Image Charge Research Articles

Microscopic Origin of Twist-Dependent Electron Transfer Rate in Bilayer Graphene.

Using molecular simulation and continuum dielectric theory, we consider how electrochemical kinetics are modulated by the twist angle in bilayer graphene electrodes. By establishing a connection between the twist angle and the screening length of charge carriers within the electrode, we investigate how tunable metallicity modifies the statistics of the electron transfer energy gap. Constant potential molecular simulations show that the activation free energy for electron transfer increases with screening length, leading to a non-monotonic dependence on the twist angle. The twist angle alters the density of states, tuning the number of thermally accessible channels for electron transfer and the reorganization energy by affecting the stability of the vertically excited state through attenuated image charge interactions. Understanding these effects allows us to express the Marcus rate of interfacial electron transfer as a function of the twist angle in a manner consistent with experimental observations.

Coarse-grained polarizable soft solvent models, with applications in dissipative particle dynamics.

We critically examine a broad class of explicitly polarizable soft solvent models aimed at applications in dissipative particle dynamics. We obtain the dielectric permittivity using the fluctuating box dipole method in linear response theory and verify the models in relation to several test cases, including demonstrating ion desorption from an oil-water interface due to image charge effects. We additionally compute the Kirkwood factor and find that it uniformly lies in the range gK≃0.7-0.8, indicating that dipole-dipole correlations are not negligible in these models. This is supported by the measurements of dipole-dipole correlation functions. As a consequence, Onsager theory over-predicts the dielectric permittivity by 20%-30%. The mean square molecular dipole moment can be accurately estimated with a first-order Wertheim perturbation theory.

Image charge effects under metal and dielectric boundary conditions

Image charge effects under metal and dielectric boundary conditions

Two- and Three-Dimensional Ag-Au Nanoalloying: Heterogeneous Building Blocks Enhancing Near-Field Focusing.

In this study, we report a strategy for fabricating binary surface-enhanced Raman scattering (SERS) substrates composed of plasmonic Pt@Ag and Pt@Au truncated-octahedral (TOh) dual-rim nanoframes (DNFs) functioning as a "nanoalloy." Pt TOh frameworks act as a scaffold to develop nanoarchitectures with surface decoration using plasmonically active materials (i.e., Au or Ag), resulting in identical sizes and shapes for the two distinct plasmonic elements, facilitating the fabrication of a "nanoalloy" blend of two shape-complex building blocks. The structural complexity from the dual-rim on (111) facets, combined with the mirror charge effect (i.e., enhanced polarization between Ag and Au) at the interface of heterogeneous components, significantly amplifies SERS activity. We carefully investigated near-field focusing of binary SERS substrates through single-particle and bulk SERS measurements corroborated by finite element method (FEM) calculations. Crucially, we developed free-standing superpowders (SPs) in which each heterogeneous building block formed micron-sized supercrystals with adjustable component ratios. These plasmonic SPs were applied to contaminated areas for analyte detection, demonstrating their potential for practical SERS applications.

Adsorption of polyelectrolytes in the presence of varying dielectric discontinuity between solution and substrate.

We examine the interactions between polyelectrolytes (PEs) and uncharged substrates under conditions corresponding to a dielectric discontinuity between the aqueous solution and the substrate. To this end, we vary the relevant system characteristics, in particular the substrate dielectric constant ɛs under different salt conditions. We employ coarse-grained molecular dynamics simulations with rodlike PEs in salt solutions with explicit ions and implicit water solvent with dielectric constant ɛw = 80. As expected, at low salt concentrations, PEs are repelled from the substrates with ɛs < ɛw but are attracted to substrates with a high dielectric constant due to image charges. This attraction considerably weakens for high salt and multivalent counterions due to enhanced screening. Furthermore, for monovalent salt, screening enhances adsorption for weakly charged PEs, but weakens it for strongly charged ones. Meanwhile, multivalent counterions have little effect on weakly charged PEs, but prevent adsorption of highly charged PEs, even at low salt concentrations. We also find that correlation-induced charge inversion of a PE is enhanced close to the low dielectric constant substrates, but suppressed when the dielectric constant is high. To explore the possibility of a PE monolayer formation, we examine the interaction of a pair of like-charged PEs aligned parallel to a high dielectric constant substrate with ɛs = 8000. Our main conclusion is that monolayer formation is possible only for weakly charged PEs at high salt concentrations of both monovalent and multivalent counterions. Finally, we also consider the energetics of a PE approaching the substrate perpendicular to it, in analogy to polymer translocation. Our results highlight the complex interplay between electrostatic and steric interactions and contribute to a deeper understanding of PE-substrate interactions and adsorption at substrate interfaces with varying dielectric discontinuities from solution, ubiquitous in biointerfaces, PE coating applications, and designing adsorption setups.

