Molecular orientation and electronic properties of oriented organic thin films by means of low-energy electron transmission.

利用低能电子透射（LEET）技术研究了长烷基分子薄膜、花生酸镉的Langmuir-Blodgett （LB）薄膜、聚二乙炔的LB薄膜、四四辛烷的蒸发薄膜以及相关分子薄膜的分子取向与电子输运性质的关系。测量了薄膜厚度、结晶度、温度和沉积时间对LEET光谱的影响。我们发现：(1)分子的正常取向程度取决于膜的厚度，当膜的厚度大于4个分子层时，取向更好；(2)较薄的膜（2个分子层）的取向较差，这是结晶度较差的结果；(3)导带底部位于真空水平以上0.4-0.5 ev，由于分子振动的热激发，能量位置通过加热转移到低能侧。（三）低温时热电子注入薄膜的有效效率较大；（四）电子通过带隙的传输不能用隧道理论来解释。此外，利用同步辐射对18-五元康酮（硬脂酮）取向薄膜进行了角度分辨光发射实验。该方法同时观察了正常分子取向和能带色散。

We investigated the correlation between the molecular orientation and the electron- transport properties for thin films of long alkyl molecules, Langmuir-Blodgett (LB) films of cadmium arachidate, LB films of polydiacetylene, evaporated films of tetratetracontane and thin films of related molecules, with low-energy electron transmission (LEET) technique. LEET spectra were measured as a function of film thickness, crystallinity, temperature and time after the film deposition.We found that (i) degree of normal molecular orientation depends on the film thickness and the orientation becomes better for the thickness larger than 4 molecular layers, (ii) a poor orientation for thinner films, 2 molecular layers, is a results of poor crystallinity, (iii) the bottom of the conduction band is located at 0.4-0.5 ev above the vacuum level and the energy position shifts to low energy side by heating due to thermal excitation of molecular vibrations, (iii) the effective efficiency of the hot- electron injection into the film is larger at low temperature, and (iv) the electron transmission through the band gap can not explained by the tunnel theory.Further, angle-resolved photoemission experiments were performed with synchrotron radiation for oriented thin films of 18-pentatriacontanone (stearone). The normal molecular orientation and energy band dispersion were observed simultaneously with this method.