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Confined low dimensional charges with high density such as two-dimensional electron gas (2DEG) at interfaces and charged domain walls in ferroelectrics show great potential to serve as functional elements in future nanoelectronics. However, stabilization and control of low dimensional charges is challenging, as they are usually subject to enormous depolarization fields. Here, we demonstrate a method to fabricate tunable charged interfaces with ~77°, 86° and 94° head-to-head polarization configurations in multiferroic BiFeO3 thin films by grain boundary engineering. The adjacent grains are cohesively bonded and the boundary is about 1 nm in width and devoid of any amorphous region. Remarkably, the polarization remains almost unchanged near the grain boundaries, indicating the polarization charges are well compensated, i.e., there should be two-dimensional charge gas confined at grain boundaries. Adjusting the tilt angle of the grain boundaries enables tuning the angle of polarization configurations from 71° to 109°, which in turn allows the control of charge density at the grain boundaries. This general and feasible method opens new doors for the application of charged interfaces in next generation nanoelectronics.

Forty planar superconducting undulators (SCUs) of 4 m length will be used in Shanghai High Repetition rate XFEL and Extreme Light (SHINE) facility. As the longest SCU in developing of the worldwide, they could generate photon in energy range of 10–25 keV. NbTi/Cu wires with the diameter 0.6 mm and the ratio of Cu to NbTi 0.9 have been chosen as the fundamental element of superconducting coils. Coils temperature must be below 5 K when operating current reaches 400 A. Meanwhile, the magnetic gap need to be less than 5 mm to obtain the peak field of 1.58 T. Therefore, beam chamber installation is infeasible in such a small gap. Two parallel copper foils close to the surface of magnetic poles have been designed to shield beam heat load. Furthermore, four liquid Helium tubes located symmetrically around magnetic structure have been designed to cool the copper foils used as beam channel. There is no cryocooler on the SCU because liquid Helium is provided by the cryogenic plant of SHINE.

Defects exist ubiquitously in crystal materials, and usually exhibit a very different nature from the bulk matrix. Hence, their presence can have significant impacts on the properties of devices. Although it is well accepted that the properties of defects are determined by their unique atomic environments, the precise knowledge of such relationships is far from clear for most oxides because of the complexity of defects and difficulties in characterization. Here, we fabricate a 36.8° SrRuO3 grain boundary of which the transport measurements show a spin-valve magnetoresistance. We identify its atomic arrangement, including oxygen, using scanning transmission electron microscopy and spectroscopy. Based on the as-obtained atomic structure, the density functional theory calculations suggest that the spin-valve magnetoresistance occurs because of dramatically reduced magnetic moments at the boundary. The ability to manipulate magnetic properties at the nanometer scale via defect control allows new strategies to design magnetic/electronic devices with low-dimensional magnetic order., We manipulate the local magnetic properties of SrRuO3 at a grain boundary, providing new strategies to design atomic-sized magnetic/electronic devices with low-dimensional magnetic order via defect engineering.

Reversible switching of non-180° ferroelastic domains that largely alters the local strain distribution enables many electromechanical, electromagnetic and electroacoustic applications. However, in thin films, the ferroelastic domain walls are usually believed to be immobile because of the interface clamping and/or dislocation pinning. Here, using in situ and aberration-corrected transmission electron microscopy, we directly observe reversible switching of individual 90° domains in dislocation-free PbTiO3 thin films and uncover the weakened interface clamping effect. We find the tetragonality is suppressed to ∼1.017 while the polarization vectors rotate 45° in the a-domain near the interface. These huge structural distortions at the interface is mainly responsible for the weakened clamping effect and thus the ability to switch ferroelastic domains. The switching is fully reversible (i.e., either electric field or mechanical stress can re-establish the erased domain) regardless of polarization orientation of the c-domain matrix. Phase-field modeling also shows excellent agreement with experimental observations. Our study reveals the mechanism of controllable and reversible ferroelastic domain switching, enabling the design of new actuators, sensors, and electromagnetic devices.

Prelithiation has important applications to convert a nonlithiated cathode or anode materials into a controllably lithiated state required for developing advanced Li-ion batteries. However, most of...

Here, by using atomically resolved scanning transmission electron microscopy and electron energy loss spectroscopy, we investigate the structural and chemical evolution of Li3V2(PO4)3 (LVP) upon the high-voltage window (3.0–4.8 V). We find that the valence of vanadium gradually increases towards the core corresponding to the formation of electrochemically inactive Li3-xV2(PO4)3 (L3-xVP) phases. These Li-deficient phases exhibit structure distortion with superstructure stripes, likely caused by the migration of the vanadium, which can slow down the lithium ion diffusion or even block the diffusion channels. Such kinetic limitations lead to the formation of Li-deficient phase along with capacity loss. Thus, the LVP continuously losses of electrochemical activity and Li-deficient phases gradually grow from the particle core towards the surface during cycling. After 500 cycles, the thickness of active LVP layer decreases to be ∼ 5–20 nm. Moreover, the micromorphology and chemical composition of solid electrolyte interphase (SEI) have been investigated, indicating the thick SEI film also contributes to the capacity loss. The present work reveals the structural and chemical evolution in the cycled electrode materials at an atomic scale, which is essential to understand the voltage fading and capacity decaying of LVP cathode.

