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Keywords:

  • electric field;
  • charge transfer;
  • SERS;
  • DFT

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Computational details
  5. Results and discussion
  6. Conclusions
  7. Acknowledgements
  8. References

The influence of a static external electric field on surface-enhanced Raman scattering is investigated by calculating the Raman spectra and excited state properties of pyridine–Au20 complex with the density functional theory and time-dependent density functional theory method. The external electric field with orientation parallel (positive) or antiparallel (negative) to the permanent dipole moment is respectively applied on the complex. This field slightly changes the equilibrium geometry and polarizabilities, which results in shifted vibration frequencies and selectively enhanced Raman intensities. The changes of charge transfer (CT) excited states in response to the electric field are visualized by employing the charge difference densities. Further, the energy of charge transfer transition is tuned by electric field to be resonant or not with the incident light, leading to the Raman intensities are enhanced or not enhanced. At the same time, the intensities of vibration modes are sensitive to the orientation of the field. The positive electric field enhances the totally symmetric ring breathing mode (~1009 cm−1) but suppresses the trigonal ring breathing mode (~1051 cm−1). On the contrary, the mode at 1051 cm−1 is more enhanced than the mode at 1009 cm−1 when the negative electric field is applied on the complex. The Raman spectra could be modulated by tuning the strength and direction of the electric field. Copyright © 2013 John Wiley & Sons, Ltd.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Computational details
  5. Results and discussion
  6. Conclusions
  7. Acknowledgements
  8. References

Since it was discovered, surface-enhanced Raman scattering (SERS) has attracted increasing attention because of the extremely high sensitivity on detecting surface species and powerful application on fingerprint vibrational spectroscopy in qualitative and quantitative analysis.[1-7] Although SERS is not always a fingerprint,[8, 9] its high sensitivity is still attractive and widely applied. This high sensitivity results from the huge enhancement effect, which can reach several orders of magnitude compared to the normal Raman scattering (NRS). Nowadays, two major mechanisms are accepted to give birth to this extreme enhancement. The first one is electromagnetic field enhancement mechanism, in which the incident light is resonant with surface plasmon,[2, 10] resulting in strong local enhancement of the electric field of both incoming and scattered radiation. The second one is charge transfer (CT) enhancement mechanism[5, 6] arising from the resonance between the incident light and the new CT transition states. These new CT transition states are produced by the interaction between metal and adsorbed molecule.

A great number of experimental and theoretical researches have been reported focusing on the enhancement mechanism of SERS.[11-19] All kinds of SERS-active experimental systems have been developed to optimize enhancement for applications, such as electrochemically roughened electrodes,[11] metal colloids produced via chemical reduction,[20] and vapor-deposited metal island films,[21] which are usually used in SERS studies as substrates. Several different factors have been demonstrated to affect the magnitude of the surface enhancement, which contain surface roughness features, surface material, analyte, laser excitation wavelength, and so on. Theoretically electrodynamics methods such as discrete dipole approximation[22] and finite-difference time-domain[23] have been successfully and widely used for a more realistic evaluation of the enhanced local electric field around metal particles. However, these methods are incapable of determining the variation in Raman intensity with molecular normal mode because they completely ignore the adsorbed molecule and the specific interaction between the molecule and the metal surface. To address a complete picture of SERS, it is necessary to consider the electronic structure of the molecules. Numerous researchers have examined CHEM effects in SERS by employing semiempirical, density functional theory (DFT), or second-order perturbation theory (MP2) methods. Issues that impact the SERS enhancement have been investigated including the binding geometry of the adsorbate,[24] the effects of adsorption on various noble and transition metal surfaces,[25] the influence of charged atoms at the metal surface,[26] the effect of co-adsorbed chloride anions,[27] the size effect of nanoclusters on SERS,[28] and the dependence of metals and excitation wavelengths on the SERS intensities.[29]

Although many researches concerning on the issues that influence on the SERS have been published, there are few papers discussing the electric field effect on the SERS. In 2003, Arenas et al.[30] studied the role of the electrode potential in the CT mechanism of SERS by justifying the existence of a parameter β, which reduces the effectiveness of the electrode potential in order to shift the excited CT level. Recently, Sriram et al.[31] demonstrated the use of silver nanotextured electrode pairs to study the influence of low frequency (5 mHz–1 kHz) oscillating electric fields on the SERS spectra of thiophenol. Theoretically, Johansson[32] provided an illustrative ab initio calculations of Raman spectra of pyridine (Py) and Py in metal–molecule complexes with silver, in order to investigate the influence of the external electric field on the Raman shift and intensities. Also, the external electric field and current dependent Raman spectra have been experimentally studied.[33-36] To gain a deep insight into the effect of electric field on SERS, especially on Raman resonance processes, further experimental and theoretical studies are needed.

