以步進式掃描傅氏轉換光譜儀 研究光激發金奈米粒子之瞬態紅外放光

Monitoring the transient thermal infrared emission of gold nanoparticles upon photoexcitation with step–scan Fourier–transform spectrometer

劉佳陵(Liu, Jia Ling)

指導教授 ： 朱立岡

國立清華大學/理學院/化學系/碩士(2016年)

摘要

金奈米粒子具有表面電漿共振的光學性質，隨著形狀、粒徑與環境的不同而改變其共振吸收波長在可見光至紅外光波段。而金奈米粒子吸收光能後得轉換成熱能，此光熱轉換的特性使得金奈米粒子被視為奈米加熱源的最佳材料。然而目前並沒有直接觀察金奈米粒子受光激發後所產生之瞬態溫度的方法。本研究將藉由時間解析傅立葉紅外光譜儀收集光激發金奈米粒子之熱紅外放光，建立一套偵測金奈米粒子於電子與聲子達平衡後奈秒時域下之瞬態溫度。吾人實驗概念以圖一表示，偵測以能量為25 mJ cm-2之532 nm脈衝雷射(5 ns)激發三種不同保護基(檸檬酸鈉、溴化十六烷基三甲銨與聚乙二醇單甲醚硫醇)之金奈米粒子，比較其紅外放光與標準黑體輻射，估計光激發金奈米粒子之瞬態溫度可達200 oC以上。此外，將雷射能量調整為19 mJ cm-2激發三種不同粒徑以溴化十六烷基三甲銨保護之金奈米粒子，得到瞬態溫度隨著粒徑增大而下降，與米氏理論所提出之金奈米粒子光熱轉換效率隨粒徑變化的趨勢相符。此外，以奈秒脈衝雷射激發金奈米粒子的過程，會因為溫度上升導致表面熔化以及其形貌的改變，其形變程度與保護基種類和粒徑大小有關係。當保護基與金奈米粒子間形成共價鍵，保護基不會因溫度上升而遠離粒子表面，使之互熔聚集而產生形變；若兩者間僅以靜電作用力吸附，隨溫度上升愈多保護基遠離粒子表面而產生愈嚴重之形變。由於步進式時間解析傅立葉紅外放光光譜法所具備多重波長特性與次微秒之時間解析能力，使得吾人得擷取奈秒時域下金奈米粒子之瞬態溫度。故此特性可作為一非接觸性之光譜溫度計，將可應用於研究其他金屬奈米粒子的光熱效應。

關鍵字

步進式掃描傅氏轉換光譜儀 ； 金奈米粒子 ； 瞬態紅外放光

並列摘要

The photothermal effect of the gold nanoparticles capped with different molecules, including citrate, cetyltrimethylammonium bromide (CTAB), and methoxyl-polyethylene glycol thiol (mPEG), has been investigated by collecting the time-resolved thermal infrared emission with a step-scan Fourier-transform interferometer. Upon the photoexcitation of the gold nanoparticles with nanosecond pulse laser (25 mJ cm–2 from a 5-ns pulsed laser) at 532 nm, the transient infrared emission lasts about 1μs, referring to the thermal relaxation from gold nanoparticle to the surroundings. By comparing the infrared emission contours in 90–120 ns with the blackbody radiation spectrum, the temperatures can reach up to 400 oC as the 24-nm mPEG-AuNP. Moreover, the photoexcitation of 35 nm CTAB-AuNP reveals higher transient temperature than that of 55 nm and 89 nm CTAB-AuNP, consisting with the prediction by Mie theory that the smaller nanoparticles possesses higher contribution of absorption in the extinction coefficient. In addition, gold nanoparticles upon photoexcitation bring about melting below melting temperature. Electron microscopy shows the morphology change before and after photoexcited gold nanoparticles. The time-resolved emission spectroscopy has the advantage of the duplexity on the temporal capability and broadband spectroscopic window and makes it a promising non-contact tool to quantify the transient photothermal event of miscellaneous metallic nanostructures.

並列關鍵字

step–scan Fourier–transform spectrometer ； gold nanoparticles ； transient thermal infrared emission

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