Imaging the Optical Fields of Functionalized Silver Nanowires through

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Imaging the Optical Fields of Functionalized Silver Nanowires through Molecular TERS Ashish Bhattarai, Kevin Thomas Crampton, Alan G. Joly, Libor Kovarik, Wayne P Hess, and Patrick Z. El-Khoury J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b03324 • Publication Date (Web): 05 Dec 2018 Downloaded from http://pubs.acs.org on December 6, 2018

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The Journal of Physical Chemistry Letters

Imaging the Optical Fields of Functionalized Silver Nanowires through Molecular TERS

Ashish Bhattarai,1 Kevin T. Crampton, 1 Alan G. Joly,1 Libor Kovarik,2 Wayne P. Hess,1 and Patrick Z. El-Khoury1,*

1

Physical Sciences Division and 2Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA

*

[email protected]

ABSTRACT We image 4-mercaptobenzonitrile-functionalized silver nanowires (~20 nm diameter) through tipenhanced Raman scattering (TERS). The enhanced local optical field-molecular interactions that govern the recorded hyperspectral TERS images are dissected through hybrid finite-difference time-domain-density functional theory simulations. Our forward simulations illustrate that the recorded spatio-spectral profiles of the chemically functionalized nanowires may be reproduced by accounting for the interaction between orientationally averaged molecular polarizability 1

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derivative tensors and enhanced incident/scattered local fields polarized along the tip axis. In effect, we directly map the enhanced optical fields of the nanowire in real space through TERS. The simultaneously recorded atomic force microscopy (AFM) images allow a direct comparison between our attainable spatial resolution in topographic (13 nm) and TERS (5 nm) imaging measurements performed under ambient conditions. Overall, our described protocol enables local electric-field imaging with few nm precision through molecular TERS, and it is therefore generally applicable to a variety of plasmonic nano-structures.

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Tip-enhanced Raman scattering (TERS) combines the utility of scanning probe microscopy and the greatly enhanced Raman scattering cross-sections of molecules in the vicinity of plasmonic metals to achieve high resolution chemical imaging.1,2,3,4 This technique, and its many variants, have been utilized to demonstrate chemical mapping with nanometer precision5,6 (or better7,8,9) under different experimental conditions. In the ultra-high resolution regime, which requires the stability of ultra-high vacuum and cryogenic temperatures, orientationally-locked individual molecules may be interrogated with atomically terminated scanning tunneling microscopy probes. In this limit, recent works suggest that the observables are governed by the interaction between localized optical fields and intra-molecular currents that vary on the Å scale.9,10 When carried out under ambient laboratory conditions in the single/few-molecule regime, a wide range of thermally accessible molecular orientations, conformations, and adsorption geometries all contribute to the recorded optical spectra, and thus, present unique challenges for interpreting TERS spectral images.6 These considerations are particularly relevant for elucidating local optical fields from TERS spectra, which is this subject of this work. Through a series of previous reports from our group, we described measurements aimed at gauging the nature,11,12 magnitude,6,13 and vector components14 of local fields involved in ambient TERS measurements. Of particular relevance to our current work is a recent report, in which a plasmonic tip functionalized with a few molecules was used to image the vector components of electric fields sustained at the edges of Au(111) terraces.14 Sensitivity to the vector components of the local fields was possible by virtue of the tensorial nature of ultrasensitive and/or high spatial resolution TERS (effective probing volumes on the order of 1-2 nm3). The previously described approach augments the information content in TERS electric field imaging measurements to include vector components. Nonetheless, extracting field information from ultrasensitive TERS

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spectra minimally requires knowledge of the molecular polarizabilities and an assumed/measured molecular orientation.14 Herein, we demonstrate a complementary approach to TERS electric field imaging that involves addressing small ensembles of aromatic thiols (4-mercaptobenzonitrile, MBN) that are chemisorbed onto silver nanowires (~20 nm diameter). The recorded TERS spectra reflect the ensemble-averaged optical response of all molecules in the probed volume, which we show to be laterally localized to the effective diameter of our TERS probe (~5 nm). Averaging the optical response simplifies the general problem of gleaning fields and molecular orientations through a typically large set of tensor elements, in effect, greatly reducing the experimental parameter space. This approach thus allows us to address general/open questions about the attainable spatial resolution and fundamental molecular-local optical field interactions that are broadcasted through TERS spectral images. We find that the optical response directly maps the accessible z-component of the local electric field of a chemically functionalized silver nanowire.