Fluorographites (CFx ) are ultrahigh-energy-density cathode materials for alkaline-metal primary batteries. However, they are generally not rechargeable. To elucidate the reaction mechanism of CFx cathodes, in situ transmission electron microscopy characterizations and ab initio calculations are employed. It is found that it is a two-phase mechanism upon K/Na/Li ion insertion; crystalline KF (crystalline NaF nanoparticles and amorphous LiF) is generated uniformly within the amorphous carbon matrix, retaining an unchanged volume during the discharge process. The diffusivity for K/Na/Li ion migration within the CFx is ≈2.2-2.5 × 10-12 , 3.4-5.3 × 10-12 , and 1.8-2.5 × 10-11 cm2 s-1 , respectively, which is comparable to the diffusivity of K/Na/Li ions in liquid-state cells. Encouraged by the in situ transmission electron microscopy (TEM) results, a new rechargeable all-solid-state Li/CFx battery is further designed that shows a part of the reversible specific discharge capacity at the 2nd cycle. These findings demonstrate that a solid-state electrolyte provides a different reaction process compared with a conventional liquid electrolyte, and enables CFx to be partly rechargeable in solid-state Li batteries.

Fluorographites (CFx) are potential cathode materials for alkaline metal primary batteries with ultrahigh energy densities. To elucidate the reaction mechanism and structural evolution of CFx cathodes, we combine in situ transmission electron microscopy and ab initio calculations and discover a two-phase mechanism upon Li/Na/K ion insertion. Amorphous LiF (crystalline NaF and KF nanoparticles) are generated uniformly within the amorphous carbon matrix, which retains an unchanged volume during the discharge process. The diffusivity for K/Na/Li ion migration within the CFx is approximately 232.9 nm2/s, 479.7 nm2/s, and 2133.0 nm2/s, respectively, which is comparable to the diffusivity of Li/Na/K ions in liquid-state cells. During charging, amorphous LiF and crystal NaF decompose into alkali metal and F2. Our results reveal no substantial volume change and good ion diffusion kinetics, demonstrating the applicability of all-solid-state M/CFx (M = Li/Na/K) primary batteries. This new understanding may promote the design and development of better CFx-based batteries.While lithium ion batteries (LIBs) are widely applied in portable electronics and electric vehicles, cheaper, more advanced battery systems with higher energy and power densities, and improved safety performance remain in demand.1-5 Among the materials employed in cathode systems, conversion-type materials, such as halides, chalcogenides, and oxides, demonstrate high specific capacities, offering lithium-metal batteries with high energy densities.4,6,7 For example, fluorinated graphite (CFx, x = 1) cathodes exhibit a high theoretical energy density and power density of 3725 Wh kg-1 and 9313 Wh L-1, respectively 6, which can be used in Li primary cells. 8 The practical energy density in Li/CF1.0 cells reaches 2600 Wh kg-1, which is much higher than that in Li/I2, Li/SOCl2, Li/MnO2, Li/Ag2CrO4, and Li/CuS systems.9,10 CFx also delivers a discharge specific capacity of 1061 mAh/g (1439 Wh kg-1) with a discharge plateau of 2.4 V11,12 in a Na/CFx system, and 749 mAh g-1 (1869 Wh kg-1) with a discharge voltage plateau of 3.0 V in a K/CFx system13, which makes CFx applicable for multiple alkali ion batteries. Moreover, the irreversible conversion of CFx into LiF and carbon during discharge, due to the high dissociation energy of LiF (6.1 eV), means the decomposition of LiF by charging alone is not possible.9,10,12 Yazami et al. first demonstrated the reversible electrochemical reaction of CFx with lithium in F ion batteries with a reversible capacity of ~120 mAh g-1.14 Later, Liu et al. reported a reversible Na/CFx battery with a specific capacity of 786 mAh g-1.11 However, these batteries suffer from poor rate performance at low temperatures15, initial voltage delay during the discharge process8 and large heat generation at high discharge rates, which limit their application in harsh environments.9,16 Therefore, a detailed study of the reaction mechanisms is urgently required for the further optimization. Various studies have been conducted to understand these battery systems using different techniques.8,16-20 For example, in situ XRD results suggested a simultaneous formation of a CF(x-y)-Li+ intermediate phase and crystal LiF15,18, while other studies reported a formation of amorphous LiF followed by recrystallization to crystalline LiF.17,19 The crystalline LiF is generated in an orientation that relates to the absorption energy of the solvents on the LiF surface.16 The reversibility and reaction mechanism of CFx in Na/CFx batteries was studied using softX-ray absorption spectroscopy (SXAS) and nuclear magnetic resonance (NMR) techniques, which revealed reversible conversions between CFx and NaF.12 The liquid electrolyte was reported to act as an ion conductor and solution medium to dissolve and aggregate alkali fluoride crystals in a M–CFx (M = Li, Na, and K) system, resulting in large crystalline alkali fluorides.13 Despite intensive efforts focusing on the reaction mechanism, the structural evolution and reaction pathway of CFx in Li/Na/K batteries remains unclear. In this study, we use in situ transmission electron microscopy (TEM) with high spatial/temporal resolution to probe the phase transformation, intermediate phase, and volume change in real time (Figure 1a).21,22 We find that a two-phase reaction occurs during alkali ion intercalation, and the diffusivity of K/Na/Li ion intercalation in CFx is approximately 232.9 nm2/s, 479.7 nm2/s, and 2133.0 nm2/s, respectively. In situ electron diffraction patterns show the formation and even distribution of crystalline KF and NaF nanoparticles and amorphous LiF in the amorphous carbon matrix, leading to no volume change. Upon the insertion of K with a large ionic radius, the interlayer spacing of CF increases, and while only subtle changes are observed during Na/Li ion insertion, both are confirmed through density functional theory (DFT) calculations. Moreover, we find both amorphous LiF and crystalline NaF decompose into alkali metal and F2 during the charging process under vacuum. The results show no volume change and good ion diffusion kinetics during ion intercalation, suggesting that the all-solid-state M/CFx (M = Li/Na/K) primary batteries have broad applicability.

