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Nanoscale Dehydrogenation Observed by Tip-Enhanced Raman Spectroscopy Songpol Chaunchaiyakul, Agung Setiadi, Pawel Krukowski, Francesca Celine Inserto Catalan, Megumi Akai-Kasaya, Akira Saito, Norihiko Hayazawa, Yousoo Kim, Hideji Osuga, and Yuji Kuwahara J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03352 • Publication Date (Web): 31 Jul 2017 Downloaded from http://pubs.acs.org on July 31, 2017

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The Journal of Physical Chemistry

Nanoscale Dehydrogenation Observed by TipEnhanced Raman Spectroscopy Songpol Chaunchaiyakul,*,†,‡ Agung Setiadi, † Pawel Krukowski, †, § Francesca Celine I. Catalan,‡ Megumi Akai-Kasaya, † Akira Saito, † Norihiko Hayazawa,‡ Yousoo Kim,‡ Hideji Osuga,# Yuji Kuwahara† †

Department of Precision Science and Technology, Graduate School of Engineering, Osaka

University, 2-1 Yamada-oka, Suita 565-0871, Japan ‡

Surface and Interface Science Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198,

Japan #

Department of Materials Science and Chemistry, Faculty of Systems Engineering, Wakayama

University, Sakaedani 930, Wakayama, 640-8510, Japan

ABSTRACT

This study shows evidence of nanoscale dehydrogenation occuring during tip-enhanced Raman spectroscopy (TERS) measurements. The near-field TERS spectra obtained locally on a self-assembled monolayer of 2,13-bis-(aldehyde)[7]-thiaheterohelicene molecules showed vibrational frequencies in good agreement with that predicted by density functional theory

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calculations, except for the L-mode at ~2000 cm-1, which is ascribed to a C≡C triple bond, implying that the benzene rings had been dehydrogenated during the experiments. Using peak analysis, incorporated with the molecular adsorption orientation deduced from high resolution STM imaging, we conclude that one side benzene ring protruding upwards was dehydrogenated as a result of pyrolysis, with the Ag tip serving as both local heat source and catalyst.

INTRODUCTION Combining a scanning probe microscope and a Raman spectrometer, referred to as tipenhanced Raman spectroscopy (TERS), provides the opportunity to explore the chemical properties of nanoscale materials beyond the optical diffraction limit. A spatial resolution of up to 0.6 nm has been shown using TERS based on a scanning tunneling microscope (STM) operating under ultrahigh vacuum (UHV) and low temperature conditions.1 Such high spatial resolution enables the chemical analysis of various nanostructures2−4 including single molecules adsorbed on surfaces.5−8 The main enhancement mechanism in TERS originates from the property of nanoscopic metallic protrusions which can act as a nanoantenna, collecting energy from far-field incident light in the form of localized surface plasmon polaritons (SPP) and emitting an enhanced electromagnetic field in its proximity. In gap-mode TERS, the electromagnetic field confined between the tip and the metallic surface is predicted to enhance the Raman signal by up to four orders in comparison to that enhanced by an isolated tip.9 The above gap-mode enhancement effect is also present in surface-enhanced Raman spectroscopy (SERS), occurring in the junction between two metallic nanoparticles.10 Although the enhancement mechanisms for TERS and SERS are similar, the former offers position controllability as well as imaging, thus providing more insight in the study of


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nanoscale chemical properties. A common feature observed in TERS and SERS is the fluctuation of

signal

intensity

with

time

('blinking'),

particularly

evident

in

single-molecule

investigation.6−8,11,12 The origin of plasmon-enhanced Raman intensity blinking has thus far been attributed to molecular diffusion,8 orientation,1,5 and photochemical processes.6,7,11 Since these fluctuations occur due to nanoscale processes, their origins can provide useful insight into singlemolecular processes. For instance, Duyne et al. demonstrated the detection of single molecules via isotopic substitution,6 and Zenobi et al. verified the single molecular photochemical processes of single-molecule TERS using statistical means.7 Such nanoscale chemical changes may complicate data acquisition for fundamental studies using TERS, but the fact that it is capable of causing chemical changes in the molecular scale could lead to a wide range of applications. For instance, Zhang et al. utilized plasmon-induced hot electrons to break the bonds within molecules in a controllable fashion. 13,

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Such plasmon-assisted chemical reactions are

rapidly gaining interest in the field of on-surface synthesis. In this study we provide evidence of a TERS-induced chemical reaction in organic molecules immobilized by a self-assembled monolayer (SAM) on a Au(111) surface to prevent molecular diffusion. In a previous study, a method of fabricating a SAM of (M)-2,13bis(hydroxymethyl)[7]-thiaheterohelicene (referred to as (M)-[7]TH-diol, hereafter) molecules was developed.15 Scanning tunneling spectroscopy measurements on the SAM revealed identical electronic structures between the isolated molecules, disordered regions, and the trenches between neighbouring twin rows, whereas the electronic structure at the center of the twin rows was significantly different, leading to the conclusion that strong intermolecular interaction exists between pairs of molecules forming the twin rows, whereas the outer rim of the twin rows exhibits strong interaction with the substrate. Thiophenol groups can form a strong S-Au bonding


