Highly Planar and Completely Insulated Oligothiophenes: Effects of π-Conjugation on Hopping Charge Transport.

Elucidating the nature of long-range intramolecular charge transport in π-conjugated molecules is of great importance for the development of organic electronic materials. However, the effects of the degree of π-conjugation on the hopping charge transport have not been experimentally explored so far owing to the lack of π-conjugated backbones with different conjugation degrees and several-nanometer lengths. Here we develop highly planar and completely insulated oligothiophenes between 0.85 and 9.64 nm in length. As compared to distorted oligothiophenes, single-molecule conductance measurements of the planar molecules show (i) a smaller activation energy and larger electrical conductance in the hopping transport regime and (ii) a shift in crossover between tunneling and hopping conduction toward a short molecular length. Theoretical calculations indicate that small reorganization energies and narrow energy gaps derived from the planar backbones result in these superior characteristics. This study reveals that the planarity of π-conjugation has significant advantages for hopping charge transport.

Thermoelectricity at the molecular scale: a large Seebeck effect in endohedral metallofullerenes.

Single molecule devices provide a unique system to study the thermoelectric energy conversion at an atomistic level and can provide valuable information for the design of organic thermoelectric materials. Here we present a comprehensive study of the thermoelectric transport properties of molecular junctions based on C(82), Gd@C(82), and Ce@C(82). We combine precise scanning tunneling microscope break-junction measurements of the thermopower and conductance with quantitatively accurate self-energy-corrected first-principles transport calculations. We find that all three fullerene derivatives give rise to a negative thermopower (n-conducting). The absolute value, however, is much larger for the Gd@C(82) and Ce@C(82) junctions. The conductance, on the other hand, remains comparable for all three systems. The power factor determined for the Gd@C(82) based junction is so far the highest obtained for a single-molecule device. Although the encapsulated metal atom does not directly contribute to the transport, we show that the observed enhancement of the thermopower for Gd@C(82) and Ce@C(82) is elucidated by the substantial changes in the electronic- and geometrical structure of the fullerene molecule induced by the encapsulated metal atom.

Thiophene-based Tripodal Anchor Units for Hole Transport in Single-Molecule Junctions with Gold Electrodes.

Molecule-metal junctions are inevitable for the realization of single-molecule electronics. In this study, we developed new tripodal anchors with electron-rich aromatic rings to achieve robust contact with gold electrodes, an effective hybridization of the π orbital with gold electrodes (π channel), and hole transport through π-channel hybridization. Cyclic voltammetry and X-ray photoelectron spectroscopy measurements of the monolayers indicated that the thiophene-based tripodal molecule exhibits anchoring characteristics as expected. The electrical conductance of thiophene-anchored bistripodal molecules using the scanning tunneling microscope (STM)-based break junction technique confirmed the formation of molecular junctions. The Seebeck coefficient of this compound estimated from thermoelectric voltage measurements using a STM was determined to be a positive value, which indicates that the charge carriers are holes. On the contrary, the corresponding pyridine-anchored molecules showed electron transport. These results reveal the versatility of π-channel tripodal anchors for the control of charge-carrier type in single-molecule electronics.

Thermopower of benzenedithiol and C60 molecular junctions with Ni and Au electrodes.

We have performed thermoelectric measurements of benzenedithiol (BDT) and C60 molecules with Ni and Au electrodes using a home-built scanning tunneling microscope. The thermopower of C60 was negative for both Ni and Au electrodes, indicating the transport of carriers through the lowest unoccupied molecular orbital in both cases, as was expected from the work functions. On the other hand, the Ni-BDT-Ni junctions exhibited a negative thermopower, whereas the Au-BDT-Au junctions exhibited a positive thermopower. First-principle calculations revealed that the negative thermopower of Ni-BDT-Ni junctions is due to the spin-split hybridized states generated by the highest occupied molecular orbital of BDT coupled with s- and d-states of the Ni electrode.

Universal temperature crossover behavior of electrical conductance in a single oligothiophene molecular wire.

We have observed and analyzed a universal temperature crossover behavior of electrical conductance in a single oligothiophene molecular wire. The crossover between the Arrhenius-type temperature dependence at high temperature and the temperature-invariant behavior at low temperature is found at a critical molecular wire length of 5.6 nm, where we found a change from the exponential length dependence to the length-invariant behavior. We have derived a scaling function analysis for the origin of the crossover behavior. After assuring that the analysis fits the explanation of the Keldysh Green's function calculation for the temperature dependence, we have applied it to our experimental results and found successfully that our scaling function gives a universal description of the temperature dependence for all over the temperature range.