Hotspot at Dimer, Small Aggregate and 2D Nanoassembly by Near-Field Scanning Optical Spectroscopy

Nanoassembly of noble metal nanoparticles facilitates surface plasmon confinements, and such localizations can coalesce and percolate due to a small variation in local nanogeometry. However, realizing such confinement by conventional microscopic and spectroscopic techniques has been challenging until now. We report near-field optical confinement observed by an aperture near-field scanning optical microscopy (a-NSOM). Three typical constructs of gold nanoparticles, viz., archetype dimer, nanoassembly of a few nanoparticles, and long-range two-dimensional (2D) nanoassembly, were investigated. Surface-sensitive techniques such as near-field surface-enhanced Raman scattering (SERS) and near-field surface-enhanced two-photon-induced photoluminescence (TPI-PL) have been recorded using a-NSOM. Well-correlated optometrology was confirmed amongst the SERS, TPI-PL, and shear-force topographies of the abovementioned constructs tapping simultaneously at the very same spot of interest.<br/>Distinctive aggregates and 2D nanoassemblies of gold nanoparticles (~100 nm diameter) were fabricated by the “sandwich technique”, wherein the droplet on a silane-treated treated cover-slip was sandwiched by another glass slide. Monomers, dimers and nano-assemblies of various sizes were confirmed by a field-emission scanning electron microscope. The specimens were spin-coated with Raman active dyes, Rhodamine 6G. A fs-Ti: sapphire laser (λ= 800nm, &lt;100fs, 80MHz) and He-Ne laser (λ=632.8nm) were coupled to the a-NSOM to carry out TPI-PL and SERS measurements. Au-coated tapered optical fiber was used to carry the excitations and act as an apertured probe (aperture ~ 50–100 nm) to scan the specimen. The emitted signals were collected by an objective lens and transferred to an avalanche photodiode (for single-channel detection) and/or to a polychromator-CCD (for multichannel detection).<br/>As expected, optical signals from SERS and TPI-PL were found to be confined and localized at the interstitials of dimers, whereas coalescence and hybridization of such nearby confinements for nanoassemblies were observed. For long-range 2D nano-assembly, optical confinements were more complex and homogenously distributed apart from the edge effect in this context. Finite-difference time-domain (FDTD) simulations were carried out to validate the results obtained in this investigation. FDTD models were designed so that the geometries and parameters closely represent the constructs under investigation and the a-NSOM setup. Such a direct observation with high spatial resolution is required to comprehend the origin of the localized electromagnetic (EM) field at the "hotspot" and the EM amplification factor in surface-sensitive optical processes.<br/>The author acknowledges the Interdisciplinary Research Center for Sustainable Energy Systems (IRC-SES), Research Institute, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia (Grant # INRE2318).