Photocurrent Measurements Research Articles

Spatially Resolved Photo-Response of a Carbon Nanotube/Si Photodetector

Photodetectors based on vertical multi-walled carbon nanotube (MWCNT) film-Si heterojunctions are realized by growing MWCNTs on n-type Si substrates with a top surface covered by Si3N4 layers. Spatially resolved photocurrent measurements reveal that higher photo detection is achieved in regions with thinner MWCNT film, where nearly 100% external quantum efficiency is achieved. Hence, we propose a simple method based on the use of scotch tape with which to tune the thickness and density of as-grown MWCNT film and enhance device photo-response.

Facile and Effective Band Gap Engineering of 2D Ruddlesden–Popper Perovskites with Improved Structural and Optoelectronic Properties

Incorporation of alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) into 3D metal-halide perovskite crystal structures has resulted in improved crystallinity and film morphology, reduced trap density, and enhanced charge carrier transport. However, due to intrinsic structural instability associated with 3D perovskites, their alkali metal ion-doped semiconductors are not a suitable option for real device optoelectronics. In recent years, Ruddlesden–Popper (RP)-phase quasi-two-dimensional (2D) metal halide perovskites have drawn considerable attention due to their tunable energy band gap, large electron and hole diffusion lengths (∼10 μm), and lower recombination rates compared to pristine 2D metal halide perovskites (n = 1). However, alkali metal ion-incorporated RP perovskite systems are not explored much; therefore, we demonstrate the incorporation of Rb+, using the RbI precursor, into the quasi-2D RP perovskite (BA)2(MA)Pb2Br7 (BA = C4H9NH3; MA = CH3NH3), which has also resulted in the successive replacement of Br– with I– in the PbBr6 network. Subsequent replacement of Br– with I– finely tuned the energy band gap from 2.92 to 2.43 eV, while keeping the phase of the pristine lead bromide RP perovskite intact, as confirmed by X-ray diffraction investigations. MA+ cations available at the outer edge sites got partially exchanged with Rb ions, which in turn has regulated the perovskite crystal growth, whereas the excess Rb+ alloyed with replaced Br– to form RbBr, which resulted in the formation of thinner grain boundaries and thus showed improved charge transport. Transient photocurrent measurements performed for RbI-doped perovskites (lateral configuration photodetector: fluorine-doped tin oxide (FTO)/RP-perovskite/FTO of channel width ∼150 μm), for 35 cycles under standard Xe-lamp (100 mW/cm2) illumination, have shown strong and fast photocurrent response with large photoresponsivity (3.92 × 10–4 A/W). Comparison of structural, optical, and optoelectronic properties of RbI-doped RP perovskites is carried out with the mixed RP perovskites and has further confirmed that Rb+ incorporation plays an important role in enhancing the crystal growth.

Graphitic Layered Structures Enhancing TiNT Catalyst Photo-Electrochemical Behaviour

The increasing knowledge in nanoscience and materials technology promoted the development of advanced materials with enhanced and unusual properties suitable for sustainable applications ranging from energy to environmental purposes. Here are presented some results from our current investigations on composite semiconducting materials. The investigated composites have been prepared from different nitrogen precursors and thin films of TiO2 nanotubes. The synergy between hetero-structures based on graphitic-C3N4 and thin films of titania nanotubes obtained by anodisation was studied. The composites have been characterised with several complementary techniques to evidence the relation between photo-behaviour and the composition of the samples. This study allows new insights into the nature of the specific enhanced properties due to this synergy among the two compounds. The g-C3N4/TiNT heterojunctions showed enhanced photo-electrochemical properties observed from the photocurrent measurements. The as-prepared composites have been investigated as cathode materials in the electrocatalytic reduction of oxalic acid (OX), evidencing the capability of tuning the reaction toward glycolic acid with respect to the pristine TiNT array. The observed Faradic efficiency (FE) for the composites follows the trend: TiNT-U6 > TiNT-M6 > TiNT-MU18. TiNT-U6 shows the best performances (FEGC = 63.7%; FEGO = 15.5%; OX conversion = 61. 4%) after 2 h of reaction. The improved photo-electrochemical properties make these materials suitable for H2 production, solar-light-driven water splitting, and CO2 reduction applications.

