Phthalocyanine Nanocrystallites Thin Film - American Chemical Society

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J. Phys. Chem. C 2007, 111, 7352-7365

Templating Effects and Optical Characterization of Copper (II) Phthalocyanine Nanocrystallites Thin Film: Nanoparticles, Nanoflowers, Nanocabbages, and Nanoribbons Santanu Karan and Biswanath Mallik* Department of Spectroscopy, Indian Association for the CultiVation of Science, 2A and 2B, Raja S. C. Mullick Road, JadaVpur, Kolkata-700 032, India ReceiVed: January 14, 2007; In Final Form: March 13, 2007

The recent emergence of molecular ultrathin films as candidates for functional electronic materials and photo catalyst has prompted numerous investigations on their crystalline structure and thin film formation. This article describes the role of the effect of the nature of substrate and substrate temperatures in molecular organization of copper (II) phthalocyanine (CuPc) on gold coated quartz substrates using conventional vapor deposition at high vacuum (∼10-6 Torr). Surprisingly, the 100 nm thick CuPc films on the quartz substrates are as highly ordered as on the polycrystalline gold-coated quartz substrates. Importantly, the molecular orientation in the two cases is radically different. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) observations of the ordered crystalline films indicated that domains are grown from the bottom to the top of the film and are densely packed with little grain boundary. X-ray diffraction (XRD) pattern shows a sharp intense peak at high substrate temperature due to the β-phase of CuPc. The crystal structure was dependent on the substrate temperature. UV-vis absorption spectra of the ordered solid films were recorded, showing unique dependences on the molecular alignments depending on the substrate and film deposition temperature. The curving of nanoribbons upon exposure to an electron beam was noticed from the changes in FESEM images. The fractal dimension of the assembly of nanostructures, in the films deposited at different temperatures on various substrates, has been estimated from FESEM images. The effects of substrates and substrate temperature on the surface morphology, optical properties, and fractal dimension have been discussed.

1. Introduction Metal phthalocyanines (MPcs) consisting of a central metallic atom bound to a π-conjugated ligand. They belong to a wellstudied class of molecules and attracted a great deal of attention for a wide variety of applications, such as dyes,1 light emitting diodes (LEDs),2 solar cells,3,4 and field-effect transistors.5 MPcs are of interest as an organic semiconductor,6,7 electronically active molecules, and also because of its similarity to biological molecules such as chlorophyll and hemeoglobin. These materials have shown many interesting properties; e.g., these are chemically and thermally very stable,8,9 and most of these can easily form ordered thin films and exhibit photoconductivity,3,4 show catalytic activity,10 etc. Among a number of different MPcs, copper (II) phthalocyanine (CuPc) has been most extensively studied.4,11-20 Such studies include the use of CuPc as solar cells,4 color sensor,11 electroluninescent device,12 interfacial properties,14-17 etc. Recently we have reported the surface morphology of CuPc molecules deposited at room temperature (30 °C) on polycrystalline gold substrates and the formation of CuPc nanoparticles and nanoflowers.21 Such studies have indicated that the morphology of the CuPc film depends on the substrate, heat-treatment temperatures, and condition of substrates during film deposition. Attempts have been made to use MPc films as molecular components in a number of electronic and optoelectronic devices.3,4,22 It has been reported that the structure, morphology, electronic, and optical properties of the films are crucial for * Corresponding author. Tel: +91-33-2473-4971. Fax: + 91-33-24732805. E-mail: [email protected].

their technological applications23 and the ordering and orientation of the molecules are very important factor for the device efficiency.23 Anisotropic electrical transport properties are often observed in these molecules, and they are caused by the preferred orientation of the molecules. It has been thought24 that the anisotropy of the transport properties has to be taken into account for the device characteristics and its influence on the device characteristics was demonstrated in detail. From the view point of technological applications of MPcs, the question arises whether both of the advantagessthe low cost and the higher performance due to the ordered filmssare accessible in allorganic devices. The properties of CuPc and other variants of MPcs make them good examples for such usage. An understanding of the molecular orientation, electronic structure, and morphology of thin film systems is a prerequisite for being able to tune their electronic as well as optoelectronic properties. The molecular orientation and electronic structure of MPcs have been investigated experimentally, in various forms and on different substrates such as conducting polymer,25 silicon,26 gold,27-29 etc., as well as theoretically.30-32 Such studies have indicated that the orientation of the grains depends on the nature of the substrate, the deposition technique, heat-treatment temperatures, and conditions of the substrates during film deposition. As an example, CuPc was reported to lie flat on the Au (100) surface, but to stand upright on the polycrystalline surface16,19 while deposited under identical conditions. Tang et al.20 using vapor deposition technique, reported the sub micrometer size ribbon of copper phthalocyanine on porous aluminum oxide substrate.

