2,7-二辛基[1]苯并噻吩并[3,2-b]苯并噻吩/Ni(100)的界面能级结构随薄膜厚度的演化

Thickness-dependent electronic structure of the interface of 2,7-dioctyl[1]benzothieno[3,2-b][1] benzothiophene/Ni(100)

利用紫外光电子能谱、X射线光电子能谱以及原子力显微镜系统研究了2,7-二辛基[1]苯并噻吩并[3,2-b]苯并噻吩(C8-BTBT)生长在单晶Ni(100)上的能级结构随着薄膜厚度的演化以及薄膜的生长方式.发现第一层C8-BTBT平躺生长且与Ni基底发生了化学吸附反应.从第二层起分子直立生长且呈现岛状生长模式.这种平躺至直立的分子取向转变,导致薄膜的能级结构在第一层与第二层间发生阶梯式的变化,真空能级与最高占据能级同步下降.此后能带结构随着薄膜厚度的增加逐渐向下弯曲,功函数随着膜厚的增加而减小.同时还发现由于直立生长的C8-BTBT其层间电导率较差导致实验中的能级未能收敛.实验结果提示对基于Ni和C8-BTBT的自旋器件需要插入缓冲层并尽可能减少C8-BTBT的层数.

Combining ultraviolet photoemission spectroscopy, X-ray photoemission spectroscopy （XPS） and atomic force mi- croscopy （AFM）, we perform a systematic investigation on the correlation of energy level alignment, film growth and molecular orientation of 2,7-dioctyl[1]benzothieno-[3,2-b][1]benzothiophene （C8-BTBT] on Ni（100）. The molecules lie down at the first layer and are partly devulcanized by the substrate. Chemical adsorption of reaction products of sulfur atoms on the Ni substrate and the evaporation of the hydrocarbon products into vacuum make the C/S ratio as low as 11.5 ： 1 in the XPS of the initially deposited C8-BTBT film of 1-4 A thickness, far less than the stoichiometric of 15 ： 1. With the thickness increasing from 4 to 8A, there are sharp downward shifts of Evac, HOMO and core levels of C 1s, S 2p, and a sharp increase of C/S rato,which can be asoribed to the change of molecular orientations from lying down at 4 A to standing up at 8 A. From 8 A onward,the C/S ratio increases steadily till it reaches 15 ： 1. The energy levels show relatively less changes when the thickness increases from 8 to 32 A. When the thickness increases over 32A, the energy band starts bending downward apparently because of the charging effect during the photoelectron emission processes. The poor conductivity along the standing alkyl chain of C8-BTBT is the main cause for the charging. The standing up configurations of the C8-BTBT molecules are confirmed by the AFM investigation in which the heights of the upper layers of C8-BTBT are around 30 A, close to the length of the long c-axis. AFM image also indicates that the molecules tend to grow into islands for larger thickness, which is consistent with the slower decrease of the In（I/Io） of Ni 2p3/2 with the C8-BTBT film thickness. Our results suggest that a buffer layer be inserted between Ni and CS-BTBT and the thickness of the C8-BTBT film be controlled as thin as possible in related devices.