Charge Transport Measurements Research Articles

Tuning Ion Mobility in Lithium Argyrodite Solid Electrolytes via Entropy Engineering.

The development of improved solid electrolytes (SEs) plays a crucial role in the advancement of bulk-type solid-state battery (SSB) technologies. Recently, multicomponent or high-entropy SEs are gaining increased attention for their advantageous charge transport and (electro)chemical properties. However, a comprehensive understanding of how configurational entropy affects ionic conductivity is largely lacking. Herein we have investigated a series of multication-substituted lithium argyrodites with the general formula Li6+x[M1aM2bM3cM4d]S5I, with M being P, Si, Ge, and Sb. Structure-property relationships related to ion mobility were probed using a combination of diffraction techniques, solid-state nuclear magnetic resonance spectroscopy, and charge-transport measurements. We present, to the best of our knowledge, the first experimental evidence of a direct correlation between occupational disorder in the cationic host lattice and lithium transport. By controlling the configurational entropy through the composition, high bulk ionic conductivities up to 18 mS cm-1 at room temperature were achieved for optimized lithium argyrodite compositions. Our results indicate the possibility of improving ionic conductivity in ceramic ion conductors via entropy engineering, unlocking the compositional limitations for the design of advanced electrolytes and opening up new avenues in the field.

Mass Inversion at the Lifshitz Transition in Monolayer Graphene by Diffusive, High-Density, On-Chip Doping.

Experimental setups for charge transport measurements are typically not compatible with the ultrahigh vacuum conditions for chemical doping, limiting the charge carrier density that can be investigated by transport methods. Field-effect methods, including dielectric gating and ionic liquid gating, achieve too low a carrier density to induce electronic phase transitions. To bridge this gap, we developed an integrated flip-chip method to dope graphene by alkali vapor in the diffusive regime, suitable for charge transport measurements at ultrahigh charge carrier density. We introduce a cesium droplet into a sealed cavity filled with inert gas to dope a monolayer graphene sample by the process of cesium atom diffusion, adsorption, and ionization at the graphene surface, with doping beyond an electron density of 4.7 × 1014 cm-2 monitored by operando Hall measurement. The sealed assembly is stable against oxidation, enabling measurement of charge transport versus temperature and magnetic field. Cyclotron mass inversion is observed via the Hall effect, indicative of the change in Fermi surface geometry associated with the Liftshitz transition at the hyperbolic M point of monolayer graphene. The transparent quartz substrate also functions as an optical window, enabling nonresonant Raman scattering. Our findings show that chemical doping, hitherto restricted to ultrahigh vacuum, can be applied in a diffusive regime at ambient pressure in an inert gas environment and thus enable charge transport studies in standard cryogenic environments.

Resolving charge transfer mechanisms in molecular tunnel junctions using dynamic charge transfer and static current–voltage measurements

Comparing the tunnelling decay coefficient β values obtained through the core-hole clock and junction J(V) measurements allow for the identification of the most likely orbital involved in charge transport measurements, derived from DFT calculations.

All-inorganic flexible epitaxial SrNbO3/mica thin films with ultrahigh figure of merit as indium-free transparent conductors

Nowadays, flexible transparent conductors (TCs) have been widely applied for solar cells, stretchable integrated circuits, sensor networks, and human-machine interaction devices. Recently, nondoped correlated metallic oxides (CMOs) have aroused great interest as indium-free TCs. In this work, an all-inorganic flexible CMO thin film with an ultrahigh figure of merit (FOM) value was fabricated using a simple preparation technology: the direct deposition of epitaxial SrNbO3 thin films on a stratiform mica substrate via a pulsed laser deposition (PLD) process. From the measured optical transmittance (Tr) spectra in conjunction with the ellipsometic data, it is found that SrNbO3 film possesses a superior visible light transparency compared with commonly investigated SrVO3 due to its larger bandgap. The charge transport measurements indicate that the dc electrical conductivity (σ) of SrNbO3 is higher than that of SrVO3, attributed presumably to a weaker electron correlation and therefore a smaller effective electron mass (m*) in SrNbO3. Bending cyclic tests reveals high mechanical durability and bending stability in SrNbO3 films. This detailed investigation demonstrates a great potential of the all-inorganic flexible epitaxial SrNbO3/mica film as an indium-free TC with high performance for future optoelectronic applications.

