Supporting Information for: Dependence and Quantitative Modeling

Transcription

1 Supporting Information for: Tip-Enhanced Raman Voltammetry: Coverage Dependence and Quantitative Modeling Michael Mattei, Gyeongwon Kang, Guillaume Goubert, Dhabih V. Chulhai, George C. Schatz, Lasse Jensen, and Richard P. Van Duyne*,, Department of Chemistry and Applied Physics Program, Northwestern University, Evanston, Illinois 60208, United States Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States S1. Calculation of Raman Spectra Using Time-Dependent Density Functional Theory The optimized geometry and normal modes of NB-d0, NB-d3, and NB-d5 were obtained with the B3LYP/6-311g* level of theory using NWChem Version Vibrational frequencies were scaled by a factor of 0.98 to account for missing anharmonicity in the simulations. The resonance Raman scattering of each of these isotopes were simulated using a time-dependent wave packet formalism, 2 with the Franck-Condon factors and transition dipole derivatives, needed for the Herzberg-Teller contributions, obtained via three-point numerical differentiation of the excited states energies and transition dipoles, respectively, along mass-weighted normal mode coordinates.

2 Figure S1: (a) Photograph and (b) Schematic of the custom Delrin electrochemical cell used in this work. The ITO working electrode secured via O-ring seal into the bottom of the cell using a stainless steel plate. Copper tape is placed between the ITO and the O-ring to make external electrical contact. The ITO is exposed to the working solution through a 15 mm diameter hole in the center of the cell. The Ag/AgCl wire reference and Pt counter enter the cell through 1 mm holes in the side and are fixed in place with Hysol epoxy. Three electrodes are connected to a green screw terminal in the bottom right of panel (a), which then connects to the potentiostat. Figure S2: 2 e -, 1 H + Redox mechanism of NB at ph=7.1.

3 Figure S3: (a) CVs of NB on ITO measured at different scan rates (3 mm NB incubation concentration). (b) Peak separation as a function of scan rate. The inverse of the slope of the linear fitting (red line) yields an ensemble rate constant. (k 0 = 10.4 s -1 ) (c) Cathodic peak current as a function of scan rate. The linear relationship indicates the reaction of a surface bound species.

4 Figure S4: Illustrations of the four most intense Raman active modes in NB. The cm -1 mode primarily involves breathing of the central oxazine ring, and corresponds to the 591 cm -1 mode used in the TERS CVs. Table S1: Mode assignment for the four vibrational modes of NB in Figure S4. Experimental mode (cm -1 ) Simulated mode (cm -1 ) Assignment Symmetric central ring distortion Asymmetric ring distortions Phenyl ring C-H bending Symmetric phenyl ring stretching

5 Figure S5: Representative TERS voltammograms constructed from the integrated intensity of the 591 cm -1 mode for 10 µm, 100 µm, and 1 mm incubation concentrations in the left, center and right part of the figure, respectively. The red and blue arrows indicate the direction of the scan for cathodic and anodic waves, respectively. The cathodic sweep (blue dots) and anodic sweep (red dots) are offset for clarity. The black lines are guides for the eye.

6 Figure S6: A TERS CV for which the signal does not return during the anodic sweep (red lines). This behavior is observed in <5% of TERS CVs, and may be caused by reductive desorption or scavenging of the reduced or intermediate species in side reactions (e.g. reaction with residual oxygen). Figure S6 shows a rare example of a TERS CV in which the signal does not return during the anodic sweep, likely due to reductive desorption or quenching by side reactions. Diffusion of the molecule out of the tip-sample junction could also contribute to signal loss. Additionally, surface diffusion, desorption and readsorption of molecules during TERS CV could contribute to the intensity fluctuations discussed in relation to Figure S14. However, the multicycle TERS CV in Figure S12 suggests that the same molecule is probed at least for the duration of one full voltammetric cycle. To investigate the influence of diffusion, desorption and readsorption on the EC-TERS signal, high-resolution EC-STM experiments are underway. Molecular dynamics simulations of NB in the tip-sample junction, accompanied by TDDFT Raman calculations, are also being carried out to explain this behavior.

7 Figure S7: Two fitted curves with different k 0 values for the same TERS CV. (k 0 =5 s -1 for red dotted curve and 75 s - 1 for black solid curve). The two fits are indistinguishable despite more than an order of magnitude difference in k 0. For a scan rate of 0.01 V/s, there is no significant change in the shape of the curve for k 0 >1 s -1, and there is a large error in the determination of k 0.

8 Figure S8: TER spectra of NB-d 0 (blue), NB-d 3 (red), and NB-d 5 (black) simulated using TD-DFT. The structure of each isotopologue is shown on the right. Figure S8 shows calculated TER spectra for NB and two isotopologues in order to assess the feasibility of an isotopologue single-molecule proof. Of the two simulated isotopologues, the necessary deuterated precursor is only available for the d3 compound. For both deuterated species, the 591 cm -1 mode shifts by < 3 cm -1, and cannot be used to reliably differentiate between the deuterated and protonated forms. The 1640 cm -1 mode does shift by ~14 cm -1 for the d5 compound compared to d0. However, no deuterated precursor is available to synthesize the d5 compound. Further, the signal-to-noise ratio of the 1640 cm -1 mode in the measured TERS spectra for the d0 compound is, on average, ~3, indicating that this mode cannot be used as a reliable indicator of single-molecule detection.

