Effect Of Surface States Research Articles

Synergistic Effects of Surface States and Deep Energy Level Traps on InGaAs p-i-n Diodes under Proton Irradiation

Synergistic Effects of Surface States and Deep Energy Level Traps on InGaAs p-i-n Diodes under Proton Irradiation

Activation/ Deactivation of Surface States of Heterostructure (Anatese-TiO2/Au) Via Addition of Materials with High Band Gap, Rutile TiO2 (R-TiO2) or Low Band Gap Silicon (Si), and Fabrication Strategy

The addition of high band gap or low band gap material not only modulates the optical absorption cross-section but also affects the surface state reactivity of heterostructure towards the photoelectrochemical (PEC) water splitting. Further, the deposition strategy to assemble the heterostructure also affects the effectiveness of the heterostructure. To explore such issues, we fabricated the two sets of heterostructures containing (A-TiO2, R-TiO2) and (Si, A-TiO2) via the click chemistry approach through two different strategies, i.e., layer-by-layer heterostructure (LLH), and randomly deposited heterostructure. In LLH strategy, only the addition of high band gap material R-TiO2 enhances the reactivity of the surface states. On the other hand, the addition of the low band gap material Silicon (Si) hampers the reactivity of the surface states of the heterostructure deposited via the LLH. Further, by changing the deposition strategy to RDH, the reactivity of surface states diminishes by adding either the high band gap or the low band gap material. These observations were rationalized from the framework of band structure/band-alignment in the heterostructure and also via electrochemical impedance spectroscopy (EIS).In LLH, the addition of the A-TiO2 quenches electrons from the conduction band (CB) of R-TiO2, and the holes of A-TiO2 are transferred to R-TiO2. The holes from R-TiO2 participate in activating the surface states present at the surface. On the other hand, the addition of silicon to LLH, having CB above the CB of A-TiO2, does not allow the electrons to transfer from A-TiO2 to Si; hence, the CB electrons from A-TiO2 transfer to surface states, which enhances the energy level of surface states, making it more negative than the VB of Si. The negative shift of surface states of A-TiO2 makes it the recombination center. Interestingly, changing the fabrication strategy from LLH to RDH makes the nature of surface states from reactive to recombination states due to the interfacing of all materials with the underlying substrates.EIS was utilized to measure and analyze the low-frequency capacitance due to the reactive surface state. The low-frequency capacitance of LLH with A-TiO2 shows the maxima at onset potential, confirming the role of reactive surface states. On the contrary, low-frequency capacitance increases monotonically for LLH with Si addition, showing the effect of recombination surface states. Overall, the study provides pointers on the interplay between band positions and electron-hole separation in heterostructures and how they can be effectively used to enhance surface reactivity in PEC water splitting.

Effect of Different Surface States of Electrolytic Copper Foils on the Performance of Lithium-Ion Batteries

Currently, the field of lithium-ion batteries faces an urgent challenge, which is how to effectively inhibit the growth of dendrites, thereby improving the coulombic efficiency of plated/stripped lithium on copper foils and enhancing the cycling stability of the batteries. In this paper, the surface of copper foil is roughened by annealing post-treatment, which in turn improves the interfacial adhesion between the copper foil and the active material. This treatment helps the anode current collector to form a flat graphite electrode sheet and a uniform solid electrolyte interphase film, reducing the growth of lithium dendrites as well as extending the cycle life of the batteries. Among them, the annealed electrode exhibits extremely high roughness (0.533 μm) and elongation (9.91%), with the initial discharge capacity of the prepared lithium battery reaching as high as 384.22 mAh g−1. It also maintains good cycling performance at different rates, which confirms the gain effect of surface roughening on battery capacity.

Advancements and Challenges in the Integration of Indium Arsenide and Van der Waals Heterostructures.

The strategic integration of low-dimensional InAs-based materials and emerging van der Waals systems is advancing in various scientific fields, including electronics, optics, and magnetics. With their unique properties, these InAs-based van der Waals materials and devices promise further miniaturization of semiconductor devices in line with Moore's Law. However, progress in this area lags behind other 2D materials like graphene and boron nitride. Challenges include synthesizing pure crystalline phase InAs nanostructures and single-atomic-layer 2D InAs films, both vital for advanced van der Waals heterostructures. Also, diverse surface state effects on InAs-based van der Waals devices complicate their performance evaluation. This review discusses the experimental advances in the van der Waals epitaxy of InAs-based materials and the working principles of InAs-based van der Waals devices. Theoretical achievements in understanding and guiding the design of InAs-based van der Waals systems are highlighted. Focusing on advancing novel selective area growth and remote epitaxy, exploring multi-functional applications, and incorporating deep learning into first-principles calculations are proposed. These initiatives aim to overcome existing bottlenecks and accelerate transformative advancements in integrating InAs and van der Waals heterostructures.

