Several days ago, I introduced the existence of a new optical technique named ECOM. My first concern was to demonstrate its sensitivity by testing the technique in air. It corresponds to the conditions in which graphene platelet manufacturing and deposits are characterized. However, major graphene applications rely on electrochemistry, which requires to use graphene layers in (salted) water.
Here I will demonstrate the sensitivity of the ECOM technique when observing in water graphene monolayers supported by a solid surface.
graphene in water – image 1
The profile displayed in image 1 shows the image intensity variation along the colored arrow. Since each step corresponds to one graphene layer, one can check that this intensity is a linear function of the layer number. It is therefore possible to identify the number of layers covering a given region by measuring this intensity, or by counting steps when starting from a known situation. Notice that the height z of one step is 0.335 nm, while the total length L of the yellow line is about 120 µm. The z/L ratio is therefore 0.3/120 000 = 2,5 10-6 . By comparison, it would be equivalent to consider variations in height of 2,5 cm along a 10 km route. What is actually amazing is the flatness of each plateau between two steps in the absence of any image treatment other than overall contrast and lighting adjustments. For the above mentioned linear relationship between thickness and intensity, it is possible to convert the same image into topographic representation, as done in image 2. The z dimension (height) is considerably enlarged compared to x and y. Otherwise, the steps would not be seen in the image.
graphene in water – image 2
The next pictures, image 3 and upper image 4, present three series of side by side images of a same region of the sample. Graphene oxide platelets were deposited on the solid from evaporation of a first water drop, then covered again with water after deposition and observed and then observed again after the second drying. The left and right images were captured during, respectively, the second and the third steps.
graphene in water – image 3
graphene in water – image 4
Graphene sheets observed in water much resemble those observed in air. One can check that graphene monolayers do not dissolve in water. One could still doubt of it, however, when looking at the lower picture in image 4, where a graphene platelet seems to disappear. This is obviously not due to graphene dissolving, but to graphene taking off from the surface, probably related to some non identified invasive molecule creeping in between the two materials. When enlarging this zone, one may distinguish some kind of short noodles in the upper part. I remember to have seen such noodles in some experiment with highly confined water, where the authors were explaining that water was breaking down in filaments. I have no idea of what are the noodles. I just notice that graphene is no more visible when such structures appear.
The main point is that graphene sheets can be viewed in water, which corresponds to the conditions where they could be used as electrodes.