A revolution in novel nanoparticles and colloidal building blocks has been enabled by recent breakthroughs in particle synthesis. These new particles are poised to become the 'atoms' and 'molecules' of tomorrow's materials if they can be successfully assembled into useful structures. Here, we discuss the recent progress made in the synthesis of nanocrystals and colloidal particles and draw analogies between these new particulate building blocks and better-studied molecules and supramolecular objects. We argue for a conceptual framework for these new building blocks based on anisotropy attributes and discuss the prognosis for future progress in exploiting anisotropy for materials design and assembly.

Extensive molecular dynamics simulations are performed on a glass-forming Lennard-Jones mixture to determine the nature of the cooperative motions occurring in this model fragile liquid. We observe stringlike cooperative molecular motion ("strings") at temperatures well above the glass transition. The mean length of the strings increases upon cooling, and the string length distribution is found to be nearly exponential. [S0031-9007(98)05583-5] PACS numbers: 61.20.Lc, 61.43.Fs, 64.70.Pf The concept of cooperative molecular motion [1,2] is commonly invoked to rationalize dramatic changes in the transport properties of liquids as they are cooled toward their glass transition [3]. In the heuristic Adam and Gibbs model [1] of supercooled liquids, relaxation occurs through "cooperatively rearranging regions" which grow with decreasing temperature. A more rigorous treatment of collective motion in liquids by Zwanzig and Nossal emphasizes the occurrence of momentum density excitations whose lifetime grows as the temperature is lowered [4]. Modecoupling theory (MCT) [5] attributes the slowing down of particle motion at low temperatures to "backflow" collective particle motion which eventually causes a structural arrest of the liquid dynamics. However, there has been no direct experimental observation of these kinds of cooperative motion.Computer simulations offer advantages over experiments on real liquids for the investigation of collective particle motion. In molecular dynamics the position and velocity of all the particles are known at all times. As a consequence, correlation functions quantifying the motion of particular subsets of particles can be readily determined. Recent experiments [6,7] and simulations [8,9] have identified dynamical heterogeneity in supercooled liquids and spin glasses [10]. It is natural to suppose that cooperative motion might be associated with this dynamical heterogeneity. A well studied Lennard-Jones (LJ) system, recently introduced to study the dynamics of simple supercooled liquids, provides a particularly good model to test for cooperative motion, since its properties have been well characterized [11] and evidence for dynamical heterogeneity has already been identified for this model [9].In this Letter, we test whether cooperative molecular motion occurs in this model fragile glass-forming liquid. We find that molecular motion indeed becomes increasingly collective upon cooling. However, the regions involved in this motion are not compact, as usually supposed [1,12], but instead form stringlike structures. The average string length increases with decreasing temperature, and the string length distribution is nearly exponential.We performed extensive molecular dynamics simulations of a three dimensional binary mixture (80:20) of 8000 LJ particles where the interaction parameters [13] are chosen to prevent demixing and crystallization [11]. Ten temperatures T between 0.550 and 0.451 above the fitted mode-coupling temperature T c ഠ 0.431 [11] are studied. We emphasize that this temperatu...

Predicting structure from the attributes of a material's building blocks remains a challenge and central goal for materials science. Isolating the role of building block shape for self-assembly provides insight into the ordering of molecules and the crystallization of colloids, nanoparticles, proteins, and viruses. We investigated 145 convex polyhedra whose assembly arises solely from their anisotropic shape. Our results demonstrate a remarkably high propensity for thermodynamic self-assembly and structural diversity. We show that from simple measures of particle shape and local order in the fluid, the assembly of a given shape into a liquid crystal, plastic crystal, or crystal can be predicted.

