Chemical Mechanism and Tunability of Surface-Enhanced Raman

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Chemical Mechanism and Tunability of Surface-enhanced Raman Scattering of Pyridine on Heteronuclear Coinage Metal Diatomic Clusters: A Density Functional Study Lei Chen, Zhengqiang Li, Yan Meng, Ming Lu, ZhiGang Wang, and Rui-Qin Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp401462t • Publication Date (Web): 30 May 2013 Downloaded from http://pubs.acs.org on June 1, 2013

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The Journal of Physical Chemistry

Chemical Mechanism and Tunability of Surfaceenhanced Raman Scattering of Pyridine on Heteronuclear Coinage Metal Diatomic Clusters: A Density Functional Study Lei Chen, †, ‡,⊥ Zhengqiang Li, † Yan Meng, ‡,|| Ming Lu, † Zhigang Wang, *,‡,|| and Rui-Qin Zhang, *,⊥,||

†

Key Laboratory for Molecular Enzymology and Engineering of Ministry of Education, College

of Life Sciences, Jilin University, Changchun, 130012, China ‡

Institute of Atomic and Molecular Physics, Jilin University, Changchun 130012, China

⊥

Department of Physics and Materials Science and Centre for Functional Photonics (CFP), City

University of Hong Kong, Hong Kong SAR, China ||

Beijing Computational Science Research Center, Beijing 100084, P. R. China

ABSTRACT As surface-enhanced Raman scattering (SERS) substrates, coinage bimetallic clusters offer high SERS enhancement and potential for applications in precision molecule detection. Here, we elucidate the chemical mechanism and reveal the tunability of the SERS of a pyridine molecule

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(Py) on coinage bimetallic substrate by investigating various possible heteronuclear coinage metal diatomic clusters (HCMDCs) interacting with the pyridine molecule. We obtain the Raman spectra, molecular orbital, and binding property of each HCMDC-Py system using density functional theory. We find that the Raman characteristic peaks, adsorption energy, charge transfer, geometry parameters, and molecular orbital distribution are all closely related to the compositions and binding sites of the HCMDC. The results show that the synergistic effect of different coinage metal atoms in the HCMDC plays an important role in SERS enhancement, which varies with the component and adsorption site of the HCMDC, thus providing the possibility of optimizing the SERS substrate by tuning the bimetallic cluster composition and adsorption site.

Keywords: SERS, heteronuclear coinage metal diatomic clusters (HCMDCs), pyridine molecule, density functional theory, resonance

1. INTRODUCTION Since the surface-enhanced Raman scattering (SERS) was first discovered on a coarse silver electrode in 1974,1 coinage metals have become the key elements for SERS detection. Due to their unique physical and chemical properties, coinage metals have attracted increasing interest in a variety of research fields, including surface analytical chemistry, nanotechnology, biotechnology, and forensic science.2-5 It is generally accepted that the larger enhancement factor of SERS is mainly caused by two mechanisms,6, 8 with the first being electromagnetic mechanism which is due to the strong surface plasmon resonance between the metallic surface and incident light7, 8 and the second being chemical mechanism which is caused by charge transfer transition between the metal cluster and molecule.6, 8 It has been found that core-shell


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nanostructured coinage bimetallic clusters have many advantages over single metal ones.9-14 In particular, silver-core gold-shell nanostructured substrates offer excellent biocompatibility and high SERS enhancement owing to the presence of the gold metal and the silver metal, respectively. The plasmon absorption peak of coinage bimetallic clusters has been found to vary with the proportion of different components.15 As SERS substrates, coinage bimetallic clusters were recently found to show broader variations in electronic excited states than single metal clusters, allowing the desired SERS peak to be achieved through controlling the cluster size and the ratio of the component coinage metals in the coinage bimetallic clusters.10, 11, 16 Coinage bimetallic clusters are significantly different from single metal clusters in terms of their electronic property; they evidently show synergistic effect because of the chemical bonding between different elements.15 It is well known that excitation light plays a decisive role in SERS detection, while the difference in electronic structure between a coinage bimetallic cluster and a single metal cluster can result in a change in the transition energy of the SERS substrate material. This change can exceed the limits that can be reached by controlling the size and morphology of single metal clusters. Although optimal SERS enhancement based on experimental excitation light can be achieved by adjusting the components of coinage bimetallic clusters, and the experimental SERS enhancement changes with the proportions of different components in coinage bimetallic clusters,9 the mechanism of SERS based on bimetallic clusters is not clear in terms of the relationship among the bimetallic type, adsorption conformation, and electronic structure. Therefore, a detailed theoretical study of the relationship between the enhancement of SERS spectra and the sample adsorption sites on the bimetallic cluster is crucial to figuring out the enhancement mechanism of the bimetallic cluster.


