Ehsan Rahimi

Quasi-classical modeling of molecular quantum-dot cellular automata multidriver gates

Molecular quantum-dot cellular automata (mQCA) has received considerable attention in nanoscience. Unlike the current-based molecular switches, where the digital data is represented by the on/off states of the switches, in mQCA devices, binary information is encoded in charge configuration within molecular redox centers. The mQCA paradigm allows high device density and ultra-low power consumption. Digital mQCA gates are the building blocks of circuits in this paradigm. Design and analysis of these gates require quantum chemical calculations, which are demanding in computer time and memory. Therefore, developing simple models to probe mQCA gates is of paramount importance. We derive a semi-classical model to study the steady-state output polarization of mQCA multidriver gates, directly from the two-state approximation in electron transfer theory. The accuracy and validity of this model are analyzed using full quantum chemistry calculations. A complete set of logic gates, including inverters and minority voters, are implemented to provide an appropriate test bench in the two-dot mQCA regime. We also briefly discuss how the QCADesigner tool could find its application in simulation of mQCA devices.

Characterization of responsivity and quantum efficiency of TiO 2 —Based photodetectors doped with Ag nanoparticles

There is a motivation to reduce the thickness of photodetectore to improve the efficiency of extracting excuted carrier. However, reducing the thickness of thin film also reduces the amount of light absorbed. To alleviate this problem we can use light trapping. Ag Nanoparticles with specific diameters are exposed to improve optical characteristics of TiO2-based photodetectors. Optical properties of photodetectors depend on optical properties of nanoparticles. There for photodetectore application we need to identify which of these properties (scattering, absorption) are beneficial and then design suitable nanoparticle to maximize these effect while minimizing unwanted optical properties. Nanoparticles diameter is usually between 5–100nm. If the diameter of nanoparticles is about more than 100nm, light will not be absorbed efficiently, and when it is smaller than 5nm, charge transition through channels fades and scattering is increased. Effect of nanoparticles on absorption coefficient, quantum efficiency, and responsivity of photodetectors based on TiO2 are discussed in this paper.

Quantum-Dot Cellular ROM: A nano-scale level approach to digital data storage

In todaypsilas world of digital communication, high capacity high performance digital data storage is an ultimate solution to preserve sheer volume of data. Nanotechnology emerges to bring new digital media with high capacity and high performance, where digital data is stored at nano-scale. In this paper a quantum dot cellular automata ROM (QDCROM) has been designed and simulated. This ROM has a very simple structure but great specifications enabling us to store 2.5 GBits of digital data per cm2.

Radius of effect in molecular quantum-dot cellular automata

Molecular quantum-dot cellular automata (mQCA) is a promising technology for building molecular computers, where binary information is represented by the charge configuration within molecular redox centres. Molecular QCA devices are coupled by electric fields and no current flows in the circuits, in theory. The Coulomb forces, which cause electrons to move from one redox centre to another within a cell, may result in faults in neighbouring cells as well. A simple electrostatic model is presented that describes the interactions of double-dot molecules within an effective radius-of-effect (RoE). The proposed model can be applied to approximate the RoE in device simulation, as well as determining a reliable cell-spacing in mQCA circuits as to avoid faults. Compared with full quantum chemistry calculations, which are of high computational cost, this simple model provides a rapid and straightforward tool for application in reliable mQCA device and circuit design.

A Parallelized and Pipelined Datapath to Implement ISODATA Algorithm for Rosette Scan Images on a Reconfigurable Hardware

Unsupervised clustering is a powerful technique that can be used for distinguishing the real target from the false targets such as flares in the images formed by infrared sensors in missiles. The ISODATA is such an algorithm that could be used in infrared guided missiles detectors, since the algorithm itself computes the number of clusters or in other words the number of flares. Although the only drawback of the ISODATA is the extension in the processing time of the algorithm while the missile approaches the target and the number of detected clusters varies frequently, we can still take advantage of the algorithm by speeding up the most time consuming parts. In our approach to identify and locate the time consuming parts of the algorithm, first a profiling on a software implementation of the ISODATA algorithm has been carried out. The results show that over 60 percent of the complete execution time of the algorithm is consumed in computation of the distance from cluster centers. In this paper we propose a pipelined and parallelized datapath for hardware implementation of the algorithm in order to speed up the distance computation process and overcome the problem.

Analysis of Super-Gaussian Ultra-Short Pulse Propagation in Nonlinear Optical Fibers

In todays overwhelming world of data, ultra-wideband communication systems are the inevitable parts of the communication society that has faced scientists with challenging and new problems. Appearance of nonlinear effects in optical fiber communication systems due to wideband data transmissions with the aid of ultra-short pulses has recently attracted a lot of publicity. In this paper, a thorough study of nonlinear effects both in time and frequency domain has been carried out to analyze the propagation of super-Gaussian ultra-short pulses in nonlinear optical fibers. The results show different behavior of nonlinear optical fibers at specific lengths, frequency and levels of nonlinearity. Moreover, with the aid of negative third order dispersion parameter, the frequency shift in the output profile has been successfully compensated.

Dark Current Reduction in ZnO-Based MSM Photodetectors with Interfacial Thin Oxide Layer

In this paper the current transport mechanism of ZnO-based metal-semiconductor-metal ultraviolet photodetectors with various contact electrodes is discussed and simulated. The simulation is based on the thermionic emission theory and tunneling effects. It was found that the lowest dark current attributes to the Ru contact electrode. Moreover, it is shown that in order to achieve a large Schottky barrier height on ZnO and more reduction of dark current, one can insert a thin oxide layer between contacts and ZnO layer. The influence of the thickness of the insulator layer on the dark current of the MIS photodetector has also analyzed.