[Show abstract][Hide abstract] ABSTRACT: Nucleation is an important stage in the growth of crystals. During this stage, the structure and orientation of a crystal are determined. However, short time- and length-scales make nucleation poorly understood. Micrometer-sized colloidal particles form an ideal model system to study nucleation due to more experimentally accessible time- and length-scales and the possibility to manipulate them individually. Here we report experiments and simulations on nucleation in the bulk of a hard-sphere fluid, initiated by seed structures configured using optical tweezers. We find that the defect topology of the critical nucleus determines the crystal morphology. From the growth of the crystals beyond the critical nucleus size, new insights into the role of defects in crystal growth were gained that are incompatible with the assumption of equilibrium growth. These results explain the complex crystal morphologies observed in experiments on hard spheres.
[Show abstract][Hide abstract] ABSTRACT: A method is presented to tune the holes in colloidal masks used for nanolithography. Using a simple wet-chemical method, a thin layer of silica is grown on masks of silica particles. The size of the holes is controlled by the amount of tetraethoxysilane (TEOS) added. More accurate tuning of the hole size is possible in the presence of a calibrated seed dispersion of silica colloids. We demonstrate modified masks that were used to create arrays of metal nanoparticles with a size ranging from 400 nm, for unmodified masks, down to tens of nanometers. The method is easy-to-use, fast, and inexpensive.
[Show abstract][Hide abstract] ABSTRACT: A method is presented to control the in-plane ordering, size, and interparticle distance of nanoparticles fabricated by evaporation through a mask of colloidal particles. The use of optical tweezers combined with critical point drying gives single-particle position control over the colloidal particles in the mask. This extends the geometry of the colloidal masks from (self-organized) hexagonal to any desired symmetry and spacing. Control over the mask's hole size is achieved by MeV ion irradiation, which causes the colloids to expand in the in-plane direction, thus shrinking the size of the holes. After modification of the mask, evaporation at different angles with respect to the mask gives additional control over structure and interparticle distance, allowing nanoparticles of different materials to be deposited next to each other. We demonstrate large arrays of metal nanoparticles with dimensions in the 15-30 nm range, with control over the interparticle distance and in-plane ordering.
[Show abstract][Hide abstract] ABSTRACT: D micromanipulation and imaging of col- loids has been achieved by combining optical tweezers and confocal microscopy using two microscope objectives. Arrays of up to several hundred time-shared traps have been cre- ated using acousto-optical deflectors. In addi- tion, two axially separated trapping planes were addressed using a Pockels cell, making trapping and dynamic manipulation of 3D colloidal structures possible. Structures of high refractive index core-shell colloidal par- ticles were trapped inside a concentrated dis- persion of index-matched particles, thereby creating a nucleus for colloidal crystalliza- tion. Using confocal microscopy, the effects of such structures upon the non-trapped rest of the dispersion could be analysed quantita- tively in 3D.
[Show abstract][Hide abstract] ABSTRACT: A setup is described for simultaneous three-dimensional manipulation and imaging inside a concentrated colloidal dispersion using (time-shared) optical tweezers and confocal microscopy. The use of two microscope objectives, one above and one below the sample, enables imaging to be completely decoupled from trapping. The instrument can be used in different trapping (inverted, upright, and counterpropagating) and imaging modes. Optical tweezers arrays, dynamically changeable and capable of trapping several hundreds of micrometer-sized particles, were created using acousto-optic deflectors. Several schemes are demonstrated to trap three-dimensional colloidal structures with optical tweezers. One combined a Pockels cell and polarizing beam splitters to create two trapping planes at different depths in the sample, in which the optical traps could be manipulated independently. Optical tweezers were used to manipulate collections of particles inside concentrated colloidal dispersions, allowing control over colloidal crystallization and melting. Furthermore, we show that selective trapping and manipulation of individual tracer particles inside a concentrated dispersion of host particles is possible as well. The tracer particles had a core–shell geometry with a high refractive index material core and a lower index material shell. The host particles consisted of the same material as the lower index shells and were fluorescently labeled. The tracer particles could be manipulated without exerting forces on the host particles because the mixture was dispersed in a solvent with the same refractive index as that of the host particles. Using counterpropagating tweezers strongly scattering particles that could not be trapped by conventional single-beam optical tweezers were trapped and manipulated.
Review of Scientific Instruments 10/2004; · 1.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Optical tweezers were used to crystallize colloidal dispersions without
manipulating colloids on a single particle level. Using "optical
gradient" or "high-frequency dielectrophoretic" forces, we demonstrate
control over local particle concentration. This control can be used to
induce crystallization and melting in two- and three-dimensional
colloidal dispersions using single beam gradient optical tweezers. In
our setup, two microscope objectives (one above and one below the
sample) allow independent three-dimensional manipulation and imaging of
the structure formation inside the sample. We demonstrate
crystallization near a wall for a range of particles sizes, refractive
index contrasts and numerical apertures. In a colloidal mixture of
tracer particles with a high refractive index core and low refractive
index host spheres dispersed in a host refractive index matching
solvent, control over the tracers can indirectly lead to controlled
crystallization of the host particles.
