Amir A. Farajian

Hydrogen compounds of group-IV nanosheets

The structural and electronic properties of the hydrides of silicene and germanene have been studied using ab initio calculations. The trend for the M–H (M=C, Si, and Ge) bond lengths, and corresponding bond energies, is consistent with the atomic size trend, and comparable to those of MH4 hydrides. Band structures were also obtained for the buckled configuration, which is the stable form for both silicene and germanene. Upon hydrogenation, both silicane (indirect gap) and germanane (direct gap) are semiconducting.

Electronic and transport properties of N-P doped nanotubes

Electronic properties of a doped zigzag nanotube are investigated by a self-consistent tight-binding method. We propose that a doped nanotube with donor atoms on one side and acceptors on the other can function as a nano diode. It is shown that a potential step in the tube, created by two different types of doping in this case, causes the nonlinear rectifying effect.

The effects of defects on the conductance of graphene nanoribbons

The quantum conductance of graphene nanoribbons that include vacancy and adatom-vacancy defects is studied for both armchair and zigzag edge structures. The conductance is calculated by using the Greens function formalism combined with a tight-binding method for the description of the system. Our results reveal that, owing to the localized states that appear near the defect sites, the conductance of the defected nanoribbons generally decreases. We show that details of the conductance reduction depend on the structure of the defect, its distance from the ribbon edges, and the ribbon width. While some defect structures cause the conductance of the ribbon to vanish, some other defects have no effect on the conductance at the Fermi energy.

THEORETICAL STUDY OF DONOR–SPACER–ACCEPTOR STRUCTURE MOLECULE FOR STABLE MOLECULAR RECTIFIER

Recently, field of molecular electronics has attracted strong attention as a “post-silicon technology” to enable future nanoscale electronic devices. To realize this molecular device, unimolecular rectifying function is one of the most fundamental requirements using nanotechnology. In the present study, the geometric and electronic structures of alkyl derivative C37H50N4O4 (PNX) molecule, (donor – spacer – acceptor), a candidate for a molecular rectifying device, has been investigated theoretically using ab initio quantum mechanical calculations. The results suggest that in such donor-acceptor molecular complexes, while the lowest unoccupied orbital concentrates on the acceptor subunit, the highest occupied molecular orbital is localized on the donor subunit. After the optimization of the structure by B3LYP/6-31(d), the approximate potential differences for the optimized PNX molecule have been estimated at the B3LYP/6-311++G(d,p) level of theory, which achieves quite good agreement with experimentally reported results.

Theoretical study of phthalocyanine–fullerene complex for a high efficiency photovoltaic device using ab initio electronic structure calculation

Abstract Many fullerene-based supramolecules have been proposed as potential organic photovoltaic devices, with their electrochemical and photo-electrochemical properties measured under light illumination. Phthalocyanine possesses good electron-donating properties due to its large easily ionised π-electron system, whereas fullerene is good π-electron acceptor which can be connected with other organic molecules. A phthalocyanine–fullerene-based supramolecular system is therefore a potential material candidate for a photovoltaic cell due to its large and flexible absorption combined with electrical properties similar to an inorganic semiconductor. We investigated the geometric and electronic structure of phthalocyanine–fullerene supramolecule using an ab initio quantum mechanical calculation. The results suggest that the lowest unoccupied molecular orbital (LUMO) state of this supramolecule localized on the fullerene and the highest occupied molecular orbital (HOMO) state is localized on half of the phthalocyanine. The energy difference of localized LUMO levels strongly depended on the functional group attached to the phthalocyanine and the structure of the supramolecule.

Ab Initio study of dopant insertion into carbon nanotubes

Ab initio total energy calculation and molecular dynamics simulation on the process of dopant insertion into carbon nanotubes are carried out on the basis of the all-electron mixed basis approach within the local density approximation. First, an upper bound for the height of the potential barrier which is seen by typical alkali metals (Na and K) going through the center of a hexagonal ring of the nanotube is estimated to be 40 eV for Na and 90 eV for K. Next, such an insertion process is simulated with a suitable kinetic energy of the dopant (70 eV for Na and 150 eV for K). It is observed that the carbon atoms are pushed to open the hexagonal ring wider and the dopant passes through. After encapsulation, the hexagonal ring restores its initial configuration, while the impact shock propagates along the nanotube and gradually decays.

Encapsulation of cesium inside single-walled carbon nanotubes by plasma-ion irradiation method

Positive Cs ions are irradiated to a negatively biased substrate, which is covered with the dispersed single-walled carbon nanotubes (SWNTs) and immersed in the Cs plasma. Field emission type transmission electron microscopy (FE-TEM) and Z-contrast technique by scanning TEM (STEM) are used for the precise observation. FE-TEM gives high resolved images of structurally modified SWNTs such as irreversible bending of tube bundles, tube dislocation, and cutting due to the positive Cs ion irradiation. Moreover, this structural modification becomes severe with an increase in the acceleration energy of irradiating Cs ions. Especially, the Cs encapsulation inside SWNTs is directly observed, the configuration of which is confirmed to comprise three varieties by FEG-TEM and STEM. In addition to these experimental results, ab initio study is used to investigate the static stable configurations due to Cs ion irradiation.

Gate-induced switching and negative differential resistance in a single-molecule transistor: emergence of fixed and shifting states with molecular length.

The quantum transport of a gated polythiophene nanodevice is analyzed using density functional theory and nonequilibrium Greens function approach. For this typical molecular field effect transistor, we prove the existence of two main features of electronic components, i.e., negative differential resistance and good switching. Ab initio based explanations of these features are provided by distinguishing fixed and shifting conducting states, which are shown to arise from the interface and functional molecule, respectively. The results show that proper functional molecules can be used in conjunction with metallic electrodes to achieve basic electronics functionality at molecular length scales.

Silicene nanoribbons as carbon monoxide nanosensors with molecular resolution

Applications based on silicene as grown on substrates are of high interest toward actual utilization of this unique material. Here we explore, from first principles, the nature of carbon monoxide adsorption on semiconducting silicene nanoribbons and the resulting quantum conduction modulation with and without silver contacts for sensing applications. We find that quantum conduction is detectably modified by weak chemisorption of a single CO molecule on a pristine silicene nanoribbon. This modification can be attributed to the charge transfer from CO to the silicene nanoribbon and the deformation induced by the CO chemisorption. Moderate binding energies provide an optimal mix of high detectability and recoverability. With Ag contacts attached to a ∼1 nm silicene nanoribbon, the interface states mask the conductance modulations caused by CO adsorption, emphasizing length effects for sensor applications. The effects of atmospheric gases—nitrogen, oxygen, carbon dioxide, and water—as well as CO adsorption density and edge-dangling bond defects, on sensor functionality are also investigated. Our results reveal pristine silicene nanoribbons as a promising new sensing material with single molecule resolution.

Electron transport of nanotube-based gas sensors: An ab initio study

The effect of physisorption of NO2 gas molecules on quantum transport properties of semiconducting carbon nanotubes is studied using ab initio calculations and Green function formalism. The results show that the conductance change is mainly due to the electric dipole moment of NO2. It is also shown that upon exposure of nanotube to different concentrations of gas, the common feature is the shift in conductance toward lower energies. This suggests that physisorption of NO2 will result in a decrease (increase) in conductance of p-type (n-type) nanotubes with Fermi energies close to the edge of valence and conduction band.