Atomic Vacancies Research Articles

Defect dependent electronic properties of WTe2: a first-principles study

Abstract Tungsten ditelluride (WTe2) possesses fascinating electronic structures and exceptional properties that make it highly suitable for use in cutting-edge devices. Defects in WTe2 can have a significant influence on its properties, both in advantageous and disadvantageous ways. Thus, a precise classification is crucial to fully comprehend the potential impacts. Here we report a thorough investigation of the electronic characteristics of intrinsic defects, including point defects, in monolayer WTe2 using first-principles calculations based on density functional theory. Our research suggests that the presence of point defects can cause a notable shift in electronic properties, resulting in a metallic behaviour. This is due to the interesting phenomenon of Fermi-level changing near the band edges. Our research findings indicate that the energy required to form a vacancy in a Te atom is lower compared to that of a vacancy in a W atom. Based on the findings, it appears that Te atom vacancies are more likely to be generated during the synthesis process. Defects like the Te vacancy and Mo substitution in the pristine monolayer of WTe2 result in a subtle reduction in the band gap, while still maintaining its characteristics as a direct band gap semiconductor. Our study reveals that the electronic properties of monolayer WTe2 can be significantly altered by the presence of vacancy defects. This discovery highlights the exciting potential of WTe2 as a promising platform for various electronic applications. Our research is anticipated to have a beneficial impact on the comprehension and control of the characteristics of WTe2, thus expediting the development of nanomaterials in various fields.

Threefold Impact on Reaction Coordinate Mapping of Zirconium Based Tri‐Chalcogenide Pseudo‐Monolayers as Emerging Ultrathin Catalysts for Water Splitting

AbstractThe continuous quest of finding optimum catalysts for photo‐electrocatalytic (PEC) water splitting has been revolving around 2D ultrathin materials because of the fact of exploiting both the surface area. In this work, a threefold impact is envisaged route to enhance the activity both in terms of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in the emerging Zirconium Tri‐sulfides (ZrS3) based pseudo‐monolayers. Due to the unique and distinct electronic properties of the tri‐chalcogenide systems as compared to the di‐chalcogenide counterpart of Zirconium, one could afford to explore the implications of not only single‐atom functionalization and vacancy defect, but also the external strain to expedite the bifunctional catalytic activity in this promising material. Rigorous electronic structure calculations have been performed for the mentioned threefold effect on not only the band‐edge alignment, but going beyond to it by further constructing the reaction coordinate mapping corresponding to HER and OER mechanism. The repercussion of external strain in addition to the single atom functionalization and vacancy defect on the adsorption free energies of the prime intermediates of HER and OER reaction has been further correlated with the work function variation in this pseudo‐monolayer system.

Accelerated elimination of residual B2 phase in TiAl-based alloys by low-temperature annealing plus stretching

Low-temperature annealing plus stretching is an efficient method to accelerate the elimination of B2 phase in TiAl-based alloys. The stretching stress can produce radial compressive stress on atom and promote the formation of atomic vacancies, both of which promote the atom diffusion, thus accelerating the elimination of B2 phase.

Effect of Atomic Vacancy Defect in h – BN Monolayer on Electronic Properties: A DFT Study

Abstract h-BN is a 2D material which also has considerable development potential in several fields. 2D materials such as h-BN are basically insulating materials but have unique properties and it would be good if they could be developed, especially to meet future energy needs or in the development of electronic devices. Therefore, the properties of h-BN are certainly needed to support this, one of which is the magnetic properties of h-BN. In this research, we modified the h-BN structure by providing defects in the form of vacancy defects by removing atoms in the h-BN layer. We modeled a monolayer h-BN with a supercell size of 4 x 4 x 1, and our calculations were carried out with Quantum ESPRESSO, which is based on density functional theory. The results we obtained showed the emergence of magnetic moment values in h-BN due to defects in the form of atomic vacancies. Furthermore, this research successfully illustrates how vacancy defects in h-BN can modify the band structure, leading to changes in electron behavior around the Fermi energy level.

