Phthalocyanines (Pc) form a family of planar organic molecules with a metal ion (or 2 hydrogen atoms) at its center. These molecules are thermally and chemically stable. It was found that phthalocyanines have diverse properties, which include optical properties (photoconductivity, dyes, photosensitizers in photodynamic cancer therapy), and electronic properties (gas, pressure, humidity sensors, photovoltaic applications, and electroluminescence devices) [1-4].

Fig. 1: Copper phthalocyanine molecule with 32 carbon (grey), 8 nitrogen (blue), 1 copper (red), and 16 hydrogen (cyan) atoms. The molecule is planar for light metals, such as copper.

Fig. 2: Atomic force microscopy images for iron phthalocyanine (FePc) deposited on a substrate kept at a) room temperature (30 C) and b) at 200 C. The grains are elongated and show molecularly flat plateaus in the case of the heated substrate. If deposited at room temperature, the grains are small and round.

The advance of thin film deposition of organics (OMBE) allows a control of the structural quantities, such as film thickness, grain size, and molecule orientation with respect to the substrate. For example, two parameters to control the structure of phthalocyanines are the substrate temperature during deposition and the type of substrate the phthalocyanine is sublimed onto. The resulting film morphology often impacts the electronic properties [5]. We are using high-resolution x-ray diffraction combined with quantitative refinement calculations to determine the precise orientation and stacking alignments of phthalocyanine molecules in thin films [6,7]. The electronic properties of these organic semiconductors are explored for different geometric configurations, which include: sandwich (perpendicular current) [8], b) interdigitated electrodes (parallel current) [9], and c) organic field effect transistors (OFET). We find that chemical sensors (ChemFETs) for thin films have a superior chemical sensitivity than thicker films. The chemical response to various analytes (methanol, DMMP, H2O2, nitrobenzene, etc) is measured in a flow system with precise temperature and flow concentration control [10-13].

Fig. 3: Phthalocynine chemical sensor prepared at UCSD consisting of 3 different types of metal organic layers (NIPc, CuPc and H2Pc). This design allows the detection and quantification (spectroscopy) of different chemical compounds.

Fig. 4a: Sensor response at 50ºC for different doses of H2O2. Sensor is exposed to the analyte for 300 seconds each time. The analyte concentration is depicted in the figure. Fig. 4b: Sensor response at 50ºC for different doses of DMMP. Sensor is exposed to the analyte for 300 seconds each time. The analyte concentration is depicted in the figure.

[1] E.A. Lawton, The Thermal Stability of Copper Phthalocyanine, J. Phys. Chem., 62, 384 (1958).

[2] C.C. Leznoff and A.B.P. Lever, Phthalocyanines: properties and applications Vol. 1-4, New York, NY : VCH, 1989-c1996.

[3] C. M. Allen, W. M. Sharman and J. E. van Lier, Current status of phthalocyanines in the photodynamic therapy of cancer, J. Porphyrins Phthalocyanines, 5, 161–169, 2001.

[4] C.D. Dimitrakopoulos, D. J. Mascaro, Organic thin-film transistors: A review of recent advances, IBM J. Res. & Dev. 45(1) 11 (2001).

[5] J. Bartolomé, F. Bartolomé, L. M. García, G. Filoti, T. Gredig, C. N. Colesniuc, I. K. Schuller, Highly Unquenched Orbital Moment in Textured Fe-phthalocyanine Thin Films, J. C. Cezar, Phys. Rev. B, 81, 195405 (2010).

[6] G. Liu, T. Gredig, I. K. Schuller, Origin of the Anomalous X-ray Diffraction in Phthalocyanine films, EPL, 83, 56001, (2008).

[7] Casey W. Miller, A. Sharoni, G. Liu, C. N. Colesniuc, B. Fruhberger, and Ivan K. Schuller, Quantitative structural analysis of organic thin films: An x-ray diffraction study, Phys. Rev. B 72, 104113 (2005).

[8] C. N. Colesniuc, R. Biswas, S. Hevia, A. Balatsky, and I. K. Schuller, Exponential Behavior of the Ohmic Transport in Organic Films, Phys. Rev. B, 83, 085414 (2011).

[9] K. A. Miller, R. D. Yang, M. J. Hale, J. Park, B. Fruhberger, C. N. Colesniuc, I. K. Schuller, A. C. Kummel and W. C. Trogler, Electrode Independent Chemoresistive Response for Cobalt Phthalocyanine in the Space Charge Limited Conductivity Regime,Jour. Phys. Chem. B 110, 361 (2006).

[10] J. Park, J. E. Royer, C. N. Colesniuc, F. I. Bohrer, A. Sharoni, S. Jin, I. K. Schuller, W. C. Trogler, A. C. Kummel, Ambient Induced Degradation and Chemically Activated Recovery in Copper phthalocyanine Thin Film Transistors, J. Appl. Phys., 106, 034505 (2009).

[11] R. D. Yang, J. Park, C. N. Colesniuc, I. K. Schuller, J. E. Royer, W. C. Trogler, A. C. Kummel, Analyte Chemisorption and Sensing on n- and p-channel Copper phthalocyanine Thin-film Transistors, J. Chem. Phys., 130, 164703 (2009).

[12] R. Yang, T. Gredig, C. Colesniuc, Ivan K. Schuller, J. Park, B. C. Trogler, A. C. Kummel, Ultrathin organic transistors for chemical sensing, Appl. Phys. Lett. 90, 263506 (2007).

[13] F. I. Bohrer, C. N. Colesniuc, J. Park, M. E. Ruidiaz, I. K. Schuller, A. Kummel, W. C. Trogler, Comparative Gas Sensing in Cobalt, Nickel, Copper, Zinc, and Metal-free phthalocyanine Chemiresistors, JACS, 13, .478 (2009).

(c) 2007 Ivan K. Schuller       -       designed by Thomas Gredig