We present the results of a large scale molecular dynamics computer simulation study in which we investigate whether a supercooled Lennard-Jones liquid exhibits dynamical heterogeneities. We evaluate the non-Gaussian parameter for the self part of the van Hove correlation function and use it to identify "mobile" particles. We find that these particles form clusters whose size grows with decreasing temperature. We also find that the relaxation time of the mobile particles is significantly shorter than that of the bulk, and that this difference increases with decreasing temperature.Recent NMR experiments have shown that the relaxation in supercooled liquids is not homogeneous, i.e. that there are regions in space in which the relaxation of the particles is significantly faster (or slower) than the average relaxation of the system [1]. Subsequently, this result has been supported by optical spectroscopy, forced Rayleigh scattering and further NMR experiments [2]. However, these types of experiments are unable to determine the nature of these "dynamical heterogeneities," and consequently details such as size are

Molecular simulations are performed to study the self-assembly of particles with discrete, attractive interaction sites - "patches" - at prescribed locations on the particle surface. Chains, sheets, rings, icosahedra, square pyramids, tetrahedra, and twisted and staircase structures are obtained through suitable design of the surface pattern of patches. Our simulations predict that the spontaneous formation of two-dimensional sheets and icosahedra occurs via a first-order transition while the formation of chains occurs via a continuous disorder-to-order transition as in equilibrium polymerization. Our results show how precise arrangements of patches combined with patch "recognition" or selectivity may be used to control the relative position of particles and the overall structure of particle assemblies. In this context, patchy particles represent a new class of building block for the fabrication of precise structures.

In their physical dimensions, surface chemistry, and degree of anisotropic interactions in solution, CdTe nanoparticles are similar to proteins. We experimentally observed their spontaneous, template-free organization into free-floating particulate sheets, which resemble the assembly of surface layer (S-layer) proteins. Computer simulation and concurrent experiments demonstrated that the dipole moment, small positive charge, and directional hydrophobic attraction are the driving forces for the self-organization process. The data presented here highlight the analogy of the solution behavior of the two vastly different classes of chemical structures.

Relaxation in supercooled liquids above their glass transition and below the onset temperature of ''slow'' dynamics involves the correlated motion of neighboring particles. This correlated motion results in the appearance of spatially heterogeneous dynamics or ''dynamical heterogeneity.'' Traditional two-point time-dependent density correlation functions, while providing information about the transient ''caging'' of particles on cooling, are unable to provide sufficiently detailed information about correlated motion and dynamical heterogeneity. Here, we study a four-point, time-dependent density correlation function g 4 (r,t) and corresponding ''structure factor'' S 4 (q,t) which measure the spatial correlations between the local liquid density at two points in space, each at two different times, and so are sensitive to dynamical heterogeneity. We study g 4 (r,t) and S 4 (q,t) via molecular dynamics simulations of a binary Lennard-Jones mixture approaching the mode coupling temperature from above. We find that the correlations between particles measured by g 4 (r,t) and S 4 (q,t) become increasingly pronounced on cooling. The corresponding dynamical correlation length 4 (t) extracted from the small-q behavior of S 4 (q,t) provides an estimate of the range of correlated particle motion. We find that 4 (t) has a maximum as a function of time t, and that the value of the maximum of 4 (t) increases steadily from less than one particle diameter to a value exceeding nine particle diameters in the temperature range approaching the mode coupling temperature from above. At the maximum, 4 (t) and the ␣ relaxation time ␣ are related by a power law. We also examine the individual contributions to g 4 (r,t), S 4 (q,t), and 4 (t), as well as the corresponding order parameter Q(t) and generalized susceptibility 4 (t), arising from the self and distinct contributions to Q(t). These contributions elucidate key differences between domains of localized and delocalized particles.

Using extensive molecular dynamics simulations of an equilibrium, glass-forming Lennard-Jones mixture, we characterize in detail the local atomic motions. We show that spatial correlations exist among particles undergoing extremely large ("mobile") or extremely small ("immobile") displacements over a suitably chosen time interval. The immobile particles form the cores of relatively compact clusters, while the mobile particles move cooperatively and form quasi-one-dimensional, string-like clusters. The strength and length scale of the correlations between mobile particles are found to grow strongly with decreasing temperature, and the mean cluster size appears to diverge at the mode-coupling critical temperature. We show that these correlations in the particle displacements are related to equilibrium fluctuations in the local potential energy and local composition.