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

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.


Introduction
In the simplest form, MPcs (C 32 H 18 N 8 M) consist of four isoindole groups connected by nitrogen atoms forming an 18 pelectron ring structure, with two covalent bonds and two coordination bonds chelating a metal or metalloid center (Fig. 1).  1,2 Additionally, trivalent and tetravalent metal cations allow for the introduction of axial substituents providing an additional handle for tuning material properties. The choice of metal and the inclusion of peripheral, bay, or axial functionalization groups can strongly inuence the physical and chemical properties of MPcs facilitating specic material tailoring. The extensive delocalization of the p-electron system and the exceptional stability of MPcs has resulted in their use for a myriad of applications since their discovery in 1907 and the rst patent in 1929. 1,2 Historically, due to their vibrant blue, purple, or green colour, MPcs have been, and are still, used as commercial colourants in paints, plastics, textiles, printing inks, dyes, and even some food colouring. 3 Noncolourant applications have included catalysts, lubricants, indicators, and semiconductors, with recent interest focusing on more advanced applications. 3 The ability of MPcs to form highly ordered thin-lms coupled with their chemical and mechanical stability has led to their use as the active layer in a number of electrochemical and photo-electrochemical sensors for drug analysis and the detection of pharmaceutical products, 4 gas sensing including the detection of alcohol vapours, 5-8 cannabinoid sensing, 9 and gamma radiation sensing. 10 MPc thin-lms are also a vastly growing area of research for emerging organic electronic devices having found promising success in organic photovoltaics, 11,12 thin-lm transistors, 13,14 and light emitting diodes. 15 In this review, we focus on the formation of MPc thin-lms and their physical properties. The rst section considers how thin-lms of MPcs are formed from solid material by physical vapour deposition (PVD), highlighting the general principals of the nucleation and growth of organic small molecules, kinetic and thermodynamic considerations, and effects of deposition parameters. The second section focuses on MPc thin-lms formed from solution, with a discussion on the relevant nucleation principles and a comparison of solution deposition methods. The third section illustrates the general microstructure of MPc thin-lms with an examination of the commonly seen packing motifs, polymorphs, and lm morphologies. The fourth section focuses on specic physical properties of MPc thin-lms, mainly the optical absorption and vibrational properties which are most relevant to emerging photophysical MPc applications. Lastly, the nal section reviews some of the most relevant and promising synchrotron based X-ray techniques which can be used to characterize and study MPc thin-lms.

Physical vapour deposition
Small molecule thin-lms are commonly fabricated by PVD, where under high vacuum (10 À6 to 10 À8 torr) the solid deposit material is heated above its sublimation temperature creating a vapour which then condenses on a target substrate. Numerous PVD techniques exist that employ different heating sources/ mechanisms or different processing conditions but in all cases no vapour phase chemical reaction occurs such that thin-lms are produced strictly through physical means. As the vapour reaches the substrate, thin-lm formation proceeds through the nucleation and growth of molecules of the deposited material. [16][17][18][19] While on the substrate, the free energy of the deposited molecules is reduced from that of the vapour phase, creating a low-density distribution, randomly diffusing among surface lattice sites. [16][17][18][19] Molecules in this distribution may then diffuse across the substrate until they are lost by one of several processes (Fig. 2). The molecules may re-evaporate back into the vapour phase (desorption), nucleate to form 2D or 3D growth, aggregate into existing nucleation clusters, get captured at defect sites, or diffuse into the substrate (interdiffusion). [18][19][20][21] For perfectly at surfaces molecule capture at defect sites and interdiffusion are excluded from these possibilities, however in practice due to imperfections on the substrate these process oen occur. 16,20,21 Aer the initial formation of nucleation clusters, rearrangement to more thermodynamically stable forms can also occur. This can include mixing of different species, and shape changes caused by surface diffusion or coalescence brought on by post deposition treatments such as annealing. Thus, diffusion processes occur at several stages of thin-lm formation, including the formation, mobility, and rearrangement of nucleation clusters. 16,[19][20][21]

Thermodynamics and kinetics
Nucleation occurs in the beginning stages of phase change when a new phase forms from a prior parent phase oen as a result from a change in temperature that triggers vapourphase condensation, solidication, or solid-state phase transformations. [17][18][19] In thin-lm formation the initial nucleation stage oen dictates the resulting grain structure, lm morphology, and thin-lm properties. The principal theories of inorganic thin-lm growth can largely be used to model the nucleation behaviour of organic small molecules, however some fundamental differences do exist. Most notably, inorganic atoms are assumed to be isotropic in shape such that the orientation of the atom relative to the substrate is irrelevant, whereas many organic small molecules are highly anisotropic and thus the strength of the molecule-molecule and moleculesubstrate interaction will depend on their orientation to the substrate. 20,22,23 Additionally, inorganic lm growth relies on strong covalent or ionic bonds, whereas organic materials rely on van der Waals interactions. 22,23 For the vapour deposition of thin-lms the thermodynamic driving force for nucleation is the difference between the chemical potential of organic molecules in the vapour phase (m v ) and crystalline phase (m c ). 16,17,20,24 The Gibbs free energy change (DG) needed to form a nite-sized crystal composed of a number, j, of molecules can be described by: where the rst term (ÀjDm) describes the thermodynamic driving force, dened as the difference in chemical potentials Dm ¼ m c À m v , and the second term describes the energy associated with creating or adding to a new surface. 16,17,20,24 The term g i corresponds to the surface energy associated with surface i with an area A i . 16,17,20,24 Eqn (1) gives the macroscopic relationship in terms of free energy, between crystal size and surface energies and is a reasonable approximation of nucleation behaviour. In general, the barrier to nucleation where the surface energy effects are greatest (DG*) can be determined by setting the derivation of eqn (1) with respect to the number of molecules (j) to zero, this represents the point at which thermodynamic equilibrium is achieved.
However, due to the anisotropic nature of organic molecules, nucleation is oen governed by kinetic processes rather than thermodynamic ones. 20,22,23 Therefore, thin-lm growth is better described as a non-equilibrium kinetic process resulting in a macroscopic state that is dependent on the respective rates of the different physical processes illustrated in Fig. 2. 20,21 Atomistic theories of nucleation describe the role of individual atoms, or molecules, during the initial stages of thin-lm formation. 17,18,20,21 An important advantage of the atomistic models is that nucleation can be expressed in terms of measurable parameters such as deposition rate and substrate temperature, instead of quantities such as DG* and g i , whose values cannot be known with certainty or easily estimated. 17,20,21 By this approach, the most simplied kinetic rate equation relating the time dependent change in monomer cluster density to surface processes is given by the following: where N 1 is the monomer density, _ R is the deposition rate, s s is the length of time atoms remain on the substrate before desorption, N i is the critical concentration of clusters per unit area of size i, and K 1 is a second-order rate constant. 17 Eqn (2) states that the monomer density change with time is given by the deposition rate, minus the desorption rate, minus the rate at which two monomers combine to form a dimer, minus the loss in monomer population due to their capture by larger clusters. 17,18,20,21 This equation can be generalized further to dene the rate equation for clusters of i size: where the rst term expresses the increase in rate caused by the attachment of monomers to smaller i À 1 sized clusters, and the second term describes the decrease in rate due to formation of larger i + 1 sized clusters. 17,19 While eqn (2) and (3) are valuable in understanding the basic kinetics of nucleation, the inclusion of surface diffusion terms, coalescence, and transient and steady-state solutions offer a much more complete account of nucleation events, however increases the mathematical and physical complexity of these models greatly. More rigorous kinetic models can be found in other works. 18,20,21,23,[25][26][27]

