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Title Page

Abstract

Contents

Abbreviations 19

Chapter 1. Introduction 20

1.1. Surface functionalization of nanomaterials 20

1.2. Nanomaterials 22

1.3. MLD of polymers 26

1.4. Molecule surface interactions 28

1.5. The main goals of the research 29

1.6. Scope of the research 30

1.7. Organization of this dissertation 30

1.8. Contributor, funding, and copy rights 31

References 32

Chapter 2. Computational Techniques 35

2.1. A few on computational chemistry 35

2.2. Quantum chemistry 36

2.2.1. Density functional theory (DFT) 36

2.2.2. Modeling substrate surface for calculations 40

2.2.3. Geometry optimization 41

2.2.4. Quantum chemistry programs 42

2.2.5. Computational output analysis 42

References 44

Chapter 3. Molecular adsorption and doping of Hafnium and Zirconium Dichalcogenides 47

3.1. Introduction 47

3.2. Methods 50

3.3. Results and discussion 54

3.3.1. Electronic properties of 1T and 1H Zr (Hf) dichalcogenides 54

3.3.2. NH₃ and NO₂ adsorption on trigonal prismatic (1H) and octahedral (IT) phase 63

3.3.3. Charge transfer and Band structure 73

3.4. Conclusions 83

References 84

Chapter 4. Quantum chemical study on atomic layer deposition of Al₂O₃ on TMDCs 89

4.1. Introduction 89

4.2. Methods 93

4.3. Results and discussion 94

4.3.1. Geometry optimization of MoS₂, WS₂, WSe₂ and h-BN 94

4.3.2. Properties of precursor interactions with the substrate surface 100

4.3.3. Reaction energy, barrier energy and desorption energy 103

4.4. Conclusions 105

References 106

Chapter 5. Dissociative adsorption of gas molecules on 2D nanomaterials 111

5.1. Introduction 111

5.2. Methods 114

5.3. Results and discussion 116

5.3.1. Structural parameters optimization of graphene, silicene and germanene materials 116

5.3.2. Nature of the substrate's surface and physical adsorption 118

5.3.3. Reaction energy at the Ortho, meta, and para positions 120

5.4. Conclusions 124

References 125

Chapter 6. Overall Concluding Remarks 128

6.1. Conclusions 128

6.2. Perspectives 131

Appendix 133

Appendix A. Published article 133

A1. Effect of molecular backbone structure on vapor phase coupling reactions between diiso (thio) cyanates with diamines, diols, and dithiols 133

Appendix B. Dimer-DMAI Vs. Monomer TMA for Al₂O₃ deposition 154

List of Tables

Table 3.1. The optimized lattice parameters (a) of Trigonal prismatic (1H) and Octahedral... 57

Table 3.2. Calculated indirect band gap values for trigonal prismatic (1H) and octahedral... 61

Table 3.3. The Preferable adsorption site, orientation of gas molecules, adsorption energy (Eads), adsorption-distance (d) and the...[이미지참조] 66

Table 3.4. The Preferable adsorption site, orientation of gas molecule, adsorption energy (Eads), adsorption-distance (d) and the...[이미지참조] 67

Table A1. Differences in Ea and △E of the coupling reactions with aromatic versus... 145

List of Figures

Figure 1.1. show the band filling diagram of nanomaterials 24

Figure 1.2. depicts optimized structure of (a) representative structure of monoatomic... 25

Figure 1.3. depicts methods of film deposition steps (a) ALD (b) MLD 27

Figure 2.1. Classification of methods used in computational calculations 39

Figure 3.1. 2D Zr (Hf) dichalcogenides,(a) Adsorption sites on trigonal prismatic (1H)... 53

Figure 3.2. depicts the unit cell and Brillouin zone of Zr/Hf(S, Se, Te) 56

Figure 3.3. Band structures of 1H (top) and 1T (bottom) pristine 2D Zr (Hf)... 59

Figure 3.4. Adsorption configurations and orientations of NH₃ (top-row) and NO₂... 65

Figure 3.5. Adsorption configurations and orientations of NH₃ (top-row) and NO₂... 69

Figure 3.6. Adsorption energy comparison of (a) NH₃ (b) NO₂ on trigonal prismatic (1H)... 72

Figure 3.7. Comparisons of charge transfers with respect to adsorption energy for (a)... 75

Figure 3.8. Charge density difference plots for NH₃ (top) and NO₂ (bottom) interacting... 77

Figure 3.9. Charge density difference plots for NH₃ (top) and NO₂ (bottom) interacting... 78

Figure 3.10. Band structures of pristine, NH₃ and NO₂ adsorbed monolayer trigonal... 81

Figure 3.11. Band structures of pristine, NH₃ and NO₂ adsorbed monolayer octahedral (1T)... 82

Figure 4.1. optimized lattice parameters of a unit cell (a) MoS₂ (b) WS₂ (c) WSe₂ (d) h-BN 96

Figure 4.2. optimized K-vector points of a unit cell (a) MoS₂ (b) WS₂ (c) WSe₂ (d) h-BN 97

Figure 4.3. optimized ENCUT energies of a unit cell (a) MoS₂ (b) WS₂ (c) WSe₂ (d) h-BN 99

Figure 4.4. summarizes interaction energy during (a) physisorption (b) chemisorption 102

