Four dimensional-scanning transmission electron microscopy study on relationship between crystallographic orientation and spontaneous polarization in epitaxial BiFeO3.

Spontaneous polarization and crystallographic orientations within ferroelectric domains are investigated using an epitaxially grown BiFeO3 thin film under bi-axial tensile strain. Four dimensional-scanning transmission electron microscopy (4D-STEM) and atomic resolution STEM techniques revealed that the tensile strain applied is not enough to cause breakdown of equilibrium BiFeO3 symmetry (rhombohedral with space group: R3c). 4D-STEM data exhibit two types of BiFeO3 ferroelectric domains: one with projected polarization vector possessing out-of-plane component only, and the other with that consisting of both in-plane and out-of-plane components. For domains with only out-of-plane polarization, convergent beam electron diffraction (CBED) patterns exhibit "extra" Bragg's reflections (compared to CBED of cubic-perovskite) that indicate rhombohedral symmetry. In addition, beam damage effects on ferroelectric property measurements were investigated by systematically changing electron energy from 60 to 300 keV.

Large bi-axial tensile strain effect in epitaxial BiFeO3 film grown on single crystal PrScO3.

A BiFeO3 film is grown epitaxially on a PrScO3 single crystal substrate which imparts ~ 1.45% of biaxial tensile strain to BiFeO3 resulting from lattice misfit. The biaxial tensile strain effect on BiFeO3 is investigated in terms of crystal structure, Poisson ratio, and ferroelectric domain structure. Lattice resolution scanning transmission electron microscopy, precession electron diffraction, and X-ray diffraction results clearly show that in-plane interplanar distance of BiFeO3 is the same as that of PrScO3 with no sign of misfit dislocations, indicating that the biaxial tensile strain caused by lattice mismatch between BiFeO3 and PrScO3 are stored as elastic energy within BiFeO3 film. Nano-beam electron diffraction patterns compared with structure factor calculation found that the BiFeO3 maintains rhombohedral symmetry, i.e., space group of R3c. The pattern analysis also revealed two crystallographically distinguishable domains. Their relations with ferroelectric domain structures in terms of size and spontaneous polarization orientations within the domains are further understood using four-dimensional scanning transmission electron microscopy technique.

Short range biaxial strain relief mechanism within epitaxially grown BiFeO3.

Lattice mismatch-induced biaxial strain effect on the crystal structure and growth mechanism is investigated for the BiFeO3 films grown on La0.6Sr0.4MnO3/SrTiO3 and YAlO3 substrates. Nano-beam electron diffraction, structure factor calculation and x-ray reciprocal space mapping unambiguously confirm that the crystal structure within both of the BiFeO3 thin films is rhombohedral by showing the rhombohedral signature Bragg's reflections. Further investigation with atomic resolution scanning transmission electron microscopy reveals that while the ~1.0% of the lattice mismatch found in the BiFeO3 grown on La0.6Sr0.4MnO3/SrTiO3 is exerted as biaxial in-plane compressive strain with atomistically coherent interface, the ~6.8% of the lattice mismatch found in the BiFeO3 grown on YAlO3 is relaxed at the interface by introducing dislocations. The present result demonstrates the importance of: (1) identification of the epitaxial relationship between BFO and its substrate material to quantitatively evaluate the amount of the lattice strain within BFO film and (2) the atomistically coherent BFO/substrate interface for the lattice mismatch to exert the lattice strain.

Understanding the Surface Properties of Halide Exchanged Cesium Lead Halide Nanoparticles.

