Ring-type Se clusters have been deposited on the C-plane of a graphite crystal and examined by STM images which were found to consist of 0.72nm×0.85nm rectangular lattices with individual molecules of 0.53nm in diameter. Se ring-type clusters sitting on each lattice point are most probably 6-membered. The STM images were taken with the bias voltage V_T between probe vs. sample as a parameter. For -0.7V<V_T<+0.7V the patterns of the rectangular lattice remains similar, but an order-disorder transition takes place at -0.065V; whereas V_T at -0.7V, the regular arrangement suddenly collapses to a random distribution of larger clusters consisting of their diameter in a range up to 5nm. The process of the structural change in the lattice system is not reversible with respect to the bias voltage. These experimental results are well explained by flrst-principles theoretical calculation on the structure and electronic properties of Se cluster using the DV-SCM method
