Formation of sub-surface silver nanoparticles in silver-doped sodium–lead–germanate glass J. M. Ferna ´ndez Navarro • J. Toudert • Y. Rodrı ´guez-Lazcano • B. Mate ´ • M. Jime ´nez de Castro Received: 30 October 2012 / Accepted: 16 April 2013 Ó Springer-Verlag Berlin Heidelberg 2013 Abstract The formation of silver nanoparticles in 60GeO 2 –20PbO–20Na 2 O bulk glass doped with 0.15 wt% of Ag has been studied by optical methods in the near ultraviolet-to-near infrared and mid-infrared ranges. A clear optical absorption band, which grows when increasing the annealing temperature, is observed around 460 nm, as a consequence of the surface plasmon resonance in the Ag nanoparticles. From the simultaneous analysis of optical transmittance and spectroscopic ellipsometry spectra in the near ultraviolet-to-near infrared range, it is demonstrated that the nanoparticles are surprisingly formed only in a thin layer (some tens of nm thick) underneath the sample sur- faces. The potential of such a simultaneous optical analysis for determining the localization of the nanoparticles in glasses of any nature is underlined. Based on the results of a complementary mid-infrared spectroscopy characterization, the processes involved in silver migration to the surfaces and further aggregation to form nanoparticles are discussed. 1 Introduction Nanocomposite materials consisting of metal nanoparticles (NPs) embedded in bulk glasses are of great interest for a large variety of applications because of their linear and nonlinear optical properties with high potential for optical and optoelectronic devices [1]. The importance of surface excitations in nanometre-sized metallic particles leads to changes in their physical properties [2]. Their fundamental property used for optical applications is the excitation of surface plasmon resonances (SPRs) [1, 3] in the free- electron metallic grains (collective oscillations of the conduction electrons), which lead to enhanced optical absorption/scattering cross sections and near-field inten- sity. The SPR’s spectral features, such as the resonance wavelength and bandwidth, together with the topology of the enhanced near-field, depend on the NP size, shape and spatial arrangement in the glass matrix [4]. Controlling these structural parameters is required for achieving the desired applications. For instance, one can take advantage of the intense near-field or scattering at the SPR for enhancing the far-field luminescence intensity of rare-earth (RE) ions embedded in the glass, provided they are located at the close vicinity of Ag or Au NPs [5, 6] whose SPR must peak at the absorption or emission wavelength of the RE. Several methods to obtain bulk glass with embedded metal NPs have been used [7–9]. Besides using the sol–gel methods in a few cases [7], most of the studied glasses (tellurites, germanates, silicates) have been obtained by the melting procedure. Ag or Au NPs are obtained either by Na–Ag ion exchange in the already produced glass samples [6, 9, 10] or by adding salts to the mixture of reagents to be melted, in both cases performing a further thermal annealing process in order to reduce the Ag ? or Au 3? ions for nucleation of silver or gold NPs [11]. The optical response in the near ultraviolet-to-near infrared range of noble metal-doped glasses has been extensively charac- terized by transmittance measurements, evidencing a SPR J. M. Ferna ´ndez Navarro  J. Toudert  M. J. de Castro (&) Laser Processing Group, Instituto de O ´ ptica ‘‘Daza de Valde ´s’’ (IO), Consejo Superior de Investigaciones Cientı ´ficas (CSIC), Serrano 121, 28006 Madrid, Spain e-mail: miguel.jimenez@csic.es Y. Rodrı ´guez-Lazcano  B. Mate ´ Grupo de Fı ´sica Molecular de Atmo ´sferas y Plasmas, Departamento de Fı ´sica Molecular, Instituto de Estructura de la Materia (IEM), Consejo Superior de Investigaciones Cientı ´ficas (CSIC), Serrano 121, 28006 Madrid, Spain 123 Appl. Phys. B DOI 10.1007/s00340-013-5458-6