Capacitance Spectroscopy Research Articles

Optimizing pH sensor performance with PANI/PMMA composite thin films: Impedimetric and capacitive transduction approaches

The continuous search for novel materials to meet the requirements of modern technological applications has led to the widespread use of polyaniline (PANI) composites for sensing purposes. Although research has been carried out on both chemical sensors using PANI/polymer composites and chemical sensors with impedimetric/capacitive transduction using PANI composites, there is still a gap in the use of PANI/polymer composites in impedimetric and capacitive transduction platforms for pH sensing. In this study, the influence of composite thin films consisting of PANI and poly(methyl methacrylate) (PMMA) on the sensitivity and linearity of pH sensors based on electrochemical impedance and capacitance spectroscopy (EIS/ECS) was evaluated. The sensitivity and linearity of the devices showed a dependence on the polymer content. For PANI:PMMA equal to 30:70, the EIS and ECS sensitivity reached 12.6 ± 2.7 and 18.7 ± 4.9 %/pH, respectively, after reaching its minimum value for the 50:50 sample. Similarly, the linearity values for the 30:70 sample were 92.7 % and 99.8 % for EIS and ECS, respectively. We were able to encapsulate the PANI in the PMMA matrix, which improved the control of ion diffusion and analyte access to the active redox quinoid rings on the PANI. As a result, the saturation effect of the polymer was reduced. By adjusting the relative content of PANI and PMMA, the structure and properties of the composite can be controlled, directly affecting the sensor parameters. These materials have potential applications in sensors for various fields such as food, biomedical and environmental monitoring, with the ability to tailor their properties for optimal response.

(Invited) Dynamic Modeling of Photocurrents in Organic Thin-Film Transistors

The development of high performance, thin and flexible light and radiation sensors for real-time detection of ionizing radiation at affordable cost is of interest in many application areas, ranging from medical diagnostics and therapy to astrophysics, high energy physics and industrial testing, including civil radiation protection.The use of organic semiconductor technology could meet the requirements of large area, conformability and portability, light weight and low power operation.In the field of organic photodetectors, phototransistors (OPT) have gained significant attention due to their ability to combine high sensitivity, switching function, and intrinsic amplification without noise problems in a single device, thus allowing significant simplification of the external signal conditioning circuitry [1].The operation of an OPT under light illumination is governed by the trapping of minority carriers, resulting in a reduction of the threshold voltage which determines an increase in the majority carrier density and, consequently, an increase of the photocurrent [2].The increase of photocurrent in organic semiconductors has been attributed by several authors to an increase in the trapping of minority carriers induced by light exposure. In fact, the light-induced creation of deep traps for carriers has already been experimentally demonstrated in the analysis of several organic semiconductors used in different optoelectronic applications [3]. Moreover, TCAD simulations show that the photocurrent can be assumed linearly dependent upon the total amount of generated traps, at least for the range of radiation exposures used in this work.In this work, we present a dedicated kinetic model capable of correctly describing the photocurrent behavior in dinaphtho[2,3-b:2′,3′-f]thieno[3,2-b]thiophene (DNTT) based OPTs operating in subthreshold condition. The OPTs have been used as direct light detectors and in an indirect flexible proton detector that has been fabricated by coupling a 500 μm thick scintillating plastic film (a polysiloxane-based matrix carrying a primary dye and a wavelength shifter) directly on top of the OPT’s passivation layer.By analyzing the response of the OPTs in light and in the proton detectors we note that:1 - the relaxation in dark of the photocurrent (fig. A) is well described by a stretched exponential with an exponent β almost constant (around 0.5) and a characteristic time τs that strongly depends on the exposure conditions (fig. B).2 - dynamic photocurrent measurements under repeated exposures show a systematic drift due to the build-up of a persistent photocurrent component that decays with a characteristic times of the order of 105 s, in addition to the swift component that has recovery times in the range of a few seconds (fig. C).To model these effects, we propose a set of rate equations similar to the one already presented by Street et al [3], suitably modified to reproduce the dynamics of the creation and recovery of two different types of defects with distributed recovery activation energies. The consideration of two different types of defects allows the reproduction of the swift and persistent photocurrent components, while the stretched exponential behavior of the photocurrent decay is reproduced by introducing a distribution in the recovery activation energies.The proposed new model is able to quantitatively reproduce the dynamic photoresponse for a wide range of exposure intensities under both photon and proton fluxes [4]. As can be seen in fig. C, the curves computed by the model with the fitted parameters superimpose well on the experimental ones, correctly reproducing the rise and fall dynamics of the exposure response and the progressive build-up of a persistent photocurrent.The model also provides some valuable physical insights to explain some features of the dynamic photoresponse.This work has been funded by the Italian National Institute of Nuclear Physics – INFN -5th commission, under the “FIRE” project (2019-2023). Lucas, T. Trigaud, C. Videlot-Ackermann, Organic transistors and phototransistors based on small molecules: Organic transistors and phototransistors, Polym. Int. 61 (2012) 374–389. https://doi.org/10.1002/pi.3213Basiricò, L. et al. Direct X-ray photoconversion in flexible organic thin film devices operated below 1 V. Nat Commun 7, 13063 (2016).Street, R. A., Yang, Y., Thompson, B. C. & McCulloch, I. Capacitance Spectroscopy of Light Induced Trap States in Organic Solar Cells. J. Phys. Chem. C 120, 22169–22178 (2016).S. Calvi, L. Basiricò, S. M. Carturan, I. Fratelli, A. Valletta, A. Aloisio, S. De Rosa, F. Pino, M. Campajola, A. Ciavatti, L. Tortora, M. Rapisarda, S. Moretto, M. Verdi, S. Bertoldo, O. Cesarini, P. Di Meo, M. Chiari, F. Tommasino, E. Sarnelli, L. Mariucci, P. Branchini, A. Quaranta, B. Fraboni, “Flexible fully organic indirect detector for megaelectronvolts proton beams”, npj Flexible Electronics (2023) 7:5; https://doi.org/10.1038/s41528-022-00229-w. Figure 1

