Eur. Phys. J. B 45, 317–324 (2005) DOI: 10.1140/epjb/e2005-00203-7 T HE EUROPEAN P HYSICAL JOURNAL B Optical characterization of the polymer embedded alloyed bimetallic nanoparticles V.I. Belotelov 1 , G. Carotenuto 2 , L. Nicolais 2 , G.P. Pepe 3, a , and A.K. Zvezdin 4 1 M.V. Lomonosov Moscow State University, Faculty of Physics, 119992, Leninskie gori, Moscow, Russia 2 Italian National Research Council, Naples, 80125, P.le Tecchio 80, Naples, Italy 3 Universit`a di Napoli “Federico II”, Dip. Scienze Fisiche, 80125, P.le Tecchio 80, Naples, Italy 4 General Physics Institute, RAS, 119991, Vavilov St., 38, Moscow, Russia Received 9 March 2005 / Received in final form 11 April 2005 Published online 6 July 2005 – c  EDP Sciences, Societ`a Italiana di Fisica, Springer-Verlag 2005 Abstract. A theoretical approach for the calculation of the bimetallic nanoparticles absorption spectra has been developed as an extension of the Mie theory in which nanoparticle dielectric function is found by the weighted linear combination of the dielectric functions for particles made of the corresponding pure met- als. In the frame work of the theoretical model an expression for the resonance light absorption frequency were derived taking into account the interband transitions in the dielectric functions. We propose a simple method for the on-line monitoring of the bimetallic nanoparticles composition based on the measurement of the absorption peak position. Elaborated theoretical approach was used to investigate the polymer em- bedded Ag/Au nanoparticles which were prepared by reducing gold and silver salts (HAuCl4 and AgNO3, respectively) by ethylene glycol in presence of poly(vinyl pyrrolidone) (PVP) at room temperature. Calcu- lated absorption spectra for the Ag/Au nanoscopic systems showed good agreement with the experimental data. Temporal evolution of the Ag/Au nanoparticles has also been investigated by this approach. PACS. 78.66.Bz Metals and metallic alloys – 78.67.Bf Nanocrystals and nanoparticles – 73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations) 1 Introduction Recently, there has been much attention paid to the cre- ation and investigation of bimetallic alloyed nanopar- ticles owing to their interesting surface plasma band energy [1–3], catalytic [4], non-linear optical properties, especially THG (third harmonic generation), and mag- netic properties [5,6], which are different from the ones of the individual metals. The magnetic properties of col- loidal ferromagnetic nanoparticles have been studied for their application to ferrofluids [7–9]. Bimetallic nanoclus- ters can also be widely exploited for use in biological label- ing, surface enhanced Raman scattering (SERS), etc. [10]. The intense research in this field is also motivated by the search for new multifunctional materials that will allow designing of the modern miniature electronic and optical devices for ultra fast data communication and optical data storage. Nanoelectronic devices prepared from careful con- struction of two- or three-dimensional layers of bimetallic nanoparticles are now a hot topic. Among different types of the bimetallic alloys Ag/Au nano-systems receive the most significant attention be- a e-mail: gpepe@na.infn.it cause of their unique optical and electrochemical prop- erties [2,3,11,12]. The intrinsic properties of a nano-structured metal are mainly determined by its size, shape, composition, crys- tallinity, and structure. In principle, accurate control of any one of these parameters allows for fine tuning of the properties of nano-structured material-based devices. One of the main metallic nanoclusters optical features is that they have broad absorption band in the visible re- gion of the electromagnetic spectrum [13]. Fluid and poly- mer systems of the noble metals copper, gold, and silver show a very intense colors, which are absent for the bulk materials. Their origin is attributed to the surface plas- mons – collective oscillation of the free conduction elec- trons induced by the incident electromagnetic radiation. The frequency position of the surface plasmon resonance in the alloyed nanoclusters is strongly dependent on the alloy composition and can be tuned finely by the compo- sition appropriate choice [2]. The latter is of prime impor- tance in connection to the possible applications for the light-stable color filters [14], polarizers [15], optical sen- sors [16], etc. Several methods have been used to prepare bimetallic nanoparticles, including radiolytic [17], alcohol [18], cit- rate [12], photolytic [19], and borohydride [20] reduction,