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Figure 1. General topographic features of the chemically functionalized Ag nanowires and selected TERS spectra. (A) AFM topography of an Ag nanowire drop-casted onto templatestripped Au and (B) scanning transmission electron microscopy (STEM) image of a selected Ag nanowire on lacey carbon. The STEM image reveals nanometric variations in MBN coverage. (C) TERS spectra (633 nm excitation) acquired when the tip is brought into contact with the wire (red) and the Au substrate (black). The general topographic features of our chemically functionalized silver nanowires are shown in Figure 1. We utilize the same Au-coated Si tips for both AFM and TERS, which are simultaneously carried out using an intermittent contact mode AFM feedback mechanism.15 The tips are prepared by sputter coating Au (100 nm) on commercial Si AFM probes (Nanosensors, ATEC, sub-20 nm cone radius), which nominally broadens apex diameter in the absence of nanometric corrugation.6 Through AFM measurements, we were nonetheless able to fully resolve the nanowire width in AFM, which was independently determined through STEM imaging to be ~20 nm (Figure 1 B). The deconvolution factor for identical Gaussian profiles then places an upper limit on the diameter of the tip apex, which we estimate to be 13 nm for the nanowire shown in Figure 1 A. Simultaneously acquired TERS spectra are bright, indicating a high degree of field localization at the apex. TERS point spectra acquired on and off the nanowire (633 nm continuous 5



wave excitation) are presented in Figure 1 C. The former is consistent with standard/plasmonenhanced Raman scattering from the reporter molecule (4-mercaptobenzonitrile), which we have previously assigned.6,14 On the substrate itself, which consists of freshly cleaved template-stripped Au, the spectrum is similar to the retracted tip spectrum. Namely, it exhibits a weak but measurable background that may be assigned to electronic Raman scattering (ERS) of the Au continuum electrons.16,17

Figure 2. TERS images, spectra and cross-sections of a chemically functionalized Ag nanowire. (A) Averaged TERS spectrum acquired on the nanowire. The shaded regions highlight the major spectral features (molecular lines) and correspond to the cross-sections and normalized images shown in (B) and (C-F), respectively. The dashed lines in panels C-E define the directions of the cuts shown in (B).

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TERS images of the functionalized Ag nanowire presented in Figure 1A are summarized in Figure 2. These images were generated by integrating over selected spectral windows at each of the vibrational resonances and the background (Figure 2A). TERS cross-sectional line cuts exhibit a well-defined twin-peak structure, reaching maxima near the edges of the nanowire and featuring a pronounces dip towards its center (see Figure 2 C). Note that this data was recorded using a lateral step size of 2.5 nm. For larger step sizes, the nanowire cross-section coalesces into a single Gaussian-like intensity distribution (see Figure S1). The general profiles of the TERS signals are consistent among the various vibrational modes and the non-resonant ERS background (see Figure S2). We note that the intensity of the ERS signal is a fraction (~30%) of the enhanced molecular response. Molecular TERS signals track the relative intensities of the vibrational lines with high fidelity, as illustrated by the normalized spectrum and corresponding spatial cross sections that are presented in Figure 2 A,B respectively. Moreover, for a given mode, variations in the relative intensities of the peaks that are observed in the spatial profiles reflect varying degrees of molecular coverage. This is evidenced by nanometric variation in molecular densities that are apparent in the recorded STEM images (Figure 1 B). In addition to the major features described above, the TERS images exhibit a persistent and reproducible peak/dip at the outer edges of the nanowire that is absent in the topographic AFM image (Figure 1 A). Although not obvious in the molecular TERS images, which emphasize the nanowire response itself, this feature is symmetric about the nanowire center (see Figure S3). This arises as a result of the finite aspect ratio of the tip and our experimental geometry. Namely, as the tip is positioned close to the nanowire edge, the apex and shaft are simultaneously in contact with the nanowire and substrate, which results in overall signal increase. On the edge itself, the apex is no longer in contact with either the nanowire or the substrate, which effectively leads to signal

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decrease. The effect is a natural outcome of contact-mode TERS and our experimental geometry, and will therefore be neglected in the ensuing analysis.