Eliminating Resistance-Capacitance Coupling Shielding for Depicting the Defect Landscape in Perovskite Solar Cells by Capacitance Spectroscopy.

Capacitance spectroscopy techniques have been widely utilized to evaluate the defect properties in perovskites, which contribute to the efficiency and operation stability development for perovskite solar cells (PSCs). Yet the interplay between the charge transporting layer (CTL) and the perovskite on the capacitance spectroscopy results is still unclear. Here, they show that a pseudo-trap-state capacitance signal is generated in thermal admittance spectroscopy (TAS) due to the enhanced resistance capacitance (RC) coupling caused by the carrier freeze-out of the CTL in PSCs, which could be discerned from the actual defect-induced trap state capacitance signal by tuning the series resistance of PSCs. By eliminating the RC coupling shielding effect on the defect-induced capacitance spectroscopy, it is obtain the actual defect density which is 4-folds lower than the pseudo-trap density, and the spatial distribution of defects in PSCs which reveals that the commonly adopted interface passivators can passivate the defects about 60 nm away from the decorated surface. It is further revealed that phenethylammonium ions (PEA+) possess a better passivation capability over octylammonium ions (OA+) due to the deeper passivation depth for PEA+ on the surface defects in perovskite films.

Effects of thermal treatment on the electrical behavior of titanium dioxide thin-films

AbstractElectrochemical biosensors made of titanium dioxide have generated significant interest due to their high sensitivity and selectivity for the detection of a variety of diseases. This study focuses on the impact of thermal treatment in an oxygen environment on the electrical properties of titanium dioxide thin films deposited on silicon substrates using pulsed laser deposition; the films were subjected to temperatures of $$\varvec{100\,\,^{\circ }C}$$ 100 ∘ C , $$\varvec{400\,\,^{\circ }C}$$ 400 ∘ C , and $$\varvec{600\,\,^{\circ }C}$$ 600 ∘ C . The primary objective was to explore how thermal processing influences the electrical characteristics of the thin films, with the goal of enhancing their capacitance and thereby improving the performance of sensors developed by functionalizing these films; X-ray photoelectron spectroscopy revealed the presence of Ti$$^{4+}$$ 4 + , Ti$$^{3+}$$ 3 + and TiO$$_x$$ x species in the untreated thin film and in the heat-treated films, with an increase in TiO$$_x$$ x species observed after oxygen heat treatment. Atomic force microscopy demonstrated a decrease in the surface homogeneity of the films at elevated treatment temperatures, which corresponded to a reduction in capacitive behavior; this was further corroborated by Nyquist plots derived from electrochemical capacitance spectroscopy (ECS). The study found that the electrochemical capacitance at lower frequencies decreased significantly from $$\varvec{1.02\,\, \mu Fcm^{-2}}$$ 1.02 μ F c m - 2 to $$\varvec{0.25\,\, \mu Fcm^{-2}}$$ 0.25 μ F c m - 2 when the treatment temperature was increased to $$\varvec{600\,\,^{\circ }C}$$ 600 ∘ C

