Thickness-Dependent Air-Exposure-Induced Phase Transition of CuPc Ultrathin Films to Well-Ordered One-Dimensional Nanocrystals on Layered Substrates

Highly ordered organic crystals are important building blocks for future high-performance organic nanodevices. Here we report a feasible way to produce arrays of well-ordered 1D copper phthalocyanine (CuPc) nanocrystals by using molybdenum disulfide (MoS2) or highly oriented pyrolytic graphite (HOPG) as substrates. The growth behaviors of CuPc on MoS2(0001) as well as on HOPG and corresponding effects of air exposure were systematically investigated by means of in situ photoemission spectroscopy (PES) and low-energy electron diffraction (LEED), combined with ex situ atomic force microscopy (AFM), surface X-ray diffraction (SXRD), and Raman spectroscopy. PES and LEED results show that CuPc molecules adopt a face-on configuration at thickness up to 4.8 nm, while AFM and SXRD results show that they adopt an edge-on configuration to form 1D nanocrystals in films thicker than 2.4 nm. Detailed analyses show that the formation of these 1D nanocrystals is closely related to air exposure, thicknesses, and growth temperature. Such 1D CuPc nanocrystals can be further optimized by tuning growth conditions and may have great potential for use in high-performance organic devices.