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Search Results: 1 - 10 of 58 matches for " Narjes Gorjizadeh "
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Chemical Functionalization of Graphene Nanoribbons
Narjes Gorjizadeh,Yoshiyuki Kawazoe
Journal of Nanomaterials , 2010, DOI: 10.1155/2010/513501
Abstract: We review the electronic properties of graphene nanoribbons functionalized by various elements and functional groups. Graphene nanoribbons are strips of graphene, the honeycomb lattice of carbon with hybridization. Basically nanoribbons can be classified into two categories, according to the geometry of their edge, armchair, and zigzag, which determine their electronic structure. Due to their fascinating electronic and magnetic properties many applications has been suggested for these materials. One of the major methods to use graphene nanoribbons in future applications is chemical functionalization of these materials to make an engineering on their band gap. In this review, we introduce various types of modifying graphene nanoribbons to meet their promising applications. 1. Introduction After discovery of graphene [1], it has been considered as basic material for the future nanoelectronic devices. Its unique properties, such as massless Dirac fermion behavior [2–4], half-integer quantum Hall effect [2, 5], and high-carrier mobility [2] make it a promising candidate for application in nanoelectronics and spintronics devices [2, 5–9]. Graphene nanoribbons can be constructed as strips of graphene, with quasi1D structures. So far, Nanoribbons with widths up to 2?nm have been fabricated experimentally [10–12]. Geometrically, two main types of nanoribbons with two edge shapes can be cut from a hexagonal lattice of graphene: zigzag edge and armchair edge [13–16]. Different types of ribbons are specified by their edge geometry and width. The width is labeled by an integer which counts the number of carbon chains between the two edges. Figure 1 shows the two types of ribbons with their width indices. The two ribbon characteristics, that is, edge geometry and width, are the key parameters which determine the electronic properties of the ribbons [7, 17–19]. Figure 1: Graphene nanoribbons with armchair (a) and zigzag (b) edges. In each case, index denotes the width of the ribbon, and ribbon axis is the vertical direction. The earliest theoretical studies of graphene nanoribbons, using a simple tight-binding method, predicted that of the armchair nanoribbons, whose width index satisfies ( is an integer), are metallic [17], and another are semiconductor with band gaps depending on their width, while all zigzag nanoribbons are metallic, a similar behavior as carbon nanotubes (CNTs). A characteristic peak in the density of states (DOS) of zigzag nanoribbons near and slightly below Fermi energy is also predicted [16, 20]. But recently the first principle studies based
Interface Effects on Tunneling Magnetoresistance in Organic Spintronics with Flexible Amine-Au Links
Narjes Gorjizadeh,Su Ying Quek
Physics , 2013, DOI: 10.1088/0957-4484/24/41/415201
Abstract: Organic spintronics is a promising emerging field, but the sign of the tunneling magnetoresistance (TMR) is highly sensitive to interface effects, a crucial hindrance to applications. A key breakthrough in molecular electronics was the discovery of amine-Au link groups that give reproducible conductance. Using first principles calculations, we predict that amine-Au links give improved reproducibility in organic spintronics junctions with Au-covered Fe leads. The Au layers allow only states with sp character to tunnel into the molecule, and the flexibility of amine-Au links results in a narrow range of TMR for fixed number of Au layers. Even as the Au thickness changes, TMR remains positive as long as the number of Au layers is the same on both sides of the junction. Since the number of Au layers on Fe surfaces or Fe nanoparticles can now be experimentally controlled, amine-Au links provide a route towards robust TMR in organic spintronics.