Figure 3. Selected TERS image and average cross-sectional profile compared to the simulated response. (A) 1580 cm-1 TERS image of the Ag nanowire and (B) cross-section extracted along the direction defined by the dashed line (A) (black curve) and profile simulated using eq. 1 (red curve). The recorded TERS profiles can be readily understood by recalling TERS selection rules, i.e., the scattering tensor that governs Raman activity in the TERS geometry15 𝑆

∝ ∑ 𝐸 ⃗𝛼 Ω 𝐸 ⃗

(1)

in which 𝛼 is the molecular polarizability derivative tensor of the nth vibrational Eigenstate, 𝛺=𝛼,𝛽,𝛾 comprises the Euler angles that define the 3D molecular orientation relative to the vector components of the enhanced incident and scattered local electric fields, 𝐸 ⃗. The requirement for incident and scattered polarization along the tip axis in TERS (𝐸 ⃗ in the laboratory frame) is well understood, and it has been rationalized elsewhere.15,18 In our present work, the ~5 nm-thick (on average) MBN layer that is chemisorbed onto the silver wire leads to an ensemble averaged optical response, which contracts the observables to the intensities and depolarization ratios of the 8


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vibrational states. Numerically, this involves averaging the molecular polarizability tensors in 3D space,19 which leads to frequency/vibrational resonance-dependent hyperspectral TERS image slices that only vary in their overall intensities. Experimental TERS images traced at different vibrational resonances probe the same field component, as shown above (see Figure 2 C-F). Using the above-described formalism, it is possible to simulate the experimental hyperspectral TERS image and cross-sectional profiles (Figure 3B). To this end, we derive the local optical fields that are sustained in the immediate vicinity of the silver nanowire upon 633-nm irradiation from finite difference time-domain simulations (FDTD) that emulate the experimental construct and geometry, as elaborated in the methods section. Frequency-dependent molecular polarizability derivative tensors were separately computed using tools of density functional theory calculations (DFT, B3LYP20,21,22/Sadlej-PVTZ23,24 level of theory). The result obtained by solving eq. 1 is compared to its experimental analogue in Figure 3 B. In this analysis, the lateral extent of the field localization, i.e. the tip geometry, is accounted for through Gaussian convolution and 𝐸 ⃗ was evaluated at various heights given by the topographic AFM map. We find that the simulated 5-nm (FWHM) broadened cross-sectional profile best reproduces the experimentally observed twin peak structure in terms of the TERS signal rise, decay, and peak to trough ratios.

Figure 4. Comparison of the lateral resolution observed in the AFM and TERS cross-sectional profiles. The solid blue and red lines correspond to the best Gaussian fits to the topographic and TERS cross-sectional profiles. 9



To ascertain our inference of 5 nm spatial resolution in TERS, we compare the spatial resolution observed in AFM and TERS, which are extracted using Gaussian fits to the two crosssectional line cuts. Note, the TERS signal probes the physical extent of the z component of the electric field sustained by the 20 nm nanowire. We extract the FWHM of the TERS signal by focusing on the contrast between the peaks in the measured electric field profile. Compared to the simulated field profile, in which the peak-to-dip ratio is 7:1, the TERS contrast is broader by 50 % (ratio of 3:1). This is consistent with a lateral resolution of ~4 nm. As mentioned above, the simultaneously acquired AFM fully resolves the nanowire, which, in the absence of sharper topographic features, places an upper limit of 13 nm on the absolute tip diameter (Gaussian convolution factor of 0.7 x 18 nm). This breakdown, and approximate factor of 2 difference between the upper limit on the tip diameter and observed TERS resolution is consistent with analytical treatments of the effective spatial resolution in TERS.3 Several conclusions can be drawn on the basis of our results and analysis. The persistence of the requirement for incident/scattered light polarized along the tip axis for TERS observation suggests that the overall enhancement and local polarization is dictated by the resonant plasmonic excitation of the tip. A comparison between our experimental high-resolution TERS images and hybrid FDTD-DFT simulations (Figure 3) allows us to infer a spatial resolution on the order of 5 nm beginning with what may be regarded as a coarse tip (~100 nm). In this regard, nanometric corrugations at the apex of the sputtered tip provide an efficient route to both improved resolution and field enhancement in TERS. Tip modification throughout the measurements may also lead to tip-restructuring, particularly when contact mode AFM feedback is used in TERS. Both corrugations and tip restructuring throughout the measurement are difficult to control. Nonetheless, performing the measurement in intermittent contact mode seems to circumvent the 10


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problem (at least throughout the course of our measurements), as evidenced by the observation of reproducible TERS cross-sections from the redundant (in 2D) nanowire imaged in this work. Beyond the use of a redundant target, simultaneous AFM measurement that place an upper limit of 13 nm on the tip dimeter are pre-requisite to ascertaining that the nominally coarse tip is in effect rather sharp.

