Spatial Resolution of Coherent Cathodoluminescence Super-Resolution Microscopy

Abstract

We investigate the nanoscale excitation of Ag nanocubes with coherent cathodoluminescence imaging spectroscopy (CL) to resolve the factors that determine the spatial resolution of CL as a deep-subwavelength imaging technique. The 10-30 keV electron beam coherently excites localized plasmons in 70 nm Ag cubes at 2.4 and 3.1 eV. The radiation from these plasmon modes is collected in the far-field together with the secondary electron intensity. CL line scans across the nanocubes show exponentially decaying tails away from the cube that reveal the evanescent coupling of the electron field to the resonant plasmon modes. The measured CL decay lengths range from 8 nm (10 keV) to 12 nm (30 keV) and differ from the calculated ones by only 1-3 nm. A statistical model of electron scattering inside the Ag nanocubes is developed to analyze the secondary electron images and compare them with the CL data. The Ag nanocube edges are derived from the CL line scans with a systematic error less than 3 nm. The data demonstrate that CL probes the electron-induced plasmon fields with nanometer accuracy.

Figure 1

(a) High-resolution HAADF-STEM image of a 70 nm Ag nanocube. (C) CL spectra (30 keV) taken at the center and a corner of a 70 nm Ag nanocube on a 15 nm Si₃N₄ membrane. (b) CL spectra (30 keV) taken at the center and a corner of a 70 nm Ag nanocube on a 15 nm Si₃N₄ membrane. (c) Top, simultaneously collected SE intensity and CL intensity (1.3–5.4 eV spectral range, 1 nm step size) collected in a 25 nm wide rectangular region across the nanocube; bottom, laterally integrated CL and SE intensities along the nanocube. Solid lines are model fits. The dashed vertical lines indicate the nanocube boundaries derived from the SE (blue dashed line) and CL (red dashed line) models.

Figure 2

Monte Carlo simulations of electron scattering in a 70 nm Ag nanocube (10 and 30 keV). (a, c) Distribution of inelastic scattering events for the electron beam incident at the center of the top facet (dots) and probability that a SE generated at a certain lateral position x′ escapes from the nanocube integrated over the nanocube height, per primary electron (blue dashed line, σ_beam = 0.85 nm; green dashed line, σ_beam = 5.1 nm). (b, d) Simulated probability that a secondary electron is generated and escapes from the nanocube for two effective beam widths (blue dots, σ_beam = 0.85 nm; green dots, σ_beam = 5.1 nm) as a function of the lateral position x of the incident electron beam, and model fits (blue and green drawn lines). The open circles indicate the nanocube edge as derived from the fits. The orange vertical dashed lines indicate the particle edges used in the Casino simulation.

Figure 3

MNPBEM calculations for 70 nm Ag cubes. (a) 30 keV CL spectra for electrons incident at the center or a corner. (b) CL intensity as a function of beam position away from the nanocube, integrated over the 1.6–3.5 eV spectral range used in the measurements in Figure 1c, for 10, 15, 20, and 30 keV electrons. The drawn lines are exponential fits to the data.

Figure 4

CL decay length L away from the cubes. Histograms of the decay length L derived by fitting the model for I_CL to 159 measured CL line scans as in Figure 1c. The green vertical lines show the decay lengths derived from the BEM calculations in Figure 3b. The dashed lines are Gaussian fits to the histograms.

Figure 5

(a) SE and CL line scans at 10 keV with model fits for I_cube and I_CL. The vertical dashed lines indicate the nanocube edges derived from the two fits, with the difference Δd indicated. (b) Histograms of Δd for 159 measurements for four different energies. The dashed lines are Gaussian fits through the data.