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The six-membered heterocyclic aromatic compounds containing a nitrogen atom exist widely in DNA bases, amino acids, proteins, and other biological molecules. In this work, the pyridine molecule, which is a six-membered ring aromatic compound containing a nitrogen atom, is used as an adsorbate interacting with the bimetallic cluster in order to study the SERS spectra. It is well known that the pyridine molecule has been experimentally proved to be vertically adsorbed on the metal substrate via the nitrogen atom,17, 18 which has been widely adopted in the subsequent quantum chemistry calculations.17, 18, 19, 20 A model containing two metal atoms as the SERS substrate has been used to simulate the local interactions between the metal and the molecules, and the calculated results agree well with an experiment.19, 20 The diatomic cluster is the smallest and is widely used to investigate the physical and chemical properties of a bimetallic material.16, 21, 22 Therefore, theoretically studying the interaction of the pyridine molecule adsorbed on a bimetallic cluster and the SERS spectra of a pyridine-bimetal system using a diatomic substrate model is a very effective and feasible approach to take. In this work, we investigate the SERS mechanism of a pyridine molecule (Py) adsorbed on heteronuclear coinage metal diatomic clusters (HCMDCs, i.e. Au-Ag, Au-Cu, and Ag-Cu) with different excitation light wavelengths using density functional theory (DFT), with a focus on the chemical contribution to the surface enhancement. We characterize the interaction between the HCMDC and the pyridine molecule by calculating the binding energy, charge transfer, and geometry parameters. We aim to promote the application of SERS technology based on bimetallic substrates in the identification and detection of biomolecules containing a six-membered heterocyclic compound with a nitrogen atom. We show that unlike single coinage metal substrate, a synergistic effect of different coinage metal atoms in the bimetallic cluster can


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change the transition energy. Therefore, we predict that the optimal SERS substrate based on bimetallic material can be produced with the use of proper excitation wavelengths.

2. METHOD We specifically adopted six interaction configurations with respect to different adsorption sites between the pyridine and three HCMDCs including Au-Ag-Py, Ag-Au-Py, Au-Cu-Py, Cu-AuPy, Ag-Cu-Py, and Cu-Ag-Py complexes. Fig. 1 presents the structures of the HCMDC-Py complexes. We assumed that the pyridine molecule interacts with the HCMDCs along the bond axis with the same C2v symmetry point group as that of the pyridine molecule. All calculations were performed using density function theory (DFT) methods implemented in the Gaussian03 package.23 The geometric optimizations and vibrational analysis of the HCMDC-Py complexes were carried out by employing Becke’s three-parameter hybrid exchange functional and Lee, Yang, and Parr’s exchange functional (B3LYP),24, 25 both of which were shown to be adequate describing the interaction of transition metal-organic molecule complexes.19, 26 The basis set for the atoms in the pyridine was 6-311+G (d, p), while for all of the coinage metal atoms, the valence electrons and the internal shells were described by the basis function LANL2DZ.16, 18, 23, 27, 28

Vibrational frequency analysis was performed based on the optimized geometries and

showed that all of the electronic ground states were stable. In our calculation, the frequencies between 400-1800 cm-1 were scaled with a factor of 0.9688.29 The transition energies of the excited states of the HCMDC-Py complexes were calculated using the time-dependent density functional theory (TD-DFT) method with the same functional and basis sets. Absolute off- and on-resonance Raman intensities were estimated in terms of the


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derivative of the polarizability with respect to a given normal coordinate. For Stokes scattering, the differential Raman scattering cross section (DRCS) was given by19, 27, 30, 31

