Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session P16: Focus Session: Molecular-Scale Electronics III |
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Sponsoring Units: FIAP Chair: Shashi Karna, Army Research Laboratories Room: Baltimore Convention Center 312 |
Wednesday, March 15, 2006 11:15AM - 11:27AM |
P16.00001: Molecular sensing using point contact conductivity modulation Adam Dickie, Robert Wolkow The electrical properties of semiconductors are sensitive to external influences, such as the adsorption of gaseous molecules. For single crystal Si surfaces, the change in conductivity induced by molecular adsorption is a very small fraction of the bulk conductivity, precluding their use as efficient sensors. Here we show that point contacts on Si surfaces in UHV environments can overcome this fundamental limitation, through the use of minority-carrier-induced conductivity modulation. Point contacts made to clean, low-doped $n-$Si(100) produce significant surface inversion layers. The inversion layer minority-carrier population is exponentially dependent upon surface charge. Slight increases in the surface charge density, from gas molecule adsorption, are detected as large increases in sample conductivity, as electrons flow in to balance the positive hole space charge. The sensitivity of this simple device structure is so high that physisorption of inert gas molecules such as He, N$_{2}$, and Ar can be detected as conductivity increases of 2 -- 100{\%}; the specific response is proportional to the molecular ionization potential. Decreasing the point contact size, from micro- to nano- to atomic-scale, increases device sensitivity because of increased minority-carrier injection ratios. [Preview Abstract] |
Wednesday, March 15, 2006 11:27AM - 11:39AM |
P16.00002: Humidity dependence of molecular tunnel junctions with an AlOx/COOH- interface Xiaohang Zhang, Stephen McGill, Peng Xiong We have studied the electron transport in planar tunneling junctions with aluminum oxide and an organic self-assembled monolayer (SAM) as the tunnel barrier. The structure of the junctions is Al/AlOx/SAM/(Au, Pb) with a junction area of $\sim $ 0.4mm$^{2}$. The organic molecules investigated include mercaptohexadecanoic acid (MHA), hexadecanoic acid (HDA), and octadecyltrichlorosilane (OTS); all of which form ordered SAMs on top of aluminum oxide. The use of a superconducting electrode (Al) enables us to determine unambiguously that these are high-quality tunnel junctions. For junctions incorporating MHA, the transport behavior is found to be strongly humidity dependent. The resistance of these junctions drops more than 50{\%} when placed in dry nitrogen and recovers when returned into the ambient. The same drop also occurs when the sample is placed into a vacuum, and backfilling the vacuum with either dry N2 or O2 has negligible effect on the resistance. For comparison, junctions with HDA show the same humidity dependence, while OTS samples do not. Since both MHA and HDA have carboxylic groups and OTS does not, the results suggest that water molecules at the AlOx/COOH- interface play the central role in the observed behavior. Inelastic tunneling spectroscopy (IETS) has also been performed to understand the role of water. This work was supported by a FSU Research Foundation PEG grant. [Preview Abstract] |
Wednesday, March 15, 2006 11:39AM - 11:51AM |
P16.00003: Signatures of magnetism in an individual Mn12O12 molecule probed by single-electron tunneling. Moon-Ho Jo, Jacob E. Grose, Daniel C. Ralph, Kanhayalal Baheti, Jeffrey R. Long, Wenjie Liang, Mandar M. Deshmukh, Hongkun Park We report low-temperature electron transport through individual molecular clusters, Mn12O12(O2C-R)16(H2O)4, [Mn12O12], where R is -CH3 and -CHCl2. Energy level spectroscopy with single-electron tunneling probes the ground state spin of the individual Mn12O12 molecules, and exhibits signatures of their magnetism. In particular the absence of the spin degeneracy is manifested as an energy splitting between low-lying energy manifolds of the ground state spin at zero-magnetic field, and it signifies the magnetic anisotropy of an individual Mn12O12 molecule. We also discuss the influence of this anisotropy to the electron tunneling spectrum in the presence of a magnetic field. [Preview Abstract] |
Wednesday, March 15, 2006 11:51AM - 12:03PM |
P16.00004: ESR-STM Spectrometer for Paramagnetic Molecular Adsorbates on Surfaces Paolo Messina, Matteo Mannini, Andrea Caneschi, Dante Gatteschi, Lorenzo Sorace, Paolo Sigalotti, Cristian Sandrin, Paolo Pittana, Yishay Manassen ESR-STM is a technique able to detect noise at the Larmor frequency in the tunnelling current associated with the spin dynamics of a single paramagnetic center on the surface. Several questions concerning details of this phenomenon in different magnetic fields and tunnelling currents, and for different paramagnetic centers are still debated. In this paper we describe the construction and the testing of an instrument able to detect the ESR-STM signal from organic paramagnetic molecules (DPPH and BDPA) deposed on Au(111) at different magnetic fields. First results on these molecules are presented. [Preview Abstract] |
