First principles calculations of indium impurity-cadmium vacancy complex in CdTe

First principles calculations are used to study stability of the complex formed by indium impurity and cadmium vacancy in CdTe. Formation energies and transition energy levels of the cadmium vacancy, indium impurity and their complex in different charge states are calculated using supercell method within density functional theory in the local density approximation. From the analysis of binding energy of the complex it is found that formation of the complex is favorable and the neutral and single charged states of the complex are stable. The studies of the formation energy as a function of the Fermi level show that interaction between the shallow indium impurity and cadmium vacancy results in the Fermi level pinning near the middle of the semiconductor band gap and leads to the formation of semi-insulating material

Study of melting and crystallization process of CsPbBr3 by differential thermal analysis

The crystalline CsPbBr3 was synthesized from CsBr (6N) and PbBr2 (5N) by the mechanochemical method with further fusion in quartz ampoule at 640-650 °С. After synthesis, the structure and chemical composition of the obtained material was confirmed by energy-dispersive X-ray spectroscopy (EDX) and X-ray diffraction analysis. The melting and crystallization of the obtained perovskite were investigated by the differential thermal analysis (DTA) with heating/cooling rates of 1, 5 and 10 °C/min. Measurements were carried on the self-constructed DTA-setup with S-type thermocouples in the range of 450-590 °C. Each cycle of heating/cooling was repeated three times to confirm the accuracy of the results obtained. A decrease in the melting point from ~ 568.1 °C to ~ 566.2 °C was demonstrated with an increase in the heating rate from 1 °C/min. up to 10 °C/min. respectively. Probably, it's due to the approach to equilibrium conditions of phase transformations at lower heating rates. We recorded an additional-endothermic effect during CsPbBr3 melting. This may indicate a complex process of melting the compound. The thesis of a two-stage melting mechanism of CsPbBr3 perovskite with an initial stage of fragmentation of the crystalline structure and subsequent dissolution of crystalline phase residues is proposed. It is reported that with increasing of the melt heating above a certain "critical" temperature (579-585 °C), its homogenization occurs, and the crystallization temperature is set at 540-550 °C for the heating/cooling rate of 1 ° C/min. and 538-543 °C for the rate of 5-10° C/min. All obtained data confirm the assumption of a two-step melting process of CsPbBr3 perovskite, and the relatively constant crystallization temperature after a critical point of overheating may also indicate a certain structure of the melt of the compound with short-range order in the arrangement of the structural units of the compound in the liquid phase.</jats:p

Transferrin-Associated Lipoplexes as Gene Delivery Systems: Relevance of Mode of Preparation and Biophysical Properties

Abstract The successful application of gene therapy depends highly on understanding the properties of gene carriers and their correlation with the ability to mediate transfection. An important parameter that has been described to improve transfection mediated by cationic liposomes involves association of ligands to cationic liposome–DNA complexes (lipoplexes). In this study, ternary complexes composed of 1,2-dioleoyl-3-(trimethylammonium) propane:cholesterol, plasmid DNA and transferrin (Tf, selected as a paradigm of a ligand) were prepared under various conditions, namely, in medium with different ionic strengths (HEPES-buffered saline [HBS] or dextrose), at different lipid/DNA (+/–) charge ratios and using different modes for component addition. We investigated the effect of these formulation parameters on transfection (in the absence and presence of serum), size of the complexes, degree of DNA protection and extent of their association with cells (in terms of both lipid and DNA). Our results show that all the tested parameters influenced to some extent the size of the complexes and their capacity to protect the carried genetic material, as well as the levels of cell association and transfection. The best transfection profile was observed for ternary complexes (Tf-complexes) prepared in high ionic strength solution (HBS), at charge ratios close to neutrality and according to the following order of component addition: cationic liposomes–Tf–DNA. Interestingly, in contrast to what was found for dextrose–Tf-complexes, transfection mediated by HBS-Tf-complexes in the presence of serum was highly enhanced

High-temperature electrical properties of undoped Cd1-xMnxTe (x = 0.05-0.55) crystals under cadmium vapor pressure

