**5. Raman investigations of cluster systems**

#### **5.1. Raman scattering of gas-phase clusters**

**Figure 7.** (A) Undistinguishable Raman of a "PSU" document from both a printer and a copier (a); SERS spectrum of the document from the printer (Xerox Phaser 8560DN PS, Genuine XEROX Solid Ink, black) (b), compared with that from a copier (RICOH, Aficio, MP 7001) (c). (B) SERS examination of four handwritten samples, by coating the gold

Recently, Tian et al. have further pullulated this method in help of shell-isolated nanoparticleenhanced Raman scattering (SHINERS) technique [113, 114], as sketched in **Figure 8**. For a typical SHINERS system, Au nanoparticles were coated with ultrathin silica shells and sowing on probed surfaces, where the Au core provides SERS signal enhancement while the silica shell shields the metal core from direct contact with analyte molecules (i.e., prevents the contamination of the chemical system under study) [115], which differs from general SERS sampling method, simply by adding analytes onto SERS-active substrates or directly mixing the target solution with metal colloids [115–118]. The SHIERS 'smart dust' on the analyte surfaces was demonstrated of practical use in a few interdisciplinary research fields, such as inspecting pesticide residues on food and fruit, examining drug security and environment protection

**Figure 8.** A sketch showing the in-situ probing of biological structures by SHINERS. Reproduced with permission

accurately and rapidly, and characterizing biological structures.

clusters on the ink-area.

42 Raman Spectroscopy and Applications

from *Nature* 2010. [61, 62]

Raman spectroscopy can also provide essential vibrational information of clusters, although only a few investigations have been published so far. For this study, coherent anti-Stokes Raman spectroscopy (CARS) takes advantages of its excellent sensitivity as applied to investigations of isolated water clusters [119–121], but the nonlinearity of CARS experiments remains a challenge to the assignment of cluster sizes and structure identification. It is worth mentioning that Raman activity of gas-phase clusters is associated with free-rotating especially at high temperature, thus the internal rotational motions could largely differ from that obtained by matrix-assisted vibrational spectroscopic measurements where the molecule/clusters could be partially or wholly clamped. Among others, Otto et al. [122] reported a study of cold water oligomers utilizing spontaneous Raman scattering (SRS) technique and revealed the vibrational dynamics of water molecules, as shown in **Figure 9**. Analysis of Raman and IR activities of the hydrogen bond-mediated water clusters seeded in rare-gas expansion molecule beam provides insights into the excitonic OH oscillator coupling, as well as vivid information of ultrafast intermolecular energy transfer (which was often suppressed in femtosecond experiments for the condensed phase due to isotopic dilution). Such investigations from gas-phase chemistry enable to determine weakened or intensified Fermi resonance between OHstretching and OH-bending motion of hydroxide radicals.

**Figure 9.** Raman and IR spectra of mixed D2O/rare gas expansions. Raman spectra were scaled to the same monomer scattering intensity at the point of measurement in the respective blocks. Lower part: Raman spectra recorded using a 0.5 m monochromator. Basically, the clustering extent increases from bottom to top. Upper part: similar conditions but recorded using a 1 m monochromator. The clustering extent increases from bottom to top. Reproduced with permission from Ref. [122].

#### **5.2. Raman scattering from monolayer-protected clusters**

While expanded Raman investigations of gas-phase clusters remain a reasonable challenge, there are several publications addressing the Raman activities of monolayer-protected clusters (MPCs). Considering that the mid infrared region of the spectrum mainly reveals information about the ligand structures (e.g., C─H, N─H, O─H bond etc.) and their interactions with the metal core [123, 124], Raman spectroscopy actually has its advantages to identify low frequency vibrational modes such as S-Au-S stretching, wagging, scissoring, rocking, and twisting. These vibrations are expected to be weak in IR spectrum due to the low polarity (i.e., IR non-active) but likely prominent in the Raman spectrum (i.e., Raman active) [125, 126]. **Figure 10** presents a typical example of Au38 and Au25 MPCs, where the vibrations of two clusters containing monomeric (SR-Au-SR) and dimeric (SR-Au-SR-Au-SR) gold-thiolate staples in the metalligand interface are addressed. Raman activities of these clusters at different charge state with different protection ligands illustrated influences of cluster sizes and composition with respect to the monomeric and dimeric moieties [127].

**Figure 10.** (A) Calculated (a) and experimental (b) Raman spectrum of the Au38(SCH3)24 and Au38(2-PET)24 cluster, respectively. The experimental spectrum is cut off at 170 cm–1, the cutoff point of our optical filter. Radial and tangential Au–S modes of the staples are schematically represented. Radial vibrations of the long staples are responsible for bands with high intensity. Modes associated with the short staples (symmetric and antisymmetric stretching and tangential vibrations) have lower Raman intensity. (B) RCF-corrected Raman spectra of the low-frequency region of Au25(CamS)18 (a), Au25(2-PET)180 (b), and Au25(2-PET)18– (c). The clusters were measured coated on a rotating glass slide. Reproduced with permission from Ref. [127].

In addition to the small number of experimental Raman investigations of clusters, there are vast theoretical studies relating to cluster systems [128–136]. For example, utilizing timedependent density functional theory (DFT) calculations, Chen et al. [137] conducted a detailed Raman study of pyridine adsorbed on M@Au12 and M@Ag12 (M = Mo, W) clusters. They found that, the calculated Raman intensity of pyridine on M@Ag12 at charge transfer (CT) transition excitations were twice as that for pyridine on M@Au12, as shown in **Figure 11**, and the energies used for SERS excitations (in the region of 1.63–2.10 eV) were largely different from each other. Calculated interactions between the core and shell produced varying and strong CT transitions from the metal clusters to pyridine, which was demonstrated to be responsible for the altered optical properties. Also found was that, the complexes of pyridine on silver-caged clusters are largely tunable with the core compared to gold-caged clusters, providing insights to the silver and gold clusters even at the same sizes.