(ω0 − ωi ) h  dσ  (2π ) I Raman =   = Si 2 45 8π cωi 1 − exp(−hcωi / kBT )  dΩ i 4

 dα S i = 45  dQi

2

  dγ   + 7    dQi 

4

(1)

2

(2)

Here, ω0 and ωi are the frequencies of the incident light and the ith vibrational mode, and h, c, kB, and T are the Planck constant, light speed, Boltzmann constant, and Kelvin temperature, respectively. For the SERS calculations, the incident light had wavelengths of 785 nm, 514 nm and 488 nm, which are commonly used in Raman spectra measurement. Si is the Raman scattering factor (in Å4/amu) that can be directly obtained using the DFT calculations, and dα/dQi and dγ/dQi are derivatives of the isotropic and anisotropic polarizability of the ith vibrational mode, respectively. The binding energy between the Py and HCMDC of each complex can be expressed as

HCMDC + Py → HCMDC − Py

BE = −(EHCMDC−Py − EPy − EHCMDC )

(3)

,

(4)

where EHCMDC (EPy) represents the energy of the HCMDC (Py) obtained with the B3LYP method on fully optimized geometries.


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Figure 1. Configurations of three HCMDCs and the corresponding six adsorption structures of HCMDC-Py complexes and the NBO charge for the metal atoms.

3. RESULTS and DISCUSSION 3.1. Raman spectra of the HCMDC-Py complexes Based on a number of constructed initial adsorption structures, we obtained six stable structures of the pyridine molecule adsorbed on three HCMDCs (Au-Ag, Au-Cu, and Ag-Cu); these structures are denoted as Au-Ag-Py, Ag-Au-Py, Au-Cu-Py, Cu-Au-Py, Ag-Cu-Py, and Cu-AgPy, respectively. First, the lowest three singlet excitation states of every stable structure were calculated using the TD-DFT method. We simulated the static Raman spectra (Fig. 2) and Raman spectra (Figs. 3-5) of every stable structure based on formula (1) using 785 nm, 514 nm, and 488 nm excitation wavelengths.


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Table 1 not only lists the transition energies and oscillator strengths for the three lowest singlet excited states in the six stable HCMDC-Py complexes but also presents the characteristics of the charge transfer transition for every excited state. Compared with the transition energies of the pyridine adsorbed on the single coinage metal cluster for the S1 excited state (1.79 eV for Cu2Py, 2.09 eV for Ag2-Py, 2.87 eV for Au2-Py),27 we found that the transition energies for S1 excited states clearly change. The transition energies of these configurations of the pyridine interacting at different adsorption sites of the same HCMDC are different, and these changes are closely dependent on the composition of the HCMDC and the adsorption site. For example, the transition energies of the S1 excited state of Cu-Au-Py and Au-Cu-Py, the two complexes of pyridine adsorbed on the Cu-Au HCMDC, are respectively 2.74 eV and 2.36 eV, differing by 0.38 eV.

Table 1. Excitation energies (in eV) and oscillator strengths of the three lowest singlet states of HCMDC-Py complexes calculated at the TD-B3LYP/6-311+G(d, p) (C, N, H) /ECP-Lanl2DZ (M) level; IE and CT denote an electronic transition happening in the intracluster excitation and a transition belonging to the CT (charge transfer) from a metal cluster to the pyridine, respectively.

Cu-Au-Py

Au-Cu-Py

Cu-Ag-Py

Ag-Cu-Py

Ag-Au-Py

Au-Ag-Py

(CT)

(CT)

(CT)

(CT)

(CT)

(CT)

2.74; 0.0003

2.36; 0.0002

2.14; 0.0003

1.75; 0.0003

2.66; 0.0003

2.55; 0.0003

(CT)

(CT)

(CT)

(CT)

(CT)

(CT)

S2

2.78; 0.0991

2.90; 0.0006

2.65; 0

2.34; 0

3.20; 0

3.10; 0

S3

(CT)

(IE)

(IE)

(CT)

(IE)

(IE)

