SERS in PAH-Os and gold nanoparticle self-assembled multilayers

NICOLAS TOGNALLI; ALEX FAINSTEIN; ERNESTO JULIO CALVO; CECILIA BONAZZOLA; LIA PIETRASANTA; MARIANO CAMPOY-QUILES; PABLO ETCHEGOIN

JOURNAL OF CHEMICAL PHYSICS

American Physical Society

Año: 2005 vol. 123 p. 447071 - 447079

0021-9606

We present a surface enhanced Raman scattering (SERS) study of poly(allylamine) modified with Os(byp)2ClPyCHO (PAH-Os) and gold nanoparticles self-assembled multilayers (PAH-Os+(Aunanoparticles/ PAH-Os)n, n = 1 and 5). AFM and variable angle spectroscopic ellipsometry measurements indicate that the first nanoparticle layer grows homogenously by partially covering the substrate without clustering. Analysing the samples thickness and roughness we infer that the growth process advances thereafter by filling with nanoparticles the interstitial spaces between the previously adsorbed nanoparticles. After five immersion steps the multilayers reach a more compact structure. The more effective interaction between near gold nanoparticles plasmons provides a new optical absorption around 650 nm which, in addition, allows a more effective SERS process in that spectral region than at the single plasmon resonance (530 nm). We compare electronic resonance Raman and SERS amplification mechanisms analyzing Raman resonance scans and Raman intensity micromaps. As a function of nanoparticle coverage we observe dramatic changes in the Raman intensity scans, with maxima that shift from the electronic transitions, to the plasmon resonance, and finally to the coupled-plasmon absorption. The Raman micromaps, on the other hand, evidence huge intensity inhomogeneities probably related to “hot-spots”. At the same time, the Raman spectra evolve from a strong functional group enhancement selectively in the electronic resonance case, to the observation of vibrations from the whole molecule at SERS resonance. Finally, we present some qualitative numerical discrete dipole approximation (DDA) results that model the interaction between gold nanoparticles and help to understand the origin of the coupled-plasmon absorption and red-shifted Raman hot-spots.2ClPyCHO (PAH-Os) and gold nanoparticles self-assembled multilayers (PAH-Os+(Aunanoparticles/ PAH-Os)n, n = 1 and 5). AFM and variable angle spectroscopic ellipsometry measurements indicate that the first nanoparticle layer grows homogenously by partially covering the substrate without clustering. Analysing the samples thickness and roughness we infer that the growth process advances thereafter by filling with nanoparticles the interstitial spaces between the previously adsorbed nanoparticles. After five immersion steps the multilayers reach a more compact structure. The more effective interaction between near gold nanoparticles plasmons provides a new optical absorption around 650 nm which, in addition, allows a more effective SERS process in that spectral region than at the single plasmon resonance (530 nm). We compare electronic resonance Raman and SERS amplification mechanisms analyzing Raman resonance scans and Raman intensity micromaps. As a function of nanoparticle coverage we observe dramatic changes in the Raman intensity scans, with maxima that shift from the electronic transitions, to the plasmon resonance, and finally to the coupled-plasmon absorption. The Raman micromaps, on the other hand, evidence huge intensity inhomogeneities probably related to “hot-spots”. At the same time, the Raman spectra evolve from a strong functional group enhancement selectively in the electronic resonance case, to the observation of vibrations from the whole molecule at SERS resonance. Finally, we present some qualitative numerical discrete dipole approximation (DDA) results that model the interaction between gold nanoparticles and help to understand the origin of the coupled-plasmon absorption and red-shifted Raman hot-spots.n, n = 1 and 5). AFM and variable angle spectroscopic ellipsometry measurements indicate that the first nanoparticle layer grows homogenously by partially covering the substrate without clustering. Analysing the samples thickness and roughness we infer that the growth process advances thereafter by filling with nanoparticles the interstitial spaces between the previously adsorbed nanoparticles. After five immersion steps the multilayers reach a more compact structure. The more effective interaction between near gold nanoparticles plasmons provides a new optical absorption around 650 nm which, in addition, allows a more effective SERS process in that spectral region than at the single plasmon resonance (530 nm). We compare electronic resonance Raman and SERS amplification mechanisms analyzing Raman resonance scans and Raman intensity micromaps. As a function of nanoparticle coverage we observe dramatic changes in the Raman intensity scans, with maxima that shift from the electronic transitions, to the plasmon resonance, and finally to the coupled-plasmon absorption. The Raman micromaps, on the other hand, evidence huge intensity inhomogeneities probably related to “hot-spots”. At the same time, the Raman spectra evolve from a strong functional group enhancement selectively in the electronic resonance case, to the observation of vibrations from the whole molecule at SERS resonance. Finally, we present some qualitative numerical discrete dipole approximation (DDA) results that model the interaction between gold nanoparticles and help to understand the origin of the coupled-plasmon absorption and red-shifted Raman hot-spots.530 nm). We compare electronic resonance Raman and SERS amplification mechanisms analyzing Raman resonance scans and Raman intensity micromaps. As a function of nanoparticle coverage we observe dramatic changes in the Raman intensity scans, with maxima that shift from the electronic transitions, to the plasmon resonance, and finally to the coupled-plasmon absorption. The Raman micromaps, on the other hand, evidence huge intensity inhomogeneities probably related to “hot-spots”. At the same time, the Raman spectra evolve from a strong functional group enhancement selectively in the electronic resonance case, to the observation of vibrations from the whole molecule at SERS resonance. Finally, we present some qualitative numerical discrete dipole approximation (DDA) results that model the interaction between gold nanoparticles and help to understand the origin of the coupled-plasmon absorption and red-shifted Raman hot-spots.