Ewa Grzanka

Magnetic and optical properties of GaMnN magnetic semiconductor

Microcrystalline Ga1−xMnxN samples with Mn content up to x=0.005 were grown by an ammonothermal method and were studied using various techniques. X-ray diffraction showed characteristic diffraction lines for hexagonal GaN phase mixed with a small contribution (<5%) from the Mn3N2 phase. Raman spectra exhibited characteristic peaks of pure GaN and modes that could be associated with Mn-induced lattice disorder. Electron spin resonance and magnetization measurements were consistent with the dominant Mn2+(d5) configuration of spin S=5/2 which is responsible for the observed paramagnetic behavior of the GaMnN material.

Analysis of Short and Long Range Atomic Order in Nanocrystalline Diamonds with Application of Powder Diffractometry

Abstract Fundamental limitations, with respect to nanocrystalline materials, of the traditional elaboration of powder diffraction data like the Rietveld method are discussed. A tentative method of the analysis of powder diffraction patterns of nanocrystals based on the examination of the variation of lattice parameters calculated from individual Bragg lines (named the “apparent lattice parameter”, alp) is introduced. We examine the application of our methodology using theoretical diffraction patterns computed for models of nanocrystals with a perfect crystal lattice and for grains with a two-phase, core-shell structure. We use the method for the analysis of X-ray and neutron experimental diffraction data of nanocrystalline diamond powders of 4, 6 and 12 nm in diameter. The effects of an internal pressure and strain at the grain surface are discussed. The results are based on the dependence of the alp values on the diffraction vector Q and on the PDF analysis. It is shown, that the experimental results lend a strong support to the concept of a two-phase structure of nanocrystalline diamond.

Nanocrystals: Breaking limitations of data analysis

Abstract A series of “virtual powder diffraction experiments” was made on models of small single crystals and nanocrystals with the core-shell structure. The results of those experiments were elaborated with application of standard methods of data analysis routinely used for reciprocal and real space analyses of polycrystalline materials. It is shown that the assumption of a uniform crystal structure of nano-materials is not justified and, therefore, application of routine procedures of collection and elaboration of diffraction data may lead to misinterpretation of the experiments and to incorrect conclusions about their structure. Tentative ways of using powder diffraction data to learn about the structure of nanocrystals with different atomic architecture of the core and of the surface of the grains are discussed. A need for elaboration of a model of the atomic structure of an individual nanograin with a non-uniform structure is discussed. An alternative approach to diffraction studies of nanocrystals by presenting the “footprints” of materials under study in the form of plots showing distribution of the experimental apparent lattice parameters as a function of diffraction vector Q, or bond length distribution as a function of r-distances derived from PDF function is suggested.

Morphology and luminescence properties of zinc oxide nanopowders doped with aluminum ions obtained by hydrothermal and vapor condensation methods

In this paper, we analyze the influence of Al doping on microstructure and light emission efficiency of ZnO nanoparticles obtained by the hydrothermal method and after vapor condensation method, where vaporization of the precursor powders was caused by high solar energy and subsequent deposition. We report an increase of lattice parameters with increasing level of doping as well as large emission enhancement, which we relate to surface passivation of recombination mechanisms and/or plasmon mechanism of photoluminescence stimulation.

Application of the apparent lattice parameter to determination of the core-shell structure of nanocrystals

In this review work we discuss applicability of Bragg scattering to examination of nanocrystals. We approximate the structure of nanograins by a commonly accepted core-shell model. We show that, for principal reasons, the Bragg equation is not applicable directly to nanocrystals. We use the Bragg relation through application of the apparent lattice parameter (alp) concept which we use to evaluate quantitatively the core-shell model. We also introduce a new parameter of the structure, Equivalent Cubic Lattice Parameter (EClp), which quantifies deviation of the real (trigonal) lattice from its parent fcc structure due to the lattice deformation (e.g. by the stacking faults). We show examples of an analysis of experimental X-ray and neutron diffraction data based on the alp methodology and on the theoretical patterns calculated for various core-shell models.

Effect of pressure on synthesis of Pr-doped zirconia powders produced by microwave-driven hydrothermal reaction

A high-pressure microwave reactor was used to study the hydrothermal synthesis of zirconia powders doped with 1 mol % Pr. The synthesis was performed in the pressure range from 2 to 8 MPa corresponding to a temperature range from 215°C to 305°C. This technology permits a synthesis of nanopowders in short time not limited by thermal inertia of the vessel. Microwave heating permits to avoid contact of the reactants with heating elements, and is thus particularly well suited for synthesis of doped nanopowders in high purity conditions. A mixture of ZrO2 particles with tetragonal and monoclinic crystalline phases, about 15nm in size, was obtained. The p/T threshold of about 5-6MPa/265-280°C was necessary to obtain good quality of zirconia powder. A new method for quantitative description of grain-size distribution was applied, which is based on analysis of the fine structure of the X-ray diffraction line profiles. It permitted to follow separately the effect of synthesis conditions on the grain-size distribution of the monoclinic and tetragonal phases.