10.1021/jp070302o CCC: $37.00 © 2007 American Chemical Society Published on Web 05/02/2007

Copper Phthalocyanine Nanocrystallites Thin Film

J. Phys. Chem. C, Vol. 111, No. 20, 2007 7353

Figure 1. FESEM images of sputtered gold nanoparticles: (a) as deposited gold on quartz (inset is the same image at higher magnification); (b) thin film morphology of Au-500 showing islands and pores; (c) aggregated gold nanoparticles of Au-500 (inset shows an individual island and pore at higher magnification); (d) spherical and elliptical clusters of different sizes of gold in Au-750 (inset shows one large spherical gold cluster surrounded by small gold nanoparticles). Contact mode 3-D AFM image of gold substrates Au-500 (e) and Au-750 (f).

To date, R-phase CuPc nanorods4 and submicrometer-sized β-phase CuPc ribbons20 have been reported. Many electronic and optoelectronic devices rely on the growth of molecular thin films structures on metal electrodes. However, there has been no systematic study of the growth of CuPc nanostructures on metal electrode materials using simple vacuum evaporation technique. For the better technological applications of MPcs as organic semiconductors, electronically active organic molecules, and components for molecular electronics, it is required to investigate, under various experimental conditions, the interactions of these molecules with various electrode materials like gold, silver etc. We have tried to form crystalline CuPc in several ways, such as, starting with bare quartz, using gold template, varying the substrate temperature, etc. In the present work, we have partially overcome the shortcomings of making crystalline CuPc using a very simple vacuum deposition technique and have achieved

well-organized crystalline CuPc. Additionally, R-phase nanoribbons at lower substrate temperature and long β-phase CuPc nanoribbons at higher substrate temperature have been found. The R-phase cabbage and flower like structures were found on gold template for film deposition at lower substrate temperatures. The obtained morphologies were strongly dependent on the substrate temperature and the nature of the gold template. The fractal dimension of the assembly of nanostructures in the films has been estimated from FESEM images. The effects of substrates and substrate temperature on the surface morphology, optical properties, and fractal dimension have been studied extensively. 2. Experimental Section Gold single crystal with (111) surface orientation was used as a target. Clean gold surfaces were prepared on the surface of clean quartz plates by D.C. sputtering (7 kV, 10 mA) for 4

7354 J. Phys. Chem. C, Vol. 111, No. 20, 2007

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Figure 2. FESEM images of vacuum deposited CuPc thin films at room temperature: (a) CuPc nanoparticles deposited on quartz and (b) at higher magnification; (c) dense and aggregated CuPc nanoparticles deposited on Au-500 and (d) at higher magnification; (e) spherical and elliptical CuPc nanoflowers of different sizes deposited on Au-750 and (f) CuPc nanoflower at higher magnification.

min followed by annealing for 1 h at 500 and 750 °C under vacuum. The substrates annealed at 500 and 750 °C are referred as Au-500 and Au-750, respectively. The CuPc source powder (in β form, dye content ∼97%, obtained from Aldrich, U.S.A.) contained in a molybdenum boat was resistively heated in the high vacuum chamber. The source CuPc powder was used after repeated degassing of the source prior to deposition. Deposition was noted to occur when the chamber pressure could no longer reduce to the base pressure and the current through the boat was kept constant. All CuPc films were deposited at a chamber pressure of ∼10-6 Torr. The thickness of the deposited CuPc films and the rate of deposition were maintained at 100 nm and 0.6 Å/s, respectively using a quartz crystal microbalance (Hitech, model DTM-101). The temperature of the substrates during deposition of CuPc was adjusted in the range of 30280 °C using a high-precision temperature controller (Eurotherm

2404, U.K.). The thin films have been characterized by optical absorption, X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). X-ray diffraction (XRD) study of the CuPc films deposited at different substrate temperatures were recorded at a scan rate of 0.05° per s by using a Seifert XRD 3000P Difractrometer with Cu-KR radiation (0.15418 nm). UV-vis scanning spectrophotometer (UV-2401 PC, Shimadzu, Japan) was used to record the electronic absorption spectra of the films at room temperature. FESEM (Model: JSM-6700F, JEOL, Japan) was used to record the scanning electron micrograph images of the CuPc thin films. The collected sample from a thin film was dispersed in water and cast on carbon coated copper grid and dried in vacuum for overnight. The images were then taken by a Transmission Electron Microscope (TEM) (Model: JEM-2010, JEOL, Japan). To estimate the surface



Figure 3. FESEM images of vacuum deposited CuPc thin films at 85 °C: (a) CuPc nanoribbons deposited on quartz and (b) at higher magnification; (c) aggregated CuPc nanoparticles deposited on Au-500 and (d) at higher magnification; (e) CuPc nanocabbages of different sizes deposited on Au-750 and (f) CuPc nanocabbage at higher magnification.

