Vacancy Trapping Research Articles

Ligand Engineering of CsPbI3 Quantum Dots for Efficient Solar Cells

Perovskite quantum dots (PQDs) have gained significant attention due to their exceptional optical and electronic properties, rendering them promising materials for various optoelectronic applications, such as single-junction solar cells because of the multiple exciton generation effects. To improve the electrical properties of CsPbI3 PQDs, ligand engineering was explored by introducing short-chain ligands to exchange the original long-chain ligands on the PQD surface during the purification process. However, the short-chain ligands suffer from low solubility in the anti-solvent, resulting in insufficient ligand exchange. To address this issue, we developed an in-situ ligand exchange (ILE) strategy to promote the replacement of long-chain ligands with short-chain ones, leading to improved surface chemistry. In this work, 4-methoxyphenethylammonium iodide (CH3O-PEAI) was demonstrated as the short-chain ligand in the ILE strategy, thereby modifying halide vacancy trap states, and promoting carrier transportation and extraction. As a result, the ILE-CsPbI3-based solar cells achieved a higher power conversion efficiency of 14.4% with a high open circuit voltage (VOC) of 1.23 V and exceptional long-term durability due to higher hydrophobicity.

Effect of Ge addition on the clustering of naturally-aged Al–Mg–Si–Cu alloy

Ge addition effect on the delaying natural aging of Al–Mg–Si–Cu alloy was investigated. Natural aging kinetic and cluster size was suppressed by the Ge addition. Using atom probe tomography, it was found that Ge refines cluster evolution by disturbing Mg-enrichment of Si–Si cluster as well as enhancing vacancy trapping. Segregation of Ge at the interface of cluster was also regarded as potential barrier to growth of the clusters.

In-situ construction of Schottky junctions with synergistic interaction of oxygen vacancies in Mo@MoO3 nanosheets for efficient N2 photoreduction

Defect engineering is a promising technique that can activate nitrogen by injecting electrons into the anti-bonding molecular orbital of nitrogen through anion vacancies. Moreover, compared to a single component, Schottky junctions can effectively overcome rapid electron recombination, thereby significantly enhancing photoelectron utilization efficiency. In this work, we prepared Mo modified MoO3 nanosheets with abundant oxygen vacancies by a solvothermal method and investigated the effect of synergistic interactions between Schottky junctions and oxygen vacancies on the photocatalytic performance of N2 reduction reaction (NRR). The photocatalytic nitrogen fixation performance of Mo@MoO3 nanosheets (MoO3-6) reached 50.78 μmol·g−1·h−1 without any scavenger, which was about 3 times that of commercial MoO3. The Schottky barrier creates a built-in electric field generating charge transfer channels during the photocatalytic reaction, while the oxygen vacancies trap electrons to activate N2, and together with Mo, broaden the light absorption range of the catalyst, facilitating more efficient transfer of excited electrons to the active site. The synergetic advantages of Schottky junctions and oxygen vacancies are exploited in advancing the photocatalytic effect, providing new opportunities and challenges for the development of metal oxide-based materials.

Defect mechanisms, oxygen vacancy trapping ability in rare‐earth doped BaTiO3 from first‐principles and thermodynamics

AbstractThe defect mechanisms of rare earth (RE) doped BaTiO3 have a strong impact on the electrical performance of the multilayer ceramics capacitors (MLCCs). Oxygen vacancy is the main reason for the device degradation over longtime use, while the effect of the doping strategy on controlling the oxygen vacancies is not yet quantitatively understood. In this work, the grand canonical thermodynamic defect model based on first‐principle calculations is applied to evaluate the defect mechanism of RE‐doped BaTiO3 under practical experimental condition. The charge compensation and prior site occupancy of RE are found not only associated with ionic size but also exhibit transitions with oxygen partial pressure and doping concentration. Furthermore, the oxygen vacancy trapping ability of RE ions is evaluated from the perspectives of thermodynamics and kinetics. The migration barrier among first nearest oxygen sites dramatically changed depending on the RE site occupancy. The large trapping ability is contributed by the relatively large negative binding energy of the defect complex and comparable RE concentrations substituted on Ba and Ti sites. The two conditions can be achieved in amphoteric ions doped systems, while in pure donor doped BT only one of these conditions can be satisfied. Although the self‐compensated defect complexes exhibit the highest binding energy, the trapping ability contributed by different defect complexes (, , REBa − RETi) is generally comparable in these systems. This feature of amphoteric RE ions accounts for the improvement of the lifetime and reliability of MLCCs.

