雷射聚焦單一金奈米造成溫度上升引發二維組裝的結構之研究

Abstract

奈米粒子特定的排列結構使其可應用在表面科學、微製造技術、生物技術、實驗室晶片以及化學感測原件上。然而奈米粒子的組裝在許多不同的奈米尺度下，至今仍受到許多製造上的限制。因此必須建立出一套新的方法可以控制奈米粒子組裝在特定的位置上。我們的方法其過程包含了(1)藉由連續波雷射聚焦在選定的金奈米粒子上造成局部的溫度上升，(2)熱由金奈米粒子傳至周圍的介質以及懸浮在溶液中的奈米粒子，(3)溫度梯度所引起的對流將溶液中的奈米粒子帶近至金奈米粒子周圍，並形成二維的組裝結構。 這篇論文中，我們也探討了不同的奈米粒子的組裝情形。在考慮了上述局部加熱及對流所引起的組裝過程，我們成功地將此方法分別應用在各種不同的奈米粒子溶液中，並形成其他奈米粒子的二維組裝。 我們證明並展示雷射能量、照射時間以及溶液中粒子的濃度如何影響組裝的機率和大小。我們的結果明顯地顯示出組裝機率有能量以及粒子濃度的依存性，而組裝大小則有能量及照射時間的依存性。較高的能量提供了較廣大的加熱區並提高對流的效率，而較高的粒子濃度有助於積聚更多的奈米粒子形成組裝。實驗的結果也支持了我們在局部加熱金奈米使溫度提高上的理論計算，而這部份是根據雷射參數、熱傳導係數及吸收截面的計算。由以上實驗及計算的結果，我們可以藉由操控不同的雷射能量、照射時間及粒子濃度達到控制組裝生成的機制，更可進一步地藉由操控雷射光束在基材上特定的位置產生二維組裝的結構。

Specifically ordered architecture composed of nanoparticles provides applications such as surface science, microfabrication, biotechnology, lab-on-chip, and chemical sensors. However, the methods of assembling nanoparticles in fabricating various nanoscale architectures so far are still limited. Therefore, it is necessary to develop a new method to control the positions and integrations of the nanoparticles in a certain architecture. This study covers the processes of (i) local temperature elevation on a target gold nanoparticle by tightly focused continuous-wave laser illumination, (ii) heat transfer from the gold nanoparticle to surrounding medium and then to suspended nanoparticles in the colloidal solution, and (iii) convection flow induced by gradient of temperature in the solution that bring about the nanoparticles into close vicinity area around the target gold nanoparticle, forming two-dimensional assembly formation. In this study, we observed the assembly of different suspended nanoparticles in aqueous solution. By considering the above mentioned processes of the local temperature elevation and convection flow, we have succeeded in developing a new method which can form a two-dimensional assembly formation of various nanoparticles suspended in aqueous solution. We have demonstrated that how the parameters of continuous-wave laser beam and colloidal solution such as laser power, irradiation time, and particle density affect on the probability and size of the two-dimensional assembly formation. The present results clearly show that the assembly probability depends on laser power and particle density, and the assembly size depends on laser power and irradiation time. A higher laser power provides a wider heated area and enhances convection flow more efficiently, and a higher particle density contributes to accumulate more nanoparticles to form the assembly. This experimental results are supported by our numerical calculations of local temperature elevation on the target gold nanoparticle based on focused laser beam, thermal conductivity, and absorption cross section. Based on overall experimental and numerical results, by varying laser power, irradiation time, and particle density we can control the assembly formation. Further, with this method we can also control the spatial position of the assembly on the substrate by adjustment of the position of the laser beam.