Understanding Of Charge Transport Research Articles

Understanding charge transport and dielectric relaxation properties in lead-free Cs2ZrCl6 nanoparticles.

In the exploration of perovskite materials devoid of lead and appropriate for capturing solar energy, a recent finding has surfaced concerning Cs2ZrCl6. This compound has attracted interest as a potential candidate, displaying advantageous optical and electrical features, coupled with remarkable durability under environmental stresses. This research outlines the effective production of non-toxic metal halide nanoparticles of Cs2ZrCl6 using the gradual cooling technique. Thorough examinations have been conducted to explore the structural, optical, and dielectric traits. Over the frequency range of 101-106 Hz, the dielectric constant, loss factor, electric modulus, and electrical conductivity of Cs2ZrCl6 exhibit a strong dependence on temperature. The Nyquist plot confirms the distinct contributions of grains and grain boundaries to the total impedance. In the high-frequency region, the dielectric constant tends to increase with temperature. In accordance with the modified Kohlrausch-Williams-Watts (KWW) equation, an asymmetric nature corresponding to the non-Debye type is observed in the electric modulus spectra at different temperatures. Furthermore, the imaginary part of the electric modulus spectrum shifts from the non-Debye type towards the Debye type with increasing temperature, despite not obtaining an exact Debye response. The frequency-dependent behavior of AC conductivity has been modeled using Joncher's universal law. The conduction mechanism within the Cs2ZrCl6 compound is attributed to the small polaron tunneling model (NSPT). Furthermore, Cs2ZrCl6 has the potential to function as an energy harvesting device due to its elevated dielectric constant combined with minimal dielectric loss.

Interaction strength in molecular junctions consisting of π-stacked antiaromatic molecules

The interaction force was measured as the adhesive force acting between norcorrole molecules anchored to the probe surface and the sample surface. This study reveals substantial interactions in π-stacked antiaromatics at the single-molecule scale.

Coexistent, Competing Tunnelling, and Hopping Charge Transport in Compressed Metal Nanocluster Crystals.

Understanding charge transport among metal particles with sizes of approximately 1 nm poses a great challenge due to the ultrasmall nanosize, yet it holds great significance in the development of innovative materials as substitutes for traditional semiconductors, which are insulative and unstable in less than ∼10 nm thickness. Herein, atomically precise gold nanoclusters with well-defined compositions and structures were investigated to establish a mathematical relation between conductivity and interparticle distance. This was accomplished using high-pressure in situ resistance characterizations, synchrotron X-ray diffraction (XRD), and the Murnaghan equation of state. Based on this precise correlation, it was predicted that the conductivity of Au25(SNap)18 (SNap: 1-naphthalenethiolate) solid is comparable to that of bulk silver when the interparticle distance is reduced to approximately 3.6 Å. Furthermore, the study revealed the coexisting, competing tunneling, and incoherent hopping charge transport mechanisms, which differed from those previously reported. The introduction of conjugation-structured ligands, tuning of the structures of metal nanoclusters, and use of high-pressure techniques contributed to enhanced conductivity, and thus, the charge carrier types were determined using Hall measurements. Overall, this study provides valuable insight into the charge transport in gold nanocluster solids and represents an important advancement in metal nanocluster semiconductor research.

Efficient calculation of electronic coupling integrals with the dimer projection method via a density matrix tight-binding potential.

Designing organic semiconductors for practical applications in organic solar cells, organic field-effect transistors, and organic light-emitting diodes requires understanding charge transfer mechanisms across different length and time scales. The underlying electron transfer mechanisms can be efficiently explored using semiempirical quantum mechanical (SQM) methods. The dimer projection (DIPRO) method combined with the recently introduced non-self-consistent density matrix tight-binding potential (PTB) [Grimme et al., J. Chem. Phys. 158, 124111 (2023)] is used in this study to evaluate charge transfer integrals important for understanding charge transport mechanisms. PTB, parameterized for the entire Periodic Table up to Z = 86, incorporates approximate non-local exchange, allowing for efficient and accurate calculations for large hetero-organic compounds. Benchmarking against established databases, such as Blumberger's HAB sets, or our newly introduced JAB69 set and comparing with high-level reference data from ωB97X-D4 calculations confirm that DIPRO@PTB consistently performs well among the tested SQM approaches for calculating coupling integrals. DIPRO@PTB yields reasonably accurate results at low computational cost, making it suitable for screening purposes and applications to large systems, such as metal-organic frameworks and cyanine-based molecular aggregates further discussed in this work.

