Graphene

Graphene has been recognized as a versatile optical material for photonic and optoelectronic applications, such as ultrafast lasers, solar cells, optical modulators, photodetectors, light emitting devices and metamaterials. In our laboratories, we aim to fabricate devices in which graphene enhances the stability and life-times of such devices. We are also interested in using graphene to fabricate test-beds to study the interplay between photonics, electronics and plasmonics.

Graphene as a part of the electrode in molecular electronics

The performance of molecular junctions is limited by a trade-off between stability and structure. Stable covalent monolayers are often defective in structure while non-covalent SAMs have well-defined structures but mostly with limited stability. We used monolayer graphene as the bottom electrode to fabricate stable and high quality SAM based tunnel junctions. These SAMs are formed on graphene via non-covalent bonding without altering the structure of graphene. The junctions are stable against voltage cycling for 1200 cycles, aging for 35 days, and retain currents for 100,000 s in ambient conditions. Unlike metal-thiolate bonds, the amine-graphene bonds are stable and do not oxidize in air. These findings demonstrate that SAMs of alkylamines on graphene are a promising platform for molecular electronics and pave the way to new types of molecular electronic junctions based on 2D materials. A new class of molecular tunnel junctions can be envisaged by functionalising the molecular back-bone of amines, as a consequence of this work (see Adv. Mater. 2016, 28, 631–639).

Fig 1: Schematic illustration of the Cu//Graphene//H2NCn//GaOx/EGaIn tunnel junction with β-plot and the J (V) curve. Structures based on molecular dynamics (performed by Damien Thompson) studies which show a highly stable platform of Amines on Graphene. This work was highlighted by Advanced Materials as a cover.

Graphene as a protection barrier in electronics

Graphene is mechanically robust enough to withstand the harsh fabrication conditions typically encountered in the fabrication of semiconductor devices (thermal evaporation at high temperatures and high vacuum conditions). We used ballistic electron emission microscopy (BEEM) in collaboration with Cedric Troadec to probe the buried interfaces of semiconductor/metal interfaces with nanometer spatial resolution. We found that the presence of graphene at the interface of n-Si(111)/Au and n-Si(111)/Cu does not change the Schottky barrier height and energetics of these interfaces, i.e. graphene is electronically transparent while functioning as a physical barrier against the interdiffusion of materials(see ACS Appl. Mater. Interfaces, 2014, 6 (22), pp 20464–20472.). Hence, graphene can be seen as a protection barrier for applications in electronic devices.

Fig 2: Schematic of the graphene protection barrier for n-Si(111)/Au and n-Si(111)/Cu devices where graphene prevents unwanted doping of the Si by the Au and Cu upon heating. BEEM spectra obtained for the n-Si(111)/Au (15 nm) and the n-Si(111)/Gr/Au (15 nm) interfaces.