مجله پژوهش فیزیک ایران
سال هفدهم شماره 3 (تابستان 1396)

Hybrid solar cells combine organic and inorganic materials with the aim of utilizing the low cost cell production of organic photovoltaics (OPV) as well as obtaining other advantages, such as tuneable absorption spectra, from the inorganic component. Whilst hybrid solar cells have the potential to achieve high power conversion efficiencies (PCE), currently obtained efficiencies are quite low. The design of the inorganic material used as the electron acceptor in hybrid solar cells, particularly the electronic structure, is crucial to the performance of the device. Hence, this paper, nanorods ZnO have been synthesized by using zinc acetate dihydrate and polyvinylpyrrolidone (PVP) as starting materials. The calcined powders in air at 600 oC for 1 hour have been characterized by XRD, TEM and SEM. So, with nanorods ZnO as electrode in inverted polymer solar cells,the average performance of devices with open circuit voltage ,short circuit current density, fill factor,and power conversion efficiency are measured are measured as 0.61V, 8.7 mA/cm2, 0.58 and 3.01%, respectively. The results indicate that the structure of ZnO nanorod can effectively serve as an electrode for inverted polymer solar cells.

In the present work, the transition matrix elements as well as differential and total scattering cross-sections for positronium formation in Positron-Hydrogen atom collision and hydrogen formation in Positronium-Hydrogen ion collision, through the charge transfer channel by Two-Centre Close-Coupling method up to a first order approximation have been calculated. The charge transfer collision is assumed to be a three-body reaction, while the projectile is a plane wave. Additionally, the hydrogen and positronium atoms are assumed, initially, to be in their ground states. For the case of charge transfer in the scattering of positron by hydrogen atoms, the differential cross sections are plotted for the energy range of 50eV to 10keV, where the Thomas peak is clearly observable. Finally, the total scattering cross-section for the charge transfer in the collision of Positron-Hydrogen and Positronium-Hydrogen ion are plotted as a function of projectile energies and compared with other methods in the literature.

In this research, barium hexaferrite samples with BaZn0.6Zr0.3X0.3Fe10.8O19 (X=Ti,Ce,Sn) composition were synthesized via mechanical activation method and were evaluated by simultaneous thermal analysis (STA), X-ray diffraction (XRD), field emission electron microscopy (FE-SEM) and vibrating sample magnetometer (VSM). All of the synthesized samples were almost single phase and with average particles size of about 450 nm and 250 nm for samples without and with dopant respectively. Significant change in magnetic properties of barium hexaferrite were observed with effect of substitution of Fe ions. According to the results maximum magnetic saturation (33.1 emu/g) and minimum coercivity force (8.14 Oe) were related to samples with composition of BaZn0.6Zr0.3Ti0.3Fe10.8O19 and BaZn0.6Zr0.3Sn0.3Fe10.8O19 respectively.

