Logarithmic behavior of the conductivity of electron-beam deposited granular $\mathrm{Pt}∕\mathrm{C}$ nanowires

We found that granular $\mathrm{Pt}∕\mathrm{C}$ composite nanowires have a logarithmic temperature dependence of conductivity in a wide range of temperatures. The logarithmic dependence can be explained by Coulomb interaction between Pt grains in a conductive carbon matrix. We stress the difference of the conductivity mechanism in the composite nanogranular material and known mechanisms in bulk metals/semiconductors and low-dimensional systems, including logarithmic dependence in systems with two-dimensional weak localization. Our observations show that local voltage fluctuations between grains result in the $\log \left(T\right)$ dependence for the samples with high conductivity (annealed nanowires), while the samples with low conductivity (as fabricated nanowires) appear to be insulators with a Coulomb gap.

Figure 1

A SEM micrograph of the $\mathrm{Pt}∕\mathrm{C}$ composite nanowires deposited over the $\mathrm{Si}∕{\mathrm{Si}}_3{\mathrm{N}}_4$ substrate with the premanufactured $\mathrm{Au}∕\mathrm{Ti}$ leads: (a) the circuit layout for two-probe and four-probe transport measurements. (b) and (c) The magnified SEM images taken at 52° to the sample surface. Bar sizes are $2\mu \mathrm{m}$ (a), $500\mathrm{nm}$ (b), and $100\mathrm{nm}$ (c).

Figure 2

HRTEM micrographs of the $\mathrm{Pt}∕\mathrm{C}$ nanowire, deposited onto a ${\mathrm{Si}}_3{\mathrm{N}}_4$ membrane: (left) the image of as deposited material; (right) the image taken after the thermal annealing. Darker regions indicate the Pt crystallites, the bar sizes are $5\mathrm{nm}$. The insets show the diffraction patterns.

Figure 3

(Color online) The influence of the rapid thermal annealing on the wire resistivity: (left) two-probe and (right) four-probe room-temperature resistivity vs the annealing temperature. The source-drain voltage was $50\mathrm{mV}$, which is within the linear current-voltage characteristics of the nanowires. The resistivity of $\sim 20–100\Omega \mathrm{cm}$ for as-deposited wires decreased by four orders of magnitude with progressive thermal annealing up to $500°\mathrm{C}$. Both panel show consistent changes, which proves the effect to be independent of contact resistance.

Figure 4

(Color online) (Main panel) The logarithmic temperature dependence of the conductivity of the annealed nanowire. (Inset) The raw experimental data on resistivity vs $T$ (from the room temperature down to $4\mathrm{K}$ in four-probe geometry). The source-drain voltage was $50\mathrm{mV}$ and the $\log \left(T\right)$ dependence was observed in a wide range of $T$—up to $200\mathrm{K}$.