Journal of Physics D: Applied Physics

Tu, Wei-Chen; Huang, Chun-Ying; Fang, Chang-Wen; Lin, Ming-Yi; Lee, Wen-Chieh; Liu, Xiang-Sheng; Uen, Wu-Yih

doi: 10.1088/0022-3727/49/49/49LT02pmid: N/A

Developing a transparent and cost-effective electrode for a textured and large-scale optoelectronic device is an important requirement for high-throughput products. Here, we propose a costly fabrication procedure using reduced graphene oxide (rGO) hybrid materials composed of rGO, Au nanoparticles (AuNPs) and Ag nanowires (AgNWs) top electrodes for structured Si solar cells via a spin coating method. This work overcomes the obstacle of graphene damage during the transferred process and provides a simple way to form large-scale graphene-based films on textured surfaces. Due to the spin-coated rGO being uniform along with AgNW frameworks and plasmonic AuNPs, the pyramidal Si solar cell exhibits a significant improved efficiency of 10.75% compared with solar cells using pure rGO flakes as the top electrodes. Our study realizes the rGO hybrid materials deposited on a textured surface and has great potential for integration into transparent and structured devices for next-generation industrial production.

Liu, Dingxin; Yang, Aijun; Wang, Xiaohua; Chen, Chen; Rong, Mingzhe; Kong, Michael G

doi: 10.1088/0022-3727/49/49/49LT01pmid: N/A

In this letter, a 1D fluid model has been used to study the electron heating and particle transport in dual frequency atmospheric-pressure helium capacitive discharge with a high-frequency (HF) voltage of 10 MHz and a low-frequency (LF) voltage of 1 MHz. The electric field is decoupled to three components: the HF, the LF and the direct current (DC) ones, and they have much different effects on the plasmas. The eletrons in plasma bulk are mainly heated by the HF electric field, while in plasma sheath they are heated and cooled by the LF and DC electric fields, respectively. With a fixed total input power, the increase of LF power leads to great enhancement of the electrode fluxes of electrons and ions, especially for the energetic electrons of Te  >  2 eV, because more power is dissipated in the vicinity of electrodes and the inelastic collision is more pronounced. Therefore, the particle transport on the treated sample can be greatly enhanced without additional gas heating in dual frequency plasmas, which meets the application requirements more compared to the single frequency plasmas.

Shigeta, Masaya

doi: 10.1088/0022-3727/49/49/493001pmid: N/A

This article presents a discussion of the ideas for modelling turbulent thermal plasma flows, reviewing the challenges, efforts, and state-of-the-art simulations. Demonstrative simulations are also performed to present the importance of numerical methods as well as physical models to express turbulent features. A large eddy simulation has been applied to turbulent thermal plasma flows to treat time-dependent and 3D motions of multi-scale eddies. Sub-grid scale models to be used should be able to express not only turbulent but also laminar states because both states co-exist in and around thermal plasmas which have large variations of density as well as transport properties under low Mach-number conditions. Suitable solution algorithms and differencing schemes must be chosen and combined appropriately to capture multi-scale eddies and steep gradients of temperature and chemical species, which are turbulent features of thermal plasma flows with locally variable Reynolds and Mach numbers. Several simulations using different methods under different conditions show commonly that high-temperature plasma regions exhibit less turbulent structures, with only large eddies, whereas low-temperature regions tend to be more turbulent, with numerous small eddies. These numerical results agree with both theoretical insight and photographs that show the characteristics of eddies. Results also show that a turbulence transition of a thermal plasma jet through a generation-breakup process of eddies in a torch is dominated by fluid dynamic instability after ejection rather than non-uniform or unsteady phenomena.