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with the substrate, attaching the molecule to the substrate, while the intermolecular interaction with adjacent molecules can prevent molecular orientation. However, as depicted in the aforementioned study, an enantiopure SAM would lead to different molecular orientations in each pair of molecules. To ensure a more uniform orientation between enantiomers, a racemic SAM using (P,M)-2,13-bis-(aldehyde)[7]-thiaheterohelicenes (hereafter referred to as [7]THaldehyde) was fabricated in this study. The racemic SAM consisted of twin rows of molecules similar to those in our previous study. The SAM was found to be intact when illuminated by laser during TERS measurements. The Raman spectrum of an isolated [7]TH-aldehyde molecule was calculated to compare with the TERS spectrum. Interestingly, a vibrational mode appeared at ~2000 cm-1, which does not match with the calculated spectrum. The above vibrational mode can be ascribed to the a C≡C triple bond stretching mode which is absent in the molecular structure of the [7]TH-aldehyde molecule, which provides clear evidence for chemical changes in the molecular structure. Several molecular structures were postulated including C≡C triple bonds and each corresponding Raman spectrum calculated. These results could provide useful insights into the properties of near-field enhancement by TERS, and create a new field of singlemolecular chemistry studies. EXPERIMENTAL AND THEORETICAL METHODS All experiments were carried out within a UHV-STM system operating at a temperature of 79 K (Unisoku Co. Ltd., USM1400). Scanning controls consisted of a combination of a Nanonis BP 4.5 controller and a RHK-SPM 100 high-voltage amplifier. Topographic images were analyzed by Gwyddion16 and WSxM.17 Commercially available chemically etched Ag tips (Unisoku Co. Ltd.) were used for both STM imaging and TERS measurements. Submolecular resolution imaging was carried out using a PtIr tip (Unisoku Co. Ltd.). The Au(111) substrate


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(Georg Albert PVD) was cleaned by repeated cycles of Ar+-ion sputtering and subsequent annealing. The cleanliness of the substrate was verified by STM imaging prior to molecular deposition. The [7]TH-aldehyde molecules were deposited from a Knudsen cell (Kitano Seiki Co.) maintained at 510±2 K onto the Au(111) substrate with the temperature maintained at 355±20 K. The STM-TERS experimental apparatus was implemented using a side illumination setup. A 532 nm continuous-wave laser beam was directed to the STM tip through an objective lens with a numerical aperture (NA) of 0.29, and the Raman scattered photons were collected through the same objective lens and directed to the CCD spectrometer (SOL Instruments Ltd.) via an optic fiber using a Nanofinder optics setup (Tokyo Instruments, Inc.). To avoid artifacts such as stray light from outside the system, the optics ensemble is covered, and all light sources are turned off during measurements. Further details on the optical alignment have been given in a previous study.3 The SERS substrate was prepared by depositing 8-nm thick Ag onto a micro glass slide in a thermal evaporator (K-Science Inc. KS-807RK) at an evaporation rate of 0.5 Å/s. The [7]TH-aldehyde powder was put onto the substrate, then grinded gently by pressing another micro glass slide. SERS measurements were performed using a 100x, NA0.9 Nikon LU Plan Fluor EPI objective lens, and a 532 nm laser source. Geometry optimization and Raman activity calculations were performed using density functional theory (DFT) with the B3LYP hybrid functional and 6-311++G(d, p) basis sets implemented in the Gaussian09 program package.18 The intensities of the Raman spectra were converted from the calculated Raman activities using the following equation19,20 which is available in the Chemcraft software:21 2 ℎ − × = × ℎ 45 8 1 − exp −


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where, , , , ℎ, , , and are Raman activities, frequency of the ith band, frequency of the incident laser, Planck constant, speed of light, Boltzmann constant, and temperature, respectively. RESULTS AND DISCUSSIONS A [7]TH-aldehyde molecule consists of four thiophene rings and three benzene rings, arranged alternatingly into two enantiomeric helical structures with an aldehyde group attached at either end (Fig. 1(a)). Racemic [7]TH-aldehyde was used to ensure a more uniform orientation between enantiomers in the SAM. Additionally, whereas (M)-[7]TH-diols have a relatively large optical bandgap of 3.02 eV, as estimated from the UV-vis absorption spectrum (Fig. 1(b)), [7]TH-aldehyde has a bandgap of 2.64 eV, closer to the predicted maxima of the field enhancement factor of an Ag nanoparticle excited by a 532 nm laser.22 It has been shown that near-field Raman scattering intensity can be further enhanced by tuning the 'gap-mode' SPP wavelength to match that of target vibrational frequency.5 Furthermore, since the four thiophene rings may form a S-Au bonding with the Au(111) surface, chemical enhancement via the charge transfer effect is possible. However, since a bias voltage of 0.8 V was used for all TERS measurements in this study, the charge transfer effect was assumed to be negligible.


