Chunlin Chen

Magnetoelectric quasi-(0-3) nanocomposite heterostructures

Magnetoelectric composite thin films hold substantial promise for applications in novel multifunctional devices. However, there are presently shortcomings for both the extensively studied bilayer epitaxial (2-2) and vertically architectured nanocomposite (1-3) film systems, restricting their applications. Here we design a novel growth strategy to fabricate an architectured nanocomposite heterostructure with magnetic quasiparticles (0) embedded in a ferroelectric film matrix (3) by alternately growing (2-2) and (1-3) layers within the film. The new heteroepitaxial films not only overcome the clamping effect from substrate, but also significantly suppress the leakage current paths through the ferromagnetic phase. We demonstrate, by focusing on switching characteristics of the piezoresponse, that the heterostructure shows magnetic field dependence of piezoelectricity due to the improved coupling enabled by good connectivity amongst the piezoelectric and magnetostrictive phases. This new architectured magnetoelectric heterostructures may open a new avenue for applications of magnetoelectric films in micro-devices.

Atomic-scale structure and properties of highly stable antiphase boundary defects in Fe3O4

The complex and intriguing properties of the ferrimagnetic half metal magnetite (Fe3O4) are of continuing fundamental interest as well as being important for practical applications in spintronics, magnetism, catalysis and medicine. There is considerable speculation concerning the role of the ubiquitous antiphase boundary (APB) defects in magnetite, however, direct information on their structure and properties has remained challenging to obtain. Here we combine predictive first principles modelling with high-resolution transmission electron microscopy to unambiguously determine the three-dimensional structure of APBs in magnetite. We demonstrate that APB defects on the {110} planes are unusually stable and induce antiferromagnetic coupling between adjacent domains providing an explanation for the magnetoresistance and reduced spin polarization often observed. We also demonstrate how the high stability of the {110} APB defects is connected to the existence of a metastable bulk phase of Fe3O4, which could be stabilized by strain in films or nanostructures.

Misfit accommodation mechanism at the heterointerface between diamond and cubic boron nitride

Diamond and cubic boron nitride (c-BN) are the top two hardest materials on the Earth. Clarifying how the two seemingly incompressible materials can actually join represents one of the most challenging issues in materials science. Here we apply the temperature gradient method to grow the c-BN single crystals on diamond and report a successful epitaxial growth. By transmission electron microscopy, we reveal a novel misfit accommodation mechanism for a {111} diamond/c-BN heterointerface, that is, lattice misfit can be accommodated by continuous stacking fault networks, which are connected by periodically arranged hexagonal dislocation loops. The loops are found to comprise six 60° Shockley partial dislocations. Atomically, the carbon in diamond bonds directly to boron in c-BN at the interface, which electronically induces a two-dimensional electron gas and a quasi-1D electrical conductivity. Our findings point to the existence of a novel misfit accommodation mechanism associated with the superhard materials.

Fluorine in Shark Teeth: Its Direct Atomic-Resolution Imaging and Strengthening Function**

Atomic-resolution imaging of beam-sensitive biominerals is extremely challenging, owing to their fairly complex structures and the damage caused by electron irradiation. Herein, we overcome these difficulties by performing aberration-corrected electron microscopy with low-dose imaging techniques, and report the successful direct atomic-resolution imaging of every individual atomic column in the complex fluorapatite structure of shark tooth enameloid, which can be of paramount importance for teeth in general. We demonstrate that every individual atomic column in shark tooth enameloid can be spatially resolved, and has a complex fluorapatite structure. Furthermore, ab initio calculations show that fluorine atoms can be covalently bound to the surrounding calcium atoms, which improves understanding of their caries-reducing effects in shark teeth.

Two-dimensional electron gas at the Ti-diffused BiFeO3/SrTiO3 interface

Oxide heterostructures with the broken translational symmetry often trigger a two-dimensional quantum confinement and associated unique electronic properties that cannot be observed in bulk constituents. Particular interest is devoted to the formation of two-dimensional electron gas (2DEG) at heterointerfaces between two insulators, which offers a fertile ground for fabricating advanced electronic devices. Here, we combine atomic force microscopy, transmission electron microscopy, and atomistic first-principles calculations to demonstrate that the (100) BiFeO3/SrTiO3 interface takes on a metallic nature and a 2DEG is generated at this interface. Our findings also reveal that the electronic reconstruction due to the polar discontinuity and the variation in valence state of Ti arising from diffusion of Ti cations in SrTiO3 to Fe sites of BiFeO3 are critical to the formation of 2DEG at the heterointerface.