Nucleation density
For vapour deposited materials the rate of heterogeneous nucleation, dened as the number of stable clusters that form per unit volume per unit time, is a function of the deposition rate, substrate temperature, substrate surface properties, intermolecular interactions, and molecule-surface interactions. 16,17,20,24 Greater nucleation rates typically result in ned grained thin-lm morphologies due to the large number of crystallites that nucleate on the substrate in a short period of time. 16,17 Conversely, if the nucleation rate is low large crystal growth is favoured. 16,17 In terms of the energetic contributions, the energetic barrier to diffusion (E diff ), energetic barrier to desorption (E des ), and thermodynamic barrier (DG*) are critical to heterogeneous nucleation and thin-lm growth. 16,17,20,24 Considering these energetic terms, the nucleation density (N D ) of stable clusters is given by eqn (4): where a is a constant related to the critical cluster size, k is Boltzmann's constant, T s is the substrate temperature, and E i is the crystal disintegration energy dened as the energy required to disintegrate a critical cluster containing i molecules into i separate molecules. 16,17,20,24 For systems with a low crystallization driving force, E i is approximately equal to negative the crystal formation energy which can be approximated by E i ¼ (ÀE des + E diff + DG*) for the vapour deposition of most organic small molecules. 16,17,20,24 Thus the three energetic barriers (diffusion barrier, desorption barrier and thermodynamic barrier) directly impact the nucleation density. The relationship between the energetic terms of eqn (4) and surface interactions of the substrate show that the processes illustrated in Fig. 2 (diffusion, desorption, and nucleation) are therefore a function of the interaction between the substrate and deposit material. 16,17,24

Growth modes
Thin-lm formation is generally characterized by three basic growth modes: island (Volmer-Weber), layer-by-layer (Frank-Vander Merwe), and Stranski-Krastanov (SK) growth. Island growth occurs when molecules of the deposited material are more strongly attracted to each other than to the substrate, resulting in 3D growth (Fig. 3i). [16][17][18]20,24 Layer-by-layer growth exhibits the opposite characteristics as the molecules are more strongly attracted to the substrate resulting in planar 2D sheet formation oen referred to as epitaxial growth (Fig. 3ii). [16][17][18]20,24 SK growth describes the formation of one or more complete monolayers where subsequent 2D growth is unfavourable and 3D island growth continues (Fig. 3iii). [16][17][18]20,24 Typically, organic thin-lms, such as those composed of MPcs, experience SK growth. The relationship between growth mode, surface energy of the deposited material, and the substrate can be related by eqn (5): where g s is the surface energy of the substrate, g* is the interfacial surface energy between the deposited material and substrate, g d is the surface energy of the deposited material, and q is the contact angle (Fig. 3iv). [16][17][18]20 In the case of layer-bylayer growth the deposited material wets the substrate and q z 0, therefore, g s $ g* + g d . [16][17][18]20 However, for island growth the opposite is true and q > 0, therefore, g s < g* + g d . [16][17][18]20 SK growth combines features of both island and layer-by-layer growth where initially g s > g* + g d until island formation occurs. [16][17][18]20 2.5 Effect of deposition parameters The formation of thin-lms by PVD is a complex process that can be inuenced by many factors such as material properties, deposition parameters, and environmental constraints resulting in lm microstructure ranging from the formation of single crystal, polycrystalline, to amorphous lms. From eqn (4) nucleation density is largely reliant on substrate temperature and deposition rate. Due to the Arrhenius nature of eqn (4), at elevated substrate temperatures the overall barrier to heterogeneous nucleation is reduced. 16,17,20,24 At high substrate temperatures, molecules have increased kinetic energy and are able to easily migrate to lower energy sites creating nucleation points, resulting in polycrystalline structures with large crystallites and fewer grain boundaries. 24,28,29 This phenomenon has been well documented in MPcs 28-36 which at room temperature exhibit ne grained morphologies, whereas large rod-like bers occur at increasing substrate temperatures, as exhibited by scanning electron microscopy (SEM) and atomic force microscopy (AFM) images of copper phthalocyanine (CuPc) in Fig. 4. 28,30 For fabrication of MPc thin-lms by PVD, substrate temperatures between 30-120 C are commonly used leading to morphologies with large regular crystals and minimal grain separation, which tends to be preferable for various applications. 28,31-33 At greater substrate temperatures (>200 C) the sticking coefficient of the deposited material is reduced and nucleation is limited, resulting in a sparse network of very large crystallites separated by wide gaps (Fig. 4i). 30,31,[34][35][36] At very low temperatures (<0 C) the surface mobility and diffusion are decreased such that molecules lack the energy required to nd favourable nucleation cites, and amorphous lms are formed, as illustrated by low temperature depositions of pentacene. 23,37,38 In addition to substrate temperature, deposition rate effects the nucleation density and subsequently thin-lm formation by determining the density of molecules diffusing on the surface. Increasing the deposition rate increases the rate of nucleation, as more molecules can interact to form a stable cluster in a dened area per unit time, oen leading to smaller and denser crystallites. 23,[39][40][41] Conversely, decreasing the deposition rate decreases nucleation density as this allows more time for incoming molecules to migrate to a favourable orientation prior to the arrival of additional molecules. 23,39-41 Low deposition rates typically lead to large crystallites, and fewer grain boundaries. 28,29,39,40 Similar to substrate temperature, the effects of deposition rate on the fabrication of a variety of MPc thin-lms has been extensively studied 28,39-41 with Fig. 5 displaying the effects on CuPc lms. 40 If the deposition rate is very high, growth becomes kinetically dominated and typically low crystallinity, polycrystalline, or amorphous lm formation is observed. 28 For the fabrication of MPc thin-lms deposition rates of 0.01-5Å s À1 are commonly used as they generally result in morphologies with large crystallites, favourable p-p stacking, connected grain boundaries, and greater crystallinity, which are typically desired for many solid state applications. 28,[39][40][41] Physical surface roughness and surface chemistry of the substrate can have a signicant impact on nucleation and thin-lm formation. Areas of high surface roughness decrease the barrier for heterogeneous nucleation by decreasing the diffusion distance of molecules. 16,[42][43][44][45][46][47] This results in small grain formation, enhanced defects, and oen a different molecular orientation relative to the substrate as exhibited by thin-lms of CuPc (Fig. 6i). [42][43][44] Altering the surface chemistry of the substrate through self-assembled monolayers (SAMs) is a common strategy to inuence the morphology and crystallinity of small molecule thin-lms. SAMs are highly ordered 2D structures consisting of a head group, terminal group, and linker. The head group typically has a specic affinity for a substrate which facilitates spontaneous monolayer formation. 48 The most common SAMs used in thin-lm engineering are thiols on gold, silanes on silicon oxide (SiO 2 ), and phosphonic acids on metal oxides. 48 Using SAMs to modify the substrate can affect the uniformity, morphology, packing structure, and molecular orientation of the resulting thin-lm, as seen by the growth of CuPc on bare SiO 2 versus SiO 2 treated with trichloro (octyl)silane (OTS) (Fig. 6ii). 45,48,49 As discussed, the surface energy of the substrate will greatly inuence the initial nucleation behaviour of the deposited material and determine the nal growth mode. 17,20,45,48 By using SAMs to selectively tune surface energy, island, layer-by-layer, and SK growth can be achieved using the same deposited material and fabrication conditions. 50,51 Overall PVD is an effective fabrication method, already employed in industry and used for the engineering of thin-lms with tunable molecular structures.

