Photoluminescence of Silica Gel.docx

1、Photoluminescence of Silica GelPhotoluminescence of Silica GelMoke Mao 1568610AbstractSilica Gel is a common material that is economic and easy to produce. It is widely used as desiccant for its great surface area and absorption capability. Recently the photoluminescence of silica gel has brought gr

2、eat interest with successful results in tuning the emission of the luminescence. The property of the silica gel and the mechanism of photoluminescence are introduced in this review with detailed model of ODC model which explain the photoluminescence in silica gel; Various methods and factors to tune

3、 the photoluminescence are introduced with specific detail of the experiment that using Ag doping to obtain near white light photoluminescence.Silica GelSilica gel is a granular, vitreous, porous form of silicon dioxide made synthetically from sodium silicate.It was obtained as a scientific curiosit

4、y as early as the 1640s and was used in World War I for the adsorption of vapors and gases in gas mask canisters.In World War II, silica gel was indispensable in the war effort for keeping penicillin dry, protecting military equipment from moisture damage, as a fluid cracking catalyst for the produc

5、tion of high octane gasoline, and as a catalyst support for the manufacture of butadiene from ethanol, feedstock for the synthetic rubber program.The synthetic route for producing silica gel was patented by chemistry professor Walter A. Patrick at Johns Hopkins University in 1918.Silica gel has a ve

6、ry high specific surface area around 800 m2/g andan average pore size of 2.4 nanometers thus it has a strong affinity for water molecules. It is useful as a desiccant (drying agent), often described as absorbing moisture, which may be appropriate when the gels microscopic structure is ignored, as in

7、 silica gel packs or other products. However, material silica gel removes moisture by adsorption onto the surface of its numerous pores rather than by absorption into the bulk of the gel.Silica gel is also used in food additive, chromatography and plastic surgery implants.Recently, the photoluminesc

8、ence properties of silica gels brought up great interests because its economic, efficiency and tunable. It has great great potential in lighting industry.Mechanism PhotoluminescencePhotoluminescence (PL) is light emission from any form of matter after the absorption of photons. It is one of many for

9、ms of luminescence (light emission) and is initiated by photo-excitation1.Following excitation various relaxation processes typically occur in which other photons are re-radiated.Fig.1 Excitation and relaxation of photoluminFig.1 shows a typical process of excitation and relaxation of PL. When a sem

10、iconductor is excited with a light-source that provides photons with an energy larger than the band gap2, the incoming light excites a polarization that can be described with the semiconductor Bloch equations. Once the photons are absorbed, electrons transit to conduction band and leave holes in val

11、ence bands with finite momenta. The excitations then undergo energy and momentum relaxation towards the band gap minimum. Typical mechanisms are Coulomb scattering and the interaction with phonons. Finally, the electrons recombine with holes and emit photons with the same energy as the band gap as t

12、he semiconductor.Photoluminescence in Silica GelIt is well established that the photosensitivity is connected to a light-induced transformation of pre-existing point defects in glass matrix or to a creation of new ones3. Particular roles are played by specific defects, connected to partially reduced

13、 Si or Ge species, the so-called oxygen deficiency centers (ODCs).Oxygen-Deficiency Centers The structural models of Si- and Ge-related ODCs remain controversial.The main reason for this situation is simple: ODCs are diamagnetic and thus invisible to electron paramagnetic resonance (EPR) techniques.

14、 EPR has provided so far most of the structural information on defects in SiO2-based glasses. The three fundamental centers which form the basis of the present understanding of defects in vitreous silica, the E-center4, the peroxy radical (POR)5and the non-bridging oxygen hole center (NBOHC)6have be

15、en all discovered and characterized by EPR.The main optically active oxygen deficiency related defects in silica are the family of different paramagnetic E centers and two kinds of diamagnetic oxygen deficiency centers, most com- monly denoted as ODC(I) and ODC(II).Fig.2 shows the typical models of

16、oxygen deficiency centers.Fig. 2. Some variants of E centers in SiO2. 3(A) fragment of a perfect lattice;(B) asymmetrically relaxed oxygen vacancy with an unpaired electron localized in a sp3-like orbital of a single Si atom (the classic model of E1 center in -quartz or E center in silica); (C) refined model for E1 or E centers shown in its lowest-energy state featuring an outwards relaxation of the positively charged Si at