Optical Spectroscopy and XRD Study of Molecular Orientation

Article pubs.acs.org/JPCC

Optical Spectroscopy and XRD Study of Molecular Orientation, Polymorphism, and Phase Transitions in Fluorinated Vanadyl Phthalocyanine Thin Films Tamara V. Basova,*,† Vitaly G. Kiselev,‡,§ Ilya S. Dubkov,† Florian Latteyer,∥ Sergei A. Gromilov,† Heiko Peisert,∥ and Thomas Chassè∥ †

Nikolaev Institute of Inorganic Chemistry SB RAS, 3 Lavrentiev Ave., 630090 Novosibirsk, Russia Voevodsky Institute of Chemical Kinetics and Combustion SB RAS, 3 Institutskaya Str., 630090 Novosibirsk, Russia § Novosibirsk State University, 2 Pirogova Str., 630090 Novosibirsk, Russia ∥ Institute of Physical and Theoretical Chemistry, University of Tübingen, D-72074 Tubingen, Germany ‡

S Supporting Information *

ABSTRACT: The molecular arrangement and phase transitions in the vanadyl hexadecafluorophthalocyanine (VOPcF16) thin films grown by physical vapor deposition have been studied using in situ X-ray diffraction, atomic force microscopy, and optical spectroscopy techniques (UV, IR, and Raman). The complete transition from the low-temperature linear cofacial structure to the slipped dimeric one occurs in the temperature range 160−220 °C. This conversion was found to be irreversible upon cooling the VOPcF16 film back to 20 °C. The structural transformation leads to decrease of the in-plane conductivity of the film by 2 orders of magnitude. According to the polarized Raman spectroscopy measurements, the mean tilt angles between the VOPcF16 species and the substrate surface were 59 ± 5° and 30 ± 5° in the as-deposited and annealed films, respectively. For the sake of comparison, the structure of the thin films of vanadyl phthalocyanine (VOPc) was also studied. The mean tilt angle between the VOPc species and the substrate surface was found to be 77 ± 5°, in good agreement with existing experimental data (∼70°). All intense bands in the experimental IR and Raman spectra of VOPcF16 and VOPc were assigned using DFT calculations (B3LYP) and the 15N isotopic shifts in the vibrational spectra of VOPc.

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INTRODUCTION

Chart 1

Because of their remarkable performance, air stability, and low cost, metallophthalocyanines (MPcs) are very promising species for organic electronics.1 Among them, oxometal phthalocyanines, e.g., vanadyl and titanyl phthalocyanines (VOPc and TiOPc, respectively), exhibit high photoconductivities and unusual nonlinear optical properties.2,3 This renders VOPc and TiOPc to be promising species for the design of organic optical switches.4 In contrast to other 3d-metal phthalocyanines, e.g., M(II)Pc (M = Mg, Cu, Co, Ni), VOPc (Chart 1) is a nonplanar polar molecule with a vanadyl group arranged perpendicular to a macrocycle.5−7 Therefore, the molecules of VOPc in the crystalline lattice are prone to twodimensional π−π stacking, which results in shorter intermolecular distances in comparison with planar phthalocyanines, larger π−π stacking overlap, and, ultimately, high mobility of charge carriers.6 This opens up the prospect of application of this species in high-performance organic thin film field-effect transistors (OFET).8−12 It should be noted that VOPc exhibits a specific polymorphism5,6 which differs significantly from the most part of planar phthalocyanines. For the planar Pcs, the α and β phases © XXXX American Chemical Society

Received: December 20, 2012 Revised: March 19, 2013

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dx.doi.org/10.1021/jp4016257 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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techniques based on the polarization-dependent Raman spectroscopy.

are typically comprised of the molecules arranged at different stacking angles between the Pc moieties and a stacking axis.13 The VOPc films also exist in two main polymorphic modifications.5,6 However, the crystal structures of these phases are noticeably different. The films deposited at room temperature have a monoclinic structure (phase I).14 The transition to the triclinic structure (phase II, P1̅ space group)15−18 occurs upon postannealing of the samples at the temperatures >100 °C. Apart from this, the phase II films can be obtained by means of deposition at elevated substrate temperatures (>60 °C).16−18 Moreover, the dominating polymorphic form is strongly dependent on the evaporation rate: e.g., the triclinic form was obtained at room temperature at the rate of 0.05 nm s−1 and the monoclinic form at 9 nm s−1, respectively.19 Note that the properties of the films comprised of various phases might differ significantly. For example, the strong dependence of the nonlinear optical properties of VOPc films on the crystalline phase has been reported.20−22 The thirdorder nonlinear optical susceptibility varied by a factor of 2−4 for different types of the crystalline polymorphs of VOPc.20−22 The current vs voltage (I−V) characteristics and charge carrier mobility of VOPc films were also dependent on the film structure and morphology.23,24 In contrast to the case of VOPc, much less is known about the structure and properties of f luoro-substituted oxometal phthalocyanines. At the same time, the thin films of metal hexadecafluorophthalocyanines (MPcF16) are of significant interest as air-stable n-channel semiconductors.25−27 For instance, the electron mobility is 0.03 cm2 V−1 s−1 in the pure films of CuPcF16 (50−60 nm thickness) deposited on ndoped silicon substrate is 0.03 and 0.08 cm2 V−1 s−1 in the airstable OFET multilayer structures based on CuPcF16.25,28 Moreover, the transistors on the basis of single crystalline CuPcF16 nanoribbons have a charge carrier mobility of 0.2 and 0.35 cm2 V−1 s−1 with the use of SiO229 and air/vacuum30 as the dielectric layers, respectively. The epitaxial growth of VOPcF16 films on the NaCl, KCl, and KBr substrates deposited by the organic molecular beam deposition (OMBD) technique has also been studied.31 The anisotropic optical properties of VOPcF16 films deposited on KBr and fused silica substrates were investigated by spectral ellipsometry.32 While the various phases of VOPc films have been studied in detail,5,6 the polymorphism and phase transitions in the films of f luorinated VOPc derivatives have never been considered in the literature, to the best of our knowledge. At the same time, as discussed above, the electrical and optical properties of the thin films are determined by the polymorphic modification and molecular orientation in the film. Therefore, the molecular arrangement and morphology of the film play a crucial role in particular device applications. In the present contribution, we address the phase transitions of the VOPcF16 (Chart 1) thin films upon annealing in the temperature range 20−220 °C by in situ X-ray diffraction (XRD), atomic force microscopy (AFM), and a variety of optical spectroscopy techniques. For the sake of comparison, the properties of VOPc (Chart 1) films have also been investigated. It is noteworthy that no VOPcF16 crystals of appropriate size have been grown so far. Therefore, in contrast to the conventional consideration employing X-ray diffraction methods, the orientation of VOPcF16 molecules relative to the substrate surface have been studied in detail using the

