Some of the most interesting research into 2D materials involves their electronic properties. The electronic properties determine how charge carriers (like electrons) behave within them, and this then dictates how the materials will perform in electronic devices. The details of the electronic properties can be seen in the electronic structure of the material, and studying this structure attracts significant research efforts.
The electronic structure of many of the new 2D materials has been investigated, and attention now turns to how heterostructures behave. Heterostructures are the result of stacking (to form a structure) two or more different materials (hence hetero-). It will be heterostructures that make devices, not single materials on their own. Therefore, understanding how they interact, particularly how their electronic structures interact, will be essential for electronic devices.
The big challenge of looking at heterostructures is that they are very small. The most reliable way of making them currently involves mechanical exfoliation. This is where tape is used to peel off single flakes that are then place on top of each other. But the flakes are only a few microns across usually, and so the stacked areas end up even smaller. Techniques that look at areas this small are still under rapid development.
Schematic of a heterostructure. A single layer of WSe2 is placed on top of MoSe2. The heterostructure is then the overlapping region. In this experiment, the heterostructure was placed on graphite because it is flat and helps stop the heterostructure from charging.
This is particularly the case for investigations of the electronic structure. The most robust technique to give accurate, detailed information on the electronic structure is angle-resolved photoemission (ARPES). Here light is shined onto the surface, which causes electrons to be photoemitted. Measuring the properties of these electrons after emission gives information about their properties in the solid. But the light shining on the surface normally covers about a millimetre. This will not work with a heterostructure that is 1000 times smaller.
Recent developments have enabled the use of a focused beam of light down to a spot only microns across. Using this technique, called microARPES, we can put the light on the different regions of the heterostructure.
Panel A is an optical image of the heterostructure, which is about 5 µm long, near the blue H. The other panels show details of the electronic structure around this heterostructure. A key result is shown in panels F and G. In F, there are two lines labelled as W and M, whereas in G, it looks like there is only one line in the same place. This demonstrates that the heterostructure interacts more strongly when they are aligned to each other.
In our recent paper we used microARPES to study the electronic structure of a heterostructure made from MoSe2 and WSe2. The ARPES beam could then be placed onto the different regions of the sample so the electronic structure of the individual layers can be measured, as well as the heterostructure. With this we found that the two layers did interact with each other and changes in their electronic structure were observed.
These results show that it could be possible to tune the electronic band structure to give the specific properties required to fit an application. This band engineering will help design heterostructures to start to fabricate ultrathin transistors or LEDs.