Defect Concentration Research Articles

Performance optimization of eco-friendly CH3NH3SnI3 based perovskite solar cell employing CH3NH3SnBr3 as hole transport layer by SCAPS-1D device simulator

This study focuses on the theoretical aspects of third-generation perovskite solar cells (PSC), with the aim of replacing traditional silicon-based counterparts. With potential for higher efficiency and low manufacturing costs, perovskite cells offer unique crystallographic structures allowing adjustments to photoluminescence wavelength. This research addresses challenges in cost-effective solar spectrum utilization and optimization of parameters, device architecture, and materials for high-efficiency cells. In this study, we simulated a perovskite-based solar cell (CH3NH3SnI3) using solar cell capacitance simulator-one dimension simulator under AM 1.5G illumination. The chosen electron transport layer is TiO2, and hole transport layer is CH3NH3SnBr3. The simulation explores variations in layer thickness, defect concentration, interface defects, doping concentration and electron affinity. Additionally, we analyzed the impact of back metal contact work function and temperature variations. Results indicate optimal absorber layer thickness at 0.5µm. Reduced defect concentrations, increased doping concentration and a higher work function for the back contact, enhance efficiency of PSC. The initial parameters yielded a 19.79% efficiency based on base values before optimization, which increased to 26.66% after optimization. According to the latest NREL data, the highest reported efficiency for PSC is 26.1%. This research provides insights into perovskite-based solar cell design for enhanced efficiency.

Breaking the size limitation of nonadiabatic molecular dynamics in condensed matter systems with local descriptor machine learning

Nonadiabatic molecular dynamics (NA-MD) is a powerful tool to model far-from-equilibrium processes, such as photochemical reactions and charge transport. NA-MD application to condensed phase has drawn tremendous attention recently for development of next-generation energy and optoelectronic materials. Studies of condensed matter allow one to employ efficient computational tools, such as density functional theory (DFT) and classical path approximation (CPA). Still, system size and simulation timescale are strongly limited by costly ab initio calculations of electronic energies, forces, and NA couplings. We resolve the limitations by developing a fully machine learning (ML) approach in which all the above properties are obtained using neural networks based on local descriptors. The ML models correlate the target properties for NA-MD, implemented with DFT and CPA, directly to the system structure. Trained on small systems, the neural networks are applied to large systems and long timescales, extending NA-MD capabilities by orders of magnitude. We demonstrate the approach with dependence of charge trapping and recombination on defect concentration in MoS2. Defects provide the main mechanism of charge losses, resulting in performance degradation. Charge trapping slows with decreasing defect concentration; however, recombination exhibits complex dependence, conditional on whether it occurs between free or trapped charges, and relative concentrations of carriers and defects. Delocalized shallow traps can become localized with increasing temperature, changing trapping and recombination behavior. Completely based on ML, the approach bridges the gap between theoretical models and realistic experimental conditions and enables NA-MD on thousand-atom systems and many nanoseconds.

Enhancing the photovoltaic efficiency of CZTSSe thin-film solar cells via Ag-doping induced defect modulation

Defect modulation is one of the key factors in determining the performance of solar cells, and the presence of defects tends to adversely affect carrier recombination and transport, thus having a direct impact on both cell stability and efficiency. Ag doping can modulate the defects, but in-depth studies on the mechanism of the effect are needed. Therefore, an in-depth study of the effect of Ag incorporation on the defect modulation mechanism of Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells is of great practical significance for understanding the mechanism of action of Ag doping and further optimising the cell performance. In this study, (AgxCu1-x)2ZnSn(S,Se)4 (0 ≤ x ≤ 1) (ACZTSSe) solar cells were successfully prepared by N, N-dimethylformamide (DMF) solution, which have the characteristics of reducing defects and enhancing photovoltaic performance. After the introduction of Ag, the absorber formed a crystal structure with few holes and penetrating top and bottom. And the electrical performance of the solar cell is significantly improved, J0 is reduced from 4.6 × 10−5 A/cm2 to 2.7 × 10−6 A/cm2, and its contribution to the photovoltaic conversion efficiency (PCE) improvement reaches 142%. This indicates that the introduction of Ag helps to optimize the quality of the absorber, thereby reducing the concentration of interfacial and bulk defects. It also reduces carrier recombination and improves carrier lifetime from 0.86 ms to 1.75 ms. Ultimately increasing our cell efficiency from 7.55% to 10.64% when the Ag content reached 5%. These results clearly show that we have successfully achieved the modulation of defects in CZTSSe solar cells by Ag doping.

