Bias-Dependent Chemical Enhancement and Non-Classical Stark

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Spectroscopy and Photochemistry; General Theory

Bias-Dependent Chemical Enhancement and Non-Classical Stark Effect in Tip-Enhanced Raman Spectromicroscopy of CO-Terminated Ag Tips Rebecca L. M. Gieseking, Joonhee Lee, Nicholas Tallarida, Vartkess Ara Apkarian, and George C. Schatz J. Phys. Chem. Lett., Just Accepted Manuscript • Publication Date (Web): 21 May 2018 Downloaded from http://pubs.acs.org on May 21, 2018

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The Journal of Physical Chemistry Letters

Bias-Dependent Chemical Enhancement and Non-Classical Stark Effect in Tip-Enhanced Raman Spectromicroscopy of CO-Terminated Ag Tips

Rebecca L. M. Gieseking,1 Joonhee Lee,2 Nicholas Tallarida,2 Vartkess Ara Apkarian,2* George C. Schatz1*

1

Department of Chemistry, Northwestern University

2145 Sheridan Road, Evanston, Illinois 60208, United States

2

Department of Chemistry, University of California at Irvine Irvine, California 92697, United States

* Corresponding authors: [email protected], (949)-824-6851; [email protected]; (847)-491-5657

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 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

Abstract Tip-enhanced Raman spectromicroscopy (TERS) with CO-terminated plasmonic tips can probe Ångstrom-scale features of molecules on surfaces. The development of this technique requires understanding of how chemical environments affect the CO vibrational frequency and TERS intensity. At the scanning tunneling microscope junction of a CO-terminated Ag tip, we show that rather than the classical vibrational Stark effect, the large bias dependence of the CO frequency shift is due to ground-state charge transfer from the Ag tip into the CO π* orbital softening the C-O bond at more positive biases. The associated increase in Raman intensity is attributed to a bias-dependent chemical enhancement effect, where a positive bias tunes a chargetransfer excited state close to resonance with the Ag plasmon. This change in Raman intensity is contrary to what would be expected based on changes in the tilt angle of the CO molecule with bias, demonstrating that the Raman intensity is dominated by electronic rather than geometric effects.

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In recent years, there has been great interest in developing spectroscopic techniques with both high sensitivity and good spatial resolution. Tips functionalized with a CO molecule have achieved atomic-scale spatial resolution in both AFM1,2 and STM.3,4 More recently, this high spatial resolution has been extended to tip-enhanced Raman spectromicroscopy (TERS) using a plasmonic Ag tip functionalized with a CO molecule.5,6 TERS imaging takes advantage of the large TERS enhancement factors of 1012 for the CO stretching mode and the high sensitivity of the CO vibrational frequency to chemical environment, revealing Ångstrom-scale features of atoms and molecules adsorbed on a surface. The spectroscopic information in these TERS images, including not only the CO frequency but also the line intensity and width, can yield insight into electrostatic potentials, intramolecular charge distributions, and intermolecular forces.6

Although the primary enhancement mechanism in TERS is thought to be an electromagnetic mechanism (EM) related to near-field enhancement of the electric field of light,7,8 chemical contributions can be significant9–11 and are less understood. Enhancement via the chemical mechanism (CM) involves two interconnected mechanisms: (1) changes in the molecular ground-state electronic structure due to interactions with the plasmonic nanostructure (GS), and (2) introduction of excited states with significant charge-transfer character between the metal and molecule (CT), which can strongly enhance the Raman intensity when these states are near resonance with the incident light.12,13

The CO-terminated plasmonic tip provides a unique platform to gain fundamental understanding of the TERS enhancement, particularly the effects of bias. Although surface-enhanced Raman



scattering (SERS) and TERS enhancement factors have long been known to vary in the presence of an electrochemical potential,14,15 to date these effects have been shown in solvent environments where it is challenging to separate molecular effects from the influence of environmental factors such as solvent interactions, double layer effects, and coverage changes.15– 18

Previous single-molecule electrochemical SERS and TERS studies have focused primarily on

larger molecules, and observations of bias dependence have focused primarily on full-electron redox reactions.17,19–22 In contrast with those wet experiments, dry conductive break junction experiments have recognized that vibrational frequencies Stark shift upon partial charge transfer to the molecule, and enhanced Raman coincides with the onset of photocurrent.23,24 The same conclusion was made in fishing mode TERS.25 The intensity fluctuations observed in the junction were primarily attributed to molecular reorientation upon charge transport. Here, we present the bias dependence of the vibrational frequency and TERS intensity of a single CO molecule adsorbed on an Ag tip under ultrahigh vacuum. We show that the vibrational frequency shift is controlled by charge transfer from Ag to CO, and rather than a geometric effect (molecular reorientation), the bias dependence of the intensity can be explained by a biasdependent chemical enhancement related to tuning a charge-transfer excited state close to resonance with the incident light.

