Tip-Enhanced Raman Spectromicroscopy on the Angstrom Scale

ACS Nano , 2017, 11 (11), pp 11393–11401. DOI: 10.1021/acsnano.7b06022. Publication Date (Web): October 5, 2017. Copyright © 2017 American Chemical...
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Tip-Enhanced Raman Spectromicroscopy on the Angstrom Scale: Bare and CO-Terminated Ag Tips Nicholas Tallarida, Joonhee Lee,* and Vartkess Ara Apkarian* Department of Chemistry, University of California at Irvine, Irvine, California 92697-2025, United States S Supporting Information *

ABSTRACT: The tip is key to the successful execution of tip-enhanced Raman scattering (TERS) measurements in the single molecule limit. We show that nanoscopically smooth silver tips, batch produced through field-directed sputter sharpening, reliably attain TERS with enhancement factors that reach 1013, as measured by the Raman spectra of single CO molecules attached to the tip apex. We validate the bare tips by demonstrating spectromicroscopy with submolecular spatial resolution and underscore that TERS is a near-field effect that does not obey simple selection rules. As a more gainful analytical approach, we introduce TERS-relayed molecular force microscopy using CO-terminated tips. By taking advantage of the large Stark tuning rate of the CO stretch, molecular structure and charges can be imaged with atomic resolution. As illustration, we image a single Ag atom adsorbed on Au(111) and show that the adatom carries +0.2e charge. KEYWORDS: tip-enhanced Raman spectroscopy, spectromicroscopy, force microscopy, scanning tunneling microscopy, field-directed sputter sharpening, CO-terminated tip, silver tip borrowing from the fields of cavity optomechanics20 and quantum electrodynamics.21 Remarkably, the experimental arrangement that yielded TERS with submolecular resolution was not substantively different from others in use,22 and the submolecular spatial resolution of the original demonstration was not reproduced under seemingly similar conditions, using the same molecule as the TERS reporter.23 In the sophisticated instrumentation used for TER-sm, the fabrication of the STM tip is the variant that is not standardized in practice.24 As such, we give it center-stage here: We describe a method for the batch processing of nanoscopically smooth silver tips, which invariably couple farfield radiation to the single cavity mode of the STM junction and enable TER-sm on the angstrom-scale. We preview the enabled measurements and highlight the fundamental challenge posed by the capability: In contrast with well-defined selection rules of vibrational spectroscopy carried out on ensembles with plane waves, single molecule SERS/TERS is a tensorial nearfield scattering determined by molecular orientation and light fields sculpted by the junction cavity.19,25 It was recognized early on that, in this limit, field gradients lead to multipolar Raman scattering and that ultimately even multipolar

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y now, it is well established that plasmonic nanojunctions allow vibrational spectroscopy with single molecule sensitivity, both in frequency1−3 and time domain,4 through the surface enhanced Raman scattering (SERS) effect discovered nearly four decades ago.5,6 In the intervening period, there has been an explosion of developments and applications of the effect, among which SERS combined with scan-probe microscopy has heralded the ultimate frontier in vibrational spectromicroscopy (sm), under the rubric of tip-enhanced Raman scattering (TERS).7−10 Despite the advances, the fundamentals of the effect remain an active area of research, as summarized in recent expansive reviews.11−13 Most notable in this effort was the demonstration of TER-sm with submolecular spatial resolution, carried out on individual molecules at the junction of a scanning tunneling microscope (STM).14 While enhanced local fields are necessary to boost the single molecule signal to a detectable level, localization of light is essential to limit the observation to just one molecule in TER-sm. This is accomplished in the STM using an atomically terminated silver needle to concentrate light at its apex. The attained submolecular resolution has confirmed that atomistic detail in junction morphology defines the local fields15−18 and has led to advances in theory to treat plasmonics in this nonclassical limit, through ab initio spectroscopy coupled to atomistic plasmonics,17,19 and photodynamics in nano/pico-cavities by © 2017 American Chemical Society

Received: August 23, 2017 Accepted: October 5, 2017 Published: October 5, 2017 11393

