Much of our experience with thermal energy takes place in the kitchen. For example, we can boil water in kettle, and in doing so understand the properties of water and its contribution to the storage and release of energy in the kettle as a whole. Whether it is a macroscopic kettle of water or a micron-scale piece of metal, we can learn about its properties and microscopic dynamics by giving it energy. Here in the QMO Lab, we study a material known as graphene by creating a bath of hot charge carriers with a laser pulse and observing their behavior.

Graphene is a material made up of carbon atoms bonded together into a sheet of hexagons which is only one atom thick (Figure 1). Graphene’s structure and thinness give its electrons many novel properties. One such property is the ability of electrons in graphene to have a much higher temperature than the surrounding carbon atoms when they are excited by (i.e. absorb) light. By studying the properties of these photo-excited electrons we hope to better understand graphene’s interaction with light.

When a pulse of laser light arrives at a piece of graphene, some of the light can be absorbed by the negatively charged electrons and excite them to a higher energy. This leaves behind positively charged holes, the absence of electrons. With the excess energy from the light, the electrons and holes can move around inside the graphene layer. Within the first trillionth of a second after excitation these charge carriers form a hot bath of electrons and holes. Then, much like a pot of hot water, this bath of electrons and holes will eventually cool back down over a longer period of time. The entire process of heating and cooling back down happens over only a few trillionth of a second. This means the process completes about ten million times faster than a lightning strike. This short-lived nature makes it challenging to determine how hot the bath of electrons and holes is. Fortunately, we can overcome this challenge by taking advantage of the high temperature the electrons and holes have through careful design of our nanodevice.

The higher the temperature of the electrons and holes, the higher the energy they can reach. In metals, even a small amount of extra energy allows the electrons and holes to move around. In contrast, electrons and holes in insulators require a large amount of energy to move which they normally do not have. However, if the electrons and holes can reach a high enough energy they too can start to move and contribute to an electric current in an insulator. Given we know the type of material we are dealing with, by observing the strength of this electric current, we can determine how many of these high energy electrons and holes there are and therefore the temperature they must have to reach this number. We can use this effect on graphene by introducing an insulator next to it so that the electrons and holes can move through only if they have a high enough energy.

To explore the temperature of the electrons and holes, we construct a multilayered sample consisting of two layers of graphene separated by a piece of hexagonal boron nitride (Figure 2). This hexagonal boron nitride layer is an insulator and acts as a filter which allows only the higher energy electrons and holes to pass from one layer of graphene to another. The resulting movement of the electrons and holes between the layers produces an electric current. By stacking the layers vertically, we can collect the hot electrons and holes as an electric current quickly at the site of the laser excitation rather than slowly at the electrical contacts on the edges of the graphene which are much further away. This allows us to collect the hot electrons and holes before they completely cool down. From changes in this laser generated current, we can infer the changes in temperatures of the electrons and holes.

By observing the changes to the temperature of the electrons and holes, we may better understand the ways which energy can flow, be stored in, or transported out of graphene, and using Multi Parameter Dynamic Photoresponse Measurement technique we can study them in great detail. This may give us a practical understanding of how light can produce an electronic response in graphene, thus revealing possible interactions these electrons and holes may have with their environment, or with each other. This research may also reveal possible applications in technology such as graphene-based photodetectors which can be smaller and more efficient than before.