Distribution Of Charge Carriers Research Articles

Monte Carlo simulation of initial and volume ion recombination correction factor in plane parallel ionization chambers exposed to ion beams

Objective. Accurate reference dosimetry with ionization chambers (ICs) relies on correcting for various influencing factors, including ion recombination. Theoretical frameworks, such as the Boag and Jaffe theories, are conventionally used to describe the ion recombination correction factors. Experimental methods are time consuming, the applicability may be limited and, in some cases, impractical to be used in clinical routine. The development of simulation tools becomes necessary to enhance the understanding of recombination under circumstances that may differ from conventional use. Before progressing, it is crucial to benchmark novel approaches to calculate ion recombination losses under known conditions. In this study, we introduce and validate a versatile simulation tool based on a Monte Carlo scheme for calculating initial and volume ion recombination correction factors in air-filled ICs exposed to ion beams with clinical dose rates. Approach. The simulation includes gaussian distribution of ion positions to model the distribution of charge carriers along the chamber volume. It accounts for various physical transport effects, including drift, diffusion, space charge screening and free electron fraction. To compute ion recombination, a Monte Carlo scheme is used due to its versatility in multiple geometries, without exhibiting convergence problems associated with numerically solved procedures. Main results. The code is validated in conventional dose rates against Jaffe’s theory for initial recombination and Boag’s theory for volume recombination based on parameters derived from experimental data including proton, helium and carbon ion beams measured with a plane parallel IC. Significance. The simulation demonstrates excellent agreement, typically 0.05% or less relative difference with the theoretical and experimental data. The current code successfully predicts ion recombination correction factors, in a large variety of ion beams, including different temporal beam structures.

Photoluminescence of dense arrays of InGaPAs/InGaAs quantum dots formed by substitution of group V elements

One drawback of self-organized quantum dots is their low optical gain. The development of new shaping methods that would allow higher gain, for example, due to a higher surface density of the QD array, remains an important task to date. In the present work, arrays of quantum dots have been formed by substituting phosphorous atoms with arsenic ones in a thin InGaP epilayer. Based on transmission electron microscopy data, the surface density was estimated to be about 1.3×1012 cm-2, which is one of the highest values for quantum dots on GaAs. The influence of an additional InGaAs epilayer (quantum well) on the luminescence properties of quantum dots was investigated in a wide temperature range and different optical pumping levels. It was found that placing the quantum well under the layer of quantum dots has little effect on the central emission wavelength, while quantum dots covered with the InGaAs demonstrate a strong red shift of the luminescence spectrum. A transition from a nonequilibrium to an equilibrium distribution of charge carriers over quantum-dot states, related to the lateral transport of charge carriers over quantum dot array, similar to that previously observed in In(Ga)As Stranski-Krastanow quantum dots, was revealed. This is manifested in a change in temperature dependencies of the linewidth and peak position of the photoluminescence band. For the structures under study, this transition occurs at a temperature of about 125 K. Rapid thermal annealing of the structures allows to increase the integrated photoluminescence intensity of quantum dots up to 4 times, while the maximum of the spectrum remains almost unchanged, provided that the annealing temperature does not exceed 620 oC.

Negative corona discharge strategy for efficient quantum dot light-emitting diodes.

In colloid quantum dot light-emitting diodes (QLEDs), the control of interface states between ZnO and quantum dots (QDs) plays a vital role. We present a straightforward and efficient method using a negative corona discharge to modify the QD film, creating a dipole moment at the interface of QDs and magnesium-doped ZnO (ZnMgO) for balanced charge carrier distribution within the QDs. This process boosts external quantum efficiencies in red, green, and blue QLEDs to 17.71%, 14.53%, and 9.04% respectively. Notably, optimized devices exhibit significant enhancements, especially at lower brightness levels (1000 to 10,000 cd·m-2), vital for applications in mobile displays, TV screens, and indoor lighting.

Distance Dependence of the Energy Transfer Mechanism in WS_{2}-Graphene Heterostructures.

We report on the mechanism of energy transfer in Van der Waals heterostructures of the two-dimensional semiconductor WS_{2} and graphene with varying interlayer distances, achieved through spacer layers of hexagonal boron nitride (h-BN). We record photoluminescence and reflection spectra at interlayer distances between 0.5 and 5.8nm (0-16 h-BN layers). We find that the energy transfer is dominated by states outside the light cone, indicative of a Förster transfer process, with an additional contribution from a Dexter process at 0.5nm interlayer distance. We find that the measured dependence of the luminescence intensity on interlayer distances above 1nm can be quantitatively described using recently reported values of the Förster transfer rates of thermalized charge carriers. At smaller interlayer distances, the experimentally observed transfer rates exceed the predictions and, furthermore, depend on excess energy as well as on excitation density. Since the transfer probability of the Förster mechanism depends on the momentum of electron-hole pairs, we conclude that, at these distances, the transfer is driven by nonrelaxed charge carrier distributions.