Bolometric detection of Josephson inductance in a highly resistive environment

The Josephson junction is a building block of quantum circuits. Its behavior, well understood when treated as an isolated entity, is strongly affected by coupling to an electromagnetic environment. In 1983, Schmid predicted that a Josephson junction shunted by a resistance exceeding the resistance quantum RQ = h/4e2 ≈ 6.45 kΩ for Cooper pairs would become insulating since the phase fluctuations would destroy the coherent Josephson coupling. However, recent microwave measurements have questioned this interpretation. Here, we insert a small Josephson junction in a Johnson-Nyquist-type setup where it is driven by weak current noise arising from thermal fluctuations. Our heat probe minimally perturbs the junction’s equilibrium, shedding light on features not visible in charge transport. We find that the Josephson critical current completely vanishes in DC charge transport measurement, and the junction demonstrates Coulomb blockade in agreement with the theory. Surprisingly, thermal transport measurements show that the Josephson junction acts as an inductor at high frequencies, unambiguously demonstrating that a supercurrent survives despite the Coulomb blockade observed in DC measurements.

Understanding interfacial conductance in non-stoichimetric Ca x Ta y O3−δ /SrTiO3

In search of novel conducting oxide heterointerfaces, we previously uncovered an distinctive quasi two-dimensional electron gas (q-2DEG) type behaviour in non-stoichimetric CaxTayO3-δ/SrTiO3 heterostructure. However, the underlying mechanism remained enigmatic. In this study, we delve into the intricate interplay of growth conditions, stoichiometry, and transport properties of these heterostructures. Using (Ca0.5TaO3)2 Target and the pulsed laser deposition technique, we grow the epitaxial thin films while systematically varying growth parameters, inculding laser energy density, oxygen pressures, and post-deposition annealing. Structural analysis unveiled a notable presence of oxygen vacancies in the as-grown films, while annealed samples exhibited an oxygen surplus. Building upon these findings, our comprehensive charge transport measurements revealed that while oxygen vacancies do contribute to conductivity, the polar catastrophe model takes precedence as the primary source of interfacial conductance in these heterostructures. This study provides valueable insights into the behavior of these innovative heterostructures, paving the way for future advancements in the field.

Mechanistic studies of Yb2O3/HAT-CN connection electrode in tandem semiconductor devices

The optically transparent connecting electrode is much desired in fabrication of tandem optoelectronic devices. Yet, optically transparent materials, such as oxides, are electrically insulating. In this work, we show that low work function oxides Yb2O3 combing with high work function 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) molecule can be used as effective connecting electrodes to make high performance tandem organic light emitting diodes with negligible voltage loss. For instance, in a tandem device with two emission zones, yielding a brightness of 100 cd/m2, the voltage required is 5.3 V, which is approximately twice that of a single emission zone device. To gain insights into the band alignment of this electrode, we conducted the measurements, including ultraviolet photoelectron spectroscopy to analyze the electronic structures of occupied valence and gap states and reflection electron energy loss spectroscopy to study the unoccupied states. To understand the charge transport and injection behavior of this electrode, we conducted variable temperature charge transport measurements. Our findings reveal the presence of localized gap states within the Yb2O3/HAT-CN structure. These gap states effectively form a conduction pathway for facilitating the transport of charge carriers. At higher temperatures (≥200 K), charge transport is primarily limited by the Efros–Shklovskii type of hopping conduction through the localized states in the Yb2O3. Conversely, at lower temperatures (<200 K), the electrical current is limited by the properties of HAT-CN. These discoveries suggest that localized gap states at the oxides/organic heterojunctions can be effectively utilized in the fabrication of tandem semiconductor devices.

