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4.1 self assembling of znpcs on zno surface 51MPMD (Figure 4.2 right) methods. The interaction betweenthe molecule and the substrate vanishes at distances largerthan about 8 Å. These results are in agreement with theliterature [56].Figure 4.2.: Comparison between the structure of a ZnPc molecule relaxedon the ZnO surface by performing DFT (left) orMPMD (right) calculations. (The DFT figure is takenfrom [4]).4.1.2 Aggregation of ZnPc on ZnOThe photophysics of ZnO functionalized by ZnPcs is affectedby temperature, molecular concentration, and theZnO surface morphology [5, 97, 98] and these effects arerelated to the tendency of Pcs to aggregate at the interface[98]. Aggregation can occur during the synthesis [99] ordue to thermally activated molecule diffusion on the surface[56]. Furthermore, ZnPcs aggregates have electrochemical,spectroscopic, photophysical, and conductive propertiesdifferent from those of the corresponding monomers.Two kinds of aggregates have been identified accordinglyto their optical absorption properties.• In the J-type aggregates the molecules are parallel ina head-to-tail (HT) alignment along the [010] crystallographicdirection as in Figure 4.3 left.• The H-type aggregates, where the molecules give riseto a parallel configuration, can be further dividedinto two groups [5]:– face-to-face aligment (FF), as in Figure 4.3 middle;– slipped cofacial alignment (SC) as in Figure 4.3right.