XFEL Microcrystallography of Self-Assembling Silver n-Alkanethiolates

New synthetic hybrid materials and their increasing complexity have placed growing demands on crystal growth for single-crystal X-ray diffraction analysis. Unfortunately, not all chemical systems are conducive to the isolation of single crystals for traditional characterization. Here, small-molecule serial femtosecond crystallography (smSFX) at atomic resolution (0.833 Å) is employed to characterize microcrystalline silver n-alkanethiolates with various alkyl chain lengths at X-ray free electron laser facilities, resolving long-standing controversies regarding the atomic connectivity and odd-even effects of layer stacking. smSFX provides high-quality crystal structures directly from the powder of the true unknowns, a capability that is particularly useful for systems having notoriously small or defective crystals. We present crystal structures of silver n-butanethiolate (C4), silver n-hexanethiolate (C6), and silver n-nonanethiolate (C9). We show that an odd-even effect originates from the orientation of the terminal methyl group and its role in packing efficiency. We also propose a secondary odd-even effect involving multiple mosaic blocks in the crystals containing even-numbered chains, identified by selected-area electron diffraction measurements. We conclude with a discussion of the merits of the synthetic preparation for the preparation of microdiffraction specimens and compare the long-range order in these crystals to that of self-assembled monolayers.

Chemical crystallography by serial femtosecond X-ray diffraction

Inorganic-organic hybrid materials represent a large share of newly reported structures, owing to their simple synthetic routes and customizable properties1. This proliferation has led to a characterization bottleneck: many hybrid materials are obligate microcrystals with low symmetry and severe radiation sensitivity, interfering with the standard techniques of single-crystal X-ray diffraction2,3 and electron microdiffraction4-11. Here we demonstrate small-molecule serial femtosecond X-ray crystallography (smSFX) for the determination of material crystal structures from microcrystals. We subjected microcrystalline suspensions to X-ray free-electron laser radiation12,13 and obtained thousands of randomly oriented diffraction patterns. We determined unit cells by aggregating spot-finding results into high-resolution powder diffractograms. After indexing the sparse serial patterns by a graph theory approach14, the resulting datasets can be solved and refined using standard tools for single-crystal diffraction data15-17. We describe the ab initio structure solutions of mithrene (AgSePh)18-20, thiorene (AgSPh) and tethrene (AgTePh), of which the latter two were previously unknown structures. In thiorene, we identify a geometric change in the silver-silver bonding network that is linked to its divergent optoelectronic properties20. We demonstrate that smSFX can be applied as a general technique for structure determination of beam-sensitive microcrystalline materials at near-ambient temperature and pressure.

Investigation of Nucleation and Growth at a Liquid-Liquid Interface by Solvent Exchange and Synchrotron Small-Angle X-Ray Scattering

Hybrid nanomaterials possess complex architectures that are driven by a self-assembly process between an inorganic element and an organic ligand. The properties of these materials can often be tuned by organic ligand variation, or by swapping the inorganic element. This enables the flexible fabrication of tailored hybrid materials with a rich variety of properties for technological applications. Liquid-liquid interfaces are useful for synthesizing these compounds as precursors can be segregated and allowed to interact only at the interface. Although procedurally straightforward, this is a complex reaction in an environment that is not easy to probe. Here, we explore the interfacial crystallization of mithrene, a supramolecular multi-quantum well. This material sandwiches a well-defined silver-chalcogenide layer between layers of organic ligands. Controlling mithrene crystal size and morphology to be useful for applications requires understanding details of its crystal growth, but the specific mechanism for this reaction remain only lightly investigated. We performed a study of mithrene crystallization at an oil-water interfaces to elucidate how the interfacial free energy affects nucleation and growth. We exchanged the oil solvent on the basis of solvent viscosity and surface tension, modifying the dynamic contact angle and interfacial free energy. We isolated and characterized the reaction byproducts via scanning electron microscopy (SEM). We also developed a high-throughput small angle X-ray scattering (SAXS) technique to measure crystallization at short reaction timescales (minutes). Our results showed that modifying interfacial surface energy affects both the reaction kinetics and product size homogeneity and yield. Our SAXS measurements reveal the onset of crystallinity after only 15 min. These results provide a template for exploring directed synthesis of complex materials via experimental methods.

