Interaction Of Charge Carriers Research Articles

Biexciton Emission in CsPbBr3 Nanocrystals: Polar Facet Matters.

The metal halide perovskite nanocrystals exhibit a remarkable tolerance to midgap defect states, resulting in high photoluminescence quantum yields. However, the potential of these nanocrystals for applications in display devices is hindered by the suppression of biexcitonic emission due to various Auger recombination processes. By adopting single-particle photoluminescence spectroscopy, herein, we establish that the biexcitonic quantum efficiency increases with the increase in the number of facets on cesium lead bromide perovskite nanocrystals, progressing from cube to rhombic dodecahedron to rhombicuboctahedron nanostructures. The observed enhancement is attributed mainly to an increase in their surface polarity as the number of facets increases, which reduces the Coulomb interaction of charge carriers, thereby suppressing Auger recombination. Moreover, Auger recombination rate constants obtained from the time-gated photon correlation studies exhibited a discernible decrease as the number of facets increased. These findings underscore the significance of facet engineering in fine-tuning biexciton emission in metal halide perovskite nanocrystals.

Inter‐ and Intra‐Device Variation and Correlation of Hyperfine Interactions in Micron‐Scale Organic Light‐Emitting Diodes

AbstractThe spin‐dependent properties of opto‐electronic devices, such as sensors, displays, and photovoltaics, are a key contributor to performance metrics such as sensitivity and efficiency. As these devices are pushed to smaller scales, an understanding of how the microscopic variations in their spin‐dependent properties impact the macroscopic scale is becoming increasingly important. In this study, the hyperfine interactions of charge carriers are investigated within a series of co‐polymer thin‐film organic light‐emitting diodes (OLEDs), with each set of devices having a different physical size. Using spatially resolved measurements, significant variation in hyperfine interactions within microscopic thin‐film OLEDs is found. The domain size of spatially correlated hyperfine regions is characterized within these devices, finding characteristic scales of several microns. Finally, multiple device averaged magneto‐electroluminescence (MEL) responses from arrays of identical devices are simultaneously measured, to probe the influence of microscopic variation on the macroscopic hyperfine properties. It is found that smaller devices typically display a smaller device‐averaged hyperfine interaction. These findings shed light on the importance of spin dynamics in optoelectronic devices and provide insights for improving their performance.

Possible Topological Hall Effect in a Spin Gapless Semiconducting Mn2CoAl Heusler Compound

Topological Hall effect (THE), induced by the interaction of charge carriers with mesoscopic or microscopic noncoplanar spin structures, holds promising applications in the field of spintronics. In the present study, a giant THE of about 2 μΩ cm at room temperature is reported in a bulk spin gapless semiconducting Mn2CoAl cubic Heusler compound. Temperature‐dependent investigation of magneto‐transport data reveals that the Mn2CoAl has the large THE over a wide temperature range of 2–300 K. The alternating current (AC) susceptibility as a function of magnetic field exhibits a smooth and continuous response rather than any kink or anomaly, suggests that the observed THE in the Mn2CoAl compound results from the interaction of charge carriers with the microscopic noncoplanar spin texture. The observed THE as a function of temperature follows the same behavior as the magnetocrystalline anisotropy (MCA) of the cubic Mn2CoAl, indicating the competition of the MCA with ferromagnetic and antiferromagnetic exchange interactions as the origin of the noncoplanar spin texture and hence THE. Micromagnetic simulations further support the emergence of noncoplanar spin structure as a result of the competition between different energies.

Advancing perovskite solar cell performance: Enhanced efficiency and stability through superimposed PbS QDs