Tailoring Interlayer Charge Transfer Dynamics in 2D Perovskites with Electroactive Spacer Molecules.

The family of hybrid organic-inorganic lead-halide perovskites are the subject of intense interest for optoelectronic applications, from light-emitting diodes to photovoltaics to X-ray detectors. Due to the inert nature of most organic molecules, the inorganic sublattice generally dominates the electronic structure and therefore the optoelectronic properties of perovskites. Here, we use optically and electronically active carbazole-based Cz-Ci molecules, where Ci indicates an alkylammonium chain and i indicates the number of CH2 units in the chain, varying from 3 to 5, as cations in the two-dimensional (2D) perovskite structure. By investigating the photophysics and charge transport characteristics of (Cz-Ci)2PbI4, we demonstrate a tunable electronic coupling between the inorganic lead-halide and organic layers. The strongest interlayer electronic coupling was found for (Cz-C3)2PbI4, where photothermal deflection spectroscopy results remarkably reveal an organic-inorganic charge transfer state. Ultrafast transient absorption spectroscopy measurements demonstrate ultrafast hole transfer from the photoexcited lead-halide layer to the Cz-Ci molecules, the efficiency of which increases by varying the chain length from i = 5 to i = 3. The charge transfer results in long-lived carriers (10-100 ns) and quenched emission, in stark contrast to the fast (sub-ns) and efficient radiative decay of bound excitons in the more conventional 2D perovskite (PEA)2PbI4, in which phenylethylammonium (PEA) acts as an inert spacer. Electrical charge transport measurements further support enhanced interlayer coupling, showing increased out-of-plane carrier mobility from i = 5 to i = 3. This study paves the way for the rational design of 2D perovskites with combined inorganic-organic electronic properties through the wide range of functionalities available in the world of organics.

Oxygen Reduction on Organic Mixed Ionic-Electronic Conducting Polymer Electrodes

Organic mixed ionic-electronic conductors (OMIECs) are a class of conjugated semiconducting polymers with tunable electronic and ionic transport properties enabled through polaron-forming ion insertion redox reactions. The energy to form these conductive polaronic states can be controlled through rational design of the polymer backbone to enable predominantly electron/cation (n-type) or hole/anion (p-type) transport. Simultaneously, electrolyte uptake into the bulk of the electrode can be controlled through incorporation of polar/non-polar sidechains. The ability to tune the energy of the redox-active states, the majority charge carrier, and the local reaction environment offers an opportunity to independently optimize activity and selectivity in electrochemical energy conversion processes with a single-phase electrode.In this talk, I will discuss our efforts to develop OMIECs as electrocatalysts for the oxygen reduction reaction. The electronic and chemical origins of reactivity are interrogated through pH-dependent electroanalytical characterization, operando spectroscopy, charge-transport measurements, and ab initio/microkinetic simulations. The nature of the polaronic states provide a generalized framework to understand pathway selectivity towards the 2-electron H2O2 or 4-electron H2O product and serve as a design principle in developing this emerging class of metal-free electrocatalysts.

Identifying Pauli spin blockade using deep learning

Pauli spin blockade (PSB) can be employed as a great resource for spin qubit initialisation and readout even at elevated temperatures but it can be difficult to identify. We present a machine learning algorithm capable of automatically identifying PSB using charge transport measurements. The scarcity of PSB data is circumvented by training the algorithm with simulated data and by using cross-device validation. We demonstrate our approach on a silicon field-effect transistor device and report an accuracy of 96% on different test devices, giving evidence that the approach is robust to device variability. Our algorithm, an essential step for realising fully automatic qubit tuning, is expected to be employable across all types of quantum dot devices.

Spin-selectivity effect of G-quadruplex DNA molecules.

