AVS2004 Session OF+EM-WeM: Molecular and Organic Films and Devices - Electronics
Wednesday, November 17, 2004 8:20 AM in 304C
OF+EM-WeM-1 Self-assembled Monolayers in Organic Electronic Devices
G. Horowitz, P. Lang (ITODYS, University Denis Diderot, France); W. Kalb (RWTH Aachen, Germany); M. Mottaghi, A. Roumiantseva, A. Yassar (ITODYS, University Denis Diderot, France)
The use of molecular materials as active component in electronic devices has recently experience considerable interest. Organic electronic devices are fabricated by piling up several layers on top of each other. Because of that structure, the role of interfaces is crucial in the performance of the devices. An elegant way of controlling the quality of interfaces is the use of self-assembled monolayers (SAMs), which consists of a single layer of molecules chemisorbed on a surface. The molecules are most often based on long alkyl chains that tend to self-assemble to form highly ordered single layers. One end of the chains is substituted with a group capable of inducing chemisorption on the surface, while the other end can be used to control the quality of the modified surface. This technique will be exemplified by two approaches. The first one is a transistor made of a thin film of pentacene deposited on an alumina layer that serves as the gate dielectric. We show that the performance of the transistor is strongly influenced by the modification of the alumina surface by a SAM of a fatty acid. Correlation is made between the structure of the pentacene film and the properties of the transistor. In the second example, the SAM is made of hybrid molecules comprising an alkyl chain connected to a conjugated part (e.g., thiophene or acene). In that case, the SAM constitues the very heart of the device, which may open the way to electronics at the molecular level.
OF+EM-WeM-3 AFM Study of @beta@-7T Oligothiophene Films on Mica: Humidity-Dependent Mechanical Properties and Structure
J.Y. Chen, I. Ratera, D.F. Ogletree, M. Salmeron (Lawrence Berkeley National Laboratory); AR Murphy, JMJ Frèchet (University of California, Berkeley)
Langmuir-Blodgett films of @beta@-7T@footnote1@ oligothiophene, a molecule with potential molecular electronic applications, were transferred to mica and studied with contact AFM. Mono and multilayer films exhibited structural and mechanical properties that depend on humidity, temperature and applied force as well as the LB transfer pressure. We found that monolayers were adsorbed through the carboxylic acid group, exposing the alkyl chains to the air interface. Two structural phases of the molecular film were observed. In the "A" phase the molecules were fully stretched and the film had a height of 2 nm. This phase was stable at high humidity and under low loads. The "B" phase had a height of 1.2 nm and was stable under dry conditions and high loads. Domains of both A and B phases were observed in the same larger islands. Changing humidity reversibly modified the A:B ratio and the sizes of the domains. Heating the samples resulted in an irreversible decomposition of the continuous film islands into small aggregates. @FootnoteText@ @footnote1@ @beta@-7T: a 7 thiophene oligomer with two alkyl side chains and an acetic acid group, 3'',4''''-didecyl-5,2';5',2'';5'',2''';5''',2''''; 5'''',2''''';5''''',2''''''-heptathiophene-4'''-acetic acid.
OF+EM-WeM-4 Fabrication of High Performance O-TFT Devices with Long Term Stability by using Atmospheric Plasma Treament and Passivation Layer
W.J. Kim, H.K. Baik (Yonsei University, Korea)
We report the effect of a surface treament of the various dielectric layers by using atmospheric plasma on the device performance. We also employ the SiO based passivation layer to the top of fabricated devices with plastic-based substrates for the long term stability. With cost-effective atmospheric glide arc plasma treatment, the surface of dielectric layers was modified to fit more well to the organic active layer resulting in the high value of saturation current and field effect mobility. The O-TFT device with our SiO based passivation layer shows good device performance even after it has been exposed to Air in long period of time. Both high performace and long term stablity of the O-TFT devices could be achieved by our cost-effective method.
