Determining Compositional Variation in Silicon-Metal Alloys by Parsing SEM/EDS Hyperspectral Images.

High-temperature differential scanning calorimetry was used to understand the thermal properties of Si-rich metal­silicon alloys. Insoluble metals (A and B) were found to produce an alloy with discrete ASi2 and BSi2 dispersed phases. In contrast, metals that form a solid solution result in a dispersed phase that has a composition of AxB1−xSi2, where x varies continuously across each inclusion. This complex composition distribution is putatively caused by differences in the solidification temperatures of ASi2 versus BSi2. Though this behavior was observed for several different combinations of metals, we focus here specifically on the Cr/V/Si system. To better understand the range and most probable element concentrations in the dispersed silicide domains, a method was devised to generate histograms of their Cr and V concentrations from energy-dispersive X-ray spectroscopy hyperspectral images. Varying the Cr/V/Si ratio was found to change the shape of the element histograms, indicating that the distribution of silicide compositions that form is controlled by the input composition. Adding aluminum was found to result in dispersed phases that had a single composition rather than a range of Cr and V concentrations. This demonstrates that aluminum can be an effective additive for altering solidification kinetics in silicon alloys.

Non-Fluorinated, Superhydrophobic Binder-Filler Coatings on Smooth Surfaces: Controlled Phase Separation of Particles to Enhance Mechanical Durability.

There has been a recent drive to develop non-fluorinated superhydrophobic coatings due to the toxicity, cost, and environmental impact of perfluorinated components. One of the main challenges in developing superhydrophobic coatings in general and non-fluorinated superhydrophobic coatings in particular is optimization of mechanical durability, as the rough asperities required for maintaining superhydrophobicity tend to be easily removed by abrasion. Although rough and self-similar hydrophobic surfaces composed of loosely adhered particles or highly porous structures tend to produce excellent superhydrophobicity, they have low inherent mechanical durability and their longevity under real conditions is compromised. To address this issue, this work investigates the addition of a polymeric matrix material (the binder) to hydrophobic nanoparticles (the filler) to produce spray-coated superhydrophobic surfaces with improved inherent mechanical durability. Hansen solubility parameters were used to tune the interactions between the binder, filler, and solvent used to deliver the coating. It was found that lowering the binder/filler miscibility and using a poor solvent mixture generates more surface roughness, thereby lowering the minimum filler load required to achieve superhydrophobicity. This leads to an overall more inherently durable system that remains hydrophobic for thousands of light abrasion cycles.

Tough, Transparent, Photocurable Hybrid Elastomers.

We investigated polydimethylsiloxane/poly(methyl methacrylate) (PDMS/PMMA) interpenetrating polymer networks (IPNs) by both sequential and simultaneous syntheses. In the sequential IPN, the PDMS network was first thermally cured after which methyl methacrylate was swelled in and UV photopolymerized in situ. The simultaneous IPN consists of a one-pot, single-step UV cure of both components. Pure shear fracture and tensile tests were used to extract the Young's modulus, critical fracture strain, and fracture energy of the materials at varying PMMA fractions (up to 50 wt %). At high PMMA fractions, a maximum increase in Young's modulus (42×) and fracture energy (21×) was observed with little sacrifice in the optical properties and the extensibility of notched samples. The Krieger-Dougherty model for particle reinforcement was fit to the modulus data as a function of the PMMA fraction and showed good agreement. The optical properties and microstructure of the IPNs were investigated by UV-visible light transmission, small-angle X-ray scattering (SAXS), and atomic force microscopy (AFM). As the weight fraction of PMMA increased, the simultaneous IPN became less transparent, while the sequential material showed the opposite trend. In the sequential IPN, the minority phase size decreased with increasing PMMA fraction, while it was constant for the simultaneous IPN. Therefore, it was concluded that the sequential IPN transparency is controlled by the size of the PMMA domains, but the simultaneous IPN transparency is controlled by the PMMA fraction. SAXS and AFM also showed evidence of bicontinuous network formation in the simultaneous IPN, which may affect the optical and mechanical properties.

Semiquantitative Atomic Force Microscopy-Infrared Spectroscopy Analysis of Chemical Gradients in Silicone Optical Waveguides.