Magnetism of Edge Modified Nano Graphene
Norio Ota,Narjes Gorjizadeh,Yoshiyuki Kawazoe
Physics , 2011,
Abstract: In order to study a magnetic principle of carbon based materials, multiple spin state of zigzag edge modified graphene molecules are analyzed by the first principle density functional theory to select suitable modification element. Radical carbon modified C64H17 shows that the highest spin state is most stable, which arises from two up-spin's tetrahedral molecular orbital configuration at zigzag edge. In contrast, oxygen modified C59O5H17 show the lowest spin state to be most stable due to four spins cancellation at oxygen site. Boron modified C59B5H22 have no {\pi}-molecular orbit at boron site to bring stable molecular spin state to be the lowest one. Whereas, C59N5H2 have two {\pi}-electrons, where spins cancel each other to give the stable lowest spin state. Silicon modified C59Si5H27 and Phosphorus modified C59P5H22 show curved molecular geometry due to a large atom insertion at zigzag site, which also bring complex spin distribution. Radical carbon and dihydrogenated carbon modification are promising candidates for designing carbon-based magnetic materials.
Magnetic Counting Rule of Radical Carbon Edge Nano Graphene
Norio Ota,Narjes Gorjizadeh,Yoshiyuki Kawazoe
Physics , 2011,
Abstract: In order to explain room-temperature ferromagnetism of graphite-like materials, this paper offers a new magnetic counting rule of radical carbon zigzag edge nano graphene. Multiple spin state analysis based on a density function theory shows that the highest spin state is most stable. Energy difference with next spin state overcomes kT=2000K suggesting a room-temperature ferromagnetism. Local spin density at a radical carbon shows twice a large up-spin cloud which comes from two orbital with tetrahedral configuration occupied by up-up spins. This leads a new magnetic counting rule to give a localized spin Sz=+2/2 to one radical carbon site, whereas Sz= -1/2 to the nearest carbon site. Applied to five model molecules, we could confirm this magnetic counting rule. In addition, we enhanced such concept to oxygen substituted zigzag edge occupied by four electrons.
Multiple spin state analysis of magnetic nano graphene
Norio Ota,Narjes Gorjizadeh,Yoshiyuki Kawazoe
Physics , 2011,
Abstract: Recent experiments indicate room-temperature ferromagnetism in graphite-like materials. This paper offers multiple spin state analysis applied to asymmetric graphene molecule to find out mechanism of ferromagnetic nature. First principle density functional theory is applied to calculate spin density, energy and atom position depending on each spin state. Molecules with dihydrogenated zigzag edges like C64H27, C56H24, C64H25, C56H22 and C64H23 show that in every molecule the highest spin state is the most stable one with over 3000 K energy difference with next spin state. This result suggests a stability of room temperature ferromagnetism in these molecules. In contrast, nitrogen substituted molecules like C59N5H22, C52N4H20, C61N3H22, C54N2H20 and C63N1H22 show opposite result that the lowest spin state is the most stable. Magnetic stability of graphene molecule can be explained by three key issues, that is, edge specified localized spin density, parallel spins exchange interaction inside of a molecule and atom position optimization depending on spin state. Those results will be applied to design a carbon-base ferro-magnet, an ultra high density 100 tera bit /inch2 class information storage and spintronic devices.
Multiple spin state analysis applied to graphite-like carbon-based ferromagnetism
Norio Ota,Narjes Gorjizadeh,Yoshiyuki Kawazoe
Physics , 2011,
Abstract: Recent experiments indicate room-temperature ferromagnetism in graphite like materials. This paper offers an multiple spin state analysis to find out the origine of ferromagnetism in case of nano meter size graphene molecule.First principle density function theory calculation (DFT-GGA with 631-G basis set) is applied to nano meter size asymmetric graphene fifteen molecules. Major results are,(1) Dihydrogenated zigzag edge molecule like C64H27 show that the most stable (lowest molecular energy) spin state is the highest one as Sz=5/2. Examples for spin density map of Sz=1/2,3/2 and 5/2 is shown in Fig.1. In other molecules like C56H24, C64H25, C64H22 and C64H23 also show the highest spin state most stable as shown in Fig.2. Energy difference between most stable spin state and next one overcome temperature difference 1000K,which suggests a stability of room temperature ferromagnetism. (2) Radical carbon zigzag edge molecules are also analysed. As illustrated in Fig.3, in every five molecule, also the highest spin state is most stable. (3) In contrast, nitrogen substituted molecules like C59N5H22, C61N3H22 etc. show opposite result,that is, the lowest spin state is most stable as shown in Fig.4. There are following three key issues to bring those results. (A) Edge specified localized spin arrangement. (B) Up-Up (also Down-Down) complex spin pairs inside of molecule. (C) Optimized atom position rearrangement depend on the spin state. Detailed mechanism will be discussed in the Symposium. Multiple spin state analysis is very useful to design carbon based ferro-magnet and also to design new spintronic devices.
