Anomalous transmission of disordered arrays of silver nanoclusters in the near- and mid-IR regions

Abstract

Arrays of nanometer sized clusters are attracting the interest of researchers because of the wide range of their possible applications [1]. The optical properties of arrays of metallic nanoparticles are of special inter est [2][3][4][5][6]. Significant work is also at present being done on studying composite structures consisting of arrays of nanoparticles made from rare earth metals that, in turn, are separated by wide gap semiconduc tors [7][8][9][10][11][12][13][14][15]. On the one hand, the alternation of metallic and semiconducting layers offers the possibil ity of creating a photonic bandgap. On the other hand, such morphology allows one to circumvent the cap ture of charge carriers by the structural defects of materials. This, in its turn, will result in the introduc tion of a novel class of subwavelength devices such as waveguides, sensors, and biosensors [16][17][18]. How ever, optical effects observed in nanocomposite mate rials still require a detailed study.In the present paper, we demonstrate the difference between the transmission spectra of a ZnO/Ag struc ture (single and double period) in which Ag was present either in the form of a cluster array or in the form of a continuous film. Equal amounts of silver were used in both cases.Different methods for the production of nanocom posites are known [7-14, 18, 19]. Our technique for the synthesis of Ag/ZnO structures is based on stan dard deposition processes (magnetron sputtering and vacuum evaporation) with subsequent annealing in vacuum.Glass for VR L photographic plates was used as a substrate for the deposition of films. Its thickness and surface roughness were 1200 µm and ~3 nm, respec tively. This type of glass cuts off part of the spectrum in the region of wavelengths below ~300 nm, while its transmission in the rest of the studied range (250-2500 nm) is about 90%. Prior to the experiments, this glass was cleaned with a mixture of sulfuric acid and hydrogen peroxide, rinsed with deionized water, and dried in isopropyl alcohol vapors.Zinc oxide films were deposited in vacuum via magnetron sputtering of the corresponding target by argon ions following a preparatory ionic cleaning of the substrate surface. The vacuum in the chamber was no worse than 7 × 10 -5 Torr, and the working gas (Ar) pressure was 7 × 10 -3 Torr. This method for the depo sition of ZnO was chosen for the potential to fabricate films with reproducible parameters [20].An array of silver clusters was formed via vacuum evaporation and condensation of small amounts of material from a molybdenum evaporator located at a distance of 20 cm from the substrate. The residual pressure of gases in the vacuum chamber was no higher than 2 × 10 -5 Torr. The experiments on determining the thickness of thicker silver layers fabricated from weighed portions of material with a mass of 100-500 mg were performed in advance. The layer thick ness was measured with the use of an atomic force microscope. A calibration dependence of the silver film thickness on the mass of material was plotted based on t...