Cathodoluminescence and phase-change functionality of metallic nanoparticles

Abstract

Nanoscale resolution cathodoluminescence (CL) has been used to demonstrate and investigate the functionality of nanoparticle-based components for future nanophotonic phase-change memory and optical antenna applications. An integrated experimental system based on a scanning electron microscope was developed for the fabrication and in situ characterization of nanoparticles. It was equipped with an atomic beam source for gallium nanoparticle growth, a liquid-nitrogen-cooled cryostat to control substrate temperature in the range from 90 to 315 K and a spectroscopic CL detection system to enable the analysis of electron-beam-induced light emission from nanoparticles across the wavelength range from 350 to 1150 nm.

A new technique of light-assisted, size-controlled growth of gallium nanoparticles from an atomic beam has been developed. Through coupling to surface plasmons in nanoparticles, infrared radiation controls the adsorption/desorption rate of gallium atoms on the particles' surface. The experiments revealed a decrease in mean particle diameter (from 68 to 45 nm) with increasing infrared excitation intensity (from 160 to 630 W·cm-2) during deposition, and the production of larger particles with a narrower size distribution for longer deposition times.

Gallium nanoparticle phase-change memory provides an important possibility to achieve small element size and low energy consumption. For the first time, it has been shown that information can be written into the structural phase of bistable gallium nanoparticles by electron beam excitation and readout achieved via measurements of their CL emission. Change of up to 20 % in CL emission intensity was observed following low fluence (> 35 fJ/nm2) electron-beam-induced, solid-to-liquid phase switching of a monolayer of 60 nm particles. Selective electron beam addressing and CL readout of individual memory elements (comprising less than 50 particles each), within a gallium nanoparticle film, have been also demonstrated. Numerical modeling of CL emission from gallium nanoparticles, performed using the boundary element method, qualitatively reproduces the experimentally observed effects.

Optical antennae are expected to become essential elements of future nanophotonic circuits. For the first time, it has been demonstrated that electron-beam-excited pairs of coupled gold nanorods can act as transmitting optical antennae; i.e. they can efficiently convert the energy from a nanoscale excitation (created by a focused 40 keV electron beam) into far-field visible radiation. Enhanced light emission was observed for electron beam injection points in the vicinity of the junction between coupled nanorods, illustrating the increased local density of electromagnetic states in such areas.

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Identifiers

ORCID for N.I. Zheludev: orcid.org/0000-0002-1013-6636

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