Performance limiting inhomogeneities of defect states in ampere-class Ga2O3 power diodes

Impacts of spatial charge inhomogeneities on carrier transport fluctuations and premature breakdown were investigated in Schottky ampere-class Ga2O3 power diodes. Three prominent electron traps were detected in Ga2O3 epilayers by a combination of the depth-resolved capacitance spectroscopy profiling and gradual dry etching. The near-surface trap occurring at 1.06 eV below the conduction band minimum (EC), named E3, was found to be confined within a 180 nm surface region of the Ga2O3 epilayers. Two bulk traps at EC − 0.75 eV (E2*) and at EC − 0.82 eV (E2) were identified and interconnected with the VGa- and FeGa-type defects, respectively. In the framework of the impact ionization model, employing the experimental trap parameters, the TCAD simulated breakdown characteristics matched the experimental breakdown properties well, consistently with inverse proportionality to the total trap densities. In particular, the shallowest distributed E3 trap with the deepest level is responsible for higher leakage and premature breakdown. In contrast, Ga2O3 Schottky diodes without E3 trap exhibit enhanced breakdown voltages, and the leakage mechanism evolves from variable range hopping at medium reverse voltages, to the space-charge-limited conduction at high reverse biases. This work bridges the fundamental gap between spatial charge inhomogeneities and diode breakdown features, paving the way for more reliable defect engineering in high-performance Ga2O3 power devices.

Deuteration-enhanced neutron contrasts to probe amorphous domain sizes in organic photovoltaic bulk heterojunction films

An organic photovoltaic bulk heterojunction comprises of a mixture of donor and acceptor materials, forming a semi-crystalline thin film with both crystalline and amorphous domains. Domain sizes critically impact the device performance; however, conventional X-ray scattering techniques cannot detect the contrast between donor and acceptor materials within the amorphous intermixing regions. In this study, we employ neutron scattering and targeted deuteration of acceptor materials to enhance the scattering contrast by nearly one order of magnitude. Remarkably, the PM6:deuterated Y6 system reveals a new length scale, indicating short-range aggregation of Y6 molecules in the amorphous intermixing regions. All-atom molecular dynamics simulations confirm that this short-range aggregation is an inherent morphological advantage of Y6 which effectively assists charge extraction and suppresses charge recombination as shown by capacitance spectroscopy. Our findings uncover the amorphous nanomorphology of organic photovoltaic thin films, providing crucial insights into the morphology-driven device performance.

Capacitive immunosensing at gold nanoparticle-decorated reduced graphene oxide electrodes fabricated by one-step laser nanostructuration

Nanostructured electrochemical biosensors have ushered in a new era of diagnostic precision, offering enhanced sensitivity and specificity for clinical biomarker detection. Among them, capacitive biosensing enables ultrasensitive label-free detection of multiple molecular targets. However, the complexity and cost associated with conventional fabrication methods of nanostructured platforms hinder the widespread adoption of these devices. This study introduces a capacitive biosensor that leverages laser-engraved reduced graphene oxide (rGO) electrodes decorated with gold nanoparticles (AuNPs). The fabrication involves laser-scribed GO-Au3+ films, yielding rGO-AuNP electrodes, seamlessly transferred onto a PET substrate via a press-stamping methodology. These electrodes have a remarkable affinity for biomolecular recognition after being functionalized with specific bioreceptors. For example, initial studies with human IgG antibodies confirm the detection capabilities of the biosensor using electrochemical capacitance spectroscopy. Furthermore, the biosensor can quantify CA-19-9 glycoprotein, a clinical cancer biomarker. The biosensor exhibits a dynamic range from 0 to 300 U mL−1, with a limit of detection of 8.9 U mL−1. Rigorous testing with known concentrations of a pretreated CA-19-9 antigen from human fluids confirmed their accuracy and reliability in detecting the glycoprotein. This study signifies notable progress in capacitive biosensing for clinical biomarkers, potentially leading to more accessible and cost-effective point-of-care solutions.

