A Three-Channel Spectrometer for Wide-Field Imaging of Anisotropic Plasmonic Nanoparticles

Abstract

A three-channel spectrometer (3CS) based on a commercial digital camera was developed to distinguish among tens of large (>100 nm), anisotropic plasmonic particles with various shapes, orientations, and compositions on a surface simultaneously. Using band pass filters and polarizers, the contrast of 3CS images could be enhanced to identify specific orientation and composition characteristics of gold and gold-silver nanopyramids and as well as the direction of the longest arm of gold nanostars.

Figure 1. Scheme using a digital camera as a three-channel spectrometer (3CS)

(A) DF scattering setup. (B) Simulation of single-particle scattering spectra of two different anisotropic particles (indicated by ◆ and ○) with scattering intensities that are similar at longer wavelengths but different at shorter wavelengths. (C) Simulation of a wide-field image of two different anisotropic particles (◆ and ○). (D) RGB channel intensities from the VIScam and NIRcam images using MATLAB. The histograms demonstrate the relative RGB intensities for the two types of particles.

Figure 2. Calibration measurement to distinguish between tip-up and tip-down orientations in Au nanopyramids

(A) Single-particle spectra of Au nanopyramids (d = 350 nm, t = 20 nm) with a tip-up (U) () and tip-down (D) (▲) orientation. (B) VIScam image and R/G values for five Au pyramids. The particles scatter very similar colors and have very similar R/G values. (C) NIRcam image and R/G values for the same image in (B). The particles still scatter very similar colors, although the R/G values for pyramids 1 and 2 begin to separate from pyramids 3, 4, 5. (D) NIRcam image with 850 nm (± 20 nm) BP filter and R/G values for the same image in (B). The R/G values separate into two distinct regions that correspond to U and D. The threshold value was set at 3.2. All optical images are 12 μm × 9.3 μm.

Figure 3. Determination of nanopyramid orientation in a larger field of view

(A) VIScam image of thirty-five (d = 350 nm, t = 20 nm) nanopyramids. (B) NIRcam image of the same area. (C) NIRcam image of (B) with 850 nm (± 20 nm) BP with orientations identified based on a threshold of 3.2 from Figure 2. (D) Histogram of R/G from (C).

Figure 4. Determination of Au and Au/Ag/Au pyramids on the same substrate

(A) Single-particle DF scattering spectra of Au (d = 350 nm, t = 40 nm) and Au/Ag/Au (d = 350 nm, t = 10/20/10 nm) pyramids. Spectra are offset for clarity. The dashed lines indicate the location of the BP filters (580 nm and 850 nm) used to refine the 3CS images. Insets: SEM images of Au and Au/Ag/Au pyramids in U and D orientations. Images are 400 nm × 400 nm. (B) VIScam image with a 580-nm BP filter, where the G-channel intensity for thirteen Au and Au/Ag/Au pyramids is shown. (C) Histogram of G intensities separate into two regions corresponding to either Au or Au/Ag/Au. (D) NIRcam image with 850-nm BF filter enables assignment of U and D for the Au pyramids.

Figure 5. Determination of geometrical features of small NPs

(A) VIScam image of three AuNSs and their RGB-channel intensities. (B-D) SEM images and polar plots for each AuNS. Dashed lines highlights the longest arm of each structure. Polar plots of R intensities from NIRcam and 850-nm BP filter were obtained every 10° from 0° to 360° (integration time of 30 s). The lobbed-feature correlates to a dipolar LSP resonance.