Langmuir-Blodgett Films of Potential Unidimensional Organic Rectifiers
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LANGMUIR-BLODGETI FILMS OF POTENTIAL UNIDIMENSIONAL ORGANIC RECTIFIERS # ROBERT M. METZGER * AND CHARLES A. PANETTA** * Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama, 35487-0336, USA ** Department of Chemistry, The University of Mississippi, University, MS 38677, USA ABSTRACT The progress of the Organic Rectifier Project (ORP) is reviewed. Several molecules of the type D-a-A (D=organic one-electron donor, a = covalent saturated bridge, A = organic oneelectron acceptor) have been synthesized, to test the Aviram-Ratner Ansatz that they could be onemolecule thick rectifiers of electrical current. Many of these molecules (a-= urethane) self-assemble as Langmuir-Blodgett films; the cyclic voltammetry peaks confirm that these molecules preserve the good D and A characteristics. Calculations confirm that the zwitterionic state D+-a-A- is much below the state D--a-A+, as expected. Preliminary tests of electrical rectification have failed. With the installation of a new scanning electron microscope, the Aviram-Ratner Ansatz will soon be put to a final and definitive test. INTRODUCTION The Organic Rectifier Project (ORP) is inspired by the Ansatz of Ari Aviram, Mark A. Ratner, and coworkers [1-3] that a single organic molecule D-a-A could rectify electrical current, because the zwitterionic state D+-c--A- should lie maybe 1 eV above the neutral ground state D-a-A, but 3 to 4 eV lower than the zwitterionic state D+-a-A- . This asymmetry would allow facile electron conduction from the A end through the molecule to the D end. If the molecule were to be placed between two conventional metallic layers M, and M2 , then the device M1 D-a-A IM2 (where I denotes an interface) would be a rectifier of electrical current of molecular dimensions. This rectifier, if realized, would be one of the most dramatic advances of the infant field of molecular electronics. Such a rectifier would have high speed (> 1 ns) , but a working length (5 nm) much smaller than conceivable for Si or GaAs devices. The progress of the ORP [4-25] has been summarized elsewhere [5,8,10,16,19-21,23,25], and is here reviewed again; for more details, see Ref. [25]. METHODS FOR ASSEMBLY OF D-a-A MOLECULAR DEVICES Until the recent advent of the scanning tunneling microscope (STM: vide infra) there was no way of addressing electronically a single molecule; thus one had to find a method to assemble and test a one-molecule thick D-a-A rectifier. The three methods that seemed promising were (i) the Langmuir-Blodgett technique [26-32], (ii) the covalent bonding of molecules to silanized electrodes [33], or (iii) the covalent bonding of silanized molecules to hydroxyl-coated electrodes [341. Full monolayer coverage of a surface is claimed by (i) and (iii), but not by (ii). The ORP chose the LB method. This often required the attachment of "greasy chains" to the D end, at the possible price of speedy electron transfer through the saturated chain; chains as short as possible were sought. Table I shows which D-a-A molecules (1-10) form LB monolayers. (
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