Ming-Dung Fu

Superior Contact for Single-Molecule Conductance: Electronic Coupling of Thiolate and Isothiocyanate on Pt, Pd, and Au

One of the critical issues for the realization of molecular electronics is the development of ideal molecule-electrode contacts that render efficient charge transportation and thus attenuate the unwanted voltage drop and power loss. The conductance at the single-molecule level has long been expected to be correlated strongly with the electrode materials. However, other than gold, systematic studies of a homologous series of molecules to extract the headgroup-metal contact conductance (G(n=0)) have not been reported. Carefully examined herein are the conductances of alkanedithiols anchored onto electrode materials of Au and Pt as well as the conductances of alkanediisothiocyanates on Au, Pd, and Pt by utilizing the method of STM-BJ (scanning tunneling microscopy break junction). In comparison with Au substrate, Pd and Pt are group 10 elements with stronger d-orbital characteristics, and larger local density of states near the Fermi level. The model compounds, SCN(CH(2))(n)NCS (n = 4, 6, and 8), are studied because the isothiocyanate (-NCS) headgroup is a versatile ligand for organometallics, an emerging class of molecular wires, and can bind to substrates of noble metals to complete a metal-molecule-metal configuration for external I-V measurements. Also studied include alkanedithiols, one of the most scrutinized systems in the field of single-molecule conductance. The results show that the conductance for single molecules bridged between a pair of Pt electrodes is about 3.5-fold superior to those between Au electrodes. On all electrode materials, observed are two sets of conductance values, with the smaller set being 1 order of magnitude less conductive. These findings are ascribed to the degree of electronic coupling between the headgroup and the electrode.

Extended Metal-Atom Chains with an Inert Second Row Transition Metal : [Ru5(μ5-tpda)4X2] (tpda2-= tripyridyldiamido dianion, X = Cl and NCS)

EMACs (extended metal-atom chains) offer a unique platform for the exploration of metal-metal interactions. There has been significant advances on the synthesis of EMACs, such as lengthening the chains up to 11 metal atoms thus far, integrating naphthyridine moieties for tuning the charge carried at metal centers, and manipulation of metal-metal interactions. However, the metal centers in EMACs hitherto are limited to first row transition metals which are more labile than those relatively inert ones with electrons filled in the 4d and 5d shells. In this Communication, the synthesis, crystallographic, magnetic, and electrical conducting studies of [Ru5(mu5-tpda)4Cl2] and [Ru5(mu5-tpda)4(NCS)2], the first pentanuclear EMACs of second-row transition metal, are reported.

On the tuning of electric conductance of extended metal atom chains via axial ligands for [Ru3(μ3-dpa)4(X)2]0/+ (X = NCS−, CN−)

The influence of a pi-acid cyanide axial ligand on the metal-metal interactions of [Ru(3)(mu(3)-dpa)(4)(X)(2)](0/+) (X = NCS(-), CN(-)) is manifested by the measurements of single-molecule conductance coupled with in situ electrochemical control.

Conductance of Tailored Molecular Segments: A Rudimentary Assessment by Landauer Formulation

One of the strengths of molecular electronics is the synthetic ability of tuning the electric properties by the derivatization and reshaping of the functional moieties. However, after the quantitative measurements of single-molecule resistance became available, it was soon apparent that the assumption of negligible influence of the headgroup-electrode contact on the molecular resistance was oversimplified. Due to the measurement scheme of the metal--molecule-metal configuration, the contact resistance is always involved in the reported values. Consequently the electrical behavior of the tailored molecular moiety can only be conceptually inferred by the tunneling decay constant (βn in Rmeasured = R(n=0)e(βnN), where N is the number of repeated units), available only for compounds with a homologous series. This limitation hampers the exploration of novel structures for molecular devices. Based on the Landauer formula, we propose that the single-molecule resistance of the molecular backbones can be extracted. This simplified evaluation scheme is cross-examined by electrode materials of Au, Pd, and Pt and by anchoring groups of thiol (-SH), nitrile (-CN), and isothiocyanate (-NCS). The resistance values of molecular backbones for polymethylenes (n = 4, 6, 8, and 10) and phenyl (-C6H4-) moieties are found independent of the anchoring groups and electrode materials. The finding justifies the proposed approach that the resistance of functional moieties can be quantitatively evaluated from the measured values even for compounds without repeated units.

The First Heteropentanuclear Extended Metal‐Atom Chain: [Ni+Ru25+Ni2+Ni2+(tripyridyldiamido)4(NCS)2]

This study develops the first heteropentametal extended metal atom chain (EMAC) in which a string of nickel cores is incorporated with a diruthenium unit to tune the molecular properties. Spectroscopic, crystallographic, and magnetic characterizations show the formation of a fully delocalized Ru2(5+) unit. This [Ru2]-containing EMAC exhibits single-molecule conductance four-fold superior to that of the pentanickel complex and results in features of negative differential resistance (NDR), which are unobserved in analogues of pentanickel and pentaruthenium EMACs. A plausible mechanism for the NDR behavior is proposed for this diruthenium-modulated EMAC.

The effect of molecular conformation on single molecule conductance: measurements of π-conjugated oligoaryls by STM break junction

Measurements of molecular break junction reveal quantitatively the correlation between the single-molecule conductance and the conformation of pi-conjugated molecules with 6-18 conjugated double bonds.

Fine tuning of pentachromium(II) metal string complexes through elaborate design of ligand

New pentachromoium metal string complexes [Cr5(μ5-L)4X2] (X = Cl−, L = dppzda2− (1), dpzpda2− (2); X = NCS−, L = dppzda2− (3), dpzpda2− (4)) were designed and synthesized through pyrazine-modulation of tripyridyldiamine ligand. X-Ray crystallographic studies revealed a linear metal chain structure consisting of two quadruple Cr–Cr bonds and a separated high spin Cr(II) at an end in crystallized form. A quintet ground state was observed for all pentachromium(II) molecules by magnetic study with g values of 2.04–2.18. While the electronic structure remained unchanged after the modification of ligands, electrochemistry showed a significant change in the molecular orbital energy levels of metal string molecules. Observation of the first oxidation peak of 1 at +0.57 V and of 2 at +0.73 V revealed that these complexes are quite resistant to oxidation. Single molecular conductance measurements showed that the complex exhibited good electronic conductance.