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Figure 1 (a) Molecular structure of (M)- and (P)-[7]TH-aldehyde shown on the left and right, respectively. (b) UV-vis absorption spectra of (M)-[7]TH-diol (red) and [7]TH-aldehyde (green) dissolved in chloroform. The arrows indicate the onset of the absorption spectra at 3.02 and 2.64 eV for (M)-[7]TH-diol and [7]TH-aldehyde, respectively. (c) Submolecular resolution imaging of a SAM of racemic [7]TH-aldehyde on the Au(111) surface, obtained using a PtIr tip. The molecular model depicts the orientation of the two enantiomers. The bias voltage and tunneling


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current is 0.8 V and 50 pA, respectively. Carbon, hydrogen, oxygen, and sulfur atoms are represented by grey, white, red, and yellow spheres, respectively. In this study, a racemic mixture of [7]TH-aldehyde molecules was deposited onto the Au(111) substrate maintained at a temperature of approximately 355 K in order to fabricate the SAM, as was carried out with enantiopure [7]TH-diol molecules.15 However, in contrast with our previous study, full coverage of self-assembled twin rows was observed without any disordered regions, as shown in Fig. 2(a). From submolecular resolution imaging, we postulate an adsorption model in which each molecule is oriented in such a way that it spirals outwards from the substrate surface, as depicted in Fig. 1(c). The bright protrusions in Fig. 1(c) may be attributed to the aldehyde group at one end of the molecule. Regarding the stability of the SAM, its periodicity can still be observed with the laser illuminated onto the tip-sample nanocavity, but without the twin rows, as shown in Fig. 2(b). Considering that the one-dimensional geometry of the tip apex results in poor thermal conductivity, the locally generated heat is bound to be dispersed inefficiently from the tip apex.23, 24 The UHV environment also prevents radiative heat dissipation. Therefore, the change in periodicity likely results from perturbations upon the molecules under the influence of the tip due to the local temperature gradient surrounding the tip apex. The laser power was set to within the range of 1‒1.5 mW at the source, which is the minimum power at which far-field spectral peaks become distinguishable over background noise. Note that in our system, roughly 50% power attenuation from the laser source to the tip was estimated. After focusing the laser to the tip apex, the tip was approached to within tunneling distance from the sample surface in order to check the magnitude and consistency of the enhancement. Figure 2(c) shows the time lapse of the far-field (0‒30 s), and near-field TERS


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(31‒60 s) spectra, corresponding to when the tip is retracted 100 nm, and is within tunneling range, respectively. The averaged near-field spectrum at t = 35 s and 45 s, far-field spectrum, and Raman spectrum of an isolated [7]TH-aldehyde molecule simulated by DFT calculations is shown in Fig. 2(d). The highest peaks were observed within the range of 1000−2000 cm-1, which largely correspond to the in-plane vibrational modes of each of the benzene and thiophenol rings. The peaks at ~1175 (v1) and ~1200 cm-1 (v2) correspond to the combination of the C-H rocking modes of the thiophene and benzene rings, as expected from the molecular structure.


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Figure 2 STM topographic of the self-assembled monolayer of [7]TH-aldehyde (a) without and (b) with laser irradiated onto the tip-sample nanocavity. The size of both STM images is 13.7 x 13.7 nm2. The bias voltage and tunneling current is 0.8 V and 30 pA, respectively. (c) Time lapse


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of the far-field (0−30 s) and near-field spectra (31−60 s). (d) Near-field spectra from (c) at times 35 and 45 s, along with the averaged far-field spectrum, and calculated Raman spectrum. The peaks at ~1403 (v5) and ~1437 cm-1 (v6) also correspond to the C-H rocking modes, which occurs simultaneously among all the benzene rings. The peaks at ~1320 (v3), ~1350 (v4) and ~1579 cm-1 (v9) correspond to the C-C stretching modes within the benzene rings, while the peak at ~1538 cm-1 (v8) corresponds to the C-C stretching within the thiophene rings. The peak at ~1740 cm-1 (v10), not observed in the experimental data, corresponds to the in-phase and outphase C-O stretching modes of the two aldehyde groups. The much weaker vibrational modes observed in the range of 0 to 1000 cm-1 (not shown) mainly belong to the out-of-plane breathinglike vibrations of the whole molecule. The fact that in-plane vibrations appear much stronger than out-of-plane vibrations is attributed to the molecular orientation with respect to excitation polarization. In general, the experimental and theoretical spectra coincide well in terms of peak positions. The interesting point, however, is the emergence of Raman active modes at ~1480 cm1

