Title Page
Contents
ABSTRACT 14
Ⅰ. Introduction 17
1.1. Introduction of two-dimensional materials 17
1.1.1. Graphene 19
1.1.2. Molybdenum disulfide (MoS₂) 23
1.1.3. Other two-dimensional materials; Perovskite oxide nanosheets 27
1.2. Devices based on two-dimensional materials 31
1.2.1. Electronical devices based on two-dimensional materials 32
1.2.2. Devices using magnetic properties 34
1.2.3. Synaptic devices based on two-dimensional materials 37
Ⅱ. Properties of functionalized two-dimensional materials 40
2.1. Introduction 40
2.2. AFM lithography on graphene and MoS₂ 42
2.2.1. Experimental section 44
2.2.2. Raman measurement for functionalized graphene 46
2.2.3. MFM measurement of graphene and oxidized graphene 49
2.2.4. Magnetic properties of oxidized graphene measured by MOKE and PEEM-XMCD 54
2.2.5. STXM measurement of MoS₂ and oxidized MoS₂ 58
2.3. Two-dimensional thin film; 2D perovskite nanosheet 60
2.3.1. Experimental section 61
2.3.2. XPS Measurement of 5-layer nanosheets 62
2.4. Conclusion 63
Ⅲ. Synaptic transistors based on charge trapping in two-dimensionally heterostructure 64
3.1. Introduction 64
3.2. Experimental section 67
3.3. Electrical properties 69
3.3.1. Hysteretic electrical properties depending on gate bias 69
3.3.2. Mechanism of charge trapping 73
3.4. Synaptic properties 75
3.5. Conclusion 84
Ⅳ . Conclusion 85
References 86
Appendix 98
Abstract (in Korean) 99
Figure 1-1. The schematic diagram of all graphitic forms. Graphene is a 2D building material for carbon materials of all other dimensionalities. 18
Figure 1-2. (a) Raman spectra of pristine (top) and defected (bottom) graphene (b) the graph of D-peak position as a function of excitation energy,... 20
Figure 1-3. Model diagram of transition metal atom absorbed on single and double vacancies in graphene. (a) side view and (c) top view of metal atom on... 21
Figure 1-4. (a) Schematics of a multilayer WS₂/graphene heterostructure device, (b) optical image of a completed device with multiple Hall bar junction... 22
Figure 1-5. Schematic and AFM image of single layer MoS₂ (a) Three- dimensional structure of MoS₂, (b) AFM image of a single layer MoS₂ deposited... 23
Figure 1-6. Schematics of the energy band diagram in the ambient environment when (a) no gate bias stress, (b) positive gate bias stress, and (c)... 25
Figure 1-7. Atomic structures of oxide nanosheet and graphene 27
Figure 1-8. Images of perovskite oxide nanosheets. (a and b) Atomic structure and (c and d) AFM image with height profile of LaNb₂O₇ and Ca₂Nb₃O₁₀,... 30
Figure 1-9. (a) Top view of metal semiconductor field-effect transistor (MESFET) with 10-layer MoS₂ channel, NiOx top-gate and Au/Ti source/drain electrodes in...[이미지참조] 33
Figure 1-10. CrI₃/CrCl₃ heterostructure and MOKE measurement. (a) Schematics of the magnetic ground states in bilayer CrI₃ and few layer CrCl₃... 35
Figure 1-11. MCD microscopy of twisted bilayer CrI₃. (a-d) Magnetic field dependence of MCD of a natural bilayer CrI₃ with twist angle 0°, 1.2°, 4°, and... 36
Figure 1-12. (a) Illustration of the synaptic behavior by the artificial memtransistor using the gate terminal for pre-synapse and source-drain terminal for post-... 39
Figure 2-1. Schematic of graphene locally oxidized by using AFM lithography, which has ferromagnetic order 42
Figure 2-2. (a) Optical microscope image of exfoliated graphene with Cr/Au electrode. Oxidized graphene areas with various lithography conditions are... 45
