High Defect Concentration Research Articles

Investigating the atomic structures and electronic properties of WS2 thin films with sulfur vacancies via a neural network potential-aided first-principles study

Transition metal dichalcogenides are promising materials for high-performance electronics, whereas the impact of defects on their electronic properties remains elusive. Here, we employ neural network potentials (NNPs) constructed from density functional theory (DFT) data to investigate defect-laden WS2 thin films. Molecular dynamics simulations reveal that at low defect concentrations (S/W ratio of 1.9), single sulfur vacancies are predominant. Conversely, at high defect concentrations (S/W ratio of 1.7), complex defects with short lifetimes appear. Additionally, DFT results indicate that the band gap persists at S/W = 1.9 but disappears at 1.7, aligning with observed device degradation at high defect concentrations.

Self-Etching Synthesis of Superhydrophilic Iron-Rich Defect Heterostructure-Integrated Catalyst with Fast Oxygen Evolution Kinetics for Large-Current Water Splitting.

Developing catalysts with rich metal defects, strong hydrophilicit, and extensive grain boundaries is crucial for enhancing the kinetics of electrocatalytic water oxidation and facilitating large-current water splitting. In this study, we utilized pH-controlled etching and gas-phase phosphating to synthesize a flower-like Ni2P-FeP4-Cu3P modified nickel foam heterostructure catalyst. This catalyst features pronounced hydrophilicity and a high concentration of Fe defects. It exhibits low overpotentials of 156 mV and 210 mV at current densities of 10 and 100 mA cm-2 respectively, and maintains stability for up to 200 h at 100 mA cm-2 with only 7.3% degradation, showcasing outstanding electrocatalytic water oxidation performance. Furthermore, when integrated into a Ni2P-FeP4-Cu3P/NF||Pt-C/NF electrolyzer, it achieves excellent overall water splitting performance, reaching current densities of 10 and 400 mA cm-2 at just 1.47 V and 1.73 V, respectively, and operates stably for 60 h at 500 mA cm-2 with minimal degradation. Analysis indicates that high-valence oxyhydroxides/phosphides of Ni, Fe, and Cu act as the primary active species. The presence of abundant Fe defects enhances electron transfer, strong hydrophilicity improves electrolyte contact, and numerous grain boundaries synergistically modulate the activation energy between active sites and oxygen-containing intermediates, significantly improving the kinetics of water oxidation.

Supercapacitor devices based on multiphase MgTiO3 perovskites doped with Mn2+ ions

Recently, perovskites have become a hotspot for researchers attempting to exploit metal and oxygen vacancies in structures of the form MTiO3, facilitating the convenient electron/hole migration, thus displaying interesting properties. Magnesium Titanate (MgTiO3) is a prominent part of the perovskite class, exhibiting remarkable electrical, thermal, and chemical properties. Undoped and Mn-doped MgTiO3 samples were obtained using a solid-state reaction starting from previously synthesized MgO and TiO2 powders, which were separately doped with different Mn ion concentrations. The resulting multiphase materials with a major MgTiO3 phase were thoroughly morpho-structurally analyzed employing XRD, STEM, Raman, PL, XPS, and EPR spectroscopy. The electrochemical results indicate that they show superior performance when used as electrode materials for supercapacitor application due to the high defect concentration as shown in EPR and PL spectroscopy and the ferroelectric behavior observed in XPS and XRD. When used in symmetric and asymmetric supercapacitor devices, they show promising results, with specific capacity values reaching up to 109 F/g for the symmetric and 609 F/g for the asymmetric devices, while energy and power density values reached 84.7 Wh/kg and 90.8 kW/kg respectively, proving a great potential in the energy storage field.

Fracture Mechanisms and Crack Propagation in Monolayer Ti3C2Tx under Nanoindentation: The Influence of Surface Terminations and Vacancy Defects.

