Search results for "crystal growth"
showing 10 items of 130 documents
MOCVD growth of CdO very thin films: Problems and ways of solution
2016
Abstract In this paper the growth of CdO by the MOCVD technique at atmospheric pressure has been studied in order to achieve very thin films of this material on r-sapphire substrates. The growth evolution of these films was discussed and the existence of a threshold thickness, below which island-shaped structures appear, was demonstrated. Some alternatives to reduce this threshold thickness have been proposed in the frame of the analysis of the crystal growth process. The morphology and structural properties of the films were analyzed by means of SEM and HRXRD. High-quality flat CdO samples were achieved with thicknesses up to 20 nm, which is five times thinner than the values previously re…
Crystal growth of Hg1−xMnxSe for infrared detection
2007
In this work, we report on the successfully growing Hg"1"-"xMn"xSe bulk crystals using a mixed, travelling heater method and Bridgman method, two-step procedure. Firstly, and with the aim of reducing Hg high pressure related to the high temperature synthesis reaction between the components in elemental form, HgSe crystals were synthesized and grown by the cold travelling heater method. Secondly, previously sublimated Mn and Se were incorporated to complete the desired composition. Then, the Bridgman growth was carried out by heating the alloy at a temperature of about 880^oC and lowering it at rate of 1mm/h through a gradient of 25^oC/cm. The Hg"1"-"xMn"xSe crystals were characterized by sc…
The effect of elastic strain on the microstructure of free surfaces of stressed minerals in contact with an aqueous solution
2001
The influence of gradients in bulk elastic strain energy on the dissolution and growth behaviour of minerals in rocks is commonly considered negligible. We experimentally observed, however, that regular arrays of macroscopically visible etch grooves may develop on the originally smooth free surfaces of soluble crystals held in an undersaturated aqueous solution if the crystals are only elastically stressed. These grooves are oriented perpendicular to the compressive stress. They disappear soon after the stress is taken off. The formation of the grooves is well explained by recent theories on the instability of the surface of stressed solids. Development of such instabilities could significa…
Preferred Growth Direction by PbS Nanoplatelets Preserves Perovskite Infrared Light Harvesting for Stable, Reproducible, and Efficient Solar Cells
2020
Formamidinium-based perovskite solar cells (PSCs) present the maximum theoretical efficiency of the lead perovskite family. However, formamidinium perovskite exhibits significant degradation in air. The surface chemistry of PbS has been used to improve the formamidinium black phase stability. Here, the use of PbS nanoplatelets with (100) preferential crystal orientation is reported, to potentiate the repercussion on the crystal growth of perovskite grains and to improve the stability of the material and consequently of the solar cells. As a result, a vertical growth of perovskite grains, a stable current density of 23 mA cm(-2), and a stable incident photon to current efficiency in the infr…
Crystal growth and structural remarks on malonate-based lanthanide coordination polymers
2016
The synthesis, structural characterization and thermal study of new coordination polymers (CPs) of formula [Ln2(mal)3(H2O)5]·2H2O [Ln = Ho (1·2H2O), Tb (1a), Dy (1b), Er (1c) and Yb (1d); mal = malonate], [Ln2(mal)3(H2O)6] [Ln = Sm (2) and Ce (2a)], [Ce2(mal)3(H2O)6]·2H2O (3·2H2O) and [Ce2(mal)3(H2O)3]·2H2O (4·2H2O) are presented. Complexes 1–4 have been also characterized by single crystal X-ray diffraction. The structure of 2 was previously reported (Elsegood, M. R. J., Husain, S., Private Communication, 2014) and it is very close to that of 3. In the light of these results and those previously reported in the literature for malonate-containing lanthanide(III) complexes, a detailed overvi…
Theory of the growth mode for a thin metallic film on an insulating substrate
2002
We have developed a novel theory predicting the growth mode of a thin metallic film on an insulating substrate. This combines ab initio electronic structure calculations for several ordered metal/insulator interfaces (varying both coverage and substrate lattice constant), with a thermodynamic approach based on microscopic calculations. We illustrate this approach for Ag film deposited on MgO(0 0 1) substrate. Ab initio calculations predict high mobility of adsorbed silver atoms on the perfect magnesia surface even at low temperatures. Our theoretical analysis clearly demonstrates that the growth of metallic islands is predominant at the initial stage of silver deposition, which agrees with …
Secondary structure of peptides. 4:13C-Nmr CP/MAS investigation of solid oligo- and poly(L-alanines)
1983
Primary and tertiary amine-initiated polymerizations of L-alanine-N-carboxyanhydride (L-Ala-NCA) were conducted at 20 or 100°C in a variety of solvents. The 75.5-MHz 13C-nmr CP/MAS spectra of the resulting poly(L-alanines) revealed that all samples contain both α-helix and pleated-sheet structures. Depending on the reaction conditions the α-helix content varied between ca. 1 and 99%. Reprecipitation from aprotic nonsolvents does not change the α-helix/β-sheet ratio, indicating that this ratio is thermodynamically controlled. Since relatively large amounts of oligopeptides of degree of polymerization (DP) 4–6 can be extracted by means of acetic acid, it is concluded that (a) most poly(L-alan…
On the mechanism of the interaction between oxygen and close-packed single-crystal aluminum surfaces
2003
Abstract Using periodic first principles simulations we investigate the interaction of oxygen molecules with both regular Al(111) and Al(001) surfaces as well as a stepped Al(111) substrate. The limitation of this approach is the use of thin metallic slabs with a limited range for their coverage by adsorbed oxygen. The advantage is the detailed modeling that is possible at an atomic level. On the regular Al(111) surface, we have been able to follow the oxidation process from the approach of O 2 molecules to the surface, through the chemisorption and absorption of O atoms, up to the formation of first Al 2 O 3 formula units. An energetically feasible mechanism for the formation of these Al 2…
Computational stability of an initially radial solution of a growth/dissolution problem in a nonradial implementation
1991
We consider a free boundary problem modelling the growth/dissolution of a crystal. The aim is to investigate the following question: Does the solution to the crystal growth problem posed in two dimensions with radially symmetric initial and boundary condition evolve as a radially symmetric solution?
Mechanisms of polymer crystallization from flowing solutions
1978
The recent experimental results on flow-induced crystallization by Pennings and coworkers show that extremely rigid polyethylene fibers can be obtained in a shear flow. On the other hand, the mechanism by which these flow-induced crystals are produced is still open to investigation. In this work a few aspects of fibrous crystal growth are theoretically investigated. The molecular dynamics of chains partly attached to the crystal and partly immersed in the solution is considered. The influence of temperature and of geometrical factors is also discussed.