Figure 2-3. Raman spectra of graphene and graphene oxidized by applying 5 V, 7 V, and 10 V during AFM lithography, respectively. 48
Figure 2-4. AFM image (leftmost) and Raman mapping images (from the left, D peak range, D' peak range, and 2D peak range, respectively) of graphene with... 48
Figure 2-5. AFM image (top) and MFM images (middle and bottom) of oxidized areas formed by applying voltages of 10V (left), 7V (middle), and 5V (right), respectively. 51
Figure 2-6. Line profiles of MFM phase signal, which are obtained from the MFM images. 51
Figure 2-7. AFM topography and MFM phase images of MoS₂ with oxidized areas, which were measured by using a MFM tip with (a) upward and (b and c)... 53
Figure 2-8. Out-of-plane MOKE hysteresis loops measured for the oxidized graphene areas 56
Figure 2-9. AFM and PEEM images of the single-layer graphene containing locally oxidized areas fabricated by AFM lithography. 57
Figure 2-10. XMCD spectra for oxidized graphene areas with three different tip- sample bias voltages. 57
Figure 2-11. STXM data at Mo edge region (385 - 425 eV) of oxidized MoS₂ by using applied voltage of (a) 10 V and (b) 5 - 7 V, and (c) pristine MoS₂,... 59
Figure 2-12. STXM data at O edge region (525 - 550 eV) of oxidized MoS₂ by using applied voltage of (a) 10 V and (b) 5 - 7 V, and (c) pristine MoS₂, respectively. 59
Figure 2-13. STXM data at S edge region (158 - 185 eV) of oxidized MoS₂ by using applied voltage of (a) 10 V and (b) by 5 - 7 V, (c) pristine MoS₂, respectively. 59
Figure 2-14. X-ray photoelectron spectra and their fitting results for 5 layers of (a) SNO and (b) SCNO-0.2. 62
Figure 3-1. (a) A schematic picture and (b) optical image of a MoS₂/SCNO transistor. 67
Figure 3-2. Raman spectra of MoS₂ on SNO (blue) and SCNO-0.2 (pink) nanosheets. 68
Figure 3-3. Transfer characteristic of MoS₂ FET on SCNO-0.2 at Vds=0.01 V in the gate voltage sweep ranging from -30 V to 30 V, -40 to 40 V, and -50 V to...[이미지참조] 71
Figure 3-4. Transfer characteristic of MoS₂ FET on SCNO-0.1 at Vds=0.01 V in the gate voltage sweep ranging from -30 V to 30 V, -40 to 40 V, and -50 V to...[이미지참조] 71
Figure 3-5. Transfer characteristic of MoS₂ FET on SNO at Vds=0.01 V in the gate voltage sweep ranging from -30 V to 30 V, -40 to 40 V, and -50 V to 50 V....[이미지참조] 72
Figure 3-6. Variation of the memory window size (△Vth) as a function of Vgs max of MoS₂ FET on SCNO-0.2 (blue circle), SCNO-0.1 (green circle), and SNO...[이미지참조] 72
Figure 3-7. Schematic diagram of the mechanism for the hysteretic field effect under gate voltage sweep applied to MoS₂ FET on (a) SCNO-0.2, (b) SCNO-... 74
Figure 3-8. The synaptic potentiation/depression processes of MoS₂ FET on SCNO-0.2 (pulses with -8 V/3 V amplitude and 5 ms width) Each... 76
Figure 3-9. The synaptic potentiation/depression processes of MoS₂ FET on SCNO-0.1 (pulses with -10 V/8 V amplitude and 10 ms width). 77
Figure 3-10. The synaptic potentiation/depression processes of MoS₂ FET on SNO (pulses with -35 V/30 V amplitude and 100 ms/50 ms width). 77
Figure 3-11. A typical EPSC of an MoS₂ /SCNO-0.16 synaptic device triggered by a pair of pre-synaptic spikes with a pulse voltage of -15 V, pulse duration of... 80
Figure 3-12. The PPF index dependent on time interval of two consecutive input spikes. 81
Figure 3-13. The EPSCs stimulated by the series of nine input pulses with different frequencies. 82
Figure 3-14. Change in EPSC amplitude rate (A₉/A₁) as a function of pre synaptic spike frequency. 83