Binghui Ge

Scalable shear-exfoliation of high-quality phosphorene nanoflakes with reliable electrochemical cycleability in nano batteries

© 2016 IOP Publishing Ltd. Atomically thin black phosphorus (called phosphorene) holds great promise as an alternative to graphene and other two-dimensional transition-metal dichalcogenides as an anode material for lithium-ion batteries (LIBs). However, bulk black phosphorus (BP) suffers from rapid capacity fading and poor rechargeable performance. This work reports for the first time the use of in situ transmission electron microscopy (TEM) to construct nanoscale phosphorene LIBs. This enables direct visualization of the mechanisms underlying capacity fading in thick multilayer phosphorene through real-time capture of delithiation-induced structural decomposition, which serves to reduce electrical conductivity thus causing irreversibility of the lithiated phases. We further demonstrate that few layer-thick phosphorene successfully circumvents the structural decomposition and holds superior structural restorability, even when subject to multi-cycle lithiation/delithiation processes and concomitant huge volume expansion. This finding provides break through insights into thickness dependent lithium diffusion kinetics in phosphorene. More importantly, a scalable liquid-phase shear exfoliation route has been developed to produce high-quality ultrathin phosphorene using simple means such as a high-speed shear mixer or even a household kitchen blender with the shear rate threshold of ∼ 1.25 × 104 s-1. The results reported here will pave the way for industrial-scale applications of rechargeable phosphorene LIBs.

Determining polarity and dislocation core structures at atomic level for epitaxial AlN/(0001)6H-SiC from a single image in HRTEM

The polarity of epitaxial AlN film grown on (0001)6H-SiC and dislocation core structures in the film have been studied using a 200 kV LaB6 high-resolution transmission electron microscope of point resolution about 0.2 nm. A posterior image processing technique, the image deconvolution, was utilized to transform a single [21¯1¯0] image that does not intuitively represent the structure into the projected structure map. The adjacent Al and N projected atomic columns with the interatomic distance 0.109 nm can be distinguished from each other by analyzing the image contrast change with the sample thickness based on the pseudo-weak phase object approximation. This makes possible to derive the polarity and core structures of partial dislocations in the epitaxial AlN film at atomic level from a single image without relying on any other additional structure information. The atomic configurations for two partial dislocations containing a 10-atom ring and a 12-atom ring, respectively, have been attained. The method is available for II-VI and other III-V compounds. Its principle and procedure are briefly introduced.

Observation of metallic indium clusters in thick InGaN layer grown by metal organic chemical vapor deposition

Pure metallic indium clusters of 10–50nm are identified in In0.37Ga0.63N film grown by metal organic chemical vapor deposition based on analysis of x-ray diffraction, transmission electron microscopy, selected area diffraction, and high resolution transmission electron microscopy (HRTEM). The in-plane orientation relationships are InGaN[11−20]‖metallic indium [0−10], InGaN [1−100]‖metallic indium [−101], and InGaN [0001]‖metallic indium [101] along the growth direction. The rocking curve of indium (101) diffraction shows a large full width at half maximum of 3060arcsec, which is consistent with the small size of the indium clusters observed in HRTEM.

The significant role of Mg stoichiometry in designing high thermoelectric performance for Mg3(Sb,Bi)2-based n-type Zintls

Complex structures with versatile chemistry provide considerable chemical tunability of the transport properties. Good thermoelectric materials are generally extrinsically doped semiconductors with optimal carrier concentrations, while charged intrinsic defects (e.g., vacancies, interstitials) can also adjust the carriers, even in the compounds with no apparent deviation from a stoichiometric nominal composition. Here we report that in Zintl compounds Mg3+xSb1.5Bi0.5, the carrier concentration can be tuned from p-type to n-type by simply altering the initial Mg concentration. The spherical-aberration-corrected (CS-corrected) high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping analysis show that the excess Mg would form a separate Mg-rich phase after Mg vacancies have been essentially compensated. Additionally, a slight Te doping at Bi site on Mg3.025Sb1.5Bi0.5 has enabled good n-type thermoelectric properties, which is comparable to the Te-doped Mg-rich sample. The actual final composition of Mg3.025Sb1.5Bi0.5 analyzed by EPMA is also close to the stoichiometry Mg3Sb1.5Bi0.5, answering the open question whether excess Mg is prerequisite to realize exceptionally high n-type thermoelectric performance by different sample preparation methods. The motivation for this work is first to understand the important role of vacancy and then to guide for discovering more promising n-type Zintl thermoelectric materials.

