Papers by Willem van Enckevort
physica status solidi (c), 2010
Crystal Growth & Design, 2015
The incorporation of impurities during protein crystallization is one of the main obstacles that ... more The incorporation of impurities during protein crystallization is one of the main obstacles that prevents the growth of high quality crystals. Mass transport has been shown to affect the incorporation of impurities. Here we used a special growth configuration that enables the simultaneous investigation of the two main means of mass transport, diffusion and convection, under otherwise identical conditions. Two polymorphic forms of hen egg-white lysozyme were crystallized using this configuration in the presence of different types of impurities at various degrees of contamination. We found that even in the presence of impurities that are not easily segregated, the diffraction quality of crystals grown under diffusion-limited conditions is better than that of those grown in the presence of natural convection. The results also reveal a significant difference in impurity uptake for the different polymorphic forms of the same protein. The combined results show that also in the presence of large fractions of impurities, the more perfect crystals grow when the rate of accretion of molecules is slow and orderly, as accomplished under diffusion-limited conditions.
Crystal Growth & Design, 2007
... (24) Graswinckel, WS; van Enckevort, WJP Leunissen, ME Kaminski, DM; Shah, IA In 't ... more ... (24) Graswinckel, WS; van Enckevort, WJP Leunissen, ME Kaminski, DM; Shah, IA In 't Veld, M.; Algra, RE; Vlieg, E., to be published, 2007. (25) Leunissen, ME; Graswinckel, WS; van Enckevort, WJP; Vlieg, E. Cryst. Growth Des. 2004, 4, 361−367. ...
Diamond and Related Materials, 2010
In this study, {0001} oriented GaN crystals have been grown on freestanding, polycrystalline diam... more In this study, {0001} oriented GaN crystals have been grown on freestanding, polycrystalline diamond substrates using AlN and GaN nucleation layers (NLs). XRD measurements and SEM analysis showed that the application of a thin AlN NL gives the best structural results, because AlN has a thermal expansion coefficient in between GaN and diamond and thus delocalizes the stress to two interfaces. The optical quality of the layers, investigated with Raman microscopy and photoluminescence spectroscopy, is similar. Although no lateral epitaxy is obtained, new insight is gained on the nucleation of GaN on diamond substrates facilitating the growth of GaN epilayers on polycrystalline diamond substrates.
Diamond and Related Materials, 2009
In this study GaN has been grown on nano-crystalline diamond substrates utilizing metal-organic c... more In this study GaN has been grown on nano-crystalline diamond substrates utilizing metal-organic chemical vapour deposition (MOCVD). It is shown that the growth of closed GaN films onto synthetic diamond substrates is feasible, when applying the correct buffer layer and growth parameters. XRD measurements showed that the GaN formed is of wurzite structure and polycrystalline, but the high intensity of the (0002) diffraction peak indicates a preferential crystallite orientation. This preferred [0001] orientation was confirmed by SEM analysis. The optical quality of the deposited GaN layer was investigated using cathodoluminescence and showed a large yellow luminescence peak. This work comprises a first step in preparing heterogeneous layers and GaN devices with a diamond heat sink as a substrate, facilitating the thermal management of these devices.
