在Cd(0001)基底上制备单斜相的红荧烯薄膜

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

利用有机分子束沉积和超高真空-低温扫描隧道显微镜,研究了红荧烯在Cd(0001)表面上的自组装和薄膜生长过程.研究发现:在低覆盖度下单个红荧烯分子斜躺在基底表面,表现出3个不同亮度的突起,并出现了手性特征.随着覆盖度的增加,红荧烯形成自组装的单层结构,每个分子仍保持斜躺的姿态,但单个分子的形貌非常依赖偏压,在高偏压下表现出3个突起,在低偏压下表现出4个突起.之后,第二层的红荧烯发生了取向相变:分子由斜躺变为站立,且在第二层出现了两种不同的结构.第一种是链状分子结构,第二种是一个新的单斜相.特别是,此前曾有人利用第一原理计算预言了该单斜相的存在,但一直没有得到实验证实.此次研究成果可以为制备新型红荧烯发光器件提供新的选择和思路.

The self-assembly 2 D thin films of rubrene molecules on a metallic Cd（0001） surface have been investigated using a lowtemperature 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 4 d 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.