Strongly Quantum-Confined Blue-Emitting Excitons in Chemically Configurable Multiquantum Wells

Light matter interactions are greatly enhanced in two-dimensional (2D) semiconductors because of strong excitonic effects. Many optoelectronic applications would benefit from creating stacks of atomically thin 2D semiconductors separated by insulating barrier layers, forming multiquantum-well structures. However, most 2D transition metal chalcogenide systems require serial stacking to create van der Waals multilayers. Hybrid metal organic chalcogenolates (MOChas) are self-assembling hybrid materials that combine multiquantum-well properties with scalable chemical synthesis and air stability. In this work, we use spatially resolved linear and nonlinear optical spectroscopies over a range of temperatures to study the strongly excitonic optical properties of mithrene, that is, silver benzeneselenolate, and its synthetic isostructures. We experimentally probe s-type bright excitons and p-type excitonic dark states formed in the quantum confined 2D inorganic monolayers of silver selenide with exciton binding energy up to ∼0.4 eV, matching recent theoretical predictions of the material class. We further show that mithrene's highly efficient blue photoluminescence, ultrafast exciton radiative dynamics, as well as flexible tunability of molecular structure and optical properties demonstrate great potential of MOChas for constructing optoelectronic and quantum excitonic devices.

Anisotropic 2D excitons unveiled in organic-inorganic quantum wells

Two-dimensional (2D) excitons arise from electron-hole confinement along one spatial dimension. Such excitations are often described in terms of Frenkel or Wannier limits according to the degree of exciton spatial localization and the surrounding dielectric environment. In hybrid material systems, such as the 2D perovskites, the complex underlying interactions lead to excitons of an intermediate nature, whose description lies somewhere between the two limits, and a better physical description is needed. Here, we explore the photophysics of a tuneable materials platform where covalently bonded metal-chalcogenide layers are spaced by organic ligands that provide confinement barriers for charge carriers in the inorganic layer. We consider self-assembled, layered bulk silver benzeneselenolate, [AgSePh]_∞, and use a combination of transient absorption spectroscopy and ab initio GW plus Bethe-Salpeter equation calculations. We demonstrate that in this non-polar dielectric environment, strongly anisotropic excitons dominate the optical transitions of [AgSePh]_∞. We find that the transient absorption measurements at room temperature can be understood in terms of low-lying excitons confined to the AgSe planes with in-plane anisotropy, featuring anisotropic absorption and emission. Finally, we present a pathway to control the exciton behaviour by changing the chalcogen in the material lattice. Our studies unveil unexpected excitonic anisotropies in an unexplored class of tuneable, yet air-stable, hybrid quantum wells, offering design principles for the engineering of an ordered, yet complex dielectric environment and its effect on the excitonic phenomena in such emerging materials.

Laser-sculptured ultrathin transition metal carbide layers for energy storage and energy harvesting applications

Ultrathin transition metal carbides with high capacity, high surface area, and high conductivity are a promising family of materials for applications from energy storage to catalysis. However, large-scale, cost-effective, and precursor-free methods to prepare ultrathin carbides are lacking. Here, we demonstrate a direct pattern method to manufacture ultrathin carbides (MoC_x, WC_x, and CoC_x) on versatile substrates using a CO₂ laser. The laser-sculptured polycrystalline carbides (macroporous, ~10–20 nm wall thickness, ~10 nm crystallinity) show high energy storage capability, hierarchical porous structure, and higher thermal resilience than MXenes and other laser-ablated carbon materials. A flexible supercapacitor made of MoC_x demonstrates a wide temperature range (−50 to 300 °C). Furthermore, the sculptured microstructures endow the carbide network with enhanced visible light absorption, providing high solar energy harvesting efficiency (~72 %) for steam generation. The laser-based, scalable, resilient, and low-cost manufacturing process presents an approach for construction of carbides and their subsequent applications.

Transition metal carbides are attractive for electrochemical energy storage and catalysis, but cost effective preparation on a large scale is challenging. Here the authors use a direct pattern method to fabricate transition metal carbides for supercapacitors and solar energy harvesting for steam generation.

