Weidong Luo

Oscillatory Mass Transport in Vapor-Liquid-Solid Growth of Sapphire Nanowires

Growing Nanowires In vapor-liquid-solid (VLS) growth of nanowires, the liquid phase acts as a transporter to bring material from the gas phase to the growing solid. By heating a single crystal of sapphire in a high-resolution transmission microscope, Oh et al. (p. 489) monitored the growth of sapphire (α-Al2O3) nanowires out of an aluminum droplet. The liquid aluminum brings oxygen to the growing wire surface, in alternating growth and dissolution reactions at the edge of the wire. The oscillation created an optimum face at the self-catalytic site for atomic stacking and regenerated the junction between the VLS phases, allowing growth of the nanowire. High-resolution transmission electron microscopy is used to identify oscillatory growth of a sapphire nanowire. In vapor-liquid-solid (VLS) growth, the liquid phase plays a pivotal role in mediating mass transport from the vapor source to the growth front of a nanowire. Such transport often takes place through the liquid phase. However, we observed by in situ transmission electron microscopy a different behavior for self-catalytic VLS growth of sapphire nanowires. The growth occurs in a layer-by-layer fashion and is accomplished by interfacial diffusion of oxygen through the ordered liquid aluminum atoms. Oscillatory growth and dissolution reactions at the top rim of the nanowires occur and supply the oxygen required to grow a new (0006) sapphire layer. A periodic modulation of the VLS triple-junction configuration accompanies these oscillatory reactions.

Interface control of bulk ferroelectric polarization

The control of material interfaces at the atomic level has led to novel interfacial properties and functionalities. In particular, the study of polar discontinuities at interfaces between complex oxides lies at the frontier of modern condensed matter research. Here we employ a combination of experimental measurements and theoretical calculations to demonstrate the control of a bulk property, namely ferroelectric polarization, of a heteroepitaxial bilayer by precise atomic-scale interface engineering. More specifically, the control is achieved by exploiting the interfacial valence mismatch to influence the electrostatic potential step across the interface, which manifests itself as the biased-voltage in ferroelectric hysteresis loops and determines the ferroelectric state. A broad study of diverse systems comprising different ferroelectrics and conducting perovskite underlayers extends the generality of this phenomenon.

Atomic-Scale Compensation Phenomena at Polar Interfaces

The interfacial screening charge that arises to compensate electric fields of dielectric or ferroelectric thin films is now recognized as the most important factor in determining the capacitance or polarization of ultrathin ferroelectrics. Here we investigate using aberration-corrected electron microscopy and density-functional theory to show how interfaces cope with the need to terminate ferroelectric polarization. In one case, we show evidence for ionic screening, which has been predicted by theory but never observed. For a ferroelectric film on an insulating substrate, we found that compensation can be mediated by an interfacial charge generated, for example, by oxygen vacancies.

Aberration-corrected scanning transmission electron microscopy: from atomic imaging and analysis to solving energy problems.

The new possibilities of aberration-corrected scanning transmission electron microscopy (STEM) extend far beyond the factor of 2 or more in lateral resolution that was the original motivation. The smaller probe also gives enhanced single atom sensitivity, both for imaging and for spectroscopy, enabling light elements to be detected in a Z-contrast image and giving much improved phase contrast imaging using the bright field detector with pixel-by-pixel correlation with the Z-contrast image. Furthermore, the increased probe-forming aperture brings significant depth sensitivity and the possibility of optical sectioning to extract information in three dimensions. This paper reviews these recent advances with reference to several applications of relevance to energy, the origin of the low-temperature catalytic activity of nanophase Au, the nucleation and growth of semiconducting nanowires, and the origin of the eight orders of magnitude increased ionic conductivity in oxide superlattices. Possible future directions of aberration-corrected STEM for solving energy problems are outlined.

Atomic-Resolution Imaging of Spin-State Superlattices in Nanopockets within Cobaltite Thin Films

Certain cobalt oxides are known to exhibit ordered Co spin states, as determined from macroscopic techniques. Here we report real-space atomic-resolution imaging of Co spin-state ordering in nanopockets of La(0.5)Sr(0.5)CoO(3-δ) thin films. Unlike the bulk material, where no Co spin-state ordering is found, thin films present a strain-induced domain structure due to oxygen vacancy ordering, inside of which some nanometer sized domains show high-spin Co ions in the planes containing O vacancies and low-spin Co ions in the stoichiometric planes. First-principles calculations provide support for this interpretation.

Prominent electrochromism through vacancy-order melting in a complex oxide

Electrochromes are materials that have the ability to reversibly change from one colour state to another with the application of an electric field. Electrochromic colouration efficiency is typically large in organic materials that are not very stable chemically. Here we show that inorganic Bi(0.9)Ca(0.1)FeO(3-0.05) thin films exhibit a prominent electrochromic effect arising from an intrinsic mechanism due to the melting of oxygen-vacancy ordering and the associated redistribution of carriers. We use a combination of optical characterization techniques in conjunction with high-resolution transmission electron microscopy and first-principles theory. The absorption change and colouration efficiency at the band edge (blue-cyan region) are 4.8×10(6) m(-1) and 190 cm(2) C(-1), respectively, which are the highest reported values for inorganic electrochromes, even exceeding values of some organic materials.

Massive Dirac surface states in topological insulator/magnetic insulator heterostructures

Topological insulators are new states of matter with a bulk gap and robust gapless surface states protected by time-reversal symmetry. When time-reversal symmetry is broken, the surface states are gapped, which induces a topological response of the system to electromagnetic field--the topological magnetoelectric effect. In this paper we study the behavior of topological surface states in heterostructures formed by a topological insulator and a magnetic insulator. Several magnetic insulators with compatible magnetic structure and relatively good lattice matching with topological insulators

Optical transmission enhacement through chemically tuned two-dimensional bismuth chalcogenide nanoplates

{\rm Bi_2Se_3}, {\rm Bi_2Se_3}, {\rm Sb_2Te_3}

Two-dimensional chalcogenide nanoplates as tunable metamaterials via chemical intercalation.

are identified, and the best candidate material is found to be MnSe, an anti-ferromagnetic insulator. We perform first-principles calculation in

Thiolated Gold Nanowires: Metallic versus Semiconducting

{\rm Bi_2Se_3/MnSe}