Characterization and field emission properties of lanthanum monosulfide nanoprotrusion arrays obtained by pulsed laser deposition on self-assembled nanoporous alumina templates

摘要

Three distinct types of nanostructures—nanodomes, nanodots, and nanowires—have been simultaneously self-assembled by pulsed laser deposition of lanthanum monosulfide on anodic alumina films containing hexagonal arrays of pores about 50 nm wide and 500 nm deep. The nanostructures have been characterized by x-ray diffraction, atomic force microscopy (AFM), and field emission scanning electron microscopy (FE-SEM). Nanodomes preferentially grow on the boundary separating regions (grains) of the alumina template that have near perfect pore ordering, and their density is ～10~9/cm~2. The diameter of a nanodome at the base is about 100 nm and their aspect ratio (height/diameter at the base) is between 1 and 3. Additionally, nanodots nucleate on top of the alumina walls that separate adjacent pores. They have a diameter of ～50 nm, a density equal to the pore density (10~(10)/cm~2), and an aspect ratio less than 1. Finally, cross sectional FE-SEM images of the templates indicate that LaS nanowires grow inside the pores with a density of 10~(10)/cm~2. They have a diameter of 50 nm and a maximum length equal to the length of the pores (～500 nm). The field emission properties of the LaS nanodomes and nanodots have been analyzed via the scanning anode field emission microscopy technique (SAFEM). For a fixed SAFEM probe to cathode distance, the applied voltage necessary to extract the same FE current is found to be ～3.5 times less for a LaS thin film deposited on alumina templates compared to the value recorded for LaS thin films deposited on Silicon substrates. Assuming a LaS work function of ～1 eV (as recorded for LaS thin films grown on silicon substrates), a field enhancement factor of ～5.8 is extracted for the nanoscale emitters from Fowler-Nordheim plots of the FE data. The field enhancement effect accrues from the concentration of electric field lines at the tip of the nanodome and nanodot emitters. The value of 5.8 can be correlated to an aspect ratio of 2.7 for the dominant emitter, which is in good agreement with the aspect ratio of the tallest nanodomes observed in AFM measurements.