Tip-Enhanced Raman Spectromicroscopy of Co(II) - ACS Publications

Subscriber access provided by LAURENTIAN UNIV

Article

Tip-Enhanced Raman Spectromicroscopy of Co(II)Tetraphenylporphyrin on Au(111): Toward the Chemists' Microscope Joonhee Lee, Nicholas Tallarida, Xing Chen, Pengchong Liu, Lasse Jensen, and Vartkess Ara Apkarian ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b06183 • Publication Date (Web): 04 Oct 2017 Downloaded from http://pubs.acs.org on October 5, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Nano is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Tip-Enhanced Raman Spectromicroscopy of Co(II)-Tetraphenylporphyrin on Au(111): Toward the Chemists' Microscope

Joonhee Lee1†, Nicholas Tallarida1†, Xing Chen2, Pengchong Liu2, Lasse Jensen2* and Vartkess Ara Apkarian1*

1. Department of Chemistry, University of California, Irvine, Irvine, California, 92697, United States 2. Department of Chemistry, Pennsylvania State University, University Park, Pennsylvania 16802, United States

ABSTRACT: Atomically terminated, nanoscopically smooth silver tips effectively focus light on the Å-scale, allowing tip-enhanced Raman spectromicroscopy (TER-sm) with single molecule sensitivity and submolecular spatial resolution. Through measurements carried out on Cobalt-tetraphenylporphyrin (CoTPP) adsorbed on Au(111), we highlight peculiarities of vibrational spectromicroscopy with light confined on Å-scale. Field 1

ACS Paragon Plus Environment

ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

gradient driven spectra, orientational fingerprinting, sculpting of local fields by atomic morphology of the junction, are elucidated through measurements that range from 2D arrays at room temperature to single molecule manipulations at 5 K. Notably, vibrational Stark tuning within molecules, reflecting intramolecular charge distributions, becomes accessible when light is more localized than the interrogated normal modes. The Stark images of CoTPP reveal that it is saddled, and the distortion is accompanied by charge transfer to gold through the two anchoring pyrroles.

Keywords: tip-enhanced Raman spectroscopy, spectromicroscopy, confined light, Stark shift, field-gradient driven Raman, quadrupolar scattering, scanning tunneling microscopy

To the extent that molecules can be regarded as networks of balls (atoms) and springs (bonds), they can be characterized by the frequencies of their internal motions. This is the realm of vibrational spectroscopy, which remains one of chemists' sharpest tools for fingerprinting molecules and characterizing their inner workings. As such, the ability to record vibrational spectra of individual molecules, by sounding off individual atoms within molecules, can be regarded as the chemists' ultimate microscope – the chemiscope for short. The task is challenging for several reasons. The universally applicable method for recording vibrational spectra is based on the Raman effect, which involves the excitation of molecules with light and recording the spectrum of re-radiation by the vibrating molecule. The effect is feeble. To obtain a useful level of signal, of 103 photons/s, it is necessary to irradiate a molecule at intensities of terrawatt/cm2. Moreover, 2


Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

photons are much larger than atoms. The Abbe principle recognizes that the wavelength of the photon limits the focus of the optical microscope to a spot of ~ λ/2, with λ ∼ 500 nm in the visible. To reach the 0.1 nm size scale of the atom, photons must be squeezed down to a thousandth of their size. These challenges are met by resorting to plasmonic nanoantennas.1 The nanoscale analog of the antenna used by Hertz, consisting of a pair of metallic nanospheres separated by a gap of ~ 1 nm, is prototypical.2 By coupling light to surface charge oscillations (plasmons) of the dipolar nanoantenna, electromagnetic fields are effectively focused at the nanojunction. The large field enhancement due to confinement is sufficient to record Raman spectra of individual molecules,3–5 through the surface enhanced Raman scattering (SERS) effect, which was discovered nearly four decades ago.6 Although there has been an explosion of developments and applications of SERS, 7 the fundamentals of the effect remain the subject of active research. The primary challenge is now recognized to stem from the atomistic detail in junction morphology that shapes the local fields,8 therefore the optical response that determine the SERS signal.9 This is precisely the sensitivity required to image the internal motions of individual molecules; at the same time, spectroscopy with confined light is poorly understood and characterization or control of nanojunctions with atomistic precision is a challenge. Resorting to the more controllable junction of a scanning tunneling microscope (STM), under the rubric of Tip-enhanced Raman spectroscopy (TERS),10 has led to accelerated discovery through TER-spectromicroscopy (TER-sm).11,12 Noteworthy among these was the realization of TERS with submolecular spatial resolution13 – an effect that was not anticipated by then standard classical models of tip plasmons.14 In the sub-nanometer 3


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

junction gaps necessary to reach single molecule sensitivity, quantum effects cannot be dismissed.15–17 The dynamics of optically driven electrons, which tunnel across junctions, must be explicitly treated to describe the relevant local fields.18,19 Atomistic electrodynamics simulations,20 coupled with quantum chemical treatments of the molecule and the scattering process,21,22 are necessary for reliable descriptions. Moreover, given the strong fields, field and matter states can become entangled.23,24 The quantum electrodynamics of plasmonic cavities, which confine photons, electrons and the molecule under study, provides a frontier in science that must be mastered along the way to develop the chemiscope. We use cobalt(II)-tetraphenylporphyrin (CoTPP) to carry out a joint experimental and theoretical study of TERS at the STM junction of a silver tip and an atomically flat Au(111) substrate. CoTPP has been previously investigated through both STM,25, TERS,26 and theory.27 It saddles upon adsorption, whereby the phenyl groups twist and tilt successive pyrroles up and down (see Fig. S1 in supporting information). The saddled molecule contains the necessary geometric features to scrutinize principles of vibrational spectromicroscopy with confined light. As an important milestone, we show room temperature TER-sm at the same resolution as STM, but now with vibrational hyperspectral information. The spectra are thermally activated. At 80 K, they degrade to field gradient driven modes, which Stark tune. This allows TERS-relayed vibrational Stark microscopy to image intramolecular charge distributions, with prior examples of such information extracted using atomically resolved KPFM.28,29 At 5 K, we manipulate individual molecules to highlight the tensorial nature of single molecule spectroscopy with confined light, and we show that the spatial resolution of TERS reaches the 1 Å4