Investigation of vacancy trapping by solutes during quenching in aluminum alloys

In aluminum alloys, excess vacancies often form pairs or clusters with solute atoms. This is commonly referred to as vacancy trapping by solutes. Such trapping results in a change in the number of mobile or free vacancies, which in turn affects solute diffusion and precipitate formation during heat treatment. In the current work, such trapping is investigated indirectly in the Al-Sn binary model alloy via studying the dependence of Frank loop formation on the availability of free vacancies. Current experimental results show that the addition of 100 at. ppm. Sn to aluminum leads to a significant reduction in the number density of quenched-in Frank loops. Modeling of mono- and divacancy trapping by solutes during quenching demonstrates the influences of the vacancy-solute binding energy, solute concentration, and temperature on the trapping process. The results reveal that the binding between vacancies and solutes significantly reduces the concentration of free vacancies after quenching in Al-Sn.

Interaction of irradiation defects with precipitates in CuCrZr and Cu-1Fe alloys

In this work, we employ transmission electron microscopy and helium ion irradiation to study the interactions between the radiation-induced point defects and the incoherent/semi-coherent precipitates in CuCrZr and Cu-1Fe alloys. Both thin foil and bulk implantation were used to explore the interactions of radiation defects with interfaces. The irradiated defects show a strong interface character dependence in both implanted CuCrZr and Cu-1Fe alloys after similar level of radiation damage. Large-sized voids appear around the incoherent Cr precipitates, while no visible cavities are formed around precipitates in Cu-1Fe alloy. Interestingly, three types of semi-coherent Fe precipitates exhibit various irradiation damage mitigation mechanisms, such as twin boundaries migration, short range elements redistribution and precipitate abnormal growth, which are caused by radiation defect-mediated dislocation glide, element diffusion and mass transportation. The high sink efficiency of the incoherent precipitates trap vacancies and leads to the generation of submicron-sized voids. In contrast, the semi-coherent precipitates can regulate the motion of point defects and effectively enhance the recombination rate of vacancies and interstitials.

First principles analysis on void-reduction mechanism and impact of oxygen in nitrogen-doped CZ-Si crystal

During silicon (Si) crystal growth with the Czochralski method, nitrogen (N) doping at a concentration of about 1013–1015 atoms/cm3 decreases the size of void defects. This is related to the trapping of vacancies (Vs) by N atoms. The effect by N-doping is weakened with an increase in oxygen (O) concentration. The stable structures and formation processes of N-V and N-O-V complexes were investigated systematically by conducting density functional theory calculations, and the void-reduction mechanism by N-doping and the impact of O atoms were clarified. The results indicate that two interstitial N atoms form N2 which reacts with Vs to change its structure to N2-V and N2-V2. The dissociation of a V from stable N-V complexes requires a large amount of energy. Furthermore, the stabilized O atoms around the N atoms in N-O-V complexes restrict the movement of Si atoms. The energy required for a V to dissociate from N-O-V complexes is reduced, resulting in an increase in the number of free Vs. This is the reason O atoms weaken the void-reduction effect by N-doping.

Illuminating Trap Density Trends in Amorphous Oxide Semiconductors with Ultrabroadband Photoconduction

AbstractUnder varying growth and device processing conditions, ultrabroadband photoconduction (UBPC) reveals strongly evolving trends in the defect density of states (DoS) for amorphous oxide semiconductor thin‐film transistors (TFTs). Spanning the wide bandgap of amorphous InGaZnOx (a‐IGZO), UBPC identifies seven oxygen deep donor vacancy peaks that are independently confirmed by energetically matching to photoluminescence emission peaks. The subgap DoS from 15 different types of a‐IGZO TFTs all yield similar DoS, except only back‐channel etch TFTs can have a deep acceptor peak seen at 2.2 eV below the conduction band mobility edge. This deep acceptor is likely a zinc vacancy, evidenced by trap density which becomes 5‐6× larger when TFT wet‐etch methods are employed. Certain DoS peaks are strongly enhanced for TFTs with active channel processing damage caused from plasma exposure. While Ar implantation and He plasma processing damage are similar, Ar plasma yields more disorder showing a ≈2 × larger valence‐band Urbach energy, and two orders of magnitude increase in the deep oxygen vacancy trap density. Changing the growth conditions of a‐IGZO also impacts the DoS, with zinc‐rich TFTs showing much poorer electrical performance compared to 1:1:1 molar ratio a‐IGZO TFTs owing to the former having a ∼10 × larger oxygen vacancy trap density. Finally, hydrogen is found to behave as a donor in amorphous indium tin gallium zinc oxide TFTs.

