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Effect of the Molecule-Metal Interface on the Surface Enhanced Raman Scattering of 1,4-Benzenedithiol Sho Suzuki, Satoshi Kaneko, Shintaro Fujii, Santiago Marqués-González, Tomoaki Nishino, and Manabu Kiguchi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b10385 • Publication Date (Web): 23 Dec 2015 Downloaded from http://pubs.acs.org on December 29, 2015

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The Journal of Physical Chemistry

Effect of the Molecule-Metal Interface on the Surface

Enhanced

Raman

Scattering

of

1,4-Benzenedithiol Sho Suzuki, Satoshi Kaneko*, Shintaro Fujii, Santiago Marqués-González, Tomoaki Nishino, Manabu Kiguchi*

Department of Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1 W4-10 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

1

ACS Paragon Plus Environment


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Abstract The influence of the number of molecule-metal interactions on the surface enhanced Raman scattering (SERS) spectroscopy of 1,4-benzenedithiol (BDT) was investigated. For this purpose, a series of SERS-active samples were prepared featuring one or two molecule-metal interfaces. Molecules were adsorbed on the surface of a rough Au substrate, or sandwiched between Au nanoparticles (NPs) and a flat Au(111) substrate in a ‘sphere-plane’ disposition. In the presence of the Au surface(s), vibrational energy and intensity of the SERS spectra differs significantly from the bulk. Molecule-metal charge transfer upon chemisorption weakens intra-molecular bonds resulting in the observed red shift of the breathing and C=C stretching modes. This effect was found to be more pronounced for samples with multiple molecule-metal interfaces. In addition, the SERS spectra of BDT featured additional b2 signals not present in the bulk spectra. Chemical enhancement of the b2 modes takes place by means of photo-induced charge transfer from an occupied molecular orbital to an unoccupied metal orbital. Analysis of the normalized SERS intensity revealed a larger scattering enhancement for the samples with a sphere-plane disposition arising from the stronger electromagnetic enhancement effect via plasmonic localization of optical fields. Complementary studies on 4-aminobenzenethiol (ABT) support these findings.

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1. Introduction Surface enhanced Raman scattering (SERS) spectroscopy has been employed in a variety areas including: biology; environment; agriculture; and fundamental science as a highly sensitive molecular characterization technique.1-5 Scattering enhancement in SERS is the product of two independent contributions: electromagnetic (EM); and chemical (CM).6-8 The EM effect is the result of photon-molecule coupling via plasmonic localization of optical fields on metallic nanostructures. On the other hand, the CM effect takes place through charge transfer resonances between metallic and molecular electronic states, largely influenced by the molecule-metal interface. Combined action of both EM and CM effects, can result in enhancement scattering factors ca. 1013 enabling single-molecule SERS studies.9,10

For example, the

single-molecule SERS spectrum of crystal violet and rhodamine 6G have been reported using metal nanoparticles (NPs).9-11 Time-resolved simultaneous SERS and conductance measurements have been performed on individual molecules suspended between two metallic electrodes revealing crucial information on molecular dynamics in single-molecule junctions.12 Furthermore, SERS signal from molecules adsorbed on well-defined single crystalline facets was observed for samples in which metal nanoparticles are deposited on the self-assembled monolayer (SAM).13, 14 The SERS spectra of chemisorbed molecules typically feature marked energy shifts and additional signals corresponding to vibrational modes not observable in the bulk Raman spectrum.12,15,16 The influence of the metal-molecule interaction can be easily observed when comparing the SERS spectra of CO and N2 adsorbed on Ag with their respective bulk Raman spectrum, the Raman profile of CO/Ag undergoes greater changes in both energy and intensity than the SERS spectra of N2/Ag.17 The SERS 3