S1


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2.98; 0.0022

2.99; 0

2.84; 0.1302

2.88; 0.0002

3.34; 0.5456

3.34; 0.1840

Next, we discuss the static Raman spectra and Raman spectra of every stable structure of the HCMDC-Py complexes at excitation light wavelengths of 785 nm, 514 nm, and 488 nm. This article mainly discusses the five totally symmetric vibrational modes, which are denoted as the ν6a, ν1, ν12, ν9a, and ν8a modes. Fig. 2 presents the simulated static Raman spectra of the pyridine and HCMDC-Py complexes. The Raman frequency shift and intensity of these five symmetric vibrational modes are listed in Table S3. The data in Table S3 indicates that the Raman intensity of HCMDC-Py is generally as twice as that of a pure pyridine molecule.

Figure 2. Simulated static Raman spectra of the pyridine molecule and HCMDC-Py complexes at the B3LYP/6-311+G(d, p) (C, N, H) /ECP-Lanl2DZ (HCMDC) level for a differential Raman scattering cross section, with intensity shown in units of 10-30 cm2/sr and wave number in cm-1.


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The spectrum has been broadened by a Lorentzian function with a width of 10 cm-1. The different colors represent different adsorption sites: red for Au atoms, blue for Ag atoms, and magenta for Cu atoms. (A) pyridine; (B) two configurations of the Au-Ag HCMDC interacting with pyridine; (C) two configurations of the Au-Cu HCMDC interacting with the pyridine; (D) two configurations of the Ag-Cu HCMDC interacting with the pyridine.

The Raman spectra of the pyridine molecule adsorbed on the Au-Ag, Au-Cu and Ag-Cu HCMDC clusters at excitation wavelengths of 785 nm, 514 nm, and 488 nm are presented in Figs. 3-5. The Raman frequency shift and intensity of the ν6a, ν1, ν12, ν9a, and ν8a vibrational modes are listed in Table S4. As the 785 nm excitation wavelength is far away from all of the electronic excited states of all of the stable structures, the intensity of the Raman spectra for 785 nm is the weakest compared to the other two excitation wavelengths.

Figure 3. Simulated Raman spectra of the pyridine interacting with Au-Ag HCMDC at 785 nm, 514 nm, and 488 nm incident light calculated at the B3LYP/6-311+G(d, p) (C, N, H) /ECPLanl2DZ (HCMDC) level for a differential Raman scattering cross section, with intensity shown in units of 10-30 cm2/sr and wave number in cm-1.


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The Raman spectra of the pyridine molecule adsorbed on the Au-Ag HCMDCs are presented in Fig. 3. For the Au-Ag-Py complex, the intensity of Raman spectra at 488 nm wavelength is far greater than that of 785 nm and 514 nm excitation wavelengths and is four orders of magnitude enhanced. According to Table 1, the excitation wavelengths of 785 nm and 514 nm are far away from the excited states of the Au-Ag-Py complex and thus the intensities of the corresponding Raman spectra are very weak, whereas the 488 nm excitation wavelength is almost in resonance with the transition energy of the S1 excited state of Au-Ag-Py and thus results in a great enhancement effect of a factor of up to 104. TD-DFT calculations and NBO analysis show that the S1 excited state is responsible to the charge transfer transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO)) of Au-Ag-Py complex. Meanwhile, we find that the HOMO is totally distributed on the HCMDC and the LUMO is located on the pyridine molecule. It is obviously in line with the mechanism of “intensity borrowing”, which was reported in previous literatures.8, 19 A molecular orbital diagram of the charge transfer transition at resonance at 488 nm excitation wavelength is also presented in Figure 3. For the Ag-Au-Py complex, there is almost no evident enhancement of the Raman intensity corresponding to the 785 nm, 514 nm, and 488 nm wavelengths because the three excitation wavelengths are all away from its electronic excited states. There are great differences between the Raman spectra of the Au-Ag-Py and Ag-Au-Py complexes at the 488 nm excitation wavelength, with the intensity of former being three orders of magnitude higher than the latter. Fig. 4 presents the Raman spectra of the pyridine molecule adsorbed on the Ag-Cu HCMDC for the 785 nm, 514 nm and 488 nm excitation wavelengths. For the Cu-Ag-Py configuration of


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the pyridine molecule interacting with the Ag site, the intensity of the Raman spectra excited by 514 nm and 488 nm is almost the same as that excited by 785 nm. None of these three excitation wavelengths can greatly enhance the Raman intensity of the pyridine molecule because none of them are in the resonant range of the transition state of the Cu-Ag-Py complex. For the Ag-CuPy configuration, although the energies of the 514 nm and 488 nm excitation wavelengths are between the S2 and S3 excited states, none of them can evidently enhance the Raman intensity of the pyridine molecule due to the deviations from the excitation wavelength and the transition energy.