Defects-Rich Heterostructures Trigger Strong Polarization Coupling in Sulfides/Carbon Composites with Robust Electromagnetic Wave Absorption

Defects-rich heterointerfaces integrated with adjustable crystalline phases and atom vacancies, as well as veiled dielectric-responsive character, are instrumental in electromagnetic dissipation. Conventional methods, however, constrain their delicate constructions. Herein, an innovative alternative is proposed: carrageenan-assistant cations-regulated (CACR) strategy, which induces a series of sulfides nanoparticles rooted in situ on the surface of carbon matrix. This unique configuration originates from strategic vacancy formation energy of sulfides and strong sulfides-carbon support interaction, benefiting the delicate construction of defects-rich heterostructures in MxSy/carbon composites (M-CAs). Impressively, these generated sulfur vacancies are firstly found to strengthen electron accumulation/consumption ability at heterointerfaces and, simultaneously, induct local asymmetry of electronic structure to evoke large dipole moment, ultimately leading to polarization coupling, i.e., defect-type interfacial polarization. Such “Janus effect” (Janus effect means versatility, as in the Greek two-headed Janus) of interfacial sulfur vacancies is intuitively confirmed by both theoretical and experimental investigations for the first time. Consequently, the sulfur vacancies-rich heterostructured Co/Ni-CAs displays broad absorption bandwidth of 6.76 GHz at only 1.8 mm, compared to sulfur vacancies-free CAs without any dielectric response. Harnessing defects-rich heterostructures, this one-pot CACR strategy may steer the design and development of advanced nanomaterials, boosting functionality across diverse application domains beyond electromagnetic response.

Defects passivation in chloride-iodide perovskite solar cell with chlorobenzylammonium halides

Despite promising optoelectronic properties, it is a matter of fact that ion migration is unavoidable in chloride-iodide-based perovskite solar cells due to a radius mismatch between chlorine and iodine. Local defects like atomic vacancies or accumulation of atoms might occur due to ion migration in chloride-iodide-based perovskite thin film. Again, atomic vacancies are prevalent in solution-processed perovskite thin film. Here we have investigated the passivation of these defects with two organic halide passivators named 4-chlorobenzylammonium chloride and 4-chlorobenzylammonium bromide. They passivate both the surface and bulk of the perovskite thin film. The surface of the perovskite thin film is passivated with the bulky organic benzylammonium cations. The bulk of the perovskite thin film is passivated with the diffusion of chlorine or bromine. With different combinations of the two passivators, we vary the chlorine range from 50% to 100% and the bromine range from 25% to 50%. We have found the champion cell performance for 75% chlorine and 25% bromine combination. The enhancement of device efficiency was around 15% compared to the control device and the device stability was also considerably enhanced.

Endowing 1T'-ReS2 Nanosheets with Sonopiezoelectric Property by Theoretical-Guided Vacancy-Manipulated Peierls Distortion for Tumor Ferroptosis Therapy.

Sonopiezoelectric therapy harnesses piezoelectric materials to efficiently generate destructive reactive oxygen species when exposed to ultrasound. This innovative approach shows promise for tumor treatment by combining precise targeting of tumor sites through noninvasive ultrasound control with high reactive oxygen species generation capabilities via the piezoelectric effect. This study utilizes a theoretical-guided method to manipulate atomic vacancy defects and regulate the Peierls distortion in 1T'-ReS2 nanosheets, thereby imparting them with sonopiezoelectric properties not inherent to the original material. Furthermore, the plentiful unsaturated sites of ReS2 nanosheets endow them with excellent catalase- and peroxidase-mimicking activities. The reactive oxygen species generation by the engineered ReS2 nanosheets also leads to the depletion of glutathione. These capabilities are leveraged for tumor ferroptosis therapy via the classical pathway involving the 7-member 11-glutathione-GPX4 signaling axis, alongside the downregulation of dihydroorotate dehydrogenase and ferritin levels and the upregulation of fatty acid CoA ligase 4 expression. This showcases the innovative approach and potential applications of employing 1T'-ReS2 nanosheets in cancer treatment through theoretical design and materials engineering.

