Крещение Крыма

Цель статьи - определить место и значение фестивной культуры в условиях социальных трансформаций

Type-II CdSe/CdTe Core/Crown Semiconductor Nanoplatelets

We have synthesized atomically flat CdSe/CdTe core/crown nanoplatelets (NPLs) with thicknesses of 3, 4, and 5 monolayers with fine control of the crown lateral dimensions. In these type-II NPLs, the charges separate spatially, and the electron wave function is localized in the CdSe core while the hole wave function is confined in the CdTe crown. The exciton’s recombination occurs across the heterointerface, and as a result of their spatially indirect band gap, an important emission red shift up to the near-infrared region (730 nm) is observed with long fluorescence lifetimes that range from 30 to 860 ns, depending on the type of interface between the core and the crown. These type-II NPLs have a high quantum yield of 50% that can be further improved to 70% with a gradient interface. We have characterized these novel CdSe/CdTe core/crown NPLs using UV–vis, emission, and excitation spectroscopy, X-ray diffraction, energy-dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy

Optimized Synthesis of CdTe Nanoplatelets and Photoresponse of CdTe Nanoplatelets Films

We study in detail the synthesis of CdTe nanoplatelets. Three populations of nanoplatelets with a thickness defined with atomic precision are obtained. We show that CdTe nanoplatelets can be extended laterally to obtain nanosheets with lateral dimensions in the micrometer range. We present the study of the reaction conditions for the formation of CdTe nanoplatelets and for their lateral extension. The reaction products are analyzed with optical spectroscopy, transmission electron microscopy, and small-angle X-ray scattering. We investigate the electro-optical properties of films formed with CdTe nanoplatelets, and we show that their current photoresponse is better than the one of comparable films formed with CdTe spherical nanocrystals

Synthesis of Zinc and Lead Chalcogenide Core and Core/Shell Nanoplatelets Using Sequential Cation Exchange Reactions

We present the synthesis of a novel type of chalcogenide nanoplatelets. Starting from CdS core or CdSe/CdS core/shell nanoplatelets, we use sequential cation exchange to copper and then to either zinc or lead to obtain ZnS and PbS core or ZnSe/ZnS and PbSe/PbS core/shell structures. The procedure preserves well the 2D geometry of the nanoplatelets, provided that they are more than 6 monolayers (∼1.8 nm) thick. The core/shell structure is also well conserved during the cation exchange as verified by TEM images. The nanoplatelets exchanged with Zn crystallize in a zinc blende structure, like the initial Cd-based material, whereas when Pb is used, the final nanoplatelets have a rock-salt crystal structure. We explored the copper cation exchange process using energy dispersive X-ray spectrometry with 1 nm resolution on nanoplatelets standing on their edges, and we show that copper ions diffuse uniformly from the outside of the nanoplatelet to the inside during the exchange

Electrolyte-Gated Field Effect Transistor to Probe the Surface Defects and Morphology in Films of Thick CdSe Colloidal Nanoplatelets

The optical and optoelectronic properties of colloidal quantum dots strongly depend on the passivation of their surface. Surface states are however difficult to quantify using optical spectroscopy and techniques based on back gated field effect transistors are limited in the range of carrier density that can be probed, usually significantly below one charge carrier per particle. Here we show that electrolyte gating can be used to quantitatively analyze the increase of defects in a population of nanoparticles with increasing surface irregularities. We illustrate this method using CdSe nanoplatelets that are grown in their thickness using low temperature layer-by-layer method. Spectroscopic analysis of the samples confirm that the nanoplatelet thickness is controlled, on average, with atomic precision, but structural analysis with transmission electron microscopy shows that the number of surface defects increases with the nanoplatelet thickness. The amount of charge defects is probed quantitatively using electrolyte-gated field effect transistor (EFET). We observe that the threshold voltage of the EFET increases with the NPL thickness, in agreement with the structural analysis. All samples displayed n-type conduction with strong current modulation (subthreshold swing slope of 100 mV/decade and on/off ratio close to 10⁷). We also point out that an efficient electrolyte gating of the film requires a fine control of the nanoparticle film morphology

Nature and Decay Pathways of Photoexcited States in CdSe and CdSe/CdS Nanoplatelets

The nature and decay dynamics of photoexcited states in CdSe core-only and CdSe/CdS core/shell nanoplatelets was studied. The photophysical species produced after ultrafast photoexcitation are studied using a combination of time-resolved photoluminescence (PL), transient absorption (TA), and terahertz (THz) conductivity measurements. The PL, TA, and THz exhibit very different decay kinetics, which leads to the immediate conclusion that photoexcitation produces different photophysical species. It is inferred from the data that photoexcitation initially leads to formation of bound electron–hole pairs in the form of neutral excitons. The decay dynamics of these excitons can be understood by distinguishing nanoplatelets with and without exciton quenching site, which are present in the sample with close to equal amounts. In absence of a quenching site, the excitons undergo PL decay to the ground state. In nanoplatelets with a quenching site, part of the initially produced excitons decays by hole trapping at a defect site. The electron that remains in the nanoplatelet moves in the Coulomb potential provided by the trapped hole