A new phase of rubrene monoclinic crystals on Cd(0001)

The self-assembly 2D thin films of rubrene molecules on a metallic Cd(0001) surface have been investigated using a low-temperature scanning tunneling microscopy. Compared with other precious metals, the divalent hexagonal close packed metal Cd are less reminiscent of a free electron metal because of the strong repulsive interaction between the 4d state and the conduction bands. It manifests some anisotropic behaviors in thermal conductivity, electron mean free path, and superconducting gaps. So in this paper, Cd was used as a substrate to study rubrene. First, isolated rubrene molecules reveal a tilted orientation on the substrate surface and exhibit three distinct protrusions with chiral feature. When the rubrene coverage increased to 0.4 ML, rubrene molecules begin to assemble into monolayer structure on the Cd(0001) substrate. Rubrene molecules adopt a side-lying orientation, and each rubrene molecule takes the form of zigzag with three bright spots. Upon further deposition of rubrene to 1.5 ML, the orientation transition from titled standing-up monolayer to standing-up monoclinic crystals was observed. There are two kinds of structures in the second layer. The first one is chainlike structure, where rubrene molecules are supported by two phenyls and the planes of tetracene are parallel to each other. The plane of tetracene backbone is perpendicular to the substrate plane. Another novel structure in the second layer of rubrene film is a new monoclinic phase. Furthermore, the rubrene monoclinic phase has a slight compressive strain in the a -axis direction, whereas the compressive strain in the c -axis direction becomes more pronounced. In addition, we noticed that one c -axis of a - c plane of monoclinic phase is parallel to the crystal orientation of the substrate. In particular, this monoclinic phase of rubrene was predicated by DFT calculation but has never been confirmed in experiment.