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profile of CO/Ag features signal broadening and large energy shifts ca. 30 cm-1, while N2/Ag displays narrow peaks and minor energy shifts of about 2 cm-1. Furthermore, the CO/Ag SERS spectrum is about fifty times more intense than that of the N2/Ag. This remarkable difference in the SERS activity in two molecules with nearly identical Raman cross sections can only be explained in terms of the CM effect, and thus the molecule-metal interaction, being much stronger for CO/Ag than for the N2/Ag pair. The effect of the strength of the molecule-metal interaction on the Raman spectra has been studied for surface-adsorbed molecules.3-5 Further change is expected for molecules interacting with more than one metallic surface in metal-molecule-metal junctions. For example, it is known that molecules bridging the gap between two metallic electrodes behave differently from their counterparts that are adsorbed on the surface of a metallic substrate.18,19 More specifically, H2 is known not to dissociate upon adsorption on Au surfaces, however H2 dissociation has been reported in Au-H2-Au junctions employing Au electrodes.20 Despite the known influence of the molecule-metal interaction, a limited number of studies have investigated the effect of multiple molecule-metal interfaces. In this study, the influence of the number of molecule-metal interfaces in the SERS spectra of BDT is studied using two different SERS-active sample designs (Figure 1a,b). The first consists of a SAM on an electrochemically roughened Au substrate featuring a single molecule-Au interface (Figure 1a). The second, featuring two metal-Au interfaces was prepared by deposition of Au−NPs onto a SAM on a flat Au substrate (sphere-plane samples: Figure 1b). Experiments revealed a significant difference in both energy and intensity in the Raman profile of BDT depending on the number of molecule-metal interfaces of the SERS-active samples. In addition, the SERS spectra of BDT featured a 4


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number of non-totally symmetric b2 vibrational modes not observed the bulk Raman spectrum. The analysis of the absolute intensity and relative intensity normalized by the totally symmetric a1 modes revealed a comparable contribution of the CM effect for both SERS-active sample designs. On the other hand, the EM scattering enhancement effect was found to be much larger for the sphere-plane samples. Finally, similar studies performed on 4-aminobenzenethiol (ABT) confirmed these findings.

Figure 1. Schematic view of (a) BDT molecular film on an electrochemically roughened Au substrate and (b) BDT molecules sandwiched between Au nanoparticles (NPs) and a flat Au(111) substrate in a ‘sphere-plane’ disposition. (c) Scanning electron microscope (SEM) image of the BDT sphere-plane sample. The scale bar is 500 nm, Au−NPs (Ø ~ 50 nm).

2. Experimental The SERS-active samples employed in this work were prepared according to the following procedure.13 The rough Au substrates were prepared from Au sheets 0.1 mm thick (Nilaco Corp.) and roughened electrochemically. The Au substrate was employed as a working electrode and the potential was repeatedly swept between -0.3 V and 1.5 V

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(vs. Ag/AgCl) in 0.1 M KCl aqueous solution. This process is known to etch the Au surface increasing its roughness and SERS activity.8 On the other hand, the flat Au surface was prepared from Au(111) substrates reconstructed through flame annealing and quenching.21 The BDT molecules were allowed to self-assemble by immersion of the Au substrates in a 1 mM BDT or ABT solution in EtOH at room temperature for 24 hours. The sphere-plane samples were prepared by immersion of the SAM-covered Au(111) substrates in colloidal NPs aqueous solutions for 24 hours. Colloidal solutions of citrate-reduced Au-NPs (diameter~ 50 nm) were purchased from Tanaka Kikinzoku Corp and used as received. Bulk Raman and SERS spectra were collected using a NanoFinder 30 Raman microprobe (Tokyo Instruments) with a near-infrared laser (λex = 785 nm, 70 mW) as an excitation light and 1 s integration times. The laser beam was focused onto the samples using an objective lens with 50× magnification and 0.95 numerical aperture. The irradiated area of the substrate was 1 µm2 approximately.