roughness of the thin films, the contact mode Atomic Force Microscopy (AFM) images were taken using NT-MDT (Solver PRO-M), Moscow, Russia. 3. Results and Discussion 3.1. Preparation of Gold Template. The FESEM and AFM images of the gold thin films are shown in Figure 1. Figure 1a shows the FESEM image of as prepared gold nanoparticles on quartz. The particles are almost spherical in shape and the average size is around 30 nm. The same image at higher magnification is shown in the inset of Figure 1a, which gives a little insight about the upper surface of the thin film. The FESEM image in Figure 1b shows the morphology of the annealed (at 500 °C) gold thin film (Au-500). Clearly there is some islands (300-500 nm) of gold nanoparticles and some

pores (150-200 nm) which are distributed in random order, but the distribution of the islands and pores are same throughout the whole film. Figure 1c shows the aggregated gold nanoparticles of Au-500. The inset of Figure 1c shows such islands (white spots) and pores (black spots) at higher magnification. The region between islands and pores is covered by nanoparticles of gold with average size of around 30-40 nm. The FESEM image in Figure 1d shows the morphology of the gold thin film annealed at 750 °C (Au-750). Here we have found some bigger particles of some spherical and elliptical shapes and with different sizes ranging from 100 to 500 nm. The inset of Figure 1d represents one such spherical particle of size nearly 320 nm surrounded by nanosized particles with sizes from 10 to 20 nm at higher magnification. Panels e and f of Figure 1 represents the contact mode AFM three-dimensional images for


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Figure 4. FESEM images of CuPc thin films deposited at 155 °C: (a) CuPc nanorods deposited on quartz and (b) vertical nanorods at higher magnification; (c) CuPc nanoribbons deposited on Au-500 and (d) twisted nanoribbons at relatively higher magnification; (e) more dense packing of CuPc nanoribbons on Au-750 and (f) twisted nanoribbons at higher magnification.

Au-500 and Au-750 surfaces, respectively. The calculated rms roughness of Au-750 is significantly higher than that of Au500 substrate (rms roughness of Au-500 is 2.1 nm and that of Au-750 is 85.0 nm). 3.2. CuPc Thin Films and FESEM Images. 3.2.1. Deposition at 30 °C. FESEM images of CuPc thin films deposited on various substrates at room temperature (30 °C) are shown in Figure 2. The image of CuPc thin film deposited on bare quartz is shown in Figure 2a, which shows clearly almost uniform distribution of some nanosized particles. Figure 2b shows the same image of nanoparticles (of size 25-30 nm) at higher magnification. In Figure 2c, dense and aggregated packing of CuPc nanoparticles has been found to be deposited on Au-500 substrate at room temperature. Figure 2d represents the stacking of nanoparticles at relatively higher magnification. The average particle size estimated is nearly 40-60 nm. In Figure 2e some

flower-like structures of CuPc on substrate Au-750 at room temperature with different shapes and sizes ranging from 200 nm to 1 µm have been found to be deposited regularly. The residual position on the film has been found to be uniformly covered by the nanoparticles. Figure 2f shows one such nanoflower of size (diameter) around 900 nm. The flower consists of some nanorods of CuPc of diameter nearly 25 nm and of few nanometers in length. The structures of such nanoflowers arise due to the interaction of CuPc molecules with annealed gold template (Au-750) having spherical and elliptical particles of various sizes ranging from few nano to micrometer. The initial nucleation of CuPc molecules occurs at room substrate temperature on the gold template and form flower like structure through the process of self-organization. 3.2.2. Deposition at 85 °C. FESEM images of CuPc thin films deposited at 85 °C substrate temperature are shown in Figure



Figure 5. FESEM images of CuPc thin films deposited at 200 °C: (a) CuPc nanoribbons deposited on quartz and (b) twisting and cross linking of nanoribbons; (c) CuPc nanoribbons deposited on Au-500 and (d) bending of nanoribbon like half circle; (e) branches of CuPc nanoribbons on Au-750 and (f) some nanoribbons at higher magnification.

