以奈米結構設計增強光與物質交互作用之研究

Abstract

本論文研究了透過兩種途徑增強光與物質相互作用的奈米結構設計：1.利用雙曲超材料 (HMM) 調控色散關係，以傳播high-k體等離子激元 (VPP/BPP) 並調節光子態密度； 2.利用自組裝極性單分子層進行界面偶極子工程，以定向排列偶激子。借助 VPP，我們在有機太陽能電池中實現了更高的激子生成，從而提高了光捕獲效率。透過將 VPP 與金奈米粒子的局域表面等離子體共振 (LSPR) 相干耦合，或在 ITO 上形成化學吸附的 P3HT-COOH 單分子層，經開爾文探針力顯微鏡 (KPFM) 和紫外光電子能譜 (UPS) 驗證，我們展示了加速的 Förster 共振能量轉移、增強的自發輻射能量轉移、增強的自發輻射能量和降低雷射閾值。我們的研究成果分為四個主要主題，並總結如下：

1.基於雙曲超穎材料的非傳統有機太陽能電池結構 雙曲超穎材料（HMM）因其可強化光和物質交互作用、調控光子態密度並產生體電漿極化子（BPP），近年備受關注。本研究首次將 HMM 應用於有機太陽能電池（OSCs）並展現優異效能。所設計之 HMM 結構由多對 MoO3/Au 疊層組成，並在主動層的主要吸收波段呈現雙曲色散行為。為了有效將 BPP 耦合進入主動層，我們將 HMM 的第一對層設計為電池結構的一部分，兼作電洞傳輸層與陽極。引入 HMM 的 OSC（HMM-OSCs）相較於僅使用一對 MoO3/Au 的相似裝置，其能量轉換效率（PCE）提升約 29.4%；與採用全反射電極的傳統 OSC 相比，PCE 亦提升約 11%。與傳統 OSC 相較，其效能增益主要來自短路電流密度在寬廣波段的提升，源於光生電荷轉換效率提高；依據光電流分析與穩態光致發光（PL）量測，這可歸因於激子生成與解離機率較高。實驗觀察與理論模擬相符，兩者皆顯示所設計之 HMM 結構可增強局部電場強度與激子生成率。因此，所設計之 HMM-OSCs 能成功將 BPP 導入器件運作，對提升光吸收效率產生顯著影響。所提出的設計可作為 HMM 擴展至其他光電元件應用的指引。 2.藉由局域表面電漿共振與體電漿子極化子之量子相干耦合增強光和物質交互作用 雙曲超穎材料（HMMs）以其強健的光和物質交互作用與產生體電漿極化子（VPP）的能力而著稱；金奈米粒子（Au NPs）則可藉由局域表面電漿共振（LSPR）形成強烈的局部電場，進一步強化此交互作用。然而，在同一器件中同時結合 LSPR 與 VPP 的作法仍少有探討。本研究提出並實驗證實金奈米粒子耦合之 HMM（NPCHMMs），將 Au NPs 與 HMM 整合，以增強隨機雷射作用並降低雷射門檻。相較於純 HMM 與純 Au NPs，NPCHMMs 的雷射強度分別提升約 6.6 倍與 8 倍，同時雷射門檻降低約 47%。根據費米黃金定律（Fermi’s golden rule）之計算，NPCHMMs 的躍遷速率超過由 HMM 與 Au NPs 各自貢獻之躍遷速率代數和，揭示了 VPP 與 LSPR 躍遷矩陣元素之間的相干耦合效應。研究結果顯示，NPCHMMs 為實現高效能光學與光電元件（如雷射、光電電晶體等）之有前景平台，適用於多種應用領域。 3.透過表面電漿與體電漿子極化子之相干耦合強化福斯特共振能量轉移 福斯特共振能量轉移（FRET）已廣泛應用於生物感測、發光元件與有機太陽能等領域，如何提升其效率一直備受關注。本研究首次提出並實作一種將金屬奈米粒子（NPs）所誘發之表面電漿（SP）與雙曲超穎材料（HMMs）所支援之體電漿極化子（VPP）整合於同一器件的作法，藉由表面電漿耦合體電漿極化子（SPCVPP）效應來強化 FRET。值得注意的是，在採用供受體為 CdSe/ZnS 量子點（QD）對之 SPCVPP 樣品中，FRET 效率可達 83.5 ± 0.1%。此外，為進一步探討 SPCVPP 效應，我們將其工作原理延伸至雷射作用研究，發現受激輻射因 SPCVPP 耦合而顯著增強，並以混合藍/紅發光 QDs 為例，相較於僅具 Au NPs 的相同 QD 對，雷射門檻降低 63%。有趣的是，SPCVPP 器件在 FRET 效率與雷射發射強度方面的提升，均明顯優於單獨 HMM 或單獨 Au NP 的效果，且超越兩者增強效應之簡單代數相加。其根源可由 SP 與 VPP 的相干耦合來理解：此耦合提升了 FRET 效率並放大了受激輻射的躍遷速率。根據費米黃金律，對應於 SP 與 VPP 之哈密頓量躍遷矩陣元素相干耦合的理論計算，與實驗觀察高度一致，驗證了本研究方法之正確性。整體而言，SPCVPP 效應揭示了驅動 FRET 增強與雷射作用的底層機制，為開發高效率先進光子裝置提供了重要科學洞見。 4.以自組裝極化單分子層輔助之激子排列與能量轉移相干耦合提升發光 基有效控制能量轉移與激子動力學是推進光電與雷射裝置的關鍵。本研究顯示，藉由自組裝單分子層（SAM）的輔助，使排列良好的激子與共振能量轉移（RET）產生相干耦合，可大幅提升量子點（QDs）/金奈米粒子（Au NPs）複合系統之光學表現。具體而言，poly[3-(6-carboxyhexyl) thiophene-2,5-diyl]（P3HT-COOH） 在 ITO 上形成之極化單分子層，已由 Kelvin 探針力顯微鏡（KPFM）、紫外光電子能譜（UPS）及其他量測證實。相較於旋塗且偶極隨機取向之 P3HT-COOH 薄膜，QDs 置於該極化 SAM 上時，其光致發光（PL）強度提升約 55%，且激子壽命縮短至約 4.5 ns。此外，系統於 42 W∙cm-2 即出現低門檻隨機雷射，而在旋塗薄膜上則未觀察到。進一步地，由於 P3HT-COOH 發光與 QDs 吸收之光譜重疊，系統內部存在共振能量轉移。因此，整體性能之提升可歸因於極化 SAM 所促成之激子良好排列與 RET 的協同效應，兩者共同導致相干耦合並觸發輻射復合的超輻射（superradiance），最終實現隨機雷射行為。此一作法為提升光電與光子元件效能提供了穩健而有效的策略。

