Self-Organized Lead(II) Sulfide Quantum Dots Superlattice
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Self-Organized Lead(II) Sulfide Quantum Dots Superlattice José Maria C. da Silva Filho1, Victor A. Ermakov1, Luiz G. Bonato2, Ana F. Nogueira2, Francisco C. Marques1 1 2
Institute of Physics “Gleb Wathagin”, University of Campinas Institute of Chemistry, University of Campinas
ABSTRACT We show that superlattice (SL) of PbS quantum dots (QD) can be easily prepared by drop casting of colloidal QD solution onto glass substrate and the ordering level can be controlled by the substrate temperature. A QD solution was dropped on glass and dried at 25, 40, 70 and 100ºC resulting in formation of different SL structures. X-ray diffractograms (XRD) of deposited films show a set of sharp and intense peaks that are higher order satellites of a unique peak at 1.8 degrees (two theta), which corresponds, using the Bragg’s Law, to an interplanar spacing of 5.3 nm. The mean particles diameter, calculated through the broadening of the (111) peak of PbS using the Scherrer’s formula, were in agreement with the interplanar spacing. Transmission electron microscopy (TEM) measurements were also used to study the SL structure, which showed mainly a face centered cubic (FCC) arrangement of the QD. The photoluminescence (PL) spectrum of QD in the SL showed a shift toward lower energy compared to one in solution. It can be attributed to the fluorescence resonant energy transfer (FRET) between neighbors QD´s. Moreover, we observed greater redshift of PL peak for film with lower drying temperature, suggesting that it has a more organized structure. INTRODUCTION Nanomaterials, like quantum dots (QD) and nanoparticles (NP), have become subject of great interest for material researchers due to their unique optical and quantum properties, such as intense photoluminescence, quantum confinement, tunable band-gap and relaxed Ramanscattering selection rules. Among these nanomaterials, lead(II) sulfide QD (PbSQD) have occupied a featured position due to their small bulk band-gap (0.41 eV), large Bohr radius (18 nm) and high charge mobility [1–4]. These characteristics have enabled many applications in LED’s, long wavelength telecommunications, near-infrared (NIR) detectors, and solar cells [5– 9]. The relatively easy surface passivation of PbSQD has stimulated their use as building blocks in self-assembled systems. The interest in superlattice (SL) fabrication is mainly due to the enhancement of QD properties and appearance of new collective properties arising from interaction between neighbors QD [4,10,11]. In this work, we show that optical and structural properties of QD SL depend on drying temperature of colloidal QD solution, since different drying temperatures lead to different drying times. These dependences were revealed through X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) measurements.
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