OF+EM-WeM-5 Pentacene Thin Film Transistors
G.G. Malliaras (Cornell University)
The growth of pentacene films on oxide surfaces plays a major role in determining the device performance. A combination of synchrotron x-ray diffraction and atomic force microscopy was used to probe this interface. In-plane diffraction from films down to one monolayer thick was observed, which allowed to probe the early stages of film growth. Depositions at various substrate temperatures and deposition rates were found to yield films with crystallite sizes from hundreds of nanometers to tens of microns. The performance of these films in thin film transistors was investigated. The scaling of the transistor characteristics down to nanometer size channel lengths is discussed. Finally, applications in sensors are demonstrated.
OF+EM-WeM-7 OMBD of Organic Semiconductors on Metal Surfaces: Structural and Electronic Properties
G. Witte, C. Wöll (Ruhr-Universität Bochum, Germany)
The promising potential of using organic semiconductor materials as active layers for organic electronic applications and the increasing interest in molecular electronics have expressed an urgent necessity of understanding the molecular microstructure and growth properties of ordered organic films. Of particular interest for the fabrication of thin films organic field effect transistors are polycyclic aromatic hydrocarbons which reveal a large variety of structures upon growth on inorganic substrates.@footnote 1@ Here we report results of a comprehensive growth study of pentacene and perylene films on various metal surfaces. By combining LEED, HAS, XPS, NEXAFS, TDS and AFM we were able to characterize the molecular microstructure developing upon film growth. In all cases a characteristic molecular reorientation from a substrate controlled interface phase towards a bulk-like thick film phase was obtained. On particular surfaces such as Cu(110) even epitaxial film growth was achieved.@footnote 2@@footnote 3@ The studied organic films revealed further a pronounced dewetting which favours the formation of crystalline islands upon deposition. Moreover, the electronic properties of thin pentacene films on various metal surfaces were characterized by UPS. It was found that the magnitude of the interface dipole moment is not directly related to the adsorption energy of the molecules at the metal surface but is caused to some extend by an exchange like coupling mechanism. @FootnoteText@ @footnote 1@ G. Witte and Ch. Wöll, J. Mat. Res. (focuss issue Organic Electronics, 2004).@footnote 2@ S. Lukas et. al., ChemPhysChem 5, 266 (2004).@footnote 3@ S. Söhnchen et al., J.Chem. Phys. in print (2004).
OF+EM-WeM-8 A Soluble Photopatternable Pentacene Precursor for Use in Thin Film Transistors
K.P. Weidkamp (University of Wisconsin-Madison); A. Afzali, R.M. Tromp (IBM T.J. Watson Research Center); R.J. Hamers (University of Wisconsin-Madison)
The practical application of pentacene as an organic semiconductor has been hampered by its lack of solubility in common solvents. Here we report the synthesis and characterization of a photopatternable, soluble pentacene precursor. This precursor, based on an N-sulfinylcarbamate adduct of pentacene, is converted back to pentacene at relatively low temperatures in the presence of acid catalyst and can be patterned by way of chemical amplification in the presence of photoacid generator. After UV illumination, a short, low-temperature post-exposure bake is used to convert the precursor to pentacene and the unexposed area is then washed away with solvent. A very short higher temperature anneal helps to form a crystalline film of pentacene that has mobilities in the range of 10@super -2@ cm@super 2@ V@super -1@ s@super -1@. Features can be patterned as small as 10 µm, small enough for use in many applications such as active matrix displays.
