A technique in nanofabrication in which nanometre-scale devices are built up atom by atom and molecule by molecule by either self-assembly or atomic manipulation. Self-assembly, simulating the way in which biological systems build molecules and cells, has been explored to fabricate semiconductor quantum dots, magnetic single domain bits, and bio-chips. Atomic manipulation using scanning-probe microscopy has the unique capability of precise placement of individual atoms, but the process is time-consuming.