Phenalenyls as tunable excellent molecular conductors and switchable spin filters

We demonstrate a new class of molecules for exceptional performance in molecular electronics and spintronics. Phenalenyl-based radicals are stable radicals whose electronic properties can be tuned readily by heteroatom substitution. We employ density functional theory-based non-equilibrium Green's function (NEGF-DFT) calculations to show that this class of molecules exhibits tunable spin- and charge-transport properties in single molecule junctions. Our simulations identify the design principles and interplay between unusually high conductivity and strong spin-filtering: Paired with moderate conductance ($10^{-3} G_0$), two of the four radicals investigated exhibit above 80% spin filter efficiency that is moreover tunable via bias control. Conversely, two radicals that make modest spin filters are excellent conductors with a low bias conductance reaching $0.48 G_0$. This is made possible by the unusually good alignment of the singly occupied or unoccupied molecular orbital with the Fermi level of the electrodes, overcoming the limitations of Fermi level pinning that typically plague molecular electronics. We show that this interplay between excellent conductance and high spin-filter efficiency is determined by the energy alignment between the singly (un)occupied molecular orbital and the Fermi level of the electrodes, and that for phenalenyls this can be readily controlled with judicious heteroatom substitution.