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Nearly monodisperse CdSe quantum dots of 1.8 nm in size were synthesized and capped with a surface monolayer of 1-thioglycerol. The optical properties of thin films made from these matrix-free close-packed quantum dots were studied at different temperatures and electric field strengths and compared with isolated quantum dots of the same size embedded in a PMMA matrix. The broadening and red shift of the optical transitions observed in absorption and excitation spectroscopy for the ensemble of close-packed quantum dots is explained by the formation of collective electronic states between interacting nanocrystals. The reversible collapse of these extended electronic states into localized states was demonstrated for the close-packed quantum dots by applying an external electric field.

This work presents a comprehensive study of electroabsorption in CdSe colloidal quantum dots, nanorods, and nanoplatelets. We experimentally demonstrate that the exposure of the nanoplatelets to a dc electric field leads to strong broadening of their lowest-energy heavy-hole absorption band and drastically reduces the absorption efficiency within the band. These are results of the quantum-confined Stark and Franz–Keldysh effects. The field-induced change in the nanoplatelets’ absorption is found to be more than 10 times the change in the absorption by the quantum dots. We also demonstrate that the electroabsorption by the nanorods is weaker than that by the quantum dots and nanoplatelets and reveal an unusual dependence of the differential absorption changes on the nanoplatelet thickness: the thicker the nanoplatelet, the smaller the change.

We examine collective effects in an ensemble of close-packed ultrasmall CdSe quantum dots by comparison of optical properties of dense and diluted thin film composites. Studies of absorption spectra, photoluminescence excitation spectra and electroabsorption of diluted and condensed structures along with numerical simulation confirmed that in a dense ensemble extended electron states occur by analogy with Anderson transitions in disordered solids. Therefore evolution from isolated quantum dots to quantum dot solids in case of strong confinement results in a drastic modification of optical and electronic properties. * ) Supported by Volkswagen-Stiftung. phys. stat. sol. (b) 224, No. 2, 393-396 (2001)

We have demonstrated a difference in the nature of the effect of a strong external electric field (>10 5 V/cm) on the photoluminescence of cadmium selenide nanoparticles of different shapes. We have determined a correlation between the magnitude of the external electric field and the average photoluminescence decay time for two types of nanoparticles: "quantum dots" and nanorods. We discuss the mechanism for the effect of an electric field on the photoluminescence of both types of nanoparticles.Keywords: "quantum dots", nanorods, cadmium selenide, kinetics of photoluminescence, polarization, electric field, Stark effect. Introduction.Optoelectronic devices designed on the basis of nanostructures that are controllable by an external electric field and that contain quantum-confined semiconductor nanoparticles allow for full integration of the optical component and conventional semiconductor microelectronics components. Effective control of the optical absorption and luminescence of such structures using an external electric field, as predicted by theory, can be fully realized in structures such as "quantum dots" [1, 2]. Chemically synthesized semiconductor nanocrystals, as representatives of this type of structure, also exhibit a dependence of the optical properties on the magnitude of the external field (the quantum-confined Stark effect for absorption [3], and also the Stark shift and quenching of luminescence on emission of light [4]). Luminophores based on such nanostructures in combination with blue light-emitting diodes are of special practical interest for designing economical sources of white light [5]. In contrast to "quantum dots" (called nanocrystals in the following),ànanorods have quantum confinement only in two spatial dimensions, which is why there is a difference in the manifestation of the effect of an external electric field and polarization selectivity of the optical properties relative to the field vector.Using an external electric field to control photoluminescence (PL) of semiconductor nanocrystals and nanorods requires detailed study of the spectral and kinetic characteristics of such structures as a function of the magnitude of the applied external electric field. The structures best studied to date are based on cadmium selenide compounds. However, the specific details of the mechanisms for radiative and nonradiative recombination in nanorods in the presence of a strong external electric field have not been fully identified. In particular, the difference in the mechanisms for quenching of photoluminescence for nanorods and nanocrystals, and also the effect of the orientation of nanorods relative to the external field on their photoluminescence kinetics, still remain insufficiently studied. Study of quenching of photoluminescence for single nanorods in an external electric field also has not provided a clear answer to the question concerning the possible mechanism of action for the field on radiative recombination of electron×donor pairs in nanoparticles [6][7][8][9][10].The aim of this work ...