Glass can be formed naturally; for example in volcanoes or when lightning strikes a sandy beach and it has been produced by humans for thousands of years. Glasses can take on many different appearances, colors, and properties. Thus, it may be surprising that our understanding of the exact structure of glass is not complete, even after such a long time. Very often, you will hear glasses described as amorphous. This word is derived from Greek and means without shape. So we have a rough idea that glass is somehow shapeless. In the following we will see, how we can investigate the atomic arrangement in glass using modern techniques.
Why is it so difficult to see the atomic structure of glass?
We will now first take a look at what is arguably the most important technique for structure investigation, especially at what it can and cannot tell us. It will help us to understand why the structure of glass is so difficult to unravel. Then it will be clearer how a workaround had to be and eventually was found to reveal the structure of something amorphous, namely silica glass.
The behavior of materials is often explained with their structure and the arrangement of atoms or molecules. Sometimes we can indirectly get information on the tiny building blocks from the way the macroscopic material looks. For example, a rock salt crystal of high purity will appear like a cube with ragged edges. That is because the atoms are arranged in a cubic pattern and the material breaks along the crystal planes.
An often-used method to find out crystal structures on the atomic level is X-ray diffraction, which is based on the interaction of light with the sample material. In a crystal, the atom positions are equivalent, so the light-matter interaction will also be equivalent for each position. That makes it easy to detect the signal which is uniform and strong. You can find out more about X-ray diffraction in this excellent online tutorial from the University of Cambridge.
What happens when the material is not crystalline? When the material has many non-equivalent positions, the interaction of light and sample has a slightly different outcome for each position. This results in many slightly different diffraction beams, and an overall signal that is neither very intense nor very clear. That is the reason why the structure of glass is not very well understood today; with X-ray diffraction, a glass sample delivers unclear, diffuse signals. The main information that X-ray diffraction can give us about glass is that it is not a crystalline material!
Exploring the atoms with surface science techniques!
Another family of techniques has more recently been developed, which allows us to get information on the atoms’ positions of a material. Scanning probe microscopes use a fine tip (or probe) that scans across a sample surface. At every pixel, the interaction between tip and sample is detected. Depending on the sample material, different tip-sample interactions can be studied, for example the force (in atomic force microscopy, AFM) or the amount of electrons tunneling between them (in scanning tunneling microscopy). The value from each pixel is combined in the measurement computer and provides a microscopy image of the surface structure. This way, it is possible to find the positions of atoms on the surface. This youtube video provides more information on STM.
This type of microscopy works best when the sample is very flat. An ultra-flat glass sample can be created by a physical vapor deposition technique, which is shown schematically in Fig. 1. [1,2] This sample preparation starts with a very clean metal crystal that is highly ordered and therefore has a very flat surface. In an ultra-high vacuum chamber, a small partial pressure of pure oxygen is provided. A very small amount of vaporized silicon is deposited on the surface and then oxidized at high temperature. The temperature and pressure during each preparation step has to be controlled precisely. The result is an ultra-thin silicon dioxide film that can be imaged very well with a scanning probe microscope.
In the last panel of Fig. 1, the structure of the thin silica film is shown schematically. It is formed by two layers of tetrahedral building blocks. One such tetrahedron is shown separately on the right side. The tetrahedra are connected with each other at their corners via oxygen bridges. Each silicon atom is bound to four oxygen atoms and each oxygen atoms is bound to two silicon atoms. In imaging the surface with atomic resolution, we can begin to understand the structure. With scanning probe microscopy, we look at the film from the top side. Depending on the specific conditions, we can reveal the positions of the oxygen atoms or the silicon atoms. 
Shown in Fig. 2 are two high-resolution STM images of the silica bilayer, revealing the different structures that occur in this system (adapted from ). On the left hand side, the Si atoms form a hexagonal pattern, which can also be called honeycomb lattice. On the right hand side, we see a more complex structure. The atoms are arranged in loops of varying size, without long range order. This arrangement is typically called an amorphous structure. What we see here is atomic resolution of a 2D glass!
Fig. 2: The atomic arrangement in two-dimensional glass (silica bilayer). Left: crystalline form, right: amorphous form. Adapted from Burson et al., Journal of Chemical Education, DOI: 10.1021/acs.jchemed.5b00056, available as open access publication .
The ability to image amorphous structures with atomic resolution allows us to study such systems in much greater detail than before. We can count the different ring sizes and investigate how they are arranged into larger structures.  We can even use them to steer diffusion of single atoms through the film.  By systematically introducing other elements, we can now study silicates containing aluminum,  iron  and other mixed compounds, thus gaining a better understanding of this important material class.
Author: Christin Büchner, Copyright: CC-BY
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