In this work, we calculated the SERS properties of Py absorbed on gold (Au20) cluster under an external electric field to study the influence of electric field. A uniform static electric field, F, was added basically parallel to the metal–molecule contact and perpendicular to the metal surface. The impacts of electric field on bonding, frequency shifts, CT states, and Raman intensities were investigated by employing DFT and time dependent DFT (TD-DFT) methods. The Raman intensities of vibrational modes were selectively enhanced because of the electric field of different field strength and orientation that is applied on Py–Au20 complex.

Computational details

  1. Top of page
  2. Abstract
  3. Introduction
  4. Computational details
  5. Results and discussion
  6. Conclusions
  7. Acknowledgements
  8. References

All the calculations were performed using the gaussian 09 program.[37] The ground state geometries of Py–Au20 complex under an external electric field of different field strength and orientation were optimized by employing DFT method[38] with BP86 functional.[39, 40] The basis sets for carbon, nitrogen, and hydrogen atoms were 6–311 g++ (d, p). For the Au atoms, the valence electron and the internal shells were described by the basis functions, LANL2DZ, and the corresponding relativity effective core potentials.[41] Computations of vibrational frequencies were carried out in presence of field to confirm local minimality of the optimized structures in field and to yield the Raman spectrum perturbed by the applied field. Here, the external electric field employed is a uniform, bipolar one by means of which we investigate the general trends in the SERS properties. The applied fields were made to orient along (positive) and opposite (negative) to the x-axis (Fig. 1), which is parallel to the metal–molecule Au–N bond and perpendicular to the metal surface. Throughout this work, the atomic units are used for field strengths (1 au = 51.42 V/image). The fields of magnitude ~10−3 to ~10−2 au were applied on Py–Au20 complex. All of the Raman modes were scaled by 1.022 for the direct comparison between experimental and theoretical results. To delve into the effect of electric field on excited state properties, excitation energies and the oscillator strengths were calculated by employing TD-DFT method[42] using BP86 functional with the same basis set as the ground state. Moreover, charge difference densities (CDDs)[43-45] method was employed to visualize the changes in photoexcitation CT because of the effect of an external electric field.

image

Figure 1. The geometry of pyridine–Au20 complex and the orientation of the external electric field applied on the complex.

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Results and discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Computational details
  5. Results and discussion
  6. Conclusions
  7. Acknowledgements
  8. References

Geometries and charge transfer

As a surface sensitive probe molecule, Py has been studied extensively on various metal surfaces. Golab et al.[46] provided strong evidence that Py ring is perpendicular to the metal surface on the basis of theoretical and experimental studies. Experimental SERS spectra of Py on Au electrode at positive potential as also reported in the work of Golab et al., Xia et al., Kudelski and Bukowska, Ohta and Yagi,[46-49] and Aikens and Schatz[50] also investigated the absorption and SERS spectra of Py interacting with Au20 cluster. In their work, two binding modes were adapted, including bonding to a vertex atom (denoted the V-complex) and bonding to one of the four faces (denoted the S-complex). In this work, the latter is chosen to model Py adsorbed on metal surface to study the electric field effect on SERS. The optimized geometry is shown in Fig. 1. When the external field is applied on the complex, a reorientation of the molecule relative to the metal surface occurs to achieve equilibrium. The geometry changes with Py ring still perpendicular to the surface accompanying the Au–N bond elongating or shortening. The changes in the bond lengths and CT of Py–Au20 complex are summarized in Table 1. For F = 0 au (field-free), the bond length and CT are consist with the results in the work of Aikens and Schatz.[50] The Au–N bond length increases when positive electric field is applied. On the contrary, negative field makes the bond length decrease. Moreover, the bond somewhat elongates as the value of positive electric field increases, while it shortens as the negative field strength increases. At the same time, the applied electric field causes the charge redistributed on the complex, resulting in the electronic charge transferred from Py to Au20 changing. The negative electric field and positive electric field respectively promotes and prevents the electronic charge transferred from Py to Au20 cluster. Increasing negative field leads to more electronic charge transferred to Au20. At the positive field, the transferred electronic charge decreases when field strength increases.