Solution deposition
Solution deposition of organic small molecules involves the dissolution of the deposit material into an organic solvent  where it can then be deposited onto a substrate by one of four main methods: drop casting (also referred to as dispensing), spin coating, printing, and meniscus-guided coating (Fig. 7). As the solvent evaporates the solution becomes supersaturated, driving nucleation and crystal growth, to form a thin-lm. Compared to PVD, the nucleation and growth of solution deposited materials is more complex due to added solventvapour, solvent-substrate, solute-solvent, and solute-substrate interactions. 52 Additionally, control over the formation of thin-lms by solution processes is limited due to the rapid progression of nucleation, crystallization, and growth stages that can occur in a matter of seconds. 53 Drop casting and spin coating are common lab scale techniques used to deposit material on small area substrates. Drop casting involves depositing solution droplets onto a stationary substrate with controlled droplet size and momentum, where the solvent is le to slowly evaporate, leading to the formation of a thin-lm. 52 As no outside forces are applied, nucleation begins along the edge of the droplet with crystal growth occurring in the direction of the contact line recession. Drop casting can oen lead to non-uniform deposition since the recession of the contact line is typically irregular. Spin coating is a more consistent fabrication method used to create uniform thin-lms by dropping solution onto a rotating substrate which simultaneously spreads the solution by rotational forces while quickly evaporating the solvent. 52 Printing is a broad denition of different deposition techniques, however it typically refers to large area solution processing methods that do not primarily rely on directional shearinduced alignment such as meniscus-guided coating. 52 Inkjet printing is one of the most common and popular printing methods where a jet of solution is ejected from a chamber by a piezoelectric or thermal actuator and is deposited onto a substrate. 52 Similar to inkjet printing, spray coating ejects solution from a nozzle where small droplets are formed by aerosolization with an inert gas and are deposited onto the substrate. 52 Inkjet printing and spray coating are particularly useful as their compatibility with roll-to-roll manufacturing facilitates effective high throughput fabrication.
Meniscus-guided coating methods are scalable large area techniques that use the linear movement of either the substrate, or coating tool, to fabricate thin-lms with uniformly aligned crystal growth. 52,54 Dip coating, involving the vertical withdrawal of a substrate from a solution bath, blade coating, involving the use of a at rectangular edge to spread solution across a substrate, and slot die coating, involving the ow of solution through an orice and shaping device onto a horizontally moving substrate are common examples of meniscus-guided coating methods. 52,54 The alignment and size of the growing crystallites relies on the shear force directing solution ow and is largely inuenced by the speed at which movement occurs.