3. The image of CuPc thin film deposited on bare quartz is shown in Figure 3a, which shows clearly the dense and uniform packing of nanosized ribbons. Figure 3b shows the same image at higher magnification which contains some horizontally aligned nanoribbons of size 25-30 nm and of 200 nm in length. In Figure 3c we can see the dense packing of CuPc nanoparticles deposited on Au-500 at 85 °C substrate temperature. Figure 3d represents the stacking of nanoparticles at relatively higher magnification. The average particle size estimated is nearly 50 nm. In Figure 3e some cabbage-like structures of CuPc on Au750 at 85 °C substrate temperature with different shapes and sizes ranging from 100 to 800 nm has been found to be deposited regularly. The residual position on the film has been found to be uniformly covered by the CuPc nanoribbons. Each cabbage consists of some aggregated nanoribbons of CuPc. Figure 3f shows one such cabbage of size around 650 nm. The structures

of such nanocabbages arise due to the interaction of CuPc molecules with annealed gold template (Au-750) having spherical and elliptical particles of various sizes. The nucleation of CuPc molecules at relatively higher deposition temperature causes to aggregate the molecules along the direction parallel to the gold template and form cabbage like structure. 3.2.3. Deposition at 155 °C. FESEM images of CuPc thin films deposited at 155 °C substrate temperature are shown in Figure 4. The image of CuPc thin film deposited on bare quartz is shown in Figure 4a, which shows clearly the dense and nonuniform packing of nanorods. Figure 4b shows the same image at higher magnification which contains some vertically aligned nanorods of edges nearly 25 nm. Figure 4c shows the horizontal and vertical packing of CuPc nanoribbons deposited on Au-500 at 155 °C substrate temperature. Figure 4d represents few twisted nanoribbons at relatively higher magnification. The


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Figure 6. FESEM images of CuPc thin films deposited at 250 °C: (a) nanoribbons and flakes like structure of CuPc on quartz and (b) nanoribbons at higher magnification; (c) CuPc deposited on Au-500 and (d) at higher magnification; (e) CuPc nanoribbons on Au-750 and (f) nanoribbons at higher magnification.

width of the nanoribbon is nearly 20 nm, the breadth 100 nm, and the length around 1 µm. In Figure 4e more dense packing of CuPc nanoribbons on Au-750 at 155 °C substrate temperature with different length ranging from 500 nm to 3 µm has been found to be deposited regularly. Figure 4f shows some nanoribbons of width nearly 45 nm and breadth around 200 nm at higher magnification. At this higher substrate temperature the nucleation depends a little on the surface structure of gold template and they give uniform distribution of nanoribbons at 155 °C substrate temperature. 3.2.4. Deposition at 200 °C. FESEM images of CuPc thin films deposited at 200 °C substrate temperature are shown in Figure 5. The image of CuPc thin film deposited on bare quartz is shown in Figure 5a, which shows clearly the dense and uniform packing of nanoribbons. Figure 5b shows some interpenetrating and twisted nanoribbons of length ranging from

1 to 5 µm. Figure 5c shows the more dense packing of CuPc nanoribbons deposited on Au-500 at 200 °C substrate temperature. Figure 5d represents some nanoribbons of length ranging from 2 to 8 µm at relatively higher magnification. The breadth of the nanoribbons is nearly 100 to 300 nm and width is around 40 nm. Nanoribbon bents like half circle has also been found from the figure. Figure 5e shows the branches of CuPc nanoribbons on Au-750 at 200 °C substrate temperature with different length ranging from 500 nm to 3 µm. Figure 5f shows some aggregated nanoribbons of width nearly 45 nm and breadth around 300 nm at higher magnification. At 200 °C substrate temperature the gold template acts as a growth center for CuPc nanoribbons of different types depending on the annealing temperature. 3.2.5. Deposition at 250 °C. FESEM images of CuPc thin films deposited at 250 °C substrate temperature are shown in



Figure 7. FESEM images of CuPc thin films deposited at 280 °C: (a) nanoribbons and flakes like structure of CuPc on quartz and (b) nanoribbons at higher magnification; (c) CuPc deposited on Au-500 and (d) at higher magnification; (e) CuPc nanoribbons on Au-750 and (f) nanoribbons at higher magnification.

Figure 6. The image of CuPc thin film deposited on bare quartz is shown in Figure 6a, which shows clearly the dense and uniform packing of nanoribbons and nano flakes like structure of CuPc. Figure 6b shows the nanoribbons at higher magnification. The ribbons are of length ranging from 1 to 5 µm and of breadth varying from 80 to 200 nm. Figure 6c also shows the same type of structure of CuPc deposited on Au-500 at 250 °C substrate temperature. The nanoribons having larger breadth varying from 120 to 250 nm are shown in the Figure 6d, at higher magnification. Figure 6e shows the CuPc nanoribbons on Au-750 at 250 °C substrate temperature with different length ranging from 500 nm to 3 µm. Figure 6f shows some nanoribbons of breadth around 200 nm at higher magnification. 3.2.6. Deposition at 280 °C. FESEM images of CuPc thin films deposited at 280 °C substrate temperature are shown in