The experiments are carried out with a

12

C18O terminated silver tip above an atomically flat

Au(111) substrate. A nanoscopically smooth tip is prepared prior to the attachment of CO.5 The bias dependence of the CO vibrational frequency is measured at fixed gap distances set at 0.1 nA and 1 nA with a common bias of +2 V. The stretching frequency decreases with increasing bias, as seen in Figure 1. The dependence of the frequency shift Δ̅ is dominated by a linear


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coefficient of -18.3 cm-1/V, along with a quadratic contribution of -2.0 cm-1/V2. While the linear coefficient is fairly robust, ~10 % tip-to-tip variation, the quadratic coefficient is sensitive to insitu tip processesing.6 The translation between the applied bias and local field is obtained through the gap dependence of the frequency shift. A careful analysis yields ̅ / -160 cm1

/(V/Å) for the linear Stark tuning rate (STR).6 The softening of vibrational modes due to an

applied bias is similar to that observed for other molecules such as fullerenes in junctions.24,26

Figure 1. Vibrational frequency of 12C18O attached to a silver tip as a function of bias. The sense of the applied bias and associated field is shown by the schematic insets. The continuous line is the fit to the data (1 nA, +2 V) in a quadratic form, with extracted parameters: a = -2.0 cm-1/V2, b = -18.3 cm-1/V.

The CO Raman intensity, measured with 634 nm (1.96 eV) incident light, is also bias dependent. In Figure 2a, we show measurements performed at a junction consisting of a CO-terminated silver tip and an atomically flat Au(111) substrate. In Figure 2b, we show measurements performed with the CO-terminated tip placed over the central Zn atom and one of the pyrroles of



a Zn-etioporphyrin (ZnEtio) molecule adsorbed on the Au(111) substrate. All three measurements show the same trend of increasing Raman intensity at more positive biases and relatively flat intensities at negative biases. This trend is consistently observed through ordered and randomized bias ramp sequences, which rules out unrelated temporal changes in the junction during the measurements. To our knowledge, this is the first clear demonstration of bias dependence of the TERS intensity of a single molecule under controlled conditions. Indeed, there is a ~80 % drop in overall TERS intensity between bare gold and ZnEtio. This can be understood as the drop in optical field when a capacitive load (polarizable molecule) is added to the junction.6 Neverthless, the functional form of the bias dependence is nearly the same on gold and ZnEtio, as can be seen by the comparison in Figure 2. The insensitivity of the observed bias dependence to the substrate suggests that the variation of the TERS intensity with bias is a property of the CO-terminated silver tip alone.


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Figure 2. TERS intensity of CO attached to a silver tip. (a) Bias dependence of intensity on the bare Au(111) surface. The intensity, which is fairly flat at negative bias, nearly doubles between 0 and 2 V. Four color-coded spectra are selected from the bias dependence and displayed in the inset. Each data point is integrated for 20 sec. (b) Bias dependence measured on a ZnEtio molecule. Each point is measured three times with 20 sec integration time. The gap distance is set at 0.1 nA, +1.2 V. The inset is the topography of ZnEtio with indicated measurement points (Zn and lobe) on which bias is ramped.

We now turn to computational models to understand the bias dependence of the CO TERS frequency and intensity, focusing on a model system of the CO molecule adsorbed on a tetrahedral Ag20 cluster (Figure 3a). Ag20 has been widely used in computational studies of SERS,27–29 since its sharp absorption peak shares many characteristics with plasmons in larger nanostructures.30,31 We first compare geometries where the CO molecule is bound to either one



of the tip atoms of the Ag20 tetrahedron (Vertex) or to the central atom of one of the triangular faces (Surface). To model the bias, we apply a uniform electric field along the axis of the Ag20 cluster; this electric field is converted to a bias assuming a 12 Å internuclear distance between the Ag tip and the counterelectrode, comparable to the experimental distance. We note that the computed bias range (-3.1 to +3.7 V) is significantly larger than the experimental range (-2 to +2 V).


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Figure 3. (a) Vertex and Surface geometries of the Ag20-CO complex at zero bias. The blue arrow indicates the direction of the applied electric field. Bias dependence of (b) the C-O vibrational frequency and (c) the Hirshfeld charge on CO at the BP86/TZP level.

