Adding ethylene glycol (EG) to a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) solution improves the crystallinity of the PEDOT and the ordering of the PEDOT nanocrystals in solid films. The carrier-mobility enhancement is confirmed by using ion-gel transistors combined with in situ UV-vis-NIR spectroscopy.

We have measured high-resolution X-ray photoelectron spectroscopy (XPS) spectra of 1-octadecanethiol (ODT) self-assembled monolayers (SAMs) on a Au(111) surface in order to investigate the Au-S binding properties at the initial stage of SAM growth and after the desorption of the ODT from the Au surface. For the SAM prepared by the 1 min immersion into the ODT solution, we found new sulfur species around 161 eV and assigned it to isolated sulfur without C-S cleavage. We also confirmed the presence of a similar 161 eV peak in the S(2p) spectra after desorption of the ODT molecules. Furthermore, we observed a peak shift in the carbon 1s (C(1s)) peak, depending on the surface coverage of the ODT. In addition to the C(1s) peak of 285.1 eV which might correspond to alkyl chains of densely packed ODT molecules, another lower binding energy peak at 284.3 eV appeared after annealing. This lower C(1s) binding energy peak formation suggested that some of the alkyl chains for the remaining ODT molecules might be oriented parallel to the Au surface after annealing.

We have used high-resolution X-ray photoelectron spectroscopy (XPS) to investigate the molecular dependence on sulfur chemical states of organosulfur monolayers of alkanethiol, dialkyl disulfide, monosulfide, thiophene, and aromatic thiols on a Au(111) surface. When monosulfides or thiophenes adsorbed on the Au surface, the S(2p) peaks appeared between 161 and 164 eV, and peak fitting revealed that these organosulfurs exhibited almost the same peaks as those of alkanethiol and dialklyl disulfide monolayers. The monolayer made from monosulfide with shorter alkyl chains exhibited almost the same S(2p) XPS spectrum as the typical alkanethiol or dialkyl disulfide monolayers. Another S(2p3/2) peak appeared for aromatic derivatized thiol SAMs at around 161 eV, in addition to strong doublet S(2p) peak observed at 162.0 and 163.3 eV in the S(2p) spectra. The 161 eV peak was observed even in the S(2p) XPS spectra of alkanethiol or dialkyl disulfide monolayers, at the initial stage of monolayer growth or after low-temperature (∼ 100 °C) annealing of low molecular density alkanethiol SAMs. We consider that this 161 eV peak can be formed without molecular decomposition as well as due to the atomic sulfur produced by C-S cleavage.

We have studied electrical conduction of conjugated molecules with phenyl rings embedded into alkanethiol self-assembled monolayers (SAMs), to investigate the molecular structural effect on the electrical conduction. Scanning tunneling microscope (STM) images of this surface revealed that the conjugated molecules with phenyl rings adsorbed mainly on defects and domain boundaries of the pre-assembled alkanethiol (nonanethiol C9) SAM and formed conjugated domains. In the case of conjugated molecules with one or three methylene groups between the sulfur and phenyl rings, the measured height of the conjugated molecular domains depended on their lateral sizes, while a strong dependence was not observed in the case of conjugated molecules without a methylene group. By analyzing size dependence on the height of the conjugated molecular domain, we could evaluate the electronic conductivity of the molecular domains. As a result of the analysis, to increase the vertical conduction of the molecular domains, one methylene group was found to be necessary between the sulfur and aromatic phenyl rings. Local barrier heights on the conjugated molecular domains in all the cases were larger than on the C9 SAM surface, suggesting that the increase in the vertical conductivitity is not likely to be due to the lowering of the local barrier height, but can be attributed to the conjugated molecular adsorption. X-ray photoelectron spectra (XPS) and ultraviolet light photoelectron spectra (UPS) revealed that the carrier density among conjugated molecular SAMs does not depend on the number of methylene groups between the sulfur and phenyl rings, suggesting that the higher vertical conduction of conjugated molecules with one methylene group can probably be attributed to higher transfer probability of carriers during the STM measurements.

