Structural and Electric-Optical Properties of Zinc Phthalocyanine

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Structural and Electric-Optical Properties of Zinc Phthalocyanine Evaporated Thin Films: Temperature and Thickness Effects Antonio A. Zanfolim,†,‡ Diogo Volpati,† Clarissa A. Olivati,† Aldo E. Job,† and Carlos J. L. Constantino*,† Faculdade de Cieˆncias e Tecnologia, Unesp UniV Estadual Paulista, Presidente Prudente-SP, 19060-900 Brazil, and UniVersidade Estadual do Mato Grosso do Sul, UEMS, Dourados-MS, Brazil ReceiVed: January 29, 2010; ReVised Manuscript ReceiVed: June 3, 2010

It is known that the molecular architecture plays a fundamental role in the electrical and optical properties of materials processed in the form of thin films. Here, zinc phthalocyanine (ZnPc) thin films were fabricated through the vacuum thermal evaporation technique (PVD, physical vapor deposition) up to 50 nm thickness with the objective of determining their molecular architecture and some electrical and optical properties. Structurally, the results showed a uniform growth of the films depending on how the evaporation is performed (step-by-step or straightforward). The uniform films present a molecular organization dominated by the ZnPc macrocycle ring forming almost 90° in relation to the substrate surface. These films are crystalline (R-form) and possess molecular aggregates in the form of dimers (or higher order of aggregates) and monomers. Such aggregates are seen at the nanometer scale; however, at the micrometer scale, the films are morphologically homogeneous. In relation to the optical properties, it was observed that these films, besides absorbing in the ultraviolet-visible region, present a photoluminescence when irradiated with the 785 nm laser line. In terms of electrical properties, it was determined an electrical conductivity of ca. 10-10 S/m and a significant photoconducting activity. Finally, a dependence of the molecular organization, crystallinity, and optical properties on the film annealing (and thickness) was investigated, and the sensitivity of the ZnPc PVD films against gasoline vapor was tested as proof-of-principle. 1. Introduction The search for new materials that can improve performance and reduce cost of manufacturing electronic devices has been increasing in recent years. In this sense, the organic materials, as a substitute or complement to inorganics, have provided suitable results. Within the existing classes of organics, the metallic phthalocyanines (MPc’s) are well-known by their semiconducting properties besides exhibiting thermal and chemical stability.1,2 The MPc’s have been extensively applied in electronic devices such as sensors,3 solar cells,4 nonlinear optics,5 diodes,6 transistors,7 and photodetectors.8 For instance, MPc’s have been tested in P-N junctions for applications in solar cells, leading to excellent conversion efficiency.9,10 It must be also mentioned the role played by different metallic atoms at the center of the Pc ring, which leads to a broad range of absorption and emission of these materials within the UV-visibleNIR spectrum,11,12 besides distinct electrochemical properties when applied as modifier electrodes in electrical catalysis,13,14 which is of great interest for applications in technological devices. The MPc’s have also been applied as dyes for textile shading processes15 or ink jet printing.16 Besides, MPc’s have also been investigated in photodynamic therapy (PDT), which is applied in certain cancer treatments.17 The MPc’s are compounds that contain highly conjugated macrocycles, consisting of four isoindol units connected by nitrogen atoms in the wing position. They have as general characteristics a flat and symmetrical molecular structure besides * To whom correspondence should be addressed. E-mail: case@ fct.unesp.br. † Unesp Univ Estadual Paulista. ‡ Universidade Estadual do Mato Grosso do Sul.

exhibiting polymorphism.1 Because most of the electronic devices based on organic semiconductors use these materials as transducers in the form of thin films, it is particularly relevant to the technological applications the possibility of the MPc’s to form thin films such as Langmuir-Blodgett (LB),18-20 selfassembly or layer-by-layer (LbL),21-23 vacuum evaporated,24-26 casting,27 spin-coating,28 electrodeposited,29 and so on. Besides, because the structural arrangement of the molecules in the thin film plays an important role on their own electrical and optical properties, the determination of the thin film molecular architecture is an issue to be considered, especially in the case of MPc’s.30 However, many of the MPc’s are difficult to dissolve using conventional organic solvents, which leads to changes in their molecular structures by adding functional groups to the macrocycle to make the MPc’s soluble.31 For instance, nothing was found in the literature on layer-by-layer or drop casted films for ZnPc, which are the most common approaches to produce thin films from solutions. Thus, the physical vapor deposition (PVD) technique, taking advantage of the high thermal stability of MPc’s, is a suitable alternative for the fabrication of thin films of MPc’s without requiring changes in their molecular structures. Zinc phthalocyanines (ZnPc’s) in the form of PVD films have been investigated, and it is found that their structural and electric-optical properties are deeply dependent on experimental conditions such as evaporating rate, thickness, substrate, and annealing. For instance, Yanagi et al.32 showed that ZnPc films deposited at 3 nm/min under 10-4 Pa on glass (0.33 nm), NaCl (1.60 nm), and highly oriented pyrolytic graphite (HOPG) (1.40 nm) substrates exhibited different molecular orientations being perpendicular on glass, parallel on NaCl, and tilted on HOPG

