Anisotropies in the electrical properties of rod-like aggregates of liquid crystalline phthalocyanines: Direct current c
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Samir K. Cherian, Lynn LaRussa, Bernard Kippelen,c) Benoit Domercq,c) and David L. Mathined) Department of Optical Sciences and Department of Electrical and Computer Engineering, University of Arizona, Tucson, Arizona 85721
David F. O’Brien and Neal R. Armstrongd) Department of Chemistry, University of Arizona, Tucson, Arizona 85721 (Received 4 November 2003; accepted 12 April 2004)
The direct current (dc) conductivities and organic field-effect transistor (OFET) characteristics of a class of octa-substituted liquid crystalline (discotic mesophase) phthalocyanines (Pcs) are discussed. These molecules self-organize into columnar aggregates with large coherence lengths (up to approximately 300 nm). Langmuir–Blodgett films of these molecules were horizontally transferred to either interdigitated microelectrodes (IME) or OFET substrates, so that current flow could be measured either parallel or perpendicular to the column axis. Twenty-eight bilayer films of these Pcs on the IME substrates showed anisotropies in dc conductivity up to 50:1, whereas similar Pc films showed anisotropies in field effect mobilities of approximately 10:1, for a variety of W/L ratios (source/drain dimensions and spacing). Field-effect mobilities of 1 to 5 × 10−6 cm2·V−1·s−1 were determined from OFET measurements, along the Pc column axis, whereas charge mobilities measured from the space charge limited current regime on the IME substrates were in the range of 10−4 cm2·V−1·s−1. Conductive tip atomic force microscopy measurements on the apprximately 500-nm length scale showed that the conductivity anisotropy can be as high as 1000:1 when the Pc columns are intimately contacted to an adjacent Au bond pad.
I. INTRODUCTION
Many liquid crystalline discotic mesophase materials self-organize from common solvents into coherent columnar, rod-like aggregates, and have become of interest as potential solution-processed molecular electronic materials.1–20 Side-chain modified phthalocyanines, porphyrins, hexabenzocoronenes, and triphenylenes have been created that show reasonable liquid crystalline (LC) transition temperatures (K → Dh) and can be readily processed into thin films. The side chains surrounding
a)
Current address: Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, England. b) Current address: National Institute for Standards and Technology, Gaithersburg, MD. c) Current address: Electrical Engineering, Georgia Tech University, Atlanta, GA 30332. d) Address all correspondence to these authors. e-mail: nrau.arizona.edu DOI: 10.1557/JMR.2004.0278 J. Mater. Res., Vol. 19, No. 7, Jul 2004
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the discotic molecular core help control self-organization into these rod-like columns, and because of their closepacking and electrically insulating nature, their presence may eventually provide for dense lateral integration of organic field-effect transistor (OFET) devices, preventing significant electrical cross-talk between adjacent columns. These side chains, however, may also create
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