The concepts developed in our group in relation to molecular self-assembly on surfaces, in particular on graphite, and in molecular electronics, are now being evaluated in view of the unique properties of graphene. Graphene is a single atom thick crystal composed of carbon arranged in a honeycomb lattice. Since its discovery in 2004, this two-dimensional material has gained significant attention of the scientific community due to its extraordinary electronic, optical and mechanical properties. One current key challenge in graphene research is to tune its charge carrier concentration, i.e., p- and n-doping graphene. Functionalization of graphene by physisorbed self-assembled monolayers (SAMs) of organic molecules is a promising approach to achieve uniform and controlled doping. Till this date, there are only a few reported studies about doping of graphene by SAMs.

We are investigating the potential of molecular self-assembly on graphene to control doping.

1.Molecular self-assembly on graphene

(A) Molecular structure of DBA-DA25. (B) High resolution STM image of honeycomb pattern of DBA-DA25 assembled at the interface between epitaxial graphene (E-G/SiC) and TCB. The inset shows the corresponding Fast Fourier transform of E-G/SiC substrate. (C) Molecular model of the honeycomb pattern; unit cell: a = 7.0 ± 0.2 nm, b = 7.1 ± 0.2 nm and γ = 60 ± 3°. The DBA-DA network was found to be rotated by about 23° with respect to the underlying graphene lattice. (D) Large-area STM image of honeycomb pattern of DBA-DA25 assemblies at TCB and E-G/SiC interface. (E) Enlarged STM image of the area marked with the white rectangle in (D), showing that the 2D porous network seamlessly crosses a step. Blue triangles highlight the DBA cores. Blue dashed lines indicate the alkoxy chains in between the DBA cores. Imaging conditions: Iset = 120 pA; Vset = −850 mV.

AFM images of DBA-DA25 self-assembled monolayer on E-G/SiC under ambient conditions. (A) Height image and (B) the corresponding phase image, showing a continuous honeycomb structure crossing over a ~ 0.7 nm high step edge on E-G/SiC. (C) The enlarged AFM phase image of the same area marked with the white square box in (B) clearly showing the DBA honeycomb network without any discontinuity at the step edge. (D and E) demonstrate that DBA-DA25 porous networks comply with a steep slope of the graphene substrate. Line profiles along the black line (1) and the red line (2) in (D) are shown below the STM images, in which profile 1 indicates that the periodicity of the pores is about 6.9 ± 0.2 nm and profile 2 shows the significant topographical changes of the steep slope, respectively.

2.Modification of the electronic properties of graphene

Figure. STM images of self-assembled oleylamine (OA) on HOPG (as 'model' surface for graphene), (A) large-scale image and (B) high resolution image. The images were taken under ambient conditions, Iset = 80 pA, Vset = −650 mV. (C) Tentative model of the organization of OA molecules. To examine the electronic interaction effects of OA SAM in graphene, back-gated graphene field-effect transistor devices were characterized electrically. Figure shows the transfer curves of drain current (Ids) vs back-gate voltage (Vg) for a graphene-FET device before and after OA-SAM modification.

(A) Schematic showing the graphene device decorated with well self-assembled OA molecules and an optical micrograph of the FET used in this study. (B) Ids-Vg characteristics of a graphene FET device before , after several OA treatments and after OA removal taken at a source-drain bias (Vds) of 5 mV in ambient conditions; (C) Charge concentration as a function of device treatment steps. The black curve of sample 1 is extracted from the data in (B); (D) Evolution of charge carriers mobility at different device treatment steps taken at |n|= 1 x 1012 cm-2