Porous Structure Enhances the Longitudinal Piezoelectric Coefficient and Electromechanical Coupling Coefficient of Lead-Free (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3.

The introduction of porosity into ferroelectric ceramics can decrease the effective permittivity, thereby enhancing the open circuit voltage and electrical energy generated by the direct piezoelectric effect. However, the decrease in the longitudinal piezoelectric coefficient (d33) with increasing porosity levels currently limiting the range of pore fractions that can be employed. By introducing aligned lamellar pores into (Ba0.85Ca0.15)(Zr0.1Ti0.9)O3, this paper demonstrates an unusual 22-41% enhancement in the d33 compared to its dense counterpart. This unique combination of high d33 and a low permittivity leads to a significantly improved voltage coefficient (g33), energy harvesting figure of merit (FoM33) and electromechanical coupling coefficient ( ). The underlying mechanism for the improved properties is demonstrated to be a synergy between the low defect concentration and high internal polarizing field within the porous lamellar structure. This work provides insights into the design of porous ferroelectrics for applications related to sensors, energy harvesters, and actuators.

First-principles study of vacancy-impurity (C, N, O) interactions in vanadium

Abstract Interstitial impurities of C, N, and O typically exist in V and its alloys, which have significant effects on the structure and properties of the materials due to their strong interaction with the vacancies. In this study, the structure and energy properties of vacancy-impurity complexes in V are comprehensively investigated by first principles calculations. The effects of impurity concentration and temperature on the concentration distribution of the complexes were studied using statistical methods. The results showed that the Vac n X 2 complexes exhibit a high sequential binding energy. There is an evident mutual attraction between the vacancy and the O atom, and it has a higher affinity than the C or N atom. The simple defect complexes of Vac 1 X 1–2 have always been the most abundant type of complex defect in V. The trapping effect of vacancy on C, N, and O atom is very significant at low temperatures rather than at high temperatures. This study would broaden the understanding of vacancy-impurity interactions in vanadium and provide new insights into the influence of C, N, and O on vacancy defect concentrations.

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.

Enhancing durable electrical conductivity in multi‐walled carbon nanotubes‐epoxy composites via laser repetition rate nanojoining for flexible electronics

AbstractWith the development of nanotechnology, laser matter interaction has become intriguing for many applications, including the manufacturing of flexible electronics to improve interface behaviors. Employing high laser repetition rate nanojoining transfer patterning, this study ensures the robust and consistent stability of electrical properties of MWCNTs. This procedure meticulously eliminates contaminants from the interface between flexible PET substrates and carbon nanotube‐containing epoxy, significantly influencing the degree of alteration in composite material and interface behaviors. By increasing the laser repetition rate from the original sample to the sample irradiated by a laser with a repetition rate of 80 kHz, the defect concentration in carbon nanotubes decreases from 0.448 × 106/nm2 to 0.39376 × 106/nm2, respectively. The relationship between defect concentration and electrical conductivity in semiconductor carbon nanotubes under laser irradiation is multifaceted and context‐dependent. Optimizing the laser repetition rate is crucial in defining the kind and density of semiconductor carbon nanotubes. This study found an electrical conductivity of 3.6799 × 10−4 S/m at a laser repetition rate of 80 kHz. Laser ablation nanojoining with a high laser repetition rate is poised to become a key recycling method for plastic materials in future industries. When MWCNTs are used in processing new materials, this method not only enhances the electrical and mechanical properties of recycled plastics but also creates high‐performance composites with added value. Additionally, the quick nature of this process helps minimize toxicity in material processing by reducing exposure time and the need for harmful chemicals. These composites exhibit superior properties suitable for advanced applications such as flexible electronics, sensors, and nanocomposite materials, contributing to both sustainability and economic value in various industrial sectors.

Field-Driven Solid-State Defect Control of Bilayer Switching Devices.

We develop a framework for controlling and investigating reversible ionic transfer between two solid metal oxides layers by examining field-driven changes in electrical properties of the thin film bilayer oxide system Pr0.1Ce0.9O2/La1.85Ce0.15CuO4 (PCO/LCCO). We show that we can reversibly redistribute oxygen ions by applied voltage in a highly controlled and reversible fashion near ambient temperatures over large oxygen ion activity limits, which, for the first time, is directly interpretable by defect chemical models. This allowed us to determine how defect concentrations in each layer systematically varied with voltage and the subsequent impact on each film's conductance. These results showcase the relevance and applicability of defect chemical models, traditionally considered only at elevated temperatures, to the development of bilayer devices of importance to neuromorphic memory applications. This allows for a more systematic approach for studying and understanding the solid-solid exchange process in electrochemically controlled microelectronic devices. Moreover, our work sets the foundation for the development of large-area field-driven defect-controlled bilayer switching devices with potential application to a broad array of functionally modulated devices.

