Spin Physics of Excitons in Colloidal Nanocrystals

We present a review of spin-dependent properties of excitons in semiconductor colloidal nanocrystals. The photoluminescences (PL) properties of neutral and charged excitons (trions) are compared. The mechanisms and the polarization of radiative recombination of a “dark” (spin-forbidden) exciton that determines the low-temperature PL of colloidal nanocrystals are discussed in detail. The radiative recombination of a dark exciton becomes possible as a result of simultaneous flips of the surface spin and electron spin in a dark exciton that leads to admixture of bright exciton states. This recombination mechanism is effective in the case of a disordered state of the spin system and is suppressed if the polaron ferromagnetic state forms. The conditions and various mechanisms of formation of the spin polaron state and possibilities of its experimental detection are discussed. The experimental and theoretical studies of magnetic field-induced circular polarization of PL in ensembles of colloidal nanocrystals are reviewed.     

Nonlinear screening of high-Z grains and the formation of Coulomb lattices in colloidal plasmas

Liquid–solid phase transitions in colloidal plasmas are studied within the Yukawa model with nonlinearly screened effective potentials. It is shown, that there is a minimal charge asymmetry needed for the crystallization of a Coulomb lattice and its value depends on the packing fraction of the colloidal component. The smaller the packing fraction, the higher the charge asymmetry, and, respectively, the stronger the coupling in the colloidal component must be in order to reach the solid state.     

An investigation on fluorescence quenching of certain porphyrins by colloidal CdS

Certain porphyrin derivatives namely meso-tetraphenylporphyrin (TPP), meso-tetrakis(4-carboxyphenyl)porphyrin (TCPP), meso-tetrakis(4-sulfonatophenyl)porphyrin (TSPP) were examined as sensitizers for colloidal CdS. The interaction of these porphyrins and colloidal CdS were studied by absorption, infrared, steady state and time resolved fluorescence spectroscopy and transient absorption techniques. The apparent association constants (K_app) resulting from adsorption of porphyrins on CdS surface were calculated from both absorption and fluorescence studies and they agree well. Using all the spectroscopic measurements we confirmed that the interaction between porphyrins and colloidal CdS occurs through ground state complex formation and the quenching follows static mechanism.     

On the nonequilibrium phase transition in protein

Considerations are presented that support the idea that the nonequilibrium phase transition in protein is accompanied by the growth of a colloidal nanocrystal (or the vitrified phase of a liquid crystal). To date, the dynamic transition forms of protein, which are “the source of the catalytic power of enzymes,” have been poorly understood. In our experiments, we observed the dehydration (drying) of the colloid solution of protein in an open (far from thermodynamic equilibrium) one-component protein-water system. The protein in this state is found to acquire properties typical of matter self-organization, including the universal properties of colloidal nanocrystals of different nature, namely, nonequilibrium chaotic dynamics with self-replicability, autocatalysis, coherency, autowave fluctuations, synchronism, fractality, 3D epitaxial growth (stacking) of films, nucleation giving rise to blocks (cells) with shell nuclei, etc. It then follows that our realistic model of the nonequilibrium state of protein during growth of its colloidal nanocrystal provides an opportunity of studying the dynamics of the structural and energy-information features of the transition and solid colloidal nanocrystalline phases of protein. In addition, it allows researchers to gain fundamentally new information about the energy characteristics of protein under abiotic and biotic conditions.     

Coupling between aging and convective motion in a colloidal glass of Laponite

We study thermal convection in a colloidal glass of Laponite in formation. Low concentration preparation are submitted to destabilizing vertical temperature gradient, and present a gradual transition from a turbulent convective state to a steady conductive state as their viscosity increases. The time spent under convection is found to depend strongly on sample concentration, decreasing exponentially with mass fraction of colloidal particles. Moreover, at fixed concentration, it also depends slightly on the pattern selected by the Rayleigh Bénard instability: more rolls maintain the convection state longer. This behavior can be interpreted with recent theoretical approaches of soft glassy material rheology.     

Aggregation kinetics in a model colloidal suspension

We present molecular dynamics simulations of aggregation kinetics in a colloidal suspension modeled as a highly asymmetric binary mixture. Starting from a configuration with largely uncorrelated colloidal particles the system relaxes by coagulation-fragmentation dynamics to a structured state of low-dimensionality clusters with an exponential size distribution. The results show that short-range repulsive interactions alone can give rise to so-called cluster phases. For the present model and probably other, more common colloids, the observed clusters appear to be equilibrium phase fluctuations induced by the entropic intercolloidal attractions.     