DOI: 10.1021/acsnano.7b06022 ACS Nano 2017, 11, 11393−11401

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Figure 1. Scanning electron micrographs of two-stage electrochemically etched tips before (a, c, e) and after (b, d, f) batch processing via fielddirected sputter sharpening (FDSS). The process eliminates nanoscopic surface roughness and sharpens the apex, with memory of the original gross structure. (g) The reduction in tip length is due to head-on sputtering.

expansions may fail.26 This was emphatically illustrated in measurements at a fusing plasmonic nanojunction, where it could be shown that field-gradient driven quadrupolar and magnetic Raman transitions, that are otherwise 103−105 times weaker than normal (dipolar) Raman, can dominate single molecule spectra.27 While important new science is accessed, the analytical utility of vibrational microscopy is compromised. A more gainful approach toward analytical spectromicroscopy is suggested through TERS-relayed molecular force microscopy (TERS-mfm) with single molecule functionalized tips, which we introduce here. We demonstrate TERS of COterminated silver tips and use the spectral shift of the CO stretching mode to map out intermolecular forces, therefore structure, with atomic resolution. The approach is closely related to implementations in atomic force microscopy (AFM) and STM using CO-terminated tips,28,29 which have yielded molecular structure with the highest spatial resolution to date. In TERS-mfm, the molecular spring replaces the mesoscopic AFM cantilever. The 8 orders of magnitude increase in operating frequency of the transducer allows instantaneous force sensing through the spectral domain, with quality factor enhanced by the multiplexing advantage, as in super-resolution microscopy.30 Beside mechanical forces, electric forces and charge distributions within individual molecules can be mapped31 by the large Stark tuning rate of the CO stretch,32 which is magnified on tipped Ag.33 Indeed, the differential Raman scattering cross section of CO is small.34 As such, observation of the single molecule requires dramatic field enhancement,27 of order >1012, which is routinely attained on the tips we describe. The very large enhancement factors, which arise from the combination of electromagnetic and chemical mechanisms,35 distinguish TERS of CO chemisorbed on the tip. At plasmonic junctions, this involves tunneling of charge transfer plasmons through the molecular bond.36 A formal treatment of the process37 and explicit time-dependent density functional simulations of the field driven tunneling current have been presented.33

produces relatively sharp Ag tips (r < 100 nm) with improved microscopic smoothness. However, the surface remains rough on the nanoscopic scale, as seen in the SEM images of Figure 1. Hot spots on nanoscopic asperities can be sufficiently bright to produce single molecule SERS.41 While focused ion beam milling can be used to superficially polish and shape individual silver tips, the process is laborious, and implanted ions degrade the surface conductivity of silver and scatter surface plasmons.42 Instead, we have adapted the field-directed sputter sharpening (FDSS) technique43 for batch processing silver tips. The result is depicted in Figure 1. Despite variations in gross shapes and cone radii, these optically smooth tips invariably concentrate visible light to a single nanometric spot, as will be validated below. Field-Directed Sputter Sharpening. The concept of FDSS, which was introduced and implemented by Schmucker et al. to produce PtIr, W, and HfB2 tips with nanometric sharpness,43 is illustrated in Figure 2. The tip is showered head-

RESULTS AND DISCUSSION Silver Tips. The dielectric properties of silver make it the material of choice for fashioning bright plasmonic tips. Although electrochemically etched silver tips serve well for STM purposes,38 in contrast to gold and tungsten, silver surfaces roughen in the process. Both electrochemically etched and vapor deposited silver tips are plagued by nanoscopic asperities, nanocrystalites, and surface irregularities due to redeposition of ions during etching and corrosion.39 To simplify, standardize, and optimize tip fabrication, we have previously developed an automated two-stage electrochemical etching and electropolishing procedure.40 The technique

Figure 2. Ion trajectories overlaid on the electric field map of a positively biased tip modeled as a cone: incident Ar+-ion energy = 2 keV, counter bias = +500 V, cone semi-angle α = 16.2°, initial lateral displacements of the incoming ions from the principal axis are 5 and 50 nm. Scale bar = 500 nm.