Organic-inorganic hybrid perovskites (OIHPs) have attracted extensive research interest as a promising candidate for efficient and inexpensive solar cells. Transmission electron microscopy characterizations that can benefit the fundamental understanding and the degradation mechanism are widely used for these materials. However, their sensitivity to the electron beam illumination and hence structural instabilities usually prevent us from obtaining the intrinsic information or even lead to significant artifacts. Here, we systematacially investigate the structural degradation behaviors under different experimental factors to reveal the optimized conditions for TEM characterizations of OIHPs by using low-dose electron diffraction and imaging techniques. We find that a low temperature does not slow down the beam damage but instead induces a rapid amorphization for OIHPs. Moreover, a less severe damage is observed at a higher accelerating voltage. The beam-sensitivity is found to be facet-dependent that a (100) exposed MAPbI3 surface is more stable than (001) surface. With these guidance, we successfully acquire the atomic structure of pristine MAPbI3 and identify the characterization window that is very narrow. These findings are helpful to guide future electron microscopy characterization of these beam-sensitive materials, which are also useful for finding strategies to improve the stability and the performance of the perovskite solar cells., Comment: 19 pages, 5 figures

Free-standing electrodes with high electrical conductivity, good deformability and durability are critical for flexible electronics, especially in field of wearable energy storage devices. Here, three dimensional (3D) Ti3C2Tx MXene/reduced graphene oxide (rGO)/carbon (MGC-500) hybrid electrode fabricated by the simple template method is presented. Commercial melamine foam (MF) worked as template not only enables the Ti3C2Tx/rGO nanosheets to form a porous architecture, but also introduces the heteroatom nitrogen into the Ti3C2Tx/rGO nanosheets during the annealing process. The as-prepared MGC-500 electrode shows a gravimetric capacitance of 276F g−1 at a current density of 0.5 A g−1. When the MGC-500 hybrid electrodes are assembled into an all solid-state supercapacitor, it shows a stable electrochemical performance at different compressive strains. Notably, we find that the MGC-500 foam electrode coated with PVA-H2SO4 gel electrolyte can be compressed into flexible film at 80% of compression. And the supercapacitor devices assembled by the film also exhibit stable capacitance under different modes of deformations such as bending and twist. The developed template approach offers a simple strategy to fabricate free-standing Ti3C2Tx MXene electrode for energy storage devices that can withstand varied deformations, and also can be extended to other members of the large MXene family.

Hollow nanostructures have witnessed an increasing interest in emerging fields, including energy storage electrodes, nanocatalysis, photonics, and biomedical carriers. The fundamental mechanisms for hollowing conversion chemistry of complex nanoalloys are indispensable to grow such nanostructures with controlled compositions and functionalities. Here, we study the Kirkendall effect on magnetic Fe–Co bimetallic alloys with the formation of hollow nanostructures for exchange interactions. The rich diffusion kinetics of binary Fe–Co alloys accompanied by its conversion from solid, core–shell to hollow nanostructures have been captured through X-ray diffraction pattern and electron microscopy. We further report the cation exchange improved magnetic performance of hollow nanostructures by increasing the crystallinity.