Shear-driven solidification of dilute colloidal suspensions

We show that shear-induced solidification of dilute charge-stabilized colloids is due to the interplay between shear-induced formation and breakage of large non-Brownian clusters. While their size is limited by breakage, their number density increases with shearing time. Upon flow cessation, the dense packing of clusters interconnects into a rigid state by means of grainy bonds, each involving a large number of primary colloidal bonds. The emerging picture of shear-driven solidification in dilute colloidal suspensions combines the gelation of Brownian systems with the jamming of athermal systems.     

UV-Visible Photoluminescence of TiO2 Nanoparticles Prepared by Hydrothermal Method

TiO₂ colloidal nanoparticles and nanocrystals are prepared by hydrolysis of titanium isopropoxide employing a surfactant-free synthetic hydrothermal method. The synthesized samples are characterized by X-ray diffraction (XRD), HRTEM and FTIR. The XRD study confirms that the size of the colloidal nanoparticle is around 4?nm which the HRTEM analysis indicates the sizes of the colloidal nanoparticles are in the range of 2.5?nm. The fluorescence property of the TiO₂ colloidal nanoparticles studied by the emission spectrum confirms the presence of defect levels caused by the oxygen vacancies. We have observed new emission bands at 387?nm,421?nm, 485?nm, 530?nm and 574?nm wavelengths, first one (387?nm) being emission due to annihilation of excitons while remaining four could be arising from surface states. The emission spectrum of annealed nanocrystallites is also having these four band emissions. It is observed that the surface state emission basically consists of two categories of emission.     

Critical volume fraction and size for a colloidal cluster to nucleate

With the aid of the critical size of colloidal cluster,the critical volume fraction of phase transition of colloidal system is determined by the principle of entropy maximum and Carnahan-Starling(CS) state equation in this paper.In our discussion,no parameter is introduced externally,and our results are in good agreement with the experimental results.     

Giant colloidal diffusivity on corrugated optical vortices

A single colloidal sphere circulating around a periodically modulated optical vortex trap can enter a dynamical state in which it intermittently alternates between freely running around the ringlike optical vortex and becoming trapped in local potential energy minima. Velocity fluctuations in this randomly switching state still are characterized by a linear Einstein-like diffusion law, but with an effective diffusion coefficient that is enhanced by more than 2 orders of magnitude.     

Gel-forming patchy colloids and network glass formers: thermodynamic and dynamic analogies

This article discusses recent attempts to provide a deeper understanding of the thermoreversible “gel” state of colloidal matter and to unravel the analogies between gels at the colloidal level and gels at the molecular level, commonly known as network-forming strong liquids. The connection between gel-forming patchy colloids and strong liquids is provided by the limited valence of the inter-particle interactions, i.e. by the presence of a limit in the number of bonded nearest neighbors.     

Colloids as model systems for metals and alloys: a case study of crystallization

Metallic systems are widely used as materials in daily human life. Their properties depend very much on the production route. In order to improve the production process and even develop novel materials a detailed knowledge of all physical processes involved in crystallization is mandatory. Atomic systems like metals are characterized by very high relaxation rates, which make direct investigations of crystallization very difficult and in some cases impossible. In contrast, phase transitions in colloidal systems are very sluggish and colloidal suspensions are optically transparent. Therefore, colloidal systems are often discussed as model systems for metals. In the present work, we study the crystallization process of charged colloidal systems from the very beginning. Charged colloids offer the advantage that the interaction potential can be systematically tuned by a variation of the particle number density and the salt concentration. We apply light scattering and ultra-small angle x-ray scattering to investigate the formation of short-range order in the liquid state even far from equilibrium, crystal nucleation and crystal growth. The results are compared with equivalent studies on metallic systems.     

Life at ultralow interfacial tension: wetting, waves and droplets in demixed colloid-polymer mixtures

Mixtures of colloids and polymers display a rich phase behavior, involving colloidal gas (rich in polymer, poor in colloid), colloidal liquid (poor in polymer, rich in colloid) and colloidal crystal phases (poor in polymer, highly ordered colloids). Recently, the colloidal gas-colloidal liquid interface received considerable attention as well. Due to the colloidal length scale the interfacial tension is much lower than in the atomic or molecular analog (nN/m instead of mN/m). This ultra-low interfacial tension has pronounced effects on the kinetics of phase separation, the colloidal gas-liquid profile near a single wall and the thermally induced fluctuations of the interface. The amplitudes of these thermally excited capillary waves are restrained by the interfacial tension and are for that reason of the order of the particle diameter. Therefore, in molecular systems, the capillary waves can only be seen indirectly in scattering experiments. In colloidal systems, however, the wave amplitudes are on a (sub) micrometer scale. This fact enables the direct observation of capillary waves in both real space and real time using confocal scanning laser microscopy. Moreover, the real space technique enables us to demonstrate the strong influence of interface fluctuations on droplet coalescence and droplet break up.     