Atomic vacancies of molybdenum disulfide nanoparticles stimulate mitochondrial biogenesis

Diminished mitochondrial function underlies many rare inborn errors of energy metabolism and contributes to more common age-associated metabolic and neurodegenerative disorders. Thus, boosting mitochondrial biogenesis has been proposed as a potential therapeutic approach for these diseases; however, currently we have a limited arsenal of compounds that can stimulate mitochondrial function. In this study, we designed molybdenum disulfide (MoS2) nanoflowers with predefined atomic vacancies that are fabricated by self-assembly of individual two-dimensional MoS2 nanosheets. Treatment of mammalian cells with MoS2 nanoflowers increased mitochondrial biogenesis by induction of PGC-1α and TFAM, which resulted in increased mitochondrial DNA copy number, enhanced expression of nuclear and mitochondrial-DNA encoded genes, and increased levels of mitochondrial respiratory chain proteins. Consistent with increased mitochondrial biogenesis, treatment with MoS2 nanoflowers enhanced mitochondrial respiratory capacity and adenosine triphosphate production in multiple mammalian cell types. Taken together, this study reveals that predefined atomic vacancies in MoS2 nanoflowers stimulate mitochondrial function by upregulating the expression of genes required for mitochondrial biogenesis.

Sub-Boltzmann Switching, Hysteresis-Free Charge Modulated Negative Differential Resistance FinFET.

The ever-increasing power consumption in integrated circuits has raised concerns about the relentless doubling of transistor density in chips and cost drop per combinational/sequential circuits. To address the physical limit of thermionic emission carrier transport (i.e., subthreshold swing >60 mV/decade at 300 K), alternative charge-transport mechanisms or the implementation of functional substances have been attempted but without appreciable success. One such choice is to take advantage of negative differential resistance with the activation of localized electrons or migration of atom and oxygen vacancies to extend the capabilities of Si-transistors. However, inconsistency in current during forward/reverse bias sweep is confronted as a notable weak point. This work proposes an eye-catching solution to modulating potential distribution between a resistance switching layer and a transistor by employing charge trapping within a hafnium zirconium oxide layer. This approach introduces features advancing the potential of "More Moore" technologies.

In Situ Growth of Suspended Zirconene Islets Inside Graphene Pores

AbstractExperiments using a transmission electron microscope decomposed zirconium acetylacetonate with an electron beam, forming zirconium nanoparticles on graphene. Continued electron irradiation transformed these nanoparticles into atomically thick zirconium islets (zirconene islets) within the graphene lattice. The electron beam caused zirconium atom dislocations and vacancies that are rapidly refilled, a process repeating until the vacancies evolved into zirconium nanoribbons before breaking. This study offers insights into the electron‐driven growth and degradation of zirconene islets, showcasing a method to fabricate freestanding zirconenes for use as atomically thin coatings in extreme environments.

Low-Defect-Density Monolayer MoS2 Wafer by Oxygen-Assisted Growth-Repair Strategy.

Atomic chalcogen vacancy is the most commonly observed defect category in two dimensional (2D) transition-metal dichalcogenides, which can be detrimental to the intrinsic properties and device performance. Here a low-defect density, high-uniform, wafer-scale single crystal epitaxial technology by in situ oxygen-incorporated "growth-repair" strategy is reported. For the first time, the oxygen-repairing efficiency on MoS2 monolayers at atomic scale is quantitatively evaluated. The sulfur defect density is greatly reduced from (2.71 ± 0.65) × 1013 down to (4.28 ± 0.27) × 1012 cm-2, which is one order of magnitude lower than reported as-grown MoS2. Such prominent defect deduction is owing to the kinetically more favorable configuration of oxygen substitution and an increase in sulfur vacancy formation energy around oxygen-incorporated sites by the first-principle calculations. Furthermore, the sulfur vacancies induced donor defect states is largely eliminated confirmed by quenched defect-related emission. The devices exhibit improved carrier mobility by more than three times up to 65.2 cm2 V-1 s-1 and lower Schottky barrier height reduced by half (less than 20 meV), originating from the suppressed Fermi-level pinning effect from disorder-induced gap state. The work provides aneffective route toward engineering the intrinsic defect density and electronic states through modulating synthesis kinetics of 2D materials.

