Film Formation of Non-Planar Phthalocyanines on Copper (I) Iodide

Structural templating is frequently used in organic photovoltaic devices to control the properties of the functional layers and therefore improve efficiencies. Modification of the substrate temperatures has also been shown to impact the structure and morphology of phthalocyanine thin films. Here we combine templating by copper iodide and high substrate temperature growth and study its effect on the structure and morphology of two different non-planar phthalocyanines, chloroaluminium (ClAlPc) and vanadyl (VOPc) phthalocyanine. X-ray diffraction, atomic force microscopy and low energy ion scattering show that both the morphology and the structure of the films are starkly different in every case, highlighting the versatility of phthalocyanine film growth. this paper we present the growth behaviour of chloroaluminium phthalocyanine (ClAlPc) on CuI (111) thin films at elevated surface temperatures. We investigate ClAlPc film formation on CuI, with increasing thickness using atomic force microscopy (AFM) and low energy ion scattering (LEIS) and compare it to another non-planar phthalocyanine, vanadyl phthalocyanine (VOPc). The structural templating effect of CuI on ClAlPc is starkly different from that reported for VOPc and this is reflected in the structure and morphology of each.

Film formation of non-planar phthalocyanines on copper(I) iodide † Introduction Structural templating layers are widely employed in organic electronic devices as a method to alter the structure and morphology of organic semiconductor lms. [1][2][3][4] Many structural templating layers, both organic and inorganic, have been shown to provide improvements in device parameters. [5][6][7][8][9] Copper(I) iodide has attracted particular attention for its templating effects in small molecule photovoltaic devices. 1,2,10 Of particular interest is the difference in the effect of CuI on phthalocyanine thin lms depending on their planarity and the chemical identity of the central metal atom/moiety. Additionally there is little understanding of the lm formation processes of structurally templated phthalocyanine layers. 11 Our previous work has shown that growth onto templating surfaces held at elevated temperatures produces lms which have structures and morphologies that are vastly different to their room temperature counterparts. 12 In this paper we present the growth behaviour of chloroaluminium phthalocyanine (ClAlPc) on CuI (111) thin lms at elevated surface temperatures. We investigate ClAlPc lm formation on CuI, with increasing thickness using atomic force microscopy (AFM) and low energy ion scattering (LEIS) and compare it to another nonplanar phthalocyanine, vanadyl phthalocyanine (VOPc). The structural templating effect of CuI on ClAlPc is starkly different from that reported for VOPc and this is reected in the structure and morphology of each.

Experimental details
Copper iodide 98% (Sigma Aldrich, UK) was used as received and evaporated from a home built evaporator at 310 C at a rate of 0.3Å s À1 as measured by a calibrated quartz crystal microbalance. VOPc 80% (Acros Organics, BE) and ClAlPc 85% (Sigma Aldrich, UK) were both triply puried by thermal gradient sublimation and the resulting crystals were used for growth from a home built evaporator at 370 C and 390 C, respectively, and at a rate of 0.3Å s À1 . All lms were grown in a custom built ultra-high vacuum (UHV) chamber with a base pressure of 3 Â 10 À9 mbar in which inorganic and organic materials were sublimed onto elevated temperature substrates. A K-type thermocouple mounted close to the sample and calibrated using an optical pyrometer was used to measure the substrate temperature. The substrates used were 10 Â 10 mm pieces of thermally oxidized silicon (100) single crystal (IDB technologies, UK) cleaned via sonication in Decon-90/de-ionised water mix, acetone and isopropanol. These were dried in a stream of dry nitrogen and cleaned in UV-ozone before being loaded into vacuum. Thin lm X-ray diffraction (XRD) patterns were measured using a PANalytical X'Pert Pro MRD diffractometer with monochromatic Cu Ka 1 radiation. Atomic Force Microscopy (AFM) images were recorded using an MFP-3D AFM (Oxford Instruments Asylum Research, Santa Barbara, USA) in AC mode (tapping mode) using Olympus AC240-TS silicon tips. Low Energy Ion Scattering (LEIS) was carried out using an IONTOF Qtac 100 LEIS instrument. A 3 keV He primary ion beam was rastered over a 500 mm 2 area, with an ion beam current of 4400 nA. Total scan time was 100 s and the ion beam dose delivered to the sample surface was 2.76 Â 10 12 ions. The scattered primary ions were collected over an energy range of 500 to 3000 eV, in order to identify the scattering peaks of the Cu and I at 2379 and 2674 eV, respectively.

