Graphene, a two-dimensional sheet of carbon atoms, is a remarkable material. It is strong, transparent, flexible, and an excellent conductor of electricity. These properties make it an attractive material for a wide range of applications that include solar cells, touch screens, and even tennis racquets. When graphene sheets are stacked, graphite is formed. Understanding the behavior of electrons within graphite could have implications for light harvesting in photosynthetic systems and may also explain how graphite is turned into diamond when energized by a light pulse from a laser. We found that shining intense laser light on graphite immediately creates a dense gas of electrons with temperatures comparable to that of the surface of the sun. Remarkably, the electron temperature achieved depends on the color of the light, with less energetic photons creating hotter temperatures.
Our experiment exposes graphite to an ultrafast, wavelength-tunable laser. We measure the energy of electrons emitted from the graphite as the laser wavelength changes from ultraviolet (290 nm) to infrared (880 nm). As the wavelength decreases, the energy of electrons increases dramatically. This counterintuitive result comes from the excited electrons in graphite scattering within 25 fs to create a temperature as high as 5500 K. We argue that the response comes not from single electrons absorbing multiple photons but from electrons scattering off of one another within the dense electron gas in graphite.
We predict that similar behavior might show itself in other materials composed of one-atom-thick layers such as graphite. The hot electron gas might also lead to efficient chemistry on the surface of some graphitic materials and could explain how laser light can transform graphite into nanodiamonds.