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EXPERIMENTAL AND COMPUTATIONAL DETAILS 1. Synthesis. The vanadyl phthalocyanine (VOPc) was prepared using “anhydride” methods from the phthalic anhydride, urea, and V2O5.33 The 15N-isotopologue of the vanadyl phthalocyanine (VOPc-15N) was synthesized by the same procedure using the urea containing 98% 15N (Aldrich). Vanadyl hexadecafluorophthalocyanine (VOPcF16) was synthesized by heating a 4:1 mixture of a sublimed 3,4,5,6tetrafluorophtalo-1,2-dinitrile (Aldrich) and V2O5 powder in a vacuum-sealed glass tube (10−3 Torr). The tube was placed in a furnace, heated up to 220 °C, and kept at this temperature for 7 h. After cooling down to room temperature, the tube was opened and the mixture was washed by ethanol and acetone to remove soluble organic impurities. 2. Deposition of Thin Films. The thin films of VOPc and VOPcF16 on glass and Si(100) substrates were obtained by physical vapor deposition under ultrahigh-vacuum conditions (10−8 mbar). The temperature of the substrate was about 60 °C. The nominal thickness of all films was estimated using a quartz microbalance and turned out to be about 50 nm. The asdeposited films were annealed in situ from the room temperature to 220 °C at heating rate of 2 K/min. 3. Instruments. The Raman spectra were recorded with a Labram HR 800 spectrometer (HORIBA Jobin Yvon). The spectra have been measured in a backscattering geometry using a BX 41 microscope. An Ar laser was used for excitation. The spectral resolution of various diffraction gratings was 1.5−3 cm−1. Laser filters were applied to attenuate the laser power and to prevent the damage of the sample. The infrared spectra of the powders in polyethylene and KBr pellets and of the deposited films on Si substrates were recorded using a Vertex 80 FTIR spectrometer. The UV−vis spectra of solution and thin films of VOPcF16 were recorded with a UV−vis−NIR scanning spectrophotometer (Shimadzu UV−vis-3101PC ) in the spectral range 400−900 nm. X-ray diffraction measurements were performed using an automatic diffractometer DRON-3 M (R = 192 mm, Cu Kα line, Ni filter, a scintillation detector with amplitude discrimination, Soller slits with aperture of 2.5° on primary and reflected beams) in the 2θ region 5°−60° with a scanning step of 0.03°. Atomic force microscopy (AFM) studies were performed with a Digital Instruments Nanoscope III Multimode AFM at ambient conditions directly after the preparation and annealing of the films. Tapping mode was applied to avoid damaging of the organic film. Current−voltage measurements were performed using a Keithley 236 electrometer equipped with a microprocessor controlled measuring system. For the measurements of the in-plane film conductivity, the VOPcF16 films were deposited onto glass substrates with interdigitated Al electrodes (an interelectrode distance L = 60 mm and a width of electrode overlap W = 3.125 mm). All electrical measurements were performed in air at room temperature. 4. DFT Computations. The IR and Raman spectra of the VOPc and VOPcF16 and their 15N-isotopologues were calculated at the B3LYP/6-311++G(2df,p) level of theory.34,35 The numerical integration of the exchange-correlation terms of the density functionals was carried out using ultrafine grids. All calculations were performed using Gaussian 09 suite of programs.36 The experimental wavenumbers below 150 cm−1 were not considered because of the strong mixing of collective B



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nm) rise up (Figure 1). It is worth mentioning that the X-ray photoelectron spectra (XPS) of the pristine and postannealed films are identical (Supporting Information, Figure S1). This fact indicates that the extent of thermal degradation of the samples throughout annealing is negligible. Meanwhile, the lattice spacing d = 0.31 nm coincides well with the reported distance between the Pc macrocycles in VOPc and TiOPc single crystals.14,39,40 These facts give evidence of the slipped dimeric stacking structure (Figure 2b) of the annealed films of VOPcF16.14,39,41 It is also worth mentioning that the distance between the Pc macrocycles in the linear cofacial stacking structure reported for the haloaluminium phthalocyanines (0.36 nm)42,43 is larger than in the case of slipped dimeric stacking structure. The position of the reflection peak of low intensity at 2θ = 14.42° (d = 0.62 nm) matches well with the lattice constant c = 0.62 nm corresponding to the tetragonal structure of the VOPcF16 crystalline film deposited on the KBr and KCl substrates.31 The ab-plane of this tetragonal cell is oriented parallel to the (100) surface of the substrates, and the c-axis is vertical to the substrate surface. Assuming the similar tetragonal structure of the VOPcF16 films in our case, the similar molecular orientation in the films could be proposed: viz., the c-axis is vertical to the Si(100) substrate surface, and the VOPcF16 molecules are parallel to the surface (Figure 2b). It is instructive to compare the XRD patterns of the VOPcF16 with their counterparts of the VOPc films. As mentioned in the Introduction, the two crystalline phases are typical of VOPc films.5,6 Recall that the VOPc layers deposited at low temperatures (usually below room temperature) have a monoclinic structure (phase I).44 Consequently, the XRD patterns of the VOPc films of phase I have two peaks at 2θ = 26.0° and 12.8°, corresponding to the interstack spacings of 0.34 and 0.69 nm, respectively. This fact suggests the perpendicular orientation of the backbone axis with respect to the substrate surface plane.16 Transformation to the triclinic structure (phase II) takes place either upon annealing above 100 °C or if the substrates are kept at the temperatures above 60 °C during the film deposition.16−18,45,46 It should be emphasized that both polymorphs of VOPc films have the same slipped dimeric stacking structure similar to Figure 2b. The main difference between the structures of the two phases lies in the distances between the neighboring molecules in the I and II polymorphs.46,47 In contrast, the VOPcF16 polymorphs have noticeably different stacking patterns: namely, linear in the asdeposited VOPcF16 films (Figure 2a) and slipped in the films after annealing (Figure 2b). Therefore, the XRD data clearly indicate that the phase transition occurs upon annealing of the VOPcF16 films at the temperatures higher than 160 °C. This

