以掃描穿隧顯微鏡在石墨表面上製作奈米結構

Abstract

近年來，掃描穿隧顯微鏡在製作表面奈米結構的研究上，有許多的實驗結果。利用瞬間脈衝的高電壓，造成表面原子轉移，以製作坑洞與山丘之結構，再以一般工作電壓掃描地貌，觀察表面上所製造的奈米結構，是較常見的方式。

高同向性熱解石墨具有易製備新鮮表面，且容易取得大範圍原子級平整區域的優點。另一方面，金探針相較於其他常用探針材料，具有較低的離子化場蒸發條件，因此常被用來製作奈米尺度的表面結構。

本次實驗嘗試在大氣下，以金探針在石墨表面上製做出坑洞與山丘。我們發現不論在樣品上施加正電壓或負電壓，皆先造成樣品表面形成坑洞，再出現金山丘；且正電壓脈衝與負電壓脈衝，製造奈米結構所需要的門檻電壓大小不同。此外，我們觀察到金原子團在石墨表面並不穩定，且較易停留在臺階或表面坑洞邊緣。

了解奈米結構產生之機制後，我們以金探針在石墨表面蝕刻出寬度40至50 奈米，深度約1奈米的線狀凹槽，並且發現：使用適當的積分回饋可連續蝕刻超過十條以上之完整的凹槽。此次研究成功地展示出：以金探針在石墨表面製造單點山丘、單點坑洞、線狀凹槽等不同幾何形狀的奈米結構。

In recent years, it has been demonstrated to fabricate nanometer structures by using scanning tunneling microscope. A high voltage pulse has to be employed to generate pit and mound structures on surfaces. To make nanostructures, we used surfaces of highly oriented pyrolytic graphite due to its easily refreshable and atomically flat surface. In addition, gold was used as a tip to make nanostructures, since it has a low threshold field-evaporation voltage.

In this study, we attempted to manufacture pits and mounds on the graphite surface with a gold tip in air. In both positive and negative sample bias, we found that the pits formed in lower voltage and the gold mounds occurred in higher voltage. The threshold voltages for positive and negative polarity are different. On the other hand, we observed that the gold mounds could be easily bound to the step edges and pits but not to the flat graphite surface.

To test the mechanism for nanostructure formation, we adopted a high sample bias from +4.0 to +6.4 V to etch grooves with a width of 40 - 50 nm and a depth of ~1 nm on graphite surfaces. In addition to the parameters of pulse voltage, duration, and polarities, we discovered that an appropriate integral gain is necessary to etch continuously. We successfully demonstrated the formation of mounds, pits, and grooves in the nanoscale on graphite surface with a gold tip by using scanning tunneling microscopy.