Unimolecular rectification was detected between oxide-free Au electrodes for a Langmuir−Blodgett (LB) monolayer of the zwitterionic D+-π-A- molecule hexadecylquinolinium tricyanoquinodimethanide, C16H33Q−3CNQ. The top gold pad was deposited by a process that cools the metal vapor before deposition. The maximum rectification ratio is 27.5 at 2.2 V (the average rectification ratio is 7.55). The currents are as large as 9.04 × 104 electrons molecule-1 s-1. The result reinforces previous work with oxide-bearing Al electrodes, but the currents with Au electrodes are larger by 3−5 orders of magnitude. Rectification was also seen for a nine-monolayer Z-type LB film between similar Au electrodes but not for a monolayer of arachidic acid. The direction of enhanced electron current is from the negatively charged dicyanomethylene end of C16H33Q−3CNQ to the quinolinium ring, as predicted by the Aviram−Ratner analysis. However, there is a good fit to the behavior expected for quantum conduction dominated by a single molecular level. The best-fit energy for this level is 1.31 ± 0.25 eV above the Fermi level of the Au electrode.

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Asymmetries were observed across a monolayer of dimethylanilinoaza[C 60 ]fullerene, (DMA-NC 60 , 1) sandwiched between gold electrodes of relatively large size (0.265 mm 2 ). Two modes of behavior are observed: (1) a sigmoidal and slightly asymmetric behavior, bespeaking of a moderate unimolecular rectifier (rectification ratio of about 2), and (2) above a threshold voltage V 1 (≈0.6 to 1.0 V), a dramatic increase of current to 0.3 to 1 A (as high as 1.36 × 10 7 electrons molecule -1 s -1 at 1.5 V), followed by ohmic behavior from V 1 to a relatively smaller negative bias V 2 (≈ -0.5 V to -0.6 V). At more negative potentials (e.g., at -1.5 V) the current is very small (a few µA). This high asymmetry in current persists for between 10 and 20 cycles of voltage scan. This increased, but ohmic conductivity is probably due to defects that grow at domain boundaries, since this behavior is not seen when very small electrodes (1 µm 2 area) are used. The defects could be stalagmitic filaments of gold which grow from the bottom electrode above V 1 but are broken at the negative bias V 2 , or else they could be due to some unknown electrochemical couple. This device is vaguely reminiscent of Zener diodes or varistors: if operated between, say, + 2 V and -2 V, it is a super-rectifier, with a rectification ratio of up to 20 000 at 1.5 V.

X-ray diffraction data from water-supported monolayers of fatty acids with chain lengths from 19 to 22 is presented. The structures of the tilted mesophases L2′, L2, and Ov are characterized in detail. The contributions to the unit cell distortion from the tilt and the ordering of the backbone planes of the molecules are separated. It is shown that at the swiveling transition L2′–L2, not only the tilt azimuth but also the packing of the backbone planes change discontinuously. We demonstrate that the tilting transition LS–L2 is accompanied by the ordering of the backbone planes and may be discontinuous. Evidence is presented for a herringbone ordering transition within the L2 region. The distortions are related to symmetry of the phases and described by the order parameters responsible for tilt and herringbone ordering of the backbone planes of the molecules.