172 research outputs found
Growth and Characterization of Multiferroic BaTiO3-CoFe2O4 Thin Film Nanostructures
Multiferroic materials which display simultaneous ferroelectricity and magnetism have been stimulating significant interest both from the basic science and application point of view. It was proposed that composites with one piezoelectric phase and one magnetostrictive phase can be magnetoelectrically coupled via a stress mediation. The coexistence of magnetic and electric subsystems as well as the magnetoelectric effect of the material allows an additional degree of freedom in the design of actuators, transducers, and storage devices. Previous work on such materials has been focused on bulk ceramics.
In the present work, we created vertically aligned multiferroic BaTiO3-CoFe2O4 thin film nanostructures using pulsed laser deposition. Spinel CoFe2O4 and perovskite BaTiO3 spontaneously separated during the film growth. CoFe2O4 forms nano-pillar arrays embedded in a BaTiO3 matrix, which show three-dimensional heteroepitaxy. CoFe2O4 pillars have uniform size and spacing. As the growth temperature increases the lateral size of the pillars also increases. The size of the CoFe2O4 pillars as a function of growth temperature at a constant growth rate follows an Arrhenius behaviour. The formation of the BaTiO3-CoFe2O4 nanostructures is a process directed by both thermodynamic equilibrium and kinetic diffusion. Lattice mismatch strain, interface energy, elastic moduli and molar ratio of the two phases, etc., are considered to play important roles in the growth dynamics leading to the nanoscale pattern formation of BaTiO3-CoFe2O4 nanostructures.
Magnetic measurements exhibit that all the films have a large uniaxial magnetic anisotropy with an easy axis normal to the film plane. It was calculated that stress anisotropy is the main contribution to the anisotropy field. We measured the ferroelectric and piezoelectric properties of the films, which correspond to the present of BaTiO3 phase. The system shows a strong coupling of the two order parameters of polarization and magnetization through the coupled lattices. This approach to the formation of self-assembled ferroelectric/ferro(ferri-)magnetic nanostructures is generic. We have created similar nanostructures from other spinel-perovskite systems such as BiFeO3-CoFe2O4, BaTiO3-NiFe2O4, etc., thus making it of great interest and value to a broad materials community
On-column 2p bound state with topological charge \pm1 excited by an atomic-size vortex beam in an aberration-corrected scanning transmission electron microscope
Atomic-size vortex beams have great potential in probing materials' magnetic
moment at atomic scales. However, the limited depth of field of vortex beams
constrains the probing depth in which the helical phase front is preserved. On
the other hand, electron channeling in crystals can counteract beam divergence
and extend the vortex beam without disrupting its topological charge.
Specifically, in this paper, we report atomic vortex beams with topological
charge \pm1 can be coupled to the 2p columnar bound states and propagate for
more 50 nm without being dispersed and losing its helical phase front. We gave
numerical solutions to the 2p columnar orbitals and tabulated the
characteristic size of the 2p states of two typical elements, Co and Dy, for
various incident beam energies and various atomic densities. The tabulated
numbers allow estimates of the optimal convergence angle for maximal coupling
to 2p columnar orbital. We also have developed analytic formulae for beam
energy, convergence-angle, and hologram dependent scaling for various
characteristic sizes. These length scales are useful for the design of
pitch-fork apertures and operations of microscopes in the vortex-beam imaging
mode.Comment: 30 pages, 7 figures, Microscopy and Microanalysis, in pres
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Dynamic deformability of individual PbSe nanocrystals during superlattice phase transitions
The behavior of individual nanocrystals during superlattice phase transitions can profoundly affect the structural perfection and electronic properties of the resulting superlattices. However, details of nanocrystal morphological changes during superlattice phase transitions are largely unknown due to the lack of direct observation. Here, we report the dynamic deformability of PbSe semiconductor nanocrystals during superlattice phase transitions that are driven by ligand displacement. Real-time high-resolution imaging with liquid-phase transmission electron microscopy reveals that following ligand removal, the individual PbSe nanocrystals experience drastic directional shape deformation when the spacing between nanocrystals reaches 2 to 4 nm. The deformation can be completely recovered when two nanocrystals move apart or it can be retained when they attach. The large deformation, which is responsible for the structural defects in the epitaxially fused nanocrystal superlattice, may arise from internanocrystal dipole-dipole interactions
Evidence for power-law frequency dependence of intrinsic dielectric response in the CaCuTiO
We investigated the dielectric response of CaCuTiO (CCTO) thin
films grown epitaxially on LaAlO (001) substrates by Pulsed Laser
Deposition (PLD). The dielectric response of the films was found to be strongly
dominated by a power-law in frequency, typical of materials with localized
hopping charge carriers, in contrast to the Debye-like response of the bulk
material. The film conductivity decreases with annealing in oxygen, and it
suggests that oxygen deficit is a cause of the relatively high film
conductivity. With increase of the oxygen content, the room temperature
frequency response of the CCTO thin films changes from the response indicating
the presence of some relatively low conducting capacitive layers to purely
power law, and then towards frequency independent response with a relative
dielectric constant . The film conductance and dielectric
response decrease upon decrease of the temperature with dielectric response
being dominated by the power law frequency dependence. Below 80 K, the
dielectric response of the films is frequency independent with
close to . The results provide another piece of evidence for an
extrinsic, Maxwell-Wagner type, origin of the colossal dielectric response of
the bulk CCTO material, connected with electrical inhomogeneity of the bulk
