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Very weak bonds to artificial atoms formed by quantum corrals

Bonding to a quantum corral Chemical bonds generally form between electronic states of atoms; in principle, other electronic states could also form bonds. Stilp et al. found that the electronic states created within quantum corrals, large rings of iron atoms on a copper surface, can form chemical bonds with metal atoms on an atomic force microscope tip. The corral states form from many electrons but have a large spatial extent compared with an atomic orbital. The covalent bond to a 48-atom corral state had an energy of just 5 millielectron volts. Science, abe2600, this issue p. 1196 The chemical bond between the surface states formed by a quantum corral and the tip of an atomic force microscope was measured. We explored the bonding properties of the quantum corral (a circle of 48 iron atoms placed on a copper surface) reported by Crommie et al. in 1993, along with variants, as an artificial atom using an atomic force microscope (AFM). The original corral geometry confines 102 electrons to 28 discrete energy states, and we found that these states can form a bond to the front atom of the AFM with an energy of about 5 millielectron volts. The measured forces are about 1/1000 of typical forces in atomically resolved AFM. The confined electrons showed covalent attraction to metal tips and Pauli repulsion to CO-terminated tips. The repulsion at close distance was evident from the response of corral states created by deliberately placing single iron atoms inside the corral. The forces scaled appropriately with a 24-atom corral.