Substrates effects on the growth behavior of Copper tetra-tert-butyl phthalocyanine films studied by atomic force microscopy

Abstract

Copper tetra-tert-butyl Phthalocyanine (CuTTBPc) was vacuum deposited onto substrates of hydrophilic glass, hydrophobic silanized-glass, and one layer CuTTBPc LB film. The effects of substrates on the growth behavior and film characteristics of CuTTBPc were studied by atomic force microscopy as well as XRD and dynamic contact angle analyzer. The results show that, in the early growth stage, the island density and coverage ratio of CuTTBPc are small on hydrophilic glass surface. This result is caused from the weak interaction of CuTTBPc molecules to the glass which leads to a small nucleation rate and rougher morphology in the early growth stage. On the contrary, the nucleation rate of CuTTBPc on the silanized-surface is high and thus a much smoother film comprises densely distributed fine-grain clusters was observed. This fact indicates the higher adhesive force of CuTTBPc molecules to the silanized-surface. On the LB film, the CuTTBPc molecules are arranged in aggregative domains which are separately distributed on the glass surface. These domains act as active sites for the nucleation and growth of the later deposition process and thus, high density clusters were found in the early growth stage. The XRD results demonstrate that the film grown on glass has higher degree of crystalline structure than the others which is resulted from the distinction of the initial growth stage.

Introduction

Due to the photoconducting and the semiconducting characteristics of Phthalocyanine (Pc) compounds [1], [2], these compounds have attached considerable interest in various applications including photoconductors, photovoltaic cell elements, non-linear optics, electrocatalysis, gas sensor [3], [4], [5], [6], etc. Many of these applications are based on the properties of the Pc in thin film form. Various techniques have been used to prepare the Pc films [7], [8], [9], [10], [11], and among them, thermal sublimation [12], [13], [14] and Langmuir–Blodgett deposition [15], [16], [17], [18] are two of the most common methods. The Langmuir–Blodgett technique offers the advantages of preparing ultrathin organic multilayers with densely packed structure and precise control thickness. However, difficulties have been encountered in obtaining high quality LB films of Pcs, and thus, various means by using the transfer promoters [19], [20], [21], [22] or by substrate modification have been adopted to improve the deposition quality.

Due to the rather small solubility of Pc in organic solvent, especially for the compounds without peripheral substitution, the fabrication by the wet method is limited. Contrary to the ‘wet’ procedure of LB film technique, the ‘dry’ characteristic of vacuum deposition has been used widely in the fabrications of electronic devices. Since the Pc compounds have features of thermal/chemical stability, the vacuum deposition provides an alternative method to fabricate the Pc thin films [4], [5], [6], [7], [12], [13], [14].

Previous studies had shown that the characteristics for the applications of Pc films are related to the crystal structure and the morphology of the thin films which were determined not only by the films preparing methods but by the deposition parameters [23], [24], [25], [26]. The effects of substrates are one of the crucial parameters in controlling the film characteristics. Previous studies on the vacuum deposited films have shown that the film morphology, crystalline structure, as well as other macroscopic properties have intimate relationship with the types of substrates [5], [26], [27]. By using various substrates, the interaction between the deposit and the substrates is varied and thus, various nucleation and growth behaviors of the deposit film will result.

For the LB technique, it was found that a hydrophobic silanized-glass substrate can be used to transfer multilayer of Copper tetra-tert-butyl Phthalocyanine (CuTTBPc) LB films, while it cannot be carried out on a hydrophilic glass surface [28]. This distinction arouses the study of growth phenomena of CuTTBPc thin films on hydrophilic glass and silanized-glass surfaces in the vacuum deposition process. Besides, when a layer of CuTTBPc LB film previously existed on the substrate, the role of this LB film in the later deposition process is also interesting and was studied in this work. The initial nucleation and growth behavior were studied by observing the film morphology of early growth stage via atomic force microscopy. In addition, the crystalline structure and surface wettability of the films were also analyzed by XRD and dynamic contact angle (DCA) analyzer, respectively.