10.1021/jp1008913  2010 American Chemical Society Published on Web 06/24/2010

Properties of ZnPc Evaporated Thin Films substrate surfaces. Besides, in the same work, it was shown how these different orientations affect the optical properties leading to displacements of the absorption band maxima and changes in their relative intensities. Gaffo et al.33 using 80 nm ZnPc films deposited under 10-6 Torr on glass and ZnSe (evaporation rate not given) showed a phase transition from R to β upon film annealing at 200 °C for 3 h in ambient atmosphere. The annealing treatment also contributed to improve the uniformity of the film. Gould34 revealed that films deposited at room temperature (100 nm thickness, evaporation rate and substrate not given) usually are in the metastable R-phase. However, they undergo a phase transition to β-form by annealing (250-300 °C for 2 h), and these structural features present different conductivity values. Zeyada and El-Nahass35 demonstrated how the annealing (613 K for 2 h) rearranges the ZnPc molecular stacks, improving the crystallinity. It was also shown that the as-deposited films on glass substrate (evaporation rate at 0.5 nm/s under 2 × 10-4 Pa, thickness not given) present a nanocrytallite structure whose orientation is affected by annealing, leading to an increase of the dielectric permittivity. Still, in the study of Zeyada and ElNahass, it was found an ohmic conduction at low voltages and exponential trap space charge limited conduction at higher voltages for Au/ZnPc/Au sandwich structures. Besides, the dc conductivity decreases with increasing film thickness (52 and 540 nm) and increases for temperatures ranging from 400 up to 435 K. Senthilarasu et al.36 observed that the degree of crystallinity and grain size increase with the film thickness while the substrate temperature influences the film quality (evaporation rate at 1 Å/s under 6 × 10-6 mbar and thickness ranging from 10 up to 1000 nm). Senthilarasu et al.37 also observed a smooth surface and crystalline nature with the crystallite size being less than 100 nm for 30 nm ZnPc films deposited at 1 Å/s under 10-6 mbar. Besides, a metastable R- to stable β-phase transformation was observed when the films were deposited at higher substrate temperatures (200 °C). In this work, considering the application of organic semiconductors in the form of thin films whose electrical and optical properties are strongly influenced by their molecular architecture, ZnPc PVD films were fabricated and structurally characterized by different techniques. The molecular architecture of the PVD films was determined in terms of growth at the nanometer thickness scale (UV-vis absorption (ex situ) and quartz crystal balance (in situ)), morphology at nano- (AFM) and micrometer (micro-Raman) scales, molecular organization (FTIR via transmission and reflection modes), and crystallinity (X-ray diffraction). In addition, the determination of the electrical conductivity of ZnPc films and photoconducting effects was performed, and some effects regarding temperature and film thickness on molecular organization and crystalline structure were investigated. The effect of the growth methodology (evaporation in steps of 10 nm or straightforward) is also discussed. The final idea is to generate subsidies for applications of these films in electronic devices based on organic semiconductors and gas sensors. 2. Experimental Section 2.1. PVD Films. ZnPc was acquired from Kodak (MW 577.91 g/mol) and used as purchased. The ZnPc PVD films were grown using the vacuum thermal evaporation technique in a Boc Edwards model Auto 306 machine. The growth process consists of placing the ZnPc powder in a metallic boat (Ta in this case, melting point of 3017 °C), where an electric current is passed through. The substrate and the quartz crystal balance

J. Phys. Chem. C, Vol. 114, No. 28, 2010 12291 are placed parallel and positioned 15 cm above the Ta boat. The evaporation process is performed within a vacuum chamber under 10-7 Torr. The electric current was adjusted slowly from 0.0 up to 2.2 A (10 V) to allow heating the Ta boat until reaching a temperature of approximately 410 °C measured with a thermopar. Then the ZnPc starts evaporating, and when a rate between 0.5 and 1 nm/s is reached, the quartz crystal balance is brought to zero value and the shutter that protects the substrate is opened, allowing the growth of the PVD film until the desirable mass thickness.25,38,39 It is important to mention that these ZnPc PVD films were grown up to 50 nm in steps of 10 nm because, when trying to deposit films with 20, 30, 40, and 50 nm through one step, the uniformity (absorbance vs mass thickness) was lost. Figures SI1, SI-2 and SI-3 show, respectively, UV-vis absorption spectra recorded for different mass thicknesses, UV-vis absorption spectra recorded for 40 nm PVD films grown in distinct days, and atomic force microscopy (AFM) images which reveal the effects of the one step (straightforward) methodology. Figures SI-1 and SI-3 must be compared with Figures 2 and 3 of this Article, respectively. Complementarily, for subsidiary experiments, a ZnPc PVD film with 400 nm was also grown through steps of 10 nm. 2.2. Characterization Techniques. Thermogravimetric (TG) measurements were carried out using NETZSCH equipment (model 209), 5.0 mg of ZnPc powder, N2 atmosphere (20 mL/ min), and heating rate at 10 °C/min until 900 °C. UV-vis absorption spectroscopy was performed in a Varian model Cary 50 spectrophotometer between 190 and 800 nm for ZnPc PVD films with thicknesses from 10 up to 50 nm grown with steps of 10 nm onto quartz substrates. The Fourier transform infrared (FTIR) spectra were recorded using a Bruker model Vector 22 spectrometer between 600 and 4000 cm-1, spectral resolution of 4 cm-1, and 128 scans for 40 nm ZnPc PVD films deposited simultaneously onto ZnSe (transmission mode) and Ag mirror (reflection mode) in N2 atmosphere and ZnPc powder dispersed in KBr pellet (1 g of ZnPc: 330 g of KBr). The Raman scattering spectra were collected using a micro-Raman Renishaw model in-Via spectrograph, coupled to a Leica optical microscope equipped with a 50× objective lens leading to a spatial resolution at ca. 1 µm2. The spectrograph is equipped with laser lines at 514.5, 633, and 785 nm and grating with 1200 and 1800 grooves/mm, leading to spectral resolution at ca. 4 cm-1. Different powers of the incoming laser (µW range) and accumulations were used to improve the signal/noise ratio. X-ray diffraction was conducted using a Rigaku diffractometer for ZnPc powder and for 40 and 400 nm PVD films deposited onto BK-7 glass. The AFM images were obtained with a Digital Instrument model Nanoscope IV instrument via tapping mode for scanning areas of 11 × 11 µm2 and 550 × 550 nm2 for 40 nm ZnPc PVD films deposited onto BK-7 glass previously heated for 2 h at 600 °C to decrease its irregularities, leading to a final roughness at 2.3 Å. The I × V dc measurements were carried out using Keithley 238 equipment for 100 nm ZnPc PVD films deposited onto Au interdigitated electrodes (capacitors) to determine their conductivity. These PVD films were also illuminated with a halogen lamp at 17 mW/cm2 to investigate the photoconductivity. 3. Results and Discussion 3.1. Powder Thermal Stability: TG. The TG technique was used to determine the thermal stability of ZnPc powder, since the preparation of PVD films involves heating the material until its evaporation. Figure 1 shows the characteristic TG curve with