Results and discussion
Structure and morphology of ClAlPc on CuI (111) Thin lms of ClAlPc (50 nm), CuI (111) (30 nm) and bilayers of ClAlPc (50 nm)/CuI (30 nm) were prepared at elevated substrate temperatures in accordance with the methodology reported previously and in the experimental section. 13 Our previous work has shown that the (111) orientation of CuI is responsible for the structural templating interaction and therefore the focus of our investigations has been on (111) oriented lms. [13][14][15] X-ray diffractograms ( Fig. 1) of 50 nm ClAlPc lms grown on SiO 2 at T sub ¼ 155 C show two peaks, at 2q ¼ 6.8 and 27.1 . According to the crystal structure of ClAlPc reported by Wynne (CCDC No. 1134071) these peaks correspond to the (001) and (004) orientations respectively. 16 These peaks suggest the molecules of ClAlPc adopt a "lying down" orientation with the plane of the molecule parallel to the surface. AFM topography images of these lms ( Fig. 1) show small faceted grains and exhibit a root mean square roughness (R q ) of 26.8 nm. For comparison the equivalent lm grown at ambient substrate temperature exhibits no peaks in its X-ray diffractogram (Fig. S1 †). Morphologically the lm has a low R q (3.2 nm) and is comprised of small spherical grains (Fig. S1 †).
Equivalent ClAlPc lms grown sequentially on to 30 nm (111) oriented lms of CuI are structurally distinct. At T sub ¼ 25 C diffraction measurements show a peak at 2q ¼ 25.9 which can be assigned to the (04 1) plane (Fig. S2 †). The presence of this peak suggests that the ClAlPc molecules are adopting a "standing up" orientation with their ligand molecular planes perpendicular to the substrate. The morphology of the ClAlPc lm is similar to that on SiO 2 in identical conditions. The lm is comprised of small spherical grains and has a low R q of 3.5 nm (Fig. S2 †). In contrast 50 nm ClAlPc thin lms grown on a 30 nm (111) oriented CuI lms at T sub ¼ 155 C exhibit three diffraction peaks in corresponding X-ray diffractograms (Fig. 2).
These peaks are observed at 2q ¼ 13.4 , 27.0 and 55.8 and can be assigned to the (111), (222) and (444) orientations of ClAlPc. The {111} Miller planes imply the ClAlPc molecules adopt an orientation in-between "lying down" and "standing up" relative to the substrate. In addition to the change in structure caused by growth at an elevated substrate temperature, the morphology of the ClAlPc lm exhibits a change (Fig. 2) with larger, more rectangular grains present and an R q of 27.4 nm.
The change in morphology and structure on increasing growth temperature of structurally templated ClAlPc lms is similar to the behaviour observed in thin lms of VOPc on CuI. 17 Although templating at an elevated substrate temperature results in both phthalocyanine molecules exhibiting a change in structure, the resulting structures of both thin lms are signicantly different. XRD measurements of templated VOPc lms show an adoption of two distinct molecular orientations, one perpendicular to the plane of the substrate the other parallel. In addition to the differences in structure between the two molecules the growth mechanisms for both molecules are different as will be seen below.

Film formation of ClAlPc on CuI (111)
Thin lms of ClAlPc were grown (T sub ¼ 155 C) at a range of thicknesses (1, 2.5, 5, 10, 25 nm) onto 30 nm (111) oriented CuI lms. AFM was used to probe the growth mode of the phthalocyanine lm by examining the morphological evolution at these intermediate points. At the lowest thickness of ClAlPc (1 nm (R q ¼ 6.76 nm)) there are few discernible features which can be attributed to the phthalocyanine layer (Fig. 3) and the morphology is characteristic of the CuI rst layer. 15 At ClAlPc thickness of 2.5 nm the onset of lm formation can be observed as additional grain boundaries appearing on grains of CuI. This becomes more pronounced at 5 nm, where CuI grains appear bisected by dark grain boundary features with seemingly random orientation with respect to the grain edges. At 2.5 nm there is a large increase in the roughness of the lms to 15.3 nm and at 5 nm the R q remains similar at 14.5 nm. At greater thicknesses (10 nm/25 nm (R q ¼ 16.9 and 14.5 nm respectively)) there is a reduction in the size of the visible ClAlPc grains, or at least the size of areas dened by the darker features. These observations lead to the conclusion that ClAlPc lms forms as islands on top of the existing ClAlPc layers suggesting a Stranski-Krastanov growth mode. 18 Film formation of VOPc on CuI (111) As with ClAlPc, thin lms of VOPc were grown (T sub ¼ 155 C) at a range of thicknesses (1, 2.5, 5, 10, 25 nm) on to 30 nm (111) oriented CuI lms. The difference in growth modes between  ClAlPc and VOPc on CuI is stark even at 1 nm VOPc thickness (Fig. 3). Small islands of VOPc form at random points on the CuI lm at a thickness of 1 nm (R q ¼ 3.42 nm). These islands are mostly rectangular, appear to grow across CuI grain boundaries and are separated by bare grains of CuI. At 2.5 nm the number of islands increases resulting in a similar R q of 3.9 nm. With increasing thickness the aspect ratio of the islands of VOPc decreases. Above 5 nm thickness there is a signicant increase in the roughness of the lms and the size and height of the islands whilst the number of islands does not signicantly increase. For 5 nm, 10 nm and 25 nm VOPc lms the R q values are 6.6, 11.0 and 18.2 nm respectively. At 25 nm there are still areas of the CuI lm which appear bare as the islands of the VOPc have not yet coalesced. Although it is possible that continuous layers of VOPc form over the CuI lm the visibility of VOPc islands at thicknesses as low as 1 nm suggests that a Volmer-Weber growth mode is dominant in the formation of VOPc lms on CuI.
Despite the similarities in molecular structure between the non-planar phthalocyanines there are signicant differences between their structural and morphological behaviours which are highlighted here in their different growth modes and morphologies on thin lm CuI. The lm formation behaviour of the non-planar metal phthalocyanines studied here is distinct from that of planar metal-centred and metal-free phthalocyanines. When iron(II) phthalocyanine is grown using identical procedures on CuI templating layers a single molecular orientation is inferred from XRD and a single crystallite morphology is observed in AFM. 15 In the case of metal-free phthalocyanine (H 2 Pc) polymorphism is observed and multiple crystallite morphologies are evident within thin lms. 12 These comparisons clearly demonstrate the differences between lm formation processes in molecules including the same phthalocyanine ligand, but with different atoms or chemical moieties in the central four-fold imidazole cavity.

