The bacterium Geobacter sulfurreducens requires the expression of conductive protein filaments or pili to respire extracellular electron acceptors such as iron oxides and uranium and to wire electroactive biofilms, but the contribution of the protein fiber to charge transport has remained elusive. Here we demonstrate efficient long-range charge transport along individual pili purified free of metal and redox organic cofactors at rates high enough to satisfy the respiratory rates of the cell. Carrier characteristics were within the orders reported for organic semiconductors (mobility) and inorganic nanowires (concentration), and resistivity was within the lower ranges reported for moderately doped silicon nanowires. However, the pilus conductance and the carrier mobility decreased when one of the tyrosines of the predicted axial multistep hopping path was replaced with an alanine. Furthermore, low temperature scanning tunneling microscopy demonstrated the thermal dependence of the differential conductance at the low voltages that operate in biological systems. The results thus provide evidence for thermally activated multistep hopping as the mechanism that allows Geobacter pili to function as protein nanowires between the cell and extracellular electron acceptors.

This paper describes the synthesis, formation mechanism, and mechanical property of multilayered ultrathin Pd nanosheets. An anisotropic, Hanoi Tower-like assembly of Pd nanosheets was identified by transmission electron microscopy and atomic force microscopy (AFM). These nanosheets may contain ultrathin Pd layers, down to single unit cell thickness. Selected area electron diffraction and scanning transmission electron microscopy data show the interconnected atomically thick layers stacking vertically with rotational mismatches, resulting in unique diffractions and Moiré patterns. Density functional theory (DFT) calculation with van der Waals correction (DFT+vdW) shows the adsorption of Pd4(CO)4(OAc)4 on Pd(110) surface (Ead = -5.68 eV) is much stronger than that on Pd(100) (Ead = -4.72 eV) or on Pd(111) (Ead = -3.80 eV). The adsorption strength of this Pd complex is significantly stronger than that of CO on the same Pd surfaces. The DFT+vdW calculation results suggest a new mechanism for the observed anisotropic growth of nanosheets with unusually high aspect ratio, in which the competitive adsorptions between Pd4(CO)4(OAc)4 complex and CO on various surfaces result in a favored growth along the ⟨110⟩ directions and inhibition along ⟨111⟩ directions. The mechanical property of these multilayered Pd nanosheets was studied using AFM and nanoindentation techniques, which indicate multilayered nanosheets show more plastic deformation than the bulk in response to an applied force.

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