Image Charge Research Articles

Extending the dynamic range of electronics in a Time Projection Chamber

Abstract When Time Projection Chambers (TPCs) are used in low to intermediate heavy ion collisions, the mass and momentum range of the emitted particles cover a wide range in energy losses. Many TPC readout electronics currently only have a single gain output with a fixed dynamic range. In a recent set of experiments using the SAMURAI Pion-Reconstruction and Ion-Tracker (S π RIT) TPC, it was important to simultaneously measure relativistic pions and heavy ion tracks from the same collisions. As the ionization from a track’s energy loss is collected and multiplied by the anode wires, a distribution of image charges is induced on the TPC read-out pads. If the avalanche on a wire is large enough, the charge collected by pads directly underneath will saturate the readout electronics; pads farther away in the distribution will not be saturated. Using these unsaturated pads and the known pad distribution function, we can estimate the charge on saturated pads, increasing the dynamic range by a factor of 5.

Stern–Gerlach splitting of low-energy ion beams

We present a feasibility study with several magnetic field configurations for creating spin-dependent forces that can split a low-energy ion beam by the Stern–Gerlach (SG) effect. To the best of our knowledge, coherent spin-splittings of charged particles have yet to be realised. Our proposal is based on ion source parameters taken from a recent experiment that demonstrated single-ion implantation from a high-brightness ion source combined with a radio-frequency Paul trap. The inhomogeneous magnetic fields can be created by permanently magnetised microstructures or from current-carrying wires with sizes in the micron range, such as those recently used in a successful implementation of the SG effect with neutral atoms. All relevant forces (Lorentz force and image charges) are taken into account, and measurable splittings are found by analytical and numerical calculations.

A generalized self-consistent model for quantum tunneling current in dissimilar metal-insulator-metal junction

We study the current density-voltage (J − V) characteristics of dissimilar metal-insulator-metal (MIM) nanoscale tunneling junctions using a self-consistent quantum model. The model includes emissions from both cathode and anode, and the effects of image charge potential, space charge and exchange correlation potential. The J − V curves span three regimes: direct tunneling, field emission, and space-charge-limited regime. Unlike similar MIM junctions, the J − V curves are polarity dependent. The forward (higher work function metal is negatively biased) and reverse (higher work function metal is positively biased) bias J − V curves and their crossover behaviors are examined in detail for various regimes, over a wide range of material properties (work function of the electrodes, electron affinity and permittivity of the insulator). It is found that the asymmetry between the current density profiles increases with the work function difference between the electrodes, insulator layer thickness and relative permittivity of the insulator. This asymmetry is profound in the field emission regime and insignificant in the direct tunneling, and space charge limited regimes.

Electrical surface charge patterns induced by droplets sliding over polymer and photoresist surfaces

We have studied the generation of surface charge distributions on polymer surfaces due to the deposition, motion and evaporation of liquid droplets. Using liquids with different static dielectric constants, we found that the magnitude of the surface charge conforms well to the Boltzmann relation for the dissociation probability of ions in solution, indicating that the phenomenon is electrokinetic in nature. The origin of the sensitive dependence of the surface charge on the substrate thickness was identified to be an image charge effect. Experiments on polycarbonate and chemically-amplified photoresist yield surface charges of opposite polarity. By varying the photoresist composition, we identified the leaching of negative quencher ions as the reason for the charge inversion.

Efficient dynamic simulations of charged dielectric colloids through a novel hybrid method.

Modern particle-based simulations increasingly incorporate polarization charges arising from spatially nonuniform permittivity. For complex dielectric geometries, calculation of these induced many-body effects typically requires numerical solvers based upon boundary-element methods, which very significantly increase the required computational effort. For the special case of dielectric spheres, such as colloids or nanoparticles, we recently proposed a semianalytical spectrally accurate hybrid method that combines the method of moments, the image-charge method, and the fast multipole method. The hybrid method is efficient and accurate even when dielectric spheres are closely packed. Here, we extend the method to the evaluation of direct and induced electrostatic forces and demonstrate how this can be incorporated in molecular dynamics simulations. The choice of the relevant numerical parameters for molecular dynamics simulations is discussed in detail, as well as comparisons to the boundary-element method. As a concrete example, we examine the challenging case of binary crystal structures composed of close-packed dielectric colloidal spheres.

High-Capacity Electrostatic Ion Trap with Mass Resolving Power Boosted by High-Order Harmonics.

