Journalof Sol-GelScienceand Technology8, 1023-1028(1997) 9 1997KluwerAcademicPublishers. Manufacturedin The Netherlands.
Fabrication of PbS Nanoparticles Embedded in Silica Gel by Reverse Micelles and Sol-Gel Routes N. PELLEGRI AND R. TRBOJEVICH
Ceram. Mat. Lab., FCEeI-IFIR-UNR, Av. Pellegrini 250, 2000 Rosario, Argentina O. DE SANCTIS
Laboratorio de Materiales Ceramicos-FCEeI IFIR-U.N.R., Av. Pellegrini 250, 2000 Rosario, Argentina
[email protected] K. KADONO
Osaka National Research Institute, Midorigaoka, Ikeda City, Osaka 563, Japan
Abstract. PbS doped-silica gels showing a visible absorption onset were prepared by the sol-gel method. PbS nanoparticles with strong quantum-confinement effect were obtained from sodium sulfide and lead nitrate by the reverse micelle method. Chemical parameters such as the water/surfactant and the [Pb2+]/[S2-] ratios play a very important role in the PbS particle size and in their absorption threshold. The PbS nanoparticles were dispersed in a hydrolyzed solution of TEOS and converted to homogeneous gels after heating. The absorption threshold of PbS doped-gel is blue shifted compared to the one of the as-prepared PbS particles. The non-linear optical properties of the PbS nanoparticle solution were measured by degenerate four-wave mixing and the X(3) value was estimated to be 1.95 10-H esu.
Keywords:
1.
PbS doped-silica gel, reverse micelles, sol-gel process, quantum confinement effect, non-linearity
Introduction
Non-linear optical properties of glasses have become the focus of growing scientific and technological interest. Recently, much attention has been paid to semiconductor colloid doped-glasses, because of their high optical non-linearity [1]. The large non-linearity of semiconductor nanoparticles is suggested to come from quantum confinement effect of electronic states. For the practical uses of semiconductor doped-glasses, the stability of the non-linear properties against laser irradiation is one of the most important factors as well as the response time. The use of dispersed media to synthesize nanoparticles in situ has growth considerably in the past few
years. Langmuir-Blodget films [2], vesicles [3], double layers or reverse micelles [4] were all used for preparing nanoparticles. CdS [5] and PbS [6] nanoparticles with intermediate and strong quantum-confinement effect, respectively, were synthesized in reverse micelles. Although the nanoparticle doped-glasses have been prepared by many routes, the sol-gel route overcomes many difficulties exhibited by almost all the other techniques such as: high temperature processing or inhomogeneous distributions of the dopants [7]. The aim of this work has been to develop a combined route for the synthesis of nanoparticles dispersed in a transparent matrix, that is based both on the reverse miceUes synthesis of PbS nanoparticles and on the solgel synthesis of the host matrix.
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Experimental
2.1. Synthesis of PbS Particles The PbS particles were prepared by mixing two 0.25M AOT (Sodium dioctyl sulfosucinate)-heptane reverse micelles containing Na2S and Pb(NO3)2, respectively, and with the same ionic concentration and water/AOT ratio (W = [H20]/[AOT]). The salts were introduced via their aqueous solution. By mixing different volumes of cationic and anionic reverse micelles at constant W, the [pb2+]/[S 2-] ratio (hereafter defined as X) was ranged between 2 and 0.5. The final PbS concentration was 4.7 10-3M for all samples. A rapid formation of PbS particles was observed after mixing both ionic micelles. In addition to the PbS micelles, CdS micelles were prepared in similar conditions.
2.2. PbS Doped-Gel Preparation Gels containing 0.9 wt% PbS were prepared by solgel route starting from a mixture of tetraethoxysilane (TEOS) and dimethylformamide (DMF). To this mixture, another one made of water, ethanol and nitric acid was added dropwise under stirring at room temperature (molar ratio TEOS/H20/C2HsOH/HNO3 = 1 : 1 : 2 : 1 : 5 . 1 0 - 3 ) . After obtaining a clear solution, a reflux treatment was performing at 70~ during 4 hours. The heptane of the PbS colloidal solution (W = 4, X ----- 1) was eliminated. The residue was dispersed in ethanol. This solution was mixed with the hydrolyzed TEOS solution. Immediately after obtaining a homogeneous solution, a mixture of ammonia and ethanol was added under stirring at 0~ After casting into polystyrene molds and gelificating at room temperature, the gels (covered with aluminum foils) were held at 50~ during 24 hours, aging at 85~ during 48 hours and 72 hours after perforating the aluminum foils. The dried gels were heated at a heating rate of 2.5~ Isothermal treatments were performed at 150~ and 170~ for 8 and 24 hours, respectively.
mixing (DFWM) arrangement. The experimental setup for DFWM geometry is described elsewhere [8]. A frequency-doubled mode-locked Nd : YAG laser (~. = 532 nm) was used. The pulse duration, the energy and the area at the sample position were 8 ns, 1 MW/cm 2 and 0.07 cm 2, respectively. All beams were vertically polarized and the probe beam had an incidence angle of 20 ~ with respect to the forward beam. The phase conjugate reflectivity was detected by a streak camera (Hamamatsu Photonics M1953). Silica cuvette of 1 nun thickness were used as containers. 3.
Results and Discussion
3.1. Optical Absorption Spectra of PbS Nanoparticle Solutions Figure 1 shows the absorption spectra of the PbS particles in AOT/heptane solutions, which are strongly blue shifted from the bulk PbS absorption spectra (onset at 3100 nm). This shift is due to the quantum confinement effect at the PbS particles. As the electron-hole pair wave is confined by potential wells of small lateral dimensions, a perturbation of the electron structure of the semiconductor appears, the conduction and valence bands become discrete and the gap energy increases. The absorption onset is assigned to transitions from the highest hole subband to the lowest electron subband level. This behavior produces a size-dependent blue shift of the exciton absorption. It is thus possible to relate the average size of the particles to their absorption threshold. In spite of the strong quantum confinement, the absorption spectra of the PbS particles does not indicate well-resolved discrete absorption peaks as it is observed in the CdS particles (W = 4 and X = 2, insert in Fig. 1). The lack of any excitonic structures should be attributed to a wide particle size distribution. The very long tails in the absorption spectra of PbS nanoparticles are also produced by that distribution. This makes the precise determination of the band gap energy (Eg) difficult. However, neglecting the tails, Eg can be estimated by means of
ot = (hv - Eg) 1/2
(1)
2.3. Optical Characterization The UV-visible absorption spectra were carried out using a Perkin Elmer ~.2S Model UV-Visible spectrophotometer between 200 and 1100 nm. The third order nonlinear optical properties were measured by a phase conjugation degenerate four-wave
where ot is the absorption coefficient Table 1 shows Eg and the mean particle size. The size of the PbS particles is derived from the value of Eg using Wang's relationship [9]. The absorption onset moves towards the visible wavelength range, as the water content decreases from
Fabrication of PbS Nanoparticles
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