Thermodynamics and kinetics
When a solution is introduced onto a substrate surface, solvent evaporates, increasing the concentration of the solution until it becomes supersaturated and the dissolved molecules begin to precipitates to form a thin-lm. The formation of precisely controlled thin-lms with specic grain structures and morphologies remains a challenge for solution processing due to the rapid nucleation and growth steps. The same thermodynamic principals that describe PVD apply to solution deposition such that the thermodynamic driving force for nucleation is the difference between the chemical potential of organic molecules in the liquid phase (m l ) and crystalline phase (m c ). 55 In the case of solution deposition, Dm corresponds to the difference between the concentration of the solution at equilibrium (C N ) and the concentration during growth (C), which can be expressed as a function of the substrate temperature (T s ): 55 Thermodynamically, C and T s are the two thermodynamic parameters that determine the nucleation and growth of crystallites during solution deposition, however, similar to PVD, solution deposition methods are largely governed by kinetic processes and rates of crystallization. 56 In the case of solution deposition, the kinetic driving force for nucleation is the rate of solvent evaporation which directly determines the rate of crystallization, and is thus key to the fabrication of consistent small molecule thin-lms. 52,57 Due to variations in the respective solution processing methods the governing principals for the rate of solvent evaporation will be method specic.
Drop casting and printing techniques use the release, impact, and spreading of one or more solution droplets that may form a continuous thin-lm before drying or may dry individually to create a thin-lm composed of many islands. Controlling the rate of solvent evaporation, and thus the nucleation and growth stages, depends solely on the solvent and substrate properties as no external rotational or shear forces are applied. [58][59][60] The solution and substrate surface properties can inuence the deposition by causing splashing, spreading, receding, and/or rebounding. [61][62][63] Additionally, temperature and concentration gradients within solution droplets can lead to coffee ring and Marangoni effects, leading to poorly controlled lm formation. 59,60 Thin-lm formation by spin coating can be accurately represented when the evaporation rate of the solvent, the viscosity increase resulting from the increase in solute concentration, the surrounding vapour phase above the substrate, and the solvent's properties are taken into account. 64,65 The simplest and earliest models describing liquid ow on a rotating surface are formulated with three main assumptions: (i) the gas and liquid phases are Newtonian uids; (ii) the uid ow is axially symmetric and laminar; and (iii) the rotating surface is at and extends innitely. 64,65 It is widely accepted that the early stages of spin coating are ow dominated while late stages are dominated by the rate of solvent evaporation. At the transition point, when evaporation and ow become equal, the evaporation rate (n e ) depends on the rotational speed (u), yielding: 64-68 This simple relationship has been observed experimentally using polymer thin-lms with only small reported variations in the exponent value. 64,65,[68][69][70][71][72] However, as solvent evaporates the physical properties of the solution change, inducing non-Newtonian behavior. More rigorous models describing the spin coating process take into account heat and momentum transfer by including the effects of solution viscosity and solvent volatility. 65,70,71 The two stage ow dominated and evaporation dominated process of spin coating has been corroborated with experimental data from spin coated small molecule thin-lms by in situ grazing-incidence wide-angle X-ray scattering (GIWAXS). 73,74 These experiments show how the rapid ow dominated crystallization stage, which occurs over a sub-second time scale, is independent of the rotational speed, and the slower more gradual evaporation dominated stage is rotation speed dependant. 73,74 Therefore, the rate of solvent evaporation during spin coating can be described by eqn (7). Meniscus-guided coating methods depend mainly on solvent properties and coating speed. Solvent evaporation is dominate in the meniscus region leading to supersaturation, precipitation, and ultimately to nucleation. However, most meniscusguided methods use an external shear force to enhance thin-lm uniformity and crystallite alignment. The intensity of this force, determined by the coating speed (n c ), can be divided into two categories: fast coating speeds (n c z 1 mm s À1 ) and slow coating speeds (n c z 1-100 mm s À1 ). Fast coating speeds where solution is spread out by shear forces and solvent evaporation is separated from the meniscus region is known as the Landau-Levich-Derjaguin (LLD) deposition regime where solvent evaporation is a function of n c . [75][76][77] At slow coating speeds deposition corresponds to the evaporation regime where n c is approximately equal to n e of a pinned drop of solution that is receding primarily due to evaporative mass loss. 77 Thus, in contrast to the LLD regime where solvent evaporation is separate from thin-lm deposition, the evaporation regime is complicated by the interactions between solvent evaporation, uid ow, and lm formation. 75-77 A number of models have been purposed to describe n e most of which take on the general form of eqn (8). 54,77,78 Here V m is the molar volume of the liquid solvent, DS vap is the entropy of vapourization of the solvent, T b is the boiling point of the solvent at atmospheric pressure, and A is a single tting parameter combining all temperature independent variables. 77,78

Effect of deposition parameters
Solution deposition processes can produce wide variations in thin-lm microstructure depending on solution concentration, solvent type, substrate temperature, and substrate surface chemistry. Solution concentration inuences thin-lm coverage, such that at low concentrations low coverage submonolayer formation is observed, whereas at increasing concentrations the coverage and interconnectivity increase with the formation of mesh layers and multilayers. This phenomena has been documented in spin coated and dip coated CuPc thin-lms where, at low solution concentration, CuPc molecules form a sub-monolayer of interconnected ribbons typically 20-50 nm wide, approximately 100 nm in length, and 1 nm thick (Fig. 8). [79][80][81] As the concentration of CuPc in the deposited solution increases, multiplayer formation is observed, however complete coverage for a single layer is never achieved due to the anisotropic nature of CuPc which effects surface diffusion and subsequent nucleation. [79][80][81] Solvent choice plays an important role in the formation of thin-lms by solution deposition. As discussed, the rate of solvent evaporation directly determines the crystallization rate, dictating the nal thin-lm morphology and microstructure. Solvents with a faster rate of evaporation generally leads to lms with a greater surface roughness due to the occurrence of well separated clusters. Solvents with high evaporation rates, such as chloroform, can lead to the formation of these clusters since the rapidly evaporating solvent leaves little time for surface mobility or diffusion of the molecules on the substrate. This oen results in lower aggregation and lms with a non-coalesced morphology. Solvents with low evaporation rates, such as dimethylformamide, facilitate greater molecular mobility on the surface due to the longer evaporation time and oen results in a more highly packed and ordered lm. This has been demonstrate with tetrakis-(isopropoxy-carbonyl)-copper phthalocyanine (TIP-CuPc) 82 and a number of other semiconducting small molecules. 58,[83][84][85] The choice of solution processing method will have signicant inuence on thin-lm microstructure. A recent study by Gojzewski et al., exhibited the differences in CuPc lm formation by drop casting, spin coating, dip coating, and spray coating (Fig. 9). 79 The authors used CuPc dissolved in tri-uoroacetic acid (TFA) that immediately spreads to cover the hydrophilic surface of SiO 2 to form a liquid lm. Upon drop casting, outward capillary ow from the center of the drop brings dissolved CuPc molecules to the edge, creating the morphology shown in Fig. 9i, a. Spin coating using the same solution produced a multi-layer formation of nanoribbons similar to that of drop casting (Fig. 9i, b), however the added rotational force increases the rate of solvent evaporation creating a rougher lm surface (Fig. 9ii). 79 Dip coating yielded similar lm characteristics (roughness, coverage and lm volume) to drop casted lms, however exhibited a unique morphology consisting of a sub-monolayer mesh-like lm made of long, asymmetrically curved and interconnected nanoribbons approximately 600 nm wide where the CuPc molecules were orientated in-plane to the substrate (Fig. 9i, c). 79 Spray coated lms displayed a similar morphology and comparable surface roughness, coverage, and lm volume to spin coated lms with large rod-like CuPc aggregates (Fig. 9i, d). 79 Due to the added rotational force during spin coating, noticeable differences in lm morphology between the two fabrication methods are expected. However, as discussed, morphology is dependent on the rate of solvent evaporation. The specic fabrication parameters used for spray coating and spin coating in this case allows for sufficient TFA evaporation to create lms of large rodlike CuPc aggregates. 79 This further corroborates the relationship between thin-lm microstructure and solvent evaporation as the driving force for the nucleation and growth of solution deposited thin-lms.