Void swelling in additively manufactured 316L stainless steel with hafnium composition gradient under self-ion irradiation

Compositionally-graded austenitic 316L stainless steel (SS) samples with five different Hafnium (Hf) concentrations (up to 1 wt.%) were additively manufactured by directed energy deposition and then were irradiated using 5 MeV Fe2+ ions to 50 displacements per atom (dpa) at 500, 550, and 600 °C, respectively. A rastering beam was used to ensure homogenous irradiations for the large areas of ∼1.40 cm2. Composition-dependent void evolution was evaluated. Void size, void number density, and void swelling all tend to decrease with increasing Hf concentration at all three temperatures. When the nominal Hf concentration increases to 1 wt.% in the doped additively manufactured (AM) 316L SS, the void swelling is over an order of magnitude lower than that in pure AM 316L. Atom probe tomography results showed that about 0.14 wt.% Hf dissolved in the matrix at the 1 wt.%. Hf nominal concentration. The suppression of void swelling by Hf addition shows qualitative agreement with the numerical calculation based on the vacancy trapping mechanism, indicating that the oversized Hf reduces the steady state vacancy concentration, and hence the incubation period of swelling is extended, resulting in a dramatic difference in void swelling. Other factors including grain boundaries, dislocation and dislocation cells, Hf-rich particles, and delta ferrite are secondary to this mechanism. This work shows that additive manufacturing enabled microalloying is promising for developing void swelling resistant materials.

Deuterium trapping behavior in tungsten surface due to low-energy ion irradiation

Deuterium (D) irradiation at 421–597 K was performed on polycrystalline tungsten with 100 eV D ions up to a fluence of 4.64 × 1023 m−2. A supersaturated layer in the ion stopping range (< 20 nm) with the concentration of 1.2 at.% under the present surface resolution of ∼ 12 nm, a shallow minimum D concentration area of > 100 nm and a subsequent significant increase in the sub-surface were revealed by combining D (12C, D) 12C elastic recoil detection and D (3He, p) 4He nuclear reaction analysis. The first-principle calculations indicated that the trapping of vacancies for hydrogen (H) atoms can greatly reduce the recombination probability of Frenkel pairs and then act to stabilize the Frenkel pair. The stabilizing effect can be enhanced with increasing number of H atoms. Furthermore, the vacancies from Frenkel pairs can trap more surrounding H atoms, resulting in further H enrichment which could account for the formation of supersaturated layers. The typical location of the blister cavities coincides rather well with the subsequent significant increase in the sub-surface area. D-induced damage in the supersaturated layer and the sub-surface layers was examined by combining scanning electron microscope, transmission electron microscope and positron annihilation measurements.

Precursor mediated and defect engineered ZnO nanostructures using thermal chemical vapor deposition for green light emission

We used thermal chemical vapour deposition to synthesize zinc oxide nanostructured thin films on n-type silicon substrate using zinc oxide (ZnO) and zinc chloride (ZnCl2) powder precursors in conjunction with bulk graphite powder. The synthesized ZnO nanostructured thin films structures are polycrystalline with a relatively enhanced c-axis orientation for ZnO powder precursor with respect to that of ZnCl2 precursor. The scanning electron microscopic analysis substantiate the higher c-axis orientation of ZnO nanostructures, grown using ZnO powder precursor. The photoluminescence (PL) and UV–Vis measurements suggest the presence of zinc vacancies, causing the green emission from both nanostructured thin films. The measured bandgap of ZnO nanostructured thin film using ZnO powder precursor is slightly higher than that of ZnCl2 powder precursor. The capacitance-voltage and Mott-Schottky measurements showed the n-type characteristics with 6.3 × 1016 cm−3 and 2.7 × 1015 cm−3 carrier densities for ZnO nanostructured thin films prepared using ZnO and ZnCl2 precursors, respectively. The current-voltage measurements showed relatively lower current for ZnO nanostructured thin films, synthesized using ZnCl2 precursor, signifying the presence of zinc vacancies trap sites and is also consistent with Mott-Schottky measurements, showing lower carrier concentrations with respect to that of ZnO precursor derived ZnO nanostructured thin films. Thus, controlled Zn vacancies in ZnO nanostructured thin films may lead to the efficient green light emitters, useful for green light colour emission applications.

