Elsevier

Thin Solid Films

Volume 598, 1 January 2016, Pages 170-176
Thin Solid Films

Annealing assisted structural and surface morphological changes in Langmuir–Blodgett films of nickel octabutoxy phthalocyanine

https://doi.org/10.1016/j.tsf.2015.11.065Get rights and content

Highlights

  • Langmuir–Blodgett (LB) films of phthalocyanine subjected to thermal annealing.

  • Structural transformation and coexistence of polymorphs in LB films

  • Surface morphology changes from nanoscale grains to elongated crystallites.

  • Reduction of fractal dimension and circularity index reveals asymmetric growth.

Abstract

We report our studies on thin films of metallo-phthalocyanine (MPc), Nickel(II)1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (NiPc(OBu)8) transferred in a well defined thermodynamic state over a self assembled monolayer (octadecyl trichlorosilane)/SiO2/Si substrate using the Langmuir–Blodgett (LB) method. The films are characterized using differential scanning calorimetry (DSC), grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM) techniques. DSC studies on powdered samples in the bulk indicate enantiotropic solid–solid phase transition. GIXD studies on the as-deposited LB film show a Bragg peak indicating crystallinity of the thin film. Annealing (373 K) results in reduction of lattice spacing (1.21 Å) signifying changes in molecular packing within the unit cell. At this stage, an additional Bragg peak is observed which grows at the expense of the former one and they coexist between 373 K and 423 K. A discontinuity in lattice spacing from 20.73 to 15.12 Å with annealing indicates clearly a structural change of the underlying crystalline lattice. Correspondingly, the surface morphology images obtained using AFM show, with annealing, a transformation from spherical granular morphology to elongated, flat crystallites suggesting asymmetric growth process. Statistical parameters of the grain extracted from the AFM images show that the size, fractal dimension and circularity are affected by annealing. Based on these studies, we infer the structural and surface morphological changes of the meta-stable phase (Form I) to the stable phase (Form II) in annealed LB films of phthalocyanine.

Introduction

Phthalocyanines exhibit polymorphism in bulk [1], [2] as well as in thin films [3], [4], [5], [6], [7], [8], [9], [10]. Different polymorphs in thin films can be obtained by deposition at elevated substrate temperatures [11], [12], [13], [14]. The substrate also influences the growth of polymorphs [15], [16]. For example, the interaction of copper phthalocyanine with the Cu(111) substrate results in a variety of polymorphs, α, β, γ, δ whose structures are very different [17]. In general, two main polymorphs (α and β) are formed when deposited on weakly interacting substrates with α (β) phase exhibiting grain-like (elongated) structures [18], [19], [20]. Though both of these phases are monoclinic and form the herringbone structure, they differ in their tilt [19]. Thermal annealing of phthalocyanine thin films has a significant effect on the formation of polymorphs [21], [22]. It is shown that annealing at higher temperatures leads to a polymorphic transition from monoclinic to triclinic structure [23], [24]. The polymorphs of phthalocyanine when subjected to electrical, magnetic and optical stimuli respond differently. In addition, the orientation (edge-on or face-on arrangement) of phthalocyanine molecules with respect to the substrate can alter the electrical [25] and magnetic response [26]. The control of the texture and crystallinity [27], surface morphology [28], grain size [29] and understanding their role is essential as it can potentially affect the characteristic parameters resulting in poor performance of the device.

Recently, alkoxy substituted metallo-phthalocyanine has generated a lot of interest among various research groups. The presence of peripheral alkoxy substituted group not only improves the solubility in common organic solvents but also perturbs the geometry of metallo-phthalocyanine resulting in unique absorption properties in the near-infrared region [30]. This extended absorption in the near-ir region is tapped in organic photovoltaics and act as potential candidates for use in photodynamic therapy [31]. The higher sensitivity of the central metal atom of metallo-phthalocyanine toward adsorption of gas is also used to develop gas sensors [32].

The metallo-phthalocyanine of our interest, Nickel(II) 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (NiPc(OBu)8), is known to exhibit interesting static and dynamic photophysical properties [33]. In this article, we have investigated the Langmuir–Blodgett film (60 layers) of NiPc(OBu)8 deposited in the edge-on configuration on self assembled monolayer (octadecyl trichloro silane)/SiO2/Si substrate in its pristine form and annealed films. We have characterized this system using differential scanning calorimetry (DSC), grazing incidence X-ray diffraction (GIXD) and atomic force microscopy (AFM) techniques. The evidence of polymorphism on powdered samples in bulk is obtained from DSC studies.

GIXD studies show crystalline nature of the as-deposited film. The evolution of the Bragg peak is followed with annealing temperature and we attribute the decrease in the lattice spacing to the change in molecular packing of the unit cell till 423 K. At this stage, another Bragg peak (at higher angle) appears at 373 K which coexists with the former peak till 423 K. We infer a solid–solid phase transition in thin film of NiPc(OBu)8 from the discontinuity in lattice spacing. The AFM images corroborate the above inference wherein a change in surface morphology from granular (as-deposited) to elongated crystallites (post annealing) is observed. Further, we discuss how thermal annealing impacts the granular morphology by analyzing the fractal dimension, circularity and mean grain size. Our studies show that annealing at higher temperatures (> 423 K) leads to transformation of the metastable (Form I) phase to the stable phase (Form II).

Section snippets

Materials

The metallophthalocyanine, Nickel(II) 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (NiPc(OBu)8, CAS: 155773-71-0) and Octadecyltrichlorosilane (OTS, CAS: 104817) have been purchased from Sigma Aldrich and used as received.

Differential scanning calorimetry (DSC)

Differential scanning calorimetry (DSC 8000, Perkin Elmer) studies on the powdered sample of phthalocyanine have been undertaken under Nitrogen atmosphere (flow rate of 20 mL/min). Before the run, the instrument was calibrated using a standard (Indium). The sample

Results and discussion

The differential scanning calorimetry on powdered samples of NiPc(OBu)8 for both heating and cooling cycles is shown in Fig. 1. An enantiotropic (solid to solid) phase transition occurs during heating and cooling cycles at 369.6 K and 313.5 K, respectively suggesting the occurrence of polymorphs. The enthalpy change, ΔH, for the heating (endothermic) and cooling (exothermic) cycles were determined to be 12.5 kJ/mol (at 369.6 K) and − 16.8 kJ/mol (at 313.5 K), respectively. This corresponds to an

Conclusions

Thin films of Nickel(II)1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine on hydrophobic (OTS coated) SiO2/Si substrates show different structural and morphological behaviors upon annealing at different temperatures. The differential scanning calorimetry studies show an enantiotropic transition and provide evidence for the occurrence of polymorphs in bulk. The grazing incidence X-ray diffraction studies on the thin Langmuir–Blodgett films of NiPc(OBu)8 reveal useful insights on the

Acknowledgments

The authors are indebted to Prof. K. A. Suresh and Prof. H. L. Bhat for useful suggestions. The authors thank Dr. S. Krishna Prasad for the DSC measurements.

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