on with 2 keV Ar+-ions while positively biased at 100−500 V. The positive surface charge density of the tip provides a retardation field that conforms to the taper by the lightning rod effect. Explicitly, the retarding potential on a cone can be described by the fractional-order Legendre polynomial: Φ(r , θ ) ≃ Ar νPv(cos θ ) 11394

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ACS Nano with ν = 0.247 for the exemplary half-cone angle of α = 16.2°, and coefficient A determined by the boundary condition at large r.44 The associated electric field, in spherical coordinates, Er = −

dΦ ≃ −vAr ν − 1Pv(cos θ ), dr

Eθ = −

1 dΦ ≃ −Ar ν − 1sin θP′v (cos θ ) r dθ

memory of initial shape, and the sharpest tip has a cone radius of 16 nm. Highly polished Ag surfaces are highly reactive. Tips tarnish upon exposure to air during transfer to the UHV chamber of the STM. The formation of an adventitious diamond-like carbon film can be recognized by the broad Raman band in the range from 1200 to 1700 cm−1 seen in Figure 4a (red trace).45

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determines the trajectory of an ion subject to the force: F = qE. For a given cone angle and impinging ion energy, the counter bias can be adjusted to produce ion trajectories that graze the taper, to produce ion-streaming on the inclined surface for optimal polishing without dulling the apex. For the cone in Figure 2, this is accomplished at a counter bias ratio of Vcone/Vion = 0.1−0.3. In practice, the asperity decorated surface dictates the intricate local fields and FDSS retains memory of the initial morphology. As such, the procedure greatly benefits from starting with uniform, electrochemically polished tips. The key consideration in the design of a batch processor is the maintenance of the symmetry of ion streamlines on tips held in close proximity. The schematic of our design to batch-process seven tips is shown in Figure 3. Its operating conditions and simulations of its computer-aided design are provided in the supplement. The outcome is illustrated by the scanning electron micrographs in Figure 1: The finished surface is smooth on the nm-scale, the tips are sharpened subject to

Figure 4. Junction plasmon resonances. (a) Raman spectrum of the unprocessed tip (red) shows a carbonaceous residue, which is removed by in situ FDSS (black). (b) Cavity mode resonances after reshaping the tip apex through voltage pulsing: The EL spectrum (black) is recorded at a sample bias of +2.8 V, and current of 0.1 nA. The Raman spectrum of the junction (red trace) shows the same resonance (excited at 532 nm, 5 μW/μm2, 10 s accumulation time). The resonance blue-shifts (blue trace) by 16 nm (419 cm−1), and its intensity halves upon retracting the tip by 7.8 Å from the tunneling set point (0.1 nA, +0.1 V, gap ∼6 Å). (c) Gap dependence of EL spectra: The plasmon resonance blue-shifts by 11 nm as the tip is retracted by 2.6 Å, by lowering the set point current from 2.0 nA (red) to 0.01 nA (blue) at VB = +2.91 V. Figure 3. Assembly for field-directed sputter sharpening. (a) Threedimensional view of the setup used to perform batch FDSS on up to seven tips. (b) Sectional view of the setup: Grounded sections are in white, while biased sections are in dark gray. The GND structure (aluminum) is connected to the grounded vacuum chamber (flange) via the metallic standoffs. The tips are biased through their contact with the holder, which, in turn, is connected to a setscrew and a bias feedthrough (shown in (a)). Ions are shot through narrow holes in the GND structure to sputter the tips. Inset: The GND structure with holes providing individual tips with separate ground surfaces and ion channels. (c) The assembly, made of molybdenum and tantalum, used to perform FDSS in the UHV TERS-STM chamber using existing sputtering housing (blue).

