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Structural Control of Emission, Polarization, and Light Extraction in Solution-Processed Luminescent Thin Films

  • 전규담 (Gyudam Jeon, 아주대학교 일반대학원)

초록/요약

This thesis investigates structural-control strategies for solution-processed luminescent thin films to improve their emission efficiency, light extraction, optical gain, and polarization behavior. Such films are promising platforms for optoelectronic, display, sensing, and photonic applications, but their optical properties depend strongly on film morphology, energy transfer, scattering, light extraction, and optical feedback as well as on the intrinsic emitters. Three systems were studied: carbon dot （CD）/polymer films, mechanoluminescent （ML） elastomer films, and quasi-two-dimensional （quasi-2D） perovskite films. First, blue- and green-emitting CDs were dispersed in a poly（vinyl alcohol） matrix. Förster resonance energy transfer from the blue to the green CDs enhanced the green emission, and one-dimensional periodic nanostructures transferred by nanoimprint lithography produced grating-mediated outcoupling. The combined effects enhanced the photoluminescence and gave angle-dependent directional emission. Second, light extraction from ZnS:Cu–polydimethylsiloxane ML films was improved using a flexible nematic liquid-crystal （NLC）–polyurethane overlayer. Phase-separated liquid-crystal droplets generated haze-based scattering through refractive-index mismatch, and transmittance and haze measurements confirmed that this scattering was broadband across the visible range. This scattering redistributed internally trapped photons and improved outcoupling while retaining mechanical flexibility. Incorporating a red-emitting dye extended the strategy to green-to-red wavelength conversion. Finally, naphthylmethylammonium （NMA）-based quasi-2D perovskite– poly（ethylene oxide） （PEO） composite films were studied as optical gain media. The quasi-2D structure provides exciton confinement and energy funneling, while the NMA spacer reduces triplet-related losses. PEO content and spin-coating speed were optimized to control phase distribution and morphology, and anti-solvent treatment and poly（methyl methacrylate） passivation improved film quality and suppressed non-radiative recombination. Distributed-feedback （DFB） nanostructures provided wavelength-selective optical feedback, while polarization-dependent absorbance and polarizing optical microscope observations suggested preferential in-plane optical anisotropy associated with the nanostructured morphology. This anisotropic response is consistent with grating-guided transition-dipole alignment inferred from optical measurements, while the polarization direction of the DFB lasing output is considered to be governed mainly by cavity-mode selection and polarization- dependent optical feedback. The amplified spontaneous emission and DFB lasing, usually demonstrated under pulsed excitation, were extended to CW excitation, where PMMA-passivated DFB films achieved clear CW DFB lasing while still requiring further improvement in operational stability.

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목차

Chapter 1 Introduction 1
1.1 Carbon Dots 1
1.2 Mechanoluminescence 3
1.3 Perovskites 4
1.4 Outline of Thesis 6
Chapter 2 Structural Design of Carbon Dot Thin Films for Energy-Transfer-Mediated Emission Control 8
2.1 Introduction 8
2.2 Experimental Section 15
2.2.1 Synthesis of B-CDs and G-CDs 15
2.2.2 Fabrication of CD/PVA/Nanostructure Films 16
2.3 Results and Discussion 18
2.3.1 Measurement Setup and Characterization Methods 18
2.3.2 Morphology and Chemical Characterization of B-CDs and G-CDs 20
2.3.3 Optical Properties of B-CDs and G-CDs in Aqueous Solutions 25
2.3.4 Emission Color Tuning of B-CD/G-CD Composite Films: Drop-Cast Films 26
2.3.5 Emission Color Tuning of B-CD/G-CD Composite Films: Spin-Coated Films 28
2.3.6 Energy-Transfer-Mediated Emission Control in CD/PVA Composite Films 31
2.3.7 Nanostructure-Assisted Angle-Dependent PL of CD/PVA Composite Films 34
2.4 Conclusions 41
Chapter 3 Light Extraction Enhancement in Mechanoluminescent Elastomer Films 42
3.1 Introduction 42
3.2 Experimental Section 48
3.2.1 Fabrication of ZnS:Cu–PDMS Films 48
3.2.2 Fabrication of PU and NLC-PU Films 49
3.2.3 Fabrication of DCM-Doped PU and NLC-PU Films 50
3.3 Results and Discussion 51
3.3.1 Measurement Setup and Characterization Methods 51
3.3.2 Morphological and Strain-Induced Optical Properties of PU and NLC-PU Films 52
3.3.3 Strain- and Polarization-Dependent Transmittance of PU and NLC-PU Films 55
3.3.4 Thickness-Dependent Total Transmittance and Haze Characteristics of PU and NLC-PU Films 58
3.3.5 ML and PL Characteristics of ZnS:Cu–PDMS Films with PU and NLC-PU Overlayers 61
3.3.6 Morphological and Optical Characteristics of DCM-Doped PU and NLC-PU Films 64
3.3.7 Broadband Optical Characteristics of DCM-Doped PU and NLC-PU Films 66
3.3.8 ML and PL Characteristics of ZnS:Cu–PDMS Films with DCM-Doped PU and NLC-PU Overlayers 68
3.4 Conclusions 71
Chapter 4 Continuous-Wave Lasing in Quasi-2D Perovskite Thin Films 72
4.1 Introduction 72
4.2 Experimental Section 82
4.2.1 Synthesis of Quasi-2D Perovskite Precursor and PEO Solution 82
4.2.2 Fabrication of Pristine Quasi-2D Perovskite Thin Films 82
4.2.3 Film Enhancement via Anti-Solvent Treatment and PMMA Passivation 83
4.2.4 Fabrication of DFB Nanostructures for CW Lasing 85
4.3 Results and Discussion 87
4.3.1 Measurement Setup and Characterization Methods 87
4.3.2 PEO-Assisted Phase Distribution and PL Enhancement in NMA-Based Quasi-2D Perovskite–PEO Composite Films 89
4.3.3 Optimization of Spin-Coating and Anti-Solvent Treatment for PL Enhancement in NMA-Based Quasi-2D Perovskite–PEO Composite Films 92
4.3.4 PMMA-Assisted Surface Passivation and PL Enhancement in NMA-Based Quasi-2D Perovskite–PEO Composite Films 95
4.3.5 Processing-Dependent Morphological and Structural Characteristics of NMA-Based Quasi-2D Perovskite–PEO Composite Flat Films 97
4.3.6 Processing-Dependent ASE Characteristics of NMA-Based Quasi-2D Perovskite–PEO Composite Flat Films under 355 nm Pulsed Excitation 103
4.3.7 Structural and Morphological Characterization of Nanostructured NMA-Based Quasi-2D Perovskite–PEO Composite Films 106
4.3.8 Optical and Polarization-Dependent Properties of Nanostructured NMA-Based Quasi-2D Perovskite–PEO Composite Films 110
4.3.9 DFB Lasing Characteristics under 355 nm Pulsed Excitation in Nanostructured NMA-Based Quasi-2D Perovskite–PEO Composite Films 116
4.3.10 DFB Lasing Characteristics under 405 nm CW Laser Pumping in Nanostructured NMA-Based Quasi-2D Perovskite–PEO Composite Films 121
4.4 Conclusions 129
Chapter 5 Conclusions 130
List of Publications 132
References 133
Chapter 1 133
Chapter 2 134
Chapter 3 139
Chapter 4 143

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