Apr 4, 2012
End-contacted graphene nanoribbons outperform side-contacted ones
The electronic and magnetic properties of nanostructures depend not only on their intrinsic structure, but also on the way they are attached to the macroscopic world through external contacts (electrodes). This study covers both these aspects – it takes into account geometrical dimensions (aspect ratio) and orientation of graphene nanoribbons (GNR), and also deals with the nature of GNR/electrode junctions. Ideally, GNR/electrode junctions can be modeled as either side-contacted or end-contacted (see the sketch below). Although the former is experimentally more relevant, the latter is used more frequently in theoretical studies just for numerical convenience (to save computer time and memory). Here both models are put on an equal footing, which makes it possible to compare directly the respective transport characteristics – conductance, shot noise Fano factor and giant magnetoresistance (GMR).
The adopted computational method combines Green’s function technique and the Landauer-type phase-coherent transport formalism. The nanostructures are modeled with atomic-size precision. In the SC case a modified recursion method has been developed, with the first iteration step corresponding to the vertical transport among the electrode-supported GNR and the electrodes. Next, the recursion involves consecutive periodicity units of the suspended segment.
It turns out that typically the high aspect ratio (wider than long) EC systems have higher conductance and lower Fano factor than their SC counterparts. This is visualized in the figure for the parallel alignment of the ferromagnetic electrodes (↑↑). A similar behavior also holds for the antiparallel configuration (↑↓).
This type of graphene nanoribbon is advantageous for miniaturization and can potentially serve as high-performance interconnects in carbon-based integrated circuits. Remarkably, at the same time the EC GNRs also exhibit a fairly large GMR, although the less conductive SC systems usually show even larger GMR values.
More details can be found in the journal Nanotechnology.
About the author
The studies were carried out in the Institute of Molecular Physics at the Polish Academy of Sciences in Poznań. The author, Prof. Stefan Krompiewski, is head of the department Theory of Nanostructures. Together with his collaborators he works on electronic transport and magnetic properties of nanostructures and quantum dots. Using theoretical methods, ranging from analytical models through semi-empirical ones up to ab initio band computations, the team covers all of the main transport regimes (ballistic, Coulomb blockade and Kondo effect). Hot topics include contact resistivity problems and the impact of edge magnetism, as well as surface rippling on electronic spectra and electrical performance of graphene nanostructures.