Ionic doping and migration in solids underpins a wide range of applications including lithium ion batteries, fuel cells, resistive memories, and catalysis. Here, by in situ transmission electron microscopy technique we directly track the structural evolution during Li ions insertion and extraction in transition metal dichalcogenide 1T-V1+xSe2 nanostructures which feature spontaneous localized superstructures due to the periodical interstitial V atoms within the van der Waals interlayers. We find that lithium ion migration destroys the cationic orderings and leads to a phase transition from superstructure to nonsuperstructure. This phase transition is reversible, that is, the superstructure returns back after extraction of lithium ion from LiyV1+xSe2. These findings provide valuable insights into understanding and controlling the structure and properties of 2D materials by general ionic and electric doping.

The performance of alkali-metal-ion batteries largely depends on the migration behavior of alkali metal ions in the electrodes. Probing the atomic structure of the reaction interface and the dynamic process during ion transport in the electrodes will help better understand the underlying electrochemical mechanisms and inspire rational electrode designs. In this study, by combining in situ transmission electron microscopy (TEM) and aberration-corrected scanning TEM (STEM), we track the reversible lithium ion transport in MoS2 nanostructures to reveal the atomic structure and dynamic behaviors of the reaction interface. We find that lithium ions insertion triggers complex phase transformations. Three different phases co-exist at the interface: a pristine 2H phase, a 1T phase with a shrank lattice constant of − 3.3% ( ± 2.3%), and a distorted 1T phase (called 1Tˊ phase) with an expanded lattice constant of 5.5% ( ± 2.5%). The atomically resolved Z-contrast image shows that the expanded 1Tˊ phase has distorted Mo arrangements. Furthermore, the lithium ions migration causes defects at the reaction front, and the diffusion on the surface is faster than that inside, forming a core-shell structure at the reaction interface. The diffusivity of lithium ions is directly measured to be ~1000–30,000 nm2/s, which is significantly higher than that of sodium insertion (~10–20 nm2/s). The atomic-scale observations of lithium-ion-migration-induced complex structural evolutions would help understand the properties of MoS2 nanostructures and shed light on the design of alkali-metal-ion batteries with general transition-metal dichalcogenide electrodes.

Quasi-one-dimensional semiconducting materials have attracted increasing attention due to their excellent ability to downscale the size of transistors. However, in quasi-one-dimensional nanowire (NW) transistors, their surface and interface properties play a very important role mainly due to the large surface-to-volume ratio of NWs and surface scattering, which degrade their carrier mobility. Herein, we developed a new method to cover the channel surface of InAs NW field effect transistors (FETs) with Y2O3/HfO2 layers to improve their electrical properties. We successfully fabricated nine FETs and measured their electrical properties, which improve after depositing the Y2O3/HfO2 layers, including an increase in on-state current, decrease in off-state current, increase in transconductance, increase in electron mobility and decrease in subthreshold swing. By comparing the properties of Y2O3/HfO2-covered devices with that of the FETs fabricated without the Y2O3 covering or without annealing, we prove that it is the combined Y2O3/HfO2 layers instead of only the Y2O3 or HfO2 layer that improve the electrical properties of the FETs. The Cs-corrected high-resolution scanning transmission electron microscopy study demonstrates that Y can actually diffuse through the native oxide layer (confirmed to be InOx) and reach the surface of the InAs NWs. Our results indicate that the desirable characteristics of Y2O3 and the surface passivation by HfO2 improve the electrical properties of the InAs NW FETs, in which Y2O3 plays an important role to modify and stabilize the interface between the InAs NWs and the outside dielectric layer. Furthermore, this method should also be applicable to other III-V materials.

Quasi van der Waals (vdW) epitaxy, a pioneering epitaxy of sp3 -hybridized semiconductor films on sp2 -hybridized two-dimensional (2D) materials, provides a way, in principle, to achieve single-crystal epilayers with preferred atom configurations that are free of substrate. Unfortunately, this has not been experimentally confirmed in the case of the hexagonal semiconductor III-nitride epilayer until now. Here, for the first time, we report that the epitaxy of gallium nitride (GaN) on graphene can tune the atom arrangement (lattice polarity) through manipulation of the interface atomic configuration, where GaN films with gallium and nitrogen polarity were achieved by forming C-O-N-Ga(3) or C-O-Ga-N(3) configurations, respectively, on artificial C-O surface dangling bonds by atomic oxygen preirradiation on trilayer graphene. Furthermore, an aluminum nitride (AlN) buffer/interlayer leads to unique metal polarity due to the formation of an AlON thin layer in a growth environment containing trace amounts of oxygen, which explains the open question why those reported wurtzite III-nitride films on 2D materials always exhibit metal polarity. The reported atomic modulation through interface manipulation provides an effective model for hexagonal nitride semiconductor layer grown on graphene, which definitely promotes the development of novel semiconductor devices. This article is protected by copyright. All rights reserved.

