AVS2004 Session OF+NS-ThM: Molecular Electronics

Thursday, November 18, 2004 8:20 AM in Room 304C

Thursday Morning

Start	Invited?	Item
8:20 AM		OF+NS-ThM-1 Molecular Engineering to Test the Mechanism of Conductance Switching for a Variety of Conjugated Molecules A.M. Moore, B.A. Mantooth, A.A. Dameron, Z.J. Donhauser (The Pennsylvania State University); J.W. Ciszek, F. Maya, Y. Yao, J.M. Tour (Rice University); P.S. Weiss (The Pennsylvania State University) Phenylene ethynylene oligomers have been studied as candidates for molecular electronic devices using scanning tunneling microscopy.^1-6 These molecules were inserted into host alkanethiolate self-assembled monolayers for isolation and individual addressability. Many different hypotheses and theoretical predictions have been put forth to describe conductance switching.3-4 We have tested several of these through variations in the molecular design of our molecular switches and have concluded that the only mechanism consistent with all the switching data are that changes in the molecule-substrate bond hybridization leads to the observed conductance changes. ¹Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. 2001, Science 292, 2303. ²Donhauser, Z. J.; Mantooth, B. A.; Pearl, T. P.; Kelly, K. F.; Nanayakkara, S. U.; Weiss, P. S. 2002, Jpn. J. Appl. Phys. 41, 4871. ³Mantooth, B. A.; Weiss, P. S. 2003, Proc. IEEE 91, 1785. ⁴Moore, A. M.; Dameron, A. A.; Mantooth, B. A.; Yao, Y.; Ciszek, J. W.; Maya, F.; Tour, J. M.; Weiss, P. S. 2004, Nanotechnology In press. ⁵Lewis, P. A.; Inman, C. E.; Yao, Y. X.; Hutchison, J. E.; Tour, J. M.; Weiss, P. S. In Preparation. ⁶Dameron, A. A.; Ciszek, J. W.; Tour, J. M.; Weiss, P. S. In Preparation.
8:40 AM		OF+NS-ThM-2 Room Temperature Molecular Memory Observed from a Nanowell Device N. Gergel, N. Majumdar (University of Virginia); K. Keyvanfar, N. Swami (University of Virgina); L.R. Harriott, J.C. Bean (University of Virginia) Researchers are debating whether the electrical switching behavior observed from some molecular devices can be attributed exclusively to the molecules¹. We tested an OPE molecule with a nitro side group. This molecule showed electrical switching behavior when tested in a nanowell device at room temperature. This behavior was not seen when testing a simple conjugated molecule that lacked the nitro group. The test was performed in a nanowell device that consisted of a monolayer of molecules self-assembled on an area of gold 10-40 nm in diameter and capped with titanium and gold². The I-V characteristics of the nitro molecule clearly showed two distinct conductivity states with a current ratio of 5 to 1 at room temperature. The experimental data showed that at a particular threshold voltage, the output current changed from a high current state to a low current state. This change in state was reversed with the opposite applied voltage. Hysteresis was not observed when testing a similar conjugated molecule without the nitro group at room temperature. Thus, the switching behavior could be attributed to changes in the conductivity of the nitro molecules due to the applied voltage. Other groups have reported seeing negative differential resistance behavior (NDR) in the I-V characteristics of the nitro molecule³. We saw similar peaks to those reported. However, our investigations indicate that this behavior is not reversible without the application of a negative threshold voltage. For this reason, these molecules may not be suitable for classic NDR circuits (e.g. Goto pairs⁴). This hysteretic behavior may nevertheless have device potential. ¹ R.F. Forrest, Science 302, 556 (2003). ²N. Majumdar, et al, submitted to J. Vac. Sci. Tech. ³ M.A. Reed, et al, Applied Physics Letters 78, 3735 (2001). ⁴ G. Rose, and M. Stan, IEEE NANO, San Fransisco, Aug (2003).
9:00 AM	Invited	OF+NS-ThM-3 Metal/Molecule/Metal and Metal/Molecule/Semiconductor Device Structures D. Janes (Purdue University) This talk will describe the development and electrical characterization of two classes of molecular electronic components. The first class of structures involves metal-molecule-metal systems with pre-formed metal contacts, primarily lateral break junctions formed either by electromigration or by shadow evaporation. A number of molecular species have been studied using these structures, including short aromatic thiols and short DNA double strands with thiol bonding groups at each end. The electrical characteristics of these devices indicate that strong coupling between the contacts and the molecular species can be realized. The second class of devices involves metal/molecule/semiconductor device structures, which are lithographically defined and fabricated using an indirect evaporation technique for the metal (top) contact and p+ GaAs for the bottom contact. In these structures, the electronic conduction between the metal and semiconductor can be modulated by choice of molecular species. Several alkyl thiol and aromatic thiol molecules have been employed in order to determine the effects of molecular length, conjugation and intrinsic dipole moment. The current-voltage characteristics and conductance versus temperature both indicate that the molecular layers change the transport mechanism, generally involving a lower effective barrier height than that of a metal/semiconductor Schottky barrier. These results reflect previous studies in which nanoscale metal/molecule/semiconductor structures exhibited low resistance contacts, implying that effective coupling and control of the surface electrical properties can be achieved using a molecular layer.¹ A simple model for the conduction has been developed, utilizing our prior studies on surface Fermi level unpinning in GaAs structures.² ¹ T. Lee, et al., APL 76, 212 (2000). ² S. Lodha, et al., Appl. Phys. Lett. 80, 4452 (2002).