Investigation of the H2 dissociation and strengthening mechanism in vacancy-induced graphene

Atomic vacancies play a crucial role in hydrogen storage and local strengthening of a substance with a mechanism that is elusive. First-principles examinations of monolayer graphene unveiled the following: (i) the neighboring carbon atoms move radially away from the vacancy center with a 3–10% C–C bond contraction, generating a circumferential density ribbon of energy and electrons; (ii) the dense electrons within the ribbon polarize the edge atoms into dipoles pointing to the center; (iii) the polarized electrons coupled together to for the vacancy dipole, resulting in the Dirac-Fermion resonant peak at Fermi energy and the energy density gai raise the local strength; and (iv) the vacancy dipole induces the adsorbed H2 molecule into H+-H- dipole to stabilize H2 adsorption. The findings of vacancy reinforcement, dipole formation, and stable H2 adsorption should provide a promising mechanism contributing to the advancement of hydrogen storage theory.

Audio Signal-Stimulated Multilayered HfOx/TiOy Spiking Neuron Network for Neuromorphic Computing.

As the key hardware of a brain-like chip based on a spiking neuron network (SNN), memristor has attracted more attention due to its similarity with biological neurons and synapses to deal with the audio signal. However, designing stable artificial neurons and synapse devices with a controllable switching pathway to form a hardware network is a challenge. For the first time, we report that artificial neurons and synapses based on multilayered HfOx/TiOy memristor crossbar arrays can be used for the SNN training of audio signals, which display the tunable threshold switching and memory switching characteristics. It is found that tunable volatile and nonvolatile switching from the multilayered HfOx/TiOy memristor is induced by the size-controlled atomic oxygen vacancy pathway, which depends on the atomic sublayer in the multilayered structure. The successful emulation of the biological neuron's integrate-and-fire function can be achieved through the utilization of the tunable threshold switching characteristic. Based on the stable performance of the multilayered HfOx/TiOy neuron and synapse, we constructed a hardware SNN architecture for processing audio signals, which provides a base for the recognition of audio signals through the function of integration and firing. Our design of an atomic conductive pathway by using a multilayered TiOy/HfOx memristor supplies a new method for the construction of an artificial neuron and synapse in the same matrix, which can reduce the cost of integration in an AI chip. The implementation of synaptic functionalities by the hardware of SNNs paves the way for novel neuromorphic computing paradigms in the AI era.

Sulfur vacancy riched pure 2H phase VS2 for efficient electrocatalytic hydrogen evolution

Vanadium disulfide (VS2), a typical metallic layered transition metal chalcogenide (TMC), has been widely concerned for its high electrical conductivity and reactivity in electrochemical energy storage and conversion applications. The 2H phase VS2 is expected to exhibit high electrocatalytic activity and be more stable for hydrogen evolution reaction (HER) than the 1T phase structure. But at present, there still lacks a facile method to prepare pure 2H phase VS2. Herein, we successfully prepared pure 2H–VS2 through a simple one-step low-temperature hydrothermal method. We have investigated the effect of the hydrothermal reaction conditions (including the proportion of raw materials and hydrothermal reaction temperatures) on the phase composition and microstructure. Under the optimal condition of 140°C-24 h with a vanadium sulfur ratio of 1:5, a nano-flower-like 2H–VS2 material with high crystallinity and rich sulfur vacancy defects was obtained. Leveraging these structural advantages, as a demonstration, the product exhibits excellent electrocatalytic HER activity (with η10 of 181 mV, Tafel slope of 52 mV dec−1, and J0 of 67.9 × 10−3 mA cm−2) and stability (it can continuously produce hydrogen for 20 h at a current density of 50 mA cm−2 with ultra-small increased potential of less than 5 mV). This work provides a new way for constructing atomic vacancy defects in layered TMCs for efficient electrocatalysis and is also expected to promote the research of the basic properties of high phase purity TMCs.

Dual sulfur vacancy pairs functionalized SnS2 for efficient photocatalytic molecular oxygen activation

Defect engineering is considered an effective means to improve the photocatalytic performance of semiconductors. However, current research on defects mainly focuses on single atom vacancies, lacking research on atomic vacancy pairs. In this work, we used SnS2 as the research model and constructed SnS2 nanostructures with rich S vacancies and dual S vacancy pairs, respectively. By comparing the photocatalytic activation O2 performance of SnS2 under different defect effects, it was found that dual S vacancy pairs have better performance optimization characteristics than S vacancies. Combining experiments and theoretical calculations, it is confirmed that dual S vacancy pairs have significant advantages over S vacancies in regulating the band structure, improving the separation efficiency of photogenerated charges and the migration rate of charge carriers, as well as activating the interface reaction of O2. In addition, when the catalyst is used for the removal of organic matter in water, it can be found that the optimization effect of dual defects on performance has good sustainability. This work contributes to a deeper understanding of the application scope of defect engineering and provides new ideas for the development of highly active photocatalysts.

