We present a theoretical study of the optical response of silver clusters, Ag n , where n = 4, 8, 10, 20, complexed with the aryl thiols FC 6 H 4 S − and CH 3 C 6 H 4 S − in an aqueous solution. The absorption spectra are found to be strongly modified by the adsorption of aromatic thiols with a red shift of the plasmon-like band and the emergence of new excitations due to charge-transfer transitions between thiols and the metal cluster. Our results highlight the influence of the molecular orientation of thiol ligands relative to the cluster surface on the excitations. We also analyze the appropriateness of substituting a thiol molecule by a SH group. Calculations have been performed using the time-dependent densityfunctional theory (TDDFT).
■ INTRODUCTIONThe interaction between light and metal nanoparticles is a topic of great fundamental and technological interest with potential applications in many fields, such as nanoelectronics, nanooptics, and biosciences. 1 In particular, the noble metal nanoparticles are characterized by their ability to support surface plasmons. However, their optical properties depend, among others, on the particle's shape 2−5 and the particle's size. 6−15 For example, small silver clusters have molecular-like optical transitions, and their spectra are composed from several narrow or broad peaks whose the positions depend on the number of atoms that compose the cluster. 8,16−18 As silver clusters grow, they lose these molecular properties and develop a large, unique absorption band in the UV−vis region. The emergence of this plasmon-like band has been shown to occur for clusters of about 20 silver atoms. 12,19 The plasmon band is red shifted as the cluster's size increases. 6,7,9,20,21 While medium-sized clusters (several tens of atoms) present a main band near 4 eV; 8,9,22,23 the nanoparticles of about 8 nm in diameter show a strong optical absorption band near 3.2 eV. 7,21 Let us note that the optical properties can be also modified by mixing several elements together. 23−27 Large efforts are engaged in the synthesis methods in order to control the size, shape, and atomic arrangement of metal nanoparticles, on which depend their physical and chemical properties. As free metal nanoparticles are inherently unstable against thermal and chemical decomposition, one had to develop techniques to stabilize them. When metal clusters are produced in the gas phase, they can be accumulated and stabilized in a protective solid matrix. 8,18,28 If they are synthesized in solution, the stabilization of clusters may be ensured by the surface adsorption of strong ligands, like thiols, resulting in thiolate monolayer-protected clusters, consisting of a metallic core that is surrounded by ligands. 29−34 The adsorbed molecules provide original and controllable properties to the clusters, mainly in the field of optical absorption or emission, and enable chemical functionalization. However, the optical properties of metal nanoparticles can be affected by the adsorbate. 14,29,35−37 The plasmon resonances o...
