Understanding 2D materials with pencil and paper.
Two-dimensional (2D) materials are a subclass of nanomaterials that are becoming increasingly popular for their electronic, optical, and mechanical properties. These materials are considered two-dimensional because of their atomically thin, planar geometry.
To understand the origins of 2D materials, you might envision a ream of printer paper. The ream is made up of many pieces of planar paper stacked on top of one another. Each sheet of paper shares physical and chemical properties with the aggregate ream. Both are white and good writing surfaces. Both the single sheet of paper and the ream will burn when exposed to a flame in air. However, there are other properties that are unique to the sheet sheet of printer paper. The single sheet is flexible and can be torn easily. The thin sheet of paper is more transparent when held up to light. Furthermore, the ream of paper is relatively loosely bound. The addition of some energy – say, dropping the ream on the floor – will cause the stack to split up into individual pieces of paper.
In this analogy, the individual sheets of paper represent 2D materials; the whole stack is what we call a layered crystal. While the layered crystal and 2D material share some properties, there are more interesting electronic, optical, and mechanical phenomena that emerge when we “exfoliate” a layered crystal down to the atomically thin, single-layer limit. This exfoliation is achieved because the bonds between in-plane atoms (that is, the connections within a single sheet of paper) are much stronger than the out-of-plane, van der Waals bonds (the pieces of paper are loosely stuck to one another).
The most well-known layered crystal is graphite – the material that is found in pencil lead. Graphite is made up of layers of carbon atoms. When you write with a pencil, portions of the graphite crystal slip past one another due to the weak van der Waals bonds. This is a very crude exfoliation process; the portions of graphite that are left behind on the paper are still thousands-to-millions of times thicker than a single layer of the carbon atoms. However, with a little bit of scotch tape, graphite can be thinned to graphene – a single layer of carbon atoms.