Raman amplification in the ultra-small limit of Ag nanoparticles on SiO2 and graphene: Size and inter-particle distance effects

• Published in: Materials & design Vol. 192; p. 108702
• Main Authors: Cortijo-Campos, Sandra, Ramírez-Jiménez, Rafael, Climent-Pascual, Esteban, Aguilar-Pujol, Montserrat, Jiménez-Villacorta, Félix, Martínez, Lidia, Jiménez-Riobóo, Rafael, Prieto, Carlos, de Andrés, Alicia
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.07.2020; Elsevier
• Subjects: Localized surface plasmon resonances; Silver nanoparticle (arrays); Surface enhanced Raman scattering; Localized surface plasmon resonances; Silver nanoparticle (arrays); Surface enhanced Raman scattering
• ISSN: 0264-1275; 1873-4197
• DOI: 10.1016/j.matdes.2020.108702

Size, shape and hot spots are crucial to optimize Raman amplification from metallic nanoparticle (NPs). The amplification from radius = 1.8 ± 0.4 nm ultra-small silver NPs was explored. Increasing NP density redshifts and widens their plasmon that, according to simulations for NPs arrays, is originated by the reduction of the interparticle distance, d, becoming remarkable for d ≤ R. Inter-particle interaction red-shifts (>130 nm) and widens (>90 nm) the standard plasmon of non-interacting spherical particles. Graphene partly delocalizes the carriers enhancing the NIR spectral weight. Raman amplification of graphene phonons is moderate and depends smoothly on d while that of Rhodamine 6G (R6G) varies almost exponentially due to their location at hot-spots that depend strongly on d. The experimental correlation between amplification and plasmon position is well reproduced by simulations. The amplification originated by the ultra-small NPs is compared to that of larger particles, granular silver films with 7 < R < 15 nm grains, with similar extinction values. The amplification is found to be larger for the 1.8 nm NPs due to the higher surface/volume ration that allows higher density of hot spots. It is demonstrated that Raman amplification can be efficiently increased by depositing low density layers of ultra-small NPs on top of granular films. [Display omitted] •High amplification capability of silver ultra-small (radius R~1.8 nm) spherical nanoparticles with ligand-free surface.•The reduction of inter-nanoparticle distance, d, red-shifts (>130 nm) and widens (> 90nm) the Ag plasmon.•Raman signal amplification of Rhodamine and graphene vary very differently on d due to spatial electric field distribution.•Amplification for 1.8 nm nanoparticles is higher than for larger grains due to higher available hot-spots density.•Combining granular films with an extremely low-density layer of ultra-small nanoparticles further enhances amplification.