Formation and valence band density of states of nonspherical Cu nanoparticles deposited on Si(100) substrate

Copper thin films consisting of nanosized particles were deposited on a Si(100) single-crystal substrate by laser ablation in an argon atmosphere. In order to obtain isolated particles with nanometer sizes, the film was sputtered by low-energy Ar+ ions. The valence band density of states of the film was in situ investigated by X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS) during the Ar+ ion sputtering. The size of the particles formed after sputtering was in the range between 3 and 6 nm in lateral dimension and 2-3 nm in height, as measured by transmission electron microscopy. In the as-deposited state the photoelectron spectrum of the thin layer was identical to what was observed in the bulk. At the end of thinning by Ar+ ion sputtering the 3d valence states were rehybridized with respect to the bulk. states, which indicates that the lower binding energy part of the d band (around 2.5 eV) is the most sensitive for size reduction. The Fermi edge became undetectable by photoemission. These data show that the valence band of small particles are affected by both the size and geometry of the particles. Supporting experiments showed that the effect is attributed to neither copper silicide formation nor intermixing.