Yan, Xin; Cao, Penghui; Tao, Weiwei; Sharma, Pradeep; Park, Harold S

doi: 10.1088/0022-3727/49/49/493002pmid: N/A

Modeling physical phenomena with atomistic fidelity and at laboratory timescales is one of the holy grails of computational materials science. Conventional molecular dynamics (MD) simulations enable the elucidation of an astonishing array of phenomena inherent in the mechanical and chemical behavior of materials. However, conventional MD, with our current computational modalities, is incapable of resolving timescales longer than microseconds (at best). In this short review article, we briefly review a recently proposed approach—the so-called autonomous basin climbing (ABC) method—that in certain instances can provide valuable information on slow timescale processes. We provide a general summary of the principles underlying the ABC approach, with emphasis on recent methodological developments enabling the study of mechanically-driven processes at slow (experimental) strain rates and timescales. Specifically, we show that by combining a strong physical understanding of the underlying phenomena, kinetic Monte Carlo, transition state theory and minimum energy pathway methods, the ABC method has been found to be useful in a variety of mechanically-driven problems ranging from the prediction of creep-behavior in metals, constitutive laws for grain boundary sliding, void nucleation rates, diffusion in amorphous materials to protein unfolding. Aside from reviewing the basic ideas underlying this approach, we emphasize some of the key challenges encountered in our own personal research work and suggest future research avenues for exploration.

Zablotskii, V; Lunov, O; Kubinova, S; Polyakova, T; Sykova, E; Dejneka, A

doi: 10.1088/0022-3727/49/49/493003pmid: N/A

A general interest in biomagnetic effects is related to fundamental studies of the influence of magnetic fields on living objects on the cellular and whole organism levels. Emerging technologies offer new directions for the use of high-gradient magnetic fields to control cell machinery and to understand the intracellular biological processes of the emerging field of nanomedicine. In this review we aim at highlighting recent advances made in identifying fundamental mechanisms by which magnetic gradient forces act on cell fate specification and cell differentiation. The review also provides an analysis of the currently available magnetic systems capable of generating magnetic fields with spatial gradients of up to 10 MT m−1, with the focus on their suitability for use in cell therapy. Relationships between experimental factors and underlying biophysical mechanisms and assumptions that would ultimately lead to a deeper understanding of cell machinery and the development of more predictive models for the evaluation of the effects of magnetic fields on cells, tissue and organisms are comprehensively discussed.

Moreno-Ramírez, L M; Blázquez, J S; Law, J Y; Franco, V; Conde, A

doi: 10.1088/0022-3727/49/49/495001pmid: N/A

The determination of the magnetocaloric magnitudes (specific magnetic entropy change, ΔsM, and adiabatic temperature change, ΔTad) from heat capacity (cH) measurements requires measurements performed at very low temperatures (~0 K) or data extrapolation when the low temperature range is unavailable. In this work we analyze the influence on the calculated ΔsM and ΔTad of the usually employed linear extrapolation of cH from the initial measured temperature down to 0 K. Numerical simulations have been performed using the Brillouin equation of state, the Debye model and the Fermi electron statistics to reproduce the magnetic, lattice and electronic subsystems, respectively. It is demonstrated that it is not necessary to reach experimentally temperatures very close to 0 K due to the existence of certain starting temperatures of the experiments, the same for ΔsM and ΔTad, that minimize the error of the results. A procedure is proposed to obtain the experimental magnitudes of ΔsM and ΔTad with a minimum error from cH data limited in temperature. It has been successfully applied to a GdZn alloy and results are compared to those derived from magnetization measurements.

Piccinini, E; Brunetti, R; Bordone, P; Rudan, M; Jacoboni, C

doi: 10.1088/0022-3727/49/49/495101pmid: N/A

The electric response of Ovonic devices to a time-dependent voltage is analysed by means of a charge-transport model previously proposed by the authors. The numerical implementation of the model shows that the features of the I(V) characteristics depend not only upon the external bias but also on more complex effects due to the interplay between intrinsic microscopic relaxation times and the inevitable parasitic elements of the system. Either stable or oscillating solutions are found, according to the position of the load line. The model also allows for speculation on the potential of Ovonic materials in the design of selector devices for two-terminal non-volatile memories.