Silicon is now well-recognized to be a promising alternative anode for advanced lithium-ion batteries because of its highest capacity available today; however, its insufficiently high Coulombic efficiency upon cycling remains a major challenge for practical application. To overcome this challenge, we have developed a facile mechanochemical method to synthesize a core–shell-structured Si/polyphenylene composite (Si/PPP) with a n-type conductive PPP layer tightly bonded in a planar orientation to the surfaces of Si nanocores. Because of its compactness and flexibility, the outer PPP layer can protect the Si core from contacting the electrolyte and maintaining the structural stability of electrode/electrolyte interface during cycles. As a result, the Si/PPP anode demonstrated a high reversible capacity of ∼2387 mAh g–1, a stable cycleability with 88.5% capacity retention over 500 cycles, and, particularly, a high Coulombic efficiency of 99.7% upon extended cycling, offering a new insight for future developme...

Organic-inorganic hybrid perovskites (OIHPs) have generated considerable excitement due to their promising photovoltaic performance. However, the commercialization of perovskite solar cells (PSCs) is still plagued by the structural degradation of the OIHPs. Here, the decomposition mechanism of OIHPs under electron beam irradiation is investigated via transmission electron microscopy, and a general decomposition pathway for both tetragonal CH3 NH3 PbI3 and cubic CH3 NH3 PbBr3 is uncovered through an intermediate superstructure state of CH3 NH3 PbX2.5 , X = I, Br, with ordered vacancies into final lead halides. Such decomposition can be suppressed via carbon coating by stabilization of the perovskite structure framework. These findings reveal the general degradation pathway of OIHPs and suggest an effective strategy to suppress it, and the atomistic insight learnt may be useful for improving the stability of PSCs.

Organic–inorganic hybrid perovskites are promising candidates for the next-generation solar cells. Many efforts have been made to study their structures in the search for a better mechanistic understanding to guide the materials optimization. Here, we investigate the structure instability of the single-crystalline CH3NH3PbI3 (MAPbI3) film by using transmission electron microscopy. We find that MAPbI3 is very sensitive to the electron beam illumination and rapidly decomposes into the hexagonal PbI2. We propose a decomposition pathway, initiated with the loss of iodine ions, resulting in eventual collapse of perovskite structure and its decomposition into PbI2. These findings impose important question on the interpretation of experimental data based on electron diffraction and highlight the need to circumvent material decomposition in future electron microscopy studies. The structural evolution during decomposition process also sheds light on the structure instability of organic–inorganic hybrid perovskites in solar cell applications., Hybrid perovskites are highly promising for photovoltaic applications, but they are prone to decomposition. Here, the authors probe the stability of CH3NH3PbI3 films in a transmission electron microscope, defining the threshold conditions to avoid damage under the electron beam, and describing a decomposition pathway.

Recent theoretical predictions and angle-resolved photoemission spectroscopy measurements have shown that single crystal Cd3As2 is a three-dimensional topological Dirac semimetal possessing linear dispersions along all three momentum directions. Nanoscale topological Dirac semimetal structures have a large surface-to-volume ratio and provide a platform to explore its topological surface states. Here we report the synthesis of high quality Cd3As2 single crystalline nanoplates and nano-octahedrons via a vapor–solid growth mechanism. Triangular and hexagonal nanoplates with lateral dimensions ranging from several hundred nanometers to tens of micrometers are obtained. The top facets are (112), consistent with the natural cleavage plane of Cd3As2 single crystal. The synthesized Cd3As2 nano-octahedrons are enclosed by the {112} facets. A photovoltaic effect is demonstrated from a Cd3As2 nanoplate/metal electrode interface, suggesting potential applications in self-powered photodetection.

Although the size effect in ferroelectric thin films has been known for long time, the underlying mechanism is not yet fully understood and whether or not there is a critical thickness below which the ferroelectricity vanishes is still under debate. Here, we directly measure the thickness-dependent polarization in ultrathin PbZr0.2Ti0.8O3 films via quantitative annular bright field imaging. We find that the polarization is significantly suppressed for films, Understanding ferroelectricity at reduced dimensions will be important for future sub-nanoscale devices based on ferroelectrics. Using high resolution electron microscopy; Gao et al., observe the existence of a measurable polarization at a thickness of just 1.5-unit cells

Helium ion microscope (HIM) has presented an outstanding ability to image and nanofabricate thin film and two-dimensional materials with high precision. However, the concomitant damage and implantation induced by focused helium ion beam should influence the imaging quality and nanomachining efficiency inevitably, especially for bulk samples. In this work, the authors investigated the generation process of damages at nanoscales in single crystalline bulk silicon caused by ions implantation in HIM using transmission electron microscopy. The dependence of implantation and damage on ion dose, ion energy, and beam current was also discussed and analyzed. It was found that the damage should be originated from the local defects caused by ion implantation and the crystal structure could be gradually destroyed and transform into amorphous silicon with the generation and growth of subsurface nanobubbles as ion dose increased. The local concentration of implanted helium ion was found as a universal factor to impact on the damage level and the size of nanobubbles directly. These findings not only shed lights on the effective imaging and nanofabrication of HIM but also provide a further understanding in the nuclear irradiation area.Helium ion microscope (HIM) has presented an outstanding ability to image and nanofabricate thin film and two-dimensional materials with high precision. However, the concomitant damage and implantation induced by focused helium ion beam should influence the imaging quality and nanomachining efficiency inevitably, especially for bulk samples. In this work, the authors investigated the generation process of damages at nanoscales in single crystalline bulk silicon caused by ions implantation in HIM using transmission electron microscopy. The dependence of implantation and damage on ion dose, ion energy, and beam current was also discussed and analyzed. It was found that the damage should be originated from the local defects caused by ion implantation and the crystal structure could be gradually destroyed and transform into amorphous silicon with the generation and growth of subsurface nanobubbles as ion dose increased. The local concentration of implanted helium ion was found as a universal factor to impact ...

