show that intrinsic defect concentration can be tuned during the synthesis of colloidal metal oxide nanocrystals by the addition of metal salts. Although not incorporated in the nanocrystals, the metal salts dissociate at high temperatures, promoting the dissociation of carboxylate ligands from metal precursors, leading to the introduction of oxygen vacancies. For example, the concentration of oxygen vacancies can be controlled up to 9% in indium oxide nanocrystals. This method is broadly applicable as we demonstrate by generating intrinsic defects in metal oxide nanocrystals of various morphologies and compositions.

New insights into intrinsic point defects of delafossite CuAlO2 as optoelectronic materials

AbstractDelafossite CuAlO2 has great potential in optoelectronics applications. The intrinsic point defects play the decisive roles in the optoelectronic properties and application of CuAlO2. However, there are still disputes about their relevant underlying physical mechanisms at the atomic scale. Here, the formation of intrinsic point defects of CuAlO2 and their effects on optical absorption spectra and electronic structure have been systematically investigated using first‐principles calculations. In oxygen‐rich and oxygen‐poor chemical environments, acceptor‐type defects such as Cu vacancy and Cu substitution on the Al site can spontaneously form and produce p‐type conductivity at room temperature. This is the important physical basis for CuAlO2 to be widely used as transparent conductive oxide films. The shallow acceptor‐type levels produced by Cu substitution on Al site and O vacancy, and the shallow donor‐type levels produced by interstitial Cu, are favorable for the separation of photo‐generated electron–hole pairs, which can be suitable for applications in photocatalysis and photovoltaics. The Cu or Al vacancy can produce delocalized energy band at the top of valence band and pinning Fermi level at the same time, which may make CuAlO2 a degenerate semiconductor and produce local surface plasmon resonance effect. Al substitution on Cu site and interstitial Al or O form complex deep defect energy bands in the forbidden band and produce significant multiband optical absorption in the visible–NIR region. They are expected to become alternative materials for photoluminescence and photoelectric detection. This work demonstrates the importance of intrinsic defect engineering for optimizing optoelectronic properties and thus promoting the discovery of novel optoelectronic materials.