Surface Morphology of Annealed Lead Phthalocyanine Thin Films

Transcription

1 Surface Morphology of Annealed Lead Phthalocyanine Thin Films ABSTRACT P.Kalugasalam * a, Dr.S.Ganesan b a. Department of Physics, TamilNadu College of Engineering, Coimbatore b. Department of Physics, Government College of Technology, Coimbatore. Kaluguindian@yahoo.co.in The thin films of Lead Phthalocyanine (PbPc) on glass substrates were prepared by Vacuum deposition. The thickness of the films was 450 nm. The sample annealed in high vacuum at 373 K temperature. The sample has been analysed by X-ray diffraction, scanning electron microscopy and atomic force microscopy in order to get structural and surface morphology of the PbPc thin film. The formation of XRD patterns of PbPc shows a triclinic grains (T) seen along with monoclinic (M) forms of PbPc. The sample is annealed at 373 K temperatures; the film shows peaks that assigned to the triclinic phase. SEM and AFM are the best tools to investigate the surface smoothness and to find the grain size of the particles. The grain size is calculated for all films of different thicknesses. The annealed AFM micrograph shows that the surface of the films consists of large holes. The annealed AFM image indicates a smooth surface. It is very clear that the grain size decreases with increase in the annealing temperature. The roughness also decreases with the increase in film annealing temperature. Annealed film leads to the oxidation of the phthalocyanine with oxygen absorbed or diffused. Therefore, the heat is responsible for the increase in film thickness. Since the films expand, it is believed that the porosity is increased. Keywords: Phthalocyanine, X ray diffraction, Atomic Force microscopy, Scanning Electron Microscopy, monoclinic and triclinic. 1. Introduction In the research on Phthalocyanines, attention has been given to Lead Phthalocyanine (PbPc) because of its characteristic structural and electrical properties. Lead Phthalocyanine is an attractive material for device applications in various field such as Opto electronics, gas sensors, OLEDs, FETs, etc. [1-3]. PbPc was chosen as a model compound since it has somewhat higher specific conductivity compared to conventional divalent Pc materials [4,5]. Metal-Pc films which consist of amorphous and/or polycrystal states have been used generally as carrier transport layers in organic devices. OTiPc thin film also has a shuttle-cock shaped molecular structure similar to PbPc [6]. The microcrystalline structure of thin films is found to have a high influence on the optical, electrical, mechanical and other film properties. The physical properties of thin films are known to differ widely from those of bulk materials. This is evidently connected with the small size of the crystallites formation on the film and with the large number of defects such as dislocations, vacancies, stacking faults, grain boundaries, twins etc in particular. The method of film deposition, the electrical and thermal conditions of the substrates also influence the structure of the deposited films. The study of the structure is particularly important when coupled with data concerning the film formation processes and with physio-chemical properties of the film. Our previous studies of the metal Phthalocyanines indicated that the sensitivity and the other characteristics were strongly influenced not only by the nature of the central metal species but also by the crystal structure and/ or crystallinity[7,8]. Additionally, the crystal structure and size have been shown to vary with the length of the operating time at higher temperature and lead phthalocyanine has been shown to get deformed following the reaction with oxidizing molecules [9]. Therefore it is preferable not to raise the operating temperature. In this paper, we will report on the surface studies of PbPc thin films prepared by Vacuum deposition technique. Information on the surface characteristics of the vaccum evaporated lead phthalocyanine thin films can be obtained by scanning electron microscope (SEM) and atomic force microscope (AFM). ISSN:

