Modified optical absorption of molecules on metallic nanoparticles at sub-monolayer coverage

Abstract

Enhanced optical absorption of molecules in the vicinity of metallic nanostructures is key to a number of surface-enhanced spectroscopies and of great general interest to the fields of plasmonics and nano-optics. Yet, experimental access to this absorbance has long proven elusive. We here present direct measurements of the intrinsic absorbance of dye-molecules adsorbed onto silver nanospheres, and crucially, at sub-monolayer concentrations where dye-dye interactions become negligible. With a large detuning from the plasmon resonance, distinct shifts and broadening of the molecular resonances reveal the intrinsic properties of the dye in contact with the metal colloid, in contrast to the often studied strong-coupling regime where the optical properties of the dye-molecules cannot be isolated. The observation of these shifts together with the ability to routinely measure them has broad implications in the interpretation of experiments involving resonant molecules on metallic surfaces, such as surface-enhanced spectroscopies and many aspects of molecular plasmonics.Over the last two decades, the optical properties of metallic nanoparticles (NPs) of various sizes and shapes [1] [18][19][20]. Many of these existing and emerging applications are underpinned by the fact that the optical (electronic) absorption of molecules on the surface of metallic NPs is enhanced. But spectral changes induced by molecular adsorption are often ignored because of the experimental challenge of measuring surface absorbance spectra on nanoparticles, despite early attempts more than 30 years ago [21].This question is not directly addressed in the great number of recent studies devoted to the topic of strongcoupling between plasmons and molecules [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]; in this regime, the plasmon-molecule interaction is evidenced by a typical anti-crossing of the two resonances as a function of detuning [25,33], but in such a strongly interacting system the molecular response cannot be isolated. Moreover, in such studies the dye concentration is often large (typically monolayer coverage and above) to maximize dye/plasmon interactions. Dye-dye interactions cannot therefore be neglected and are expected to induce resonance shifts of the dye layer independently of any plas- * Eric.LeRu@vuw.ac.nz monic effects; in fact many studies specifically work with J-aggregates rather than isolated dyes [22, 25-27, 31, 33]. As a result, the intrinsic effect (chemical and/or electromagnetic) of the NP on an isolated adsorbed molecule cannot be elucidated. To this aim, we will show that it is necessary to measure the absorbance of the dye when the dye and plasmon resonances are detuned (to avoid dye-plasmon interaction effects) and at low surface coverage (to avoid dye-dye interaction effects). This low surface coverage poses a significant experimental challenge because the dye absorbance is then very small and obscured by the large optical response (absorption and scattering) of the NPs.We here propose to measure NP/d...

A spectroelectrochemical investigation of nanoparticle and molecular resonances in surface enhanced Raman scattering from crystal violet and malachite green

“…Our spectra show close agreement with previously reported SERS spectra for CV under similar conditions . The absorption maximum of CV adsorbed on silver nanoparticles (in an aqueous environment) is 502 nm (shifted from 588 nm for CV in solution phase) . The absorption maximum of the silver nanoparticles is approximately 420 nm.…”

“…Charge‐transfer transitions are typically invoked to explain the intensity profile. However, the absorption spectra for CV adsorbed on Ag nanoparticles (under typical SERS conditions) show only a single band that is assigned to the π − π * transition of CV . Spectra for other dyes (R6G, Nile Blue) show similar behaviour.…”

“…Absorption spectra of molecules adsorbed on metal nanoparticles often show strong coupling between molecule and plasmon states . Recently, absorption spectra have been recorded for plasmon and molecule states that are well‐separated in energy and at sub‐monolayer coverage, conditions that are often employed in single‐molecule SERS . Because of the large nanoparticle transition strength, the spectra are completely dominated by the nanoparticle absorption and the molecular transitions appear as a small shoulder on the nanoparticle absorption band shifted in energy with respect to solution phase spectra, with the extent of the energy shift largely determined by the solvatochromic nature of the molecule.…”

“…For large τ , mixing is strong and the electronic states have both dye and nanoparticle character. Given the absorption spectra suggest only weak coupling under SERS conditions, strong coupling will not be considered here.…”

Investigating Molecule-plasmon Interactions in Chemically-functionalized Metal Nanoparticles Using Monochromated EELS

“…Among other difficulties, the intense scattering arising from the metal constitutes a challenging technical issue for the study of molecule to metal or metal to molecule charge transfer (CT) states by optical methods. Notably, the direct measurement of the modified optical absorption of molecules on plasmonic nanoparticles has only been reported within the last year [2]. Novel approaches that can be used to gain access to the molecular electronic and vibrational states and to the plasmonic response of plasmonic particles on a nanometer length scale could yield a paradigm shift in thinking about molecule-plamon interactions and surface-enhanced optical spectroscopies.…”