Adaptive modeling optimized by the data fusion strategy: Real-time dying cell percentage prediction using capacitance spectroscopy.

The previous research showcased a partial least squares (PLS) regression model accurately predicting cell death percentages using in-line capacitance spectra. The current study advances the model accuracy through adaptive modeling employing a data fusion approach. This strategy enhances prediction performance by incorporating variables from the Cole-Cole model, conductivity and its derivatives over time, and Mahalanobis distance into the predictor matrix (X-matrix). Firstly, the Cole-Cole model, a mechanistic model with parameters linked to early cell death onset, was integrated to enhance prediction performance. Secondly, the inclusion of conductivity and its derivatives over time in the X-matrix mitigated prediction fluctuations resulting from abrupt conductivity changes during process operations. Thirdly, Mahalanobis distance, depicting spectral changes relative to a reference spectrum from a previous time point, improved model adaptability to independent test sets, thereby enhancing performance. The final data fusion model substantially decreased root-mean squared error of prediction (RMSEP) by around 50%, which is a significant boost in prediction accuracy compared to the prior PLS model. Robustness against reference spectrum selection was confirmed by consistent performance across various time points. In conclusion, this study illustrates that the data fusion strategy substantially enhances the model accuracy compared to the previous model relying solely on capacitance spectra.

Electrochemistry at 2D and 3D nanoelectrodes: The interplay between interface kinetics and surface density of states

Heterogenous Electron Transfer (HET) at electrode-electrolyte interfaces depends strongly on the morphological features or geometry of the nanostructure used for modifying the electrode surface. A swift HET results in faster interface kinetics, which has significant impact on the development/calibration of electrochemical devices like biomolecular sensors, supercapacitors, batteries and electrochromic platforms. The interface electrochemistry depends strongly on the electronic Density of States (DOS) of electrode materials. Over the past years, the 2D electron gas nanomaterials - primarily graphene, Graphene Oxide (GO) and Reduced Graphene Oxide (RGO), have garnered significant interest in electrochemical applications due to promising DOS features. However, the electroanalytical dependency on DOS of 3D nanostructures such as Zinc Oxide Tetrapods (ZnOT) is yet unexplored. The current work focusses on a comparative electrochemical analysis of interface kinetics at RGO (2D) and ZnOT (3D) coated screen printed electrodes, with the intention of selecting the suitable material geometry. The analyses were performed using Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). While the dependence of HET on DOS of RGO and ZnOT nanomaterials were studied using both DFT analysis and impedance-derived capacitance spectroscopy – the latter giving insights on quantum capacitance. It was observed that, the 2D RGO nanostructures exhibit higher surface DOS near the Fermi level, along with a high quantum capacitance (∼345 nF) as compared to 3D ZnOT (∼276 nF). This results in enhanced HET at the former, thereby indicating its suitability in developing futuristic electrochemical devices for various applications as desired.

Electrochemical capacitive dengue aptasensor using NS1 in undiluted human serum.

Non-structural 1 (NS1) is a protein biomarker that can be found in blood in the early stages of dengue and related infections (Zika and Chikungunya). This study aims to develop a biosensor to selectively quantify NS1 using DNA aptamer co-immobilized on gold electrodes with 6-(ferrocenyl)hexanethiol (FCH) using electrochemical capacitive spectroscopy. This technique uses a redox probe (FCH) immobilized on the self-assembled monolayer to convert impedance into capacitance information. The developed platform was blocked with bovine serum albumin before NS1 exposure and the ratio between aptamers and FCH was optimized. Theaptasensor was tested using commercial NS1 serotype 4 in phosphate-buffered saline and commercial undiluted human serum. Using the optimum applied potential provideshigh sensitivity (3 and 4 nF per decade) and low limit of detection (30.9 and 41.8fg/mL) with a large linear range (10pg to 1µg/mL and 10pg to 100ng/mL, respectively). Both results exhibit a residual standard deviation value < 1%. The results suggested that this aptasensor was capable of detecting NS1 in the clinical range and can be applied to any other specific aptamer with FCH, opening the path for label-free miniaturized point-of-care devices with high sensitivity and specificity.

