Elsevier

Synthetic Metals

Volume 223, January 2017, Pages 199-204
Synthetic Metals

Effect of order and disorder on degradation processes of copper phthalocyanine nanolayers

https://doi.org/10.1016/j.synthmet.2016.11.024Get rights and content

Highlights

  • Morphology impact on air-induced chemical degradation products of 50 nm-CuPc layers.

  • Surface topographical order/disorder caused by deposition rate.

  • Characterization of morphological homogeneity by AFM, SEM, XRD.

  • Air-induced chemical structure changes studied by EDX and Raman spectroscopy.

  • Raman peak features mirrored CuPc surface ordering and tendency for degradation.

Abstract

The impact was examined of surface ordering of 50 nm-thick copper phthalocyanine (CuPc) layers on the layer's susceptibility to ambience-induced degradation processes. The surface morphology of CuPc layers obtained by physical vapor deposition with different deposition rates, 0.01 nm/s (r1) and 0.02 nm/s (r2), was diagnosed applying atomic force and scanning electron microscopes. The images exhibited compact, ordered surface topography with crystallites of homogeneous geometry for a layer with r1 while randomly distributed bigger crystallites on a rougher and more expanded surface for a layer with r2. X-ray diffraction revealed the α-form of phthalocyanine, mostly with an orientation of the a axis perpendicular to the substrate plane. Mean grain size in bulk was slightly larger for CuPc with r2. Energy dispersive X-ray spectroscopy demonstrated an increase of C/Cu and N/Cu elemental ratios compared to the expected composition for both layers but significantly more pronounced for layer with r2. Morphological features and traces of CuPc-air interaction were mirrored also in the Raman spectra. Samples with r2 exhibited an increased peak width, and their peaks were shifted compared to samples with r1, which was attributed to surface disorder. The Raman spectra exhibited the appearance of additional peaks of oxidation products indicating Csingle bondOsingle bondC, Cdouble bondO and Nsingle bondO bonds, with intensities coinciding to an increased carbon and nitrogen content. More intensive peaks were recorded for layers obtained with higher deposition rate, proving their stronger susceptibility to environment-induced degradation processes.

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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