Charge Accumulation Layer Research Articles

Theoretical study on photocatalytic hydrogen generation performance of two-dimensional GaN/ReS2 heterostructure

One effective way to solve energy crises and environmental problems is to find efficient photocatalysts to decompose water and produce hydrogen. In this work, the electronic structure and optical properties of GaN/ReS2 van der Waals heterostructure were studied by first-principles calculations. The findings demonstrate that the GaN/ReS2 heterostructure is an indirect band gap semiconductor with a band gap of 1.901 eV, exhibiting stability and type II band alignment characteristics. This ensures the spatial separation of photoexcited electron-hole pairs. At the heterostructure interface, a minor built-in electric field extends from the charge depletion layer of GaN to the charge accumulation layer of ReS2. Moreover, the heterostructure's visible light absorption coefficient is significantly enhanced compared to a single two-dimensional material. These calculations suggest that the GaN/ReS2 heterostructure holds potential as a photocatalytic material, offering theoretical guidance for future experimental research.

General Capacitance Upper Limit and Its Manifestation for Aqueous Graphene Interfaces

Double-layer capacitance (Cdl) is essential for chemical and biological sensors and capacitor applications. The correct formula for Cdl is a controversial subject for practically useful graphene interfaces with water, aqueous solutions, and other liquids. We have developed a model of Cdl, considering the capacitance of a charge accumulation layer (Cca) and capacitance (Ce) of a capacitance-limiting edge region with negligible electric susceptibility and conductivity between this layer and the capacitor electrode. These capacitances are connected in series, and Cdl can be obtained from 1/Cdl = 1/Cca + 1/Ce. In the case of aqueous graphene interfaces, this model predicts that Cdl is significantly affected by Ce. We have studied the graphene/water interface capacitance by low-frequency impedance spectroscopy. Comparison of the model predictions with the experimental results implies that the distance from charge carriers in graphene to the nearest molecular charges at the interface can be ~(0.05–0.1)nm and is about a typical length of the carbon-hydrogen bond. Generalization of this model, assuming that such an edge region between a conducting electrode and a charge accumulating region is intrinsic for a broad range of non-faradaic capacitors and cannot be thinner than an atomic size of ~0.05 nm, predicts a general capacitance upper limit of ~18 μF/cm2.

Effects of charge rearrangement on interfacial contact resistance of TiO2/graphite from first-principles calculations

Titanium bipolar plates have been widely employed in fuel cells, due to the high resistance achieved by the oxide film (TiO2). However, that also results in high interfacial contact resistance between the titanium and the gas diffusion layer of graphite, reducing the cells’ efficiency significantly. To improve the conductivity properties, the effects of thirty-five metallic dopants on the contact resistance and electrical conductivity were studied based on the first-principles merged with the Schottky-Mott theory and Boltzmann transport equation. The results show that the contact resistance depends on the charge distribution induced by graphite. The contact resistance depends on the conductivity when the charge depletion and accumulation layer are distributed in the space-charge region. If the charge rearrangement occurs in the heterojunction as a whole, the contact resistance is determined by the electronic injection. In addition, the single conduction path between titanium, oxygen and graphite atoms disappears in the TiO2 doped with Zn, Cd, etc., because those dopants have stronger electron affinity in the heterojunction. While a stronger built-in electric field is formed that can further enhance the electronic conductivity.

Electrically tunable conducting oxide metasurfaces for high power applications.

Active metasurfaces designed to operate at optical frequencies are flat optical elements that can dynamic, subwavelength-scale wavefront control of reflected or transmitted light. The practical and fundamental power-handling limits of active metasurfaces at high pulse energies and high average powers determine the potential applications for these emerging photonic components. Here, we investigate thermal performance limits of reflective gate-tunable conducting oxide metasurfaces illuminated with high power density laser beams, for both continuous wave (CW) and pulsed laser illumination. Our gate-tunable metasurfaces use indium tin oxide (ITO) as an active material, which undergoes an epsilon-near-zero (ENZ) transition under applied electrical bias. We experimentally show that under CW illumination, there is no significant change in the electrically tunable metasurface optical response for high irradiances ranging from 1.6 kW/cm2 to 9.1 kW/cm2 when the illuminating laser beam diameter is 7μm. Even under an applied bias, when over 60% of the incoming light is absorbed in a 1 nm-thick charge accumulation layer within ITO, the local temperature rise in the metasurface is modest, supporting its robustness for high-power applications. Additionally, we theoretically show that in the ENZ regime, the metasurface reflectance can be increased by a factor of 10 by replacing the active ITO layer with cadmium oxide (CdO). Thus conducting oxide metasurfaces can tolerate the power densities needed in higher power applications, including free space optical communications, to light detection and ranging (LiDAR), as well as laser-based additive manufacturing.