Chirality-induced spin selectivity has been attracting extensive interest in recent years and is demonstrated in a variety of chiral molecules, all of which arise from inherent molecular chirality. Here, we first propose a theoretical model to study the spin-dependent electron transport along guanine-quadruplex (G4) DNA molecules, connected to two nonmagnetic electrodes, by considering the molecule-electrode contact and weak spin-orbit coupling. Our results indicate that the G4-DNA molecular junctions exhibit pronounced spin-selectivity effect, and the asymmetric contact-induced external chirality, instead of the inherent molecular chirality, dominates their spin filtration efficiency. Furthermore, the spin-selectivity effect is robust against the disorder and hold in a wide range of model parameters. These results could be checked by charge transport measurements and provide an alternative way to improve the spin-selectivity effect of chiral nanodevices.

Non-equilibrium thermodynamics in a single-molecule quantum system

Thermodynamic probes can be used to deduce microscopic internal dynamics of nanoscale quantum systems. Several direct entropy measurement protocols based on charge transport measurements have been proposed and experimentally applied to single-electron devices. To date, these methods have relied on (quasi-)equilibrium conditions between the nanoscale quantum system and its environment, which constitutes only a small subset of the experimental conditions available. In this paper, we establish a thermodynamic analysis method based on stochastic thermodynamics, that is valid far from equilibrium conditions, is applicable to a broad range of single-electron devices and allows us to find the difference in entropy between the charge states of the nanodevice, as well as a characteristic of any selection rules governing electron transfers. We apply this non-equilibrium entropy measurement protocol to a single-molecule device in which the internal dynamics can be described by a two-site Hubbard model.

Jellybean Quantum Dots in Silicon for Qubit Coupling and On-Chip Quantum Chemistry.

The small size and excellent integrability of silicon metal-oxide-semiconductor (SiMOS) quantum dot spin qubits make them an attractive system for mass-manufacturable, scaled-up quantum processors. Furthermore, classical control electronics can be integrated on-chip, in-between the qubits, if an architecture with sparse arrays of qubits is chosen. In such an architecture qubits are either transported across the chip via shuttling or coupled via mediating quantum systems over short-to-intermediate distances. This paper investigates the charge and spin characteristics of an elongated quantum dot-a so-called jellybean quantum dot-for the prospects of acting as a qubit-qubit coupler. Charge transport, charge sensing, and magneto-spectroscopy measurements are performed on a SiMOS quantum dot device at mK temperature and compared to Hartree-Fock multi-electron simulations. At low electron occupancies where disorder effects and strong electron-electron interaction dominate over the electrostatic confinement potential, the data reveals the formation of three coupled dots, akin to a tunable, artificial molecule. One dot is formed centrally under the gate and two are formed at the edges. At high electron occupancies, these dots merge into one large dot with well-defined spin states, verifying that jellybean dots have the potential to be used as qubit couplers in future quantum computingarchitectures.

Charge transport and low-frequency conductance noise in metal-nanoparticle embedded one-dimensional conducting polymer nanotubes: multiple resistive switching phenomena

Metal-nanoparticle embedded benzene tetracarboxylic acid-doped polyaniline (MBDP) nanotubes have been investigated by charge transport measurements and low-frequency 1/f conductance noise spectroscopy for resistive switching (RS) applications. To improve the RS properties, MBDP nanotubes are prepared by in-situ reduction of silver ions (Ag+) on the surface of the benzene tetracarboxylic acid-doped polyaniline (BDP) nanotubes with the silver nanoparticles (Ag NPs). The Ag NPs act like conformal active materials inside BDP nanotubes to effectively control the charge transport by modifying electron-phonon and electron-electron interactions. The RS device on the optimized MBDP (in Ag/MBDP/Ag – configuration) shows a considerable OFF/ON ratio with excellent RS characteristics. The transport results and conductance noise spectroscopy indicate that RS phenomena originated from current-driven space charge limited carrier drift and Columb blocked in transport channels. The RS devices exhibit no noticeable change in RS characteristics even after eighteen months of storage in ambient conditions. Furthermore, the MBDP samples significantly reveal two unipolar switching under varying applied currents and can be used to implement diverse functionalities, including symmetric complementary resistive switching (CRS). This work suggests a suitable route for advancing symmetric CRS characteristics for conducting polymer-based RS memory devices.