9:40 AM		OF+NS-ThM-5 Room Temperature Negative Differential Resistance Measured through Molecular Monolayers Adsorbed to Silicon Surfaces with Ultra-high Vacuum Scanning Tunneling Microscopy N.P. Guisinger, R. Basu, M.E. Greene, A.S. Baluch, M.C. Hersam (Northwestern University) In recent years, substantial progress has been made in the emerging field of molecular electronics. In particular, metal-molecule-metal junctions have been widely studied. In this paper, a continued study of charge transport through molecule-semiconductor junctions is considered. The presence of the energy band gap in semiconductors provides opportunities for resonant tunneling through individual molecules, leading to interesting effects such as room temperature negative differential resistance (NDR).¹ In this study, the ultra-high vacuum scanning tunneling microscope was used to probe charge transport through two different molecular monolayers adsorbed to the Si(100) substrate. I-V measurements were taken on monolayers of TEMPO and cyclopentene for both degenerately doped n-type and p-type Si(100) substrates. Initial I-V measurements through the TEMPO monolayer exhibited a suppression of NDR behavior relative to previously reported transport through isolated molecules.¹ I-V measurements were also taken on isolated cyclopentene molecules, as well as on cyclopentene monolayers. The cyclopentene monolayers similarly exhibited a suppression of the observed NDR behavior relative to transport through isolated molecules. For both molecular monolayers, the suppression of the peak-to-valley ratio (PVR) has been measured to exceed a 47 percent reduction compared to observed PVRs of isolated molecules. The resulting NDR suppression in both monolayers indicate that the local environment surrounding the molecules strongly influences charge transport. In addition to molecular monolayers, initial studies of transport through isolated TEMPO molecules adsorbed to both degenerate and non-degenerate Si(111) will be discussed. ¹N. P. Guisinger, M. E. Greene, R. Basu, A. S. Baluch, and M. C. Hersam, "Room temperature negative differential resistance through individual molecules on silicon surfaces," Nano Letters, 4, 55 (2004).
10:00 AM		OF+NS-ThM-6 Two Distinct Types of Switching Behavior in a Single Molecule A.S. Blum, J.G. Kushmerick, C.H. Patterson (Naval Research Laboratory); J.C. Yang (Duke University); J.C. Henderson, Y. Yao, J.M. Tour (Rice University); R. Shashidhar (Geo-Centers, Inc.); B.R. Ratna (Naval Research Laboratory) There is recent controversy surrounding the ability of molecules to function as switches in molecular electronic devices. We report the observation of two distinct types of switching in matrix isolated and complete monolayers of bipyridyl-dinitro-oligophenylene-ethynylene (BPDN). Extensive measurements in a scanning tunneling microscope (STM) demonstrate both stochastic and voltage driven switching in this molecule, representing the first description of two distinct types of switching in a single molecule. While stochastic switching has been reported for several molecular systems, we argue that the observed voltage controlled switching is a distinct physical event specific to BPDN. Furthermore, consistent switching behavior measured with both a scanning tunneling microscope and a crossed-wire tunnel junction demonstrates that the switching is intrinsic to the molecule and is not an artifact of the measurement system.
10:20 AM		OF+NS-ThM-7 Electrical and Mechanical Contacts at the Atomic Scale: a Combined UHV STM/AFM Study Y. Sun, M. Henrik, S. Schaer, Y. Miyahara, A.-S. Lucier, M.E. Ouali, P. Grutter (McGill University, Canada); W. Hofer (University of Liverpool, United Kingdom) Understanding electrical contacts is widely considered as one of the central issues in molecular electronics. As a first step, we have measured simultaneously at the atomic scale the interaction forces and the currents between a sharp tungsten tip and a Au(111) sample using a combined ultra-high vacuum scanning tunnelling and atomic force microscope (UHV STM/AFM). Close correlation between conductance and interaction forces were observed in the regimes from weak coupling to strong interaction. In particular, the electrical and mechanical points of contact are defined as a result of the observed barrier collapse and adhesive bond formation, respectively. The points of contact as defined by force and current measurements coincide within measurement error. We find experimentally that at contact the very front atoms of the tip apex experience repulsive forces, while the total interaction force remains attractive as a consequence of competing interaction decay lengths. Ab-initio calculations of the current as a function of distance were performed for our experimental tip-sample system. We find that in the weak coupling regime the calculated electrical current as a function of distance is in quantitative agreement with experimental results only if tip and sample relaxation effects are taken into account. The calculated relaxation of the tip apex atoms is 50-100 pm. We conclude that force effects of different decay lengths cannot be excluded if a detailed understanding of atomic scale contacts is to be achieved.
