氧化鈦奈米線為主體之異質結構與硒化物 核殼量子點:載子動力學及光電轉換應用

Abstract

半導體奈米異質結構因能帶結構的差異，產生高效載子分離性質與優越的光催化能力，可應用於產生各種太陽燃料。另一方面，將具有核殼結構的硒化物量子點應用於發光二極體，可使元件發光具波長可調性與色彩高飽和度之優勢。因此，光能與電能間的轉換，本質上皆與材料內部的載子傳輸現象有關，本論文主要針對對半導體異質結構與核殼硒化物量子點進行載子動力學分析，探討結構內部載子轉移現象與其光電轉換應用間的相互關聯性，本論文分為三部份主題進行探討: 第一部份是以NaxH2-xTi3O7單晶奈米線為主要研究材料，並在其表面接枝Au奈米顆粒，使光激發電子可由NaxH2-xTi3O7傳遞至Au端，達到載子分離的特性，並經由最適化成份比例調整後，使NaxH2-xTi3O7-Au在可見光照射下表現高光催化活性，接著以光沈積法可進一步在所接枝的Au顆粒上成長一薄層的Cu2O，形成NaxH2-xTi3O7-Au-Cu2O之Z-scheme奈米異質結構。在此能階結構中，Au可作為載子傳輸板，而展現高度載子分離特性與優異的光催化氧化還原能力。本研究利用時間解析螢光光譜技術，來對發生在此Z-scheme奈米異質結構界面間的載子傳輸行為做定量化描述，以建立樣品之相對能帶結構、界面載子動力學與載子被導出利用效率之間的關連性。 第二部份的主題是研究金屬奈米顆粒敏化半導體材料，其金屬的表面電漿共振(surface plasmon resonance, SPR)吸收與光催化性質的關聯性。當金屬奈米顆粒(例如Ag及Au)經由SPR吸收可見光後，最外層的電子可被激發至高能態，並產生具氧化還原力的光催化活性。經由調控Ag奈米顆粒修飾於SiO2奈米球表面之尺寸與分佈密度，可使SPR吸收產生寬化及紅移的現象。隨著Ag的SPR吸收範圍越寬時，所展現的可見光催化活性就越高。另一方面，調控修飾TiO2奈米線表面之Au奈米顆粒的形貌後，將TiO2-Au奈米異質結構應用於光電化學水分解時，Au奈米晶體經由SPR吸收後產生的高能態電子，可進一步注入TiO2的傳導帶，同時在Au端留下類似電洞的正電位狀態做為額外載子來源，來增進TiO2在可見光照射下的水分解效能，且提升效能之可見光區間可對應至Au奈米結構之SPR吸收波長。此研究也同時發現，Au奈米顆粒的接枝可有效修飾TiO2的表面狀態，使TiO2在紫外光照射下進行水分解時減少載子經由缺陷複合的機率，進而提升TiO2光陽極的工作效率。有鑒於TiO2表面狀態對於光催化水分解之影響，本研究進一步開發一簡易的前軀物處理法，可有效地在rutile TiO2奈米線表面包覆厚度約2 nm的anatase TiO2殼層，此殼層可有效修飾rutile TiO2奈米線的表面狀態，並提升TiO2在光電化學系統中的水分解效能。 第三部份的主題為開發熱注射合成系統，製備具核殼結構之硒化物量子點，隨著反應時間的調控，此量子點產物會因殼層成份擴散進入核心而產生即時合金程序，使得量子點成份改變而發出各種不同顏色的螢光。經相關的分析結果顯示，反應初期量子點之組成成份為核殼CdSe/ZnSe，隨著反應時間拉長，其核心晶體成份經過與殼層的合金化轉變為Cd1-xZnxSe以至最後接近ZnSe。利用簡易的發光元件組裝製作，可製備出對應螢光發光波段的高亮度電致發光元件，說明此研究製備之Cd1-xZnxSe核殼量子點，可作為顯示器中薄型光源使用，以提供高飽和色彩的彩度 。

