Andrei V. Petukhov

Long-range orientation and atomic attachment of nanocrystals in 2D honeycomb superlattices

Nanoparticle lattices and surfaces The challenge of resolving the details of the surfaces or assemblies of colloidal semiconductor nanoparticles can be overcome if several characterization methods are used (see the Perspective by Boles and Talapin). Boneschanscher et al. examined honeycomb superlattices of lead selenide nanocrystals formed by the bonding of crystal faces using several methods, including high-resolution electron microscopy and tomography. The structure had octahedral symmetry with the nanocrystals distorted through “necking”: the expansion of the contact points between the nanocrystals. Zherebetskyy et al. used a combination of theoretical calculations and spectroscopic methods to study the surface layer of lead sulfide nanocrystals synthesized in water. In addition to the oleic acid groups that capped the nanocrystals, hydroxyl groups were present as well. Science, this issue p. 1377, p. 1380; see also p. 1340 Metal-chalcogenide nanocrystals undergo necking and large-scale atomic rearrangements when forming a surface lattice. [Also see Perspective by Boles and Talapin] 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.

Kinetic Control of Metal–Organic Framework Crystallization Investigated by Time‐Resolved In Situ X‐Ray Scattering

Metal–organic frameworks (MOFs) are among the most sophisticated nanostructured solids: they often possess high surface areas and pore volumes, with the possibility of finetuning their chemical environment by either selecting the appropriate building blocks or by postsynthetic functionalization. For many frameworks, flexibility of the lattice allows them to undergo a significant transformation in solid state.[1] All these features make MOFs a special class of solids with the potential of transcending many common limitations in different technological disciplines, such as ferromagnetism,[2] semiconductivity, gas separation,[3] storage,[4] sensing,[5] catalysis,[ 6] drug delivery,[7] or proton conductivity.[8] However, the crystallization mechanism of these complex structures is far from understood. Notwithstanding the plethora of publications that present new MOFs,[9] and the effectiveness of the high-throughput approach,[10] serendipity still governs the synthesis of new structures.

Self-assembly of Colloidal Cubes via Vertical Deposition

The vertical deposition technique for creating crystalline microstructures is applied for the first time to nonspherical colloids in the form of hollow silica cubes. Controlled deposition of the cubes results in large crystalline films with variable symmetry. The microstructures are characterized in detail with scanning electron microscopy and small-angle X-ray scattering. In single layers of cubes, distorted square to hexagonal ordered arrays are formed. For multilayered crystals, the intralayer ordering is predominantly hexagonal with a hollow site stacking, similar to that of the face centered cubic lattice for spheres. Additionally, a distorted square arrangement in the layers is also found to form under certain conditions. These crystalline films are promising for various applications such as photonic materials.

Microradian X‐ray diffraction in colloidal photonic crystals

Ultra-high-resolution small-angle X-ray scattering in various colloidal photonic crystals is reported. It is demonstrated that an angular resolution of about two microradians is readily achievable at a third-generation synchrotron source using compound refractive optics. The scheme allows fast acquisition of two-dimensional X-ray diffraction data and can be realised at sample-detector separations of only a few metres. As a result, diffraction measurements in colloidal crystals with interplanar spacings larger than a micrometre, as well as determination of the range of various order parameters from the width of the Bragg peaks, are made possible.

Microporous Niobia-Silica Membrane with Very Low CO2 Permeability

A sol-gel-derived microporous ceramic membrane with an exceptionally low permeability for CO(2) from gaseous streams was developed and characterized. The sols were prepared from a mixture of niobium and silicon alkoxide precursors by acid-catalyzed synthesis. Microporous films were formed by coating asymmetric gamma-alumina disks with the polymeric sol (Si/Nb=3:1), followed by calcination at 500 degrees C. The membrane consists of a 150-nm-thick layer with a Si/Nb atomic ratio of about 1.5. The single-gas permeance of small gas molecules such as H(2), CH(4), N(2), and SF(6) decreases steadily with kinetic diameter. Hydrogen, helium, and carbon dioxide follow an activated transport mechanism through the membrane. The permeance of CO(2) in this membrane is much lower than that in pure silica, and its behavior deviates strongly from the general trend observed with the other gases. This is attributed to a relatively strong interaction between CO(2) and adsorption sites in the niobia-silica membrane.

Phase behavior of colloidal silica rods

Recently, a novel colloidal hard-rod-like model system was developed which consists of silica rods [Kuijk et al., JACS, 2011, 133, 2346]. Here, we present a study of the phase behavior of these rods, for aspect ratios ranging from 3.7 to 8.0. By combining real-space confocal laser scanning microscopy with small angle X-ray scattering, a phase diagram depending on concentration and aspect ratio was constructed, which shows good qualitative agreement with the simulation results for the hard spherocylinder system. Besides the expected nematic and smectic liquid crystalline phases for the higher aspect ratios, we found a smectic-B phase at high densities for all systems. Additionally, real-space measurements on the single-particle level provided preliminary information on (liquid) crystal nucleation, defects and dynamics in the smectic phase.

In situ study of the formation mechanism of two-dimensional superlattices from PbSe nanocrystals

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.

Semiconductor nanorod self-assembly at the liquid/air interface studied by in situ GISAXS and ex situ TEM.

We study the self-assembly of colloidal CdSe/CdS nanorods (NRs) at the liquid/air interface combining time-resolved in situ grazing-incidence small angle X-ray scattering (GISAXS) and ex situ transmission electron microscopy (TEM). Our study shows that NR superstructure formation occurs at the liquid/air interface. Short NRs self-assemble into micrometers long tracks of NRs lying side by side flat on the surface. In contrast, longer NRs align vertically into ordered superstructures. Systematic variation of the NR length and initial concentration of the NR dispersion allowed us to tune the orientation of the NRs in the final superstructure. With GISAXS, we were able to follow the dynamics of the self-assembly. We propose a model of hierarchical self-organization that provides a basis for the understanding of the length-dependent self-organization of NRs at the liquid/air interface. This opens the way to new materials based on NR membranes and anisotropic thin films.

Helical colloidal sphere structures through thermo-reversible co-assembly with molecular microtubes.

Self-assembly is ubiquitous in nature, science, and technology and provides a general route to achieve order from disorder at various length scales.[1] Extensive effort has been exerted to molecular and colloidal self-assembly, where molecules and colloids, respectively, organize into larger-scale ordered structures. Although these two research areas have developed separately to a great extent, their combination would be very promising. Nature, for instance, utilizes hierarchical selfassembly across different length scales to construct complex, dynamic functional entities such as cells. Here we bridge the nano- and microscale by the hierarchical co-assembly between molecules and colloids, where molecular self-assembly induces the self-assembly of colloids into ordered structures.

Live encapsulation of a Keggin polyanion in NH2-MIL-101(Al) observed by in situ time resolved X-ray scattering

The templating effect of the Keggin polyanion derived from phosphotungstic acid (PTA) during the synthesis of NH(2)-MIL-101(Al) has been investigated by means of in situ SAXS/WAXS. Kinetic analysis and structural observations demonstrate that PTA acts as a nucleation site and that it stabilizes the precursor phase NH(2)-MOF-235(Al). Surprisingly kinetics of formation are little changed.