(v7) and ~2000 cm-1 (v11), which are unforeseen in the calculated Raman spectrum of the

isolated molecule. For clarity, our discussion focused on v11, since v7 – being tightly surrounded by other vibrational modes – is difficult to distinguish. Since there is no Raman activity in the range of 1750−2900 cm-1, it is believed that v11 was newly introduced by the experimental conditions. Additionally, such a strong peak at ~2000 cm-1 never having been observed when investigating other samples, it is believed to be to a new vibrational mode induced in the [7]TH-aldehyde molecular structure. Considering the elements within a [7]TH-aldehyde molecule, this peak may have emerged as a result of a C≡C triple bond. A straightened C≡C triple bond, such as that in acetylenes, has a Raman active stretching mode in the range of 2100−2140 cm-1, which redshifts


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when bent, due to a reduced degree of overlap between the in-plane orbitals.25,26 This implies that the molecules had been dehydrogenated, allowing the triple bond to form, possibly owing to the effect of the radiative heat and intense electromagnetic field at the tip.23, 24 Given that the molecular structure is not altered as a whole, it is evident that a C≡C triple bond can only be localized between the three pairs of carbon atoms on the outer perimeter of the three benzene rings. We thus refer to v11 as the L-mode, a localized vibrational mode which has also been observed at the edges of armchair graphene nanoribbons (AGNR) synthesized on a Au(111) surface.27,28 To compare with the TERS spectrum, the Raman spectra (Fig. 3(a)) of an isolated pristine [7]TH-aldehyde molecule (Fig. 3(b, i)), and of the isolated [7]TH-aldehyde molecule with the L-mode localized at the side (L1) (Fig. 3(b, ii)), center (L2) (Fig. 3(b, iii)), and on all three benzene rings (L3) (Fig. 3(b, iv)) were calculated. The summary of the calculated and measured vibrational frequencies, along with their corresponding optimized molecular geometry, is shown in Fig. S1 of the supporting information. According to the calculated spectra, the L1and L2-modes result in peaks at 1996 and 2012 cm-1, respectively. In the case of all three Lmodes, as in Fig. 3(b, iv), two peaks were observed at 2000 and 2015 cm-1. The TERS spectrum showed a peak at ~1980 cm-1. It is postulated that the discrepancies between the experimental and calculated spectra owe to the contrast between real and simulated conditions surrounding the molecule. For reference, the SERS spectrum obtained with a 17.1 µW incident laser power density and an acquisition time of 100 s is also shown in Fig. 3(a). Clearly, peaks v5 and v6 of the TERS spectrum differ from that of the SERS and DFT calculations. Since the molecules are uniformly oriented in the TERS measurements (Fig. 1(c)), the C-H bonds of the benzene ring closest to the tip apex may have undergone two processes: dehydrogenation, and Raman


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scattering enhancement, the latter being the reason for the intense v5 and v6 peaks. This is not the case for SERS measurements, since the molecules are randomly oriented. Our discussion was centered around the assignment of the L-modes. The two highest peaks v5 and v6 (at ~1403 and ~1437 cm-1, respectively), can be assigned to the in-phase and outphase C-H rocking vibrations between the side and middle benzene rings of the [7]TH-aldehyde molecule. That v5 and v6 were observed shows that the L-mode is not localized within all the benzene rings of the molecules in the vicinity enhanced by the plasmonic hotspot. The problem is to determine which rings in the molecule had been converted. The differences between the calculated L1-, L2-, and L3-modes will now be discussed.

Figure 3 (a) Experimental TERS spectrum (green) averaged from near-field spectra in Fig. 2(c), SERS spectrum (dark blue) of [7]TH-aldehyde powder (17.1 µW, 100 s), and calculated Raman


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spectra of pristine [7]TH-aldehyde (orange), and (b) the [7]TH-aldehyde with the L-mode localized at the side (L1), center (L2), and all three L-modes together (L3), corresponding to the molecular models in (i), (ii), (iii), and (iv), respectively. Note that the C-H bonds in the thiophene and aldehyde groups are not taken into account for simplicity. The discontinuity around 1750 cm-1 in the SERS spectrum is due to the spectrometer grating. The v3 and v4 peaks correspond to Csp2-Csp2 stretching vibrations in the middle and the side benzene rings, respectively. The presence of a sharp peak at ~1316 cm-1 in the experimental spectrum is assigned to v3 and suggests that the middle benzene ring remains unperturbed. A small shoulder of v5 at ~1356 cm-1 can be assigned to v4, and also suggests the existence of unperturbed side benzene rings, although with much lower magnitude. This implies that the L1mode is dominant, but even so certain discrepancies between the experimental conditions and the DFT calculations may affect the analysis. Peak v8, corresponding to the Csp2-Csp2 stretching vibrations in the two thiophene rings at each end of the molecule, is thus considered as a reference peak, since it is present in all cases and also in the experimental spectrum. Peak v9, corresponding to the Csp2-Csp2 stretching vibrations in the side benzene rings, exists merely as a small shoulder of v8. This further supports the assumption that it is the side benzene rings which have been dehydrogenated. In addition, as previously mentioned, submolecular resolution STM images (Fig. 1(c)) revealed that the molecules in the SAM are oriented such that one side benzene ring protrudes upwards from the substrate surface. It is postulated that the benzene ring pointing upwards is closest to the tip apex, is thus exposed to the heat, and undergoes pyrolytic dehydrogenation with the Ag tip serving as catalyst. The L1-mode is therefore observed. However, the other parts of the molecule are still within the enhancement range of the electromagnetic field emitted from the tip, and therefore v3 is observed from the middle benzene