Packing motifs
The growth mode and packing structure of inorganic thin-lms is well understood by reason of the strong covalent bonds, and the inherent isotropic shape of inorganic atoms. In contrast, due to the anisotropic geometry and weak van der Waals forces of organic molecules more variable crystallite growth, molecular packing structures, thin-lm textures, and morphologies are observed. 86,87 Molecular packing can not only impact the solid state properties of organic molecules but it can also effect the thermodynamic, kinetic, mechanical, electrical, and optical properties of the nal thin-lm. 88 The identication and clas-sication of different packing structures is therefore crucial for applications in various industries including pharmaceuticals, 89 organic semiconductors, 90 pigments, 91 and explosives. 92 Conjugated aromatic small molecules have been known to form two main crystal packing structures: herringbone and p-stacked (Fig. 10). 93 The herringbone structure exhibits altering face-toedge and face-to-face molecular packing, and mainly occurs in planar MPcs such as CuPc, silicon phthalocyanine (SiPc) and zinc phthalocyanine (ZnPc), whereas the p-stacked conguration exhibits face-to-face packing and is adopted by non-planar MPcs such as titanyl phthalocyanine (TiOPc), chloro-aluminum phthalocyanine (AlClPc), and lead phthalocyanine (PbPc). 93 Polymorphism refers to the ability of molecules to form multiple distinct crystal structures. Controlling polymorphism in organic thin-lms is challenging since p-conjugated molecules typically have similar cohesive energies and a low kinetic barrier to solid-solid transformations, making polymorphs difficult to isolate and stabilize. 88 Common methods of obtaining different polymorphs in thin-lms is through varying lm thickness, temperature, surface chemistry and post deposition processes such as thermal and solvent annealing. 88 The identication of polymorphs and the differences in morphological, structural, and spectroscopic properties have been documented through electrical conductivity measurements, 94,95 optical absorption spectra, 96-99 electron paramagnetic resonance spectroscopy (EPR), 94  The polymorphic character of MPcs was rst reported by Hamm and Norman in 1948 for CuPc 102 and has since been extensively studied in a number of MPcs. 33,101,[103][104][105][106][107][108][109] MPcs are known to exists in various polymorphic forms identied as a, b, g, d, 3, and x-phases with the metastable a-phase and stable bphase being the most common and commercially signicant. 94,100,101 The phase transition from a to b occurs in most MPc thin-lms through exposure to temperature (200-300 C) 97,99,100,104 or organic solvents, 110,111 and is characterized by a change in tilt angle between planes and the degree of pelectron overlap (Fig. 11i and ii). 96,112 The stable b-phase is monoclinic in structure and forms long crystallite needles, 113 whereas the metastable a-phase has been reported to be tetragonal, 114 orthorhombic, 115 or monoclinic 94 in structure, and generally forms into spherical crystallites. As an example, Fig. 11 highlights some of the differences between aand bphase CuPc polymorphs. For both polymorphs the CuPc molecules align in the herringbone packing structure with a 65 angle between molecules and the b axis for a-phase CuPc and a 45 angle for b-phase CuPc. 96 The larger angle of a-phase CuPc results in increased p-electron overlap and is likely the reason for the higher conductivity displayed by this polymorph. 94,95 The XRD pattern of aand b-phase CuPc (Fig. 11iii) shows the distinct crystallographic differences between polymorphs. a-Phase CuPc exhibits a primarily polycrystalline structure with crystallites preferentially oriented with their (001) planes (approximately 2q ¼ 7 ) parallel to the substrate. 97,98,100 In the  case of b-phase CuPc alignment in the (20À1) direction is preferred as seen by the high intensity peak at approximately 2q ¼ 9 . 97,98,100 Through Raman spectroscopy differences in the vibrational frequencies of aand b-phase CuPc are shown in Fig. 11iv. Vibrational shis between polymorphs can be observed in ve Raman bands with the largest differences exhibited by the n 52 vibration (Cu-N deformation), n 14 vibration (C-H bending of the benzene ring), and the n 28 vibration (stretching of the phthalocyanine macrocycle). 91 Differences in CuPc packing structure determine solid state properties such as conductivity, optical absorbance, and even colour which are critical for determining appropriate use in some applications. aand b-phase CuPc are oen used as organic semiconductors in electronic devices with particular interest in a-phase CuPc due to the high carrier mobility and high-frequency capacitance and conductance demonstrated by a-phase CuPc OTFTs, 116 and aphase CuPc-Si hetero-structures. 117 Additionally, in the ink industry the most widely used blue pigments are CuPc based, with a-(purple), b-(green-blue), and 3-phase (red) CuPc being the most popular in printing inks, paints, plastics, and textiles. 91,118 4.2 Thin-lm morphologies MPcs will form different surface morphologies depending on the molecular structure and corresponding packing. Fig. 12 displays AFM images of a number of MPc thin-lms deposited by PVD onto heated substrates, including planar and nonplanar structures and divalent, trivalent, and tetravalent metal inclusions. The planar divalent MPcs, such as ZnPc, CuPc, cobalt phthalocyanine (CoPc), iron phthalocyanine (FePc), and magnesium phthalocyanine (MgPc) exhibit comparable morphologies with ribbon-like grains of similar structure and shape with only small variations in grain size. 119 Typically, ribbon-like grain morphologies are observed for lms deposited on heated substrates whereas smaller more cylindrical shapes are observed at lower substrate surface temperatures. 32,33,120,121 The non-planar trivalent and tetravalent MPcs, such as AlClPc, TiOPc, PbPc, and vanadyl phthalocyanine (VOPc) have much larger rectangular plate-like features owing to their different pstacked packing structure. 119,[122][123][124][125][126] Additionally, these four MPc thin-lms have a greater surface roughness and lower surface area to volume ratio compared to the planar divalent MPc thin-lms. 119,[122][123][124][125] Unlike metal center, uorination of the outside ring (F x MPc, x ¼ 4, 8, 16) yields little effect on the morphology of MPc thin-lms as studied in peruorinated CuPc and ZnPc. 9,[127][128][129] In general the addition of uoro molecules to the outside ring slightly alters grain size however, the packing structure and grain shape remain analogous to non-uorinated MPcs. 9,[127][128][129] The packing and resulting thin-lm morphology of MPcs can also be greatly altered through the inclusion of axial substituents as demonstrated by the AFM images of R 2 -SiPc presented in Fig. 13. R 2 -SiPc thin-lms with phenoxy and uorophenoxy groups fabricated by PVD (Fig. 13ii and iii) show two distinct morphologies either consisting of small regular circular grains or more elongated rectangular grains depending on the structure of the phenoxy substituent. 130,131 Additionally, R 2 -SiPc molecules with alkyl axial substituents fabricated by solution processing (Fig. 13iv) highlight how the alkyl chain length, branching position and symmetry affect thin-lm morphology; creating lms with either small dense cylindrical grains, large interconnected grains, or very large plate-like features. 132 By changing only the axial substituent, wide variations in thin-lm morphologies are observed, where in general, it is hypothesized that large substituents alter molecular packing resulting in morphologies with sizeable features as demonstrated in R 2 -SiPcs and other conjugated small molecules. [132][133][134] In electronic devices, morphology has been shown to impact the charge carrier mobility of transistors, 24,135,136 the power conversion efficiency of solar cells, [137][138][139] and the performance of light emitting diodes. 140 Additionally, the mechanical stability of thin-lms, including the exibility and sensitivity to stress and strain, will affect the degree of reorganization in lm morphology with mechanical deformation. [141][142][143] In particular, lms with large grains and broad boundaries are more susceptible to mechanical deformation as the formation of wide interconnected cracks are more prevalent compared to lms with smaller grains and a smoother surface morphology. 141,143,144 5. Physical properties of metal phthalocyanines

