本研究透過有機金屬化學氣相沉積系統在藍寶石基板上生長 7 層量 子點結構，並且在其表面沈積 Au 奈米顆粒，以利用局部表面電漿 效應製作表面增益拉曼散射(surface-enhanced Raman spectroscopy, SERS)感測結構，檢測 rhodamine 6G (R6G) 與 crystal violet (CV) 分子。我們採用的雙分析物檢測法(bianalyte method)，是目前單分子檢測最常用的方式，因為這種方式取決於 大量(1024 筆)的光譜統計結果，而非少數特定的點所產生的閃爍 訊號。雙分析物檢測法的機制，源自於極小的熱點尺寸(< 10nm)， 只能偵測到其中一種分析物的拉曼訊號。由於熱點的範圍很小，且 尺寸擴張不易，只有非常稀少的分子能夠產生有效的拉曼訊號。我 們發現，在量子點 SERS 晶片的表面，可以偵測到大量的單分子訊 號，與傳統雙分析物檢測法的結果不同，這是因為量子點表面的 SERS 熱點面積大增，足以覆蓋兩個或多個單分子，可同時增強多個 單分子的拉曼訊號。這種大面積的熱點擴張，是由於每個金奈米粒 子上與下方的量子點產生的局部表面電漿耦合，因此吸附在 SERS 晶片表面的任何單分子，都能落在擴大的熱點範圍內。;Surface-enhanced Raman spectroscopy (SERS) substrates were built with 7-repeat InGaN quantum wells (QWs) to detect single molecules of rhodamine 6G (R6G) and crystal violet (CV). The QWs were grown by metal-organic chemical vapor deposition (MOCVD) on sapphire substrates. To induce the localized surface plasmon resonance (LSPR), the QWs were decorated by Au nanoparticles. The bianalyte proof is currently the most adopted approach for single-molecule detection since it is a statistical result from the analysis of thousands of spectra, rather than the conclusion based on the blinking signals observed at few selected spots. The bianalyte principle for single molecule detection is rooted in an inherent nature of SERS, i.e., the ultrasmall (< 10 nm) hotspot (or the metallic nano-cavity), which can only intensify the Raman signal from one of the two analytes. Since it is extremely difficult to control the size of the nano-cavity in a scalable manner, only a very limited portion (< 1%) of the diluted molecules, being closest to the tiny hotspot, can yield detectable Raman signals. In this work, we demonstrate the single-molecule signals prevailing on the entire probed area, which reverses the rule for single-molecule detection with the bianalyte method. The result was achieved by making the hotspot big enough to cover two or more single molecules, leading to simultaneous enhancement of their Raman signals. The unprecedented hotspot expansion was accomplished by coupling the localized surface plasmons at every Au nanoparticle with the electrons confined within the underneath InGaN quantum dots (QDs). This configuration allows all of the dense Au nanoparticles to become the hotspots. Thus, any single molecule adsorbed on the SERS substrate can be easily detected by the Au-QD complexes, resulting in the preferential observation of mixed events in the bianalyte statistics.
