Search for: vacancies
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    Kondo resonance from p-wave hybridization in graphene

    , Article Journal of Physics Condensed Matter ; Vol. 26, Issue. 41 , 2014 ; SSN: 09538984m Jafari, S. A ; Tohyama, T ; Sharif University of Technology
    The p-wave hybridization in graphene present a distinct class of Kondo problem in pseudogap Fermi systems with bath density of states (DOS) ρ0(ε) ∝ |ε|. The peculiar geometry of substitutional and hollow-site ad-atoms, and effectively the vacancies allow for a p-wave form of momentum dependence in the hybridization of the associated local orbital with the Dirac fermions of the graphene host which results in a different picture than the s-wave momentum independent hybridization. For the p-wave hybridization function, away from the Dirac point we find closed-form formulae for the Kondo temperature TKwhich in contrast to the s-wave case is non-zero for any value of hybridization strength V of... 

    Modeling of the mutual effect of dynamic precipitation and dislocation density in age hardenable aluminum alloys

    , Article Journal of Alloys and Compounds ; Volume 683 , 2016 , Pages 527-532 ; 09258388 (ISSN) Khani Moghanaki, S ; Kazeminezhad, M ; Sharif University of Technology
    Elsevier Ltd  2016
    A model has been proposed to capture the complex strain rate effect on dynamic precipitation of GP zones in an age-hardenable aluminum alloy. The contributions of vacancies and dislocations to dynamically formed GP zones have been specified in the model. It has been demonstrated that the proposed model is capable for predicting the contribution of each dynamic precipitation mechanisms, accurately, which are acting during deformation. Furthermore, the vacancy and dislocation evolutions during deformation have been considered in this modeling. The effect of strain rate by considering different mechanisms of dynamic precipitation of GP zones has been studied and confirmed by experimental data... 

    Details of a theoretical model for electronic structure of the diamond vacancies

    , Article Proceedings of the 9th International Conference on New Diamond, Tokyo, 26 March 2004 through 29 March 2004 ; Volume 13, Issue 11-12 , 2004 , Pages 2125-2130 ; 09259635 (ISSN) Saani, M. H ; Vesaghi, M. A ; Esfarjani, K ; Shafiekhani, A ; Sharif University of Technology
    A new model to calculate electronic states of the diamond vacancies has been developed using many body techniques. This model is based on physical assumptions of previous molecular models but does not use configuration interaction. Present model allows an accurate and unified treatment of electronic levels and related eigen functions for diamond vacancies, in addition to transition energies of the first dipole-allowed transitions in the neutral (V0) and negatively charged (V-) vacancies, GR1 and ND1 band. For the first time, we calculated their optical transition intensities. For obtaining these results, we solved a generalized form of the Hubbard Hamiltonian, which consists of all... 

    The formation and dissociation energy of vacancies in cementite: A first-principles study

    , Article Nuclear Instruments and Methods in Physics Research, Section B: Beam Interactions with Materials and Atoms ; Volume 502 , 2021 , Pages 157-163 ; 0168583X (ISSN) Mehrdad Zamzamian, S ; Amirhossein Feghhi, S ; Samadfam, M ; Sharif University of Technology
    Elsevier B.V  2021
    Because of the possibility of various types of vacancies in cementite due to its crystalline structure, the focus of this paper was only on vacancies. In this regard, the formation energies of single, two, three and four vacancies of over than 120 different cases were calculated using first-principles method. For the case of single vacancy, the results were in three values of ~1.63, 1.39 and 0.78 eV according to iron vacancies at general positions, iron vacancies located on mirror planes and carbon vacancies in the interstitial positions, respectively. The results for the case of two, three and four vacancies were between from 2.10 to 3.34 eV, from 3.92 to 5.10 eV and from 4.77 to 6.33 eV,... 

    Pull-in behaviors of carbon nanotubes with vacancy defects and residual stresses

    , Article Journal of Computational and Theoretical Nanoscience ; Vol. 11, issue. 1 , 2014 , pp. 153-159 ; ISSN: 15461955 Fakhrabadi, M. M. S ; Rastgoo, A ; Ahmadian, M. T ; Sharif University of Technology
    The paper deals with investigation of the effects of vacancy defects on the pull-in behaviors of the carbon nanotubes. The influences of single, double and triple vacancies in different peripheral and longitudinal positions are studied. The results reveal that the vacancy defects drastically reduce the pull-in charge because they weaken the nanostructure. Moreover, the effects of residual stresses on the pull-in and vibrational properties of the carbon nanotubes are scrutinized. The influences of the angular deviations from the parallel positions of the CNT and ground plate during the fabrication process are also reported. Copyright  