**5.2. Raman scattering from monolayer-protected clusters**

44 Raman Spectroscopy and Applications

to the monomeric and dimeric moieties [127].

Au25(CamS)18 (a), Au25(2-PET)180

Reproduced with permission from Ref. [127].

While expanded Raman investigations of gas-phase clusters remain a reasonable challenge, there are several publications addressing the Raman activities of monolayer-protected clusters (MPCs). Considering that the mid infrared region of the spectrum mainly reveals information about the ligand structures (e.g., C─H, N─H, O─H bond etc.) and their interactions with the metal core [123, 124], Raman spectroscopy actually has its advantages to identify low frequency vibrational modes such as S-Au-S stretching, wagging, scissoring, rocking, and twisting. These vibrations are expected to be weak in IR spectrum due to the low polarity (i.e., IR non-active) but likely prominent in the Raman spectrum (i.e., Raman active) [125, 126]. **Figure 10** presents a typical example of Au38 and Au25 MPCs, where the vibrations of two clusters containing monomeric (SR-Au-SR) and dimeric (SR-Au-SR-Au-SR) gold-thiolate staples in the metalligand interface are addressed. Raman activities of these clusters at different charge state with different protection ligands illustrated influences of cluster sizes and composition with respect

**Figure 10.** (A) Calculated (a) and experimental (b) Raman spectrum of the Au38(SCH3)24 and Au38(2-PET)24 cluster, respectively. The experimental spectrum is cut off at 170 cm–1, the cutoff point of our optical filter. Radial and tangential Au–S modes of the staples are schematically represented. Radial vibrations of the long staples are responsible for bands with high intensity. Modes associated with the short staples (symmetric and antisymmetric stretching and tangential vibrations) have lower Raman intensity. (B) RCF-corrected Raman spectra of the low-frequency region of

In addition to the small number of experimental Raman investigations of clusters, there are vast theoretical studies relating to cluster systems [128–136]. For example, utilizing timedependent density functional theory (DFT) calculations, Chen et al. [137] conducted a detailed Raman study of pyridine adsorbed on M@Au12 and M@Ag12 (M = Mo, W) clusters. They found

(b), and Au25(2-PET)18– (c). The clusters were measured coated on a rotating glass slide.

**Figure 11.** Raman spectra with CT excitation of the (A) Mo@Au12-Py complex, (B) W@Au12-Py complex, (C) Mo@Ag12- Py complex, and (D) W@Ag12-Py complex. Differential cross sections are in units of 10–30 cm2 /sr and wavenumbers are in cm–1. Spectra have been broadened by a Lorentzian having a width of 20 cm–1. Reproduced with permission from Ref. [137].

Regarding to the interesting CT of silver cluster with small organic molecules, recently Chen et al. [138] have given a study to the interactions between tetracyanoquinodimethane (TCNQ) and two typical silver clusters Ag13 and Ag20, as shown in **Figure 12**. It was found that charge transfer from silver clusters to TCNQ molecules initiates the Ag─N bond formation at selective sites giving rise to different isomers of the Ag13-TCNQ and Ag20-TCNQ complexes. From a spectroscopic analysis for the two CT complexes mainly on Raman and infrared activities, vivid illustration of electron cloud interactions and the behavior of TCNQ adsorbed on silver clusters was comprehensively demonstrated, along with frontier molecular orbital (FMO) and natural bond orbital (NBO) patterns. The calculated Raman activity for a TCNQ molecule of Ag20 was found consistent with experimental Raman measurement of TCNQ molecules on single-crystal Ag(1 1 1) surface. Further efforts in this field regarding to clusters and complex molecular aggregates are expected to clarify the charge-transfer interactions within building blocks of granular materials [138–140].

**Figure 12.** (Left) Raman spectra of TCNQ molecules. Calculated spectra of a free TCNQ molecule (blue, top) and surface-adsorbing Ag13-TCNQ (purple) Ag20-TCNQ (green) complexes; and experimental spectra of TCNQ on Ag(1 1 1) surface of silver single crystal (red).Interactions of tetracyanoquinodimethane with silver clusters Ag13 and Ag20 are demonstrated by first-principles calculations and Raman/IR spectroscopy. (Right) Natural bond orbital (NBO) donoracceptor (overlap) interactions between N and Ag atoms in Ag (cluster)-TCNQ complexes.

#### **6. Conclusions**

In this chapter, Raman spectroscopy is demonstrated of importance in solving scientific issues relating to molecule aggregates and cluster systems profiting from its spectral fingerprints by which aggregation states, phase transition and cluster structures can be identified. Raman theory for aggregated molecules is simply introduced based on molecular exciton theory and Raman scattering enhancement at the formation of vibro-excitonic levels. Next, we summarize the research advances toward both plasmon-free Raman and SERS systems, such as aggregation-enhanced Raman scattering (AERS), resonance Raman (RR) effect from aggregation, magnetic-field trapped Raman scattering (MFTRS), shell-isolated nanoparticle-enhanced Raman scattering (SHINERS), and Raman probes for aerosols, etc. With the development of scientific instrumentation, the importance of Raman spectroscopy toward precise-sized molecule aggregates and cluster systems will be more clearly embodied, enabling to step toward interdisciplines of cluster science, molecular science, material science and surface science.

#### **Author details**

Zhixun Luo\* and Jiannian Yao

\*Address all correspondence to: zxluo@iccas.ac.cn

Institute of Chemistry, Chinese Academy of Sciences, Beijing, China