This surface contamination is completely removed by additional FDSS cycles inside the STM chamber, as illustrated by the spectrum in Figure 4a (black trace). The design of the tip holder for in situ FDSS is illustrated in Figure 3c, and operating conditions are given in the supplement. Cavity Plasmons. The cavity plasmon resonance can be observed through electroluminescence (EL) and electronic Raman scattering on the empty junction. EL arises from inelastic scattering of tunneling electrons at the protruding atom of the tip apex, with far-field radiation colored by the junction plasmon.46,47 Raman process is driven by intraband 11395

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Figure 5. TER-sm of a CoTPP monolayer on Au(111). (a) Constant current STM topography showing the CoTPP array (I = 0.1 nA, Vb = 9.8 mV). (b) Simultaneously acquired TER-sm image of the prominent vibrational peak at 1280 cm−1 (integrated between 1244 and 1304 cm−1, 1 s acquisition time per 1 Å × 1 Å pixel, 634 nm excitation with ∼5 μW/μm2 incident intensity). (c) Line profiles along a dashed line in (a) and (b), showing the anticorrelation between the STM (black) and TER-sm (red) maps. (d) Degraded spectrum recorded on one pixel. The images have been low-pass filtered for clarity. Scale bar = 1 nm.

four different molecules share the local-field gradient. This is consistent with recent explicit simulations of TERS driven by field gradients.19 The dominant effect in the anticorrelation between spectral intensity and tip height is the confinement of the cavity mode. The 2 Å variation in the apparent height of the molecule in Figure 5c leads ∼30% modulation in intensity of both vibrational lines and the background void of vibrational lines. The latter is directly associated with the cavity resonance, which on the angstrom-scale is defined by both tunneling junction and conductance of the intervening molecule at optical frequencies. In effect, the observed submolecular spatial resolution of TER-sm derives from confinement of the cavity mode localized at the tip apex. We defer the analysis of modespecific vibrational information contained in the images. It should be noted, however, that the signal is more localized than the probed normal modes. Therefore, the mode specific image (Figure 5b) is at the ultimate spatial resolution of relevance to vibrational spectromicroscopy in molecular matter. This comes at a price. TERS of CoTPP/Au(111) was previously studied under ambient conditions, but without spatial resolution.51 The molecules deposited from the liquid phase appear in ordered and disordered domains. The two domains show very different spectra, and remarkably, neither agrees with the present 80 K spectrum. In effect, the vibrational fingerprint of the molecule is lost. Single Molecule TERS. Richer vibrational spectra can be observed for molecules that do not freeze in a planar adsorption state. In Figure 6a, we show TERS of a single 1,2-di(4-pyridyl) ethylene (BPE) molecule adsorbed on Au(111) at 80 K. Although BPE is mobile on gold at this temperature, a molecule can be trapped at the STM junction. The observed vibrational line intensities fall by ∼50% upon 1 Å retraction of the tip, and the spectrum and cavity mode disappear when the tip is retracted by 17 nm. BPE is a common nonresonant SERS reporter.3 Its surface-enhanced Raman, hyper-Raman, and IR spectra have been previously reported along with the normal modes,52 and we have previously relied on BPE as the reporter in our time-resolved4,53 and polarization selected54 SERS studies on nanodumbbells. Although the observed lines in Figure 6a can be assigned, the line intensities are entirely different from those of the ensemble, and the commonly reported stationary SERS spectra are presented in Figure 6b. The most intense Raman lines of CC stretching modes above 1600 cm−1 seen in Figure 6b are absent in the TERS spectrum of Figure 6a. The most intense TERS line at 990

