Systems and methods for synthesizing molecules on a substrate surface are disclosed. In one aspect, a molecule synthesizing system includes a crossbar array with a planar arrangement of crossbar junctions. Each crossbar junction is independently switchable between a high-resistance state and a low-resistance state. The system also includes a slab with a first surface and a second surface parallel to the first surface. The second surface is disposed on the crossbar array. A current applied to a crossbar junction in a high-resistance state creates an adjacent heated site on the first surface for attaching thermally reactive molecules for molecular synthesis.