Colloidal TiO2 nanocrystals with engineered defectivity and optical properties.

Partially reduced forms of titanium dioxide (sometimes called "black" titania) have attracted widespread interest as promising photocatalysts of oxidation due to their absorption in the visible region. The main approaches to produce it rely on postprocessing at high temperatures (up to 800 °C) and high pressures (up to 40 bar) or on highly reactive precursors (e.g., TiH2), and yield powders with poorly controlled sizes, shapes, defect concentrations and distributions. We describe an approach for the one-step synthesis of TiO2 colloidal nanocrystals at atmospheric pressure and temperatures as low as 280 °C. The temperature of the reaction allows the density of oxygen vacancies to be controlled by nearly two orders of magnitude independently of their size, shape, or colloidal stability. This synthetic pathway appears to produce vacancies that are homogeneously distributed in the nanocrystals, rather than being concentrated in an amorphous shell. As a result, the defects are protected from oxidation and result in stable optical properties in oxidizing environments.

THE COMPOSITION OF IMPURITIES AND DEFECTS IN Cd1-XMgXTe:In, NECESSARY TO ENSURE STABLE DETECTOR PROPERTIES

A model study of the promising new material Cd0.92Mg0.08Te:In, intended for X-ray and gamma radiation detectors operating at room temperature, was carried out. The paper highlights the results of quantitative studies of the influence of the content of impurities and structural defects on the electrophysical and detector properties of Cd0.92Mg0.08Te:In. An analysis of the calculated values of resistivity ρ and concentrations of free charge carriers, life time of non-equilibrium electrons τn, and holes τp, charge collection efficiency η with different composition of impurities and defects in this material at temperature T = 298 K was carried out. The optimal ranges of energy and concentration of alloying deep donor, which ensure a stable high-resistive state and acceptable values of η, are established. Compensation of cadmium vacancies with indium admixture was studied. Assumption was made regarding possibility of increasing the operating time of the detector having semi-insulating properties and great charge collection efficiency. A direction for further research has been formulated in order to clarify the nature of a suitable doping deep donor that ensures stable properties of the detector.

Surface nitrogen defect modified crystalline carbon nitride spheres with boosted carrier dynamics for efficient solar hydrogen production

Engineering of the defective structure and crystallinity of semiconductor photocatalysts to promote carrier dynamics has attracted considerable attention. This study presents the design of surface nitrogen defect modified crystalline carbon nitride spheres (QCN) with synergistically boosted surface and bulk carrier separation through a rapid thermal treatment coupled with a flux assisted strategy. Using supramolecular self-assembled polymer as the precursor, spherical crystalline CN heterojunctions are firstly prepared by a molten salt method to improve the separation of bulk carriers. Abundant nitrogen defects are then introduced onto the surface of crystalline CN through the following quick thermal treatment in air, which can generate a higher exciton density and drive rapid migration of surface carriers by acting as effective electron sinks. Furthermore, the defect level also improves the visible light utilization and provides more active centers for reactant adsorption and activation. As a result, QCN samples exhibit excellent performance in photocatalytic hydrogen evolution, with an optimal hydrogen production rate 4.3 times higher than that of pristine bulk CN under visible light irradiation. Moreover, the photocatalytic activities of QCN are dependent on the concentration of nitrogen defects, which can be flexibly adjusted via thermal treatment temperature. Such a two-step regulation strategy combining the distinctive advantages of surface defect structure and high crystallinity is beneficial for synergistic enhancement of photocatalytic performance, and provides new insights for sustainable energy production in the future.

Design Strategies for High-Performance NH3-SCO Catalysts: Identifying and Modulating Direct Anchoring Sites for Ag on TiO2.