Spherical image potentials for an N-charged particle system

The potential energy interaction between a system of N -charged particles and a spherical conducting surface is obtained by means of direct integration of the electrostatic energy density. Making use of the image charge method, we show that the full potential can be split into a sum of one-body central potentials and two-body contributions related to particle–particle and particle–sphere interactions. Analytical expressions are provided for all contributions. The spherical surface of radius R can be considered to correspond to a cavity inside a conductor or a metallic ball, with two different electric conditions, namely the grounded and isolated charged cases. By taking the limit R → ∞ of the spherical case, one recovers the image potential for an N -charged particle system interacting with a planar conductor. While the final results for the potential do not appear as unexpected, the provided analytical expressions should help in diverse physical applications modelled by a number of charged particles interacting close to a spherical surface.

Effects and optimizations of image charge technology on the imaging performance of capacitive division image readout MCP detectors

A finite element model is proposed to investigate the impact of image charge technology on the imaging performance of capacitive division image readout (C-DIR) devices. Through detailed simulation analysis, we explore the charge density of the induction layer and each electrode layer in the multilayer capacitive anode. The position nonlinearity is calculated for various resistances per square of the induction layer and substrate thicknesses. The simulation results indicate a strong linear correlation between the position nonlinearity and substrate thickness, and an exponential decay relationship with the logarithmic resistance per square of the induction layer. The C-DIR detector is experimentally tested for imaging nonlinearity and counting rate, with the resistance per square of the induction layer ranging from 0.005 to 1000 MΩ, and the substrate thickness ranging from 1 to 5 mm. The experimental results validate the simulation conclusions and reveal the impact of the “charge accumulation effect” and “shielding effect” on the imaging performance of the detector, as well as the “imaging compression” phenomenon. Optimized image charge readout technology enables the C-DIR detector to achieve an imaging nonlinearity of 1.53% in the experiment.

Orientational dynamics of the water layer adjacent to Au surface accelerated by polarization effect.

The orientation and rearrangement of water on a gold electrode significantly influences its physicochemical heterogeneous performance. Despite numerous experimental and theoretical studies aimed at uncovering the structural characteristics of interfacial water, the orientational behavior resulting from electrode-induced rearrangements remains a subject of ongoing debate. Here, we employed molecular dynamics simulations to investigate the adaptive structure and dynamics properties of interfacial water on Au(111) and Au(100) surfaces by considering a polarizable model for Au atoms in comparison with the non-polarizable model. Compared to the nonpolarizable systems, the polarization effect can enhance the interaction between water molecules and the gold surface. Unexpectedly, the rotational dynamics directly associated with the orientational behavior of water adjacent to the gold surface is accelerated, thereby reducing the hydrogen bond lifetime. The underlying mechanism for this anomalous phenomenon originates from the polarization effect, which induces the attraction of the positive hydrogen atoms to the surface by the negative image charge. This leads to a change in orientation that disrupts the hydrogen bonds in the first water layer and subsequently accelerates reorientation dynamics of water molecules adjacent to the gold surface. These results shed light on the intricate interplay between polarization effects and water molecule dynamics on metal surfaces, establishing the foundation for the rational regulation of the orientation of interfacial water.

Leakage current as a probe into the mechanics of carrier transport in insulating composite polymers

Amorphous composite polymers are widely used as insulators in microelectronics due to their high dielectric strength, mechanical robustness, and thermal stability. However, organic–inorganic composite systems suffer from undesirable performance and accelerated degradation due to leakage current (JTot). Unfortunately, the underlying mechanism of JTot and its components (e.g., ionic and electronic constituents) are inadequately understood, particularly in extreme use conditions (e.g., high humidity and temperature). In this study, we use numerical simulation and experimental JTot data (in amorphous epoxy polymer with silica fillers) to (i) unify the electrostatic model for JTot in composite polymers, (ii) illustrate that the early part of JTot (i.e., external current) is primarily due to the image charge associated with ion transport/ localization (Jion) near the metallic contacts, (iii) demonstrate that the accumulated counter-ions reduce the barrier for electronic charge injection (by band bending) and facilitate electronic injection from the metals (Jelec), and (iv) provide an algorithm for the in situ ion transport characterization of composite insulators by exploiting Jion. This work provides new insights regarding the leakage current mechanism and how it can be used as a probe into the complex transport mechanisms of the composite material.