Graded-index separate confinement heterostructure InGaN laser diodes

We demonstrate graded-index-separate-confinement-heterostructure InGaN laser diodes (GRINSCH) grown by metal organic vapor phase epitaxy. In this type of structure, the optical mode is confined close to the active region by cladding layers with linearly changing Al content. The virtue of this structure lies in simultaneous improvement of injection efficiency through the funneling effect and mode confinement. After a careful modeling we demonstrate that GRINSCH InGaN laser indeed possess very encouraging properties. Low threshold current density of around 3.5 kA/cm2 accompanied by high differential gain (dG/dJ = 14.93 cm kA−1) make these devices an interesting alternative for the classical step-index structure.

Magnetic properties of ZnMnO nanopowders solvothermally grown at low temperature from zinc and manganese acetate

The authors demonstrate that nanometer size ZnMnO nanopowders, grown from zinc and manganese (II) acetates at low temperatures under microwave radiation, are free of Mn clusters and the inclusion of Mn oxides. These nanopowders show a strong paramagnetic phase with only a weak antiferromagnetic contribution due to Mn–Mn interactions.

Enhancement of optical confinement factor by InGaN waveguide in blue laser diodes grown by plasma-assisted molecular beam epitaxy

The influence of InxGa1−xN waveguide on the properties of blue (λ = 450 nm) laser diodes grown by plasma-assisted molecular beam epitaxy was studied. The threshold current density was reduced by 50% (to 3.6 kA/cm2) when the indium content was increased from x = 0.04 to 0.08. This is explained by a substantial enhancement of the optical confinement factor for a high-In-content waveguide, which increases the differential modal gain. Furthermore, we observed a decrease in optical losses when Mg-doped layers were separated from the active region by a thicker waveguide. This is attributed to the lower overlap between the optical mode and absorptive Mg-doped layers.

Microwave – Hydrothermal Synthesis of Nanocrystalline Pr - Doped Zirconia Powders at Pressures up to 8 MPa

Nanocrystalline praseodymium doped zirconia powders were produced using a microwave driven hydrothermal process under pressures up to 8 GPa. The ai m of the work was to evaluate the effect of synthesis conditions on the phase composition and g rain size of nanopowders of zirconia with Pr in solid solutions having Pr contents of: 0, 0.5, 1, 5 and 10 mol %. Introduction Application of microwaves (MW) in chemical synthesis has attrac ted onsiderable attention [1]. The advantage using MW as an energy source is primarily the possi bility of carrying out the processes at a much higher rate than during conventional heating. Zirc onia is an important ceramic material with useful mechanical, thermal, optical and electrica l properties. Recently much research effort has been dedicated to study nanocrystalline ceramics, since they may display a range of novel properties, including enhanced plasticity [2,3]. Therefore it is of gr eat importance to develop versatile synthesis methods for producing nanocrystalline ceramic powders. The aim of this work was to explore the possibility of synthesizing nanopowders in a MW driven reaction in aqueous solution under pressures of up to 8 MPa. The tests we re performed with nanocrystalline ZrO2 powders doped with Pr, which might be an interesting pigment [4,5] or oxyg en storage material [6]. It’s production by hydrothermal methods has been extensively studied [4,5,7,8]. Experimental methods Powders containing praseodymium in the 0.5–10 mol% range were obtained by addi ng praseodymium(III) nitrate (Pr(NO3)3*H2O, Carlo Erba) to a 0.5M ZrOCl 2 aqueous solution. The solutions were neutralized with NaOH 1 M to pH 10. 40 ml of the solution was poured into the Teflon vessel of the microwave reactor of volume 110 ml. The reacti ons were carried out using a MW reactor from Plazmatronika Ltd. The system operates at 2.45 GH z and can deliver up to 250 W of unpulsed MW power to the reaction fluid. The power level was automati cally adjusted to the maximum pressure, which pre-set for each experiment. The maximum accessible pressure is 10 MPa. The typical ramp time to a pressure of 4 MPa was 5 min and the cooling down time was 10 min. When the reaction was completed the solid phase was separated from the solution by filtering and the solids were washed free of salts with distilled water and isopropanol. One run produced approximately 0.5 g of powder which was annealed in air at 200 C for 0.5h after synthesis. Solid State Phenomena Online: 2003-06-20 ISSN: 1662-9779, Vol. 94, pp 193-196 doi:10.4028/www.scientific.net/SSP.94.193