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This article describes the use ofembossing and “cut-and-stack”methods of assembly, to generate microfluidic devices from omniphobicpaper and demonstrates that fluid flowing through these devices behavessimilarly to fluid in an open-channel microfluidic device. The porosityof the paper to gases allows processes not possible in devices madeusing PDMS or other nonporous materials. Droplet generators and phaseseparators, for example, could be made by embossing “T”-shapedchannels on paper. Vertical stacking of embossed or cut layers ofomniphobic paper generated three-dimensional systems of microchannels.The gas permeability of the paper allowed fluid in the microchannelto contact and exchange with environmental or directed gases. An aqueousstream of water containing a pH indicator, as one demonstration, changedcolor upon exposure to air containing HCl or NH3gases. [ABSTRACT FROM AUTHOR]

Paramagnetic ionic liquids (PILs) provide new capabilities to measurements of density using magnetic levitation (MagLev). In a typical measurement, a diamagnetic object of unknown density is placed in a container containing a PIL. The container is placed between two magnets (typically NdFeB, oriented with like poles facing). The density of the diamagnetic object can be determined by measuring its position in the magnetic field along the vertical axis (levitation height, h), either as an absolute value or relative to internal standards of known density. For density measurements by MagLev, PILs have three advantages over solutions of paramagnetic salts in aqueous or organic solutions: (i) negligible vapor pressures; (ii) low melting points; (iii) high thermal stabilities. In addition, the densities, magnetic susceptibilities, glass transition temperatures, thermal decomposition temperatures, viscosities, and hydrophobicities of PILs can be tuned over broad ranges by choosing the cation-anion pair. The low melting points and high thermal stabilities of PILs provide large liquidus windows for density measurements. This paper demonstrates applications and advantages of PILs in density-based analyses using MagLev. [ABSTRACT FROM AUTHOR]

This paper describes physical-organic studies of charge transport by tunneling through self-assembled monolayers (SAMs), based on systematic variations of the structure of the molecules constituting the SAM. Replacing a -CH2CH2- group with a -CONH- group changes the dipole moment and polarizabiity of a portion of the molecule and has, in principle, the potential to change the rate of charge transport through the SAM. In practice, this substitution produces no significant change in the rate of charge transport across junctions of the structure AgTS-S(CH2)mX(CH2)nH//Ga2O3/EGaJn (TS = template stripped, X = -CH2CH2- or -CONH-, and EGaIn = eutectic alloy of gallium and indium). Incorporation of the amide group does, however, increase the yields of working (non-shorting) junctions (when compared to n-alkanethiolates of the same length). These results suggest that synthetic schemes that combine a thiol group on one end of a molecule with a group, R, to be tested, on the other (e.g., HS~CONW~R) using an amide-based coupling provide practical routes to molecules useful in studies of molecular electronics. [ABSTRACT FROM AUTHOR]

This paper compares charge transport across self-assembled monolayers (SAMs) of n-alkanethiols containing odd and even numbers of methylenes. Ultraflat template-stripped silver (AgTS) surfaces support the SAMs, while top electrodes of eutectic gallium-indium (EGaIn) contact the SAMs to form metal/SAM//oxide/EGaIn junctions. The EGaIn spontaneously reacts with ambient oxygen to form a thin (∷1 nm) oxide layer. This oxide layer enables EGaIn to maintain a stable, conical shape (convenient for forming microcontacts to SAMs) while retaining the ability to deform and flow upon contacting a hard surface. Conical electrodes of EGaIn conform (at least partially) to SAMs and generate high yields of working junctions. Ga2O3/EGaIn top electrodes enable the collection of statistically significant numbers of data in convenient periods of time. The observed difference in charge transport between n-alkanethiols with odd and even numbers of methylenes-the "odd-even effect"-is statistically discernible using these junctions and demonstrates that this technique is sensitive to small differences in the structure and properties of the SAM. Alkanethiols with an even number of methylenes exhibit the expected exponential decrease in current density, J, with increasing chain length, as do alkanethiols with an odd number of methylenes. This trend disappears, however, when the two data sets are analyzed together: alkanethiols with an even number of methylenes typically show higher J than homologous alkanethiols with an odd number of methylenes. The precision of the present measurements and the statistical power of the present analysis are only sufficient to identify, with statistical confidence, the difference between an odd and even number of methylenes with respect to J, but not with respect to the tunneling decay constant, β, or the pre-exponential factor, J0. This paper includes a discussion of the possible origins of the odd-even effect but does not endorse a single explanation. [ABSTRACT FROM AUTHOR]

