Pulsed laser deposition of crystal polyaniline thin films

Surface and Coatings Technology 125 (2000) 388–391 www.elsevier.nl/locate/surfcoat Pulsed laser deposition of crystal polyaniline thin films Z.M. Ren a, Y.F. Lu *, Z.H. Mai a, S.C. Ng b, P. Miao b, S.I. Pang c, J.P. Wang c, T.C. Chong c a Laser Microprocessing Laboratory Department of Electrical Engineering and Data Storage Institute, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore b Department of Chemistry, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore c Data Storage Institute, 5 Engineering Drive 1, Singapore 117608, Singapore Accepted 28 June 1999 Abstract Thin films of polyaniline (PBT ), a kind of polyheterocyclic compound with hydrogen function groups, were deposited by KrF excimer ablation in a vacuum chamber. The laser pulse fluence was selected at 2 J cm−2 with a pulse duration of 25 ns. The polymer used in our experiments bears the basic structure and frame of many polyheterocyclic compound ramifications. The deposition of such a polymer should be helpful for future research when other kinds of function radicals are attached to polyaniline to realise different applications. The structural and topographic properties of the deposited thin films were analysed by atomic force microscope and X-ray diffraction. The deposited thin films were observed to have good crystal properties and be composed of crystalline cubes with a uniform size of 0.1 mm. © 2000 Elsevier Science S.A. All rights reserved. Keywords: AFM; Excimer laser; Polymer; Pulsed laser deposition; Thin film; XRD 1. Introduction There is currently an increased interest in the application of conducting polymers and polymer-based optoelectronic materials due to their unique properties. The fabrication of the polymer light-emitting device (PLED) has received a lot of attention in recent years [1–4]. In many practical applications, polyheterocyclic compounds such as polyaniline (PBT ) and polythiophenes are as basic materials with different kinds of function groups. In industrial applications, polymers are normally in their bulk forms of alloys and blends. Besides the bulk forms, the applications of polymers in the forms of thin-films are needed in miniaturization and integration technologies. Some fabrication methods of polymer thin films have been investigated such as vacuum evaporation [5–7], plasma polymerization [8,9], synchrotron radiation photodecomposition [10], spin coating [1–4,11], chemical vapour deposition [12], photo decomposition [13] and plasma sputtering [14] etc. Pulsed laser deposition (PLD) is another suitable deposition for fabricating polymer thin films on various kinds * Corresponding author. Tel.: +65-874-2118; fax: +65-779-1103. E-mail address: [email protected] ( Y.F. Lu) of substrates [15–17]. In comparison to other methods, PLD has mainly two advantages. The first is its faithful transfer of target materials to substrate surfaces without obvious changes in composition. Since the ablated radicals normally have the same compositional ratio to the target, PLD can keep the electronic properties of the original target materials. The second concerns the energetic radicals in the ablated plume. These radicals with certain energies are beneficial to the formation of ideal crystalline structures in the deposited thin films. In thin film deposition, polyheterocyclic compounds are usually difficult to find which are suitable solvents for spin-coating. Physical vapour deposition (PVD) always involving ion/plasma processing sometimes becomes an alternative deposition technique. However, since ion-beam processing of polymer solid surfaces can cause serious damage to polymer molecules and thus make the structures difficult to handle and affect the properties of deposited polymers, PLD seems to be an attractive deposition method since it avoids destruction of the target materials. In our experiment, we intend to deposit a kind of polyheterocyclic compound, polyaniline (PBT ), by KrF pulsed excimer laser ablation of a solid target. The chemical structure of polyaniline (PBT ) used in our experiments is shown in Fig. 1. 0257-8972/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 58 2 - 4 Z.M. Ren et al. / Surface and Coatings Technology 125 (2000) 388–391 Fig. 1. Chemical structure of PBT used in the experiment. In our compound, we have X=sulphur (S). The function group is R=hydrogen (H ). The function group plays an important role with the electrical properties of the polymer. The conductivity and optoelectronic properties of the compound depend strongly on the selection of the functional junction groups. The polymer used in our experiments has the basic frame of many polyheterocyclic compound ramifications. The deposition of such a polymer should be helpful for future research when other kinds of function groups are attached to PBT. This deposition will be targeted at fabricating polyaniline thin films with good crystal properties. 2. Experimental Polyaniline thin films were deposited in a PLD system with a background vacuum of 1×10−6 Torr. The distance between the target and the substrate was 4 cm. The target was rotated by an external motor so as to provide each pulse with a fresh surface. The target was a piece of pure PBT. A KrF excimer laser with wavelength of 248 nm and a pulse duration of 25 ns was used as light source to ablate the PBT target. Its repetition rate was 10 Hz. The fluence of the KrF excimer laser was kept at 5 J cm−2. The deposition rate was in the range of 50–150 nm min−1. The thickness of the deposited thin films was in the range of 1–2 mm. The substrate temperature was automatically controlled by a digital controller and can be set up to 900°C. Before deposition, the silicon substrate wafers were cleaned by acetone in an ultrasonic bath. The measurement of the surface morphology of 389 deposited thin films was performed on an Autoprobe CP atomic force microscopy (AFM ) system from Park Scientific Instruments. All images were obtained in a taping mode with standard Si tips with normal radius of 10–20 nm. X-ray diffraction ( XRD) measurements were performed on a Philips X’Pert-MRD system. Cu Ka irradiation was used as the X-ray source in the diffraction measurements. 