Multivariate analysis to evaluate indium behavior at the copper phthalocyanine thin film

Abstract

In this paper, we demonstrate the application of multivariate chemometric analysis to enhance information available from photoelectron spectra for the metal-organic interface: indium on copper phthalocyanine (CuPc). The previous studies conducted by Aristov et al. [Phys. Rev. B 72, 165318 (2005)] and Ivanco et al. [Phys. Rev. B 81, 115325 (2010)] reported inconsistent and contradictory findings about the chemical reaction between In and CuPc at the initial stages of interface formation. The study of changes in ultraviolet photoelectron spectra (UPS) by principal component analysis (PCA) provides evidence for the coexistence of two types of indium phases, “reactive” and “non-reactive,” whose behavior is of importance for devices based on multilayer metal-organic semiconductors. The key information about the metal-organic chemical reaction has been gained from a PCA biplot. This characteristic graph simplifies interpretation of the UPS spectra and enables to estimate the number of pronounced peaks, their energetic positions, and the strength of correlation between them. To detect one of the indium phases, not clearly observed in the experimental spectra, we propose a new concept based on PCA methodology for an enhancement of the UPS spectra resolution. Moreover, we confirm a co-existence of different indium phases by the multivariate curve resolution alternating least square (MCR) analysis on the experimental data. We have found satisfactory agreement between the identified model and experimental data.