Gap Density Research Articles

Multiple valence states of Fe boosting SERS activity of Fe3O4 nanoparticles and enabling effective SERS-MRI bimodal cancer imaging

Developing novel nanoparticle-based bioprobes utilized in clinical settings with imaging resolutions ranging from cell to tissue levels is a major challenge for tumor diagnosis and treatment. Herein, an optimized strategy for designing a Fe3O4-based bioprobe for dual-modal cancer imaging based on surface-enhanced Raman scattering (SERS) and magnetic resonance imaging (MRI) is introduced. Excellent SERS activity of ultrasmall Fe3O4 nanoparticles (NPs) was discovered, and a 5 × 10−9 M limit of detection for crystal violet molecules was successfully obtained. The high-efficiency interfacial photon-induced charge transfer in Fe3O4 NPs was promoted by multiple electronic energy levels ascribed to the multiple valence states of Fe, which was observed using ultraviolet–visible diffuse reflectance spectroscopy. Density functional theory calculations were utilized to reveal that the narrow band gap and high electron density of states of ultrasmall Fe3O4 NPs significantly boosted the vibronic coupling resonances in the SERS system upon illumination. The subtypes of cancer cells were accurately recognized via high-resolution SERS imaging in vitro using the prepared Fe3O4-based bioprobe with high sensitivity and good specificity. Notably, Fe3O4-based bioprobes simultaneously exhibited T1-weighted MRI contrast enhancement with an active targeting capability for tumors in vivo. To the best of our knowledge, this is the first report on the use of pure semiconductor-based SERS-MRI dual-modal nanoprobes in tumor imaging in vivo and in vitro, which has been previously realized only using semiconductor–metal complex materials. The non-metallic materials with SERS–MRI dual-modal imaging established in this report are a promising cancer diagnostic platform, which not only showed excellent performance in early tumor diagnosis but also possesses great potential for image-guided tumor treatment.

Design of Hybrid Magnetic Stage for a Magnetic–Pneumatic Levitation System

This article presents a magnetic–pneumatic levitation system for gravity offload system of large deployable mechanisms, which combines the attractive force of magnetic stage and thrust force of air-bearing and is characterized by inner stability. The article focuses on the design of the hybrid magnetic stage. The equivalent magnetic circuit network method is used to analyze the electromagnetic (EM)-permanent hybrid magnetic field, the distribution of magnetic flux density of the air gap is solved, and then the magnetic force is obtained. To improve the design efficiency, the two-step design is adopted. First, the air gap is determined by sweeping the reluctance ratio to reduce the influence of parameter deviation of cast iron on the magnetic force. Then, the other parameters of the hybrid magnetic stage are optimized based on the multi-objective particle swarm optimization (MOPSO) algorithm. Through the comparison between the results of finite-element simulation and prototype experiments, the accuracy of the model is verified, and the expected design goal is achieved.

Performance prediction of 2D vertically stacked MoS2-WS2 heterostructures base on first-principles theory and Pearson correlation coefficient

The gas sensor made of two-dimensional (2D) material heterojunction has superior gas sensitivity, but the response mechanism has not been systematically investigated for a long time. In this work, the changes in total density of states (TDOS), partial density of states (PDOS), band gap (BG), and differential charge density (DCD) are investigated by first-principles calculations, revealing the response mechanism of NO2 adsorption by V-MoS2-WS2. The theoretical response value and recovery time of the sensor are described by the Boltzmann transport theory. The theoretical response recovery time and response value are analyzed by Pearson correlation coefficient, and the main influencing factors are BG and electron transfer. Through bader charge analysis, it is found that the electron transfer in the response process of the gas sensor is not only the transfer between the material surface and gas but also that the internal metal atoms provide electrons to be transferred to the gas molecules through the non-metal atoms. This discovery effectively explains the transient and steady-state processes of sensor response, fills the gap in the response mechanism of gas sensors prepared by 2D material heterojunction, and lays the theoretical foundation for the development of 2D material heterojunction gas sensors.

