The Control of the Expansion or Compression of Colloidal Crystals Lattice with Salt Solution.

Tuning the lattice spacing or stop bands holds great significance in the design and application of materials with colloidal crystals. Typically, particle surface modifications or the application of external physical fields are needed. In this study, we demonstrated the ability to expand or compress the lattice of colloidal crystals simply by utilizing a salt solution, without the need for any special treatments to the colloidal particles. We found that by only considering the diffusiophoresis effect we cannot explain the reversion of lattice expansion to lattice compression with the increase in the salt concentration and that the diffusioosmotic flow originating from the container wall must be taken into account. Further analysis revealed that variations in the salt concentration altered the relative amplitudes between diffusiophoresis and diffusioosmosis through changing the zeta potentials of the particles and the wall, and the competition between the particle diffusiophoresis and wall diffusioosmosis lay at the center of the underlying mechanism.

Template-induced crystallization of charged colloids: a molecular dynamics study.

By using a large enough number of particles and implementing a parallel algorithm on the CUDA platform, we have performed brute-force molecular dynamics simulations to study the template-induced heterogeneous crystallization in charged colloids. Six kinds of templates, whose patterns include the planes of fcc(100), fcc(110), fcc(111), bcc(100), bcc(110) and bcc(111), have been implanted into the middle of the simulation box. Except the fcc(111) template, whose structure benefits not only fcc but also hcp crystals resulting in a similar behavior to homogeneous crystallization, bcc-type templates favor the formation of bcc crystals and bcc-like precursors while fcc-type templates favor the formation of fcc crystals and fcc-like precursors. Therefore, for fcc(100) and fcc(110) templates, heterogeneous crystallization will definitely result in a fcc crystallite. However, the results of heterogeneous crystallization that are induced by bcc-type templates are subtly different at different state points. At the state points where the interaction strength of charged colloids is weak and the fcc phase is thermodynamically stable, the bcc crystals formed with the promotion of bcc-type templates are not stable so as to tend to transform into fcc or hcp crystals. When the interaction strength of charged colloids is high, the predominant bcc crystals formed with the promotion of bcc-type templates can always persist within the time scale of simulation although not bcc but fcc crystals are thermodynamically stable.

Light Amplification in Fe-Doped CsPbBr3 Crystal Microwire Excited by Continuous-Wave Laser.

Electrically pumped halide perovskite laser diodes remain unexplored, and it is widely acknowledged that continuous-wave (CW) lasing will be a crucial step. Here, we demonstrate room-temperature amplified spontaneous emission of Fe-doped CsPbBr3 crystal microwire excited by a CW laser. Temperature-dependent photoluminescence spectra indicate that the Fe dopant forms a shallow level trap states near the band edge of the lightly doped CsPbBr3 microcrystal. Pump intensity-dependent time-resolved PL spectra show that the introduced Fe dopant level makes the electron more stable in excited states, suitable for the population inversion. The emission peak intensity of the lightly Fe-doped microwire increases nonlinearly above a threshold of 12.3 kW/cm2 under CW laser excitation, indicating a significant light amplification. Under high excitation, the uniform crystal structure and surface outcoupling in Fe-doped perovskite crystal microwires enhanced the spontaneous emission. These results reveal the considerable promise of Fe-doped perovskite crystal microwires toward low-cost, high-performance, room-temperature electrical pumping perovskite lasers.

Microfluidic Synthesis, Doping Strategy, and Optoelectronic Applications of Nanostructured Halide Perovskite Materials.

Halide perovskites are increasingly exploited as semiconducting materials in diverse optoelectronic applications, including light emitters, photodetectors, and solar cells. The halide perovskite can be easily processed in solution, making microfluidic synthesis possible. This review introduces perovskite nanostructures based on micron fluidic channels in chemical reactions. We also briefly discuss and summarize several advantages of microfluidics, recent progress of doping strategies, and optoelectronic applications of light-sensitive nanostructured perovskite materials. The perspective of microfluidic synthesis of halide perovskite on optoelectronic applications and possible challenges are presented.

Correction to "Molecular Dynamics Simulation of the Soret Effect on Two Binary Liquid Solutions with Equimolar n-Alkane Mixtures".

[This corrects the article DOI: 10.1021/acsomega.1c04926.].

Molecular Dynamics Simulation of the Soret Effect on Two Binary Liquid Solutions with Equimolar n-Alkane Mixtures.