According to the suddenly compression of the matters in some regions of the compressible fluids, the density and temperature suddenly increases, and shockwaves can be produced. The cooling of post-shock region and non-idealness of the equation of state, $p=(k_B/mu m_p)rho T (1溸) equivmathcal{K}rho T (1竖 R)$, where $mu m_p$ is the relative density of the post-shock gas and $Requiv rho_2 / rho_1$ is the non-idealness parameter, may affect on the shocked gases. In this article, we study the effects of both cooling timescale and non-idealness of the shocked gases, on the relative density of the post-shock region. For simplicity, the shock is assumed planar and steady in which the deceleration is negligible and there is no any instabilities through the cooling layer. Conservation of mass, momentum, and energy across the shock front are given by the Rankine-Hugoniot conditions. The most important factor through the shock is the energy lost per unit mass during the shock process, $Q=frac{n_2 Lambda}{mu_2 m_p} t_{dur}$, where $Lambda (erg cm^{-3} s^{-1}$ is the cooling function at the post-shock region with density $n_2} and mean particle mass $mu_2 m_p$, and $t_{dur}$ is the duration time of the post-shock process. Accurate determination of the cooling timescale requires specifying the elemental abundance of the post-shock region, but a simple estimate can be obtained using $t_{cool}approx k_B T_2/(n_2Lambda)$. Eliminating the $n_2 Lambda$, we approximately have $Q/c^2approx lambda T$, where $c equiv sqrt{K_1 T_1}$ is the pre-shock sound speed, $lambda equiv t_{dur}/t_{cool}$ and $T equiv K_2 T_2/K_1 T_1$.
We would be interested to consider the collision of two gas sheets with velocities $v_0$ in the rest frame of the laboratory. Defining the Mach number as $M_0 equiv v_0/c$, we obtain a third degree polynomial equation for $R$, with coefficients as functions of the three parameters $eta$, $lambda$, and $M_0$. We numerically solved this three degree polynomial equation to obtain $R$.The results for adiabatic case ($lambda=0$), with ideal ($eta=0$) and non-ideal ($eta neq 0$) mono-atomic ($gamma_1 = gamma_2 = 5/3$) gas are shown in the Fig.1. In the ideal case, the strong supersonic shockwave ($M_0 rightarrow infty$) leads to $R approx 4$. Considering of non-ideal parameter ($eta neq 0$) increases the pressure of the post-shock region so that the shock fronts move faster. In this way, for each $M_0$, the relative density of the post-shock non-ideal gas decreases in respect to the ideal case. The cooling shockwaves with low cooling timescale ($lambda=1$) and fast cooling timescale ($lambda=10$) are shown in the Fig.2. The results show that the relative density of post-shock gas, $R$, increases with increasing the Mach number, $M_0$, and asymptotically reaches to a value which depends on the two other parameters $eta$ and $lambda$. With increasing of the energy lost per unit mass during the shock process, $Q$ (i.e., increasing of $lambda$), the post-shock gas has more chance for condensation and increasing of its relative density, while including the non-ideal effects (i.e., increasing of $eta$) reduces this chance.

In this article, M2 factor of a thin disk laser with unstable resonator in two regimes is calculated numerically. In the first case, thermal effects are ignored and the parameters of beam width, divergence angle, radius of wave front curvature and finally, M2 factor of laser beam are calculated by using generalized beam parameters. These calculations show that the beam quality of the laser is dependent on the resonator magnification parameter in disk laser. In the second case, thermal effects are considered. In this regime, by using analytical formula for distribution of temperature in crystal and the main contributors to the OPD, M2 factor of thin disk laser is calculated numerically. When the thermal effects are considered, calculations show that the beam quality of thin disk laser is degraded in respect to the condition in which thermal effect was ignored, and also the output power is reduced extremely.

In this investigation nickel-cadmium ferrite nanoparticle with stoichiometric composition of Ni0.3Cd0.7Fe2O4 was synthesized by Sol-gel auto- combustion method. In order to study the effect of particle size on physical properties of samples, the powder samples was annealed at temperatures 350, 400, 450 and 500 ○ C for 3h. Structural, morphological and magnetic properties of samples were analyzed using X- ray diffraction (XRD), field emission scanning electron microscope (FESEM), vibrating sample magnetometer (VSM) and ac susceptibility. XRD data revealed spinel mono-phase formation and crystallite size was estimated in the range of 17- 35 nm, using sherrer’s equation which also confirmed by FESEM. The VSM results indicate that magnetization increases by increasing particle size. Using the results of ac susceptibility measurements and analysis by the Neel- Brown, Vogel-Fulcher and critical slowing down methods, indicates that the samples annealed at temperatures of 350 and 400 ○ C are super-paramagnet at room temperature and have super-spin glass behavior at low temperatures.

In the frame work of relativistic density functional theory, using full potential local orbital band structure scheme (FPLO), the magnetic properties of single 3d transition metals (3d-TM) adsorbed on 2D hexagonal boron nitride (2D h-BN) are investigated. Binding energies between 3d-TM adatoms and 2D h-BN in three different compositions, local spin magnetic moments of 3d-TM and total spin magnetic moments per supercell, orbital magnetic moments and spin orbit coupling energies are calculated. In this study, three different magnetic relativistic methods the so-called scalar relativistic (SR), full relativistic (FR) and full relativistic plus an orbital polarization correction (OPC) are used. Results of nonmagnetic binding energies in the nonmagnetic SR method indicate that with the exception of Sc other 3d-TM adatoms can bind to BN surface. While, the results of magnetic binding energies in the spin-polarized SR approach show that Sc, Cr and Mn cannot bind on the surface of 2D h-BN. In addition, there is shown that the behavior of spin magnetic moments of 3d-TM adatoms are depended on their geometric positions due to their different crystal fields. Moreover, it is shown that Co in the top of N atoms and Fe adatoms in the top of B atoms with 1.23 (1.92) and 0.89 (1.72 ) have a large orbital magnetic moments in the FR(OPC) approaches due to their massive spin-orbit coupling effects, respectively. These so large values of orbital magnetic moments are promising the existence of large magnetic anisotropy energies.