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ring, and v4 and v9 are observed from the unperturbed side benzene ring, which, being furthest from the tip, exhibits smaller intensities. Peaks v5 and v6 are particularly strong because vibration of the C-H bond is not as restricted as the C-C bonds within the molecule. It is thus concluded that the L1-mode is the main contributor to the experimental spectrum. Even so, the possibility of a mixture of different L-modes must be considered. Depending on the distribution of the thermal and electromagnetic fields, certain modes may be excited, whereas some molecules may remain unperturbed, if they are not significantly heated by the tip, or are too far for the tip to catalyze the pyrolitic hydrogenation reaction. Thus far in our TERS experimental setup, a maximum spatial resolution of approximately 7.6 nm (data not shown) has been achieved using single-walled carbon nanotubes. Assuming that the electromagnetic and thermal field distribution is of a perfectly circular shape originating from the tip axis, the enhancement region should cover an area of 45.36 nm2. From the STM images of the selfassembled monolayer, the average coverage of molecules on the surface is approximately 7 molecules per 10 nm2. Therefore, as a rough estimation, a minimum of 32 molecules are contributing to the TERS spectrum. Owing to heat distribution,23 the molecules close to the center of the hotspot would be exposed to more intense heat and undergo pyrolitic dehydrogenation, whereas molecules further away may remain unperturbed while still contributing to the TERS spectrum. Nevertheless, knowledge of exact tip apex geometry and the spatial distribution of the evanescent electromagnetic field is necessary for a more accurate estimation. For the C≡C triple bond to occur, the hydrogen atoms have to be detached from two adjacent carbons of the benzene ring in the [7]TH-aldehyde molecule. The cyclodehydrogenation of hydrogen atoms from structures containing benzene rings has been widely studied in recent


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years, owing to increasing interest in the bottom-up fabrication materials such as graphene nanoribbons (GNR).29 The GNR fabrication procedure by pyrolysis is carried out in two distinct steps, the first being dehalogenation, and the second dehydrogenation. In both aforementioned steps, a noble metal substrate serves as catalyst.29,30 It has also been demonstrated that the tunneling electrons alone can cause detachment of terminal hydrogen atoms of GNRs, provided that a sufficient bias voltage is applied.30 Generally, the termini of GNRs are usually zigzag edges, which have a lower energy barrier for dehydrogenation in comparison to armchair edges.28 It has also been determined that the L-mode appears at a frequency of 1450 cm-1 instead of 2000 cm-1.27 Additionally, the L-mode only exists at the edges, and does not contribute to the G-mode as does the sp2 carbon atoms far from the edges. As seen in a study by Huang et al., the L-mode at ~2000 cm-1 was experimentally observed from AGNRs on a Ag(111) surface, and showed a noticeably smaller intensity ratio compared to that of the G-mode.28 Their experimental results also verify that far-field incident laser is capable of detaching hydrogen atoms from the edges of AGNRs lying flat on metallic surfaces. Our study focuses on the detachment of hydrogen atoms by a near-field source during TERS experiments. In order to investigate the thermal threshold for the dehydrogenation process, we carried out power-dependent SERS studies with the [7]TH-aldehyde molecules on a rough Ag subsrtrate. However, as the laser power was increased from 4.5 to 450 µW, we observed the emergence of the D- and G-bands (Figs. S2 and S3, supporting information), associated with amorphous carbon, indicating that the molecules could decompose before dehydrogenation occurs, since this was done in ambient conditions. Hence, the power-dependent SERS studies cannot be compared to the TERS data, which was carried out under low-temperature under UHV conditions. We thus explain our results


