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- Type of Document: M.Sc. Thesis
- Language: Farsi
- Document No: 43466 (05)
- University: Sharif University of Technology
- Department: Electrical Engineering
- Advisor(s): Faez, Rahim
- Abstract:
- In this thesis the effects of impurities and different defects on electrical and magnetic properties of monolayer and bilayer graphene is investigated. Calculations are performed using density function theory. Alloying monolayer graphene with boron and nitrogen the energy gaps of 0.7 and 0.6 eV are obtained, respectively. Moreover, we show that BN is a good candidate for opening the band gap in graphene. Alloying monolayer graphene with BN the energy gap of 1.2 eV is obtained. Another way of opening band gap is to somehow break the symmetry between layers of the bilayer graphene. By alloying one layers of bilayer graphene with B, N or B-N, although the symmetry of layers is broken, each layer behaves separately. Therefore the energy gap, which is opened in alloyed layer, will be closed with the density of state of an intrinsic layer. However upon alloying both layers with B, N and BN impurities the energy gaps of 0.6, 0.5 and 1.2 eV were gained, respectively. In the case of B and N, because of interlayer interaction, the energy gap in bilayer grphene is less than that in monolayer graphene. If we alloy one layer with N and another with B the energy gap will be closed, since the interlayer distance increases and the energy gap is opened in different energies in each layer, therefore the energy gap of structure is closed. These structures are nonmagnetic, because when the graphene is alloyed with impurities like N and B, π orbital of these impurities is located in parallel with π orbitals of structure, and due to overlapping of these orbitals the imbalance in spins are balanced and the structure remains nonmagnetic. Upon introducing monovacancy in monolayer graphene, a magnetic moment of 0.57 is obtained. This magnetization is because due to an imbalance of the spin of electrons in π orbitals of the structure. In bilayer graphene with a monovacancy in one of the layers, the magnetization is lower than monolayer one. Interlayer ineraction is the main reason of this feature. Upon introducing monovacancy in each layer, the structure can become magnetic or nonmagnetic depending on vacancy type (A or B). If we position N atom on top of monolayer graphene, the structure becomes magnetic. The electron positioned in π orbital of N is located vertically with π orbitals of structure without any overlapping between them, therefore the structure becomes magnetic. In the case of B, since π orbital of B is empty the structure remains nonmagnetic. If N and B are positioned among two layers, these structures become magnetic, with the difference that the boron is bonded with two layers but nitrogen is bonded with one layer. When B-N is located between two layers the magnetic moment of 2 is earned, in this structure every layer is separately magnetic. Inaddition, the band gap of 1.2 eV is obtained for this structure. therefore, the last structure can be used in both electronic and spintronic applications.
- Keywords:
- State Density ; Density Functional Theory (DFT) ; Band Structure ; Band Gap ; Magnetization ; Monolayer Graphene ; Bilayer Graphene
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