Investigation of light-induced carrier behaviors in monolayer MoS₂ with Kelvin probe force microscopy
- 주제(키워드) MoS , KPFM , Photogating
- 주제(DDC) 621.042
- 발행기관 아주대학교
- 지도교수 박지용
- 발행년도 2021
- 학위수여년월 2021. 8
- 학위명 박사
- 학과 및 전공 일반대학원 에너지시스템학과
- 실제URI http://www.dcollection.net/handler/ajou/000000031104
- 본문언어 영어
- 저작권 아주대학교 논문은 저작권에 의해 보호받습니다.
초록/요약
In this thesis, we analyzed the properties of 1D and 2D nanomaterials using Kelvin Probe Force Microscopy (KPFM) and Atomic Force Microscopy (AFM), which are based on Scanning Probe Microscopy (SPM). And also, various nanomaterials and nanodevices required for the research were synthesized and fabricated. First, the surface potential distribution of monolayer Molybdenum disulfide (MoS₂) flake was mapped by KPFM measurement. Typically, the photoresponsivity of monolayer MoS₂ increases under light illumination. By applying these properties to a photonic device, the distribution of charges in monolayer MoS₂ flake due to illumination was investigated. When light is irradiated on monolayer MoS₂, the work function increase, and the observed change in the surface potential distribution may be related to charge carrier generation, diffusion, and recombination in MoS₂ under light illumination. The polarity of surface potential changes points to the trapping of photogenerated holes at the interface between MoS₂ and the substrate as a major mechanism for the photoresponse in monolayer MoS₂. The temporal response of the surface potential changes is compatible with the time constant of monolayer MoS₂ photodetectors. We also investigate spatial inhomogeneity in the surface potential changes at the low light intensity that is related to the defect distribution in MoS₂. In addition, we observed that the effects of MoS₂/SiO₂ interfacial separation, passivation and P-doping were enhanced by chemical treatment. Finally, we measured the photoresponse of MoS₂ in a nitrogen environment to confirm the effects of adsorption and photodesorption of water or oxygen molecules. Next, single-walled carbon nanotube (SWCNTs) and multi-walled carbon nanotube (MWCNTs) devices embedding a polymer matrix were measured using Atomic Force Microscopy (AFM). We investigate the interaction of SWCNTs and MWCNTs embedded in a polymer matrix [Poly(methyl methacrylate) (PMMA)] with Ar plasma, which results in the formation of PMMA nanostructures as CNTs act as an etching mask. Due to the difference in the Ar ion sputtering yields between the CNTs and PMMA, PMMA lines with a width similar to that of the CNTs (for SWCNTs) or as high as 80 nm (for MWCNTs) could be obtained. After repeatedly exposing the CNT/PMMA film to Ar plasma, IV characteristics and Raman spectrum changes were investigated to confirm the etching mechanism. We follow the etching process by investigating changes in IV characteristics and Raman spectra of CNTs after each exposure to Ar plasma, which shows progressive defect generations in CNTs while they maintain structural integrity long enough to act as an etching mask for PMMA underneath. The PMMA nanostructured patterns can be transferred to other polymer substrates, such as nanoimprinting.
more목차
Chapter 1 Introduction 1
Chapter 2 Theoretical background 7
2.1 1D and 2D nanomaterials 7
2.1.1 Carbon nanotube 7
2.1.2 Molybdenum disulfide 12
2.2 Atomic force microscopy 16
2.2.1 Principle of AFM 16
2.2.2 Non-contact mode AFM 18
2.2.3 Contact mode AFM 21
2.3 Kelvin probe force microscopy 22
2.3.1 Introduction to KPFM 22
2.3.2 Fundamentals of KPFM 23
2.4 References 27
Chapter 3 Imaging Spatial Distribution of Photogenerated Carriers in Monolayer MoS₂ with Kelvin Probe Force Microscopy 31
3.1 Introduction 31
3.2 Experimental Methods 33
3.2.1 KPFM measurements 33
3.2.2 Growth of MoS₂ 35
3.2.3 Growth characterizations 35
3.2.4 Device fabrication & Electrical characterizations 35
3.2.5 Post-deposition of Sulfur 36
3.3 Results and Discussion 37
3.3.1 Photoresponse of MoS₂ 37
3.3.2 KPFM measurements of MoS₂ 38
3.3.3 Temporal response of surface potential 46
3.3.4 Surface potential changes according to the intensity of light 48
3.4 Conclusion 51
3.5 References 52
Chapter 4 Photogating effect of MoS₂ in chemical treatment and environmental control 58
4.1 Introduction 58
4.2 Experimental Section 60
4.2.1 Growth of MoS₂ 60
4.2.2 Chemical treatment 60
4.2.3 Measurement in N₂ environment 61
4.3 Results and Discussion 61
4.3.1 Photogating effect in Chemical treatment 61
4.3.2 Photogating effect in N₂ environment 65
4.4 Conclusion 68
4.5 References 69
Chapter 5 Carbon Nanotubes as an Etching Mask for the Formation of Polymer Nanostructures 72
5.1 Introduction 72
5.2 Experimental Section 74
5.2.1 CVD growth of CNTs 74
5.2.2 Deposition of CNTs from dispersion solution 74
5.2.3 Device fabrication 75
5.2.4 Transfer process and Ar plasma etching process 75
5.2.5 Characterization of CNTs and devices 75
5.2.6 Preparation for TEM observation 76
5.3 Results and Discussion 77
5.3.1 Formation of Polymer nanostructures using SWCNTs 77
5.3.2 Estimation of the nanostructure profile from AFM images 88
5.3.3 Formation of polymer nanostructures using MWCNTs 90
5.3.4 Etching results for bundled SWCNTs 92
5.3.5 Formation nanoscale trenches with h-PDMS 93
5.4 Conclusion 95
5.5 References 96
Chapter 6 Summary and future work 102
6.1 Summary 102
6.2 Future work 103