Page 5 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

limit. The range of observed effects can be rationalized through simulations using the atomic electrodynamics model20,30 to describe confined fields; and the dressed tensor formalism,31 to treat spectroscopy with confined light in terms of local field/field gradient enhancement driven through derivatives of electric dipole-dipole, electric dipolequadrupole, and electric quadrupole-quadrupole polarizabilities. Notwithstanding the necessary approximations, the combined experiment and theory study yields deep insights on the salient features and prospects of TER-sm.

Results & Discussion TER-sm At room temperature, CoTPP forms an ordered monolayer on Au(111), as seen in the simultaneously recorded STM and hyperspectral TERS images in Fig. 1a-c. The latter consist of complete spectra recorded on each 1.4 Å × 1.4 Å pixel with an acquisition time of 1 s/pixel. The TERS spectrum (Fig. 1d) disappears upon 1 nm retraction of the tip (Fig 1e). The junction gap dependence of the signal intensity can be approximated as exponential, with a decay constant of 3.5 Å. The scatter in the retraction data is due to motion of the molecule, both thermal and lift-off by the van der Waals attraction to the tip.32 The spectrum (Fig. 1d) consists of three distinct components: the exponentially decaying background of electronic Raman scattering on the metal at shifts larger than 1700 cm-1 (better shown in Fig. 6c), a flat background with cut-off near 1600 cm-1, and a set of thermally broadened molecular vibrational lines within this window. The presented spatial maps are the spectrally integrated TERS intensity (Fig. 1b) and the mode specific map of the vibration at 1576 cm-1 (Fig. 1c). The mode specific vibrational images of all 5


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

eight observed lines are provided in Fig. S3 in SI. They show subtle variations with respect to the STM topography. Inspection of the images in Fig. 1a-c is sufficient to infer that the spatial resolution in the TER-sm is nearly the same as the STM.

6


Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 1. Vibrational spectromicroscopy at 300 K. (a) Topographic image (57×57 Å2) recorded at 10 pA, +0.15 V. (b) Simultaneously recorded Raman map integrated over the full window (300-1700 cm-1). (c) Mode-specific map of the highest frequency vibration (integrated over the 1550-1600 cm-1 window colored by light pink and blue bands). (d) Comparison between TERS of CoTPP on Au(111) and simulated spectrum. Calculated 7


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

normal modes nearest to the color-coded windows are illustrated accordingly. (e) Exponential decay of TERS intensity integrated over the 700-1650 cm-1 window. The dashed line indicates the level for zero intensity. Images are filtered due to the thermal noise at 300 K (raw images are presented in Fig. S4 in SI).

A satisfactory reproduction of the spectrum is possible under the assumption of electric field polarized along the tip z-axis, and molecules tilted out of plane. The best match is obtained upon tilting the molecule out of the substrate plane by 32° relative to the surface normal, followed by 42° rotation in the tilted molecular plane. The field active normal modes dominate the spectrum, and at room temperature, the thermal population of molecules that tilt out of the plane is observable.33,34 The observed vibrations are normal modes that are principally localized on the phenyl rings, as illustrated in Figure 1. Nevertheless, there are subtle spatial variations in the mode specific images (see SI). The maps of the 1252 cm-1 (1220 cm-1 calculated) and 1576 cm-1 (1592 cm-1 calculated) mode are generally anti-correlated with the location of the molecules, while the map of the mode at 1092 cm-1 (absent in the calculations) is closely correlated. The 1550 cm-1 is the only mode that does not involve phenyl vibrations. It is mainly localized on the pyrroles, which in the saddled molecule pull out of the porphyrin plane.

8


Page 9 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 2. Raman signal of confined cavity mode. (a) Overlay of the STM topography (black contours) and the cavity mode map (red contours, integrated over the cyan band in Fig. 1d, 320-500 cm-1). (b) Line profiles of topography (black) and cavity map (red) taken along the two diagonal lines in (a), which are displaced by 4 Å.

Confined field, photon and signal The enhanced background without vibrational peaks tracks the STM topography with high fidelity. This is clarified by the superimposed contour maps of the STM topography and the background (cyan band in Fig. 1d) map in Fig. 2a, and associated line-cuts shown in Fig. 2b. The background is assigned to the cavity mode, namely, the confined optical field in the junction. Its intensity perfectly tracks the STM topography, but with a diagonal displacement of ~4 Å. The apparent displacement of the optical signal from the terminal tip Ag atom by the dimensions of one atom, provides a direct measure of the lateral confinement of the TERS signal to an area of ~ 16 Å2. Together with the decay length of the intensity with the vertical tip retraction, a signal confinement volume of ~ 100 Å3 can be inferred. It is tempting, though not sensible to assign this volume to confinement of a photon, since the local field that would arise from one such photon, 9


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

= ℏ/2 = 1.2 /Å , would ionize any molecule. The local field can be estimated from the observed scattered intensity and the Raman cross section of the molecule to be hundred times smaller, ~5×10-3 V/Å. Only a small fraction by volume, 104