Scavenging Energy and Information through Dynamically Regulating the Electrical Double Layer

AbstractThe electrical double layer (EDL) between solids and liquids serves as the primary interface for ionic‐electronic coupling and is pivotal in nanoscale phenomena, governing electric field effects, ion transport, surface interactions, etc. Dynamically regulating the EDL through mechanical or electrostatic methods can influence charge carrier behavior, thereby impacting energy scavenging and storage processes. This regulation enabled efficient energy scavenging by governing ionic migration and optimizing charge carrier concentration at the interface, presenting a novel avenue to achieve efficient energy and information flow. Here, various scavenging energy and information devices through dynamically regulating the EDL are systematically reviewed. They are classified into three groups by regulating the distribution and movement of charge carriers throughout the entire EDL, diffuse layer, and Debye length range. The review provided a comprehensive overview of the operating principles, influencing factors, output characteristics, and typical applications, along with a discussion on future challenges. This holistic examination offers researchers valuable insights for evaluating their applicability in various scenarios.

Modeling of semiconductor heterostructures for energy converters and sensors

A set of modeling programs for constructing a sequence of energy zones of heterojunctions is presented for analyzing the distribution of charge carriers in the heterostructure and internal characteristics, for describing the processes of charge transfer and accumulation. Wolfram Mathematica analytical system and Delphi programming language were used. The main elements of materials are semiconductors, metals of contact structures and injection regions of nonequilibrium carriers. The programs allow determining the structural characteristics of materials, active zones and spatial charge regions, calculating quasi-Fermi levels and built-in potentials, as well as the efficiency of heterostructures in general and for separation-charge collection, charge accumulation, determining the type of metallization of barrier or ohmic contact.

A novel dual-parameter proximity and touch sensor using SiO2 nanoparticles and NaCl with commercial acrylic-based encapsulation

This study shows the development and analysis of a novel capacity proximity sensor (CPS) based on a sensing layer made up of a mixture of silicon dioxide nanoparticles (SiO2) and sodium chloride (NaCl), and an encapsulation layer based on a commercial acrylic-based varnish. The encapsulated and non-encapsulated proximity sensors were characterised using impedance spectroscopy (IS), revealing that the resulting impedimetric and capacitance responses exhibit different sensitivities and working sensing ranges. The non-encapsulated sensor presents impedimetric and maximum capacitive sensitivities of 0.0775 cm−1 and -0.9831 cm−1, respectively, within a 2–14 cm sensing range. In contrast, the encapsulated CPS shows maximum impedimetric and capacitive sensitivities of 0.3447 cm−1 and −3.349 cm−1, respectively, and an operation sensing range of 0–3 cm. The results show a 75% decrease in the total sensing range that could be attributed to: (i) a reduction of the effective sensing area due to a reduction of the roughness as demonstrated by SEM analysis, (ii) insulation effects limiting the impact of the material under test (MUT) on the charge carriers distribution, and (iii) decreased charge carrier density involved in the sensing process. Despite the reduced operational range, the encapsulation layer maintains the dual-parameter sensing capabilities, preserves the integrity of the sensing layer, and enables its dual functionality as a proximity and touch sensor. The reported comparison between the encapsulated and non-encapsulated CPSs highlights the effects of the encapsulation layer. The encapsulated version introduces a simple, fast, and cost-effective novel approach for developing CPSs that outperforms some reported CPSs in terms of reliability due to its dual-parameter sensing capability and sensitivity.

Overcoming the Unfavorable Effects of "Boltzmann Tyranny:" Ultra-Low Subthreshold Swing in Organic Phototransistors via One-Transistor-One-Memristor Architecture.

Organic phototransistors (OPTs), as photosensitive organic field-effect transistors (OFETs), have gained significant attention due to their pivotal roles in imaging, optical communication, and night vision. However, their performance is fundamentally limited by the Boltzmann distribution of charge carriers, which constrains the average subthreshold swing (SSave ) to a minimum of 60mV/decade at room temperature. In this study, an innovative one-transistor-one-memristor (1T1R) architecture is proposed to overcome the Boltzmann limit in conventional OFETs. By replacing the source electrode in an OFET with a memristor, the 1T1R device exploits the memristor's sharp resistance state transitions to achieve an ultra-low SSave of 18mV/decade. Consequently, the 1T1R devices demonstrate remarkable sensitivity to photo illumination, with a high specific detectivity of 3.9 × 109 cm W-1 Hz1/2 , outperforming conventional OPTs (4.9 × 104 cm W-1 Hz1/2 ) by more than four orders of magnitude. The 1T1R architecture presents a potentially universal solution for overcoming the detrimental effects of "Boltzmann tyranny," setting the stage for the development of ultra-low SSave devices in various optoelectronic applications.