Study of the Interaction Between Reduced Graphene Oxide and NO2Gas Molecules via Density Functional Theory (DFT)

Electronic properties such as density of state, energy gap, HOMO (the highest occupied molecular orbital) level, LUMO (the lowest unoccupied molecular orbital) level and density of bonds, as well as spectroscopic properties like infrared (IR), Raman scattering, force constant, and reduced masses for coronene C24, reduced graphene oxide (rGO) C24O5and interaction between C24O5and NO2gas molecules were investigated. Density functional theory (DFT) with the exchange hybrid function B3LYP with 6-311G** basis sets through the Gaussian 09 W software program was used to do these calculations. Gaussian view 05 was employed as a supplementary software to investigate the geometrical structure of C24, C24O5and the interaction of C24O5with NO2gas molecule. It shows the energy gap of coronene C243.5 eV because the effect of quantum confinement and the Coulomb interaction geometry greatly influence the quasi-particle band gap and C24Oxwhere x = 1–7 was from 0.89 eV to 1.6862 eV a function of number of oxygen atoms and compared with the experiment value of graphene oxide which was between 1 eV and 2.2 eV. The spectroscopic properties were compared with the experiment value of graphene, graphene oxide and NO2longitudinal optical (LO) modes of 1 585, 1 582 and 1 600 cm−1, respectively. The transition state of the interaction of rGO with nitrogen dioxide and Gibbs energy, enthalpy, activation entropy and reaction at various temperatures between 25°C and 100°C were calculated. The activation energy of C24O5with nitrogen dioxide decreases with increasing temperature.

Quantitatively Deciphering Electronic Properties of Defects at Atomically Thin Transition-Metal Dichalcogenides.

Defects can locally tailor the electronic properties of 2D materials, including the band gap and electron density, and possess the merit for optical and electronic applications. However, it is still a great challenge to realize rational defect engineering, which requires quantitative study of the effect of defects on electronic properties under ambient conditions. In this work, we employed tip-enhanced photoluminescence (TEPL) spectroscopy to obtain the PL spectra of different defects (wrinkle and edge) in mechanically exfoliated thin-layer transition metal dichalcogenides (TMDCs) with nanometer spatial resolution. We quantitatively obtained the band gap and electron density at defects by analyzing the wavelength and intensity ratio of excitons and trions. We further visualized the strain distribution across a wrinkle and the edge-induced reconstructive regions of the band gap and electron density by TEPL line scans. The doping effect on the Fermi level and optical performance was unveiled through comparative studies of edges on TMDC monolayers of different doping types. These quantitative results are vital to guide defect engineering and design and fabrication of TMDC-based optoelectronics devices.

Temperature dependent carrier transport in few-layered MoS2: from hopping to band transport

Understanding the carrier transport mechanisms is critical for electronic devices based on 2D semiconductors. Here, using a two-terminal device configuration, we show that the carrier transport behaviours in chemical vapour deposited few-layer MoS2 transition from resonant tunnelling to hopping, and eventually to band transport as the temperature increases from 5 K to 370 K. Specifically, the transport in the channel is dominated by resonant tunnelling when T < 30 K is reflected in the temperature-independent conductance. At 50 K < T < 110 K, the channel conductance exhibits a dependence of exp(T 1/2), a signature of Efros–Shklovskii type variable range hopping (VRH). At 110 K < T < 160 K, carrier transport behaves in a transition region with potential attribution to Mott-type VRH. At 160 K < T < 210 K, the nearest neighbour hopping mechanism is confirmed by the linear dependence from the resistance curve derivative analysis. For VRH, the localization length, hopping distance and energy, Coulomb gap energy and density of states are extracted. At T > 210 K, the carrier transport is dominated by thermally activated band transport based on AC conductance and mobility analysis. These findings are significant for revealing the material properties for future 2D semiconductor device applications.

