用低溫掃描穿隧電流顯微鏡研究奈米金顆粒之電子能譜

Abstract

本實驗利用超高真空低溫掃描穿隧電流顯微鏡觀察奈米金顆粒之表面影像與其電子能譜，並比較不同尺寸、不同基板、不同溫度下奈米金顆粒之電性特徵差異。實驗上所使用之奈米金顆粒是利用熱蒸鍍之鈍氣凝結法鋪撒於基板上，再經由真空加熱處理使其凝聚為單一球體。我們使用掃描穿隧能譜觀察奈米金顆粒之庫倫阻滯現象，以雙穿隧接面之orthodox理論對這些奈米金顆粒之電流-電壓特徵曲線擬合，得到兩個接面之等效電阻值與電容值。

經比較奈米金顆粒於液態氮溫度與室溫下電子能譜之特徵差異，發現室溫下之奈米金顆粒電性行為較接近金塊材之行為。其次再比較不同顆粒尺寸之奈米金顆粒電子能譜的差異，也發現奈米金顆粒隨著尺寸上升，其電性行為逐漸接近塊材。此外，我們觀察不同基板與溫度下奈米金顆粒之電子能譜，認為在此雙穿隧接面情況下，所有量測到之奈米金顆粒的電子能譜均包含了金奈米顆粒與基板的貢獻，並非單純反應奈米金顆粒電性。最後，我們觀察到奈米金顆粒之費米面能態密度值因尺寸變大而增加，符合久保理論之預測

We present an experimental study for geometric and electronic properties of Au nanoparticles grown on HOPG and Au(111) substrates by scanning tunneling microscope in a ultra-high vacuum. The nanoparticles were fabricated by using inert-gas condensation method. We obtained current voltage (I-V) behaviors of a single Au nanoparticle as a function of the particle size and temperatures. The I-V curves showed Coulomb blockade behaviors and they were fitted with the double-tunneling-junction model. The equivalent capacitances and resistances can be estimated through fitting.

Comparing the I-V curves of Au nanoparticles at room temperature and at 77 K, we concluded that high-temperature I-V behaviors are close to that of Au bulk. In addition, we found that the I-V curves of large Au nanoparticles were close to that of Au bulk as well. We investigated the electronic spectrum of Au nanoparticles on different substrates and at temperatures. We believe that the tunneling spectrum come from both the Au nanoparticles and the substrate. Moreover, we discovered discrete density of states, which were proposed in Kubo theory, in Au nanoparticles.