RESEARCH ARTICLE JOURNAL OF NANOSCIENCE AND NANOTECHNOLOGY Morphology and Current–Voltage Characteristics of Nanostructured Pentacene Thin Films Probed by Atomic Force Microscopy Serkan Zorba, Quoc Toan Le, Neil J. Watkins, Li Yan, and Yongli Gao Department of Physics and Astronomy, University of Rochester, Rochester, New York 14627, USA Atomic force microscopy was used to study the growth modes (on SiO 2 , MoS 2 , and Au substrates) and the current–voltage (I–V) characteristics of organic semiconductor pentacene. Pentacene lms grow on SiO 2 substrate in a layer-by-layer manner with full coverage at an average thickness of 20 Å and have the highest degree of molecular ordering with large dendritic grains among the pen- tacene lms deposited on the three different substrates. Films grown on MoS 2 substrate reveal two different growth modes, snowake-like growth and granular growth, both of which seem to compete with each other. On the other hand, lms deposited on Au substrate show granular structure for thinner coverages (no crystal structure) and dendritic growth for higher coverages (crystal struc- ture). I–V measurements were performed with a platinum tip on a pentacene lm deposited on a Au substrate. The I–V curves on pentacene lm reveal symmetric tunneling type character. The eld dependence of the current indicates that the main transport mechanism at high eld intensities is hopping (Poole–Frenkel effect). From these measurements, we have estimated a eld lowering coefcient of 9077 10 ƒ6 V ƒ1=2 m 1=2 and an ideality factor of 18 for pentacene. Keywords: Nanostructured Pentacene Film, Growth Modes, Morphology, Current–Voltage Charac- teristics, Atomic Force Microscopy, CP-AFM. 1. INTRODUCTION For the last one and a half decades we have witnessed how the atomic force microscope 1 has become an indis- pensable tool in the area of science and technology for the study of a whole range of material properties in the nanoscale regime with very high spatial resolution. The atomic force microscope, with its ever expanding family, enables us to characterize and correlate material proper- ties such as topographical, mechanical, electrical, thermal, and optical. Molecular electronics is one of the major elds in which the atomic force microscope is being employed widely to investigate the material properties of organic semiconductors that are used as active materials in organic thin lm transistors (OTFTs) and organic light emitting diodes (OLEDs). Among these organic semiconductors, pentacene is one of the most promising materials due to its relatively high electron and hole mobility. 2 It is well known that the growth mode of pentacene will affect the mobil- ity considerably. 3 The high mobility of pentacene results partly from the fact that it forms a crystalline structure with high molecular ordering, as observed in atomic force microscopy (AFM) and X-ray diffraction experiments. 3 Author to whom correspondence should be addressed. This makes pentacene a model organic semiconductor for charge carrier transport studies of organic materials. In this article, we present the results of our recent study of the morphology of pentacene lms deposited on SiO 2 , MoS 2 , and Au substrates and the I–V characteristics of pentacene deposited on a Au substrate using AFM. We investigated the morphology of pentacene lms to see how the ordering of the pentacene lms changes with dif- ferent substrates. It is well known that organic molecules deposited onto an inorganic substrate can result in three different growth scenarios. 4 First, if the substrate is chemically inert, the inuence of the substrate on the growth of the organic lm is minimal. Thus the organic molecules will move freely on the surface and form large, separated crystals after adsorption (Volmer–Weber mode or three-dimensional island growth). Second, if the substrate has highly reac- tive bonds, the organic molecules will be strongly bound to the substrate as soon as they hit the surface (Frank–Van Der Merwe mode or layer-by-layer growth). This will pre- vent the self-ordering mechanism within the organic lms that is crucial for ordered growth; hence, disorder will result. The third scenario is a moderate combination of the rst two schemes; that is, the interaction between the substrate and the organic molecules occurs in such a way that the substrate has strong enough bonding to dictate its J. Nanosci. Nanotech. 2001, Vol. 1, No. 3 © 2001 by American Scientic Publishers 1533-4880/2001/03/317/005/$17.00+.25 doi:10.1166/jnn.2001.050 317