Halide Ion Migration and its Role at the Interfaces in Perovskite Solar Cells

AbstractLead‐halide perovskite solar cells (PSCs) based on unusual semiconductors made with ions, have shown impressive improvement in photovoltaic performance in few years exceeding nowadays 25 % power conversion efficiency. However, PSCs suffer from a lack of stability and show significant hysteresis in current‐voltage curves, which are impeding commercialization. We confirmed the importance of halide ion migration in the hysteresis effect which has direct consequences on device efficiency. Using impedance spectroscopy, in addition to the geometrical capacitance found at high frequency for fresh samples without bias, we observed a second capacitance at low frequency after ageing or under bias. This second capacitance is interpreted as a charge accumulation layer at interfaces, which can be promoted by the presence of grain boundaries. Through glow‐discharge optical emission spectroscopy elemental depth profiles, we found that under dark conditions, iodide ions diffuse through the electron transport layer versus ageing time. These ions interact chemically with the front‐end electrode after four weeks and form silver iodide.

Pixelated Photoinduced Discharge Readout X-Ray Detector Using 450-nm LED Line Beam

This paper describes a study on a novel X-ray detector with photoinduced discharge (PID) readout devices. Especially, we demonstrated optical scanning X-ray imaging devices based on intrinsic hydrogenated amorphous silicon (i-a-Si:H) PID readout layer, on which a pixelated metal layer, molybdenum, is formed as a charge accumulation layer of 200 μm pitch for clearly defining the pixel in the lateral and longitudinal directions. We decided to use i-a-Si:H as a readout layer instead of amorphous selenium(a-Se) layer because the material of a-Se does not withstand standard wet fabrication processes like lift-off wet process. An X-ray absorption layer of 500 μm thick a-Se is formed by a thermal evaporation process. To optically switch on the i-a-Si:H PID readout layer, we use a narrow line-beam with a beam width of 50 μm consisting of a blue LED array, optical films, and GRIN lens array. The pixelated PID readout X-ray detector is 512×512 image resolutions with a 200 μm pitch, and the overall active dimension is 10.24 cm ×10.24 cm. The sensitivity of the pixelated PID readout X-ray detector is 302 pC/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ·mR at Normal X-ray Tube conditions (IEC66220-1). According to MTF measurement results, the value of MTF0.1(10%) is measured at near 2.5 1/mm in the column direction scanning while the value is measured at 1.9-2.5 1/mm in the row direction scanning.

Improved Thermoelectric Power Factor of InGaZnO/SiO₂ Thin Film Transistor via Gate-Tunable Energy Filtering

Progress on improving the thermoelectric power factor (PF) of InGaZnO thin films by various surface treatment techniques has been seriously hindered by the innate coupled but opposing relationship between its electrical conductivity ( σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> ) and Seebeck coefficient (S). Therefore, proposing unique solutions to decouple these properties is crucial in advancing its thermoelectric performance. This study demonstrates that the S - σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> coupled relationship can be suppressed by integrating InGaZnO in an InGaZnO/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> thin film transistor, leading to a significantly enhanced PF compared to pristine InGaZnO thin films. An exponential increase in σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> was observed without the typical severe decline in S. An extremely narrow charge accumulation layer near the InGaZnO/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface could have formed under a positive applied gate voltage ( V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">G</sub> ), causing a dramatic reduction of the thermoelectrically active region to an almost two-dimensional surface. By utilizing the Kamiya-Nomura percolation modeling, it was observed that energy filtering occurs at the InGaZnO/SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> interface. This likely causes a change in the shape of the potential barriers, which leads to the suppression of the S - σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</sub> coupling relationship.

