Nanoindentation-enhanced tip-enhanced Raman spectroscopy

The continuous development and ever-expanding applications of tip-enhanced Raman spectroscopy (TERS) and nano-imaging attest to the power and promise of this technique.^1–4 TERS combines the chemical selectivity and unique properties of Raman scattering with the nanoscale resolution and sensitivity that accompany optical field localization and enhancement at the apex of a plasmonic probe. In the realm of molecular TERS, it is now well-appreciated that utilizing a metallic substrate can further localize and enhance the local optical fields in the so-called gap-mode TERS configuration.^1–4 Indeed, the spectroscopic signatures and images of individual molecules (and their vibrational states) have been recorded via gap-mode TERS.⁵ 

Much emphasis in the community has been put on optimizing the attainable spatial resolution and detection limit of ambient TERS.^1–4,6 That said, the focus has mostly been on the micro- and nanoscopic makeup of the plasmonic probe. Whereas the first dictates the far field response/resonance, the second governs the attainable spatial resolution in TERS. The substrates of choice for most prior gap-mode TERS measurements consist of flat/single crystalline gold and silver substrates.^5,7 Indeed, flat samples facilitate STM-based TERS measurements, chiefly targeting flat molecules such as porphyrins, wherein the atomic limit becomes possible in Raman imaging.^5,7 More recently, there has been a number of atomic force microscopy (AFM)-based TERS spectral imaging measurements that targeted chemically functionalized plasmonic nanoparticles.⁸ Such structures were found to support few-nm spatial resolution in ambient TERS spectral imaging, especially in the case of faceted nanoparticles and nanocubes.^9,10 The attainable spatial resolution was often limited by the spatial extent of local optical fields defined by the interaction between the plasmonic probe and a nanoparticle.⁸ Note that recording Raman spectra with few-nm spatial resolution necessitates that the optical signal arises from a small number of molecules. In a sense, high spatial resolution measurements and few/single molecule detection sensitivity are analogous in this context.

This work builds on and expands the scope of TERS mapping of plasmonic nanostructures. Specifically, we take advantage of imprinted plasmonic nanostructures prepared using pulsed-force nano-lithography^11,12 to enhance TERS in several aspects. First, we obtain a 50X increase (on average) in TERS signal levels compared to TERS from a flat region of the substrate. This allows enhanced TERS-based detection and chemical identification. Second, we show that the nano-indented regions sustain nano-localized local optical fields that may be exploited for (i) high spatial resolution TERS nano-imaging and (ii) inducing/tracking plasmon-enhanced/induced chemical transformations taking place in ultra-confined volumes. Related to (i), we demonstrate down to ∼1 nm spatial resolution (pixel-limited) in TERS imaging. As for (ii), we illustrate that the nano-indented regions support plasmon-induced chemical transformations of 4-nitrothiophenol (NTP). Specifically, we show that we can distinguish between NTP and its dimer (4,4′-dimercaptoazobenzene or DMAB) with a pixel-limited spatial resolution (4 nm). The latter again attests to the high degree of local field confinement using the nanostructured we describe below.

Figure 1 shows correlated AFM (a) and hyperspectral dark field micrographs (b) of an array of nano-indented structures. Nano-lithographic patterns shown in this figure were obtained using a silicon probe. Throughout the course of nano-indentation, the silicon probe is deformed, which leads to modified indentation patterns. The latter can be traced through both high-resolution AFM imaging as well as optically, i.e., through hyperspectral dark field scattering [see Fig. 1(c)]. Indeed, sharp nano-triangular patterns prior to tip deformation lead to a single resonance centered at ∼530 nm, whereas additional red-shifted resonances are observable when the patterns deviate from the triangular shape. The aforementioned time-dependent tip (and hence nanoindentation pattern) modification can be avoided using diamond probes, as shown in the supplementary material. On the basis of the above observations, the indentation patterns trace the shape of the probes, which in the case of diamond remain unchanged after several days of using the same probe.¹¹ 

