Electron transport in a $\mathrm{Pt}\text{−}\mathrm{CO}\text{−}\mathrm{Pt}$ nanocontact: Density functional theory calculations

We have performed first-principles calculations for the mechanic and electric properties of pure Pt nanocontacts and a Pt contact with a single CO molecule adsorbed. For the pure Pt contacts we see a clear difference between point contacts and short chains in good agreement with experiments. We identify a tilted bridge configuration for the $\mathrm{Pt}\text{−}\mathrm{CO}\text{−}\mathrm{Pt}$ contact, which is stable and has a conductance close to $0.5G_0$ $(G_0=2e^2∕h)$, and we propose that this structure is responsible for an observed peak at $0.5G_0$ in the conductance histogram for Pt exposed to a CO gas. We explain the main features of the transmission function for the $\mathrm{Pt}\text{−}\mathrm{CO}\text{−}\mathrm{Pt}$ contact, and show that the conductance is largely determined by the local $d$ band at the Pt apex atoms.

Figure 1

Supercells used to model the two considered structures: (i) A point-contact, and (ii) a 1-atom chain. The electrode displacement $d_z$ is defined as the distance between the (111) surfaces.

Figure 2

Conductance (triangles) and total energy (circles) for the Pt point contact as a function of electrode displacement $d_z$.

Figure 3

Conductance (triangles) and total energy (circles) as a function of electrode displacement, $d_z$.

Figure 4

Conductance (triangles) and total energy (circles) as a function of electrode displacement $d_z$. The conductance trace is divided into an upper and a lower part, corresponding to the CO in the upright bridge and tilted bridge configuration, respectively. $A$ and $B$ labels the configurations just before and just after CO has tilted, respectively.

Figure 5

Conductance traces for different values of the electrode stiffness, $k_s$. The size of $k_s$ determines the length of the conductance plateau at $0.5G_0$: a stiff electrode (large $k_s$) results in a long plateau, while a soft electrode (small $k_s$) results in a short or no plateau.

Figure 6

Typical transmission function for the upright bridge (configuration $A$ in Fig. 4) and the tilted bridge (configuration $B$). The dotted line is the transmission function obtained using Siesta DZP/PBE. Both transmission functions ($A$ and $B$) have a resonance at the Fermi level which can be related to the density of states of the so called group orbitals.

Figure 7

(Color online) The PDOS of the $2{\pi }^{*}$ states $\mid a⟩$ and $\mid b⟩$ together with the PDOS of the group orbital in the weakly coupled right lead. The inset shows an isosurface plot of the MO $\mid a⟩$ (transparent) and its corresponding left and right group orbitals (solid). The PDOS for the MOs (circles and stars) is quite flat around the Fermi level, while the PDOS of two group orbitals (full and dashed lines) both have a peak at the Fermi level. It is this peak that gives rise to the resonance in the transmission function.