Amir A. Farajian

Wright State University, Dayton, Ohio, United States

Are you Amir A. Farajian?

Claim your profile

Publications (80)136.52 Total impact

  • Tim H. Osborn, Amir A. Farajian
    [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Nano Research 06/2014; · 7.39 Impact Factor
  • Kirti K. Paulla, Amir A. Farajian
    [Show abstract] [Hide abstract]
    ABSTRACT: We study detection of CO and CO2 gas molecules by change in quantum conductance of armchair graphene nanoribbons (AGNR) with a width of 1 nm. Quantum conductance modulations are calculated by using the second-order Møller–Plesset (MP2) method and density functional theory (DFT) for geometry optimization and a hybrid approach for electronic structure calculations. We determine stable and metastable physisorption orientations of gas molecules with varying concentrations. Our MP2-calculated binding energies relate 8.33% and 16.33% surface coverages of CO and CO2, respectively, to 1.72 × 104 and 497 ppm. With such concentrations, adsorption of molecules results in conductance characteristics shifts on the order of few meV. As the concentrations detected in experiments are much less, other mechanisms including substrate and/or carrier gas doping as well as adsorption on defects or electrodes may contribute toward gas sensing using graphene plates. We also discuss temperature effects and propose possible methods for improving gas detection by GNRs.
    The Journal of Physical Chemistry C. 06/2013; 117(24):12815–12825.
  • Kirti K Paulla, Amir A Farajian
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the electronic and magnetic structures of bilayer graphene nanoribbons (BGNRs) beyond the conventional AA and AB stackings, by using density functional theory within both local density and generalized gradient approximations (LDA and GGA). Our results show that, irrespective of the method chosen, stacking arrangements other than the conventional ones are most stable, and result in significant modification of BGNR characteristics. The most stable bilayer armchair and zigzag structures with a width of ∼1 nm are semiconducting with band gaps of 0.04 and 0.05 eV, respectively. We show mechanical shift evolution of magnetic states and the emergence of magnetization upon mechanical deformation in bilayer zigzag GNRs. Band gap dependence on mechanical shift can be used to design accurate nanosensors.
    Journal of Physics Condensed Matter 03/2013; 25(11):115303. · 2.22 Impact Factor
  • Tim H. Osborn, Amir A. Farajian
    [Show abstract] [Hide abstract]
    ABSTRACT: We explore the adsorption characteristics and stability of lithium on silicene from first principles. Our work shows that lithium adsorption could provide a unique method for isolating a stable silicene-based material while inducing a bandgap. We explore the energetics, temperature dependent dynamics, phonon frequencies, and electronic structure associated with lithium chemisorption on silicene. Our results predict the stability of completely lithiated silicene sheets (silicel) in which lithium atoms adsorb on the atom-down sites on both sides of the silicene sheet. Stability is confirmed by molecular dynamics simulations conducted at elevated temperatures and real phonon frequencies for all k-values. Upon complete lithiation, the band structure of silicene is transformed from a zero-gap semiconductor to a 0.368 eV bandgap semiconductor. This new, uniquely stable, two-atom-thick, semiconductor material could be of interest for nanoscale electronic devices.
    The Journal of Physical Chemistry C. 10/2012; 116(43):22916–22920.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The electronic transport through bent single-wall carbon nanotubes is studied. Using a four-orbital per atom tight-binding model, the relaxed configurations of a (10, 0) semiconducting nanotube at different bending angles are first obtained. The optimized structures are then used in calculating conductance and current–voltage characteristics of the systems. These results are used to establish a correspondence between the mechanical deformation and transport properties, with potential applications in, e.g., nanoswitches. The source of the switching behavior is explained in terms of the localized states within the bent region.
    International Journal of Nanoscience 11/2011; 03(01n02).
  • Source
    03/2011; , ISBN: 978-953-307-152-7
  • [Show abstract] [Hide abstract]
    ABSTRACT: The effect of electron-phonon (e-ph) interaction on the conductance of carbon chains is investigated by a non-equilibrium Green's function technique combined with a four-orbitals-per-atom tight-binding Hamiltonian. The optimized structure of the chain is found to be the semiconducting polyyne type (···-C≡C-C≡C-···). Our results show that the conductance of a carbon chain attached to two fixed contacts decreases due to e-ph interaction, and this reduction is stronger for longitudinal phonon modes which decrease the hopping energy between carbon atoms. Study of individual phonon modes reveals that emission of longitudinal phonons is stronger than that of transverse modes at room temperature, while absorption of transverse phonons is dominant. Conductance at finite temperature is also studied by considering the overall phonon effects; this shows that the reduction of the conductance is stronger at higher temperatures. The results are explained on the basis of the unique features of the carbon chain band structure.