crystal lattice modes and internal vibrations in this range. The Raman intensities were not calculated because of the resonant nature of the Raman spectra obtained using the excitation wavelength in the visible spectral range.

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RESULTS AND DISCUSSION 1. X-ray Diffraction and Atomic Force Microscopy Studies of the Structure and Morphology of VOPcF16 Films. As the first step of the film structure studies, we used the X-ray diffraction (XRD) technique. The XRD patterns of the as-deposited VOPcF16 films and while the films underwent gradual heating up to 220 °C are given in Figure 1. It is seen

Figure 1. X-ray diffraction patterns of VOPcF16 films annealed at different temperatures.

that the XRD patterns of the as-deposited VOPcF16 films at 20 °C display only one diffraction peak at 2θ = 6.19° corresponding to a lattice spacing of 1.43 nm. This pattern is closely reminiscent of CuPcF16 films deposited in the temperature range 30−120 °C.37 In that case, the similar single diffraction peak at 2θ = 6.14° was assigned to the (100) lattice plane37 by analogy with unsubstituted MPc, and the edge-on orientation of the CuPcF16 molecules was proposed.25,38 On the basis of these facts, we hypothesized the same edge-on orientation (Figure 2a) of VOPcF16 molecules with respect to the silicon surface plane. Nevertheless, it should be pointed out that the crystal structures of both CuPcF16 and VOPcF16 compounds have not been reported, and more detailed studies are indeed necessary to clarify this issue. Note that the annealing of VOPcF16 films leads to the remarkable changes in the XRD pattern. The changes become noticeable at the temperatures ∼160 °C (Figure 1). The diffraction peak at 2θ = 6.19° almost disappears while the new peaks at 2θ = 14.42° (d = 0.62 nm) and 2θ = 29.01° (d = 0.31

Figure 2. Proposed structures of the VOPcF16 films before (a) and after (b) annealing. C



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Figure 3. AFM images and the roughness profiles (along the green section) of the VOPcF16 films: as-deposited (a), after heating at 180 °C (b), and at 220 °C (c).

filled with the crystallites of square shape with the average size 60−100 nm (Figure 3c). The rms roughness of the annealed film was 8.53 nm (cf. the cross sections below the AFM images in Figure 3). Note that an increase of roughness upon thermal and chemical annealing is typical of metal phthalocyanine films.51 It is also worth mentioning that the complete transformation of the crystallites from elongated to squaredshape form after annealing is consistent with disappearance of the diffraction peak at 2θ = 6.19° in the XRD pattern at 220 °C (Figure 1). Finally, annealing at the temperatures >220 °C does not lead to any appreciable changes in the surface morphology. Thus, the XRD and AFM data give firm evidence that annealing of as-deposited VOPcF16 films leads to a phase transition and changes in the morphology. However, it is clearly seen that more detailed information (particularly, the molecular orientation relative to the substrate surface) cannot be extracted from the XRD and AFM measurements. As mentioned in the Introduction, the structure of VOPcF16 single crystals has not been resolved; therefore, the XRD provides only a set of various lattice spacings in thin films. In order to rationalize the proposed low- and high-temperature structures and, eventually, to obtain more detailed insight into the structural transformations of the films, we used optical spectroscopy techniques. 2. UV−vis Absorption Spectroscopy of the VOPcF16 Films. The optical absorption spectra of the samples undergoing gradual annealing (the temperature step was 10 °C) are shown in Figure 4. It is seen that the UV−vis absorption spectrum of the as-deposited VOPcF16 film on a glass slide consists of a Q band at 660 nm with a shoulder at 720 nm (Figure 4). Note that the Q band is blue-shifted in comparison with the corresponding spectrum of VOPcF16 in solution (∼720 nm).7 In general, the split of the Q band and its shift from the transition in solution in the absorption spectra of phthalocyanine films is due to π−π interactions between adjacent molecules along the stacking axis.46 Furthermore, the absorption properties in the visible range are very sensitive to the crystalline structure, which determines the extent of the π−π overlap between the orbitals of neighboring molecules.46 According to the molecular exciton theory,52 the spectrum of Figure 4 is typical of a cofacial parallel arrangement of phthalocyanine macrocycles (Figure 2a). This fact agrees well with the XRD results given in the previous section (cf. Figure 1