Molecular dynamics is employed to simulate the Soret effect on two binary liquid solutions with equimolar mixtures: normal pentane (n-pentane, nC-5) and normal heptane (n-heptane, nC-7) molecules plus normal decane (n-decane, nC-10) and normal pentane molecules. Moreover, two coarse-grained force field (the CG-FF) potentials, which may depict inter-/intramolecular interactions fairly well among n-alkane molecules, are developed to fulfill such investigations. In addition, thermal diffusion for the mass fraction of each of these n-alkane molecules is simulated under an effect of a weak thermal gradient (temperature difference) exerting on solution systems from their hot to cold boundary sides. Finally, quantities of the Soret coefficient (SC) for two binary solutions are calculated by means of the developed CG-FF potentials, so as to improve the calculation rationality. As a result, first, it is found that molecules with light molar masses will migrate toward the hot boundary side, while those with heavy molar masses will migrate toward the cold boundary one ; second, the SC quantities indicate that they match relevant experimental determinations fairly well, i.e., trends of these SC quantities show inverse proportionality to the thermal gradient on the systems.

Evolution of concentration and phase structure of colloidal suspensions in a two-ends-open tube during drying process.

We investigated the evolution of concentration and phase structure of colloidal suspensions in a two-ends-open tube during drying process. The volume fraction and crystal structure of suspension in the capillary tube were determined by reflection spectrometer during drying process. Our experimental results show: (a) evaporation takes place in two directions of the tube, though much stronger in one direction than the other; (b) during drying process, colloidal suspension column along the tube can be divided into four regions, namely, the close packed region, concentrated region, initial concentration region and dilution region. A new model describing the evolution of concentration profile was proposed and the calculated results based on the model are in good agreement with the experimental ones. According to solute conservation, we also present a simple way to estimate the concentration of close packed region.

Entire crystallization process of Lennard-Jones liquids: A large-scale molecular dynamics study.

By using a graphics processing unit-accelerated parallel algorithm on a compute unified device architecture platform, we perform large-scale molecular dynamics simulations in a Lennard-Jones system to observe the entire crystallization process, including metastable stage, critical nuclei formation, and the stage of crystal growth. Although the intermediated precursors that play a role in determining the polymorphs are predominantly bcc ordered, the polymorph selection is rather different at different stages. The precursors that have a relatively high orientational order will be on average in a denser region than uniform liquids, but microscopically the crystal nucleation happens without a density change. The average density of nuclei first increases significantly, and then almost keeps independent on the crystallite size after the growing post-critical nucleus becomes large enough. With such a large enough system, the crystal growth rate is able to be calculated directly by doing a linear fit to the temporal evolution of growing crystallite size. The obtained value of the growth rate indicates that the actual crystal growth in the Lennard-Jones system where the crystal-liquid interface has several kinds of structures is possibly driven by both collision-controlled and diffusion-controlled mechanisms.

Determination of Bulk Modulus for a Colloidal Crystal with Highly Charged Particles by DC Electric Field.

In this study, bulk modulus of a colloidal crystal formed by highly charged particles is experimentally determined by applying direct current electric field. A theoretical expression is also proposed to independently predict the bulk modulus based on van't Hoff's law of osmotic pressure and the theory of Ohshima. The experimental result thus obtained agrees well with the theoretical expectation. In addition, results from both above-mentioned methods coincide with that inferred from the static structure factor.

Crystal nucleation and metastable bcc phase in charged colloids: A molecular dynamics study.

The dynamic process of homogenous nucleation in charged colloids is investigated by brute-force molecular dynamics simulation. To check if the liquid-solid transition will pass through metastable bcc, simulations are performed at the state points that definitely lie in the phase region of thermodynamically stable fcc. The simulation results confirm that, in all of these cases, the preordered precursors, acting as the seeds of nucleation, always have predominant bcc symmetry consistent with Ostwald's step rule and the Alexander-McTague mechanism. However, the polymorph selection is not straightforward because the crystal structures formed are not often determined by the symmetry of intermediate precursors but have different characters under different state points. The region of the state point where bcc crystal structures of large enough size are formed during crystallization is narrow, which gives a reasonable explanation as to why the metastable bcc phase in charged colloidal suspensions is rarely detected in macroscopic experiments.

Polymorph selection and nucleation pathway in the crystallization of Hertzian spheres.

The crystallization process of Hertzian spheres is studied by means of molecular dynamics simulations in an NPT ensemble where the total number of particles N, the pressure P, and the temperature T are kept constant. It has been observed that the bond orientational ordering rather than the translational ordering (density) plays a primary role. The crystal polymorphs are determined by the state points. Under the conditions of small supercooling, the system is likely to be nucleated into crystals that have a preference for the metastable bcc structure, which can be regarded as a manifestation of the Alexander-McTague mechanism. In contrast, small nuclei are found to have a preference for fcc symmetry under conditions of a high degree of supercooling. Prestructured precursors that act as seeds and wet on the nuclei during nucleation always have a high degree of bcc-like ordering, despite different state points. The results above may provide a clue to the understanding of the crystallization process in core-softened particles.

Structural and dynamical anomalies of soft particles interacting through harmonic repulsions.

