Nature of the Anomalous Size Dependence of Resonance Red Shifts in Ultrafine Plasmonic Nanoparticles

Abstract

Plasmonic red shifts of nanoparticles are commonly used in imaging technologies to probe the character of local environments, and the understanding of their dependence on size, shape, and surrounding media has therefore become an important target for research. The red shift of plasmon resonances changes character at about 8−10 nm of size for spherical gold nanoparticles�above this value, the red shift progresses linearly with particle size, while below this size, the red shift changes nonlinearly and more strongly with size. Using an atomistic discrete interaction model, we have studied the special properties of the nanoparticle surface layers and discovered its importance for ultrafine plasmonic nanoparticles and their red shifts. We find that the physical origin for the specific properties inherent to the surface layer of atoms near the nanoparticle boundary is related to the anisotropy of the local environment of atoms in this layer by other atoms. The anisotropy changes the conditions for light-induced nonlocal interactions of neighboring atoms with each other and with the incident radiation compared to the atoms located in the particle core with isotropic nearest surroundings by other atoms. The local anisotropy of the nanoparticle crystal lattice is a geometric factor that increases toward its boundary and that is the most fundamental factor underlying the physical differences between the nanoparticle surface layer and the core material. It is shown that the inflexion point at 8−10 nm is due to a change in the dominant physical origin of the red shift�from chaotization of atomically light-induced dipoles within the surface layer in the case of ultrafine nanoparticles to retardation effects for large nanoparticles in which the relative volume of the surface layer decreases rapidly to a negligible value with increasing nanoparticle size. The patterns revealed are the basis for predicting the manifestation of surface layer effects in ultrafine plasmonic nanoparticles of different shapes and composed of different plasmonic materials.

Synthesis and Characterization of Ultra‐Small Gold Nanoparticles in the Ionic Liquid 1‐Ethyl‐3‐methylimidazolium Dicyanamide, [Emim][DCA]

“…[37] Therein the smallest gold nanoparticles with a diameter of 8.9 nm display an absorption maximum at 517 nm. Recently, the surface plasmon red-shift size dependence in the diameter range 3 to 10 nm has been reported by Sørensen et al [38] Therein, the particles with the smallest diameter of 3.13 nm only show a very weak and broad band at around 500 nm. In general, no surface plasmon resonance could be found in gold nanoparticles smaller than 2 nm in diameter.…”

Multijet Gold Nanoparticle Inks for Additive Manufacturing of Printed and Wearable Electronics

“…UV–vis was employed to evaluate the size of gold NPs as the surface plasmon resonance (SPR) peak position red shifts with the decreasing size of gold NPs . Multiple batches of the gold NPs were produced and combined to yield the final ink.…”

“…The droplet diameter, in the case of inkjet printers, depends on the nozzle orifice diameter, which is 21.5 μm. Ink jet printers are capable of forming stable droplets when the Z number is between 1 and 14 by using a cosolvent ink system for Newtonian fluids Figure S2 shows viscosity, surface tension, and Z number for different combinations of solvent systems, demonstrating precise control over the parameters.…”

“…UV−vis was employed to evaluate the size of gold NPs as the surface plasmon resonance (SPR) peak position red shifts with the decreasing size of gold NPs. 35 Multiple batches of the gold NPs were produced and combined to yield the final ink. As the particles are kept in suspension after synthesis and washing procedures, absorbance spectra were obtained for various concentrations of the gold NPs so that Beer's law could be utilized to determine the concentrations of the final ink by using the absorbance intensity of the SPR peak.…”

“…Ink jet printers are capable of forming stable droplets when the Z number is between 1 and 14 by using a cosolvent ink system for Newtonian fluids. 35 Figure S2 shows viscosity, surface tension, and Z number for different combinations of solvent systems, demonstrating precise control over the parameters. Table S1 shows the variation of ink parameters as a function of cosolvent system.…”