Unusual Janus Bi2TeSe2 Topological Insulators Displaying Second-Harmonic Generation, Linear-in-Temperature Resistivity, and Weak Antilocalization.

Well-established knowledge about inversion-symmetric Bi2TexSe3-x topological insulators characterizes the promising new-generation quantum device. Noticeably, the inversion asymmetric phase containing different surface electronic structures may create an extra topological phenomenon pointing to a new device paradigm. Herein, Janus Bi2TeSe2 single-crystal nanosheets with an unconventional stacking sequence of Se-Bi-Se-Bi-Te are realized via chemical vapor deposition growth, which is clarified by atomically resolved AC-STEM and elemental mapping. An obvious polarization-dependent second-harmonic generation with a representative 6-fold rotational symmetry is detected due to the broken out-of-plane mirror symmetry in this system. Low-temperature transport measurements display a strange metal-like linear-in-temperature resistivity. Anomalous conductance peaks under low magnetic fields induced by the weak antilocalization effect of topological surface states and the two-dimensional transport-dominated anisotropic magnetoresistance are revealed. These findings correlate the Janus Bi2TeSe2 phase with emerging physics topics, which would inspire fresh thoughts in well-developed Bi3TexSe3-x topological insulators and open up opportunities for exploring hybrid nonlinear optoelectronic topological devices.

A review of advancements in theoretical simulation of oxygen reduction reaction and oxygen evolution reaction single-atom catalysts

Aqueous electrolytes metal-air batteries have garnered widespread attention owing to their safety, low cost, and non-toxic properties. Yet, their development is hindered by slow reaction efficiency. To address this issue, extensive exploratory work has been conducted by researchers. Among them, single-atom catalysts (SACs) represent a significant research direction and a hot topic for the metal-air battery catalysts. Theoretical calculations have been instrumental in predicting and screening high-activity SACs. Besides, the efforts have been made to investigate the performance of SACs for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, we specifically analyze the development and prospects of SACs for ORR/OER. The theoretical research on SACs can be divided into two stages. The first stage involves exploring the mechanism of ORR/OER reactions, also known as ideal condition theoretical simulations. The exploration scope can be categorized into four types: 1) transition metals loaded on different substrates; 2) different transition metals loaded on the same substrate; 3) the impact of the coordination environment on the catalytic efficacy of SACs; and 4) the application of machine learning in the field of catalysts. The second stage involves simulating experimental environments, where researchers begin to consider the effects of voltage, solvation, pH, surface states, and microscopic molecular dynamics on catalysts. This work comprehensively reviews the current status of SACs for ORR/OER and provides suggestions for the development of SACs in the next stage.

Effect of surface states and in-plane deformation on transport properties of charge carriers in bismuth films

Since the energy spectrum of bismuth charge carriers is highly sensitive to mechanical deformations, studying their influence, leading to an increase or decrease in the overlap of actual extrema, on the transport properties of charge carriers in ultrathin bismuth films makes it possible to investigate the combination of manifestations of the quantum size effect and metallic surface states. In this work, the temperature dependencies of the electrical and galvanomagnetic properties of thin bismuth films (10–1250 nm) are studied under conditions of in-plane tensile and compressive deformation. Deformations are created as a result of the difference in the coefficients of thermal expansion of the film and substrate materials. Silicon with an oxidized surface, mica, and polished cleavage (111) CaF2 are used in order to create either tensile and compressive deformations. The presence of film deformations is confirmed by XRD. The magnitude of deformation is calculated based on the CTEs of the materials. The absence of mechanical stress relaxation or the formation of additional defects during temperature measurements is confirmed by the absence of hysteresis of properties in the range of 77–300 K. Based on the experimental results, the mobilities and concentration of charge carriers are calculated within the framework of the two-band model. As the film thickness decreases to less than 100 nm, an increase in the charge carrier concentration is observed, which is associated with metallic surface states. It is shown that the magnitude of the effect of deformation on the concentration of charge carriers in films of all thicknesses remains unchanged.