Crossing losses in silicone optical waveguides are related to the magnitude and spatial extent of the waveguide refractive index gradient. When processing conditions are altered, the refractive index gradient can vary substantially, even when the formulation remains constant. Controlling the refractive index gradient requires control of the concentration of small molecules present within the core and clad layers. Developing a fundamental understanding of how small molecule migration drives changes in crossing loss requires the ability to examine chemical functionality over small length scales, which is a natural fit for atomic force microscopy-infrared spectroscopy (AFM-IR). In this work, AFM-IR spectra from model bilayer stacks are initially examined to understand molecular migration that occurs from heating the core and clad layers. The results of these model studies are then applied to photopatterned waveguide builds, where structure-function relationships are constructed between values of crossing loss and the concentration of C-H and O-H functionalities present in the core and clad layers. Results show that small molecule evaporation and migration are competing processes that need to be controlled to minimize crossing loss.

Measuring relative barrier heights in molecular electronic junctions with transition voltage spectroscopy.

Though molecular devices exhibiting potentially useful electrical behavior have been demonstrated, a deep understanding of the factors that influence charge transport in molecular electronic junctions has yet to be fully realized. Recent work has shown that a mechanistic transition occurs from direct tunneling to field emission in molecular electronic devices. The magnitude of the voltage required to enact this transition is molecule-specific, and thus measurement of the transition voltage constitutes a form of spectroscopy. Here we determine that the transition voltage for a series of alkanethiol molecules is invariant with molecular length, while the transition voltage of a conjugated molecule depends directly on the manner in which the conjugation pathway has been extended. Finally, by examining the transition voltage as a function of contact metal, we show that this technique can be used to determine the dominant charge carrier for a given molecular junction.

Tracing electronic pathways in molecules by using inelastic tunneling spectroscopy.

Using inelastic electron tunneling spectroscopy (IETS) to measure the vibronic structure of nonequilibrium molecular transport, aided by a quantitative interpretation scheme based on Green's function-density functional theory methods, we are able to characterize the actual pathways that the electrons traverse when moving through a molecule in a molecular transport junction. We show that the IETS observations directly index electron tunneling pathways along the given normal coordinates of the molecule. One can then interpret the maxima in the IETS spectrum in terms of the specific paths that the electrons follow as they traverse the molecular junction. Therefore, IETS measurements not only prove (by the appearance of molecular vibrational frequencies in the spectrum) that the tunneling charges, in fact, pass through the molecule, but also can be used to determine the transport pathways and how they change with the geometry and placement of molecules in junctions.

Vibronic coupling in semifluorinated alkanethiol junctions: implications for selection rules in inelastic electron tunneling spectroscopy.

Determining the selection rules for the interaction of tunneling charge carriers with molecular vibrational modes is important for a complete understanding of charge transport in molecular electronic junctions. Here, we report the low-temperature charge transport characteristics for junctions formed from hexadecanethiol molecules having varying degrees of fluorination. Our results demonstrate that C-F vibrations are not observed in inelastic electron tunneling spectroscopy (IETS). Because C-F vibrations are almost purely dipole transitions, the insensitivity to fluorine substitution implies that Raman modes are preferred over infrared modes. Further, the lack of attenuation of the C-H vibrational modes with fluorine substitution suggests that either the scattering cross section is not an additive quantity or the physical position of a vibrational mode within the junction influences whether the transition is observed in IETS.

Transition from direct tunneling to field emission in metal-molecule-metal junctions.

Current-voltage measurements of metal-molecule-metal junctions formed from pi-conjugated thiols exhibit an inflection point on a plot of ln(I/V(2)) vs 1/V, consistent with a change in transport mechanism from direct tunneling to field emission. The transition voltage was found to scale linearly with the offset in energy between the Au Fermi level and the highest occupied molecular orbital as determined by ultraviolet photoelectron spectroscopy. Asymmetric voltage drops at the two metal-molecule interfaces cause the transition voltage to be dependent on bias polarity.

Correlation between HOMO alignment and contact resistance in molecular junctions: aromatic thiols versus aromatic isocyanides.

Understanding electron transport in metal-molecule-metal (MMM) junctions is of great importance for the advancement of molecular electronics. Critical factors that determine conductivity in a MMM junction include the nature of metal-molecule contacts and the electronic structure of the molecular backbone. We have studied the electronic transport property and the valence electronic structure on rigid, conjugated oligoacenes of increasing length with either thiol (-S) or isocyanide (-CN) linkers using conducting probe atomic force microscopy (CP-AFM) and ultraviolet photoelectron spectroscopy (UPS). We find that for these conjugated systems the Au-CN contact is more resistive than Au-S. The difference in contact resistance correlates with UPS measurements that show the highest-occupied molecular orbital (HOMO) of the isocyanide series is lower in energy (relative to the Fermi level of Au) than the HOMO of the thiol series, indicating the presence of a higher tunneling barrier at the contact for the isocyanide-linked molecules. By contrast, the difference in the HOMO positions for the two series of molecules does not appear to affect the length dependence of the junction resistance (i.e., the beta value = 0.5 A-1).