Journal of Crystal Growth, 2007
In this paper, a study is presented on the effect of In on the surface morphology of GaN grown by... more In this paper, a study is presented on the effect of In on the surface morphology of GaN grown by HVPE. Experiments are performed with N 2 and H 2 as the carrier gasses, both with and without In present in the reactor. The adding of In increases the morphological quality of the grown layers; this effect is most strongly observed for N 2 as the carrier gas. It is found that adding In reduces the growth rate and also increases the steepness of the growth hillocks on the surface. The step velocity, which is calculated from hillock slopes and the growth rate, decreases upon adding In. Without In, trails are visible across the surface where steps are distorted by passing a dislocation outcrop. With In present this pinning is still present, however, the trails do not form. Two possible explanations for this phenomenon are an increased surface diffusion due to a mono-or bi-layer of In on the surface and the slower step motion when In is present. r
Journal of Crystal Growth - J CRYST GROWTH, 2007
Journal of Applied Physics, 2011
ABSTRACT By an extensive investigation of the principal growth parameters on the deposition proce... more ABSTRACT By an extensive investigation of the principal growth parameters on the deposition process, we realized the epitaxial growth of crystalline wurtzite GaN thin films on single crystal (001) diamond substrates by metal organic chemical vapor deposition. From the influence of pressure, V/III ratio, and temperature, it was deduced that the growth process is determined by the mass-transport of gallium precursor material toward the substrate. The highest temperature yielded an improved epitaxial relationship between grown layer and substrate. X ray diffraction (XRD) pole figure analysis established the presence of two domains of epitaxial layers, namely (0001) 〈 10 0〉 GaN∥ (001)[110] diamond and (0001) 〈 10 0〉 GaN∥ (001) [10] diamond, which are 90∘ rotated with respect to each other. The presence of these domains is explained by the occurrence of areas of (2×1) and (1×2) surface reconstruction of the diamond substrate. When applying highly misoriented diamond substrates toward the [110] diamond direction, one of the growth domains is suppressed and highly epitaxial GaN on (001) diamond is realized.
J Phys Chem B, 2007
We show that consistency of the transition probabilities in a lattice Monte Carlo (MC) model for ... more We show that consistency of the transition probabilities in a lattice Monte Carlo (MC) model for binary crystal growth with the thermodynamic properties of a system does not guarantee the MC simulations near equilibrium to be in agreement with the thermodynamic equilibrium phase diagram for that system. The deviations remain small for systems with small bond energies, but they can increase significantly for systems with large melting entropy, typical for molecular systems. These deviations are attributed to the surface kinetics, which is responsible for a metastable zone below the liquidus line where no growth occurs, even in the absence of a 2D nucleation barrier. Here we propose an extension of the MC model that introduces a freedom of choice in the transition probabilities while staying within the thermodynamic constraints. This freedom can be used to eliminate the discrepancy between the MC simulations and the thermodynamic equilibrium phase diagram. Agreement is achieved for that choice of the transition probabilities yielding the fastest decrease of the free energy (i.e., largest growth rate) of the system at a temperature slightly below the equilibrium temperature. An analytical model is developed, which reproduces quite well the MC results, enabling a straightforward determination of the optimal set of transition probabilities. Application of both the MC and analytical model to conditions well away from equilibrium, giving rise to kinetic phase diagrams, shows that the effect of kinetics on segregation is even stronger than that predicted by previous models.
Physics Letters A, 1998
The morphological instabilities of two-dimensional hexagonal crystals in lipid monolayers are stu... more The morphological instabilities of two-dimensional hexagonal crystals in lipid monolayers are studied. Fluorescence microscopy indicates that beyond a critical size the faceted crystal develops into dendrites with unusual tip growth behavior, which is not consistent with the current theory of dendritic growth. The morphological transitions in relation to the driving force for two-dimensional crystal growth are also studied.
Physical Review Letters, 1998
In this Letter we report a unique fractal aggregation behavior of NH 4 Cl crystallites grown in a... more In this Letter we report a unique fractal aggregation behavior of NH 4 Cl crystallites grown in agarose gel. The fractal branches consist of faceted crystallites with remarkable correlations in their crystallographic orientations, which are formed by successive heterogeneous nucleations. Unlike the conventional fractal growth, this aggregation process occurs at a low growth driving force with strong crystallographic anisotropy, which leads to the ordering in the fractal branches. [S0031-9007(98)
Physical Chemistry Chemical Physics, 2013
Self-assembled monolayers of meso-5,10,15,20-tetrakis(undecyl)porphyrin copper(II) on a graphite/... more Self-assembled monolayers of meso-5,10,15,20-tetrakis(undecyl)porphyrin copper(II) on a graphite/1-octanoic acid interface have been studied by Scanning Tunnelling Microscopy. Four distinct polymorphs were observed, varying in their unit cell size. Arrays of unit cells of the various polymorphs seamlessly connect to each other via shared unit cell vectors. The monolayers are not commensurate, but coincident with the underlying graphite substrate. The seamless transition between the polymorphs is proposed to be the result of an adaptation of the molecular conformations in the polymorphs and at the boundaries, which is enabled by the conformational freedom of the alkyl tails of these molecules.
