Self-Assembled Molecular Films of Alkanethiols on Graphene for Heavy Metal Sensing

We report a comparative study involving the formation of self-assembled molecular films by two types of alkanethiols (1-octadecanethiol and 1-dodecanethiol) on graphene grown via chemical vapor deposition, for heavy metal sensing applications. Scanning tunneling microscopy measurements confirm that the alkanethiol molecules can form localized, ordered molecular films on single-layer graphene despite the presence of structural and chemical irregularities. To test and compare the sensory characteristics associated with graphene functionalized by 1-octadecanethiol and 1-dodecanethiol, graphene-based field effect transistors are fabricated via photolithography on silicon dioxide substrates. Devices based on graphene functionalized with 1-octadecanethiol are successfully employed to demonstrate the detection of mercury and lead ions at the 10 ppm level via Dirac point shifts, with a notable difference in response associated with the use of different heavy metal ions. On the other hand, devices based on graphene functionalized with 1-dodecanethiol exhibit p-type character, before and/or after exposure to heavy metal ions, complicating their use in heavy metal sensing in a straightforward fashion via Dirac point shifts.