Plasmon ruler with angstrom length resolution

Abstract

We demonstrate a plasmon nanoruler using a coupled film nanoparticle (film-NP) format that is well-suited for investigating the sensitivity extremes of plasmonic coupling. Because it is relatively straightforward to functionalize bulk surface plasmon supporting films, such as gold, we are able to precisely control plasmonic gap dimensions by creating ultrathin molecular spacer layers on the gold films, on top of which we immobilize plasmon resonant nanoparticles (NPs). Each immobilized NP becomes coupled to the underlying film and functions as a plasmon nanoruler, exhibiting a distance-dependent resonance red shift in its peak plasmon wavelength as it approaches the film. Due to the uniformity of response from the film-NPs to separation distance, we are able to use extinction and scattering measurements from ensembles of film-NPs to characterize the coupling effect over a series of very short separation distances-ranging from 5 to 20 Å-and combine these measurements with similar data from larger separation distances extending out to 27 nm. We find that the film-NP plasmon nanoruler is extremely sensitive at very short film-NP separation distances, yielding spectral shifts as large as 5 nm for every 1 Å change in separation distance. The film-NP coupling at extremely small spacings is so uniform and reliable that we are able to usefully probe gap dimensions where the classical Drude model of the conducting electrons in the metals is no longer descriptive; for gap sizes smaller than a few nanometers, either quantum or semiclassical models of the carrier response must be employed to predict the observed wavelength shifts. We find that, despite the limitations, large field enhancements and extreme sensitivity persist down to even the smallest gap sizes.

Figure 1

Film-coupled nanoparticle (film-NP) plasmon nanorulers (PNRs). (A) An ensemble of film-NP PNRs is created by coupling gold NPs directly to a gold film. NPs become coupled to their electromagnetic image induced within the film and the film-NP plasmon resonance red-shifts as the spacing between the NPs and film decreases. (B) Self assembled monolayers (SAMs) of amine-terminated alkane thiols are used to create ultra-thin gaps for film-NP PNR measurements in the Ångstrom regime. The cartoon in (B) is drawn to approximate scale and shows a 60-nm gold NP separated from a 30-nm gold film by an amine thiol SAM with a 2-carbon alkane chain length (n). A series of amine thiols with n of 2, 3, 6, 8, 11, and 16 were used to create film-NP spacer layers ranging from 5 – 20 Å.

Figure 2

A series of amine-terminated alkane thiols with 2, 3, 6, 8, 11, and 16 carbons were used to create film-NP spacer layers. (A) Ellipsometric thicknesses of the layers depicted by the open circles are found to be systematically skewed relative to theoretical thicknesses of the layers with the amine thiols standing straight up on the gold film (short dashed line) as well as the case when the amine thiols are tilted by 30° relative to the normal of the gold film surface (long dashed line). Theoretical lengths of amine-terminated alkane thiols are depicted standing straight up on the gold film (B) and tilted 30° relative to the gold surface normal (C).

Figure 3

Images of the series of film-NP samples using amine thiol spacer layers of varying alkane carbon chain lengths (n, denoted by Cn). (A) A series of dark field microscope images (100x 0.9 NA) showing optically un-resolved 60-nm gold NPs immobilized to underlying gold film using amine thiol spacer layers of varying thicknesses. Scanning electron microscope images in (B) showing a 4x-magnified field of view relative to the dark field images in (A) reveal the NP surface coverage on the various samples (see Results and Discussion for surface coverage statistics). High NP surface coverage was targeted for the film-NP plasmon nanoruler demonstration so as to be able to use ensemble film-NP measurements to efficiently determine the average film-NP plasmonic response to the spacer layer thickness.

Figure 4

Spectroscopic response of the film-NP samples to variation of the amine thiol spacer layer thickness. Normalized ensemble scattering spectra (A, with instrument ray diagram in upper C inset) and extinction spectra (B, with instrument ray diagram in lower C inset) both show the expected blue-shifting plasmonic response of the film-NP samples to increasing spacer layer thickness. Peak centroids were calculated for each set of data and plotted against the number of carbons in each of the amine thiol spacer layers in (C) and against the theoretical amine thiol SAM thicknesses in (D). The scattering (open circles) and extinction (closed squares) data plotted against film-NP spacer layer show the ability to discriminate between the number of carbon atoms in the amine thiol chain length. The same data plotted against separation distance shows the ability to discern Ångstrom-scale changes in spacer thickness. S and D stand for source and detector, respectively.

Figure 5

Spectroscopic response of the film-NP samples with spacer layers of varying thickness from 5 Å – 27 nm. Film-NP scattering (open circles) and extinction (closed squares) data shown in Figure 3 from the thin amine thiol samples is combined with data from samples using thicker polyelectrolyte spacer layers. (A) Scattering and extinction data from the film-NP samples displays a non-linear trend with increasing film-NP separation distance. (B) Same data plotted on a log₁₀ – log₁₀ scale and following a linear trend, which is a signature of a power law function. Linear regression of this data produces best-fit functions of y = −0.07652x + 2.8489 (R² = 0.99622) for the scattering data (dashed line) and y = −0.07719x + 2.83434 (R² = 0.99337) for the extinction data (solid line). These functions are also displayed in (A) as the power functions y = 706.155x^−0.07652 for scattering (dashed line) and y = 682.873x^−0.07719 for extinction (solid line).