Surface Molecular Vibrations as a Tool for Analyzing Surface Impurities in Copper Phthalocyanine Organic Nanocrystals
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Surface Molecular Vibrations as a Tool for Analyzing Surface Impurities in Copper Phthalocyanine Organic Nanocrystals K. Nauka, Y. Zhao, Hou T. Ng and E.G. Hanson Hewlett-Packard Laboratories, Hewlett-Packard Company, 1501 Page Mill Road, Palo Alto, CA 94304, USA.
ABSTRACT Surface sensitive attenuated total reflectance IR measurements have revealed small but measurable changes of certain vibrational frequencies of the surface molecules of copper pthalocyanine nanocrystals. These changes are due to electrostatic and electrodynamic interactions between the copper phthalocyanine surface molecules and external vicinal molecular species. The observed frequency shift can be changed by modifying the population of vicinal molecular entities. The observed frequency changes were confirmed by the molecular modeling of selected molecular polar species either chemically bound or vicinal to the copper phthalocyanine molecules. The observed vibrational frequency shifts can be used as an analytical tool for probing the surfaces of organic nanocrystals. INTRODUCTION Copper phthalocyanine (CuPc) forms exemplary organic nanocrystals with a complex crystal structure consisting of intricate arrangement of weakly interacting molecules. A CuPc molecule consists of a planar porphyrin-like structure with a central Cu atom and four peripheral benzene rings. Because of S-conjugation of the central carbon atoms, CuPc exhibits semiconductor behavior that has found a number of potential applications in organic electronics [1]. In addition, the beneficial arrangement of its molecular orbitals provides for a visible energy gap making CuPc a popular cyan pigment used in a variety of applications ranging from printing on a paper to fabric dyeing [2]. For these reasons, CuPc nanocrystals with uniform and wellcontrolled sizes are readily available commercially. However, various levels of impurities are typically present in these materials. Electrostatic and electrodynamic interactions between an organic molecule and its vicinal chemical species are known to affect its basic structural and electronic properties, as in the case of solvatochromism [3]. It is conceivable that these interactions could approach the bonding strength of surface molecules within an organic crystal, like CuPc, and could modify molecular arrangement within the surface region of a crystal. This work aimed at detecting such modifications and, in particular, changes of vibrational frequencies of the surface molecules within the CuPc nanocrystals.
EXPERIMENTAL DETAILS A number of commercially available pigment-quality CuPc materials were used in this study. They differed in terms of impurities either in the form of undesirable residues originating from the pigment manufacturing process or intentionally introduced to control properties of the pigment powder. The amount of impurities was quantified using the photoelectron spectroscopy and IR absorption [4]. In addition, pure CuPc (triply sublimated) was also studied. All samples consist of ȕ-CuPc nanocrystallites havi
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