Table 1. Bond distances (R(N–Au)), electronic charge transferred from gold cluster to pyridine (Q (Py [RIGHTWARDS ARROW] Au)), and polarizabilities (αxx, αyy, αzz) of py–Au20 complex at various field strengths
Electric Field/auR(N–Au)/ÅQ (Py [RIGHTWARDS ARROW] Au)/eαxx/auαyy/auαzz/au
−0.012.1690.3101102.64795.29770.67
−0.0052.2160.2311002.43794.24770.21
02.3020.169952.98798.36776.15
0.0052.4270.127927.99805.74783.27
0.012.5590.074930.47815.35792.07

Normal Raman spectra

The effect of field on the vibration frequency and Raman intensity is presented in Fig. 2 and Table 2. In the field-free case, vibration modes at 1009, 1051, and 1610 cm−1 dominate in the NRS spectrum. Because of the applied field, the frequencies of several modes shift significantly. For F = 0.005 au, the largest changes are red-shifts of 9, 7, 9, and 9 cm−1 for the modes at 645, 1009, 1051, and 1610 cm−1 relative to the field-free values. When the field strength increases to 0.01 au, these four modes are red-shifted by 18, 17, 15, and 19 cm−1, respectively. In the case of negative field, these modes are blue-shifted by 7, 4, 5, and 3 cm−1 for F = −0.005 au and 12, 3, 6, and −4 cm−1 for F = −0.01 au. The phenomenon that vibration frequencies shift response to the applied electric field is commonly referred to as vibrational stark effect (VSE).[1, 51] In addition, the Raman intensities of vibration mode change as the applied field alters, which is defined as vibrational intensity effect.[52, 53] The changes of intensities are more complicated than those of vibration frequencies. For the mode at 1009 and 1610 cm−1, the intensity is slightly enhanced at positive fields but is weaken at negative fields. On the contrary, positive fields weaken the intensities, while negative fields enhance the intensities for the mode at 1051 cm−1. While the remaining mode intensities are nearly constant as applied field alters. In brief, applied field selectively enhances vibrational modes, which can be attributed to static chemical enhancement. When the external electric field is applied on the Py–Au20 complex, the electronic charge redistributes in the complex to response the field, resulting in the changes in polarizabilities. Table 1 shows the effect of electric field on polarizabilities. The component along x-axis changes significantly, while the other two components almost remain unchanged, because the field is applied parallel or antiparallel to the x-axis.

image

Figure 2. Normal Raman spectra of Py–Au20 complex at applied field strength, (a) F = −0.01 au, (b) F = −0.005 au, (c) F = 0.0 au, (d) F = 0.005 au, and (e) F = 0.01 au, respectively.

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Table 2. Variation in vibrational frequencies of Py–Au20 complex at various field strengths (in au)
SpeciesF = −0.01F = −0.005F = 0F = 0.005F = 0.01Experimentala
  1. a

    Experimental assignment of SERS spectra of pyridine from Ref 54.

6a (A1)657652645636626636
5 (B1)960961958951942938
1 (A1)10121013100910029911009
12 (A1)105710561051104210351035
18a (A1)108810881086108410811065
15 (B2)121512091201119111801158
9a (A1)123812411239123412291213
8a (A1)160616131610160115881597

