Modification of Molecular Conductance by in Situ Deprotection of Thiol-Based Porphyrin.

Acetylthio-protected free base porphyrins are used to form scanning tunneling microscope-molecular break junctions. The porphyrin molecules are deprotected in situ, before the self-assembly. Two types of molecular junctions are formed in the junctions: Au-S-Por-SAc-Au and Au-S-Por-S-Au. Lower conductance values and higher conductance values are observed. Computational modeling attributes the lower conductance to the Au-S-Por-SAc-Au junctions and the higher conductance to the Au-S-Por-S-Au junctions. First-principles calculation suggests that the reduced conductance in the protected porphyrin originates from the presence of the acetyl end groups (-COCH3), rather than from the elongation of the sulfur-gold (S-Au) bonds at the tip-molecule interface.

Temperature-dependent and bistable current-voltage measurements in zinc porphyrin molecular junctions.

We report bistability in current-voltage curves from di(PEP)PorZn in an electromigrated molecular junction. Bistability was observed at ±0.3 V at 300 K but did not occur at 4 K. No bistability was identified at 300 K for another porphyrin molecule (di(Xyl)PorZn), where the phenyl-ethnyl-phenyl (PEP) side groups were replaced with a flexible p-xylene. Molecular dynamics simulations show that bistability may be due to conformation changes related to the fluctuation of the dihedral angle surrounding the zinc and/or the rotation of the porphyrin central plane of the molecule. Results suggest that other mechanisms may play a role in the current-voltage characteristics observed.

Effects of nanoscale surface roughness on the resistivity of ultrathin epitaxial copper films.

The knowledge on the influence of surface roughness and the electron-phonon (el-ph) interaction on electrical transport properties of nanoscale metal films is important from both fundamental and technological points of view. Here we report a study of the temperature dependent electron transport properties of nanoscale copper films by measuring temperature dependent electrical resistivity with thickness ranging from 4 to 500 nm. We show that the residual resistivity, which is temperature independent, can be described quantitatively using both measured vertical surface root-mean-square roughness and lateral correlation length in the nanoscale, with no adjustable parameter, by a recent quasi-classical model developed by Chatterjee and Meyerovich (2010 Phys. Rev. B 81 245409-10). We also demonstrate that the temperature dependent component of the resistivity can be described using the Bloch-Grüneisen equation with a thickness dependent el-ph coupling constant and a thickness dependent Debye temperature. We show that the increase of the el-ph coupling constant with the decrease of film thickness gives rise to an enhancement of the temperature dependent component of the resistivity.

Stable aqueous dispersions of noncovalently functionalized graphene from graphite and their multifunctional high-performance applications.

We present a scalable and facile technique for noncovalent functionalization of graphene with 1-pyrenecarboxylic acid that exfoliates single-, few-, and multilayered graphene flakes into stable aqueous dispersions. The exfoliation mechanism is established using stringent control experiments and detailed characterization steps. Using the exfoliated graphene, we demonstrate highly sensitive and selective conductometric sensors (whose resistance rapidly changes >10,000% in saturated ethanol vapor), and ultracapacitors with extremely high specific capacitance (∼ 120 F/g), power density (∼ 105 kW/kg), and energy density (∼ 9.2 Wh/kg).