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Figure 1. TG and DTG curves recorded for the ZnPc powder obtained under N2 atmosphere and heating rate at 10 °C/min. Inset: molecular structure of the ZnPc.

TABLE 1: Mass Loss and Assignments for the ZnPc Powder T (°C)

mass loss (%)

assignments

212-497 497-662 662-800

4.5% 62.60% 25.52%

organic material C32H16N8 residual mass Zn3N2

the ZnPc mass loss as a function of temperature and the first derivative of this variation (DTG). Table 1 displays the values of the mass loss and the respective assignments.40 The TG curve in Figure 1 reveals a high thermal stability of ZnPc, since until 500 °C less than 5% of the mass is lost and the maximum rate of material degradation occurs at ca. 652 °C (DTG). At temperatures exceeding 800 °C, the ZnPc is already completely degraded with residual mass of less than 10%. 3.2. Film Growth, Aggregates, and Optical Properties. It is known that the thickness and the molecular organization influence the final properties of the thin films. So it is desirable that these films have thickness and growth monitored in a way that similar amounts of material are deposited per unit of thickness (nm). Figure 2 presents the UV-vis absorption spectra

Figure 2. UV-vis absorption spectra recorded for ZnPc PVD films with different mass thicknesses (steps of 10 nm). Inset A: absorption x mass thickness for the PVD films. Inset B: fluorescence for the 40 nm ZnPc film excited with the 785 nm laser line.

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Figure 3. (a) Height and (b) amplitude AFM images for the 40 nm ZnPc PVD film (steps of 10 nm). (c) Height AFM image in 3D whose step was produced by removing the film along a line by scratching. (d) Profile present in 2D.

for ZnPc PVD films with 10, 20, 30, 40, and 50 nm mass thicknesses evaporated in successive steps of 10 nm onto quartz plates. The UV-vis spectra are characteristic of phthalocyanines with the bands B at lower wavelengths and Q at higher wavelengths. The B band (or Soret) with a maximum at 340 nm and a shoulder at 293 nm points to the coexistence of monomers and dimers (or higher order of aggregates).1 The B band (or Soret) with a maximum at 340 nm and a shoulder at 293 nm is assigned to πfπ*.41 On the other hand, it is found in the literature both assignments for the Q-band, that is, πfπ*42 and nfπ*.41 However, there is a good agreement that the maximum at lower wavelength (633 nm in this case) is related to dimers and higher orders of aggregates and that at higher wavelength (699 nm in this case) is related to monomers.42-44 Besides, a linear growth of the maximum absorption at 340 nm with the thickness of the films is seen in inset A in Figure 2, which indicates that similar quantities of material have been transferred to the substrate, as desirable. Complementary to UV-vis absorption, inset B in Figure 2 shows the fluorescence of the ZnPc PVD films with 40 nm using the 785 nm laser line as the exciting irradiation whose maximum emission is at the near-infrared (ca. 915 nm). According to Ogunsipe et al.,45 the fluorescence found for phthalocyanines and their derivatives is assigned to the monomeric species. 3.3. Film Surface Morphology at Nano- and Micrometer Scales. The morphology of the ZnPc PVD films deposited on treated glass substrates was studied at the nanometer scale using the AFM technique. In Figure 3a and b are shown images of height and amplitude, respectively. In Figure 3c is presented the topography of the film surface area in three dimensions where a step produced by removing the film along a line is observed. The profile produced by removing the film is shown in two dimensions in Figure 3d. This procedure allowed inferring the average thickness of the ZnPc PVD film, which is in good agreement with the value read in situ by using the quartz crystal balance (40 nm) within the evaporator machine. Furthermore, it is observed a homogeneous distribution of molecular aggregates varying between 20 and 70 nm in size for the analyzed surfaces whose average roughness of the film surface (rms) was around 9 nm. Results reported for PVD films with distinct thicknesses and central metallic atoms such as 200 nm of CuPc on Si46 and 10 nm of FePc on glass48 presented similar molecular aggregates at nanometer scale.

