Anomalous Two-Dimensional Organic Molecular Crystal Formation in an Evaporating Droplet with Different Vibrational Characteristics of the Surface and the Bulk

Two-dimensional (2D) crystals of organic molecules, bearing in-plane van der Waals interactions and having unique properties, are capable of multifunctional applications. However, there is a significant challenge to prepare 2D crystalline organic materials by conventional evaporation induced crystallization method from saturated solution. Herein, by utilizing the novel evaporation induced ‘coffee-ring’ deposition process, 2D crystals of organic molecules have been realized at the three-phase contact line (TPCL) of the evaporating droplet. We prepared 2D crystals of methyl orange and methylene blue at the TPCL of the evaporating droplets having aqueous solutions of the molecules, respectively. The thin films consisted of 2D layer-by-layer arrangements of molecules in the crystal structures as revealed from the refinement of X-ray diffraction data. Moreover, different intensity and red-shift of vibration peak positions in the Raman spectra of the interior of the crystal compared to surface indicated that the vibrational characteristics of the surface of crystal is different from that of the interior of crystal. Importantly, Raman spectroscopic probe on the 3D crystals of the molecules, when crystalized from the super-saturated aqueous solutions using conventional technique, also indicated the different nature of the vibrational characteristics of the surface as compared to the interior bulk hinting at the generality of the observations.