Figure 7. The image of CuPc thin film deposited on bare quartz is shown in Figure 7a, which shows clearly the dense and uniform packing of nanoribbons and flakes like structure of CuPc. Figure 7b shows the nanoribbons at higher magnification. The ribbons are of length ranging from 2 to 5 µm and of breadth nearly 100 nm. Figure 7c also shows the same type of structure of CuPc deposited on Au-500 at 280 °C substrate temperature. The nanoribons having breadth nearly 120 nm are shown in the Figure 7d, at higher magnification. Figure 7e shows only the CuPc nanoribbons on Au-750 at 280 °C substrate temperature with different length ranging from 500 nm to 5 µm. Figure 7f shows some nanoribbons of breadth around 70-200 nm at higher magnification. 3.3. Effects of Electron Beam Exposure. We have studied the effect of electron beam exposure on the shape of nanoribbons


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Figure 8. Representative FESEM images illustrating the curving of CuPc nanoribbons, deposited on Au-500 at 200 °C, upon electron beam exposure. Panel (a-d) depicting the sequential changes. The insets illustrate the flexibility of as deposited CuPc nanoribbons with high curvature.

using field emission scanning electron microscope (FESEM). The FESEM image of CuPc nanoribbons deposited on Au-500 at 200 °C is shown in Figure 8a. Upon repeated exposure of nearly 30 s to an electron beam at a relatively lower voltage of 3 kV, the nanoribbons become bended and twisted as shown in Figure 8b-d. Figures 8b-d represent the sequential change in the shape of the nanoribbons upon repeated exposure to an electron beam. The change in shape under electron beam exposure is marked with white arrows in the corresponding FESEM images. The inset of Figure 8a,b illustrates the flexibility of CuPc nanoribbons in as-grown samples showing high curvature of nanoribbons. Actually, excellent flexibility of nanoribbons has also been exhibited by their bending like “half circle” (Figure 5d). It has also been shown by other workers33 that exposure to an electron beam causes curving and the coiling of thin nanoribbons of CuPc. They have mentioned that for thicker ribbons the straight geometry is the stable shape.33 In contrast, in the present study curving and coiling of the thick nanoribbons of CuPc have been noticed upon electron beam exposure. Depending on the dimension of the nanoribbons, the bending and twisting of nanoribbons could be due to the formation of stable minimized energy structure.33 Heating effect of the sample could be another reason for bending and twisting of the nanoribbons upon exposure of electron beam.33 3.4. TEM Image Analysis. TEM image of CuPc thin film deposited on Au-750 at 155 °C substrate temperature has been shown in Figure 9. Figure 9a shows the TEM image of some nanoribbons uniformly distributed throughout the mess. The length and breadth are measured around 100 nm and 1 µm respectively. The inset of Figure 9a shows the selected area electron diffraction (SAED) pattern of a single nanoribbon of R-phase CuPc. Figure 9b represents the TEM image of CuPc

thin film deposited on Au-750 at 280 °C. Here we found some pentapod like structure of CuPc nanoribbons. Figure 9c represents one such pentapod and a twisted nanoribbon, the inset of which shows the twisted portion of the nanoribbon of width nearly 23 nm. Figure 9d represents the high-resolution TEM (HRTEM) of a single CuPc nanoribbon. The HRTEM image exhibits clear fringes parallel to the nanoribbon axis with fringe spacing of 3.12 Å, which is in good agreement with the interplanar spacing of (212) plane of β-phase CuPc. The single crystalline nature of the CuPc nanoribbon can be appreciated from the SAED pattern in the bottom right inset of Figure 9d. It can be observed from Figure 9d that the long nanoribbons of β-phase CuPc grow along the b-axis, which is the stacking axis of the CuPc molecules, similar to the previously reported result.20 TEM observations of the ordered crystalline films indicated that domains are grown from the bottom to the top of the film and are densely packed with little grain boundary. 3.5. XRD Measurements. Figures 10 and 11 shows the X-ray diffraction patterns for CuPc thin films deposited on bare quartz and Au-750 respectively. The representative curves in each figure give the pattern for different substrate deposition temperatures. The lower panel of Figure 11 represents the X-ray diffraction for the starting β-phase CuPc powder. It is known that CuPc has three dominant different crystal phases: R-,β-, and χ-phases.34, 35 The most commonly studied forms are the R and β polymorphs. The stable β-phase of CuPc thin film arises when heated at high temperature and when stored in certain organic solvents.34 The structure of the R-phase is known34 to be an orthorhombic crystal having unit cell dimensions: a ) 25.92 Å, b ) 3.79 Å, and c ) 23.92 Å. The structure of β-phase is a monoclinic crystal with the following unit cell dimensions: a ) 14.68 Å, b ) 4.98 Å, c ) 19.6 Å, and γ ) 121.5°.34 It can



Figure 9. TEM image of CuPc nanoribbons deposited on Au-750: (a) R-phase CuPc nanoribbons deposited at 155 °C (inset shows the SAED pattern of a single nanoribbon); (b) β-phase CuPc nanoribbons deposited at 280 °C; (c) pentapod like structure of CuPc nanoribbon (inset shows twisted region of a nanoribbon); (d) HRTEM of a single CuPc nanoribbon (bottom right inset represents the corresponding SAED pattern).