3. Results and discussion Figure 2 shows representative SERS spectra of BDT in both sphere-plane (Figure 2a) and rough Au samples (Figure 2b) together with the bulk Raman signal obtained from powder samples (Figure 2c). The bulk Raman spectrum features major peaks at 329, 600, 750, 900, 1050, 1087, 1200 and 1566 cm-1 in good agreement with previously reported studies. 4 The most prominent peaks at 329, 1087 and 1566 cm-1 correspond to the ν6a (deformation-coupled C-S stretching mode), ν1 (ring breathing mode) and ν8a (C=C stretching vibrational mode) respectively. A more detailed assignment of the Raman-active BDT vibrational modes is provided in the Supporting Information (Table S1).4 In addition to the marked signal broadening observed for both SERS-active 6


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samples, the SERS spectrum corresponding to the rough Au sample ν6a, ν1 and ν8a shifted to 345, 1060, and 1555 cm-1, respectively. By comparison, the same BDT vibrational modes in the sphere-plane sample shift to 350, 1060, and 1550 cm-1, respectively. Importantly, a number of additional peaks appeared in the spectra of both SERS-active samples at 490, 800, 1300, and 1400 cm-1. Assuming a C2v symmetry for BDT, these signals correspond to b2 vibrational modes ν16b, ν17b, ν3 and ν19b that are not active in the bulk Raman spectra. Due to the weakness of the signals at 800, 1300 and 1400 cm-1 the energy of these vibrational modes could not be precisely determined.

Figure 2. Representative examples of BDT (a) SERS spectrum in a sphere-plane sample, (b) SERS spectrum on a rough Au substrate and (c) bulk Raman spectrum (powder). Solid arrows indicate the BDT b2 vibrational modes observed in both SERS-active samples that are silent in the bulk Raman spectrum.

Changes in the vibrational energy of the three most prominent peaks in the BDT Raman spectra were quantitatively evaluated from over 50 different samples. Figure 3 shows the vibrational energy of ν8a, ν1, and ν6a modes in the bulk, rough Au, and sphere-plane samples. The BDT the C=C stretching (ν8a) and the ring breathing (ν1) 7



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modes were found to shift to lower energies (Figure 3a, b), deformation-coupled C-S stretching mode (ν6a) (Figure 3c) followed the opposite trend. In all cases, the largest variations in vibrational energy was obtained from the sphere-plane samples, following the shift trend bulk < rough Au < sphere-plane.

Figure 3. The Raman shift of BDT molecule in bulk, on a rough Au substrate and in sphere-plane sample. The vibration modes are (a) C=C stretching mode (ν8a), (b) ring breathing mode (ν1) and (c) deformation-coupled C-S stretching mode (ν6a). When a molecule chemisorbs on the surface of a metallic substrate a certain degree of charge transfer takes place from the molecular bonding orbital (e.g. HOMO) to the metal unoccupied state, and from metal occupied state to the antibonding orbital (e.g. LUMO).22 While this process secures the molecule to the metal surface, it typically results in the weakening of molecular bonds. This effect fits with the vibrational energy trends observed for the Raman-active modes of BDT. While dissociation of the S-H bond and formation of the S-Au bond results in a blue shift of the deformation-coupled C-S stretching mode (ν6a); both C=C stretching mode (ν8a) and ring breathing (ν1) shift to lower energies highlighting a weakening of the intramolecular forces. Furthermore, the larger energy variations observed for the sphere-plane samples featuring an additional molecule-metal interface confirms that the effects of chemisorption in the structural and electronic properties of BDT increase with an increasing number of Au-S 8


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bonds. Density Functional Theory (DFT) calculations were performed using a streamlined model for the bulk, rough Au, and sphere-plane samples (Fig. S1, and Table S1). Theoretical calculations were found to be in good agreement with previously reported studies23 and to adequately describe the experimentally observed vibrational energy trends i.e. bulk < rough Au < sphere plane for the ν6a and inversely for the ν8a and ν1 vibrational modes.

The appearance of the new b2 vibrational modes in the SERS spectra that are not observable in the bulk Raman spectrum can be explained in terms of Lombardi’s model. 24-25

