Substrates effects on the growth behavior of Copper tetra-tert-butyl phthalocyanine films studied by atomic force microscopy
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.
Section snippets
Experimental methods
CuTTBPc was purchased from Aldrich Chem. Co. (purity >97%) and used without further purification. Optical glass plates with dimensions of 24×32×0.2 mm3 were used as substrates. Each glass plate was cleaned ultrasonically in succession with detergent, trichloroethylene, methanol, and pure water, and was then dipped in 0.1 M NaOH solution for approximately 10 h. After this pre-cleaning process, the advancing contact angle of water on the glass surface was nearly 0° as examined by a DCA analyzer.
Films grown on glass surface
The surface morphologies of CuTTBPc films of various thicknesses grown on hydrophilic glass surfaces are shown in Fig. 1. The related parameters concerning these surface morphologies as determined from the AFM data are listed on Table 1. The hydrophilic glass surface, as shown in Fig. 1a, has a very smooth topography. The root-mean-square roughness of this surface is estimated to be 0.35 nm. When a film of mean thickness of 1 nm is grown on the glass, outstanding islands of low density are
Conclusions
In the thermal deposition, the growth of CuTTBPc molecules on hydrophilic glass was found to follow the island growth mode due to the weak interaction between the deposit and the substrate. The nucleation rate and cluster density are small in the initial stage and thus, larger grains and higher crystallite structure resulted in the later growth stage. On the silanized-glass surface, the high adhesive force of CuTTBPc molecules to the substrate promotes the nucleation events. In the initial
Acknowledgements
The support of this research by the National Science Council of the Republic of China through Grant No. NSC 90-2214-E-006-028 is gratefully acknowledged.
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