conversion was found to be irreversible upon cooling the VOPcF16 film to 20 °C. Electrical properties of the films are also in a good agreement with the proposed molecular organization. The room temperature dc lateral conductivities of the pristine and annealed VOPcF16 films (the details are presented in the Supporting Information, Figure S2) are σ∥ = 6 × 10−4 and 2 × 10−6 Ω−1 cm−1, respectively. The pronounced decrease (by 2 orders of magnitude) of the lateral conductivity upon film annealing correlates well with the changes in the film structure (cf. Figure 2). It is well-known that the conductivity of molecular semiconductor films is determined by electronic coupling (transfer integral) between adjacent molecules,48 which, in turn, correlates to a good extent with the π−π overlap between the Pc moieties.48,49 It is clear that in the as-deposited film (Figure 2a) this overlap is indeed much higher in the direction parallel to the substrate surface because of the standing-on orientation of Pc macrocycles. To get deeper insight into the structural transformations of the VOPcF16 films, we also studied their morphology at various temperatures using atomic force microscopy (AFM) technique. Figure 3 shows the morphology of the as-deposited (Figure 3a) films of VOPcF16 and the films after heating up to 180 °C (b) and 220 °C (c). It is seen from Figure 3a that the as-deposited films consist of relatively small distinct elongated grains with the average length ∼30−40 nm. The rms film roughness in this case was 1.96 nm. The remarkable changes in morphology occur upon annealing at 160 °C; the average grain size increased to ∼50−60 nm. Let us remember that the new peaks occur in the XRD patterns of the films at this temperature (cf. Figure 1). Furthermore, the films annealed at 180 °C consist of two clearly distinguishable types of domains, viz., the elongated grains and the crystallites of square and rectangular shape (Figure 3b). Note that the rms roughness of the samples increased to 8.11 nm (Figure 3b). As seen from Figure 1, both polymorphs coexist at this temperature. Therefore, the phase transition is accompanied by elongation of the grains of initial phase prior to transformation. The similar transformation of crystallites was observed in the case of CuPc and H2Pc.50,66 Formation of the new phase of VOPcF16 films begins at uniformly distributed discrete nucleation centers, viz., domains of square and rectangular shape (Figure 3b). After 30 min annealing of the samples at 220 °C the surface was completely D



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films.54−56 These techniques are particularly valuable in the case of compounds with unknown crystal structure because the commonly employed X-ray diffraction methods cannot be applied for detailed investigations in this case. Note, however, that the vibrational spectroscopy methods (in our case, polarized Raman spectroscopy) can give insight into the structure of thin films only provided the detailed assignment of the vibrational spectra of the deposited species is available. Unfortunately, in the particular case of vanadyl phthalocyanine and its derivatives, the existing assignments of the vibrational spectra are entirely based on the experimental IR and Raman data and empirical force field calculations.54,55 Moreover, to our knowledge, the vibrational spectra of VOPcF16 have not been discussed in the literature. 3.1. Experimental and Quantum Chemical Study of the Vibrational Spectra of VOPcF16 and VOPc. At first, we scrutinized the vibrational spectra of the VOPc and VOPcF16. In order to assign unambiguously all intensive bands in the experimental IR and Raman spectra, we used both quantum chemical calculations (DFT) and the 15N isotopic shifts in the vibrational spectra of VOPc. This approach has recently been shown to be particularly useful for assignment of the vibrational bands of the large conjugated phthalocyanine macrocycles.57,58 The VOPc and VOPcF16 are pseudoplanar molecules of C4v point group symmetry (Chart 1). The geometries of all compounds under study optimized at the B3LYP/6-311+ +G(2df,p) level of theory are given in the Supporting Information (Table S1). Assuming a C4v point group for the VOPc and its derivatives comprised of 58 atoms, the corresponding vibrational representation reads as Γ = 23A1 + 19A2 + 21B1 + 21B2 + 42E. The A1, B1, B2, and E modes are Raman-active, and the A1 and E modes are IR-active. The calculated bond lengths and bond angles of the VOPc molecule coincide fairly well (within 0.02 Å, Supporting Information, Table S1) with the corresponding experimental values.6 The experimental and calculated IR spectra of VOPc, its 15Nisotopologue (VOPc-15N), and VOPcF16 are shown in Figures 5 and 6. The far-IR spectra of VOPc and VOPcF16 are given in the Supporting Information (Figure S3). The experimental Raman spectra of VOPc and VOPcF16 are presented in Figure 7. The comparison of the experimental and

Figure 4. UV−vis absorption spectra of the VOPcF16 films in the temperature range 20−220 °C.

and the discussion therein). The weaker shoulder at ∼720 nm (Figure 4) indicates the presence of an amorphous fraction in the film. Note that a similar spectrum was also obtained for a ZnPcF16 film on a quartz substrate.53 Moreover, similar UV−vis spectra have been reported31 for VOPcF16 films deposited on quartz substrates. The authors31 concluded that the molecules within the cofacial stacks of VOPcF16 have predominantly a standing-on orientation with respect to the substrate surface. At the same time, the average tilt angle of ∼56° for VOPcF16 molecules on a silicon substrate was estimated from spectroscopic ellipsometry measurements.32 No changes have been observed in the UV−vis spectra of the VOPcF16 films heated up to temperatures below 160 °C. When the VOPcF16 films were heated to temperatures >160 °C in air (cf. the discussion of Figures 1 and 3), the intensity of the Q band at 660 nm decreased, while the shoulder at 720 nm shifted to 758 nm and its intensity increased (Figure 4). The shape of the Q band in the spectra at 220 °C corresponds to a head-totail arrangement of the chromophore−phthalocyanine macrocycles52 (Figure 2b), while the films at 20 °C had a cofacial (face-to-face) arrangement of the molecules (Figure 2a). It is worth mentioning that a similar red-shift in the UV−vis spectra has been reported earlier for the VOPcF16 films with tetragonal structure deposited on the KCl and KBr substrates at room temperature.31 Thus, it is reasonable to propose the similar tetragonal structure of the annealed films in our case. It is therefore seen that the results of the UV−vis studies are in agreement with the above-discussed XRD and AFM data and confirm that the phase transition in the VOPcF16 films occurs at ∼160−180 °C. The comparison of the UV−vis spectra of VOPcF16 with the counterparts for similar species gives qualitative evidence of the proposed structure of the two polymorphs (Figure 2). However, these techniques cannot give quantitative insight into the film structure: e.g., the inclination angle of the phthalocyanine moieties with respect to the substrate surface cannot be determined using the abovediscussed experimental techniques. At the same time, the orientation of the species in the film is crucial for electrophysics and applications. To clarify this issue, we applied the approach based on the polarization-dependent Raman spectroscopy. 3. Study of the VOPcF16 Film Structure Using Vibrational Spectroscopy Techniques. Among the huge variety of experimental techniques, vibrational spectroscopies (IR and especially Raman) have been shown to be very useful for studies of the surface structure and properties of thin

