Exceptional Spatial Variation of Charge Injection Energies on Plasmonic Surfaces

Charge injection into a molecule on a metallic interface is a key step in many photoactivated reactions. We employ the many-body perturbation theory and compute the hole and electron injection energies for CO2 molecule on an Au nanoparticle with ∼3,000 electrons and compare it to results for idealized infinite surfaces. We demonstrate a surprisingly large variation of the injection energy barrier depending on the precise molecular position on the surface. Multiple "hot spots," characterized by low energy barriers, arise due to the competition between the plasmonic coupling and the degree of hybridization between the molecule and the substrate. The charge injection barrier to the adsorbate on the nanoparticle surface decreases from the facet edge to the facet center. This finding contrasts with the typical picture in which the electric field enhancement on the nanoparticle edges is considered the most critical factor.

Going ballistic: a novel characterization for the electronic energy gap

The energy gap of molecular semiconductors is a critical parameter in molecule-based devices because it fundamentally determines the operating principle and device performance, such as the charge transfer driving force. Hence, an accurate quantification of the energy gap of a molecular semiconductor is essential. The hot electron technique, which is implemented using a hot electron transistor and ballistic electron emission microscope, has been verified as the most powerful characterization in recent years. By monitoring the charge transport on the metal/molecule interface, an electron-injection barrier (or LUMO) and hole-injection barrier (or HOMO) can be achieved, which contributes to the electronic energy gap determination. In this review, a comprehensive comparison of these two techniques was made. It helps us to select a suitable method in specific situations and provides a profound comprehension of the charge transport process in molecular semiconductor and molecule-based devices.

Anti-ohmic nanoconductors: myth, reality and promise

The recent accomplishment in the design of molecular nanowires characterized by increasing conductance with length has led to the origin of an extraordinary new family of molecular junctions referred to as "anti-ohmic" wires. Herein, this highly desirable, non-classical behavior, has been examined for molecules long-enough to exhibit pronounced diradical character in their ground state within the unrestricted DFT formalism with spin symmetry breaking. We demonstrate that highly conjugated acenes signal higher resistance in an open-shell singlet (OSS) configuration as compared to their closed-shell counterparts. This anomaly has been further proven for experimentally certified cumulene wires, which reveals phenomenal modulation in the transport characteristics such that an increasing conductance is observed in the closed-shell limit, while higher cumulenes in the OSS ground state yield regular decay of conductance.

Metallic-semiconducting transition and spin polarized-unpolarized transition in a single molecule with a negative Poisson's ratio

Different from conventional materials, structures with a negative Poisson's ratio (NPR) contract/expand laterally under a longitudinal compressive/tensile strain, usually exhibiting peculiar features. Through first-principles calculations, we investigate the electronic and transport properties of Pd₉B₁₆ molecules. Its Poisson's ratio is found to be negative under uniaxial strain along a specific direction. By contacting with Au nanowires, atomic Au chains and atomic C chain electrodes, two kinds of transitions for transmission states could be realized by the modulation of the strain and the contacting site, i.e., metallic-semiconducting transition and spin polarized-unpolarized transition. Further analysis shows that it is the suppression and shifting of density of states, caused by the strain or contacting electrodes, that trigger the transitions. Those findings combine NPR and spintronics at the single-molecule level, which may throw light on the development of nanoelectronic devices.