This report describes a characterization study of the surfaces of CsPbBr3 and CsPbBr3- xI x perovskite nanoparticles (NPs) obtained via a simultaneous purification and halide exchange (HE) postsynthetic processing technique. We studied composition-dependent NP-ligand interactions via diffusion ordered NMR (DOSY) and quantified resulting photoluminescence quantum yield (QY) as a function of halide exchange as well as ligand exchange. Importantly, ligand binding strength and QY were found to decrease when successive purification and/or halide/ligand exchange steps were taken without careful concurrent additions of acid and base ligands. This suggests that ligands added during postsynthetic processing steps are localized at the surface of the NP, passivating open surface sites. Further, we show that CsPbBr3- xI x with increasing CsPbI3 character, obtained via the same method, have decreasing ligand density, from 6.4 to 1.4 to 0.2 nm-2, indicating the composition-dependence of surface ligand binding, which also has consequences on the QY of the resulting mixed-halide NPs. These results shed further light on the importance of ion-ligand moiety additions during purification and halide exchange of highly emissive CsPbBr3 NPs to maintain their as-synthesized properties, as well as the intrinsic differences in surfaces binding and photostability between near-unity QY CsPbBr3 and mixed-halide CsPbBr3- xI x NPs.

Exceptional Lithium Storage in a Co(OH)2 Anode: Hydride Formation.

Current lithium ion battery technology is tied in with conventional reaction mechanisms such as insertion, conversion, and alloying reactions even though most future applications like EVs demand much higher energy densities than current ones. Exploring the exceptional reaction mechanism and related electrode materials can be critical for pushing current battery technology to a next level. Here, we introduce an exceptional reaction with a Co(OH)2 material which exhibits an initial charge capacity of 1112 mAh g-1, about twice its theoretical value based on known conventional conversion reaction, and retains its first cycle capacity after 30 cycles. The combined results of synchrotron X-ray diffraction and X-ray absorption spectroscopy indicate that nanosized Co metal particles and LiOH are generated by conversion reaction at high voltages, and Co xH y, Li2O, and LiH are subsequently formed by hydride reaction between Co metal, LiOH, and other lithium species at low voltages, resulting in a anomalously high capacity beyond the theoretical capacity of Co(OH)2. This is further corroborated by AIMD simulations, localized STEM, and XPS. These findings will provide not only further understanding of exceptional lithium storage of recent nanostructured materials but also valuable guidance to develop advanced electrode materials with high energy density for next-generation batteries.

Tensile stress effect on epitaxial BiFeO3 thin film grown on KTaO3.

Comprehensive crystal structural study is performed for BiFeO3 (BFO) film grown on KTaO3 (KTO) substrate using transmission electron microscopy (TEM) and x-ray diffraction (XRD). Nano-beam electron diffraction (NBED) combined with structure factor calculation and high resolution TEM images clearly reveal that the crystal structure within BFO thin film is rhombohedral BFO, i.e., bulk BFO phase. Epitaxial relationship found by NBED indicates the BFO film grows in a manner that minimizes lattice mismatch with KTO. It further suggests BFO film is under slight biaxial tensile stress (~0.35%) along in-plane direction. XRD reveals BFO lattice is under compressive stress (~1.6%), along out-of-plane direction as a result of the biaxial tensile strain applied along in-plane direction. This leads to Poisson's ratio of ~0.68. In addition, we demonstrate (1) why hexagonal notation rather than pseudocubic one is required for accurate BFO phase evaluation and (2) a new XRD method that shows how rhombohedral BFO can readily be identified among other phases by measuring a rhombohedral specific Bragg's reflection.

Elucidation of crystal and electronic structures within highly strained BiFeO3 by transmission electron microscopy and first-principles simulation.

Crystal and electronic structures of ~380 nm BiFeO3 film grown on LaAlO3 substrate are comprehensively studied using advanced transmission electron microscopy (TEM) technique combined with first-principles theory. Cross-sectional TEM images reveal the BiFeO3 film consists of two zones with different crystal structures. While zone II turns out to have rhombohedral BiFeO3, the crystal structure of zone I matches none of BiFeO3 phases reported experimentally or predicted theoretically. Detailed electron diffraction analysis combined with first-principles calculation allows us to determine that zone I displays an orthorhombic-like monoclinic structure with space group of Cm (=8). The growth mechanism and electronic structure in zone I are further discussed in comparison with those of zone II. This study is the first to provide an experimentally validated complete crystallographic detail of a highly strained BiFeO3 that includes the lattice parameter as well as the basis atom locations in the unit cell.