be seen from Figures 10 and 11 that the CuPc thin films formed at 30 °C are amorphous on Au-750 and little R-phase crystalline on quartz. At substrate temperature 85 °C and 155 °C only one prominent diffraction peak appears at θ ) 6.7-6.8° corresponding to the (200) lattice plane of R-phase of CuPc.34 For the thin film deposited at 200 °C the diffraction peak remains at θ ) 6.7-6.8°. Thus, the crystal structure of CuPc thin films deposited for different substrate temperatures ranging from 30 to 200 °C are all in the R-phase and is independent of gold template, although the starting CuPc powder was in the β-phase. Above 200 °C substrate temperature only the β-phase of CuPc is noticed. 3.6. Absorption Measurements. The electronic absorption spectra of the sample films deposited on quartz, Au-500 and Au-750 at different substrate temperatures were recorded at room temperature. The optical absorption spectra of CuPc thin films deposited on quartz and Au-750 are shown in Figures 12a and 13a, respectively. Each representative curve in these figures represents the spectrum for a specific substrate temperature for film deposition. It has been suggested that the UV-vis spectrum of MPcs originates from the molecular orbitals within the aromatic 18-π electron system and from overlapping orbitals on the central metal.36 In the near UV region the B-band or Soret band,36,37 representing the πfπ* transition appear with peak position in the range about 329-349 nm depending on the substrate temperature and the nature of substrate used. The absorption band in the visible region for each sample film known as the Q-band representing the πfπ* transition37 has a doublet due to Davydov splitting.38 The intensity of the peaks depends on the film deposition temperature and the nature of substrates used. The positions of the absorption peaks including the amount

Figure 10. XRD patterns of CuPc thin films deposited on quartz. The representative curves in each figure give the pattern for different substrate deposition temperatures.

of Davydov splitting for the different sample films on various substrates at different deposition temperatures are shown in Table 1. From the Table 1 and Figures 12a and 13a it is clear that the position and relative intensity of peaks and the amount of Davydov splitting depend on the temperature of film deposition and the nature of substrate used. The extent of Davydov splitting is related to the differences in relative orientation of molecules which are close enough to give electronic transitions, namely, interactions between transition


Karan and Mallik

Figure 11. XRD patterns of CuPc thin films deposited on Au-750. The representative curves in each figure give the pattern for different substrate deposition temperatures. The lower panel represents the pattern for starting β-phase CuPc powder.

Figure 13. (a) Optical absorption spectra and (b) variation of R2 with hν for the CuPc thin films deposited on Au-750; the representative curves in each figure give the pattern for different substrate deposition temperatures.

Figure 12. (a) Optical absorption spectra and (b) variation of R2 with hν for the CuPc thin films deposited on quartz; the representative curves in each figure give the pattern for different substrate deposition temperatures.

dipole moments from adjacent molecules. The amount of Davydov splitting in the case of CuPc films deposited on bare

quartz and on Au-500 is almost the same/comparable for the film deposition temperature of 30 and 280 °C. But the Davydov splitting for CuPc films deposited on Au-750 is appreciably higher except at 85 °C deposition temperature. Thus, the change in orientation of the molecules in the CuPc films deposited on Au-750 due to the change in substrate annealing condition is indicated from the differences in the extent of Davydov splitting. Recently, it has been found that substrate roughness significantly affects the orientation of molecules.39 These molecules lie on flat substrates (i.e., the molecular plane is parallel to the film surface) but they stand on rough substrates.39 Our results indicate that adsorption of CuPc molecules deposited at room temperature on Au-750 occurs in a standing/tilted geometry. Annealing of the gold substrate at 500 °C caused no essential changes in the adsorption geometry of the CuPc molecules for deposition at room temperature. The high tendency of the self-ordering of phthalocyanine molecules could be one of the main reasons for the anisotropy in film morphology depending on the substrate annealing condition and film deposition temperature. For the film deposition temperature up to 200 °C, the intensity of the higher energy (∼630 nm) maximum peak has been observed to be larger than that of the lower energy (∼693 nm) peak (Figures 12a and 13a). This behavior represents the typical features of the R-phase of CuPc.34 Above 200 °C the intensity



Figure 14. Variation of log(P) with log(S) derived from FESEM images for CuPc thin films deposited on quartz at: (a) 30, (b) 85, (c) 155, (d) 200, (e) 250, and (f) 280 °C.

of higher peak becomes smaller than the lower energy peak, which confirms the formation of β-phase of CuPc.34 The variation in absorption coefficient (R) with photon energy (hν) for band-to-band transitions can be represented as40

