Surface analysis for LiBq4 growing on ITO and CuPc film using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS)

We have investigated the morphology and surface electron states of LiBq4 deposited on ITO and CuPc/ITO, using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The AFM observations indicate that LiBq4 can form a much more uniform film on CuPc than that on ITO. Furthermore, X-ray photoelectron spectroscopy (XPS) is utilized to further demonstrate the AFM results. From the analysis of XPS, we found that LiBq4 molecules have poor thermal stability, they are seriously oxidized during depositing; but when a CuPc layer is inserted between LiBq4 and ITO film, the oxidation and surface contamination of LiBq4 are significantly reduced. It is then concluded that the introduction of a CuPc buffer layer under the LiBq4 film can improve the film quality of LiBq4.The XPS results also testified the fact that no coordination bonds between N atoms and B atoms are formed in LiBq4 molecules, which make LiBq4 to be potential blue organic light-emitting material