On the Scope of the Oxygen Plasma Treatment on FTO Electrodes for Electrochemical Processes

The cleaning and/or activation of electrodes for electrochemical purposes using the oxygen plasma treatment has been widely used because of its proven effectiveness to improve the properties of electrodes. Beyond its cleaning effect, the plasma can either create stepped and functionalized carbon-based materials which are active for specific electroanalytes and increase their capacitance [1]. Prussian Blue can also be activated by creating oxygen rich sites thus enhancing its electrochromism [2]. In the case of transparent conductive oxides (TCOs) electrodes such as FTO and ITO-coated glass this treatment has proven to be suitable in the preparation of spray-pyrolyzed titania thin films used as blocking hole layer or electron transporting layer in efficient third generation solar cells [3]. However, the role of this treatment on TCOs when used for electrochemical purposes has not been deeply explored yet. The reason to carry out this study lies in the great number of electrochemical processes that are usually employed for the growth of wide and middle band gap semiconductor metal oxides and chalcogenides with applications in photoelectrochemical and photovoltaic cells [4].To assess the role of the oxygen plasma, FTO was chosen as TCO reference material and the effect of different time exposure and power of plasma was analysed by electrochemical polarization in aqueous media for both anodic and cathodic directions. In the former, PbO2 electrodeposition and oxygen evolution reaction (OER) were considered. In the later, hydroxide generating reactions that usually precipitate in presence of transition metal cations were studied: i) the oxygen reduction reaction (ORR, Figure 1), ii) hydrogen peroxide reduction. Further, the reduction of S8 in DMSO media to sulphide ion was also considered as an aprotic media to growth chalcogenides. Interestingly, all these electrochemical reactions exhibited an increased activation overpotential. The behaviour of treated FTO for OER demonstrated the as implanted oxyanions do not act as source of molecular oxygen as previously claimed [5]. This was verified by the poorly response of ORR after anodic polarizations in acid media.Besides, contact angle measurements allowed to estimate the energetic of the FTO surfaces being these energies correlated with open circuit potential values. Capacitance (Mott-Schottky plots), XPS, UPS and UV-Vis spectroscopy were used to study the semiconductor energy levels of both treated and untreated FTO electrodes. The oxyanion implantation in the oxygen vacancies would be responsible of the sharply diminution of the ORR wave. However, the electrochemical response of the well-known Ferri/Ferro couple only suffered changes under highly aggressive treatments, i.e. for long times or power. Meanwhile, XRD data has shown only slight changes in the structural features of FTO. We can conclude that oxygen plasma treatment is not suitable in FTO for electrochemical purposes, contrary to other deposition methods. Surprisingly, an exception in metals deposition (Ni and Zn) was found.

Capacitive spectroscopy as transduction mechanism for wearable biosensors: opportunities and challenges

Wearable sensors would revolutionize healthcare and personalized medicine by providing individuals with continuous and real-time data about their bodies and environments. Their integration into everyday life has the potential to enhance well-being, improve healthcare outcomes, and offer new opportunities for research. Capacitive sensors technology has great potential to enrich wearable devices, extending their use to more accurate physiological indicators. On the basis of capacitive sensors developed so far to monitor physical parameters, and taking into account the advances in capacitive biosensors, this work discusses the benefits of this type of transduction to design wearables for the monitoring of biomolecules. Moreover, it provides insights into the challenges that must be overcome to take advantage of capacitive transduction in wearable sensors for health.Graphical abstract

Capacitive Spectroscopy of Deep Levels in Silicon with Samarium Impurity

The effect of thermal treatment on the behavior of samarium atoms introduced into silicon during the growth process was studied using the method of transient capacitive deep-level spectroscopy (DLTS). It has been shown that various high-temperature treatments lead to the activation of samarium atoms in the bulk of n-Si and the formation of deep levels. The energy spectrum of deep levels arising during heat treatments has been determined. The dependence of the efficiency of formation of these levels in n‑Si&lt;Sm&gt; on the processing temperature has been studied. It was found that the higher the content of samarium atoms in the bulk of silicon at the same high-temperature treatment temperature, the higher the concentration of the deep level EC–0.39 eV. From this, we can conclude that the EC–0.39 eV level is associated with the activation of samarium atoms in the n-Si&lt;Sm&gt; volume.

