不同形狀的金奈米粒子磁性之研究

貴金屬奈米粒子在吸收光譜中會產生特性吸收帶，此特性稱為定點化表面電漿共振波帶，其波長的位置與吸收度對外在環境的改變相當敏感，若要增加貴金屬奈米粒子的感測靈敏性，可以從粒子形狀作選擇，也可利用光波導多次內反射來增加光學路徑，同時也可透過貴金屬奈米粒子的表面官能化修飾增加感測器的選擇性。最常見的是應用覆蓋劑來控制奈米粒子的尺寸大小，例如：界面活性劑、聚合物…等。近年來奈米粒子在尺寸大小及形狀方面被有效的控制，大多是採用模板、界面活性劑或其他的物理方法，去限制奈米粒子的成長，有效的控制奈米粒子的形狀及大小。而在製備金屬奈米棒與奈米線方面是一項高挑戰的任務，在製作方面有兩個主要的論點：首先是利用表面能去支援球形的奈米粒子形成，第二是大多數的金屬是高度勻稱的立方晶格結晶體，因此無論是利用軟式或硬式的模板都可以製備出棒狀的金屬奈米粒子。本研究是利用液相晶種還原法(Seed-Mediate growth method)來製備不同形狀的金奈米粒子，其晶種還原法是以CTAB作為界面活性劑，再分別製作出晶種液與成長液，並且藉由改變晶種液的劑量或成長液中硝酸銀(AgNO3)濃度，來控制金奈米粒子的長寬比。此外亦可利用成長液中的檸檬酸AA(Ascorbic acid)劑量，來修飾金奈米粒子的表面，製作出不同形狀的金奈米粒子。其主要的方式是先配置晶種液，晶種液中包含了HAuCl4 、CTAB、NaBH4 ，其中 CTAB是包覆劑，主要是限制金粒子成長的大小在10 nm以下，而NaBH4 是一種強還原劑，另外再配置生長液，生長液中包含了HAuCl4、CTAB、ascorbic acid 、 AgNO3，其中CTAB 是包覆劑而ascorbic acid是一種弱還原劑。在實驗中我們改變硝酸銀(AgNO3)濃度和晶種劑量，來改變Au金屬奈米粒子的形狀。實驗所得Au金屬奈米粒子，分別是以紫外光及可見光吸收光譜（UV/Vis）、穿透式電子顯微鏡（TEM）、X射線繞射儀（XRD）、能量分散式光譜儀（EDS），低溫電阻量測系統及超導量子干涉儀（SQUID）等儀器做吸收光譜、尺寸、形狀、結構、成分、結晶分向、低溫電性及磁性等分析。經由測量得知改變硝酸銀濃度(0.01M~0.05M)，當濃度越高時長波長的吸收光譜會有明顯的藍位移現象(785nm~759nm)，棒狀的金奈米粒子長寬比亦會變小，產率也會下降，棒狀的奈米粒子的平均長度為20nm~50nm，當硝酸銀為0.01M時，棒狀奈米粒子的產率最高。經由測量得知改變晶種(0.01ml~0.5ml)，當晶種劑量越多時，長波長的吸收光譜會有明顯的藍位移現象(785nm~714nm)，當劑量小於0.03ml或大於0.03ml時，棒狀的奈米粒子長寬比會變小，產量會下降，棒狀奈米粒子的平均長度為20nm~60nm，當金種量為0.03ml與0.5ml時可產出99%的棒狀奈米粒子與球形的奈米粒子，在磁性方面可得知棒狀的奈米粒子在溫度5K下有弱鐵磁性的存在。圓形的金奈米粒子無論是否有界面活性劑在溫度5K及低場時，雖然都會呈現弱鐵磁性，但易受被覆有機物分子與不純物等的影響。

