Electron-Donor Dye Molecule on ZnO(10(1)over-bar0), (0001), and (000(1)over-bar) Studied by Photoelectron Spectroscopy and X-ray Absorption Spectroscopy

Interaction between an air-stable orange-colored dye molecule, acridine orange base (AOB), and single-crystal surfaces of ZnO(10 (1) over bar0), (0001), and (000 (1) over bar) is examined by synchrotron-radiation-excited photoelectron spectroscopy and X-ray absorption spectroscopy. AOB adsorbs molecularly on the ZnO surfaces to form chemical bonds with the surface O atoms. AOB tends to lie flat on the surfaces, whereas a more stand-up orientation is preferred on the AOB-crowded surfaces. AOB adsorption induces downward bending of the ZnO band with the largest amount on ZnO(000 (1) over bar), followed by the ZnO(10 (1) over bar0) and ZnO(0001) surfaces, indicating an electron donor property of AOB. The energy level alignment at the AOB/ZnO interface is determined and is found to be characterized by a type-II heterojunction, which favors the charge transfer of the excited electron from AOB to ZnO.