    Composition lines of the visible band of synthetic diamond

    , Article Japanese Journal of Applied Physics, Part 1: Regular Papers and Short Notes and Review Papers ; Volume 42, Issue 5 A , 2003 , Pages 2749-2751 ; 00214922 (ISSN) Vesaghi, M. A ; Shafiekhani, A ; Horiuchi, K ; Sharif University of Technology
    Japan Society of Applied Physics  2003
    The visible band (band A) of photoluminescence spectra of high-purity synthesized diamond is analyzed by the deconvolution technique. A set of eight lines with distinct peak energies are found. The peak energy and the width of these lines were either constant or varied very slightly with temperature. The amplitude of the lines are significantly temperature dependent. The closeness of the temperature at which the amplitudes of these lines reach their minimum to the temperature at which the free-exiton emission is maximum, is an indication of the competition between these two effects  

    Jahn-Teller Effect in Diamond Vacancy Under Stress

    , M.Sc. Thesis Sharif University of Technology monfared, Mohammad (Author) ; Vesaghi, M. A (Supervisor) ; Babamoradi, M (Co-Advisor)
    Electron states of diamond vacancy under stress were studied theoretically. The generalized Hubbard model is used to calculate the electrons energy levels. The results shows that the degeneracy of some levels (T, with triple space degeneracy) reduced to levels with lower degeneracy (E, with double space degeneracy and A, without degeneracy). Besides this spiting of degeneracy the energy of all states changed and therefore the transition energy between these states changed too. Jahn-Teller effect was observed  

    Effect of defects on the local shell buckling and post-buckling behavior of single and multi-walled carbon nanotubes

    , Article Computational Materials Science ; Volume 79 , November , 2013 , Pages 736-744 ; 09270256 (ISSN) Eftekhari, M ; Mohammadi, S ; Khoei, A. R ; Sharif University of Technology
    The local buckling behavior of perfect/defective and single/multi-walled carbon nanotubes (CNTs) under axial compressive forces has been investigated by the molecular dynamics approach. Effects of different types of defects including vacancy and Stone-Wales (SW) defects and their configurations on CNTs with different chiralities at room temperature are studied. Results show that defects largely reduce the buckling stress and the ratio of immediate reduction in buckling compressive stress of the defective CNT to the perfect one, but have little influence on their compressive elastic modulus. SW defects usually reduce the mechanical properties more than vacancy defects, and zigzag CNTs are... 

    Fracture analysis of monolayer graphene sheets with double vacancy defects via MD simulation

    , Article Solid State Communications ; Volume 151, Issue 17 , 2011 , Pages 1141-1146 ; 00381098 (ISSN) Ansari, R ; Motevalli, B ; Montazeri, A ; Ajori, S ; Sharif University of Technology
    Carbon nanostructures such as carbon nanotubes (CNTs) and graphene sheets have attracted great attention due to their exceptionally high strength and elastic strain. These extraordinary mechanical properties, however, can be affected by the presence of defects in their structures. When a material contains multiple defects, it is expected that the stress concentration of them superposes if the separation distances of the defects are low, which causes a more reduction of the strength. On the other hand, it is believed that if the defects are far enough such that their affected areas are distinct, their behavior is similar to a material with single defect. In this article, molecular dynamics... 

    Stable local moments of vacancies, substitutional and hollow site impurities in graphene

    , Article Journal of Physics Condensed Matter ; Volume 27, Issue 15 , 2015 ; 09538984 (ISSN) Mashkoori, M ; Jafari, S. A ; Sharif University of Technology
    Institute of Physics Publishing  2015
    The two-sublattice nature of graphene lattice in conjunction with three-fold rotational symmetry, allows for the p-wave hybridization of the impurity state with the Bloch states of carbon atoms. Such an opportunity is not available in normal metals where the wave function is scalar. The p-wave hybridization function V (-k) appears when dealing with vacancies, substitutional adatoms and the hollow site impurities while the s-wave mixing on graphene lattice pertains only to the top site impurities. In this work, we compare the local moment formation in these two cases and find that the local moments formed by p-wave mixing compared to the s-wave one are robust against the changes in the... 

