Sharif Digital Repository

Searching for images of the same object or scene in a large number of images is a major problem in computer vision. It has many applications specially in the search engines.For the goal of efficient image search, we need descriptors that are not only discriminative, but also short and need small amount of memory.In this thesis we analyze the image search methods in two categories: The first methods are based on converting the existed descriptors such as gist into a compact binary code. The second methods are based on building short descriptors, especially by some modifications in the framework of bag of features descriptor.Finally we will introduce a novel descriptor "bag of codes" which... 

In the first part of thesis, the electropolymerization of pyrrole was performed in the presence of new coccine (NC) as a dopant anion on the surface of the electrode precoated with SWCNT. The modified electrode was used to study the voltammetric response of salbutamol (SAL). The results showed a remarkable increase in the anodic peak current of SAL in comparison to the bare GCE. The surface morphology PPY/CNT/GCE was thoroughly characterized by scanning electron microscopy (SEM) and cyclic voltammetry (CV) techniques. Under the optimized analysis conditions, the modified electrode showed two linear dynamic ranges of 0.02 – 0.1 μM and 0.1-10 μM with a detection limit of 6 nM for the... 

The project scheduling problem is one of the classic problems in project management that includes a range of optimization problems. One of the limitations we face in project scheduling problem is the resource constraint limitation. In many studies in the literature review, problems are based on the false assumption that a resource can be transferred between activities of a project or different projects without spending time and cost. The resource availability method in many fundamental researches in the context of resource constrained project scheduling problem (RCPSP) is considered as resource investment problems (RIP); while in many real world projects resources are rented for projects and... 

In this thesis the effects of impurities and different defects on electrical and magnetic properties of monolayer and bilayer graphene is investigated. Calculations are performed using density function theory. Alloying monolayer graphene with boron and nitrogen the energy gaps of 0.7 and 0.6 eV are obtained, respectively. Moreover, we show that BN is a good candidate for opening the band gap in graphene. Alloying monolayer graphene with BN the energy gap of 1.2 eV is obtained. Another way of opening band gap is to somehow break the symmetry between layers of the bilayer graphene. By alloying one layers of bilayer graphene with B, N or B-N, although the symmetry of layers is broken, each... 

In this work, we have simulated graphene nanoribbon based photodetector and impact of changing the width, and Boron-Nitride doping and Stone-Wales defects on the optical properties of GNR has study. The energy band structure of GNR with nearest-neighbor approximation in a tight binding model calculated. To correspond with experiment and accuracy, we need to consider the impact of third nearest neighbors and edge bond relaxation. By Using the band structure, we calculated joint density of state and optical matrix elements and obtain inter band selection rule for A-GNR and Z-GNR. Then, using the Fermi’s golden rule, we investigate optical properties of GNR such as optical conductivity and... 

In this thesis, a field effect transistor (FET) using armchair graphene nanoribbon as the channel is simulated, and the effects of changing nanoribbon width and length, as well as adding defects, are also studied. To obtain the Hamiltonian matrix and the energy band structure of graphene nanoribbon, tight binding method is used in which the first and third neighbor approximation is considered. Also, to maximize accuracy, we also considered the edge bond reconstruction. To obtain the transport characteristics of the transistor, such as the transmission coefficient and the density of states (DOS), Poisson and Schrodinger equations are solved self-consistently. We used the nonequilibrium... 

Magnetization have an important role in spintronic devices based on graphene. In this study the effect of stone-wales and vacancy defect on electrical and magnetic properties of zigzag and armchair graphene nanoribbons were investigated. Calculations are performed using tight-binding and hubard model combining with Green's function techniques. When the balance between A and B sub-lattices gets disturbed, magnetization will be appeared. In stone-wales defect we have local magnetization because of changing the position of sub-lattices and the total magnetization is zero. In addition the stone-wales defect can change and modify the band structures and band gaps. Vacancy of carbon atom in... 

Solar cells are one of the most promising clean and readily available energy sources. Organic solar cells as a new generation of solar cells, have attracted strong attention in recent years, due to the advantages of flexibility, thinness, and simple manufacturing process. This work focuses on the electrical processes in organic solar cells and approaches for enhancing the efficiency of solar cell by employing two-dimensional drift-diffusion model. At the first step. We investigate the role of different parameters such as mobility (considering different recombination mechanisms), active layer thickness, light intensity, barrier injections and energetic disorder on the performance of single... 

Organic photodetectrors(OPD) are new generation of photodetectors that use of organic materials to convert optical signal to electrical current. Organic photodetectors in recent decade have witnessed significant improvements and many recent research allocated to this photodetectors. This OPD’s have important application so that the inorganic photodetectors have some problems in that application , for example: wide area imagination , biomedical sensors , flexible application and . . . .This work focus on the design and simulation of a panchromatic photodetector that use of new material and have high bandwidth and low dark current. One of application of this OPD is to use in optical... 

