Title Page

Contents

Abstract 22

Chapter 1. Introduction 24

1-1. Hot carriers 24

1-1-1. Excited carrier dynamics 24

1-1-2. Hot carrier dynamics 26

1-1-3. Hot carrier applications 28

1-2. Hot carriers in 2D semiconductors 32

1-2-1. Hot carrier physics in 2D materials 32

1-2-2. Hot carrier extraction from 2D semiconductors 33

1-3. 2D material/metal electrode interface 36

1-3-1. Electrode engineering 36

1-3-2. Electrode contact interfaces 37

1-4. Experimental method 41

1-4-1. Transient absorption spectroscopy 41

1-4-2. vdW metal mask 42

Chapter 2. Photophysics in 2D semiconductor/metal interfaces 46

2-1. Hot carrier transfer in monolayer TMD/metal heterostructures 46

2-1-1. Introduction 46

2-1-2. Experimental method 48

2-1-3. Results and Discussion 50

2-1-4. Conclusion 52

2-2. Carrier dynamics in bulk MoS₂/Au interfaces 56

2-2-1. Introduction 56

2-2-2. Experimental method 59

2-2-3. Results and Discussion 61

2-2-4. Conclusion 68

2-3. Evidence of hot carrier extraction in bulk MoS₂ 76

2-3-1. Introduction 76

2-3-2. Experimental method 78

2-3-3. Results and Discussion 79

2-3-4. Conclusion 81

2-4. Electrode engineering modulated absorption 85

2-4-1. Introduction 85

2-4-2. Experimental method 87

2-4-3. Results and Discussion 88

2-4-4. Conclusion 93

Chapter 3. 2D optoelectronic devices 99

3-1. Photovoltaic devices with vdW electrode engineering 99

3-1-1. Introduction 99

3-1-2. Experimental method 101

3-1-3. Results and Discussion 103

3-1-4. Conclusion 107

3-2. Dark current suppression in 2D Vertical device 111

3-2-1. Introduction 111

3-2-2. Experimental method 113

3-2-3. Results and Discussion 114

3-2-4. Conclusion 119

3-3. Silicon photodiode compatible photodetector 125

3-3-1. Introduction 125

3-3-2. Experimental method 128

3-3-3. Results and Discussion 129

3-3-4. Conclusion 134

3-4. SWIR photodetector via electrode-engineered midgap absorption 142

3-4-1. Introduction 142

3-4-2. Experimental method 145

3-4-3. Results and Discussion 145

3-4-4. Conclusion 151

References 159

논문요약 170

Figure 1-1-1. Schematic of the excited carrier dynamics of a general 2D semiconductor. 30

Figure 1-1-2. Power conversion efficiency of hot carrier solar cellas a function of absorber bandgap at various carrier temperatures. 30

Figure 1-1-3. Schematic of the ultrafast optoelectronic working based on the hot carriers. 31

Figure 1-1-4. Schematic of the wavelength and spin photocurrent achieved via extracting the hot carriers. 31

Figure 1-2-1. Hot carrier transport measurement with the pump-probe microscopy. 34

Figure 1-2-2. PL and transient absorption data demonstrating the hot carrier transfer from the MoS₂ to Au nanoparticles. 35

Figure 1-3-1. Ideal Schottky barrier achieved via the metal electrode transfer method. 39

Figure 1-3-2. The contact interface engineering for modulating the band hybridization in the interface. 39

Figure 1-3-3. The schematic of the interface condition between MoS₂ and metal electrode in ideal case and in real devices. 40

Figure 1-4-1. The schematic of the pump-probe transient reflection setup and the mechanism of the transient reflection signals. 43

Figure 1-4-2. The equipment setup and materials for the metal film transfer with the probe tip. 44

Figure 1-4-3. The representative process for the van der Waals metal mask assisted treatment. 44

Figure 1-4-4. The cross-section scanning transmission electron microscopy (STEM) image demonstrating the van der Waals contact between 2D material... 45

Figure 2-1-1. The schematic and the optical microscope image of the sample for the transient absorption spectroscopy. 53

Figure 2-1-2. (a, b) The transient absorption spectrum and the kinetics data in pristine WSe₂ and WSe₂/Au heterostructure. 53

Figure 2-1-3. The schematic of the carrier dynamics in the pristine WSe₂ and the WSe₂ on Au. 54

Figure 2-1-4. (a) The schematic of the MoS₂ sample which contacted its half part with Ti. (b, c) The kinetics data of A and B exciton in MoS₂ and MoS₂ Ti. 54

Figure 2-1-5. (a)The transient reflection mapping of pristine bulk MoS₂. (b) The steady state reflection and transient reflection spectrum at 2 ps delay time.... 55

Figure 2-1-6. The schematic of the photocarrier dynamics in the bulk MoS₂. 55

Figure 2-2-1. Schematic of the sample fabrication with the probe tip assisted metal film transfer method. 69

Figure 2-2-2. (a) Schematic of the interface structure in the sample. (b) Optical microscope image of the sample measured from the transparent... 69