Properties of ZnPc Evaporated Thin Films

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Figure 4. (a) Optical image with 500× magnification for the 40 nm ZnPc PVD film (steps of 10 nm). In the center of the image is shown a line with 100 µm where RRS spectra were collected point-by-point with step of 1 µm. (b) One of the RRS spectra (633 nm) collected from the line shown in (a). (c) All 101 RRS spectra collected from the line shown in (a) composing the called line Raman mapping (chemical image).

The micro-Raman technique was used to determine the morphology at the micrometer scale of the 40 nm ZnPc PVD film deposited onto a quartz plate. The 633 nm laser line was applied to collect spectra along a line of 100 µm every 1 µm. Figure 4a shows the optical image (500× magnification) of the film surface where the line on which the Raman spectra were collected is highlighted in the center of the optical image. One of these spectra is given in Figure 4b, which is named resonance Raman scattering (RRS) spectrum since the 633 nm laser line is in full resonance with the UV-vis absorption of the ZnPc PVD film (Figure 2). Figure 4c brings the called chemical image since it contains all the 101 RRS spectra collected along the line highlighted in Figure 4a. The difference in intensity for the RRS bands is approximately 8.5%. Therefore, it can be observed from both optical image and Raman mapping (chemical image) that the ZnPc PVD film is pretty homogeneous morphologically (absence of changes in height or domains) and chemically (same profile for the Raman signal). The latter is consistent with the AFM data (roughness and aggregate size). The defect in the middle of the mapping was intentionally produced by increasing the laser power to stress the homogeneity of this film. Similar homogeneity was found for PVD films of FePc47 evaporated under the same conditions of the ZnPc presented here. 3.4. Film and Powder Crystallinity. The phthalocyanine molecules in thin films are usually stacked, forming columns with the ring tilted in relation to the column vertical axis. The most common polymorphic forms are the metastable R and the stable β.48 The interplanar distance for both forms is coincident (3.4 Å). The differences are in the angle of the molecule in relation to the column vertical axis being 26.5° for the R-form and 45.8° for the β-form and in the lattice parameters being a ) 23.9 Å and b ) 3.8 Å for the R-form and a ) 19.4 Å and b ) 4.79 Å for the β-form.49 The crystalline structure is monoclinic with two MPc molecules per unit cell for β-form,50 while there is some uncertainty in relation to the R-form. Robinson and Klein51 suggested a tetragonal structure; Assour52

Figure 5. (a) X-ray diffraction obtained for the ZnPc powder and 400 nm PVD film. (b) FTIR spectra recorded for the ZnPc powder in KBr pellet and 40 and 400 nm PVD films. (c) Raman spectra recorded for the ZnPc powder and 40 and 400 nm PVD films. All the ZnPc PVD films were grown in steps of 10 nm.

proposed the orthorhombic one, while Ashida et al.53 suggested the monoclinic one, which is more accepted. Figure 5a presents the X-ray diffractograms for the ZnPc powder and the 400 nm PVD film deposited onto a glass substrate. X-ray measurements were also taken for the 40 nm ZnPc PVD film; however, diffractograms were not acquired due to the low thickness. Thus, the 400 nm PVD film was used to support the discussions made below through FTIR and Raman scattering regarding crystallinity for the 40 nm ZnPc PVD film. Analyzing the ZnPc diffractograms in Figure 5a and based on the work by El-Nahass et al.,54 the powder was identified as β-phase. On the other hand, the 400 nm PVD film displays only the peak at ca. 2θ ) 6.94° corresponding to the (200) plane, which suggests the R-phase based on Ashida et al.,53 Uyeda et al.,55 and Debe et al.50 These authors showed that the structure

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Figure 6. (a) FTIR spectra recorded for the ZnPc powder in KBr pellet and 40 nm PVD films deposited onto ZnSe (transmission mode) and onto Ag mirror (reflection mode) in steps of 10 nm. (b) Illustration of the surface selection rules with the electric field orientation, in both cases transmission and reflection. (c) Illustration of the ZnPc molecular organization in the PVD films.

of vacuum evaporated ZnPc films is also monoclinic with the spacing of 1.3 nm between the (200) planes. The FTIR technique can also be used to identify phthalocyanine polymorphic forms.56 The main characteristics of the spectra that distinguish the different crystalline forms of MPc’s are in the region between 700 and 800 cm-1.56,57 Figure 5b presents the FTIR spectra for ZnPc powder in KBr pellet and for PVD films with 40 and 400 nm evaporated onto ZnSe. The intense peak at 727 cm-1 for the powder corresponds to the C-H angular deformation out-of-plane, and according to Sindu Louis et al.58 this band can give information on the crystalline structure of the ZnPc. Based on results of El Nahass et al.54 and Sindu Louis et al.,58 the peak position at 727 cm-1 for the ZnPc powder indicates that this sample is in the β-phase. The peaks at 779 and 876 cm-1 that are also assigned to C-H angular deformation out-of-plane indicate that the ZnPc powder is in the β-phase according to El-Nahass et al.59 On the other hand, the peak position at 721 cm-1 also assigned to C-H angular deformation out-of-plane and that appears for both 400 and 40 nm ZnPc PVD films indicates that the ZnPc is in the R-phase based on El-Nahass et al.59 and Gordan et al.60 The FTIR results for either powder or PVD film are in full agreement with the X-ray diffraction discussed in Figure 5a. The Raman scattering technique was also used to confirm the ZnPc polymorphic forms suggested by X-ray and FTIR techniques. The Raman spectra in Figure 5c for ZnPc powder and for PVD films with 40 and 400 nm evaporated onto ZnSe present certain differences comparing powder and PVD films. For instance, the bands at 420 and 717 cm-1 are present only in the powder spectrum, and some other bands, which are marked by dashed squares in Figure 5c, have their relative intensities strongly affected when comparing powder and PVD films. On the other hand, the similarity between 40 and 400 nm PVD films suggests that both have the same crystalline

