Nanometal Skin of Plasmonic Heterostructures for Highly Efficient Near-Field Scattering Probes

Abstract

In this work, atomic force microscopy probes are functionalized by virtue of self-assembling monolayers of block copolymer (BCP) micelles loaded either with clusters of silver nanoparticles or bimetallic heterostructures consisting of mixed species of silver and gold nanoparticles. The resulting self-organized patterns allow coating the tips with a sort of nanometal skin made of geometrically confined nanoislands. This approach favors the reproducible engineering and tuning of the plasmonic properties of the resulting structured tip by varying the nanometal loading of the micelles. The newly conceived tips are applied for experiments of tip-enhanced Raman scattering (TERS) spectroscopy and scattering-type scanning near-field optical microscopy (s-SNOM). TERS and s-SNOM probe characterizations on several standard Raman analytes and patterned nanostructures demonstrate excellent enhancement factor with the possibility of fast scanning and spatial resolution <12 nm. In fact, each metal nanoisland consists of a multiscale heterostructure that favors large scattering and near-field amplification. Then, we verify the tips to allow challenging nongap-TER spectroscopy on thick biosamples. Our approach introduces a synergistic chemical functionalization of the tips for versatile inclusion and delivery of plasmonic nanoparticles at the tip apex, which may promote the tuning of the plasmonic properties, a large enhancement, and the possibility of adding new degrees of freedom for tip functionalization.

Figure 1. Schematic process of SAM coating of Si-AFM tip by BCP micelles loaded with AgNPs and Ag@AuNPs.

(a) In step (i), PS-b-P4VP micelles are loaded with AgNPs, mainly in the core of P4VP copolymer. (ii) Next, overgrowth of AgNPs in presence of an excess of Ag⁺ in solution, produces bigger NPs; Ag-BCP nanocomposites are purified by density gradient centrifugation to select micelles with larger NPs in the core. In the next step (iii), Au precursor is added to the solution of micelles preloaded with AgNPs; AuNPs, formed with excess of NaBH₄, were evident in the BCP micelles by EDX analysis (Fig. S1) and UV-vis spectroscopy (Fig. 2), and morphologically evident in the PS shell at TEM inspection reported in panel (b). Therefore, we tentatively describe this structure as consisting of AgNPs mainly in the P4VP core and AuNPs mainly in the PS shell ( as Ag@AuNPs). Finally, both solutions of BCP with AgNPs or Ag@AuNPs are used to coat Si-AFM tips by dip coating. (b) TEM micrographs (inverted colormap) of the three steps of the process depicted in panel (a). In particular, the modification of the size of the NPs, evident from the comparison of the consecutive SAMs spin-coated on glass, pointed out the formation of AgNPs of 15–20 nm surrounded by smaller Ag seed satellites in the P4VP core (ii). An outer shell that we ascribe to tiny AuNPs ( nm) appears in (iii) as described in the main text. Top insets are magnified regions of the corresponding bottom scans. Scalebars are 50, 150 and 100 nm in (i), (ii) and (iii), respectively, whereas scalebar = 25 nm in all top insets.

Figure 2. UV-vis characterization.

Absorbance and reflectance spectra of nanoislands obtained from templates of BCP micelles loaded with clusters of AgNPs and bimetallic Ag@AuNPs with amount ratios Ag: Au as indicated in the legend (from EDX, Fig. S1). UV-vis curves were acquired in transmission (extinction) and reflection (scattering) on films spin-coated on glass coverslips, hence determining the absorbance contribution. For AgNPs, a large scattering contribution is measured and peaked at 500 nm against absorption peaked at 420 nm (red lines). For Ag@AuNPs, there is a significant variation in both absorption and scattering coefficients. The main scattering band, initially peaked at 480 nm (yellow line), progressively moves to 575 nm (cyan line) and then to ~630 nm (violet line) extending in the infrared. These bands are ascribed to additional clustering of AuNPs mixed with AgNP aggregates.

Figure 3. SEM characterization of Si-AFM probes coated with SAMs of metal nanoislands.