Multilayer Random Sequential Adsorption

In this work, we present a variant of the multilayer random sequential adsorption (RSA) process that is inspired by orthogonal resource sharing in wireless communication networks. In the one-dimensional (1D) version of this variant, the deposition of overlapping rods is allowed only if they are assigned two different colors, where colors are symbolic of orthogonal resources, such as frequency bands, in communication networks. Owing to a strong spatial coupling among the deposited rods of different colors, finding an exact solution for the density of deposited rods of a given color as a function of time seems intractable. Hence, we propose two useful approximations to obtain the time-varying density of rods of a given color. The first approximation is based on the recursive use of the known monolayer RSA result for the indirect estimation of the density of rods for the multilayer version. The second approximation, which is more accurate but computationally intensive, involves accurate characterization of the time evolution of the gap density function. This gap density function is subsequently used to estimate the density of rods of a given color. We also consider the two-dimensional (2D) version of this problem, where we estimate the time-varying density of deposited circles of a given color as a function of time by extending the first approximation approach developed for the 1D case. The accuracy of all the results is validated through extensive Monte Carlo simulations.

Thickness Dependence of Electronic Structure and Optical Properties of F8BT Thin Films.

Electronic devices based on polymer thin films have experienced a tremendous increase in their efficiency in the last two decades. One of the critical factors that affects the efficiency of polymer solar cells or light emitting devices is the presence of structural defects that controls non-radiative recombination. The purpose of this report is to demonstrate a non-trivial thickness dependence of optoelectronic properties and structure (dis)order in thin conductive poly(9,9-dioctyfluorene-alt-benzothiadiazole), F8BT, polymer films. The UV-Vis absorption spectra exhibited blue shift and peak broadening; significant changes in 0–0 and 0–1 radiative transition intensity was found in photoluminescence emission spectra. The density of state (DOS) was directly mapped by energy resolved-electrochemical impedance spectroscopy (ER-EIS). Satellite states 0.5 eV below the lowest unoccupied molecular orbital (LUMO) band were revealed for the thinner polymer films. Moreover, the decreasing of the deep states density in the band gap manifested an increment in the material structural ordering with increasing thickness. Changes in the ratio between crystalline phases with face-on and edge-on orientation of F8BT chains were identified in the films by grazing-incidence wide angle X-ray scattering technique. A thickness threshold in all investigated aspects of the films at a thickness of about 100 nm was observed that can be attributed to the development of J-H aggregation in the film structure and mutual interplay between these two modes. Although a specific structure–property relationship thickness threshold value may be expected for thin films prepared from various polymers, solvents and under different process conditions, the value of about 100 nm can be generally considered as the characteristic length scale of this phenomenon.

Effect of Concentrated Growth Factor on Distraction Osteogenesis of Dental Implant Distractors

The periosteum of bone segments can be destroyed during the installation of a dental implant distractor (DID) device, resulting in bone defects on the exposed side of the distraction gap. The purpose of this animal experiment is to explore the application of concentrated growth factor (CGF) in DID surgery, which can induce angiogenesis and osteogenesis and improve osteogenesis defects caused by periosteum loss on the exposed side of bone segments. CGF is the latest generation of platelet concentrate. Twenty-four DID devices inserted into the tibias of 8 goats were evenly divided into the CGF and control groups. Following 10days of distraction and a 12-week consolidation period, all 8 animals were euthanized to retrieve their tibias. The distraction gap between each segment was measured at 5 points, and the average value was taken as the computed tomography (CT) value of the distraction gap at 4, 8, and 12weeks after distraction. The vascular density and trabecular bone volume of each DID distraction gap were determined and statistically compared between the 2 groups. The CT value of the distraction gap increased gradually in control groups at 4, 8, and 12weeks after distraction to 319.3±14.6, 449.3±34.4, and 614.0±15.6 HU in the control group and 368.3±8.8, 544.5±12.3, and 661.0±8.1 HU in the CGF group. The trabecular bone volume was 281.7±16.5 and 209.7±21.6μm2 in the CGF group and control group. The vascular density was 17.7±2.1 and 11.7±1.9 in the CGF group and control group. Statistically significant differences were observed in the CT value (P=.002), vascular density (P=.023), and trabecular bone volume (P=.010) between the CGF and control groups. The application of an autogenous CGF membrane in DID surgery repaired bone defects caused by osteolysis around osteotomy segments.