structure while the differences between these PVD films and powder suggest these samples (powder and PVD films) have different crystalline arrangements. The latter findings support which was determined through X-ray and FTIR (Figure 5a and b). 3.5. Film Molecular Organization. The ZnPc molecular organization in the PVD films was determined by FTIR in the transmission and reflection modes. Figure 6a presents the FTIR for ZnPc in KBr pellet and 40 nm ZnPc PVD films deposited onto ZnSe (transmission mode) and onto Ag mirror (reflection mode). The assignments for the main ZnPc FTIR bands are given in Table 2 based on several articles since different assignments are found for the same vibration.20,26,42,58,60-72 The FTIR spectrum of the powder is given as a reference for a system with a random molecular organization.73 Therefore, the differences found in the relative intensities of several FTIR bands when comparing both powder and PVD film (transmission mode) allow concluding that the PVD film is anisotropic in terms of molecular organization. The latter is confirmed by the significant inversion of the relative intensities of the bands at 721 and 752 cm-1 comparing both transmission and reflection FTIR spectra for the 40 nm PVD films. This anisotropy is induced by the technique of fabricating the film,47 and the specific molecular organization can be determined combining the FTIR data and the surface selection rules,29,74 which are illustrated in Figure 6b and briefly described as follows: (i) in the transmission mode, the electric field of the incident radiation is parallel to the surface of the substrate (E|) since the radiation beam is propagating perpendicularly to the substrate surface; (ii) in the reflection mode, considering the metal used (Ag) and the radiation incident angle (80°), the electric field is polarized preferentially perpendicular to the substrate surface (E⊥); (iii) the intensity (I) of the absorbed radiation is given by the scalar product between the electric field (E) and the variation induced



TABLE 2: Assignments of the Main FTIR Bands for ZnPc Powder in KBr Pellet and in 40 nm PVD Film on ZnSe powder (cm-1)

PVD (cm-1)

assignments

727 752 779

721 752

C-H out-of-plane angular deformation26,42,58,61-63 Pc ring20,62,64,65 CNC pyrrole stretching66 C-H out-of-plane angular deformation42,60,67 CN stretching67,68 benzene breathing62 C-H out-of-plane angular deformation67 C-H in-plane angular deformation;62 CN pyrrole stretching;67 isoindol deformation64 C-H in-plane angular deformation65,67,68 pyrrole stretching68 C-H in-plane angular deformation58,63,67,68 C-H in-plane angular deformation20,58,66 CN in-plane deformation67,68 C-H in-plane angular deformation;20 CN stretching;66 CN isoindol stretching67,68 CC in-plane angular deformation;42 C-H in-plane angular deformation42,66 pyrrole stretching62,67,68,70,71 C-H in-plane angular deformation66 isoindol stretching;64 CNC stretching, pyrrole expansion, C-H in-plane angular deformation42 isoindol stretching;72 C-H in-plane angular deformation67,68 isoindol stretching;65 CC benzene stretching62

876 1059

1062

1087 1117 1164

1091 1119 1165

1285

1286

1332 1409

1333 1417

1453 1482

1456 1487

in the molecular dipole moment (µ′) by the incident radiation, that is, I ) b E ·b µ′. Therefore, the dipole moments whose µ′ is parallel to the surface substrate will be stronger in the transmission mode (E| is parallel to µ′), and those dipole moments whose µ′ is perpendicular to the surface substrate will be stronger in the reflection mode (E⊥ is parallel to µ′). In the case of the FTIR spectra for the ZnPc PVD films (transmission and reflection modes), the main differences are found for the relative intensities of the bands at 721, 752, 1119, and 1333 cm-1. The bands at 721 and 1119 cm-1 are assigned to C-H angular deformation out-of-plane and in-plane, respectively (in-plane and out-of-plane refer to the macrocycle plane of the ZnPc). The bands at 752 and 1333 cm-1 are mainly assigned to deformation of the macrocycle ring and stretching of the pyrrole group in the plane of the macrocycle, respectively. This suggests that the ZnPc molecules are preferentially organized in the PVD films with the macrocycle ring practically perpendicular to the surface substrate as illustrated in Figure 6c. The latter is determined based on the fact that the out-ofplane C-H band (721 cm-1) dominates the spectrum in the transmission mode and becomes weaker in the reflection mode. Besides, the in-plane bands at 1119 and 1333 cm-1 present an opposite trend; that is, they dominate the spectrum in the reflection mode and become weaker in the transmission mode. Several works regarding MPc molecular organization in PVD films26,47,73,75-77 report that these molecules usually assume a preferred orientation; however, the specific molecular organization depends on factors such as the metallic central atom and the experimental conditions of the film fabrication such as deposition rate and substrate temperature. 3.6. Film Electrical Properties (dc). The electrical conductivity for 100 nm ZnPc PVD film deposited onto Au interdigitated electrodes, parallel contact (inset in Figure 7a), was determined through dark current × voltage curves (I × V dc), which are shown in Figure 7a. A linear trend, characteristic of ohmic behavior, is observed. A similar result was found by Rajesh and Menon78 studying electrical and optical properties of MnPc PVD films. It is known that the conductivity and driving mechanism depend on the nature of the metal contacts.79 Here, the conductivity was calculated from a linear adjustment of the I × V curve using the model developed by Olthuis et al.,80 and a value at 1.2 × 10-10 S/m was obtained, which is in agreement with literature for thermally evaporated ZnPc films.81 The same ZnPc PVD film deposited onto Au interdigitated electrodes was used for photoconductivity measurements. Figure