Polarity-dependent homo-epitaxy on (0001)-Zn and (000)-O surfaces of cleaved ZnO microwires was investigated by in situgrowth in ESEM and DFT simulations. ZnO monomers adsorption, adatoms desorption and chemisorption were simulated to understand the explicit mechanism.

In situ epitaxial growth of high-oriented ZnO nanowires on annealed zinc film was realized in an environmental scanning electron microscope. Energy dispersive spectra, X-ray diffraction and photoluminescence measurements demonstrated that a thin layer of [001] ZnO was formed on the surface of zinc film after annealing. Formation of a ZnO layer on annealed zinc film caused dense [001] oriented nanowires compared to the sparse random nanowires grown on unannealed zinc film. Possible growth mechanism for the nanowires grown on annealed and unannealed film was discussed. This work offers a novel approach for effective epitaxial growth of high-oriented ZnO nanowires and helps to understand the growth mechanism of ZnO nanowires.

Ni3Al-hBN composite powders were manufactured by spray drying technology, and then plasma sprayed to form the coatings. The influence of hBN addition amount on the flowability and apparent density of the composite powders, as well as the mechanical and tribological properties of the as-sprayed coatings was evaluated. The results indicate that the spherical powders with uniformly distributed compositions are successfully manufactured by spray drying technology. Both the flowability and apparent density of the Ni3Al-hBN powders, as well as the bond strength and hardness of the composite coatings decrease with the increase of hBN content. Addition of hBN less than 10 wt.% reduces the friction coefficient and brittle fracture of the coatings, which is beneficial to improve the tribological properties of the Ni3Al-hBN composite coatings. However, high hBN addition, damaging the bond in the coating, will deteriorate the wear resistance of the coating. The Ni3Al-hBN composite coating containing 10%hBN shows the optimum properties combining strength and hardness with tribological properties. (C) 2010 Elsevier B.V. All rights reserved.

NiCr clad BaF2 center dot CaF2 fluoride eutectic powders were added into chromium carbide-nichrome feedstock to improve the tribological properties of NiCr-Cr3C2 coating, and the structures, mechanical, and ball-on-disk sliding wear performance of the coating were characterized. The results show that NiCr cladding can effectively decrease the density and thermophysical difference between the feedstock components, while alleviate the decarburization and oxidization of the constituent phases, and form the coating with a uniform and dense microstructure. However, the addition of BaF2 center dot CaF2 has a negative effect on mechanical properties of the coating. When the temperature reaches 500 A degrees C, the BaF2 center dot CaF2 eutectic is soften by the heat and smeared by the counterpart, thus the low shear stress lubricating film forms between the contact surface, that improves the tribological properties dramatically. At this temperature, the dominant wear mechanisms also change from splats spallation and abrasive wear at room temperature to plastic deformation and plawing by the counterpart. Within the temperature range from 600 to 800 A degrees C, the friction coefficient, the wear rates of NiCr/Cr3C2-10% BaF2 center dot CaF2 coating and its coupled Si3N4 ball are 20%, 40%, and 75% lower than those of the NiCr/Cr3C2 coating, respectively. The NiCr/Cr3C2-BaF2 center dot CaF2 coating shows superior wear performance to the NiCr/Cr3C2 coating without lubricant additive.

Mn-doped ZnO nanobelts have been synthesized via the vapor phase evaporation method, which exhibited ferromagnetism at room temperature. Electron energy-loss spectroscopy was used to investigate the chemical state of Mn dopants. It revealed that the Mn species had the chemical valence of +2. Meanwhile, the fine structures of the Mn L(23) edges indicated that the Mn dopant was located at the center of an oxygen octahedron but not an oxygen tetrahedron. This suggested that the Mn dopants did not substitute on the Zn sites as expected and sub-nanoscale MnO clusters had been formed in the synthesized ZnO nanobelts.

The authors show the real-time growth of ZnO nanowalls and nanowires on zinc particles via in situ observation in an environmental scanning electron microscopy. It was observed that a ZnO polycrystalline film is first deposited on zinc particles. The nanowires started to grow when the nanowalls had just formed and they grew epitaxially on the junctions of the nanowalls. The nanowalls and the nanowires grew together until the source of zinc was exhausted. The vapor-solid mechanism is deemed to be the growth mechanism as it quantitatively accounts for the growth speed of the nanowalls and nanowires observed in the experiment. Cathodoluminescence reveals that the growth at low zinc concentration leads to blue emission from defects, which may be zinc vacancies.