    Journal of Physics Condensed Matter 02/2011; 23(7):075301. · 2.22 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Silicene, a silicon equivalent of graphene, is a newly synthesized nanostructure with unique features and promising potential. Using density functional theory, the geometries and energetics of partially hydrogenated silicene (hydrogenation ratios between 3.1 and 100 atom%) are calculated. We find that the hydrogenation energy increases with the hydrogenation ratio, reaching 3.01 eV/H for complete hydrogenation. Molecular dynamics simulations reveal the stability of the adsorption configurations. Our results show that partial and patterned hydrogenation, achievable through exposing silicene to hydrogen gas with various densities and/or masking techniques, provide the attractive possibility of metal/semiconductor/insulator functionality within the same silicon nanosheet.Graphical abstractView high quality image (110K)Highlights► Density functional geometries and energetics of partially hydrogenated silicene. ► Stability assessment via molecular dynamics simulations. ► Possibility of engineering silicene’s electronic and transport properties. ► Metal/semiconductor/insulator functionality within the same silicon nanosheet.
    Chemical Physics Letters 01/2011; 511:101-105. · 2.15 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Direct ultrasonication of graphite particles dispersed in water, in the presence of a surfactant, is a promising way to produce pristine nanoscale graphene platelets (NGPs) without graphite intercalation or oxidation. We investigate possible exfoliation mechanisms, specifically those involving sodium dodecylbenzenesulfonate (SDBS) surfactant, and compare their corresponding energies. The model includes interlayer van der Waals interactions and a force-field approach capable of treating charged surfactant and solvent. Our calculations reveal the significant role of SDBS in liquid-phase NGP production, through a locking mechanism that prevents restacking.
    The Journal of Physical Chemistry C. 11/2010; 114(49).
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: 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 MH <sub>4</sub> 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.
    Applied Physics Letters 11/2010; · 3.79 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The structural configurations and electronic properties of polythiophene-cyclodextrin (PT-CD) inclusion complexes have been investigated by a combined quantum mechanics and molecular mechanics method. The results show that the structure of n-type PT in CDs has a quinoidlike form. In the cases of β-cyclodextrins and cross-linking α-cyclodextrins the electronic structure of polythiophene is almost the same as that of polythiophene in free space. The dopants are located outside the CDs, and hence for realization of a doped polymer chain it is important to control the separation distance between CDs, which can be easily achieved in the case of a molecular nanotube of cross-linking α-CDs.
    Molecular Crystals and Liquid Crystals 07/2010; 406(1):1-10. · 0.58 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The structure, stability and electronic properties of nanosheets of group-IV elements were studied using density-functional theory. The nature of bonding with hydrogen was investigated by analyzing the electron density distribution and by calculating the binding energy.
  • [Show abstract] [Hide abstract]
    ABSTRACT: The energies and temperature-dependent dynamics of hydrogen chemisorption on a silicon nanosheet were studied using density functional theory and molecular-dynamics (MD) simulations. Energy calculations were performed by utilizing generalized-gradient approximation with the Perdew-Burke-Ernzerhof exchange correlation functional. The adsorption energies of hydrogen on the silicon nanosheet were calculated for different hydrogenation ratios corresponding to weight percents between 0 and 3.59 %. The preferred adsorption configurations were determined based on these energy calculations. MD simulations revealed the stability of adsorption configurations, and possible transitions between them, at different temperatures.
  • [Show abstract] [Hide abstract]
    ABSTRACT: We calculate the quantum transport of a nanoelectronic gas sensor for various adsorption orentations of the gas molecules. The nanosensor employs electronic transport properties of a carbon nanotube exposed to NO2 molecules. The calculations are based on ab initio electronic structures, combined with the Green's function formulation of Landauer's transport theory. Our results show that different energetically equivalent orientations of the NO2 molecules result in different details of transport characteristics. The main features of transport modulation, however, are the same for all the orientations. Implications for nanotube-based gas sensors are discussed.
  • Japanese Journal of Applied Physics 01/2010; 49(11):5103. · 1.07 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: 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 Green's 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.