Growth Control of InP/ZnSe Heterostructured Nanocrystals.

The morphology of heterostructured semiconductor nanocrystals (h-NCs) dictates the spatial distribution of charge carriers and their recombination dynamics and/or transport, which are the main performance indicators of photonic applications utilizing h-NCs. The inability to control the morphology of heterovalent III-V/II-VI h-NCs composed of heavy-metal-free elements hinders their practical use. As a case study of III-V/II-VI h-NCs, the growth control of ZnSe epilayers on InP NCs is demonstrated here. The anisotropic morphology in InP/ZnSe h-NCs is attributed to the facet-dependent energy costs for the growth of ZnSe epilayers on different facets of InP NCs, and effective chemical means for controlling the growth rates of ZnSe on different surface planes are demonstrated. Ultimately, this article capitalizes on the controlled morphology of InP/ZnSe h-NCs to expand their photophysical characteristics from stable and pure emission to environment-sensitive one, which will facilitate their use in a variety of photonic applications.

Impact of crystallinity and grain density on the charge-carrier distribution and transport in organic semiconductors

The crystalline and amorphous phases often coexist in the organic active layers of their electronic devices, which further undergo morphological changes over time. Understanding of the characteristics of the microscopic processes in polycrystalline organic films is essential for optimizing organic semiconductors and their devices, particularly in the context of flexible electronics. Here, we systematically investigate the charge-carrier distributions and transport in polycrystalline organic films, focusing on the impact of the crystallinity and grain density. These polycrystalline morphology data were generated from an efficient Monte Carlo method, which were then incorporated into molecular-level device simulations that can describe the microscopic charge-transport processes. These results demonstrate the distributions and transport characteristics of charge carriers in different phases of the polycrystalline organic semiconductor films, as well as the influencing factors. Importantly, we show that the influence of a polycrystalline morphology becomes more pronounced at low driving voltages, which has been the subject of significant research efforts. Our study also revealed a decrease in mobility at low crystallinities, a phenomenon that was not previously anticipated.

Pattern-illumination time-resolved phase microscopy and its applications for photocatalytic and photovoltaic materials.

Pattern-illumination time-resolved phase microscopy (PI-PM) is a technique used to study the microscopic charge carrier dynamics in photocatalytic and photovoltaic materials. The method involves illuminating a sample with a pump light pattern, which generates charge carriers and they decay subsequently due to trapping, recombination, and transfer processes. The distribution of photo-excited charge carriers is observed through refractive index changes using phase-contrast imaging. In the PI-PM method, the sensitivity of the refractive index change is enhanced by adjusting the focus position, the method takes advantage of photo-excited charge carriers to observe non-radiative processes, such as charge diffusion, trapping in defect/surface states, and interfacial charge transfer of photocatalytic and photovoltaic reactions. The quality of the image sequence is recovered using various informatics calculations. Categorizing and mapping different types of charge carriers based on their response profiles using clustering analysis provides spatial information on charge carrier types and the identification of local sites for efficient and inefficient photo-induced reactions, providing valuable information for the design and optimization of photocatalytic materials such as the cocatalyst effect.

Carrier modulation of one-dimensional MAPbxSr1-x(IyCl1-y)3 core-shell perovskite nanowires.

Low dimensional hybrid organic-inorganic perovskites (HOIPs) have become one of the most promising materials in solar cells, photodetectors, and lasers due to their low exciton binding energy, high bipolar carrier mobility, and long carrier lifetime. The effective separation and collection of photo-generated electrons and holes have always been crucial for a perovskite as a working medium for optoelectronic devices. However, the surface state of pristine perovskite nanowires causes recombination of electrons and holes at the edge of the energy band, leading to deactivation of charge carriers. In this work, we constructed MAPbxSr1-xI3 core-shell nanowires, which adjust the band gap and control the spatial distribution of charge carriers by changing the Pb composition ratio, achieving spontaneous separation of electron-hole pairs to avoid surface recombination. In addition, MAPbxSr1-x(IyCl1-y)3 core-shell nanowires with different component ratios (x, y) can be constructed by further doping the Cl element, and the spatial distribution of charge carriers in the nanowires can be flexibly manipulated to achieve exchange between type-I and type-II band alignment orders. This study provides a feasible method for enhancing the carrier separation of organic-inorganic hybrid perovskites and improving the performance of optoelectronic devices.