This study investigates enhancing efficiency in thin-film photovoltaic devices by incorporating varying-sized superimposed PbS colloidal quantum dots (CQDs). These CQDs offer tunable near-infrared absorption, multiple exciton generation effects, and solution processability, rendering them promising for improved performance. Superimposing QDs of different sizes extends the device's absorption spectrum beyond conventional limits, utilizing tailored absorption coefficients for broader sunlight wavelength utilization and increased solar energy conversion efficiency. Each QD group exhibits a unique absorption coefficient, with a peak at its central wavelength from the Schrödinger equation solution. Spectral analysis reveals strategic enhancement in light absorption across visible and near-infrared regions, broadening the perovskite absorption range beyond 800 nm. These QDs enhance both infrared and visible spectra, improving light harvesting. Summing the absorption of single QD groups creates a continuous spectrum, enhancing perovskite film stability and morphology. Rigorous optical analysis and comprehensive electrical modeling investigate charge carrier interactions within the photovoltaic framework. The study verifies around 24 % efficacy enhancement compared to QD-absent configurations, surpassing uniformly dispersed QDs by 11 %. Optical and electrical analyses yield a short-circuit current of 26.7 mA/cm2, open-circuit voltage of 1.13 V, fill factor of 0.88, and overall efficiency of 25.12 % in superimposed configurations. Results underscore the significance of varied QD dimensions and layered structures for enhanced absorption, offering a promising route for advancing thin-film solar technology. The study showcases the potential of PbS CQDs with varying radii in photovoltaics, contributing to improved efficiency and stability, with implications for the future of solar energy technology.

The fundamental 1/f noise in monolayer graphene

A lower bound on the voltage noise power spectrum set by quantum indeterminacy exists in any conducting material. This bound is calculated explicitly for monolayer graphene taking into account graphene pseudospin/valley band structure. It is found to be of the form characteristic of the observed [Formula: see text] noise. It is shown that the deviation of [Formula: see text] from unity is a result of the piezoelectric interaction of charge carriers with acoustic phonons. A comparison with the measured [Formula: see text] noise in graphene and graphene-based heterostructures shows that the observed noise intensity is near the quantum bound. It is demonstrated that the recently observed sharp peak in [Formula: see text] at small gate voltages in the field-effect transistors based on graphene/boron nitride heterostructure is well reproduced by the theory, and is to be attributed to the out-of-plane piezoelectric activity in this heterostructure. A simple kinetic model is proposed to describe the effect of the charge carrier trapping on [Formula: see text].

Ultrafast Charge Transfer Mechanism in Van der Waals Heterostructures of Topological Insulators with 2D Materials

Hybridizing topological materials (TM) with two-dimensional (2D) materials has resulted in interesting structures, such as van der Waals heterostructures (vdW), having potential in optoelectronics. However, the performance of devices made out of such structures relies on the charge carrier interactions among the layers, which establishes the basic trade-off between the kinetically controlled charge carrier dynamics at the interface and the performance of vdW-based devices. In this perspective, three vdW are fabricated using a wet transfer technique, and the structures are probed to study the charge carrier and phonon dynamics among the TM (a few layered Bi2Te3) and 2D systems. The three different vdW fabricated in this article are graphene/Bi2Te3 (G/BT), fluorographene/Bi2Te3 (FG/BT), and monolayer MoS2/Bi2Te3 (MS/BT). Raman spectroscopy establishes the formation of all of these vdW and indicates the occurrence of the charge transfer (CT) phenomenon among them, which is further probed using ultrafast pump–probe spectroscopy. The charge carrier and phonon dynamics reveal the presence of the occurrence of the CT mechanism in all of the vdW. These spectra are further used to model the bands between different types of heterointerfaces, such as the Schottky junction between graphene and Bi2Te3 and the type II heterojunction between FG/BT and MS/BT. With the help of band alignment and charge carrier and phonon dynamics studies presented, a unique CT mechanism and excitonic peak tunability in vdW are proven, where such information is essential in developing highly sensitive optoelectronic and spintronic devices

Universal scaling laws for charge-carrier interactions with quantum confinement in lead-halide perovskites

Lead halide perovskites open great prospects for optoelectronics and a wealth of potential applications in quantum optical and spin-based technologies. Precise knowledge of the fundamental optical and spin properties of charge-carrier complexes at the origin of their luminescence is crucial in view of the development of these applications. On nearly bulk Cesium-Lead-Bromide single perovskite nanocrystals, which are the test bench materials for next-generation devices as well as theoretical modeling, we perform low temperature magneto-optical spectroscopy to reveal their entire band-edge exciton fine structure and charge-complex binding energies. We demonstrate that the ground exciton state is dark and lays several millielectronvolts below the lowest bright exciton sublevels, which settles the debate on the bright-dark exciton level ordering in these materials. More importantly, combining these results with spectroscopic measurements on various perovskite nanocrystal compounds, we show evidence for universal scaling laws relating the exciton fine structure splitting, the trion and biexciton binding energies to the band-edge exciton energy in lead-halide perovskite nanostructures, regardless of their chemical composition. These scaling laws solely based on quantum confinement effects and dimensionless energies offer a general predictive picture for the interaction energies within charge-carrier complexes photo-generated in these emerging semiconductor nanostructures.