3. Results and discussion The deposited PBT thin films were set for AFM measurements so as to characterise their surface morphologies and structures. The AFM results are shown in Fig. 2(a–c) corresponding to substrate temperatures of room temperature, 100 and 200°C, respectively. The deposited thin films appear to be composed of cubic nanocrystals. The cubic nanocrystals are very obvious in Fig. 2, regardless of the different substrate temperatures. A detailed comparison among these three figures shows that the stack structure of PBT nanocrystals appears to be improved when substrate temperature is elevated to 200°C. The size distribution is very uniform for the sample deposited at a substrate temperature of 200°C while a low substrate temperature leads to some irregular crystal structures. All the cubic grains have the same size of 100 nm for the thin film deposited at a substrate temperature of 200°C whilst a substrate temperature of 100°C leads to a slightly larger size of 110 nm. A three-dimensional AFM surface profile of the thin film deposited at a substrate temperature of 100°C is shown in Fig. 3. The cubic shapes of the nanocrystals are very obvious, demonstrating the formation of crystalline thin films. Crystal structures are seldom obtained in most depositions of polymer thin films due to the random orientation of the long molecular chains. The formation of crystalline in the deposited thin films also suggests some changes in molecular composition and structure in the resulting thin films as compared to the original target. Fig. 2. AFM surface profiles of PBT thin film deposited at substrate temperatures of (a) room temperature, (b) 100, and (c) 200°C. 390 Z.M. Ren et al. / Surface and Coatings Technology 125 (2000) 388–391 Fig. 5. FTIR spectra of deposited thin films at different substrate temperatures of room temperature, 100 and 200°C. Fig. 3. Three-dimensional AFM surface scanning profile of a deposited PBT thin film. X-ray diffraction measurements were carried out so as to assist the characterization of the deposited thin films. Fig. 4 shows the result of XRD measurements of the thin film deposited at a substrate temperature of 200°C. The diffraction peaks appear to be very sharp at 2h=38.3, 44.5, 78.2 and 82.3°. There are also two peaks from the Si(100) substrate in the spectrum. These sharp diffraction peaks confirm crystalline structures inside the deposited thin films. Some crystal orientations with different d-spacings can be calculated from these diffraction maxima: 0.24, 0.20, 0.12 and 0.11 nm, respectively. This series of d-spacings does not fit any allotropes of carbon such as graphite and diamond. The crystallines in the deposited thin films should be composed of carbon atoms and other elements such as H and S which are the functional groups in the original PBT target. The incorporation of H and S atoms into the carbon crystalline frameworks determines the lattice characters differing from other allotropes. In Fig. 4, two other broad peaks to the low end of diffraction angle at 2h=12.0 and 19.4° are also quite obvious, corresponding to d-spacings of 0.74 and 0.46 nm, respectively. These two peaks with large lattice parameters possibly originate from crystal structures of polymeric compounds that can be traced to the starting PBT target. In our deposition, despite of the random stack of micro-size cubes in the deposited thin films, the surfaces still appear to be smooth with a surface roughness <10 nm as measured by alpha-step. The nanocrystallines inside the deposited polymer thin films exert great influence on their properties, especially conductivity and optoelectronic behaviour. The composition of uniform cubic nanocrystallines of the thin film should be easy to analyse and thus can readily control the optoelectronic and conductive properties of the PBT thin films. The electronic properties of the deposited thin films were investigated by Fourier transform infrared (FTIR) spectroscopy. The FTIR results are shown in Fig. 5. There is no obvious difference among the three FTIR spectra of the samples deposited at room temperature, 100 and 200°C, respectively. In the spectra, the peak at 2644 cm−1 is related to the heterocycle composed of four carbon and one sulphur atoms. The elevating of substrate temperature therefore does not have influence on the chemical binding structures inside the polythiophene nanocrystals in the deposited thin films although it can reduce the size of the cubes as well as compress their size distributions as indicated by the previous AFM measurements. 4. Conclusions Fig. 4. XRD result of a thin film deposited at a substrate temperature of 200°C. Through our experiments, we know that thin films of PBT can be deposited on Si(100) substrates by KrF pulsed excimer laser deposition. The deposited thin films were proven to be purely composed of cubic nanocrystals instead of amorphous structures which happened in most other physical deposition of polymer thin films. The size of the nanocrystals decreases slightly with the elevating substrate temperature up to 200°C. The nanocrystals in the deposited thin films by 200°C substrate temperature have a uniform size of 100 nm. Deposited thin films exhibit good crystal properties with strong and sharp XRD peaks. The substrate temperature Z.M. Ren et al. / Surface and Coatings Technology 125 (2000) 388–391 does not have much influence on the chemical binding structures inside the deposited thin films. Acknowledgements The authors would like to express their thanks to Mr. Goh Yeow Whatt and Miss Koh Hwee Lin for their technical assistance in this research work. References [1] S. Tasch, E.J.W. List, C. Hochfilzer, G. Leising, P. Schlichting, U. Rohr, Y. Geerts, U. Scherf, K. Mullen, Phys. Rev. B 56 (8) (1997) 4479. [2] K. 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