硒化鉛量子點在石墨基板上之二維成長行為和溫度之相關性

Abstract

本實驗觀察了溫度對硒化鉛量子點在石墨基板上的成長情形之影響。硒化鉛顆粒原本溶在甲苯當中，直徑為10奈米。我們以鹵素燈將石墨基板加熱，調整燈和基板的距離以控制基板的溫度。當到達所要求的溫度後，將奈米顆粒溶液滴在基板上進行成長。樣品成長完成後，以原子力顯微鏡觀察島嶼的分布情形，並且以軟體分析之。實驗觀察了四種不同溫度的成長情形，分別為70℃、100℃、130℃和150℃。我們發現，當溫度高時，成長出的二維島嶼趨向於分散，溫度低時，島嶼趨向於聚集。我們認為成長的機制有兩種可能。第一種可能是：包覆在顆粒外圍的保護基在受熱脫附的過程中扮演溶劑的角色，使得奈米顆粒在其中以膠體顆粒的方式進行成長，而成長的時間則是由溫度所決定。我們推測第二種可能的機制則是由二維Lennard-Jones相圖來決定，溶液中的顆粒隨著溫度和覆蓋度的不同而呈現出相圖中各個不同的相，當溶劑揮發後，顆粒則固著在基板上。另外，當基板溫度在100℃~130℃時，我們觀察到spinodal分解的現象，顯示出在這個溫度區間內，整個二維粒子系統中的能障已降低或消失。

We have studied the influence of temperature on the growth of 10 nm-PbSe quantum dots on graphite. The PbSe SQD were dispersed in toluene solution and then deposited onto a fresh graphite surface which was pre-heated by the halogen lamp. The growth temperatures were controlled by tuning the distance between the substrate and the lamp. The topography was then taken by using atomic force microscope (AFM) with contact and tapping modes and analyzed with the software, NIP. We have observed four kinds of samples which temperatures were 70℃, 100℃, 130℃, and 150℃. The islands tended to be disassociated at higher temperature and aggregated at lower temperature. We conjecture that there are two possible mechanisms for the growth process. First, the islands formed during the desorption of the capping agent. As the colloidal particles clustering, the island size is dependent on the growth time. For our system, the growth time is corresponding to the desorption rate of the capping agent and dominated by the temperature. Second, the particles dispersed in the solution form a two-dimensional gas system which is governed by the 2D Lennard-Jones phase diagram. The island morphology representing fluid, gas+liquid or gas+solid phase depends on the temperature and the coverage. As the solvent drying out, the morphology is fixed on the substrate. We have also observed the spinodal decomposition at 100℃~130℃. This means that the 2D particle system becomes barrierless under this temperature.