, of one photon is ever present at the junction. The observed localization of the signal is consistent with the field confinement

expected from the atomically terminated tip presented in Figure 3. We use an icosahedral gold nanoparticle consisting of 2057 atoms against a planar gold slab to calculate the local fields. The dimensions and gap independent resonance of the structure, which matches the experiment, 2.32 eV (534 nm), is shown in Fig. S5. The strongest and most confined near field is obtained at a slightly lower energy than the plasmonic resonance.21 At the depicted junction gap of 5.5 Å, which corresponds to the experiment, the hot spot ~ 6 Å (Fig. of the Au-atom terminated tip has a lateral width at half maximum of /

S5b), with associated field gradients that are more tightly confined. Taking into account the quartic dependence of the signal on the local field, the predicted lateral confinement of the field-induced signal is ~ 3 Å, in agreement with the experiment. The signal is localized down to the size of an atom, nearly four orders of magnitude sharper than allowed by the Abbe principle of optics, and significantly smaller than the spatial extent of vibrational normal modes that extend over the molecule.

10


Page 11 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 3. Sculpted junction fields. Cross-sectional views of near field and field gradient distributions are illustrated for a flat gold tip (a-b), atomically terminated tip (c-d), and CO-terminated tip (e-f).

Silence and activation of TERS with CO Upon cooling to 80 K, the molecular TERS signal disappears. Due to the improved thermal stability and planarity of the cold molecules on the substrate, sharp STM images of the molecular array can be recorded, as in Figure 4a. However, the simultaneously collected TERS images are completely featureless. Indeed, if the scattering was electronically resonant via the Q-band, then cancellation of the in-plane transition dipole via its image in the gold mirror could explain the total silence. However, the excitation wavelength at 634 nm is removed from the ← transition of CoTPP, which occurs at 530 nm (Fig. S2). Flattening the molecule from its room temperature tilt into planarity 11


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

reduces the signal by a factor of 5-10, but this does not explain the complete silence of the spectra given the dynamic range of detection. In the depicted junctions (Fig. 3c-d) the hot spots of field and field gradient miss the aromatic macrocycle, which remains in the electron spill-out region where the electric field is screened. Not included in the model is the Schockley surface state (SS) of gold,35 which provides a nearly free 2D electron density, famously visualized by the construction of quantum corrals.36 The coupling of the slouched molecule with the SS is suspected as the origin of the silence, which would also explain the prior TERS imaging of planar porphyrins with bulky substituents that raise the macrocycle plane.13 Consistent with this picture, the vibrational spectra are activated upon introducing a 2D lattice gas of mobile molecules; ostensibly, by destroying the SS, or by lifting molecules out of the zone of silence. Upon introducing CO molecules, which are mobile on gold at 80 K but eventually desorb, complete vibrational spectra of CoTPP emerge. This is illustrated in the simultaneously recorded images of Fig. 4b,c: The STM image is streaky, due to the CO gas; and the simultaneously recorded TERS data show no correlation with the STM image or any spatial features. However, the bright streaks that appear sporadically during the scan carry the sharp and rather complete vibrational spectrum of the cold molecule (Fig. 4d, trace 1). The activated spectra are stable when probed with a stationary tip, as illustrated by the TERS trajectory in Fig. 4e. The lines are instrument limited to FWHM = 6.7 cm-1, with fluctuations in center frequency of order 1 cm-1. The large dynamic range of the spectra identifies over 30 vibrational lines, with direct lineage to the room temperature spectrum (see traces 3 and 4 in Fig. 4d). Their single molecule TERS nature is established by the retraction dependence – the spectrum disappears upon 5 Å retraction 12


Page 13 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

of the tip (Fig. 4d, trace 5). That the intercalated CO is only loosely attached to the junction or to CoTPP, is demonstrated by the trajectory in Fig. 4f: The activated spectrum is lost as soon as the tip is scanned. Despite the high S/N ratio of the activated spectra, we do not observe the characteristic CO vibrational line, which we see when it binds to the apex at 5 K (see below). The possible role of CO as a local field spoiler is considered by comparing the field of an empty cavity and one in which a single CO molecule or a single Au atom is introduced (Fig. 3c-h). The field lines and field gradients are sculpted quite differently in each case. Their coexistence would generate significant inhomogeneity on the molecular scale and destroy the extended SS.

Figure 4. CO as a cavity spoiler. (a) Topography of CoTPP monolayer at 80 K with resolved phenyl groups. (b) Degraded topography of CoTPP due to CO lattice gas on the 13


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

surface. (c) Simultaneously recorded Raman map with flashes as CO molecules pass by. Each pixel is the Raman intensity integrated from 600 to 1650 cm-1. (d) Extracted spectra from the map (c), 1 and 2, highlight the flashes. Acquisition time is 1 s. Spectra 4 and 5 are recorded with a stationary tip engaged and retracted by 5

, respectively (10 s

integration). The 80 K spectra 1 and 4 agree with the spectrum acquired at room temperature, 3. (e) TERS trajectory while the tip is stationary. (f) Disappearance of Raman signal upon initiating a scan.