Here, the Raman tensor elements are represented by α = A + B + C, where A is a

Frank-Condon contribution, and B and C represent Herzberg-Teller contributions corresponding to photo-induced molecule-to-metal and metal-to-molecule charge transfer transitions, respectively. Both Herzberg-Teller terms enhance non-totally symmetric modes. Previous thermopower measurements of BDT in single-molecule junctions revealed that frontier orbitals of BDT lie at -1.2 (HOMO) and 2.5 eV (LUMO) with respect to the Fermi level of Au.26 Taking into consideration the energy of the incident laser used in this study (785 nm ~ 1.6 eV) molecule-to-metal charge transfer transitions corresponding to the B term, can contribute to the enhancement of the Raman scattering process. According to Lombardi’s model, the term B is nonvanishing only when all of the following requirements are satisfied: i) the transition from the molecular ground state to the excited state is allowed; ii) photo-induced transitions from the molecular orbital to the metal unoccupied state are allowed; and iii) the direct product of ΓM × ΓK × ΓQ contains the totally symmetric representation, where ΓM, ΓK and ΓQ are the irreducible representation of charge transfer state, excited state and the 9



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vibrational mode, respectively. Assuming C2v symmetry for the BDT molecule, HOMO and LUMO are b1 and a2 in symmetry respectively. The transition from the molecular ground state to the excited state is allowed (b1 × a2 = B2). Taking into account that the molecular axis is perpendicular to the Au surface, the symmetry of the charge transfer state is a1. Therefore, ΓM × ΓK × ΓQ contains the totally symmetric representation for b2 mode. Under these circumstances, the three requirements to make the B term of the Herzberg-Teller contribution nonvanishing are satisfied, thus the vibrational modes with b2 symmetry are enhanced. Comparison of the scattering enhancement of the ν16b mode (b2 in symmetry) between rough Au and sphere-plane BDT samples, the signal enhancement was found to be largest for the latter (Figure 4a). Likewise, the intensity of the aforementioned BDT ring breathing mode ν1 (a1 in symmetry) is also larger for the sphere-plane samples (inset Fig. 4(b)). Figure 4b shows the relative intensity of the ν16b normalized to the intensity of the ν1 mode. After normalization of the spectra, the intensity of the ν16b mode was nearly identical for both the rough Au and sphere-plane samples.

Figure 4. Intensity variation of the ν16b (b2 in symmetry) for bulk (silent), rough Au, and sphere-plane samples. (b) Relative intensity of the ν16b (b2 in symmetry) normalized to the intensity of the ν1 ring breathing mode (a1 in symmetry). Inset: Intensity of the ν1 vibrational mode. 10


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As it was previously discussed, the scattering enhancement of the vibrational modes in SERS-active samples is the product of the EM and CM contributions. Considering a C2v symmetry for BDT, the totally symmetric a1 modes are mainly enhanced by the EM effect. On the other hand, both EM and CM effects should contribute to enhancement of the non-totally symmetric b2 modes. Assuming that both a1 and b2 vibrational modes are equally enhanced by the non-selective EM effect.1 The nearly identical normalized intensity of the ν16b mode for both the rough Au and the sphere-plane samples can only originate from a comparable degree of CM enhancement. The BDT molecular orbital effectively hybridizes with the Au orbital, which is sufficient to induce the photo-induced charge transfer from HOMO to Au unoccupied state even when featuring a single molecule-metal interface. While the effective surface areas are larger for the rough Au samples than for the sphere-plane systems, the larger scattering enhancement of the latter arises from a larger EM contribution.

In order to further consolidate these findings, similar studies were performed on 4-aminobenzenthiol (ABT). Contrary to the symmetrically substituted BDT, ABT can bind to two Au surfaces via thiol and amino anchoring groups. Figure 5a-c shows representative SERS spectra of ABT in both sphere-plane samples (Figure 5a) and rough Au (Figure 5b) together with the bulk Raman signal obtained from powder samples (Figure 5c). The bulk Raman spectrum of ABT presents peaks at 500 (ν6a), 600 (ν12), 800 (ν10a), 1081 (ν7a), 1200 (ν9a), and 1585 (ν8a) cm-1 in good agreement with previously reported studies (Table S2). 15 Similarly to BDT, in addition to a noticeable signal broadening, a series of new signals appeared in the SERS spectra at 1140 (ν9b), 1400 (ν3), and 1430 (ν19b) cm-1 for both the rough Au and sphere-plane samples. 11