material.Comment: v4: RevTeX, two-column, 9 pages, 7 figures; title modified, minor
content change in p.7, reference adde
The influence of leader–signaled knowledge hiding on tourism employees’ work withdrawal behavior: A moderated mediating model
Even though organizations encourage the dissemination of knowledge and information among organizational members, the phenomenon of knowledge hiding still exists widely in organizations. The consequences of leader-signaled knowledge hiding are more destructive to the workplace than the consequences of employees’ knowledge hiding. It is particularly necessary to explore the influence mechanism of leader-signaled knowledge hiding on employees’ work behavior. Drawing on Conservation of Resources theory, this study establishes a moderated mediation model with emotional exhaustion as a mediating variable and supervisor-subordinate guanxi as a moderating variable. This study focuses on the consequences of leader-signaled knowledge hiding and divides leader-signaled knowledge hiding into self-practiced knowledge hiding and explicit knowledge hiding. Based on the results of 440 questionnaires from tourism employees, it is shown that leader-signaled knowledge hiding has a positive impact on employees’ work withdrawal behavior. Specifically, leader’s self-practiced knowledge hiding has a greater direct impact on employees’ work withdrawal behavior, while leader’s explicit knowledge hiding has a greater direct impact on employees’ emotional exhaustion. Emotional exhaustion plays a key mediating role in the relationship between leader-signaled knowledge hiding (i.e., self-practiced knowledge hiding and explicit knowledge hiding) and employees’ work withdrawal behavior. Supervisor-subordinate guanxi significantly moderates the positive relationship between leader-signaled knowledge hiding (i.e., self-practiced hiding and explicit knowledge hiding) and employees’ emotional exhaustion. This study is an extension of previous research on knowledge hiding. The results provide a reference for leaders to deal with knowledge hiding and improve organizational knowledge management ability
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In Situ TEM Study of the Degradation of PbSe Nanocrystals in Air
PbSe
nanocrystals have attracted widespread attention due to a
variety of potential applications. However, the practical utility
of these nanocrystals has been hindered by their poor air stability,
which induces undesired changes in the optical and electronic properties.
An understanding of the degradation of PbSe nanocrystals when they
are exposed to air is critical for improving the stability and enhancing
their applications. Here, we use in situ transmission electron microscopy
(TEM) with an environmental cell connected to air to study PbSe nanocrystal
degradation triggered by air exposure. We have also conducted a series
of complementary studies, including in situ environmental TEM study
of PbSe nanocrystals exposed to pure oxygen and PbSe nanocrystals
in H2O using a liquid cell, and ex situ experiments, such
as O2 plasma treatment and thermal heating of PbSe nanocrystals
under different air exposure. Our in situ observations reveal that
when PbSe nanocrystals are exposed to air (or oxygen) under electron
beam irradiation, they experience a series of changes, including shape
evolution of individual nanocrystals with the cuboid intermediates,
coalescence between nanocrystals, and formation of PbSe thin films
through drastic solid-state fusion. Further studies show that the
PbSe thin films transform into an amorphous Pb rich phase or eventually
pure Pb, which suggest that Se reacts with oxygen and can be evaporated
under electron beam illumination. These various in situ and ex situ
experimental results indicate that PbSe nanocrystal degradation in
air is initiated by the dissociation and removal of ligands from the
PbSe nanocrystal surface
Recent progress in thermoelectric nanocomposites based on solution-synthesized nanoheterostructures
Thermoelectric materials, which can convert waste heat into electricity, have received increasing research interest in recent years. This paper describes the recent progress in thermoelectric nanocomposites based on solution-synthesized nanoheterostructures. We start our discussion with the strategies of improving the power factor of a given material by using nanoheterostructures. Then we discuss the methods of decreasing thermal conductivity. Finally, we highlight a way of decoupling power factor and thermal conductivity, namely, incorporating phase-transition materials into a nanowire heterostructure. We have explored the lead telluride–copper telluride thermoelectric nanowire heterostructure in this work. Future possible ways to improve the figure of merit are discussed at the end of this paper
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Difficult Measurements of Materials Systems at Cryogenic Temperatures: Cryo-EELS and Cryo-4D-STEM
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Revealing of the Activation Pathway and Cathode Electrolyte Interphase Evolution of Li-Rich 0.5Li2MnO3·0.5LiNi0.3Co0.3Mn0.4O2 Cathode by in Situ Electrochemical Quartz Crystal Microbalance.
The first-cycle behavior of layered Li-rich oxides, including Li2MnO3 activation and cathode electrolyte interphase (CEI) formation, significantly influences their electrochemical performance. However, the Li2MnO3 activation pathway and the CEI formation process are still controversial. Here, the first-cycle properties of xLi2MnO3·(1- x) LiNi0.3Co0.3Mn0.4O2 ( x = 0, 0.5, 1) cathode materials were studied with an in situ electrochemical quartz crystal microbalance (EQCM). The results demonstrate that a synergistic effect between the layered Li2MnO3 and LiNi0.3Co0.3Mn0.4O2 structures can significantly affect the activation pathway of Li1.2Ni0.12Co0.12Mn0.56O2, leading to an extra-high capacity. It is demonstrated that Li2MnO3 activation in Li-rich materials is dominated by electrochemical decomposition (oxygen redox), which is different from the activation process of pure Li2MnO3 governed by chemical decomposition (Li2O evolution). CEI evolution is closely related to Li+ extraction/insertion. The valence state variation of the metal ions (Ni, Co, Mn) in Li-rich materials can promote CEI formation. This study is of significance for understanding and designing Li-rich cathode-based batteries
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