Angle-resolved X-ray photoelectron spectroscopy intensity modeling of SiNx ultrathin layer grown on Si (100) and Si (111) substrates by N2 plasma treatment

In this work, we investigate the silicon nitridation process using a N2 plasma Glow Discharge Source (GDS) in an ultra-high vacuum chamber. In situ Angle Resolved X-ray Photoelectron Spectroscopy (ARXPS) measurements were performed to determine various chemical environments of the surface species. The main goals of this study are determination of the film chemical composition, to estimate its thickness and identify various phenomena taking place during the nitridation process under different experimental conditions. To achieve these objectives, we developed an ARXPS intensity model for an ultrathin SiNx overlayer. Based on the ARXPS results, which indicate that the silicon nitride film is not homogenous, we adopted a bi-layer model composed of two SixNyOz layers with varying thickness and chemical composition. The model is built from analysis of photoelectron intensities, considering a comparison between experimental and theoretical data. This bi-layer model revealed that the silicon nitride layer consists of a surface layer composed of approximately equal amounts of silicon and nitrogen (50% each), and an interfacial layer containing varying amounts of oxygen depending on experimental parameters. The presence of oxygen is due to residual air contamination in the vacuum chamber and/or to Si native oxide. The estimated thickness of silicon nitride film is less than 9 nm, depending on the nitridation time and temperature.The model developed in this work enabled us to study the effects of initial surface state (as received or chemically cleaned), Si substrate crystallographic orientation (such as (100) and (111)), and the nitridation temperature (room temperature and 500°C). The diffusing species (N+, N2+, O+), which play a crucial role during nitridation, were found to be influenced by temperature, treatment time, and substrate orientation. The diffusion phenomenon competed with other processes such as NO desorption, leading to a significant impact on formation of the SiNx film. Furthermore, we compared the thickness obtained from the ARXPS model using cross-sectional High-Resolution Transmission Electron Microscopy (HR-TEM) images. Additional Low-Energy Electron Diffraction (LEED) measurements indicated that the SiNx film was amorphous.

Ammonia vapour detection at room temperature via carbon quantum dots based facile system

With widespread industrial use of ammonia and its potential hazards to both human health and the environment, ensuring reliable detection has become paramount. In the present work, we report a study on a novel and highly sensitive gas sensing material, composed of carbon quantum dots (CQDs) for the detection of ammonia vapor at room temperature. The CQDs were synthesized at 200 °C using a hydrothermal single-step process. To investigate the morphology and chemical composition, CQDs were characterized by Fourier transform infrared spectroscopy, Transmission Electron Microscopy, photoluminescence, and UV–Vis spectroscopy. A thin-film of CQDs was then deposited on an ITO-coated glass substrate for the detection and quantification of ammonia vapor (NH3) by monitoring the response under ambient conditions. By corresponding change in conductance, the change in resistance of the film is investigated. The enhanced properties of sensing of the ammonia samples may be due to the combined effect of localized electronic surface states of CQDs and chemical sensitization caused by the catalytic properties of an active functional group on the CQDs surface. Notably, this sensor exhibits higher sensitivity, excellent response time, and greater stability to ammonia vapours compared to previous reports.

Magnetochiral anisotropy induced nonlinear planar Hall effect in topological insulator surface states

Magnetochiral anisotropy induced nonlinear planar Hall effect in topological insulator surface states

The effects of heat treatment and surface state on the corrosion resistance of laser powder bed fusion 304L stainless steel in 3.5 wt% NaCl solution

The corrosion resistance of laser powder bed fusion (LPBF) 304L stainless steel (SS) in 3.5 wt% NaCl solution was evaluated at room temperature through potentiodynamic polarization and electrochemical impedance spectroscopy tests. For electropolished samples, annealing at 1200 °C slightly enhanced the corrosion resistance due to changes in crystal orientation and inclusion composition, and reduced residual strain. In contrast, annealing at 1050 °C significantly reduced the corrosion resistance mainly due to the disappearance of dislocation cells and the remained residual dislocations. Residual stress/strain and martensitic transformation induced by sandpaper grinding can also deteriorate the corrosion resistance. Only the as-received sample shows stable passivation regardless of the surface conditions.

Small-signal equivalent circuit model of GaN-based nanodiodes at low temperature including trap-related low frequency dispersion

The small-signal equivalent circuit of GaN-based self-switching diodes has been obtained, which apart from the intrinsic R‖C branch, generally used to describe the diode performance, needs new elements to describe the low-frequency dispersion of the impedance originated by the presence of surface and bulk traps. The proposed model allows us to reproduce not only the high-frequency results (extracted from S-parameter measurements in the 40 MHz–43.5 GHz range) at room temperature, but also the low-frequency impedance measurements (75 kHz–30 MHz) at cryogenic temperatures down to 70 K. These new elements are a self-inductance associated to the effect of surface states (typical of a device with a high surface-to-volume ratio) and an extra series R–C branch modeling the influence of the bulk traps.

