PbS Quantum Dots in a Porous Matrix: Optical Characterization Aleksandr P. Litvin, † Peter S. Parfenov, † Elena V. Ushakova, † Anatoly V. Fedorov, † Mikhail V. Artemyev, ‡ Anatol V. Prudnikau, ‡ Valery V. Golubkov, § and Alexander V. Baranov* ,† † National Research University of Information Technologies, Mechanics and Optics, 197101 Saint Petersburg, Russia ‡ Institute for Physico-Chemical Problems, Belarusian State University, 220030 Minsk, Belarus § I.V. Grebenschikov Institute of Silicate Chemistry, Russian Academy of Sciences, 199155 Saint-Petersburg, Russia ABSTRACT: We propose simple and practical method for creation of quantum dot (QD) systems in a porous matrix. The commercial filter paper is soaked in the colloidal solution of PbS QDs in carbon tetrachloride followed by drying. The samples prepared by the method demonstrate linear depend- encies of optical density and photoluminescence intensity on the QD concentration, excellent homogeneity, and reprodu- cibility. A red-shift of QD photoluminescence spectrum after their infiltration into the matrix and energy transfer between QDs of different sizes indicate formation of the close-packed QD system. Optical properties and stability of the close-packed PbS quantum dot systems are investigated at room temperature in a wide range of QD sizes. A strong reduction of average QD photoluminescence lifetime from 435 to 55 ns with decreasing QD diameter from 3.0 to 7.4 nm has been found. A blue-shift of the photoluminescence spectra accompanied by increasing the photoluminescence lifetime observed for small and medium QDs with the sample storage indicates decreasing the QD size due to oxidation of their surface. ■ INTRODUCTION Over the past decades close-packed systems of semiconductor nanocrystals (quantum dots, QDs) have attracted much attention due to possibility to utilize them in a variety of optoelectronic devices, such as photovoltaic elements, 1 LEDs and lasers, 2 and photodetectors. 3 Because of size-dependent optical properties, broad absorption spectra with high extinction and narrow emission, high quantum efficiency, and excellent photostability semiconductor QDs can replace traditional materials of optoelectronics like organic dyes or bulk semiconductors. 4 QDs for near-infrared (NIR) photonics, such as PbS and PbSe QDs, attract a particular attention. 5 These QDs possess a number of unique properties, such as high charges mobility, large Bohr radii, small and equal effective masses of electrons, and holes. 6 PbS QDs demonstrate unusual luminescence properties, particularly large photoluminescence lifetimes 7-11 and significant Stokes shift. 8-10,12-14 An anomalous size dependence of PbS QDs luminescent properties has been recently explained by the existence of the size-dependent in-gap state and by the phonon-induced transitions between the fundamental and in-gap states. 15 Theoretical study of thermal transitions occurring with both decrease and increase in energy at room or higher temperature was made by Rukhlenko et al. 16 and Leonov et al. 17,18 Blended PbS QDs of different sizes deposited on substrates or embedded in polymer films and porous matrix are very promising as efficient broadband absorbers and emitters for various applications in the NIR region, including solar energy conversion and communica- tion. 19-25 A particular feature is that in the close-packed nanocrystal systems, or QD solids, with more or less pronounced nanocrystal size distribution appropriate conditions for Fö rster resonant energy transfer (FRET) are realized. 26-31 The FRET, an entirely nonradiative process caused by dipole-dipole interaction, 32 can change considerably optical and electrical properties of close-packed nanocrystal systems. Therefore, the FRET effects on energy spectra and dynamics of electron transitions in systems of close-packed QDs of different sizes are of great interest. There is a lack of knowledge on these topics especially for close-packed QDs of lead chalcogenides. Weak environmental stability restricts utilization of the lead chalcogenides QD solids. PbS QDs are known to be more stable under ambient conditions than PbSe QDs. 33 QDs of this type are usually capped by organic passivating agents such as oleic acid and trioctylphosphine. 5 This type of surface passivation is not efficient in case of dry QD solids and leads to degradation of optical properties of QDs in particular due to oxidation of QD. 34 If oxidation is a prevailing process, it results in decreasing of QD size and blue-shifts in absorption and emission spectra. Numerous efforts have been made to improve the stability of PbS QD systems. Moreels et al. 35 proposed a modified Cademartiri synthesis 36 to get stable PbS QDs in a wide range of sizes, but there is no information about dry close- packed PbS QDs systems based on this method. Ihly et al. 37 examined an atomic layer deposition (ALD) technique to cover Received: March 6, 2013 Revised: May 21, 2013 Published: May 22, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 12318 dx.doi.org/10.1021/jp402287b | J. Phys. Chem. C 2013, 117, 12318-12324