10:40 AM		OF+NS-ThM-8 Structure of Self-Assembled Monolayers on Platinum D.Y. Petrovykh (University of Maryland, Naval Research Laboratory); H. Kimura-Suda, A. Opdahl, L.J. Richter, R.D. Van Zee, M.J. Tarlov (National Institute of Standards and Technology); L.J. Whitman (Naval Research Laboratory) We studied formation of self-assembled monolayers (SAMs) on polycrystalline platinum thin films using X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), spectroscopic ellipsometry, and contact angle measurements. In particular we are interested in properties of SAMs on Pt with respect to their possible use as a substrate for Si-compatible molecular electronics. We find that SAMs formed on piranha-cleaned Pt from ethanoic solutions of n-alkanethiols have initial quality comparable to or better than that achieved under other conditions. The FTIR and XPS data indicate that films are formed with nearly normal orientation of alkane chains, have higher packing density than comparable SAMs/Au, and remain stable in ambient air for 3-5 days. XPS spectra of the S 2p region also show that SAMs/Pt are distinctly multicomponent. The main component, with the lowest binding energy, unambiguously corresponds to alkanethiol molecules adsorbed on Pt in a configuration similar to that for SAMs/Au. The minority higher binding energy components are not affected by exposure to good solvents and thus appear to correspond to different binding configurations related to surface roughness and oxidation (rather than to physisorbed molecules). Our results suggest that the use of oxide-free and atomically smooth Pt substrates may be necessary to attain a single-component, high-quality SAM on Pt.
11:00 AM		OF+NS-ThM-9 Oligomer Length Dependent Study of Metal-Molecule Interactions in Model Molecular Wire Systems C.D. Zangmeister, S.W. Robey, R.D. Van Zee (NIST) Interactions at the molecule-contact interface perturb the molecular orbitals important for electron transport in conjugated systems. These perturbations are particularly important for the nanometer size junctions applicable in molecular-scale electronics. This study looks at the variation of the occupied electronic structure of fully conjugated model molecular wire compounds as a function of molecular length. Specifically, the number of phenyl rings was varied from a single ring to three rings in unsubstituted phenylene ethynylene monolayers chemisorbed on Au using ultraviolet photoemission spectroscopy. This provides a qualitative picture of the extent of perturbation of the electronic structure due to thiol coupling and the variation of the molecular π levels important for electron transport as a function of the degree of conjugation. These data show a shift towards the Au Fermi levels in the π levels by more than an eV as the conjugation length is increased. These observations will be discussed in terms of previous electron transport investigations of these compounds adsorbed on Au.
11:40 AM		OF+NS-ThM-11 Electrical Properties of DNA Characterized by Conducting-Atomic Force Microscopy C. Nogues, S.R. Cohen, S. Daube, R. Naaman (Weizmann Institute of Science, Israel) DNA has been widely promoted as the key component of future molecular devices, due to its unique assembly and recognition properties. Specifically, the strong interaction between complementary base-pair sequences on interacting DNA strands can be utilized to self-assemble a desired structure in a molecular circuit. The most basic building blocks of such circuits can be formed through the hybridization of two single strands into a double one, and the specific binding of functionalized DNA strands to gold electrodes using the chemical thiol-gold linkage. Such manipulations can exploit the ease in which DNA strands can be synthesized, and modified chemically. Despite these advantages, the electrical properties of individual strands of DNA have yet to be reproducibly characterized, due to the inherent difficulties in reliably accessing and measuring single molecules. We have attacked this problem by developing protocols for reproducible formation and characterization of DNA monolayers, and then probing their electrical functioning using conducting atomic force microscopy (cAFM). The electrical contact to the DNA was made by chemical binding to a gold electrode on one end, and to a gold nanoparticle on the other. Thus, repeatable measurements of the conductivity of individual DNA strands have been performed. Meaningful differences could be detected between conductivity in single- and double-strand DNA. The single strand DNA was found to be insulating over the range of -2 to +2 V, while the double strand DNA passes significant current outside a 3 eV gap.