In this work,the physical properties of KTP and RTP single-crystals have been investigated by performing accurate total energy calculations in the framework of density functional theory by using the full-potential linearized augmented plane wave method. The effects of Rb substitution on structural, electronic and optical properties of KTP are discussed. The structural properties have been calculated by using different exchange correlation including LDA, PBE, WC and PBEsol. Also PBEsol approximation and and more accurate approximation mBJ are employed to calculate the energy gap values. The Pseudoinversion values of both crystals have been calculated by using PseudoSymmetry software . Rb substitution effect on pseudosymmetry of KTP and also relation between second-order susceptibility of crystals and the Pseudoinversion values are discussed. The optical coefficients such as refractive index, birefringence values and absorption coefficients have been calculated by using the dielectric function. The anisotropy in the linear optical properties of KTP and RTP crystals have been demonstrated. Then calculated results have been compared.

A comparative study of magnetic properties of MnFe2O4 ferrite nanoparticles prepared by two different methods has been reported. The first sample (S1) was synthesized by thermal decomposition of metal nitrates. And the second sample (S2) was prepared by solvothermal method using Tri-ethylene glycol (TEG). Magnetic hysteresis loops at 300 and 5 K; magnetization and AC susceptibility measurements versus temperature confirmed the effective role of TEG on the magnetic properties of nanoparticles. The results showed that, at 300 K the saturation magnetization (MS) of S2 sample is 46% greater than that of S1 sample. At 5 K, the difference in MS of the samples raised to 60%. AC susceptibility measurements at different frequencies and also magnetization versus temperature under field cooling and zero field cooling processes revealed that, the TEG molecules influence the surface spins order of S2 sample. The sample S1 showed strongly interacting superspin glass state, while the sample S2 consists of weakly interacting superparamagnetic nanoparticles.

In this study, the unsteady double-diffusive mixed convection in the one and two-sided lid-driven cavities are studied and compared together using numerical SIMPLE algorithm. Uniform but different temperatures and mass concentrations are assumed at horizontal walls. The used fluid (pure air) was assumed incompressible and Newtonian. The used numerical method is firstly validated against previously published numerical results. Then, the numerical simulations were carried out for almost a wide range of Richardson number, whereby the study of various convection regimes would be possible. Results show that the convection heat and mass transfer are reduced with increasing Richardson number. The conductive mode of heat transfer is enhanced by increasing Ri value, so cavities are quasi-conductive domains when fluid flow was dominated by free convection. It was also observed that the heat and mass are better transferred in two-sided lid-driven cavities with respect to the other ones which was due to the extra mechanical forces imposed by the two-sided lids movement. The velocity profile variations demonstrate that the flow is decelerated with increasing Richardson number especially near the bottom horizontal wall. It has also been found that both of entropy generation and total kinetic energy reduce as either Richardson number enhances or two-sided lids movement reduces to one-sided lid movement.

In this paper, we used green's function approach in microscopic theory to investigate a resonant tunneling diode (RTD). We introduced the detailed Hamiltonian for each part of the photovoltaic p-i-n system, then by calculating the green's function components in tight-binding approximation, we calculate local density of states and current-voltage characteristic of the p-i-n structure. Our results show a non-Ohmic behavior and negative differential resistance in RTD. As a result of a longitudinal electric field, the local density of states varies by changing the applied potential. Moreover, we study the effect of changing the physical parameters on the current of the device. Entering quantum dots in the middle of device causes a negative differential resistance, which is a consequence of resonant tunneling phenomenon.