Figure 5. Experimental and DFT calculated IR spectra of VOPc and of its 15N-substituted analogue (VOPc-15N). E



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modes in the spectra of VOPc have remarkable isotopic shifts (about 6 cm−1, Table S2) and therefore involve also isoindole ring deformations. Similar to this case, the assignment of the Raman modes at 591 and 1033 cm−1 to benzene deformations and CH bending, respectively,56 do not agree with our results. The isotopic shifts for these modes are high, viz., about 8 and 11 cm−1, respectively (Table S2), and therefore, the nitrogen atoms are also involved in the corresponding normal vibrations. Furthermore, the Raman band observed at 948 cm−1 has been assigned to VO stretching.56 However, the isotopic shift for this mode is 19 cm−1 (Table S2). At the same time, the DFT calculations predict the VO stretching mode at 1059 cm−1 (Table S2) almost without any isotopic shift. In the experimental Raman spectrum, the only fundamental with a small isotopic shift appears in this region at 1002 cm−1 (Table S2). The vibrational spectra of VOPcF16 have not been studied before. The benzene ring stretching mode in the experimental IR spectrum of VOPcF16 (Figure 6) is located at 1637 cm−1 (cf. 1609 cm−1 in the case of VOPc, Figure 5). The wavenumber range 1200−1400 cm−1 in the experimental IR spectra of the VOPcF16 derivative is dominated by two strong fundamentals at 1270 and 1330 cm−1 (Figure 6). These modes are mainly assigned to isoindole and pyrrole deformations (Table S3). DFT calculations predict the VO stretching modes of VOPcF16 at 1065 cm−1, and the corresponding fundamental lies at 1020 cm−1 (Table S3 and Figure 6). The totally symmetric band (A1) corresponding to a macroring breathing and V−Nα stretching is located at 593 cm−1 in the Raman spectrum of VOPc (Figure 7 and Table S2). Upon F-substitution the position of this band shifts to 589 cm−1 (Table S3). The most intense bands in the far-IR spectrum of VOPc (100−450 cm−1, Figure 5) were observed at 357 and 368 cm−1. These bands, along with the V−Nα stretching and benzene ring vibrations, can be assigned to the inner ring deformations (Table S2). The A1 bands at 262 and 353 cm−1 corresponding to out-of-plane macroring vibrations become more prominent in the far-IR spectrum of VOPcF16 (Table S3). Apart from this, in the case of VOPcF16 the contribution of C−C−F bending and C−F stretching vibrations to the normal coordinates of several modes (e.g., at 750, 965, and 1150 cm−1) become noticeable in comparison with the VOPc case (Tables S2 and S3). 3.2. Effects of Phase Transition on the IR and Raman Spectra of the VOPcF16 Films. After detailed assignment of vibrational spectra described in the previous section, we studied the IR and Raman spectra of the films annealed at different temperatures. These spectra are presented in Figures 8 and 9, respectively. It is worth mentioning that both IR and Raman spectra of the as-deposited VOPcF16 films differ from those of the VOPcF16 powder while the spectra of the films after annealing, in contrast, resemble those (Figure S4). This is not surprising because the powder was purified by gradient sublimation in vacuum and deposited in the hot zone of the gradient furnace. Therefore, we propose that the films after annealing at 220 °C have the same phase composition as the powder. As was discussed before, the annealing of the as-deposited films leads to the change of the film structure. These structure changes, in turn, have an effect on the vibrational spectra of the films. The positions of the bands corresponding to stretching vibrations of isoindole fragment in the range at 1460−1510 cm−1 in the IR spectra are very sensitive to the crystalline structure of VOPcF16 (Figure 8). The two bands located at

Figure 6. Experimental and DFT calculated IR spectra of VOPcF16.

Figure 7. Experimental Raman spectra of VOPc, its analogue VOPc-15N, and VOPcF16.

15

N-substituted

calculated wavenumbers of the most intense vibrations of VOPc, isotopic shifts, and their assignments are given in Table S2. The assignment of experimental bands was primarily based on the DFT calculated values and isotopic shifts upon 15N substitution. In the case of ambiguity, the IR intensity data were also used. Similar data on VOPcF16 are given in Table S3. As seen from Tables S2 and S3 and Figures 5−7, the experimentally measured vibrational fundamentals of VOPc molecules coincide well with the DFT theoretical predictions. The rms differences between the calculated and experimental values were 20 and 10 cm−1 for wavenumbers in the case of VOPc and VOPcF16, respectively, and about 1 cm−1 for isotopic shifts in VOPc. The experimental results of the present study are in a fairly good agreement with the reported data on the IR bands of VOPc.56 However, we propose new assignments of some Raman-active modes. The experimental data along with the theoretical results obtained in the present work allowed to assign not only totally symmetric A1 and E but also the B1 and B2 Raman modes of vanadyl phthalocyanines as well. For instance, the Raman-active modes at 1197 and 1210 cm−1 have been proposed to involve C−H bendings.56 On the contrary, our isotopic substitution data show that the corresponding F