Absorption properties
The unique ultraviolet-visible (UV-vis) absorption spectra of MPcs are a result of their extensively conjugated p-electron systems and the overlapping orbitals of the central metal. UV-vis spectroscopy is oen performed on liquid samples which display sharp, well dened peaks. However, solvent coordination and aggregation can result in peak shis uncharacteristic to the MPc itself. 145 Additionally, solid state UV-vis absorption includes effects of thin-lm molecular packing and crystal structures that are not visible in solution. MPcs typically display two strong absorption bands, one in the UV region of 280-350 nm known as the B (Soret) band, and the stronger, oen better resolved, band in the visible region between 550-750 nm known as the Q band (Fig. 14i). [146][147][148] For most planar MPcs, the B band displays three peaks and two shoulders as exhibited by CuPc in Fig. 14i, 147 whereas non-planar MPcs display one to two broad peaks as seen in chloro-gallium phthalocyanine (GaClPc) and VOPc lms (Fig. 14iii). 149,150 In the low energy region of the B band (around 288 nm) changes to the absorption spectra between MPcs is thought to be due to orbital overlap of the phthalocyanine ring and the central metal. 146,147 The high intensity peak in the low energy B band region exhibited in CuPc, CoPc, FePc, and nickel phthalocyanine (NiPc) suggests dband association with the central metal, arising to p-d transitions as a result of the partially occupied d-orbitals of these metals. 146,147 Changes in the higher energy region of the B band (210-275 nm) are thought to be a result of d-p* transitions. 146,147 For all MPcs, the Q band region displays a single peak with characteristic Davydov splitting. 99,100,146,[151][152][153] In contrast, metal free phthalocyanine (H 2 Pc) can exhibit a split Q band due to asymmetry of the isoindole nitrogen atoms. 148,154 The Q band has been interpreted in terms of p-p* excitation between bonding and anti-bonding molecular orbitals. 99,146,147,155 The high energy peak of the Q-band has been assigned to the rst pp* transition on the MPc macrocycle with the low energy peak explained as either a second p-p* transition, 155 an excitation peak, 156 a vibrational internal interval, 157 or a surface state. 157 The extent of Davydov splitting observed in the Q band is related to the degree of available molecules able to participate in electronic transitions, in particular interactions between the transition dipole moments from adjacent molecules. 99 Davydov splitting is common in all MPcs, however is more prominent in lms which adopt a herringbone packing structure as seen by the spectra of CuPc and GaClPc in Fig. 14. 151,152 This is also evident by Q band shis and intensity changes of aand b-MPcs UV spectra (Fig. 14ii), demonstrating how packing angle and therefore the degree of p-electron overlap alters Q band absorption. 153 Several factors can inuence Q and B band absorption, mainly the metal center and the inclusion of substituent groups. MPcs with different metal centers can lead to a Q band shi of around 100 nm as a function of metal size, coordination, and oxidation state. MPcs with closed-shell metals (lithium, magnesium, and zinc) typically exhibit a red shied (bathochromic shi) maximum Q band peak, while open-shelled metals (iron, cobalt, and ruthenium) display a more blue shifted (hypsochromic shi) maximum peak due to stronger interactions with the phthalocyanine macrocycle. 148,151 The chemical tunability of MPcs facilitates the functionalization in the peripheral, bay, and axial positions providing control over the physical, optical, and electronic properties. Functional groups can generally be categorized as electron withdrawing, such as sulfonyl, carboxyl, and uoro groups, and electron donating, such as amino, alkoxy, and alkyl groups. Peripheral substituents with electron withdrawing character typically result in a red shied Q band, whereas electron donating groups have little effect on the Q band absorption in solution samples. 148,151 However, the addition of substituent groups may impact the molecular packing in thin-lms and thus result in changes to the absorption spectra of solid samples. Additionally, functionalization at the bay position tends to result in a greater change in the absorption spectra of MPcs compared to similar groups in the peripheral position. 148,151 The addition of axial substituents to MPcs will similarly affect the absorption spectra by altering the molecular packing resulting in shied peaks of different intensity exhibited by the thin-lm UV-vis spectra of axially substituted R 2 -SiPc shown in Fig. 14iv. 132,158 The general trends relating MPc functionalization and absorption properties may not always hold true since the effects of added substituent groups will depend on the individual nature, number, and position of the group.

