1027 ISSN 1062-8738, Bulletin of the Russian Academy of Sciences: Physics, 2018, Vol. 82, No. 8, pp. 1027–1029. © Allerton Press, Inc., 2018. Original Russian Text © A.G. Shmelev, A.V. Leontyev, D.K. Zharkov, V.G. Nikiforov, R.R. Shamilov, I.V. Kryukov, V.S. Lobkov, V.V. Samartsev, 2018, published in Izvestiya Rossiiskoi Akademii Nauk, Seriya Fizicheskaya, 2018, Vol. 82, No. 8, pp. 1133–1135. Effect of Quantum Size on the Luminescent Properties of Quantum Dots Based on Cadmium Halcogenides A. G. Shmelev a, *, A. V. Leontyev a , D. K. Zharkov a , V. G. Nikiforov a , R. R. Shamilov b , I. V. Kryukov c , V. S. Lobkov a , and V. V. Samartsev a a Zavoisky Physical-Technical Institute, Federal Research Center “Kazan Scientific Center of the Russian Academy of Sciences,” Kazan, 420029 Russia b Kazan National Research Technological University, Kazan, Tatarstan, 420015 Russia c Federal Research Center “Crystallography and Photonics,” Moscow, 119421 Russia *e-mail: sgartjom@gmail.com Abstract—The quantum size effects of colloid water-organic media synthesized nanocomposites (CdSe–CdS core–shell) in toluene solution is studied. Femtosecond fluorescence up-conversion spectroscopy is used to establish that quantum dots are characterized by a biexponential decay of luminescence kinetics: the decay times are 1.8 and 26.8 ps for nanoparticles with average sizes of 2 nm and 4.5, and 68 ps for nanoparticles with an average size of 2.9 nm. DOI: 10.3103/S1062873818080385 INTRODUCTION Semiconductor quantum dots (QDs) find wide practical application due to their unique chemical and physical properties [1]. These materials are character- ized by a quantum size effect on the bandgap width, and this makes them attractive for manufacturing optoelectronic radiators (light-emitting diodes (LEDs)), single-electron transistors (SETs), quantum dot (QD-LED) displays, lasers, and so on [2]. CdSe and CdS semiconductor nanocrystallites are of special interest among the known semiconductor QDs due to their unique optical properties [3]. The evolution of photoinduced electrons and holes (and thus the kinet- ics of luminescence) depend strongly on the size of a nanocrystallite and the properties of its surface. Detected optical responses generally have picosecond relaxation times, due to processes of exciton–photon interaction [4]. They are accompanied by Auger recombination processes [5], which cause fast subpi- cosecond decays in the observed optical responses. It is known from the literature [3] that QDs display mul- tiexponential luminescence decay with characteristic times of up to several hundreds of nanoseconds, depending on their size. Different ways of synthesizing semiconductor nanoparticles and controlling their size distribution are described in the literature [6]. One problem in syn- thesizing them is their aggregation into clusters several tens and hundreds of nanometers in size. This strongly suppresses the quantum size effect and prevents the application of clusters in optoelectronics and other fields of nanotechnics. To solve this problem, we use the core–shell method, which allows us to synthesize 2 and 2.9-nm QDs composed of CdSe cores coated with CdS shells. Such nanoparticles are much less prone to aggregation processes and retain all the opti- cal properties caused by the quantum size effect. In this work, we used femtosecond fluorescence up-conversion spectroscopy and common steady- state luminescence and absorption spectroscopy to determine the optical properties of quantum dots. EXPERIMENTAL CdSe nanoparticles were synthesized using the col- loid approach in water–organic media [7]. We used water-soluble ions of cadmium and selenium as ions source for QD synthesis (e.g., cadmium acetate and sodium selenosulfate). Oleic acid was selected as a sta- bilizer to obtain QDs dispersible in organic solvents (e.g., toluene, hexane, and chloroform). The organic component needed to dissolve oleic acid in this medium was ethanol or glycerin. CdSe nanocrystal- lites were crystallized at different temperatures, depending on the required optical characteristics. CdSe nanocrystallites with a luminescence maxi- mum at 500 nm were synthesized in a water–ethanol medium at a temperature of 50°C and a synthesis time of 1.5 h. They were further purified by reprecipitation from an ethanol–hexane mixture. The luminescence maximum of synthesized QDs was detected at 500 nm upon excitation at a wavelength of 340 nm (Fig. 1a). The absorption spectrum of QDs contained a broad wing corresponding to exciton absorption in the