For the first time, the results of a study of the high-temperature electrical characteristics of undoped Cd1-xMnxTe single crystals in a wide composition range x = 0.05-0.55 are described. For this purpose, a study of Hall effect was made at the temperatures of 723–1073 K and isothermal and temperature dependences of electrical parameters were constructed. At 300 K, all the samples studied had a p-type conductivity, and Cd1-xMnxTe crystals with x = 0.02 and 0.15 had a low resistivity (102-103 Ohm×cm) while with x = 0.3 and 0.55 they had a relatively high ρ~ 107 Ohm×cm, which is due to an increase in the band gap with an increase in the MnTe content. The sample with the lowest Mn content (x = 0.02) showed the instability of the impurity-defect system during the first heating-cooling cycles: the conductivity type changed from p- to n- above 750 K and the mobility gradually increased as a result of the introduction of Cd atoms from the gas phase to the interstitial positions of the lattice. In this sample, during the first heating, the influence of donor impurities is noticeable, since the experimental line lg[e-] is significantly (~ 1-1.4 orders of magnitude) higher than that for model (undoped) CdTe, probably due to the donor behavior of Mn atoms. The crystal with the highest MnTe content (Cd0.45Mn0.55Te) also demonstrated the instability of the impurity-defect system, which was reflected in the decrease in the electrical conductivity after heat treatment at 773-873 K by almost one order of magnitude, which can be explained by a superposition of the results of the interaction of donor Cd atoms introduced from the gas phase with its native acceptor point defects (compensation) and the interaction of impurities between the matrix and Te inclusions. It has been established that a peculiarity of Cd1-xMnxTe crystals with a high content MnTe (x = 0.15, 0.55, and partially 0.3) is the inverse dependence of the carrier concentration on the cadmium vapor pressure caused by the onset of mixed conductivity of these crystals at high temperatures (T&gt; 873 K). Accordingly, for these crystals, the value 1/RH at the moment of the onset of mixed conductivity characterizes the conditional rather than real mobility of the charge carriers. On the pressure dependences of the charge carrier concentration, the corresponding dependences are lower than the undoped CdTe (especially at low temperatures) for samples with x≥0.15 indicating that the introduction of a significant amount of MnTe (x≥0.15) leads to a decrease in the charge carrier concentration by 0.5-0.8 orders of magnitude (at ~ 773 K). This is due to the influence of Mn atoms, which form a stronger bond with Te than Cd, and therefore the generation of electrons requires more energy than in pure CdTe.</jats:p

Electrical Instability of CdTe:Si Crystals

Results of Hall effect measurements of cadmium telluride crystals, doped by silicon (dopant concentration in the melt was 1018 - 1019 cm-3), allowed to classify the studied samples and the conditions under which probably the definite crystal and impurity states are realized. We have found the distinction between 3 type of CdTe:Si crystals: (1) low-resistance p-type crystals with shallow acceptors, in which Si impurity is localized mainly in the large inclusions; (2) semi-insulating crystal with deep acceptors and submicron size dopant precipitates that are source/drain for interstitials Sii - shallow donors; and (3) low-resistance crystals in which the n-type conductivity is provided by shallow donors: Sii (and/or SiCd). Therefore the silicon is responsible for n-type conductivity of doped samples, introducing as a donor Siі and provides semi-insulating state by forming deep acceptor complexes (SiCd-VCd2-)- with (Еv + 0.65 eV). Besides, the submicron silica precipitates, that have a tend to "dissolution" at relatively low temperatures, can act as electrically active centers. Keywords: cadmium telluride, silicon, doping, electrical properties, impurity, precipitates.</span

Peculiarities of Electrical Characteristics of Semi-Insulating CdTe-Cl crystals

Electrical properties of semi-insulating CdTe-Cl crystals, grown by the vertical Bridgman and the travelling heater method, have been studied. It is found that the travelling heater method provides electron conductivity of the crystals, and the vertical Bridgman method - hole conductivity. Specific resistance of the samples is of (108-109) Ohm*cm at 300 K, and Hall mobility of the holes and electrons is of (45 - 55) cm2/V*sec. and (10 - 20) cm2/V*sec., respectively. Very low values of electron mobility and an exponential temperature dependence of µn are due to drift barriers with a height of εb ≈ 0,20 eV. Formation of the barriers is caused by the fluctuations of the potential relief resulting from the microheterogeneity of the defect-impurity system. Quasi-photochemical reactions that reduce electron mobility after photo-excitation have been observed in n-CdTe-Cl samples. In p-CdTe-Cl samples, neither drift barriers, nor quasi-photochemical reactions were detected. Key words: transport phenomena, scattering of charge carriers, cadmium telluride.</span