Electrical detection of phase changes in adsorbed neutral dipolar molecules on graphene

Graphene is a very promising material for sensing application because its transport properties are highly sensitive to adsorbates on its surface. Here, we study the carrier-density-dependent resistance of bilayer graphene to neutral dipolar adsorbates under ultra-high vacuum condition. Halocarbon molecules with known dipole moment are deposited on graphene at $\sim$ 20 K. After deposition of a few monolayers of molecules, the resistance of graphene near the Dirac point is measured as a function of carrier density (tuned by gate voltage) and temperature, from 20 K to room temperature. We observe negligible shifts of the gate voltage of maximum resistance, indicating negligible charge transfer from adsorbate to graphene. In the temperature-dependent-resistance curve, a sharp step-like increase and decrease in resistance occur at $\sim$ 45 K and $\sim$ 65 K, respectively. We relate these abrupt changes in resistance to phase transitions in the adsorbate overlayer. The same molecules adsorbed on graphite are known to exhibit a complex temperature - coverage phase diagram. We will discuss the relationship between graphene resistance and the phases of molecules on graphite. This work was supported by the NSF-MRSEC at the University of Maryland, DMR 0520471