Tuning Charge Transport of Oligopeptide Junctions via Interfacial Amino Acids†

Comprehensive SummaryThe charge transport through peptides can imitate the corresponding processes in more complicated proteins, enabling us to develop high‐performance bioelectronic devices and to understand the mechanisms of biomolecular recognition and information transfer. While charge transport modulation through individual peptides has been achieved via various covalent strategies, the intermolecular modulation is still very challenging, which may capture the charge transport between proteins. To tackle this challenge, we used well‐defined self‐assembled monolayers (SAMs) of oligopeptides as a model to imitate the interface of proteins and explored an interfacial amino acid strategy for charge transport modulation. We showed that non‐covalently interfaced charged amino acids (e.g., arginine) effectively attenuated the charge transport of glutamic acid terminated polyglycine peptide SAMs. By analyzing the relationship of the charge transport with the molecular frontier orbital relative to the Fermi energy level of the electrode, the molecule‐electrodes coupling (Γ), and the trends in skewness and kurtosis with voltage and the dielectric constant (εr), we showed that the attenuation was from the decreased Γ and the reduced polarizability. We present an efficient strategy to modulate the charge transport of oligopeptide‐SAM junctions by intermolecular interactions, which will advance our understanding of charge transport in biological systems and facilitate developing future electronics.

Revisiting the Mechanisms of Charge Transport in Solutions of Redox-Active Molecules Using Computer Simulations: When and Why Do Analytical Theories Fail?

Understanding charge transport is essential for the development of energy-storage applications. This work introduces a new theoretical methodology to model diffusive charge transport in solutions of redox-active molecules by combining Langevin dynamics for the spatial degrees of freedom and a master-equation formalism to describe the electron-hopping events between redox molecules. The model is used to analyze the effects of the concentration of the redox molecules and the strength of the intermolecular interactions on the charge-transport mechanism. In the past, the rate of charge transport has been modeled with the analytical Dahms-Ruff equation; however, this is a mean-field equation, whose range of validity has not been tested with less approximate theories. We show that the Dahms-Ruff equation fails to quantitatively predict the diffusion coefficient for charge transport for large concentrations of the redox species and high bimolecular electron-transfer rates, i.e., the most relevant conditions for energy-storage applications. Under these conditions, the diffusion coefficient for charge transport obtained from simulations is larger than that predicted from the Dahm-Ruff equation because of the formation of transient clusters of redox molecules. Also, intermolecular interactions, which are not taken into account by the Dahms-Ruff equation, play a central role in the charge transport of redox species. We show that the apparent diffusion coefficient experiences a maximum with respect to the strength of the intermolecular attractions. This maximum is traced back to the formation of clusters and their two opposite effects on the diffusion coefficient: electron hopping is fast within a cluster but inefficient between neighboring clusters.

High‐Temporal‐Resolution Characterization Reveals Outstanding Random Telegraph Noise and the Origin of Dielectric Breakdown in h‐BN Memristors

AbstractMemristor‐based electronic memory have recently started commercialization, although its market size is small (~0.5%). Multiple studies claim their potential for hardware implementation of artificial neural networks, advanced data encryption, and high‐frequency switches for 5G/6G communication. Application aside, the performance and reliability of memristors need to be improved to increase their market size and fit technology standards. Multiple groups propose novel nano‐materials beyond phase‐change, metal‐oxides, and magnetic materials as resistive switching medium (e.g., two‐dimensional, nanowires, perovskites). However, most studies use characterization setups that are blind to critical phenomena in understanding charge transport across the devices. Here an advanced setup with high temporal resolution is used to analyze current noise, dielectric breakdown growth, and ambipolar resistive switching in memristors based on multilayer hexagonal boron nitride (h‐BN), one of the most promising novel nano‐materials for memristive applications. The random telegraph noise in pristine memristors and its evolution as the devices degrade, covering ~7 orders of magnitude in current with consistent observation, is studied. Additionally, an ambipolar switching regime with very low resistance down to 50Ω and its connection with a telegraph behavior with high/low current ratios >100, linked to a thermally‐driven disruption of a metallic nanofilament, is shown.

