Nanophotoactivity of Porphyrin Functionalized Polycrystalline ZnO Films.
Kelvin probe force microscopy in darkness and under illumination is reported to provide nanoscale-resolved surface photovoltage maps of hybrid materials. In particular, nanoscale charge injection and charge recombination mechanisms occurring in ZnO polycrystalline surfaces functionalized with Protoporphyrin IX (H2PPIX) are analyzed. Local surface potential and surface photovoltage maps not only reveal that upon molecular adsorption the bare ZnO work function increases, but also they allow study of its local dependence. Nanometer-sized regions not correlated with apparent topographic features were identified, presenting values significantly different from the average work function. Depending on the region, the response to the light excitation is different, distinguishing two relaxation processes, one faster than the other. This behavior can be explained in terms of electrons trapped closed to the molecule-semiconductor interface or electrons pushed into the ZnO bulk, respectively. Moreover, the origin of these differences is correlated with the H2PPIX-ZnO bonding and molecules configuration and aggregation. The chenodeoxycholic acid (CDCA) coadsorption leads to a more homogeneous surface potential distribution, confirming the antiaggregate effect of this additive, while the surface photovoltage is mostly dominated by the slow relaxation component. This work reveals the complexity of real device architectures with ill-defined surfaces even in a relatively simple system with only one type of dye molecule and hightlights the importance of nanoscale characterization with appropriate tools.
Centro de Física de Materiales (CSIC-UPV/EHU), Material Physics Center (MPC) and Donostia International Physics Center , 20018 San Sebastian, Spain.
Celia
C
Rogero
Centro de Física de Materiales (CSIC-UPV/EHU), Material Physics Center (MPC) and Donostia International Physics Center , 20018 San Sebastian, Spain.
David F
DF
Pickup
Departamento Física, Facultad de Química (Campus Espinardo), Universidad de Murcia , E-30100 Murcia, Spain.
Jaime
J
Colchero
Materials Department, IK4-CIDETEC , 20009 San Sebastian, Spain.
Eneko
E
Azaceta
Materials Department, IK4-CIDETEC , 20009 San Sebastian, Spain.
Ramón
R
Tena-Zaera
Departamento Física, Facultad de Química (Campus Espinardo), Universidad de Murcia , E-30100 Murcia, Spain.
Elisa
E
Palacios-Lidón
Kelvin probe force microscopy; ZnO; dye−semiconductor interface; porphirin; surface photovoltage