Size onset of metallic behavior in neutral aluminum clusters.

The excited state lifetimes of neutral (Al)n clusters up to ∼1 nm in diameter in size, where n ≤ 43, are systematically measured with femtosecond time-resolved mass spectrometry. The onset of metallic behavior is identified as a distinct change in the relaxation behavior initiated with single ultraviolet (400 nm) photon excitation. The experimentally measured excited state lifetimes gradually decrease with size for small molecular scale clusters (n < 10) before becoming indistinguishable for larger clusters (n > 9), where the measurements are comparable to electron-lattice relaxation time of bulk Al (∼300 fs). Particularly intense, or magic, Aln clusters do not exhibit any significant excited state lifetime behavior. Time-dependent density functional theory quantify the excited state properties and are presented to show that dynamics are strongly tied to the excited state charge carrier distributions and overlap, rather than detailed changes related to changes in the cluster's electronic and geometric structure. The consistency in excited state lifetimes for clusters larger than n = 9 is attributed to the hybridization of the s- and p-orbitals as well as increasing delocalization. Al3 exhibits unique temporal delay in its transient behavior that is attributed to a transition from triangular ground state to linear structure upon excitation.

Odnovremennoe nablyudenie tsiklotronnogo rezonansa dyrok i elektronov v dvoynoy kvantovoy yame HgTe/CdHgTe v usloviyakh effekta “opticheskogo zatvora”

Spectral studies of the photoconductivity in the temperature range of T = 5–70 K, as well as studies of the magneto-absorption and magnetotransport at T = 4.2 K, have been performed in a HgTe/CdHgTe heterostructure with a double quantum well under an “optical gate” effect. Studies of magneto-absorption spectra under the controlled optical exposure have made it possible to observe absorption lines caused by both the cyclotron resonances of electrons and holes simultaneously. The coexistence of electrons and holes in the HgTe/CdHgTe double quantum well with a relatively large bandgap (~80 meV) indicates the appearance of a strongly inhomogeneous light-induced distribution of charge carriers in the plane of the structure. Experimental results obtained clearly demonstrate disadvantages of the control of the Fermi level positions in heterostructures with HgTe/CdHgTe quantum wells by means of the optical gate.

Simultaneous Observation of the Cyclotron Resonances of Electrons and Holes in a HgTe/CdHgTe Double Quantum Well under “Optical Gate” Effect

Spectral studies of the photoconductivity in the temperature range of T = 5–70 K, as well as studies of the magneto-absorption and magnetotransport at T = 4.2 K, have been performed in a HgTe/CdHgTe heterostructure with a double quantum well under an “optical gate” effect. Studies of magneto-absorption spectra under the controlled optical exposure have made it possible to observe absorption lines caused by both the cyclotron resonances of electrons and holes simultaneously. The coexistence of electrons and holes in the HgTe/CdHgTe double quantum well with a relatively large bandgap (~80 meV) indicates the appearance of a strongly inhomogeneous light-induced distribution of charge carriers in the plane of the structure. Experimental results obtained clearly demonstrate disadvantages of the control of the Fermi level positions in heterostructures with HgTe/CdHgTe quantum wells by means of the optical gate.

In-situ synthesis of fluoropolymer-grafted Nanohybrid electrolyte as ion reservoir for Li-metal batteries

Heterogeneity in ion flux of electrolyte materials largely dictates the growth of lithium dendrites and performance of lithium metal anode, which is considered “holy grail” to enable safe rechargeable batteries with ultrahigh energy density. This challenge is even more prominent in polymer-based electrolytes, which thereby leads to uneven ion distribution and mobility of ionic charge carriers, disturbing electrodeposition. In this work, we propose an ion-immobilization strategy achieved by an in-situ grafted nanohybrid fluoropolymer electrolyte, which takes advantages of unique weak solvation of fluoropolymers to tether anions to solvation structure of Li+. Such regulation in conjunction with the well dispersed nanometric aggregates, enables polymer electrolytes serving as ion reservoirs with strengthened weakly-solvating effect. As such, we suppressed formation of dendritic lithium by inducing uniform ion distribution, which significantly extended cell lifetime to over 1900 h in electrodeposition and prolonged charge-discharge cycles of in-situ lithium metal batteries up to 4.6 V at ambient temperature. This work demonstrates a new approach to regulate interfacial chemistry of Li anode and access high-performance polymer-based electrolytes.

Elemental-Doping-Induced Orbital Redistribution in Conjugated Polymer Boosts Charge-Transfer-Mediated Singlet Oxygen Production.