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comparison with the spectrum of annealed VOPcF16 film (about 100 cm−1) might be attributed to intermolecular interactions in different polymorphic modifications of the films before and after annealing. The VO stretching mode is much stronger in the spectrum of the films before heating with a cofacial (face-to-face) arrangement of the molecules (Figure 2). This fact agrees well with a preferential flat-on orientation of VOPcF16 molecules with respect to the substrate surface suggested on the basis of XRD data (Figures 1 and 2). Apart from the VO stretching mode, the bands located at 268 and 357 cm−1 are also very intense in the Raman spectrum of as-deposited film while being rather weak in the spectrum of the film after annealing (Figure 9). These two bands were assigned to the out-of-plane bending vibrations involving central metal and isoindole moieties (Table S3). Therefore, it is clearly seen that the VO stretching mode and the modes involving the coordination center of the VOPcF16 are very sensitive to the packing of VOPcF16 molecules in the different polymorphs. 3.3. Study of the Molecular Orientation in the Thin Films of VOPc and VOPcF16 by Polarization-Dependent Raman Spectroscopy. Having studied in detail the vibrational spectra of VOPcF16, we were able to get insight into molecular orientation in the films using the polarized Raman spectra of the films in parallel (z(xx)z)̅ and cross (z(xy)z)̅ polarizations.60,61 To describe the molecular orientation, the Euler coordinates were used.62 The detailed description of the methodology employed is given elsewhere.58,59 Since no azimuthal anisotropy of the depolarization ratio ρ = I(z(xy) z̅)/I(z(xx)z)̅ was observed, we applied the model for azimuthally averaged molecular orientation.63 Therefore, the transformation from molecular to substrate coordinates with the subsequent azimuthal averaging over the angle φ reads as

Figure 8. IR spectra of the VOPcF16 films recorded in the temperature range 20−220 °C.

Figure 9. Raman spectra of VOPcF16 films recorded in the temperature range 20−220 °C.

I

−1

1478 and 1492 cm in the IR spectrum of as-deposited film shift to 1484 and 1497 cm−1, respectively, after heating to 220 °C (Figure 8). The increase of the relative intensity of the band at 1484 cm−1 was also observed. Other sensitive IR bands are located at 1313 and 1139 cm−1 spectrum of as-deposited VOPcF16 film. Their positions shift to 1318 and 1132 cm−1 after annealing of the film (Figure 8). These bands were assigned to pyrrole deformations along with the Cβ−Cβ stretching and isoindole deformations, respectively (Table S3). On the other hand, the Raman band of VOPcF16, which is the most sensitive to changes of the crystalline structure, lies at 1512 cm−1 (Figure 9). After heating its position shifts to 1523 cm−1. Note that the Raman bands of TiOPc films in the spectral range 1505−1530 cm−1 were also found to change noticeably upon the phase transition.59 It is also seen from Figures 8 and 9 that the intensities of several bands are strongly dependent on the temperature. The most relevant signature of the phase transition is the Raman band at 901 cm−1 (Figure 9). The intensity of this band decreases upon heating and it totally disappears at 220 °C (Figure 9). At the same time, the appearance of new peak at 1023 cm−1 with increasing intensity is observed (Figure 9). In accordance with the DFT predictions, we hypothesize that both bands correspond to the same out-of-plane VO stretching vibration (Table S3). A similar tendency was observed in the IR spectra of VOPcF16 (Figure 8). The IR band at 903 cm−1 of the as-deposited film disappears upon heating with simultaneous appearance of the band at 1020 cm−1. A very large red shift in

/⊥

=

∫0

2π

(e i ·(R(φ , θ , ψ )T ·A1 / ⊥ ·R(φ , θ , ψ )) ·es / ⊥)2 dφ (1)

where ei and es are the unit electric vectors of the incident (i) and scattered (s) light with respect to the polarization vector (in the Porto notation, z(xx)z̅ (||) and z(xy)z̅ (⊥)).64 In the coordinate system employed, the Euler angle θ corresponds to the tilt angle between the molecular and substrate planes (cf. the inset of Figure 10). It has already been shown63 that the depolarization ratios ρ for the Raman modes of particular symmetry are strongly dependent on the θ angle. Therefore, the value of θ can be derived from (1) using the experimentally measured depolarization ratios for the modes of the corresponding symmetry type. Figure 10 represents the polarized Raman spectra of the VOPcF16 films recorded in the parallel (z(xx)z)̅ and cross (z(xy)z)̅ polarizations of incident and scattered light at 20 °C (a) and after heating at 220 °C (b). For the sake of comparison, the polarized Raman spectra of VOPc films (Figure 10c) deposited under the same experimental conditions are also given. The symmetry types of all Raman modes were determined on the basis of DFT calculations discussed above. For orientation analysis we opted for the A1, B2, and E modes, which are clearly separated from other bands in the experimental Raman spectra of VOPc and VOPcF16 (Figure 9, Tables S2 and S3). The depolarization ratios of the selected bands in the spectra of VOPc and VOPcF16 films and the corresponding Euler angles θ are given in Table 1. G