scattering on Fermi electrons of the metal, which generates a gap-independent background that decays exponentially as a function of Stokes shift48 and a gap-sensitive resonance, which is shown in Figure 4b. The close agreement between EL and Raman spectra and the gap dependence of the resonances (Figure 4b and 4c) indicate that the cavity mode is being seen.49 The red shift of the resonance as the gap is reduced identifies the binding dipolar plasmon formed by the coupling of the tip plasmon with its mirror image in the metallic substrate.47 The spectral shift of the optically detected resonance upon an atomic-scale variation in the STM junction validates the TERS utility of a tip, since it establishes that the nanocavity mode is coupled to the far-field and is the sole source of the observed Raman scattering. FDSS processed tips show this signature, which is absent in unprocessed electrochemically etched silver tips. Although resonances of tips with different cone radii vary, they can be tuned to some extent by reshaping the apex by applying voltage pulses (100 ms, 3−10 V) in the tunneling regime. The resulting shape of the apex in use is unknown beyond its atomic protrusion, which is deduced from the sharpness of topographic STM images. Indeed, cavity modes evolve and tips deteriorate over time and dull after crashing with the substrate. Nevertheless, tip performance can be restored by voltage pulsing. TER-sm. We carry out TER-sm on a two-dimensional (2D) array of Co(II)-tetraphenylporphyrin (Co-TPP) molecules prepared on Au(111) at 80 K. The simultaneously recorded constant current STM topography and TER-sm image are shown in Figure 5a,b. The latter is a spectral slice of the hyperspectral image, consisting of complete Raman spectra recorded on 1 Å × 1 Å pixels (1 s/pixel, I0 ∼ 5 μW/μm2, λ = 634 nm). The Raman spectrum of the flat aromatic molecules, with polarizability dominated by the in-plane modes, is strongly degraded, as seen in Figure 5d. The line spectrum rides over the exponentially decaying Raman response of the metal electrons. It is limited to the cavity resonance window, which cuts off near 1600 cm−1. The integrated intensity under the prominent line at 1280 cm−1, which is a field gradient driven mode localized on the phenyl rings,50 is used to construct the image in Figure 5b. Inspection of the images and the line cuts shown in Figure 5c clarifies that the spatial resolution in the STM, and TER-sm is nearly identical. Remarkably, the line cuts are anticorrelated, and the contrast in the TER-sm (Figure 5b) is inverted relative to the topography (Figure 5a). The TERS signal peaks when the tip is placed between molecules, where phenyl groups from 11396

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cm−1 is not visible in the normal spectra (Figure 6b). With field fixed along the tip z-axis, and Raman restricted to dipolar scattering, the 990 cm−1 can be made to dominate (see Figure 6c) for the molecular orientation shown in Figure 6d. However, the rest of the intensity pattern does not match. As in partially fused nanojunctions,27 the spectrum is dominated by the odd parity modes driven by field gradients. This is a consequence of spectroscopy with confined light, where the local wavevector qi given by the spatial Fourier components of the field reaches submolecular length scales, |qr| ≥ 1.54 Given knowledge of molecular orientation and scattering tensor, the spectra can, in principle, be inverted to reconstruct the local field.27 In practice, this is not a trivial task. Moreover, in this limit, multipolar expansion of the polarizability tensor becomes suspect,19,26 and quantum treatments of the plasmon and molecule on an equal footing will be necessary. This detracts from the aim of vibrational spectromicroscopy, since a given molecule no longer has a single vibrational fingerprint. CO-Terminated Tips. A great deal of structural uncertainty is eliminated in TERS of CO-terminated Ag tips. Now, a single diatomic reporter is aligned along the maximal z-component of the local E-field, with its signal localized to the 1.1 Å bond length of the molecule placed at 2.6 Å from the apex atom.33 In measurements carried out at 5 K on a Au(111) surface, we transfer a single CO molecule to the Ag tip through voltage pulsing. CO is weakly bound on Ag, preferentially attaching to the vertex atom on Ag tips.33 The TERS of the stretching mode of an attached 12C18O molecule is shown in the inset of Figure 7a. The line width is instrument limited (FWHM = 2.6 cm−1), and the integrated line intensity, which reaches count rates R = 103 photons/s, decays as a function of gap distance. The data

Figure 6. Tensorial nature of a single BPE Raman spectrum. (a) TER spectra of a single BPE molecule trapped at the STM junction. The intensity of the vibrational lines and background of the cavity resonance decay upon 1 Å tip retraction, by dropping the STM current from 0.8 nA (red) to 0.1 nA (black), and completely disappear upon retracting by 17 nm (gray). (b) Comparison between simulated Raman spectrum (blue) and SERS of BPE on a gold nanodumbbell (red).55 The molecule, incident, and scattered optical fields are aligned along the x-axis. (c) Comparison between a simulated spectrum of BPE oriented in Euler angles (α = 78°, β = 235°, γ = 187°) (blue) and the TERS in (a) (red). (d) Junction geometry of the simulated spectrum in (c), with incident and scattered fields assumed to be along the tip z-axis. The simulation is performed at the B3LYP/6-31G(d) level using Gaussian 09.56