Monolayer MXenes are a novel class of two-dimensional transition metal carbides/nitrides with fascinating physicochemical properties. Despite recent advances in the study of MXenes' mechanical properties, a comprehensive understanding of the fundamental physical mechanisms that affect fracture due to surface terminations and vacancy defects in MXenes under nanoindentation remains largely unexplored. Here, we address this gap using molecular dynamics simulations and nanoindentation theory to investigate the effects of surface terminations and vacancy defects on the fracture behavior of Ti3C2Tx MXenes. By inducing the rupture of monolayer MXenes through nanoindentation, we find that bare Ti3C2 exhibits brittle fracture behavior. The presence of surface terminations and vacancy defects reduces the load-carrying capacity and flexibility of MXenes. Interestingly, surface terminations increase the stiffness of the structure, while vacancy defects have the opposite effect. We also find that high concentrations of surface oxidation impart ductile fracture characteristics to MXenes and increase the maximum crack length at failure. Additionally, defects exceeding the critical concentration can effectively prevent brittle crack propagation by causing frequent crack deflection and blunting crack tips. Combining these findings, we propose a new strategy to synergistically enhance the fracture toughness of MXenes through high concentrations of surface oxidation and vacancy defects exceeding the critical concentration without significantly affecting strength and stiffness, thereby avoiding catastrophic failure in MXene monolayers due to brittle fracture. This work provides fundamental insights into the mechanical properties and fracture mechanisms of monolayer MXenes.

Enhancing charge extraction in BiVO4 photoanodes by ZrCl4 treatment of SnO2 hole-blocking layers

In the search for more efficient and sustainable photoelectrochemical devices, BiVO4 is nowadays one of the best-performing photoanode material, with favourable band structure for water oxidation. However, BiVO4 photoanodes face challenges such as poor charge transport and slow kinetics. To address these issues, SnO2 films are commonly used as hole blocking layers, reducing recombination rate and enhancing charge lifespan and overall productivity. Yet, this method encounters problems like high defect concentrations at the SnO2/BiVO4 interface and pinholes in the SnO2 layer, which lead to charge recombination. In this study, we explore a ZrCl4 treatment to improve the effectiveness of SnO2 as a hole-blocking layer in BiVO4 photoanodes. Our findings, supported by detailed optoelectronic characterization and continuous and modulated electrochemical analysis, reveal that ZrCl4 treatment significantly enhances the hole-blocking properties of SnO2. This treatment results in a 37 % increase in photocurrent density at 1.23 VRHE and a 40 mV shift in the onset voltage, demonstrating a substantial improvement in overall photoanode efficiency.

Deepening the Understanding of Carbon Active Sites for ORR Using Electrochemical and Spectrochemical Techniques.

Defect-containing carbon nanotube materials were prepared by subjecting two commercial multiwalled carbon nanotubes (MWCNTs) of different purities to purification (HCl) and oxidative conditions (HNO3) and further heat treatment to remove surface oxygen groups. The as-prepared carbon materials were physicochemically characterized to observe changes in their properties after the different treatments. TEM microscopy shows morphological modifications in the MWCNTs after the treatments such as broken walls and carbon defects including topological defects. This leads to both higher surface areas and active sites. The carbon defects were analysed by Raman spectroscopy, but the active surface area (ASA) and the electrochemical active surface area (EASA) values showed that not all the defects are equally active for oxygen reduction reactions (ORRs). This suggests the importance of calculating either ASA or EASA in carbon materials with different structures to determine the activity of these defects. The as-prepared defect-containing multiwalled carbon nanotubes exhibit good catalytic performance due to the formation of carbon defects active for ORR such as edge sites and topological defects. Moreover, they exhibit good stability and methanol tolerances. The as-prepared MWCNTs sample with the highest purity is a promising defective carbon material for ORR because its activity is only related to high concentrations of active carbon defects including edge sites and topological defects.

Fabrication of nitrogen-hyperdoped silicon by high-pressure gas immersion excimer laser doping