Influence of carbon on hydrogen retention in molybdenum for nuclear material application: A first-principles investigation

• Systematic DFT simulations for the effect of carbon on the H retention in Mo. • Impurity carbon cannot trap H atoms in perfect Mo. • Impurity carbon has little effect on the H vacancy capturing in Mo. • Mo-carbon layer prevents the H diffusion and permeation in Mo. Mo has been applied to nuclear material, in which H and carbon impurities are unavoidable. In view of this, we have studied the effect of carbon on the H retention in Mo using first-principles simulations. In perfect Mo, there is always repulsion between H and carbon, and impurity carbon cannot capture H atoms. With the appearance of vacancy in Mo, vacancy and carbon-vacancy cluster trap seven and six H atoms, respectively. This means that impurity carbon has little effect on the H vacancy capturing. Finally, we explore the H diffusion by considering the presence of one Mo-carbon layer in Mo. Away from the Mo-carbon layer, H jumps along the optimal route with a diffusion barrier of 0.12 eV. As H moves close and passes through the Mo-carbon layer, the H diffusion barrier is increased to 0.73 eV. Therefore, the repulsive interaction between H and carbon can increase the H energy barrier in the vicinity of the Mo-carbon layer, which prevent the H diffusion and permeation in Mo. The current results can explain the promoting mechanism of bubble formation due to impurity carbon implantation and help us design future Mo-based nuclear material.

Microscopic origin of the acceptor removal in neutron-irradiated Si detectors - An atomistic simulation study

The improved radiation hardness of p-type Si detectors is hindered by the radiation-induced acceptor removal process, which is not fully understood yet. Through atomistic modeling of displacement damage and dopant interactions, we analyze the acceptor removal under neutron irradiation, providing physical insight into its microscopic origin. Our results show that the fast decay of the effective dopant concentration (Neff) at low irradiation fluences is due to B deactivation caused by Si self-interstitials. The intriguing increase of the acceptor removal parameter with the initial dopant concentration (Neff,0) is explained by the limited number of mobile Si self-interstitials that survive annihilation and clustering processes. The sublinear dependence of the removal parameter on Neff,0 is associated to the inhomogeneity of damage for low Neff,0 and the formation of B-interstitial clusters with several B atoms for high Neff,0. The presence of O and C modifies B deactivation mechanisms due to the key role of BiO defects and the trapping of vacancies and Si self-interstitials, but for the impurity concentrations analyzed in this work ([O] >> [C]) it has little effect on the overall amount of removed acceptors. At high irradiation fluences, the reported increase of Neff is attributed to the formation of defect-related deep acceptors. From the analysis of the defect concentrations resulting from neutron irradiation and the occupancy of small clusters with acceptor levels reported in literature, we point out the tetra-vacancy cluster as one of the main contributors to Neff with negative space charge.

Influence of the Composition and Vacancy Concentration on Cluster Decomposition Behavior in Al-Si-Mg Alloy: A Kinetic Monte Carlo Study.

The influence of cluster composition and the addition of vacancies on the decomposition behavior of clusters during artificial aging in Al–Si–Mg alloys were analyzed according to the kinetic Montel Carlo model. Clusters with a balanced composition (Mg/(Mg + Si) = 0.5) were the most difficult to decompose. In addition, the cluster decomposition was slower when more vacancies were added to the cluster. Among Si, Mg, and vacancies, vacancies most significantly affect decomposition. The clusters with Mg/(Mg + Si) ≤ 0.4 strongly trap vacancies, which can be classified as hardly decomposable vacancy-rich clusters. The clustering behavior during natural aging and the effect of pre-aging were analyzed using the Kinetic Monte Carlo model. Pre-aging slows down cluster formation due to the lowered vacancy concentration. In addition, the overall composition of the clusters changes to easily decomposable clusters after pre-aging. Thus, not only is the number of clusters reduced but also the clusters are more easily decomposable when pre-aging is performed.