Asymmetric nano graphene model applied to graphite-like room-temperature ferromagnetism
Norio Ota,Narjes Gorjizadeh,Yoshiyuki Kawazoe
Physics , 2011,
Abstract: Room temperature ferromagnetic materials composed only by light elements like carbon, hydrogen and/or nitrogen, so called carbon magnet, are very attractive for creating new material categories both in science and industry. Recently several experiments suggest ferromagnetic features at a room temperature, especially in graphite base materials. This paper reveals a mechanism of such ferromagnetic features by modeling nanometer size asymmetric graphene molecule by using both a semi-empirical molecular orbital method and a first principle density function theory. Asymmetrically dihydrogenated zigzag edge graphene molecule shows that high spin state is more stable in total molecular energy than low spin state. Proton ion irradiation play an important role to create such asymmetric features. Also, nitrogen contained graphite ferromagnetism is explained by a similar asymmetric molecule model.
Asymmetric graphene model applied to graphite-like carbon-based ferromagnetism
Norio Ota,Narjes Gorjizadeh,Yoshiyuki Kawazoe
Physics , 2011,
Abstract: Several experiments have recently found room-temperature ferromagnetism in graphite-like carbon based materials. This paper offers a model explaining such ferromagnetism by using an asymmetric nano-graphene. Our first typical model is C48H24 graphene molecule, which has three dihydrogenated (-CH2) zigzag edges. There are several multiple spin states competing for stable minimum energy in the same atomic topology. Both molecular orbital and density function theory methods indicate that the quartet state(S=3/2) is more stable than that of doublet (S=1/2), which means that larger saturation magnetization will be achieved. We also enhanced this molecule to an infinite length ribbon having many (-CH2) edges. Similar results were obtained where the highest spin state was more stable than lower spin state. In contrast, a nitrogen substituted (-NH) molecule C45N3H21 demonstrated opposite results. that is, the lowest spin state(S=1/2) is more stable than that of highest one(S=3/2), which arises from the slight change in atom position.
Lithium doped graphene as spintronic devices
Narjes Kheirabadi
Physics , 2015,
Abstract: Generating spintronic devices has been a goal for the nano-science. We have used density function theory to determine magnetic phases of single layer and bilayer lithium doped graphene nanoflakes. We have introduced graphene flakes as single molecular magnets, spin on/off switches and spintronic memory devices. To aim this goal, adsorption energies, spin polarizations, electronic gaps, magnetic properties and robustness of spin-polarized states have been studied in the presence of dopants and second layers. We find that for bilayer SMMs with two layers of different sizes the highest occupied molecular orbital and the lowest unoccupied molecular orbital switch between the layers. Based on this switch of molecular orbitals in a bilayer graphene SMM, spin on/off switches and spintronic memory devices could be achievable.
The Effects of Price Elasticity Dynamics on a Firm’s Profit
Ali Jazayeri,Narjes Jazayeri
Iranian Journal of Management Studies , 2011,
Abstract: This paper studies the dynamic behavior of price elasticity and its effects on the overall profit.Although price elasticity has a significant effect on sales, its dynamics have not been examined sofar in pricing models. In this paper, a simple pricing model is suggested in which, price elasticity isconsidered dynamic. The suggested pricing model is concerned with a monopolist that its objectiveis to maximize profit by determining the optimal price. Dynamics of price elasticity is described bya quadratic model, with product lifetime as the single dependent variable. By solving the modelusing the theory of optimal control, a system of differential equations is obtained which can be usedto find the optimal price trajectory. Finally, an example is provided to show how the dynamicbehavior of price elasticity can influence the firm's overall profit.
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