Oriented attachment of synthetic semiconductor nanocrystals is emerging as a route for obtaining new semiconductors that can have Dirac-type electronic bands such as graphene, but also strong spin-orbit coupling. The two-dimensional (2D) assembly geometry will require both atomic coherence and long-range periodicity of the superlattices. We show how the interfacial self-assembly and oriented attachment of nanocrystals results in 2D metal chalcogenide semiconductors with a honeycomb superlattice. We present an extensive atomic and nanoscale characterization of these systems using direct imaging and wave scattering methods. The honeycomb superlattices are atomically coherent and have an octahedral symmetry that is buckled; the nanocrystals occupy two parallel planes. Considerable necking and large-scale atomic motion occurred during the attachment process.

Oriented attachment of PbSe nanocubes can result in the formation of two-dimensional (2D) superstructures with long-range nanoscale and atomic order. This questions the applicability of classic models in which the superlattice grows by first forming a nucleus, followed by sequential irreversible attachment of nanocrystals, as one misaligned attachment would disrupt the 2D order beyond repair. Here, we demonstrate the formation mechanism of 2D PbSe superstructures with square geometry by using in situ grazing-incidence X-ray scattering (small angle and wide angle), ex situ electron microscopy, and Monte Carlo simulations. We observed nanocrystal adsorption at the liquid/gas interface, followed by the formation of a hexagonal nanocrystal monolayer. The hexagonal geometry transforms gradually through a pseudo-hexagonal phase into a phase with square order, driven by attractive interactions between the {100} planes perpendicular to the liquid substrate, which maximize facet-to-facet overlap. The nanocrystals then attach atomically via a necking process, resulting in 2D square superlattices.

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We present a general theoretical method for deriving effective susceptibilities for (non)linear optical scattering processes of arbitrary order using the reciprocity principle. This method allows us to formulate a generalized treatment of nonlinear optical scattering and deduce selection rules independent of the precise mechanism of light-matter interaction. We particularize this approach to second-order sum frequency scattering from an inhomogeneous medium and consider the limiting cases of small particle scattering, refractive index matched (Rayleigh-Gans-Debye) scattering, small refractive index contrast (Wentzel-Kramers-Brillouin) scattering and correlated scattering. We compare the derived expressions to experimental results of sum frequency scattering from monodisperse particles in suspension with varying sizes.

Field-induced structures in a ferrofluid with well-defined magnetite nanoparticles with a permanent magnetic dipole moment are analyzed on a single-particle level by in situ cryogenic transmission electron microscopy (2D). The field-induced columnar phase locally exhibits hexagonal symmetry and confirms the structures observed in simulations for ferromagnetic dipolar fluids in 2D. The columns are distorted by lens-shaped voids, due to the weak interchain attraction relative to field-directed dipole-dipole attraction. Both dipolar coupling and the dipole concentration determine the dimensions and the spatial arrangement of the columns. Their regular spacing manifests long-range end-pole repulsions that eventually dominate the fluctuation-induced attractions between dipole chains that initiate the columnar transition. DOI: 10.1103/PhysRevLett.97.185702 PACS numbers: 64.70.Nd, 75.50.Mm, 82.70.Dd Nanoparticles dispersed in a solvent and with a sufficiently large permanent magnetic dipole moment selfassemble into a variety of magnetic equilibrium structures such as (flux-closure) rings and wormlike, branched dipole chains [1,2]. The morphology of these clusters, formed in absence of an external field, has been examined in detail, together with a determination of pair correlation functions and chain-length distributions [3]. In contrast, much less is known about the structural transitions induced by an external (homogeneous) magnetic field for fluids of permanent dipoles. Interestingly, magnetic colloids in an external field are nevertheless frequently encountered in practical applications [4] and biomedicine [5,6].The structure formation and phase behavior of colloidal systems in reduced dimensions is not necessarily equivalent to that of three-dimensional (3D) systems [7][8][9]. In particular, for permanent dipolar spheres confined to two dimensions (2D) a field-induced transition to a columnar phase with local hexagonal symmetry was predicted [10], although a conclusive experimental real-space analysis is still lacking. Elongated iron-particle clusters have been imaged [1] but the irregular particle shape and the bidisperse size distribution obstruct the wanted single-particle analysis. Parallel structures have also been observed for maghemite ferrofluids dried in the presence of a homogeneous field [11,12]. However, we have shown elsewhere that drying procedures may drastically change structure morphology [2]. Moreover, dipole interactions in conventional ferrofluids are in general too weak for a realistic comparison to the purely dipolar spheres from simulations.In this Letter, we report unequivocal real-space evidence for the predicted columnar phase transition [10] from in situ cryo-TEM images of monodisperse magnetic colloids with dominating dipolar interactions. The particle positions are confined by a 2D film whereas the dipole orientations can thermally fluctuate in 3D. Our imaging results, in addition, allow to quantify positional and angular interparticle correlations showing, among other things, a pr...

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A novel application of vibrational sum frequency generation (VSFG) is developed to study the molecular properties of the surface of submicron particles in suspension. The Rayleigh-Gans-Debye scattering theory is extended to extract the local molecular response from the macroscopic nonlinearly scattered spectral intensity. These results demonstrate the use of VSFG to investigate quantitatively the surface molecular properties of submicron particles, dispersed in solution. It provides information on the order and density of alkane chains and allows us to determine the elements of the local second-order surface susceptibility.

MOFs scattering away: The mechanism behind the multistep synthesis of two metal–organic frameworks sharing the same metal and organic precursors was revealed by in situ time‐resolved small‐ and wide‐angle X‐ray scattering. Key factors governing the crystal assembly could be established (see picture: C gray, H white, N blue, O red, Al yellow, Cl green), including solvent, temperature, and precursor concentration.