In recent years, research on hyperdoped semiconductors has accelerated, displaying dopant concentrations far exceeding solubility limits to surpass the limitations of conventionally doped materials. Nitrogen defects in silicon have been extensively investigated for their unique characteristics compared to other pnictogen dopants. However, previous practical investigations have encountered challenges in achieving high nitrogen defect concentrations due to the low solubility and diffusivity of nitrogen in silicon, and the necessary non-equilibrium techniques, such as ion implantation, resulting in crystal damage and amorphisation. In this study, we present a single-step technique called high-pressure gas immersion excimer laser doping (HP-GIELD) to manufacture nitrogen-hyperdoped silicon. Our approach offers ultrafast processing, scalability, high control, and reproducibility. Employing HP-GIELD, we achieved nitrogen concentrations exceeding 6 at% (3.01 × 1021 at/cm3) in intrinsic silicon. Notably, nitrogen concentration remained above the liquid solubility limit to ~1 µm in depth. HP-GIELD’s high-pressure environment effectively suppressed physical surface damage and the generation of silicon dangling bonds, while the well-known effects of pulsed laser annealing (PLA) preserved crystallinity. Additionally, we conducted a theoretical analysis of light-matter interactions and thermal effects governing nitrogen diffusion during HP-GIELD, which provided insights into the doping mechanism. Leveraging excimer lasers, our method is well-suited for integration into high-volume semiconductor manufacturing, particularly front-end-of-line processes.

(Invited) Bulk Traps in Layered 2D Gate Dielectrics

Ultra-thin ferromagnets, when coupled with magnetoelectric or multiferroic materials, could potentially enable highly energy-efficient electric field control of the magnet for use in nanoelectronic memories. Substitutional doping of magnetic impurities in monolayer transition metal dichalcogenides (TMDs) may be a promising way to create 2D ferromagnets but, according to theoretical calculations, require high doping levels (10-20 %) to achieve above room temperature (RT) Curie temperature. Room-temperature ferromagnetism has been reported for very low doping levels (0.1-1%), in conflict with the theoretical calculations, and always in the presence of high concentrations of defects, making it unclear if the dopants alone are responsible for this ferromagnetism. In this work, we use a combination of molecular beam epitaxy (MBE), plan-view TEM, XRD, XPS, Raman, and magnetometry to study iron and vanadium doping in monolayer WSe2. We show that optimally grown monolayers with up to 35% doping and low defect density show no ferromagnetism. Interestingly, ferromagnetism is observed when these monolayers contain a significant amount of selenium vacancies (Sevac), intentionally created via a post-growth heat treatment process, and magnetism is seen to scale with heating time/vacancy concentration. Moreover, even undoped WSe2 shows similar ferromagnetism for Sevac > 1014 cm-2. This ferromagnetism can later be quenched by filling these vacancies via Se annealing. For films with low Se vacancy concentration, TEM analysis reveals significant clustering of the V and Fe dopants which likely suppresses the ferromagnetism predicted by theory - a problem that has previously plagued III-V based dilute magnetic semiconductors as well. We go so far as to claim that all of the above room-temperature ferromagnetism reports in the literature thus far are due to Se vacancies and not magnetic doping.

Insight into the configuration of hard-soft carbon composites for high performance sodium-ion batteries

Hard carbon materials are considered suitable anode materials for sodium-ion batteries due to their abundant defect sites and large interlayer distance, which are respectively beneficial to the adsorption/desorption and insertion/extraction behaviors of Na+ during the electrochemical process. However, the poor electrical conductivity resulting from the intrinsic short-range ordered structure in the carbon skeleton and limited active sites significantly hinders their electrochemical performance. Herein, hard and soft carbon composites with various configurations are developed using a layer-by-layer coating method with SiO2 nanospheres as templates. These composites, including the hard carbon coated soft carbon composite structure (SC@HC), soft carbon coated hard carbon composite structure (HC@SC), and uniformly mixed soft and hard carbon composites (HSC), are prepared through confined-carbonization under the outermost SiO2 layer. The resulting hollow nanospheres possess ultra-small diameters (approximately 50 nm), ultra-thin wall thicknesses (5-6 nm), and different hard/soft carbon layer arrangements. Evaluation of sodium storage properties showed that the combination of soft and hard carbon outperformed pure soft or hard carbon, with HC@SC configuration offering materials with larger interlayer spacing, higher defect concentration, and a more complete conductive network. This enhances sodium storage capacity (293 mAh g-1@1 A g-1) and rate performance (242 mAh g-1@5 A g-1). The findings of this study present a novel approach to designing soft and hard carbon composites for high-performance sodium-ion batteries.