Alternatively, the spectra may be activated by chemical binding of CO. As many as two CO molecules bind to the central Co atom of CoTPP, with two possible binding geometries: axial binding on opposite sides and bridge binding with both carbonyls on the same face. The optimized structures are shown in Fig. 5, along with predicted spectra, generated by dressing molecular polarizabilities with field and field gradient and adjusting their orientation. (Details in Table S1 of SI). The comparison between experiment and simulation is informative. Although the direct lineage between the 80 K and room temperature bare molecule is established, the best agreement in Fig. 5 is with the axial dicarbonyl. The contradiction illustrates that in the single molecule limit, intensity patterns in spectra can be more strongly influenced by orientation than chemical structure. Here, the axially bound CO leads to tilting the molecule and raising its slouch. As a result, the in-plane normal modes are enhanced (e.g., 1342 cm-1 mode) relative to bare CoTPP and bridging dicarbonyl. The calculations clarify that the local field dominates the spectra ( /∇ = 14 Å). Either geometry could in principle lift the macrocycle out of the zone of silence. 14


Page 15 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 5. CO activated spectrum at 80 K and simulations assuming (a) bare Co-TPP and its dicarbonyl with (b) axial and (c) bridge binding. The spectra are dominated by field contributions.

Vibrational Stark microscopy - polarization of the pyrroles TERS microscopy, albeit with a degraded vibrational spectrum, becomes possible once the CO gas subsides. The simultaneously recorded STM and TERS images are shown in Figure 6a-b along with the observed spectrum in Fig. 6c. An adequate reproduction of the spectrum is obtained assuming quadrupole-quadrupole scattering in extremely confined light, /∇= 1.4 Å. Note that the contrast in the TERS images of Fig. 6 is inverted. The TERS intensity is largest when the tip traces the metal intervening the molecules. This is in part due to the conductivity of the junction, which controls field confinement and it is consistent with field gradient driven scattering, which has previously been demonstrated 15


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

to be more efficient when the tip is placed outside the molecule.21 Despite the weak signal, spatially well-resolved images can be obtained by mapping out the Gaussian fit to the vibrational line at 1270 cm-1, with advantage analogous to that of super-resolution in fluorescence microscopy.37 A given line now generates two maps, one of intensity and one of frequency shift, as shown for the magnified area scan in Fig. 4f-g. The spatial detail in the frequency shift image is significantly higher than in the STM. The intensity map shows that the signal is peaked in the intermolecular pockets where phenyl groups converge. The vibrational frequency shift arises from the Stark effect, due to the local electrostatic field, F, sampled by the vibration. The effect is first order in dipolar transitions of odd (u) symmetry, Δ = ∆/ℎ, second order in strictly Raman active modes of even (g) symmetry, Δ = ∆ /2ℎ.38 The large vibrational shift of 10 cm-1 observed across the image, identifies an odd transition that scatters into the far field through the local field gradient. We show in Fig. S7 in SI that the 1270 cm-1 mode becomes the main spectral line when |∇/| ~ 2. The large field gradients alter the traditional Raman selection rule so that quadupolar scattering dominates. Since the measurement is carried out at a small bias, Vb = 10 mV, the source of the electrostatic field is dominated by the local contact potential. Due to the difference in workfunctions, a contact potential of 0.57 V can develop between Au (111) and Ag tip if we assume [111] termination.39 Stark tuning of a mode within the molecule implies variation of charge density, therefore potential, inside the molecule. The largest frequency shift, 10 cm-1 relative to gold, occurs on the raised pyrroles of the saddled molecule. The lower pyrroles appear inequivalent due to the extent of their contacts to gold, while the cobalt center appears at an intermediate potential. Clearly, the molecule is 16


Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

charged; evidently, by electron transfer to gold through the anchoring pyrroles. These findings agree with the recent theoretical analysis of CoTPP on gold, which predicts nearly 1 electron transferred to gold, almost entirely from the anchoring pyrroles.27 In effect, through vibrational Stark tuning, super-resolved TER-sm maps out polarization and submolecular charge distributions. An example of such a map was produced through Kelvin-probe force microscopy using CO terminated tips.28 There, the force neutrality point as a function of applied potential is used to map the intramolecular charge distribution. Here, a normal mode of the targeted molecule is probed with the bare tip, with spatial resolution more localized than the mode itself (see Fig. 2g).

Figure 6. Field gradient driven TER-sm and intramoleculear Stark imaging at 80 K. (a) STM topography of CoTPP lattice at 0.1 nA, 9.8 mV. (b) Raman image anti-correlated with topography produced by integrating 1270 cm-1 mode intensity. (634 nm laser at 5 uW/um2, 1 sec accumulation per pixel). (c) Degraded TERS spectrum. (d) Background17


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

subtracted experimental spectrum and simulation. (e) Zoomed-in topography. (f-h) Super-resolution TER-sm. The 1270 cm-1 mode is fitted to a Gaussian (h), and the extracted intensity and frequency shift are mapped in (f) and (g), respectively. The high pyrroles of saddled CoTPPs are resolved in the frequency map. (h) Line profiles from two pixels in (g). Scale bars are 1 nm long. Images are filtered due to invasive imaging conditions. Raw images are presented in Fig. S6 in SI.

Isolated molecules on terraces, at 5 K The governing principles of TER-sm are tested through measurements on isolated single molecules immobilized at 5 K on the Au(111) substrate. The manipulation sequence on the flat terrace is summarized in Fig. 7a-c. In the scanned area we see three molecules, but TERS is silent. When the tip is placed on one of the phenyl groups, we only see the electronic Raman continuum of the tip, void of molecular lines (orange trace in first panel). Upon pressing on the phenyl ring (VB = -2 mV, I = 4 nA), a rather complete vibrational spectrum emerges (blue trace).