Heterostructured Au/Pd-M (M = Au, Pd, Pt) nanoparticles with compartmentalized composition, morphology, and electrocatalytic activity.

The synthesis, processing, and galvanic exchange of three heterostructured nanoparticle systems is described. The surface accessibility and redox potential of a Au/Pd-Ag dumbbell nanoparticle, where a Au/Pd core/shell region, and a silver region make up the domains, was used to prepare the new nanostructures with controlled composition, morphology, and microstructure. Results indicate that the silver domain was particularly susceptible to galvanic displacement, and was exchanged to Au/Pd-M (M = Au, Pd, Pt). Interestingly, the dumbbell morphology remained after exchange, and the silver region was transformed to hollow, parachute, or concentric domains respectively. The morphology and microstructure change was visualized via TEM and HRTEM, and the composition changes were probed via STEM-EDS imaging and XPS. The electrocatalytic activity of the Au/Pd-M towards methanol oxidation was studied, with results indicating that the Au/Pd-Pt nanoparticles had high activity attributed to the porous nature of the platinum domains.

Polymer-graphene nanocomposites as ultrafast-charge and -discharge cathodes for rechargeable lithium batteries.

Electroactive polymers are a new generation of "green" cathode materials for rechargeable lithium batteries. We have developed nanocomposites combining graphene with two promising polymer cathode materials, poly(anthraquinonyl sulfide) and polyimide, to improve their high-rate performance. The polymer-graphene nanocomposites were synthesized through a simple in situ polymerization in the presence of graphene sheets. The highly dispersed graphene sheets in the nanocomposite drastically enhanced the electronic conductivity and allowed the electrochemical activity of the polymer cathode to be efficiently utilized. This allows for ultrafast charging and discharging; the composite can deliver more than 100 mAh/g within just a few seconds.

Radial elasticity of multi-walled boron nitride nanotubes.

We investigated the radial mechanical properties of multi-walled boron nitride nanotubes (MW-BNNTs) using atomic force microscopy. The employed MW-BNNTs were synthesized using pressurized vapor/condenser (PVC) methods and were dispersed in aqueous solution using ultrasonication methods with the aid of ionic surfactants. Our nanomechanical measurements reveal the elastic deformational behaviors of individual BNNTs with two to four tube walls in their transverse directions. Their effective radial elastic moduli were obtained through interpreting their measured radial deformation profiles using Hertzian contact mechanics models. Our results capture the dependences of the effective radial moduli of MW-BNNTs on both the tube outer diameter and the number of tube layers. The effective radial moduli of double-walled BNNTs are found to be several-fold higher than those of single-walled BNNTs within the same diameter range. Our work contributes directly to a complete understanding of the fundamental structural and mechanical properties of BNNTs and the pursuits of their novel structural and electronics applications.

Radial mechanical properties of single-walled boron nitride nanotubes.

The radial mechanical properties of single-walled boron nitride nanotubes (SW-BNNTs) are investigated by atomic force microscopy. Nanomechanical measurements reveal the radial deformation of individual SW-BNNTs in both elastic and plastic regimes. The measured effective radial elastic moduli of SW-BNNTs are found to follow a decreasing trend with an increase in tube diameter, ranging from 40.78 to 1.85 GPa for tube diameters of 0.58 to 2.38 nm. The results show that SW-BNNTs have relatively lower effective radial elastic moduli than single-walled carbon nanotubes (SWCNTs). The axially strong, but radially supple characteristics suggest that SW-BNNTs may be superior to SWCNTs as reinforcing additives for nanocomposite applications.

Reversal of Hall-Petch effect in structural stability of PbTe nanocrystals and associated variation of phase transformation.

