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The colloidal epitaxy utilizing a patterned substrate is used to fabricate colloidal crystals of the same structure and lattice spacing with the substrate, which is an effective ...technique for creating desired nanoscale architectures. However, this technique has been mainly limited to a single-component system. The colloidal epitaxy is versatile if multicomponent colloidal crystals can be produced, which is inspired by our previous study regarding binary colloidal crystals (b-CCs) fabricated at the edge of single-component crystals.
We have examined various particle size combinations of binary colloidal mixture and substrates for heteroepitaxial growth of b-CCs. Colloidal crystallization was achieved through depletion attraction induced by added polymers.
We demonstrated heteroepitaxial growth of b-CCs on the foreign colloidal crystals as the substrate. Under depletion attraction, deviation from equilibrium interparticle distance because of lattice mismatch between the substrate and epitaxial layers induces strain energy among the particles, yielding the b-CCs to attain minimum strain energy. Various types of b-CCs are created by adjusting the particle size ratio and polymer concentration. The heteroepitaxial growth technique enables the fabrication of complex multicomponent colloidal crystals that greatly facilitate versatile applications of the colloidal crystals.
For the versatile potential applications of colloidal crystals, precisely controlling their growth is required to achieve properties such as high crystallinity and large-area crystals. Because ...colloidal crystallization is a self-assembly process of dispersed particles in a solution, solution flow directly and markedly changes the behavior of particles. Thus, the effects of solution flow on the growth of colloidal crystals were investigated in the present study. We found three different effects of solution flow on the growth of colloidal crystals: enlarging the first layer, facilitating the growth of superlattice structures, and forming a new circular packing structure. Specifically, in the single-component system, because the flow speed is lower closer to the bottom of the cell, the second and further layers dissolve owing to the large flow speed, whereas the first layer remains undissolved at the appropriate flow speed. The dissolved particles (particles that are detached from the crystals and returned back into the aqueous medium) are transported near the first layer, where they facilitate the growth of the first layer. In a binary system, when colloidal crystals with different particles are neighboring each other, the flow dissolves the surface of each crystal, which forms a dense, melt-like phase between crystals, from which a superlattice structure such as AB2 grows. The flow often moves the second layer more than the first layer because the flow speed varies with the distance from the bottom. This causes the second layer to slide above the first layer of the neighboring crystals composed of different particle sizes, which transform from the initial face-centered cubic structure of the first layer into a circular pattern with strain. These findings contribute to establishing a sophisticated control method for growing colloidal crystals.
The effect of point defects on the Curie temperature (Tc) of LiNbO3 (LN) was investigated by combining Tc measurements with an analysis of the defect structures of LN doped with impurities having ...various valences. The data show that Tc of congruent LN increases with the impurity concentration up to around 3 and 2 mol% for divalent and trivalent impurities, respectively, whereas it decreases continuously with increased concentrations of tetravalent impurities. These Tc variations were examined with respect to the defect structures of impurity‐doped LN, which are expressed in the form chemical formulae using Kröger‐Vink notation. The defect structures of divalent and trivalent impurity‐doped LN are {LiLi×1−5x−2yNbLi∙∙∙∙xMLi∙yVLi′4x+y}NbNb×OO×3 and {LiLi×1−5x−3yNbLi∙∙∙∙xMLi∙∙yVLi′4x+2y}NbNb×OO×3, respectively (NbLi: Nb at Li sites; VLi: vacancies at Li sites, MLi: impurities at Li sites, and Li/Nb = the congruent ratio). The defect structure in the case of tetravalent impurities is {LiLi×1−5x+yNbLi∙∙∙∙xVLi′4x−y}{NbNb×1−yMNb′y}OO×3. Analyses of the defect structures indicated that the NbLi concentration decreases with divalent or trivalent impurity doping, which increases Tc. In contrast, the NbLi concentration increases with tetravalent impurity doping, which decreases Tc. In addition, the divalent or trivalent impurity concentrations at which the NbLi concentration becomes zero were found to correspond to the concentrations at which Tc is maximized, suggesting that Tc of LN depends on the NbLi concentration.
Due to their tunable material properties, binary colloidal crystals (BCCs) are highly desired for many of the same applications as colloidal crystals. Here, we have investigated the detailed growth ...process of BCCs with attractive interparticle interactions in which depletion forces were induced between particles via added polymer. An AB2 superlattice phase (A, large; B, small) was grown under various solution conditions, including various large-to-small particle ratios and polymer concentrations. The solution composition at which the AB2 phase initially nucleated shifted toward an A-rich composition as the polymer concentration increased because the free energy curves of AB2 and other phases displayed different trends. This indicates that interparticle interactions played an important role and depended on the particle structure. In situ observations revealed that BCCs typically grew via one-dimensional heteroepitaxy, which differs from conventional colloidal epitaxial growth, demonstrating a novel technique to control the growth of BCCs.
