Far-infrared spectra of mesoporous ZnS nanoparticles J. Traji  c a, * , M. Rom  cevi  c a , N. Rom  cevi  c a , B. Babi  c b , B. Matovi  c b , P. Bal  a  z c a Institute of Physics, University of Belgrade, 11080 Belgrade, Serbia b Institute Vinca, University of Belgrade, 11000 Belgrade, Serbia c Institute of Geotechnics, Slovak Academy of Sciences, 043 53 Ko sice, Slovakia article info Article history: Received 12 February 2016 Received in revised form 16 March 2016 Accepted 2 May 2016 Available online 11 May 2016 Keywords: Nanostructures Electron-phonon interactions Light absorption and reflection abstract ZnS nanoparticles were synthesized mechanochemically by high-energy milling, with three different milling times (5 min, 10 min and 20 min). Nitrogen adsorption method was used for examining specific surface area and texture of obtained powders. It was found that all samples are completely mesoporous. The optical properties were studied by far-infrared spectroscopy at room temperature in spectral region of 50e600 cm 1 . The analysis of the far-infrared reflectivity spectra was made by the fitting procedure. The dielectric function of ZnS nanoparticles is modeled as a mixture of homogenous spherical inclusions in air by the Maxwell-Garnet formula. In the analysis of the far-infrared reflection spectra, appearance of combined plasmon-LO phonon modes (CPPMs) with high phonon damping are observed, which causes decrease of coupled plasmon-phonon frequencies. © 2016 Elsevier B.V. All rights reserved. 1. Introduction Research on semiconductor nanoparticles has significant sci- entific and practical interest because of their unique optical and electrical properties [1e4]. Zinc sulfide (ZnS) is an important IIeVI semiconductor which semiconductor nanoparticles has been investigated extensively because of its broad spectrum of potential applications such as in catalysts, cathode-ray tubes (CRT), field emission display (FED) phosphors for a long time. It can also be used for electroluminescent devices and photodiodes [5e12]. The differences between the nanoparticles and bulk particles are caused by a high surface to volume ratio, which induces the structural and electronic changes. These differences depend on particle sizes, shape and surface characteristics. The decrease of particle sizes causes an extremely high surface area to volume ratio. The enhanced surface area increases surface states, which change the activity of electrons and holes, and affects the chemical reaction dynamics. Consequently, much research on ZnS nanoparticles and their physicochemical properties has been carried out and various methods have been used for the preparation of these nanoparticles [13e16]. The plasmons of free carriers and the longitudinal-optical (LO) phonons interact via their macroscopic electric fields, and as the result the coupled LO phonon-plasmon modes (CPPMs) appears. The coupling of elementary excitations in solids has been investi- gated by many authors, and the phenomenological approach to this problem is formulated by several authors [17,18]. The most of published studies are devoted to the interaction of a single phonon with effective plasmons as well as the influence of the plasmon damping on the CPPM [19,20]. Our intention is to use far-infrared spectroscopy to study the fundamental properties of the coupled plasmon-phonons modes in the ZnS nanoparticles as well as to investigate these coupled modes under different phonon damping conditions. In this paper, we present the results obtained by using far- infrared spectroscopy (FIR) to study optical properties of the ZnS nanoparticles which are mechanochemical synthesized using high- energy milling. Specific surface area and texture of obtained nanoparticles were examined using nitrogen adsorption method. The dielectric function of ZnS nanoparticles is modeled as a mixture of homogenous spherical inclusions in air, by the Maxwell-Garnet formula. 2. Samples preparation and characterization Mechanochemical synthesis of ZnS nanoparticles was per- formed in a Pulverisette 6 planetary mill. The milling condition were: 50 balls of 10 mm diameter; weight charge of total powder mixture in the mill was 14.2 g, ball charge in the mill was 360 g, material of milling chamber and balls was tungsten carbide and * Corresponding author. E-mail address: jelena@ipb.ac.rs (J. Traji c). Contents lists available at ScienceDirect Optical Materials journal homepage: www.elsevier.com/locate/optmat http://dx.doi.org/10.1016/j.optmat.2016.05.004 0925-3467/© 2016 Elsevier B.V. All rights reserved. Optical Materials 57 (2016) 225e230