Metal phthalocyanines: thin-film formation, microstructure, and physical properties

Abstract

Metal phthalocyanines (MPcs) are an abundant class of small molecules comprising of a highly conjugated cyclic structure with a central chelated metal ion. Due to their remarkable chemical, mechanical, and thermal stability MPcs have become popular for a multitude of applications since their discovery in 1907. The potential for peripheral and axial functionalization affords structural tailoring to create bespoke MPc complexes for various next generation applications. Specifically, thin-films of MPcs have found promising utility in medical and electronic applications where the need to understand the relationship between chemical structure and the resulting thin-film properties is an important ongoing field. This review aims to compile the fundamental principles of small molecule thin-film formation by physical vapour deposition and solution processing focusing on the nucleation and growth of crystallites, thermodynamic and kinetic considerations, and effects of deposition parameters on MPc thin-films. Additionally, the structure-property relationship of MPc thin-films is examined by film microstructure, morphology and physical properties. The topics discussed in this work will elucidate the foundations of MPc thin-films and emphasize the critical need for not only molecular design of new MPcs but the role of their processing in the formation of thin-films and how this ultimately governs the performance of the resulting application.

Fig. 1. Schematic diagram of MPc structure with elements that form phthalocyanine complexes.

Fig. 2. Schematic diagram of nucleation and growth processes on a substrate.

Fig. 3. Diagram of (i) island, (ii) layer-by-layer, (iii) SK thin-film growth, and (iv) relevant surface energies.

Fig. 4. (i) SEM micrographs of CuPc thin-films deposited at (a) room temperature, (b) 150 °C, (c) 200 °C, and (d) 300 °C and (ii) XRD spectra of CuPc thin-films deposited at various temperatures. Adapted with permission from ref. . Copyright© 2002 Elsevier Science B. V. (iii) AFM images (1 μm × 1 μm), of CuPc thin-films deposited on SiO₂ and Si substrates at substrate temperatures of 30 °C and 105 °C. Adapted with permission from ref. . Copyright© 2013 Elsevier B. V.

Fig. 5. (i) XRD pattern of CuPc thin-films, (ii) AFM images (1 μm × 1 μm) of CuPc thin-films, and (iii) crystal size (D), dislocation density (d) and lattice microstrain (e) of CuPc thin-films, deposited at various deposition rates. Adapted with permission from ref. . Copyright © 2012 Elsevier Ltd.

Fig. 6. (i) AFM images CuPc thin-films and cross sectional diagrams CuPc molecules deposited on (a) rough and (b) smooth Si(111)–H surfaces. Adapted with permission from ref. . Copyright© 1996 American Vacuum Society. (ii) AFM images of CuPc thin-films and diagrams of CuPc molecules deposited on (a) bare SiO₂, and (b) OTS treated SiO₂. Adapted with permission from ref. . Copyright© 2015 American Chemical Society.

Fig. 7. Diagram of solution processing methods mentioned in text.

Fig. 8. AFM images (5 μm × 5 μm) of CuPc thin-films fabricated by (i) spin coating and (ii) dip coating at various concentrations of CuPc solution on SiO₂ substrates. (i) Adapted with permission from ref. . Copyright© 2020 the Authors under the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License. (ii) Adapted with permission from ref. . Copyright© 2015 Elsevier B. V.

Fig. 9. (i) AFM images (5 μm × 5 μm) of CuPc thin-films fabricated by (a) drop casting, (b) spin coating, (c) dip coating, and (d) spray coating. (ii) Mean roughness (R_a), substrate coverage fraction (S_CuPc), and film volume (V_CuPc) for CuPc films deposited by solution processing methods. All films were fabricated from 1.5 × 10⁻³ mol L⁻¹ CuPc solution on SiO₂ substrates. Adapted with permission from ref. . Copyright© 2020 the Authors under the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License.

Fig. 10. Schematic diagram of herringbone crystal packing represented by CuPc and π-stacked crystal packing represented by TiOPc.