Impurity accumulation at the grain boundaries in multicrystalline Si (mc-Si) was investigated by in situ observation of the crystal/melt interface, analysis of the grain boundary characteristics, and ...measurement of impurity concentrations. The impurity concentration was higher at grain boundaries that formed a groove at the crystal/melt interface than that at regions which did not form a groove at the crystal/melt interface. We conclude that groove formation at the crystal/melt interface is the cause of impurity accumulation at the grain boundary.
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•111- and 100-oriented grains alternately form as growth proceeds in wedge-shaped cell.•Stripe patterns form between n-layer 100- and n + 1-layer 111-oriented grains.•The stripe ...pattern is formed by continuous change of particle configuration.•The interval of the strip pattern becomes long as the number of layers increase.
The structural evolution of growing thin colloidal crystals in a confined space via the convective assembly technique has been investigated. The thin colloidal crystals were grown in a wedge-shaped cell, where the height of the cell increased with increased crystal growth. Triangle and square patterns, denoted as 111- and 100-oriented grains, respectively, were formed alternately as the height of the cell increased. The structural transformation was associated with an increase in the number of layers when the n-layer 100-oriented grains changed to n + 1-layer 111-oriented grains. Between the different grain structures, a stripe pattern was observed, which was a transitional region, where particle configuration gradually changed. The structural transformation occurred through the continuous change of particle configuration rather than through the abrupt formation of a grain boundary. The interval of the strip pattern lengthened as the number of layers increased, which is understood to be the structure with the highest packing density. The findings of the study give a better insight into convective assembly in a confined space, and also contribute to the greater structural control of colloidal crystals, useful for a number of applications.
Particle interaction is a critical parameter for growth kinetics of colloidal crystals. The equilibrium concentration and step free energy are strongly dependent on the particle interaction. The ...growth mechanism of the attractive system of colloidal crystals is similar to that of vapor or solution growth, where crystals grow by incorporating diffusing ad-particles on the terrace into kinks of steps (Terrace-Step-Kink model). Here, we have applied the theory of crystal growth to experimentally determine the binding energy that originates from the particle interaction. We focus on the relationship between kink distance and bond energy, which is described by BCF (Burton-Cabrera-Frank) theory. The value of the step free energy determined by the kink distance measurement agrees well with the values obtained from other measurements, including nucleation rate and critical radius. The obtained value also accounts well for the change in step velocity of two-dimensional islands, which is due to the Gibbs–Thomson effect. The measurement of binding energy contributes significantly to the fundamental study of colloidal science as well as to technology for growing high quality colloidal crystals.
The relationship between protein crystal quality and growth kinetics was assessed by measuring the normal growth rates vs supersaturation of the (110) and (101) faces of dislocation-free tetragonal ...hen egg white lysozyme crystals at three precipitant concentrations, with NaCl as the precipitant. Assuming a two-dimensional birth and spreading nucleation mechanism, an increase in the surface free energy of the step edges was realized with increasing NaCl concentration, as established by decreases in the full width at half-maximum values of X-ray diffraction rocking curves obtained from crystals. These results demonstrate that controlling the surface free energy of the step edges is an important aspect of obtaining high-quality protein crystals. This work also proposes a mechanism for the observed improvements in the crystal quality, based on the reduced incorporation of impurities into the steps during crystal growth.
Nucleation and growth of two-dimensional (2D) islands on a terrace are the dominant growth mechanisms of colloidal crystals whose particle interaction is attractive. The step velocity, v step, of the ...2D islands at various area fractions, ϕarea, and polymer concentrations, C p, has been investigated. At low C p (weak attractive interaction between particles), there is a nearly linear relationship between v step and ϕarea, whereas it is parabolic for high C p (strong attraction). Depending on the C p, two manners of kink generation at a step are observed: 1D nucleation under strong attraction and association of mound formation under weak attraction. As a result of mound formation, abundant kinks are created at steps, resulting in a linear relationship between v step and ϕarea, whereas the relationship is parabolic for step propagation associated with 1D nucleation. Though the kink site is the most favored site for particles to be incorporated into crystals, weak attractive interaction makes the step front site an incorporation site as well, and this latter process is the main mechanism for mound formation. This study is the first to elucidate the relationship between step kinetics and the kink generation mechanism of colloidal crystals, and these new findings significantly contribute to better control of the growth of colloidal crystals.
