Formation and Evolution of Single-Molecule Junctions

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.

Figure 1

(a) Normalized linear conductance histograms of 1,4-butanediamine (1, bin size ${10}^{-5}{G}_0$), 1,6-hexanediamine (2, bin size ${10}^{-6}{G}_0$), and 1,4-bis (dimethylphosphino) butane (3, bin size ${10}^{-5}{G}_0$). Inset: Sample conductance traces (offset horizontally for clarity) that show a molecular step for each molecule. (b)–(d) Normalized 2D histograms for molecules 1, 2, and 3, respectively (100 bin/decade against $0.007\mathrm{nm}/\mathrm{bin}$). (e) Normalized log-bin conductance histograms for 1, 2, and 3.

Figure 2

(a) Conductance step length distribution for diamino alkanes with 4, 6, 8, and 11 ${\mathrm{CH}}_2$ groups in the chain. Crosses indicate the 95th percentile for each distribution. (b) Conductance step length (95th percentile) as a function of the number of ${\mathrm{CH}}_2$ groups for alkanes with the three links studied.

Figure 3

Calculated adiabatic 1,4-butanediamine junction elongation trace. (a) Relaxed junction geometries at four different positions. (b) Binding energy relative to relaxed, isolated Au pyramids and molecule. (c) External applied force calculated from the derivative of junction energy with respect to elongation. (d) N-Au bond lengths: upper N to the Au atom on the second layer (+), to the Au atom on the bottom layer (□), and the lower N-Au bond (▵). (e) Conductance.

Figure 4

Calculated adiabatic 1,4-bis(dimethylphosphino)-butane junction elongation trace. (a) Relaxed junction geometries at four different positions. (b) Binding energy. (c) External applied force. (d) P-Au bond length: upper (+) and lower (▵) pyramids. (e) Conductance.