High Efficiency Photocathodes Research Articles

Switchable Multiplex Photoelectrochemical Immunoassay of Aβ42 and Aβ40 Based on a pH-Responsive i-Motif Probe and Pyrene-Based MOF Photocathode.

In accurately diagnosing Alzheimer's disease (AD) and distinguishing AD from other dementia, the concentration ratio of amyloid-beta 42 (Aβ42) to Aβ40 is more reliable than the concentration of Aβ42 alone. For the multiplex PEC assay, generating an independent photocurrent of multiple targets on a single interface is a great challenge. Herein, an i-motif-based switchable sensing approach is proposed to construct a pH-regulated multiplex PEC immunosensor for Aβ42 and Aβ40 by using Bi-TBAPy as an efficient photoactive cathode material. An independent photocurrent signal of Aβ42 and Aβ40 is produced through the regulation of the electron-transfer tunneling distance by a pH-dependent configuration transition of the i-motif DNA. In a 96-well plate, immunological recognition of Aβ42 (or Aβ40) coupled with an enzymatic catalytic reaction produces an acidic (or alkaline) lysis solution, which triggers the formation and unravelment of the i-motif structure. The above configuration transition regulates the distance between Au NPs labeled SH-DNA and Bi-TBAPy, leading to PEC signal switching. Smart integration of the pH-responsive switchable DNA probe with a high-efficiency photocathode enables the precise monitoring of Aβ42 and Aβ40 at a single interface in a wide detection range (10 fg/mL ∼ 1 μg/mL and 1 pg/mL ∼ 1 μg/mL) with detection limit of 4.5 fg/mL and 0.52 pg/mL, respectively. The proposed i-motif-based switchable sensing strategy paves a new avenue for a multiplex PEC assay on a single interface, showing great prospects in bioanalysis and early disease diagnosis.

Surface Assistant Charge Separation in PEC Cu2S-Ni/Cu2O Cathode.

Fabrication of a high efficiency photocathode is a challenging issue in photoelectrocatalysis (PEC). In this work, a Cu2S-Ni/Cu2O photocathode was constructed via electrodeposition followed by a two-step overlayer deposition procedure including direct-current magnetron sputtering (DCMS) and ion exchange reaction. We found that the presence of Ni in the inner-layer could not only affect the morphology but also enhance the formation rate of the outer-layer Cu2S. The XPS results indicate that the Ni exist as NiOx instead of Ni0. The photocurrent of Cu2S-Ni/Cu2O achieved 2 times of it on the pristine Cu2O. The charge dynamic characterizations, including electrochemical impedance spectroscopy (EIS), Tafel slopes, and photoluminescence (PL) spectra, demonstrated that the Ni can promote the hydrogen evolution reaction follow the Heyrovsky reaction, while Cu2S shows a crucial role on the surface charge separation. At last, surface photovoltage microscopy (SPVM) technology was used to reveal the function of each overlayer. It gives direct evidence for the charge transportation pathway in the system and explains the function of each component.

Graphene-based copper oxide thin film nanostructures as high-efficiency photocathode for p-type dye-sensitized solar cells

Graphene-based p-type dye-sensitized solar cells (p-DSSCs) have been proposed and fabricated using copper oxide urchin-like nanostructures (COUN) as photocathode with an FeS2 counter electrode (CE). COUN composed of Cu2O core sphere and CuO shell nanorods with overall diameters of 2 to 4 μm were grown by a simple hydrothermal method with self-assemble nucleation. It was figured out that the formation of copper oxide core/shell structures could be adjusted by an ammonia additive leading to pH change of the precursor solution. In addition to a photocathode, we also demonstrated FeS2 thin films as an efficient CE material alternative to the conventional Pt CEs in DSSCs. FeS2 nanostructures, with diameters of 50 to 80 nm, were synthesized by a similar hydrothermal approach. FeS2 nanostructures are demonstrated to be an outstanding CE material in p-DSSCs. We report graphene/COUN as photocathode and Pt/FeS2 as CE in p-DSSCs, and results show that the synergetic combination of electrodes in each side (increased interconnectivity between COUN and graphene layer, high surface area, and high catalytic activity of FeS2) increased the power conversion efficiency from 1.56% to 3.14%. The excellent performances of COUN and FeS2 thin film in working and CEs, respectively, make them unique choices among the various photocathode and CE materials studied.

