Preliminary Data: Oxidized Copper

In our video, Nancy talked about how we use a vacuum chamber to keep our sample clean. Here, we are going to talk about how we clean and prepare our sample before we use it for our experiment. Our sample preparation consists of cleaning the single crystal Cu(100) substrate with multiple cycles of argon ion sputtering and annealing at 600°C (or 1112 °F). "What is argon ion sputtering?" you might ask. Sputtering – which is typically performed with argon ions, because they are heavy, but do not react with asample – can be compared to industrial sandblasting. Industrial sandblasting uses a stream of sand to remove paint or debris from a metallic surface. In our vacuum chamber, we replace the grains of sand with argon ions. By propelling the ions toward the dirty Cu(100), we can remove thin layers of dirt and rust from our sample and have a clean and rust-free copper substrate. Similar to the industrial sandblasting process, the argon ion sputtering tends to leave the copper surface rough, so we anneal our sample to make the surface smooth. An analogy that I like to use for this process is the heating of candle wax. By melting and cooling, you can give a knobby piece of wax a flat and smooth surface. When heated very close to the melting temperature of copper, which is 1,085 °C (or 1,984°F), atoms on the copper sample surface can move around and rearrange themselves to make a flatter and smoother surface when they cool down. After the last annealing cycle of the sample preparation, we cool the copper sample to 300°C (or 572 °F) and expose it to 1x10^-6 millibar of oxygen gas for 5 minutes. (For comparison, atmospheric pressure is about 25,000 millibars, so this is a really small amount of oxygen gas.)  By exposing the clean copper to a low pressure of oxygen like this, we can control the oxidation such that only the first layer of the copper sample is oxidized. We used this single layer of oxidized copper as simple model of the more complex catalyst. The sample is then transferred into a low temperature STM (5 Kelvin or -452 °F) with a pressure of 1x10^-10 millibar. Figure (a) below shows the topographical STM image of the oxidized Cu(100) surface. The bright dots that form the rails and rungs of the ladder-like features are copper atoms. Two oxygen atoms sit in the dark gaps separating the rungs. A model of the oxidized Cu(100) surface is pictured in Figure (b) for comparison with the STM image. In both figures, the red box highlights the positions of four individual copper atoms.