RSC Advances View Article Online PAPER View Journal | View Issue Surface-enhanced Raman scattering of pyrazine on Au5Al5 bimetallic nanoclusters† Cite this: RSC Adv.,2017,7,12170 Quanjiang Li,a Qianqian Ding,ab Weihua Lin,b Jiangcai Wang,b Maodu Chen*a and Mengtao Sun*bc In this study, we theoretically investigated the Raman and absorption spectra of pyrazine adsorbed on Au5Al5 bimetallic nanoclusters by a time-dependent density functional theory (TD-DFT) method. The surface-enhanced resonance Raman scattering (SERRS) spectra of pyrazine absorbed on different isomers and sites of the Au5Al5 cluster were simulated. The visualization of orbital transitions in electronic transitions was used to analyze the enhancement mechanism of SERRS spectroscopy. Compared with those of isolated pyrazine excited at 598 nm, the SERRS of pyrazine–Au–Au4Al5- a excited at the same incident light can be enhanced on the order of 104, which is a typical charge transfer (CT) resonance excitation and charge transfer from substrate to pyrazine. Due to the fact that the intensity of ultraviolet SERRS can be significantly enhanced to 1.2 Â 106 A4 per amu for pyrazine– Creative Commons Attribution 3.0 Unported Licence. Au–Au4Al5-a model at 280 nm, the Au5Al5 cluster may be a good candidate for research of the Received 15th December 2016 ultraviolet SERRS materials. Other key factors that can change the intensity of SERRS include the Accepted 2nd February 2017 resonance excitation wavelength, oscillator strength of the electronic excited state, metal–molecule DOI: 10.1039/c6ra28240g binding site and structure of the substrate cluster. Hence, the optical properties of complexes can be rsc.li/rsc-advances tuned by varying these factors. 1. Introduction incident light radiates on the nanostructures causing charge transfer (CT) between the metal clusters and adsorbate mole- This article is licensed under a Raman signals can be greatly enhanced when the molecules are cules, which results in some Raman peaks being selectively – absorbed on rough metallic surfaces.1–4 As a molecular detec- enhanced by a factor of 105.15 18 The method of visualization of tion tool with high sensitivity, surface-enhanced Raman scat- orbital transitions in electronic excitations of a molecule– 16,19 Open Access Article. Published on 20 February 2017. Downloaded 10/5/2021 1:22:26 PM. tering (SERS) overcomes the disadvantages of conventional nanostructure system has been developed in recent years, Raman spectra, and plays an important role in the studies of where detailed information about the electronic transitions can spectroscopic properties and electronic structures of mole- be easily obtained and analyzed. Then, the contributions of cules.5–11 There are two generally accepted enhanced mecha- intra-metallic excitation and charge transfer excitation can be nisms for SERS: the electromagnetic eld enhancement and the distinguished. charge transfer enhancement. For the electromagnetic eld In principle, all metals can enhance local electromagnetic enhancement mechanism, the excitations in the metal cluster elds, and the intensity of SERS mainly depends on the ability can cause collective oscillation of electrons, which results in of metals to support strong LSPR. Since the discovery of rough local surface plasmon resonance (LSPR), and then the molec- sliver substrates that strongly enhance signals of Raman ular Raman signals can be strongly enhanced by a factor of spectra, extensive research of SERS has focused on coinage 1011.7,9,12–14 However, for the charge transfer enhancement, the metal.1,2,5,8,12,15,20,21 With the exploration of metal substrates, transition metal substrates have been theoretical designed and experimentally synthesized to enhance the SERS of adsorbate aKey Laboratory of Materials Modication by Laser, Electron, and Ion Beams, Ministry molecules, which are mainly based on the electromagnetic eld of Education, School of Physics and Optoelectronic Technology, Dalian University of enhancement mechanism, such as Pt, Ru, Rh, Pd, Fe, Co, Technology, Dalian 116024, People's Republic of China. E-mail: [email protected] Ni.3,22–25 In addition, bimetallic nanoparticles have been widely b Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface Science, investigated due to their unique properties on spectroscopic School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China. E-mail: [email protected] and electronic structures. In recent years, numerous studies on cBeijing National Laboratory for Condensed Matter Physics, Beijing Key Laboratory for SERS have shown that the optical properties of molecules can be Nanomaterials and Nanodevices, Institute of Physics, Chinese Academy of Sciences, affected by the multicomponent substrates. Moreover, the SERS Beijing, 100190, People's Republic of China of molecules absorbed on different bimetallic nanoclusters † Electronic supplementary information (ESI) available. See DOI: were simulated and analyzed, such as planar nanocluster 