The Effect of Oxygen Heteroatoms on the Single Molecule Conductance of Saturated Chains

Abstract

Single molecule conductance measurements on alkanedithiols and alkoxydithiols (dithiolated oligoethers) were performed using the STM-controlled break junction method in order to ascertain how the oxygen heteroatoms in saturated linear chains impact the molecular conductance. The experimental results show that the difference in conductance increases with chain length, over the range studied. Comparisons with electronic structure calculations and previous work on alkanes indicate that the conductance of the oligoethers is lower than that of alkane chains with the same length. Electronic structure calculations allow the difference in the conductance of these two families of molecules to be traced to differences in the spatial distribution of the molecular orbitals that contribute most to the conductance. A pathway analysis of the electronic coupling through the chain is used to explain how the difference in conductance between the alkane and oligoether molecules depends on the chain length.

Defining the Value of Injection Current and Effective Electrical Contact Area for EGaIn-Based Molecular Tunneling Junctions

“…The length used (2 Å) represents the variation of the length of the tunnel gap (±1.0 Å, that is, ∼ ±σ log ) calculated from the average standard deviation of log|J| (σ log = 0.3) considering β = 0.75 Å −1 for nalkanethiolates. We used a 900 nm × 900 nm STM image of a Ag TS -SC 10 substrate that did not present valleys or asperities that would require, for the estimation of α SAM , that we take into account the partial adaptation of the Ga 2 O 3 /EGaIn tip to the topography of the surface.…”

“…The values of A elec /A geo for these different techniques are unknown; in addition, a number of factors other than the value of the ratio A elec /A geo might contribute to the variability of the values of J 0 within the consensus range; these parameters include the nature of the top-and bottom-contacts (e.g, covalent or van der Waals), the nature of the materials used as electrodes, and the presence of contaminants. We recommend using SAMs of SC 10 and SC 16 as standards for the calibration of values of J(V).…”

“…We estimated this value of J 0 by extrapolating the value of J 0 (∼1 × 10 −5 A·mol −1 ) measured for single nalkanethiolates in STM-based junctions [5][6][7]10 (Figure 8) to the value of J 0 for the theoretical number of molecules per cm 2 of the SAM (on Au(111), this number is ∼4.5 × 10 14 mol·cm…”

Modified Superexchange Model for Electron Tunneling Across the Terminated Molecular Wire

“…For such a monolayer, the anchor groups play very important roles in controlling the tunneling current between the substrate and the tip. In the nonresonant charge transmission regime, where the MOs of the wire are not occupied by the transferred electrons/holes, the current decays exponentially with the increase in molecular length …”

“…In the nonresonant charge transmission regime, where the MOs of the wire are not occupied by the transferred electrons/holes, the current decays exponentially with the increase in molecular length. [15][16][17][18][20][21][22][23] For theoretical description of a coherent electron/hole tunneling, the Landauer-Buẗtiker expression is mostly used. [2,[24][25][26][27] However, its integral form is not quite convenient for the experimentalists.…”