Vera V. Butova

Atomic and electronic structure of CdS-based quantum dots

The ab initio computer design of the CdS-based quantum dots and the cobalt doped CdS quantum dots is carried out. The characteristics features of the atomic and electronic structures of semiconductor colloidal quantum dots on CdS of different sizes are studied, and the effect of cobalt impurity atoms is estimated. We have proved the sensitivity of the X-ray absorption near-edge structure (XANES) method for the verification of the nanosized atomic structural parameters calculated by the methods of computer modeling for small-scale quantum dots of the CdS family, and for the determination of the local environment parameters of the cobalt atom in the quantum dot.

Synthesis and structure modeling of ZnS based quantum dots

Quantum dots (QDs) based on zinc sulfide are synthesized by a microwave method in an aqueous medium using dioctyl sodium sulfosuccinate (DS) or 4,4′-bipyridine (BP). Based on the analysis of X-ray diffraction profiles the conclusion is drawn that QDs obtained have a structure of cubic zinc blende with an average particle size of 5.6 nm for the ZnSDS sample and 4.8 nm for ZnSBP. Transmission electron microscopy images show the presence of spherical aggregates of particles only for ZnSDS. FTIR data indicate the presence of sulfate ions in both samples; DS remains in the sample, facilitating the QD agglomeration, while BP is effectively washed out. From the optical diffuse reflectance spectra the band gap is estimated, which turns out to be larger than the expected one due to the presence of elemental sulfur in the samples and partial oxidation of the QD surface. The QD structure based on ZnS particles is also modeled in the work. The possibility to employ X-ray absorption near-edge spectroscopy for the verification of atomic structural parameters around zinc sites in QDs based on zinc sulfide is demonstrated.

Local atomic and electronic structure of quantum dots based on Mn- and Co-doped ZnS

Solid solutions of zinc sulfide with manganese and cobalt are synthesized. Based on the analysis of X-ray diffraction profiles the conclusion is drawn about the formation of a hexagonal wurtzite type structure in the synthesized quantum dot (QD) solutions. The average crystallite sizes are 8 nm and 22 nm for the samples with manganese and cobalt respectively. Results of IR and optical spectroscopy are consistent with the powder X-ray diffraction and X-ray fluorescence data. The question about particle aggregation in isopropanol and DMF solutions is considered. The QD structures based on ZnS particles doped with Mn and Co transition metal atoms are modeled. The possibility to apply X-ray absorption near edge structure (XANES) spectroscopy to verify the atomic structure parameters around the positions of doping transition metal atoms in QDs of the ZnS family is shown. Partial densities of ZnS:Mn and ZnS:Co electronic states are calculated.

Analysis of the local atomic structure of quantum dots of the CdS family

The microwave synthesis of quantum dots based on CdS has been performed, the temperature (T 180°C and 150°C) and synthesis duration (10 min and 5 min) have been varied. The analysis of the peak broadening in X-ray diffraction has shown that the average particle size in the synthesized samples is 10.02 nm for the CdST = 180°C sample and 5.22 nm for the CdST = 150°C sample. For both synthesis temperatures particles of sphalerite phase are formed but the sample CdST = 180°C contained some impurity of wurtzite phase too. CdK-XANES spectra in the standard samples and quantum dots have been recorded using a Rigaku R-XAS X-ray absorption laboratory spectrometer. The theoretical analysis of the CdK-XANES spectra of bulk samples of CdS and CdS nanoparticles has been performed. It has been shown that the theoretical difference spectra between bulk CdS and CdS with decreased lattice parameters demonstrate the same tendency as the experimental difference spectra between bulk CdS and the quantum dot samples under study. It has been shown that the theoretical CdK-edge HERFD-XANES spectrum in CdS demonstrates considerably more detailed structure pointed to the need of the analysis of experimental CdK-edge HERFDXANES spectra to pick out more precise information on local atomic structure parameters of small semiconducting quantum dots.

Structure and magnetic properties of pure and samarium doped magnetite nanoparticles

In this paper, a study of pure and doped samarium magnetite nanoparticles synthesized using a microwave synthesis in aqueous solution was performed. The shape, size and structure of the pure and samarium doped magnetite nanoparticles were determined by X-ray diffraction, transmission electron microscopy, X-ray absorption spectroscopy and Mössbauer spectroscopy. The magnetic properties of the nanoparticles were investigated using a vibrating sample magnetometer. It was found that the samarium doped magnetite nanoparticles were superparamagnetic with high saturation magnetization. The doping with a small amount of samarium allowed to reduce the size of nanoparticles, their size distribution, increase resistance to oxidation and improve their magnetic characteristics.

Characterization of local atomic structure in Co/Zn based ZIFs by XAFS

The local atomic structure in bimetallic Co/Zn zeolitic imidazolate frameworks (ZIFs) was studied using X-ray Absorption Fine Structure (XAFS) spectroscopy and theoretical calculations. The experimental Co K-edge and Zn K-edge XANES (X-ray Absorption Near Edge Structure) spectra of Zn1-xCoxC8H10N4 samples (x = 0.05, 0.25, 0.75) synthesized by microwave synthesis were compared with the data for the ZIF-67 (x=1) and ZIF-8 (x=0). Theoretical XANES spectra for the bimetallic ZIFs were calculated. It was shown that in bimetallic ZIFs the Co and Zn atoms have the similar local environment.