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Vapor sensors with both high sensitivity and broad detection range are technically challenging yet highly desirable for widespread chemical sensing applications in diverse environments. Generally, an increased surface‐to‐volume ratio can effectively enhance the sensitivity to low concentrations, but often with the trade‐off of a constrained sensing range. Here, an approach is demonstrated for NH3 sensor arrays with an unprecedentedly broad sensing range by introducing controllable steps on the surface of an n‐type single crystal. Step edges, serving as adsorption sites with electron‐deficient properties, are well‐defined, discrete, and electronically active. NH3 molecules selectively adsorb at the step edges and nearly eliminate known trap‐like character, which is demonstrated by surface potential imaging. Consequently, the strategy can significantly boost the sensitivity of two‐terminal NH3 resistance sensors on thin crystals with a few steps while simultaneously enhancing the tolerance on thick crystals with dense steps. Incorporation of these crystals into parallel sensor arrays results in ppb–to–% level detection range and a convenient linear relation between sheet conductance and semi‐log NH3 concentration, allowing for the precise localization of vapor leakage. In general, the results suggest new opportunities for defect engineering of organic semiconductor crystal surfaces for purposeful vapor or chemical sensing. [ABSTRACT FROM AUTHOR]

We share our perspective that a simple analytical model for electron tunneling in molecular junctions can greatly aid quantitative analysis of experimental data in molecular electronics. In particular, the single-level model (SLM), derived from first principles, provides a precise prediction for the current–voltage (I–V) characteristics in terms of key electronic structure parameters, which in turn depend on the molecular and contact architecture. SLM analysis thus facilitates understanding of structure–property relationships and provides metrics that can be compared across different types of tunnel junctions, as we illustrate with several examples. [ABSTRACT FROM AUTHOR]

The relationship between hole density and conductivity in electrochemically gated polythiophene films is examined. The films are integrated into electrolyte‐gated transistors (EGTs), so that hole accumulations can be electrochemically modulated up to ≈0.4 holes per thiophene ring (hpr). Polythiophenes include poly(3‐alkylthiophenes) (P3ATs) with four different side chain lengths – butyl (P3BT), hexyl (P3HT), octyl (P3OT), or decyl (P3DT) – and poly[2,5‐bis(3‐dodecylthiophen‐2‐yl)thieno[3,2‐b]thiophene] (PBTTT) and poly(3,3′′′‐didodecyl[2,2′:5′,2′′:5′′,2′′′‐quaterthiophene]‐5,5′′′‐diyl) (PQT). Analysis of the drain current – gate voltage (ID–VG) and gate current – gate voltage (IG–VG) characteristics of the EGTs reveals that all six polythiophene semiconductors exhibited reversible conductivity peaks at 0.12 – 0.15 hpr. Conductivity is suppressed beyond ≈0.4 hpr.The maximum carrier mobilities of the P3AT semiconductors increase, and hysteresis of the conductivity peaks decreases, with increasing alkyl side‐chain length. PBTTT and PQT with reduced side chain densities exhibit the largest hysteresis but have higher hole mobilities. The results suggest that at ≈0.4 hpr, a polaronic sub‐band is filled in all cases. Filling of the sub‐band correlates with a collapse in the hole mobility. The side‐chain dependence of the peak conductivity and hysteresis further suggests that Coulombic ion‐carrier interactions are important in these systems. Tailoring ion‐carrier correlations is likely important for further improvements in transport properties of electrochemically doped polythiophenes. [ABSTRACT FROM AUTHOR]

The feasibility of creating top- and bottom-gate pentacene organic field-effect transistors (OTFTs) of comparable performance through the vapor-phase deposition of parylene onto existing pentacene OTFTs on Si/SiO2 is demonstrated. The dependence of the electrical properties on carrier concentration and temperature suggests that, despite differences in interface chemistry and roughness, similar mechanisms are responsible for limiting transport both within the channel and at the metal-organic contacts in both top- and bottom-gate devices.

Alkanethiol tunnel junctions are studied using conducting-probe atomic force microscopy to determine the causes of variability in measured resistance behavior. It is observed that controlling parameters like substrate roughness, tip chemistry, presence of solvent, and extensive tip usage can reduce junction resistance measurements.

The possibility to detect single bond rupture forces in discrete charge-transfer complexes by atomic force microscopy is described. The strong and reversible nature of the CT bond is the key factor in its detection.

A metal insulator metal junction having the structure metal/SAM//Sam mercury in which the two metal-organic interfaces are formed under mild conditions is described. The results demonstrate that a single redox center asymmetrically placed in a metal-insulator-metal junction can cause the rectification of current and indicate that a fixed dipole in the insulating region of a metal-insulator-metal junction is not required for rectification.

Pull-off forces for chemically modified atomic force microscopy tips in contact with flat substrates coated with receptor molecules are calculated using a Johnson, Kendall, and Roberts contact mechanics model. The expression for the work of adhesion is modified to account for the formation of discrete numbers of chemical bonds ([n.sub.Bonds]) between the tip and substrate. The model predicts that the pull-off force scales as [n.sub.Bonds.sup.1/2], which differs from a common assumption that the pull-off force scales linearly with [n.sub.Bonds]. Periodic peak progressions are observed in histograms generated from hundreds of computed pull-off forces. The histogram periodicity is the signature of discrete chemical interactions between the tip and substrate and allows estimation of single-bond rupture forces. The effects of solvent, probe tip radius, and chemical binding probability on the detection of single-bond forces are examined systematically. A dimensionless parameter, the effective force resolution, is introduced that serves as a quantitative predictor for determining when periodicity in force histograms can occur. The output of model is compared to recent experimental results involving tips and substrates modified with self-assembled monolayers. An advantage of this contact mechanics approach is that it allows straightforward estimation of solvent effects on pull-off forces.

It has been possible to make temperature and gate voltage dependent measurements on single grain boundaries in the organic semiconductor sexithiophene (6T). It was found that single grain boundaries between gains of 6T can be isolated between electrodes and characterized. Charge transport in these structures is restricted by the grain boundary.