Electron sources driven by femtosecond laser have important applications in many aspects, and the research about the intrinsic emittance is becoming more and more crucial. The intrinsic emittance of polycrystalline copper cathode, which was illuminated by femtosecond pulses (FWHM of the pulse duration was about 100 fs) with photon energies above and below the work function, was measured with an extremely low bunch charge (single-electron pulses) based on free expansion method. A minimum emittance was obtained at the photon energy very close to the effective work function of the cathode. When the photon energy decreased below the effective work function, emittance increased rather than decreased or flattened out to a constant. By investigating the dependence of photocurrent density on the incident laser intensity, we found the emission excited by pulsed photons with sub-work-function energies contained two-photon photoemission. In addition, the portion of two-photon photoemission current increased with the reduction of photon energy. We attributed the increase of emittance to the effect of two-photon photoemission. This work shows that conventional method of reducing the photon energy of excited light source to approach the room temperature limit of the intrinsic emittance may be infeasible for femtosecond laser. There would be an optimized photon energy value near the work function to obtain the lowest emittance for pulsed laser pumped photocathode. (C) 2018 Author(s).

Structures of the two-dimensional atomic nuclei on ZnO (0001)-Zn and (0001)-O polar surfaces were studied by first principles density functional theory. The polarity-dependent nucleation dynamics was investigated by simulating two-dimensional (2D) nuclei consisting of 1-8 ZnO monomers on both polar surfaces. According to total energy calculations, average binding energy per ZnO monomer of the surface nuclei was analyzed to investigate if the nucleation and growth will proceed reasonably in physics. We found nucleation on (0001)-Zn surface was easier than that on (0001)-O surface. By using atomistic thermodynamics analysis, we calculated the Gibbs free energy of formation of these nuclei and made a comparison between the two polar surfaces. On (0001)-Zn surface, the critical Gibbs free energy of formation is much lower than that on (0001)-O surface under the same supersaturation, which leads to a much larger ZnO growth rate and rougher morphology, in accordance with experimental results. In addition, energetic analysis of nucleation at real thermodynamic conditions was achieved by introducing the temperature-and pressure-dependent chemical potentials of ZnO precursors. (C) 2017 Elsevier B.V. All rights reserved.

Recent theoretical predictions and angle-resolved photoemission spectroscopy measurements have shown that single crystal Cd3As2 is a three-dimensional topological Dirac semimetal possessing linear dispersions along all three momentum directions. Nanoscale topological Dirac semimetal structures have a large surface-to-volume ratio and provide a platform to explore its topological surface states. Here we report the synthesis of high quality Cd3As2 single crystalline nanoplates and nano-octahedrons via a vaporsolid growth mechanism. Triangular and hexagonal nanoplates with lateral dimensions ranging from several hundred nanometers to tens of micrometers are obtained. The top facets are (112), consistent with the natural cleavage plane of Cd3As2 single crystal. The synthesized Cd3As2 nano-octahedrons are enclosed by the {112} facets. A photovoltaic effect is demonstrated from a Cd3As2 nanoplate/metal electrode interface, suggesting potential applications in self-powered photodetection.

The formation mechanism of experimentally observed epitaxial morphologies on ( 000 +/- 1) polar surfaces of ZnO microwire was studied by density functional theory simulation of atomic step models on both polar surfaces. In situ observations by environmental scanning electron microscopy were employed to control and detect the morphology evolutions during the epitaxial growth process. Edge formation energies for representative step models were calculated as a function of Zn chemical potential. The energetically favorable step structures were determined under different conditions, which could be related to certain surface morphologies regarding the aspects of crystallography and growth kinetics. Our experiments can be well explained by theoretical simulations.

Polarity-dependent homo-epitaxy on (0001)-Zn and (000 (1) over bar)-O surfaces of cleaved ZnO microwires was investigated by in situ growth in ESEM and DFT simulations. ZnO monomers adsorption, adatoms desorption and chemisorption were simulated to understand the explicit mechanism.