Localized surface plasmon-induced vibrational excitations in the surface-enhanced Raman scattering using two-dimensional array of silver nanocubes

Abstract

We have investigated the nanoscale structural and vibrational properties of polyvinylpyrrolidone (PVP)-covered silver nanocubes (AgNCs) array. The ordered AgNCs array fabricated on Au(111) by the Langmuir–Blodgett method was utilized for a surface-enhanced Raman scattering (SERS) platform. The local arrangements and the facet structure of AgNCs were observed by various types of microscopies, which revealed that AgNCs form an ordered monolayer and exhibit an atomically flat facet. The vibrational peaks associated with PVP were clearly observed in the Raman spectra owing to the SERS effect by AgNCs. We found that two types of C=O stretching peaks appear in the Raman spectra, of which the appearance patterns depend on the excitation energy of the incident laser. The simulations based on the finite-difference time domain method imply that highly ordered 2D AgNCs enable us to excite localized surface plasmon modes, such as a single particle mode and a gap mode, of AgNCs selectively, leading to vibrational excitation of PVP existed at the surface and the gap of AgNCs.

Thermally and photoinduced structural and chemical changes of a silver nanocube array on Au(111)

“…The height analysis indicated that the root mean square (rms) roughness of the AgNC surface is 0.32 nm. This means that the facets of AgNC are almost atomically at, as was pointed out in a previous paper 27. Fig.3(a) shows the Raman spectrum of PVPcovered AgNCs/Au(111) at room temperature.…”

“…The Au(111) substrate was obtained by evaporating Au (purity, 99.99%) on a freshly cleaved mica surface under vacuum (3 Â 10 À4 Pa), followed by annealing at 500 C. [41][42][43] The 2D array of AgNCs was fabricated on Au(111) by the LB method. 27 We evaluated the shape and array structures of AgNCs by SEM (Hitachi High-tech, SU8000) and AFM (SII, SPI3700, contact mode). Raman spectroscopy (JASCO, NRS5100) was performed at room temperature under an atmosphersic condition using a laser of 532 nm wavelength (spot diameter, 1 mm, power, 0.01 mW), a 600 lines per mm diffraction grating, and a Â100 objective (N.A.…”

“…The details of the assignments of the vibrational peaks have been reported in the previous paper. 27 Although the thickness of PVP is small (4-6 nm), the SERS effect of AgNCs enables us to observe vibrational peaks of PVP. 27 Next, this sample was annealed in UHV to induce the structural change of AgNCs.…”

“…20,21 Thermal and photoinduced structural changes of NPs are oen accompanied by a shi in LSPR wavelength and are proposed to control the plasmon properties of NPs. 4,5,7,[19][20][21][22][23][24] Recently, a silver nanocube (AgNC) has drawn much attention since it has a higher electric eld enhancement [25][26][27][28][29][30][31] than and a superior catalytic activity to AgNP of spherical shape. 32,33 Unique nanoscale properties, such as single-molecule detection by the SERS effect, 25 superior catalytic activity on (100) facets 32,33 and plasmonic chemical reactions, [34][35][36] and brightening of an organic light-emitting device, 37 have been realized by utilizing AgNCs.…”

“…39,40 As described above, although shape changes of isolated AgNCs have been well studied, those of AgNCs in an array have not been reported. Several research groups pointed out that unique characteristics, such as the enhanced SERS effect 26,29 and selective excitation of localized surface plasmon (LSP) modes, 27 appear in the AgNC array. Thus, it is highly expected that the optical properties of AgNC arrays can be tuned by changing the size and alignment of AgNCs.…”