Growth Kinetics of Highly Crystalline Gold Thin Films

The project described here was designed for undergraduates interested in learning about surface chemistry and some of the techniques employed in this field of research. While the underlying idea of the project is to teach undergraduates about surface chemistry, we do anticipate that it will lead to new, publishable results. Once the work is complete, we will also implement selected portions of the experiment in the undergraduate Physical Chemistry Laboratory course as an introduction to surface chemistry and vacuum techniques.

Thin films of gold are utilized in a number of areas of science as, for example, substrates for self–assembled monolayers and chemical sensors. A common method of growing gold thin films is thermal evaporation and is illustrated below.

        PICTURE

In this example, the gold shot contained in the alumina crucible is thermally evaporated in a high vacuum chamber by heating it to near it's melting point. The resulting "spray" of gold deposits on a substrate, typically mica, located above the gold. To the naked eye, the gold film looks flat and uniform. On a microscopic scale, however, the film consists of crystalline grains of gold that are "glued" together by amorphous gold. The grains are typically 10 to 100 nm in diameter, as revealed by scanning electron microscopy or scanning tunneling microscopy. The quality of the gold film, measured by the size and crystallinity of the grains, depends on several factors, notably the temperature of the substrate during growth, the temperature after growth and the length of time the substrate is held at that temperature, the flux of gold atoms striking the substrate, the flatness and cleanliness of the substrate, and the ambient environment of the substrate during and after growth. All of these factors are controllable, and thus the quality of the gold film can be optimized.

The goal of the current study is two–fold: 1) to provide a methodology for the growth of highly crystalline gold films that is easily reproducible, and 2) to probe the effects of various ambient gases on the growth kinetics of the gold film. Previous work by other research groups has focussed on the first goal.

Method

The experimental procedure involves two parts.

The gold film is grown on a mica substrate in a high vacuum chamber whose base pressure is 2 x 10^–8 torr. The chamber is homemade and was assembled by two undergraduates. It is evacuated using a 1200 L sec^–1 diffusion pump with a refrigerant–cooled cold baffle. A homemade sample holder, constructed out of molybdenum, is heated by a tantalum heater wire cemented to it's back.

The film is analyzed with a scanning tunneling microscope (STM), which is an instrument capable of providing atomically resolved images of a surface.

Present State of Project