Figure 7. Chemical speciation of a single Ag atom and CO molecule using a CO-terminated Tip. (a) Decay of the integrated intensity of the CO vibrational line (inset) as a function of gap distance is fitted to an exponential decay (red, τ = 3.55 ± 0.14 Å) and power law (blue, n = 2.07 ± 0.06). Inset: A CO spectrum recorded at 1.2 V. (b) Schematic of the measurement. (c) Topographic image (1.2 V, 0.1 nA) of a Ag atom and CO molecule. (d−g) TERS relayed molecular force microscopy (0.37 V. 1.0 nA): (d) Map of CO stretch frequency obtained via a Gaussian fit. The frequency red-shifts over the Ag atom and blue-shifts over the CO molecule with respect to the Au(111) surface. When integrating the Raman intensity, the silver atom appears in the spectral window (e) 2042−2055 cm−1 while the CO molecule is best mapped in the window (f) 2072−2077 cm−1. (g) A composite image of (e) and (f). Images are low pass filtered and low intensity background noise is masked for clarity. Scale bar = 1 nm. 11397

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ACS Nano can be represented by an exponential decay of 3.6 Å (Figure 7a, red), consistent with TERS determined by the tunneling current of Fermi electrons through the π*-orbital of CO.33 In this limited range of measurement, between 5.5 and 11.5 Å, the decay of the signal can also be fit to a power law, R ∝ z−2.1, as shown by the blue fit in the log−log plot in Figure 7a. Under the quartic law of TERS, the implied local-field scales as EL ∝ R1/4 ∝ z−0.525, in agreement with the recently derived law of enhancement in gap distances, beyond the classical limit of the original derivation.57 The field experienced by the molecule can be obtained from the effective enhancement factor, Ef: Ef =

R = 1013 ηN Ωdσ /d Ω

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assuming the gas phase scattering cross section, dσ/dΩ = 3.3 × 10−31 cm2/sr,34 incident photon flux N determined by the softly focused 634 nm laser to 5 μW/μm2, collection angle Ω = 2.7 sr, and efficiency η = 0.1.41 Accordingly, the field localized on the molecule is EL = E0(Ef)1/4 = 0.01 V/Å, and the signal is confined to a volume of ≤100 Å3, as measured by its vertical decay length of 3.6 Å and lateral resolution in the TER-sm map of ∼5 Å. TERS-mfm is illustrated in Figure 7b. Rather than the nearfield TER-sm of targeted molecules (Figure 5), TERS of the CO terminus on the scanning tip is used to map out intermolecular forces. To illustrate the principles, we consider a single Ag atom evaporated onto the Au(111) substrate (Figure 7b). As seen in the STM topography (Figure 7c), an additional CO molecule is bound to the atom and locked in orientation. This can be more clearly seen in the TERS-mfm image of Figure 7g, which is a composite constructed by mapping the CO vibrational line intensity in two different spectral windows. At the applied junction bias of 0.37 V, the 12 18 C O frequency on Au peaks at 2068 cm−1, and the redshifted window (2042−2055 cm−1) yields the image of the Ag atom (Figure 7e), while the blue edge of the CO peak (2072− 2077 cm−1) images the CO molecule (Figure 7f). Chemically speciated images with atomic resolution are obtained in the different spectral windows. At the 5.5 Å gap fixed by the STM set point, electrostatic forces dominate the interaction. Ascribing the spectral shifts entirely to the first-order Stark effect with a tuning rate of 150 cm−1/V/Å,33 and assuming fields due to point charges, E(r) = δe/r2, the spectral shifts identify a positively polarized Ag atom with a net charge of +0.2e relative to Au, and a negatively charged CO molecule with a net charge of −0.1e relative to Au. The unusual binding geometry of the CO suggests that it is dominated by the interfacial dipole at the adatom-Au junction and the quadrupole of CO. In addition, the polarization of the surface electrons of Au in response to the Ag adatom can be seen. This is illustrated by the discretely sampled spectra as a function of distance from the Ag atom, recorded 10 Å apart (see Figure 8a) with an integration time of 30 s. The intensity of the CO vibration is anticorrelated with the topographic image of herringbone structure (Figure 8b): It is enhanced on the FCC domain compared to the HCP domain. The frequency shifts monotonically as a function of distance (Figure 8c), by 1 cm−1 over 40 Å, corresponding to a potential drop of 0.2 mV/Å. The potential drop relative to gold, ΔV, measures the local workfunction, which is affected up to at least 50 Å away from the adatom. While the analysis can be refined along lines presented for molecular force imaging with CO-terminated tips,58 the extracted images of atomic polarization illustrate that in