Figure 2-2-3. (a, b) The atomic force microscope image of the flat Au and the rugged Au, respectively. 70

Figure 2-2-4. The scanning transmission electron microscope images of the MoS₂ and rugged Au interface. 70

Figure 2-2-5. The absorption spectra of the four different regions in the sample on Figure 2-2-2. 71

Figure 2-2-6. (a) The schematic for demonstrating the fermi level shift in the MoS₂ after contact with different Au. (b) The band bending condition of the... 71

Figure 2-2-7. The transient reflection mapping at different delay times. 72

Figure 2-2-8. (a)The △R kinetics data of the four different regions in the sample in Figure 2-2-7. (b) The extracted decay constant from Figure 2-2-8a. 72

Figure 2-2-9. The △R kinetics data of the MoS₂ and MoS₂/Au heterostructures in the sample demonstrated in Figure 2-2-2b. 73

Figure 2-2-10. The kinetics of transient reflection of sample #3. (a)The △R kinetics data of the MoS₂ and MoS₂/Au heterostructures. (b) The optical... 73

Figure 2-2-11. The kinetics of transient reflection of sample #4 which includes pristine MoS₂, MoS₂/pt, MoS₂/Au and MoS₂/Ag. 74

Figure 2-2-12. The schematic of the carrier dynamics happens in MoS₂ and MoS₂/Au interfaces. 74

Figure 2-2-13. The normalized △R kinetics of the MoS₂ and MoS₂/rugged Au corresponding to Figure 2-2-8a. 74

Figure 2-2-14. The hot electron transfer dynamics from Au to MoS₂ in different interfaces. (a) The schematic of the pump-probe reflection... 75

Figure 2-2-15. The schematic of hot electron dynamics of MoS₂ and MoS₂/Au interfaces. 75

Figure 2-3-1. The schematic of the carrier transport measurement with the pump-probe transient reflection microscopy. 82

Figure 2-3-2. Normalized spatial distributed △R signal in each time delay in pristine MoS₂ and MoS₂/Au interfaces. 83

Figure 2-3-3. Carrier transport in pristine MoS₂ and MoS₂/Au interfaces in the ultrashort timescale up to 1.4 ps extracted from Figure 2-3-2. 83

Figure 2-3-4. (a) Schematic of the photocarrier extraction from the MoS₂ to Au electrode within the lateral device structures. Bottom panel demonstrate the... 84

Figure 2-3-5. (a) The schematic of the carrier transport in the pristine MoS₂ and MoS₂ on rugged Au. (b) The schematic of the carrier transport in the MoS₂... 84

Figure 2-4-1. The schematic of the metal electrode deposition and defect generation in the interfaces. 94

Figure 2-4-2. The schematic of MoS₂/metal electrode interface fabricated with different electrode integration methods. 94

Figure 2-4-3. The absorption spectrum of three different MoS₂/Au electrode interfaces in Figure 2-4-2. 95

Figure 2-4-4. The cross section scanning transmission electron microscope image of three different MoS₂/Au interfaces demonstrated in Figure 2-4-2. 95

Figure 2-4-5. (a) The optical microscope image of the MoS₂ with its half area deposited with Au. (b) The absorbance in the MoS₂ with and without the... 96

Figure 2-4-6. The absorption spectrum of the ultrathin Au deposited on the MoS₂/Au heterostructure. The metal mask was used for covering half area... 96

Figure 2-4-7. (a) schematic of the sample which includes the ultrathin Au layers fabricated with all-van der Waals integration method. (b) The absorption... 97

Figure 2-4-8. The AFM image of the ultrathin Au layer deposited on MoS₂. The thickness of the Au layer is 2.3 nm and it remains ultraflat with this thin thickness. 97

Figure 2-4-9. (a) The optical microscope image as well as the thickness of the samples. (b) The absorption spectrum of the sample displayed in Figure 2-4-... 98

Figure 2-4-10. (a) The schematic of the intermediate band absorption in the defective interfaces. (b) The schematic of Fabry-Perot like cavity formed in... 98

Figure 3-1-1. The schematic of the PN junction formation in the channel due to the van der Waals electrode induced carrier doping. 108

Figure 3-1-2. (a) The atomic force microscope image of the template-striped Ag surface. (b) The atomic force microscope image of the template-striped Pt surface. 108

Figure 3-1-3. (a) The OM image of the device. (b) The current-voltage curve measured in the device showing the diode characteristics. 108

Figure 3-1-4. (a) The current-voltage curve of the device under dark and 1062 nm laser excitation. (b) The time dependent current sampling with the... 109

Figure 3-1-5. The schematic of the vertical photovoltaic device fabrication via the van der Waals metal mask and van der Waals electrode transfer. 109

Figure 3-1-6. (a) The schematic of the device structure. (b) The OM image of the vertical photovoltaic device. (c) The atomic force microscope image of... 109

Figure 3-1-7. The current-voltage curve of the device under dark and light conditions. 110