The adsorption energy of the CO molecule at zero bias (no applied electric field) is 0.20 eV in the Vertex geometry and 2 × 10-3 eV in the Surface geometry, as computed using density functional theory (DFT) at the BP86/TZP32,33 level (see SI for full computational details). In the Vertex geometry, the CO molecule is tilted 121° from the axis of the Ag20 cluster; this tilt allows π-backbonding between the CO molecule and the Ag20 cluster, strengthening the interaction. Throughout the studied bias range, the binding energy at the Vertex site remains greater than 0.19 eV, whereas the binding energy at Surface sites is consistently smaller than 10-2 eV. Thus, we expect CO to bind preferentially to the tip vertex. Since the experimental tip is atomically terminated, which is verified by the large Stark tuning rate34 and the clarity of the topographic images obtained with the bare tip prior to picking up the CO,35 it provides a binding environment similar to our Vertex model.

To understand the origins of the vibrational frequency shift of the adsorbed CO molecule, we examine the effects of bias for the isolated CO molecule (parallel to the field) and the Ag20-CO complex. The isolated CO molecule has a well-known STR,36 as shown in Figure 3b. However, the effect is a factor of 4 smaller than the experimentally observed shift for tip-attached CO. In the Ag20-CO complex, the computed STR is significantly larger. The dominant contribution to this shift is ground-state CT from the Ag tip to the CO π* orbital, leading to increasingly negative charge on the CO molecule with increasing bias as shown in Figure 3c. This weakens the CO bond and thereby decreases the vibrational frequency with increasing bias.



Although inclusion of the Ag20 cluster significantly increases the STR of the CO stretch, this change is overestimated relative to the experimental value by a factor of 1.7 due to the limitations of our model. The small size of the Ag20 cluster (~1 nm) as a model for a larger nanoscale tip may limit accuracy. The well-known tendency of GGA functionals to overestimate CT37 may also contribute to overestimation of the frequency shift. In addition, the bias in our computational model is treated as uniform electric field applied across all space, not just across the gap between the two electrodes, which may lead to an overestimation in the variation in CT to the CO molecule. Despite these differences, our model demonstrates that the large change in CO vibrational frequency cannot be explained by the classical Stark effect and instead relies on bias-dependent ground-state CT between CO and the Ag electrode. Although not stated explicitly, the same mechanism appears to be operative in recent DFT calculations of the COSTM junction.34

To understand the bias dependence of the Raman intensity, we examine two possible effects: (1) bias-dependent geometric changes and (2) bias-dependent electronic structure changes. As described earlier, the CO molecule is significantly tilted from the axis of the Ag20 tip. The CO molecule adsorbs through a combination of σ donation from CO into the Ag20 cluster and π backbonding involving electron transfer from Ag20 into CO; the different directionalities of these interactions strongly influence the CO tilt. As the bias becomes more positive, the CO molecule bears more negative charge (Figure 3c), corresponding to a larger degree of π backbonding and a lesser extent of σ donation. Thus, the Ag-C-O angle becomes smaller with increasing bias (Figure 4). The tilt of the Ag-C bond from the axis of the Ag cluster, labeled Ag-Ag-C, remains between 162° and 173° across the range of biases studied and does not change systematically


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with bias. Thus, the total tilt of CO from the axis of the Ag cluster, which corresponds to the direction of incident light polarization, follows the same overall trend as the Ag-C-O angle. We note that this change is counter to the electrostatic torque experienced by a dipole in an electric field with oxygen as the negatively charged end of the dipole in the Ag20-CO complex, demonstrating that bonding effects dominate over electrostatic effects in determining the molecular tilt.



Figure 4. (a) Definitions and (b) bias dependence of angles related to the CO tilt in the Ag20CO complex at the BP86/TZP level. (c) Geometric contribution to the CO Raman intensity at 6 K as a function of bias at the INDO/SCI level.

