Defects In Colloidal Crystals Research Articles

Computer Simulations of Crystal Growth Using a Hard-Sphere Model

A review of computer simulation studies on crystal growth in hard-sphere systems is presented. A historical view on the crystallization of hard spheres, including colloidal crystallization, is given in the first section. Crystal phase transition in a system comprising particles without bonding is difficult to understand. In the early days, therefore, many researchers did not accept such crystalline structures as crystals that should be studied in the field of crystal growth. In the last few decades, however, colloidal crystallization has drawn attention because in situ observations of crystallization process has become possible. Next, simulation studies of the crystal/fluid interface of hard spheres are also reviewed. Although colloidal crystallization has now been recognized in the crystal growth field, the stability of the crystal–fluid coexistence state has still not been satisfactorily understood based on a bond-breaking picture, because of an infinite diffuseness of the interfaces in non-bonding systems derived from this picture. Studies of sedimentary colloidal crystallization and colloidal epitaxy using the hard-sphere model are lastly reviewed. An advantage of the colloidal epitaxy is also presented; it is shown that a template not only fixes the crystal growth direction, but also improves the colloidal crystallization. A new technique for reducing defects in colloidal crystals through the gravity effect is also proposed.

Inhibited/enhanced fluorescence of embedded fluorescent defects by manipulation of spontaneous emission based on photonic stopband

We demonstrate inhibition and enhancement of fluorescence of embedded fluorescent defects in colloidal crystals incorporated by two-photon polymerization.

Gravitational Tempering in Colloidal Epitaxy To Reduce Defects Further

Less-defective colloidal crystals can be used as photonic crystals. To this end, colloidal epitaxy was proposed in 1997 as a method to reduce the stacking defects in colloidal crystals. In this method, face-centered cubic (fcc) (001) stacking is forced by a template. In fcc (001) stacking, in contrast to fcc {111} stacking, the stacking sequence is unique, and thus the stacking fault can be avoided. Additionally, in 1997, an effect of gravity that reduces the stacking disorder in hard-sphere (HS) colloidal crystals was found. Recently, we have proposed a gravitational tempering method based on a result of Monte Carlo (MC) simulations using the HS model; after a colloidal crystal is grown in a relatively strong gravitational field, the defects can be reduced by decreasing the gravity strength and maintaining it for a period of time. Here, we demonstrate this method using MC simulations with a programmed gravitation. The dramatic disappearance of defect structures is observed. Gravitational tempering can complement gravitational annealing; some defect structures that accidentally remain after gravitational annealing (keeping the colloidal crystal under gravity of a considerable constant strength) can be erased.

Coherent x-ray imaging of defects in colloidal crystals

Coherent x-ray diffractive imaging (CXDI) was applied to reveal the structure of colloidal crystals. The colloidal sample was illuminated by a coherent x-ray beam through a $7\text{ }\ensuremath{\mu}\text{m}$ pinhole aperture. The resulting diffraction patterns contain several Bragg peaks and an additional interference structure between the peaks due to the coherent illumination of a finite part of the sample. The inversion of these diffraction patterns reveals the arrangement of colloidal particles in a face-centered cubic (fcc) lattice as well as defects in the form of stacking faults in the (111) planes.

Fabrication of high-quality colloidal crystal films by vertical deposition method integrated with a piezoelectric actuator

In recent years the self-assembling of monodispersed colloidal spheres has gained much attention due to their potentials in many fields. Though many self-assembling methods have been put forward, the existence of many defects in colloidal crystals is still an open question restricting the dimensions of ordered domains and the performance thereof in many applications. In this paper we introduced a modified vertical deposition method to improve film quality of colloidal crystal films. A piezoelectric actuator was integrated with the conventional vertical deposition method, which excited the substrate vibrating along the vertical direction. The vibratory amplitude and frequency of the actuator could be controlled by the driving voltage. Our experimental observations indicated that, grown at proper driving parameters, frequency of 300 Hz and vibratory amplitude of 310 nm in our current observations, film quality could be greatly improved with less defects. The mechanism was also briefly illuminated.