Figure 4. Simulated Raman spectra of the pyridine interacting with the Cu-Ag HCMDC at 785 nm, 514 nm and 488 nm incident light at the B3LYP/6-311+G(d, p) (C, N, H) /ECP-Lanl2DZ (HCMDC) level for a differential Raman scattering cross section, with intensity shown in units of 10-30 cm2/sr and wave number in cm-1.


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Figure 5. Simulated Raman spectra of the pyridine interacting with the Cu-Au HCMDC at 785 nm, 514 nm and 488 nm incident light at the B3LYP/6-311+G(d, p) (C, N, H) /ECP-Lanl2DZ (HCMDC) level for a differential Raman scattering cross section, with intensity shown in units of 10-30 cm2/sr and wave number in cm-1.

The Raman spectra of the pyridine molecule adsorbed on the Au-Cu HCMDC excited at 785 nm, 514 nm, and 488 nm wavelengths are shown in Fig. 5. For the Cu-Au-Py and Au-Cu-Py complexes, both the 514 nm and 488 nm excitation wavelengths are far away from the transition energy according to Table 1. Therefore, the intensities of the Raman spectra excited with these two wavelengths are similar to the intensity obtained at 785 nm and involve no significant enhancement effect. Through a comprehensive comparison, it can be seen that both a different adsorption site of each HCMDC and the same adsorption site at different excitation wavelengths can lead to a change in the enhancement effect of Raman intensity, and the large enhancement factor appears only in the condition of resonance. For the same excitation wavelength, the same adsorption site


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of a different HCMDC can also cause different Raman intensity enhancement. From the above analysis, it can be seen that for the same excitation wavelength, not only a different HCMDC but also a different adsorption site of the same HCMDC can result in a different enhancement effect. In further understanding Figs. 3-5, we found that the enhancement effect caused by the same adsorption site of different HCMDCs is different. Both the Au-Ag-Py and Cu-Ag-Py offer structures with an Ag as the adsorption site, and there is little difference between the intensity of the Raman spectra for these two HCMDC-Py complexes at 514 nm and 785 nm excitation wavelengths. However, at the 488 nm excitation wavelength, the intensity of the Raman spectra for the Au-Ag-Py structure is obviously amplified with an enhancement factor of up to 104 because the energy of the 488 nm excitation wavelength is resonant with the S1 excited state of the Au-Ag-Py structure. The resonance effect leads to a dramatic enhancement of the Raman signal. However, there is almost no change in Raman intensity due to the Ag-Cu-Py because the 488 nm excitation light is not resonant with the transition energy of its excited states. The enhancement effect closely depends on the excitation wavelength even if the adsorption site of different HCMDCs is same. So, the SERS enhancement effect of the HCMDC substrate is determined collectively by the excitation light, binding site, and composition of the HCMDC. Finally, the presentation in Figs. 2-5 somewhat obscures the fact that the shapes of the Raman spectra are primarily influenced by the metal atom at the attachment site. The similarity between Au-Cu-Py and Ag-Cu-Py, etc., is particularly striking for static Raman spectra (see Fig. 2). But even at higher excitation energies, the attachment site has the largest influence. This finding is in line with previous results obtained for larger bimetallic clusters.32 An exception is the spectrum of the Au-Ag-Py complex at 488 nm, in which resonance enhancement is significant and leads to an altered Raman spectral shape.