9 Figure S9: Histogram of peak potentials extracted from fitting TERS CVs at all coverages. Potential histograms for cathodic waves (blue) and anodic waves (red) were fitted to Gaussian distributions (black curves). Peak positions are ± V for cathodic wave and ± V for anodic wave. FWHM values are 168 ± 64 mv for cathodic wave and 90 ± 3 mv for anodic wave. Figure S10: (a) AFM image of a representative ITO surface with a Au(111) nanoplate for reference, taken with an Au coated Si tip. Many grains (~ nm diameter) are visible on the ITO surface, while the single crystal Au surface is very smooth and uniform. The blue and red lines correspond to the height profiles in panel b. (b) AFM height profile of two paths in panel a. (c) RMS roughness averaged over multiple AFM height profiles over ITO (black) and the Au nanoplate (yellow). The ITO surface is highly heterogeneous, with an RMS roughness of 1.75 nm. The Au surface is atomically flat.

10 Figure S11: Histogram of peak separation extracted from each TERS CV. The histogram was fitted to a Gaussian distribution (black curve) which is centered at ± V and has a FWHM of 121 ± 29 mv. Figure S12: TERS CVs for three consecutive potential cycles with experimental parameters identical to Figure 2 in the main text and an incubation concentration of 1 mm. Each of the six segments of the CV were individually fitted

11 to the Laviron model. In five of the segments (~80%) the TERS CVs fit better to a two-molecule model (compared to ~30% of 1 mm TERS CVs in Table 2 in the main text). Further, the first two cathodic cycles show nearly identical behavior. This indicates that the same two molecules are probed for most of the three-cycle CV, and that molecular diffusion, tip drift, and desorption/readsortion of molecules during the CV do not lead to detection of multiple different molecules during a single cycle. Figure S13: The cathodic segment of a TERS CV fitted to Laviron s model before (a) and after (b) TERS intensity spike removal. TERS intensity fluctuations are a well-documented behavior associated with few- and singlemolecule detection. 3, 4 In some TERS CVs, anomalously intense spectra were observed on the intensity plateaus at which no electrochemistry occurs (See Figure S14). These unusually high intensities may be attributed to additional NB molecules temporarily entering the tip-sample junction or transient reorientation of the NB molecule such that the molecular electronic transition dipole is strongly aligned in the direction of the enhanced field (tip axis). 6 To ensure proper fitting to Laviron s model, we removed these intensity spikes before fitting. For each point in a TERS CV, we determined average and standard deviation (σ) of the intensity of 30 points on either side of the point in question. If the point in question was more than 2σ larger than the average, it was removed from the data set before fitting. This method is similar to a

12 moving boxcar or Savitzky Golay filter, but only removes anomalously intense points, without smoothing the remaining data. Figure S14: Selected TERS spectra (left) at the indicated points along the TERS CV (right). The significant scatter in the TERS CV can largely be attributed to intensity fluctuations commonly observed in single- and few-molecule TERS experiments, rather than low signal-to-noise ratio (SNR) in the individual spectra. 4 The SNR for the 591 cm -1 mode in each spectrum, using the root-mean-square noise near the peak, are 20 (black), 75 (green), and 23 (orange).

13 Figure S15: (a) CVs corresponding to Figure 1(a) in the main text before background subtraction. (b) A plot of coverage and estimated number of molecules under the tip vs incubation concentration (corresponding to the values in Table 1 in the main text). References 1. Valiev, M.; Bylaska, E. J.; Govind, N.; Kowalski, K.; Straatsma, T. P.; Van Dam, H. J. J.; Wang, D.; Nieplocha, J.; Apra, E.; Windus, T. L.; de Jong, W. A. Comput. Phys. Commun. 2010, 181, (9), Silverstein, D. W.; Jensen, L. J. Chem. Phys. 2012, 136, (6), Sonntag, M. D.; Chulhai, D.; Seideman, T.; Jensen, L.; Van Duyne, R. P. J. Am. Chem. Soc. 2013, 135, (45), Sonntag, M. D.; Klingsporn, J. M.; Garibay, L. K.; Roberts, J. M.; Dieringer, J. A.; Seideman, T.; Scheidt, K. A.; Jensen, L.; Schatz, G. C.; Van Duyne, R. P. J. Phys. Chem. C. 2012, 116, (1), Zeng, Z.-C.; Huang, S.-C.; Wu, D.-Y.; Meng, L.-Y.; Li, M.-H.; Huang, T.-X.; Zhong, J.- H.; Wang, X.; Yang, Z.-L.; Ren, B. J. Am. Chem. Soc. 2015, 137, (37), 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. Nano Lett. 2016, 16, (6),