Photoluminescence variations in organic fluorescent crystals by changing the surface energy of the substrate

Organic fluorescent crystals were obtained using single-benzene-based diethyl 2,5-dihydroxyterephthalate (DDT) molecules through crystallization from a droplet of the DDT solution on an Au substrate. To control the size of the DDT crystals, the surface energy of the Au substrate was modified with air plasma treatment, producing a hydrophilic surface and a hydrophobic self-assembled monolayer (SAM) coating. The size of DDT crystals increased as the surface energy of the substrate decreased. The averaged cross-section area of the DDT crystals on the Au substrates increased in the order of the air-plasma-treated substrate (∼23.43 μm²) < pristine substrate (∼225.6 μm²) < hydrophobic SAM-coated substrate (∼2240 μm²). On the other hand, the main emission of the DDT crystals redshifted from blue to green as the crystal size increased, which is related to the aggregation of the DDT crystals. Moreover, the coffee-ring effect during the DDT crystallization was hindered by controlling the solvent evaporation conditions. As examples of the application of the proposed technique, patterned DDT crystals were obtained using selectively patterned hydrophobic and hydrophilic substrates.