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corresponding Euler angles θ for the as-deposited (Figure 10a) and annealed VOPcF16 films (Figure 10b) were estimated to be 59 ± 5° and 30 ± 5°, respectively (Table 1). Hence, these results undoubtedly confirm the above-discussed qualitative assumptions made on the basis of XRD and UV−vis spectroscopy. The as-deposited VOPcF16 films have a predominant standing-on orientation of the molecules relative to the substrate surface, while the annealed films have a preferential flat-on orientation (cf. the corresponding mean tilt angles θ ∼ 60° and ∼30°, Table 1). In the case of VOPc film, the Euler angle θ is 77 ± 5° (Table 1). Note that the structure of the similar VOPc films (phase II, P1̅ space group) was studied using the related Raman spectroscopy technique.56 In a good agreement with our results, the mean tilt angle of VOPc species was found to be ∼70°.56 4. VOPcF16 Film Structure and Comparison with Similar Metal Phthalocyanines. The above-discussed results indicate that the VOPcF16 films exist in two polymorphic forms. In general, several types of polymorphism are typical of metal (MPc) and oxometal phthalocyanines. For instance, 3d-metal phthalocyanines (M = Cu(II), Co(II), Ni(II)) exist in two polymorphic modifications, α- and β-, with different stacking angles between the Pc molecules and the stacking axis.65−69 Apart from this, the I- and II-phases of the oxometal phthalocyanines (TiOPc and VOPc)46,47 have different overlapping arrangements of the neighboring molecules. In contrast, according to our data, the two polymorphs of VOPcF16 have two different stacking modes, viz., linear cofacial in the lowtemperature structure and slipped dimeric in the hightemperature one (Figure 2). Note that such type of polymorphism is also typical of lead phthalocyanine.70 The relative orientation of neighboring molecules in the film and, ultimately, the dominating polymorphic form are primarily determined by weak interactions (viz., π−π overlap and electrostatic repulsion) of the adjacent moieties. Stronger electrostatic repulsion between the electronegative oxygen atom of vanadyl group and fluorine substituents results in new packing motifs in VOPcF16 films in comparison with the unsubstituted VOPc. Recall that the phase transition in the VOPcF16 films is irreversible. Therefore, the low-temperature linear cofacial structure obtained by physical vapor deposition (Figure 2a) is metastable and exists due to kinetic reasons. The slipped dimeric modification formed upon annealing (Figure 2b) is thermodynamically more favorable. This stabilization can be attributed to a higher extent of π−π intermolecular interactions in the slipped dimeric structure. As seen from Figure 2b), each VOPcF16 molecule has four neighbors and the intermolecular distance between the species is 0.31 nm. As was discussed above, the phase transition in the VOPcF16 occurs in the temperature range ∼160−220 °C. It is noteworthy that the phase changes in the films of MPc (M = Co, Ni, Cu, Zn) become noticeable at higher temperatures.71 For instance, the incomplete phase change from α-CuPc to βCuPc occurs after heating for 3 h at ca. 240 °C.72 The similar transition in CoPc takes place at 300 °C.73 At the same time, the temperatures of phase transition in the films of nonplanar phthalocyanines (VOPc, TiOPc, PbPc) were found to be lower than those in the films of the planar MPc species. For example, the phase transition in PbPc films occurs in the temperature range 130−190 °C.74 This fact might be attributed to the

Figure 10. Polarized Raman spectra of the as-deposited VOPcF16 thin film (a), VOPcF16 thin film after annealing at 220 °C (b), and asdeposited VOPc thin film (c) on silicon substrates. The inset shows the Euler coordinates employed: φ corresponds to the rotation around the substrate Z-axis, θ to the rotation around the molecular X′-axis (the tilt angle between Z and Z′), and ψ to the rotation around the molecular Z′-axis.

Table 1. Depolarization Ratios ρ(ij/ii) of the Active Modes in the Raman Spectra of VOPcF16 and VOPc Films and the Estimated Euler Angles θa film VOPcF16 as-deposited (20 °C)

VOPcF16 after annealing (220 °C)

VOPc phase II

a

wavenumber, cm−1

sym irrep

ρ(ij/ii)

589 730 1056 1482 1193 589 730 1023 1056 1136 1482 1193 593 681 836 724 1033 1195 785 1588 1107

A1 A1 E E B1 A1 A1 A1 E E E B1 A1 A1 A1 E E E B2 B2 B1

0.17 0.16 1.02 1.00 0.56 0.02 0.02 0.01 0.78 0.63 0.50 1.00 0.28 0.30 0.27 1.03 0.99 1.00 1.00 0.99 0.36

Euler angle θ, deg 59 ± 5

30 ± 5

77 ± 5

The Euler angles are defined in accordance with Figure 10.

It is seen that the intensities of several bands (e.g., at 589, 730, and 1056 cm−1) in the polarized Raman spectra of the asdeposited VOPcF16 films (Figure 10a) differ noticeably from those of the VOPcF16 films after annealing (Figure 10b). Consequently, the depolarization ratios of these bands change upon annealing as well (Table 1). Using these values, the H



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similar molecular arrangement of the VOPcF16 and PbPc films discussed above. It should be emphasized that the film structures of planar hexadecafluorinated metal phthalocyanines MPcF16 (M = Cu(II), Zn(II)) are also sensitive to the thermal annealing and temperature of the substrate surface.37,75 Similar to our case, the deposition at the elevated substrate temperature and annealing of the CuPcF16 films led to the change of morphology and size of crystallites. In contrast, the XRD patterns of the CuPcF16 films did not change significantly. Only a tiny shift in the main diffraction peak from 2θ = 6.10° (d = 1.45 nm) to 2θ = 6.31° (d = 1.40 nm) occurs upon annealing. This slight modification was due to the change of molecular tilt angle within the cofacial stacks of the molecules.76 At the same time, the structural transformations observed in the VOPcF16 films lead to a pronounced difference in the XRD patterns (Figure 1), UV−vis (Figure 4), and vibrational spectra (Figures 8 and 9).

was also supported by the Tubingen University Computing Center (ZDV). V.G.K. appreciates the support of this work by the Government of Novosibirsk region.

