Effect of Annealing on the Nanostructure Formation in Alkoxy Substituted Phthalocyanine Thin Films

Vacuum deposited 2,3,9,10,16,17,23,24-octakis (octyloxy) phthalocyanine (H2PcOC8) thin films on glass substrates have exhibited a change on their surface morphology with the post deposition annealing temperature under normal atmosphere. These films have been characterized by optical absorptions and Scanning Electron Microscopy. SEM images also have shown nano-rod like structures for the samples annealed at different temperatures. The variation of optical band gap with annealing temperature is determined. The direct and allowed optical band gap energy has been evaluated from the α versus hυ plots. The electrical conductivity of the films at various heat treated samples are also studied. The activation energies are determined from the Arrhenius plots of lnσ versus 1000/T . It shows variation with the annealing temperature.


Introduction
Phthalocyanine dyes, due to their spectroscopic and photoelectric properties, have been extensively studied and can be applied in many fields: physics, medicine, chemistry and other fields.The electrical and optical properties of Pc's are determined by central metal ion and side groups [1].H 2 PcOC 8 is obtained by substituting eight alkoxy chains in the peripheral (2,3,9,10,16,17,23,24) positions of the phthalocyanine.Its chemical formula is C 96 H 146 N 8 O 8 .Metal free phthalocyanines (H 2 Pc) and metal substituted phthalocyanines (MPc) are organic conjugated macrocycles structurally similar to biologic molecules as the porphyrins and the chlorophyll with intense colour, high thermal and chemical stability.
Their optoelectronic properties and their ability to form well defined crystals [2], ordered films and nanostructures [3][4][5] motivated many researchers with fundamental interest concerning the molecular solid and organic-inorganic heterostructure [6][7][8][9][10][11][12] as well as applied interest like solar cells [13] and active semiconducting materials in gas sensors [14].Very recently these compounds have been successfully tested for the detection of volatile organic compounds and sedative drugs by optical techniques also [15][16] .Mpc's with excellent growth properties and chemical stability, are some of the most promising candidates to be used in the fabrication of organic thin film transistors which can be used as smart cards, electronic identification tags and active matrix displays [17][18][19][20][21].These commercially available materials have the great advantage to be easily sublimed resulting in high purity thin films without decomposition.High vacuum evaporation has become the most widely used technique for the deposition of MPc films [22].The microcrystalline structure of these film is found to have profound influence on the optical, electrical, mechanical and other film properties.The physical properties of these films are known to differ widely from those of the bulk materials.This is evidently connected with small size of the crystallites forming the film and in particular, with large number of defects such as dislocations, vacancies, stacking faults and grain boundaries etc.The method of film deposition and electrical and thermal conditions of the substrates also influence the structure of the deposited film.It is known that the orientation of the grains depends on the choice of the substrate, the effect of doping , the deposition technique and the heat treatment of the film.The substrate temperature during deposition directly affects the crystal structure and morphology of thin film.Bao.et al [23] reported that the grains of CuPc thin film are improved by increasing the substrate temperatures.This improves electronic properties of the films by increasing the mobility of holes and electrons, and by reducing the density of various traps.The surface morphology and optical properties of the thin film plays an important role in the optoelectronic device applications.Little data is available on the surface morphology and optical properties of H 2 PcOC 8 thin films.This paper brings out the results of the effect of annealing temperature on the optical properties and surface morphology of H 2 PcOC 8 thin films.

Experimental
Spectroscopically pure H 2 PcOC 8 powder procured from Sigma Aldrich Company (USA) is used as the source material for the preparation of thin films.The molecular structure of the material is given in fig.1.Thin films of suitable thickness are prepared by vacuum sublimation from a resistively heated molybdenum boat using a "Hind -Hi-Vac 12 A4" coating system, Commercially available micro glass slides are used as substrates for the thin films.The powder is sublimed under high vacuum with a base pressure of 10 -5 Torr to form H 2 PcOC 8 thin films.The thickness of the films are determined by the quartz-crystal oscillation monitor `and are crosschecked by multiple beam interferometer technique (MBI) [24].The deposited films are annealed in air for one hour in a programmable temperature controllable furnace.The reflectance and absorption spectra are recorded in the wavelength range 200-900nm using JASCO V-750 spectrophotometer.JEOL JSM-6390 Scannig electron microscope is employed to analyze the surface morphology of the film surface.The electrical measurements are carried out using a programmable Keithley electrometer(Model No.617) for all the samples.Vacuum deposited Silver with an interelectrode distance of 0.7 cm, is used as the ohmic contact.To avoid any possible contamination, conductivity measurements are performed in vacuum at 0.133Pa and in a dark chamber to reduce the photoconductive contribution.