A model study on superfluidity of a unitary Fermi gas of atoms interacting with a finite-ranged potential

We calculate Bardeen–Cooper–Schrieffer (BCS) state of a unitary Fermi gas of atoms interacting with the finite-ranged Jost-Kohn potential which has been recently shown to account for the resonant interactions (2019 J. Phys. B: At. Mol. Opt. Phys. 52 165004). Using exact scattering solution of the potential, we derive two-body T-matrix element which is employed to construct the BCS Hamiltonian in momentum space. We present results on the energy- and range-dependence of the pairing gap and superfluid density and the range-dependence of the chemical potential for a wide variation of the scattering length including the unitary regime. In the zero range limit our calculated gap at the Fermi energy is found to be nearly equal to that calculated in mean-field theory with contact potential. The mean gap averaged over the full width at half maximum of the gap function in the zero range and unitary limits is found to be 0.42E F which is quite close to the recent result of the quantum Monte-Carlo simulation (2018 Phys. Rev. A 97 013601). The chemical potential in the zero range limit also agrees well with that for the contact potential.

An Efficient Modeling of Air Gap and Nonlinear Analysis of Magnetic Equivalent Circuit for Large Turbogenerators Under No-Load Condition

Fast and accurate modeling of large turbogenerators is of great importance for optimal design and commercial value. Therefore, a 2-D nonlinear magnetic equivalent circuit (MEC) model considering saturation and rotor motion is proposed in this article. When the node-based MEC adopts relaxation iteration for nonlinear calculation, it is not easy to converge by using traditional interpolation function and piecewise fitting of the core <i>B&#x2013;H</i> or <inline-formula> <tex-math notation="LaTeX">$u_{r} (B)$ </tex-math></inline-formula> curve. The main contribution of this article is to find that using continuous fitting function is of great significance for the convergence of nonlinear iteration. At the same time, a simple and effective MEC of air gap is also provided. The flux density of air gap and stator tooth obtained by the proposed method is in good agreement with results of the finite element. No-load characteristics were also verified by the finite element and experiment. The calculation time of a cycle takes about 700 s by the finite element, and however, MEC only takes about 2 s.

Treatment of electroplating industry wastewater: a review on the various techniques.

Water pollution by recalcitrant compounds is an increasingly important problem due to the continuous introduction of new chemicals into the environment. Choosing appropriate measures and developing successful strategies for eliminating hazardous wastewater contaminants from industrial processes is currently a primary goal. Electroplating industry wastewater involves highly toxic cyanide (CN), heavy metal ions, oils and greases, organic solvents, and the complicated composition of effluents and may also contain biological oxygen demand (BOD), chemical oxygen demand (COD), SS, DS, TS, and turbidity. The availability of these metal ions in electroplating industry wastewater makes the water so toxic and corrosive. Because these heavy metals are harmful to living things, they must be removed to prevent them from being absorbed by plants, animals, and humans. As a result, exposure to electroplating wastewater can induce necrosis and nephritis in humans and lung cancer, digestive system cancer, anemia, hepatitis, and maxillary sinus cancer with prolonged exposure. For the safe discharge of electroplating industry effluents, appropriate wastewater treatment has to be provided. This article examines and assesses new approaches such as coagulation and flocculation, chemical precipitation, ion exchange, membrane filtration, adsorption, electrochemical treatment, and advanced oxidation process (AOP) for treating the electroplating industry wastewater. On the other hand, these physicochemical approaches have significant drawbacks, including a high initial investment and operating cost due to costly chemical reagents, the production of metal complexes sludge that needs additional treatment, and a long recovery process. At the same time, advanced techniques such as electrochemical treatment can remove various kinds of organic and inorganic contaminants such as BOD, COD, and heavy metals. The electrochemical treatment process has several advantages over traditional technologies, including complete removal of persistent organic pollutants, environmental friendliness, ease of integration with other conventional technologies, less sludge production, high separation, and shorter residence time. The effectiveness of the electrochemical treatment process depends on various parameters, including pH, electrode material, operation time, electrode gap, and current density. This review mainly emphasizes the removal of heavy metals and another pollutant such as CN from electroplating discharge. This paper will be helpful in the selection of efficient techniques for treatment based on the quantity and characteristics of the effluent produced.