This dissertation investigates designed nanostructures that enhance light–matter interactions through two routes: (i) dispersion engineering with hyperbolic metamaterials (HMMs) to access high-k volume/bulk plasmon polaritons (VPP/BPP) and tailor the photonic density of states, and (ii) interfacial-dipole engineering with self-assembled polar monolayers to align excitons. With the assistance of VPP, we achieve higher exciton generation in organic solar cells to enhance light harvest efficiency. By coherently coupling VPPs with localized surface plasmon resonance (LSPR) from Au nanoparticles or forming chemisorbed P3HT-COOH monolayers on ITO, as verified by Kelvin probe force microscopy (KPFM) and ultraviolet photoelectron spectroscopy (UPS), we demonstrate accelerated Förster resonance energy transfer, enhanced spontaneous emission, and low-threshold random lasing. Our results are classified into four main topics and summarized as follows:

1. Unconventional Organic Solar Cell Structure Based on Hyperbolic Metamaterial HMM has attracted considerable attention due to its enhanced light-matter interaction for tuning photonic density of states and producing bulk plasmon polariton (BPP). In this thesis, we demonstrate the application of HMM in organic solar cells (OSCs) with superior performance for the first time. The designed HMM structure composed of multiple pairs of MoO3/Au stacks possesses a hyperbolic dispersion behavior in the primary light absorption regime of the photoactive material. To effectively couple BPP into the photoactive layer, the first pair of the HMM structure is designed to be a portion of the OSC structure, serving as the hole transport layer and anode. The unconventional HMM incorporated OSCs (HMM-OSCs) exhibit an ~29.4% enhancement in power conversion efficiency (PCE) relative to the similar OSCs using one pair of MoO3/Au and an ~11% improvement in PCE as compared to conventional OSCs with fully reflective electrode. Compared to conventional OSCs, the performance improvement is primarily from the improved short circuit current density from a broad wavelength range of the enhanced photon-to-charge conversion efficiency due to the higher exciton generation and dissociation probability as suggested by the photocurrent analysis and steady-state photoluminescence measurements. The experimental observation agrees well with that inferred from theoretical simulation for the enhancement in the local electric field and exciton generation rate for the designed HMM structure. Therefore, the designed HMM-OSCs can successfully couple BPP into operation with a large impact on enhancing light absorption efficiency. Our design principle can serve as a useful guideline for the application of HMM in other optoelectronic devices.

