用STM第一次观察到聚乙烯晶片的折迭链结构

FOLDED CHAIN STRUCTURES OF THE POLYETHYLENE CRYSTAL LAMELLAE OBSERVED WITH STM FOR THE FIRST TIME

通常人们认为聚乙烯薄膜是绝缘体,聚乙烯晶片的表面形貌是分子链的折迭部分,多是无规排列的,不能用扫描隧道显微镜(STM)研究聚乙烯薄膜的表面形貌。我们研究了聚乙烯薄膜的电学特性和结构特性,极薄的聚乙烯晶片在强电场下,有nA级的传导电流,因此可以用STM分析聚乙烯晶片的表面形貌。我们用STM首次观察到了聚乙烯晶片表面折迭链的周期结构。

Usually, the polyethylene thin film is considered an insulator and the topography of thepolyethylene crystal lamellae is a randomly folded chain, so that scanning tunneling micro-scope (STM) can not be used to analyze the surface structures of the polyethylene thin films.We have studied the electronic properties and the structure properties of the polyethy-lene thin films. Very thin poltethylene crystal lamellae can conduct current of the order ofmagnitude of nA under higher electrical field. So the topography of polyethylene crystallamellae can be analyzed with STM. We have observed periodic structures of the folded chainof the polyethylene crystal lamella surface for the first time. We have prepared the polyethylene thin films with the ionized cluster beam (ICB) depo-sition technique. The principle and characteristic of preparing organic thin films have beenstated in Reference[3]. The perfect crystal lamellae have been prepared with the ICB tech-nique. We have prepared polyethylene thin films on glass, Ni, Mo, Si, stainless steel andgraphite substrates. We have analyzed these samples with transmission electron microscope (TEM). The (001)plane of the polyethylene crystal lamellae parallels the substrate surface, see Fig. 1. We have observed the topography of the polyethylene crystal lamellae with STM. Theconditions of the STM are the follows: bias V_b=-67mV, tunneling current I_t=1.2nA andscanning area A=5. 5nm×3. 8nm. The topography of the polyethylene crystal lamellae isshown in Fig. 2. There are good periodic structures in the vertical direction and periodicstructures of two rows in the lateral direction on the right hand side of Fig. 2. Accordingto these structures, the size of the polyethylene subunit cell of the (001) plane we obtained is that a=0.76nm, b=0.54nm. This result is consistent with that of the TEM. Since thesize of the polyethylene subunit cell is that a=0. 745nm,b=0.497nm, c=0. 252nm, the topo-graphy of the polyethylene thin films in Fig. 2 is the folded chain structure of the polyethy-lene crystal lamellae on the (001) plane.