Strong polarization enhancement in asymmetric three-component ferroelectric superlattices.

Theoretical predictions--motivated by recent advances in epitaxial engineering--indicate a wealth of complex behaviour arising in superlattices of perovskite-type metal oxides. These include the enhancement of polarization by strain and the possibility of asymmetric properties in three-component superlattices. Here we fabricate superlattices consisting of barium titanate (BaTiO3), strontium titanate (SrTiO3) and calcium titanate (CaTiO3) with atomic-scale control by high-pressure pulsed laser deposition on conducting, atomically flat strontium ruthenate (SrRuO3) layers. The strain in BaTiO3 layers is fully maintained as long as the BaTiO3 thickness does not exceed the combined thicknesses of the CaTiO3 and SrTiO3 layers. By preserving full strain and combining heterointerfacial couplings, we find an overall 50% enhancement of the superlattice global polarization with respect to similarly grown pure BaTiO3, despite the fact that half the layers in the superlattice are nominally non-ferroelectric. We further show that even superlattices containing only single-unit-cell layers of BaTiO3 in a paraelectric matrix remain ferroelectric. Our data reveal that the specific interface structure and local asymmetries play an unexpected role in the polarization enhancement.

Near-edge X-ray absorption fine structure spectroscopy as a tool for investigating nanomaterials.

We have demonstrated near-edge X-ray absorption fine structure (NEXAFS) spectroscopy as a particularly useful and effective technique for simultaneously probing the surface chemistry, surface molecular orientation, degree of order, and electronic structure of carbon nanotubes and related nanomaterials. Specifically, we employ NEXAFS in the study of single-walled carbon nanotube and multi-walled carbon nanotube powders, films, and arrays, as well as of boron nitride nanotubes. We have focused on the advantages of NEXAFS as an exciting, complementary tool to conventional microscopy and spectroscopy for providing chemical and structural information about nanoscale samples.

Molecular beam-controlled nucleation and growth of vertically aligned single-wall carbon nanotube arrays.

The main obstacle to widespread application of single-wall carbon nanotubes is the lack of reproducible synthesis methods of pure material. We describe a new growth method for single-wall carbon nanotubes that uses molecular beams of precursor gases that impinge on a heated substrate coated with a catalyst thin film. In this growth environment the gas and the substrate temperature are decoupled and carbon nanotube growth occurs by surface reactions without contribution from homogeneous gas-phase reactions. This controlled reaction environment revealed that SWCNT growth is a complex multicomponent reaction in which not just C, but also H, and O play a critical role. These experiments identified acetylene as a prolific direct building block for carbon network formation that is an order of magnitude more efficient than other small-molecule precursors. The molecular jet experiments show that with optimal catalyst particle size the incidence rate of acetylene molecules plays a critical role in the formation of single-wall carbon nanotubes and dense vertically aligned arrays in which they are the dominant component. The threshold for vertically aligned growth, the growth rate, the diameter, and the number of walls of the carbon nanotubes are systematically correlated with the acetylene incidence rate and the substrate temperature.

Synthetic nanoscale elements for delivery of materials into viable cells.

Arrays of vertically aligned carbon nanofibers (VACNFs) provide structures that are well suited for the direct integration and manipulation of molecular-scale phenomena within intact, live cells. VACNFs are fabricated via a combination of microfabrication techniques and catalytic plasma-enhanced chemical vapor deposition. In this chapter, we discuss the synthesis of VACNFs and detail the methods for introducing these arrays into the intracellular domain of mammalian cells for the purpose of delivering large macromolecules, specifically plasmid DNA, on a massively parallel basis.

Nonequilibrium interlayer transport in pulsed laser deposition.

We use time-resolved surface x-ray diffraction measurements with microsecond range resolution to study the growth kinetics of pulsed laser deposited . Time-dependent surface coverages corresponding to single laser shots were determined directly from crystal truncation rod intensity transients. Analysis of surface coverage evolution shows that extremely fast nonequilibrium interlayer transport, which occurs concurrently with the arrival of the laser plume, dominates the deposition process. A much smaller fraction of material, which is governed by the dwell time between successive laser shots, is transferred by slow, thermally driven interlayer transport processes.

Imperfect surface order and functionalization in vertical carbon nanotube arrays probed by near edge X-ray absorption fine structure spectroscopy (NEXAFS).

Probing surface order as well as the degree of structural modification in carbon nanotube systems is of fundamental importance for incorporation of these materials into practical functional devices. The current study pertains to the analysis of the surface order of vertically-aligned single-walled and multi-walled carbon nanotube arrays of varying length and composition by means of near-edge X-ray fine structure spectroscopy (NEXAFS). Both NEXAFS and scanning electron microscopy (SEM) studies concluded that the nanotubes in these samples were oriented vertically to the plane of the surface. However, NEXAFS polarization analysis provided a more quantitative and nuanced description of the surface structure, indicative of far less localized surface order, an observation partially attributed to misalignment and bending of the tubes. Moreover, it was demonstrated by NEXAFS that the surface order of the arrays was imperfect and relatively independent of the height of the nanotube arrays. In addition, we have shown that NEXAFS can be used to correlate the extent of chemical functionalization and oxygenation with disruption of the electronic and physical structure of nanotubes embedded in array motifs.

Nanowire array gratings with ZnO combs.

Diffraction gratings are mainly manufactured by mechanical ruling, interference lithography, or resin replication, which generally require expensive equipment, complicated procedures, and a stable environment. We describe the controlled growth of self-organized microscale ZnO comb gratings by a simple one-step thermal evaporation and condensation method. The ZnO combs consist of an array of very uniform, perfectly aligned, evenly spaced and long single-crystalline ZnO nanowires or nanobelts with periods in the range of 0.2 to 2 microm. Diffraction experiments show that the ZnO combs can function as a tiny three-beam divider that may find applications in miniaturized integrated optics such as three-beam optical pickup systems.

Persistent step-flow growth of strained films on vicinal substrates.

We propose a model of persistent step flow, emphasizing dominant kinetic processes and strain effects. Within this model, we construct a morphological phase diagram, delineating a regime of step flow from regimes of step bunching and island formation. In particular, we predict the existence of concurrent step bunching and island formation, a new growth mode that competes with step flow for phase space, and show that the deposition flux and temperature must be chosen within a window in order to achieve persistent step flow. The model rationalizes the diverse growth modes observed in pulsed laser deposition of SrRuO3 on SrTiO3.