DFT study of the role and possible contribution of defects to lithium metasilicate [formula omitted] luminescence

Lithium metasilicate (Li2SiO3) has recently been shown to display outstanding stimulated luminescence properties, which are governed by defects and/or impurities. Here, the possible contribution and character of native point and complex defects to stimulated luminescence of lithium metasilicate have been investigated employing calculations based on the density functional theory (DFT). Under high concentrations of acceptor defects, isolated lithium interstitials were found to be the defects with the lowest formation energy (0.41 eV) but it is shown that they cannot contribute to luminescence. On the other hand, Oi−2 are the most favorable and stable defects under high concentrations of donor defects. F-centers and oxygen peroxides were identified as first- and second-order processes of point defects, which would explain experimental observations in the literature. Bridging oxygen vacancies were shown to introduce shallow electron traps able to trap up to four electrons. On the contrary, non-bridging oxygen vacancies lead to the formation of deep color centers with the capability of trapping up to two carriers/center, which could be the final state for the radiative process of luminescence. These results provide guidance for the interpretation of experimental measurements of luminescence.

MoS2-based polarization-sensitive photodetectors with asymmetric plasmonic structures and decreased detection time

This study analyzes the response time and sensitivity of photosensors that use 2D MoS2 semiconductor films with a high concentration of defects resulting from technological process sequences. In the developed photosensors, we introduce asymmetric plasmonic gratings providing their sensitivity enhancement as well as light polarization sensitivity. The photodetectors, as initially fabricated, exhibit an exceptionally long response time, surpassing a duration of a thousand seconds. This prolonged response time significantly impedes the practical application of these detectors. Nevertheless, the implementation of a specifically designed algorithm enables a dramatic reduction in detection time, by several orders of magnitude in comparison to the response time. It is based on a mathematical model that accounts for the dynamic features of these devices. Our study demonstrates that photodetectors, when enhanced with asymmetric plasmonic gratings, exhibit significantly improved performance metrics. These include a photosensitivity of approximately 60 mA/W and a degree of linear polarization at 0.78. Furthermore, the implementation of our proposed algorithm, which optimizes the calculation of detection time, dramatically reduces the characteristic detection time from over a thousand seconds to just around 10 s.

Improvement of the Thermoelectric Properties of p-Type Mg3Sb2 by Mg-Site Double Substitution.

A P-type Mg3Sb2-based Zintl phase compound has been considered a promising candidate for thermoelectric applications. Alloying, which introduces a high concentration of point defects, is particularly effective in scattering phonons and reducing lattice thermal conductivity. Herein, alloying in p-type Mg2.995Na0.005Sb2 via the introduction of elements like Yb, Eu, Ca, and Ba was realized, and the room-temperature lattice thermal conductivity has been effectively reduced to ∼1.1 W m-1 K-1. To further intensify the phonon scattering, two groups of elements (Eu and Cd, and Yb and Cd) were chosen for heavy alloying at the Mg site, and the lattice thermal conductivity of Mg1.49Eu0.5Cd1Na0.01Sb2 was further reduced to ∼0.45 W m-1 K-1. Eventually, a peak zT as high as ∼1.0 was achieved at 773 K, and the compound outperforms the previously reported p-type Mg3Sb2 compounds.

Super-Assembled Multilayered Mesoporous TiO2 Nanorockets for Light-Powered Space-Confined Microfluidic Catalysis.

In the field of sustainable chemistry, it is still a significant challenge to realize efficient light-powered space-confined catalysis and propulsion due to the limited solar absorption efficiency and the low mass and heat transfer efficiency. Here, novel semiconductor TiO2 nanorockets with asymmetric, hollow, mesoporous, and double-layer structures are successfully constructed through a facile interfacial superassembly strategy. The high concentration of defects and unique topological features improve light scattering and reduce the distance for charge migration and directed charge separation, resulting in enhanced light harvesting in the confined nanospace and resulting in enhanced catalysis and self-propulsion. The movement velocity of double-layered nanorockets can reach up to 10.5 μm s-1 under visible light, which is approximately 57 and 119% higher than that of asymmetric single-layered TiO2 and isotropic hollow TiO2 nanospheres, respectively. In addition, the double-layered nanorockets improve the degradation rate of the common pollutant methylene blue under sustainable visible light with a 247% rise of first-order rate constant compared to isotropic hollow TiO2 nanospheres. Furthermore, FEA simulations reveal and confirm the double-layered confined-space enhanced catalysis and self-propulsion mechanism.

