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Title Page

Abstract

Contents

I. Multiferroicity in atomic Van der Waals heterostructures 7

I.1. Introduction 7

I.2. Computational Method 8

I.3. Result and Discussion 9

I.4. Conclusion 17

I.5. References 17

II. Edge Functionalized Graphene Nanoribbons with tunable band alignments for Carrier Transport Interlayer in Organic-Inorganic Perovskite Solar Cells 20

II.1. Introduction 20

II.2. Computaional Method 21

II.3. Results and Discussion 23

II.4. Conclusion 26

II.5. References 27

List of Figures

Figure 1. Van der Waals Cr2Ge2Te6/In₂Se₃ heterostructure and magnetoelectric coupling. a Lateral unit cell size is chosen to...(이미지참조) 10

Figure 2. Spin-polarized and Cr d orbital-decomposed band structures of Cr2Ge2Te6/In₂Se₃ heterostructures, a and b. The...(이미지참조) 12

Figure 3. Calculated magnetocrystalline anisotropy of Cr2Ge2Te6 in the heterostructure versus the vdW interlayer distance....(이미지참조) 14

Figure 4. Magnetoelectric effect in In₂Se₃, mediated by the magnetic proximity to Cr2Ge2Te6. a Projected spin density of...(이미지참조) 15

Figure 5. Structures of (a) MAPbI₃(I₄/mcm) (b) H-GNR (c) F-GNR and (d) Clean MAPbI₃(001) 22

Figure 6. Band structures of (a) MAPbI₃ (b) H-GNR (c) F-GNR 23

Figure 7. Band alignments of (a) H-GNR and (b) F-GNR 24

Figure 8. (a) Optimized structure (b) Band structure and Density Of States of H-GNR/MAPbI₃ 25

Figure 9. (a) Optimized structure (b) Band structure and Density Of States of F-GNR/MAPbI₃ 26

초록보기

 The DFT (Density Functional Theory) method has been a powerful tool to investigate the electronic structures of molecules and atoms, especially been very useful in solid state physics. Here, the DFT method has been applied to study two interesting topics; Van der Waals Multiferroicity and Perovskite Solar cell.

Materials that are simultaneously ferromagnetic and ferroelectric – multiferroics – promise the control of disparate ferroic orders, leading to technological advances in microwave magnetoelectric applications and next generation of spintronics. Single-phase multiferroics are challenged by the opposite d-orbital occupations imposed by the two ferroics, and heterogeneous nanocomposite multiferroics demand ingredients’structural compatibility with the resultant multiferroicity exclusively at inter-materials boundaries. Here, we propose the two-dimensional heterostructure multiferroics by stacking up atomic layers of ferromagnetic Cr2Ge2Te6 and ferroelectric In₂Se₃, thereby leading to all-atomic multiferroicity. Through first-principles density functional theory calculations, we find as In₂Se₃ reverses its polarization, the magnetism of Cr2Ge2Te6 is switched, and correspondingly In₂Se₃ becomes a switchable magnetic semiconductor due to proximity effect. This unprecedented multiferroic duality (i.e., switchable ferromagnet and switchable magnetic semiconductor) enables both layers for logic applications. Van der Waals heterostructure multiferroics open the door for exploring the low-dimensional magnetoelectric physics and spintronic applications based on artificial superlattices.

In hybrid organic-inorganic halide perovskite solar cells, organic graphene nanoribbons (GNRs) can be a good candidate as carrier extraction interlayers owing to the capability of tuning the band edge energy levels through varying the size of width and the type of edge functionalization. Here, by using the density function theory (DFT) method, the electronic structures of H or F edge functionalized armchair type GNRs on MAPbI₃(001) are calculated. It is shown that the H/F-passivated GNR with width of seven carbon atoms exhibits a good band alignment for hole/electron extraction. Exploiting the unzipping synthesis of carbon nanotubes in the solution phase, our result provides a facial strategy for efficient carrier extraction.

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