Techniques for directing the assembly of metal or semiconductor quantum dots into novel superstructures have been extensively pursued over the past decades. Recent interest has been drawn toward 1-dimensional nanoscale building blocks such as nanotubes, nanowires, and nanorods. If these one-dimensional nanoscale building blocks can be ordered and rationally assembled into appropriate 2-dimensional architectures, they will offer fundamental scientific opportunities for investigating the influence of size and dimensionality with respect to their collective optical, magnetic, and electronic properties, as well as many other technologically important applications. 2-dimensional (2D) nanorod monolayer assembly has been studied using Langmuir-Blodgett technique in our laboratory. During the experiments, the nanorod colloidal suspension was spread dropwise on the water surface of a Langmuir-Blodgett trough. The nanorod layer was then slowly compressed. At different stages during the compression, the nanorod assemblies at the water-air interface were transferred carefully onto transmission electron microscope (TEM) grids covered with continuous carbon thin film. At low densities, the nanorods form raft-like aggregates (generally 3 to 5 rods) by aligning side-by-side due to the directional capillary force and van der Waals attraction. However, the aggregates are dispersed on the subphase surface in a mostly isotropic state. As the monolayer is compressed, the nanorods start to align into a certain direction, presumably dictated by the barrier of the trough, forming a nematic phase. With further compression nanorod assemblies with smectic arrangement are obtained, which is characterized by layer-by-layer stacking of ribbon-like nanorod superstructures. After a certain pressure the monolayer breaks into multilayers, where it resumes a disordered 3-dimensional (3D) nematic configuration. This 2D isotropic, 2D nematic, 2D smectic, to 3D nematic transition can be explained mainly by volume exclusion entropy. As the density of the rods increases, it becomes increasingly difficult for the rods to point in random directions and intuitively one may expect the fluid to undergo a transition to a more ordered anisotropic phase having uniaxial symmetry. This ordering occurs to maximize the entropy of the self-assembled structure by minimizing the excluded volume per particle in the array as first proved by Onsager. Indeed, Frenkel et al. have carried out a Monte Carlo simulation on the phase behavior of two-dimensional hard rod fluids, which shows strong correlations with our experimental data despite the fact that our nanorods are not ideal hard rods. Providing suitable surface passivation chemistry, this Langmuir-Blodgett technique should be generally applicable to many nanorod, which promise interesting, tunable collective physical properties and functional 2-dimensional nanodevices.
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Bates, M. A.; Frenkel, D. J. Chem. Phys. 2000, 112, 10034.