Tuning ferroelectricity of niobium clusters by oxygen chemisorption.

The chemisorption of atomic oxygen on Nb(N) (N=2-16) has been investigated by the scalar relativistic all-electron density-functional calculations with emphasis on its effect on the ferroelectricity of Nb(N). We have shown that the binding of O atom to niobium clusters is site- and size-dependent, for which the bridge-site doping is preferred in the smaller size range from N=2 to 8 and the threefold hollow site one for the larger Nb(N) with 9 < or = N < or = 16. Though the geometrical structures of Nb(N) are modified slightly when doped with an oxygen atom, their ferroelectric properties vary considerably, depending on cluster size and the O adsorption sites, which is mainly caused by the charge transfer between the oxygen atom and niobium clusters. The addition of oxygen can enhance the ferroelectricity of Nb(N) with moderate and essentially zero moments while induce only small dipole change for those Nb(N) with large moments. Thus, the big fluctuation of the Nb(N)'s dipole moments with size is greatly suppressed by the O doping, indicating that one might expect to tune the size-dependent ferroelectricity of Nb(N) by the chemical decoration.