Synthesis and optical characterization of SiO2/Zn2SiO4:Mn nanocomposite
Introduction
Efficient luminescent materials under vacuum ultraviolet radiation and low-voltage electron bombardment, in recent years, are becoming more and more important for their actual and potential applications [1], [2], [3], [4]. Particularly nanocrystal oxides (NCOs) in glasses, have recently received much attention due to their technological applications in high performance ceramic [5], microelectronics components [6], catalysis [7], [8] and sensors [9], [10], [11], [12]. Those nanometer-size crystals, which are comparable with the bulk exciton Bohr radius, exhibit intermediate behaviour between a bulk crystal and isolated molecule [13], [14], [15]. In the nanoparticles, there is a strong spatial delocalisation of valence electrons, and therefore a small crystallite must grow fairly large to achieve the limiting bulk electronic structure. The intermediate-size clusters can have unique properties, characteristic of neither the molecule nor solid state limits. Examples include metal clusters (as predicted by free electron shell theory) and semiconductor crystallites which exhibit the bulk unit cell but have only partial band structure development. Nanoclusters have optical spectra that can be tuned in wavelength simply by varying the crystallite size. In three-dimensions, they represent an analogy to the quantum well semiconductor heterostructures that show a one-dimensional quantum size effect. The bulk optical response depends on the number of polarizable electrons per unit volume so that a high-density collection of quantum nanoclusters is required, both for achieving a basic understanding of the quantum-structure phenomena and for device development. In designing such devices, by necessity, one must consider not only the atomic construction of the individual nanoparticles but also the assembly of these into a macroscopic unit. In fact, owing to their large surface, NCOs have tendency to aggregate, which strongly affect their applications. In order to overcome this disadvantage we incorporated NCOs in silica aerogel. During the elaboration of these nanocomposites, silica solution acted as “nanoglue” to build three-dimensional silica network. The aerogel networks play an important role in determining the oxide nanocrystalline size, confining their growth and retarding their motion.
Up to now, we have fabricated undoped NCOs such as ZnO, Al2O3, SnO2 and TiO2 incorporated in silica host matrix by sol–gel method combined with a furnace firing [16], [17], [18]. The structural and optical properties of the composites were investigated. Room temperature PL spectra of the obtained nanocomposites showed a strong luminescence bands in the Vis–IR range with different behaviours depending on type of incorporated oxide. It was suggested from different analyses that the contents of the OH-related spices and non-bridging oxygen hole centres were responsible for the different bands and the energy position strongly depend on the type of oxide incorporated in silica [17], [18].
In this study, the method is applied to prepare Zn2SiO4:Mn particles embedded in silica monolith by the same protocol of sol–gel method combined with a furnace firing [16] but using for the first time, manganese doped zinc oxide nanoparticles and studied the structural and optical properties of the obtained materials and their dependencies upon temperature and power excitation density.
Section snippets
Sample preparation
The preparation of manganese doped Zn2SiO4 as colloid suspension particles in silicate host matrix (SiO2/Zn2SiO4:Mn) has been done in three steps. In the first one, nanocrystalline ZnO:Mn aerogels were prepared by dissolving 2 g of zinc acetate dehydrate (Zn(CH3COO)2·2H2O) and 0.083 g of manganese(II) chloride-4-hydrate (MnCl2·4H2O) in 14 ml of methanol under magnetic stirring for 2 h. The water for hydrolysis was slowly released by esterification of acetate with methanol. Nanoparticles aerogel
Structural studies
Fig. 1 shows typical XRD pattern of the aerogel powders. Five pronounced diffraction peaks appear at 2θ=37.07°, 40.20°, 42.40°, 55.82° and 66.81° which can be attributed to the (1 0 0), (0 0 2), (1 0 1), (1 0 2) and (1 1 0) planes of ZnO, respectively [16]. The lattice constants calculated from the XRD pattern are a=3.249 Å, c=5.205 Å, which are very close to wurtzite ZnO ones, i.e., a=3.250 Å, c=5.207 Å [19]. This result indicates that ZnO:Mn has a polycrystalline hexagonal wurtzite structure. Diffraction
Conclusion
Manganese doped zinc oxide nanoparticles aerogel were synthesized by sol–gel method from zinc acetate dihydrate as a precursor. These particles were obtained by slow hydrolysis of the precursors using an esterification reaction, followed by a supercritical drying in EtOH. The X-ray diffraction and TEM show a crystalline phase with a particle size ranging between 15 and 30 nm. Upon incorporation in SiO2 and heat treatment at 1200 °C, Zn2SiO4:Mn phase was formed in SiO2 host matrix. Room
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