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We develop a coarse-grained lattice gas model to describe drying-mediated self-assembly of nanoparticles in three dimensions. Our model is an extension of the model developed by Rabani et al. [Nature 2003, 426, 271] in two dimensions. We show that when liquid evaporation occurs layer by layer and solvation forces are strong, the resulting morphologies agree well with the predictions of the 2D model. We discuss scenarios in which the full 3D treatment is necessary and predict the formation of nanostalagmites.

The self-assembly of nanoparticles is one of the most promising techniques currently used to develop advanced materials. [1,2] The distinct properties of the nanoparticles can be integrated into assemblies of nanometer-scale building blocks, which leads to novel superstructures with unique collective properties. Most recent studies have focused on assemblies of single-component systems, such as semiconducting, [3][4][5][6][7][8][9][10][11] magnetic, [12][13][14][15][16] and metal nanocrystals. [17][18][19][20][21][22][23][24][25] The study of multicomponent materials, i.e., assemblies of mixtures of nanoparticles, has received very little attention despite their role in the development of metamaterials, [26] materials with properties arising from the controlled interactions between different nanoparticles in an assembly. Previous studies of assemblies of multicomponent nanocrystals have usually resulted in amorphous materials. [27][28][29][30] This is because of the lack of directional forces or low diffusivity of the nanoparticles during the assembly process itself. [26] An exception is the work reported by Redl et al. where mixtures of PbSe semiconducting quantum dots and Fe 2 O 3 magnetic nanocrystals were assembled into ordered three-dimensional arrays. [26] This was achieved by carefully controlling the solvent vapor pressure, thereby adjusting the rate of evaporation during the drying process. Redl et al. showed how the final superlattice crystal structure can be tuned by the size ratio of the two nanoparticle components. In the present study, an attempt is made to model and understand the physical mechanisms of the assembly of binary mixtures of nanoparticles. For this, the model presented by Rabani and co-workers [31,32] is adopted and extended to describe the drying-mediated self-assembly of binary mixtures of nanoparticles. This out-of-equilibrium problem requires the dynamic treatment of three coupled phase transformations: the evaporation of the solvent (drying), the formation of domains (coarsening), and ordering within the domains (formation of superlattices). In the language of statistical mechanics the present paper treats the dynamics of the three coupled phase transformations, two are described by a conserved order parameter and one by a non-conserved order parameter.[33]Three cases of the self-assembly of binary mixtures of nanoparticles under homogeneous and heterogeneous evaporation conditions are studied. The cases differ only in the interactions between the different nanoparticle species. For the case where the interactions between all the nanoparticles are equal, the dynamics of the model presented here and the resulting selfassembled morphologies are well described by the simplified single-component model. [31,32] However, in the other two cases studied, where the cross interactions are smaller/larger than the interactions between identical particles, it is found that the coarsening of the nanoparticle domains is strongly coupled to the superlattice formation within each domain. Furthermo...

Multi-component nano-systems are created by combining two or more different materials yielding new chemical and physical properties. Here we report the selective anisotropic growth creating hybrid nanparticles of two different material systems, a metal onto a semiconductor. We developed a simple method for the selective growth of gold tips onto the apexes of colloidal semiconductor nanorods and tetrapods. This combination provides new functionalities to the nanostructures, the most important of which is the formation of natural anchor points that can serve as a recognition element for directed self assembly and for wiring them onto electrical circuitry. At higher gold concentrations we observe a transition from two to one-sided gold growth. This behavior is attributed to a ripening process as was also studied using a theoretical model.