Wednesday, March 15, 2006 12:03PM - 12:15PM |
P16.00005: Exploring electron transport through organic monolayers using conductive tip AFM techniques Denis Scaini, Matteo Castronovo, Martina Dell'Angela, Robert Hudej, Loredana Casalis, Giacinto Scoles We follow an alternative approach to the study of Metal-molecule-Metal junctions that uses a combination of two atomic force microscopy (AFM) techniques. We use Nanografting to build a nanopatch of the molecules of interest and a second made of a reference molecule into a hosting self assembled monolayer (SAM) typically made of alkanethiols. After the tip is changed to a conductive one CT-AFM is used to characterized the whole system recording, at the same time, the system topography. Some of the advantages of this approach are the possibility to build and study a wide range of different M-m-M junctions and the in-situ control of the quality of the monolayers and patches. Results will be presented on saturated and unsaturated thiols self-assembled and nanografted on Au(111) surfaces. The results will be compared with those obtained by Liang and Scoles at Princeton using similar techniques. [Preview Abstract] |
Wednesday, March 15, 2006 12:15PM - 12:27PM |
P16.00006: A Novel Automated System for Assembling Films of Nanoparticles Edna Cardenas, Stephen Howell, Shawn Dirk, Dave Wheeler Due to their interesting properties, nanoparticle films have emerged as useful platforms for miniaturized chemical sensing. For nanoparticle sensors to become practical in real world applications, a reproducible method of assembly has to be implemented. This project focuses on robotic assembly techniques that deposit nanoparticle films on various substrates. We have developed a process to iteratively assemble and electronically characterize nanoparticle films using a custom robotic preparation system. The robot's design uses commercially available pneumatic and electronic actuators, valves and regulators to manage precision movements. Control of the robot is obtained by a custom Labview program which uses a TTL and GPIB interface to control relays, power supplies, and measurement circuitry. Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under contract DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 15, 2006 12:27PM - 1:03PM |
P16.00007: Electronic Conduction in Metal/Molecule/Semiconductor Devices Invited Speaker: In the field of molecular electronics, the contacts to the molecular elements are critical interfaces. The use of semiconductor contacts allows direct covalent bonding, provides an additional degree of freedom due to the semiconductor states, and, in certain circumstances, can minimize the effects of electrical shorting due to direct metal/substrate contacts. This talk will describe the development and electrical characterization of metal/molecule/semiconductor device structures on GaAs and Si active layers. In order to observe the conductance of the molecular species, rather than that of the semiconductor barrier, the semiconductor layers used in this study are generally highly doped. 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. In certain molecules, conductance peaks or memory/switching effects have been observed. 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. Studies on both n- and p- type substrates, including those with nanometer scale cap layers, allow the effects of the molecular and semiconductor barriers to be isolated. A basic conduction model has been developed, based on the electrostatics of the structure and thermionic-field-emission analysis of the semiconductor portion of the barrier. [Preview Abstract] |
Wednesday, March 15, 2006 1:03PM - 1:15PM |
P16.00008: Towards a 160 kBit molecular electronic memory at 10$^{11}$ Bits/cm$^{2}$ Jonathan Green, J.W. Choi, E. Johnston-Halperin, E. DeIonno, Y. Luo, A. Boukai, Y. Bunimovich, B.A. Sheriff, J.R. Heath Since its inception by Avirim and Ratner in 1974, molecular-based electronics has emerged as a promising alternative to scaled CMOS technology and its eventual integration limit. Here we present progress towards an electronically configurable, molecule-based 160,000 Bit random access memory at a Bit density approaching 10$^{11}$ Bits/cm$^{2}$. This device is based on a cross-bar architecture in which the active switching elements are bi-stable [2]-rotaxane supramolecules sandwiched between perpendicular arrays of SNAP-fabricated [1] metallic and n-Si nanowires at 34 nm pitch. Challenges in memory fabrication and testing will be discussed. [1] \textit{Science} \textbf{300}, 112 (2003); \textit{J. App. Phys.} \textbf{96}, 5921 (2004). [Preview Abstract] |
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