Comparison of Hydrogen-Induced Oxide Charges Among GaN Metal-Oxide-Semiconductor Capacitors with Al2O3, HfO2, or Hf0.57Si0.43Ox Gate Dielectrics

The effect of hydrogen on GaN metal-oxide-semiconductor (MOS) capacitors with Al2O3, HfO2, or Hf0.57Si0.43O x gate dielectrics was studied using capacitance–voltage (C–V) measurements. Hydrogen exposure shifted all the C–V curves toward the negative bias direction, and the hydrogen response of the devices was reversible. When the hydrogen-containing ambient atmosphere was changed to N2, the C–V characteristics were found to gradually revert to the initial values in N2. Application of a reverse gate bias accelerated the reversion compared with that in the absence of a bias, indicating that hydrogen was absorbed into the dielectric (Hf0.57Si0.43O x ) as positive mobile charges. This result is consistent with the direction of the shift of the C–V curves; positively charged hydrogen absorbed into a dielectric can cause a flatband voltage shift. The hydrogen-induced shift of the C–V curves varied depending on the dielectric. MOS devices with HfO2-based high-k dielectrics were found to have approximately two to four times more incorporated charges than devices with Al2O3. Under the hypothesis that oxygen vacancies (VOs) trap hydrogen, the obtained results imply that the number of VOs in HfO2-based high-k dielectrics is much larger than that in Al2O3-based dielectrics.

Electrical properties of Bi2.99(Li0.5Sm0.5)0.01Ti1-x(Mn1/3Ta2/3)xNbO9 high-temperature piezoelectric ceramics via oxygen vacancy defects tailoring

Tribismuth titanium niobium oxide (B3TiNbO9, BTNO) is one of the Aurivillius phases with the highest Curie temperature (Tc), but its piezoelectric coefficient (d33) is poor. When x = 0.045, high-temperature piezoelectric ceramics with d33 of 11.9 pC/N and Tc of 916.7 °C were prepared by (Mn1/3Ta2/3)(4+ω)+ modification of Ti4+ at B-site of Bi2.99(Li0.5Sm0.5)0.01Ti1-x(Mn1/3Ta2/3)xNbO9. The substitution will aggravate the distortion of BO6 octahedron due to the displacement of oxygen ions. The existence of Mn3+ in the dopant will act as an oxygen vacancy trap and then increase the resistivity. As a result, BTNO ceramics have enhanced piezoelectric properties as well as temperature and cycle stability. This work provides a theoretical basis for better high-temperature piezoelectric properties of A/B co-doped Aurivillius phase ceramics in the future.

Improving the performance of protonic ceramic fuel cells and electrolyzers: The role of acceptor impurities in oxide membranes

Protonic ceramic electrochemical cells (PCECs) are promising energy conversion devices offering an efficient and environmentally friendly way to produce electricity and pure hydrogen. An important challenge in the development of PCECs is the enhancement of their performance through selection of optimal dopants for acceptor-doped oxide perovskites, commonly used as electrolyte membranes. In this work, we elucidate the implications of the interaction of protons and oxygen vacancies with acceptor impurities in oxide membranes for the performance of PCECs operating in the fuel cell and electrolysis modes. The analysis relies on our recent theoretical developments on fuel cell operation and oxide hydration, as well as on the proposed statistical theory of proton hopping conduction in acceptor-doped oxides. It is shown that the interaction between ionic defects and impurities can substantially affect the characteristics of proton-conducting membranes. General relationships between the output characteristics of PCECs based on membranes with proton and hole conductivity and the energies of acceptor-bound states of ionic defects are determined. It is established that trapping of protons and oxygen vacancies reduces both proton and hole currents and effective conductivities, thereby altering the dependences of the voltage and power density on current density. We demonstrate the possibility of tuning the power density and faradaic efficiency of PCECs by doping oxide membranes using acceptor impurities with varying defect trapping energies. The presented results contribute to understanding the functioning mechanisms of protonic ceramic fuel cells and electrolyzers, and provide new fundamental criteria for the selection of acceptor impurities for oxide membranes to develop high-efficient devices.

