Effect of order and disorder on degradation processes of copper phthalocyanine nanolayers
Introduction
Promising candidates for several electronic applications emerge from the group of organic semiconductors, the metallophthalocyanines (MePc) – heterocyclic conjugated molecules with high thermal and chemical stability [1]. In the form of thin films, MePc’s have been already tested in third generation solar cells, gas sensors and advanced opto-electronics technologies [2], [3], [4], [5], [6], [7].
However, operating in a “real world” environment, the organic-based devices are inevitably subjected to the influence of the ambient conditions [8]. Particularly, exposure to surrounding air leads to the adsorption of species on the surface of organic layer. Adsorption may be accompanied by charge transfer and subsequently provoke changes of surface chemical structure, electronic and vibrational properties [9]. All air-originated changes of layer properties degraded the device efficiency, lifetime and consequently have an impact on the effective use of devices [10].
Yet, the aging of phthalocyanines and the other organic thin films was mainly investigated by means of the electrical measurements [11], [12], [13], Kelvin probe [12], and photoemission methods like e.g. photoemission yield spectroscopy [14], ultraviolet and X-Ray photoelectron spectroscopies (XPS) [15]. In these studies, the main emphasis was placed on specification of the variation of electronic and chemical properties after interaction with air over different time scales [16]. For instance, the propensity of organic layers for a strong interaction with the atmosphere depends on their morphological features, which in turn are determined during film preparation by specific conditions [17].
In this work, the products of air-induced chemical degradation of 50 nm-thick copper phthalocyanine (CuPc) films on Si(111) was investigated by energy dispersive X-ray (EDX) and Raman spectroscopy, in relation to the surface topographical ordering. The variation of surface morphology was obtained during the deposition process by changing the deposition rate. Samples’ morphologies were characterized by atomic force and scanning electron microscopies (AFM and SEM, respectively). The phase and mean crystallite size were checked by X-ray diffraction (XRD). The analysis of Raman peak shape and positioning revealed impact of samples’ morphological order/disorder. Up to now, Raman spectroscopy as fast, non-destructive and cost-effective diagnostic method [18], has been applied for MePcs mainly as the tool for distinguishing of their molecular orientation and polymorphic phase [19], [20], [21], [22]. At present, the detailed analysis of Raman peaks suggested ability of this technique to fingerprint phthalocyanine surface homogeneity and its inclination for degradation processes.
Section snippets
Sample preparation
CuPc (Copper (II) Phthalocyanine, see Fig. 1 for molecule scheme; molecular formula: C32H16N8Cu) layers with thickness of 50 nm were thermally evaporated from sublimed powder (Sigma-Aldrich, >97% purity, β-form) in high vacuum by physical vapor deposition on p-type Si(111) native substrates (BOSCH GmbH) kept at room temperature. The powder was degassed and purified in vacuum conditions at 220 °C prior deposition in order to extract residual contaminations. Substrates were pre-cleaned with acetone
Results and discussion
Fig. 2 presents the NC-AFM surface topography images recorded for samples r1 and r2 (Fig. 2a and b, respectively). The images are combined with roughness analysis histogram presenting the distribution of grain heights (Fig. 2c and d).
The images show well-developed phthalocyanine surfaces with grains of different size and orientation, dependent on deposition rate. For r1 CuPc (Fig. 2a), an ordered, compact and homogeneous topography with bent crystallites oriented parallel to the substrate plane
Summary and conclusions
The 50 nm thick CuPc layers of different surface morphology were subjected to controlled air exposure and then investigated by surface microscopic and diffraction techniques, and with Raman spectroscopy. A variation of surface morphology was obtained during the deposition process by altering the deposition rate.
The surface topography of CuPc layers was compact, with bent crystallites of homogeneous geometry, for layers deposited with lower rate. Layers deposited faster showed bigger
Acknowledgements
This work was financially supported by the Polish budget for science in years 2013–2015 (by Ministry for Science and Higher Education) within Iuventus Plus IP2012 019072 project through the Silesian University of Technology, Institute of Physics. Authors would like to acknowledge B. Breitbach for support on XRD experiment and E. Müller-Lorenz from Max Planck Institute for Iron Research, Duesseldorf, for support on SEM/EDX measurements.
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