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in analogy with GNRs adsorbed on the Ag(111) surface; in our case, with the Ag tip as the catalyst. From the above discussions, a model of the molecule under three circumstances, as depicted in Fig. 4, is illustrated. Under normal STM tunneling conditions, the [7]TH-aldehyde molecule remains unperturbed, as illustrated in Figs. 4(a), since there is insufficient heat for the reaction to occur. Alternatively, molecules far from the tip also remain unperturbed even with laser irradiated, since the tip acts as the catalyst for the dehydrogenation to occur, as illustrated in Fig. 4(b). In the case where the laser and the tip are combined, the incident laser power is confined to tip-sample nanocavity, generating a small volume of intense heat, exposing the molecules within the range of the plasmonic field of the tip apex to a sharp temperature gradient,23 causing pyrolysis to occur to the [7]TH-aldehyde molecules close enough to be catalyzed by the tip (Fig. 4(c)). This pyrolysis causes dehydrogenation of the side benzene rings in the [7]TH-aldehyde molecule, resulting in the formation of a C≡C triple bond, with the detached hydrogen atoms being adsorbed onto the surface of the STM tip which serves as a hydrogen storage (Fig. 4(c)). As mentioned above, the presence of noble metals lowers the energy barrier in the dehydrogenation process. In the case of AGNRs, since they lie flat on the substrate, the C-H bonds at the edges are easily catalyzed by the metallic substrate itself. This cannot be in the case of [7]TH-aldehydes since it is nonplanar, and thus it is more likely that the Ag tip catalyzes the dehydrogenation of the uppermost benzene ring only for the molecules in close vicinity of the tip apex. This is evident from the fact that the L-mode intensity is relatively strong, when compared to the same vibrational mode in AGNRs adsorbed on Ag(111).28 In other words, this is proof that this dehydrogenation reaction occurs in the near-field.


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Since a bent C≡C triple bond is highly unstable, it is reasonable to assume that the hydrogen atoms adsorbed on the metallic surface of the tip can recombine to form the C-H bond29 with the [7]TH-aldehyde molecules. It is therefore assumed that the conversion from pristine [7]TH-aldehyde to dehydrogenated [7]TH-aldehyde is reversible, occurring before the hydrogen atoms can combine into H2. The [7]TH-aldehyde molecule reverts back to its pristine form once the hotspot from the STM tip is removed (Figs. 4(a), (b)).

Figure 4 (a) Unperturbed [7]TH-aldehyde molecule under normal STM tunneling conditions. (b) Unperturbed [7]TH-aldehyde molecule irradiated by the laser. (c) [7]THaldehyde with the hydrogen atoms detached from the middle benzene rings when simultaneously under the effect of radiative heating from the STM tip and irradiated by the laser. Note that only one molecule is illustrated for simplicity. Carbon, hydrogen, oxygen, and sulfur atoms are represented by grey, white, red, and yellow spheres, respectively. CONCLUSIONS To summarize, in this study we show evidence of vibrational modes being induced during TERS measurements. Utilizing a SAM to immobilize the molecules on the substrate, near-field TERS spectra showing strong characteristic peaks in agreement with that predicted by DFT calculations were obtained. One unpredicted peak, however, is the L-mode, ascribed to a C≡C


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triple bond, which implies that the benzene rings had been dehydrogenated. High resolution STM imaging shows that one side benzene ring protrudes upwards close to the tip. Added to the fact that peak analysis suggests dehydrogenation of a side benzene ring, we believe that due to the side ring being closest to the tip, it undergoes dehydrogenation induced by pyrolysis with the Ag tip acting as both local heat source and catalyst. This shows that TERS has the potential to induce nanoscale chemical reactions.

AUTHOR INFORMATION Corresponding Author *(S.C.) E-mail: [email protected], [email protected] Present Addresses § Department of Solid State Physics, Faculty of Physics and Applied Informatics, University of Lodz, ul.Pomorska 149/153, 90-236 Lodz, Poland

SUPPORTING INFORMATION Summary of Raman scattering frequencies and corresponding optimized geometry, powerdependent SERS, comparison of SERS measurements from powder and dropcasted samples

ACKNOWLEDGEMENTS Funding Sources This work is supported by a Grant-in-Aid for Scientific Research (S) (24221009) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.