Evolution of extremely large magnetoresistance in a Weyl semimetal, WTe2 with Ni-doping

${\mathrm{WTe}}_{2}$, a type-II Weyl semimetal, exhibits extremely large magnetoresistance (XMR) that is attributed to perfect electron-hole compensation. Here, we studied the evolution of the electronic properties of ${\mathrm{WTe}}_{2}$ with Ni substitution at W sites. Transport measurements exhibit a metal-to-insulator transition at low temperatures with the application of magnetic field; the field-induced effect is significantly reduced in the doped sample. The XMR of about ${10}^{6}$% at 2 K in ${\mathrm{WTe}}_{2}$ is reduced to ${10}^{4}$% in the 10% Ni-doped sample. The magnetoresistance shows an ${H}^{2}$ dependence and follows the temperature dependence of classical carrier mobility. We observe a deviation from Kohler's rule in both cases, indicating the presence of multiple types of charge carriers. Quantum oscillation data exhibit a signature of four Fermi pockets (two electron and two hole pockets) in both cases, and the nontrivial nature of the electron and hole bands. Interestingly, the electron-hole compensation is nearly perfect even in the doped compound, although the size of the Fermi pockets and the charge-carrier interaction parameters are slightly changed with doping. These results reveal intriguing insight into the XMR behavior of this class of materials and the possibilities of tuning the properties of charge carriers within the XMR regime.

Generalized design principles for hydrodynamic electron transport in anisotropic metals

Interactions of charge carriers with lattice vibrations, or phonons, play a critical role in unconventional electronic transport of metals and semimetals. Recent observations of phonon-mediated collective electron flow in bulk semimetals, termed electron hydrodynamics, present new opportunities in the search for strong electron-electron interactions in high carrier density materials. Here we present the general transport signatures of such a second-order scattering mechanism, along with analytical limits at the Eliashberg level of theory. We study electronic transport, using ab initio calculations, in finite-size channels of semimetallic ZrSiS and ${\mathrm{TaAs}}_{2}$ with and without topological band crossings, respectively. The order of magnitude separation between momentum-relaxing and momentum-conserving scattering length scales across a wide temperature range make both of them promising candidates for further experimental observation of electron hydrodynamics. More generally, our calculations suggest that the hydrodynamic transport regime does not, to first order, rely on the topological nature of the bands. Finally, we discuss general design principles guiding future search for hydrodynamic candidates, based on the analytical formulation and our ab initio predictions. We find that systems with strong electron-phonon interactions, reduced electronic phase space, and suppressed phonon-phonon scattering at temperatures of interest are likely to feature hydrodynamic electron transport. We predict that layered and/or anisotropic semimetals composed of half-filled $d$ shells and light group V/VI elements with lower crystal symmetry are promising candidates to observe hydrodynamic phenomena in the future.

Coexistence of quantum diffusive and diffusive two band carriers in site disordered AgSnSbTe3

Electronic transport in materials has always been enigmatic due to the interplay between disorder and electronic correlation of varying degrees. Tuning of disorder/order of atoms in crystalline rock-salt tellurides and associated charge carrier localization/delocalization have significant impact on its thermoelectric properties. In the weak limit of disorder and charge carrier interactions, associated phenomena contribute as a correction to the conventional classical electrical conductivity which is usually observed in two-dimensional (2D) thin films but is rare in bulk three-dimensional (3D) materials. Here, we report the experimental observation of coexisting disorder enhanced electron-electron interaction (diffusive) along with the weak anti-localization (quantum diffusive) corrections to the two band classical theory of electronic transport in the 3D site disordered AgSnSbTe3. Our analysis of low temperature magneto-transport investigations along with the theoretical modelling provide the evidence of the involvement of only one type of carriers in the interaction (diffusive), while both carrier types are participating in the weak anti-localization (quantum diffusive) phenomena. Thus, this study demonstrates the fascinating multi-carrier type electronic transport with the coexistence of quantum diffusive and diffusive corrections to the classical conductivity, which should help in designing high performance material for various applications such as thermoelectrics.