Fig. 11. (i) Crystal packing structure of α-phase and β-phase CuPc. Reproduced with permission from ref. . Copyright© 2017 Elsevier B. V. (ii) α-Phase and β-phase superposition of phthalocyanine molecules along the b axis. Reproduced with permission from ref. . Copyright© 1988 American Chemical Society. (iii) XRD trace of α-phase and β-phase CuPc. Adapted with permission from ref. . Copyright© 1992 Wiley-VCH Verlag GmbH & Co. KGaA. (iv) Raman spectra of α-phase (red) and β-phase (black) CuPc. Adapted with permission from ref. . Copyright© 2010 American Chemical Society.

Fig. 12. AFM images (2.5 μm × 2.5 μm) of CoPc, AlClPc, FePc, MgPc, TiOPc, ZnPc, CuPc deposited at T_s = 140 °C. Adapted with permission from ref. . Copyright© 2019 The Royal Society of Chemistry. AFM images (1 μm × 1 μm) of VOPc deposited at T_s = 90 °C. Adapted with permission from ref. . Copyright© 2008 American Chemical Society. AFM image (2 μm × 2 μm) of PbPc deposited at T_s = 70 °C. Adapted with permission from ref. . Copyright© 2011 American Chemical Society.

Fig. 13. (i) Structure of axially substituted R₂-SiPc. AFM images of R₂-SiPcs with (ii) phenoxy, (iii) pentafluorophenoxy and (iv) alkyl axial substituents. Adapted with permission from (ii) ref. , (iii) ref. , and (iv) ref. . Copyright© 2020 American Chemical Society. Copyright© 2019 Wiley-VCH Verlag GmbH & Co. KGaA.

Fig. 14. (i) Absorption spectra of as-deposited CuPc thin-film. Adapted with permission from ref. . Copyright© 2006 Elsevier Ltd. (ii) Absorption spectra and of α- and β-phthalocyanine thin-films. Adapted with permission from ref. . Copyright© 1968 American Institute of Physics. (iii) Absorption spectra GaClPc and VOPc thin-films as-deposited (straight line) and after the thermal annealing (dotted line). Adapted with permission from ref. . Copyright© 2004 Elsevier B. V. (iv) Absorption spectra of axially substituted R₂-SiPcs thin-films. Adapted with permission from ref. . Copyright© 2020 American Chemical Society.

Fig. 15. (i) Labelling scheme for MPcs and variation in position of the Raman band identified as an ion size marker. Adapted with permission from ref. . Copyright© 2001 PCCP Owner Societies. (ii) Raman spectra of MPcs and F₁₆MPcs (M = Co, Fe, Cu, Pd, Zn, VO, Pb). Adapted with permission from ref. . Copyright© 2019 Elsevier B. V. (iii) Average angle between MPc molecule (M = Mg, Zn, Cu, Co) and substrate, and (iv) α-phase and β-phase angle maps between MgPc molecule and substrate estimated from polarized Raman spectra. Adapted with permission from ref. . Copyright© 2011 the Authors under the Creative Commons Attribution and Non-commercial License.

Fig. 16. Size scales, structural features, and relevant X-ray characterization techniques for organic thin-films. Adapted with permission from ref. . Copyright© 2012 American Chemical Society.

Fig. 17. Schematic diagram of (i) GIXS, and (ii) STXM. Adapted with permission from ref. . Copyright© 2012 American Chemical Society. (iii) 2D GIWAXS pattern and schematic diagram of molecular orientation of bis(pentafluoro phenoxy) SiPc (F10-SiPc) thin-film. Adapted with permission from ref. . Copyright© 2020 American Chemical Society. (iv) 2D GISAXS patterns and schematic diagrams of molecular orientation showing the thermal evolution of CuPc thin-films on hydrophilic and hydrophobic Si surfaces. Adapted with permission from ref. . Copyright© 2012 The Royal Society of Chemistry.

Rosemary R. Cranston

Benoît H. Lessard