Due to the difference in band structure between the constituents, semiconductor heterostructures exhibit remarkable charge separation property which is beneficial to solar fuel generation. On the other hand, the advantages of quantum dots-based light emitting diodes (QD-LEDs) include color tunability, high color saturation and high color rendering index (CRI) white lighting. The performance of both photon-to-electron and electron-to-photon conversions is closely related to the intrinsic charge carrier dynamics of the constitutes. In this dissertation, the correlation between the charge carrier dynamics and the photoelectric conversion efficiency for semiconductor heterostructures and core/shell Cd1-xZnxSe QDs was investigated. Three individual yet relevant projects were included in the dissertation: First, we demonstrated that Au-decorated NaxH2-xTi3O7 nanobelts (NaxH2-xTi3O7-Au NBs) may exhibit remarkable photocatalytic performance under visible light illumination due to the remarkable charge separation property. In order to further enhance the photocatalytic efficiency, a thin layer of Cu2O was deposited on the Au surface of the Au-decorated NaxH2-xTi3O7 NBs to form Z-scheme NaxH2-xTi3O7-Au-Cu2O nanoheterostructures. Because of the relative band alignment of the constituents, Au may mediate the carrier transfer of NaxH2-xTi3O7-Au NBs to render them enhanced photocatalytic performance. Time-resolved photoluminescence (PL) spectra were measured to quantitatively analyze the electron transfer in the Z-scheme NaxH2-xTi3O7-Au-Cu2O NBs. The carrier utilization efficiency of the samples was evaluated and the result was correlated with that of the charge carrier dynamics measurement, which may provide insightful information when using Z-scheme heterostructures in photoconversion applications. Second, we investigated the plasmonic effect of noble metal nanocrystals on the photocatalytic properties of semiconductor nanostructures. Since the surface plasmon resonance (SPR) of metal (e.g. Ag and Au) energizes the conduction electrons and excites them from the outermost bands to higher energy states, there is a great probability that these electrons can participate in chemical reactions. We developed a Ag-decorated SiO2 NSs, which exhibited significantly red-shifted and relatively broad SPR absorption spanned from visible to near-infrared region. The photocatalytic activity of Ag-decorated SiO2 NSs was corresponded with the SPR absoption ability. On the other hand, by acting as an antenna that localizes the optical energy by SPR, plasmonic Au can sensitize TiO2 to light with energy below the band gap, generating additional charge carriers for water oxidation. The photoactivity of Au-decorated TiO2 electrodes for photoelectrochemical water oxidation can be effectively enhanced in the entire UV-visible region from 300 nm to 800 nm, by manipulating the shape of the decorated Au nanostructures. The analysis results suggested that the enhanced photoactivity of Au NP-decorated TiO2 nanowires in UV region was attributed to effective surface passivation. Since the existence of surface states greatly affected the photoconversion performance of TiO2, we employed a facile precursor-treatment approach for effective surface passivation of rutile TiO2 nanowire photoanode to improve its performance in photoelectrochemical water oxidation. Last, we developed a single-step hot-injection process to synthesize core/shell Cd1-xZnxSe QDs with tunable emission wavelengths. Because of the higher reactivity of the Cd precursor, QDs whose composition was rich in CdSe were generated at the beginning of reaction. As the reaction proceeded, the later-formed ZnSe shell was simultaneously alloyed with the core, giving rise to a progressive alloying treatment for the grown QDs. During the reaction period, the continued blue shifiting emissioned Cd1-xZnxSe QDs were obtained. A LED composed of conducting polymer with Cd1-xZnxSe QDs was fabricated to test the electroluminescence properties, which show high color purity for the emissions from LED. The findings from this work also demonstrate the advantage of using the current single-step synthetic approach to obtain a batch of Cd1-xZnxSe QDs that may emit different colors in prototype LEDs.