Beyond the traditional exciton-based energy transfer, carrier-involved charge transfer provides an alternative pathway to overcoming spin restriction in photocatalytic singlet oxygen (1O2) generation. However, the charge-transfer-mediated method usually suffers from insufficient charge-carrier concentration and distribution, which are related to the electronic band structure. Herein, exemplified by polymeric carbon nitride (PCN), we propose an orbital-redistribution strategy for facilitating charge-transfer-mediated 1O2 generation. On the basis of theoretical simulation and spectroscopic characterizations, we demonstrated that the incorporation of bromine could dramatically facilitate the redistribution of the HOMO and LUMO in PCN, resulting in promoted exciton dissociation and adjacent electron/hole distribution. Benefiting from those, bromine-doped polymeric carbon nitride (Br-PCN) exhibited an obvious enhancement in the production of 1O2 via a two-step charge-transfer process. This work not only uncovers a promising method for regulating photoexcitation in polymeric photocatalysts but also sheds new light on photocatalytic 1O2 generation.

Understanding the influence of cation and anion migration on perovskite light-emitting diodes via transient response

Despite the rapid progress demonstrated in the efficiency of Perovskite light-emitting diodes (PeLEDs) in the past few years, ion migration has challenged the practical applications of these devices with undesirable hysteresis and degradation effect. Mobile ions in PeLEDs induced many unique and fast transient phenomena occurring on the time scale of microseconds to seconds and it is still far from clear how the underlying physical mechanism of ion motion-induced variation relates to the device performance. Therefore, in this work, we employ an ionic Drift–Diffusion Model (DDM) to evaluate measuring transient current response in a time scale of sub-seconds. The results show that spatial redistribution of ions within the perovskite results in dynamic electric field variation, which in turn, affects charge carrier injection and distribution. Moreover, the time delay between anion and cation migration leads to an unequal rate of charge carrier injection, hence the multi-stage behavior of the current–time response. It is also realized that the potential barrier of charge injection due to cation and anion accumulation at perovskite interfaces with electron and hole transporting layers reduces. Therefore, the facilitation of charge injection favors radiative recombination, and improved IQEs are expected at higher ion densities. It is found that the current–time response of the device gives beneficial information on cation and anion migration time scales. Choosing an appropriate scan rate in accordance with cation-related slow migration time is the first step to achieving reliable measurement procedures and hysteresis-free PeLED.

Bright Green-Emitting All-Inorganic Terbium Halide Double Perovskite Nanocrystals for Low-Dose X-ray Imaging.

Inorganic halide double perovskite (DP) nanocrystals (NCs) have attracted great attention because of their nontoxicity, mild reaction conditions, good stability, and excellent optical and optoelectronic properties. Herein, we prepare the inorganic terbium halide DP Cs2BTbCl6 (B = Na or Ag) NCs with bright green photoluminescence (PL) emission. The Na-Tb-based DP NCs exhibit better PL properties compared with the Ag-Tb-based DP NCs, which is due to Cs2NaTbCl6 NCs having a more localized charge carrier distribution on the [TbCl6]3- octahedron. The incorporation of Sb3+ dopant in Cs2NaTbCl6 NCs can construct a more efficient energy transfer process, resulting in a doubling of PL efficiency. Furthermore, Cs2NaTbCl6: Sb3+ NCs possess excellent X-ray scintillating performance with a low-dose detection limit of 140 nGyair/s, which is nearly 5 times more sensitive than the undoped NCs. The optimized NCs show great application prospects in X-ray imaging. This work helps deepen the understanding of the luminescence mechanism, excited state dynamics, and scintillation property in Tb-based DP NCs.

Influence of the Lateral Electrostatic Field on the Statistical Distribution of Charge Carriers in a Cylindrical Nanolayer of β-HgS

Analytical expressions are obtained for the wave functions and the energy spectrum of charge carriers in the β-HgS nanolayer of a cylindrical core/shell/shell β-CdS/ β-HgS/ β-CdS nanocomposite in the presence of a strong lateral uniform electrostatic field. It is shown that, under the influence of an external field, the position of the chemical potential of the electron-hole subsystem at absolute zero shifts to the bottom of the conduction band of the sample. The displacement value is determined by the intensity of the external field and increases linearly with increasing field. The concentration, internal energy, and heat capacity of the electronic subsystem of the β-HgS layer in the presence of a field are compared with similar values in the absence of a field. Calculations show that under identical conditions, the presence of an external field leads to an increase in the carrier concentration, which in turn leads to an increase in the internal energy and heat capacity of the system of electrons and holes in the layer.