The spectrum remains stationary as we

translate the tip, indicating a molecule snagged by the tip. After measuring the retraction dependent sequence of spectra (middle panel), we repeat the same area scan (Fig. 7b). The distorted image of the remaining molecules confirms that the missing Co-TPP molecule is firmly attached to the tip apex. Upon applying a voltage pulse, the molecule is dropped, and verified to be the intact CoTPP (Fig. 7c). In this manipulation, the CoTPP spectrum is replaced with the single vibrational line of CO at 2065 cm-1. Now the tip is terminated with a CO molecule, which also explains the sharper contrast in topography than in the case of the bare tip (compare Figs. 7a and 7c). Several observations are 18


Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

noteworthy about the gap dependence of the spectra of the molecule attached to the tip apex. The entire spectrum disappears upon 2.4 Å retraction (Fig. 7d), even though the molecule remains on the apex. While the vibration localized on the phenyl rings at 807 cm-1 decays, the pyrrolic mode at 1517 cm-1 increase in intensity during the first 1 Å retraction, and both decay exponentially over the next ~ 1 Å retraction (Fig. 7d). The mode selective variation in intensity arises from the relative orientation of molecule to the local field. In the simulated spectra (Fig. 7e), we employ a vertically oriented free CoTPP, and |/∇| < 0.2 Å. The field gradients play the key role in this case, which again indicates that the field gradients break the traditional Raman selection rules. However, when the molecule is lifted away by a few Å, TERS dramatically changes. A larger ratio of field and field gradient reproduces the spectrum at 2.13 Å, indicating that the field contribution becomes increasingly dominant during the tip retraction.

19


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

Figure 7. TER spectra upon transferring a CoTPP molecule to the tip at 5 K. The common set point for topographic images (104×104 Å2) is 0.1 nA and +0.485 V. (a) First image of the transfer sequence. The Ag tip is placed on the phenyl ring denoted by the asterisk (*). The TER spectrum appears upon approaching the molecule by changing the 20


Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

set point from 1 nA, +0.485 V to 4 nA, -2 mV as demonstrated at the beginning of the spectral sequence denoted by * as well. (b) Image scanned after picking up a CoTPP molecule. The disappearance of molecule is marked by a dashed circle. With molecule held at the tip apex, TER spectra are recorded as the tip is incrementally retracted and brought back to tunneling position. (c) Image taken after dropping the CoTPP by applying a -3 V pulse. In the process, the porphyrin is accidentally exchanged with a CO molecule as identified by the TERS of C-O stretch and enhanced resolution of the image. The landing position is displaced from the position where the pulse was applied (denoted by x), which shows the CoTPP was adsorbed slightly off the apex and possibly tilted. (d) Gap dependences of two vibrational peaks at 807 and 1517 cm-1, respectively. The peak intensities are obtained by integrating over shaded windows in the spectral sequence. The exponential fit to the decay of 808 cm-1 peak intensity yields 2.2 Å in decay length. (e) The simulated spectra compared with the experimental retraction dependence.

Å-scale resolved tensorial TER-sm Molecules isolated on terraces, at 5 K, cannot be imaged. They are displaced, or picked up by the tip, at the junction gaps required to see the TERS signal. In contrast, CoTPP molecules at step edges are strongly bound, and remain stationary under harsh junction conditions (4 nA, -10 mV). The stationary molecules allow the direct demonstration of the spatial resolution in TER-sm governed by field gradients. Measurements on molecules decorating a step edge are presented in Fig. 8. We zoom-in on the STM identified three phenyl groups from two different molecules that straddle the edge (Fig. 8a-c) and record hyperspectral images in constant height mode, enabled by the stability of 21


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

the junction at 5 K. The spectra observed on the three phenyls are displayed in Fig. 8e. The mode specific map of the shaded spectral window in Fig. 8d shows all three phenyls. The TERS signal on P2 arises from a 1 Å × 1 Å area, localized on the right side of the topographic image of P2 (see contour lines). The image of P3, which is closer to the tip, is larger; accordingly, the TERS signal arises from a somewhat larger area, ~2 Å × 2 Å, now localized on the left side of P3. The spatial resolution is atomic in scale and the excitation is tensorial; it depends on the orientation of molecule and confined field. Had we mapped the modes at 1500 cm-1, only P3 would have been imaged. This captures the excitement and challenge of vibrational spectromicroscopy with atomistically confined light.

22


Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

Figure 8. Hyperspectral imaging on the Å-scale, of CoTPP molecules adsorbed at a step edge. (a) Topography of molecules decorating a step edge (66×66 Å2). (b) Close-up of the area marked by the yellow box in (a). White dashed lines trace two CoTPP molecules that straddle the edge, with a pair of phenyls each on upper and lower terrace (highlighted by black dashed lines).(c) Close-up of the area indicated by the yellow oval in (b), note the scale bar. The common imaging condition for (a-c) is 0.1 nA and +0.48 V. (d) Raman map of the same area as in (c) recorded in constant height mode (4 nA, -10 mV). (e) 23


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

Spectra recorded on the three phenyl groups (P1, P2, and P3) of two different molecules. The shaded band is the spectral window used in the TERS map in (d).

Conclusions The reported measurements were selected to underscore the promise of TER-sm as the chemist's ultimate microscope. We directly demonstrate that the method can yield vibrational spectroscopy with Å-scale spatial resolution, significantly sharper than the extent of molecular normal modes, therefore at the ultimate limit of relevance to molecular matter. We have shown that cavity spectra alone lead to imaging of molecular structure with resolution comparable to STM. Beyond structure, the inner workings of molecules are accessible through vibrational Stark microscopy. Befitting a chemist's microscope, TER-sm provides structure, bond strengths, and charge distributions with submolecular resolution. In the specific example of CoTPP considered here, we show that the saddling of the molecule on Au(111) is accompanied by charge transfer through the pyrroles in contact with the metal, the central cobalt atom does not participate in the transfer, and the molecule and the surrounding gold atoms are polarized.27 The exposition also highlights the challenges that must be overcome to develop a quantitative tool with analytical utility. Spectroscopy with confined light does not obey standard selection rules. A propensity rule previously noted, is the silence of cold molecules that lie flat on metallic surfaces.40 Here we suggest that this arises from coupling to the SS. They can be activated by lifting or tilting with the tip using large tunneling currents near zero-bias, and more interestingly, the spectra can be activated by the introduction of a TERS-invisible mobile 2D gas. In the present, rather complete and 24


Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

sharp vibrational spectra of individual CoTPP molecules have been obtained, with high S/N ratio. Nevertheless, spectral interpretations are nontrivial, since multipolar scattering driven by field gradients dominate spectroscopy with confined light. And in the absence of orientational averaging, the tensorial nature of scattering makes it sensitive to 3D orientation of the targeted sample and the cavity confined fields.41 These are ultimately advantages, given atomistically characterized and controlled junctions. It may be recognized that with simple molecular reporters attached to a substrate, TERS may be used to characterize tip morphology. Alternatively, with a simple reporter molecule attached to the tip, TERS of the tip terminus can be used to probe molecular matter. The CO terminated tip, which we encountered here, is particularly valuable in this regard, since it can act as a transducer of electrostatic forces with atomic resolution, as already demonstrated in our validation of TERS-bright silver tips.42 Quite clearly, image quality and resolution will be directly related to the signal strength and acquisition speed, and significant improvement over the present can be expected through tip enhanced stimulated Raman spectroscopy, as already demonstrated in related work.43

Methods The instrumentation used in the current studies has been described previously.40 We use a cryogenic, ultrahigh vacuum STM equipped with a parabolic collector mounted on a piezoelectric stack. The focus of the parabola can be precisely centered on the tip apex by imaging electroluminescence (EL) and modeling the image obtained through the train of optical elements.40 The laser excitation is focused through an aspheric lens (focal length = 79 mm) at 45° incidence angle with an estimated spot size of ~10 µm. The 25


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

Raman spectra are recorded using continuous wave diode lasers, at 532 nm or 634 nm, with a typical intensity of 5 µW/µm2 delivered at the sample. The principle advance that has allowed us to obtain reproducible TERS results is the fabrication of sharp silver tips with nanoscopically smooth surfaces, as recently described.42 After electrochemically etching and polishing tips with an automated setup,44 we use field-directed sputter sharpening as the final process. A detailed description of the procedure has been reported.42 A good indicator of the formation of junction cavities is the observation of gap dependent shift in the EL spectra,45 and this criterion is used to establish the TERS utility of a tip when reshaped through field emission. The fine-tuning of the cavity is accomplished in operando, while monitoring the electronic Raman scattering continuum of the junction. TERS requires alignment of the cavity window with the Stokes scattering range. Either the broad resonance of the cavity window is moved toward the Stokes window, or the color of the laser is chosen to match it. In the present, the excitation wavelength was shifted from 532 nm, where no signal could be seen despite the Q-band electronic resonance of CoTPP (see Fig. S2 in SI), to 634 nm to match with the cavity window.

ASSOCIATED CONTENT

Supporting information: The supporting information is available for free of charge via the Internet at http://pubs.acs.org. 1) Optimized geometry of Co-TPP on Au(111), 2) absorption spectrum of Co-TPP, 3) mode-specific maps of all nine vibrations seen at RT, 4) raw images recorded at room temperature, 5) theoretical model of the TERS junction, 6) simulation of TERS spectra in 26


Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

the dressed tensor formalism, junction resonance, field profile and gap dependence of field strengths (Fig. S5), 7) raw images acquired at 80 K, 8) field gradient driven spectra (Fig. S7), 9) multipolar decomposition of Raman scattering spectra (Fig. S8), 10) CO binding energies to CoTPP, and predicted orientations of CoTPP and bicarbonyl CoTPP molecules at 80 K.

AUTHOR INFORMATION Corresponding Author * [email protected] * [email protected] Author Contributions †Joonhee Lee and Nicholas Tallarida equally contributed to the experimental effort, Xing Chen and Pengchong Liu carried out the modeling of the junctions and the TERS calculations, Vartkess A. Apkarian and Lasse Jensen conceived and coordinated the work, and authored the paper with input from all coauthors.

ACKNOWLEDGMENT

This research was supported by the grant of NSF Center for Chemical Innovation dedicated to Chemistry at the Space-Time Limit (CHE-1414466). We gratefully acknowledge Dr. Laura Rios for her measurements of the porphyrin absorption spectrum, and Prof. Everly Fleischer for many stimulating discussions and for donating his collection of metalloporphyrins. The cryo-UHV STM was constructed under Prof. W. 27


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Ho's

supervision.

Portions

of

this

work

Page 28 of 31

were

conducted

with

Advanced

CyberInfrastructure computational resources provided by The Institute for CyberScience at The Pennsylvania State University (http://ics.psu.edu).