(a) SEM micrograph of an Arrow^©-type AFM probe coated with clusters of AgNPs showing a uniform monolayer on the pyramidal tip with dewet regions at the base. (b) Magnified version of (a) with inset showing a region close to the apex where contrast is increased to resolve the structure of close-packed nanoislands. (c) Arrow^©-type tip coated with Ag@AuNP nanoislands: the ripped coating reveals a monolayer structure of metal nanoislands, which resembles a sort of nanometal skin on the probe. (d) Region of residual coating present on the base shaft close to the tip, where it is possible to appreciate the close-packed assembly produced by attractive forces during solvent evaporation. These are capable of producing granular patterns commensurate with the geometry of the tip apex. Please note, as for instance, the stinger-shaped assembly indicated by the arrow. Such a kind of terminations appear also on the apex of (b).

Figure 4. Numerical simulations of representative apical clusters at the tip apex of our TERS probes.

(a) Enhancement factor spatial distribution approximated as the four power of the norm E of the scattered electric field normalized to the incident radially polarized wave field, of amplitude E_o at λ = 520 nm (bottom illumination); surrounding medium index n = 1.4. The tomographic representation of the field amplification is rendered by means of 3 overlaid transparent cut planes at z = −7, 0, 8 nm and a y-planar cut 0.5 nm below AuNPs. Colormap on the left represents the surface charge density calculated on wireframed nanoparticles and tip. The enhancement factor (logscale) on the right is saturated at 10⁶: regions of such level fill and surround the apical cluster. (b) Same as in (a), with nonsaturated colormap of enhancement factor and different angle of view. Please note the chain coupling from the center to the peripherical NPs that transmits local field amplification to the surface of AuNPs. (c) Enhancement factor distribution as in (a), but for a slightly different configuration of NPs (the bigger is 12 nm) for excitation at 480 nm, with plane wave polarization along the x-axis. See text for details.

Figure 5. Experimental characterization of the TERS probes on SWCNTs.

(a) Large area topography of bundles on SWCNTS spin-coated on glass and (b) corresponding s-SNOM map. (c) Topography of a magnified region of interest and (d) corresponding phase map; 512 × 512 pixels over an area of 1 μm². (e) Tip-in and tip-out signals onto a bundle of SWCNT; the inset shows the topographic cross section along the dashed line traced in panel (c). (f) TERS map onto a region of (c), acquired with incident power of 450 μW, integration time Δt = 15 ms per pixel.

Figure 6. Bi-analyte TERS experiment.

(a) Tip-in (TERS) and tip-out (SERS) spectra onto a transparent SERS substrate covered with CV and R6G molecules (with incident radial polarization). The SERS and gap-mode TERS spectra point out only the CV component into a fixed position. Panels (b–d) show examples of typical TERS spectra of CV, R6G and mixed CV with R6G (indicated as CV AND R6G) acquired in consecutive positions of the SERS substrate. (e) Normalized integrated intensity maps, respectively, of the main bands of CV (blue color) and R6G (red color) with overlaid colormaps and coincidence map of the product of the intensities of CV and R6G (dark level = 0, white level = 1). The map was taken with 256 × 170 pixels over an area of 3 μm × 2 μm, with integration time Δt = 15 ms (per pixel). (f) Histogram of the counts of single molecule events of CV, R6G and mixed events. The low coincidence rate points out a single-molecule statistics. (g) Centroidal map of the regions of connected pixels in which one spectral species (CV, R6G or mixed) appear consecutively detected along the scan in (f).

Figure 7. TERS characterization on a thick biosample: spore of Bacillus subtilis.

(a) Phase map of B. subtilis spore. (b) Tip-in, trace and retrace TERS spectra onto the B. subtilis spore. See text for details.

Figure 8. s-SNOM measurements on a patterned gold nanostructure.

(a) Topography and integrated Rayleigh intensity (s-SNOM) along the cross section indicated in (b), acquired in contact mode with a Ag@AuNP coated tip, with incident power of 100 nW at 532 nm and integration time of 15 ms per pixel, with incident radial polarization, over an area of 1 μm² (256 × 256 pixels). (c) Detail of closely spaced gold nanopillars (topography and s-SNOM), extracted from a scan area of 350 × 350 pixels over 4 μm² (step of 5.7 nm), measured with excitation laser power of 150 nW at 785 nm, with radial polarization and integration time of 12 ms per pixel. All colormaps represent signal amplitudes leveled to the minimum.