Closing the loop between microstructure and charge transport in conjugated polymers by combining microscopy and simulation

A grand challenge in materials science is to identify the impact of molecular composition and structure across a range of length scales on macroscopic properties. We demonstrate a unified experimental-theoretical framework that coordinates experimental measurements of mesoscale structure with molecular-level physical modeling to bridge multiple scales of physical behavior. Here we apply this framework to understand charge transport in a semiconducting polymer. Spatially-resolved nanodiffraction in a transmission electron microscope is combined with a self-consistent framework of the polymer chain statistics to yield a detailed picture of the polymer microstructure ranging from the molecular to device relevant scale. Using these data as inputs for charge transport calculations, the combined multiscale approach highlights the underrepresented role of defects in existing transport models. Short-range transport is shown to be more chaotic than is often pictured, with the drift velocity accounting for a small portion of overall charge motion. Local transport is sensitive to the alignment and geometry of polymer chains. At longer length scales, large domains and gradual grain boundaries funnel charges preferentially to certain regions, creating inhomogeneous charge distributions. While alignment generally improves mobility, these funneling effects negatively impact mobility. The microstructure is modified in silico to explore possible design rules, showing chain stiffness and alignment to be beneficial while local homogeneity has no positive effect. This combined approach creates a flexible and extensible pipeline for analyzing multiscale functional properties and a general strategy for extending the accesible length scales of experimental and theoretical probes by harnessing their combined strengths.

The electrons' journey in thick metal oxides

Originally introduced in electronic manufacturing to replace the SiO2 insulating layer, metal oxides are now extensively used in a multitude of electronic devices. Understanding charge transport mechanisms in metal oxides is of paramount importance for device optimization; however, a detailed and self-consistent discussion of electron conduction at all applied electric fields is lacking in the literature. In this work, we investigated the conduction mechanisms in three model systems, Al2O3, HfO2, and Al-doped HfO2 metal–insulator–metal capacitors, determining the path that the electrons travel within the metal oxide. Traps properties are extracted from experimental current–voltage characteristics using the Ginestra® simulation software. Furthermore, the analysis allowed to visualize the location of traps most involved in the conduction and the dominant transport mechanisms at each applied electric field. Despite the different oxide properties, a similar trend was recognized at low electric fields, the electron transport through the oxide is negligible, and the dominant contribution to the measured current is ascribed to the charge/discharge of traps located near the metal/oxide interfaces, leading to displacement currents. At high electric fields, the transport of electrons occurs through the defect rich oxides in the two following ways: if a large density of traps is energetically located near the electrodes Fermi level (as in HfO2), the electrons tunnel from trap to trap until they reach the anode; otherwise, when traps are closer to the conduction band (as in Al2O3 and AlHfO), the electrons tunnel from the cathode into one trap and then into the oxide conduction band, interacting only with traps near the cathode.

Achieving Multiple Quantum-Interfered States via Through-Space and Through-Bond Synergistic Effect in Foldamer-Based Single-Molecule Junctions.

The construction of multivalued logic circuits by multiple quantum-interfered states at the molecular level can make full use of molecular diversity and versatility, broadening the application of molecular electronics. Understanding charge transport through different conducting pathways and how they interact with each other in molecules with a secondary structure is an indispensable foundation to achieve this goal. Herein, we elucidate the synergistic effect from through-space and through-bond conducting pathways in foldamers derived from ortho-pentaphenylene by the separate modulation on these pathways. The shrinkage of central heterocycles' sizes allows foldamers to stack with larger overlap degrees, resulting in level-crossing and thus transformation from constructive quantum interference (CQI) to destructive quantum interference (DQI) in a through-space pathway. The alteration of central heterocycles' connection sites enhances through-bond conjugation, leading to amplified contribution from a through-bond pathway. The enhanced through-bond pathway destructively interferes with the through-space pathway, exerting a suppression effect on transmission. Therefore, four quantum-interfered states of through-space and through-bond combination are generated, including through-space CQI-dominated states, through-space DQI-dominated states, through-space CQI states with through-bond suppression, and through-space DQI states with through-bond suppression. These findings enable us to regulate charge transport within high-order structures via multiple conducting pathways and provide a proof of concept to construct multivalued logic circuits.

Charge Carrier Transport Mechanism in Ta2 O5 , TaON, and Ta3 N5 Studied by Applying Polaron Hopping and Bandlike Models.

TaON and Ta3 N5 are considered promising materials for photocatalytic and photoelectrochemical water splitting. In contrast, their counterpart Ta2 O5 does not exhibit good photocatalytic performance. This may be explained with the different charge carrier transport mechanisms in these materials, which are not well understood yet. Herein, we investigate the charge transport properties in Ta2 O5 , TaON, and Ta3 N5 by polaron hopping and bandlike models. First, the polaron binding energies were calculated to evaluate whether the small polaron occurs in these materials. Then we performed calculations to localize the excess carriers as small polarons using a hybrid density functional. We find that the small polaron hopping is the charge transfer mechanism in Ta2 O5, whereas our calculations indicate that this mechanism may not occur in TaON and Ta3 N5 . We also investigated the bandlike model mechanism by calculating the charge carrier mobility of these materials using the effective mass approximation, but the calculated mobility is not consistent with experimental results. This study is a first step towards understanding charge transport in oxynitrides and nitrides and furthermore establishes a simple rule to determine whether a small polaron occurs in a material.