A form of electrostatic ion trap mass analyzer, named the orbital frequency analyzer (OFA), has been developed. The ions in the analyzer are trapped around the middle plane and orbit around the central axis perpendicular to the middle plane with high-ellipticity and precessing trajectories. The orbital frequency of the ions in this device has been optimized to be independent of ions' energy so that the image charge signal picked up by some of the field-forming circular/ring electrodes can be used to produce a mass spectrum after Fourier transform data processing. Spectra acquired by the OFA are rich of high-order harmonics, which offer higher mass resolving power than that for fundamental frequency components. The experiment shows that the resolving power is proportional to the harmonic order and exceeds 150 k for mass-to-charge ratio ( m/ z) of 526 Th and the transient length of 500 ms. Using high-order harmonics, an isotopic cluster of a heavy protein was resolved with a shorter transient length. The transient signals from different pick-up electrodes give different waveform shapes, and therefore, their harmonic peak distributions in a frequency spectrum are different, thus allowing the removal of unwanted harmonic peaks. The preliminary results also show a wide dynamic range of the analyzer.

Image charge detection statistics relevant for deterministic ion implantation

Image charge detection is a non-perturbative pre-detection approach for deterministic ion implantation. Using low energy ion bunches as a model system for highly charged single ions, we experimentally studied the error and detection rates of an image charge detector setup. The probability density functions of the signal amplitudes in the Fourier spectrum can be modelled with a generalised gamma distribution to predict error and detection rates. It is shown that the false positive error rate can be minimised at the cost of detection rate, but this does not impair the fidelity of a deterministic implantation process. Independent of the ion species, at a signal-to-noise ratio of 2, a false positive error rate of 0.1% is achieved, while the detection rate is about 22%.

Exciton Fine Structure in Perovskite Nanocrystals.

The bright emission observed in cesium lead halide perovskite nanocrystals (NCs) has recently been explained in terms of a bright exciton ground state [ Becker et al. Nature 2018 , 553 , 189 - 193 ], a claim that would make these materials the first known examples in which the exciton ground state is not an optically forbidden dark exciton. This unprecedented claim has been the subject of intense experimental investigation that has so far failed to detect the dark ground-state exciton. Here, we review the effective-mass/electron-hole exchange theory for the exciton fine structure in cubic and tetragonal CsPbBr3 NCs. In our calculations, the crystal field and the short-range electron-hole exchange constant were calculated using density functional theory together with hybrid functionals and spin-orbit coupling. Corrections associated with long-range exchange and surface image charges were calculated using measured bulk effective mass and dielectric parameters. As expected, within the context of the exchange model, we find an optically inactive ground exciton level. However, in this model, the level order for the optically active excitons in tetragonal CsPbBr3 NCs is opposite to what has been observed experimentally. An alternate explanation for the observed bright exciton level order in CsPbBr3 NCs is offered in terms of the Rashba effect, which supports the existence of a bright ground-state exciton in these NCs. The size dependence of the exciton fine structure calculated for perovskite NCs shows that the bright-dark level inversion caused by the Rashba effect is suppressed by the enhanced electron-hole exchange interaction in small NCs.

Influence of image charges on the impurity-related optical properties of near-surface quantum wells under a transverse electric field

The simultaneous effects of an external electric field and image charges on the hydrogen-like impurity-related optical properties of a GaAs/AlGaAs near-surface quantum well with a parabolic potential have been investigated. The oscillator strengths and the linear and the third-order nonlinear optical absorption coefficients have been investigated as the functions of incident photon energy, impurity position, and electric field strength. Numerical results show that the inclusion of image charges leads to an increase in the oscillator strength of about two times for the system under study. This effect is especially notable for large values of an electric field. It is also shown that the influence of image charges leads to a blue shift and a significant increase in peak values of the total absorption coefficient. Our results show a remarkable dependence of optical properties on both the electric field and the impurity position. We believe that a significant increase in the optical characteristics of near-surface quantum wells due to the influence of image charges should be taken into account in future developments.

Assembled Films of Sn-Doped In2O3 Plasmonic Nanoparticles on High-Permittivity Substrates for Thermal Shielding

This study presents the infrared (IR) plasmonic responses of assembled films consisting of Sn-doped In2O3 nanoparticles (ITO-NP films) placed on different substrates. The reflectance properties of the NP films are found to depend on substrate permittivity that affects the types of solar thermal-shielding. Increasing the substrate permittivity provides high reflectance in the IR range resulting from the plasmon modes and the interference effect in the NP films, as demonstrated through changes in the optical properties by tuning substrate permittivity. In particular, the plasmon modes played an important role in enhancing reflectance in the near-IR range. Induced image charges in the substrates produced field interactions with the plasmon modes in the NP films that are formed along the in-plane and out-of-plane directions, a consideration further verified by the strong image charges induced by metallic substrates consisting of ITO films. On the other hand, the interference effects contributed to the reflect...

