Control growth of PbS quantum dots doped sono-ormosil
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. Pin˜ero Dpto. Fı´sica Aplicado, Univeridad de Ca´diz, Aptdo. 40, 11510 Puerto, Real (Cadiz), Spain
N. de la Rosa-Fox and A. Santos Dpto. Cristalografı´a y Mineralogı´a, Estratigrafı´a, Geodina´mica y Petrologı´a y Geoquı´mica, Universidad de Ca´diz, Aptdo. 40, 11510 Puerto, Real (Cadiz), Spain
L. Esquivias Dpto. Fı´sica de la Materia Condensada, Universidad de Ca´diz, Aptdo. 40, 11510 Puerto, Real (Cadiz), Spain (Received 31 December 2000; accepted 21 June 2001)
Semiconductor PbS quantum dots doped-SiO2 organically modified silicate (ormosil) gels were synthesised via sol-gel by using high-power ultrasounds (sonogel). The effect of PbS crystal concentration and the addition of (3-mercaptopropyl)trimethoxysilane acting as surface capping agent (SCA) were investigated. By adjustment of the SCA to lead ratio, PbS nanoparticles of different sizes and morphologies were obtained. Textural parameters were calculated from N2 physisorption isotherms. The PbS galena phase was identified by x-ray diffraction, the crystal size by high-resolution transmission electron microscopy, and the exciton confinement by ultraviolet–visible–near-infrared spectrophotometry. Crystallite mean sizes of spheres and cubes ranging from 6.5 to 10.5 nm and needles 7-nm wide and 15–20 nm long, for different PbS and SCA concentrations, were obtained. These results differ from those predicted by the effective mass approximation corroborating the band gap modifications in the smallest nanocrystals. The method allows the control of the crystal size and improves the stabilization of the PbS nanocrystals.
I. INTRODUCTION
Electronic and optical properties of IV–VI semiconductors have been the object of extensive research in the last three decades. Their narrow band gap permits the creation of a strongly quantum confined exciton (one electron–hole pair) when microcrystallites are smaller than the bulk exciton Bohr radius (aB ⳱ 18 nm for the PbS) (quantum dots).1–3 This is the case for PbS crystallites presenting a particle size smaller than 18 nm, with a threshold of absorption of 0.41 eV for the bulk material. Many approaches have been explored for the preparation of small PbS clusters and its incorporation into solid and transparent matrices.4 – 6 One of them is the use of the sol-gel process combined with colloidal chemistry.7–9 This technique avoids agglomeration permitting the control of particle size and size distribution but produces instability of the small precipitates. However, surface-capping methods10 may help to overcome the last drawback. 2572
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J. Mater. Res., Vol. 16, No. 9, Sep 2001 Downloaded: 13 Mar 2015
Gel matrices and films have been used as hosts for semiconductor quantum dots from more than 10 years, mainly because low temperatures are required to create the network. On the other hand, they present a high porosity that makes them especially active for humid condensation and oxidation, thus favoring subsequent crystal growth. In spite of these inconveniences, this issue arouses interest and recentl
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