硒化鎘/硫化鋅膠狀量子點應用於微碟共振腔雷射及能量共振轉移之研究

Colloidal CdSe/ZnS Quantum Dots- Application to Microdisk Lasers and Study on Resonance Energy Transfer

向育民(Yu-Ming Hsiang)

指導教授 ： 毛明華

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2018年)

https://doi.org/10.6342/NTU201804387

摘要

在本論文中，我們以硒化鎘/硫化鋅膠狀量子點作為主動層材料，製作出具三明治結構的量子點微碟共振腔，並利用原子層沉積技術成長氧化鋁緻密層包覆量子點，有效阻隔量子點被電漿氣體破壞並提升在空氣中的光穩定性，成功在室溫下以連續式雷射激發產生低雷射閥值，光穩定性高之紅光微碟雷射。量子點層上下及側壁皆包覆氧化鋁的直徑10μm微碟，雷射閥值約為153kW/cm2，品質因子為1280。雷射特性可藉由改變激發功率來觀察，在低於元件雷射閥值時，我們已可觀察到迴音廊模態，模態半寬隨激發功率增加而減小，此一特性證實元件從自發放光到受激放光的雷射產生過程。另外，由黃光量子點作為主動層之微碟共振腔，亦可以觀察到迴音廊模態。然而，因黃光量子點經過原子層沉積以及電漿增強式化學氣相沉積後，發光強度已大幅降低，造成共振腔內增益不夠，無法產生雷射。而由黃、紅量子點混合做為主動層之微碟共振腔，因混和時會稀釋濃度，混和後之紅光量子點濃度降低，導致在相同激發強度下，載子濃度較低，無法產生紅光雷射。若以濃度較高的黃光量子點來補償製程中的損耗，在激發強度低時，黃光模態能夠與紅光模態強度相等，隨著激發強度增加，黃光模態強度大於紅光模態，反映出能態填充的效應(band-filling effect)。
為了瞭解量子點之間Förster resonance energy transfer (FRET)效應，我們進行具有時間解析與頻譜解析之光激發光實驗，發現當光激發結束之後，波長較長的量子點發光衰減呈現延遲現象，並且波長較長的量子點具有較長的載子生命期。這些現象指出FRET效應的存在。另外，藉由改變量子點濃度來調整量子點之間的平均距離，發現隨著量子點之間平均距離增大，FRET效應會降低。最後，我們將混合量子點的微碟元件與薄膜樣品相互比較，發現由於共振腔的Purcell effect效應與非輻射複合導致各個模態處之載子生命期皆大幅縮減，不易觀察FRET效應。

關鍵字

膠狀量子點 ； 微碟雷射 ； 光穩定性

並列摘要

In this thesis, we use CdSe/ZnS colloidal quantum dots as active medium to fabricate microdisk resonators with sandwiched structure, and use atomic layer deposition (ALD) technique to passivate Al2O3 on quantum dots, which can prevent quantum dots from being destroyed by plasma gas and enhance light stability in the air. Finally, we produce red-emitting microdisk lasers with low threshold power density and high photo-stability operating in continuous wave excitation at room temperature. A 10μm-diameter microdisk laser with quantum-dot active medium encapsulated by Al2O3 in all directions exhibits a threshold power density of 153 kW/cm2 and quality factor of 1280. Lasing characteristics can be observed by changing the excitation power. At below threshold, we can observe small whispering gallery modes. The spectral width narrows as the excitation power density increases. It confirms the transition from spontaneous emission to stimulated emission of the laser device. In addition, whispering gallery modes can also be observed from the microdisks with yellow-emitting quantum dots as active medium. However, luminescence intensity of yellow-emitting quantum dots has been significantly reduced after processed by ALD and PECVD, resulting in gain in the resonant cavity insufficient to overcome loss. No lasing action has been observed from these yellow-emitting quantum dots. The microdisk with yellow-emitting and red-emitting quantum dots mixed as the active layer doesn’t show laser action because the red-emitting quantum dot density becomes lower after mixture. If higher concentration of yellow-emitting quantum dots is used to compensate their luminescence loss during the process, luminescence intensity of yellow-emitting quantum dots can be comparable to that of red-emitting quantum dots at low excitation power. As the excitation power increases, luminescence intensity of yellow-emitting quantum dots can become higher than that of red-emitting quantum dots due to band-filling effect.
In order to investigate Förster resonance energy transfer effect, we perform time resolved and spectrally resolved photoluminescence measurement. It is found that quantum dots with longer emission wavelength exhibit a delayed photoluminescence decay and their carrier lifetime is longer. These phenomena indicate the existence of FRET. Furthermore, with increasing the average distance between quantum dots by diluting quantum dot solution, FRET effect will be reduced. Finally, using yellow-emitting and red-emitting quantum dots mixed as active medium, we observe much shortened carrier lifetime in a microdisk cavity in comparison with that in the thin film. This phenomenon is attributed to the Purcell effect of the cavity and the increased nonradiative recombination due to the small cavity size. FRET effect is less likely to be observed in this case.

並列關鍵字

Colloidal quantum dots ； Microdisk lasers ； Photo-stability ； Förster resonance energy transfer (FRET)

參考文獻

1. Hoogland, S., The fuss about quantum dots. Photon. Spect. 2008, 42 (1), 80-+.

2. Lim, X., The nanolight revolution is coming. Nature 1993, vol. 115, pp. 8706-8715.

3. Konstantatos, G.; Howard, I.; Fischer, A.; Hoogland, S.; Clifford, J.; Klem, E.; Levina, L.; Sargent, E. H., Ultrasensitive solution-cast quantum dot photodetectors. Nature 2006, 442 (7099), 180-183.

4. Zeng, T. W.; Liu, I. S.; Hsu, F. C.; Huang, K. T.; Liao, H. C.; Su, W. F., Effects of bifunctional linker on the performance of P3HT/CdSe quantum dot-linker-ZnO nanocolumn photovoltaic device. Opt. Express 2010, 18 (19), A357-A365.

5. Kim, T. H.; Cho, K. S.; Lee, E. K.; Lee, S. J.; Chae, J.; Kim, J. W.; Kim, D. H.; Kwon, J. Y.; Amaratunga, G.; Lee, S. Y.; Choi, B. L.; Kuk, Y.; Kim, J. M.; Kim, K., Full-colour quantum dot displays fabricated by transfer printing. Nat. Photonics 2011, 5 (3), 176-182.