Alloying Effects on the Optical Properties of Ag–Au Nanoclusters from TDDFT Calculations

Abstract

The optical properties of alloyed AgÀAu 147-atom cuboctahedral nanoclusters are theoretically investigated as a function of composition and chemical ordering via a time-dependent density functional theory (TDDFT) approach. Compositions 37À63%, 46À54%, and 63À37%, in AgÀAu, and three types of chemical ordering, coreÀshell, multishell and maximum mixing, are considered. Additionally, the optical spectra of pure Ag clusters with several structural motifs are also studied. It is found that (a) pure Ag clusters exhibit a neater dependence of the absorption peak on the shape of the cluster than Au clusters, (b) the absorption spectrum of alloyed clusters is not strongly affected by changes in chemical ordering, possibly because of their limited size, and (c) the optical absorption peak smoothly shifts to higher energies, gets narrower, and substantially gains in intensity by increasing Ag concentration, in excellent agreement with available experimental data. An analysis of the character of the electronic transitions mostly contributing to the absorption peak allows us to rationalize the notable difference between Ag and Au in terms of optical properties and the effect of alloying.

Optical properties of nanoalloys

“…In particular in Ref. 44 not only the composition but also the chemical ordering effect on the spectrum of 147-atom bare clusters with cubo-octahedral geometry was studied, employing core-shell, multi-shell and maximum mixing candidate structures differing in their chemical ordering. It was then found that chemical ordering has a marginal effect on the plasmonic features, whose energy position and intensity is essentially governed by the composition, see Figure 1.…”

“…Surprisingly, to the best of our knowledge doping has not yet been considered for the Au 102 (SR) 44 compound which was actually the first nanocrystal molecule for which X-ray crystal structure was determined 160 163,164 . These calculations assumed that Ag-doping did not affect the structural framework of the cluster, which may be suggested by the constancy of the overall composition, and used the geometry proposed in Ref.…”

DNA-stabilized Ag–Au bimetallic clusters: the effects of alloying and embedding on optical properties

“…[22][23][24] For instance, Ag-Au bimetallic clusters seem promising in this context. [25][26][27][28][29][30][31][32][33] However, the ultimate goal is to bring these novel properties to real materials and applications. For this task, such clusters should be stable and able to function in the medium of interest, for example the living cell, be it for bio-labelling, 34 imaging, 35 medical purposes, 36,37 or as analytical sensors.…”

Surface plasmons in quantum-sized noble-metal clusters: TDDFT quantum calculations and the classical picture of charge oscillations

“…24,32,33 However, the spectra turn out to be rather sensitive even to rather subtle changes of the geometries. [6][7][8][9]23,24,26,[34][35][36][37][38][39] Using pseudopotentials and / or localized orbitals, the atomistic inhomogeneity of the metals can fully be taken into account. In recent years, calculations of intermediate-size noble metal clusters have been carried out using, in particular, time-dependent density-functional theory.…”