Optical studies
The study of optical absorption is a useful method for understanding the induced transition and providing information about the band structure in the materials.Optical studies are done to determine the energy band gap and the effect of annealing on band gap.Normally organic molecules of Pc and their derivatives exhibit anomalous optical characteristic because of their unique molecular ring structures.Fig. 2 shows the absorption spectrum of vacuum evaporated typical H 2 PcOC8 thin film of thickness 250 nm annealed at various temperatures.It is known that they possess two kinds of energy bands.The broad absorption bands at 300-400 nm and 600-900 nm are associated with the soret (B band) and the Q bands respectively.The peak wavelengths in the absorption spectrum are found at 330 nm and 620 nm and the shoulder is at 670 nm .The intensity of the higher energy peak is larger than that of the second peak.This behaviour represents the typical feature of α -phase of H 2 PcOC 8 .The intensity of the peak is found to maximum for the film annealed at 373 K. Effect of Annealing on The Nanostructure Formation 1995 The absorption coefficient α is calculated using the relation

2.303A t   (1)
where A is the absorbance of the film and t is its thickness.The optical band gap energy is calculated using the relation where E g is the optical band gap.The plot of α 2 versus hυ for H 2 PcOC 8 thin films are shown in fig. 3. The X -intercept of α 2 versus hυ graph gives the value of band gap energy [25].The variations in energy gap with different annealing temperatures are tabulated in It is observed that the band gap energy is almost the same up to 423 K.But it shows a change for the film annealed at 473 K; which may be due to the phase transition from α to β [26].The phase separation could be responsible for the degradation of the optical properties [27].As the annealing temperature increased to 473 K, the film morphology changed to nano rod like patterns, which are inter connected.These fibres grow in a direction which are parallel to the substrate surface.The length of the fibres increase with further increase in annealing temperature.The grain size is measured from the micrographs.The average grain size of the H 2 PcOC 8 thin films annealed at different temperatures are collected in table.2.As the annealing temperature increases, the grain size also increases.For the film annealed at 523 K, the breadth of the fibre is about 93 nm.

Surface morphological studies
Table 2. Grain size of the H 2 PcOC 8 thin films annealed at different temperatures.

K 65
Gas sensitivity depends both on material parameters, such as the crystal phases of the material and on film parameters such as the surface topography of the film [28].The band gaps, trap levels, and activation energies are influenced by the grain size [29].

Electrical conductivity studies
Semiconducting properties of phthalocyanines are first observed by Eley [30].In organic semiconductors, the semiconducting properties are brought about by the thermal excitation, impurities, lattice defects and non stoichiometry.Holes in the valence band and electrons in the conduction band contribute to the electrical conductivity.If we assume that the variation of mobility of the electrons and holes in an electric field with temperature is small, then conductivity, which is proportional to the number of carriers as Conductivity in phthalocyanine is due to both hopping of holes and charge transport via excited states.In such a case, the conductivity is given by where E 1 ,E 2 ,E 3 … are the thermal activation energies needed to excite the carriers to the conduction band.A,B,C are the constants.Arrhenius plots of lnσ vs (1000/T) yields a straight line whose slope can be used to determine the thermal activation energy of the film and are given in Fig. 5 .There are two linear regions for the graph, corresponding to two activation energies E 1 and E 2 .The activation energies of the H 2 PcOC 8 thin films are collected in Table .3.The intrinsic activation energy E 1 is found to increase with annealing temperature up to 423K and then reduces as shown in fig. 5.The reduction in activation energy may be attributed to the instability of the material due to heavy vibrations of the atoms.This lowering of activation energy is probably influenced by the structure of the film and therefore by the distribution of electronic tail states.Variation in the extrinsic activation energy E 2 during annealing can be attributed to the distribution of trap levels [31].

Conclusion
H 2 PcOC 8 thin films are produced by vacuum deposition.The thickness of the as deposited thin film is around 250 nm.The optical, electrical properties and surface morphology of the film at their as deposited, 323,373,423,473,523 K heat treated stages are studied.SEM micrographs indicate the tendency to form nano structure arrays of H 2 PcOC 8 by thermal evaporation method.The fine grain crystallite on the as deposited films is transformed to rod like structures as a result of coalescence and re-organization of the grains during the heat treatment.The optical transition is found to be direct allowed and the optical band gap is shifted from 3.17 to 3.12 eV.Since the interactions of the molecules are of Vander Waals type, the rearrangement of molecules alters the energy gap.The activation energy is calculated from the electrical conductivity studies.The existence of trap levels is confirmed by the presence of more than one linear portion in the ln σ versus 1000/T plots.The variation in the intrinsic and extrinsic activation energy during annealing can be attributed to the change in the position of the Fermi level and the distribution of trap levels.

Figure 5 .
Figure 5. Plot of lnσ versus 1000/T for the as deposited.

Table 1 .
Optical band gap of H 2 PcOC 8 as deposited thin film and films annealed at different temperatures.

Table 3 .
Activation energies of the H 2 PcOC 8 thin films.