Photoelectrochemical Modelling of Semiconducting Electrodes for Neural Interfacing

Semiconducting electrodes are increasingly utilised for neural interfacing applications, such as neural recording, stimulation, and photomodulation. To characterize the performance of these electrodes, photoelectrochemical analysis is often undertaken in biologically relevant electrolytes. These include electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and for photomodulation applications, photocurrent (PC) measurements. From such measurements, it is possible to deduce key properties of semiconductor surfaces, such as electrochemical impedance and capacitance, as well as mechanisms of charge transfer. To extract these parameters from the experimental data, equivalent electrical circuit modelling is often employed, but usually only for a single technique at a time which often misses key insights about the processes occurring at the electrode-electrolyte interface. Here we present an equivalent circuit model that simultaneously describes the results from CV, EIS, and PC transient measurements. Using semiconducting nitrogen-doped ultrananocrystalline diamond (N-UNCD) electrodes in saline solution, we show that the model describes physical mechanisms that occur at the interface with electrolyte, encompassing the space charge region, the electrical double layer, and the electrolyte. Using the model we are able to optimize parameters relevant for neural interfacing and suggest that this framework may assist in the characterization of other semiconducting electrodes.

Direct In Situ Measurement of Quantum Efficiencies of Charge Separation and Proton Reduction at TiO2-Protected GaP Photocathodes.

Photoelectrochemical solar fuel generation at the semiconductor/liquid interface consists of multiple elementary steps, including charge separation, recombination, and catalytic reactions. While the overall incident light-to-current conversion efficiency (IPCE) can be readily measured, identifying the microscopic efficiency loss processes remains difficult. Here, we report simultaneous in situ transient photocurrent and transient reflectance spectroscopy (TRS) measurements of titanium dioxide-protected gallium phosphide photocathodes for water reduction in photoelectrochemical cells. Transient reflectance spectroscopy enables the direct probe of the separated charge carriers responsible for water reduction to follow their kinetics. Comparison with transient photocurrent measurement allows the direct probe of the initial charge separation quantum efficiency (ϕCS) and provides support for a transient photocurrent model that divides IPCE into the product of quantum efficiencies of light absorption (ϕabs), charge separation (ϕCS), and photoreduction (ϕred), i.e., IPCE = ϕabsϕCSϕred. Our study shows that there are two general key loss pathways: recombination within the bulk GaP that reduces ϕCS and interfacial recombination at the junction that decreases ϕred. Although both loss pathways can be reduced at a more negative applied bias, for GaP/TiO2, the initial charge separation loss is the key efficiency limiting factor. Our combined transient reflectance and photocurrent study provides a time-resolved view of microscopic steps involved in the overall light-to-current conversion process and provides detailed insights into the main loss pathways of the photoelectrochemical system.

Hybrid Reduced Graphene Oxide/GaN Nanocolumns on Flexible Niobium Foils for Efficient Photoelectrochemical Water Splitting

We present the photoelectrochemical (PEC) water-splitting properties of pristine and reduced graphene oxide (rGO)-coated GaN nanocolumns (NCs) on flexible niobium (Nb) metal foils. The structural, optical, and electronic structure analyses of rGO-coated GaN-NCs on Nb foils revealed the formation of a rGO/GaN-NCs hybrid structure. Further, the valence-band studies of pure GaN-NCs shows valence-band maxima at ∼3.0 eV below the Fermi level, which decreased to ∼2.8 eV for rGO/GaN-NCs. The PEC measurement performed on pristine GaN-NCs under standard (1 Sun) conditions shows an effective photocatalytic nature with a photocurrent density of ∼60 μA/cm2 at 0.8 V (vs Ag/AgCl) in a 1 M oxalic acid electrolyte, which increases to ∼110 μA/cm2 for rGO/GaN-NCs. The efficient PEC characteristics of rGO/GaN-NCs are attributed to the effective charge separation/transport of photogenerated carriers. Furthermore, transient photocurrent measurement reveals that hybrid and pristine GaN-NCs on flexible metal foil have a fast and stable photoresponse in an aqueous solution. The development of a hybrid nitride nanostructure-based PEC device on flexible metal foils paves the way toward developing scalable PEC devices for hydrogen production applications.

Bionic opto-responsive fiber for directing neurite growth

Mounting evidence indicates that the nervous system plays a central function in pathogenesis. To study the association of the nervous system in diseases, tools that allow delicate control of neural behavior are crucial. Wireless electrical modulation of neural cells via opto-stimulation is an emerging approach to trigger and control neural behavior. Furthermore, to imitate the highly organized natural neural network, neurite outgrowth needs to be guided to achieve directionality. This work successfully combines bionic fibers with nano-groove topography and evident opto-response to guide neurite outgrowth. With excellent biocompatibility, the bionic opto-responsive fibers spatiotemporally controlled and guided PC12 neurite outgrowth. Transient photocurrent measurement revealed that this effect was attributed to anodic faradaic photocurrents. This bionic opto-responsive fiber has great potential to act as a tool to control and guide neurite outgrowth to study nervous system/disease interaction.