Defect state reinforced microwave-grown CuxO/NiO nanostructured matrix engineered for the development of selective CO2 sensor with integrated micro-heater

Monitoring of CO2 emission is highly encouraged due to its multiple facets of impacts occurring in the field of health, agriculture and environmental sectors. The bulky nature of available detection techniques paved the way for downsized sensors owing to solid-state devices such as Semiconducting Metal Oxides. Here a CuxO/NiO nano-structured matrix-based sensor development and optimization are concentrated for the spotting of CO2 level in the mixed gas environment. The sensor optimized at 250oC is highly selective to CO2 due to the catalyst-dominant defect state in the sensor matrix. The structure-based depletion region formation with a detailed mechanism behind the CO2 adsorption and desorption is explained in detail to develop a working sensor prototype. Here in this work, rather than depending on the cliché-ridden gas sensing based on the basic-type of materials (n-type/p-type/heterostructure/composites towards reducing/oxidizing gases), the dynamic characteristic of the sensor is analyzed with the charge accumulation layer (CAL) formed by the introduction of catalyst-oriented defect states and other factors such as temperature, moisture etc. An attempt is made to analyze the performance of the developed sensor based on the aforementioned factors with the aid of X-ray Photoelectron and Photoluminescence spectroscopy. Also, a sensor platform that includes the heater and Resistance Temperature Detector (RTD) microstructures is simulated (Coventorware) and fabricated. Finally, a selective CO2 sensor prototype is developed by the incorporation of the optimized sensing layer to the fabricated Sensing platform.

Density functional theory-based investigation of hydrogen adsorption on zinc oxide ([formula omitted]) surface: Revisited

Density functional theory based calculations with Hubbard correction (DFT + U) were performed to investigate the effects of varying coverage and different adsorption sites on hydrogen (H) adsorption on zinc oxide (ZnO) (101¯0) surface. Results show that H adsorption on top of oxygen (O) at low coverage (0.25 monolayer, ML) shifts the conduction band below the Fermi level and narrows the band gap. These phenomena are attributed to the charge transfer between H and the surface zinc (Zn) and O atoms. On the other hand, the H adsorption on top of Zn at low coverage (0.25 ML) shows an overlapping of H, Zn, and O states while maintaining the semiconductor nature of the system. At high coverage (1.0 ML), a charge accumulation layer on the surface forms, and the mechanisms that govern the interactions of H atoms when adsorbed exclusively on top of Zn or top of O are found to be similar with the low coverage cases. Lastly, at full coverage (2.0 ML), the effect of H on top of Zn is more evident as the system retained its semiconducting property. The adsorption energy is enhanced due to the reinforced overlapping of the H, Zn, and O states and due to the possible attraction between the adsorbed H atoms. The properties and stability of full-coverage adsorption were explained based on the findings on high- and low- coverages adsorption. The findings of the study will aid in understanding the interaction of H with the ZnO surface toward the further development of ZnO's optoelectronic applications.

Triboelectric nanogenerators from reused plastic: An approach for vehicle security alarming and tire motion monitoring in rover

The fast urbanization and speedy growth of automobiles throughout the globe requires the development of a highly reliable vehicle security system to protect against vehicle crime. Similarly, the interest in space exploration increases rapidly in many of the countries, and the rover used in space exploration operates on solar power with 6 wheels for its movement. So, it is highly desirable to find an energy harvester as well as motion tracker in space exploration vehicles. Herein, a vehicle security alert system and tire motion signaling and harvesting system are developed using a triboelectric nanogenerator (TENG) with good reliability and cheap manufacturing cost. The TENG device is made by reusing the waste plastic bottle made of PET, which serves as the substrate for housing the triboelectric active layers. The active layers composed of sandpaper made roughness created PBAT (Polybutylene adipate terephthalate, commercially available as Ecoflex-0030) film as a negative layer, copper film as a positive layer, and electrodes. A Kapton layer is introduced at the bottom of the Ecoflex film as a charge accumulation layer. The entire layers with the reused plastic substrate made the device as an eye-shaped TENG device (EYE-TENG). The device generates a maximum electrical output of voltage and current ~250 V and ~25 µA respectively with a maximum instantaneous peak power density of ~16 µW/Cm2. Furthermore, the device exhibits long term stability, durability and has potential to light up 100 LEDs. The device also demonstrates its capability is scavenging the biomechanical energy by hand and leg tapping at fast and slow motions. After that, the device was used to perform a vehicle security alarming system by placing into the vehicle tire and alarming with the help of an Arduino controller and Bluetooth module. The studies and results prove that the EYE-TENG is capable of acting as a sustainable power source as well as an alarming system and can be used as potential device in the future robotics, automobile industries, tire motion monitoring in rovers and aerospace electronics applications.