Understanding Charge Transport in Triarylmethyl-Based Spintronic Nanodevices

In this Article, we explore the potential of triarylmethyl (TAM) oligomers as devices for organic molecular spintronic applications. We use the Landauer formalism to compute global spin transport a...

Operando bulk and interfacial characterization for electrochemical energy storage: Case study employing isothermal microcalorimetry and X-ray absorption spectroscopy

The global shift to electricity as the main energy carrier will require innovation in electrochemical energy storage (EES). EES systems are the key to the “electron energy economy,” minimizing losses and increasing reliability between energy supply and demand. However, steep challenges such as cost, cycle/calendar life, energy density, material availability, and safety limit widespread adoption of batteries for large-scale grid and vehicle applications. Battery innovation that meets today’s challenges will require new chemistries, which can originate from understanding charge transport phenomena at multiple time and length scales. The advancement of operando characterization can expedite this progress as changes can be observed during battery function. This article highlights progress in bulk and interfacial operando characterization of batteries. Specifically, a case study involving Fe3O4 is provided demonstrating that combining X-ray absorption spectroscopy and isothermal microcalorimetry can provide real-time characterization of productive faradaic redox processes and parasitic interfacial reactions during (de)lithiation.Graphic abstract

Ligand Exchange and Impurity Doping in 2D CdSe Nanoplatelet Thin Films and Their Applications

AbstractThe effects of halide‐ligand exchange and Cu and Ag doping are studied on structural, optical, and electrical properties of four monolayer CdSe nanoplatelet (NPL) and NPL thin films. Combinational study shows that NH4Cl‐treatment on CdSe NPL and NPL thin films show tetragonal lattice distortion of NPL, side‐to‐side attachment between NPLs, bathochromic shift in absorption spectra, and complete quenching of band‐edge and dopant‐induced emissions. First‐principle calculations reveal that Cl creates states below valence band maximum while Ag and Cu dopants create acceptor‐like states, explaining the change of their optical property. Field‐effect transistors are fabricated to investigate the effect of doping and reduced interplatelet distance on electrical properties of CdSe NPL thin films, demonstrating Cu and Ag dopants mitigate n‐type character of CdSe NPL thin films. Temperature‐dependent electrical characterization is conducted to further understand charge transport behavior depending on the existence of dopants. This work provides scientific information on the influence of surface chemistry and impurity doping on quantum confined semiconductors and new directions for the design of high‐performance nanomaterial‐based electronic and optoelectronic devices.

Reversible Rectification of Microscale Ferroelectric Junctions Employing Liquid Metal Electrodes.

Both ferroelectric crystals and liquid metal electrodes have attracted extensive attention for potential applications in next-generation devices and circuits. However, the interface information between ferroelectric crystals and liquid metal electrodes has so far been lacking. To better understand the optoelectronic properties of microscale ferroelectric crystals (potassium tantalate niobate, KTN) and its potential integration with liquid metal electrodes (a "printing ink" for flexible electric circuit production), microscale KTN crystals sandwiched by eutectic gallium indium (EGaIn, a liquid metal) with varied contact geometries were studied. Unlike the bulk KTN crystal junctions, the microscale KTN junctions show electrical rectifying characteristics upon light illumination, and the directionality of the rectification can be reversed by increasing the ambient temperature to a few degrees. Furthermore, a strong suppression of the current upon increasing voltage, that is, the quasi-negative differential resistance, is observed when the microscale KTN is half-enclosed by the EGaIn electrode. Our results show that trapping/detrapping of carriers affected by the crystal size and the ambient temperature is the dominant physical mechanism for these observations. These results not only facilitate a better understanding of charge transport through the microscale ferroelectric crystals but also advance the design of miniaturized hybrid devices.

Guiding Students to Understand the Nanoscale Charge Transport by the Mechanically Controllable Break Junction Technique

Fabrication of molecular devices will meet the demands of the miniaturization of traditional electronic devices. Although microfabrication based on the complementary metal-oxide–semiconductor process can fabricate high-quality single-molecule devices, the cost is high, leading to a challenge to provide a hands-on experiment in the teaching practice of charge transport through single-molecule devices. Here, we demonstrate the low cost and simple procedures associated with a mechanically controllable break junction (MCBJ) technique to fabricate single-molecule devices. On the basis of the MCBJ technique, the conductance of gold–gold atomic contacts and single-molecule junctions can be readily characterized. The teaching practice helps students understand charge transport at the single-molecule level, that is, the quantum tunneling mechanisms. Moreover, the MCBJ technique study provides students the opportunity to develop interdisciplinary thinking, which is vital for understanding, designing, and developing single-molecule electronics.