    Simulation of vacancy diffusion in a silver nanocluster

    , Article Chemical Physics Letters ; Volume 498, Issue 4-6 , 2010 , Pages 312-316 ; 00092614 (ISSN) Taherkhani, F ; Negreiros, F. R ; Parsafar, G ; Fortunelli, A ; Sharif University of Technology
    The formation and diffusion of a vacancy in a silver nanocluster are studied via a combination of first-principles and statistical mechanics simulations. A 38-atom truncated-octahedral (TO) arrangement and its homologue with 37 Ag atoms and one vacancy are considered, and density-functional calculations are performed to derive the energies of the local minima and the energy barriers connecting them. These data are then used as an input for a study of the system dynamics via a kinetic Monte Carlo algorithm, evaluating site occupancies, diffusion coefficient and equilibration time. It is found that vacancy formation and diffusion represents a viable path for atom-atom exchange in these... 

    Effects of particle size, shape and crystal structure on the formation energy of Schottky vacancies in free-standing metal nanoparticles: A model study

    , Article Physica B: Condensed Matter ; Volume 406, Issue 20 , October , 2011 , Pages 3777-3780 ; 09214526 (ISSN) Delavari H., H ; Madaah Hosseini, H. R ; Simchi, A ; Sharif University of Technology
    A simplified model based on cohesive energy is proposed to estimate the formation energy of Schottky vacancies (VFE) in free-standing metal nanoparticles with BCC and FCC crystal structures. To study the effect of particle size and shape, the surface energy, elastic contraction and average coordination number of particles at the surface and core was considered. It is shown that the energy of vacancy formation in FCC nanoparticles increases with decreasing the size while the effect of particle shape (sphere, cubic and icosahedral) is marginal. In spite of this behavior, BCC nanoparticles exhibit a critical particle size at around 25 Å, at which a minimum VFE is attained. Additionally, the... 

    Effect of mono-vacant defects on the opto-electronic properties of ionic liquid functionalized hexagonal boron-nitride nanosheets

    , Article Journal of Molecular Liquids ; Volume 249 , 2018 , Pages 1172-1182 ; 01677322 (ISSN) Shakourian Fard, M ; Bayat, A ; Kamath, G ; Sharif University of Technology
    We compare and contrast the physisorption behavior of imidazolium and butyltrimethylammonium based ionic liquids (ILs) on mono-vacant nitrogen and boron defective hexagonal boron nitride nanoflakes (h-BNNF) using M06-2X/cc-pVDZ level of theory. The presence of defects on the nanoflakes results in an increase in IL binding energy by ~ 1–27 kcal/mol partly due to the lowering of the energy band in the defective nanoflakes. Imidazolium based ILs adsorb energetically more favorably on h-BNNF-VB than on h-BNNF-VN while butyltrimethylammonium based ILs prefer to adsorb on h-BNNF-VN. Upon adsorption of imidazolium ILs on the nanoflakes, an increase in both HOMO and LUMO orbital energies is observed... 

    Modeling comparison of graphene nanoribbon field effect transistors with single vacancy defect

    , Article Superlattices and Microstructures ; Volume 97 , 2016 , Pages 28-45 ; 07496036 (ISSN) Nazari, A ; Faez, R ; Shamloo, H ; Sharif University of Technology
    Academic Press  2016
    In this paper, some important circuit parameters of a monolayer armchair graphene nanoribbon (GNR) field effect transistor (GNRFET) in different structures are studied. Also, these structures are Ideal with no defect, 1SVGNRFET with one single vacancy defect, and 3SVsGNRFET with three SV defects. Moreover, the circuit parameters are extracted based on Semi Classical Top of Barrier Modeling (SCTOBM) method. The I-V characteristics simulations of Ideal GNRFET, 1SVGNRFET and 3SVsGNRFET are used for comparing with SCTOBM method. These simulations are solved with Poisson-Schrodinger equation self-consistently by using Non- Equilibrium Green Function (NEGF) and in the real space approach. The... 

    Synthesis, Characterization and Photoelectrochemical Properties of BiVO4 Porous Thin Films

    , M.Sc. Thesis Sharif University of Technology Rasouli Ardalani, Abolfazl (Author) ; Moshfegh, Alireza (Supervisor) ; Zirak, Mohammad (Co-Supervisor)
    Synthesis, characterization and photoelectrocatalytic performance evaluation of worm-like BiVO4 porous films have been conducted in this research. An in situ combustion approach was employed to prepare BiVO4 nanolayers on glass and ITO/glass by using drop casting, spin coating, dip coating and pulsed spray pyrolysis deposition techniques. The precursor solution of vanadium and bismuth include ~0.0233g ammonium metavanadate (NH4VO3) and ~0.0970g bismuth (III) nitrate pentahydrate (Bi(NO3)3.5H2O) in 2.5 mL ethylene glycol. Different deposition parameters were investigated for each of the aforementioned deposition techniques to obtain a uniformly-deposited BiVO4 nanolayer with high adhesion to... 