Vibrational properties
The vibrational properties of MPc thin-lms can elucidate changes to the conguration of the MPc macrocycle as a result of substituent groups or large central metals, and insight into the orientation and packing structure of MPc molecules relative to the substrate. The vibrational modes of MPc thin-lms assessed by both Raman and IR spectroscopy exhibit very similar spectra with the same structural trends and characteristic vibrational bands observed in powder samples and calculated data. 101,[159][160][161][162][163] Raman spectroscopy of MPc thin-lms exhibit a distinctive band pattern with vibrations under 600 cm À1 attributed to the deformation of the macrocycle ring, N-M stretching, and the deformation of isoindoles. [164][165][166] The 600-900 cm À1 vibrations are generally due to the deformation of the benzene and isoindole rings, with 1330-1445 cm À1 assigned to isoindole stretching and vibrations of the N-M and C-H bonds. [164][165][166] The most intense vibrational band observed in MPcs is around 1500 cm À1 which exhibits a clear sensitivity to changes in the central metal with a denite trend correlating metal size to shis in vibration. [159][160][161]163,164 Bands in this region correspond to the displacement of the C a -N b -C a bridges between isoindole groups in the MPc macrocycle (Fig. 15i). [159][160][161]163,164 The change in wavenumber of this band observed in different MPcs correlates to the cavity size (N a -M-N a distance) of the phthalocyanine macrocycle. 159,160 MPc cavity size varies widely depending on the central metal with four possibilities: (i) the metal is smaller than the cavity size, (ii) the metal is approximately equal to the cavity size, (iii) the metal is larger than the cavity size, and (iv) the metal is much larger than the cavity size. 159 These four scenarios result in either ring contraction, equilibrium ring geometry, ring expansion, or nonplanar geometry and ring doming. 159 Consequently, due to its high intensity this band allows for the identication of the central metal ion and orientation of MPc molecules in thin-lms. 101,162,167 Using the integral intensity obtained from polarized Raman spectra the angle between the MPc molecule and the substrate can be determined and used to ascertain the effects of fabrication parameters such as substrate temperature, deposition method, and lm thickness, and identify polymorphic phases and lm order. 101,162,167 For MPcs with a cavity diameter similar to that of H 2 Pc (3.93Å) such as CoPc, FePc, CuPc, and manganese phthalocyanine (MnPc) a planar equilibrium geometry is adopted. 159 With a cavity diameter of 3.66Å, NiPc is an example of an MPc with a metal inclusion that is smaller than the cavity of the phthalocyanine ring such that the four isoindole groups are pulled towards the metal center resulting in ring contraction. 159 Conversely, ZnPc with a cavity diameter of 3.96Å is an example where the metal is larger than the cavity of the phthalocyanine ring causing ring expansion but not large enough to result in a non-planar geometry. 159 Lastly, PbPc and tin(II) phthalocyanine (SnPc) have much larger metal centers and are pushed out of the MPc ring resulting in a nonplanar geometry and ring doming. 159 The effects of metal ion size on the MPc macrocycle are observed by shis in the vibrational band corresponding to the C a -N b -C a bridge bonds, with the wavelength noticeably decreasing with an increase in metal size. 159,160 NiPc has the most shied position at 1545 cm À1 (Fig. 15i), with all other MPcs ordered according to metal size ( Fig. 15i and ii). 159 Although ZnPc, PbPc, and VOPc have similar located bands in the lower wavenumber region, PbPc and VOPc display signicant ring doming and a non-planar geometry, suggesting that the packing structure has less of an impact on the vibration properties compared to metal ion size. 159,160 Additionally, this trend holds for uorinated MPcs as seen in Fig. 15ii, with a more dramatic shi observed in F 16 MPcs compared to their non-uorinated analogs as the addition of uoro substituents has a noticeable impact on the N a -M-N a distance. 160 Other than the metal dependant band around 1500 cm À1 , the spectra region from 1350-1500 cm À1 , known as the nger print region, changes depending on the individual MPc and can display up to six unique bands. 159,160 This region has been known to change depending on the metal center, degree of uorination, and the inclusion of substituent groups. 159,160,164 A change in metal ion band intensity in MPc lms is attributed to changes in the molecular packing and lm organization whereas band location is a result of metal ion size. 101,162 Polarized Raman spectroscopy using parallel and cross polarization allows Raman surface mapping to determine the angle distribution of MPc molecules in thin-lms and the identication of polymorphic forms (Fig. 15iii and iv). The change in MPc orientation can be observed by an increase or decrease in band intensity with different polarizations, indicating a change in angle between MPc molecules and the substrate. Szybowicz et al., demonstrated this through the polymorphic forms of various MPcs studied by polarized thin-lm Raman spectroscopy. 101,162,167 Fig. 15iii shows the average angle between MPc molecules and the substrate determined by the C a -N b -C a bridge vibration before and aer thermal annealing to induce a polymorphic phase transition. 101 For the MPcs studied an increase in angle was observed between the substrate and MPc, with a smaller increase exhibited by MPcs with a large cavity diameter (ZnPc) compared to MPcs with a cavity size similar to that of H 2 Pc (CuPc and MgPc). 101 The Raman surface map reveals additional information on the angle and orientation of MPc molecules in lms. Using MgPc as an example Fig. 15iv shows the angle distribution of molecules estimated by polarized Raman surface mapping before and aer thermal annealing. 101 Before annealing, the lm consists of the metastable aphase with molecules aligned 26-36 to the substrate while aer annealing Raman mapping shows the transition to the more stable b-phase with molecules aligned 39-46 to the substrate. 101 Through Raman and IR spectroscopy the vibrational properties of MPc thin-lms can be used to determine fundamental thin-lm characteristics such as molecular alignment and lm homogeneity, and identify MPc lms by their metal ion and polymorphic forms.