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(1) McKeown, N. B. In The Porphyrin Handbook; Kadish, K. M., Smith, R., Guilard, R., Eds.; Academic Press: San Diego, CA, 2003; Vol. 15, pp 61−124. (2) Henari, F. Z.; Callgham, J.; Blau, W. J.; Haisch, P.; Hanack, M. Pure Appl. Opt. 1997, 6, 741−748. (3) Henari, F. Z.; Davey, A.; Blau, W. J.; Haisch, P.; Hanack, M. J. Porphyrins Phthalocyanines 1999, 3, 331−338. (4) Petty, M. C. Molecular Electronics: From Principles to Practice; Wiley: Chichester, 2007. (5) Griffiths, C. H.; Walker, M. S.; Goldstein, P. Mol. Cryst. Liq. Cryst. 1976, 33, 149−170. (6) Ziolo, R. F.; Griffiths, C. H.; Troup, J. M. J. Chem. Soc., Dalton Trans. 1980, 2300−2302. (7) Handa, M.; Suzuki, A.; Shoji, S.; Kasuga, K.; Sogabe, K. Inorg. Chim. Acta 1995, 230, 41−44. (8) Wang, H. B.; Song, D.; Yang, J. L.; Yu, B.; Geng, Y. H.; Yan, D. H. Appl. Phys. Lett. 2007, 90, 253510. (9) Li, L.; Tang, Q.; Li, H.; Hu, W. J. Phys. Chem. B 2008, 112, 10405−10410. (10) Wang, L.; Liu, G.; Zhu, F.; Pan, F.; Yan, D. Appl. Phys. Lett. 2008, 93, 173303. (11) Huang, L.; Liu, C.; Qiao, X.; Tian, H.; Geng, Y.; Yan, D. Adv. Mater. 2011, 23, 3455. (12) Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu, D. Chem. Rev. 2012, 112, 2208−2267. (13) Engel, M. K. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2003; Vol. 20, pp 1−9. (14) Huang, T.-H. J. Phys. Soc. Jpn. 1987, 56, 1213−1222. (15) Hashimoto, S.; Ogawa, T.; Isoda, S.; Kobayashi, T. J. Electron Microsc. 1999, 48, 731−738. (16) Pan, Y. L.; Wu, Y. J.; Chen, L. B.; Zhao, Y. Y.; Shen, Y. H.; Li, F. M.; Shen, S. Y.; Huang, D. H. Appl. Phys. A: Mater. Sci. Process. 1998, 66, 569−573. (17) Hoshi, H.; Hamamoto, K.; Yamada, T.; Ishikawa, K.; Takezoe, H.; Fukuda, A.; Fang, S.; Kohama, K.; Maruyama, Y. Jpn. J. Appl. Phys. 1994, 33, L1555−L1558. (18) Hiller, W.; Strahle, J. Z. Kristallogr. 1982, 159, 173−183. (19) Minami, N.; Asai, M. Jpn. J. Appl. Phys. 1987, 26, 1754−1758. (20) Hosoda, M.; Wada, T.; Yamada, A.; Garito, A. F.; Sasabe, H. Jpn. J. Appl. Phys. 1991, 30, L1486−L1488. (21) Terasaki, A.; Hosoda, M.; Wada, T.; Tada, H.; Koma, A.; Yamada, A.; Sasabe, H.; Garito, A. F.; Kobayashi, T. J. Phys. Chem. 1992, 96, 10534−10542. (22) Fang, S.; Tada, H.; Mashiko, S. Appl. Phys. Lett. 1996, 69, 767− 769. (23) Pakhomov, G. L.; Pakhomov, L. G.; Travkin, V. V.; Abanin, M. V.; Stakhira, P. Y.; Cherpak, V. V. J. Mater. Sci. 2010, 45, 1854−1858. (24) Yu, X. J.; Xu, J. B.; Cheung, W. Y.; Ke, N. J. Appl. Phys. 2007, 102, 103711. (25) Bao, Z.; Lovinger, A. J.; Brown, J. J. Am. Chem. Soc. 1998, 120, 207−208. (26) Hiller, S.; Schlettwein, D.; Armstrong, N. R.; Wöhrle, D. J. Mater. Chem. 1998, 8, 945−954. (27) Tate, J.; Rogers, J. A.; Jones, C. D. W.; Vyas, B.; Murphy, D. W.; Li, W.; Bao, Z.; Slusher, R. E.; Dodabalapur, A.; Katz, H. E. Langmuir 2000, 16, 6054−6060. (28) Ling, M.; Bao, Z. Org. Electron. 2006, 7, 568−575. (29) Tang, Q.; Li, H.; Liu, Y.; Hu, W. J. Am. Chem. Soc. 2006, 128, 14634−14639. (30) Tang, Q. X.; Tong, Y. H.; Li, H. X.; Hu, W. P. Appl. Phys. Lett. 2008, 92, 083309.

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CONCLUSION In the present contribution, we investigated the structural evolution of VOPcF16 thin films grown by physical vapor deposition. To this end, optical spectroscopy, XRD, and AFM measurements have been employed. It was shown that asdeposited VOPcF16 films exhibit a linear cofacial structure with average tilt angles of Pc moieties ∼60°. Upon annealing at 160−220 °C, this structure irreversibly transforms to a slipped dimeric modification with average tilt angles ∼30°. The thermal annealing had a pronounced effect on electrical properties of the films: the in-plane conductivity of the films decreased by 2 orders of magnitude. Note that the edge-on orientation of VOPcF16 molecules with respect to substrate surface in the lowtemperature metastable phase is favorable for field effect transistors. In this case the charge migration between a source and a drain is enhanced. On the other hand, a parallel orientation of the VOPcF16 molecules in the slipped dimeric phase is expected to improve the performance of photovoltaic cells and light-emitting diodes. Moreover, it is natural to expect profoundly anisotropic charge carrier mobility in the film with a slipped dimeric structure. This also renders the films to be of particular interest for applications.

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ASSOCIATED CONTENT

S Supporting Information *

DFT optimized geometries of all species under study, complete ref 36, the assignments of the IR- and Raman-active vibrational modes, the far-IR spectra of VOPc, VOPc-15N, and VOPcF16, XPS and Raman spectra of VOPcF16 films before and after annealing and VOPcF16 powder, and current−voltage characteristics for the VOPcF16 film. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank W. Neu for technical support. The funding by DFG PE 546/5-1, CH 132/23-1, and RFBR (projects 11-0391336 and 12-03-31363) is gratefully acknowledged. This work I



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Optical Spectroscopy and XRD Study of Molecular Orientation

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