Physics

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Wet chemical etching is one of the extensively process in the fabrication of optoelectronic devices such as semiconductor lasers. In this paper the GaAs wafer etched in phosphoric acid/ hydrogen peroxide/ methanol solution at 0-4°C by dipping. The results of this experiment were studied by Dek Tak surface profile meter that shows U anisotropic etching, depth 4600 A°, roughness about 17.4 A°. Presence of methanol in the etchant results in easy and controllable etching process.

In designing an AlGaAs diode laser, by selecting different aluminum content and varying quantum well width as active layer. The laser can be operated in optimum condition in any specific wavelength, if optimum values for aluminum content and well width are selected. In this paper, the optimum conditions for laser diode operating at 800nm was investigated. The maximum optical power and minimum threshold current are criterions for optimization process. In our design, the practical semiconductor material growth limitations and optical damage threshold were considered. Finally the optimum quantum well width and aluminum content were found to be 10 nm and 0.085, respectively. In such condition, the threshold current of 545mA and optical power of 2450mW can be achieved.

Thickness and refractive index of waveguide layer determine the electro-optic characteristics in semiconductor lasers. In this research the stepped refractive index profile was replaced by suitable broadened graded refractive index in waveguide layer. The results show such design can significantly improve the vertical divergence and threshold current for laser operating in constant efficiency. In order to preventing of slop efficiency decreasing in broadened waveguide, we proposed the optimum dopant in n-section of waveguide without extra effective free carrier loss.

Laser diode beam divergence is the main parameter for beam shaping and fiber optic coupling. Increasing the waveguide layer thickness is the conventional method to decrease the beam divergence. In this paper, the broadened asymmetric waveguide is introduced to decrease the divergence without increasing the optical power. The asymmetric waveguide was used to shift the vertical optical field to n-section, which has lower free carrier loss. The main target in this research is to minimize the internal loss to avoid the disadvantage of the broadened waveguide. Finally the beam divergence was decreased to 35 degrees that is very suitable for the conventional multi-mode optical fiber coupling and the optical power was increased to 2400mW in the laser diode with 100μm stripe width and 1mm cavity length. In addition to the fiber coupling, this improvement can be used for other direct applications that need beam shaping.

Nowadays a plenty of applications of thin film in different subjects such as scientific and industrial items has been grown. Au deposition is very important in microelectronic device fabrication. In this research project we studied Au sputtering in different energies and different angles of incident ions. Results show that the measured sputtering yield confirms the results that reached by TRIM.SP software. In specific range, sputtering yield increases by increasing energy and incident angle of bombardment ion particles.

Organic-inorganic hybrid perovskite materials have captured many researchers' interest in solar cell application during the last decade. Due to their astonishing photoelectric properties and rapid increase in efficiency, these materials are promising candidates for affordable solar cell technologies and optoelectronic devices. Also, with respect to their high-power conversion efficiency (PCE), cost-effectiveness, gap tunability, longer charge diffusion length, much improved photochemical and thermal stability, formamidinium (FA)-based perovskites are one of the most promising materials as well. However, the easy formation of non-perovskite δ-phase formamidinium at low temperatures and toxic Pb is disadvantageous. It is worth mentioning that various strategies have been developed to stabilize α-phase FAPbI3 at room temperature and decrease the toxicity of lead element while retaining their good performances. The current review highlights the recent theoretical progress in stabilization, bandgap engineering, and optical performance of these promising materials.

We investigate the electronic transport properties of two types of junction based on single polyaromatic hydrocarbons (PAHs) and PAHs embedded in boron nitride (h-BN) nanoribbons, using nonequilibrium Green's functions (NEGF) and density functional theory (DFT). In the PAH junctions, a Fano resonance line shape at the Fermi energy in the transport feature can be clearly seen. In hybrid junctions, structural asymmetries enable interactions between the electronic states, leading to observation of interface-based transport. Our findings reveal that the interface of PAH/h-BN strongly affects the transport properties of the structures.

The need to reduce our reliance on fossil fuels and lessen the environmentally harmful effects of CO2 have encouraged investigations into CO2 hydrogenation to produce useful products. Transition metal carbides exhibit a high propensity towards CO2 activation, which makes them promising candidates as suitable catalysts for CO2 hydrogenation. Here, we have employed calculations based on the density-functional theory to investigate the reaction network for CO2 hydrogenation to product molecules on the tantalum-terminated TaC (111) surface, including two routes from either HCOOH* or HOCOH* intermediates. Detailed calculations of the reaction energies and energy barriers along multiple potential catalytic pathways, along with the exploration of all intermediates, have shown that CH4 is the predominant product yielded through a mechanism involving HCOOH, with a total exothermic reaction energy of −4.24 eV, and energy barriers between intermediates ranging from 0.126 eV to 2.224 eV. Other favorable products are CO and CH3OH, which are also produced via the HCOOH pathway, with total overall reaction energies of −2.55 and −2.10 eV, respectively. Our calculated thermodynamic and kinetic mechanisms that have identified these three predominant products of the CO2 hydrogenation catalyzed by the TaC (111) surface explain our experimental findings, in which methane, carbon monoxide, and methanol have been observed as the major reaction products.

