Pulsed laser deposition of lanthanum monosulfide thin films for cathode material applications.

摘要

Thin films of lanthanum monosulfide (LaS) were successfully deposited on Si and MgO substrates by pulsed laser deposition (PLD). The films were golden yellow in appearance with a mirror-like surface morphology. The thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), high resolution transmission electron microscopy (TEM), Kelvin probe (KP) and scanning anode filed emission microscopy (SAFEM). XRD analysis revealed the successful growth of the cubic rocksalt structure. AFM and TEM images revealed increasing crystallinity with increasing substrate temperature. Films grown at room temperature were comprised of nanocrystals separated by regions of amorphous material. Films grown at elevated substrate temperature in a background gas of H2S were crystalline with grain sizes that increased with increasing substrate temperature. KP characterization revealed a decrease in work function with an increase in crystallinity. The SAFEM technique was used to characterize the field emission properties. Using the conventional Fowler-Nordheim relation, the work function of LaS thin films was extracted from the slope of the plot ln(J/F2) vs. 1/F, where J is the field emission current density and F is the local applied field. This lead to an outstanding, reproducible work function value across a 1cm2 sample of ∼1.1 eV. The threshold for an emission current density of 1 mA/cm2 occurs around a 230 V/mum applied electric field across the vacuum gap.