Using the Electrode Potential to Orient Single Molecules in Nanoscale Electrical Junctions and Determine Charge Transport Mechanism

Charge transport through and between molecules is central to important processes in nature. Studying the conductivity of single molecules can contribute to a better understanding of charge transport, and also help to develop building blocks of molecular electronics, light harvesting devices, etc. We measure the conductivity of molecules using the Scanning Tunneling Microscope break-junction (STM-BJ) method that utilizes repeatedly formed circuits where one or a few molecules are trapped between two electrodes, at least one of which has nanoscale dimensions. The statistical analysis of thousands of measurements yields the conductance of single molecules.One particular interest is the role of the molecule-electrode contact and orientation in charge transport. In the simplest analysis this contact can present a substantial barrier to charge injection, which can have important consequences in devices such as dye sensitized semiconductor nanoparticle solar cells. Our most recent developments include controlling the orientation of the molecule in the junction using the electrode potential so that we can measure conductance along different molecular axes, accessing elements of the anisotropy of the molecular conductance tensor. Specifically, when the molecule is oriented flat, with the molecular plane parallel to the electrode surface, charge transport is measured perpendicular to the molecular plane. Furthermore, the electrode dependence of the molecular conductivity provides insight into the transport mechanism. This approach can be applied to molecules that do not have suitable molecular anchoring groups for traditional STM-BJ measurements, and allows us to determine the importance of anchoring groups in charge transport in molecular junctions.

(Energy Technology Division Graduate Student Award sponsored by Bio-Logic) Understanding Charge Transport for Current and Future Electrochemical Energy Storage Technologies

Advances in electrochemical energy storage are key for the integration of intermittent wind and solar energy sources into the grid, widespread adoption of electric vehicles, and development of new safe and reliable consumer devices. For grid, transportation, and electronic energy storage markets, each application has distinct needs. Therefore, developing battery technology for emerging and wide-spread applications demands greater understanding of the complex chemical and electrochemical processes that occur within rechargeable batteries. Electron transfer and ion transport are the foundation of battery function where the capacity, loaded voltage and current define the practical energy and power delivery of the system. During operation, repetitive addition and removal of electrons and ions can lead to structural change within the electrode and interfacial formation that can dramatically impact electrochemical outcomes. Thus, structural, chemical, and electrochemical characterization during operation is critically important for next generation electrochemical energy storage technologies. The research efforts highlighted in this talk use multiple advanced characterization techniques, including operando methods, to probe charge transport properties, structural changes, and interfacial evolution in multiple rechargeable battery systems. Specifically, spatiotemporal x-ray fluorescence mapping was used to quantify elemental composition near the electrode and in the electrolyte of operando cells. Isothermal microcalorimetry was used to determine heat evolution due to parasitic or decomposition reactions operando. X-ray absorption spectroscopy enabled structure and oxidation state elucidation in the absence of long-range order. X-ray diffraction was employed to track phase changes of distinct electroactive materials. Using advanced characterization, these research efforts provide insight into electron transfer and ion transport properties and their impact on application relevant outcomes (resistance, cycle life, delivered capacity) of electrochemical energy storage technologies.

Charge Traps in All‐Inorganic CsPbBr3 Perovskite Nanowire Field‐Effect Phototransistors

AbstractAll‐inorganic halide perovskite materials have recently emerged as outstanding materials for optoelectronic applications. However, although critical for developing novel technologies, the influence of charge traps on charge transport in all‐inorganic systems still remains elusive. Here, the charge transport properties in cesium lead bromide, nanowire films are probed using a field‐effect transistor geometry. Field‐effect mobilities of μFET = 4 × 10−3 cm−2 V−1 s−1 and photoresponsivities in the range of R = 25 A W−1 are demonstrated. Furthermore, charge transport both with and without illumination is investigated down to cryogenic temperatures. Without illumination, deep traps dominate transport and the mobility freezes out at low temperatures. Despite the presence of deep traps, when illuminating the sample, the field‐effect mobility increases by several orders of magnitude and even phonon‐limited transport characteristics are visible. This can be seen as an extension to the notion of “defect tolerance” of perovskite materials that has solely been associated with shallow traps. These findings provide further insight in understanding charge transport in perovskite materials and underlines that managing deep traps can open up a route to optimizing optoelectronic devices such as solar cells or phototransistors operable also at low light intensities.

Understanding charge transport in wavy 2D covalent organic frameworks

Understanding charge transport in wavy 2D covalent organic frameworks