Papers by Keyword: Nanocrystal

Abstract: Theory of electronic and optical properties of excitonic states confining in Si nanocrystals is presented. The electron and hole states are numerically computed using the atomistic empirical tight-binding Hamiltonian including the spin-orbit coupling together with the first nearest-neighboring interaction. We theoretically study the electron-hole interactions in spherical silicon hydrogenated nanocrystals by incorporating coulomb and exchange interaction into the empirical tight-binding model. The comparisons of coulomb and exchange energies with empirical pseudopotential method (EPM), tight-binding method (TB), effective-mass approximation (EMA) and ab initio calculations are quantitatively realized. Finally the energies of the excitonic ground states obtained from diagonalizing the tight-binding configuration-interaction scheme are in a good agreement with other theoretical and experimental data.

Abstract: The mesoporous CaMoO₄ microcrystals have been prepared by precipitation in an aqueous solution. The scheelite-type structure in space group I4₁/a has been verified by the XRD analysis of the final powder product. The CaMoO₄ powder is composed of the almost size-uniform erythrocyte-like microspheroids with a diameter of ~5 μm. Every particle is formed by plate-like crystals with the characteristic thickness at a few-nanometer level and a diameter of ~1-2 μm.

Abstract: Nanoclusters and nanocrystals of the room temperature magnetic spinel CuCr₂S₄ synthesized using a facile solution-based method were examined by magnetic resonance in a wide frequency range 9.6÷80 GHz and at temperatures down to 4.2 K. Decreasing of the resonance field and broadening of the resonance lines are found below ∼ 50 K for both the nanocluster and nanocube samples due to the freezing of magnetic moments of nanocubes and nanocrystalline particles constituting nanoclusters. Additional blocking temperature T_b≅300 K appears in nanoclusters due to the freezing of the magnetic moment of the entire cluster as a whole. Below this temperature, an additional low-field resonance line is found in the resonance spectra of nanoclusters at X-band.

Abstract: Silicon (Si) is currently the basis of most of nanodevice technology, therefore ultrathin materials based on Si have the great advantage of easy integration into existing circuitry. First flat silicon nanoparticles have been obtained with perfluorophenyl (PFPh) ligand coating. The size of these particles varied from 15 to 50 nm. Their thickness evaluated with the atomic force microscopy was about 3.3 nm. Based on ab initio DFT calculations we investigate the geometries and electronic structures of free-standing PFPh-stabilized 2D silicon in order to see if such systems have promising electronic and optical properties. We also examined the effect of doping PFPh-stabilized 2D silicon by the Mn atoms.

Abstract: The aggregation of urine crystallites with different sizes in the urines of 5 cases of uric acid (UA) calculi patients and 5 cases of healthy controls were comparatively investigated by means of nanoparticle size analyzer. Different sizes of urine crystallites were obtained by filtrating the urine through microporous membrane with different pore sizes (0.22, 0.45, 1.2, 3, and 8 μm), respectively. The average particle size () increased rapidly with placement time (t) in the lithogenic patients. The values of the urine crystallites of the controls increased more slowly with t. When t was increased, the autocorrelation curves in the two types of urine crystallites became less smooth and the decay of the correlation curve became slower, and decay time of different sizes of urine crystallites both for the calculi patients and the controls increased. These results indicated that the urine crystallites of the controls were more stable than those of the patients. The rapid aggregation of urine crystallites may be an important factor affecting the growth of crystallites in UA stone patients.

Abstract: The effects of potassium citrate (K₃cit) on sedimentation and aggregation of 50 nm calcium oxalate dihydrate (COD) crystals was studied in order to explore the mechanism of formation and inhibition of kidney stone. In four different systems, including pure water, urine of healthy person and CaOx stone former before and after taking medicine K₃cit, the influences of K₃cit on average size (), Zeta potential and optical density of nanoCOD suspensions were studied by nanoparticle size Zeta potential analyzer and ultraviolet spectrophotometer. It was showed that K₃cit could specifically adsorb on nanoCOD crystals, and also competed with the inhibitor molecules in urine. Due to the decrease of inhibitors in urine of patients, K₃cit can be better to fulfill its inhibitory effect, leading to the largest reduced values of the diameter of COD crystal and the optical density of urine suspension. In conclusion, the inhibitory effect of K₃cit on aggregation and sedimentation of calcium oxalate crystals is better in urine of patients, whereas the inhibitory effect has been reduced in urine of control group and patients after taking k₃cit.

Abstract: An atomic model is proposed which describes the formation of dislocations and deformation twins through a direct plus reverse martensitic transformation localized in two or several slip planes. The reasons and mechanisms for the formation of partial dislocations and mechanical twinning in nanocrystals under plastic deformation are discussed.

Abstract: Nanocrystalline bismuth titanate (Bi4Ti3O12; BTO) powders were successfully prepared by the sol-gel method, using bismuth nitrate (Bi(NO3)3·5H2O) and tetrabutyl titanate (Ti(OC4H9)4) as source materials, acetic anhydride and ethanediol as solvents. The thermal decomposition and phase inversion process of the gel precursors were studied by using differential thermal analysis (DTA). The crystal structures and microstructures of BTO powders were investigated by using x-ray diffraction (XRD), and transmission electron microscope (TEM). The crystallization of amorphous bismuth titanate has been discussed. The effect of sintering temperature on the structure and morphology of BTO was investigated. At 644 oC and above, BTO powder undergoes a phase transformation from tetragonal to orthorhombic. At 900 oC, the purified orthorhombic BTO nanocrystals were obtained.

Abstract: Objectives: The effects of sodium citrate (Na₃cit) on sedimentation and aggregation of calcium oxalate dihydrate (COD) nanocrystallites with a size of about 50 nm was studied in order to explore the mechanism of Na₃cit to inhibit calcium oxalate stone formation. Methods: The influence of concentration of Na₃cit (c(Na₃cit)) on size, Zeta potential, aggregation, and sedimentation of COD nanoparticles in aqueous solution was studied using nanoparticle size Zeta potential analyzer and ultraviolet spectrophotometer. Results: In low c(Na₃cit) (< 0.8 mmol/L), the inhibition effect of Na₃cit on aggregation and sedimentation of nanoCOD increased with the increase of c(Na₃cit); while in high c(Na₃cit), COD was gradually dissolved by Na₃cit. The former was due to the specific adsorption of citrate ions (cit^3-) on COD surface, which made the absolute Zeta-potential increased, therefore inhibited the aggregation and sedimentation of nanocrystallites; the later was because the high concentration of cit^3- made Ca²⁺ ions dissociated from the crystal surface, leading to COD dissolution. Conclusions: Na₃cit can inhibit the aggregation and sedimentation of nanoCOD in low concentration and dissolve COD in high concentration, indicating that Na₃cit can inhibit the formation of calcium oxalate stones.

Abstract: Fluorine-doped tin dioxide (FTO) nanocrystals were prepared with sol–gel method using SnCl₄·5H₂O and NH₄F as precursor material. The FTO was characterized with X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Differential Thermal Analysis and Thermal Grativity (DTA-TG) and Infrared Radiation (IR) respectively. The electrical property was measured with Hall Effect Sensor. The result of XRD and SEM shows that FTO nanocrystal size is about 20 nm and the dimension of the grain is about 300 nm. IR spectrum analysis proves fluorine doping. The crystal phase transformation was discussed with DTA-TG curve. When the sintering temperature is 450°C, the sintering time is 60 min, and the molar ratio of F to Sn is 2:10, the sheet resistance of FTO film is 107Ω/□.