On the Properties of the Si-SiO2 Transition Layer in Multilayer Silicon Structures

Capacitance spectroscopy was used to study the capacitive-voltage characteristics of multilayer structures with a Si-SiO2 transition layer in Al-SiO2-n-Si type samples fabricated by the thermal oxidation of a semiconductor. It is shown that the inhomogeneous distribution of the density of surface states is a localized electroactive center at the very semiconductor-dielectric interface, due to over-barrier charge emission or thermal ionization of impurity centers.

Comparing nanobody and aptamer-based capacitive sensing for detection of interleukin-6 (IL-6) at physiologically relevant levels

A major societal challenge is the development of the necessary tools for early diagnosis of diseases such as cancer and sepsis. Consequently, there is a concerted push to develop low-cost and non-invasive methods of analysis with high sensitivity and selectivity. A notable trend is the development of highly sensitive methods that are not only amenable for point-of-care (POC) testing, but also for wearable devices allowing continuous monitoring of biomarkers. In this context, a non-invasive test for the detection of a promising biomarker, the protein Interleukin-6 (IL-6), could represent a significant advance in the clinical management of cancer, in monitoring the chemotherapy response, or for prompt diagnosis of sepsis. This work reports a capacitive electrochemical impedance spectroscopy sensing platform tailored towards POC detection and treatment monitoring in human serum. The specific recognition of IL-6 was achieved employing gold surfaces modified with an anti-IL6 nanobody (anti-IL-6 VHH) or a specific IL-6 aptamer. In the first system, the anti-IL-6 VHH was covalently attached to the gold surface using a binary self-assembled-monolayer (SAM) of 6-mercapto-1-hexanol (MCH) and 11-mercaptoundecanoic acid. In the second system, the aptamer was chemisorbed onto the surface in a mixed SAM layer with MCH. The analytical performance for each label-free sensor was evaluated in buffer and 10% human serum samples and then compared. The results of this work were generated using a low-cost, thin film eight-channel gold sensor array produced on a flexible substrate providing useful information on the future design of POC and wearable impedance biomarker detection platforms.

Capacitance spectroscopy of InAs quantum dots inserted in an AlGaAs/GaAs HEMT for photodetector applications

We have investigated the electrical characteristics of an AlGaAs/GaAs high electron mobility transistor, in which a layer of InAs self-assembled Quantum Dots (QDs) was inserted below the 2DEG channel. A Negative Differential Capacitance (NDC) appeared in the capacitance–voltage characteristics at a bias of 1 V and at low temperatures (even at room temperature) under different illumination powers using white light bulbs. This results in an increase in negative differential conductance with the increase in frequency and optical power. This also applies to the NDC except that it decreases with increasing frequency. The numerical simulation of the energy band structure of the device confirmed that the conduction band lowers to its minimum at a special bias value of 1 V. The numerical analysis of the evolution of the energy levels in the QD-HEMT follows the appearance of multiple capacitance peaks and their behavior with the gate voltage.

Electrochemical capacitance spectroscopy based determination of antibodies against SARS-CoV-2 virus spike protein

In this study, we are reporting a novel electrochemical capacitance spectroscopy (ECS) platform designed for the sensitive and label-free detection of antibodies against severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) virus spike protein (anti-rS) in diluted blood serum. The determination of anti-rS is crucial for identification individuals who have been infected by SARS-CoV-2 virus and may have acquired immunity. The rS protein was immobilized on a screen-printed carbon electrode, which was incubated in diluted blood serum containing anti-rS antibodies. Label-free ECS was applied for the determination of interaction between immobilized rS and free-standing anti-rS. Here reported bioanalytical platform demonstrated high sensitivity and specificity in detecting anti-rS, achieving a limit of detection of 4.38 nM. This versatile platform could be further enhanced by applying various electrode materials and adapting this platform to detect antibodies against some other proteins. Our findings have significant implications for the development of affordable, scalable biosensing platforms capable to provide rapid and accurate public health screening and monitoring, particularly in the context of the coronavirus disease 2019 (COVID-19) pandemic.