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3.2. Binding between the pyridine and the HCMDC To better understand the SERS mechanism of the pyridine molecule adsorbed on the HCMDC, we analyzed the binding properties of the HCMDC-Py complexes, including binding energy, charge transfer, and binding characteristics. The physical and chemical properties of the HCMDC are evidently different from those of the single metal cluster because of the synergistic effect of different coinage metal atoms. Table 2. Binding properties between the pyridine and the HCMDC calculated at the B3LYP/6311+G(d, p) (C, N, H) /ECP-Lanl2DZ (M) level. These properties include the bond length between N and coinage metal atoms, R(N-M), in Å; Q(Py→M) in units of electron charge; the ground state NBO charge transfer between the pyridine and the HCMDC; the bond order between the N atom and the adjacent coinage metal atom; and the total binding energy △Ebind in Kcal/mol, calculated based on Eqs. (3) and (4); aReference 23, bReference 17. Configuration

△Ebind

Q(Py-M)


Bond order

R(N-M)

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Cu2-Py

21.15a

0.030

0.1770

2.030a

Ag2-Py

11.47a

0.053

0.1159

2.380a

Au2-Py

25.73a

0.112

0.2587

2.166a

Pyridine with CuAg HCMDC

Cu-Ag-Py

11.79

0.050

0.1144

2.382

Ag-Cu-Py

21.33

0.033

0.1784

2.025

Pyridine with AuCu HCMDC

Au-Cu-Py

30.45

0.049

0.2136

1.982

Cu-Au-Py

11.58

0.071

0.1522

2.327

Pyridine with AgAu HCMDC

Ag-Au-Py

11.44b

0.079

0.1639

2.311b

Au-Ag-Py

18.91b

0.068

0.0341

2.287b

Pyridine with single coinage metal clusters

In order to probe the difference between a single coinage metal cluster and a coinage bimetallic cluster, we analyzed the bond order, charge transfer, binding energy, and structure parameter (as shown in Table 2 and Fig. 1) of the stable structures and made a comparison with the binding property of pyridine interacting with a single coinage metal cluster M2 (M=Cu, Ag or Au). From the data in Table 2, the following conclusions can be drawn: Among these three HCMDCs, the Au-Cu cluster shows the largest difference when the pyridine molecule is adsorbed on its two ends, with a difference of 18.76 kcal/mol. The binding energy difference between the two configurations of the pyridine molecule interacting with the Au-Ag HCMDC is the smallest, with a value of 7.47 kcal/mol. Under the conditions of the SERS experiment, although the ligand attachment is usually assumed to be thermodynamically controlled, considering the large difference in binding energies for different metals listed in Table 2, such a precise control of the attachment site seems rather unlikely achievable with current technology. NBO analysis showed that the natural charge redistribution depends on the composition of the metal cluster and the adsorption site. The results of bond order calculations indicate that the


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covalent bond between the pyridine molecule and the HCMDCs is weak 33, 34 and that the bond order is no more than 0.3, much larger than that of an ionic bonding structure, whose bond order tends to be zero. In all of these HCMDC-Py complexes, the HCMDCs get electrons and the pyridine ring loses electrons. The Au atom in the Au-Cu-Py configuration has the most negative charge of -0.510 e, and the Cu atom has the most positive charge of 0.461 e. The differences between the pyridine interacting with the HCMDC and the pyridine interacting with the single coinage metal cluster M2 (M=Cu, Ag or Au) are shown in Table 1, which also includes the charges, binding energies, and bond lengths. The structure parameters of the pyridine adsorbed on the HCMDCs are presented in Table S1. The change of structure parameters is similar to that of the pyridine interacting with single coinage metal clusters.25, 27 The variation is no more than 0.01 Å for bond length and 1o for bond angle. There are obvious changes in the structure parameter near the metal atom. Specific changes are detailed in SI.

Figure 6. Energy levels, HOMO, HOMO-1, HOMO-2, LUMO, LUMO+1, and LUMO+2 of six adsorption structures of HCMDC-Py complexes based on three HCMDCs.