OF+EM-WeM-9 Electronic Polarization at the Pentacene - Gold Interface
F. Amy, A. Kahn (Princeton University)
Pentacene has been successfully used as a high-mobility hole transport material. One of the reasons is that the energy of relaxation of the molecular ion is smaller by at least a factor of two than in other commonly used hole transport materials@footnote 1@. However, the full benefit of this relatively high mobility can be realized only if charge carrier injection is not a significant bottleneck in the device. The pentacene/metal interface energetics are therefore of prime importance. We focus here on the specific issue of polarization and narrowing of the transport gap at the pentacene/Au interface. Tsiper et al.@footnote 2@ have shown both experimentally and theoretically for PTCDA/Au that the polarization induced by an electron (P@sub -@) or a hole (P@sub +@) resident on a molecule at the interface increases by ~0.2 eV with respect to the polarization in the bulk of the film. This increase is due to the large polarizability of the metal substrate and results in a narrowing of the transport gap at the interface. This, in turns, affects the modeling of charge injection at interfaces. In this work, we use ultra-violet and inverse photoemission spectroscopy (UPS, IPES) to measure highest occupied and lowest unoccupied molecular orbitals (HOMO, LUMO) of pentacene as a function of film thickness, from one to ten molecular layers. HOMO and LUMO levels represent the hole and electron transport levels, respectively. We find the increase in polarization at the interface to be in line with that measured on PTCDA. @FootnoteText@ @footnote 1@ N.E. Gruhn, et al. J. Am. Chem. Soc., 124, 7918 (2002)@footnote 2@ E.V. Tsiper, Z. Soos, W. Gao and A. Kahn, Chem. Phys. Lett. 360, 47 (2002) .
OF+EM-WeM-10 Planar Molecular Networks Built by 1D and 2D Polymerization
M. Stöhr, M. Wahl, M. De Wild (University of Basel, Switzerland); C.H. Galka, L.H. Gade (University of Heidelberg, Germany); T.A. Jung (University of Basel and Paul Scherrer Institute, Switzerland); H.-J. Güntherodt (University of Basel, Switzerland)
Self-assembly of molecules on surfaces directed by different supramolecular interactions has been widely explored. There are striking examples of molecular surface structures, whose formation is driven by metal co-ordination, dipolar coupling or hydrogen bonding. In contrast to these examples, our aim is the formation of covalently linked planar structures by means of polymerization confined in one or two dimensions. The perylene derivative (DPDI) we investigated belongs to a class of compounds which serve as precursors for the production of photovoltaic devices. Recent investigations using differential thermoanalysis and gravimetry demonstrated that bulk DPDI can polymerize releasing ammonia. Inspired by this observation, we tried to exploit the formation of covalent networks on metallic surfaces and to check the feasibility of such an approach for the formation of stable polymer-nanostructures. For this purpose, thin films of DPDI were prepared on Ag(111) and Cu(111) by evaporation in a UHV setup. In a first step, the supramolecular arrangements were analyzed with a home-built STM. A condensed phase with a rectangular unit cell was found on both substrates if the coverage was in the range of 1ML. After annealing to 580K, a rearrangement of the DPDI molecules into a rhombic unit cell structure was observed. This symmetry change was accompanied by the appearance of a link between individual perylene groups which we identify as covalent bonds. Further evidence, in favor of a covalent bond formation is provided by the shortening of the intermolecular distance of the ad-molecular patterns after the thermal activation. We identify these structures as arrays of 1D polymer rows. For lower coverage in the range of 0.3ML only a mobile phase was detected before annealing. However, upon annealing to 580K a stable 2D network with a honeycomb-like structure was observed which conveniently matches the angles and distances expected for the chemically feasible polymer structure.
OF+EM-WeM-11 UHV-STM/XPS Studies of ACA Molecular Assemblies on Ag(111)
B. Xu, D. Evans, B. Varughese, J. Reutt-Robey (University of Maryland)
The structures of acridine carboxylic acid (ACA) molecular films grown on Ag (111) by physical vapor deposition were characterized using UHV-STM and XPS. On large terraces (with terrace-spanning diameters exceeding 100 nm), ACA molecules form well ordered 2-d islands. The driving force for island formation is attributed to hydrogen-bond formation between ACA molecules (attractive along [1-10] ) and substrate mediated attraction along [11-2]. XPS experiments reveal that H-bond interaction involves the ring nitrogen acting as the H-bond acceptor(O-H...N). On narrow terraces (~3 nm in width) completely different ACA structures are observed. In these confined regions, ACA molecules assemble into dimers, which are further organized into coverage-dependent adlayer structures. Finally, we report the observation of bi-layer and tri-layer structures.