7b shows the I × t curve (applied voltage -5 V) for this configuration, and it is possible to observe the enhancement of ZnPc conductivity, when illuminated with a halogen lamp (∼17 mW/cm2). The photoconduction effect is clear, since the conductivity increases by about 1 order of magnitude, from 1 × 10-10 to 2.7 × 10-9 S/m. The increase and decay processes seem to show the exponential behavior, which has been found typical for organic semiconductor films.82-84 The application

Figure 7. (a) Dark I × V curves for the 100 nm ZnPc PVD film deposited onto Au interdigitated electrodes (steps of 10 nm). Inset: cartoon of the Au interdigitated electrode covered by ZnPc PVD film. (b) I × t curve for the 100 nm ZnPc PVD film deposited onto Au interdigitated electrodes when irradiated with a halogen lamp at 17 mW/cm2.

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Figure 9. Raman spectra recorded for the ZnPc powder (room temperature) and for 400 and 40 nm PVD films before and after annealing at 200 °C for 2 h. All the ZnPc PVD films were grown in steps of 10 nm.

Figure 8. (a) FTIR spectra (reflection mode) recorded for the 40 nm PVD films before and after annealing at 200 °C for 2 h. (b) FTIR spectra (transmission mode) recorded for ZnPc dispersed in KBr pellet, 40 nm PVD film before annealing and 400 nm PVD film before and after annealing. All the ZnPc PVD films were grown in steps of 10 nm.

of a band-type model for electronic conduction process in phthalocyanine films has been reported.34 In this context, the photoconductivity behavior is similar to the electronic structure in terms of a semiconducting band model, indicating photogeneration of free carriers via interband transition. After a fast initial decay, the excess of photoinduced carriers approaches the equilibrium dark with a higher rate of decay. This effect was also observed for VOPc (vanadyl phthalocyanine).85 3.7. Temperature (and Thickness) Effects. The effects that the heating may provoke in the ZnPc PVD films were investigated by FTIR and Raman scattering before and after annealing the films up to 200 °C at environmental atmosphere for 2 h. The main objective is to determine the consequences of the temperature on the film structural properties such as molecular organization, crystalline structure, and balance between molecular monomers and aggregates. The FTIR spectra in reflection mode, before and after heating, given in Figure 8a for a 40 nm PVD film show a strong increase in the relative intensity of the band at 722 cm-1 (C-H out-of-plane angular deformation) and a strong decrease in the relative intensity of the band at 752 cm-1 (macrocycle ring deformation) due to annealing. Moreover, the bands at 1064, 1093, 1120, 1166, and 1286 cm-1 (C-H in-plane angular deformation) and the band at 1335 cm-1 (pyrrol in-plane stretching) also suffered a strong reduction. These changes reveal a drastic alteration in the molecular organization of the film. According to the surface selection rules previously described, the ZnPc assumes an organization with the molecules practically parallel to the substrate surface (face-on) after annealing. In terms of the crystalline structure, it can be observed in Figure 8a that the band at 722 cm-1 is practically kept at the same wavenumber after annealing, which suggests that the crystalline structure of the film remains in R-form. On the other hand, a different trend is observed for the 400 nm PVD film.

Figure 8b presents the FTIR spectra of the ZnPc in KBr pellet and 400 nm PVD film before and after annealing for 2 h (FTIR spectra for the 40 nm PVD film is given as reference). It can be observed that the 400 and 40 nm films are predominantly in the R-form before annealing. However, the 400 nm film seems to present a predominance of the β-form after annealing, acquiring a FTIR spectrum closer to that of the ZnPc in KBr pellet. The latter suggests a reduction of the thermal stability of the film with increasing thickness. Trying to confirm what was observed by FTIR, Figure 9 shows the Raman scattering spectra for the ZnPc powder (room temperature) and for the 400 and 40 nm PVD films before and after annealing at 200 °C for 2 h. The similarity between the Raman spectra for the powder and for the 400 nm film after annealing indicates that both samples have the ZnPc in the same phase (β-form mainly). Besides, the similarity between the Raman spectra for 40 nm film before and after annealing indicates that this film does not have its crystalline structure significantly affected by annealing. Finally, the similarity between the Raman spectra for 40 and 400 nm films before annealing strongly suggests that both of these films are dominated by the same crystalline structure (R-form) before annealing. These results support the conclusions extracted from FTIR data (Figure 8b); that is, the annealing at 200 °C for 2 h induces a phase transition in the 400 nm PVD film; however, the crystallinity of the 40 nm PVD films is not affected. The changes observed for the 40 nm PVD film due to annealing at 200 °C for 2 h can also be felt by the fluorescence of the film, for instance. Figure 10 brings fluorescence spectra recorded at different temperatures (22, 75, 100, and 200 °C) and using the 785 nm laser line as the exciting irradiation. However, different from the previous characterizations where the measurements were made before and after the annealing treatment, in the fluorescence measurements the spectra were recorded in situ, that is, with the film at the indicated temperature. Another difference is that the film was left during 20 min at the indicated temperature before collecting the fluorescence spectrum. It can be seen that, even for temperatures below 200 °C and with shorter treatment time (20 min instead 2 h), the fluorescence is already affected, decreasing for higher temperatures. However, collecting spectra after the annealing cycle, that is, at room temperature (22 °C) after cooling back from 200 °C, the signal intensity recovers ca. 85% of its original intensity (curves not shown to avoid making the figure too crowded). Because thermal degradation of the ZnPc can be discarded (TG, FTIR, and Raman results) and crystalline phase



