Charge transfer and band bending at Au/Pb(Zr_(0.2)Ti_(0.8))O_3 interfaces investigated by photoelectron spectroscopy

摘要

The growth of gold layers on Pb(Zr,Ti)O_3 (PZT) deposited on SrTiO_3 is investigated by X-ray photoelectron spectroscopy in the Au thickness range 2-100 A. Two phases are identified, with compositions close to nominal PZT. The 'standard' phase is represented by all binding energies (Pb 4f, Ti 2p, Zr 3d, O 1s) sensibly equal to the nominal values for PZT, whereas the 'charged' phase exhibits all core levels are shifted by ~1 eV toward higher binding energies. By taking into account also scanning probe microscopy images together with recent photoemission results, the 'charged' phase belongs to P~((+)) regions of PZT, whereas the 'normal' phase corresponds to regions with no net ferroelectric polarization perpendicular to the surface. Au deposition proceeds in a band bending of Φ_(PZT)- Φ_(Au)~ 0.4-0.5 eV for both phases, identified as similar shifts toward higher binding energies of all Pb, Ti, Zr, 0 core levels with Au deposition. The Au 4f core level exhibits also an unusually low binding energy component 1 eV below the 'nominal' Au 4f binding energy position (metal Au). This implies the existence of negatively charged gold, or electron transfer from PZT to Au, although the 'normal' PZT phase have a higher work function, as it is derived from the band bending. Most probably this charge transfer occurs toward Au nanoparticles, which have even higher ionization energies. High resolution transmission electron microscopy evidenced the formation of such isolated nanoparticles.