Figure 4.5. Calculated reaction energy for ALD of Al₂O₃ on different 2D materials using... 104

Figure 5.1. Geometry of different two-dimensional materials 113

Figure 5.2. represents (a) Considered positions (b) Dissociation of gas molecules 115

Figure 5.3. (a) Unit cell and a 4x4 supercell sheet of graphene, germanene or silicene (b)... 117

Figure 5.4. physical adsorption energy of gas molecules on different 2D materials 119

Figure 5.5. reaction energy for the dissociation of molecules at different positions on 2D materials 122

Figure 5.6. reaction energy for the dissociation of SO₂ on different 2D materials 123

Figure A1. (a) Schematic definition of the critical points on the reaction coordinates.... 137

Figure A2. Calculated geometries of the reactants, transition states, and products. White ＝... 138

Figure A3. Correlation between experimental and calculated (a) basicity, (b) acidity; (c)... 141

Figure A4. Correlation between calculated Ea and △E values of the coupling reactions.... 143

Figure A5. Correlation of Ea and △E values with acidity and basicity of the R-(YH)₂... 147

Figure B1. depicts optimized 2D TMDCs (a) MoS₂ (b) WSe₂ (c) WS₂ (d) h-BN monolayer... 154

Figure B2. depicts overall reaction energy and desorption energy of (a) Erxn of dimer-...[이미지참조] 156

Figure B3. depicts comparison of reaction energy of dimer-DMAI and monomer-TMA... 157

Figure B4. depicts comparison of desorption energy of dimer-DMAI and monomer-... 158

Figure B5. depicts four different types of precursors used for Al₂O₃ deposition 159

초록보기

Two-dimensional materials have attracted tremendous attention due to their unique physical and chemical properties since the discovery of graphene. The novel properties of 2D materials spur fundamental studies and technological advancements for a wide range of applications such as optoelectronics, superconductors, and catalysis. Here, 2D materials (graphene, germanene, silicene and h-BN) and 2D transition metal dichalcogenides [Zr/Hf (S, Se and Te), MoS₂, WS₂ and WSe₂] have chosen for this study. All DFT calculations study in this work have carried out with slab models using Vienna ab initio simulation package (VASP, version 5.4.4).

Firstly, we have studied the molecular adsorption and doping of Zr and Hf dichalcogenides to tune their electronic properties. One possible method to precisely tune the material properties of the atomically thin two-dimensional nanomaterials is to adsorb molecules on their surfaces as non-bonded dopants. Hence, molecular adsorption of NO₂ and NH₃ on two-dimensional Hf and Zr dichalcogenides (S, Se, Te) was examined. Adsorption configuration, energy, and charge transfer properties during molecular adsorption were calculated. In addition, the effects of the molecular dopants (NH₃ and NO₂) on the electronic structure of the materials are studied. It was observed that adsorbed NH₃ molecule donates electron to the conduction band of Hf (Zr) dichalcogenides, while NO₂ received electron from valence band. The resulting band structure of the molecularly doped Zr and Hf dichalcogenides are modulated by the molecular adsorbates. This confirms that the material properties of the substrates can be tuned by introducing molecular dopants such as NH₃ and NO₂ to TMDCs.

Extending our work, we have also studied deposition of Al₂O₃ based on four different precursors (TMA, TIBA, DMAI and ATIP) to understand what is going during reactions, to suggest an alternative precursor and what factors affect the deposition process through quantum simulation study. Thin films are essential in a large variety of modern equipment and technologies. One method of depositing thin film is atomic layer deposition (ALD), which is a unique technique to deposit ultrathin films in a controlled manner. While a right choice of the precursor is needed to make the film growth possible, the choice will also together with physicochemical properties are vital. Reaction wise, it was observed that for all precursors' chemisorption reactions are energetically favorable for the deposition of Al₂O₃ on MoS₂, WS₂ and WSe₂ materials. However, ATIP is less reactive on the substrate's surface due to its fragments are larger in size and hence increase steric hindrance. This may block the nearby active sites of the substrate's surface and results poor film deposition. Apparently, TIBA & DMAI precursors can be good alternative precursors in addition to TMA to grow Al₂O₃ successfully using water as a counter reactant.

Ended, we have also worked on the tendency of graphene, germanene, silicene and h-BN 2D nanomaterials to dissociate gas molecules (NH₃, PH₃, H₂O, H₂S and SO₂). Since adsorption occurrence, intermolecular forces which include van der Waals attraction or hydrogen bonding between the molecules and the substrate's surface as well as surface reactivity are considered crucial factors. It was observed that the physical adsorption of molecules on all these 2D materials shows similar patterns. Despite the difference in energy, the dissociation reaction of molecules on graphene, germanene and h-BN surfaces are endothermic. While all gas dissociation reactions on silicene are exothermic, suggesting that it is more reactive than others. In conclusion, graphene, germanene and silicene 2D materials can be used as a catalyst to dissociate gas molecules.

Generally, 2D materials are still demanding lots of work to dig out their versatile applications in various fields. In this regard, theoretical study using density functional theory can broaden understanding and outlooks about 2D materials potential applications. We believe that this dissertation work can shed light on the insights of functionalizing two-dimensional materials via molecular adsorption.

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