Synergistic effect of sequential solvent treatment on the structural and low temperature charge transport of PEDOT:PSS films

A detailed study on the low temperature charge transport and magnetoresistance (MR) measurements of spin-coated and solvent treated poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) films has been presented. The samples were prepared by sequential treatment of PEDOT:PSS film using dimethyl sulfoxide and sulphuric acid. X-ray diffraction and Raman spectroscopy elucidate the effect of solvent treatment on the structural modifications of the film, which correlates to the enhanced conductivity values. The nature of solvent and the sequence of successive treatments using different solvents has been effectively utilized to tune the conductivity of the film over a wide range. Further, the low temperature (300–2 K) charge transport study indicates that the resistance of the film rises sharply below 50 K, which implies the samples lie in the category of disordered materials. A linear fit of ln R vs. T −1/4 for all the samples indicates that the temperature dependent resistance of solvent treated PEDOT:PSS films follow 3D variable range hopping model. The observed large positive MR behavior of the films has been discussed in terms of the wave function shrinkage of the charge carriers. The MR follows ∼B 2 and ∼B 1/3 dependence at low and high magnetic field regimes, respectively. The sequential treatment of PEDOT:PSS films shown in the present study is an effective method to enhance the electrical conductivity significantly.

Role of aggregates and microstructure of mixed-ionic-electronic-conductors on charge transport in electrochemical transistors.

Synthetic efforts have delivered a library of organic mixed ionic-electronic conductors (OMIECs) with high performance in electrochemical transistors. The most promising materials are redox-active conjugated polymers with hydrophilic side chains that reach high transconductances in aqueous electrolytes due to volumetric electrochemical charging. Current approaches to improve transconductance and device stability focus mostly on materials chemistry including backbone and side chain design. However, other parameters such as the initial microstructure and microstructural rearrangements during electrochemical charging are equally important and are influenced by backbone and side chain chemistry. In this study, we employ a polymer system to investigate the fundamental electrochemical charging mechanisms of OMIECs. We couple in situ electronic charge transport measurements and spectroelectrochemistry with ex situ X-ray scattering electrochemical charging experiments and find that polymer chains planarize during electrochemical charging. Our work shows that the most effective conductivity modulation is related to electrochemical accessibility of well-ordered, interconnected aggregates that host high mobility electronic charge carriers. Electrochemical stress cycling induces microstructural changes, but we find that these aggregates can largely maintain order, providing insights on the structural stability and reversibility of electrochemical charging in these systems. This work shows the importance of material design for creating OMIECs that undergo structural rearrangements to accommodate ions and electronic charge carriers during which percolating networks are formed for efficient electronic charge transport.

Charge Transport Measured Using the EGaIn Junction through Self-Assembled Monolayers Immersed in Organic Liquids

This paper describes measurements of charge transport by tunneling through molecular junctions comprising a self-assembled monolayer (SAM) supported by a template-stripped metal bottom electrode (MTS), which has been immersed in an organic liquid and contacted by a conical Ga2O3/EGaIn top electrode. These junctions formed in organic liquids are robust; they show stabilities and yields similar to those formed in air. We formed junctions under seven external environments: (I) air, (II) perfluorocarbons, (III) linear hydrocarbons, (IV) cyclic hydrocarbons, (V) aromatic compounds, (VI) large, irregularly shaped hydrocarbons, and (VII) dimethyl siloxanes. Several different lengths of SAMs of n-alkanethiolates, S(CH2)n-1CH3 with n = 4-18, and two different kinds of bottom electrodes (AgTS or AuTS) are employed to assess the mechanism underlying the observed changes in tunneling currents. Measurements of current density through junctions immersed in perfluorocarbons (II) are comparable to junctions measured in air. Junctions immersed in other organic liquids show reductions in the values of current density, compared to the values in air, ranging from 1 (III) to 5 orders of magnitude (IV). We interpret the most plausible mechanism for these reductions in current densities to be an increase in the length of the tunneling pathway, reflecting the formation of thin (0.5-1.5 nm) liquid films at the interface between the SAM and the Ga2O3/EGaIn electrode. Remarkably, the thickness of the liquid film─estimated by the simplified Simmons model, measurements of electrical breakdown of the junction, and simulations of molecular dynamics─is consistent with the existing observations of structured liquid layers that form between two flat interfaces from measurements obtained by the surface force apparatus. These results suggest the use of the EGaIn junction and measurements of charge transport by tunneling as a new form of surface analysis, with the applications in the study of near-surface, weak, molecular interactions and the behavior of liquid films adjacent to non-polar interfaces.