2. Enhancement of Light-Matter Interaction Induced by Quantum-Coherent Coupling between Localized Surface Plasmon Resonance and Volume Plasmon Polariton HMMs are known for their robust light-matter interaction and ability to generate VPPs. Au nanoparticles (Au NPs) enable to enhance this interaction through LSPR, creating intense local electric fields. However, combining LSPR and VPPs in one device remains unexplored. This thesis proposes and demonstrates Au NP-coupled-HMMs (NPCHMMs), integrating Au NPs with HMMs to enhance random laser action and lower the lasing threshold. NPCHMMs boost emission intensity by approximately 6.6 and 8 times compared to pure HMMs and Au NPs, respectively, with a ~ 47% reduction in lasing threshold. Based on Fermi’s golden rule, the calculated transition rate in NPCHMMs surpasses the algebraic sum of the individual transition rates derived from HMMs and Au NPs. It reveals the effect of the coherent coupling between the transition matrix elements of VPP and LSPR. This research indicates that NPCHMMs are a promising platform to create high-performance optical and optoelectronic devices, such as lasers and phototransistors, for a wide range of application in many fields.

3. Enhancement of Fluorescence Resonance Energy Transfer by Coherent Coupling in-between Surface Plasmon and Volume Plasmon Polariton Förster resonance energy transfer (FRET) has been widely utilized in various domains, spanning from biosensors to light-emitting devices and organic photovoltaics. Enhancement of the efficiency of FRET has attracted a great deal of attention. In this study, we have proposed and demonstrated a first attempt of the integration of SP and VPP induced by metal NPs with HMMs in a device that possesses surface-plasmon-coupled-volume-plasmon-polariton (SPCVPP) effect for the enhancement of FRET. Notably, an enhanced FRET efficiency as high as 83.5±0.1% has been attained for the donor-acceptor CdSe/ZnS quantum dot (QD) pairs implemented in the SPCVPP sample. Additionally, to further explore the SPCVPP effect, we adapted the working principle to the study of laser action and found that stimulated emission of the QD pairs is enhanced significantly based on the SPCVPP effect, driving by the enhancement of FRET. The lasing threshold is reduced by 63% via the SPCVPP effect for the mixed blue- and red- emitting as QDs, compared to the same QD pairs with Au NPs. Interestingly, it is found that the SPCVPP device reveals a very large enhancement of the efficiency of FRET as well as laser action emission intensity compared to individual HMM and Au NP effect, exceeding the simple algebraic sum of their individual enhancement. The underlying origin of the large enhancement can be well understood based on the coherent coupling of SP and VPP, which results in the enhanced FRET efficiency and amplifies the transition rate of the stimulated emission. Theoretical calculations according to Fermi’s golden rule for the coherent coupling of the Hamiltonian transition matrix element with respect to SP and VPP closely agree with experimental observations, confirming the validity of our approach. The SPCVPP effect sheds light on the underlying mechanisms driving the FRET enhancement and laser action, offering deep scientific insights for the development of advanced photonic devices with improved efficiency and performance.

4. Enhancement of Light Emission via Coherent Coupling of Aligned Excitons and Energy Transfer Assisted by Self-Assembled Polar Monolayer Enhancing light-matter interactions through effective control of energy transfer and exciton dynamics is essential for advancing optoelectronic devices and lasers. Herein, we demonstrate a substantial enhancement in the optical performance of quantum dots (QDs)/Au nanoparticle (Au-NPs) composite through the coherent coupling of aligned excitons and resonant energy transfer assisted by a self-assembled monolayer (SAM) of poly[3-(6-carboxyhexyl) thiophene-2,5-diyl] (P3HT-COOH). The SAM of P3HT-COOH forms a polar monolayer on indium-tin-oxide (ITO), as evidenced by KPFM, UPS, and several other measurements. It is found that the photoluminescence (PL) intensity of QDs on this polar SAM dramatically increases by ~ 55% with a shortened exciton lifetime of ~4.5 ns in contrast to those deposited on a spin-coated P3HT-COOH thin film with random arranged dipoles. Additionally, the emergence of random lasing behavior occurs at a relatively low threshold of 42 W∙cm-2 whereas it is absent on the spin-coated thin film. Further, resonant energy transfer is involved for the spectral overlap between the emission of P3HT-COOH and the absorption of QDs. Therefore, the improved optical performance can be attributed to the synergistic effects of energy transfer and coherent coupling of well aligned excitons in QDs assisted by the polar SAM, which triggers the superradiance process of the radiative recombination and enables to induce random lasing behavior. This innovative approach highlights a robust strategy for enhancing the performance of optoelectronic and photonic devices.