Theoretical study of aerosol particle electroscavenging by clouds

This paper describes a theoretical model which computes the collection efficiency of aerosol particles by droplets, due to the combined action of dynamic (inertia, weight and drag) and electrophoresis forces acting on an aerosol particle of radius 0.004≤a≤1.3μm around a droplet of radius 15≤A≤100μm. The electrostatic forces are defined following the concept of image charges. In the given particle range, the Brownian motion must be considered and was consequently added to the model. A novel approach is developed, based on the Langevin's theory and solved using an Itô process. Results of electroscavenging related to natural atmospheric ionisation, as well as for strongly charged aerosol particles released after a nuclear accident, are presented with pressure and temperature representative of the mid-troposphere (-17°C, 540 hPa). The droplet charges considered in the paper are representative of weakly and strongly electrified clouds. The values of collection efficiency computed are convenient for incorporation into cloud models and to study scavenging of aerosol particles by clouds whether for climate, pollution or nuclear safety issues.

Analysis of Silicon Channel Waveguide Thermo-Optic Switches by the Image Charge Method

Employing the “image charge” method, we theoretically study the influence of key physical parameters on the thermo-optic characteristics of a channel waveguide silicon-on-insulator (SOI) Mach–Zehnder Switch. The power consumption and spatial temperature profile of such structures are given as explicit functions of various structural, thermal, and optical parameters, offering physical insight not available in finite-element simulations. Agreement with finite-element simulations is demonstrated. The trends of switching power versus various parameters provide quantitative guide for further device optimization. This approach may also be useful for efficient modeling of an integrated optical circuit containing a large number of thermo-optic devices.

Unveiling the Interfacial Effects for Enhanced Hydrogen Evolution Reaction on MoS2 /WTe2 Hybrid Structures.

Using the MoS2 -WTe2 heterostructure as a model system combined with electrochemical microreactors and density function theory calculations, it is shown that heterostructured contacts enhance the hydrogen evolution reaction (HER) activity of monolayer MoS2 . Two possible mechanisms are suggested to explain this enhancement: efficient charge injection through large-area heterojunctions between MoS2 and WTe2 and effective screening of mirror charges due to the semimetallic nature of WTe2 . The dielectric screening effect is proven minor, probed by measuring the HER activity of monolayer MoS2 on various support substrates with dielectric constants ranging from 4 to 300. Thus, the enhanced HER is attributed to the increased charge injection into MoS2 through large-area heterojunctions. Based on this understanding, a MoS2 /WTe2 hybrid catalyst is fabricated with an HER overpotential of -140 mV at 10 mA cm-2 , a Tafel slope of 40 mV dec-1 , and long stability. These results demonstrate the importance of interfacial design in transition metal dichalcogenide HER catalysts. The microreactor platform presents an unambiguous approach to probe interfacial effects in various electrocatalytic reactions.

Spontaneous Radiation of a Two-Level System Confined in a Reflective Spherical Shell Quantum Dot

Using a first-order time-dependent perturbation theory, we calculate the spontaneous emission rate of a two-level system trapped between perfectly reflecting concentric spheres. The emitter is represented by a two-level monopole coupled to a Hermitian massless scalar field satisfying Dirichlet boundary conditions in such quantum-confined low-dimensional structure. We obtained the appropriate Green’s function evaluated in worldline of the atom which incorporates contributions from an infinite set of variable image charges. We provide an analytical expression for the decay rate to investigate the radiation process of the trapped atomic system. We perform a broad analysis of the dependence of the decay rate for different relations between the radii of spheres and the emitted radiation energy. We unveil regimes of strong suppression of the spontaneous emission rate as well as the development of irregular oscillations as a function of the quantum of emitted energy.

Ultrafast Dynamic Microscopy of Carrier and Exciton Transport.

We highlight the recent progress in ultrafast dynamic microscopy that combines ultrafast optical spectroscopy with microscopy approaches, focusing on the application transient absorption microscopy (TAM) to directly image energy and charge transport in solar energy harvesting and conversion systems. We discuss the principles, instrumentation, and resolutions of TAM. The simultaneous spatial, temporal, and excited-state-specific resolutions of TAM unraveled exciton and charge transport mechanisms that were previously obscured in conventional ultrafast spectroscopy measurements for systems such as organic solar cells, hybrid perovskite thin films, and molecular aggregates. We also discuss future directions to improve resolutions and to develop other ultrafast imaging contrasts beyond transient absorption.