Time-Dependent Charge Carrier Transport with Hall Effect in Organic Semiconductors for Langevin and Non-Langevin Systems.

The time-dependent charge carrier transport and recombination processes in low-mobility organic semiconductor diodes are obtained through numerical simulations using the finite element method (FEM). The application of a Lorentz force across the diode alters the charge transport process leading to the Hall effect. In this contribution, the Hall effect parameters, such as the Hall voltage and charge carrier concentration with varying magnetic fields, are computed for both Langevin and non-Langevin type recombination processes. The results indicate the charge carrier concentration within the diode for the Langevin system is about seven and fourteen times less while the maximum amount of extracted charge is nearly five and ten times less than that in the non-Langevin system of 0.01 and 0.001, respectively. The Hall voltage values obtained for the steady-state case are similar to the non-Langevin system of ββL=0.01. However, the values obtained for the Langevin and non-Langevin systems of ββL=1 and 0.001 exhibit anomalies. The implications of these findings advance the understanding of the charge transport and Hall effect measurements in organic semiconductors that underpins the device's performance.

High-Mobility Hole Transport in Single-Grain PbSe Quantum Dot Superlattice Transistors.

Epitaxially-fused superlattices of colloidal quantum dots (QD epi-SLs) may exhibit electronic minibands and high-mobility charge transport, but electrical measurements of epi-SLs have been limited to large-area, polycrystalline samples in which superlattice grain boundaries and intragrain defects suppress/obscure miniband effects. Systematic measurements of charge transport in individual, highly-ordered epi-SL grains would facilitate the study of minibands in QD films. Here, we demonstrate the air-free fabrication of microscale field-effect transistors (μ-FETs) with channels consisting of single PbSe QD epi-SL grains (2-7 μm channel dimensions) and analyze charge transport in these single-grain devices. The eight devices studied show p-channel or ambipolar transport with a hole mobility as high as 3.5 cm2 V-1 s-1 at 290 K and 6.5 cm2 V-1 s-1 at 170-220 K, one order of magnitude larger than that of previous QD solids. The mobility peaks at 150-220 K, but device hysteresis at higher temperatures makes the true mobility-temperature curve uncertain and evidence for miniband transport inconclusive.

CuO-based materials for thermochemical redox cycles: the influence of the formation of a CuO percolation network on oxygen release and oxidation kinetics

Thermochemical redox cycles such as chemical looping combustion (CLC) are an economically promising CO2 capture technology that rely on the combustion of a hydrocarbon fuel with lattice oxygen that is derived from a solid oxygen carrier. The oxygen carrier is typically regenerated with air. To increase the agglomeration resistance and redox stability of the oxygen carriers, the active phase is often stabilized with high Tammann temperature ceramics, resulting in the formation of so-called cermet structures. It has been hypothesized that the redox performance of the cermets depends critically on the conduction pathways for solid-state ionic diffusion and the activation energy for charge transport. Here, we investigate the influence of the formation of a percolation network on the electrical conductivity and the rate of oxidation for CeO2-stabilized Cu. We found that for oxygen carriers that contained 60 wt. % CuO, the charge transport occurred predominately via Cu/CuO conduction pathways. Below the percolation threshold of CuO, the conduction of charge carriers took place via CeO2 grains, which formed a continuous network. The measurements of charge transport and redox characteristics confirmed that the activation energy for charge transport through the cermet increased with decreasing Cu content. This indicates that the solid-state diffusion of charge carriers plays an important role during re-oxidation.