Low energy ion scattering
Low energy ion scattering measurements were carried out on 50 nm lms of both phthalocyanines on 30 nm lms of CuI and the results shown in Fig. 4. Although attempts were made to minimise surface contamination there is evidence of uorine present on the surface of both lms. Both lms also show a peak corresponding to the presence of carbon on the surface of the sample. The phthalocyanine ligand contains 32 carbon atoms and is likely the origin of this carbon peak. The inuence of small amounts of adventitious carbon from exposure to ambient environments cannot be deconvoluted from the inherent molecular signal. Both LEIS traces exhibit high-energy tails aer the uorine peaks. As there is no high-energy tail for uorine the tail must be caused by other elements present. In the case of the VOPc lms this is due to an in-depth signal resulting from the presence of iodine. 19 The high-energy tail for the ClAlPc lms is a result of an in-depth signal relating to the presence of Cl at the surface of the lms and iodine cannot be detected.
LEIS is incredibly sensitive to the atoms present in the topmost surface layers. 20 The presence of iodine in the scattering measured from the VOPc/CuI bilayers suggests that there are areas of CuI which are not covered by a phthalocyanine layer. This is reected in the corresponding AFM image of the bilayers. This conrms that the VOPc layer adopts a Volmer-Weber growth mode. As islands of VOPc have formed on top of the CuI layer there are areas of the CuI lm uncovered by VOPc islands even at higher VOPc lm coverages. Therefore ion scattering from iodine atoms present in the topmost few layers of the CuI lm is possible. If the VOPc lm formation was occurring by a growth mode which involved layer by layer growth we would not observe the signal for iodine in the scattering measurements.
In contrast scattering for the ClAlPc lms shows an in-depth signal relating to the presence of Cl at the surface. As the phthalocyanine molecules have chlorine as part of their central moiety this is unsurprising, however the lack of an in-depth signal relating to iodine, which was observed for the VOPc lms, suggests that there are no exposed areas of CuI visible to the ions at the surface. This demonstrates that the ClAlPc growth onto the CuI lm involves a continuous layer of the ClAlPc molecules forming over the surface of the CuI layer. The scattering measurements in combination with the AFM measurements suggest that the ClAlPc layer forms via a Stranski-Krastinov growth mode.

Conclusions
The structure and morphology of ClAlPc thin lms templated by CuI at ambient and elevated substrate temperatures are reported. It is observed that growth at an elevated substrate temperature results in ClAlPc thin lm morphology and structure that is vastly different from ambient substrate temperature growth. Furthermore, at high temperatures, templating changes the molecular orientation of ClAlPc on the CuI substrate.
By examining the evolution of ClAlPc lm morphology for thicknesses between 1 to 25 nm, and combining with the detailed chemical composition using LEIS, a Stranski-Krastanov growth can be identied on high temperature CuI. This is in contrast to the behaviour of VOPc, which grows via a Volmer-Weber mode. These observations highlight the need for caution when comparing phthalocyanine molecules with similar chemical structures but different central moieties, especially when out of plane groups are present. Although the phthalocyanines in this work are both non-planar molecules, with perpendicular moieties in their central cavity, their structures, morphologies and growth modes are very different.