Langmuir, 2010
Layers of a free base porphyrin have been deposited from n-heptane solution onto single crystalli... more Layers of a free base porphyrin have been deposited from n-heptane solution onto single crystalline potassium acid phthalate substrates and the self-assembled structures have been characterized by atomic force microscopy and X-ray diffraction. Depending on the concentration of the porphyrin solution, different anisotropic structures are found that are epitaxially related to the substrate, with the length axis parallel to the [001] direction of the (010) substrate surface. For undersaturated solutions, islands, cellular structures, and a hole pattern of monolayers of approximately 2.5 nm height are formed, which correspond to porphyrin molecules stacking approximately perpendicular to the substrate surface. From supersaturated solutions, multilayers are formed. At the highest concentrations, nanosized needles develop, which cover the whole substrate, creating a "monocrystalline" film with a length scale of several millimeters.
Journal of Crystal Growth, 2007
This paper presents detailed ex situ observations of spiral patterns on the ð01 0Þ face of the st... more This paper presents detailed ex situ observations of spiral patterns on the ð01 0Þ face of the stable polymorph of the 7aMNa steroid, using atomic force microscopy. In contrast to the opposite (0 1 0) face, which is rough, the ð01 0Þ face grows layer-wise involving steps down to monomolecular height. The growth spirals show symmetry induced interlacing, i.e. a regular splitting of d 010 steps into half height d 020 steps as a consequence of the 2 1 screw axis perpendicular to ð01 0Þ. In many cases differences in advancement rate of symmetrically non-equivalent layers and entropic repulsion leads to further step splitting generating steps of quarter unit cell height, d 040 . The observed step orientations are identical to the strongest periodic bond chain directions within the thinnest growth layer thickness. The screw dislocations b ¼ [0 1 0] at the spiral centers are marked by hollow cores. The edge dislocations ending on the same face do not reveal hollow cores. This is a consequence of their low stress fields, which is some 40 times smaller than that of the screw dislocations. r
Faraday Discussions, 2007
Our understanding of crystal growth continues to increase thanks to progress in theoretical model... more Our understanding of crystal growth continues to increase thanks to progress in theoretical models, computer simulations and experimental techniques. A discussion of the state-of-the-art in morphology prediction and of the determination of the solid-liquid interface structure using X-ray diffraction shows, however, that there is still a large gap between experiment and theory. We expect that computer modelling, in the form of both Molecular Dynamics simulations and first-principle calculations, will play a crucial role in filling this gap.
Crystal Growth & Design, 2009
ABSTRACT The extremely elongated growth spirals observed using atomic force microscopy on the lar... more ABSTRACT The extremely elongated growth spirals observed using atomic force microscopy on the largest {101̅} faces of the needle crystals of vapor-grown metal-free β-phthalocyanine are analyzed. An analytical model for the growth of anisotropic rectangular spirals is proposed to relate step distances and the step energies to the driving force for crystallization. The model is confronted with the experimental results, with kinetic Monte Carlo simulations, and with a growth step analysis. The model is well able to describe the observed and simulated spiral shapes. In addition, the experimental driving force for crystallization could be estimated and turns out to be reduced considerably as a result of diffusion limitation when compared to the value obtained for ideal gas behavior.
Chemical Communications, 2011
The transition from low to high density 2D surface structures of copper porphyrins at a liquid/so... more The transition from low to high density 2D surface structures of copper porphyrins at a liquid/solid interface requires specific defects at which nearly all exchange of physisorbed molecules with those dissolved in the supernatant occurs.
Calphad, 2006
Several theoretical models for the determination of kinetic phase diagrams for solid solution gro... more Several theoretical models for the determination of kinetic phase diagrams for solid solution growth from the liquid phase are presented and compared to each other. These models include a Monte Carlo simulation model, used as a reference model, a previously defined analytical model, based on linear non-equilibrium thermodynamics, and a new model, rooted in the kinetics at kink sites.
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Papers by Willem van Enckevort