Interatomic potentials, which describe interactions between elements of nanosystems, are crucial in theoretical study of their physical properties. We focus on two well known empirical potentials, i.e. Tersoff's and Brenner's potentials, and compare their performance in calculation of thermal transport in carbon nanotubes. In this way, we study the temperature and diameter dependence of thermal conductivity of single walled armchair carbon nanotube by using the mentioned interatomic potentials. We take advantage of direct non-equilibrium molecular dynamics simulation, which well resembles the experimental set up for thermal conductivity measurement. The results show that increasing the temperature increases the conductivity in contrast with diameter growth which decreases the thermal conductivity. It is important to note that both interatomic potentials describe the system behavior very well, however they lead to different conductivity values. It is found that the difference between the performance of studied potentials can be seen more obviously in longer tubes. We also observe a peak in thermal conductivity by increasing system temperature. System is deformed at T≈1000 K, when Tersoff's potential is employed for description of interactions. While its instability occurs at higher temperature (T≈1600 K), when we try to simulate system by Brenner's potential.

Carbon dioxide (CO2) hydrogenation is an energetic process which could be made more efficient through the use of effective catalysts, for example transition metal carbides. Here, we have employed calculations based on the density functional theory (DFT) to evaluate the reaction processes of CO2 hydrogenation to methane (CH4), carbon monoxide (CO), methanol (CH3OH), formaldehyde (CH2O), and formic acid (HCOOH) over the carbon-terminated niobium carbide (111) surface. First, we have studied the adsorption geometries and energies of 25 different surface-adsorbed species, followed by calculations of all of the elementary steps in the CO2 hydrogenation process. The theoretical findings indicate that the NbC (111) surface has higher catalytic activity towards CO2 methanation, releasing 4.902 eV in energy. CO represents the second-most preferred product, followed by CH3OH, CH2O, and HCOOH, all of which have exothermic reaction energies of 4.107, 2.435, 1.090, and 0.163 eV, respectively. Except for the mechanism that goes through HCOOH to produce CH2O, all favourable hydrogenation reactions lead to desired compounds through the creation of the dihydroxycarbene (HOCOH) intermediate. Along these routes, CH3* hydrogenation to CH4* has the highest endothermic reaction energy of 3.105 eV, while CO production from HCO dehydrogenation causes the highest exothermic reaction energy of −3.049 eV. The surface-adsorbed CO2 hydrogenation intermediates have minimal effect on the electronic structure and interact only weakly with the surface. Our results are consistent with experimental observations.

The increasing demand for clean fuels and sustainable products has attracted much interest in the development of active and selective catalysts for CO conversion to desirable products. This review maps the theoretical progress of the different facets of most commercial catalysts, including Co, Fe, Ni, Rh, and Ru. All relevant elementary steps involving CO dissociation and hydrogenation and their dependence on surface structure, surface coverage, temperature, and pressure are considered. The dominant Fischer–Tropsch synthesis mechanism is also explored, including the sensitivity to the structure of H-assisted CO dissociation and direct CO dissociation. Low-coordinated step sites are shown to enhance catalytic activity and suppress methane formation. The hydrogen adsorption and CO dissociation mechanisms are highly dependent on the surface coverage, in which hydrogen adsorption increases, and the CO insertion mechanism becomes more favorable at high coverages. It is revealed that the chain-growth probability and product selectivity are affected by the type of catalyst and its structure as well as the applied temperature and pressure.

The need to reduce our reliance on fossil fuels and lessen the environmentally harmful effects of CO2 have encouraged investigations into CO2 hydrogenation to produce useful products. Transition metal carbides exhibit a high propensity towards CO2 activation, which makes them promising candidates as suitable catalysts for CO2 hydrogenation. Here, we have employed calculations based on the density-functional theory to investigate the reaction network for CO2 hydrogenation to product molecules on the tantalum-terminated TaC (111) surface, including two routes from either HCOOH* or HOCOH* intermediates. Detailed calculations of the reaction energies and energy barriers along multiple potential catalytic pathways, along with the exploration of all intermediates, have shown that CH4 is the predominant product yielded through a mechanism involving HCOOH, with a total exothermic reaction energy of −4.24 eV, and energy barriers between intermediates ranging from 0.126 eV to 2.224 eV. Other ...

