2,7-二辛基[1]苯并噻吩并[3,2-b]苯并噻吩在Cu(100)上的吸附生长以及能级结构演化

Adsorption,film growth,and electronic structures of 2,7-dioctyl[1]benzothieno-[3,2-b][1]benzothiophene(C8-BTBT) on Cu(100)

结合紫外光电子能谱(UPS),X射线光电子能谱(XPS),原子力显微镜(AFM)和掠入射X射线衍射谱(GIXRD)等实验手段,系统研究了2,7-二辛基[1]苯并噻吩并[3,2-b]苯并噻吩在Cu(100)基底上的吸附、生长过程以及界面能级结构.发现第一层的分子平躺吸附于Cu(100)上形成稳定的物理吸附.随膜厚增加,分子取向转为直立于薄膜平面,生长模式转为岛状生长模式.分子取向的变化导致膜厚大于16A的薄膜的能级结构发生变化.直立取向的分子在表面形成由内向外的电偶极层,引起真空能级下降,功函数降低;而轨道电离的各向异性使得分子从平躺到直立时UPS得到的分子最高占据轨道(HOMO)峰型发生变化,且HOMO起始边向深结合能端移动整体上随着膜厚的增加,真空能级向下弯曲,HOMO下移,电离能则先减小后增大下移的能带结构利于电子从界面向表面的迁移以及空穴从表面向界面的迁移.

Using ultraviolet photoemission spectroscopy（UPS）,X-ray photoemission spectroscopy（XPS）,atomic force microscopy（AFM）,and grazing X-ray diffraction measurement（GIXRD）,we systematically investigate the correlations of interface energy level structure,film growth and the molecular orientation of 2,7-dioctyl[l]benzothieno-[3,2-b][l]benzothiophene（C8-BTBT） on Cu（100）.We find that the adsorption of the first layer of C8-BTBT molecules on Cu（100）is a stable physical one,and there is no chemical shift of the S 2p peaks of XPS and the ratio of the output of C to that of S is the same as the stoichiometric value of the molecular C8-BTBT.The heights of the steps of the upper layers of C8-BTBT in the AFM images are -30 A,close to the length of the molecular long c-axis,indicating the standing-up configuration of the upper molecules.AFM image shows that the upper molecules tend to grow into islands while the bottom molecules tend to grow into layer,suggesting an Stranski-Krastanov growth mode of multilayer C8-BTBT on Cu（100）.The GIXRD shows an out-of-plane period of 30.21 A,which consistently proves the standing-up configuration of the outer molecule layer.There is an electric dipole of 0.41 eV at the very interface pointing from the substrate copper to C8-BTBT,which will reduce the barrier for electron transport and increase the barrier for hole transport from Cu to C8-BTBT.The vacuum level（Evac） starts to bend downward after 16 A deposition,and with the increase of the thickness of the film,a total downward shift of 0.42 eV is observed.The downward shift is ascribed to the changing of molecular orientation from lying down before 16 A to standing up after 16 A,which establishes an outward-pointing layer of C H bonds and accordingly forms a dipole layer depressing the surface barrier.The shape and leading edge of the hightest occupied molecular orbit（HOMO） also change with the increase of film thickness.These changes are due to the anisotropy of electron ionization from molecular orbit.The total downward shift of the HOMO is about 0.63 eV.The downward bending of 0.42 eV for Evac and 0.63 eV for HOMO with increasing film thickness lead to a slightly decreasing ionization potential（IP） about 0.1 eV before 32 A and then an increasing IP about 0.31 eV,which finally results in a total increase of 0.21 eV for IP.The bending electronic structures facilitate electron transport from interface to surface and hole transport from surface to interface.Our investigation provides valuable information for relevant device design.