The potential energy surface for CO tilting is fairly steep at zero bias, with an energetic barrier of 0.086 eV for tilting through 180° (see SI for details); the potential energy surface becomes flatter with decreasing bias as the barrier decreases to 0.006 eV at -3.1 V. This flattening of the potential energy surface gives rise to a slight broadening of the tilt angle distribution at 6 K, with the angular range corresponding to a 90% Boltzmann probability increasing from approximately a 5° range at zero and positive biases to 15° at -3.1 V. We examine the geometric effects of the CO tilt on Raman intensities (i.e., contributions not related to charge transfer) using these Boltzmann probability distributions by computing the Raman intensity of the isolated CO molecule at the semiempirical INDO/SCI level29 as a function of tilt angle (details in the SI). At zero bias, the expected Raman intensity is roughly 1/6 that of the perfectly aligned (180° tilt angle) CO molecule. The geometric contribution to the Raman intensity becomes smaller with increasing bias as shown in Figure 4c; this change is most significant at large negative biases where the tilt angle is most strongly bias dependent. Thus, as these effects would lead to the reverse trend in Raman intensity with bias than was demonstrated experimentally in Figure 2, purely geometric effects cannot explain the experimentally observed bias dependence of the Raman intensity.

We turn now to the electronic structure of the Ag20-CO complex, focusing on the bias dependence of the CT excited states and the resulting chemical contribution to the TERS enhancement factor. We compute the Raman intensities for 2.0 eV incident light polarized along the axis of the Ag20 cluster, using a sum-over-states approach at the INDO/SCI level as we have 12 ACS Paragon Plus Environment

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recently implemented.29,38 These computations use our recently developed parameters for Ag29,31 in concert with Zerner’s INDO/S parameters39 for other atoms. We consider three models for the applied potential: 1. OESA: Our recently developed orbital energy shift approximation (OESA) approach40 in which the bias is converted to a shift in the atomic orbital energies of the Ag atoms; a +1 V bias corresponds to a +1 eV shift in the INDO parameters corresponding to the Ag atomic orbital energies. The DFT geometry and vibrational modes with no applied electric field were used for all biases in this model. 2. Electric field (reoptimized): A uniform electric field model, using the DFT geometries and vibrational modes optimized in a uniform electric field. 3. Electric field (constant geometry): A uniform electric field model, using the zero-bias geometry and vibrational modes for all INDO/SCI biases. Comparing these three computational models allows us to understand which features of the electronic structure are key to determining the bias-dependent Raman intensity and allows us to evaluate the ability of these models to explain the experimental trends. Previous modeling of the bias-dependent Raman intensity of molecules in junctions has used a non-equilibrium Green’s function approach,41 which focuses on the role of current through the molecule and shows a sharp onset of current-enhanced Raman scattering at the conduction threshold. As in our case the CO molecule is strongly coupled to only one of the two electrodes and the experimental Raman intensity shows a slow increase with bias, we expect that the static charge-transfer contributions are more significant here than the non-equilibrium effects.



The main absorption of the Ag20-CO complex is due to local plasmon-like excited states on the Ag20 cluster, which occur at 3.16-3.48 eV at zero bias and are relatively constant in energy with bias; since the Ag20 model (~1 nm) is significantly smaller than the experimental tip, this plasmonic absorption is significantly higher in energy than the ~2 eV experimental plasmon energy, so EM TERS enhancement is small in our model. In all three models, the Raman intensity is small at negative biases, and a peak in the Raman intensity is seen at a positive value of the bias as shown in Figure 5a. The value of the bias at which this peak is seen is +1.4 V, +3.4 V, and +3.1 V for models 1, 2, and 3, respectively. We note that the peak in the Raman intensity for models 2 and 3 is computed to be well beyond the maximum bias (+2 V) studied experimentally.


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Figure 5. (a) Raman intensity of the C-O stretch and (b) change in charge on the CO moiety of the Ag20-CO complex as a function of bias at the INDO/SCI level. (c) Raman intensity of the C-O stretch as a function of charge on the CO moiety.



Analysis of the excited states of the Ag20-CO complex shows that the bias dependence of the Raman intensity is due to a bias-dependent chemical enhancement, primarily related to the CT enhancement mechanism. Examination of the excited states (detailed in SI) shows that the peak in the Raman intensity in each model occurs at a bias with near-resonance between the incident light and excited states with significant Ag20 → CO CT character. The effective stabilization of the CO molecular orbitals relative to the Ag20 molecular orbitals with increasing bias both promotes ground-state CT from Ag20 to CO and stabilizes excited states with Ag20 → CO CT character.