Figure 3-1-8. The photocurrent mapping demonstrates the dependence on the bias voltage and the excitation wavelength. 110

Figure 3-1-9. The wavelength-dependent photocurrent in different bias voltages. To elucidate the bias-dependent quantum efficiency, external... 110

Figure 3-2-1. (a) The schematic of the ultrafast photocarrier extraction in the vertical devices. (b) The van der Waals electrode engineering for blocking the... 120

Figure 3-2-2. The schematic shows the van der Waals integration of all components of device with van der Waals stacking. 120

Figure 3-2-3. (a) The optical microscope image of the fabricated device. (b) The cross-section scanning transition electron microscopy image of the device.... 121

Figure 3-2-4. The current-voltage curve of the device under dark and light conditions. It is demonstrated with the semi-log scale. 121

Figure 3-2-5. The excitation power dependent photocurrent measured in our vertical photodetector. 121

Figure 3-2-6. The current-voltage curve of the device scanned with the forward and reverse mode. 122

Figure 3-2-7. (a) The optical microscope image of several different devices with the same device structure. (b, c) The corresponding current-voltage curve... 122

Figure 3-2-8. (a) The schematic of vertical devices. (b-d) The vertical photodetectors with different device configurations. 123

Figure 3-2-9. The current-voltage curve of the vertical photodetector under dark and light conditions. It was demonstrated in the linear scale with two... 123

Figure 3-2-10. The schematic of the photocurrent response under negative and positive bias voltages. 124

Figure 3-2-11. (a) The optical microscope image (top panel) and the schematic of the device (bottom panel). (b, c) The current-voltage curve of the device,... 124

Figure 3-3-1. (a) The current-voltage curve with different light power density from 24 uW cm⁻² to 1.1 W cm⁻². (b) The photocurrent (top panel) and... 135

Figure 3-3-2. The light power density dependent photocurrent measured with the time resolved photocurrent measurement. 136

Figure 3-3-3. The linear photocurrent dependence on the power density in different wavelength laser of 458 nm, 640 nm and 725 nm. 136

Figure 3-3-4. The photocurrent under the different wavelength light. 137

Figure 3-3-5. (a) The photocurrent responsivity resolved by the bias voltage and the wavelength. (b) The wavelength dependent photocurrent responsivity... 137

Figure 3-3-6. (a) The photocurrent response speed measured with the pump-probe photocurrent measurement. (b, c) The photocurrent response speed... 138

Figure 3-3-7. The comparison of the photodetector performances with other 2D material-based photodetectors and silicon photodiode. 138

Figure 3-3-8. (a) The photograph of the real device on the bendable mica substrate and the schematic of the device structure. (b) The current-voltage... 139

Figure 3-3-9. The time dependent photocurrent measurement with the light on/off with every 20 seconds under the flat, bending and after 1000 times bending. 139

Figure 3-3-10. The 3D mapping data of the dark current and photocurrent under different temperature and bias voltages. 140

Figure 3-3-11. (a) The time dependent photocurrent under different temperatures. (b) The temperature dependent current under different bias voltages. 140

Figure 3-3-12. The current-voltage curve of the device before and after annealing at 200℃ measured under the dark and light conditions. 141

Figure 3-4-1. The equipment setup for measuring the localized photocurrent of the 1310 nm and 1550 nm wavelength. (b, c) The optical microscope image... 152

Figure 3-4-2. (a) The optical microscope image of the device with the deposited electrode and transferred electrode integrated to the MoS₂. (b) The... 153

Figure 3-4-3. (a) The basic current-voltage curve measured in the device shown In Figure 3-4-2a. 153

Figure 3-4-4. The photocurrent mapping of the device under 808 nm laser light under different bias voltages. 153

Figure 3-4-5. The schematic image of the carrier dynamics in the two different electrode interfaces. 154

Figure 3-4-6. The absorption spectrum of the MoS₂ on two different electrodes in the sample is shown in Figure 3-4-2a. 154

Figure 3-4-7. (a, b) The photocurrent mappings of -1 V and 1 V bias voltage, respectively, under the 455 nm laser excitation. 155

Figure 3-4-8. (a) The current-voltage curve of the device under dark and light illumination on the deposited Au electrode side with 1310 nm and 1550 nm... 155

Figure 3-4-9. (a) The time-dependent photocurrent under 1310 nm and 1550 nm light excitation on the deposited electrode side. (b)The time-dependent... 156

Figure 3-4-10. (a) The photocurrent response speed of the device with 1310 nm and 1550 nm light excitation. (b) The linear dependence of the photocurrent... 156

Figure 3-4-11. The device fabricated with the structure of deposited Au/TMD/transferred Au with changing the TMD materials. (a-d) The current-... 157

Figure 3-4-12. The photocurrent response of the device. The large area beam covering the whole device area was illuminated both for 1310 nm and 1550 nm... 158

Figure 3-4-13. The comparison of the device performance for the 2D material-based infrared photodetectors. 158