Near atomically smooth alkali antimonide photocathode thin films

Nano-roughness is one of the major factors degrading the emittance of electron beams that can be generated by high efficiency photocathodes, such as the thermally reacted alkali antimonide thin films. In this paper, we demonstrate a co-deposition based method for producing alkali antimonide cathodes that produce near atomic smoothness with high reproducibility. We calculate the effect of the surface roughness on the emittance and show that such smooth cathode surfaces are essential for operation of alkali antimonide cathodes in high field, low emittance radio frequency electron guns and to obtain ultracold electrons for ultrafast electron diffraction applications.

Cu2O/CuO Bilayered Composite as a High-Efficiency Photocathode for Photoelectrochemical Hydrogen Evolution Reaction

Solar powered hydrogen evolution reaction (HER) is one of the key reactions in solar-to-chemical energy conversion. It is desirable to develop photocathodic materials that exhibit high activity toward photoelectrochemical (PEC) HER at more positive potentials because a higher potential means a lower overpotential for HER. In this work, the Cu2O/CuO bilayered composites were prepared by a facile method that involved an electrodeposition and a subsequent thermal oxidation. The resulting Cu2O/CuO bilayered composites exhibited a surprisingly high activity and good stability toward PEC HER, expecially at high potentials in alkaline solution. The photocurrent density for HER was 3.15 mA·cm−2 at the potential of 0.40 V vs. RHE, which was one of the two highest reported at the same potential on copper-oxide-based photocathode. The high photoactivity of the bilayered composite was ascribed to the following three advantages of the Cu2O/CuO heterojunction: (1) the broadened light absorption band that made more efficient use of solar energy, (2) the large space-charge-region potential that enabled a high efficiency for electron-hole separation, and (3) the high majority carrier density that ensured a faster charge transportation rate. This work reveals the potential of the Cu2O/CuO bilayered composite as a promising photocathodic material for solar water splitting.

Metal on metal oxide nanowire Co-catalyzed Si photocathode for solar water splitting

We report a systematic study of Si|ZnO and Si|ZnO| metal photocathodes for effective photoelectrochemical cells and hydrogen generation. Both ZnO nanocrystalline thin films and vertical nanowire arrays were studied. Si|ZnO electrodes showed increased cathodic photocurrents due to improved charge separation by the formation of a p/n junction, and Si|ZnO:Al (n+-ZnO) and Si|ZnO(N2) (thin films prepared in N2/Ar gas) lead to a further increase in cathodic photocurrents. Si|ZnONW (nanowire array) photocathodes dramatically increased the photocurrents and thus photoelectrochemical conversion efficiency due to the enhanced light absorption and enlarged surface area. The ZnO film thickness and ZnO nanowire length were important to the enhancements. A thin metal coating on ZnO showed increased photocurrent due to a catalyzed hydrogen evolution reaction and Ni metal showed comparable catalytic activities to those of Pt and Pd. Moreover, photoelectrochemical instability of Si|ZnO electrodes was minimized by metal co-catalysts. Our results indicate that the metal and ZnO on p-type Si serve as co-catalysts for photoelectrochemical water splitting, which can provide a possible low-cost and scalable method to fabricate high efficiency photocathodes for practical applications in clean solar energy harvesting.

Electron emission characterization of Mg photocathode grown by pulsed laser deposition within anS-band rf gun

Pulsed laser deposition (PLD) has been proposed several years ago as a suitable technique to deposit a pure Mg film over a radio frequency (rf) gun Cu backflange in order to obtain a high efficiency photocathode surface for the generation of high brightness electron beams. In this paper we report preliminary experimental results on the emission properties of a PLD grown Mg film within the high electric field gradients of a rf gun showing the effects of the rf conditioning process on the cathode surface. Even though a laser cleaning process should be performed on the sample surface in order to remove contaminated layers, the results presented here are very promising for the realization of a final Mg-based photocathode.

