第二型半導體奈米結構的光學探討以及利用表面電漿共振增強第一型和第二型量子點間能量轉移效率

Optical properties of type II semiconductor nanostructures and surface plasmon resonance enhanced energy transfer between type I and type II quantum dots

王群雄(Chun-Hsiung Wang)

指導教授 ： 陳永芳

國立臺灣大學/理學院/物理研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.01594

摘要

本論文的重點分為兩個部分，包含了第二型奈米結構的光學研究以及表面電漿共振增強第一型和第二型量子點間能量轉移效率。所有的實驗數據是經由光激螢光光譜儀(Photoluminescence )、螢光激發光譜(Photoluminescence excitation)、時間解析螢光光譜儀(Time-resolved photoluminescence)、吸收光譜儀(Absorption spectroscopy)以及電子顯微鏡(Scanning electron microscopy)所測量。第一部份關於第二型奈米結構的實驗結果可以幫助我們設計更有效率的量子點光電元件。第二部分關於第一型及第二型量子點能量轉移的發現則是提供了一個發可見光量子點將其激子(exciton)能能量轉移到紅外光量子點的可能性。此外藉由奈米金粒子的表面電漿共振特性，上述能量轉移現象可以被增強。以下是本論文的詳細摘要:

I. 第二型奈米結構的光學研究
1. Recombination dynamics in CdTe/CdSe type-II quantum dots
藉由變溫螢光生命週期量測，我們發現在低溫下(小於150 K) CdTe/CdSe 中的激子是屬於侷限型，而隨著溫度升高，激子開始轉變為自由型。同時我們發現CdTe/CdS粒徑大小也會改變激子的生命週期，這是因會量子侷限效應所造成的影響。此外隨著激發光強度增強，激子的生命週期也隨之減短，直接證明了常見於第二型奈米半導體中能帶彎曲的現象。

2. Direct evidence of type II band alignment in P3HT/CdSe heterostructures
除了上述CdTe/CdSe量子點外，我們發現P3HT/CdSe 有機/無機奈米混合結構也具有第二型螢光性質。藉由時間螢光解析光譜儀，我們發現光激載子會藉由電荷轉移的方式而分離，所以在界面上複合產生第二型螢光。此外，我們也發現P3HT/CdSe的螢光生命週期長達幾百奈秒，以及隨著激發光強度增強，螢光光譜會藍移。最後藉由改變CdSe粒徑大小，P3HT/CdSe的螢光光譜也會受到影響。這些實驗結果提供一個直接的光學證據證明P3HT/CdSe的能帶是屬於第二型能帶結構。
II. 表面電漿增強第一型和第二型量子點間能量轉移效率
3. Resonant energy transfer between CdSe/ZnS type I and CdSe/ZnTe type II quantum dots
藉由光激螢光時間解析螢光測量，我們發現能量轉移的現象可以存在於第一型和第二型量子點間。螢光激發光譜提供了一個明顯的證據證明第二型量子點的發光特性會受到第一型量子點影響。此外，藉由改變第一型量子點的粒徑大小，我們可以選擇性地增強第二型量子點在某些特定波長的螢光強度。因此我們要特別強調第一型和第二型量子點的螢光峰值差異極大，對於設計生物顯影感測器(biosensor)非常有幫助。
4. Surface plasmon enhanced energy transfer between type I CdSe/ZnS and type II CdSe/ZnTe quantum dots
此外我們發現金奈米粒子的表面電漿共振可以增強第一和第二型量子點間的能量轉移效率。藉由螢光激發光譜和時間螢光解析測量，我們發現當激發光能量接近金奈米粒子的表面電漿共振能量時，能量轉移效率增加效果最好。這個現象可以用激發光和金奈米粒子產生共振來解釋。增強的能量轉移效率可以增加量子點間能量轉移的有效距離，因此對於第二型量子點所以做成的生物顯影感測器有很大的幫助。
由於現今對type II量子點之研究並不多，本論文之結果當有助該行量子點之理解與未來可行的應用。

關鍵字

第二型奈米結構 ； 量子點 ； 能量轉移 ； 表面電漿共振 ； 時間解析螢光光譜儀

並列摘要

This thesis mainly focuses on two parts, including optical investigation of type II nanostructures and surface plasmon enhanced energy transfer between type I and type II quantum dots. All of our experimental results were obtained by photoluminescence (PL), photoluminescence excitation (PLE), and Time-resolved photoluminescence (TRPL) measurements. A detailed optical investigation of CdTe/CdSe and P3HT/CdSe type II nanostructures in part I help us to approach the high efficient optoelectronic devices based on colloidal quantum dots (QDs). In part II, the observation of energy transfer effect between type I and type II QDs is an original research which provides a possibility of energy transfer process between two QDs with large optical band gap difference (visible and near-infrared). In addition, we have also improved the energy transfer efficiency by the participation of gold nanoparticles (NPs).

