Interplay of non-uniform charge distribution on the electrochemical modification of graphene.

Graphene is considered a model material for surfaces because it is stable despite being composed of a single layer of carbon atoms. Although the thermal and electronic properties of graphene are well reported, the behavior of graphene sheets with the addition of charges to the structure is not well understood. Combining infrared spectroscopy, electrochemical analysis, and computational simulations, we report the effect of an electrochemically induced covalent anchoring of 4-carboxyphenyl (4-CP) units on the optical and electronic properties of graphene. Charges in graphene become concentrated at specific sites of the sheet when electrochemically perturbed and the functionalization occurs inhomogeneously along the graphene sheet. We observed that, when graphene is covalently functionalized, the resistance to heterogeneous electron transfer is increased by a factor of 1.4. Furthermore, scattering-type scanning near-field optical microscopy and atomic force microscopy show that the covalent functionalization affects drastically the optical and physical properties of the graphene/SiO2 system, especially the plasmon-phonon coupling after the functionalization. In addition, from these we infer that a comparatively higher degree of functionalization occurs near the electrode edges. These results are supported by computational simulations, which show that the covalent anchoring of 4-CP units weakens electron transfer because the charges are retained on the sp3-hybridized carbon atoms generated upon functionalization, suggesting that graphene properties are deeply influenced by the way the molecules are immobilized on its structure.

A primary battery-on-a-chip using monolayer graphene.

We present here a bottom-up approach for realizing on-chip on-demand batteries starting out with chemical vapor deposition-grown graphene. Single graphene monolayers contacted by electrode lines on a silicon chip serve as electrodes. The anode and cathode are realized by electrodeposition of zinc and copper respectively onto graphene, leading to the realization of a miniature graphene-based Daniell cell on a chip. The electrolyte is housed partly in a gel and partly in liquid form in an on-chip enclosure molded using a 3d printer or made out of poly(dimethylsiloxane). The realized batteries provide a stable voltage (∼1.1 V) for many hours and exhibit capacities as high as 15 µAh, providing enough power to operate a pocket calculator. The realized batteries show promise for deployment as on-chip power sources for autonomous systems in lab-on-a-chip or biomedical applications.