In situ TEM visualization of superior nanomechanical flexibility of shear-exfoliated phosphorene

Recently discovered atomically thin black phosphorus (called phosphorene) holds great promise for applications in flexible nanoelectronic devices. Experimentally identifying and characterizing nanomechanical properties of phosphorene are challenging, but also potentially rewarding. This work combines for the first time in situ transmission electron microscopy (TEM) imaging and an in situ micro-manipulation system to directly visualize the nanomechanical behaviour of individual phosphorene nanoflakes. We demonstrate that the phosphorene nanoflakes can be easily bent, scrolled, and stretched, showing remarkable mechanical flexibility rather than fracturing. An out-of-plane plate-like bending mechanism and in-plane tensile strain of up to 34% were observed. Moreover, a facile liquid-phase shear exfoliation route has been developed to produce such mono-layer and few-layer phosphorene nanoflakes in organic solvents using only a household kitchen blender. The effects of surface tensions of the applied solvents on the ratio of average length and thickness (L/T) of the nanoflakes were studied systematically. The results reported here will pave the way for potential industrial-scale applications of flexible phosphorene nanoelectronic devices.

Filling fraction of Yb in CoSb3 Skutterudite studied by electron microscopy

The filling fraction limit in filled Skutterudites plays a critical role in thermoelectric properties; however, the exact filling fraction limit for certain fillers is still in debate. In this work, we observed the lattice distortion, where lattice parameters a and b are unequal, in the annealed Yb-filled CoSb3. Microstructure characterization by advanced spherical aberration-corrected (CS-corrected) electron microscopy, energy-dispersive X-ray spectroscopy, and electron energy loss spectroscopy in conjunction with thermoelectric characterization clearly proves that the Yb filling fraction is higher in the annealed sample (with lattice distortion) than in the ball-milled sample (without lattice distortion). Therefore, the results indicate that lattice distortion appears when the Yb filling fraction reaches a certain critical value in CoSb3 Skutterudites. The observed lattice distortion provides an alternative approach to probe the filling fraction in filled Skutterudites.

Mapping Valence Electron Distribution of Iron-Based Superconductors using Quantitative CBED and Precession Electron Diffraction

The discovery of superconductivity with transition temperature (Tc) up to 56 K in Fe-based superconductors (FeSCs) has provided a grand opportunity to explore high-temperature superconducting materials beyond copper containing oxides. Like in the cuprates, superconductivity in FeSCs emerges when the parent antiferromagnetic phase is suppressed, typically by introduction of dopant atoms, such as Coor Nidoped BaFe2As2. Unlike the oxygen anions sitting at the middle of the Cu-Cu bonds in the cuprates, the anions in FeSCs are positioned above and below the Fe planes. This low-symmetry setup facilitates the anion polarization. The electronic oscillation of the anions has been shown to be a critical relevant degree of freedom in a one-orbital model for FeSCs. Whether this holds in real materials and how it interacts with the other active degree of freedom in the Fe planes are urgently needed to be elucidated. We address these important questions by probing the Co concentration dependence of the valence electron density distribution in Ba(Fe1-xCox)2As2 (BFCA), a prototypical FeSC using quantitative CBED (Fig. 1(a-c)). With the combination of measured low-order structure factors (SFs) and density function theory (DFT) calculated high-order SFs, we retrieve the charge density distribution for undoped (x=0, Tc=0) and optimally doped BFCA (x=0.1, Tc=22.5K) through multipole refinement. The resulting three-dimensional (3D) and two-dimensional (2D) difference electron density map (difference between experimentally measured electron density and superimposed isolated spherical atomic electron density) are shown in Fig.1(d-h). The Co concentration dependence is characterized by a significant increase in the out-of-plane component of the electronic dipole moment around the As anions and in the out-of-plane components of the quadruple moment around the Fe cations (Fig.1(i,j)), echoing the significant increase in Tc and indicating a strong dipole-quadruple interaction between As and Fe atoms. The observations show a strong correlation among Tc, the electronic polarization of the anions, interorbital charge transfer on the Fe cations, and Fe-Fe bond polarization in BFCA. These observations provide direct support for the proposal that the large polarizability of the anions is critical to iron-based superconductivity by solvating the repulsive Coulomb interaction between electrons in the Fe plane [1].