Photo-induced carrier dynamics in nano-porous TiO2 electrochemically doped with cuprous oxide

In the present paper, photo-induced carrier dynamics and charge transfer processes on porous anodic TiO2, modified with Cu2O via pulse electrodeposition, are studied with steady-state and modulated photocurrent measurements in a wide range of experimental conditions. Surface composition was estimated using XPS. The obtained oxides are p-type semiconductors that effectively absorb visible light, and rates of charge transfer and recombination are strongly dependent on synthesis conditions. A tentative mechanism of photo-induced hydrogen evolution on such materials is proposed using recent simulation results from the literature and the dependence of formal charge transfer rate constant on potential.

Preparation and performance of aerogel-based BiOI/TiO2 heterojunction photoelectrocatalytic electrodes.

TiO2/BiOI/CA electrodes with improved conductivity, reduced photoelectron-hole recombination rates, and increased reaction sites based on p-n type heterojunctions were constructed on carbon aerogels (CA) as photoelectrode substrates. Characterization based on ultraviolet-visible diffuse reflectance spectroscopy, photocurrent measurements, and impedance analysis showed that the TiO2/BiOI/CA photoelectrode with a Ti/Bi mole ratio of 0.4 exhibited the best visible light absorption, lowest photogenerated electron-hole pair recombination rate, and strongest photocatalytic degradation, with 90.4% degradation of phenol under 120 min of light. Moreover, the stability of this electrode remained at a high level. This was mainly because the energy levels of TiO2 and BiOI matched each other and the p-n heterojunction formed adjusted the energy band structure of the composite material, widened the electron transfer path, formed an internal electric field between the phase interfaces, had a higher electron transfer rate, and reduced the photogenerated electron-hole recombination rate. Since ˙OH and ˙O2- are the main active substances in the degradation of phenol, the TiO2/BiOI/CA photoelectrodes had higher degradation efficiency than BiOI/CA electrodes. This study provides a unique concept for the treatment of organic pollutant wastewater and electrode design for photoelectrocatalysis.

On-Chip Emulation and Measurement of Variable-Length Photocurrents in Sub-50nm ICs

On-Chip Emulation and Measurement of Variable-Length Photocurrents in Sub-50nm ICs

Photoinduced absorption spectroscopy (PIAS) study of water and chloride oxidation by a WO3 photoanode in acidic solution.

The mechanisms of water and chloride oxidation by a WO3 photoanode are probed by photoinduced absorption spectroscopy (PIAS) coupled with transient photocurrent (TC) measurements. Linear sweep voltammograms (LSVs) and incident photon to current efficiencies (IPCEs) are obtained, in the water oxidation electrolyte (1 M HClO4) and chloride oxidation electrolyte (3.5 M NaCl in 1 M HClO4). Other work shows that the faradaic efficiency of water oxidation to O2 in 1 M HClO4 is ca. 1.0, and that for chloride oxidation to Cl2 in 3.5 M NaCl plus 1 M HClO4 is ca. 0.62. The PIAS/TC data reveals a 0.4 order dependency of the rate of water oxidation on the steady state concentration of photogenerated surface holes, [hs+]ss, and an approximately first order dependency of the rate of chloride oxidation on [hs+]ss. Associated mechanisms and rate determining steps for water and chloride oxidation at the photoanode surface that account for these reaction orders are proposed.

Smartphone-based portable photoelectrochemical biosensing system for point-of-care detection of urine creatinine and albumin.

Creatinine and albumin are crucial biomarkers for health monitoring and their ratio in urine is an effective approach for albuminuria assessment. Herein, to address the challenges of point-of-care and efficient analysis of the biomarkers simultaneously, we developed a fully integrated handheld smartphone-based photoelectrochemical biosensing system. A miniaturized printed circuit board included a potentiostat for photocurrent measurements and single-wavelength light-emitting diodes (LEDs) for photo-excitation, which was controlled with a Bluetooth-enabled smartphone. Graphitic carbon nitride (g-C3N4)/chitosan nanocomposites were modified on a transparent indium tin oxide (ITO) electrode as photoactive materials. Creatinine was detected through chelate formation with copper ion probes, while albumin was recognized specifically by an antigen-antibody reaction based on immunoassay. The biosensing system demonstrated good linearity and high sensitivity, with detection ranges of 100 μg mL-1 to 1500 μg mL-1 for creatinine, and 9.9 μg mL-1 to 500 μg mL-1 for albumin. Spiked artificial urine samples with different concentrations were tested to confirm the practical validity of the biosensing system, where an acceptable recovery rate ranged from 98.7% to 105.3%. This portable photoelectrochemical biosensing platform provides a convenient and cost-effective method for biofluid analysis, which has an extensive prospect in point-of-care testing (POCT) for mobile health.

