Photocathodes as photoelectric conversion materials play a key role in vacuum photodetectors, accelerator-based light sources, and ultrafast electron microscopy. Since graphene has unique properties such as gas impermeability, electron transparency, a large specific surface area, and atomic thinness, the graphene/metal van der Waals heterojunction material may provide the possibility to improve the photoemission performance of metal photocathodes through surface modification. The specific Cs/O activation process was performed on the surface of graphene/copper heterojunction material. Its photoemission behaviors, including light-excited current, spectral response, and degradation characteristic, were investigated to verify the effect of surface modification on the improvement of photoemission performance. The experimental results show that, by preliminary Cs/O activation, the graphene/Cu photocathode can extend the threshold of spectral response to the near-infrared region. Furthermore, through repeated Cs/O activation, the cathode sample can possess better stability of operation. The photoelectron spectroscopy measurement results present the changes in the chemical states of surface elements and the corresponding work function after Cs/O activation and degradation. This Cs/O-activated graphene/metal photocathode can provide a new strategy to enhance the photoemission performance and stability of such photocathodes.
Electron source can be considerably benefited from the application of photocathodes with long lifetime, broad spectral response and high quantum efficiency. As graphene has exhibited potential to serve as the passivation layer to improve electron emission behavior of photocathodes due to its unique structure and properties, exploring photoemission phenomenon of metal-supported graphene material can help to develop robust photocathodes. We report the photoemission performance of the nickel-supported monolayer graphene through various activation processes. To investigate the effects of different activation processes on this composite material, we compare the photoemission behavior of the nickel-supported monolayer graphene by cesium (Cs)-only activation at room temperature, Cs-only activation under heated condition and cesium-oxygen (Cs/O) activation under heated condition. The experimental results show that these activation methods are able to promote photoemission ability of the nickel-supported monolayer graphene. Among them, the Cs/O activation process under heated condition can improve the photocurrent most significantly. This process validation of activating the nickel-supported graphene material provides a new route to enhance photoemission performance of metal photocathodes, and the activation method could be applied to the surface modification of other metal-supported graphene materials.
Underwater photoelectric detection equipment with 532 nm laser as light source needs to match specific photocathodes to achieve the purpose of high quantum efficiency and narrow-band response. NEA GaAlAs photocathodes have a series of advantages, such as high quantum efficiency, adjustable spectral response cutoff threshold and long lifetime to serve as the devices for underwater detection. However, the quantum efficiency of GaAlAs photocathodes is not high enough to meet the actual detection requirements. In addition, micro-nano structures on the surface of materials have been proved to be an effective method to improve optical absorption. In this paper, four kinds of nanostructures including square column, cylinder, square cone and cone are designed. By the finite difference time domain method, results show that the optical absorptivity of reflection-mode GaAlAs photocathodes can be effectively improved. The optical absorptivity of square cone nanostructures increases and tends to be stable with the increase of filling factor. The optical absorptivity is approximately 100% at blue-green light region including 532 nm, and has an abrupt cutoff feature. Otherwise, when the absorptivity of square cone nanostructures is high and stable, the most intense light absorption part will move to the top of nanostructures with the increase of filling factor, thus effectively shortening the electron transport distance and improving the photoemission capability.
In order to research H+ beam radiation on photoelectric performance of GaAs photocathodes used in low-light-level optoelectronic devices, based on Monte-Carlo method, quick calculation of damage, along with effects of Cs and Cs-O activation layer on ion trajectory, performance, ionization of ions and recoils are discussed. From the simulation results, the average stopping range increases with the increase of incident energy, and the dispersion varies with the incident angle, the minimum dispersion at 1 keV is obtained at 60° when Cs-O ratio is 2:1, and the minimum dispersion at 2 keV is obtained at 60° when Cs-O ratio is also 2:1. In addition, the produced vacancies increase with the incident energy while the value is almost not influenced by the incident angle, and the backscattered ions increase as the incident angle increase both in 1 keV and 2 keV cases, and decrease with the incident energy. Also, ionization dominate the H+ ion bombarding process instead of producing vacancies and phonons. With the increase of incident energy, the percentage of ionization of ions increases, while those of phonons of ions and ionization of recoils decrease. However, the corresponding percentages of ions and recoils remain nearly unchanged with the increase of incident angle and the variation of composition of Cs or Cs-O activation layer.
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