R ) Ro(hν - E)r

(1)

where Ro is a constant, E is the energy gap, and r determines the type of transitions, which is equal to 0.5, 2, or 3/2 for direct, indirect, and forbidden transitions. The dependence of (R)1/r with hν for onset gaps were plotted from the transmission spectra of CuPc thin film deposited on quartz, Au-500 and Au-750. For different values of r, the best fit was obtained for r ) 0.5 (not shown). In Figures 12b and 13b, plots of R2 versus hν are shown for quartz and Au-750, respectively. The direct allowed onset energies corresponding to various substrates for different deposition temperatures are listed in Table 1. From Table 1 it has been found that the energy gap decreases with the increase

in substrate deposition temperature for all substrates up to 200 °C (R-phase to β-phase transition). Above 200 °C the energy gap increases slightly or remains constant. The changes in energy gap at different substrate deposition temperatures are attributed to the changes in the crystal structure and morphology of these films. 3.7. Calculation of Fractal Dimension. The surface structure and surface coverage of CuPc thin film formed on quartz, Au500 and Au-750 are different, and the thin film preparation in this process has a direct relationship with their fractal dimension/ structure. The fractal dimension enables us to relate the aggregation of CuPc molecules on quartz and gold surface to a proper kinetic and growth mechanism.41 Using the FESEM images, the fractal dimension of the assembly of nanostructures (nanoparticles, nanoflowers, nanocabbages and nanoribons) in the CuPc films grown on various substrates and at different film deposition temperatures can be calculated by using the area (S)perimeter (P) relationship,42,43 which is generally used to


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TABLE 1: Positions of Absorption Peak, Davydov Splitting, Direct Energy Gap and Fractal Dimension (D′) for CuPc Thin Films Deposited on Quartz, Au-500, and Au-750 at Different Substrate Temperatures substrate temperature (°C) 30 85 155 200 250 280

template used Quartz Au-500 Au-750 Quartz Au-500 Au-750 Quartz Au-500 Au-750 Quartz Au-500 Au-750 Quartz Au-500 Au-750 Quartz Au-500 Au-750

positions of absorption peak (nm) 329.8 349.0 336.5 333.2 359.5 334.5 335.0 336.5 334.5 334.5 347.5 334.5 338.5 346.0 334.5 341.0 343.5 347.0

617.0 616.8 614.6 623.0 605.0 622.0 624.5 623.0 617.0 631.5 641.5 614.5 641.5 642.0 642.0 641.5 642.5 641.5

693.0 692.5 692.5 692.5 692.5 695.5 696.0 691.0 694.0 695.5 722.5 692.0 718.0 721.0 715.5 718.0 719.5 722.0

estimate the fractal dimension of objects/islands. The perimeterarea relationship for a set of islands can be written as,42,43

Davydov splitting (cm-1) 1777.16 1772.31 1828.78 1610.93 2088.49 1699.03 1644.99 1579.63 1798.23 1457.17 1749.63 1822.53 1660.88 1706.69 1600.08 1660.88 1665.66 1738.05

direct energy gap (eV) 1.633 1.621 1.641 1.631 1.589 1.626 1.620 1.555 1.620 1.617 1.608 1.605 1.628 1.626 1.619 1.625 1.638 1.617

1.866 1.865 1.871 1.831 1.855 1.865 1.816 1.866 1.884 1.779 1.854 1.874 1.791 1.869 1.836 1.804 1.861 1.829

3.273 3.184 3.225 3.233 3.161 3.223 3.216 3.137 3.158 3.169 3.106 3.123 3.176 3.180 3.086 3.181 3.206 3.052

fractal dimension (D′) 1.84 1.87 1.76 1.86 1.93 1.89 1.91 1.87 1.93 1.83 1.82 1.82 1.95 1.91 1.87 1.86 1.83 1.83

(2)

of the crystal aggregates comprising a particular organic pigment is strongly influenced by the geometry of the pigment crystal and the surface roughness. The fractal dimensions estimated for aggregated β-copper phthalocyanine pigment crystals were