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The band gaps (the energy difference between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)) of the HCMDC-Py complexes are closely related to the adsorption site of the pyridine molecule interacting with the HCMDC and the compositions of the HCMDC. Figs. 6 and 7 present the distribution of energy levels and shapes for the molecular orbitals (MOs), including HOMO, HOMO-1, HOMO-2, LUMO, LUMO+1, and LUMO+2. The difference in the MOs for every stable HCMDC-Py complex can be vividly described according to the adsorption site and the compositions of the HCMDCs in these two figures, and the values of the band gap for every HCMDC-Py complex are shown in Table 2S. Compared to the pyridine molecule, the band gap of the HCMDC-Py complexes reduces significantly to about 3 eV. The band gap difference between the two configurations of the pyridine molecule adsorbed on the same HCMDCs is about 0.4 eV for the Au-Cu and Ag-Cu HCMDCs and about 0.22 eV for the Au-Ag HCMDC. In contrast to the pyridine molecule adsorbed on the same site of a single coinage metal cluster (3.73 eV for Au2-Py, 2.93 eV for Ag2-Py, 2.63 eV for Cu2-Py),25, 27 the band gap of the HCMDC-Py configuration have changed from 2.36 eV to 3.34 eV.


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Figure 7. Molecular orbitals (HOMO, HOMO-1, HOMO-2, LUMO, LUMO+1, and LUMO+2) for different HCMDC-Py complexes. From the above analysis, it can be seen that compared to a single coinage metal cluster, the binding property of the HCMDC interacting with the pyridine molecule is tunable in terms of binding energy, charge transfer, bond order, and molecular orbital. The change closely depends on the adsorption site and the composition of the HCMDC. These characteristics can facilitate optimizations of the SERS substrate according to excitation light and the tunability of the SERS enhancement effect that the single coinage metal cluster cannot offer.


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4. CONCLUSIONS Our study of the Raman spectra of six stable HCMDC-Py configurations at incident wavelengths of 785 nm, 514 nm, and 488 nm and the binding property of a pyridine molecule interacting with three HCMDCs reveals the chemical mechanism and tunability of the SERS of the pyridine on the HCMDC. The Au-Ag-Py complex can give a SERS at 488 nm excitation wavelength due to the resonance effect between the excitation light and transition energy of the S1 excited state, but the 785 nm and 514 nm excitations cannot excite SERS effectively because their energy is far away from the transition energy of the excited states of the Au-Ag-Py complex. Other HCMDCPy complexes cannot give enhanced Raman spectra at 785 nm, 514 nm, and 488 nm excitations because they are far away from the transition energy of the excited states of these HCMDC-Py complexes. The Raman spectra of the pyridine molecule adsorbed on the HCMDCs vary with the choice of incident light, adsorption site, and the composition of the HCMDC. The analysis of the binding property indicates that the binding property of the pyridine adsorbed on the HCMDCs is evidently different from that of the pyridine adsorbed on a single coinage metal cluster, especially in terms of binding energy, charge transfer, and molecular orbital. This difference also exists in the configuration of different adsorption sites for the same HCMDC. In all of the complexes of the pyridine interacting with the HCMDCs, the difference between the two stable structures is the biggest for the pyridine molecule adsorbed on the Au-Cu HCMDC and the smallest for the pyridine molecule adsorbed on the Au-Ag HCMDC.

ASSOCIATED CONTENTS Supporting Information


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Optimized geometrical parameters of the pyridine and its adsorption state on various CBDAD clusters, molecular orbital analysis and band gap analysis of the CBDAD-Py complexes, static Raman vibrational analysis of the pyridine interacting with the CBDAD clusters, Raman vibrational analysis of the CBDAD-Py complexes with 785 nm, 514 nm, and 488 nm excitations. This information for this article is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *To whom correspondence should be addressed. E-mail: [email protected], Tel.: +86-431851688167; [email protected], Tel.: +852-34427849. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The work described in this paper is supported by grants from the National Science Foundation of China under grant Nos. 30870533 and the Research Grants Council of Hong Kong SAR [project No. CityU 103812]. Z. W. acknowledges the High Performance Computing Center (HPCC) of Jilin University and the NSFC under grant Nos. 11004076 and 11034003.

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TOC

Molecular orbitals of a pyridine molecule interacting at different adsorption sites on an Au-Ag HCMDC.


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Chemical Mechanism and Tunability of Surface-Enhanced Raman

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