Modeling of charging dynamics in electrochemical systems with a graphene electrode

A classical electrochemistry problem related to the polarization of a graphene electrode immersed in an aqueous solution and subjected to a small external ac voltage is revisited. The Poisson-Nernst-Planck equations with proper boundary conditions are linearized and normalized, leading to an analytical formula for the impedance of the electrochemical system containing a graphene-metal electrode pair. Electrochemical impedance spectroscopy is utilized to compare the impedance behavior of the graphene-metal electrode pair with the standard metal-metal electrode pair for a range of ion concentrations in the electrolyte. Also studied is the electrochemical capacitive spectroscopy to provide a detailed analysis related to the effects of the quantum capacitance of graphene on the total capacitive properties of the system.

Charge-trap memory effect in spray deposited ZnO-based electrolyte-gated transistors operating at low voltage

Charge-trap memory phenomena were demonstrated in an electrolyte-gated transistor (EGT) using a spray-coated zinc oxide (ZnO) active layer and a cellulose-based electrolyte. The EGT exhibited efficient programming and erasing characteristics at low voltages, shifting the threshold voltage and the magnitude of the on-current. This behavior is discussed in terms of the influence of charged trapping states at the ZnO/electrolyte interface and within the ZnO bulk. The presence of these traps leads to a shift in the mobility from 0.57 ± 0.16 cm2 V−1 s−1 in the initial state to 0.02 ± 0.01 cm2 V−1 s−1 when programmed. Retention experiments revealed improved stability of the memory state when a low positive voltage is applied to the gate, indicating that the device's characteristics are extremely sensitive to the trapping/detrapping of charges at the semiconductor/electrolyte interface. Capacitance spectroscopy measurements using planar and metal-insulator-semiconductor configurations within the same device were used to analyze the charging dynamics of the trap states at different programming states.

Schottky barrier heights and electronic transport in Ga2O3 Schottky diodes

The Schottky contact, formed at the interface between a metal and a semiconductor, is instrumental in defining the electrical properties of Schottky barrier diodes (SBDs). The characteristics of the Schottky contact are contingent on the properties of interacting metal and semiconductor properties. Herein, we studied the carrier-transport mechanisms and electrical characteristics at room and elevated temperatures. These SBDs employ pre-treated Ga2O3 thin films and either Ni or Au Schottky contacts. The SBDs pre-treated (pre-T) via annealing at 900 °C under an N2 atmosphere for the Ni contact showed highest on/off ratio at room temperature. They also demonstrated ideality factors and Schottky barrier heights (SBHs) that remained relatively stable between 298 K and 523 K. To ascertain the SBH, ideality factors (n) derived from the thermionic emission (TE) and thermionic field emission (TFE) models were used, and results were subsequently compared. Moreover, SBDs employing Ni as the anode material exhibited lower SBHs than those employing Au. The pre-T Ni SBD was best described by the TFE model, wherein the SBH and ideality factor varied by 0.14 eV and 0.13, respectively, between 298 K and 523 K. Conversely, for pre-T Au, untreated Ni, and untreated Au SBDs, neither TE and TFE provided a satisfactory fit due to the ideality factor is greater than 2 at room temperature and the variation of SBH and n with temperature. These suggests that the transport mechanism should be described by other physical mechanisms. Without pre-treatment, both the Ni and Au SBDs exhibited more significant variation in the SBH and n with temperature. SBHs values were determined using measurement of current, capacitance and x-ray photoelectron spectroscopy, and were found to depend on the interface quality, indicating inhomogeneous SBH. Our results suggest that the use of annealing pre-treatments and anode metals with low work functions holds considerable potential for reducing Schottky barrier heights in Schottky diodes, thereby enhancing their electrical performance.