Xuebing Zhao

Tunable Microwave Absorption Frequency by Aspect Ratio of Hollow Polydopamine@α-MnO2 Microspindles Studied by Electron Holography

A tunable response frequency is highly desirable for practical applications of microwave absorption materials but remains a great challenge. Here, hollow lightweight polydopamine@α-MnO2 microspindles were facilely synthesized with the tunable absorption frequency governed by the aspect ratio. The size of the hard template is a key factor to achieve the unique shape; the polymer layer with uniform thickness plays an important role in obtaining spindles with homogeneous size. With the aspect ratio increasing, the maximum reflection loss, as well as the absorption bandwidth (<-10 dB), increases and then decreases; meanwhile, the microwave absorption band shifts to the low frequency. The optimized aspect ratio of the cavity about the hollow polydopamine@α-MnO2 microspindles is ∼2.8. With 3 mm thickness at 9.7 GHz, the strongest reflection reaches -21.8 dB, and the width of the absorbing band (<-10 dB) is as wide as 3.3 GHz. Via electron holography, it is confirmed that strong charge accumulates around the interface between the polydopamine and α-MnO2 layers, which mainly contributes to the dielectric polarization absorption. This study proposes a reliable strategy to tune the absorption frequency via different aspect ratio polymer@α-MnO2 microspindles.

Uniform wurtzite MnSe nanocrystals with surface-dependent magnetic behavior

AbstractManganese selenide (MnSe) possesses unique magnetic properties as an important magnetic semiconductor, but the synthesis and properties of MnSe nanocrystals are less developed compared to other semiconductor nanocrystals because of the inability to obtain high-quality MnSe, especially in the metastable wurtzite structure. Here, we have successfully fabricated wurtzite MnSe nanocrystals via a colloidal approach which affords uniform crystal sizes and tailored shapes. The selective binding strength of the amine surfactant is the determining factor in shape-control and shape-evolution. Bullet-shapes could be transformed into shuttle-shapes if part of the oleylamine in the reaction solution was replaced by trioctylamine, and tetrapod-shaped nanocrystals could be formed in trioctylamine systems. The three-dimensional (3D) structure of the bullet-shaped nanorods has been demonstrated by the advanced transmission electron microscope (TEM) 3D-tomography technology. High-resolution TEM (HRTEM) and electron energy-loss spectroscopy (EELS) show that planar-defect structures such as stacking faults and twinning along the [001] direction arise during the growth of bullet-shapes. On the basis of careful HRTEM observations, we propose a “quadra-twin core” growth mechanism for the formation of wurtzite MnSe nanotetrapods. Furthermore, the wurtzite MnSe nanocrystals show lowtemperature surface spin-glass behavior due to their noncompensated surface spins and the blocking temperatures increase from 8.4 K to 18.5 K with increasing surface area/volume ratio of the nanocrystals. Our results provide a systematic study of wurtzite MnSe nanocrystals.

Insight into the atomic structure of Li2MnO3 in Li-rich Mn-based cathode materials and the impact of its atomic arrangement on electrochemical performance

Li-rich Mn-based cathode materials have been considered as promising candidates for next generation Li-ion batteries due to their high-energy density, low cost and non-toxicity. However, the atomic arrangement of such materials and the relationship between the microstructure and electrochemical performance are still not fully understood. In this paper, local heterogeneity in the crystal lattice is directly observed in synthesized Li2MnO3/LiMO2 (M = Ni and Mn) cathode materials. With SAED application, for the first time, we accordingly uncover that the lattice heterogeneity is induced by different Li2MnO3 atomic arrangements coexisting in the same crystal domain. The co-growth of Li2MnO3 with different orientations is proved to be a defective feature, which would induce atomic vacancy concentration in the lattice and increase the risk of layered structure collapse in the cycling process. The electrochemical test results also suggest that the composition with a relatively uniform Li2MnO3 arrangement exhibits better cycling performance (the capacity retention is as high as 95.1% after 50 cycles at 0.1C), oppositely, the coexistence of multiple complex Li2MnO3 arrangements results in poor cycling performance (the capacity retention is below 70% after 50 cycles at 0.1C). The crystal lattice structure comparison between primary and cycled is shown to manifest the effect of Li2MnO3 arrangement on the electrochemical performance and structural stability, providing one possible explanation for the capacity degradation of the Li-rich materials.

Crystal defect-mediated band-gap engineering: a new strategy for tuning the optical properties of Ag2Se quantum dots toward enhanced hydrogen evolution performance

Crystal defects have been introduced into inherently narrow-band-gap and non-toxic Ag2Se quantum dots (QDs) via a facile and efficient thermal vibration approach during synthesis and studied by using electron holography and geometric phase analysis techniques. These crystal defects consequently demonstrated a solid possibility for tuning the optical band-gaps of Ag2Se QDs, and thereby enhancing the visible-light-driven water splitting and hydrogen evolution performance of Ag2Se QD-sensitized TiO2 photocatalysts.