Excitation state properties

In order to investigate the electric field effect on excitation state properties of Py–Au20 complex, TD-DFT method is employed to calculate the excitation energies and oscillator strengths. Figure 3(a) shows the energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) at various fields. With the increase in the field strength in the direction parallel or antiparallel to the x-axis, the energy of HOMO changes slightly, while the energy of LUMO lowers rapidly. Hence, the HOMO–LUMO (HL) energy gap decreases rapidly as the field strength increases. In the field-free case, HL energy gap is 1.65 eV. When the field is applied on the complex, the HL energy gap decreases to 1.62 and 1.53 eV for F = 0.005 au and F = −0.005 au, respectively. As the field strength increases, the HL energy gap keeps on reducing to 1.19 and 1.29 eV for F = 0.01 au and F = −0.01 au, respectively. As a result, the energy of the electronic transition states varies at different field strength. In particular, the energy of the first electronic transition state reduces significantly, which involves the transition from frontier HOMO to LUMO. Figure 3(b) shows the variation in excitation energy of the ten lowest electronic transition states in response to the electric field. At the field strength of 0.005 au, the excitation energy slightly decreases while rapidly reduces when the field strength increases to 0.01 au. Similar trend is present in the case of applying field in the opposite direction.

image

Figure 3. Variation of (a) frontier orbital energies and (b) the energies of the ten lowest excited states of Py–Au20 complex as the function of electric fields applied.

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The applying field not only affects the energy of excited state but also makes the oscillator strength change. As shown in Fig. 4, positive field decreases the oscillator strength, while it is enhanced as applying field in the opposite direction. Besides, CDDs for the first excitation states of Py–Au20 complex in response to the fields applied are shown in Fig. 4. In the field-free case, the electrons mostly located on the surface atoms of Au cluster and the holes located on the rest atoms of Au cluster and Py molecule. For F = 0.005 au, the CDDs are similar with that of the field-free case. However, as the field strength increases to 0.01 au, electron and hole were delocalized on the molecule and on the Au cluster, respectively. That is, the first excited state becomes a pure CT excited state that the CT process is from metal to molecule. When the field is applied in the opposite direction, the electrons are localized on the bottom atoms of Au cluster, and the holes are localized on the surface atoms of Au cluster and the molecule. The first excited state becomes a CT excited state, in which electrons transfer from molecule to metal.

image

Figure 4. Charge difference densities for the first excitation states of Py–Au20 complex in response to the fields applied (the green and red stand for holes and electrons, respectively, and the isovalue is 4 × 10−4 au). This figure is available in colour online at wileyonlinelibrary.com/journal/jrs

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Pre-resonant Raman spectra

In Raman scattering measurements, if the wavelength of the incident light is close to an electronic excitation of the molecule, the intensity of Raman signal can be enhanced by several orders of magnitude. This progress is referred to as resonance Raman scattering. When a molecule adsorbs to metal surface, additional metal–molecule CT states might emerge because of the interaction between the molecule and the metal cluster. These new CT states could be resonant with the incident light resulting in enhanced Raman scattering. To investigate the influence of electric field on the Raman resonant progress, the pre-resonant Raman spectra were calculated. The simulated spectra of Py–Au20 complex for the 785 nm excitation wavelength are presented in Fig. 5. The selected wavelength is often used for Py molecule in experiment. In the field-free case, the spectrum is almost the same as the NRS spectrum. Because the energy of the incident light is lower than the energy of the lowest excited state S1 (733.14 nm, f = 0.0045), there is no resonance effect, and thus the relative intensities of the peaks in the spectrum are similar to those of the peaks in NRS spectrum. When the field of 0.005 au is applied on the complex, the energy of the first excited state decreases somewhat but is still higher than that of the incident light. Therefore, no interaction between incident light and excited states occurs, leading to no enhancement in Raman intensities. The same trend is obtained as applying field in the opposite direction. For larger value of the field, the energies of excited states keep on decreasing and cross the energy of excitation wavelength. As a result, the energy of exited state is tuned to be close to that of the excitation wavelength leading to pre-resonance between incident light and the excited state. The excited state whose energy is close to excitation wavelength is state S4 (750.36 nm, f = 0.0033) and S6 (770.77 nm, f = 0.0010) for F = −0.01 au and F = 0.01 au, respectively (Fig. 3 (b)). The Raman intensities are enhanced about ten times through this pre-resonance. In brief, the Raman intensities can be tuned by the applying field through modulating the resonance condition.

image

Figure 5. Calculated pre-resonant Raman spectra of Py–Au20 complex at applied field strength, (a) F = −0.01 au, (b) F = −0.005 au, (c) F = 0.0 au, (d) F = 0.005 au, and (e) F = 0.01 au, respectively, with incident light at 785 nm.