Figure 10. Fluorescence spectra recorded for the 40 nm PVD film (steps of 10 nm) at different temperatures.

transition too, it could be speculated that, besides the observed change in the molecular organization (FTIR), the balance between monomers and aggregates is slightly affected if the fluorescence is mainly assigned to monomers.45 3.8. Gas Sensor Application. Envisioning future applications as gas sensors, the sensitivity of the ZnPc PVD films were tested as proof-of-principle by exposing them to gasoline vapor. Despite all the literature available on ZnPc evaporated films, relatively few papers are found regarding their application in sensing devices.86-90 The gasoline was obtained from a gas station (commercially available) and dropped in a beaker of 20 mL. The substrates containing ZnPc PVD films were put on the top of the beaker, and this system was put inside a closed recipient (500 mL total volume) at room atmosphere conditions (23 °C and 60% humidity) for 20 min. UV-vis absorption, fluorescence, and Raman spectra were recorded right before and after this procedure, and the results are shown in Figure 11a, b, and c, respectively. It can be clearly seen that the vapor affects the optical properties (absorption and emission) of the ZnPc films. According to Aroca and Battisti,91 the changes observed in the electronic absorption spectrum are related to the oxidation of the Pc ring with the adsorbed gas acting as an electron receptor. Besides, the changes in the vibrational spectrum point to π-π interactions between the adsorbed gas and the Pc ring.92 The changes in the optical properties support the application of the ZnPc PVD films as a transducer in gas sensing devices. It is worth mentioning that similar tests carried out with NiPc PVD films did not present such changes, revealing that the central atom (Zn) also plays an important role in this case. 4. Conclusions Thin films of zinc phthalocyanine (ZnPc) have been produced by vacuum thermal evaporation (PVD technique) on different types of substrate depending on the characterization and desired application. The uniform growth of the film was confirmed by UV-vis absorption spectroscopy and quartz crystal balance when the evaporation is performed in steps of 10 nm. MicroRaman and AFM revealed that morphologically the films are fairly homogeneous microscopically while at nanometer scale it was observed the presence of molecular aggregates. The latter is consistent with the UV-vis data, which indicated the presence of monomers, dimers, and higher order of ZnPc aggregates. In addition, the FTIR showed that the molecules of ZnPc present a preferential organization forming an angle ca. 90° in relation to the surface of the substrate, besides being in the R crystalline structure according to X-ray diffraction. The dc electrical

Figure 11. (a) UV-vis absorption, (b) fluorescence, and (c) Raman scattering data obtained for ZnPc PVD films before and after exposure to gasoline vapor. All the ZnPc PVD films were grown in steps of 10 nm.

measurements for ZnPc PVD films deposited onto Au interdigitated electrodes revealed an ohmic behavior with conductivity at 1.2 × 10-10 S/m and a significant photoconductor effect. Finally, it was found that, by annealing the 40 nm films up to 200 °C, the ZnPc molecules have their molecular organization changed, assuming a face-on arrangement. The fluorescence was also affected, being reduced due to annealing. However, the crystallinity of the 40 nm films was not affected. A transition from R to β form was able to be induced only by annealing films with 400 nm thickness. Finally, as proof-of-principle, the sensitivity of the ZnPc PVD films was tested by exposing them to gasoline vapor, and the results indicated their potential application as transducer in gas sensing devices. Acknowledgment. FAPESP, CNPq, and CAPES for the financial support and Prof. Dr. Paulo Noronha Lisboa Filho from Faculdade de Cieˆncias, Unesp Univ Estadual Paulista, for the X-ray facility.