In this thesis, we simulate and investigate two transistors which both have tunneling FET (TFET) structure but the channel in the first one is graphene nanoribbon (GNR) and in the second one is a monolayer of phosphorene (PML). To describe the quantum systems of the transistors, we use the tight binding method and its models for the channel materials. In case of the PML TFET, based on the behavior of the phosphorene nanoribbon in different widths, we obtain a proper amount for the width to model the infinite-width channel. To extract the channel quantum transport, the Schrodinger-Poisson equations are self-consistently solved. In the next step, we simulate the GNR TFET and present the... 

There are a wide variety of light-triggered switches. Photoconductive semiconductor switches (PCSSs) have been investigated intensively for many applications owing to their unique advantages over other switches. The advantages of PCSSs make them a perfect choice for many important applications where high switching accuracy and high-power capability are required. Photoconductive switches are fabricated from a variety of semiconductors, including silicon carbide (SiC), gallium arsenide (GaAs) and gallium nitride (GaN). In Photoconductive semiconductor switches (PCSSs) the switching mechanism is initiated by optical illumination and laser source controls the flow of current. In the off or... 

One of the different sensing techniques for biosensors is based on surface plasmon resonance which provides high reliability and efficiency. In this project working operating principles of biosensors based on surface plasmon resonance will be examined. For investigation of sensing meachanism we assume that there an array consisting of alternating graphen ribbons which by applying light leads to an effective magnetic field coupling to the localized surface plasmon. The numerical calculations are based on the exact solution of Maxwell's equation using Fuorier Model Method and Matlab coding. In these calculations, the coefficients of transmission, reflection and absorption of light versus... 

This thesis completely introduses ferromagnetic single electron transistor while that identifies the application and types of the transistor.Master equation is used for simulationing of the transistor. This simulation is divided to two general sections.In the first section proposed thermal equilibrium the normal island and The two spin subsystems , however, are in thermal equilibrium and quantized nature of energy spectrum of a small central electrode and fluctuations in the spin accumulation were ignored , however, we simplify the problem and assume that the charging energy are independent of the electron distribution, and number of electrons in the normal island. ،hat tunnel rates can be... 

Conventionally all objects radiate most of its energy in the from of infrared waves, therefore, infrared photodetectors for detecting objects is important and more study is done on improve the detecting parameters. The proposed structures are developed from bulk to quantum well and quantum dots in recently years. In this work at reason insensitivity to incident light polarization, flexible operation wavelength, high operation temperature, inherent photovoltaic effect, low capture probability we study the structure of quantum dot infrared photodetectors and then for improve the detecting parameters structure of two layers quantum dot separated with a layer thin barrier (bilayer). And then a... 

The measurement of saturated hydraulic conductivity in conventional methods is based on the assumption of non-deformable porous media. However, this assumption is violated in most cases, especially when dealing with highly deformable soils such as collapsible and expansive soils. An alternative is to use flexible wall triaxial devices, yet the deformation characteristics during wetting and sample saturation are missed. This issue is of great importance in soils with considerable amount of volume changes (e.g., problematic soils). It should be noted that errors included in measuring sample volume changes, water inflow/outflow rates, and the creation of sidewall leakage are essential factors... 

In this thesis, the goal is to improve the efficiency of quantum and solar cell. The effect of quantum wells was simulated by Silvaco. Initially, the quantum well has been adjusted to a cell junction GaAs. Then, using quantum wells in cell junction towing and three, improving short-circuit, the returns will be 53.79% and 54.33%. This efficiency is about 10% more than the triple-junction solar cells. To improve the efficiency effects of layer thicknesses, parameters, and the number of quantum wells were studied.Tandem cell is 〖In〗_0.51 〖Ga〗_0.49 P/GaAs, and triple is 〖In〗_0.51 〖Ga〗_0.49 P/GaAs/〖In〗_0.28 〖Ga〗_0.72 As. By adjusting the quantum well and sandwiching the well layers staggered by... 

Graphene, a monolayer carbon-atomic honeycomb lattice crystal, has attracted attention due to its peculiar carrier transport properties owing to the gapless energy spectrum. Due to lack of applicable light source in terahertz (THz) band, further researches to finding materials that are suitable for semiconductor lasers in this band to be continued. In this thesis, the population inversion possibility that is the first provision of laser operation, was analyzed in graphene optical pumping situation. The analysis was done by numerical solving of carrier concentration and energy density relations.As a result of this analysis population inversion is obtained when the stimulation square pulse has... 

In this thesis, a field-effect transistor using armchair bilayer grapheme nanoribbon as channel material in simulated. In first, electronic property of bilayer graphene and bilayer graphene nanoribbons using tight binding are studied. Transport properties of transistor, sush as transsmition and density of carriers, are studied by self-consistently solving poisson equation and Non-Equilibrium Green’s Function. Finally, I-V characteristics of the transistor can be obtained using the Landauer formula.
Also, effect of Stone-Wales defect on bandstructure and transmission of armchair bilayer grapheme nanoribbon is investigated. This defect increases energy gap and broadens egergy bands in...