Rashba Metamaterials and Metasurfaces with Zero Reflectivity and Effect of Surface States in Ultrathin Metal Films.

Metals, renowned for their high reflectivity, find extensive use in various technological applications, from mirrors to optical coatings in radars, telescopes, and mobile communications. However, their potential in antireflective coatings has remained largely untapped. In this study, we demonstrate that by applying an ultrathin metallic film onto an oxide layer, we can achieve a flawless optical surface with zero reflectivity. This phenomenon has been successfully observed across various metals, including Sn, Ag, Au, Pt, Bi, and Nb, showcasing its broad applicability. The underlying principle lies in the emergence of surface states, where the Rashba effect is strong, which give rise to the formation of Rashba metamaterial and metasurface (RMM) structures. Remarkably, these RMMs can be fine-tuned to act as high-resolution Veselago lenses. To illustrate, we achieved zero reflectivity with an RMM consisting of a 1 nm thick Sn metal film on a 1 nm Ge buffer, situated on a 60 nm Al2O3/Si substrate. Similar results were observed for other metals (Pt, Au, Ag, and Nb) and semimetals (Bi) by adjusting the film thickness to 2, 3, 5, 10, and 6 nm, respectively. The revelation of RMMs with zero reflectivity (R = 0) has tremendous potential to revolutionize optical device technologies, covering renewable energy, optoelectronics, and the telecommunications industry.

Are topological insulators promising thermoelectrics?

Some of the best thermoelectric (TE) materials to date are also topological insulators (TIs). While many studies have investigated the effects of topologically-protected TI surface states on TE properties, the conditions needed to realize such effects are quite different from typical operating conditions of TE devices for, e.g., power generation and room-temperature Peltier cooling. As a result, it is still unclear what properties of TIs, especially those related to the bulk band structure, are beneficial for TE performance, if any. Here, we perform high-throughput transport calculations using density functional theory to reveal that, within the same structure type, TIs tend to outperform normal insulators as TEs when properly optimized. The calculations also indicate that the TE performance is higher for TIs with strongly inverted bands. To explain these observations, we develop models based on Boltzmann transport theory which show that warping driven by band inversion, a key characteristic of TIs, is responsible for the high TE performance of TIs. We find that warping benefits the TE performance because of reduced transport mass and effectively higher valley degeneracy. Our results show that the band inversion strength is a critical property of a TI dictating the TE performance, and we suggest potential strategies to tune the inversion strength and enhance the TE performance in TIs, such as alloying and strain engineering. The study marks TIs as serious candidates for TE applications owing to band inversion-driven warping.

3D quantum Hall effect in a topological nodal-ring semimetal

A quantized Hall conductance (not conductivity) in three dimensions has been searched for more than 30 years. Here we explore it in 3D topological nodal-ring semimetals, by employing a minimal model describing the essential physics. In particular, the bulk topology can be captured by a momentum-dependent winding number, which confines the drumhead surface states in a specific momentum region. This confinement leads to a surface quantum Hall conductance in a specific energy window in this 3D system. The winding number for the drumhead surface states and Chern number for their quantum Hall effect form a two-fold topological hierarchy. We demonstrate the one-to-one correspondence between the momentum-dependent winding number and wavefunction of the drumhead surface states. More importantly, we stress that breaking chiral symmetry is necessary for the quantum Hall effect of the drumhead surface states. The analytic theory can be verified numerically by the Kubo formula for the Hall conductance. We propose an experimental setup to distinguish the surface and bulk quantum Hall effects. The theory will be useful for ongoing explorations on nodal-ring semimetals.

Correlation between structural characteristics of edge selectively-oxidized graphite and electrochemical performances under fast charging condition

In the pursuit of ameliorating of the battery-performance degradation particularly during fast charging of lithium-ion batteries, we investigated the influence of surface-structural parameter of graphite. To explore the effect of the graphite surface state on the battery performance, we synthesized the edge selectively-oxidized graphite. The detailed analysis about the structural parameters of graphite samples revealed that the edge area increased from 3.41 to 4.26 m2 g−1 by oxidation process while preserving the specific surface area. Edge selectively-oxidized graphite samples showed the similar ratio between carbon and oxygen regardless of oxidized time, but the change of surface charge due to the different functional groups. The edge selectively-oxidized graphite exhibited a significantly improved capacity retention and rate performance compared with the bare graphite. Based on thorough examination about material properties and electrochemical responses, we corroborated a strong correlation among the electrochemical performance, edge surface area, and type of oxygen-based functional groups of graphite. Our study provides conclusive evidence that the surface nature of graphite plays a pivotal role in boosting the battery performance.