TOC graphic

REFERENCES

(1) (2) (3)

(4) (5)

Novotny, L.; van Hulst, N. Antennas for Light. Nat. Photonics 2011, 5, 83–90. Alù, A.; Engheta, N. Hertzian Plasmonic Nanodimer as an Efficient Optical Nanoantenna. Phys. Rev. B - Condens. Matter Mater. Phys. 2008, 78, 1–6. Kneipp, K.; Wang, Y.; Kneipp, H.; Perelman, L. T.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Single Molecule Detection Using Surface-Enhanced Raman Scattering (SERS). Phys. Rev. Lett. 1997, 78, 1667–1670. Nie, S.; Emory, S. R. Probing Single Molecules and Single Nanoparticles by Surface-Enhanced Raman Scattering. Science 1997, 275, 1102–1106. Le Ru, E. C.; Etchegoin, P. G. Single-Molecule Surface-Enhanced Raman 28


Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

(6)

(7) (8)

(9)

(10)

(11)

(12)

(13)

(14) (15)

(16)

(17)

(18)

(19)

(20)

Spectroscopy. Annu. Rev. Phys. Chem. 2012, 63, 65–87. Jeanmaire, D. L.; Van Duyne, R. P. Surface Raman Spectroelectrochemistry: Part I. Heterocyclic, Aromatic, and Aliphatic Amines Adsorbed on the Anodized Silver Electrode. J. Electroanal. Chem. 1977, 84, 1–20. Surface-Enhanced Raman Scattering: Physics and Applications; Kneipp, Katrin, Moskovits, Martin, Kneipp, H., Ed.; Springer Berlin Heidelberg, 2010. Zhang, P.; Feist, J.; Rubio, A.; García-González, P.; García-Vidal, F. J. Ab Initio Nanoplasmonics: The Impact of Atomic Structure. Phys. Rev. B - Condens. Matter Mater. Phys. 2014, 90, 1–5. Trautmann, S.; Aizpurua, J.; Götz, I.; Undisz, A.; Dellith, J.; Schneidewind, H.; Rettenmayr, M.; Deckert, V. A Classical Description of Subnanometer Resolution by Atomic Features in Metallic Structures. Nanoscale 2017, 9, 391– 401. Pettinger, B.; Schambach, P.; Villagómez, C. J.; Scott, N. Tip-Enhanced Raman Spectroscopy: Near-Fields Acting on a Few Molecules. Annu. Rev. Phys. Chem. 2012, 63, 379–399. Zrimsek, A. B.; Chiang, N.; Mattei, M.; Zaleski, S.; McAnally, M. O.; Chapman, C. T.; Henry, A. I.; Schatz, G. C.; Van Duyne, R. P. Single-Molecule Chemistry with Surface- and Tip-Enhanced Raman Spectroscopy. Chem. Rev. 2017, 117, 7583– 7613. Jiang, N.; Kurouski, D.; Pozzi, E. A.; Chiang, N.; Hersam, M. C.; Van Duyne, R. P. Tip-Enhanced Raman Spectroscopy: From Concepts to Practical Applications. Chem. Phys. Lett. 2016, 659, 16–24. Zhang, R.; Zhang, Y.; Dong, Z. C.; Jiang, S.; Zhang, C.; Chen, L. G.; Zhang, L.; Liao, Y.; Aizpurua, J.; Luo, Y.; Yang, J. L.; Hou, J. G. Chemical Mapping of a Single Molecule by Plasmon-Enhanced Raman Scattering. Nature 2013, 498, 82–86. Chu, P.; Mills, D. Plasmonic Response of STM Tips. Phys. Rev. B 2011, 84, 1–8. Song, P.; Nordlander, P.; Gao, S. Quantum Mechanical Study of the Coupling of Plasmon Excitations to Atomic-Scale Electron Transport. J. Chem. Phys. 2011, 134. Savage, K. J.; Hawkeye, M. M.; Esteban, R.; Borisov, A. G.; Aizpurua, J.; Baumberg, J. J. Revealing the Quantum Regime in Tunnelling Plasmonics. Nature 2012, 491, 574–577. Zhu, W.; Esteban, R.; Borisov, A. G.; Baumberg, J. J.; Nordlander, P.; Lezec, H. J.; Aizpurua, J.; Crozier, K. B. Quantum Mechanical Effects in Plasmonic Structures with Subnanometre Gaps. Nat. Commun. 2016, 7, 11495. Gieseking, R. L.; Ratner, M. A.; Schatz, G. C. Review of Plasmon-Induced HotElectron Dynamics and Related SERS Chemical Effects. ACS Symp. Ser. 2016, 1245, 1–22. Barbry, M.; Koval, P.; Marchesin, F.; Esteban, R.; Borisov, A. G.; Aizpurua, J.; Sánchez-Portal, D. Atomistic Near-Field Nanoplasmonics: Reaching AtomicScale Resolution in Nanooptics. Nano Lett. 2015, 15, 3410–3419. Chen, X.; Moore, J. E.; Zekarias, M.; Jensen, L. Atomistic Electrodynamics Simulations of Bare and Ligand-Coated Nanoparticles in the Quantum Size Regime. Nat. Commun. 2015, 6, 8921. 29