Using an in situ synchrotron X-ray diffraction technique, a pressure-induced phase transformation of PbTe nanocrystals with sizes of 13 and 5 nm up to ∼20 GPa was studied. Upon an increase of pressure, we observed that the 13 nm PbTe nanocrystals start a phase transformation from rocksalt structure to an intermediate orthorhombic structure and finally CsCl-type structure at 8 GPa, which is 2 GPa higher than that in bulk PbTe. In contrast, the 5 nm PbTe nanocrystals do not display the same type of transition with a further increased transition pressure as expected. Instead of orthorhombic or CsCl-type structure, the 5 nm PbTe nanocrystals turn to amorphous phase under a similar pressure (8 GPa). Upon a release of pressure, the 13 nm PbTe nanocrystals transform from high pressure CsCl-type structure directly to rocksalt structure, whereas the 5 nm PbTe nanocrystals remain their amorphous phase to ambient conditions. The structure stability of rocksalt-type PbTe shows a significant reversal of Hall-Petch effect. On the basis of such an observation with a critical size determination of ∼9 nm, PbTe nanocrystals appear as the first class of material that demonstrates a pressure-induced structural change from order to disorder. By sharing the insight of this reversed Hall-Petch effect with associated transition types, we tuned our experimental protocol and successfully synthesized a sample with "high-pressure metastable structure", amorphous phase at ambient pressure. This integrative study provides a feasible pathway to understand nucleation mechanism as a function of particle size and to explore novel materials with high-pressure metastable structure and unique properties under lab-accessible conditions.

Core-shell-structured magnetic ternary nanocubes.

We report a novel core-shell-structured ternary nanocube of MnZn ferrite synthesized by controlling the reaction temperature and composition in the absence of conventionally used reducing agents. The highly monodispersed core-shell structure consists of an Fe(3)O(4) core and an MnZn Ferrite shell. The observation of a Moiré pattern indicates that the core and the shell are two highly crystalline materials with slightly different lattice constants that are rotated relative to each other by a small angle. The ternary core-shell nanocubes display magnetic properties regulated by a combination of the core-shell composition and exhibit an increased coercivity and field-cooled/zero-field-cooled characteristics drastically different from those of regular MnZn ferrite nanoparticles. The ability to engineer the spatial nanostructures of ternary magnetic nanoparticles in terms of shape and composition offers atomic-level versatility in fine-tuning the nanoscale magnetic properties.

Chromium-assisted synthesis of platinum nanocube electrocatalysts.

This report demonstrates a novel strategy of chromium-assisted synthesis of platinum nanocubes as electrocatalysts for oxygen reduction reaction with enhanced specific activity.

LiMnPO4 nanoplate grown via solid-state reaction in molten hydrocarbon for Li-ion battery cathode.

Electrochemically active LiMnPO(4) nanoplates have been synthesized via a novel, single-step, solid-state reaction in molten hydrocarbon. The olivine-structured LiMnPO(4) nanoplates with a thickness of approximately 50 nm appear porous and were formed as nanocrystals were assembled and grew into nanorods along the [010] direction in the (100) plane. After carbon coating, the prepared LiMnPO(4) cathode demonstrated a flat potential at 4.1 V versus Li with a specific capacity reaching as high as 168 mAh/g under a galvanostatic charging/discharging mode, along with an excellent cyclability.

Structural relaxation of amorphous silicon carbide.

We have examined amorphous structures of silicon carbide (SiC) using both transmission electron microscopy and a molecular-dynamics approach. Radial distribution functions revealed that amorphous SiC contains not only heteronuclear (Si-C) bonds but also homonuclear (Si-Si and C-C) bonds. The ratio of heteronuclear to homonuclear bonds was found to change upon annealing, suggesting that structural relaxation of the amorphous SiC occurred. Good agreement was obtained between the simulated and experimentally measured radial distribution functions.