فهرست مطالب سمیه فتوحی

The present paper investigates the electronic and optical behavior of monolayer gallium sulfide as a transition metal monochalcogenide, doped with IV and V group atoms of the periodic table. The calculations are performed in the SIESTA software package based on density functional theory using the exchange correlation function and the generalized gradient approximations. Analysis of the electronic structure of this material shows that the pure gallium sulfide monolayer has an indirect band gap of 2.3 eV. In order to investigate the effects of impurities on this structure, the impurity atoms of groups IV and V were applied in the position of sulfur and gallium atoms. The impurity concentration of doped structures is 1.14%. The simulation results show that the presence of this impurity, depending on the type of impurity atom and its location, leads to a transition from semiconductor to metal, indirect to direct semiconductor or non-magnetic to magnetic state in this structure. For example, when Sb is doped to the gallium sulfide monolayer in the position of the sulfur atom, this system is magnetic with a direct band gap, while its replacement by the gallium atom preserves the semiconductor nature of the indirect band gap structure with less gap energy. In addition to the electronic properties, the optical properties of the alloyed structure were also analyzed. The results show that the GaS structure of the alloyed alloy with elements of the fourth and fifth groups opens the way for nanoelectronics, optoelectronics and spintronics applications.

In this paper, the electronic transport properties of graphene-like borophene as well as its doped structures with boron, carbon and nitrogen atoms are investigated using the density functional theory. Total and partial density of states, band structure, charge density, quantum conductance and current-voltage characteristic of these structures have been studied and compared. The results indicate that graphene-like borophene is a metal, and has a Dirac point with a linear dispersion relation similar graphene. Our investigations demonstrate that the Dirac point is in upper place than the Fermi level, and the doping can affect the location of Dirac point. Moreover, the current-voltage characteristics show Ohmic behavior of these structures. In doped graphene-like borophene structures, boron atoms are formed ionic bonds. In all considered structures, the current density along zigzag and armchair directions exhibit an anisotropic behavior. By 90° rotation of graphene-like borophene sheet with carbon atom, its current is controlled and this material can be used to design nanoelectronic switches. The current control with C atom doping can be used in this two-dimensional material to design nanoelectronic switches.

Design and atomic modeling of multimode sliding electromechanical switch based on bilayer armchair α-graphyne nanoribbons is presented by applying density functional theory combined with non-equilibrium Green's function. In the proposed switch structure, bottom graphyne layer is fixed and the top layer is movable. Different configurations of the layers are created by moving the top layer over the bottom layer along the horizontal axis with the displacement distances of 1.36 Å, 2.61 Å, 3.97 Å, 8.41 Å, 9.44 Å and 12.6 Å. There are six stacking modes for the proposed device, namely, Ab, Aa, AB, AB2, Aa2 and AA, respectively. These configurations cause current of the proposed switch changes significantly in each stacking mode. To better analysis, electronic transport properties of the proposed device including transmission spectrum, band structure, molecular energy spectrum, molecular projected self-consistent hamiltonian and transmission pathway are calculated. The results demonstrate that current switching ratio of the proposed device depends on the type of layers atomic configuration and varies significantly from one mode to another. Maximum switching ratio of the proposed device can reach to 34 under the bias voltage of 0.6 V when the mode of device changes from AA to Aa2. This suggests that the controlled movement of layers in bilayer graphyne nanoribbon device could be a useful method to design multimode sliding electromechanical switch in nanoelectronics field. Furthermore, the results exhibit that the proposed device in AB2, Aa and AA modes reveals negative differential resistance which provides ability of its usage in quantum tunneling device.

A seamless graphene inverter including graphene nanoribbon field effect transistor (GNRFET) and graphene interconnect is proposed. The seamless structure is suggested to eliminate the ohmic, schottky, and parasitic resistances in the junction of the traditional interconnects with the Gate, Source and Drain of GNRFET. After that, using the circuit models of the graphene devices that are used in the proposed structure, transfer matrix model of the proposed seamless graphene inverter is calculated and extracted. All of the capacitive, inductive and scattering effects are included in the assumed circuit models of the GNRFET - graphene interconnect and consequently in the overall matrix model of the seamless graphene inverter. Elimination of the ohmic, schottky and parasitic resistances causes to improve in the working speed of the proposed inverter. Extraction of the transfer matrix model of the seamless graphene inverter and calculation of its step time response, relative stability and frequency bandwidth confirms this improvement. The advantage of the transfer matrix model of the proposed inverter is that any change in the physical parameters of the graphene nanoribbons that are used in the structure can be included in the model and one can analyze the effect of it in all of the technology nodes. Using the circuit model and the extracted transfer matrix, anyone can evaluates various stability analyses such as Nyquist, Bode and Nichols together with the time-frequency responses of the graphene seamless inverter used in very large scale integrated (VLSI) circuits.