    Nanotechnology 02/2009; 20(1):015201. · 3.84 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We theoretically investigate the polarization, aggregation, and yield stress in carbon nanotube suspensions under an electric field. The nanotubes are modeled as solid rods with hemispherical ends. An exact numerical approach, which includes self-consistent Coulomb interactions within classical electrostatics, is employed to derive nanotube surface charge densities. Two essential nanotube characteristics, i.e., large aspect ratios and end contributions, are included together. The reliability of the model is demonstrated by comparing the calcu-lated emerging yields against experimental data. The onsets of system parameters can be used to control the phase transition in nanotube suspensions.
    Physical review. B, Condensed matter 05/2008; 77. · 3.77 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: We model fullerene nanocages filled with hydrogen, of the general formula Hn@Ck, and study the capacity of such endohedral fullerenes to store hydrogen. It is shown using density functional theory that for large numbers of encapsulated hydrogen atoms, some of them become chemisorbed on the inner surface of the cage. A maximum of 58 hydrogen atoms inside a C60 cage is found to still remain a metastable structure, and the mechanism of its breaking is studied by ab initio molecular dynamics simulations. Hydrogen pressure inside the fullerene nanocage is estimated for the first time and is shown to reach the values only a few times smaller than the pressure of hydrogen metallization. We provide a general relation between the hydrogen pressure and resulting C-C bond elongation for fullerene nanocages of arbitrary radii. This opens a way to assess possible hydrogen content inside larger carbon nanocages, such as giant fullerenes, where significant capacity can be reached at reasonable conditions.
    Nano Letters 04/2008; 8(3):767-74. · 13.03 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Effect of inelastic electron-phonon interaction is studied on electronic transport of semiconducting carbon chains and carbon nanotubes. Absorption and emission of individual phonon modes are investigated as well as collective modes in order to reveal the nature of the interactions and the role of vibrations in quantum transport at finite temperature. The conductance in this study is calculated using non-equilibrium Green's function formalism combined with a tight-binding Hamiltonian description. The phonon spectrum is obtained from frozen-phonon approach and the electron-phonon interaction appears in the calculations as a coupling matrix determined by atomic displacements and phonon eigenvectors. Our results show that the effect of individual electron-phonon interaction on quantum conductance depends on temperature and energy of the phonon mode, regarding absorption and emission processes. The type of the phonon mode is in fact a determining part of the interactions. Decrease of conductance due to e-ph scattering is stronger when the process is scattering of electron out of in-plane phonons which make the in-plane C-C bonds of nanotube or chain vibrate with higher length. The effect of collective modes also suggests the temperature dependent nature of the conductance of a finite size carbon chain and nanotube.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Electronic transport is among the unique physical phenomena whose applications have shaped human civilization as we know it today. From old telegrams and light bulbs to modern televisions, mobile phones, laptops, and super-computers, all make use of electronic transport. In fact, nowadays we can hardly find any significant technological product in which electronic transport is not used one way or another. This enormous technological impact is a result of basic scientific research on electron tunneling and scattering, in different environments and including various levels of interactions and correlations. The basic research in the field of electronic transport is expected to yield equally unique, and even more important, fruits in future, as the challenges of this vibrant field are ever increasing. One of the main areas of interest which has been the focus of numerous scholarly works is electronic transport at nanometer length scales. The reason is that miniaturization of electronic components has caused the device dimensions to reach nanoscale. At nanoscale, the atomistic character of the systems can no longer be treated using rather rough models applicable at micrometer length scales. Therefore, fundamentally new approaches are necessary, in both theory and experiment, to deal with electronic transport at nanoscale.
    01/2008: pages 219-241;

Publication Stats

617 Citations
136.52 Total Impact Points


  • 2008–2014
    • Wright State University
      • • Department of Mechanical and Materials Engineering
      • • Department of Physics
      Dayton, Ohio, United States
  • 1998–2011
    • Tohoku University
      • Institute for Materials Research
      Sendai-shi, Miyagi-ken, Japan
  • 2007–2008
    • Rice University
      • Department of Mechanical Engineering and Materials Science
      Houston, TX, United States
  • 2001–2002
    • National Institute of Advanced Industrial Science and Technology
      Tsukuba, Ibaraki, Japan