633 research outputs found
Sub-band level-assisted photoconduction in epitaxial BiFeO3 films
Sub-band level assisted conduction mechanisms are well known in the field of semiconducting materials. In this work, we explicitly show the validity of such a mechanism in the multiferrroic material BiFeO3 (BFO). Our study is based on two different systems of epitaxial thin films of BFO, relaxed and strained. By analyzing the spectral distribution of the photoresponse from both the systems, the role of the sub-band levels in the photoconductive phenomena becomes evident. Additionally, the influence of epitaxial strain on the trapping activity of these levels is also observed. A model is proposed by taking into account the reversal of the role of a sub-band gap level, i.e., from a trapping to a ground state
Simultaneous dynamic characterization of charge and structural motion during ferroelectric switching
Monitoring structural changes in ferroelectric thin films during electric
field-induced polarization switching is important for a full microscopic
understanding of the coupled motion of charges, atoms and domain walls. We
combine standard ferroelectric test-cycles with time-resolved x-ray diffraction
to investigate the response of a nanoscale ferroelectric oxide capacitor upon
charging, discharging and switching. Piezoelectric strain develops during the
electronic RC time constant and additionally during structural domain-wall
creep. The complex atomic motion during ferroelectric polarization reversal
starts with a negative piezoelectric response to the charge flow triggered by
voltage pulses. Incomplete screening limits the compressive strain. The
piezoelectric modulation of the unit cell tweaks the energy barrier between the
two polarization states. Domain wall motion is evidenced by a broadening of the
in-plane components of Bragg reflections. Such simultaneous measurements on a
working device elucidate and visualize the complex interplay of charge flow and
structural motion and challenges theoretical modelling
CLIMATE AND WEATHER INFLUENCE OVER THE TOURIST SECTOR IN THE LAND OF DORNA
The Land of Dorna represents a region with a climate specific to intermountain depressions and mountain areas by its geographic position, the characteristics of active subjacent surfaces and of dominant air-masses,. Climatic factors and phenomena play an important part in outlining the whole features of the territorial system and development of some particular tourist activities in this geographic region. The Land of Dorna has a lot of natural and anthropic resources that have taken many different shapes under the influence of some internal and external elements over the time. Therefore, our main goal is to identify and analyze the way in which climate and weather work on tourist activities in this part of the country
Optimization of the extraction process in order to isolate antioxidant compounds from walnut leaves
In the popular perception, nuts were considered to have high content of fats and therefore were seen as unhealthy foods, which are indicted in different cardio-vascular diseases or diabetes. This perception has changed after the lately researches, which proved their healthy fatty acid profile of the walnut and its products. The walnut (Juglans regia L.) consumption is now associated with a reduced risk of coronary heart diseases, cancer and all other oxidative stress mediated diseases. Different studies had shown that the walnut leaves contain several phenolic compounds which contribute to their biological properties. Researchers are focused on walnut because it contains an important phenolic compound called juglone and it is used in the treatment of inflammatory and infectious diseases. They also inhibit the Gram positive and Gram negative bacteria and also fungi. The present research has the priority to establish the proper method of extraction using walnut leaves and a mixture of solvents in different volumetric proportions. Primary the walnut leaves were extracted with a mixture of ethanol and water in proportions 50:50; 60:40; 70:30; 80:20. After the extracts were obtained, they were concentrated and the final extract was used to analysis
Strain-gradient mediated local conduction in strained bismuth ferrite films
It has been recently shown that the strain gradient is able to separate the light-excited electron-hole pairs in semiconductors, but how it affects the photoelectric properties of the photo-active materials remains an open question. Here, we demonstrate the critical role of the strain gradient in mediating local photoelectric properties in the strained BiFeO3 thin films by systematically characterizing the local conduction with nanometre lateral resolution in both dark and illuminated conditions. Due to the giant strain gradient manifested at the morphotropic phase boundaries, the associated flexo-photovoltaic effect induces on one side an enhanced photoconduction in the R-phase, and on the other side a negative photoconductivity in the morphotropic [Formula: see text]-phase. This work offers insight and implication of the strain gradient on the electronic properties in both optoelectronic and photovoltaic devices
Artefacts in geometric phase analysis of compound materials
The geometric phase analysis (GPA) algorithm is known as a robust and
straightforward technique that can be used to measure lattice strains in high
resolution transmission electron microscope (TEM) images. It is also attractive
for analysis of aberration-corrected scanning TEM (ac-STEM) images that resolve
every atom column, since it uses Fourier transforms and does not require
real-space peak detection and assignment to appropriate sublattices. Here it is
demonstrated that in ac-STEM images of compound materials (i.e. with more than
one atom per unit cell) an additional phase is present in the Fourier
transform. If the structure changes from one area to another in the image (e.g.
across an interface), the change in this additional phase will appear as a
strain in conventional GPA, even if there is no lattice strain. Strategies to
avoid this pitfall are outlined.Comment: 9 pages, 7 figures, Preprint before review, submitted to
Ultramicroscopy 7 April 201
Metal-Ferroelectric-Metal heterostructures with Schottky contacts I. Influence of the ferroelectric properties
A model for Metal-Ferroelectric-Metal structures with Schottky contacts is
proposed. The model adapts the general theories of metal-semiconductor
rectifying contacts for the particular case of metal-ferroelectric contact by
introducing: the ferroelectric polarization as a sheet of surface charge
located at a finite distance from the electrode interface; a deep trapping
level of high concentration; the static and dynamic values of the dielectric
constant. Consequences of the proposed model on relevant quantities of the
Schottky contact such as built-in voltage, charge density and depletion width,
as well as on the interpretation of the current-voltage and capacitance-voltage
characteristics are discussed in detail.Comment: 14 pages with 4 figures, manuscript under revision at Journal of
Applied Physics for more than 1 year (submitted May 2004, first revision
September 2004, second revision May 2005
A simple model of ac hopping surface conductivity in ionic liquids
The boundary conditions proposed to discuss the charge exchange taking place in an ionic liquid in contact with non-blocking electrodes are reconsidered in a dynamic situation. Assuming that the variation of the bulk ionic current density depends linearly on the surface value of the ionic current density, the frequency dependence of the phenomenological parameter is determined. The analysis has been performed in the framework where the relaxation times are smaller than a maximum relaxation time Ï„M, and that the response function is independent on the value of the relaxation time. Using simple physical considerations, an expression for the surface conductivity describing the ionic charge exchange at the electrode is obtained. According to our calculations, its frequency dependence is similar to that predicted for the electric conductivity in disordered materials when the mechanism is of the hopping type. From measurements of impedance spectroscopy, by the best fit of the experimental data, the temperature dependence of the hopping time, of the dc surface conductivity, and of the diffusion coefficient are derived. They are in good agreement with the theoretical predictions obtained with the random distribution of surface energy barrier. Keywords: Ionic liquids, Non-blocking electrodes, Electrical impedance spectroscopy, AC hopping surface conductivit
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