Analysis of photoelectrochemical water splitting using machine learning

In this study, an extensive dataset containing 10,560 data points obtained from 584 experiments in 180 articles for photoelectrochemical water splitting over n-type semiconductors was analyzed using machine learning techniques. After the pre-analysis of the dataset using simple descriptive statistics, association rule mining (ARM), random forest (RF) and decision tree (DT) were utilized to identify the patterns in the data and establish relations between photocurrent density and 33 descriptors including electrode materials and synthesis methods as well as the properties of irradiation and electrolyte solution. ARM was successfully employed to identify the critical foctors for band gap and photocurrent density. DT model for the band gap was quite successful (with the overall training and testing accuracies of 78% and 72% respectively) while the model was less accurate for the photocurrent density even though it is still notable (training accuracy = 61%; testing accuracy = 54%). Predictive model developed by random forest for the band gap of the electrode was also remarkably good with the root mean square error of validation and testing 0.24 and 0.27 respectively.

In Situ Formation of δ-FAPbI3 at the Perovskite/Carbon Interface for Enhanced Photovoltage of Printable Mesoscopic Perovskite Solar Cells

Formamidinium lead triiodide (FAPbI3) perovskite has attracted intensive research attention due to its ideal band gap and low defect density for photovoltaic applications. Particularly, the existence of non-photoactive δ-phase FAPbI3 has been considered detrimental to regular-structured perovskite solar cells (PSCs). Here, in hole-conductor-free triple-mesoscopic PSCs, we observe that α-phase FAPbI3 transforms to δ-phase FAPbI3 at the perovskite/carbon interface and in situ forms an α/δ-phase junction when the as-fabricated cells are exposed to humid atmosphere. The α/δ-phase junction shows a favorable band alignment and significantly suppresses the charge recombination at the interface. By controlling the relative humidity of the atmosphere, we fabricate FAPbI3-based cells that deliver a champion efficiency of 17.11% with an enhanced open-circuit voltage (VOC) of 1020 mV. This work demonstrates the potential of δ-phase FAPbI3 for benefiting the device performance of PSCs and proposes the concept of constructing perovskite-based junctions at the interface between a perovskite-absorbing layer and charge-transporting layers for enhancing the VOC of PSCs.

First-principles study on the heterostructure of twisted graphene/hexagonal boron nitride/graphene sandwich structure

In recent years, the discovery of ‘magic angle’ graphene has given new inspiration to the formation of heterojunctions. Similarly, the use of hexagonal boron nitride, known as white graphene, as a substrate for graphene devices has more aroused great interest in the graphene/hexagonal boron nitride heterostructure system. Based on the first principles method of density functional theory, the band structure, density of states, Mulliken population, and differential charge density of a tightly packed model of twisted graphene/hexagonal boron nitride/graphene sandwich structure have been studied. Through the establishment of heterostructure models twisted bilayer-graphene inserting hBN with different twisted angles, it was found that the band gap, Mulliken population, and charge density, exhibited specific evolution regulars with the rotation angle of the upper graphene, showing novel electronic properties and realizing metal–insulator phase transition. We find that the particular value of the twist angle at which the metal–insulator phase transition occurs and propose a rotational regulation mechanism with angular periodicity. Our results have guiding significance for the practical application of heterojunction electronic devices.