Suppressing Deep-Level Trap Toward Over 13% Efficient Solution-Processed Kesterite Solar Cell.

Cu2ZnSn (S,Se)4 (CZTSSe), a promising absorption material for thin-film solar cells, still falls short of reaching the balance limit efficiency due to the presence of various defects and high defect concentration in the thin film. During the high-temperature selenization process of CZTSSe, the diffusion of various elements and chemical reactions significantly influence defect formation. In this study, a NaOH-Se intermediate layer introduced at the back interface can optimize Cu2ZnSnS4 (CZTS)precursor films and subsequently adjust the Se and alkali metal content to favor grain growth during selenization. Through this back interface engineering, issues such as non-uniform grain arrangement on the surface, voids in bulk regions, and poor contact at the back interface of absorber layers are effectively addressed. This method not only optimizes morphology but also suppresses deep-level defect formation, thereby promoting carrier transport at both interfaces and bulk regions of the absorber layer. Consequently, CZTSSe devices with a NaOH-Se intermediate layer improved fill factor, open-circuit voltage, and efficiency by 13.3%. This work initiates from precursor thin films via back interface engineering to fabricate high-quality absorber layers while advancing the understanding regarding the role played by intermediate layers at the back interface of kesterite solar cells.

New Insight into the Electronic and Magnetic Properties of Sub-Stoichiometric WO3: A Theoretical Perspective

We present a theoretical investigation on the wide-band-gap semiconductor WO 3 in its room-temperature monoclinic structure. We carried out density functional theory and GGA-1/2 calculations on the bulk phase and the most stable (001) surface of the material, either in their stoichiometric form or in the presence of oxygen vacancies at various concentrations. Concerning the bulk phase, our results show how the inclusion of these defects correctly reproduces the intrinsic n-type doping of the material. The system is also found to be magnetic at reasonably high defect concentrations. As for the surface, the presence of vacancies gives rise to a magnetic behavior, whose features depend on the relative arrangement of native point defects. Oxygen vacancies are also responsible for additional tungsten oxidation states in both bulk and surface. Based on these results, we provide a rationale for the interpretation of most experimental data of this material and, possibly, other widespread transition metal oxides with similar properties and applications such as ReO 3, TiO 2, and SnO 2.

Qualitative Model of Electrical Conductivity of Irradiated Semiconductor

There is constructed a qualitative model of the electrical conductivity of semiconductors irradiated with sufficiently high-energy particles. At certain conditions (irradiation temperature and dose, and subsequent thermal treatment), high-energy particles fluence, in addition to primary and secondary point radiation defects, forms a number of nano-sized disordered regions, highly conductive (“metallic”) compared to the semiconductor matrix. Their high total volume fraction can lead to the charge major carriers’ effective Hall mobility significantly exceeding that of the matrix. Due to elastic stresses created by these disordered inclusions, a high concentration of point radiation defects tends to form defective shells. In certain temperature ranges, such nanosized core-shell structures act as capacitors storing the electric charge sufficient for the Coulomb blockade of the major current carriers. Transformation of high-conductive inclusions into low-conductive (“dielectric”) ones manifests in a noticeable decrease in effective Hall mobility. The proposed model qualitatively explains all the experimental data available on single-crystalline n- and p-type silicon irradiated with high-energy electrons and protons and isochronously annealed.

Simulation of irradiation damage in CsPbI3-based perovskite solar cells by energetic protons

Perovskite solar cells are emerging as one of the most promising energy sources for space applications. However, the displacement damage of solar cells in the space environment has gradually deteriorated their performance. In this work, the depth distribution of primary knocked-on atoms (PKA), non-ionizing energy loss and defect concentration of CsPbI3-based perovskite solar cells irradiated by 0.1–2000 MeV protons have been simulated by the Geant4 toolkit. It is found that the PKA yield shows an obvious step behavior near the layer boundary of the solar cell, and that the electron and hole transport layers have the high defect concentration whereas the CsPbI3-based perovskite layer owns the least PKA yield. In particular, in the geostationary orbit, the radiation damage effect caused by space protons to the solar cells is approximately equivalent to that irradiated by 0.18 MeV protons, which provides a good reference for ground irradiation experiments for CsPbI3-based perovskite solar cells.