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To gain deep insight into the electric field effect on Raman resonant spectra, we calculate the pre-resonant Raman spectra with incident wavelength corresponding to the excitation energy of the first excited state (Fig. 6). Compared to the NRS spectra, more vibrational modes are noticeable in pre-resonant SERS spectra. Most A1 modes including the modes at 1009, 1051, 1239, and 1610 cm−1 are enhanced more than the rest, which is consistent with the experimental measurement results reported in Zuo and Jagodzinski's work.[54] In particular, the enhancement of modes at 1009, 1051, and 1239 cm−1 is sensitive to the orientation of the electric field, which is totally symmetric ring breathing mode, C–H in-plane bending mode and trigonal ring breathing mode, respectively. As shown in Fig. 6, the spectra under positive field (red curves) have a much weaker mode at 1051 cm−1 with relative intensities less than 10% of the mode at 1009 and 1239 cm−1. Similar spectra were found in the experimental SERS spectra of Py on Au electrode at positive potential.[49] On the contrary, the relative intensities of the mode at 1009 and 1239 cm−1 are less than 10% of the mode at 1028 cm−1 in the spectra under the negative field (blue curves). That is, the positive field enhances the mode at 1009 and 1239 cm−1 stronger, while the negative field makes the mode at 1051 cm−1 more remarkable. For the mode at 1610 cm−1 (C–C stretching), the relative intensities are independent of the orientation of the applying field. Both positive and negative electric field strongly enhances this symmetric C–C stretching mode. These results indicate that the vibration mode can be selectively enhanced or suppressed by changing the orientation of the electric field applied on the complex. In general, the Raman spectra could be modulated by tuning the strength and direction of the electric field.

image

Figure 6. Calculated pre-resonant Raman spectra of Py–Au20 complex at applied field strength, (a) F = −0.01 au, λ = 868 nm; (b) F = −0.005 au, λ = 755 nm; (c) F = 0.0 au, λ = 743 nm; (d) F = 0.005 au, λ = 758 nm; and (e) F = 0.01 au, λ = 1035 nm. λ represents the wavelength of the incident light. (f) Calculated pre-resonant Raman spectrum at field strength F = 0.0052 au (corresponding to applied potential on electrode at 0.14 V), λ = 785 nm. The graph inserted at the upper right portion in (f) is the experimental SERS spectrum of pyridine on the gold electrode in 0.1 M KCl at 0.04 and 0.14 V with incident light at 785 nm from the study of Ohta and Yagi[49].

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Conclusions

  1. Top of page
  2. Abstract
  3. Introduction
  4. Computational details
  5. Results and discussion
  6. Conclusions
  7. Acknowledgements
  8. References

In summary, the influence of the electric field on the SERS from Py–Au20 complex was investigated using DFT and TD-DFT methods in this work. The changes in the structure, vibrational frequencies, energy spectrum of MOs, excitation energies, and Raman intensities have been analyzed. The distance between Py molecule and the cluster surface extends or shortens to response to the external electric field. As a result, the electronic charge transferred from Py to Au20 decreases or increases. The applied field causes vibrational frequencies blue shift or red shift because of the stark effect. TD-DFT calculation reveal that the energy of CT transition is tuning by the electric field to be or not resonant with the incident light, resulting in the different enhancement of Raman intensities. In addition, the intensities of vibration modes are sensitive to the direction of the field, which can be selectively enhanced or suppressed. Our calculated results may provide a reference to investigate the techniques for selectively enhancing or suppressing the vibration modes in SERS measurement spectra.

Acknowledgements

  1. Top of page
  2. Abstract
  3. Introduction
  4. Computational details
  5. Results and discussion
  6. Conclusions
  7. Acknowledgements
  8. References

This work was supported by the National Natural Science Foundation of China (Grant Nos. 61137005, 11374353, and 10974023), the Program for Liaoning Excellent Talents in University (Grant No. LJQ2012002), and the Program for New Century Excellent Talents in University (Grant No. NCET-12-0077).

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Computational details
  5. Results and discussion
  6. Conclusions
  7. Acknowledgements
  8. References