    Magnetization of bilayer graphene with interplay between monovacancy in each layer

    , Article Journal of Applied Physics ; Volume 114, Issue 8 , 2013 ; 00218979 (ISSN) Rostami, M ; Faez, R ; Rabiee Golgir, H ; Sharif University of Technology
    Effects of introducing two monovacancies in bilayer graphene are investigated by using spin-polarized density functional theory. Each layer of bilayer graphene has a monovacancy. Two different classifications are studied, namely, AA and AB. In AA category, vacancies in upper layer and lower layer are chosen from the same sublattices (A or B). However, in AB category, vacancies are selected from the different sublattices (A and B). Two different structures of every classification are examined in order to investigate the effects of two monovacancies on structural, electronic, and magnetic properties of bilayer graphene. Structural optimization reveals that introducing a monovacancy in every... 

    Improving ION / IOFF and sub-threshold swing in graphene nanoribbon field-effect transistors using single vacancy defects

    , Article Superlattices and Microstructures ; Volume 86 , October , 2015 , Pages 483-492 ; 07496036 (ISSN) Nazari, A ; Faez, R ; Shamloo, H ; Sharif University of Technology
    Academic Press  2015
    Graphene nanoribbon field effect transistors are promising devices for beyond-CMOS nanoelectronics. Graphene is a semiconductor material with zero bandgap and its bandgap must be changed. One of the opening bandgap methods is using graphene nanoribbons. By applying a defect, there is more increase on band gap of monolayer armchair graphene nanoribbon field effect transistor. So, by applying more than one defect, we can reach to much more increase in bandgap of graphene nanoribbon field effect transistors (GNRFET). In this paper, double-gated monolayer armchair graphene nanoribbon field effect transistors (GNRFET) with one single vacancy (1SV) defect (so-called 1SVGNRFET)are simulated and... 

    Explanation of atomic displacement around lattice vacancies in diamond based on electron delocalization

    , Article European Physical Journal B ; Volume 65, Issue 2 , 2008 , Pages 219-223 ; 14346028 (ISSN) Heidari Saani, M ; Hashemi, H ; Ranjbar, A ; Vesaghi, M. A ; Shafiekhani, A ; Sharif University of Technology
    The relationship between unpaired electron delocalization and nearest-neighbor atomic relaxations in the vacancies of diamond has been determined in order to understand the microscopic reason behind the neighboring atomic relaxation. The Density Functional Theory (DFT) cluster method is applied to calculate the single-electron wavefunction of the vacancy in different charge states. Depending on the charge and spin state of the vacancies, at outward relaxations, 84-90% of the unpaired electron densities are localized on the first neighboring atoms. The calculated spin localizations on the first neighboring atoms in the ground state of the negatively charged vacancy and in the spin quintet... 

    Effect of vacancy defects on the fundamental frequency of carbon nanotubes

    , Article 3rd IEEE International Conference on Nano/Micro Engineered and Molecular Systems, NEMS 2008, Sanya, 6 January 2008 through 9 January 2008 ; 2008 , Pages 1000-1004 ; 9781424419081 (ISBN) Pirmoradian, M ; Ahmadian, M. T ; Asempour, A ; Tajalli, S. A
    Carbon nanotubes are widely used in the design of nanosensors and actuators. Any defect in the manufactured nanotube plays an important role in the natural frequencies of these structures. In this paper, the effect of vacancy defects on the vibration of carbon nanotubes is investigated by using an atomistic modeling technique, called the molecular structural mechanics method. Vibration analysis is performed for armchair and zigzag nanotubes with cantilever boundary condition. The shift of the principal frequency of the nanotube with vacancy defect at different locations on the length is plotted. The results indicate that the frequency of the defective nanotube can be larger or smaller or... 

    Development of Nanostructured Lithium-Rich Cathode Material

    , Ph.D. Dissertation Sharif University of Technology Vahdatkhah, Parisa (Author) ; Sadrnezhaad, Khatiboleslam (Supervisor)
    This research is divided in three parts. In the first part of this research, we report the one-pot synthesis of carbonate-coated nanostructured LLO (Li2CO3@LLO) through a polyol-assisted method as a Li-ion battery cathode. Carbonate protects the cathode from adverse reactions with the electrolyte, also reduces the layered-to-spinel phase transition, thereby stabilizing the cathode structure. LLO nanostructure provides a fast Li+ diffusion. The target material exhibits excellent long-term stability with 77% capacity retention after 1000 cycles at 0.2C-rate. In the second part of research, Li(Na-doped)-Mn-Ni-O oxides were synthesized by different LiOH.H2O amount, type and amount of reducing...