Mar 25, 2013
Substrate controls doping in graphene
The electronic properties of graphene are very sensitive to the local environment in which the 2D carbon material finds itself. So say researchers at Columbia University in New York who have performed scanning tunnelling microscopy on graphene films exfoliated onto cleaved mica – an ultraflat material that contains surface electric dipoles. The finding will come in very useful for when it comes to making improved graphene-based devices.
Graphene is a single layer of carbon atoms organized in a honeycomb lattice. Scientists now know that particles, like electrons, moving through such a structure behave as though they have no mass and travel through the material at near light speeds. These particles are called massless Dirac fermions and their behaviour could be exploited in a host of applications, including transistors that are faster than any that exist today.
Because graphene is only one atom thick (it is the thinnest material known), the material takes on the same shape as the structure of the substrate on which it is grown – something that significantly affects the electronic properties of the 2D carbon sheet. For instance, graphene on silica is much “rougher” than graphene grown on ultraflat hexagonal boron nitride. In the first case, the electrons and holes in graphene pool together in large numbers thanks to long-range scattering processes, while in the second case the charge carriers collect together much less.
Mica as substrate
Mica is a phyllosilcate mineral made up of aluminium and potassium. It can easily be cleaved to produce extremely thin sheets by separating the negatively charged aluminosilicate layers from the potassium ion layers. Cleaved mica is an interesting dielectric substrate for measuring the electronic properties of graphene because it is ultraflat and possesses surface electric dipoles. As well as the ultraflat regions, the material also contains two types of plateau: one 1.5 angstroms above the flat regions (which corresponds to potassium ions at the graphene/mica interface) and the other 3.7 angstroms above the flat region (corresponding to water at the graphene/mica interface).
Graphene remains undoped immediately above the water molecules but becomes p-doped on the mica surface surrounding them. “This tells us that the water molecules are affecting how potassium ions are distributed on the mica surface and creating a negative surface environment on the mica silicate,” explained team member Scott Goncher. “In contrast, graphene becomes n-doped near the potassium ion regions, but there are two types of n-doping: there is substantially more electron doping immediately above the potassium plateau than on the neighbouring silica network, but on flat regions with no plateau and far away from the potassium plateau, we only see n-doping.”
This means that the graphene above the same flat silicate network is both electron- and hole-doped depending on its surface environment (see figure).
Hole or electron doping
“Such variability allows us to either hole-dope or electron-dope graphene,” said Goncher. “In theory, we can then create specific n-type and p-type regions in the material that could allow for phenomena such as exciton (electron-hole) dissociation, which is very useful for photovoltaics applications. This type of control has never been seen before in graphene – previous work showed that the material either had random charge puddling or no puddling at all.”
The team, led by George Flynn, obtained its results using an ultrahigh vacuum scanning tunnelling microscope (UHV-STM) with an atomically sharp tungsten metallic tip. The researchers began by applying a voltage to the tip that they then slowly brought down onto the graphene sample surface (without actually touching it) while monitoring the current flowing from the tip. At current values of 100 pA employed, the tip is in the so-called tunnelling regime as electrons tunnel from the tip end to the semi-metallic surface.
Surface topography and graphene’s electronic properties
“By then keeping the tip just a few angstroms above the sample surface and keeping this height (and thus current) constant while rastering the tip position, we are able to obtain information about the topology of the surface,” Goncher told nanotechweb.org. “If we then modulate the tip voltage (while still keeping the height constant), we obtain information on the density of states (DOS) of the graphene immediately below the tip.”
By combining these DOS measurements with the topographic information, we can see that there is an exact match between topographic features on the graphene surface and the variations in its electronic properties,” he added.
Spurred on by its findings, the team says that it is now trying to determine how different electronic environments affect reactivity and scattering on the sample surface because scattering is directly related to how fast electrons and holes move in graphene.
The current work is detailed in Nano Letters.
About the author
Belle Dumé is contributing editor at nanotechweb.org.