關鍵字

金奈米粒子

並列摘要

Noble metal nanoparticles exhibit a characteristic absorption band in the absorption spectrum, known as "localized surface plasma resonance (LSPR)". The absorbance and peak wavelength of the LSPR band are linearly dependent on the refractive index of the surrounding medium. The sensing sensitivity can be increased by using nanoparticles of different shapes and by using waveguides to increase the optical path. The nanoparticles can also be functionalized to allow the selectivity of the sensor. Common methods for size control employ capping agents, such as surfactants, ligands, polymers, or dendrimers, to confine the growth in the nanometer regime. Recently, there has been substantial progress in controlling the size and shape of nanoparticles by restricting growth obtained by the introduction of templates, surface capping agents, and other physicochemical means. Solution phase preparation of metallic nanorods and nanowires is a challenging task. The basic principle for two reasons: First, surface energy favors the formation of spherical particles. Second, most metals crystallize in highly symmetric cubic lattices. Therefore, soft and rigid templates had been used to achieve rod-shaped metal nanostructures. However, surface capping agents, such as cetyltrimethylammonium bromide (CTAB), benzyldimethylhexadecylammonium chloride, thtraoctylphosphine-oxide, oleic acid, and so forth, had been successfully used for the creation of rod-shaped nanoparticles. Short aspect ratio Au nanorods are especially interesting because of their optical properties. They exhibit the transverse as well as intense longitudinal plasma bands in the visible region of the spectrum making them promising candidates for sensing, imaging application, and delivery applications. In this study, we prepare gold nanorods by a seed-mediate growth method. In seed-mediated growth method, it starts with the synthesis of Au nano-seeds using chemical reduction of a HAuCl4 salt with a strong reducing agent such as sodium borohydride in the presence of CTAB as a capping agent to prevent particle growth. The synthesized Au nano-seeds are ~ 10 nm in diameter and served as the seeds to grow more anisotropic nanostructures. These Au nano-seeds were added in the growth solution containing more HAuCl4 salt, ascorbic acid (weak reducing agent), silver nitrate (AgNO3) and cetyltrimethylammonium bromide (CTAB). The basic principle for our method of shape-controlled synthesis involves in two steps: First, the preparation of small size (~10nm) spherical gold nanoparticles. Second, growth of the prepared spherical particle is in rod-like micelle environment. The particle size is controlled by varying the ratio of metal salt to seed dosage and silver nitrate (AgNO3) concentration, thus restricting the particle size to the nanometer regime. The amount of seed was decreased, the aspect ratio increased, and this longer wavelength plasma band gradually red-shifted and broadened. The seed performs a crucial role in the growth process. With increasing amount of seed, the rate of particle formation also increases. We employed ascorbic acid as the reducing agent of Au salts in the growth stage. Ascorbic acid is a mild reducing agent and cannot reduce the gold salt in the presence of the micelle without the presence of seed. Consequently, minimal additional nucleation occurs during particle growth. The effects of various processing parameters on the morphology and uniformity of Au nanoparticles were investigated by ultraviolet-visible absorption spectra analyses（UV/Vis）, transmission electron microscopy（TEM）, energy dispersive spectrometer（EDS）, X-ray diffraction（XRD）, and Superconducting Quantum Interference Device（SQUID） measurements. These measurements are used to analyze the size, structure, composition, crystallization, absorption spectra, low-temperature electricity and magnetic of Au nanoparticles. We change the concentration of silver nitrate (0.01M~0.05M) and observe there are obvious blue-shifted (785nm~759nm) phenomena in the absorption spectrum of the long wave section when the concentration is the higher, and the aspect ratio of gold nanorods will diminish. The output of gold nanoparticles will also decrease. The average length of nanorods is 20nm ~ 50nm when the silver nitrate is 0.01M, and the producing rate of nanorod is the highest. While increasing seed dosage from 0.01ml to 0.5ml, there are obvious blue-shifted (785nm~714nm) phenomena in the absorption spectrum of section of the long wave. When seed dosage is smaller than 0.03ml or greater than 0.03ml, the aspect ratio of gold nanorods will diminish. The output of gold nanoparticles will also decrease, and the average length of nanorods is 20nm~60nm. The nanorods and spherical nanoparticles have 99% output while seed dosage is 0.03ml and 0.5ml, respectively. There are existences of weak ferromagnetism for gold nanorods and nanoparticles. However, it is easy to be influenced by capping agents for the organic matter and impurity.