No abstract

Controlling charge transport through individual molecules and further understanding the effect of anchoring groups on charge transport are central themes in moleculebased devices. However, in most anchoring effect studies, only two, or at most three nonthiol anchoring groups were studied and compared for a specific system, i.e., using the same core structure. The scarcity of direct comparison data makes it difficult to draw unambiguous conclusions on the anchoring group effect. In this contribution, we focus on the single molecule conductance of porphyrins terminated with a range of anchoring groups: sulfonate (−SO 3 − ), hydroxyl (−OH), nitrile (−CN), amine (−NH 2 ), carboxylic acid (−COOH), benzyl (−C 6 H 6 ), and pyridyl (−C 6 H 5 N). The present study represents a first attempt to investigate a broad series of anchoring groups in one specific molecule for a direct comparison. It also is the first attempt, to our knowledge, to explore single molecule conductivity with two novel anchoring groups sulfonate (−SO 3 −) and hydroxyl (−OH). Our experimental results reveal that the single molecule conductance values of the porphyrins follow the sequence of pyridyl > amine > sulfonate > nitrile > carboxylic acid. Electron transport calculations are in agreement that the pyridyl groups result in higher conductance values than the other groups, which is due to a stronger binding interaction of this group to the Au electrodes. The finding of a general trend in the effect of anchoring groups and the exploration of new anchoring groups reported in this paper may provide useful information for molecule-based devices, functional porphyrin design, and electron transfer/transport studies.

No abstract

Multivalent ion batteries (MVIBs) provide an inexpensive and energy-dense alternative to Li-ion batteries when portability of the battery is not of primary concern. However, it is difficult to find cathode materials that provide optimal battery characteristics such as energy density, adequate charge/discharge rates, and cyclability when paired with a multivalent ion. To address this, we investigate six MnO 2 polymorphs as cathodes for MVIBs using density functional theory calculations. We find voltages as high as 3.7, 2.4, 2.7, 1.8, and 1.0 V for Li, Mg, Ca, Al, and Zn, respectively, and calculate the volume change due to intercalation. We then focus specifically on Ca and compute the energy barriers which are associated with the diffusion of the ion throughout the materials. Our findings show that the α-phase displays the most rapid diffusion kinetics for a Ca ion, with a diffusion barrier as low as 190 meV. We then investigate the potential for the five polymorphs exhibiting the highest voltage to intercalate additional atoms and demonstrate that it is energetically favorable for each to accept at least one additional Ca ion; furthermore, two of the phases can accept more than two Ca ions. However, in each case, there is also a corresponding drop in the voltage as further atoms are intercalated. We also utilize a crystal-chemistry approach to detail the structural evolution of each phase by computing the bond valence sum and effective coordination of the Mn 4+ ions upon intercalation of increasing numbers of Ca ions. Finally, by computing the electronic density of states, Bader charges, and real space charge density, we describe how the additional electrons from the Ca ions are distributed throughout the unit cell. These insights provide guidance in selecting a MnO 2 polymorph with the traits necessary for the realization of MVIBs.