Nonlinear optical response, theoretical efficiencies, and high photosensitivity of CH3NH3PbI3−xClx thin films for photonic applications

In this paper, we have investigated the nonlinear optical response and theoretical efficiency of CH3NH3PbI3−xClx thin films from the optical transmission and reflection measurements. The dispersion of the complex refractive index is evaluated using the Wemple–DiDomenico single oscillator model. The oscillator energy (E0) of CH3NH3PbI3−xClx follows by an empirical relationship with optical bandgap (Eg) as E0 ≈ 2.41 Eg for chemical dip coating, spray, and E0 ≈ 1.63 Eg for dipping deposited samples, respectively. The long wavelength refractive index, average oscillator wavelength, and oscillator strength are also determined using the Sellmeier oscillator equation. The estimated third-order nonlinear optical susceptibility is found to be the order of 10−12 esu. The incident photon and charge carrier interaction in CH3NH3PbI3−xClx is studied from the dielectric response of the samples. The charge carrier excitation is found higher at lower wavelength and experienced bulk excitation in volume while surface excitation on the surface region. The optical conductivity of CH3NH3PbI3−xClx is notably high, which leads to an increase in carrier transfer through the extrinsic halide perovskite material expedient for higher conversion efficiency. The highest theoretical efficiency of CH3NH3PbI3−xClx is estimated to be 17.4%, which is in excellent agreement with the experimental report. From photosensitivity study, it is confirmed that CH3NH3PbI3−xClx films are highly photosensitive. All these results comprehend that CH3NH3PbI3−xClx is a potential candidate for photonic applications.

Hollow TiO2/g-C3N4 nanocomposite for photodegradation of volatile organic carbons under visible-light

Hollow TiO2/g-C3N4 nanocomposite was prepared using solvothermal method. Two-dimensional g-C3N4 nanosheets were coupled with TiO2 hollow spheres at different ratios to investigate the charge-carrier interactions with the aim of enhancing the photocatalytic properties of the nanocomposite. This coupling was systematically examined by scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, photoluminescence, X-ray photoelectron spectroscopy, and UV diffuse reflectance spectroscopy. The prepared nanocomposite was used for the photodegradation of the volatile organic carbons methyl tetra-butyl ether and toluene present in aqueous solution. Elemental analysis and X-ray diffraction revealed a high-purity sample, while the UV diffuse reflectance spectroscopy demonstrated the presence of a well-defined anatase crystal phase for the TiO2 hollow sphere; and the photoluminescence measurements showed an enhancement in visible-light absorbance, with a good reduction in the electron-hole recombination rate. The performance of the nanocomposites in the photocatalytic degradation of toluene under irradiation with visible-light was evaluated. The 20/80% TiO2/g-C3N4 nanocomposite materials showed highest photocatalytic activity for toluene and methyl tetra-butyl ether, achieving a degradation of more than 90%; this is attributed to the interaction between the two surfaces in the TiO2/g-C3N4 nanocomposite, resulting in a higher performance than the individual components.

Ion transport mechanism in anhydrous lithium thiocyanate LiSCN part III: charge carrier interactions in the premelting regime.

In lithium thiocyanate Li(SCN), the temperature regime below the melting point (274 °C) is characterized by excess conductivities over the usual Arrhenius behavior (premelting regime). Here, the Schottky defect pair concentration is high, and the point defect chemistry can no longer be considered as dilute. Coulomb interactions of Schottky pairs are expected to occur lowering the formation energy of new carriers and hence leading avalanche-like to a transition into a fully defective superionic state. The respective non-linear behavior is investigated using the cube-root law approach characterized by a defect interaction parameter J, which is a measure of the effective defect-lattice energy. In the case of Li(SCN), the rather pronounced volume expansion is to be included in the model. A literature comparison with other materials emphasizes to what degree defect formation as well as defect interactions depend not only on the dominant mobile defect, but also on the respective sublattice. Overall, a quantitative description of the defect chemistry of Li(SCN) in the premelting regime is derived.

