Angle-resolved ultraviolet photoelectron spectroscopy and theoretical simulation of a well-ordered ultrathin film of tetratetracontane (n-C 44 H 90 ) on Cu(100): Molecular orientation and intramolecular energy-band dispersion
Angle-resolved ultraviolet photoelectron spectroscopy and theoretical simulation of a well-ordered ultrathin film of tetratetracontane (n-C 44 H 90 ) on Cu(100): Molecular orientation and intramolecular energy-band dispersion
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Cu(100) 上四十四烷 (n-C 44 H 90 ) 有序超薄膜的角分辨紫外光电子能谱和理论模拟：分子取向和分子内能带色散
DOI：
10.1103/physrevb.60.9046
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发表时间：
1999
影响因子：
3.7
通讯作者：
K. Seki
中科院分区：
文献类型：
--
作者：
D. Yoshimura;H. Ishii;Y. Ouchi;E. Ito;T. Miyamae;S. Hasegawa;K. Okudaira;N. Ueno;K. Seki
利用同步辐射角分辨紫外光电子能谱（ARUPS）研究了Cu（100）表面上的正十四烷（n-C44 H90; TTC）薄膜的电子结构和分子取向.采用真空蒸发法在室温下成功地制备了取向性良好的TTC薄膜。我们观察到沉积的TTC薄膜具有类似（2× 1）的低能电子衍射（LEED）图样。这一结果表明TTC分子在Cu（100）表面上以两种类型的区域存在，彼此呈矩形，其中烷基链轴沿着Cu（100）表面的[110]和[11 0]方向。偶极选择规则在正常发射ARUPS谱中的应用表明TTC的C-C-C平面平行于Cu（100）表面平面（平坦取向）。通过改变发射电子沿Cu（100）面[110]方向沿着的发射角，研究了TTC分子内能带色散。实验结果支持了LEED观察到的烷基链轴方向的结论。为了更定量地分析分子取向，我们还使用独立原子中心（IAC）近似结合从头算分子轨道（MO）计算对各种分子取向进行了角度分辨光电子能谱的理论模拟。模拟的光谱与观察到的光谱非常吻合。这些结果再次验证了所推导的分子取向，同时也证明了IAC/MO近似对不含π电子体系的化合物的理论模拟的可靠性.此外，我们观察到约-0.3 eV的吸附TTC的功函数的变化。这种功函数的降低表明在界面处形成偶极层，与传统的假设有机/金属界面处的共同真空水平的能级对准的图片相反。
The electronic structure and molecular orientation of a tetratetracontane (n− C 44 H 90; TTC) ultrathin film on a Cu (100) surface were studied by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) using synchrotron radiation. A well-oriented thin film of TTC was successfully prepared by vacuum evaporation in ultrahigh vacuum at room temperature. We observed a (2× 1)-like low-energy electron-diffraction (LEED) pattern for the deposited TTC film. This result indicates that the TTC molecules lie on the Cu (100) surface in two types of domains, rectangular to each other, in which the alkyl-chain axes are along the [110] and [11 0] directions of the Cu (100) surface. The application of the dipole selection rules to the normal-emission ARUPS spectrum revealed that the C—C—C plane of TTC is parallel to the Cu (100) surface plane (flat-on orientation). The intramolecular energy-band dispersion of TTC was examined by changing the take-off angle of emitted electron along the [110] direction of the Cu (100) surface. The observed results support the conclusion about the direction of alkyl-chain axes by LEED observation. In order to analyze the molecular orientation more quantitatively, we also performed theoretical simulations of the angle-resolved photoemission spectra using the independent-atomic-center (IAC) approximation combined with ab initio molecular-orbital (MO) calculations for various molecular orientations. The simulated spectra for flat-on orientation are in excellent agreement with the observed spectra. These results once again verify the deduced molecular orientation, and also demonstrate the reliability of theoretical simulation with the IAC/MO approximation for compounds without a π-electron system. Furthermore, we observed a work function change of about-0.3 eV by adsorption of TTC. Such a decrease of the work function indicates the formation of a dipole layer at the interface, in contrast to the traditional picture of energy-level alignment assuming a common vacuum level at the organic/metal interface.