where, D′ is the fractal dimension, the area S is the number of pixels making up a given object, the perimeter P is a count of the number of pixel edges, and k is a scaling constant. Selfsimilarity, or geometric scale-invariance, is expressed by a linear relationship between log P and log S over some range of scales. We have calculated D′ for the CuPc thin film surfaces deposited on various substrates by applying the method used for SPM image analysis.43 Commonly used models for calculating fractal dimensions from SPM images assume the scaling to be equal in the lateral directions on the surface but not necessary the same as in the normal direction.43 Such surfaces are called self-affine and differ from self-similar surfaces with equal scaling in all three directions. An evaluation of fractal dimension (D′) with the area perimeter method uses the fact that the intersection between a plane and a self-affine surface43 generates self-similar lakes or islands. The evaluated fractal dimension (D′) is the fractal dimension of the coastlines and it has a value between one and two; whereas the fractal dimension of the surface ) D′ + 1.43 The FESEM images were converted to a digitalized image similar to the approach for SPM image analysis.43 The area and apparent perimeter, neglecting islands that touch the sides of the image field, of the independent islands have been calculated throughout the whole image. The slope of the log-log perimeter-area plot for a set of objects gives the value of D′ (i.e., D′ ) 2 × slope). In Figure 14 the linear plots of log P versus log S are shown on the basis of the analysis of FESEM images corresponding to the CuPc films deposited on quartz at different temperatures. The representative curves (a-f) in Figure 14 indicate the variation with different film deposition temperatures. Linear plots of log P versus log S were also obtained (not shown) for Au-500 and Au-750 for different film deposition temperatures. The values of D′, evaluated from the linear plots of log P versus log S corresponding to the CuPc films deposited on various substrates at different deposition temperatures are shown in Table 1. The degree of crystal aggregation in organic pigments like CuPc was assessed by Mather44 using fractal nature of the pigment crystal surfaces. Mather44 considered that the geometry

Figure 15. Variation of (a) fractal dimension and (b) Davydov splitting with substrate temperature for the CuPc thin films deposited on quartz, Au-500 and Au-750, respectively.

P ) kSD′/2

Copper Phthalocyanine Nanocrystallites Thin Film less than 2, while for the unaggregated pigment crystals its value was higher than 2 (∼2.28). The present study indicates that crystal aggregation depends on the nature of substrate, surface roughness, and the film deposition temperature. The values of D′ are lower than 2 (Table 1), which agree with the value of fractal dimension estimated by a different method by Mather44 for aggregated β-copper phthalocyanine pigment crystals. The variation of fractal dimension (D′) with temperature of film deposition on quartz, Au-500 and Au-750 is shown in Figure 15a. The fractal dimension (D′) for all the substrates increases initially with the increase of film deposition temperature and above 200 °C (R-phase to β-phase transition) it decreases and fluctuates/oscillates. It is reported in the literature45-47 that fluctuations in physical parameters occur near the phase transition region in various systems. In the present experiment, a fluctuation in fractal dimension noted at various temperatures for the thin films deposited on different substrates is possibly related to the phenomenon of phase transition. Also the calculated fractal dimension is strongly dependent on the surface morphology of the films. With the increase in film deposition temperature, the aggregation of CuPc nanoparticles on different substrates becomes prominent and ultimately the structure of prominent nanoribbons are observed at about 155 °C. Above this temperature the nanoribbons remains in the same crystalline phase (β-phase), but the surface orientation of the nanoribbons becomes different for different substrate temperatures. Similar to the fractal dimension, the fluctuations/ oscillations in the Davydov splitting have also been observed (as shown in Figure 15b). Thus the results of fractal analysis agree with the results of optical measurements. 4. Conclusions In summary, it is clear that for the growth of CuPc layer by vacuum evaporation, the surface morphology of the thin film is influenced very strongly by the natures of the gold template and the substrate temperature during film deposition. The crystal structures in all cases are independent of the growth conditions used to deposit the CuPc layer. The results of fractal analysis agree with the result of optical measurements. The templating effects can be extended to structures involving the growth of multilayer heterostructures. The gold template can also be useful as an electrode in an optoelectronic device. The implication of these results for molecular device structures is very important, since it is likely that the templating effect will have a profound influence on the consequential electronic and optical properties of a heterostructures based device. Excellent flexibility and easy bending and twisting of CuPc nanoribbons upon electron beam exposure indicates that these nanostructures may represent a low-cost low-temperature alternative to inorganic nanostructures for application purposes. They could also be used to fabricate single-crystal organic field effect transistors (OFETs) on flexible substrates to meet the requirements of plastic electronics. References and Notes (1) Caronna, T.; Colleoni, C.; Dotti, S.; Fontana, F.; Rosace, G. J. Photochem. Photobiol., A 2006, 184, 135. (2) Blochwitz, J.; Pfeiffer, M.; Fritz, T.; Leo, K. Appl. Phys. Lett. 1998, 73, 729. (3) Leznoff, C. C.; Lever, A. B. P. Phthalocyanines, Properties and Applications; VCH: New York, 1993; Vol. 3. (4) Young, F.; Shtein, M.; Forrest, S. R. Nat. Mater. 2005, 4, 37. (5) Wang, D. X.; Tanaka, Y.; Iizuka, M.; Kuniyoshi, S.; Kudo, S.; Tanaka, K. Jpn. J. Appl. Phys. 1999, 18, 256.

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Phthalocyanine Nanocrystallites Thin Film - American Chemical Society

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