Ammonia (NH3) slip from diesel vehicle aftertreatment systems and internal combustion engines fueled by NH3 or NH3/H2 poses serious environmental problems. Ag-based catalysts are widely used for the selective catalytic oxidation of NH3 to N2 (NH3-SCO), and their performance is greatly dependent on the state of Ag, which is influenced by the anchoring sites on the support. Despite efforts to identify the direct anchoring sites of metal atoms on TiO2, conflicting views persist. Here, we compared the correlation between Ag dispersion and the content of hydroxyl (OH) groups or defects on TiO2 and conducted density functional theory (DFT) calculations, and the results confirmed that the surface OH groups of TiO2 serve as the direct anchoring sites for Ag. By modulating the OH group content through thermal induction, the optimal OH group content on TiO2-800 resulted in more metallic Ag nanoparticles (Ag0 NPs) in larger sizes, leading to the development of an excellent NH3-SCO catalyst. Moreover, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), kinetic studies, and DFT calculations suggested that more Ag0 NPs in larger sizes on 10Ag/TiO2-800 were conducive to O2 activation and NH3 dissociation. Our findings provide new insights for designing efficient NH3-SCO catalysts, and OH groups as direct anchoring sites could be extended to other metals and supports for the rational design of catalysts.

Defect engineering in SnO2 catalysts for the organic oxidation reaction

Defect engineering in metal oxides is an effective approach for improving advanced oxidation processes. Herein, we report that regulating the defect types present on SnO2 enables deconvolution of their distinct effects on organic oxidation. Nitrogen annealing created E′δ center and non-bridging oxygen hole center (NBOHC) defects, while optimum hydrogenation introduced oxygen vacancies, significantly enhancing catalytic oxidation performance. Based on spectroscopic analysis, extended hydrogenation times passivated NBOHCs and formed new types of defects, such as electrons trapped in oxygen vacancies, which are less catalytically active in comparison with NBOHCs. DFT indicated that oxygen vacancies lower the energy barrier for oxygen activation as well as activation of the C-H bonds in formic acid, corroborating the experimental results of enhanced catalytic activity in samples with optimized defect concentrations. The current work advances understanding of the roles different defects play in enhancing organic oxidation in the ongoing search for efficient materials for oxidation reactions.

Pulsed electric field treatment reduces the internal stress and increases the adhesion of TiAlN coating

Higher temperatures and longer treatment times are necessary in a conventional thermal treatment method, which restricts the substrate materials and treatment efficiency. In this paper, the grain size and orientation of the coating are regulated by pulsed electric field (PEF) treatment, and the adhesion of the coating is enhanced by reducing the internal stress between the grains. The findings demonstrate that the PEF can change the coating to the close-packed (111) with high conductivity and facilitate the short-range diffusion of N atoms. The effects reduce the concentration of defects and reduce the coating’s internal stress, improving the critical load of the coating. The PEF treatment provides an effective solution for low-temperature and high-efficiency treatment coatings.

Effect of Al doping on structural and optical properties of atomic layer deposited ZnO thin films

In this work, zinc oxide (ZnO) and aluminum-doped ZnO (AZO) thin films with varying Al contents were grown via atomic layer deposition on Si and glass substrates. The structural and optical properties of ZnO and AZO thin films were investigated using X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), photoluminescence (PL), and ultraviolet-visible spectroscopy. While undoped ZnO had a wurtzite crystal structure, Al2O3 layers altered the crystal structure depending on the Al content. With the increasing number of Al2O3 monolayers, the γ- Al2O3 structure became more prominent, and ZnO peaks were completely suppressed when the Zn ratio reached 10:1 and beyond. SEM images revealed that the thin films homogeneously covered the entire substrate surface. EDS mapping showed a uniform distribution of Zn, Al, and O in the structure. According to the XPS results, the thin films displayed significant peak intensities corresponding to the main components Zn, O, and Al. Zinc atoms were present only in their oxidized state, not as interstitial atoms. The O 1 s peak could be deconvoluted into three components: the oxygen-zinc bond, the oxygen-aluminum bond, and chemisorbed or OH species. In the AZO samples, a direct correlation was observed between the intensity of the Al 2p peak and the number of Al2O3 layers. Additionally, the calculated elemental ratio of Al increased from 2.582 to 23.955 at.% with more Al2O3 layers. Optical measurements revealed that ZnO and AZO thin films exhibit high transmittance in the visible region. Moreover, introducing Al2O3 layers into the supercycle led to an increasing trend in transmittance. Another improvement in the optical properties was observed as a blue shift in the absorption edge with Al doping, indicating band gap widening. The optical band gap of the ZnO thin films increased from 3.2 to 3.5 eV with Al doping. The origin and concentration of defects were evaluated by deconvoluting the PL spectra. A comparative investigation of the PL spectrum and XPS results revealed variations in defect concentration with deposition parameters and defect formation mechanism was discussed.

