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52 ternary zno/znpc/p3ht systemJ-type aggregates give rise to red shift transitions in theabsorption spectra with respect to the monomer [99], whileH-types are associated with a shift toward the blue.Figure 4.3.: Modality of aggregation of ZnPcs on ZnO. Left: headto-tailconfiguration; middle: face-to-face configuration;right: slipped cofacial configuration (figure from [5]).J-type aggregates turn out to be more energetically stablewith respect to a face-to-face aggregation [5], due tothe molecule-substrate adhesion (2.2 eV) larger than themolecule-molecule binding (1.6 eV) [5]. Therefore, the adsorptionof ZnPc molecules on the ZnO surface is morelikely to occur with respect to their stacking, and the formationof ZnPcs monolayers is energetically favored [5].At room temperature the lifetime of dimers and smallmolecular stripes is as short as a few microseconds [5].However, at high coverages, the aggregation involves morethan 50% of molecules [5] and there are portions of ZnOthat are fully covered by ZnPcs.By assuming a fully coverage of ZnO by ZnPcs, we wantto study the formation of a molecular monolayer of ZnPcs.Accordingly, we start from the single relaxed molecule (Figure4.4, top left panel) on the ZnO surface. We create andrelax a ZnPcs dimer, by putting a second molecule shiftedalong the [010] direction (Figure 4.4, top right panel) at adistance of ∼ 13 Å (4a, where a = 3.25 is the ZnO latticeconstant along this direction), providing the most stableconfiguration [5].The relaxed dimer, can be used as building block for theZnPcs stripes (Figure 4.4, top left panel) that, repeated periodicallyalong the trench grooves of the ZnO, give riseeventually to a "carpet" of ZnPcs (Figure 4.4, bottom rightpanel).