Contact dependence of the conductance of ${\mathrm{H}}_2$ molecular junctions from first principles

Using a first-principles scattering-state approach, transport properties of molecular hydrogen in Pd and Pt junctions are computed, revealing a dramatic reduction in conductance when replacing Pt with Pd contacts. This decrease originates from a change in conduction mechanism, from ballistic transport in Pt junctions to off-resonance tunneling in Pd junctions. Our findings also indicate that lead atoms in contact with ${\mathrm{H}}_2$ are as important in determining conductance as ${\mathrm{H}}_2$ itself.

Figure 1

(Color online) Model geometry used in conductance calculations: The H atoms are shown in white, and Pt or Pd atoms in gold. The interslab distance, H-H atom distance, and tip atom to hydrogen distance are determined by minimizing the total energy.

Figure 2

(Color online) Transmission spectra for ${\mathrm{H}}_2$ junctions across planar (a) Pt (111) and (b) Pd (111) leads. Left panels show transmission spectra obtained from $\Gamma$-point calculation, and dotted lines indicate the position of $E_F$. Right panels show a comparison between transmission spectra obtained with different $k$-grid samplings.

Figure 3

(Color online) Color contour plot of square of conducting eigenchannel wave function with the highest eigenvalue at $E_F$ in a plane containing the ${\mathrm{H}}_2$ molecule for the (a) $\mathrm{Pt}\text{−}{\mathrm{H}}_2$ and (b) $\mathrm{Pd}\text{−}{\mathrm{H}}_2$ systems, overlaid on a ball-and-stick model of the atomic positions. Scattering states are incident from the left, and these results are obtained from $\Gamma$-point calculations.

Figure 4

(Color online) Transmission and conducting eigenchannel density of states obtained from a $\Gamma$-point calculation projected onto the extended molecule and deep inside the lead for (a) $\mathrm{Pt}\text{−}{\mathrm{H}}_2$ and (b) $\mathrm{Pd}\text{−}{\mathrm{H}}_2$ systems. Dotted lines centered on resonant-tunneling peaks serve as a guide to the eye and the Fermi level is at the zero of energy; the inset shows the region of projection for extended molecule PDOS.

Figure 5

(Color online) Transmission spectra obtained from $\Gamma$-point calculations for Pt extended molecule (black) and Pd extended molecule (red) across (a) Pt leads and (b) Pd leads.

Figure 6

(Color online) Transmission and impurity PDOS as a function of energy for a single state impurity tight-binding model with $t_0=1$ for a range of values of $t_1$.