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Time-resolved IR Pump-probe Study of Intraband Transitions in Self-doped HgSe Colloidal Quantum Dots

초록/요약

This dissertation investigates the intraband carrier dynamics of self-doped HgSe colloidal quantum dots (CQDs) using femtosecond infrared pump–probe spectroscopy. Pump fluence dependent transient absorption signals were measured for CQDs of three different diameters (2.9, 5.3, and 7.5 nm). By varying pump fluence and particle size, transient absorption dynamics were analyzed and interpreted using a four-state kinetic model consisting of biexciton state, higher state (hs), single exciton state, and hot ground state (HGS) populations. This model quantitatively describes the rates of intraband Auger processes, internal conversion from hs to single exciton state, subsequent exciton recombination to HGS and phonon-phonon scattering to ground state. In addition, a table-top terahertz time-domain spectroscopy (THz-TDS) system was developed based on laser-driven air- plasma generation and air-biased coherent detection (ABCD). The system was validated by resolving low-frequency vibrational features in ambient air, PTFE polymer, and liquid water.

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초록/요약

본학위논문은자가도핑(self-doped) HgSe콜로이드양자점(CQD)의밴드내(intraband)전하 이동 및 이완 동역학을 femtosecond 적외선 들뜸-탐침 분광법을 통해 조사하였다. 펌프 세기 (pump fluence)와입자크기를변수로하여서로다른직경(2.9, 5.3, 7.5 nm)을갖는CQD에대한 시분해흡수스펙트럼을측정하였고,이를바탕으로쌍엑시톤상태,상위상태(higher state, hs), 엑시톤 상태(single exciton), 고온 바닥 상태(hot ground state, HGS)의 점유도(population) 변화 를 4상태 동역학 모델로 분석하였다. 글로벌 피팅을 통해 밴드내 오제 과정, 상위 상태에서 여 기 상태로의 내부 전이(internal conversion), HGS로의 여기자 재결합(excitonic recombination), 이후 바닥 상태로의 phonon-phonon 산란 속도를 정량화하였다. 또한, 레이저 유도 공기 플라 즈마 발생과 공기-바이어스 결맞음 탐지기 (ABCD)에 기반한 소형 테라헤르츠 시간영역 분광 시스템(THz-TDS)을구축하였고,이를통해상온대기, PTFE고분자,액체물등의저주파진동 응답을 성공적으로 검출하였다.

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목차

ABSTRACT i
국문 초록 ii
TABLE OF CONTENTS iii
List of Figures vi
List of Tables xiii
Chapter 1. Introduction 1
1.1 Background on Self-Doped Colloidal Quantum Dots 2
1.2 Motivation for Size- and Pump Fluence-Resolved Spectroscopy 3
1.3 Review of Prior Studies 3
1.4 Objective and Scope of this Study 5
Chapter 2. Theoretical Background on CQD states and Time-resolved Spectroscopy 6
2.1 Quantum Confinement in CQDs 7
2.2 Effective Mass Theory 9
2.3 Self-doping in HgSe Colloidal Quantum Dots 10
2.4 Infrared Absorption Spectroscopy 11
2.5 Infrared Pump–Probe Spectroscopy 11
2.5.1 Definition of Transient Absorption Signal 12
2.5.2 Signal Origin and Interpretation 14
2.5.3 Coherent Artifact 16
Chapter 3. Experimental Methods for Quantum Dot Dynamics Study 17
3.1 Sample Preparation 18
3.1.1 Synthesis of HgSe Colloidal Quantum Dots 18
3.1.2 Purification and Handling Procedures 18
3.1.3 Size Characterization 18
3.1.4 FTIR Spectra 21
3.2 Infrared Pump-probe Spectroscopy Setup 22
3.2.1 Overview of Pump-probe Configuration 22
3.2.2 Beam Geometry and Delay Line 22
3.2.3 Measurement Protocol and Data Acquisition 24
Chapter 4. Infrared Relaxation Dynamics of Self-doped HgSe Colloidal Quantum Dots 25
4.1 Overview of Investigated System and Objectives 26
4.2 Degenerate IRPP Results 27
4.3 Kinetic Modeling and Simulation 32
4.3.1 Four-State Rate Equation Model 32
4.3.2 Simulation of Transient Absorption Signal 35
4.3.3 Degenerate IRPP Fitting Results 39
4.4 Non-degenerate IRPP Results and Fitting 45
4.4.1 1Se–1Pe Resonant Pump-1Pe–1De Resonant Probe (2600 cm−1–2000 cm−1) 45
4.4.2 Off-Resonant Pump-1Se-1Pe Resonant Probe (2000 cm−1-2600 cm−1) 50
4.5 Summary 54
Chapter 5. Development of a Terahertz Time-domain Spectroscopy System 55
5.1 Introduction 56
5.2 Experimental Setup for Air-Plasma-Based THz-TDS 61
5.2.1 Comparison Between Ambient Air and N2 Environment with Vapor Peak
Assignment 62
5.2.2 THz Transmission Measurement of PTFE Pellet 67
5.2.3 THz Absorption Characteristics of Liquid Water 70
5.3 Summary 72
Chapter 6. Conclusion and Future Outlook 73
6.1 Summary of Research Outcomes 74
6.2 Implications of Quantum Dot Spectroscopy 74
6.3 Final Remarks 75
References 76

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