Part. I Optical investigation of type II nanostructures

1. Recombination dynamics in CdTe/CdSe type-II quantum dots
Recombination dynamics in CdTe/CdSe core-shell type-II quantum dots (QDs) has been investigated by TRPL spectroscopy. A very long PL decay time of several hundred nanoseconds has been found at low temperature, which can be well rationalized by the spatially separated electrons and holes occurring in a type-II heterostructure. For the temperature dependence of the radiative lifetime, the linewidth and the peak energy of PL spectra show that the recombination of carriers is dominated by delocalized excitons at temperature below 150 K, while the mixture of delocalized excitons, electrons and holes overbears the process at higher temperature. The binding energy of delocalized excitons obtained from the temperature dependence of non-radiative lifetime is consistent with the theoretical value. The energy dependence of lifetime measurements reveals a third power relationship between the radiative lifetime and the radius of QDs, the light of which can be shed by the quantum confinement effect. In addition, the radiative decay rate is found to be proportional to the square root of excitation power, arising from the change of wave function overlap of electrons and holes due to the band bending effect, which is an inherent character of a type-II band alignment.
2. Direct evidence of type II band alignment in P3HT/CdSe heterostructures
Due to inheriting advantages of both constituent materials, organic/inorganic hybrid composites have attracted increasing attention. One of the fundamental issues needed to be resolved is their band alignment, which governs most of the electrical and optical properties. Here, we report the investigation of optical transition in poly(3-hexylthiophene) (P3HT)/CdSe nano composites (NCs). It is found that the relaxation dynamics of photo-carriers in nano composites is dominated by Förster energy transfer and charge separation effects. Based on the band bending effect and the quantum confinement energy of electrons in the conduction band of CdSe quantum dots, we provide the first direct evidence of type II band alignment in P3HT/CdSe NCs. The establishment of type II transition in NCs is very useful for further design of efficient optoelectronic devices based on conjugated polymer/semiconductor hybrid systems.

Part. II Observation of energy transfer and surface plasmon enhanced energy transfer efficiency between type I and type II quantum dots

3. Resonant energy transfer between CdSe/ZnS type I and CdSe/ZnTe type II quantum dots
Fluorescence resonant energy transfer (FRET) has been clearly demonstrated between CdSe/ZnS type I quantum dots and CdSe/ZnTe type II quantum dots by PL, TRPL and PLE measurements. The result of PLE experiment provides us a concrete evidence that the emission of CdSe/ZnTe quantum dots is indeed influenced by CdSe/ZnS quantum dots. It is found that by changing the size of CdSe/ZnS quantum dots, the emission spectra of CdSe/ZnTe quantum dots can be selectively enhanced in a particular wavelength range and the emission intensity of type II quantum dots can be greatly enlarged by up to four times. Together with the large tunability of the emission energy in infrared region, our finding provides an opportunity for creating highly efficient optoelectronic devices and bio-imaging labels derived from type II quantum dots. We stress out here that the large difference of emission energies between donors and acceptors in our studied system is especially useful for the development of bio-sensors.
4. Surface plasmon enhanced energy transfer between type I CdSe/ZnS and type II CdSe/ZnTe quantum dots
FRET has been investigated between donor-acceptor pairs of type I CdSe/ZnS and type II CdSe/ZnTe QDs. An Au NPs assisted FRET enhancement was clearly demonstrated. It is found that the efficiency of the energy transfer depends on the excitation wavelength and is largest when in resonance with the Au surface plasmon mode. With the large tunability of the emission intensity in near infrared region, our finding paves an excellent route for creating highly efficient optoelectronic devices and bio-imaging labels derived from type II QDs.
Due to the limited investigation of type II QDs, the results obtained in this thesis should be very useful for their understanding as well as applications.

並列關鍵字

type II nanostructure ； quantum dots ； energy transfer ； surface plasmon resonance ； time-resolved photoluminescence

參考文獻

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