Analysis of the causes of variance in resistance measurements on metal-molecule-metal junctions formed by conducting-probe atomic force microscopy.

Alkanethiol tunnel junctions were studied using conducting-probe atomic force microscopy to determine causes of variability in measured resistance behavior. Measurements were made on Au/decanethiol/Au monolayer junctions, and effects of substrate roughness, tip chemistry, presence of solvent, extensive tip usage, applied load, and tip radius were examined. Resistance measurements yielded log-normal distributions under a variety of conditions, indicating that the origin of the variance is likely to be either changes in tunneling length or electronic overlap. Spreads in resistance values for a given tip were much less when flat, template-stripped Au substrates were used rather than rough, evaporated Au substrates. Chemical modification of tips with ethanethiol (C2) or butanethiol (C4) and performing measurements under cyclohexane were also found to reduce variance by a factor of about 2-4. Experiments performed with unmodified tips showed an increase in junction resistance over the course of hundreds of consecutive measurements, whereas junctions made with modified tips or under cyclohexane did not. Attempts to ascribe variance between tips to varying tip radii failed; however, decreases in resistance with increasing applied load on the tip contact were observed and could be interpreted in terms of conventional contact mechanics models.

Length dependence of charge transport in nanoscopic molecular junctions incorporating a series of rigid thiol-terminated norbornylogs.

Four tetrathiol-terminated norbornane homologues were synthesized and self-assembled monolayers (SAMs) of these molecules were formed on Au via adsorption from CH2Cl2. SAMs were characterized structurally via spectroscopic ellipsometry (SE), reflection-absorption infrared spectroscopy (RAIRS), Rutherford backscattering spectrometry (RBS), and X-ray photoelectron spectroscopy (XPS). Results of these analyses show that the rigid norbornylogs form monolayers that have a surface coverage slightly lower than that of alkanethiols, and that they exhibit a nonmonotonic dependence of film thickness on molecular length. Nanoscale molecular junctions incorporating these SAMs were formed and characterized electrically using conducting probe atomic force microscopy (CP-AFM). The resistances of these junctions scale exponentially with the contour length of the molecules, with beta = 0.9 A(-1), consistent with a nonresonant tunneling mechanism. Further, the resistance of norbornyl SAMs correlates well with the resistance of alkanedithiol SAMs of similar length, suggesting that the norbornyl molecules form sulfur-metal bonds on both ends of the junction.

Length-dependent transport in molecular junctions based on SAMs of alkanethiols and alkanedithiols: effect of metal work function and applied bias on tunneling efficiency and contact resistance.

Nanoscopic tunnel junctions were formed by contacting Au-, Pt-, or Ag-coated atomic force microscopy (AFM) tips to self-assembled monolayers (SAMs) of alkanethiol or alkanedithiol molecules on polycrystalline Au, Pt, or Ag substrates. Current-voltage traces exhibited sigmoidal behavior and an exponential attenuation with molecular length, characteristic of nonresonant tunneling. The length-dependent decay parameter, beta, was found to be approximately 1.1 per carbon atom (C(-1)) or 0.88 A(-)(1) and was independent of applied bias (over a voltage range of +/-1.5 V) and electrode work function. In contrast, the contact resistance, R(0), extrapolated from resistance versus molecular length plots showed a notable decrease with both applied bias and increasing electrode work function. The doubly bound alkanedithiol junctions were observed to have a contact resistance approximately 1 to 2 orders of magnitude lower than the singly bound alkanethiol junctions. However, both alkanethiol and dithiol junctions exhibited the same length dependence (beta value). The resistance versus length data were also used to calculate transmission values for each type of contact (e.g., Au-S-C, Au/CH(3), etc.) and the transmission per C-C bond (T(C)(-)()(C)).

Contact resistance in metal-molecule-metal junctions based on aliphatic SAMs: effects of surface linker and metal work function.

Using conducting probe atomic force microscopy (CP-AFM), we have formed molecular tunnel junctions consisting of alkanethiols and alkane isonitrile self-assembled monolayers sandwiched between gold, platinum, silver, and palladium contacts. We have measured the resistance of these junctions at low bias (dV/dI |V=0) as a function of alkane chain length. Extrapolation to zero chain length gives the contact resistance, R0 . R0 is strongly dependent on the type of metal used for the contacts and decreases with increasing metal work function; that is, R0,Ag > R0,Au > R0,Pd > R0,Pt. R0 is approximately 10% smaller for Au junctions with isonitrile versus thiol surface linkers. We conclude that the Fermi level of the junction lies much closer to the HOMO than to the LUMO.