Synchrotron techniques for thinfilm characterization
High performing organic thin-lm devices rely on the specic interfacial orientation and alignment of molecules to achieve optimum opto-electric properties and thus the characterization of these molecular interfaces is critical to the development of advanced devices. The variable nature of organic thin-lms can lead to an imbalance in property optimization where oen the ability to ne tune molecular structure to optimize nano-scale properties, such as intermolecular charge transfer, negatively impacts large-scale thin-lm formation properties. From molecular packing to crystallite formation, analysis of the thin-lms must be performed at various size scales in order to fully characterize the lms. Fig. 16 illustrates the relevant size scales and corresponding structural characteristics important to organic thin-lms and the synchrotron based X-ray techniques which can be used to provide information at each scale. 168 X-ray techniques using synchrotron light sources provide additional information not possible with other methods like optical microscopy, scanning probe techniques, or transmission electron microscopy (TEM). 168 The ability to select specic wavelengths and vary the incident and collection angles facilitates the resolution of nano-scale features such as bond lengths, molecular packing, and phase segregation through the entire lm, rather than strictly at the surface. Additionally, unlike lab scale X-ray methods, synchrotron X-ray techniques can be used to study weakly scattering samples due to the greater ux, brilliance, and collimation of synchrotron light sources, making them ideal for investigating organic thin-lms. 168 X-ray scattering techniques employ the distribution of incident X-rays through a sample where a fraction of the waves are diffracted and collected creating distinct diffraction patterns with high intensity peaks characteristic to the specic lm properties. The angle of the diffracted peaks provides information on the spacing between molecular planes in the lm, whereas the direction of the peaks correspond to the orientation of the planes. Grazing-incidence X-ray scattering (GIXS) is a common X-ray scattering technique where the scattering vector is directed along the sample plane and the diffracting planes are perpendicular to the sample plane. 168 GIXS can be used to analyze the bulk or surface lm properties depending on the chosen incident angle and detection method, for example signal can be collected by a point detector for high accuracy or more commonly using a 2D detector for rapid data collection over a large area with minimal sample damage (Fig. 17i). 168 Grazing-incidence wide-angle X-ray scattering (GIWAXS) and grazing-incidence small-angle X-ray scattering (GISAXS) are two of the most commonly used synchrotron techniques to investigate organic thin-lms with the ability to resolve features in the range of approximately 1Å -100 nm for GIWAXS and 1-100 nm for GISAXS. 168 2D GIWAXS patterns can be used to determine crystal packing through the size and symmetry of the unit cell by analysing peak position and intensity, crystallite size and disorder by analysing peak width, and the degree of crystallinity by analysing the integrated diffraction intensity. 168 Additionally, the molecular orientation and alignment can be determined by performing an azimuthal scan where a diffraction peak is selected and the intensity recorded while the sample is rotated about the substrate normal to determine the orientation distribution. 168 GIWAXS has been demonstrated to be useful for determining how fabrication parameters, such as annealing temperature, effect the molecular orientation of small molecules, 56,73,132 and how molecular structure effects orientation as demonstrated by R 2 -SiPc thin-lms (Fig. 17iii). 130,132 GISAXS is used to analyze the nanoscale surface morphology of polymer and multi-component thin-lms with some use in quantifying domain size in singlecomponent small molecule lms as demonstrated in Fig. 17iv which characterizes CuPc thin-lm formation using different annealing temperatures on different surfaces. [168][169][170] However, GISAXS is predominantly used to study the phase segregation and morphology in polymer and small molecule-polymer blends and is typically used in conjugation with GIWAXS in order to obtain a more complete analysis. 168,169 Scanning X-ray microscopy techniques combine X-ray scattering or spectroscopy methods with a spatially resolved rendering of an image using rasters through a focused X-ray beam (Fig. 17ii). 168,171 Scanning transmission X-ray microscopy (STXM), oen called near-edge X-ray absorption ne structure spectroscopy (NEXAFS) microscopy, is a common method which combines high resolution images with NEXAFS spectra to obtain composition and orientation maps of single and multicomponent thin-lms. 168,171 Typically STXM is used for largescale features (10 nm to 5 mm) and similar to GISAXS has found the most utility in lms consisting of polymer and small molecule-polymer blends. [171][172][173] Orientation and order mapping of single-component thin-lms is achieved by polarized STXM measurements. Different molecular orientations with respect to the polarization axis can be determined by tuning the photon energy to a specic dichroic NEXAFS resonance while measurements with a linearly or elliptically polarized X-ray beam provide contrast between molecules. 168,171 Thus, by rotating the sample about the surface normal and collecting multiple images in the same region with different in-plane polarizations, areas of varying molecular orientation can be mapped and information such as packing structure, tilt angle, and domain size can be acquired for the bulk lm. 168,171 Therefore making STXM a useful tool for large area visualization of organic thin-lms with recent use demonstrated in analyzing the composition of bis(tri-n-propylsilyl oxide) SiPc/ poly-(3-hexithiophene) blends in thin-lms. 173

Conclusion
For over 90 years MPcs have demonstrated their utility as colourants, catalysts, and semiconductors, with particular interest as thin-lm active layers in a myriad of electrical devices. With nearly endless molecular structure possibilities, the ongoing research into the physical, chemical, mechanical, electrical, and optical properties of MPc thin-lms is an evolving discipline. Understanding the building blocks in the formation of MPc thin-lms from deposition, to nucleation and lm growth, helps recognize the inuences of chemical structure and fabrication conditions on lm microstructure, morphology, and properties. Herein the fundamentals of small molecule nucleation and growth in the context of MPc thin-lms fabrication by PVD and solution processing have been discussed with focus on the thermodynamic and kinetic considerations, and how various fabrication parameters and methods effect lm formation. The structure-property relationship of MPc thin-lms was considered in terms on lm microstructure, surface morphology, and optical and vibrational absorption properties. This review provides a valuable resource for the design and application of MPc based thin-lms.

Conflicts of interest
There are no conicts to declare.