Excellent electrostatic control and gate reliability for breakdown enhanced AlGaN/GaN HEMTs with extreme permittivity BaTiO3

In this Letter, we investigated the electrostatic control capability and gate reliability of the BaTiO3 integrated AlGaN/GaN HEMTs. The fabricated HEMT exhibits a high peak maximum transconductance of 141 mS/mm, a large ON/OFF current ratio of 4.1 × 109, and a small subthreshold swing of 70.6 mV/dec, attributed to the gate leakage suppression and prominent gate coupling. These parameters demonstrate that the HEMT device with extreme permittivity BaTiO3 dielectric has the excellent electrostatic control capability. A high breakdown voltage of 1348 V was also achieved thanks to the screening of the negative gate image charges and the resulting improvement of electric field distribution by using double-layer BaTiO3 with the wrapped gate structure. Furthermore, gate bias step-stress and pulse I–V measurements show that the device exhibits a small VTH shift of 0.06 V at 2 V bias stress and a slight current collapse of ∼2.7% accompanied with a 6.0% RON increment at a quiescent drain bias VDSQ = 30 V, which benefits from the high quality of the dielectrics/AlGaN interface with Dit of 8.87 × 1012 eV−1/cm2. These findings elucidate the immense potential of extreme permittivity dielectrics engineering in achieving high-performance AlGaN/GaN power electronic devices.

Differential Capacitance of Ionic Liquid Confined between Metallic Interfaces.

We present here a detailed analysis of the electric double layer at the gold electrode/[BMIM][BF4] interface using a polarizable model for the electrode, based on our recent approach to include image charges [Geada et al. Nat. Commun. 2018, 9, 716]. A double bell (camel) shape is obtained for the differential capacitance, where the inclusion of metal polarization allows for a higher density of ions in the double layer, particularly around the maxima, thereby increasing the capacitance. The charging mechanism differs for the positive and negative electrodes, with counterion adsorption prevailing at the anode and co-ion desorption prevailing at the cathode. The charging mechanism is predominantly governed by the BF4 anions, serving as counterions and co-ions at the anode and cathode, respectively. Within the considered range of potentials, only minor changes are observed in the dynamical properties, specifically in the diffusion coefficients. Notably, it is interesting to observe that bulk properties are restored at a shorter distance from the gold surface in the case of the anode compared to the cathode.

Tunable hyperbolic polaritons with plasmonic phase-change material In3SbTe2

Abstract Hyperbolic polaritons that originate from the extreme optical anisotropy in van der Waals (vdW) crystals have gained much attention for their potential in controlling nanolight. For practical use, there has been a strong interest to develop various manipulation strategies to customize the propagation of hyperbolic polaritons on a deeply sub-diffractional scale. In this regard, phase-change materials (PCMs) that possess two phases with different refractive indices offer suitably a tunable dielectric environment. Here, we report on the tuning of hyperbolic phonon polaritons in natural vdW crystals, hexagonal boron nitride (hBN), and alpha-phase molybdenum trioxide (α-MoO3), using the plasmonic phase-change material In3SbTe2 (IST). Unlike conventional PCMs whose both phases are dielectric, IST features a metallic crystalline phase that is stable at room temperature. The coupling between polaritons with their mirror charges in the underneath crystalline IST triggers an even stronger field confinement for polaritons. Moreover, benefited from the metallicity of laser-writable crystalline IST, we show an all-optical material platform in which crystalline IST boundaries efficiently excite and focus hyperbolic phonon polaritons in α-MoO3. Our experiments highlight the possibility to obtain new degrees of freedom in polariton engineering with plasmonic PCMs, thereby expanding the toolkit of tunable nanophotonics with flexible, on-demand fabrication and reconfiguration capabilities.