Non-volatile phototransistor based on two dimensional MoTe2 nanostructures

We fabricate field-effect transistors (FETs) from low-layer molybdenum ditelluride (MoTe2) in the nanometer range and study its optoelectronic properties. In particular, we investigate the mechanisms of photocurrent generation in MoTe2 FETs using gate and bias-dependent photocurrent measurements. The results obtained show the photocurrent effects and signals generated in the MoTe2-electrode junctions, which identify the effect of amplifying the photoconductor in the off and on modes. This is different from those conventional MoTe2 FETs, which exhibited a continuous increase in photocurrent with no back gate voltage. These results play an important role in the fabrication of direction dependent field effect transistors based on MoTe2 crystals.

A contactless method to study carrier kinetics in SnS thin films

Contactless methods to study laser-induced carrier kinetics present important advantages for materials characterization without necessity for contact fabrication. For example, mobility and lifetime of photogenerated carriers are studied by the transient grating method and by the transient photoluminescence technique. In this report we will mainly discuss the Microwave Transient Reflection (MWTR) technique in the nanosecond time regime applied to thin films of undoped and Ga-doped tin sulphide, SnS.As described in detail in previous publications, the SnS thin films were produced by Chemical Bath Deposition (CBD) from appropriate precursor solutions, and characterized with respect to morphological, structural, and optoelectronic properties by optical transmission, SEM and AFM, XRD, XPS, and RBS. We also add a detailed analysis of the optical properties from Spectral Ellipsometry (SE).The kinetics of photoinduced carriers after high-power laser pulses was monitored for a series of SnS samples after different anneal procedures with temperatures up to 600 °C. Overall power law decays observed in the MWTR signals is discussed in relation to trap state distributions by applying a simple recombination and reemission model. The results are compared to conventional time-resolved photocurrent measurements, TPC, which show similar power law decays.

Stability of a pyrimidine-based dye-sensitized TiO2 photoanode in sacrificial electrolytes

Dye sensitization of semiconductor metal oxides aims to extend the light absorption range into the visible region, of particular interest for their use as photoanodes in photoelectrochemical (PEC) water splitting. Organic dyes, however, suffer from limited chemical stability when they are exposed to constant potential and irradiation in aqueous media. Thus, in this work, we evaluate a novel metal-free donor-π-acceptor (D-π-A) dye (AT-Pyri), demonstrating significant sensitization of film photoanodes based on TiO2 nanoparticles. Among a series of tested sacrificial agents, the addition of triethanolamine (TEOA) to the sodium sulphate electrolyte prevents the degradation of AT-Pyri even after several hours of operation. Combining two sacrificial agents (methanol and TEOA) results in a synergetic improvement of the PEC water-splitting activity. Notably, the stabilization of the AT-Pyri photoanode is accompanied by a substantial increase in the photocurrent over the reference TiO2 photoanode, while the associated spikes in transient photocurrent measurements disappear.

Identification of the Origin of Ultralow Dark Currents in Organic Photodiodes.

Organic bulk heterojunction photodiodes (OPDs) attract attention for sensing and imaging. Their detectivity is typically limited by a substantial reverse bias dark current density Jd . Recently, using thermal admittance or spectral photocurrent measurements, Jd has been attributed to thermal charge generation mediated by mid-gap states. Here, we report the temperature dependence of Jd in state-of-the-art OPDs with Jd down to 10-9 mA cm-2 at -0.5V bias. For a variety of donor-acceptor bulk-heterojunction blends we find that the thermal activation energy of Jd is lower than the effective bandgap of the blends, by ca. 0.3 to 0.5eV, but higher than expected for mid-gap states. Ultra-sensitive sub-bandgap photocurrent spectroscopy reveals that the minimum photon energy for optical charge generation in OPDs correlates with the dark current thermal activation energy. The dark current in OPDs is attributed to thermal charge generation at the donor-acceptor interface mediated by intra-gap states near the band edges. This article is protected by copyright. All rights reserved.

