We compile, compare, and discuss experimental results on low‐bias, room‐temperature currents through organic molecules obtained in different electrode–molecule–electrode test‐beds. Currents are normalized to single‐molecule values for comparison and are quoted at 0.2 and 0.5 V junction bias. Emphasis is on currents through saturated alkane chains where many comparable measurements have been reported, but comparison to conjugated molecules is also made. We discuss factors that affect the magnitude of the measured current, such as tunneling attenuation factor, molecular energy gap and conformation, molecule/electrode contacts, and electrode material.

Nanoscopic tunnel junctions were formed by contacting Au-, Pt-, or Ag-coated atomic force microscopy (AFM) tips to self-assembled monolayers (SAMs) of alkanethiol or alkanedithiol molecules on polycrystalline Au, Pt, or Ag substrates. Current-voltage traces exhibited sigmoidal behavior and an exponential attenuation with molecular length, characteristic of nonresonant tunneling. The length-dependent decay parameter, beta, was found to be approximately 1.1 per carbon atom (C(-1)) or 0.88 A(-)(1) and was independent of applied bias (over a voltage range of +/-1.5 V) and electrode work function. In contrast, the contact resistance, R(0), extrapolated from resistance versus molecular length plots showed a notable decrease with both applied bias and increasing electrode work function. The doubly bound alkanedithiol junctions were observed to have a contact resistance approximately 1 to 2 orders of magnitude lower than the singly bound alkanethiol junctions. However, both alkanethiol and dithiol junctions exhibited the same length dependence (beta value). The resistance versus length data were also used to calculate transmission values for each type of contact (e.g., Au-S-C, Au/CH(3), etc.) and the transmission per C-C bond (T(C)(-)()(C)).

Using conducting probe atomic force microscopy (CP-AFM), we have formed molecular tunnel junctions consisting of alkanethiols and alkane isonitrile self-assembled monolayers sandwiched between gold, platinum, silver, and palladium contacts. We have measured the resistance of these junctions at low bias (dV/dI |V=0) as a function of alkane chain length. Extrapolation to zero chain length gives the contact resistance, R0 . R0 is strongly dependent on the type of metal used for the contacts and decreases with increasing metal work function; that is, R0,Ag > R0,Au > R0,Pd > R0,Pt. R0 is approximately 10% smaller for Au junctions with isonitrile versus thiol surface linkers. We conclude that the Fermi level of the junction lies much closer to the HOMO than to the LUMO.

Alkanethiol tunnel junctions were studied using conducting-probe atomic force microscopy to determine causes of variability in measured resistance behavior. Measurements were made on Au/decanethiol/Au monolayer junctions, and effects of substrate roughness, tip chemistry, presence of solvent, extensive tip usage, applied load, and tip radius were examined. Resistance measurements yielded log-normal distributions under a variety of conditions, indicating that the origin of the variance is likely to be either changes in tunneling length or electronic overlap. Spreads in resistance values for a given tip were much less when flat, template-stripped Au substrates were used rather than rough, evaporated Au substrates. Chemical modification of tips with ethanethiol (C2) or butanethiol (C4) and performing measurements under cyclohexane were also found to reduce variance by a factor of about 2-4. Experiments performed with unmodified tips showed an increase in junction resistance over the course of hundreds of consecutive measurements, whereas junctions made with modified tips or under cyclohexane did not. Attempts to ascribe variance between tips to varying tip radii failed; however, decreases in resistance with increasing applied load on the tip contact were observed and could be interpreted in terms of conventional contact mechanics models.

Four tetrathiol-terminated norbornane homologues were synthesized and self-assembled monolayers (SAMs) of these molecules were formed on Au via adsorption from CH2Cl2. SAMs were characterized structurally via spectroscopic ellipsometry (SE), reflection-absorption infrared spectroscopy (RAIRS), Rutherford backscattering spectrometry (RBS), and X-ray photoelectron spectroscopy (XPS). Results of these analyses show that the rigid norbornylogs form monolayers that have a surface coverage slightly lower than that of alkanethiols, and that they exhibit a nonmonotonic dependence of film thickness on molecular length. Nanoscale molecular junctions incorporating these SAMs were formed and characterized electrically using conducting probe atomic force microscopy (CP-AFM). The resistances of these junctions scale exponentially with the contour length of the molecules, with beta = 0.9 A(-1), consistent with a nonresonant tunneling mechanism. Further, the resistance of norbornyl SAMs correlates well with the resistance of alkanedithiol SAMs of similar length, suggesting that the norbornyl molecules form sulfur-metal bonds on both ends of the junction.

Electrical tunnel junctions consisting of alkanethiol molecules self-assembled on Au-coated Si substrates and contacted with Au-coated atomic force microscopy tips were characterized under varying junction loads in a conducting-probe atomic force microscopy configuration. Junction load was cycled in the fashion of a standard nanoindentation experiment; however, junction conductance rather than probe depth was measured directly. The junction conductance data have been analyzed with typical contact mechanics (Derjaguin-Müller-Toporov) and tunneling equations to extract the monolayer modulus (approximately 50 GPa), the contact transmission (approximately 2 x 10(-6)), contact area, and probe depth as a function of load. The monolayers are shown to undergo significant plastic deformation under compression, yielding indentations approximately 7 Angstroms deep for maximum junction loads of approximately 50 nN. Comparison of mechanical properties for different chain lengths was also performed. The film modulus decreased with the number of carbons in the molecular chain for shorter-chain films. This trend abruptly reversed once 12 carbons were present along the backbone.

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