Exciton-exciton and exciton–charge carrier interaction and exciton collisional broadening in GaAs/AlGaAs quantum wells

Wave functions of heavy-hole excitons in GaAs/Al$_{0.3}$Ga$_{0.7}$As square quantum wells (QWs) of various widths are calculated by the direct numerical solution of a three-dimensional Schr\"odinger equation using a finite-difference scheme. These wave functions are then used to determine the exciton-exciton, exciton-electron and exciton-hole fermion exchange constants in a wide range of QW widths (5-150 nm). Additionally, the spin-dependent matrix elements of elastic exciton-exciton, exciton-electron and exciton-hole scattering are calculated. From these matrix elements, the collisional broadening of the exciton resonance is obtained within the Born approximation as a function of the areal density of excitons, electrons and holes respectively for QW widths of 5, 15, 30 and 50 nm. The obtained numerical results are compared with other theoretical works.

Scanning ultrafast electron microscopy reveals photovoltage dynamics at a deeply buried p−Si/SiO2 interface

The understanding and control of charge carrier interactions with defects at buried insulator/semiconductor interfaces is essential for achieving optimum performance in modern electronics. Here, we report on the use of scanning ultrafast electron microscopy (SUEM) to remotely probe the dynamics of excited carriers at a Si surface buried below a thick thermal oxide. Our measurements illustrate a previously unidentified SUEM contrast mechanism, whereby optical modulation of the space-charge field in the semiconductor modulates the electric field in the thick oxide, thus affecting its secondary electron yield. By analyzing the SUEM contrast as a function of time and laser fluence we demonstrate the diffusion mediated capture of excited carriers by interfacial traps.

The dominance of non-electron–phonon charge carrier interaction in highly-compressed superhydrides

The primary mechanism governing the emergence of near-room-temperature superconductivity (NRTS) in superhydrides is widely accepted to be the electron–phonon interaction. If so, the temperature-dependent resistance, R(T), in these materials should obey the Bloch–Grüneisen (BG) equation, where the power-law exponent, p, should be equal to the exact integer value of p= 5. However, there is a well-established theoretical result showing that the pure electron–magnon interaction should be manifested by p= 3, and p= 2 is the value for pure electron–electron interaction. Here we aimed to reveal the type of charge carrier interaction in the layered transition metal dichalcogenides PdTe2, high-entropy alloy (ScZrNb)0.65[RhPd]0.35 and highly-compressed elemental boron and superhydrides H3S, LaH x , PrH9 and BaH12 by fitting the temperature-dependent resistance of these materials to the BG equation, where the power-law exponent, p, is a free-fitting parameter. The results showed that the high-entropy alloy (ScZrNb)0.65[RhPd]0.35 exhibited pure electron–phonon mediated superconductivity with p = 4.9 ± 0.4. Unexpectedly, we revealed that all studied superhydrides exhibit 1.8 < p < 3.2. This implies that it is unlikely that the electron–phonon interaction is the primary mechanism for the Cooper pairs formation in highly-compressed superhydrides, and alternative pairing mechanisms, for instance, the electron–magnon, the electron–polaron, the electron–electron and other pairing mechanisms should be considered as the origin for the emergence of NRTS in these compounds.

Classifying Charge Carrier Interaction in Highly Compressed Elements and Silane.

Since the pivotal experimental discovery of near-room-temperature superconductivity (NRTS) in highly compressed sulphur hydride by Drozdov et al. (Nature 2015, 525, 73–76), more than a dozen binary and ternary hydrogen-rich phases exhibiting superconducting transitions above 100 K have been discovered to date. There is a widely accepted theoretical point of view that the primary mechanism governing the emergence of superconductivity in hydrogen-rich phases is the electron–phonon pairing. However, the recent analysis of experimental temperature-dependent resistance, R(T), in H3S, LaHx, PrH9 and BaH12 (Talantsev, Supercond. Sci. Technol. 2021, 34, accepted) showed that these compounds exhibit the dominance of non-electron–phonon charge carrier interactions and, thus, it is unlikely that the electron–phonon pairing is the primary mechanism for the emergence of superconductivity in these materials. Here, we use the same approach to reveal the charge carrier interaction in highly compressed lithium, black phosphorous, sulfur, and silane. We found that all these superconductors exhibit the dominance of non-electron–phonon charge carrier interaction. This explains the failure to demonstrate the high-Tc values that are predicted for these materials by first-principles calculations which utilize the electron–phonon pairing as the mechanism for the emergence of their superconductivity. Our result implies that alternative pairing mechanisms (primarily the electron–electron retraction) should be tested within the first-principles calculations approach as possible mechanisms for the emergence of superconductivity in highly compressed lithium, black phosphorous, sulfur, and silane.