In recent years, due to electron transport properties of nanostructures based on carbon nanotubes, a lot of attention to design electronic devices in the field of nanotechnology has attracted. There are three types of carbon nanotubes in zigzag, armchair and chiral (asymmetrical) forms. Since the types of armchair are electrically conductive, by a combination with a metal such as zinc can be achieved by various means distinct applications. In this respect, we select different layers of circular connectors on the number of atoms of 10, 20 and 30, respectively, in the systems A-Zn10-A, A-Zn20-A and A-Zn30-A, where (A: armchair). Our calculations are based on the Green's function method within tight-binding approximation in the nearest neighbors in the framework of Landauer. The results are able to predict that devices with different functions such as quantum conductor wire, negative differential resistance and rectifier design. The results may be useful in the design of electronic devices at the nanometer scale.

The first phase of the Alborz Observatory Array (Alborz-1) consists of 20 plastic scintillation detectors each one with surface area of 0.25 spread over an area of realized to the study of Extensive Air Showers around the knee at the Sharif University of Technology campus. The first stage of the project including construction and operation of a prototype system has now been completed and the electronics that will be used in the array instrument has been tested under field conditions. In order to achieve a realistic estimate of the array performance, a large number of simulated CORSIKA showers have been used. In the present work, theoretical results obtained in the study of different array layouts and trigger conditions are described. Using Monte Carlo simulations of showers the rate of detected events per day and the trigger probability functions, i.e., the probability for an extensive air shower to trigger a ground based array as a function of the shower core distance to the center of array are presented for energies above 1 TeV and zenith angles up to . Moreover, the angular resolution of the Alborz-1 array is obtained.
For experimental study of the array, Alborz-1 sub-array consists of 5 detectors on a pentagon configuration similar to the central cluster of the Alborz-1 array have been collecting data since 2014 February for 14 month in 4th floor of physics department at Sharif University of Technology. Alborz-I, made of 20 scintillation detectors is set up in a cluster layout to study the cosmic ray spectrum in the energy range of 1012 to 1016 eV. . This paper reveals the zenith angle distribution function of detected air showers by this sub-array.

We study tunneling conductance of a graphene based normal metal-insulator-superconductor (NIS) junction with Corbino disk structure. Solving Dirac-Bogolioubov- De Gennes (DBdG) equation in different regions of the junction and employing scattering approach we obtain normal and Andreev reflection coefficients of the junction. Using Blonder-Tinkham-Klapwijk (BTK) formula we calculate tunneling conductance of the junction as a function of the barrier strength of insulating region. The obtained results show that tunneling conductance of the junction oscillates as a function of the barrier strength as in the planar structure case. The tunneling conductance shows maximums at resonances which have a pi/2 phase shift with respect to the planar structure.

The melting of zigzag, armchair and chiral single walled boron nitride nanotubes (SWBNs) investigated using molecular dynamics (MD) simulation based on Tersoff-like many body potential. The MD simulation has been employed in the constant pressure, constant temperature (NPT) ensemble. The temperature and pressure of the system were controlled by Nose-Hoover thermostat and Berendsen barostat, respectively. We have computed the variation of the melting temperature with the radius of BN nanotube. The results show that the melting temperature of nanotubes increase with increasing in the size of radii, but this dependence is not the same for the various chiral angle of nanotubes. The relation of the melting point with radius for three types of nanotubes i.e. zigzag, armchair and chiral obtained. Moreover, our results show that the melting temperature of nanotubes approach a constant value at larger radii.

In this research, CdS and PbS quantum dots were applied as the light sensitizers in TiO2 based nanostructured solar cells. The PbS quantum dots could absorb a wide range of the sunlight spectrum on earth due to their low bandgap energy. As a result, the cell sensitization is more effective by application of both CdS and PbS quantum dots sensitizers. The TiO2 nanocrystals were synthesized through a hydrothermal process and deposited on FTO glass substrates as the photoanode scaffold. Then PbS quantum dots were grown on the surface of this nanocrystalline layer by a successive ionic layer adsorption and reaction (SILAR) method. The CdS quantum dots were over-grown in the next step through a similar deposition method. Finally this sensitized layer was applied as the photoelectrode of the corresponding quantum dot sensitized solar cells. The results demonstrated that the maximum efficiency was achieved for the cell with a photoanode made of co-sensitization through 2 and 6 cycles of PbS and CdS deposition, respectively. The photovoltaic parameters of this cell were measured as Jsc of 10.81 mA/cm2, Voc of 590 mv and energy conversion efficiency of 2.7±0.2%