Figure 8. (a) Line cut across the STM topography (inset) showing the height of the silver adatom and the corrugation of the Au(111) surface reconstruction (0.1 nA, 0.25 V). Integrated Raman intensity (b) and frequency of the CO peak (b) obtained via a Gaussian fit and acquired at specific points along the line cut (30 s integration, 5 μW/μm2, 634 nm). The y -axis in (c) is broken to highlight the standard errors.

addition to structure, electrostatic potentials can be mapped out as in atomically resolved Kelvin probe force microscopy.31 More generally, the vibrational frequency shift is due to the sum of intermolecular forces, F = −∂V/∂r, and force gradients, k′ = ∂2V/∂r2, acting on the anharmonic CO bond. The different ranges of electrostatic, dispersive, and Pauli exclusion forces determine the dominant interaction probed at a given distance and, hence, determine molecular shapes observed in molecular force microscopy. The advantages of a molecular transducer versus mesoscopic cantilevers derive from the dramatic increase in frequency of operation (1014 Hz versus 106 Hz) and substantial localization of the measured force, determined by 11398

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ACS Nano the vibrational amplitude of the molecular oscillator, Δr = |⟨v = 1|r|v = 0⟩| = 0.034 Å. Although the anharmonic CO stretching frequency responds to local forces through the Stark effect,59 for the sake of comparison with mechanical harmonic cantilevers, consider the figure of merit for resolvable force: 2k k′Δr = |⟨v = 1||r ||v = 0⟩| Q

Additional experimental details can be found in the Supporting Information.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06022. Computer-assisted design of the batch production assembly and operating conditions of the ex situ and in situ FDSS process (PDF)

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determined by the quality factor, Q, which reflects the accuracy with which frequency shifts can be measured. The spectral domain TERS measurement has the multiplexing advantage, where the uncertainty in frequency is determined by the goodness of the spectral fits and therefore by the signal strength. As indicated by the error bars of the discrete measurements in Figure 8c, δν0̅ ∼ 0.1 cm−1, giving a quality factor Q = ν̅0/δν0̅ = 2 × 104. Using the stiffness of the 12C18O stretching mode, k = 1.7 × 103 N/m, the forces of ∼0.5 pN can be measured according to eq 4. This favorably competes with advanced quartz cantilevers used with functionalized tips.60

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Joonhee Lee: 0000-0003-4685-3511 Vartkess Ara Apkarian: 0000-0002-7648-5230 Author Contributions

V.A.A. and J.L. conceived the idea and planned the project. N.T. and J.L. designed and constructed the FDSS batch production system. N.T. and J.L. performed experiments and recorded data. N.T, J.L., and V.A.A. cowrote the manuscript.

