Monitoring morphological changes in 2D monolayer semiconductors using atom-thick plasmonic nanocavities

Abstract

Nanometer-sized gaps between plasmonically coupled adjacent metal nanoparticles enclose extremely localized optical fields, which are strongly enhanced. This enables the dynamic investigation of nanoscopic amounts of material in the gap using optical interrogation. Here we use impinging light to directly tune the optical resonances inside the plasmonic nanocavity formed between single gold nanoparticles and a gold surface, filled with only yoctograms of semiconductor. The gold faces are separated by either monolayers of molybdenum disulfide (MoS2) or two-unit-cell thick cadmium selenide (CdSe) nanoplatelets. This extreme confinement produces modes with 100-fold compressed wavelength, which are exquisitely sensitive to morphology. Infrared scattering spectroscopy reveals how such nanoparticle-on-mirror modes directly trace atomic-scale changes in real time. Instabilities observed in the facets are crucial for applications such as heat-assisted magnetic recording that demand long-lifetime nanoscale plasmonic structures, but the spectral sensitivity also allows directly tracking photochemical reactions in these 2-dimensional solids.

Keywords: 2D-materials; molybdenum disulfide; nano-optics; nanoparticles; tunable plasmons; waveguides.

Figure 1

Scattering of NP on mirror with semiconductor nanospacers. (a) AuNP spaced above gold surface by 2D semiconductor sheet, forming MIM cavity. (b) TEM micrograph of faceted NP (scale bar 50 nm). (c) DF-STEM image of NPoM cross-section. The MIM cavity formed by facet and surface is clearly visible (scale bar 50 nm). (d) DF scattering and SEM of NP on MoS₂ (scale bar 1 μm). (e) Experimental and (f) Boundary Element Method (BEM) simulation of NPoM optical response. Labeled are resonances arising from hybridized MIM cavity modes (j₁,j₂,j₃) and the transverse single NP mode (T).

Figure 2

MIM waveguide model. (a) Dispersion of MIM waveguide with s = 1,2,3,4 modes for varying facet size. Shaded area shows experimentally accessed facet range. (b) 1D MIM waveguide with boundary conditions defined by the facet size. (c) Lateral normalized field distribution |E|² for first three modes seen in the nanogap of Figure 1d for increasing facet widths from 10 to 30 nm (top 3 rows) and vertical field distribution of 30 nm facet (bottom).

Figure 3

Optical tuning of coupled gap plasmons. (a and b) Scattering spectra of (a) MoS₂ and (b) CdSe NPoM during irradiation with 448 nm laser light. The 514 nm peak in (b) is CdSe photoluminescence. (c) BEM simulation of NPoM optical response with increasing NP facet diameter, showing the j_2,3 modes (dashed). (d) For all different NPoMs, measured spectral splitting Δλ = λ₂ – λ₃ increases with time compared to average spectral position, for CdSe and MoS₂ spacers. Gray dots indicate beginning of irradiation.