Unimolecular rectifiers: methods and challenges.

Six unimolecular rectifiers have been studied at the University of Alabama: Langmuir-Blodgett (LB) or Langmuir-Schaefer (LS), or self-assembled monolayers of these molecules show asymmetric electrical conductivity between Au or Al electrodes. These molecules are gamma-hexadecylquinolinium tricyanoquinodimethanide (Fig. 1, 2), 2,6-di[dibutylamino-phenylvinyl]-l-butylpyridinium iodide, 3, dimethylanilino-aza[C60]-fullerene, 4, fullerene-bis-[4-diphenylamino-4''-(N-ethyl-N-2-ethyl)-amino-1,4-diphenyl-1,3 -butadiene] malonate, 5, N-(10-nonadecyl)-N-(2-ferrocenyl-ethyl)-pyrenyl-methyl)pery-lene-3,4,9,10-bis(dicarboxyimide), 6, and 4,5-dipentyl-5'-methyltetrathiaful-valen-4'-methyl-oxy-2,4,5-trinitro-9-dicyanomethylenefluorene-7-(3-sulfonylpropionate), 7. Many ancillary experiments must be performed before unimolecular rectification can be fully understood. This review will focus on the fabrication techniques and the analytical tools that can help understand the asymmetric current-voltage (IV) curves. These tools include molecular orbital calculations, cyclic voltammetry, ultraviolet photoelectron spectroscopy, scanning tunneling microscopy, contact angle goniometry, ultraviolet-visible-near-infrared spectroscopy, grazing-angle Fourier transform infrared spectroscopy, surface plasmon resonance, spectroscopic ellipsometry, grazing-incidence X-ray reflectometry, core-level and valence-band X-ray photoelectron spectroscopy.