CONCLUSIONS TER-sm offers spectromicroscopy on the angstrom-scale. However, its implementation rests upon the fabrication of tips with reliable performance. FDSS processing of electrochemically etched and polished silver-tips fulfills this requirement. The process sharpens, smoothens, and cleans silver tips and demonstrably limits the far-field scattering to the sole nanocavity mode between the atomically terminated tip and substrate. Localization of the field, rather than enhancement, is the crucial consideration in single molecule sensitivity and spectromicroscopy. Tips with reliable performance accelerate discovery, as previewed by the implementations used to validate the FDSS processed tips. We show that finely tuned tips can lead to large field enhancement by confining the signal to picocavities of ≤100 Å3. A consequence of this extreme confinement is that TERS is a near-field effect, where planewave selection rules break down, and scattering driven by field gradients dominate the observable spectra. The contrast inversion between the STM and TER-sm images is a consequence, which affects spatial images. True spatial imaging is provided through TERS-mfm using CO-terminated Ag tips, which image molecular structure, forces, and local potentials with atomic resolution. Of particular value is the mapping of local workfunctions through the CO Stark shift, which gives access to charges and energetics of surfaces with atomic resolution.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This research was supported by the grant of NSF Center for Chemical Innovation dedicated to Chemistry at the SpaceTime Limit (CHE-1414466). We gratefully acknowledge Dr. Laura Rios for her contribution to the fabrication of the FDSS system, Kevin T. Crampton for recording SERS spectra of BPE, Angel Nunez for producing electrochemically etched silver tips, and former Managing Director Dr. Andre A. Schirotzek for his support of this project. REFERENCES (1) Nie, S. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, 1102−1106. (2) Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L.; Itzkan, I.; Dasari, R.; Feld, M. Single Molecule Detection Using Surface-Enhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667−1670. (3) Le Ru, E. C.; Etchegoin, P. G. Single-Molecule Surface-Enhanced Raman Spectroscopy. Annu. Rev. Phys. Chem. 2012, 63, 65−87. (4) Yampolsky, S.; Fishman, D. A.; Dey, S.; Hulkko, E.; Banik, M.; Potma, E. O.; Apkarian, V. A. Seeing a Single Molecule Vibrate through Time-Resolved Coherent Anti-Stokes Raman Scattering. Nat. Photonics 2014, 8, 650−656. (5) Fleischmann, M.; Hendra, P. J.; McQuillan, A. J. Raman Spectra of Pyridine Adsorbed at a Silver Electrode. Chem. Phys. Lett. 1974, 26, 163−166. (6) Jeanmaire, D. L.; Van Duyne, R. P. Surface Raman Spectroelectrochemistry. J. Electroanal. Chem. Interfacial Electrochem. 1977, 84, 1−20. (7) Hayazawa, N.; Inouye, Y.; Sekkat, Z.; Kawata, S. Metallized Tip Amplification of near-Field Raman Scattering. Opt. Commun. 2000, 183, 333−336. (8) Pettinger, B.; Picardi, G.; Schuster, R.; Ertl, G. Surface Enhanced Raman Spectroscopy: Towards Single Molecule Spectroscopy. Electrochemistry 2000, 68, 942. (9) Stöckle, R. M.; Suh, Y. D.; Deckert, V.; Zenobi, R. Nanoscale Chemical Analysis by Tip-Enhanced Raman Spectroscopy. Chem. Phys. Lett. 2000, 318, 131−136.

METHODS The instrument used in the present measurements has been described previously. Briefly, it consists of an ultrahigh-vacuum (base pressure 4 × 10−11 Torr) cryogenic STM equipped with a parabolic collector (collection solid angle 2.7 sr), with focus precisely aligned to the tip apex by imaging EL from the tip.41 For Raman measurements, the tip−sample junction is illuminated at 45° with a single-mode 634 nm diode laser, and the spectra are acquired with a Princeton Instruments SpectraPro 2300i spectrograph with a LN2-cooled CCD. For the tip preparation, two rounds of FDSS processing are required. First, FDSS processing was done in an external high-vacuum chamber with a beam energy of +2 keV (chamber backfilled with Ar gas at ∼5 × 10−5 Torr) and tip bias of +150 V. Second, FDSS processing was done in the main UHV STM chamber with a beam energy of +1 keV (chamber backfilled with Ne gas at ∼5 × 10−5 Torr) and sample bias of +80 V. Tips were sputtered with ∼4 × 1017 ions cm−2 during each treatment. 11399

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