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REFERENCES (1) Jiang, S.; Zhang, Y.; Zhang, R.; Hu, C.; Liao, M.; Luo, Y.; Yang, J.; Dong, Z.; Hou, J. G. Distinguishing adjacent molecules on a surface using plasmon-enhanced Raman scattering. Nat. Nanotechnol. 2015, 10, 865‒869. (2) Liao, M.; Jiang, S.; Hu, C.; Zhang, R.; Kuang, Y.; Zhu, J.; Zhang, Y.; Dong, Z. Tipenhanced Raman spectroscopic imaging of individual carbon nanotubes with subnanometer resolution. Nano Lett. 2016, 16, 4040‒4046. (3) Chaunchaiyakul, S.; Yano, T.; Khoklang, K.; Krukowski, P.; Akai-Kasaya, M.; Saito, A.; Kuwahara, Y. Nanoscale analysis of multiwalled carbon nanotube by tip-enhanced Raman spectroscopy. Carbon 2016, 99, 642‒648. (4) Vantasin, S.; Tanabe, I.; Tanaka, Y.; Itoh, T.; Suzuki, T.; Kutsuma, Y.; Ashida, K.; Kaneko, T.; Ozaki, Y. Tip-enhanced Raman scattering of the local nanostructure of epitaxial graphene grown on 4H-SiC (0001̅). J. Phys. Chem. C 2014, 118, 25809‒25815. (5) Zhang, R.; Zhang, Y.; Dong, Z. C.; Jiang, S.; Zhang, C.; Chen, L. G.; Zhang, L.; Liao, Y.; Aizpurua, J.; Luo, Y. et al. Chemical mapping of a single molecule by plasmon-enhanced Raman scattering. Nature 2013, 498, 82‒86. (6) 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.; Duyne, R. P. V. Single-molecule tipenhanced Raman spectroscopy. J. Phys. Chem. C 2012, 116, 478‒483. (7) Zhang, W.; Yeo, B. S.; Schmid, T.; Zenobi, R. Single-molecule tip-enhanced Raman spectroscopy with silver tips, J. Phys. Chem. C 2007, 111, 1733‒1738. (8) Klingsporn, J. M.; Jiang, N.; Pozzi, E. A.; Sonntag, M. D.; Chulhai, D.; Seideman, T.; Jensen, L.; Hersam, M. C.; Duyne, R. P. V. Intramolecular insight into adsorbate-substrate interactions via low-temperature, ultrahigh-vacuum tip-enhanced Raman spectroscopy. J. Am. Chem. Soc. 2014, 136, 3881‒3887. (9) Yang, Z.; Aizpurua, J.; Xu, H. Electromagnetic field enhancement in TERS configurations. J. Raman Spectrosc. 2009, 40, 1343‒1348. (10) Heeg, S.; Oikonomou, A.; Fernandez-Garcia, R.; Lehmann, C.; Maier, S. A.; Vijayaraghavan, A.; Reich, S. Plasmon-enhanced Raman scattering by carbon nanotubes optically coupled with near-field cavities. Nano Lett. 2014, 14, 1762‒1768. (11) Sonntag, M. D.; Chulhai, D.; Seideman, T.; Jensen, L.; Duyne, R. P. V. The origin of relative intensity fluctuations in single-molecule tip-enhanced Raman spectroscopy. J. Am. Chem. Soc. 2013, 135, 17187‒17192. (12) Maher, R. C.; Dalley, M.; Le Ru, E. C.; Cohen, L. F.; Etchegoin, P. G.; Hartigan, H.; Brown, R. J. C.; Milton, M. J. T. Physics of single molecule fluctuations in surface enhanced Raman spectroscopy active liquids. J. Chem. Phys. 2004, 121, 8901−8910.


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Page 21 of 28

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


(13) Sun, M.; Zhang, Z.; Kim, Z. H.; Zheng, H.; Xu, H. Plasmonic scissors for molecular design. Chem. Eur. J. 2013, 19, 14958−14962. (14) Zhang, Z.; Sheng, S.; Zheng, H.; Xu, H.; Sun, M. Molecular resonant dissociation of surface-adsorbed molecules by plasmonic scissors. Nanoscale 2014, 6, 4903−4908. (15) Chaunchaiyakul, S.; Krukowski, P.; Tsuzuki, T.; Minagawa, Y.; Akai-Kasaya, M.; Osuga, H.; Kuwahara, Y. Self-assembly formation of M‑type enantiomer of 2,13bis(hydroxymethyl)[7]-thiaheterohelicene molecules on Au(111) surface investigated by STM/CITS. J. Phys. Chem. C 2015 119, 21434‒21442. (16) Necas, D.; Klapetek, P. Gwyddion: An open-source software for SPM data analysis. Centr. Eur. J. Phys. 2012, 10, 181−188. (17) Horcas, I.; Fernandez, R.; Gomez-Rodriguez, J. M.; Colchero, J.; Gomez-Herrero, J.;Baro, A. M. WSXM: a software for scanning probe microscopy and a tool for nanotechnology. Rev. Sci. Instrum. 2007, 78, 013705. (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B. et al., Gaussian 09, Revision E.01, Gaussian, Inc., Wallingford CT, 2009. (19) Polavarapu, P. L. Ab initio vibrational Raman and Raman optical activity spectra. J. Phys. Chem. 1990, 94, 8106–8112. (20) Krishnakumar, V.; Keresztury, G.; Sundius, T.; Ramasamy, R. Simulation of IR and Raman Spectra based on scaled DFT force fields: A case study of 2-(methylthio)benzonitrile, with emphasis on band assignment. J. Mol. Struct. 2004, 702, 9–21. (21) Zhurko, G. A.; Zhurko, D. A. Chemcraft: graphical program for working with quantum chemistry computations. 2009, chemcraftprog.com (22) Geshev, P. I.; Klein, S.; Witting, T.; Dickmann, K.; Hietschold, M. Calculation of the electric-field enhancement at nanoparticles of arbitrary shape in close proximity to a metallic surface. Phys. Rev. B. 2004, 70, 075402. (23) Downes, A.; Salter, D.; Elfick, A. Heating effects in tip-enhanced optical microscopy. Opt. Express 2006, 14, 5216−5222. (24) Balois, M.V.; Hayazawa, N.; Catalan, F.C.; Kawata, S.; Yano, T.; Hayashi, T. Tipenhanced THz Raman spectroscopy for local temperature determination at the nanoscale. Anal Bioanal Chem 2015, 407, 8205 (25) Pavia, D. L.; Lampman, G. M.; Kriz, G. S. Introduction to spectroscopy: a guide for students of organic chemistry. Philadelphia: W.B. Saunders Co., 1979. (26) Gampe, C. M.; Carreira, E. M. Arynes and cyclohexane in natural product synthesis. Angew. Chem. Int. Ed. 2012, 51, 3766−3778.