A Novel Axial-Flux Dual-Stator Toothless Permanent Magnet Machine for Flywheel Energy Storage

This paper presents an alternative system called the axial-flux dual-stator toothless permanent magnet machine (AFDSTPMM) system for flywheel energy storage. This system lowers self-dissipation by producing less core loss than existing structures; a permanent magnet (PM) array is put forward to enhance the air–gap flux density of the symmetrical air gap on both sides. Moreover, its vertical stability is strengthened through the adoption of an axial-flux machine, so expensive active magnetic bearings can be replaced. The symmetry configuration of the AFDSTPMM system is shown in this paper. Then, several parts of the AFDSTPMM system are optimized thoroughly, including stator windings, number of pole pairs and the PM parameters. Further, the performance of the proposed PM array, including back-EMFs, air–gap flux density, average torque, torque ripple and over-load capacity, are compared with the Halbach PM array and spoke PM array, showing the superiority of proposed configuration. Finally, 3D simulations were made to testify for the 2D analyses.

Numerical Modeling and Evaluation of Centrifugal Pendulum Vibration Absorber for EV Motors

With the spread of electric vehicles, development of compact motors with high energy conversion efficiency is underway, and IPM motors are widely used as vehicle drive motors. However, spatial harmonics in the magnetic flux density in the air gap increase the torque ripple and cause vibration noise. Therefore, we focused on a centrifugal pendulum vibration absorber (CPA), which is effective in suppressing torsional vibration in engines. Although the suppression of low-order rotational fluctuation components has been verified, there are few previous studies of its application to higher-order rotational fluctuation components such as those of IPM motors. Therefore, in this paper, the vibration suppression effect of CPA on high-frequency torsional vibration of IPM motors is verified by multi-body dynamics analysis.

Mapping between Density of States and Energy Band Gap for Bismuth-Based Semiconductors Via Machine Learning Prediction

Mapping between Density of States and Energy Band Gap for Bismuth-Based Semiconductors Via Machine Learning Prediction

Numerical simulations of 26.11% efficient planar CH3NH3PbI3 perovskite n-i-p solar cell

The current research work identifies a novel combination of perovskite solar cell (PSC) layers that improves the effectiveness of a solar cell. In this research work, analysis and optimization of the results for a CH3NH3PbI3 perovskite-based solar cell is performed using SCAPS 1D modelling software. The impact of varying multiple characteristics including band gap, thickness, and total defect density on the effectiveness of a PSC has been studied and analysed. In terms of performance parameters, the current research work's findings are promising. The proposed structure attains an open circuit voltage (VOC) of 1.112 V along with a short circuit current density (JSC) of 26.02 mA/cm2. Consequently, the proposed simulated structure has a power conversion efficiency (PCE) of 26.11% and 86.27% value of fill factor.

Excited-state charge polarization and electronic structure of mixed-cation halide perovskites: the role of mixed inorganic-organic cations in CsFAPbI3.

Mixed-cation perovskite materials have shown great potential for sunlight harvesting and have surpassed unmixed perovskite materials in solar cell efficiency and stability. The role of mixed monovalent cations in the enhanced optoelectronic properties and excited state response, however, are still elusive from a theoretical perspective. Herein, through time dependent density functional theory calculations of mixed cation perovskites, we report the electronic structure of Cs formamidinium (FA) mixed cationic lead iodide (Cs0.17FA0.87PbI3) in comparison to the corresponding single monovalent cation hybrid perovskite. The results show that the Cs0.17FA0.87PbI3 and FAPbI3 had negligible differences in the optical band gap, and partial and total density of states in comparison to a single cation perovskite, while the effective mass of carriers, the local atomic density of states, the directional transport, and the structural distortions were significantly different. A lattice-distortion-induced asymmetry in the ground-state charge density is found, and originates from the co-location of caesium atoms in the lattice and signifies the effect on the charge density upon cation mixing and corresponding symmetry breaking. The excited-state charge response and induced polarizabilities are quantified, and discussed in terms of their importance for effective light absorption, charge separation, and final solar cell performance. We also quantify the impact of such polarizabilities on the dynamics of the structure of the perovskites and the implications this has for hot carrier cooling. The results shed light on the mechanism and origin of the enhanced performance in mixed-cation perovskite-based devices and their merits in comparison to single cation perovskites.