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Nanotechnology News from the Characterization Facilty and the Nanofabrication Center.

Characterization Facility News

Oriented Aggregation: Formation and Transformation of Mesocrystal Intermediates Revealed

Oriented aggregation is a special case of aggregation in which nanocrystals self-assemble and form new secondary single crystals.  This process has been suggested to proceed via an intermediate state known as the mesocrystal, in which the nanocrystals have parallel crystallographic alignment but are spatially separated.  We present the first direct observations of mesocrystals with size and shape similar to product oriented aggregates by employing cryo-TEM to directly image the particles in aqueous suspension.  The cryo-TEM images reveal that mesocrystals not only form but also transform to the final single crystal product while in the dispersed state.  Further, high-resolution cryo-TEM images demonstrate that the mesocrystals are composed of spatially separated and crystallographically aligned nanocrystals.

Nanoscale Reactor Engineering: Hydrothermal Synthesis of Uniform Zeolite Particles in Massively Parallel Reaction Chambers

This communication highlights the importance of controlling surface interactions and processing conditions when zeolites are synthesized hydrothermally within the confinement of a nanoporous reactor (here, three-dimensionally ordered macroporous carbon or 3DOM C).  Nanocasting processes are becoming very important for preparations of nanostructures (including zeolites) with controlled architecture.  Typically products are assumed to take on the shape of the mold used for nanocasting.  We show in our paper that the template is not so inert and that product morphologies can depend on gel composition and several other parameters (especially surface charges) that can be engineered into a hydrothermal reaction system under confinement.  We observed several interesting product morphologies for zeolite that was hydrothermally grown within macropores of inverse opal carbon, including solid spheres with corrugated surfaces, with smooth surfaces, hollow geode-like structures and needles.  These can be grown in high yield (i.e., all macropores filled) depending on specific conditions.  Concentration gradients across a monolithic porous reactor also produced morphology gradients (a kind of combinatorial synthesis).  Finally, recovery of either colloidal crystals of the zeolite spheres (materials with hierarchical porosity) or discrete, monodisperse zeolite particles was possible by calcination of the carbon reactor.  This nanoreactor engineering approach promises to bring improved control over the morphology of materials prepared by confined syntheses, and the design principles should also be applicable to hydrothermal syntheses of other materials.

Figure: SEM images illustrating the different morphologies of silicalite products obtained after hydrothermal syntheses in 3DOM nanoreactors for varying processing conditions.  The effect of repeated infiltration/hydrothermal reaction (IHT) cycles is shown in the sequence (a, b, c) for 1, 2 and 4 cycles, respectively.  The images show core cross-sections of 3DOM C monoliths with an outermost anionic polyelectrolyte layer for reactions with a high silica concentration.  To illustrate the difference in product morphology between core and edge regions of the 3DOM C monolith, corresponding edge cross-sections are shown for 1 (d) and 2 (e) IHT cycles.  The effect of the charge of the outermost polyelectrolyte layer on particle growth is shown in the sequence (c, f).  Dense spheres were produced in 3DOM C reactors with an anionic outermost polyelectrolyte (c).  Hollow geode structures of silicalite particles formed after multiple IHTs in the confinement of 3DOM C with a cationic outermost polyelectrolyte (f).  The effect of reducing the silica precursor concentration ("low concentration") is shown in the SEM images (g) after 5 IHT cycles, (h) after 7 cycles and (i) after 12 cycles for silicalite prepared in 3DOM C with an anionic outermost polyelectrolyte layer.  Scale bars: 500 nm.

Acknowledgements:
Funding was provided by the NSF (mainly by CMMI-0707610 and in parts by DMR-0704312, DMR-0212302 and CBET-0522518) and the Petroleum Research Foundation, administered by the American Chemical Society (ACS-PRF #42751-AC10).  Parts of this work were carried out in the College of Science and Engineering Characterization Facility, University of Minnesota, which receives partial support from NSF through the NNIN program.

Observation of Unusual Homo-Epitaxy in Pentacene Films and Correlated Surface Potential Domains

Figure: Height image shows a fully coalesced pentacene first monolayer and few cross-shape second layer islands.  Transverse Shear Microscopy (TSM) image reveals different signals for each of the second layer islands, indicating their different crystallographic orientations with respect to the cantilever scan vector.  The TSM signal of the cross-shape second layer crystallites establishes that the long arm is aligned along the [010] axis and the perpendicular short arm corresponds to the [100] axis..

The microstructure of ultrathin organic semiconductor films on gate dielectrics plays a pivotal role in the electrical transport performance of these films in organic field effect transistors (OFETs).  The structure of thin films of pentacene is of particular interest, as pentacene is a benchmark semiconductor for OFETs.  Using a variety of scanning probe microscopy techniques, including a unique method for Grain Orientation Mapping, we have established that vapor-deposited pentacene layers on SiO₂ substrates can exhibit homo-epitaxy, i.e., epitaxial growth of pentacene layers on pentacene.  Pentacene films grow with coincidence-II epitaxy, except for the first monolayer on SiO₂, resulting in a 4 x 4 supercell structure.  This mode of epitaxy is also referred to as geometrical coincidence.  The epitaxial growth is not ubiquitous; for example, some crystalline domains in the second pentacene monolayer have both epitaxial and non-epitaxial interactions with the underlying pentacene monolayer.  Regions of non-epitaxial growth exhibit higher friction in scanning probe measurements and also high dislocation densities, as assessed by chemical etching experiments.  Furthermore, we demonstrate using Kelvin probe microscopy that the non-epitaxial domains have a lower (more negative) surface potential, which provides a concrete connection between microstructure and electrical properties.  These experiments point to an as yet unproven but intriguing possibility, namely that homo-epitaxy, or its absence in certain domains, can give rise to electrostatic disorder in pentacene films, which in turn directly impacts hole transport at the pentacene/SiO₂ interface.

A Structural Resolution Cryo-TEM Study of the Early Stages of MFI Growth

Figure: A high-resolution cryo-TEM image of a representative crystal in synthesis sol aged for 220 days at room-temperature.  Fast Fourier transform (FFT), shown in the inset, indicates the crystal to be oriented either along [100]- or [010]-axis.  The bright and dark dots within the high-lighted perimeter represent the straight or sinusoidal pores of the MFI framework structure.  Image obtained using the FEI Tecnai G2 F30 Cryo-TEM.

In this communication, the authors present a cryogenic transmission electron microscopy (cryo-TEM) study on precursor sols prior to and during the early stages of MFI formation.  Cryo-TEM images with structural resolution were obtained and yielded new insights in MFI growth.  The importance of this study is two-fold.  First, the authors provide evidence supporting the recently proposed mechanism of evolution of nanoparticles followed by aggregative crystal growth while adding a new element.  The new element is the formation of predominantly amorphous aggregates before MFI crystallization and points to the importance of intra-aggregate rearrangements in nucleation and growth.  Second, the authors demonstrate that electron-beam sensitive materials such as zeolites can be imaged by cryo-TEM with structural resolution in their parent sols.  Similar studies for other zeolites under different conditions may reveal useful structural information for the understanding of hydrothermal nucleation and growth.

Abstract and article at Journal of the American Chemical Society