CP-AFM Molecular Tunnel Junctions with Alkyl Backbones Anchored Using Alkynyl and Thiol Groups: Microscopically Different Despite Phenomenological Similarity.

In this paper, we report results on the electronic structure and transport properties of molecular junctions fabricated via conducting probe atomic force microscopy (CP-AFM) using self-assembled monolayers (SAMs) of n-alkyl chains anchored with acetylene groups (CnA; n = 8, 9, 10, and 12) on Ag, Au, and Pt electrodes. We found that the current-voltage (I-V) characteristics of CnA CP-AFM junctions can be very accurately reproduced by the same off-resonant single-level model (orSLM) successfully utilized previously for many other junctions. We demonstrate that important insight into the energy-level alignment can be gained from experimental data of transport (processed via the orSLM) and ultraviolet photoelectron spectroscopy combined with ab initio quantum chemical information based on the many-body outer valence Green's function method. Measured conductance GAg < GAu < GPt is found to follow the same ordering as the metal work function ΦAu < ΦAu < ΦPt, a fact that points toward a transport mediated by an occupied molecular orbital (MO). Still, careful data analysis surprisingly revealed that transport is not dominated by the ubiquitous HOMO but rather by the HOMO-1. This is an important difference from other molecular tunnel junctions with p-type HOMO-mediated conduction investigated in the past, including the alkyl thiols (CnT) to which we refer in view of some similarities. Furthermore, unlike in CnT and other junctions anchored with thiol groups investigated in the past, the AFM tip causes in CnA an additional MO shift, whose independence of size (n) rules out significant image charge effects. Along with the prevalence of the HOMO-1 over the HOMO, the impact of the "second" (tip) electrode on the energy level alignment is another important finding that makes the CnA and CnT junctions different. What ultimately makes CnA unique at the microscopic level is a salient difference never reported previously, namely, that CnA's alkyne functional group gives rise to two energetically close (HOMO and HOMO-1) orbitals. This distinguishes the present CnA from the CnT, whose HOMO stemming from its thiol group is well separated energetically from the other MOs.

Tribocharging of granular materials flowing in grounded inclined tubes.

Granular materials charge as they flow through inclined tubes due to the triboelectric effect. This charging persists even when the tube is grounded and is influenced by the tube's angle. In this study, we demonstrate that the tribocharging of granular materials can be accurately replicated through numerical simulations using the patch model. Charge transfers occur between the grains and the tube from donor patches to acceptor patches, ensuring the tube does not retain charge to simulate grounded conditions. Interactions between the grains and the tube are modeled using the method of image charges. We conducted experiments to validate the numerical results. Our findings reveal that the density difference between donor patches on the grains and the tube determines the charge acquired by the granular materials. Additionally, we observed both in experiments and simulations that granular materials charge less with the tube when it is shorter or less tilted. We interpret the influence of the tube's angle and length using the patch model. This model offers valuable insights into the tribocharging mechanisms of various materials.