2 2. Experiment The powder of lead phthalocyanine (80% dye, sigma Aldrich company, Bangalore, India) was kept in a molybdenum boat of 100 amps and heated with high current controlled by a transformer. The transformer is capable of supplying 150amps at 20volts is used to provide the accessory current for heating the molybdenum source which was used for the evaporation process. Prior to evaporation, the evaporant material was carefully degassed at lower temperature for about 45 minutes with the shutter closed. Deposition of PbPc on pre-cleaned glass substrates under the pressure of 10-6 Torr was achieved by slowly varying the current. The rate of evaporation was properly controlled and maintained constant during all the evaporations. Rotary drive was employed to maintain uniformity in film thickness. The film thickness was controlled to be 450nm by Quartz crystal monitor. The adhesion of the films to the substrate seems to be extremely good. The samples prepared in a similar environment were used for studying their SEM and AFM properties. 3. Results and discussion XRD analysis of PbPc Thin Films The X-ray diffraction patterns of thermally deposited PbPc thin films with 450 nm thickness prepared at room temperature are shown in Fig. 1. The pattern shows peaks at 2θ values 6.85 o, o and o that were assigned to monoclinic (001), (320) and (111) lines respectively and a peak at 2θ value o coinciding with the triclinic (400) line. The peak values are in good agreement with the previous literature works [10, 11]. Thus deposited PbPc films at room temperature are a mixture of monoclinic and triclinic [11, 12]. These deposited films are polycrystalline in nature. The XRD patterns show weak diffraction peak for (320) monoclinic lines of PbPc. PbPc molecules in the monoclinic modification are composed of well-defined structures, which cluster themselves and consist of molecular stacks arranged in an orderly way. Within the stacks there exist two substructures which have opposite orientations of the shuttlecock Pc structure. Such a structure explains the weak diffraction peaks of monoclinic crystals [10, 11, 13 and 14]. In this phase, the molecules stack linearly to form a molecular column parallel to the c axis [11, 12]. 4 M (001) M (111) T(400) t=4500a 0 3 intensity(counts) 2 M (320) q (degrees) Fig. 1. XRD Patterns of PbPc Films for thicknesses 450 nm at 303 K The XRD pattern of PbPc films with a thickness of 450 nm at 373 K temperature is shown in Fig. 2. The peaks obtained at room temperature were assigned to the monoclinic phase with some triclinic grains. The XRD graphs at 373 K shows peaks at 2θ values 7.43 o, 14.9 o, 22.4 o and 30.3 o assigned to the triclinic (100), (200), (300) and (400) lines respectively. The results are consistent with the previous literature [10, 11]. From the XRD patterns at 373 K temperature it is concluded that as temperature increases the number of peaks for triclinic phase also increase. Annealing at 373 K changes the crystal structure to triclinic with (100) as the direction of preferential orientation. So when annealing is carried out in air at 373 K, triclinic phase becomes predominant [12]. The annealed films are also polycrystalline in nature. PbPc in the monoclinic modification is composed structures which cluster themselves and consist of molecular stacks arranged in an orderly way. Within the stacks there exist two substructures which have opposite orientations. One structure contains shuttle cocks with Pb ions below the phthalocyanine and other substructure the shuttle cocks are upside down [14]. This type of structure is the most likely explanation for the week diffraction peaks obtained from the monoclinic from the monoclinic samples. ISSN:

3 T(100) T(200) T (300) T (400) 1.50 intensity(counts) (degrees) SEM analysis of PbPc thin films Fig. 2. XRD Patterns of PbPc Films for thickness 450 nm at temperatures 373 K Fig. 3 shows surface topography of the SEM photographs of PbPc films with a thickness of 450 nm at room temperature (303 K). The SEM image shows fine particles with rod like structures [15]. The surface of the films is rough. There are powder agglomerations. The area fraction of the powder agglomeration on the film increases as the PbPc content increases. The agglomerations of PbPc powder make the film structure porous [16]. The higher the PbPc content, the more numerous are the agglomerations and the higher is the porosity. The porosity and the amount of PbPc on the film are responsible for the increase in film thickness with increasing PbPc content [17]. Fig. 3. SEM images of PbPc films for thicknesses 450 nm at 303 K Fig. 4 shows the SEM images of PbPc films with a thickness of 450 nm at 373 K annealed temperature. As the temperature increases the smoothness and uniformity of the film also increases. In annealed films, the rod like crystals deforms and become small particles as a result of the oxidation of phthalocyanine with oxygen adsorbed and /or diffused. So, what formed were not rods like crystals but small particle taking a different form from the fine particles observed earlier. Therefore, the formation of the fine particles may proceed as a result of the interaction between the absorbed oxygen and phthalocyanine. These tendencies to absorb imply that the surface of phthalocyanine particles is oxidized by annealing at 373K and the crystal growth is depressed by the formation of lead oxide on the surface. For the more the rod like crystals deform the more they become small particles as a result of the oxidation of the phthalocyanine with oxygen absorbed or diffused. Therefore, the heat is responsible for the increase in film thickness. Since the film expands, it is believed that its porosity also increases. ISSN:

4 Fig. 4. SEM images of PbPc films for thickness 450 nm at annealed at 373 K AFM Analysis of the film Interesting results have been obtained by means of AFM measurements. The two dimensional and three dimensional AFM micrographs of PbPc thin film with a thickness of 450 nm are shown in figure 5. The AFM image indicates a rough surface. Figure 5 shows the beta phase films which consist of large grain size of about nm [16]. In Figure 6, we can observe roughly cubic crystallites of 120x200 nm width. The crystallite size is roughly constant. The surface area is about 1.4 µm 2, which is 40% more than a 1x1 µm 2 scanned area. The AFM micrograph of figure 5 shows that the surface of the films consists of larger grain size and are homogeneously distributed [18]. ISSN:

5 Fig. 5 AFM micrographs of PbPc thin films of thickness 450 nm at 303 K ISSN:

6 Fig. 6. AFM micrographs of PbPc thin films of thickness 450 nm annealed at 373 K The AFM has been used to see the effect of the annealing temperature on the surface morphology of the PbPc thin films. The two dimensional and three dimensional AFM micrographs of PbPc thin film with a thickness of 450 nm annealed at 373 K are shown in figure 6. It becomes compact and the surface area decreases to 1.2 µm 2 which is 15% less than a 1.4 µm 2 area. The annealed AFM micrograph shows that the surface of the film consists of large holes, some hundred square nanometers wide and about 10,000 nm deep. Inbetween the holes, larger compact areas formed of little crystallites of 20 nm are observed. The AFM image suggests that part of the film has evaporated after the annealing at 373 K. It is very clear that the grain size decreases with increase the annealing temperature. The annealed AFM image indicates a smooth surface. The roughness also decreases with the increase in film annealing temperature [16]. 4. Conclusion Films of Lead Phthalocyanine with a thickness of 450nm are prepared by vacuum deposition method. From the XRD patterns at different temperatures, it is concluded that as temperature increases the number of peaks for triclinic phase also increases. These deposited films are polycrystalline in nature. The SEM image of lower thicknesses shows very fine particles and disordered phases. The agglomerations of PbPc powder make the film structure porous. The higher the PbPc content, the more numerous are the agglomerations and the higher is the porosity. In annealed films, the rod like crystals deforms and become small particles as a result of the oxidation of phthalocyanine with oxygen adsorbed and /or diffused. So, what formed was not rod like crystals but small particles, taking a different form from the fine particles observed earlier. The AFM image is indicating a rough surface. It shows the beta phase films which consist of large grain size about nm. We can observe roughly cubic crystallites of 120x200 nm width. The crystallite size is roughly constant. The surface area is about 1.4 µm 2, which is 40% more than a 1x1 µm 2 scanned area. The sample annealed at 373 K it becomes compact and the surface area decreases to 1.2 µm 2 which is 15% less than a 1.4 µm 2 area. The grain size decreases with increase the annealing temperature. The roughness also decreases with the increase in film annealing temperature. References [1] F.H.Moser and A.C.Thomas. 1983, The Phthalocyanines (CRC Prl. Florida, ) Vol 1&2 [2] K. Ukei, 1973, Structural Crystallography and Crystal Chemistry Acta Crystallography B, 29, [3] P.B.M.Archer, A.V.Chadwick, J.J.Miasik, M.Tamizi and J.D.Wright, 1989, Kinetic factors in the response of organometallic semiconductor gas sensors Sensors and actuators, 16, 379. ISSN:

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