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Assuming C2v in symmetry for ABT, these peaks correspond to b2 vibrational modes that are silent in the bulk Raman spectrum. Importantly, perhaps with the exception of the ν3 mode at 1400 cm-1, the b2 modes in the SERS spectra of ABT are more intense and better defined than those previously observed for BDT (see Figure 2a). When compared to BDT, the HOMO-LUMO gap is smaller for ABT, as it is the energy difference between the HOMO and Fermi level of Au.5 Hence, the photo-induced charge transfer from the HOMO to metal unoccupied state is enhanced for the ABT/Au system compared to the BDT/Au system. As the b2 modes are enhanced through this mechanism, the intensity of the b2 vibrational modes in the SERS spectra of ABT is much larger than that previously observed for BDT. Figure 5d,e show the relative vibrational energy shift of two intense peaks at 1580 cm-1 and 1080 cm-1 corresponding to ν8a and ν7a ABT modes respectively, for bulk, rough Au and sphere-plane samples. The energy shift was normalized to the vibrational energy of the given mode in the bulk Raman spectrum. Similarly to what it was previously observed for the BDT/Au system, both vibrational modes ν8a and ν7a shifted to lower energies following the trend bulk > rough Au > sphere-plane. To facilitate direct comparison, data from the BDT/Au system was included in Figure 5d,e in black. The relative Raman shift is comparatively small for the ABT/Au system. This may originate from the weaker NH2-Au interaction that resulting in a weaker modulation of the electronic and structural properties of ABT upon chemisorption. Hence the small relative Raman shift observed in the SERS spectra of the ABT/Au samples.

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Figure 5. Representative examples of ABT (a) SERS spectrum in a sphere-plane sample, (b) SERS spectrum on a rough Au substrate, and (c) bulk Raman spectrum (powder). Solid arrows indicate the appearance and position of non-totally symmetric b2 vibrational modes. Relative vibrational energy shift of (d) ν8a (1580 cm-1) and (e) ν7a (1080 cm-1) modes with respect to the vibrational energy of each mode in the bulk Raman spectrum, rough Au and sphere-plane samples (Red colored points). For comparison, the relative Raman shift of BDT modes (d) ν8a (1560 cm-1) and (e) ν1 (1060 cm-1) are shown in black.

4. Conclusion The SERS spectra of 1,4-benzenedithiol BDT adsorbed on a rough Au substrate and sandwiched between Au nanoparticles (NPs) and a flat Au(111) substrate (sphere-plane design) were investigated. On the rough Au substrate, BDT binds to the Au surface via a single Au-S bond while in the sphere-plane samples BDT binds to the two available Au surfaces through both thiol groups. The SERS spectra of BDT revealed that upon chemisorption, the ring breathing mode and C=C stretching modes shift to the lower energies while the deformation-coupled C-S stretching mode, related with the S-Au bond undergoes a marked blue shift. Importantly, all vibrational energy shifts were found to increase with the increasing number of Au-S bonds following the trend bulk < 13



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rough Au < sphere-plane. Furthermore, the SERS spectra of BDT from both the rough Au, and sphere-plane samples featured a number of additional signals not observed in the bulk Raman spectra. The additional peaks correspond to the non-totally symmetric b2 vibrational modes. The enhancement of these modes in the SERS spectra originates from a photo-induced charge transfer process from the HOMO to unoccupied metal orbital. The intensity the b2 signals were found to be larger for the sphere-plane samples featuring two metal-molecule interfaces, than for the rough Au samples with a single metal-molecule interface. After normalization of the b2 modes to the intensity of the a1 modes, both SERS-active samples featuring one or two molecule-metal interfaces show a nearly identical CM effect. On the other hand, the EM effect was much larger for the sphere-plane samples. Complementary studies performed on the SERS profile of 4-aminobenzenethiol (ABT) confirmed these findings.

ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Author [email protected] [email protected] Notes The authors declare no competing financial interest.

Supporting Information Available

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Assignment of BDT and ABT Raman-active vibrational modes. The material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENTS

This work was financially supported by Grant-in-Aid (No. 24245027, 26102013, 15K17842) from MEXT, and also supported from the Murata Science Foundation and Asahi Glass Foundation. SMG is an International Research Fellow of the Japan Society for the Promotion of Science.

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