Atomistic Mechanisms of Binary Alloy Surface Segregation from Nanoseconds to Seconds Using Accelerated Dynamics.

Although the equilibrium composition of many alloy surfaces is well understood, the rate of transient surface segregation during annealing is not known, despite its crucial effect on alloy corrosion and catalytic reactions occurring on overlapping timescales. In this work, CuNi bimetallic alloys representing (100) surface facets are annealed in vacuum using atomistic simulations to observe the effect of vacancy diffusion on surface separation. We employ multi-timescale methods to sample the early transient, intermediate, and equilibrium states of slab surfaces during the separation process, including standard MD as well as three methods to perform atomistic, long-time dynamics: parallel trajectory splicing (ParSplice), adaptive kinetic Monte Carlo (AKMC), and kinetic Monte Carlo (KMC). From nanosecond (ns) to second timescales, our multiscale computational methodology can observe rare stochastic events not typically seen with standard MD, closing the gap between computational and experimental timescales for surface segregation. Rapid diffusion of a vacancy to the slab is resolved by all four methods in tens of nanoseconds. Stochastic re-entry of vacancies into the subsurface, however, is only seen on the microsecond timescale in the two KMC methods. Kinetic vacancy trapping on the surface and its effect on the segregation rate are discussed. The equilibrium composition profile of CuNi after segregation during annealing is estimated to occur on a timescale of seconds as determined by KMC, a result directly comparable to nanoscale experiments.

The Impact of Nanoscale Percolation in Yttrium‐Doped BaZrO3 on the Oxygen Ion and Proton Conductivities: A Density Functional Theory and Kinetic Monte Carlo Study

Yttrium‐doped BaZrO3 is known for its high proton conductivity, making it an advanced energy material for various applications, like electrolyzers, fuel cells, or methane conversion cells. In the previous study, density functional theory (DFT) and kinetic Monte Carlo simulations (KMC) show that nanoscale percolation of Y ions enhances the proton mobility by providing fast migration pathways for protons. To investigate the general impact of nanoscale percolation on ionic conductivities, the same methods, DFT and KMC, are now used to calculate oxygen ion conductivity of Y‐doped BaZrO3. The results explain on a microscopic level why the macroscopic oxygen ion conductivity exhibits a higher activation energy than the proton conductivity, as known experimentally. They also show that nanoscale percolation pathways are not beneficial for oxygen ion conduction due to trapping of oxygen vacancies by single Y ions and blocking of their motion by nanoscale percolation of Y ions. Finally, the understanding and comparison of the microscopic jump processes of oxygen vacancies and of protons in acceptor‐doped BaZrO3 are used to propose a simple descriptor for the influence of a dopant on the ionic conductivities, of oxygen ions and of protons as well. This new descriptor allows an easy screening of various dopants.

Exploration of free volume behavior and ionic conductivity of PVA: x (x = 0, Y2O3, ZrO2, YSZ) ion-oxide conducting polymer ceramic composites

This article presents a comparative analysis of three distinct zirconia-inserted polyvinyl alcohol (PVA) polymer matrices. X-ray diffraction, Fourier transform infrared spectroscopy (FTIR), and atomic force microscopy (AFM) were used for the study of structural, chemical composition (functional groups), and morphological properties. Changes in IR bands revealed the interaction of nanofillers with the PVA matrix. ZrO2 added sample shows the lowest degree of crystallinity with the least free volume compared to pristine PVA. Threadlike chains and scattershot crystal morphologies were seen in AFM images, and the PVA/ZrO2 has the lowest level of roughness (1.27 nm). Positron annihilation lifetime profile showed identical behavior for τ3, τ2, and their intensities I3,I2. ZrO2 nanoparticles had a better interfacial interaction with the PVA matrix, which resulted in a higher Tg and shorter o-Ps lifetime (τ3). The positron trapping rate increased for ZrO2/PVA polymer ceramic composite (PCC), which indicated that both volume expansion and vacancy trapping effects occur when the positrons are effectively trapped at vacancies. The Nyquist plot for dielectric measurement was investigated, and an equivalent circuit model was used to evaluate variables such as carrier concentration (n), mobility (μ), and diffusion coefficient/ diffusivity (D). Ionic conductivity of 2.77 × 10−7 S cm−1 was recorded for the ZrO2/PVA sample. This might be due to a change (increase) in the carrier concentration.