Photoelectrical properties of single ZnO nanowires have been investigated using photocurrent–voltage characteristics measurements varying with excitation photon energy and temperature. It is found that persistent photoconductivity (PPC) exists, and the PPC decreases with decreasing temperature. The temperature dependence of the PPC effect indicates that thermally activated return of electrons from shallow traps is responsible for the PPC phenomenon. The photosensitivity is found to be linear with the applied voltage, and it increases with decreasing temperature. A temperature dependence of photoconductivity gain was introduced to explain the experimental results.

The texture structure of 2H-WS2 plate-like particles has been investigated via electron diffraction (ED) patterns and high-resolution transmission electron microscopy (HRTEM) images. It is found th...

To improve the wear resistance of the cylinder liner of the engines operating in desert area, the tungsten carbide-based cermet coating (WC-12wt.%Co) was produced using the supersonic plasma spraying technique. The microstructure and phase composition of the as-sprayed coating were evaluated. The effect of sand size in lubricant on the friction and wear behavior of the coating under the lubrication of oil containing sand was investigated on a ball-on-disk tribometer. Characterization of the coating shows that the coating possesses dense microstructure with low level of porosity. The decarburization of WC is limited for the high particle velocity of the supersonic plasma spraying and only a few of W2C phases are observed in the coating. With the higher hardness and the good adhesion with the substrate the sprayed coating exhibits a better wear resistance compared with the gray casting iron used in the current cylinder liner. Micro-structural examination shows that the wear of coating is dominated by micro-cutting, WC particles pull-out and sub-surface indentation-induced sub-surface cracking, while the main wear mechanism of the gray casting iron is micro-cutting and plastic deformation. (C) 2008 Elsevier B.V. All rights reserved.

We have studied the influence of C and Si ion implantation with different implantation doses on yellow luminescence (YL) from GaN. Three kinds of GaN samples were used. In their as-grown states, #1 samples had strong YL, #2 samples had no YL, while #3 samples had weak YL. Our experimental results show that: (i) after annealing at 950 degreesC, the YL intensity for Si ion implanted #1 sample decreased with increasing implantation dose, while that for C ion implanted #1 sample exhibited a reverse rule; (ii) for #2 samples, C ion implantation produced much stronger YL than Si ion implantation did after annealing at 950 degreesC; (iii) the YL intensity sequence for Si ion implanted and 950 degreesC annealed #1, #2, and #3 samples was consistent with that for the unimplanted #1, #2, and #3 samples. However, the YL intensity sequence for the C ion implanted and 950 degreesC annealed #1, #2, and #3 samples reversed with that for the unimplanted ones. In order to explain all these phenomena, we suggested a physical model which claims that the deep C center is another important origin of YL in GaN, and during C ion implantation, the C ion prefers to combine with a V-Ga to form a C-Ga.

Although the low-temperature specific heat in metallic nanocrystals has been extensively demonstrated to be always enhanced due to the surface phonon softening, it is revealed here to be anomalously depressed by the surface oxidation so that the excess specific heat can be either positive or negative as a function of temperature with respect to the counterparts of the bulk crystals. The background mechanism has been theoretically deduced to well explain these experimental phenomena by calculating and comparing the vibrational densities of states (VDOS) of face-centered-cubic-aluminum (fcc-Al) and amorphous-alumina (a-Al2O3) nanoparticles. Different from fcc-Al nanocrystals, both the surface and volume VDOSs g(ω) of a-Al2O3 nanoparticles are scaled as g(ω) ∼ ω1.5 at the low-frequency limit (ω). The effective spatial dimension for the a-Al2O3 surfaces and volume as well as the fcc-Al surfaces is thus assumed to be between 2D and 3D while it is 3D for the fcc-Al volume. The specific heat of a-Al2O3@Al nanopa...

We developed a novel approach to synthesize phosphorus (P)-doped ZnO nanowires by directly decomposing zinc phosphate powder. The samples were demonstrated to be P-doped ZnO nanowires by using scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction spectra, X-ray photoelectron spectroscopy, energy dispersive spectrum, Raman spectra and photoluminescence measurements. The chemical state of P was investigated by electron energy loss spectroscopy (EELS) analyses in individual ZnO nanowires. P was found to substitute at oxygen sites (PO), with the presence of anti-site P on Zn sites (PZn). P-doped ZnO nanowires were high resistance and the related P-doping mechanism was discussed by combining EELS results with electrical measurements, structure characterization and photoluminescence measurements. Our method provides an efficient way of synthesizing P-doped ZnO nanowires and the results help to understand the P-doping mechanism.