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(27) Zhou, J.; Dong, J. Vibrational property and Raman spectrum of carbon nanoribbon. Appl. Phys. Lett. 2007, 91, 173108. (28) Huang, H.; Wei, D.; Sun, J.; Wong, S. L.; Feng, Y. P.; Neto, A. H. C.; Wee, A. T. S. Spatially resolved electronic structures of atomically precise armchair graphene nanoribbons. Sci. Rep. 2012, 2, 983. (29) Talirz, L.; Ruffieux, P.; Fasel, R. On-surface synthesis of atomically-precise graphene N nanoribbons. Adv. Mater. 2016, 28, 6222−6231. (27) Talirz, L.; Sode, H.; Cai, J.; Ruffieux, P.; Blankenburg, S.; Jafaar, R.; Berger, R.; Feng, X.; Mullen, K.; Passerone, D. et al. Termini of bottom-up fabricated graphene nanoribbons. J. Am. Chem. Soc. 2013, 135, 2060−2063.


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Figure 1 (a) Molecular structure of (M)- and (P)-[7]TH-aldehyde shown on the left and right, respectively. (b) UV-vis absorption spectra of (M)-[7]TH-diol (red) and [7]TH-aldehyde (green) dissolved in chloroform. The arrows indicate the onset of the absorption spectra at 3.02 and 2.64 eV for (M)-[7]TH-diol and [7]THaldehyde, respectively. (c) Submolecular resolution imaging of a SAM of racemic [7]TH-aldehyde on the Au(111) surface, obtained using a PtIr tip. The molecular model depicts the orientation of the two enantiomers. The bias voltage and tunneling current is 0.8 V and 50 pA, respectively. Carbon, hydrogen, oxygen, and sulfur atoms are represented by grey, white, red, and yellow spheres, respectively. 80x157mm (300 x 300 DPI)


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Figure 2 STM topographic of the self-assembled monolayer of [7]TH-aldehyde (a) without and (b) with laser irradiated onto the tip-sample nanocavity. The size of both STM images is 13.7 x 13.7 nm2. The bias voltage and tunneling current is 0.8 V and 30 pA, respectively. (c) Time lapse of the far-field (0-30 s) and near-field spectra (31-60 s). (d) Near-field spectra from (c) at times 35 and 45 s, along with the averaged far-field spectrum, and calculated Raman spectrum. 170x187mm (300 x 300 DPI)



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Figure 3 (a) Experimental TERS spectrum (green) averaged from near-field spectra in Fig. 2(c), SERS spectrum (dark blue) of [7]TH-aldehyde powder (17.1 µW, 100 s), and calculated Raman spectra of pristine [7]TH-aldehyde (orange), and (b) the [7]TH-aldehyde with the L-mode localized at the side (L1), center (L2), and all three L-modes together (L3), corresponding to the molecular models in (i), (ii), (iii), and (iv), respectively. Note that the C-H bonds in the thiophene and aldehyde groups are not taken into account for simplicity. The discontinuity around 1750 cm-1 in the SERS spectrum is due to the spectrometer grating. 170x104mm (300 x 300 DPI)


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Figure 4 (a) Unperturbed [7]TH-aldehyde molecule under normal STM tunneling conditions. (b) Unperturbed [7]TH-aldehyde molecule irradiated by the laser. (c) [7]TH-aldehyde with the hydrogen atoms detached from the middle benzene rings when simultaneously under the effect of radiative heating from the STM tip and irradiated by the laser. Note that only one molecule is illustrated for simplicity. Carbon, hydrogen, oxygen, and sulfur atoms are represented by grey, white, red, and yellow spheres, respectively. 177x43mm (300 x 300 DPI)



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Nanoscale Dehydrogenation Observed by Tip ... - ACS Publications

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