(Invited) Defect Photoluminescence Generation in Boron Nitride Nanotubes by Chemical Functionalization Approaches

Boron nitride nanotubes (BNNTs) are structural analogues of carbon nanotubes (CNTs). They have nanocylindrical structures composed of rolled-up hexagonal boron nitride (h-BN) sheets. The BN framework consists of alternately connecting boron and nitrogen atoms with sp2 hybridization. Therein, a strong covalent bond with an ionic character is formed due to their electronegativity difference. Accordingly, BNNTs exhibit unique properties such as high mechanical strength, thermal stability, and a wide bandgap semiconducting feature [1]. The large bandgap (ca. 6 eV) is less dependent on tube diameters and chiralities, and optical absorption and emission for pristine tubes occur in deep UV regions (around 200 nm) based on the band edge transition. In typical BNNT samples, however, atomic vacancies in the BN framework are reported to provide longer wavelength photoluminescence (PL) in UV–vis regions [2].To date, chemical functionalization of single-walled CNTs (SWCNTs) have been developed to create luminescent defects emitting red-shifted and bright near-infrared PL [3-6]. In this study, we use the chemical functionalization method to produce luminescent defects in BNNTs [7].A reductive alkylation reaction for BNNTs was conducted using sodium naphthalenide and 1-bromohexane to synthesize hexylated BNNTs (h-BNNTs). In a PL spectrum of h-BNNTs, PL peaks appeared at 334 and 413 nm, which were not observed for unmodified BNNTs. X-ray photoelectron spectroscopy revealed that the hexyl group was covalently attached on the boron site of BNNTs, by which sp3 hybridized boron atoms were partly formed in the BN network as defects. The covalent attachment of the hexyl group was also confirmed by atomic-resolution transmission electron microscope observations. Chemical reaction conditions were found to be a key factor for the emission wavelength variations of the resultant defect PL. Therefore, it is revealed that the luminescent defect formation in BNNTs is achieved based on the chemical functionalization approach. The functionalized BNNTs would be applicable to advanced optical applications such as quantum light sources.[1] D. Golberg et al. ACS Nano,4, 2979 (2010). [2] L. Museur et al. J. Appl. Phys., 118, 084305 (2015). [3] T. Shiraki et al. Acc. Chem. Res., 53, 1846 (2020). [4] B. Gifford et al. Acc. Chem. Res., 53, 1791 (2020). [5] A. H. Brozena et al. Nat. Rev. Chem. 3, 375 (2019). [6] J. Zaumseil. Adv. Opt. Mater. 10, 2101576 (2022). [7] T. Shiraki et al. Chem. Lett. 52, 44 (2023). Figure 1

Engineering the electronic properties of MoTe2 via defect control

ABSTRACT The remarkable electronic properties of monolayer MoTe2 make it a very adaptable material for use in optoelectronic and nano-electronic applications. MoTe2 growth often exhibits intrinsic defects, which significantly influence the material’s characteristics. In this work, we conducted a thorough investigation of the electronic characteristics of intrinsic defects, including point defects, in monolayer MoTe2 using first-principles calculations based on density functional theory (DFT). Our findings indicate that the presence of point defects leads to the formation of n-type properties as the Fermi level situates above the conduction band. Our first-principles density functional theory calculation revealed an appearance of donor level in the band gap close to the conduction band in MoTe2. Our study signifies that the formation energy of a vacancy in a Te atom is lower than that of both a vacancy in a Mo atom and two vacancies in Te atom. This suggests that during the synthesis process, it is more probable for Te atom vacancies to be created. A defect in the pristine monolayer of MoTe2 leads to a slight decrease in the band gap, causing a transition from a direct band gap semiconductor to an indirect band gap semiconductor. The results of our study indicate that the presence of vacancy defects may modify the electronic properties of monolayer MoTe2, suggesting its potential as a new platform for electronic applications. Hence, our analysis offers significant theoretical backing for defect engineering in MoTe2 monolayers and other 2D materials, a critical aspect in the advancement of nanoscale devices with the desired functionality.