Analysis of rates of tunneling across self-assembled monolayers (SAMs) of n-alkanethiolates SCn (with n = number of carbon atoms) incorporated in junctions having structure AgTS-SAM//Ga2O3/EGaIn leads to a value for the injection tunnel current density J0 (i.e., the current flowing through an ideal junction with n = 0) of 103.6±0.3 A·cm-2 (V = +0.5 V). This estimation of J0 does not involve an extrapolation in length, because it was possible to measure current densities across SAMs over the range of lengths n = 1-18. This value of J0 is estimated under the assumption that values of the geometrical contact area equal the values of the effective electrical contact area. Detailed experimental analysis, however, indicates that the roughness of the Ga2O3 layer, and that of the AgTS-SAM, determine values of the effective electrical contact area that are ~10-4 the corresponding values of the geometrical contact area. Conversion of the values of geometrical contact area into the corresponding values of effective electrical contact area results in J0(+0.5 V) = 107.6±0.8 A·cm-2, which is compatible with values reported for junctions using top-electrodes of evaporated Au, and graphene, and also comparable with values of J0 estimated from tunneling through single molecules. For these EGaIn-based junctions, the value of the tunneling decay factor β (β = 0.75 ± 0.02 Å-1; β = 0.92 ± 0.02 nC-1) falls within the consensus range across different types of junctions (β = 0.73-0.89 Å-1; β = 0.9-1.1 nC-1). A comparison of the characteristics of conical Ga2O3/EGaIn tips with the characteristics of other top-electrodes suggests that the EGaIn-based electrodes provide a particularly attractive technology for physical-organic studies of charge transport across SAMs. [ABSTRACT FROM AUTHOR]

Self-assembled monolayers (SAMs) have emerged as a simple platform technology and hence have been broadly studied. With advances in state-of-the-art fabrication and characterization methods, new insights into SAM structure and related properties have been delineated, albeit with some discrepancies and/or incoherencies. Some discrepancies, especially between experimental and theoretical work, are in part due to the misunderstanding of subtle structural features such as phase evolution and SAM quality. Recent work has, however, shown that simple techniques, such as the measurement of static contact angles, can be used to delineate otherwise complex properties of the SAM, especially when complemented by other more advanced techniques. In this article, we highlight the effect of nanoscale substrate asperities and molecular chain length on the SAM structure and associated properties. First, surfaces with tunable roughness are prepared on both Au and Ag, and their corresponding n-alkanethiolate SAMs are characterized through wetting and spectroscopy. From these data, chain-length- and substrate-morphology-dependent limits to the odd-even effect (structure and properties vary with the number of carbons in the molecules and the nature of the substrate), parametrization of gauche defect densities, and structural phase evolution (liquidlike, waxy, crystalline interfaces) are deduced. An evaluation of the correlation between the effect of roughness and the components of surface tension (polar-γp and dispersive-γd) reveals that wetting, at nanoscale rough surfaces, evolves proportionally with the ratio of the two components of surface tension. The evolution of conformational order is captured over a range of molecular lengths and parametrized through a dimensionless number, χc. By deploying a well-known tensiometry technique (herein the liquid is used to characterize the solid, hence the term inverse tensiometry) to characterize SAMs, we demonstrate that complex molecular-level phenomena in SAMs can be understood through simplicity. [ABSTRACT FROM AUTHOR]

Thispaper investigates the influence of the atmosphere used inthe fabrication of top electrodes from the liquid eutectic of galliumand indium (EGaIn) (the so-called “EGaIn” electrodes),and in measurements of current density, J(V) (A/cm2), across self-assembled monolayers (SAMs) incorporated intoAg/SR//Ga2O3/EGaIn junctions, on values of J(V) obtained using these electrodes. A gas-tight measurementchamber was used to control the atmosphere in which the electrodeswere formed, and also to control the environment in which the electrodeswere used to measure current densities across SAM-based junctions.Seven different atmospheresair, oxygen, nitrogen, argon, andammonia, as well as air containing vapors of acetic acid or waterweresurveyed using both “rough” conical-tip electrodes,and “smooth” hanging-drop electrodes. (The manipulationof the oxide film during the creation of the conical-tip electrodesleads to substantial, micrometer-scale roughness on the surface ofthe electrode, the extrusion of the drop creates a significantly smoothersurface.) Comparing junctions using both geometries for the electrodes,across a SAM of n-dodecanethiol, in air, gave log |J|mean= −2.4 ± 0.4 for the conicaltip, and log |J|mean= −0.6± 0.3 for the drop electrode (and, thus, Δlog |J| ≈ 1.8); this increase in current density is attributedto a change in the effective electrical contact area of the junction.To establish the influence of the resistivity of the Ga2O3film on values of J(V), junctionscomprising a graphite electrode and a hanging-drop electrode werecompared in an experiment where the electrodes did, and did not, havea surface oxide film; the presence of the oxide did not influencemeasurements of log |J(V)|, and therefore did notcontribute to the electrical resistance of the electrode. However,the presence of an oxide film did improve the stability of junctionsand increase the yield of working electrodes from ∼70% to ∼100%.Increasing the relative humidity (RH) in which J(V)was measured did not influence these values (across methyl (CH3)- or carboxyl (CO2H)-terminated SAMs) over therange typically encountered in the laboratory (20%–60% (RH)). [ABSTRACT FROM AUTHOR]