ACS Nano

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(21) Liu, P.; Chulhai, D. V.; Jensen, L. Single-Molecule Imaging Using Atomistic Near-Field Tip-Enhanced Raman Spectroscopy. ACS Nano 2017, 11, 5094– 5102. (22) Duan, S.; Tian, G.; Ji, Y.; Shao, J.; Dong, Z.; Luo, Y. Theoretical Modeling of Plasmon-Enhanced Raman Images of a Single Molecule with Subnanometer Resolution. J. Am. Chem. Soc. 2015, 137, 9515–9518. (23) Roelli, P.; Galland, C.; Piro, N.; Kippenberg, T. J. Molecular Cavity Optomechanics as a Theory of Plasmon-Enhanced Raman Scattering. Nat. Nanotechnol. 2015, 11, 164–169. (24) Benz, F.; Schmidt, M. K.; Dreismann, A.; Chikkaraddy, R.; Zhang, Y.; Demetriadou, A.; Carnegie, C.; Ohadi, H.; de Nijs, B.; Esteban, R.; Aizpurua, J.; Baumberg, J. J. Single-Molecule Optomechanics In “picocavities.” Science 2016, 354, 726–729. (25) Seufert, K.; Bocquet, M.-L.; Auwärter, W.; Weber-Bargioni, A.; Reichert, J.; Lorente, N.; Barth, J. V. Cis-Dicarbonyl Binding at Cobalt and Iron Porphyrins with Saddle-Shape Conformation. Nat. Chem. 2011, 3, 114–119. (26) Domke, K. F.; Pettinger, B. In Situ Discrimination between Axially Complexed and Ligand-Free Co Porphyrin on Au(111) with Tip-Enhanced Raman Spectroscopy. ChemPhysChem 2009, 10, 1794–1798. (27) Janet, J. P.; Zhao, Q.; Ioannidis, E. I.; Kulik, H. J. Density Functional Theory for Modelling Large Molecular Adsorbate–surface Interactions: A Mini-Review and Worked Example. Mol. Simul. 2017, 43, 327–345. (28) Mohn, F.; Gross, L.; Moll, N.; Meyer, G. Imaging the Charge Distribution within a Single Molecule. Nat. Nanotechnol. 2012, 7, 227–231. (29) Albrecht, F.; Repp, J.; Fleischmann, M.; Scheer, M.; Ondráček, M.; Jelínek, P. Probing Charges on the Atomic Scale by Means of Atomic Force Microscopy. Phys. Rev. Lett. 2015, 115, 1–5. (30) Payton, J. L.; Morton, S. M.; Moore, J. E.; Jensen, L. A Hybrid Atomistic Electrodynamics–Quantum Mechanical Approach for Simulating SurfaceEnhanced Raman Scattering. Acc. Chem. Res. 2014, 47, 88–99. (31) Chulhai, D. V.; Jensen, L. Determining Molecular Orientation With SurfaceEnhanced Raman Scattering Using Inhomogenous Electric Fields. J. Phys. Chem. C 2013, 117, 19622−19631. (32) Rios, L.; Lee, J.; Tallarida, N.; Apkarian, V. A. Hovering and Twirling of Tethered Molecules by Confinement between Surfaces. J. Phys. Chem. Lett. 2016, 7, 2461–2464. (33) Shao, F.; Müller, V.; Zhang, Y.; Schlüter, A. D.; Zenobi, R. Nanoscale Chemical Imaging of Interfacial Monolayers by Tip-Enhanced Raman Spectroscopy. Angew. Chemie 2017, 129, 9489–9494. (34) Jiang, N.; Chiang, N.; Madison, L. R.; Pozzi, E. A.; Wasielewski, M. R.; Seideman, T.; Ratner, M. A.; Hersam, M. C.; Schatz, G. C.; Van Duyne, R. P. Nanoscale Chemical Imaging of a Dynamic Molecular Phase Boundary with Ultrahigh Vacuum Tip-Enhanced Raman Spectroscopy. Nano Lett. 2016, 16, 3898–3904. (35) Yan, B.; Stadtmüller, B.; Haag, N.; Jakobs, S.; Seidel, J.; Jungkenn, D.; Mathias, S.; Cinchetti, M.; Aeschlimann, M.; Felser, C. Topological States on the Gold 30


Page 30 of 31

Page 31 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Nano

(36) (37)

(38) (39) (40)

(41)

(42) (43)

(44)

(45)

Surface. Nat. Commun. 2015, 6, 10167. Crommie, M. F.; Lutz, C. P.; Eigler, D. M. Confinement of Electrons to Quantum Corrals on a Metal Surface. Science 1993, 262, 218–220. Betzig, E.; Trautman, J. K. Near-Field Optics: Microscopy, Spectroscopy, and Surface Modification beyond the Diffraction Limit. Science 1992, 257, 189– 195. Bishop, D. M. The Vibrational Stark Effect. J. Chem. Phys. 1993, 98, 3179. Michaelson, H. B. The Work Function of the Elements and Its Periodicity. J. Appl. Phys. 1977, 48, 4729–4733. Tallarida, N.; Rios, L.; Apkarian, V. A.; Lee, J. Isomerization of One Molecule Observed through Tip-Enhanced Raman Spectroscopy. Nano Lett. 2015, 15, 6386–6394. Banik, M.; El-Khoury, P. Z.; Nag, A.; Rodriguez-Perez, A.; Guarrottxena, N.; Bazan, G. C.; Apkarian, V. a. Surface-Enhanced Raman Trajectories on a NanoDumbbell: Transition from Field to Charge Transfer Plasmons as the Spheres Fuse. ACS Nano 2012, 6, 10343–10354. Tallarida, N.; Lee, J.; Apkarian, V. A. Tip-Enhanced Raman Spectromicroscopy on the Å Scale: Bare and CO-Terminated Ag Tips. Submitted. 2017. Wickramasinghe, H. K.; Chaigneau, M.; Yasukuni, R.; Picardi, G.; Ossikovski, R. Billion-Fold Increase in Tip-Enhanced Raman Signal. ACS Nano 2014, 8, 3421– 3426. Sasaki, S. S.; Perdue, S. M.; Rodriguez Perez, A.; Tallarida, N.; Majors, J. H.; Apkarian, V. A.; Lee, J. Note: Automated Electrochemical Etching and Polishing of Silver Scanning Tunneling Microscope Tips. Rev. Sci. Instrum. 2013, 84, 96109. Rendell, R.; Scalapino, D.; Mühlschlegel, B. Role of Local Plasmon Modes in Light Emission from Small-Particle Tunnel Junctions. Phys. Rev. Lett. 1978, 41, 1746–1750.

31


Tip-Enhanced Raman Spectromicroscopy of Co(II) - ACS Publications

Recommend Documents