Pt Co-catalyst nanolayer on nanostructured V-doped Fe2O3 for boosting photoelectrochemical water oxidation

A defective system of V-doped Fe2O3 with Pt as a cocatalyst was employed for photoelectrochemical (PEC) water oxidation. The contained defects in VFPt-2.5 photoanode were revealed by XPS and EPR analyses. Interestingly, the morphology of electrodeposited V-doped α-Fe2O3 is observed to be nanosized with an average diameter size of ∼12 nm and thickness of 300 nm by SEM and TEM analyses. Under light irradiation, VFPt-2.5 photoanode achieved a remarkable onset potential of 0.22 VRHE and a photocurrent density of 2.5 mA/cm2 with an applied external bias at 1.23 VRHE in 1 M KOH electrolyte solution (pH = 14). The calculated IPCE and ABPE values of VFPt-2.5 were ∼37% and 0.85%, respectively. The generated oxygen and hydrogen by VFPt-2.5//Pt were found to be 82 and 170 μmol. The stability of VFPt-2.5 reached 95.2% revealing that Pt could help to overcome the reversed effect of surface states that cause the charge recombination.

Self-Powered Photodetector Based on Ag/CH3NH3PbI3/C Asymmetric Dual-Terminal Device.

CH3NH3PbI3 is capable of exhibiting a superior photoresponse to visible light, but its self-powered devices are typically formed through p-n junctions. In this study, we fabricated a Ag/CH3NH3PbI3/C dual-terminal asymmetric electrode device using a single CH3NH3PbI3 perovskite micro/nanowire, enabling both the photoresponse and self-powered characteristics of CH3NH3PbI3 to visible light. Compared with traditional p-n junction devices, this simple device demonstrates enhanced interface photovoltaic effects by optimizing the combination of the Ag electrode with CH3NH3PbI3, resulting in superior self-powered characteristics. Under low bias voltage, the device achieves a significant on/off ratio of 103, with superior sensitivity and responsivity as well as a maximum rectification ratio of about 12. The photogenerated voltage and current reach approximately 0.8 V and 2 nA, respectively. This simple, compact, and self-powered asymmetric device exhibits great potential for applications in self-powered optoelectronics and wearable devices. This research provides a promising approach for recognizing and utilizing surface state effects in single nanoscale structures.

Experimental investigation of the effect of topological insulator on the magnetization dynamics of ferromagnetic metal: and heterostructure

We have studied the spin-pumping phenomenon in ferromagnetic metal (FM) ()/topological insulator (TI) () bilayer system to understand magnetization dynamics of FM in contact with a TI. TIs embody a spin–momentum-locked surface state that spans the bulk band gap. Due to this special spin texture of the topological surface state, the spin-charge interconversion efficiency of TI is even higher than that of heavy metals. We evaluated the parameters like effective damping coefficient (), spin-mixing conductance () and spin current density () to demonstrate an efficient spin transfer in heterostructure. To probe the effect of the topological surface state, a systematic low-temperature study is crucial as the surface state of TI dominates at lower temperatures. The exponential increase of for all different thickness combinations of FM/TI bilayers and the enhancement of effective damping coefficient () with lowering temperature confirms that the spin chemical potential bias generated from spin-pumping induces spin current into the TI surface state. Furthermore, low-temperature measurements of effective magnetization () and magnetic anisotropy field (Hk ) showed anomaly around the same temperature region where the resistivity of TI starts showing metallic behavior due to the dominance of conducting TI surface state. The anomaly in Hk can result from the emerging exchange coupling between the TI surface state and the local moments of the FM layer at the interface without any long-range ferromagnetic order in TI at the interface.

Photoluminescence enhancement of aluminum ion intercalated MoS2 quantum dots

Low photoluminescence (PL) quantum yield of molybdenum disulfide (MoS2) quantum dots (QDs) has limited practical application as potential fluorescent materials. Here, we report the intercalation of aluminum ion (Al3+) to enhance the PL of MoS2 QDs and the underlying mechanism. With detailed characterization and exciton dynamics study, we suggest that additional surface states including new emission centers have been effectively introduced to MoS2 QDs by the Al3+ intercalation. The synergy of new radiative pathway for exciton recombination and the passivation of non-radiative surface traps is responsible for the enhanced fluorescence of MoS2 QDs. Our findings demonstrate an efficient strategy to improve the optical properties of MoS2 QDs and are important for understanding the regulation effect of surface states on the emission of two dimensional sulfide QDs.