Tunable acoustic graphene plasmon enhanced nano-infrared spectroscopy

Nano-infrared spectroscopy (nano-IR) technology can exceed the diffraction limit of light, achieving infrared spectroscopic detection with a spatial resolution of about 10 nm, which is an important technical means for studying the chemical composition and structure of molecules on a nanoscale. However, the weak infrared absorption signals of nanoscale materials pose a significant challenge due to the large mismatch between their dimensions and the wavelength of infrared light. The infrared absorption signals of molecular vibrational modes are proportional to the squares of the electromagnetic field intensities at their positions, implying that higher electromagnetic field intensity can significantly improve the sensitivity of molecular detection. Acoustic graphene plasmons (AGPs), excited by the interaction between free charges in graphene and image charges in metal, exhibit strong optical field localization and electromagnetic field enhancement. These properties make AGPs an effective platform for enhancing nano-IR detection sensitivity. However, the fabrication of graphene nanostructures often introduces numerous edge defects due to the limitations of nanofabrication techniques, significantly reducing the electromagnetic field enhancement observed in experiments. Here, we use finite element simulation to theoretically propose a tunable enhanced nano-IR detection platform based on nanocavity-acoustic graphene plasmons (n-AGPs), which utilizes a graphene/air gap/gold nanocavity structure. This platform avoids needing the nanofabrication of graphene, thereby preventing defects and contamination from being introduced in processes such as electron beam exposure and plasma etching. By plotting the dispersion of n-AGP, it is found that n-AGP has a high wavelength compression capability comparable to AGP (&lt;i&gt;λ&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt;/&lt;i&gt;λ&lt;/i&gt;&lt;sub&gt;AGP&lt;/sub&gt; = 48). Additionally, due to the introduction of the gold nanocavity structure, n-AGP possess an extremely small mode volume (&lt;i&gt;V&lt;/i&gt;&lt;sub&gt;n-AGP&lt;/sub&gt; ≈ 10&lt;sup&gt;–7&lt;/sup&gt;&lt;inline-formula&gt;&lt;tex-math id="M5"&gt;\begin{document}$ {{ \lambda }}_{0}^{3} $\end{document}&lt;/tex-math&gt;&lt;alternatives&gt;&lt;graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240489_M5.jpg"/&gt;&lt;graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="13-20240489_M5.png"/&gt;&lt;/alternatives&gt;&lt;/inline-formula&gt;, &lt;i&gt;λ&lt;/i&gt;&lt;sub&gt;0&lt;/sub&gt; = 6.25 μm). By calculating the electric field intensity distribution (|&lt;i&gt;E&lt;/i&gt;&lt;sub&gt;norm&lt;/sub&gt;|) and the normalized electric field intensity spectrum (i.e. the relationship between frequency and |&lt;i&gt;E&lt;/i&gt;&lt;sub&gt;&lt;i&gt;z&lt;/i&gt;&lt;/sub&gt;|/|&lt;i&gt;E&lt;/i&gt;&lt;sub&gt;&lt;i&gt;z&lt;/i&gt;0&lt;/sub&gt;|) of the n-AGP structure, it is evident that due to the high electron density on the gold surface, electromagnetic waves can be reflected from the boundaries of the gold nanocavity and resonantly enhanced within the nanocavity. At the resonant frequency of n-AGP (1800 cm&lt;sup&gt;–1&lt;/sup&gt;), the electric field inside the cavity is enhanced by about 50 times. In contrast, at similar resonant frequencies, the electric field enhancement factor of Graphene plasmon (resonant frequency 1770 cm&lt;sup&gt;–1&lt;/sup&gt;) and AGP (resonant frequency 1843 cm&lt;sup&gt;–1&lt;/sup&gt;) are approximately 3 and 2 times, respectively, significantly lower than that of n-AGP. Furthermore, by placing a protein film (60 nm wide and 10 nm high) under the graphene, we calculate the spectral dip depths caused by Fano resonance between n-AGP and AGP with the vibrational modes of protein molecules, thereby validating the enhancement factors of different modes for protein vibrational mode infrared absorption. For the amide-I band of proteins, the detection sensitivity of n-AGP is about 60 times higher than that of AGP. Additionally, we find that by adjusting the structural parameters of the gold nanocavity, including cavity depth, width, and surface roughness, the response frequency band of n-AGP can be modulated (from 1290 to 2124 cm&lt;sup&gt;–1&lt;/sup&gt;). Specifically, as the cavity depth increases, the electric field enhancement of n-AGP is improved, and the wavelength compression capability of n-AGP decreases, causing the resonant frequency to be blue-shifted (from 1793 to 2124 cm&lt;sup&gt;–1&lt;/sup&gt;). As the cavity width increases, the resonant frequency of n-AGP is red-shifted (from 1793 to 1290 cm&lt;sup&gt;–1&lt;/sup&gt;), and the effectiveness of the gold nanocavity boundary in reflecting the resonant electric field within the cavity diminishes, resulting in a decrease in the electric field enhancement factor. With the gradual increase in the roughness of the gold nanocavity bottom, the effective depth of the gold nanocavity increases, causing the n-AGP resonant frequency to be blue-shifted (from 1793 to 1861 cm&lt;sup&gt;–1&lt;/sup&gt;) and the electric field enhancement factor to increase. Moreover, by adjusting the Fermi level of graphene (from 0.3 to 0.6 eV), we achieve dynamic tuning of n-AGP (from 1355 to 1973 cm&lt;sup&gt;–1&lt;/sup&gt;). As the Fermi level of graphene increases, the wavelength compression capability of n-AGP decreases, resulting in a blue-shift in the resonant frequency. Finally, by optimizing the structural parameters and Fermi level of n-AGP, and placing protein particles of different sizes (20, 15, and 10 nm high, all 10 nm wide) into the graphene/gold nanocavity structure, we verify the protein detection capability of n-AGP-enhanced nano-IR. We find that n-AGP can detect the vibrational fingerprint features of the amide-I band and amide-II band. For protein films (60 nm wide and 10 nm high), the sensitivity increased by approximately 300 times, and for a single protein particle (10 nm wide and 10 nm high), the sensitivity increased by approximately 9 times. This enhanced structure based on n-AGP holds promise for providing an important detection platform for nanoscale material characterization and single-molecule detection, with broad application potential in biomedicine, materials science, and geology.