Polarizable potentials for metals: The density readjusting embedded atom method (DR-EAM)

In simulations of metallic interfaces, a critical aspect of metallic behavior is missing from the some of the most widely used classical molecular dynamics force fields. We present a modification of the embedded atom method (EAM) which allows for electronic polarization of the metal by treating the valence density around each atom as a fluctuating dynamical quantity. The densities are represented by a set of additional fluctuating variables (and their conjugate momenta) which are propagated along with the nuclear coordinates. This ``density readjusting EAM'' (DR-EAM) preserves nearly all of the useful qualities of traditional EAM, including bulk elastic properties and surface energies. However, it also allows valence electron density to migrate through the metal in response to external perturbations. We show that DR-EAM can successfully model polarization in response to external charges, capturing the image charge effect in atomistic simulations. DR-EAM also captures some of the behavior of metals in the presence of uniform electric fields, predicting surface charging and shielding internal to the metal. We further show that it predicts charge transfer between the constituent atoms in alloys, leading to novel predictions about unit cell geometries in layered L$1_0$ structures.

Grounded hyperspheres as squashed wormholes

We compute exterior Green functions for equipotential, grounded hyperspheres in N-dimensional electrostatics by squashing Riemannian wormholes, where an image charge is placed in the branch of the wormhole opposite the branch containing the source charge, thereby providing a vivid geometrical approach to a method first suggested in 1897 by Sommerfeld. We compare and contrast the strength and location of the image charge in the wormhole approach with that of the conventional Euclidean solution where an image charge of reduced magnitude is located inside the hypersphere. While the two approaches give mathematically equivalent Green functions, we believe they provide strikingly different physics perspectives.

Asymptotic Formulas in the Theory of Diffraction and Transition Radiation on a Conducting Sphere

The diffraction and transition radiation of a nonrelativistic charged particle on a perfectly conducting sphere is described using the method of image charges known from electrostatics. Asymptotic formulas that describe the characteristics of the radiation in the low- and high-frequency regions are constructed. It is shown that this approach in the high-frequency limit can be extended to the case of a relativistic incident particle.

Determination of Energy-Level Alignment in Molecular Tunnel Junctions by Transport and Spectroscopy: Self-Consistency for the Case of Oligophenylene Thiols and Dithiols on Ag, Au, and Pt Electrodes.

We report detailed measurements of transport and electronic properties of molecular tunnel junctions based on self-assembled monolayers (SAMs) of oligophenylene monothiols (OPT n, n = 1-3) and dithiols (OPD n, n = 1-3) on Ag, Au, and Pt electrodes. The junctions were fabricated with the conducting probe atomic force microscope (CP-AFM) platform. Fitting of the current-voltage ( I-V) characteristics for OPT n and OPD n junctions to the analytical single-level tunneling model allows extraction of both the HOMO-to-Fermi-level offset (εh) and the average molecule-electrode coupling (Γ) as a function of molecular length ( n) and electrode work function (Φ). Significantly, direct measurements of εhUPS by ultraviolet photoelectron spectroscopy (UPS) for OPT n and OPD n SAMs on Ag, Au, and Pt agree remarkably well with the transport estimates εhtrans, providing strong support-beyond the high quality I-V simulations-for the relevance of the analytical single-level model to simple molecular tunnel junctions. Because the UPS measurements involve SAMs bonded to only one metal contact, the correspondence of εhUPS and εhtrans also indicates that the top contact has a weak effect on the HOMO energy. Corroborating ab initio calculations definitively rule out a dominant contribution of image charge effects to the magnitude of εh. Thus, the effective molecular tunnel barrier εh is determined, and essentially pinned, by the formation of a single metal-S covalent bond per OPT n or OPD n molecule.

Kirigami Mechanics as Stress Relief by Elastic Charges.

We develop a geometric approach to understand the mechanics of perforated thin elastic sheets, using the method of strain-dependent image elastic charges. This technique recognizes the buckling response of a hole under an external load as a geometrically tuned mechanism of stress relief. We use a diagonally pulled square paper frame as a model system to quantitatively test and validate our approach. Specifically, we compare nonlinear force-extension curves and global displacement fields in theory and experiment. We find a strong softening of the force response accompanied by curvature localization at the inner corners of the buckled frame. Counterintuitively, though in complete agreement with our theory, for a range of intermediate hole sizes, wider frames are found to buckle more easily than narrower ones. Upon extending these ideas to many holes, we demonstrate that interacting elastic image charges can provide a useful kirigami design principle to selectively relax stresses in elastic materials.