Nanostructured surfaces

Obtaining and properties of monolayer ensembles of isolated star-shaped Au nanoparticles
Beynik T.G., Matveevskaya N.A.
State Scientific Institution “Institute for Single Crystals” Institute for Single Crystals of NAS of Ukraine, 60 Nauky Ave, Kharkiv, 61072, Ukraine.
E-mail: beynik@isc.kharkov.ua

In recent decades branched Au nanoparticles, NPs, (nanostars, nanoflowers) have been increasingly used in many fields, including catalysis, Raman spectroscopy, etc. due to the presence of sharp branches where significant electric field strength increase is observed caused by the excitation of delocalized electrons of Au NPs by the electromagnetic wave. For today, the problem of formation of monolayer films based on metal star-shaped NPs, which correspond to the essential requirements (monodispersity of NPs with the ability of controlling their size and morphology; homogeneity of NPs distribution over the substrate surface; isolation of NPs, absence of aggregated fraction; film insensitivity to the environment), is little studied, thus the aim of this work was to obtain stable homogeneous monolayer ensembles with preservation of NPs individual properties, as well as to study the optical properties of the obtained materials.
Monolayer ensembles of star-shaped gold NPs with precisely controlled geometric parameters (shape, size, number and length of sharp branches) and filling degree were synthesized using template synthesis on functionalized glass substrates. Au NPs are characterized by a monomodal size distribution in a monolayer, the average NPs size is in the range of 50-90 nm. The obtained materials have a high distribution uniformity of isolated Au NPs in a monolayer. The mechanism of formation of monolayer ensembles is proposed. 
The study of the surface profile of monolayer ensembles with (80 ± 20) nm average NPs size showed that NPs have a three-dimensional shape, the average roughness of the films surface is 20-30 nm, which corresponds to the length of the sharp NP branches ((25 ± 5) nm). It was shown that in absorption spectra of monolayer ensembles of NPs with (80 ± 20) nm average size two maxima are observed, which correspond to the longitudinal (1010 nm) and transverse (620 nm) peaks of localized plasmon resonance. The using of the obtained materials as SERS-substrates allows significantly increasing the Raman signal intensity of Rhodamine 6G and crystal violet molecules.