Understanding ion-transfer reactions in silver electrodissolution and electrodeposition from first-principles calculations and experiments.

The electrified aqueous/metal interface is critical in controlling the performance of energy conversion and storage devices, but an atomistic understanding of even basic interfacial electrochemical reactions challenges both experiment and computation. We report a combined simulation and experimental study of (reversible) ion-transfer reactions involved in anodic Ag corrosion/deposition, a model system for interfacial electrochemical processes generating or consuming ions. With the explicit modeling of the electrode potential and a hybrid implicit-explicit solvation model, the density functional theory calculations produce free energy curves predicting thermodynamics, kinetics, partial charge profiles, and reaction trajectories. The calculated (equilibrium) free energy barriers (0.2 eV), and their asymmetries, agree with experimental activation energies (0.4 eV) and transfer coefficients, which were extracted from temperature-dependent voltage-step experiments on Au-supported, Ag-nanocluster substrates. The use of Ag nanoclusters eliminates the convolution of the kinetics of Ag+(aq.) generation and transfer with those of nucleation or etch-pit formation. The results indicate that the barrier is controlled by the bias-dependent competition between partial solvation of the incipient ion, metal-metal bonding, and electrostatic stabilization by image charge, with the latter two factors weakened by stronger positive biases. We also report simulations of the bias-dependence of defect generation relevant to nucleating corrosion by removing an atom from a perfect Ag(100) surface, which is predicted to occur via a vacancy-adatom intermediate. Together, these experiments and calculations provide the first validated, accurate, molecular model of the central steps that govern the rates of important dissolution/deposition reactions broadly relevant across the energy sciences.

Surface charge density and induced currents by self-charging sliding drops.

Spontaneous charge separation in drops sliding over a hydrophobized insulator surface is a well-known phenomenon and lots of efforts have been made to utilize this effect for energy harvesting. For maximizing the efficiency of such devices, a comprehensive understanding of the dewetted surface charge would be required to quantitatively predict the electric current signals, in particular for drop sequences. Here, we use a method based on mirror charge detection to locally measure the surface charge density after drops move over a hydrophobic surface. For this purpose, we position a metal electrode beneath the hydrophobic substrate to measure the capacitive current induced by the moving drop. Furthermore, we investigate drop-induced charging on different dielectric surfaces together with the surface neutralization processes. The surface neutralizes over a characteristic time, which is influenced by the substrate and the surrounding environment. We present an analytical model that describes the slide electrification using measurable parameters such as the surface charge density and its neutralization time. Understanding the model parameters and refining them will enable a targeted optimization of the efficiency in solid-liquid charge separation.

A review of first-principles calculation methods for defects in semiconductors

Doping and defect control in semiconductors are essential prerequisites for their practical applications. First-principles calculations of defects based on density functional theory offer crucial guidance for doping and defect control. In this paper, the developments in the theoretical methods of first-principles semiconductor defect calculations are introduced. Firstly, we introduce the method of calculating the defect formation energy and finite-size errors to the formation energy caused by the supercell method. Then, we present corresponding image charge correction schemes, which include the widely used post-hoc corrections (such as Makov-Payne, Lany-Zunger, Freysoldt-Neugebauer-van de Walle schemes), the recently developed self-consistent potential correction which performs the image charge correction in the self-consistent loop for solving Kohn-Sham equations, and the self-consistent charge correction scheme which does not require an input of macroscopic dielectric constants. Further, we extend our discussion to charged defect calculations in low-dimensional semiconductors, elucidate the issue of charged defect formation energy divergence with the increase of vacuum thickness within the jellium model and introduce our theoretical model which solves this energy divergence issue by placing the ionized electrons or holes in the realistic host band-edge states instead of the virtual jellium state. Furthermore, we provide a brief overview of defect calculation correction methods due to the DFT band gap error, including the scissors operator, LDA+&lt;i&gt;U&lt;/i&gt; and hybrid functionals. Finally, in order to describe the calculation of defect formation energy under illumination, we present our self-consistent two-Fermi-reservoir model, which can well predict the defect concentration and carrier concentration in the Mg doped GaN system under illumination. This work summarizes the recent developments regarding first-principles calculations of defects in semiconducting materials and low-dimensional semiconductors, under whether equilibrium conditions or non-equilibrium conditions, thus promoting further developments of doping and defect control within semiconductors.