High-κ Wide-Gap Layered Dielectric for Two-Dimensional van der Waals Heterostructures.

van der Waals heterostructures of two-dimensional materials have unveiled frontiers in condensed matter physics, unlocking unexplored possibilities in electronic and photonic device applications. However, the investigation of wide-gap, high-κ layered dielectrics for devices based on van der Waals structures has been relatively limited. In this work, we demonstrate an easily reproducible synthesis method for the rare-earth oxyhalide LaOBr, and we exfoliate it as a 2D layered material with a measured static dielectric constant of 9 and a wide bandgap of 5.3 eV. Furthermore, our research demonstrates that LaOBr can be used as a high-κ dielectric in van der Waals field-effect transistors with high performance and low interface defect concentrations. Additionally, it proves to be an attractive choice for electrical gating in excitonic devices based on 2D materials. Our work demonstrates the versatile realization and functionality of 2D systems with wide-gap and high-κ van der Waals dielectric environments.

Nitrogen admixture effects on growth characteristics and properties of carbon nanowalls

The plasma functionalization of carbon nanowalls (CNWs) is of great interest due to their potential applications in electron field emission and energy storage devices. In this study, the effects of nitrogen on the growth and properties of CNWs are thoroughly investigated by varying a wide range of the N2 concentrations in the CH4/H2/N2 discharge plasma. Nitrogen-doped carbon nanowalls (NCNWs) were synthesized in a single-step growth using the hydrogen radical injection plasma-enhanced chemical vapor deposition technique. The formation of dense NCNWs was found to be enhanced by nitrogen and was three times higher in the sample with a 150 sccm N2 flow rate than in the sample without N2 addition (N-0). As the N2 concentration increased from 0 to 200 sccm, the morphology of CNWs changed from relatively straight to curly-like nanowall structures. A high-resolution optical emission spectroscopic observation shows that as N2 flow increased, CN radicals became the dominant species, suppressing the formation of C2 and CH. Photoelectron spectroscopy analysis revealed the nitrogen doping effect on graphene layers at high N2 concentrations. According to Raman and Transmission Electron Microscopy analyses, the NCNWs had more branched and thicker nanowalls with higher defect concentrations than the N-0 sample. The wettability of NCNWs was improved, particularly for the sample with a N2 flow rate of 150 sccm.

Microscale characterization of an anti-cracking stone base course filled with cement stabilized macadam

Large stone base course filled with cement stabilized macadam (LSFC) is a promising solution for addressing reflection cracks in semi-rigid base asphalt pavements due to its superior crack resistance. However, challenges associated with balancing strength and crack resistance, and mechanizing high-efficiency construction have hindered broader adoption. To address these issues, Wang et al. [1] proposed an optimized mix proportion design for an anti-cracking stone base course filled with cement stabilized macadam (SFC). In this study, various microscopic techniques were employed to explore the anti-cracking mechanism of SFC on a microscopic scale, and compared with the conventional cement stabilized macadam (CSM). These techniques included examining the structural morphology and pore distribution using polarizing microscope (PM) and nuclear magnetic resonance (NMR), analysing the elemental composition and hydration reaction in non-aggregate zones using energy dispersive spectroscopy (EDS), and testing the mechanical properties using atomic force microscope (AFM). The findings revealed, compared with CSM, SFC exhibits a more tightly interlocked coarse aggregate skeleton, which, in turns, resulting in weak interfacial transition zones (ITZs) with a high concentration of pores and defects, weak bonding, and a loose structure, potentially exceeding 20 µm in thickness. The tightly interlocked coarse aggregate skeleton in SFC effectively limits material shrinkage but also leads to localized high stress and weak interfacial damage within the vulnerable ITZ region, converting shrinkage strain energy into microcrack surface energy, thereby delaying the onset of cracking and ultimately enhancing the cracking resistance of the base.