A Dual Plasmonic Photoelectrode System for Visible‐Light Photocatalysis

AbstractPlasmonic photocathodes based on plasmon‐induced charge separation (PICS) were prepared by depositing an Au nanoparticle (AuNP) ensemble on a transparent electrode and coating it with compact TiO2. Its PICS efficiency was improved by coating it further with porous TiO2 or introducing a NiO blocking layer as an underlayer of the AuNP ensemble. The former photocathode was modified further with Pt co‐catalyst nanoparticles, and combined with a PICS photoanode fabricated by depositing a AuNP ensemble on compact TiO2 and coating it with a Ni(OH)2 charge accumulation layer. The dual plasmonic photoelectrode system thus obtained exhibited a higher efficiency than the sum of the efficiencies of the single photoanode and single photocathode.

Controlled alignment of polymer chains near the semiconductor-dielectric interface

Abstract Alignment of semiconducting polymer chains is crucial for achieving desired charge transport in organic field effect transistors (OFETs). Although the transistor performance is known to be critically affected by the charge accumulation layer of the semiconducting polymer film next to the semiconductor-gate dielectric interface, it is challenging to investigate the effect of polymer chain alignment specifically at the interface due to experimental limitations. In this work, the unique capability of multilayer formation and polymer chain alignment of the floating film transfer method (FTM) is utilized to study the alignment effect on the charge transport properties of OFETs. Discrete modulation of polymer chain alignment for the top half and bottom half layers of the films was achieved in a multilayered structure. When the bottom layer, which is close to the interface with the gate dielectric, has a parallel orientation of the polymer chains and the chains in the top layer are oriented perpendicular to the source-drain direction in the OFET devices, the average hole mobility is increased by a factor of 3.3 compared to that in the opposite case where the bottom layer has a perpendicular orientation and a parallel orientation in the top layer, even though the two cases have very similar bulk polymer chain alignments. In addition, no matter which polymer chain orientation is chosen for the top layer, the polymer chain orientation direction of the bottom layer governs the overall device performance. These experimental findings evidently support that the polymer chain orientation adjacent to the semiconductor/dielectric interface is a decisive factor in charge transport of OFETs.

Ion‐Enhanced Field Emission Triboelectric Nanogenerator

AbstractAs interest in triboelectric nanogenerators (TENGs) continues to increase, some studies have reported that certain limitations exist in TENG due to high potential difference, resulting in air breakdown and field emission. In addition, with known limitations such as extremely low voltage at low external resistance, a breakthrough is required to overcome the limitations of TENG. Here, a new TENG mechanism is reported, utilizing ion‐enhanced field emission (IEFE). Using a simple IEFE‐inducing layer, which consists of a charge accumulation layer and a metal‐to‐metal contact point, electrons can flow directly to a counter electrode while generating high‐output power. Under vertical contact–separation input, the generated root mean square (RMS) power of IEFE‐TENG is 635% higher compared to conventional TENG. As the fundamental mechanism of the IEFE‐TENG is based on installing this simple IEFE‐inducing layer, the power output of existing TENGs including sliding mode types can be boosted. This new TENG mechanism can be a new standard for metal–metal contact TENGs to effectively amplify the output power and to overcome potential limitations.

Increasing surface charge density by effective charge accumulation layer inclusion for high-performance triboelectric nanogenerators

Abstract

Increasing Surface Charge Density By Intrinsic Charge Layer Inclusion for High Performance Efficient Triboelectric Nanogenerators

In recent years, mechanical transducers for energy harvesting, particularly triboelectric nanogenerators (TENGs) have been conceived as a practical power source due to its simple, compact and high efficiency for energy harvesters. Charge injection and retention in thin film dielectric layers remain critical issues in realizing TENGs as a sustainable power source. In this work, we have presented a novel technique to boost the output electrical performance of triboelectric nanogenerators by the inclusion of charge accumulation layer. This thin film layer allows to store and accumulate the charges generated by triboelectrification and refill the charges on every triboelectrification process of the vertical contact separation TENG mechanism. This increase in the stored charges has an effect of inducing many free charges on the electrodes of the device in a much larger quantity and gives rise to a much higher electrical output power. A proof of concept TENG structure is devised to validate the electrical output performance. Experimental results show that, the devised structure reaches an open circuit voltage of 600 V demonstrating 25 times enhancement than the conventional triboelectric devices in ambient conditions. The in–situ surface characterization implemented using atomic force microscopy (AFM) reveals the charge retention characteristic due to the presence of the charge accumulation layer. This technical outcome will significantly facilitate the future research in increasing the charge density to efficiently provide higher output power attractive to charging applications for portable electronics/ wearables and large-scale harvesting without massive volume. By this technique, numerous friction-based operation challenges including lack of uniform contact, material loss through wear, high sensitivity to humidity, and output power storage have been resolved.