Finally, it is evident our TERS images trace the local electric field of the

nanowire (Figure 3). This is consistent with recent reports from our group, in which different properties of localized and enhanced local optical fields were mapped via TERS.6,17 Unlike our prior ultrasensitive TERS measurements, however, the orientationally averaged response selectively exposes the z-component15,18 of the local electric field of the plasmonic nanowire. This is evidently the case under the below-described experimental conditions, but the generality of this observation for different plasmonic (e.g. SERS) substrates and resonance conditions needs to be further investigated.

Methods A 1:1 silver nanowires (806714 Aldrich) stock and 1 mM ethanolic 4-mercaptobenzonitrile (MBN, Aurum Pharmatech) solution was allowed to react for an hour. The solution is then dropcasted onto a freshly cleaved template-stripped Au substrate (Platypus Tech.). The substrate is then sonicated in ethanol for ~30-60 seconds, rigorously washed using the same solvent, and allowed to dry prior to TERS mapping. Our AFM/TERS setup is previously described elsewhere.6,17 Briefly, initial topographic AFM measurements were performed in tapping mode feedback using a silicon tip (Nanosensors, ATEC) coated with 100 nm of Au by arc-discharge physical vapor deposition (target: Ted Pella Inc., 99.99% purity). A 633 nm laser (100-200 μW) is incident onto the apex of the TERS probe

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using a 100X air objective (Mitutoyo, NA=0.7) at a ~65o angle with respect to the surface normal. The polarization of the laser is controlled with a half waveplate and is set along the long axis of the AFM probe. The scattered radiation is collected through the same objective and further filtered through a long pass filter. The resulting light is detected by a CCD camera (Andor, Newton EMCCD) coupled to a spectrometer (Andor, Shamrock 303i). A dedicated TERS imaging mode (SpecTop, patent pending from AIST-NT) was employed for simultaneous AFM-TERS mapping. Using this mode, TERS signals are collected when the tip is in direct contact with the surface with a typical force in the 10−25 nN range. A semicontact mode is then used to move the sample relative to the tip (pixel to pixel) to preserve the sharpness and optical properties of the tip and to minimize the lateral forces that otherwise perturb the substrate. Scanning transmission electron microscopy images were recorded using an 80-300 kV FEI probe Cs-corrected Titan electron microscope operating at 300 kV. The chemically functionalized nanowires were deposited on a lacey carbon-coated Cu grid for these measurements. FDTD simulations were performed using a commercially available software package (Lumerical Inc.) running on a local computer cluster. The computational models used replicate our experimental geometry by accounting for sample permittivity, laser wavelength, polarization, and angle of incidence. The calculations incorporate a silver nanowire atop a gold substrate parsed in a three dimensional simulation volume. The calculations yield the spatially resolved relative intensities of the electric field components as a function of time. Standard Fourier transforms result in the corresponding spatial and frequency resolved fields. DFT calculations were performed using a local development version of NWChem,25 with the B3LYP20-22 exchange-correlation functional in conjunction with the Sadlej-PVTZ23-24 basis set. A custom Mathematica code was used to

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generate the simulated TERS profile shown in Figure 3B. The general protocol is well described in recent works from our group.17

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AUTHOR INFORMATION Corresponding Author *[email protected] The authors declare no competing financial interest.

ACKNOWLEDGMENTS AB and LK were supported by the Department of Energy’s (DOE) Office of Biological and Environmental Research Bioimaging Technology project #69212. AGJ, KTC, PZE and WPH are supported by the US DOE, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. This work was performed in the environmental and molecular sciences laboratory (EMSL), a DOE Office of Science User Facility sponsored by BER and located at PNNL. PNNL is operated by Battelle Memorial Institute for the DOE under contract number DE-AC05-76RL1830.

Supporting Information Available: Additional TERS images recorded at various pixel densities, analysis of different TERS cross-sectional cuts, relevant FDTD simulations of the nanowire, selected area TEM images.

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Imaging the Optical Fields of Functionalized Silver Nanowires through

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