Modified Surface Structure in Copper Phthalocyanine Thin Films via Bithermal Deposition

The chemical stability and tunability of small molecules used in thin films has a wide range of applications in chemical sensing and photovoltaic devices. Highly ordered structures are sublimed onto heated substrates to improve electronic mobilities or change the sensitivity of gas sensors. Copper phthalocyanine crystals in thin films can typically grow up to 1 µm in length, often resulting in deep pinholes at grain boundaries. We explore a bithermal deposition technique involving sequential sublimation of the same material at two different substrate temperatures in situ. This method produces novel surface structures characterized via atomic force microscopy (AFM) and x-ray diffraction. The AFM analysis shows new surfaces without elongated crystals but with low long-range roughness. For bithermal films deposited at 30 °C followed by 200 °C, the x-ray diffraction shows additional optical reflections from the bithermal interface indicating a sharp interface. The findings also highlight the critical role of the first molecular layer grown onto the substrate that creates the elongated long crystals.