Jing-Hong Liang

On the Hopping Efficiency of Nanoparticles in the Electron Transfer across Self‐Assembled Monolayers

Redox reactions of solvated molecular species at gold-electrode surfaces modified by electrochemically inactive self-assembled molecular monolayers (SAMs) are found to be activated by introducing Au nanoparticles (NPs) covalently bound to the SAM to form a reactive Au-alkanedithiol-NP-molecule hybrid entity. The NP appears to relay long-range electron transfer (ET) so that the rate of the redox reaction may be as efficient as directly on a bare Au electrode, even though the ET distance is increased by several nanometers. In this study, we have employed a fast redox reaction of surface-confined 6-(ferrocenyl) hexanethiol molecules and NPs of Au, Pt and Pd to address the dependence of the rate of ET through the hybrid on the particular NP metal. Cyclic voltammograms show an increasing difference in the peak-to-peak separation for NPs in the order Au<Pt<Pd, especially when the length of the alkanedithiol increases from octanedithiol to decanedithiol. The corresponding apparent rate constants, kapp , for decanedithiol are 1170, 360 and 14 s(-1) for NPs of Au, Pt and Pd, respectively, indicating that the efficiency of NP mediation of the ET clearly depends on the nature of the NP. Based on a preliminary analysis rooted in interfacial electrochemical ET theory, combined with a simplified two-step view of the NP coupling to the electrode and the molecule, this observation is referred to the density of electronic states of the NPs, reflected in a broadening of the molecular electron/NP bridge group levels and energy-gap differences between the Fermi levels of the different metals.

Efficient conducting channels formed by the π-π stacking in single [2,2]paracyclophane molecules

The electronic transport properties of single [2,2]paracyclophane molecules directly connected to gold and platinum electrodes have been investigated both theoretically and experimentally by using first-principles quantum transport simulations and break-junction experiments. For comparison, investigations on [3,3]- and [4,4]-paracyclophanes have also been performed. Our calculations show that the strength of the π-π interaction in paracyclophanes is critically dependent on the inter-ring distance. In contrast to [4,4]paracyclophane in which the π-π interaction is very weak due to the large inter-ring distance, the π-π interaction in [2,2]- and [3,3]-paracyclophanes is rather strong and dominates the electronic transport properties. In particular, for the asymmetric Au-[2,2]paracyclophane-Au junction in which the [2,2]paracyclophane molecule is connected to each gold electrode through a Au adatom and the two Au adatoms are attached in η(1)-fashion to two carbon atoms in the benzene backbones connecting with different ethylene groups, the transmission coefficient at the Fermi level is calculated to be 1.0 × 10(-2), in excellent agreement with experiments. When the gold electrodes are replaced by platinum, the calculated transmission coefficient at the Fermi level of the symmetric Pt-[2,2]paracyclophane-Pt junction with one Pt adatom used as the linker group is increased to 0.83, demonstrating that the π-π stacking in [2,2]paracyclophane is efficient for electron transport when the molecule-electrode interfaces are electronically transparent. This is confirmed by our preliminary experimental studies on the Pt-[2,2]paracyclophane-Pt junctions, for which the low-bias junction conductance has reached 0.40 ± 0.02 G(0) (G(0) is the conductance quantum). These findings are helpful for the design of molecular electronic devices incorporating π-π stacking molecular systems.

Measurement of the Quantum Conductance of Germanium by an Electrochemical Scanning Tunneling Microscope Break Junction Based on a Jump-To-Contact Mechanism

We present quantum conductance measurements of germanium by means of an electrochemical scanning tunneling microscope (STM) break junction based on a jump-to-contact mechanism. Germanium nanowires between a platinum/iridium tip and different substrates were constructed to measure the quantum conductance. By applying appropriate potentials to the substrate and the tip, the process of heterogeneous contact and homogeneous breakage was realized. Typical conductance traces exhibit steps at 0.025 and 0.05 G0. The conductance histogram indicates that the conductance of germanium nanowires is located between 0.02 and 0.15 G0 in the low-conductance region and is free from the influence of substrate materials. However, the distribution of conductance plateaus is too discrete to display distinct peaks in the conductance histogram of the high-conductance region.