Single-Molecule Conductance of Viologen-Cucurbit[8]uril Host-Guest Complexes

The local molecular environment is a critical factor which should be taken into account when measuring single-molecule electrical properties in condensed media or in the design of future molecular electronic or single molecule sensing devices. Supramolecular interactions can be used to control the local environment in molecular assemblies and have been used to create microenvironments, for instance, for chemical reactions. Here, we use supramolecular interactions to create microenvironments which influence the electrical conductance of single molecule wires. Cucurbit[8]uril (CB[8]) with a large hydrophobic cavity was used to host the viologen (bipyridinium) molecular wires forming a 1:1 supramolecular complex. Significant increases in the viologen wire single molecule conductances are observed when it is threaded into CB[8] due to large changes of the molecular microenvironment. The results were interpreted within the framework of a Marcus-type model for electron transfer as arising from a reduction in outer-sphere reorganization energy when the viologen is confined within the hydrophobic CB[8] cavity.

Evidence for a hopping mechanism in metal|single molecule|metal junctions involving conjugated metal–terpyridyl complexes; potential-dependent conductances of complexes [M(pyterpy)2]2+ (M = Co and Fe; pyterpy = 4′-(pyridin-4-yl)-2,2′:6′,2′′-terpyridine) in ionic liquid

Extensive studies of various families of conjugated molecules in metal|molecule|metal junctions suggest that the mechanism of conductance is usually tunnelling for molecular lengths < ca. 4 nm, and that for longer molecules, coherence is lost as a hopping element becomes more significant. In this work we present evidence that, for a family of conjugated, redox-active metal complexes, hopping may be a significant factor for even the shortest molecule studied (ca. 1 nm between contact atoms). The length dependence of conductance for two series of such complexes which differ essentially in the number of conjugated 1,4-C₆H₄- rings in the structures has been studied, and it is found that the junction conductances vary linearly with molecular length, consistent with a hopping mechanism, whereas there is significant deviation from linearity in plots of log(conductance) vs. length that would be characteristic of tunnelling, and the slopes of the log(conductance)–length plots are much smaller than expected for an oligophenyl system. Moreover, the conductances of molecular junctions involving the redox–active molecules, [M(pyterpy)₂]^2+/3+ (M = Co, Fe) have been studied as a function of electrochemical potential in ionic liquid electrolyte, and the conductance–overpotential relationship is found to fit well with the Kuznetsov–Ulstrup relationship, which is essentially a hopping description.

Low variability of single-molecule conductance assisted by bulky metal–molecule contacts

A detailed study of the trimethylsilylethynyl moiety, –CCSiMe₃ (TMSE), as an anchoring group, using a combination of experiment and DFT is presented.

Gating of single molecule junction conductance by charge transfer complex formation

The solid-state structures of organic charge transfer (CT) salts are critical in determining their mode of charge transport, and hence their unusual electrical properties, which range from semiconducting through metallic to superconducting. In contrast, using both theory and experiment, we show here that the conductance of metal |single molecule| metal junctions involving aromatic donor moieties (dialkylterthiophene, dialkylbenzene) increase by over an order of magnitude upon formation of charge transfer (CT) complexes with tetracyanoethylene (TCNE). This enhancement occurs because CT complex formation creates a new resonance in the transmission function, close to the metal contact Fermi energy, that is a signal of room-temperature quantum interference.

Single-Molecule Electronics: Chemical and Analytical Perspectives

It is now possible to measure the electrical properties of single molecules using a variety of techniques including scanning probe microcopies and mechanically controlled break junctions. Such measurements can be made across a wide range of environments including ambient conditions, organic liquids, ionic liquids, aqueous solutions, electrolytes, and ultra high vacuum. This has given new insights into charge transport across molecule electrical junctions, and these experimental methods have been complemented with increasingly sophisticated theory. This article reviews progress in single-molecule electronics from a chemical perspective and discusses topics such as the molecule-surface coupling in electrical junctions, chemical control, and supramolecular interactions in junctions and gating charge transport. The article concludes with an outlook regarding chemical analysis based on single-molecule conductance.

Electrical characterization of single molecule and Langmuir-Blodgett monomolecular films of a pyridine-terminated oligo(phenylene-ethynylene) derivative

Monolayer Langmuir-Blodgett (LB) films of 1,4-bis(pyridin-4-ylethynyl)benzene (1) together with the "STM touch-to-contact" method have been used to study the nature of metal-monolayer-metal junctions in which the pyridyl group provides the contact at both molecule-surface interfaces. Surface pressure vs area per molecule isotherms and Brewster angle microscopy images indicate that 1 forms true monolayers at the air-water interface. LB films of 1 were fabricated by deposition of the Langmuir films onto solid supports resulting in monolayers with surface coverage of 0.98 × 10(-9) mol·cm(-2). The morphology of the LB films that incorporate compound 1 was studied using atomic force microscopy (AFM). AFM images indicate the formation of homogeneous, monomolecular films at a surface pressure of transference of 16 mN·m(-1). The UV-vis spectra of the Langmuir and LB films reveal that 1 forms two dimensional J-aggregates. Scanning tunneling microscopy (STM), in particular the "STM touch-to-contact" method, was used to determine the electrical properties of LB films of 1. From these STM studies symmetrical I-V curves were obtained. A junction conductance of 5.17 × 10(-5) G 0 results from the analysis of the pseudolinear (ohmic) region of the I-V curves. This value is higher than that of the conductance values of LB films of phenylene-ethynylene derivatives contacted by amines, thiols, carboxylate, trimethylsilylethynyl or acetylide groups. In addition, the single molecule I-V curve of 1 determined using the I(s) method is in good agreement with the I-V curve obtained for the LB film, and both curves fit well with the Simmons model. Together, these results not only indicate that the mechanism of transport through these metal-molecule-metal junctions is non-resonant tunneling, but that lateral interactions between molecules within the LB film do not strongly influence the molecule conductance. The results presented here complement earlier studies of single molecule conductance of 1 using STM-BJ methods, and support the growing evidence that the pyridyl group is an efficient and effective anchoring group in sandwiched metal-monolayer-metal junctions prepared under a number of different conditions.