Molecular dynamics (MD) simulations are carried out to investigate the structural and dynamical anomalies in the core-softened fluid with harmonic repulsions. At sufficiently low temperature and densities, a general scaling relation between the Rosenfeld diffusivity DR and excess entropy Sex holds true because the ultrasoft particles tend to avoid overlapping and behave like effective hard spheres. When the temperature is increased high enough, the system acts as a weakly correlated mean-field fluid so that an alternative scaling relation between temperature scaled diffusion DT and Sex does work. Interestingly, the plots of DRversus two-body excess entropy S2 approximately collapse onto a single master curve despite the structural and dynamical anomalies, which has also been observed in other core-softened fluids with bounded potential.

Structural ordering and glass forming of soft spherical particles with harmonic repulsions.

We carry out dissipative particle dynamics simulations to investigate the dynamic process of phase transformation in the system with harmonic repulsion particles. Just below the melting point, the system undergoes liquid state, face-centered cubic crystallization, body-centered cubic crystallization, and reentrant melting phase transition upon compression, which is in good agreement with the phase diagram constructed previously via thermodynamic integration. However, when the temperature is decreased sufficiently, the system is trapped into an amorphous and frustrated glass state in the region of intermediate density, where the solid phase and crystal structure should be thermodynamically most stable.

Influence of the surface charge on the homogeneity of colloidal crystals.

Five groups of suspensions composed of polystyrene particles, having similar size but different effective surface charge, were adopted to investigate the effects of surface charge and volume fraction on the homogeneity of colloidal crystals through checking the difference between D(exp) and D(uni) by reflection spectroscopy method (D(exp), D(uni) are the experimental and the expected value of the average nearest neighbor interparticle distance by assuming a uniform structure, respectively). We found volume fractions (ranging from 0.006 to 0.02) and structure types basically have no influence on the values of D(exp)/D(uni). Moreover, for crystals formed by lowly charged particles, D(exp)/D(uni) is approximately equal to 1, implying the crystals are homogeneous. With the increase of effective surface charge, D(exp) gradually deviates from D(uni) and the formed crystals become inhomogeneous. Our experimental observations are in accordance with the previous simulation results. Additionally, we also found D(exp)/D(uni) initially drops quickly with increasing effective surface charge and then it tends to an asymptotic value (~0.85), it is supposedly due to the saturation of effective charge. Our relevant computer simulations confirmed that the study scheme that using D(exp)/D(uni) as an indicator to assess the homogeneity of crystal structure is tenable and the simulation results are consistent with experiments.

Gas-liquid phase coexistence in quasi-two-dimensional Stockmayer fluids: A molecular dynamics study.

The Maxwell construction together with molecular dynamics simulation is used to study the gas-liquid phase coexistence of quasi-two-dimensional Stockmayer fluids. The phase coexistence curves and corresponding critical points under different dipole strength are obtained, and the critical properties are calculated. We investigate the dependence of the critical point and critical properties on the dipole strength. When the dipole strength is increased, the abrupt disappearance of the gas-liquid phase coexistence in quasi-two-dimensional Stockmayer fluids is not found. However, if the dipole strength is large enough, it does lead to the formation of very long reversible chains which makes the relaxation of the system very slow and the observation of phase coexistence rather difficult or even impossible.

Phase behavior of a reversibly assembling system in two dimensions: a Monte Carlo simulation study.

Using Monte Carlo simulation, we investigate the structural phase behavior of a continuum molecular model for self-assembling semiflexible equilibrium polymers in two dimensions. Particle-particle interaction is modeled via a Lennard-Jones potential with tunable anisotropic attraction. Depending on the strength of the anisotropy, we find the formation of reversible networks as well as stiff rodlike aggregates. The phase transition observed in the presence of the network structures is compared to predictions of the Tlusty-Safran defect model.

From gas-liquid to liquid crystalline phase behavior via anisotropic attraction: a computer simulation study.

The partial phase behavior of a continuum molecular model for self-assembling semiflexible equilibrium polymers is studied via Monte Carlo and molecular dynamics simulation. We investigate the transfer from ordinary gas-liquid coexistence to the appearance of liquid crystallinity driven by excluded volume interaction between rodlike aggregates. The transfer between the two types of phase behavior is governed by a tunable anisotropic attractive interaction between monomer particles. The relation to dipolar fluid models, which are also known to form reversible chains, is discussed.

Influence of molecular topology on the static and dynamic properties of single polymer chain in solution.

The influence of molecular topology on the structural and dynamic properties of polymer chain in solution with ring structure, three-arm branched structure, and linear structure are studied by molecular dynamics simulation. At the same degree of polymerization (N), the ring-shaped chain possesses the smallest size and largest diffusion coefficient. With increasing N, the difference of the radii of gyration between the three types of polymer chains increases, whereas the difference of the diffusion coefficients among them decreases. However, the influence of the molecular topology on the static and the dynamic scaling exponents is small. The static scaling exponents decrease slightly, and the dynamic scaling exponents increase slightly, when the topology of the polymer chain is changed from linear to ring-shaped or three-arm branched architecture. The dynamics of these three types of polymer chain in solution is Zimm-like according to the dynamic scaling exponents and the dynamic structure factors.