Following the fabrication and initial AFM/dark field characterization of nanohole arrays, we chemically functionalized the resulting substrate with NTP. This molecular reporter has been thoroughly investigated in the literature, largely because of its ability to dimerize to form DMAB on plasmonic metals.^8,13,14 The Raman (and TERS) spectra of both NTP and DMAB are well-known and have been properly assigned elsewhere.^14,15 Representative TERS results are shown in Fig. 2, and additional images are provided in the supplementary material. The left panel of this figure shows simultaneously recorded AFM (a) and TERS (b) maps of a small region of the sample. We observe a bright TERS response inside the imprinted nanostructure compared to flat regions of the substrate in its immediate vicinity. The latter can be gauged by comparing the spectra recorded inside vs outside the nano-pattern [see Fig. 2(c)]. The signal inside the etched-in structure is enhanced ∼50X compared to TERS spectra recorded from the flat substrate. Note that the enhancement value observed in this case comprises an average of the values we recorded from tens of nanostructures, which are spread between 1 and 2 orders of magnitude. It is also important to note that our experimental geometry leads to the observed asymmetry in the local field profiles: the upper edges of the nanostructures [see Figs. 2(a), 2(b), 2(d), and 2(e)] are brighter than the lower edges. This is because the laser is incident from the lower part of the image at an angle of ∼65° with respect to the surface normal such that the upper edge is better-exposed to the incident radiation under our experimental geometry. A similar argument was previously invoked to rationalize local optical field maps of plasmonic nanoholes, as visualized through photoemission electron microscopy measurement following grazing angle femtosecond pulse irradiation.¹⁶ 

The localization and enhancement of optical fields within the indented regions of the sample may be exploited to induce/track plasmon-induced chemical transformations in confined spatial regions. In effect, the nano-patterns we describe in this work may potentially be viewed as nano-reactors, in which plasmon-induced chemical transformations can take place. The results shown in the right panel of Fig. 2 illustrate the concept. We once again show simultaneously recorded AFM (d) and TERS (e) maps of an NTP-functionalized nanostructure. A close inspection of the recorded Raman spectra reveals that the DMAB dimer of NTP (red spectrum) is formed within the nano-hole-like structure [see Figs. 2(d)–2(f)]. Interestingly, we identify four adjacent pixels, wherein the recorded spectra alternate between NTP [black, blue, and green spectra in Fig. 2(f)] and DMAB (red trace in the same figure). This places the resolution in our recorded chemical reaction map at <4 nm (pixel limited). This is reminiscent of prior work from our group in which we used two different analytes¹⁷ or different conformations¹⁸ of the same molecule to gauge the attainable spatial resolution in TERS.

Since the resolution described above is only limited by the step size used in our TERS maps, we decreased the lateral/vertical steps in our measurements to 1 nm. The result is shown in Fig. 3, which traces the edge of one of the etched-in nanostructures. A larger area scan of the slit-like nanostructure is otherwise shown in the supplementary material. The image shown in Fig. 3(a) is essentially internally calibrated: scans along the x-direction that are taken at different y positions essentially correspond to repeating the same measurement, besides a slight offset because of the curved nature of the nanostructure (see the supplementary material). TERS cross-sectional cuts taken at two different positions marked in panel (a) are shown in panel (b) of Fig. 3. In both cases, we can resolve the rise in signal in going from the flat area of the substrate to the nanolithographic pattern with a spatial resolution of ∼1 nm. Evidently, the field varies extremely rapidly in space at the edge, which causes a dramatically fast signal rise that we can clearly resolve under our experimental conditions.

The attainable spatial resolution in our measurements can be understood by considering the corrugated nature of the sputtered gold probe used to record the TERS maps. Locally and in the small area imaged in Fig. 3(a), it appears that a nanometric asperity at the apex of the probe leads to the observed spatial resolution. Further localization and enhancement of local fields by atomic and small atomic protrusions at the apex of the tip have been recently invoked to rationalize the attainable spatial resolutions in TERS and related measurements performed at ultralow temperatures and under ultrahigh vacuum.^5,19 Our results can be understood on this basis.

In conclusion, this work describes the fabrication and multi-modal characterization of plasmonic nanopatterns etched into flat gold substrates using AFM nano-lithography. The hyperspectral Raman nano-imaging measurements reveal several exciting aspects of the local fields that are sustained within the patterns described in this work. Specifically, extreme local field localization and nano-confinement leads to dramatically enhanced TERS signals inside the patterns. Moreover, these structures can be considered nano-reactors, wherein plasmon-enhanced/induced chemical reactions can take place and, evidently, can be monitored via TERS. Overall, more systematics are warranted to describe the effect of different parameters in AFM nano-lithography on the resulting patterns, both in terms of their structures and their plasmonic properties. This will be the subject of future work.

See the supplementary material for experimental details, correlated AFM-dark field mapping of nanoholes indented using a diamond tip, and additional AFM-TERS mapping of different nanoholes at various pixel densities.

P.Z.E.-K. acknowledges support from the United States Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. C.-F.W. and B.T.O. were supported by the United States Department of Energy, Office of Science, Office of Biological and Environmental Research, through the bioimaging technology development program. Some of the instrumentation that was required to do the measurements described in this work was purchased and developed through funding from the Laboratory Directed Research and Development (LDRD) program at the Pacific Northwest National Laboratory.