Factors affecting performance of dispenser photocathodes

Usable lifetime has long been a limitation of high efficiency photocathodes in high average current accelerator applications such as free electron lasers, where poor vacuum conditions and high incident laser power contribute to early degradation in electron beam emission. Recent progress has been made in adapting well known thermionic dispenser techniques to photocathodes, resulting in a dispenser photocathode whose photosensitive surface coating of cesium can be periodically replenished to extend effective lifetime. This article details the design and fabrication process of a prototype cesium dispenser photocathode and describes in detail the dominant factors affecting its performance: activation procedure, surface cleanliness, temperature, and substrate microstructure.

External photoelectric effect and high-efficiency photocathodes in the soft-x-ray region of the spectrum

Two approaches to calculating the “true” x-ray photoemission from high-efficiency photocathodes are analyzed in the photon energy range 1–10 keV. Analytic expressions are derived for the quantum yield of true x-ray photoelectrons. The calculated values of quantum yield are compared with experimentally measured values.

High-efficiency photocathodes on the NEA-GaAs basis

We have investigated three types of GaAs NEA photocathodes-polycrystalline on the molybdenum substrate, homoepitaxial and glass-sealed (semitransparent). It was found that any of these cathodes can be a stable high-efficient source of electrons, with maximum quantum yield 20–50%.

High efficiency photocathodes for gaseous detectors of visible photons

Our latest results in the development of gaseous detectors for visible photons are presented in this paper. Quantum efficiencies of a few percent at 400 nm were routinely achieved. These detectors can operate at gas gains up to ∼1000. The main effort was focused on a study of the effect of different liquids and solid layers on the quantum efficiency of photocathodes. Some of these layers make the photocathodes more robust, while others increase the quantum efficiency. It was found, for example, that thin layers of solid and liquid Xe and Kr enhance the quantum efficiency by a factor ∼2. These solid layers can also protect the photocathodes against some impurities in the surrounding gas. After some further development gaseous detectors for visible photons will probably find applications in crystal calorimetry, noble liquid scintillation detectors, Cherenkov counters, air shower detectors and the readout of scintillating fibers for preshower and max-shower detectors.

Use of MgF_2 and LiF Photocathodes in the Extreme Ultraviolet

The photoelectric yields of 2000-A thick samples of MgF(2) and LiF have been measured at wavelengths in the range 1216-461 A. Peak values of 43% and 34%, respectively, were obtained at wavelengths around 550 A at 45 degrees incidence. Coating the cathode of a channel electron multiplier with 3000 A of MgF(2) produced no significant deterioration in the electrical properties and increased the sensitivity by factors of 1.62, 2.76, and 2.60 at wavelengths of 742 A, 584 A, and 461 A, respectively. Since the stability of response of the MgF(2) photocathodes appears to be equal to that of conventional metallic and semiconducting cathodes, it is concluded that MgF(2) would be a practical, high-efficiency photocathode for use in the extreme uv.

Sensitive high speed photodetectors for the demodulation of visible and near infrared light

Photomultipliers, solid state photodiodes and avalanche photodiodes are the preferred detectors for the demodulation of light intensity variations at visible and near infrared wavelengths. The principles of operation of these detectors will be described together with their basic construction and operational characteristics. Photomultipliers with the recently developed high efficiency photocathodes for visible and near infrared wavelengths and the high gain dynode arrangements with fast speed of response will be mentioned. The various types of silicon and germanium photodiodes with and without internal current gain will be discussed. Tradeoffs involved in the optimization of quantum efficiency, speed of response, internal current gain and sensitivity to weak light signals will be treated in detail for silicon photodiodes operating at wavelengths either in the visible or at the GaAs (≈0.7–0.9 μm) or YAG (1.06 μm) infrared laser emission lines. The capabilities of germanium avalanche photodiodes for the detection of infrared light at wavelengths up to about 1.6 μm will be mentioned.