Ionic liquid based approach for single-molecule electronics with cobalt contacts

An electrochemical method is presented for fabricating cobalt thin films for single-molecule electrical transport measurements. These films are electroplated in an aqueous electrolyte, but the crucial stages of electrochemical reduction to remove surface oxide and adsorption of alkane(di)thiol target molecules under electrochemical control to form self-assembled monolayers which protect the oxide-free cobalt surface are carried out in an ionic liquid. This approach yields monolayers on Co that are of comparable quality to those formed on Au by standard self-assembly protocols, as assessed by electrochemical methods and surface infrared spectroscopy. Using an adapted scanning tunneling microscopy (STM) method, we have determined the single-molecule conductance of cobalt/1,8-octanedithiol/cobalt junctions by employing a monolayer on cobalt and a cobalt STM tip in an ionic liquid environment and have compared the results with those of experiments using gold electrodes as a control. These cobalt substrates could therefore have future application in organic spintronic devices such as magnetic tunnel junctions.

Single-molecule conductance determinations on HS(CH2)4O(CH2)4SH and HS(CH2)2O(CH2)2O(CH2)2SH, and comparison with alkanedithiols of the same length

The acetyl-protected, thiol-terminated ethers AcS(CH(2))(4)O(CH(2))(4)SAc and AcS(CH(2))(2)O(CH(2))(2)O(CH(2))(2)SAc have been synthesised, and a range of related scanning tunnelling microscopy (STM)-based methods have been employed to fabricate and electrically characterise gold | single molecule | gold junctions involving these molecules. The single-molecule conductance values obtained are consistently found to be substantially higher (by a factor of 2-3) than the conductances of analogous alkanedithiols of similar length (HS(CH(2))(9)SH and HS(CH(2))(8)SH, respectively). A rationalisation of these findings is suggested, namely that the lone pair electrons on the oxygen atoms are substantially closer in energy to the Fermi energy of the gold leads than are the occupied and unoccupied states of methylene chains, so that the ether oxygens behave in a manner analogous to 'wells' in a double-tunnelling-barrier system. In agreement with this suggestion, the current-voltage behaviour of the monoether can be fitted using the Simmons approach, and the barrier height is found to be significantly lower than for alkanedithiols of approximately the same length.

Comparison of the conductance of three types of porphyrin-based molecular wires: β,meso,β-fused tapes, meso-Butadiyne-linked and twisted meso-meso linked oligomers

The length dependence of charge transport is evaluated in three families of porphyrin-based wires. Planar edge-fused tapes and alkyne-linked oligomers mediate efficient charge transport with exceptionally shallow distance dependence, whereas the conductances of the twisted singly linked chains decrease steeply with increasing oligomer length. The planar tapes are more conjugated than the alkyne-linked oligomers, but these two types of wires have similar conductance attenuation factors.

Long-range electron tunnelling in oligo-porphyrin molecular wires

Short chains of porphyrin molecules can mediate electron transport over distances as long as 5-10 nm with low attenuation. This means that porphyrin-based molecular wires could be useful in nanoelectronic and photovoltaic devices, but the mechanisms responsible for charge transport in single oligo-porphyrin wires have not yet been established. Here, based on electrical measurements of single-molecule junctions, we show that the conductance of the oligo-porphyrin wires has a strong dependence on temperature, and a weak dependence on the length of the wire. Although it is widely accepted that such behaviour is a signature of a thermally assisted incoherent (hopping) mechanism, density functional theory calculations and an accompanying analytical model strongly suggest that the observed temperature and length dependence is consistent with phase-coherent tunnelling through the whole molecular junction.

Functionalized Regioregular Polyalkylthiophenes for Biosensing Applications

A regioregular copolymer of 3–hexylthiophene and 3–(6–hydroxyhexan–1yl)thiophene has been functionalised with biotin hydrazide; binding of avidin to the biotin moieties causes drastic changes to the absorption spectrum of the polymer in solution, and to the electrochemistry and conductivity of the polymer in thin films.

Critical Considerations in Polymer Thin-Film Transistor (TFT) Dielectrics

We present a study of aqueous and plasma anodised aluminium oxide (Al2O3) and its performance in thin film transistors (TFTs). The current through the oxide was measured with aluminium electrodes and with one of the electrode replaced by poly(3-hexylthiophene)(P3HT). The current increased by up to 2 orders of magnitude with P3HT. The current increased further when the polymer was doped with different percentages of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). It was also found to be dependent on the thickness of the polymer film. Surprisingly, the oxide current fell to its initial value when the polymer film was removed. Two mechanisms may explain the behaviour in these devices: charge injection and/or displacement. C-V plots were obtained from the MOS capacitors and were frequency dependent. They also showed substantial hysteresis, with a lateral shift along the voltage axis. This indicates the presence of a mobile species that increases with the concentration of dopant. We deduce that much of the increased gate current is associated with displacement currents induced by ion motion.