The current-voltage (I-V) characteristics of single ZnO nanowires were measured varying with temperature and illumination. A model of the ZnO nanowire sandwiched by back-to-back diodes was utilized to explain the experimental data. Simulations of the I-V curves exhibited that the surface barrier height was independent of temperature from 180 to 290 K. This work also shows that the larger the incident laser power is, the smaller the contact surface barrier height will be. The photon induced reduction in the surface barrier height is attributed to the photogenerated holes, which result in a shielding effect on the surface trapped electrons.

Spin-dependent electron transport in individual magnetite (Fe3O4) nanowires contacted with normal metallic electrodes was investigated. Such a configured device demonstrated a spin-filter effect, that is, only the minority spin carriers can transport through the magnetite nanowire due to its negative spin polarization. An anomalous positive magnetoresistance approximately 7.5% is observed at room temperature. Moreover, the magnetoresistance can be controlled via bias voltage.

GaN nanotip triangle pyramids were synthesized on 3C-SiC epilayer via an isoelectronic In-doping technique. The synthesis was carried out in a specially designed two-hot-boat chemical vapor deposition system. In (99.999%) and molten Ga (99.99%) with a mass ratio of about 1:4 were used as the source, and pieces of Si (111) wafer covered with 400-500 nm 3C-SiC epilayer were used as the substrates. The products were analyzed by x-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy, selected area electron diffraction, Raman spectroscopy, and photoluminescence measurements. Our results show that the as-synthesized GaN pyramids are perfect single crystal with wurtzite structure, which may have potential applications in electronic/photonic devices.

The controllable deformation of nanopores was realized by moving a convergent electron beam in a high-resolution transmission electron microscope. Nanostructures with the desired geometries were successfully fabricated from the original nanopores in 100 nm-thick and 260 nm-thick Si(3)N(4) membranes. The formation dynamics is a competition process between the knock-on effect of the high-energy electron beam and surface tension driven shrinkage. This approach can be used to finely tune critical dimensions and deform nanopores to particular desired geometries with single-nanometer precision, which offers substantial opportunities in flexibly fabricating nanostructures for various applications such as nanoelectronics and nanofluidics.

In situ uniform growth of ZnO nanowires was realized and monitored at real time by heating zinc film in an environmental scanning electron microscope. Better controllability and repeatability were obtained by using zinc film as source material compared to traditionally used zinc powder. Morphology of the as-grown ZnO nanowires was found to depend on both the growth temperature and holding time. Low temperature (500 degrees C) and short growth time (approximately 20 min) favor one-dimensional nanowire growth, whereas longer holding time (40 min) or higher temperature (700 degrees C) lead to nanosheet growth. The results suggest that the zinc vapor partial pressure is vital in determining the final morphology. These results help to give more insights into the mechanism of ZnO nanowire synthesis.

Asymmetric Schottky barriers between ZnO nanowire and metal electrode have been fabricated at the two ends of the nanowire. An obvious photocurrent generated from the device at zero voltage bias can be switched on/off with quick response by controlling the light irradiation. Moreover, the device can still afford a current at zero bias after switching off light illumination, which is ascribed to the charge storage effect in single ZnO nanowires. The underlying mechanisms related to the photovoltaic effect and charge storage were discussed.

We studied the influence of adsorbates on the space-charge-limited current (SCLC) in individual ZnO nanowires through varying the bias voltage, laser illumination, and ambient pressure. In dark and air conditions, the free carriers were depleted by the surface adsorbates, and electrons injected from the electrode to the nanowire dominated the electron transport properties. Under laser illumination, the current-voltage characteristic was linear at low voltage and superlinear at high voltage, and the SCLC regime occurred at high voltages due to the surface desorption. The time response of photoconductivity further revealed the dynamic process of elimination of SCLC by desorption of oxygen molecules at the ZnO nanowire surface.

Uniform TiSi2 nanocables were synthesized on large substrates using a simple physical vapor deposition method. X-ray diffraction, scanning electron microscopy, and transmission electronic microscopy were employed to characterize the samples. It reveals that the as-grown TiSi2 nanocables are of high quality single crystal inside with thin amorphous SiO2 sheathing layer. Electrical properties of these core-shell structure nanowires are also presented. Insulativity of the outer layer and stable metallic characters of the inside single crystal TiSi2 were observed. Moreover, carrying capacity of current density is up to 1011A∕m2. The nanoscale structure and excellent electrical performance make the TiSi2 nanocables good candidate for electrical interconnection in potential nanodevices.

An experimental investigation is reported on in situ encapsulating copper nanoparticles with a nanolayer of hydrocarbon coating. Copper nanoparticles produced by the flow-levitation method are in situ encapsulated by the dielectric barrier discharge, which generates carbon and hydrogen plasmas for forming a polymerized nanolayer of hydrocarbon at the surface of Cu nanoparticles. The structural, chemical components and optical properties of the encapsulated Cu nanoparticles are characterized by transmission electron microscopy, energy dispersive x-ray spectra, x-ray diffraction, x-ray photoelectron spectra, and UV-visible absorption.

Highly ordered ultrafine nanostructures (feature size