Multi‐Point Collaborative Passivation of Surface Defects for Efficient and Stable Perovskite Solar Cells

AbstractThe inherent defects (lead iodide inversion and iodine vacancy) in perovskites cause non‐radiative recombination and there is also ion migration, decreasing the efficiency and stability of perovskite devices. Eliminating these inherent defects is critical for achieving high‐efficiency perovskite solar cells. Herein, an organic molecule with multiple active sites (4,7‐bromo‐5,6‐fluoro‐2,1,3‐phenylpropyl thiadiazole, M4) is introduced to modify the upper interface of perovskites. When M4 interacts with the perovskite surface, the active bromine (Br) site interacts with lead (Pb) at the surface to repair iodine atomic vacancy defects. The fluorine (F) site of M4 interacts with Pb to correct octahedral crystal lattice distortions and eliminate PbI defects. Additionally, sulfur–iodine (S–I) interactions reduce I–I dimerization and eliminate IPb defects. It is also calculated that the energy level of M4 aligns with the band gap, promoting charge transfer. As a result, the perovskite devices achieve an efficiency of 25.1%, a stabilized power output (SPO) of 25.0%, a voltage of 1.19 V, and a fill factor of 85.2%. The device retains 95% of its initial efficiency after 2000 h of ageing in a nitrogen atmosphere. Thus, multi‐point cooperative passivation of surface defects provides an effective method to improve the efficiency and stability of perovskite solar cells.

Effect of Surface Electron Trapping and Small Polaron Formation on the Photocatalytic Efficiency of Copper(I) and Copper(II) Oxides.

Cu2O, CuO, and mixed phase Cu2O/CuO represent promising candidates for photoelectrochemical H2 evolution due to their strong visible light absorption, earth-abundance, and chemical stability. However, the photoelectrochemical efficiency in these materials remains far below the theoretical limit, largely due to poorly understood surface electron dynamics. These dynamics depend on defect states, such as Cu atom vacancies and phase boundaries, which control electron trapping, charge carrier separation, and recombination. In this work, we study the photoinduced electron and hole dynamics at the surface of various Cu oxides using ultrafast extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy. In Cu2O we find that photoexcitation occurs as electron promotion from primarily Cu 3d valence band to Cu 4s conduction band states compared to O 2p valence band to Cu 4s conduction band states in CuO. In catalysts with a significant concentration of Cu vacancies, we observe fast electron trapping to the Cu 3d defect band occurring in less than 100 fs. In contrast, photoexcited electrons in phase pure CuO do not trap to midgap states; rather these electrons form small polarons within approximately 500 fs. Photoelectrochemical measurements of these catalysts show that Cu vacancy-mediated electron trapping correlates with a significant loss of photocurrent. Together, these results provide a detailed picture of the defect states and associated ultrafast carrier dynamics that govern the photocatalytic efficiency in widely studied Cu2O and CuO photocatalysts.

Effect of monovacancy defects on anisotropic mechanical behavior of monolayer graphene: A molecular dynamics study

The mechanical properties of monolayer graphene were studied using molecular dynamics (MD) simulation. The simulation results show that the tensile stress in the [010] crystal direction is greater than that in the [110] and [100] directions under uniaxial tensile. This confirms that graphene exhibits anisotropic behavior when subjected to external loading, resulting in changes in the Young's modulus in different directions. The calculated Young's modulus were 764.68 GPa, 532.50 GPa and 697.83 GPa for the [010], [110] and [100] directions, respectively. On this basis, the effect of defects on the mechanical properties was also considered. When there are vacancy defects, the stress concentration is linearly distributed along the Zigzag direction on both sides of the atomic vacancy defects. The effect of crack defects on the mechanical properties of graphene [010] crystal direction is the most significant. With the increase of crack size, the difference in fracture strength and fracture toughness calculated by MD simulation and the Griffith model decreases gradually. The potential energy of the non-defect model was found to be the highest, with the potential energy decreasing as the defect size increases. In addition, it was found that the strain rate also affects the stress-strain behavior of the monolayer graphene. However, when the crack size is large, the difference in the effect of strain rate on the anisotropic tensile fracture stress can be ignored.