The use of organic materials presents a tremendous opportunity to significantly impact the functionality and pervasiveness of large-area electronics. Commercialization of this technology requires reduction in manufacturing costs by exploiting inexpensive low-temperature deposition and patterning techniques, which typically lead to lower device performance. We report a low-cost approach to control the microstructure of solution-cast acene-based organic thin films through modification of interfacial chemistry. Chemically and selectively tailoring the source/drain contact interface is a novel route to initiating the crystallization of soluble organic semiconductors, leading to the growth on opposing contacts of crystalline films that extend into the transistor channel. This selective crystallization enables us to fabricate high-performance organic thin-film transistors and circuits, and to deterministically study the influence of the microstructure on the device characteristics. By connecting device fabrication to molecular design, we demonstrate that rapid film processing under ambient room conditions and high performance are not mutually exclusive.

Scanning tunneling microscopy (STM) was used to investigate the reaction of 1,3-cyclohexadiene with Si dimers on the bare Si(100) surface. Detailed, high-resolution STM images show a distribution of surface products. In addition to the intradimer [2 + 2] and [4 + 2] reaction products previously reported, two different [2 + 2] conformers are identified. Empty state STM images also show interdimer [4 + 2] reactions involving neighboring dimers in the same row and in adjacent rows. The former are shown to be the dominant surface product despite the presence of unpaired dangling bonds on two adjacent dimers. These results are interpreted in terms of a reaction mechanism in which the reduced ring strain favors the kinetic interdimer [4 + 2] product over the thermodynamically stable [4 + 2] intradimer product.

Thin-film electronic devices fabricated using organic semiconductors, insulators, and conductors are expected to greatly impact the semiconductor electronics industry by significantly reducing manufacturing costs and expanding the applicability of active thin-film electronics. With that, organic thin-film transistors (OTFTs) have been a topic of intense interest for over a decade. [1][2][3] Despite significant improvements to the electrical performance of OTFTs over this time, surprisingly little is known about the fundamental mechanisms governing charge injection and transport in the transistor channel. Scanning probe microscopy methods have become the popular tool for studying the intricate details of these systems. Early work utilized conductive-probe atomic force microscopy (AFM) measurements at discrete points within OTFTs to probe potential drops at the device contacts [4] and at grain boundaries [5,6] within the active organic layer.Following these experiments, several groups have reported the use of scanning Kelvin probe microscopy (SKPM) to characterize charge transport in a range of OTFTs. [7][8][9][10][11][12] SKPM affords unique insight into device behavior since it provides the spatially resolved surface potential map of an operating device. To date, SKPM studies have focused on the influences of contact resistance on device performance, including comparisons of top-versus bottom-contact devices [7] and the role of source and drain electrode metallurgy. [8,10] Additional SKPM studies have investigated the variation in surface potential at grain boundaries in evaporated organic thin films, similar to those used in OTFTs. [13] Perhaps most importantly, in a number of these earlier experiments it was found that the potential drop at the electrode/organic semiconductor interface can be on the order of several volts, indicating that the electrical performance of the devices in those instances is contact-limited. [4,7,8,10] In this paper, we present an SKPM study of the potential distribution in solution-processed, bottom-contact OTFTs to determine the role of film microstructure on device performance. By examining devices with a series of channel lengths, we are able to correlate the potential profile across the device and the morphology of the organic layer within the channel to the measured device mobility. Thin-film transistors prepared from spun-cast fluorinated 5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT) on pentafluorobenzene thiol (PFBT) [14] functionalized electrodes (see schematic in Fig. 1) provide an excellent platform to study potential profiles in functioning devices since the unique properties of the system lead to the formation of large (%10 mm) grains that nucleate and grow from the electrodes into the channel region. [15,16] Consistent with this grain growth, we typically observe three unique microstructures within the transistor channel: first a highly ordered channel, with large grains bridging the source-drain gap (L ¼ 5 mm), a second microstructure for which contact n...

Precision current measurements are recorded at 5 K during the approach and contact between a Pt-inked probe and the carbon-carbon double-bond region of an isolated 1,3-cyclohexadiene molecule chemisorbed on a Si(100) surface. Scanning tunneling spectroscopic data reveal systematic features in the current at specific probe-molecule separations. Aided by density functional theory calculations, we show that these features arise from interaction forces between the probe and molecule, which can be interpreted as the relaxation of the probe-molecule system prior to and during contact. DOI: 10.1103/PhysRevLett.97.098304 PACS numbers: 82.37.Gk, 39.25.+k, 68.43.ÿh, 81.16.Ta There is enormous interest in the potential use of molecules in future electronic device technologies [1][2][3]. Although molecular electronics has the potential for the highest possible integration densities, the major difficulty in advancing this technology is establishing reliable contacts with molecules [1,4]. Not only is the influence of the contacting metal not well understood but the manner in which the contact perturbs the properties of molecules is a complete unknown [2,5], and precludes the rational design of molecular devices. In this Letter we introduce a novel STM-based method to measure the contact forces and interactions between a metallic probe and a single molecule. We show that, for a Pt-metal-''inked'' STM probe and a 1,3-cyclohexadiene (1,3-CHD) molecule on the Si(100) surface, there is a repulsive barrier at large separations followed by an attractive interaction associated with contact and chemical bond formation. This method is quite general and applicable to a wide range of molecular systems.To study the interaction between a single molecule and a STM probe it is necessary to develop a reliable method to control the chemical composition of the probe. This is accomplished by placing a single crystal metal sample [in this case Pt(111)] and the Si(100) substrate s[n -type As, <5 m cm] together in a sample holder that allows each to be heated and prepared. Each sample was then characterized by LEED, exposed at room temperature to less than 0.1% of a monolayer of 1,3-CHD, cooled to 80 K, transferred into a Createc LT-STM and cooled to 5 K for imaging [see Figs. 1(a) and 1(b)] and spectroscopic analysis [see Figs. 1(c)-1(e)]. Tungsten STM probes were cleaned by electron bombardment and then inked by drawing Pt atoms from the surface onto the end of the probe (see below). Figure 1(c) shows that the current increases exponentially during the approach to the Pt(111) surface, after which there is a sudden jump associated with contact formation [6 -8]. The exponential behavior of the current I follows the well-known distance dependence of the tunneling current [9-11]:where A is the apparent tunneling barrier height, Z is the probe-surface separation, and A 2 8m e 1=2 =h. The Pt surface is locally melted in the contact area and when the probe is withdrawn there is a clear hysteresis in the current due to the formation of a Pt metal nec...