Finite-bias electronic transport of molecules in a water solution

The effects of water wetting conditions on the transport properties of molecular nanojunctions are investigated theoretically by using a combination of empirical-potential molecular-dynamics and first-principles electronic-transport calculations. These are at the level of the nonequilibrium Green’s-function method implemented for self-interaction corrected density-functional theory. We find that water effectively produces electrostatic gating to the molecular junction with a gating potential determined by the time-averaged water dipole field. Such a field is large for the polar benzene-dithiol molecule, resulting in a transmission spectrum shifted by about 0.6 eV with respect to that of the dry junction. The situation is drastically different for carbon nanotubes (CNTs). In fact, because of their hydrophobic nature the gating is almost negligible so that the average transmission spectrum of wet Au/CNT/Au junctions is essentially the same as that in dry conditions. This suggests that CNTs can be used as molecular interconnects also in water-wet situations, for instance, as tips for scanning tunnel microscopy in solution or in biological sensors.

Figure 1

(Color online) Au capacitor with water as dielectric medium. The top panel shows the unit cell used for the MD simulations, which includes ${\text{H}}_2\text{O}$ confined between the two Au electrodes. In panel (a), we show the probability to find a O (solid curve) or a H (dashed curve) atom at a given $z$ position. Panels (b)–(d) are the planar averages of the Hartree electrostatic potential ${\overline{V}}_H$ as function of position along the transport direction, $z$, for different setups: (b) entire junction, (c) junction without the electrodes, and (d) junction without water. The gray curves, merging in a shadow, are the results for 21 snapshots taken at different times after equilibration, the black solid curves are their time averages. The dashed curve in (c) shows the difference between the time averages of panels (b) and (d).

Figure 2

(Color online) DOS projected onto ${\text{H}}_2\text{O}$ for the capacitor of Fig. 1 and obtained for different values of the ASIC scaling parameter $\alpha$. The gray lines are the superimposed curves for 21 MD snapshots and the solid black one is their average. The dashed curve is the average DOS for the same MD snapshots for a system where Au is replaced by vacuum.

Figure 3

(Color online) Unit cell used for the BDT molecule in water attached to Au electrodes.

Figure 4

(Color online) Transmission coefficient for transport through BDT in water as function of energy for one MD snapshot and for different values of the ASIC scaling parameter $\alpha$. The dashed curves are for BDT in dry conditions (no water) while the solid curves are for BDT in solution.

Figure 5

Density of states projected onto ${\text{H}}_2\text{O}$ for the junction of Fig. 3 as function of energy for one MD snapshot and for different values of the ASIC scaling parameter $\alpha$.

Figure 6

(Color online) Current, $I$, as function of the bias voltage, $V$, through the BDT molecule calculated at the first MD time step and for different values of the SIC scaling parameter $\alpha$. The dashed curves are for the molecule in the dry while the solid curves are for BDT in solution.

Figure 7

(Color online) Time averaged (a) transmission coefficient and (b) DOS projected onto the ${\text{H}}_2\text{O}$ molecules for BDT attached to Au. The calculations are obtained with ASIC and $\alpha =0.7$; the solid black curves are the time averages over 201 time steps while the gray ones forming a shadow are for each of the 201 time steps. The dashed curve is for the dry situation.

Figure 8

(Color online) (a) Probability to find an O (solid curve) or an H (dashed curve) atom at a given position $y$ in the Au/BDT/Au junction, for atoms whose $x$ coordinate lies within the shadowed region. In the top panel we show a representative snapshot of the atomic configuration of the water. Note that in the first solvation layer the ${\text{H}}_2\text{O}$ molecules are oriented with the H atoms pointing toward the BDT. (b) The black curve shows the time average of the planar average over $x$ and $z$ of the electrostatic potential, ${\overline{V}}_{Hxz}$, which is averaged for $x$ laying in the shadowed region, and for the $z$ coordinate extending between the two S atoms of the BDT. The dashed green curve illustrates the difference of this planar average between the system with and the one without water. The inset is a zoom in of this difference in the region around the BDT where no water molecules are found.

Figure 9

(Color online) Histogram of ${ϵ}_{\text{HOMO}}$ extracted from the maximum in $T\left(E\right)$ at around $-2\text{eV}$ (see Fig. 7) for $\alpha =0.7$ and for 201 MD time steps. $N$ is the number of times ${ϵ}_{\text{HOMO}}$ is found in the given energy window specified by the bin width. The solid line indicates the time-averaged ${ϵ}_{\text{HOMO}}$ while the dashed red one marks the result in dry conditions.

Figure 10

(Color online) $I\text{−}V$ curve for a BDT molecule attached to gold in presence of water obtained with ASIC $\left(\alpha =0.7\right)$; the solid black curve corresponds to the average current over 21 time steps, the gray curves merging in a shadow are the $I\text{−}V$ curves of each individual configuration and the red dashed curve is for BDT in dry conditions.

Figure 11

(Color online) Au/CNT/Au junction. The top panel shows the unit cell used for the MD simulations, which includes the CNT, Au electrodes, and water molecules. In panels (a) and (b) we present the planar averages of the Hartree electrostatic potential, ${\overline{V}}_H$, as function of position along the transport direction $z$: (a) junction without water and (b) junction with water. The gray curves, merging in a shadow, are the results for all 201 MD snapshots, the black solid curves are their time averages.

Figure 12

(Color online) Transmission coefficient as function of energy for the $\text{Au}\left(111\right)/\text{CNT}\left(3,3\right){\text{-H}}_2\text{O}/\text{Au}\left(111\right)$ junction. The solid black curve corresponds to the average $T$ over 201 MD snapshots, the dashed red curve is the transmission for the same junction in the dry and the dot-dashed green line indicates the number of channels (spin degenerate) for the infinite CNT. The gray curves, merging in a shadow, are $T\left(E;V=0\right)$ for each of the MD snapshot.

Figure 13

(Color online) Histogram of the energy position of the transmission peak located at $-1.7\text{eV}$ below $E_{\text{F}}$ (see Fig. 12). The histogram has been constructed from 201 MD snapshots. $N$ is the number of times the peak is found in the given energy window specified by the bin width. The solid line indicates the time-averaged position while the dashed red one marks the result in dry conditions.

Figure 14

(Color online) Transmission coefficient as function of energy for the $\text{Au}\left(111\right)/\text{CNT}\left(8,0\right){\text{-H}}_2\text{O}/\text{Au}\left(111\right)$ junction. The solid (black) curve corresponds to the average $T$ over 41 MD snapshots, the dashed (red) curve is the transmission for the same junction in the dry, and the dot-dashed (green) line indicates the number of channels (spin degenerate) for the infinite CNT. The gray curves, merging in a shadow, are $T\left(E;V=0\right)$ for each of the MD snapshot.