The Great Chemistry Bake Off: Chocolate cake and thin films

Let’s talk about cake. Chocolate cake. Suppose you’ve been asked to make a 6-inch tall chocolate cake, but your cake pan is only 3 inches tall. What do you do? Make a layer cake! Problem solved.
 
The same solution is often applied when producing inorganic (carbon-less) thin films for the semiconductor industry using the environmentally-friendly, water-based approach pioneered by the Center for Sustainable Materials Chemistry. The chemistry works great for producing films in a certain thickness range, but sometimes thicker films are needed. In this situation, making a layered film can provide the thickness needed. 
 
Until recently, researchers assumed that layering the films this way led to a thicker film that was the same throughout, unlike when making a layer cake. This is a bit like thinking that if you took two 3-inch tall cake pans filled with batter, flipped one over and stacked it on top of the other (don’t spill!), the layers of the cake merge and you could bake a solid 6-inch tall cake. It seemed like a reasonable picture of what might happen. However, CSMC researchers recently demonstrated that the layers don’t necessarily merge together seamlessly. Instead, each layer of the film might form a crust around it that prevents it from merging completely with the other layers. It’s like the cake batter instantly starts drying and forming a crust on its surface as soon as it is poured into the pan. The crusts merge together somewhat, so the two layers are connected, but the cake is no longer uniform. 
 
Understanding how and why these crusts form is important to developing the best technology and to gain insight into the fundamental chemical mechanisms that occur during film formation. If the “thick” films produced by layering one film on top of the other are not actually uniform, that could have a profound impact on the film properties. However, inspecting the layered films to evaluate the impact of changes in processing conditions on crust formation is not nearly as easy—and certainly not as tasty!—as cutting out a slice of chocolate layer cake. Some of our “easy” experiments are like looking at an object through a frosted glass window. You can get a general sense of what it probably is, but you don’t know for sure. 
 
In our paper, we use a more complicated experimental technique to actually remove a slice of our film and look at in cross-section, just like looking at a slice of layer cake. In this case, the slice itself is only a few micrometers wide, less than a tenth the width of a human hair, and the film is far smaller than that, less than a thousandth the width of a human hair. So instead of using our eyes to look at the film, we use a high-powered transmission electron microscope to take images of the films. Even with this sophisticated instrument that lets us “see” the layers in the film, the layers can still be hard to identify. We then rely on a statistical analysis to look for features in the images that are significant relative to the noise in the images, which gives us a tool for directly examining the layers in the films. Having this tool is important as we continue our experiments to understand why the crusts develop, what impact they may have on the film properties, and how we might be able to control their presence. Once you have the right tools, science is a piece of cake!
 
Application of HAADF STEM image analysis to structure determination in rotationally disordered and amorphous multilayered films (http://dx.doi.org/10.1088/0268-1242/31/8/084003)