[Show abstract][Hide abstract] ABSTRACT: We have studied, with quantitative confocal microscopy, epitaxial colloidal crystal growth of particles interacting with an almost hard-sphere (HS) potential in a gravitational field and density matched colloids interacting with a long-range (LR) repulsive potential with a body-centred cubic (BCC) equilibrium crystal phase. We show that in both cases it is possible to grow thick, stacking fault-free metastable crystals: close-packed crystals with any stacking sequence, including hexagonal close packed (HCP), for the HS particles and face-centred cubic (FCC) in the case of the LR colloids. In accordance with recent computer simulations done for HS particles it was found that the optimal lattice constant to grow HS HCP crystals was larger than that of equilibrium FCC crystals. In addition, because of the absence of gravity, pre-freezing could be observed for the particles with the LR potential on a template of charged lines. We also argue that the ability to manipulate colloids with highly focused light, optical traps or tweezers, will become an important tool in both the study of colloidal crystallization and in making new structures. We show how cheap 2D and 3D templates can be made with optical tweezers and demonstrate, in proof of principle experiments with core-shell colloids, how light fields can generate crystal nuclei and other structures in the bulk of concentrated dispersions and how the effect of these structures on the rest of a dispersion can be studied quantitatively in 3D.
[Show abstract][Hide abstract] ABSTRACT: A general method to coat colloids with silica is described. The amphiphilic, nonionic polymer poly-(vinylpyrrolidone) (PVP) was adsorbed to various colloidal particles such as small gold colloids, gold-shell silica-core particles, small and large silver colloids, boehmite rods, gibbsite platelets, and positively or negatively charged polystyrene. After this functionalization the stabilized particles could be transferred to a solution of ammonia in ethanol and directly coated with smooth and homogeneous silica shells of variable thickness by addition of tetraethoxysilane in a seeded growth process. The length of the polymer used strongly influences the stability of the colloids and the homogeneity and smoothness of the initial silica coating. This method is especially useful for colloidal particles that cannot be covered directly with SiO2 by a Stöber-like growth process. Compared to methods known from the literature for the coating of such particles, this new method is faster and requires neither the use of silane coupling agents nor a precoating step with sodium silicate, which is poorly reproducible.
[Show abstract][Hide abstract] ABSTRACT: Photonic crystals are regular three-dimensional (3D) structures with which the propagation and spontaneous emission of photons
can be manipulated in new ways if the feature sizes are roughly half the wavelength and the coupling with the electromagnetic
radiation is sufficiently strong. ‘Early’ speculation on these new possibilities can be found in the Refs.1–4 A more recent overview can be found in Ref.5 and, of course, the other chapters in this book. A useful analogy to guide thinking about the properties and the applications
of photonic crystals is the propagation of electrons in a semiconductor in comparison to the propagation of photons scattered
by a regular 3D dielectric material. An example is the possibility of opening up a region of energy, a photonic band gap,
for which the propagation of photons is forbidden, in analogy to the electronic band gap present in semiconductors. However,
there are also important differences; for instance, the scattering of photons cannot be described well by scalar wave equations
because the polarization of light cannot be neglected. Most theoretical and experimental work for visible light applications
have until now focused on pure dielectric structures, interestingly, recent calculations have shown that metallo-dielectric
structures should also be considered as having very interesting photonic properties in the visible, including, if one neglects
absorption, a complete band gap.6–8 And even with absorption taken into account, it seems that for relatively thin photonic crystals most of the interesting
optical properties remain.8
[Show abstract][Hide abstract] ABSTRACT: The local optical density of states ~LDOS! in 340-nm-diam SiO2 spherical microcavities was calculated and
probed experimentally by measuring the luminescence decay rate at 1.54 mm of erbium ions implanted in the
colloids. To separate the effect of nonradiative processes, first the radiative decay rate of Er31 in bulk SiO2 was
determined. This was done by varying the LDOS in an Er-doped planar SiO2 film by bringing the film into
contact with liquids of different refractive index in the range n51.33– 1.57. By comparing the calculated
LDOS with the observed changes in decay rate with index, the radiative rate was found to be 54610 s21 (t
51863 ms) in bulk SiO2. This value was then used to analyze the difference in decay rate in colloids
surrounded by air or immersed in an index-matching liquid. Within the experimental error, agreement was
found between the calculated and experimentally probed LDOS in the colloids. Finally, a full determination of
the LDOS vs size in SiO2 microcavities is presented (2pR/l50.1–6.9), which shows the appearance of a
number of maxima, corresponding to the position of the electric-type resonances inside the microcavity.
Physical Review A 01/2001; 64:033807. · 3.04 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The spontaneous emission rate of an optical probe atom is strongly dependent on its optical environment. This concept is well
known in one-dimensional geometries, e.g. for an atom placed near a mirror, a dielectric interface, or in a microcavity.1,2,3,4,5,6 With the recent development of two- and three-dimensional photonic crystals it becomes possible to tailor optical modes and
the local optical density-of-states (DOS) to a much greater extent. Large effects on the spontaneous emission rate of optical
probe ions are expected in these materials.