Optimizing perovskite solar cell efficiency with Cs0.15FA0.81MA0.04PbBr0.14I2.86 - MAPbBr3 QDs Complex active layer and GuBr interface modified layer

Achieving optimal efficiency in perovskite solar cells (PSCs) necessitates the creation of a light-absorbing layer with superior crystallization, minimal defect concentration, exceptional stability and better energy level alignment. In this context, the active layer of the PSCs comprises (Cs0.15FA0.81MA0.04PbBr0.14I2.86), incorporating MAPbBr3 quantum dots doped with toluene as an anti-solvent to enhance device performance, achieving a Photovoltaic Conversion Efficiency (PCE) of 18.9 %. Furthermore, GuBr is introduced at the interface between NiOx and photovoltaic active layer for effective interface modification. These improvements lead to optimized devices, particularly with GuBr modification at doping amount of 3 mg/ml, showcasing an impressive PCE of 20.6 %. Noteworthy attributes of these devices include a short-circuit current density of 24.7 mA/cm2 and an open-circuit voltage (Voc) value of 1.02 V. Significantly, the altered film retained more than 81 % of its original effectiveness for a prolonged duration surpassing 10 days. The conclusive findings underscore the successful attainment of high efficiency and high stability through the employed design strategy in PSCs.

Coupling Phase Field Crystal and Field Dislocation Mechanics for a consistent description of dislocation structure and elasticity

This work addresses differences in predicted elastic fields created by dislocations either by the Phase Field Crystal (PFC) model, or by static Field Dislocation Mechanics (FDM). The PFC order parameter describes the topological content of the lattice, but it fails to correctly capture the elastic distortion. In contrast, static FDM correctly captures the latter but requires input about defect cores. The case of a dislocation dipole in two dimensional, isotropic, elastic medium is studied, and a weak coupling is introduced between the two models. The PFC model produces compact and stable dislocation cores, free of any singularity, i.e., diffuse. The PFC predicted dislocation density field (a measure of the topological defect content) is used as the source (input) for the static FDM problem. This coupling allows a critical analysis of the relative role played by configurational (from PFC) and elastic (from static FDM) fields in the theory, and of the consequences of the lack of elastic relaxation in the diffusive evolution of the PFC order parameter.

Synthesis and characterization of hierarchically porous carbon anodes from furfural biorefinery residues for sustainable lithium-ion batteries

This study aims to valorize abundant lignocellulosic residues from furfural biorefineries into hierarchically porous carbon anodes for sustainable lithium-ion batteries (LIBs). Simultaneously, it addresses the growing demand for high-performance anode materials by leveraging renewable waste feedstocks. Sugarcane bagasse waste residues obtained after furfural extraction were used as the precursor. The furfural-derived bagasse residue (FDBR) underwent pyrolysis at 700 °C to produce a carbon-rich char, which was further activated using KOH and steam at 900 °C for 15–60 minutes to enhance porosity development. Comprehensive physicochemical characterizations, including proximate analysis, N2 adsorption-desorption, SEM-EDX, FTIR, and TGA/DTG, and Raman spectroscopy were performed to evaluate the properties of the synthesized activated carbons. The results demonstrated the successful generation of high surface area (405.8 m2/g) activated carbons with a hierarchical pore structure comprising micropores and mesopores. SEM and EDX analyses revealed a uniform, porous morphology enriched with 10.41 % at oxygen-containing functional groups. FTIR spectra confirmed the presence of hydroxyls, carbonyls, and carboxylic acid groups, beneficial for pseudocapacitive charge storage. The Raman analysis revealed a highly disordered and amorphous carbon structure with a significant concentration of defects, as evidenced by a high-intensity ratio (ID/IG ≈ 1.2–1.4) of the disordered carbon band to the graphitic band. The activated carbons exhibited a high initial discharge capacity of 1055 mAh/g in lithium half-cells, significantly exceeding the theoretical limit of commercial graphite anodes (372 mAh/g). This superior capacity was attributed to the high surface area, hierarchical porosity, and pseudocapacitive contributions from the oxygen functionalities. However, rapid capacity fading from 1055 to ∼250 mAh/g was observed over 100 cycles at a C/2 rate. The capacity retention stabilized at ∼88 % after 47 cycles, indicating irreversible capacity losses. These limitations were ascribed to the disordered amorphous structure, instability of the solid-electrolyte interphase, lithium trapping by oxygen groups, and structural changes during cycling. Optimizing activation conditions, incorporating conductive additives/coatings, exploring alternative binders, and surface functionalization are proposed to improve long-term cycling stability. Overall, this study demonstrates the feasibility of upcycling furfural biorefinery waste into sustainable LIB anodes while highlighting challenges and potential solutions for enhancing their electrochemical performance.

(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.