| 研究課題/領域番号 |
15F15374
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| 研究機関 | 大阪大学 |
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研究代表者 |
吉矢 真人 大阪大学, 工学(系)研究科(研究院), 准教授 (00399601)
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| 研究分担者 |
ABDUL MAJID マジド 大阪大学, 工学(系)研究科(研究院), 外国人特別研究員
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| 研究期間 (年度) |
2015-10-09 – 2018-03-31
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| キーワード | Computational study / Semiconductors / Monolayer / Prediction of Properties / Doping |
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| 研究実績の概要 |
The calculations conducted to study structural stability of pure MoS2 revealed that displaced Mo and S atoms settled on regular lattice sites when full relaxation is allowed. The study of increase in vacuum thickness between adjacent layers of MoS2 revealed that Mo-S bond strengthens and thickness of MoS2 layer decreases when bulk material scales down to monolayer. Energy band gap of monolayer is higher than that of bulk MoS2.
The value of heat of formation of MoS2 monolayer was calculated which showed that thermodynamically allowed range is 0 eV (for S rich) to 1.50 eV (for Mo rich).
To design the strategy of doping into MoS2, we studied Ti (similar to Mo) and Y (being aliovalent) doped cases. The calculated values of defect formation energies indicate that doping of Ti is more feasible in MoS2 than that of Y (larger size than Ti). Moreover, formation energy was high for metallic oxides in comparison to that of metallic environment of the dopants. To start with Rare Earth (RE) doping, Ce doped MoS2 monolayer was studied in detail. Defect formation energy was calculated by changing Ce content in full compositional range from MoS2 to CeS2. The formation energy for terminal compounds was least whereas material having Mo and Ce in equal ratio exhibited the highest value.
In order to explore ways for MoS2, we examined preferred lattice site location of dopants in well-studied compound semiconductor GaN. The findings indicated that Ti relaxed 0.6 percent off-side along negative c-axis whereas Ce settled 2.7 percent off-site along negative c-axis in fully relaxed hexagonal GaN.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
1: 当初の計画以上に進展している
理由
Our previous research interests include study of modifications in electronic, optical and magnetic properties of the materials upon doping with transition metals and RE metals. It helped a lot to start and accelerate research on approved this JSPS topic.
One reason of the success is detailed optimization of computational parameters to study the materials of interest. Though it took some time but provided base to calculate reliable results. The study of variation in c-axis length of MoS2 happened to be very helpful to find a way to detach adjacent layer in MoS2 unit cell to construct monolayer. A series of calculations enabled us to know that structure of MoS2 strongly depends upon total number of electrons in the crystal. It provided motivation to dope MoS2 with dopants of different nature to tailor the properties of MoS2 for applications in optoectronics and spintronics.
Out of plethora DFT packages, the choice of code suitable to meet the challenges of the approved project directed us to Vienna Ab initio simulation package (VASP). Its plane wave basis formalism provides precise values of total energy of structures which is required to calculate formation energy of materials. It helped to explore the probability of formation of pure and doped MoS2. Furthermore, VASP enables geometrically optimize the crystals to attain minimum energy configuration which is a key for defects related calculations on doped materials. The computational experiments to know preferential dopant site in relaxed structure were very helpful before calculating meaningful results.
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| 今後の研究の推進方策 |
Foreign atoms may segregate on surface, form clusters and/or secondary phases instead of homogeneous doping in the host matrix. We therefore planned to study the seating preference of RE dopants on cationic sites of MoS2. In order to execute this, RE dopant will be allowed to relax after initial placement at different locations around substitutional lattice site.
One portion of our future planning is to calculate defect formation energy of the materials. In addition to neutral dopants, charged dopants shall also be considered in this regard to find more probable charge state of the dopant in the host. Formation energy versus Fermi energy will be plotted for the materials.
The values of bond lengths and angles, lattice constants, c/a parameter will be monitored to know the effects of doping on structure which leaves strong impact on properties of materials. A detailed analysis to explore the mechanism of exchange interactions in the materials will be carried out.
One important part of future planning is to explore thermodynamic properties of RE doped MoS2. Owing to monolayerd nature of target material (MoS2), study of thermodynamical properties will be very useful to know surface free energy and Gibbs free energy of doping. In order to do so, 'Phonopy' code will be used.
In order to utilize the material for devices, metallic contacts are needed. One of the current problems in metal/MoS2 interfaces is high contact resistance. It is planned that RE-metal/RE:MoS2 hetrostructures will be studied to explore energy level alignment and possible implications on devices.
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