10.1039/c6ra28240g 12170 | RSC Adv.,2017,7,12170–12178 This journal is © The Royal Society of Chemistry 2017 View Article Online Paper RSC Advances 26 Ac@Au7, and core@shell nanoclusters of Cr@Au12, W@Ag12, between the nitrogen and metal nanoparticles. The metal– 27–30 – Mo@Ag12, and Th@Au14. molecule models, used in this study, are shown in Fig. 1(c f), In this study, using a time dependent density functional which can be considered as models of simplied, tip-enhanced theory method, we theoretically investigated the enhancement Raman scattering.34,35 mechanism of surface-enhanced resonance Raman scattering (SERRS) spectroscopy of pyrazine absorbed on Au5Al5 bimetallic nanoclusters. We select the CT excitations and the intra- 2. Methods metallic excitations, which can strongly enhance the SERS of All calculations discussed here were performed with the 2014.07 pyrazine in the visible or the ultraviolet regions, to calculate the version of the Amsterdam density functional (ADF) suite.36 The SERRS of complexes. The electromagnetic enhancements and structures of complexes were optimized at the density func- the chemical enhancements for this model system are consis- tional theory (DFT)37 level of theory using the exchange–corre- tently treated based on a short-time approximation for the lation functional BP86.38,39 A triple-z with polarization functions Raman scattering cross-section. Due to the inert and hypo-toxic and a (TZP)40 Slater type basis set were used, where the Au(1s2– properties of Au, gold can be used as an appropriate substrate in 4f14), Al(1s2–2p) are the frozen core, and C, N and H are full medical applications. Investigations on gold substrates have electrons. Relativistic corrections of elements (Au, Al) at the shown excellent enhancement of SERRS. A study on the zeroth order regular approximation (ZORA)41–44 level have been absorption and SERS spectra of pyridine interacting with Au20 taken into account. The electronic transitions were calculated by Schatz et al. showed that the Au nanocluster can signi- 20 45 3 4 by a TD-DFT method, using the same functional and basis cantly enhance the intensity of SERS up to about 10 to 10 .31 In sets. The adiabatic local density approximation (ALDA) was addition, aluminum is a good metal substrate for ultraviolet employed in all calculations of polarizability. The SERRS SERRS. An investigation on resonance Raman spectroscopy of intensities were calculated by differential Raman scattering pyrazine absorbed on two Al clusters or one Al cluster has 3 4 46 32 cross-section (DRSC), which is given by eqn (1): Creative Commons Attribution 3.0 Unported Licence. been reported by Sun et al. It revealed that the enhancements can respectively reach up to about 104 and 105, compared with d ð2pÞ4 h ð À Þ4 I ¼ s ¼ u0 ui Raman 2 the Raman intensity of pyrazine excited at the same incident dQ i 45 8p cui 1 À expð Àhcui=kBTÞ " # light, due to intra-metallic excitation and charge transfer exci- d 2 d 2 Â 45 s þ 7 g tation. The models of Au5Al5 clusters can be seen in Fig. 1(a) and (1) dQi dQi (b). The structures of Au5Al5-a and Au5Al5-b have been demon- 33 strated to be stabilized, and Au5Al5-a is the lowest energy structure of Au5Al5 clusters. The Au5Al5 bimetallic nanoclusters For the corresponding physical quantities, h is the Planck protected by appropriate ligands are expected to have a favor- constant, c is the speed of light, kB is the Boltzmann constant, This article is licensed under a able in uence on the enhancement mechanism of SERRS for ds dg probe molecules. Pyrazine is a suitable probe molecule to and T is the Kelvin temperature. The terms and are the dQi dQi investigate the SERRS enhancement mechanism of substrates derivatives of the isotropic and anisotropic polarizability of the in metal–molecule models8,32 owing to the bonding interaction Open Access Article. Published on 20 February 2017. Downloaded 10/5/2021 1:22:26 PM. ith vibrational mode, respectively, and u0 and ui respectively Fig. 1 The models of (a) Au5Al5-a (C4V), (b) Au5Al5-b (C4V), (c) pyrazine–Au–Au4Al5-a (C2V), (d) pyrazine–Al–Au5Al4-a (CS), (e) pyrazine–Au– Au4Al5-b (C2V), (f) pyrazine–Al–Au5Al4-b (CS). This journal is © The Royal Society of Chemistry 2017 RSC Adv.,2017,7,12170–12178 | 12171 View Article Online RSC Advances Paper denote the frequencies of the incident light and the ith vibra- order to explore the SERS enhancement mechanism of pyrazine tional mode. The nite lifetime was included using a damping binding to Au5Al5 clusters, the static electronic polarizability parameter of 0.004 a.u., which was determined by the dephas- components of the four models were calculated (Table 1). These ing time of the conduction electrons and reasonable for metal calculated results of complexes were signicantly increased clusters and resonant Raman intensities.20 compared to those of pyrazine, particularly in the z-direction, which resulted in strong enhancement of the static Raman intensity of complexes.