Laser assisted molecular beam deposition of thin films of polymeric copperphthalocyanine and their characterization
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Abstract Thin films of polymeric copperphthalocyanine were fabricated via laser assisted molecular beam deposition by reacting the laser ablated plasma flume of a copper target with 1,2,3,5tetracyanobenzene entrained within a molecular beam expansion. 1,2,4,5-Tetracyanobenzene entrained within a helium carrier gas was supersonically expanded to intercept a second expansion containing gas-phase aggregates of metallic copper. These two molecular beams met at a point between the target and the substrate producing gas phase polymeric copperphthalocyanine which could be then directly deposited as a thin film coating on room temperature substrates. Such films were characterized by scanning electron microscopy and uvvisible, infrared and x-ray photoelectron spectroscopic techniques. Optical spectra of films dissolved in H2SO 4 solutions reveal that small amounts of monomeric copperphthalocyanine is also present in the grown films. All the copper in these films is found to be in Cu÷2 ionization state.
Introduction Organic polymers are considered to be one of the most hopeful materials for future application in photonics and electrooptic devices.1 3 In this regard, metallophthalocyanines have received a considerable attention because of their photocatalytic and pohoconductive properties as well as their remarkable thermal and chemical stabilities.4.s Pressed powder of the copperphthalocyanine polymer (poly-CuPc) shows p-type semiconductivity with a hole concentration of 1016 cm"3 and a mobility of 10 cm2 V1 s-', which is considered to be a significantly large value.6 Also, poly-CuPc absorbs visible light very strongly, hence, it may be useful in solar energy conversion devices. Despite these interesting properties, investigations of poly-CuPc are scarce primarily due to the difficulty in processing it in the form of thin films.7 9 To employ it in electronic switches, sensors and photoresponsive devices it is necessary to fabricate thin films of poly-CuPc with micron spatial resolution. The lack of solubility of polyCuPc in common solvents prevents the use of solution based thin films fabrication techniques 621
Mat. Res. Soc. Symp. Proc. Vol. 354 0 1995 Materials Research Society
such as spin coating, dip coating and Langmuir-Blodgett method. Also, thermal evaporation procedures can not be employed since poly-CuPc decomposes prior to melting. Hence, some attempts have been made to fabricate thin films of poly-CuPc by double source evaporation 0 technique.' Poly-CuPc can be prepared from 1,2,4,5-tetracyanobenzene (TCNB) and copper salts or copper powders as shown in the following reaction. 9.11
NCN
NC)
CN
NCM N
CuCN
Heat
N-N-
N
CN
However, the films prepared by double source evaporation of Cu and TCNB (at lxl0torr) are not poly-CuPc, but are merely mixtures of TCNB and Cu, almost without any reaction.' 0
It is necessary to heat treat the products, at elevated temperatures (350 TC), in an oxygen free atmosphere, in order to then convert them into poly-CuPc. The presence of traces of oxygen (lxl0s torr air atmosp
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