Tu, Jay F; Rajule, Nilesh; Molian, Pal; Liu, Yi

doi: 10.1088/0022-3727/49/49/495301pmid: N/A

A copper–single-walled carbon nanotube (Cu–SWCNT) metal nanocomposite could be an ideal material if it can substantially improve the strength of copper while preserving the metal’s excellent thermal and electrical properties. However, synthesis of such a nanocomposite is highly challenging, because copper and SWCNTs do not form intermetallic compounds and are insoluble; as a result, there are serious issues regarding wettability and fine dispersion of SWCNTs within the copper matrix. In this paper we present a novel wet process, called the laser surface implantation process (LSI), to synthesize Cu–SWCNT nanocomposites by mixing SWCNTs into molten copper. The LSI process includes drilling several microholes on a copper substrate, filling the microholes with SWCNTs suspended in solution, and melting the copper substrate to create a micro-well of molten copper. The molten copper advances radially outward to engulf the microholes with pre-deposited SWCNTs to form the Cu–SWCNT implant upon solidification. Rapid and non-equilibrium solidification is achieved due to copper’s excellent heat conductivity, so that SWCNTs are locked in position within the copper matrix without agglomerating into large clusters. This wet process is very different from the typical dry processes used in powder metallurgy. Very high hardness improvement, up to 527% over pure copper, was achieved, confirmed by micro-indentation tests, with only a 0.23% SWCNT volume fraction. The nanostructure of the nanocomposite was characterized by TEM imaging, energy-dispersive x-ray spectroscopy mapping and spectroscopy measurements. The SWCNTs were found to be finely dispersed within the copper matrix with cluster sizes in the range of nanometers, achieving the goal of molecular-level mixing.

Qian, H; Tong, H; Zhou, L J; Yan, B H; Ji, H K; Xue, K H; Cheng, X M; Miao, X S

doi: 10.1088/0022-3727/49/49/495302pmid: N/A

We utilized a GeTe/Sb2Te3 superlattice-like structure (SLL) to obtain a lower crystalline work function (WF) than that for GeTe. Electrostatic force microscopy measurements demonstrated the difference in crystalline WF. Due to the lower crystalline WF, the heterojunction diodes based on the SLL obtained a better crystalline electrical performance. We preformed numerical simulation to confirm that the higher number of Te dangling bonds caused by the multilayer interface and grain boundaries in the SLL is the main reason for the decrease in WF. X-ray photoelectron spectroscopy analysis indicated that more Te–O bonds formed in SLL than GeTe after atmospheric annealing. While it is easy for the Te dangling bonds to combine dipoles, more Te dangling bonds exist in SLLs.

Engelhardt, S; Mietschke, M; Molin, C; Gebhardt, S; Fähler, S; Nielsch, K; Hühne, R

doi: 10.1088/0022-3727/49/49/495303pmid: N/A

Epitaxial BaZrxTi1−xO3 (BZTO) thin films with Zr contents of x  =  0, x  =  0.12 and x  =  0.2 were grown by pulsed laser deposition on (0 0 1)-oriented single crystalline SrTiO3 substrates utilizing an additional conducting SrRuO3 buffer layer. It was found that the oxygen pressure during the deposition heavily influences the lattice constants and the microstructure of BZTO. A low of 0.01 mbar gives rise to a significant tetragonal distortion. Texture measurements reveal that an undisturbed epitaxial growth is only achieved for BZTO films prepared in 0.01 mbar oxygen. In contrast, the formation of twins was observed for higher . A detailed microstructural analysis indicates that the sample preparation in low prevents a preferential growth of columnar grains within the BZTO layers and leads to smoother film surfaces. BZTO thin films deposited with optimized deposition parameters show characteristic ferroelectric polarization behavior. The saturation polarization at room temperature declines with increasing Zr content and the characteristic ferroelectric hysteresis diminishes. Temperature-depended measurements of the relative permittivity reveal the existence of a broad transition range and a significant shift of the phase transition temperature to lower values for increasing Zr content.