Coalescence-Induced Droplet Jumping for Electro-Thermal Sensing.

Jumping droplet condensation, whereby microdroplets (ca. 1-100 μm) coalescing on suitably designed superhydrophobic surfaces jump away from the surface, has recently been shown to have a 10× heat transfer enhancement compared to filmwise condensing surfaces. However, accurate measurements of the condensation heat flux remain a challenge due to the need for low supersaturations (<1.1) to avoid flooding. The low corresponding heat fluxes (<5 W/cm2) can result in temperature noise that exceeds the resolution of the measurement devices. Furthermore, difficulties in electro-thermal measurements such as droplet and surface electrostatic charge arise in applications where direct access to the condensing surface, such as in isolated chambers and small integrated devices, is not possible. Here, we present an optical technique that can determine the experimental electro-thermal parameters of the jumping droplet condensation process with high fidelity through the analysis of jumping droplet trajectories. To measure the heat flux, we observed the experimental trajectories of condensate droplets on superhydrophobic nanostructures and simultaneously matched them in space and time with simulated trajectories using the droplet dynamic equations of motion. Two independent approaches yielded mean heat fluxes of approximately 0.13 W/cm2 with standard deviations ranging from 0.047 to 0.095 W/cm2, a 79% reduction in error when compared with classical energy balance-based heat flux measurements. In addition, we analyzed the trajectories of electrostatically interacting droplets during flight and fitted the simulated and experimental results to achieve spatial and temporal agreement. The effect of image charges on a jumping droplet as it approaches the surface was analyzed, and the observed acceleration has been numerically quantified. Our work presents a sensing methodology of electro-thermal parameters governing jumping droplet condensation.

Advancing understanding of structural, electronic, and magnetic properties in 3d-transition-metal TM-doped α-Ga2O3 (TM = V, Cr, Mn, and Fe): A first-principles and Monte Carlo study

In this paper, we investigated the properties of transition metal (TM)-doped α-Ga2O3 using first-principles calculations and Monte Carlo simulations. α-Ga2O3 is a wide-bandgap semiconductor material with enhanced performance and lower fabrication costs on sapphire substrates compared to β-Ga2O3. Doping with TMs can modify electrical transport, optical absorption, and magnetic properties, yet theoretical studies on this are scarce. Our study focused on V, Cr, Mn, and Fe impurities. We introduced a newly proposed scheme for efficiently determining the ground-state defect configuration during structural relaxation. We adopt a recent, novel image charge correction method to accurately calculate formation enthalpy and thermodynamic transition levels for spin-polarized transition metal ion doping, without employing the empirical dielectric constant. Results showed Cr ions tend to neutral substitutional Ga, while V, Mn, and Fe impurity ions tend to carry a negative charge in common n-type α-Ga2O3. Magnetic moments and spin-splitting impurity levels primarily arise from transition metal impurities and their d orbitals. We used the generalized four-state method to calculate exchange interaction constants between substitution lattice sites and identified (anti) ferromagnetic couplings at specific distances in a 120-atom supercell, which are negligible in total energy calculations. Monte Carlo simulations indicated a Curie temperature of 360 K in n-type α-Ga2O3: Mn system with 12.5% doping, suggesting intrinsic ferromagnetic ordering based on the Heisenberg model. Our study contributes to understanding TM-doped α-Ga2O3 electronic structure and magnetic properties through improved methodologies. The approach can be applied in research involving other TM-doped oxides or wide-bandgap semiconductors.