Enhancement of superconducting transition temperature in FeSe electric-double-layer transistor with multivalent ionic liquids

Electric-double-layer transistors (EDLTs), which consist of ionic liquids (IL) as gate dielectrics, have attracted keen interest owing to dramatic modulation of physical properties by high-density carrier accumulation and tuning. In this study, EDLTs based on the superconducting ultrathin FeSe film with multivalent ILs exhibit a great potential for improvement of physical properties at the interface charge accumulation layer. The superior superconducting property found in high-Tc FeSe-EDLT with divalent IL sheds new light on the importance of interface engineering via appropriate selection of ILs.

Direct Imaging of Space-Charge Accumulation and Work Function Characteristics of Functional Organic Interfaces.

The tailoring of organic systems is crucial to further extend the efficiency of charge transfer mechanisms and represents a cornerstone for molecular device technologies. However, this demands control of electrical properties and understanding of the physics behind organic interfaces. Here, a quantitative spatial overview of work function characteristics for phthalocyanine architectures on Au substrates is provided via kelvin probe microscopy. While macroscopic investigations are very informative, the current approach offers a nanoscale spatial rendering of electrical characteristics which is not possible to attain via conventional techniques. Interface dipole is observed due to the formation of charge accumulation layers in thin F16 CuPc, F16 CoPc, and MnPc films, displaying work functions of 5.7, 6.1, and 5.0 eV, respectively. The imaging and quantification of interface locations with significant surface potential and work function response (<0.33 eV for material thickness <1 nm) show also a dependency on the crystalline state of the organic systems. The work function mapping suggests space-charge carrier regions of about 4 nm at the organic interface. This reveals rich spatial electric parameters and ambipolar characteristics that may drive electrical performance at device scales, opening a realm of possibilities toward the development of functional organic architectures and its applications.

Accumulation-layer surface plasmons in transparent conductive oxides.

A rigorous analytical study of the eigenmodes supported by a charge accumulation layer within a transparent conductive oxide (TCO) is presented. The new class of surface plasmons termed accumulation-layer surface plasmons (ASPs) is introduced. Near resonance ASPs are tightly bound and display a vast effective index tunability that could be of great practical interest. The suppression of ASPs in the presence of epsilon-near zero regions is discussed.

Noise-enhanced chaos in a weakly coupled GaAs/(Al,Ga)As superlattice.

Noise-enhanced chaos in a doped, weakly coupled GaAs/Al_{0.45}Ga_{0.55}As superlattice has been observed at room temperature in experiments as well as in the results of the simulation of nonlinear transport based on a discrete tunneling model. When external noise is added, both the measured and simulated current-versus-time traces contain irregularly spaced spikes for particular applied voltages, which separate a regime of periodic current oscillations from a region of no current oscillations at all. In the voltage region without current oscillations, the electric-field profile consist of a low-field domain near the emitter contact separated by a domain wall consisting of a charge accumulation layer from a high-field regime closer to the collector contact. With increasing noise amplitude, spontaneous chaotic current oscillations appear over a wider bias voltage range. For these bias voltages, the domain boundary between the two electric-field domains becomes unstable and very small current or voltage fluctuations can trigger the domain boundary to move toward the collector and induce chaotic current spikes. The experimentally observed features are qualitatively very well reproduced by the simulations. Increased noise can consequently enhance chaotic current oscillations in semiconductor superlattices.

Local aspects of hydrogen-induced metallization of theZnO(101¯0)surface

This study combines surface-sensitive photoemission experiments with density functional theory (DFT) to give a microscopic description of H adsorption-induced modifications of the ZnO(10${\overline{1}}$0) surface electronic structure. We find a complex adsorption behavior caused by a strong coverage dependence of the H adsorption energies: Initially, O--H bond formation is energetically favorable and H acting as an electron donor leads to the formation of a charge accumulation layer and to surface metallization. The increase of the number of O--H bonds leads to a reversal in adsorption energies such that Zn--H bonds become favored at sites close to existing O--H bonds, which results in a gradual extenuation of the metallization. The corresponding surface potential changes are localized within a few nanometers both laterally and normal to the surface. This localized character is experimentally corroborated by using sub-surface bound excitons at the ZnO(10${\overline{1}}$0) surface as a local probe. The pronounced and comparably localized effect of small amounts of hydrogen at this surface strongly suggests metallic character of ZnO surfaces under technologically relevant conditions and may, thus, be of high importance for energy level alignment at ZnO-based junctions in general.