Facile preparation of 3D hierarchical coaxial-cable-like Ni-CNTs@beta-(Ni, Co) binary hydroxides for supercapacitors with ultrahigh specific capacitance

A facile chemical method for Co doping Ni-CNTs@α-Ni(OH)2 combining with an in situ phase transformation process is successfully proposed and employed to synthesize three-dimensional (3D) hierarchical Ni-CNTs@β-(Ni, Co) binary hydroxides. This strategy can effectively maintain the coaxial-cable-like structure of Ni-CNTs@α-Ni(OH)2 and meanwhile increase the content of Co as much as possible. Eventually, the specific capacitances and electrical conductivity of the composites are remarkably enhanced. The optimized composite exhibits high specific capacitances of 2861.8F g-1 at 1A g-1 (39.48F cm-2 at 15mAcm-2), good rate capabilities of 1221.8F g-1 at 20A g-1 and cycling stabilities (87.6% of capacitance retention after 5000cycles at 5A g-1). The asymmetric supercapacitor (ASC) constructed with the as-synthesized composite and activated carbon as positive and negative electrode delivers a high specific capacitance of 287.7F g-1 at 1A g-1. The device demonstrates remarkable energy density (96Whkg-1) and high power density (15829.4Wkg-1). The retention of capacitance remains 83.5% at the current density of 5A g-1 after 5000cycles. The charged and discharged samples are further studied by ex situ electron energy loss spectroscopy (EELS) analysis, XRD and SEM to figure out the reasons of capacitance fading. Overall, it is believable that this facile synthetic strategy can be applied to prepare various nanostructured metal hydroxide/CNT composites for high performance supercapacitor electrode materials.

Microstructure research for ferroelectric origin in the strained Hf0.5Zr0.5O2 thin film via geometric phase analysis

Recently, non-volatile semiconductor memory devices using a ferroelectric Hf0.5Zr0.5O2 film have been attracting extensive attention. However, at the nano-scale, the phase structure remains unclear in a thin Hf0.5Zr0.5O2 film, which stands in the way of the sustained development of ferroelectric memory nano-devices. Here, a series of electron microscopy evidences have illustrated that the interfacial strain played a key role in inducing the orthorhombic phase and the distorted tetragonal phase, which was the origin of the ferroelectricity in the Hf0.5Zr0.5O2 film. Our results provide insight into understanding the association between ferroelectric performances and microstructures of Hf0.5Zr0.5O2-based systems.

Atomic Mechanism of Interfacial-Controlled Quantum Efficiency and Charge Migration in InAs/GaSb Superlattice

A series of systematic electron microscopy imaging evidence are illustrated to prove that a high-quality interface is vital for enhancing quantum efficiency from 23 to 50% effectively, because improved crystal quality of each layer can suppress the disordered atom arrangement and enhance the carrier lifetime via decreasing the overall residual strain. The distribution width of charge rises and then falls as bias increasing, revealing the existence of an optimum operating voltage, which could be attributed to the proper energy band bending. Our results provide new insights into the understanding of the association between macro-property and microstructure of the superlattice system.

Thermal effects on current-related skyrmion formation in a nanobelt

We report an in-situ Lorentz transmission electron microscopy (LTEM) investigation to study the thermal effects on the generation of magnetic skyrmions within a nanobelt. Under an action of a moderate current pulse, magnetic skyrmions appear even in the temperature range far below the critical temperature and even at zero field. Finite element simulation reveals that the Joule heating plays an essential role in this behavior. Our results also uncover the importance of the cooling conditions in the current-related in situ LTEM research.We report an in-situ Lorentz transmission electron microscopy (LTEM) investigation to study the thermal effects on the generation of magnetic skyrmions within a nanobelt. Under an action of a moderate current pulse, magnetic skyrmions appear even in the temperature range far below the critical temperature and even at zero field. Finite element simulation reveals that the Joule heating plays an essential role in this behavior. Our results also uncover the importance of the cooling conditions in the current-related in situ LTEM research.

Insight into the split and asymmetry of charge distribution in biased M-structure superlattice

The charge distribution in real space of an insertion variant based on an InAs/GaSb superlattice for an infrared detector is illustrated by in situ electron microscopy. The localization split of positive charge can be directly observed in the InAs/GaSb/AlSb/GaSb superlattice (M-structure) rather than in the InAs/GaSb superlattice. With the applied bias increasing from 0 to 4.5 V, the double peaks of positive charge density become asymmetrical gradually, with the peak integral ratio ranging from 1.13 to 2.54. Simultaneously, the negative charges move along the direction of the negative electric field. Without inserting the AlSb layer, the charge inversion occurs in both the hole wells and the electron wells of the InAs/GaSb superlattice under high bias. Such a discrepancy between the M-structure superlattice and the traditional superlattice suggests an effective reduction of tunneling probability of the M-structure design. Our result is of great help to understand the carrier immigration mechanism of the s...