Luminescence properties of silver zinc phosphate glasses following different irradiations
Introduction
Recent interest has grown in the ability to change the optical characteristics, most notably the refractive index, of various silicate and phosphate glasses doped with silver by irradiation. Many diverse applications, waveguides [1], [2], [3], [4], gratings [5], [6], [7], data-storage media [8], 3-D microstructures [9], [10], [11] and sensors, are possible with the controlled creation of precious metal (silver, gold, or copper) nanoparticles inside various glasses. There have been numerous techniques for Ag+-doped glass fabrication including sol–gel [12], ion-exchange [13], ion implantation [14] and melting/annealing [15]. Silver clusters or metallic nanoparticles can be synthesized by different irradiation methods: electron-beam [16], γ-ray [17], [18], ultraviolet (UV) [19], [20], [21] or by femtosecond infrared (IR) laser [22], [23], [24], [25]. A simplified description of the processes taking place at the level of the material can be presented. There exists a mobility threshold of the charge carriers corresponding, energetically, to a fundamental absorption band. One can then locally define, similar to a crystal, a forbidden band [26]. Thus, after the irradiation, the electrons pass from the valence band to the conduction band of the glass. Following this photoionization, the electrons are trapped by Ag+ ions forming neutral atoms of Ag0. In most cases identification of the induced silver species is not easy. It has been proposed, based on gamma-irradiated samples, that the trapping of the free electrons forms Ag0 and other more energetically favorable species (Ag2+, Ag32+) [18]. These species have specific spectral properties. Nevertheless, in most cases these Agmx+ centers have not been clearly identified. In this work, we report a spectral study of the silver species created after different irradiation including femtosecond IR irradiation and leading to different optical signature. Comparison using different irradiation methods is presented.
Section snippets
Experimental
Samples of a phosphate glass 40P2O5–4Ag2O–55ZnO–1Ga2O3 glass (mol%) were used to study the processes occurring after irradiation. Glasses were elaborated using standard melt-quench technique. Gallium oxide has been introduced to increase the glass stability. (NH4)2HPO4, ZnO, AgNO3 and Ga2O3 were used as raw materials and placed with appropriate amount in platinum crucible. A heating rate of about 1 °C min−1 have been conducted up to 1000 °C and kept at this last temperature 1000 °C for 24–48 h. The
Silver intrinsic luminescence in zinc phosphate glass
The glass exhibits an absorption cut-off (Fig. 1) at around 280 nm mainly due to the silver ions associated absorption. Fluorescence spectroscopy have been measured to investigate the emission of silver ions corresponding to dipolar electric transition 4d10→4d9 5s1. The main feature is composed of an excitation band at 265 nm and a corresponding emission at 380 nm. Such emission has been previously attributed to isolated Ag+ silver ions. Weaker excitation band can be recorded for excitation at
Discussion
Even if the mechanisms of formation of hole and electron traps are different, in all the irradiation cases, the 500 and 620 nm peak observed in the emission spectrum is believed to be due to aggregation of Ag0 and Ag+ ions which have possibly generated additional chemically stable clusters Agmx+ such as Ag2+, Ag32+,… [18], [27]. These species have specific absorption and emission features as it has been demonstrated for instance in liquid phase [28], [29]. Up to now, still, no clear assignments
Conclusion
Femtosecond IR pulses irradiation leads to the localized formation of Agmx+ silver clusters mixing of Ag0 and Ag+, which are more stable than neutral atom Ag0. Comparisons with different irradiation method have been studied and influence of the amount of silver Ag0 electron traps on the optical response is proposed. A mechanism of formation of silver clusters has been proposed. Fluorescent structures inside the glass have been observed in the focal volume optical voxel.
Acknowledgements
This work was carried out with the support of a number of research, equipment and educational French and US grants, including the Agence Nationale de la recherche ANR (Grant ANR-05-BLAN-0212-01), the CNRS PICS Grant 3179, a FACE grant from the French Embassy in the US, and an international REU NSF Grants (EEC-0244109 and DMR-031208).
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