Mismatch-free tunnel spin contacts by photo-induced molecular functionalization of single layer graphene

A major issue preventing efficient spin injection in graphene is the mismatch between the contact and the spin impedance of the channel. This is of particular relevance for ultra-thin (0.6 nm) alumina tunnel barriers. We overcame this problem by growing the alumina barriers on graphene functionalized with phenyl radicals. The functionalization is achieved by using a laser beam to catalyse a photochemical reaction in benzoyl peroxide. The sub-micron spatial resolution enabled by the focused laser beam allows a direct comparison of different functionalization levels within a single graphene flake. Raman spectroscopy and Atomic Force Microscopy showed a dependence of the roughness of the tunnel barrier on the level of functionalization with the roughness of the barrier on functionalized graphene, reduced to half of that on pristine graphene. This functionalization mitigates the impedance mismatch problem, leading to an order of magnitude increase in the spin valve response.