Influence of hydrodynamic interactions on lane formation in oppositely charged driven colloids

The influence of hydrodynamic interactions on lane formation of oppositely charged driven colloidal suspensions is investigated using Brownian dynamics computer simulations performed on the Rotne-Prager level of the mobility tensor. Two cases are considered, namely sedimentation and electrophoresis. In the latter case the Oseen contribution to the mobility tensor is screened due to the opposite motion of counterions. The simulation results are compared to that resulting from simple Brownian dynamics where hydrodynamic interactions are neglected. For sedimentation, we find that hydrodynamic interactions strongly disfavor laning. In the steady state of lanes, a macroscopic phase separation of lanes is observed. This is in marked contrast to the simple Brownian case where a finite size of lanes was obtained in the steady state. For strong Coulomb interactions between the colloidal particles a lateral square lattice of oppositely driven lanes is stable similar to the simple Brownian dynamics. In an electric field, on the other hand, the behavior is found in qualitative and quantitative accordance with the case of neglected hydrodynamics.     

Nematic-wetted colloids in the isotropic phase: pairwise interaction, biaxiality, and defects

We calculate the interaction between two spherical colloidal particles embedded in the isotropic phase of a nematogenic liquid. The surface of the particles induces wetting nematic coronas that mediate an elastic interaction. In the weak wetting regime, we obtain exact results for the interaction energy and the texture, showing that defects and biaxiality arise, although they are not topologically required. We evidence rich behaviors, including the possibility of reversible colloidal aggregation and dispersion. Complex anisotropic self-assembled phases might be formed in dense suspensions.     

杂质对硬球系统团簇成核临界条件的影响

通过CS状态方程及熵极大原理在多元胶体系统研究了杂质对硬球系统团簇成核临界条件的影响. 极少量元素或杂质的小胶球对系统成核的临界条件的影响也是不可忽略的: 成核的临界体积分量显著地降低;此外,研究还发现杂质对成核的临界尺寸影响较少.     

Colloidal particles at fluid interfaces: Effective interactions, dynamics and a gravitation–like instability

Colloidal particles of micrometer size usually become irreversibly trapped at fluid interfaces if they are partially wetted by one phase. This opens the chance to create two–dimensional model systems where the effective interactions between the particles are possibly influenced by the presence of the interface to a great extent. We will review recent developments in the quantitive understanding of these effective interactions with a special emphasis on electrostatics and capillarity. Charged colloids of micrometer size at an interface form effective dipoles whose strength sensitively depends on the double layer structure. We discuss the success of modified Poisson–Boltzmann equations with regard to measured colloidal dipole moments. On the other hand, for somewhat larger particles capillary interactions arise which are long–ranged and analogous to two–dimensional screened Newtonian gravity with the capillary length λ as the screening length. For colloidal diameters of around 10 micrometer, the collective effect of these long–ranged capillary interactions will dominate thermal motion and residual, short–ranged repulsions, and results in an instability towards a collapsed state for a finite patch of particles. Such long–ranged interactions with the associated instability are also of interest in other branches of physics, such as self-gravitating fluids in cosmology, two–dimensional vortex flow in hydrodynamics, and bacterial chemotaxis in biology. Starting from the colloidal case we develop and discuss a dynamical “phase diagram” in the temperature and interaction range variables which appears to be of more general scope and applicable also to other systems.     

Synthesis and time-resolved photoluminescence spectroscopy of capped CdS nanocrystals

Here, we report the synthesis of colloidal CdS nanoparticles by capping with starch, phenol and pyridine. We also study the photophysical properties of CdS nanoparticles by steady state and time-resolved photoluminescence spectroscopy. The experimental results show that the relaxation of the excited state of CdS nanoparticles is composed of two different components. Our analysis suggests that the fast and slow components decay times of these capped CdS nanocrystals are due to trapping of carriers in surface state and e–h radiative recombination processes, respectively.     

Random organization in periodically driven gliding dislocations

We numerically examine dynamical irreversible to reversible transitions and random organization for periodically driven gliding dislocation assemblies using the stroboscopic protocol developed to identify random organization in periodically driven dilute colloidal suspensions. We find that the gliding dislocations exhibit features associated with random organization and evolve into a dynamically reversible state after a transient time extending over a number of cycles. At a critical shearing amplitude, the transient time diverges. When the dislocations enter the reversible state they organize into patterns with fragmented domain wall type features.