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Supporting Information Available: Figure SI-1 shows the UV-vis absorption spectra recorded for ZnPc PVD films grown with different mass thicknesses using the one step methodology (straightforward), Figure SI-2 brings UV-vis absorption spectra recorded for 40 nm PVD films grown in distinct days using either the one step methodology or the step-by-step methodology and Figure SI-3 presents AFM images for 40 nm PVD films grown using either the one step methodology or the step-bystep methodology. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Leznoff, C. C. LeVer ABP, Phthalocyanines: Properties and Applications; VCH: New York, 1993. (2) Shirota, Y. J. Mater. Chem. 2000, 10, 1–10. (3) Jakubik, W. P. Thin Solid Films 2009, 517, 6188–6191. (4) Pfuetzner, S.; Meiss, J.; Petrich, A.; Riede, M.; Leo, K. Appl. Phys. Lett. 2009, 94, 253303. (5) Medina, A.; Claessens, C. G. J. Porphyrins Phthalocyanines 2009, 13, 446–454. (6) Kato, S.; Moriyama, H.; Takahashi, K.; Pac, C. J. Mater. Chem. 2009, 19, 8403–8410. (7) Rapp, L.; Diallo, A. K.; Alloncle, A. P.; Videlot-Ackermann, C.; Fages, F.; Delaporte, P. Appl. Phys. Lett. 2009, 95, 171109. (8) Ohmori, Y.; Kajii, H. Proc. IEEE 2009, 97, 1627–1636. (9) Myers, J. D.; Tseng, T. K.; Xue, J. Org. Electron. 2009, 10, 1182– 1186. (10) Ye, R. B.; Baba, M.; Suzuki, K.; Mori, K. Thin Solid Films 2009, 517, 3001–3004. (11) Tunhoo, B.; Nukeaw, J. Mater. Res. InnoVations 2009, 13, 145– 148. (12) Benten, H.; Kudo, N.; Ohkita, H.; Ito, S. Thin Solid Films 2009, 517, 2016–2022. (13) Rodriguez-Mendez, M. L.; de Saja, J. A. J. Porphyrins Phthalocyanines 2009, 13, 606–615. (14) Alencar, W. S.; Crespilho, F. N.; Santos, M. R. M. C.; Zucolotto, V.; Oliveira, O. N., Jr.; Silva, W. C. J. Phys. Chem. C 2007, 111, 12817– 12821. (15) Bachmann, F.; Basler, R. W.; Dosenbach, C.; Jeevanath, M.; Kaeser, A.; Kramer, H.; Lant, N. J.; Miracle, G. S.; Roentgen, G. Patent No. WO2009069077-A2, 04 June 2009. (16) Fujii, T.; Hirota, K. Patent No. WO2009133668-A1, 05 Nov 2009. (17) Zhao, B. Z.; Yin, J. J.; Bilski, P. J.; Chignell, C. F.; Roberts, J. E.; He, Y. Y. Toxicol. Appl. Pharmacol. 2009, 241, 163–172. (18) Chen, S.; Liu, Y.; Xu, Y.; Sun, Y.; Qiu, W.; Sun, X.; Zhu, D. Synth. Met. 2006, 156, 1236–1240. (19) Gaffo, L.; Constantino, C. J. L.; Moreira, W. C.; Aroca, R. F.; Oliveira, O. N., Jr. Langmuir 2002, 18, 3561–3566. (20) Gaffo, L.; Constantino, C. J. L.; Moreira, W. C.; Aroca, R. F.; Oliveira, O. N., Jr. Spectrochim. Acta, Part A 2004, 60, 321–327. (21) Cooper, T. M.; Campbell, A. L.; Crane, R. L. Langmuir 1995, 11, 2713–2718. (22) Zucolotto, V.; Ferreira, M.; Cordeiro, M. R.; Constantino, C. J. L.; Balogh, D. T.; Zanata, A. R.; Moreira, W. C.; Oliveira, O. N., Jr. J. Phys. Chem. B 2003, 107, 3733–3737. (23) Woojung, C.; Naito, M.; Fujii, R.; Morisue, M.; Fujiki, M. Thin Solid Films 2009, 518, 625–628. (24) Burghard, M.; Fischer, C. M.; Schmelzer, M.; Roth, S.; Haisch, P.; Hanack, M. Synth. Met. 1994, 67, 193–195. (25) Souto, J.; De Saja, J. A.; Aroca, R.; Rodriguez-Mendez, M. L. Synth. Met. 1993, 54, 229–235. (26) Gaffo, L.; Constantino, C. J. L.; Moreira, W. C.; Aroca, R. F.; Oliveira, O. N., Jr. J. Raman Spectrosc. 2002, 33, 833–837. (27) Treacher, K. E.; Clarkson, G. J.; Ali-Adib, Z.; McKeown, N. B. Chem. Commun. 1996, 1, 73–75. (28) Komino, T.; Matsuda, M.; Tajima, H. Thin Solid Films 2009, 518, 688–691. (29) Rajaputra, S.; Sagi, G.; Singh, V. P. Sol. Energy Mater. Sol. Cells 2009, 93, 60–64. (30) Claessens, C. G.; Hahn, U.; Torres, T. Chem. Rec. 2008, 8, 75–97. (31) Wiber, J. H.; Busch, D. H. Inorg. Chem. 1965, 4, 469–471. (32) Yanagi, H.; Kouzeki, T.; Ashida, M. J. Appl. Phys. 1992, 71, 5146– 5153. (33) Gaffo, L.; Cordeiro, M. R.; Freitas, A. R.; Moreira, W. C.; Girotto, E. M.; Zucolotto, V. J. Mater. Sci. 2010, 45, 1366–1370. (34) Gould, R. D. Coord. Chem. ReV. 1996, 156, 237–274. (35) Zeyada, H. M.; El-Nahass, M. M. Appl. Surf. Sci. 2008, 254, 1852– 1858.

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JP1008913

Structural and Electric-Optical Properties of Zinc Phthalocyanine

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