The structures of semifluorinated alkanethiol self-assembled monolayers (SAMs) generated by the adsorption of CF3(CF2)9(CH2)nSH (F10HnSH, n ) 2, 6, 11, 17, 33) onto gold were investigated with atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and surface plasmon resonance spectroscopy. Images obtained by AFM of the F10H2SH SAM showed a remarkably ordered 2D hexagonal lattice with a lattice constant, a ) 5.9 ( 0.1 Å, on Au(111)/mica. As the total number of carbon atoms in the alkyl spacer groups (n) was increased, the fluorocarbon adlayer structure appeared more disordered in the AFM images; however, the thicknesses of the fluorocarbon layers estimated from the C 1s (CF3), C 1s (CF2), and F 1s XPS signal intensities were indistinguishable in all of the SAMs. In contrast, the C 1s (CH2) signals revealed that the tilt angles of the alkyl spacer groups depended strongly on n. We utilized a contrast variation SPR technique with various contacting media for an independent determination of the refractive indices and the film thicknesses of the semifluorinated SAMs. The obtained data were consistent with our AFM and XPS results, which show that the longer alkyl chains pack more densely than the shorter ones in these SAMs.

Recently published research from the National Renewable Energy Laboratory (NREL) reports that biohybrid hydrogen electrodes comprising metallic single-walled carbon nanotube (SWNT) networks and the hydrogenase from Clostridium acetobutylicum achieved a new activity record for hydrogenase-based electrode electrocatalysis. These results demonstrate that hydrogenase/ SWNT electrodes have the potential to provide a cheaper but equally efficient alternative to the precious metal catalysts, such as platinum, for application in photoelectrochemical or fuel cells. The high-performance hydrogen electrodes are based on the [FeFe]-hydroge-nase from C. acetobutylicum immobilized onto SWNT networks. The researchers prepared the electrodes with varying ratios of metallic (m-) and semiconducting (s-) SWNTs to explore the role of SWNT electronic structure in the biohybrid electrodes. Although most hydrogenase/SWNT electrodes showed improved performance in comparison to the hydrogenase immobilized directly to bulk carbon, high current densities up to 12 mA cm-2 (at-1 V vs. SHE) were achieved with hydrogenase immobilized on SWNT networks with high m-SWNT content. Using electrochemical methods, NREL researchers showed that m-SWNTs contribute to increased electrode electroactive surface available for hydrog-enase binding and improve electronic coupling between the electrode and the hydrogenase redox sites.

The electrical conduction of self-assembled monolayers (SAMs) made from conjugated molecules was measured using conductive atomic force microscopy (AFM), with a focus on the molecular structural effect on the electrical conduction. For phenylene oligomer SAMs, resistances through the monolayers increased exponentially with increases in molecular length and the decay constants of transconductance β were ca. 0.35 to 0.5 Å-1. Using an insertion technique into insulative alkanethiol SAMs, we successfully obtained single molecular resistance of terphenyl methanethiol at ca. 5.4 × 1010 Ω. We further investigated the influence of applied load on the resistances. The resistances through terphenyl SAMs increased with increases in the applied load up to 15 nN. When two or three methylene spacers were introduced between the sulfur and terphenyl groups in a series of terphenyl derivatized thiols, the monolayer resistances and β values increased extraordinarily. One explanation is that the addition of methylene spacers changed the location of the molecular orbital as a result of MOPAC calculation.

Electric conductivity of organic molecules was estimated with molecular resolution using self-assembled techniques and scanning tunneling microscopy (STM). Conjugated molecules of [1,1‘:4‘,1‘ ‘-terphenyl]-4-methanethiol (TP) were embedded in self-assembled monolayers of insulative n-alkanethiols, and when observed by STM, TP molecules appeared as protruding domains. The apparent height of the TP domains increases as the lateral size of the domains grows from 1 to 10 nm, reflecting the increase in the vertical conductance of the domains due to the lateral, intermolecular interaction. We assumed that the molecules are connected to each other with resistors for estimating the effect of intermolecular interaction on the conductance and calculated the height of conducting disks with various radii, which should roughly reproduce the size-dependent height of the TP domains observed by STM. The estimated resistance of the single TP molecule was less than 40 GΩ, and the effective lateral conductivity corresponding to the large TP domains was larger than 0.01 S/cm.