Ground-state features and spectral properties of large polaron liquids from low to high charge densities

A new variational approach is proposed at zero temperature for a finite density of charge carriers in order to study ground state features of the Frohlich model including electron-electron and electron-phonon interactions. Within the intermediate electron-phonon coupling regime characteristic of large polarons, the approach takes into account on the same footing polaron formation and polaron-polaron correlations which play a relevant role going from low to high charge densities. Including fluctuations on top of the variational approach, the electronic spectral function is calculated from the weak to the intermediate electron-phonon coupling regime finding a peak-dip-hump line shape. The spectra are characterized by a transfer of spectral weight from the incoherent hump to the coherent peak with decreasing the electron-phonon coupling constant or with increasing the particle density. Three different density regimes stem out: the first, at low densities, where the features of a single large polaron with a substantial incoherent spectral weight are not modified by charge carrier interactions; a second one, at intermediate densities, where the polaronic liquid shows a rapid crossover from incoherent to coherent dynamics; the third one, at high densities, where screening effects are so prominent that the system presents a conventional metallic phase. The results obtained in the low to intermediate density regime turn out to be relevant for the interpretation of recent tunneling and photoemission experiments in SrTiO3-based systems.

Quantifying the Charge Carrier Interaction in Metallic Twisted Bilayer Graphene Superlattices.

The mechanism of charge carrier interaction in twisted bilayer graphene (TBG) remains an unresolved problem, where some researchers proposed the dominance of the electron–phonon interaction, while the others showed evidence for electron–electron or electron–magnon interactions. Here we propose to resolve this problem by generalizing the Bloch–Grüneisen equation and using it for the analysis of the temperature dependent resistivity in TBG. It is a well-established theoretical result that the Bloch–Grüneisen equation power-law exponent, p, exhibits exact integer values for certain mechanisms. For instance, p = 5 implies the electron–phonon interaction, p = 3 is associated with the electron–magnon interaction and p = 2 applies to the electron–electron interaction. Here we interpret the linear temperature-dependent resistance, widely observed in TBG, as , which implies the quasielastic charge interaction with acoustic phonons. Thus, we fitted TBG resistance curves to the Bloch–Grüneisen equation, where we propose that p is a free-fitting parameter. We found that TBGs have a smoothly varied p-value (ranging from 1.4 to 4.4) depending on the Moiré superlattice constant, λ, or the charge carrier concentration, n. This implies that different mechanisms of the charge carrier interaction in TBG superlattices smoothly transition from one mechanism to another depending on, at least, λ and n. The proposed generalized Bloch–Grüneisen equation is applicable to a wide range of disciplines, including superconductivity and geology.

Coexistence of Short- and Long-Range Ferromagnetic Proximity Effects in a Fe/(Cd,Mg)Te/CdTe Quantum Well Hybrid Structure.

In a Fe/(Cd,Mg)Te/CdTe quantum well hybrid structure, short-range and long-range ferromagnetic proximity effects are found to coexist. The former is observed for conduction band electrons, while the latter is observed for holes bound to shallow acceptors in the CdTe quantum well. These effects arise from the interaction of charge carriers confined in the quantum well with different ferromagnets, where electrons interact with the Fe film and holes with an interfacial ferromagnet at the Fe/(Cd,Mg)Te interface. The two proximity effects originate from fundamentally different physical mechanisms. The short-range proximity effect for electrons is determined by the overlap of their wave functions with d-electrons of the Fe film. On the contrary, the long-range effect for holes bound to acceptors is not associated with overlapping wave functions and can be mediated by elliptically polarized phonons. The coexistence of the two ferromagnetic proximity effects reveals the presence of a nontrivial spin texture within the same heterostructure.