Проблемы измерения потоков энергии излучения на объектах транспортной инфраструктуры с излучающей плазмой

Purpose: Transport infrastructure objects comprise numerous technical devices containing radiating plasma. They include visible and spectral radiation sources, switching devices at traction substations, high-temperature heat exchangers and combustion chambers. During experimental study of such devices, the measurements of power and spectral composition of radiation emitted by plasma are carried out. For this, as a rule, photodiodes of small sizes are used, installed at a certain distance from plasma formation. The purpose of the present work is to establish relationship between plasma emission power and radiation flux amount incident on photodiode working surface. Methods: To solve the set task, the method of direct integration of radiation transfer equation in homogeneous plasma structure assumption, but in the absence of local thermodynamic equilibrium presence assumption, is used. The cases of reflecting and absorbing surfaces, limiting a plasma, are considered. Results: Explicit expressions are found for radiation flux which exits plasma formation surface and for flux incident on photodiode surface. The dependence for the ratio of the values of these radiation fluxes to plasma geometric sizes and optical thickness is numerically studied. For the case of reflecting surfaces that bound plasma, simple asymptotic expression is found for ratio value for the fluxes and applicability field of the expression is determined. Practical significance: The ratios, which establish relationship between plasma radiation power and power value for radiation flux incident on photodiode working surface, make it possible to solve the main task of the experimental study of plasma formations — the restoration of plasma characteristics according to photocurrent measurement results. Obtained in the work results can be used at experimental study of technical devices containing emitting plasma.

Photoelectrochemical immunoassay for thyroglobulin on nanogold-functionalized BiVO4 photoanode coupling with enzymatic biocatalytic precipitation

Methods derived from photoelectrochemical (PEC) have been constructed for immunoassays, but most involve the split-type immunoreaction modes, and thus easily cause unpredictable intermediate precision. Herein, we innovatively designed an integrated PEC immunosensing platform for the quantitative monitoring of thyroglobulin (TG) on the gold nanoparticles (AuNPs)-functionalized BiVO4 photoanode coupling with enzymatic biocatalytic precipitation (EBCP). This sensing system could simultaneously implement the immunoreaction and photocurrent measurement. Anti-TG capture antibodies were modified onto AuNPs-decorated BiVO4 photoelectrode. A sandwich-type immunoreaction was carried out in the presence of target TG using horseradish peroxidase (HRP)-conjugated anti-TG detection antibody. The carried HRP molecules catalyzed 4-chloro-1-naphthol (4-CN) to generate an insoluble benzo-4-chlorohexadienone product on the photoanode in the presence of peroxide hydrogen, thereby decreasing the photocurrent. Under optimal conditions, the PEC immunosensors gave good photocurrent responses toward target TG within the dynamic range of 0.01–10 ng mL−1 at a detection limit of 7.6 pg mL−1. Good repeatability and precision, high specificity and acceptable storage stability were acquired during the measurement. No significant differences were encountered for screening 15 human serum specimens between the developed PEC immunoassay and commercially available enzyme-linked immunosorbent assay (ELISA) method for the detection of target TG. Significantly, PEC immunosensing system offers promise for simple and cost-effective analysis of disease-related biomarkers.

Enhancing the photoconductivity and gas sensing performance of TiO2/SnO2 heterostructures tuned by the thickness of the SnO2 upper layer

In this report, polycrystalline TiO2/SnO2 heterostructures with variable SnO2 film thickness were deposited by DC sputtering. Scanning electron microscopy images show a cracked surface in all films. The latter gets more widespread as the SnO2 layer thickness increases with deposition time. Optical transmittance measurements were used to determine the thickness of the TiO2 and SnO2 polycrystalline films. Photocurrent measurements of pure SnO2 films using UVA irradiation revealed a good response for thinner SnO2 films, however, these decrease as film thickness increases. Besides, photocurrent response is enhanced for the TiO2/SnO2 heterostructures over pure SnO2 film. It is thought that a high photocurrent response can be produced due to the improved ability to separate the photoinduced electrons and holes, as well as due to suitable charge management at the TiO2 and SnO2 interface. Additionally, the large amount of active sites for the thinner SnO2 upper layer favors better room temperature gas response to ethanol than that obtained for single SnO2 films. These features make the TiO2/SnO2 heterostructure a promising candidate for room temperature gas sensors and photosensitivity applications.

Understanding bulk photovoltaic effect in type-II Weyl semimetal Td-WTe2 using polarization dependent photocurrent measurement

Topological effects in a Weyl semimetal are explored in developing self-powered photodetectors at room temperature. The observed photocurrent is attributed to a combined effect of photothermoelectric effect and bulk photovoltaic phenomenon and is found to be a non-linear optical effect that converts light into electrical current. The self-powered photoresponse at 640 nm excitation wavelength reveals the presence of a diverging Berry curvature of tungsten ditelluride (Td-WTe2) at room temperature. The different perspective of polarization dependent photocurrent spectroscopy is used to separate the photothermal current from the shift current and the circular photo galvanic response from the linear photo galvanic response.