ABSTRACT
We have measured the conductance and characterized molecule-electrode binding geometries of four pyridine-terminated molecules by elongating and then compressing gold point contacts in a solution of molecules. We have found that all pyridine-terminated molecules exhibit bistable conductance signatures, signifying that the nature of the pyridine-gold bond allows two distinct conductance states that are accessed as the gold-molecule-gold junction is elongated. We have identified the low-conductance state as corresponding to a molecule fully stretched out between the gold electrodes, where the distance between contacts correlates with the length of the molecule; the high-conductance state is due to a molecule bound at an angle. For all molecules, we have found that the distribution of junction elongations in the low-conductance state is the same, while in the high-conductance state, the most likely elongation length increases linearly with molecule length. The results of first-principles conductance calculations for the four molecules in the low-conductance geometry agree well with the experimental results and show that the dominant conducting channel in the conjugated pyridine-linked molecules is through the pi* orbital.
ABSTRACT
We measure the conductance and current-voltage characteristics of two amine-terminated molecular wires -- 4,4'-diaminostilbene and bis-(4-aminophenyl)acetylene -- by breaking Au point contacts in a molecular solution at room temperature. Histograms compiled from thousands of measurements show a slight increase in the molecular junction conductance (I/V) as the bias is increased to nearly 450 mV. Comparatively, similar conductance measurements made with 1,6-diaminohexane, a saturated molecule, demonstrate almost no bias dependence. We also present a new technique to measure a statistically defined current-voltage (I-V) curve. Application to all three molecules shows that 4,4'-diaminostilbene exhibits the largest increase in differential conductance as a function of applied bias. This indicates that the predominant transport channel for 4,4'-diaminostilbene (the highest occupied molecular orbital) is closer to the Fermi level of the metal than that of the other molecules, consistent with the trends observed in the molecular ionization potential. We find that junctions constructed with the conjugated molecules show greater noise in individual junctions and less structural stability, on average, at biases greater than 450 mV. In contrast, junctions formed with the alkane can sustain a bias of up to 900 mV. This significantly affects the statistically averaged I-V characteristic measured for the conjugated molecules at higher bias.
ABSTRACT
We analyze the formation and evolution statistics of single-molecule junctions bonded to gold electrodes using amine, methyl sulfide, and dimethyl phosphine link groups by measuring conductance as a function of junction elongation. For each link, the maximum elongation and formation probability increase with molecular length, strongly suggesting that processes other than just metal-molecule bond breakage play a key role in junction evolution under stress. Density functional theory calculations of adiabatic trajectories show sequences of atomic-scale changes in junction structure, including shifts in the attachment point, that account for the long conductance plateau lengths observed.