The electrical conductance of zigzag graphene nanoribbons (ZGNRs) is numerically investigated in the presence of different percentages of edge and middle vacancies. The vacancies are randomly distributed in the edge or innermost of ZGNRs. We take advantage of recursive Green's function under tight-binding model to study the electrical conductance. It is found that in the both vacancy types, and all widths and lengths of our samples, the conductance degrades as the vacancy concentration increases. In addition, we illustrate how transport properties are sensitive to the width and length of nanoribbon. At fixed defect concentration, the length growth leads to exponential decrease, and the width growth leads to power law increase in the conductance of ZGNRs, respectively. Under the same numerical framework, we have also studied transition from metal to insulator and vice versa.

Ranking the words in human written texts, according to their relevance to text context, plays a crucial role in many text mining tasks. Highly relevant words concentrate in some limited areas, while the irrelevant ones have nearly random spatial distribution throughout the text. But in the randomly shuffled version of the text, all word types are distributed at random. The difference between spatial distribution of words in the original version of a text and its shuffled version seems a proper criterion for word relevance ranking. In this procedure, spatial distribution of each word type in the document is defined by box counting method. Then we apply Jensen-Shannon divergence to measure the difference between probability distributions of each word in the original text and its shuffled version. This metric properly distinguishes relevant words from irrelevants without requiring any previous knowledge about text structure.

The electrical conductance of two kinds of graphene nanoflakes (GNFs) is studied numerically, using nonequilibrium Green's function. We perform the calculations on bowtie and rhombus GNFs within the nearest neighbor tight binding model. Our findings reveal the sensitivity of conductance on the orientation of zigzag edges. The results show that the conductance of the bowtie and rhombus GNFs could be tuned via enlarging the system size. Moreover, we obtain different results for conductance of the mentioned GNFs and rectangle zigzag GNF with the same size.

Electron transport in graphene/h-BN lateral hybrids with rhombus and bowtie domains is investigated. We apply non-equilibrium Green's function method under tight binding approximation to calculate electrical transmission in mentioned hybrid monolayers. Our findings reveal the importance of size and shape of doped regions in the transport through hybrid systems. Generally, we find that boron and nitrogen doping in zigzag graphene nanoflake, with both rhombus and bowtie patterns, improves its conductance. Growing size of the doped segments, in fixed system size, leads to a stable conductance level. Moreover, carbon doping in h-BN can also make a tunable semiconducting behavior. In the latter case, when the system size is kept constant, enlarging the doped carbon domains exponentially rise up the transmission.

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Highlights  Non-extensive statistical mechanics appropriately describes complex systems.  We apply Tsallis entropy to ranks the terms' relevance to document subject.  Experimental evaluations confirm the capability of this metric in keyword detection.  This metric has reliable word ranking results in comparison with previous methods.

Zipf's law, as a power-law regularity, confirms long-range correlations between the elements in natural and artificial systems. In this article, this law is evaluated for one hundred live languages. We calculate Zipf's exponent for translations of the holy Bible to several languages, for this purpose. The results show that, the average of Zipf's exponent in studied texts is slightly above unity. All studied languages in some families have Zipf's exponent lower/higher than unity. It seems that geographical distribution impresses the communication between speakers of different languages in a language family, and affect similarity between their Zipf's exponent. The Bible has unique concept regardless of its language, but the discrepancy in grammatical rules and syntactic regularities in applying stop words to make sentences and imply a certain concept, lead to difference in Zipf's exponent for various languages.

The construction of hybrid graphene/hexagonal boron nitride (hBN) nanodevices with applications in sensing could make it possible to tailor hybrid nanogap architectures. Here, we explore the advantages of hybrid graphene/hexagonal boron nitride nanogaps to improve the sensing of DNA nucleobases compared to graphene nanogaps. To this end, ab‐initio calculations are employed to investigate both the electronic properties of different graphene/hBN hybrids and the transport properties of DNA nucleobases located between aforementioned hybrid nanogaps. Our results show that hybrid nanogaps with boron interfaces shift the HOMO toward the Fermi energy leading to a better transmission coefficient in the vicinity of the Fermi energy level. In addition, nanogaps with boron interfaces and narrower graphene nanoroads enhance the sensitivity at and around the Fermi energy level for all nucleobases, especially for small ones.