To understand the differences between these three models, we analyze the ground-state CT between the Ag20 and CO moieties (Figure 5b). Model 1, based on the OESA potential, yields a bias dependence of the ground-state CT nearly a factor of 2 larger than that seen in models 2 and 3, both based on electric field potentials. This is unsurprising given the nature of the two basic approximations. In the two electric field models, the electric field is converted to the nominal bias plotted on the x axis in Figure 5a,b assuming a uniform field across a distance of 12 Å, roughly corresponding to the experimental distance between the Ag tip and the Au surface. Thus, the effective bias between the Ag20 cluster and the CO molecule is smaller than the nominal bias (between the Ag20 cluster and the implicit counterelectrode) by an amount scaled by the effective distance between the two moieties. In contrast, in the OESA model, the bias is treated as an orbital energy shift between the Ag20 and CO moieties. In this model, the effective bias between Ag20 and CO is equal to the nominal bias and thus is significantly larger than the effective bias in the other two models. When the Raman intensities are re-plotted vs. the change in CO charge (Figure 5c), all three models give peaks in the Raman intensity at similar degrees of ground-state


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CT. This suggests that the driving force toward ground-state CT determines the change in Raman intensity, and the main difference between the models is the complexity of properly converting the model parameters to experimentally relevant biases.

As mentioned earlier, our model system substantially underestimates the EM enhancement; the computed enhancement factor of 4 × 103 at zero bias is many orders of magnitude smaller than the experimental enhancement on the order of 1012. This underestimation is expected due to the limited size of the Ag20 cluster and the > 1 eV detuning of incident light from the plasmon, whereas experiments are performed using light on resonance with the plasmon. The computed enhancement factor peaks at values 30-70 times larger than the zero-bias enhancement, much larger than the factor of 2 variation in the experimental TERS intensities. This magnification of the bias dependence may have its roots in several limitations of the model system. The largest computed enhancement factors are outside the experimental bias range, and the underestimation of the EM enhancement may amplify the apparent change in intensity. In addition, the small Ag20 cluster has only a few discrete CT states involving CO. The larger tip studied experimentally is likely to have much broader CT resonances, either due to broadening of the discrete CT states into an effective band or due to mixing of the plasmon with CT states. Even though the model is simplified, it captures the critical features needed to describe the key experimental trends of increasing TERS intensity at positive biases.

In summary, understanding the TERS intensity and vibrational frequency of CO-functionalized plasmonic tips is key to further developments of TERS imaging with Ångstrom-scale resolution. We have extended measurements of the Stark effect in the Ag-CO-Au junction to a larger bias



range, showing that the frequency shift is due to the bias-dependent balance between σ bonding and π back-donation leading to ground-state charge transfer between CO and the Ag tip. The CO molecule is significantly tilted on the Ag tip, and the change in the nature of the Ag-CO interaction leads to a bias dependence of the CO tilt angle which should be taken into account in quantitative analysis of intensities and frequency shifts. The angular distribution of the tilt angle, which is manifested in observed linewidths, is also controlled by the competition between σ bonding and π back-donation. The tilt potential is softened at negative bias, increasing the sensitivity of the pendular motion of CO to lateral forces. The TERS intensity is likewise biasdependent; this dependence cannot be explained by purely geometric changes but is instead attributed to a bias-dependent chemical effect related to resonance of CT excited states with the incident light. This understanding will aid in interpreting and developing TERS imaging techniques to yield practical atomic-level insight into molecular structure and function.

Experimental Methods The experiments are carried out in an ultrahigh vacuum (base pressure 4×10-11 Torr) cryogenic scanning tunneling microscope (STM) equipped with a parabolic collector (collection solid angle 2.7 sr), with focus precisely aligned to the tip apex by imaging electroluminescence from the tip.42 For Raman measurements, the tip-sample junction is illuminated at 45° with a single-mode 634 nm diode laser focused at the tunneling junction through an aspheric lens installed inside the chamber. The Raman spectra are acquired using a 0.3 m spectrograph (Princeton, SpectraPro 2300i) equipped with a liquid nitrogen-cooled CCD. The nanoscopically smooth silver tip is prepared by field directed sputter sharpening (FDSS), following procedures described recently.20 12 18

C O is dosed onto the Au(111) surface at 6 K, forming a mixture of islands and a 2D gas.


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Upon applying voltage pulses, CO attaches to the atomically terminated apex of the silver tip, which is verified by observation of the CO TERS spectrum and enhanced spatial resolution of the STM images. The selective attachment relies on the poor binding of CO to flat silver and its affinity to the vertex of the tip.

Acknowledgments This research was supported by the grant of NSF Center for Chemical Innovation dedicated to Chemistry at the Space-Time Limit (CHE-1414466).

Supporting Information Experimental details of bias dependence on a Zn-etioporphyrin molecule, computational methodology, and potential energy surfaces, Boltzmann distributions, molecular orbitals, and excited states for the Ag20-CO model system.

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Bias-Dependent Chemical Enhancement and Non-Classical Stark

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