Atomic-Scale Observation of Migration and Coalescence of Au Nanoclusters on YSZ Surface by Aberration-Corrected STEM

Unraveling structural dynamics of noble metal nanoclusters on oxide supports is critical to understanding reaction process and origin of catalytic activity in heterogeneous catalysts. Here, we show that aberration-corrected scanning transmission electron microscopy can provide direct atomic-resolution imaging of surface migration, coalescence, and atomic rearrangement of Au clusters on an Y:ZrO2 (YSZ) support. The high resolution enables us to reveal migration and coalescence process of Au clusters at the atomic scale, and to demonstrate that the coalesced clusters undergo a cooperative atomic rearrangement, which transforms the coherent into incoherent Au/YSZ interface. This approach can help to elucidate atomistic mechanism of catalytic activities and to develop novel catalysts with enhanced functionality.

Atomic and electronic structure of the SrNbO3/SrNbO3.4 interface

We have determined the atomic-scale structure of the SrNbO3/SrNbO3.4 interface and related it to the electronic structure. Experimentally, transmission electron microscopy observations reveal that SrNbO3 and SrNbO3.4 show orientation relationships [ 1¯10]SrNbO3 // [010]SrNbO3.4 and (110)SrNbO3 // (001)SrNbO3.4, and that their interface is coherent and atomically abrupt. Theoretically, this interface is found to be strongly anisotropic in electronic structure, and takes on quasi-one-dimensional nature. We also find that the interface impacts greatly the electron occupation of Nb d orbitals, particularly dz2 orbital. The combined study represents a relevant advance in atomically bridging structures to properties of heterointerfaces.

Effect of Icosahedral Quasicrystalline Fraction and Extrusion Ratio on Microstructure, Mechanical Properties, and Anisotropy of Mg-Zn-Gd-Based Alloys

We have fabricated three types of Mg-Zn-Gd-based alloys containing the icosahedral quasicrystalline phase (I-phase) to investigate how volume fraction of the I-phase and extrusion ratio can have an impact on the microstructure, mechanical properties, and anisotropy of the as-extruded alloys. We find that grains are refined and that the ultimate tensile strength and elongation are improved as either the volume fraction of I-phase or the extrusion ratio is increased, which can be attributed to the secondary phase particle stimulate recrystallization nucleation and restrained grain boundary motion. Moreover, anisotropy is mitigated in all of the alloys as either the I-phase fraction or the extrusion ratio is increased owing to the coeffect of texture weakening and grain refinement as well as to the effect of I-phase on twinning. We also find that with the increase in the amount of the I-phase, the yield strength (YS) is decreased for the alloys extruded at a low ratio owing to the texture weakening, yet it is increased for the alloys extruded at high ratio owing to the strengthening originating from the I-phase and refined grains. The mechanical properties are improved for the alloys extruded at high ratio, which is due to their fine grains and uniform microstructure.

Comparative study on size dependence of melting temperatures of pure metal and alloy nanoparticles

A comparative study on the size dependence of the melting temperatures of pure metal and alloy nanoparticles has been carried out. It was found that the melting temperatures of Bi-Sn, In-Sn, and Pb-Sn alloy nanoparticles decreased more rapidly with decreasing particle size than those of the constituent metal nanoparticles (Bi, In, Pb, Sn). Namely, the size dependence of the melting temperature was stronger for the alloy nanoparticles than that for the constituent metal nanoparticles. Results calculated with a thermodynamic model were in good agreement with the experimental observations.

PATTERNING OXIDE NANOPILLARS AT THE ATOMIC SCALE BY PHASE TRANSFORMATION

Phase transformations in crystalline materials are common in nature and often modify dramatically properties of materials. The ability to precisely control them with a high spatial precision represents a significant step forward in realizing new functionalities in confined dimensions. However, such control is extremely challenging particularly at the atomic scale due to the intricacies in governing thermodynamic conditions with a high spatial accuracy. Here, we apply focused electron beam of a scanning transmission electron microscope to irradiate SrNbO3.4 crystals and demonstrate a precise control of a phase transformation from layered SrNbO3.4 to perovskite SrNbO3 at the atomic scale. By purposely squeezing O atoms out of the vertex-sharing NbO6 octahedral slabs, their neighboring slabs zip together, resulting in a patterning of SrNbO3 nanopillars in SrNbO3.4 matrix. Such phase transformations can be spatially manipulated with an atomic precision, opening up a novel avenue for materials design and processing and also for advanced nanodevice fabrication.