AFM scanning images of the surface of a PTCDA/p-Si specimen used in an organic/inorganic photodetector show that PTCDA grows in island shapes that are poorly distributed, with each island shaped like a round hillock. The images also show that there exist enormous defects in the PTCDA layer due to pedestal sites and other defects that appear when Si atoms shift transversely, and that the bonding condition is satisfied by the action of atom suspension bonding at the surface of the Si substrate. We infer the growth mode of PTCDA deposited onto p-Si substrates as follows. First,PTCDA molecules assemble at the defects to form three-dimensional island-like PTCDA crystal nuclei, and then by the action of delocalized big π bonding, two adjacent layers of PTCDA molecules overlap to some extent and finally island-like structures form. The PTCDA molecules and Si substrate combine by a process of the combination of benzene rings with Si atoms at the defects and of acid anhydride radicals with Si atoms at the perfect fraction of the surface. In the course of combination, although the structure of the benzene rings does not change, the chemical reaction of acid anhydt＇ide radicals and Si occurs to break off the C=O bond in the acid anhydride, and then C-Si-O and silicon oxide might be produced.
The electronic states of the surface and interface of 3,4, 9, 10-perylenetetracarboxylic dianhydride （PTCDA）/indium-tin-oxide （ITO） thin film are investigated using X-ray photoelectron spectroscopy （XPS）. A- tomic force microscopy （AFM） is also applied to investigate the pattern of PTCDA/ITO film. XPS results show that there are two main peaks,which are associated with C atoms in the perylene rings and acid anhydride groups, located at 284.6 and 288.7eV, respectively,in the Cls spectrum of the original surface. It can be deduced from the emergence of a small peak at 290.4eV in the Cls spectrum that some C atoms are oxidized by O atoms from ITO. The binding energies of O atoms in C-O bonds and C--O---C bonds are 531.5 and 533.4eV respectively. At the interface,the peak at the high binding energy in the Cls spectrum disappears,and the peak value shifts about 0.2eV to lower binding energy, There is a significant 1.5eV chemical shift to lower binding energy in the Ols spectrum. These observations indicate that perylene rings inside PTCDA molecules are combined with In vacancies in the ITO at the interface. The AFM results show that PTCDA molecules formed an island-like structure a height of about 14nm. The sizes of the crystal grains are about 100--300nm. The island-like pattern comes from the delocalized π bonds of adjacent molecules in PTCDA and the combination of vacancies in ITO with perylene rings at the PTCDA/ITO interface.
对有机/无机光电探测器PTCDA/p-Si样品的表面进行得AFM扫描看出,PTCDA呈岛状生长,各岛成圆丘状,岛的分布不均匀,PTCDA层中存在大量缺陷。原因是p-Si(100)衬底的表面原子悬挂键的作用,使硅原子横向移动满足键合需要形成台阶和其它缺陷引起的。将样品表面的XPS全谱及精细谱与表面的AFM扫描图进行对比分析,得出PTCDA在p-Si基底上的生长模式,即:PTCDA首先在缺陷处聚集,形成许多三维岛状的PTCDA晶核,然后在PTC-DA离域大π键的作用下,相邻的两层PTCDA分子存在一定程度的交叠,最终形成岛状结构。与硅衬底原子结合的过程为:苝环与缺陷处的Si原子结合,而酸酐基团与表面完整处的Si结合。结合时,苝环结构保持不变,而酸酐基团与Si发生化学反应,使酸酐中的C O键断开,形成O Si C和硅氧化物。对PTCDA/p-Si样品的界面的XPS全扫描谱和精细图谱进行分析后,进一步验证了PTCDA在p-Si基底上的生长模式。