Self-Assembly of Large-Area 2D Polycrystalline Transition Metal Carbides for Hydrogen Electrocatalysis

Low-dimensional (0/1/2 dimension) transition metal carbides (TMCs) possess intriguing electrical, mechanical, and electrochemical properties, and they serve as convenient supports for transition metal catalysts. Large-area single-crystalline 2D TMC sheets are generally prepared by exfoliating MXene sheets from MAX phases. Here, a versatile bottom-up method is reported for preparing ultrathin TMC sheets (≈10 nm in thickness and >100 μm in lateral size) with metal nanoparticle decoration. A gelatin hydrogel is employed as a scaffold to coordinate metal ions (Mo⁵⁺ , W⁶⁺ , Co²⁺ ), resulting in ultrathin-film morphologies of diverse TMC sheets. Carbonization of the scaffold at 600 °C presents a facile route to the corresponding MoC_x , WC_x , CoC_x , and to metal-rich hybrids (Mo_{2- x} W_x C and W/Mo₂ C-Co). Among these materials, the Mo₂ C-Co hybrid provides excellent hydrogen evolution reaction (HER) efficiency (Tafel slope of 39 mV dec^-1 and 48 mVj = 10 mA cm-2 in overpotential in 0.5 m H₂ SO₄ ). Such performance makes Mo₂ C-Co a viable noble-metal-free catalyst for the HER, and is competitive with the standard platinum on carbon support. This template-assisted, self-assembling, scalable, and low-cost manufacturing process presents a new tactic to construct low-dimensional TMCs with applications in various clean-energy-related fields.

Competing Roles of Crystallization and Degradation of a Metal-Organic Chalcogenolate Assembly under Biphasic Solvothermal Conditions

Metal-organic chalcogenolate assemblies have attracted recent interest as ensemble nanomaterials that contain one- or two-dimensional inorganic nanostructures in a periodic array with supramolecular isolation provided by an associated organic ligand lattice. Biphasic immiscible synthesis at liquid-liquid interfaces is a convenient way to grow crystalline d¹⁰ metal-organic chalcogenolate assemblies. However, there has been little systematic study of the role of temperature on the nucleation, growth, and stability of hybrid chalcogenolates during biphasic synthesis. Silver benzeneselenolate, a robustly blue-luminescent, lamellar metal-organic chalcogenolate assembly, was crystallized at biphasic immiscible liquid-liquid interfaces under solvothermal conditions. A positive correlation between temperature and nucleation density was observed, and the luminescence was conserved in all examples of the crystalline phase. Applying solvothermal conditions to the biphasic synthesis generally increased the lateral dimensions of the crystals and strongly favored the crystalline phase of the compound. Although thin, well-formed crystals were observed within 1 h for interfacial reactions performed at high temperatures, degradation was observed in long duration growths resulting in aggregated silver metal. A study of the thermal stability of the material via thermogravimetric analysis revealed that the decomposition is likely a redox reaction reverting the compound to silver metal and diphenyl diselenide. In situ analysis of this degradation was performed by grazing-incidence wide-angle X-ray scattering, which confirmed that the decomposition occurs in a single step with no preceding changes to the structure of the material. This work demonstrates that biphasic solvothermal methods are amenable to the synthesis of hybrid metal-organic chalcogenolate assemblies and that temperature can be used to control product morphology and lateral crystal growth at the immiscible interface.

Effective Remediation of Groundwater Fluoride with Inexpensively Processed Indian Bauxite

India represents one-third of the world's fluorosis burden and is the fifth global producer of bauxite ore, which has previously been identified as a potential resource for remediating fluoride-contaminated groundwater in impoverished communities. Here, we use thermal activation and/or groundwater acidification to enhance fluoride adsorption by Indian bauxite obtained from Visakhapatnam, an area proximate to endemic fluorosis regions. We compare combinatorial water treatment and bauxite-processing scenarios through batch adsorption experiments, material characterization, and detailed cost analyses. Heating Indian bauxite above 300 °C increases available surface area by > 15× (to ∼170 m²/g) through gibbsite dehydroxylation and reduces the bauxite dose for remediating 10 ppm F^- to 1.5 ppm F^- by ∼93% (to 21 g/L). Additionally, lowering groundwater pH to 6.0 with HCl or CO₂ further reduces the average required bauxite doses by 43-73% for ores heated at 300 °C (∼12 g/L) and 100 °C (∼77 g/L). Product water in most examined treatment scenarios complies with EPA standards for drinking water (e.g., As, Cd, Pb, etc.) but potential leaching of Al, Mn, and Cr is of concern in some scenarios. Among the defluoridation options explored here, bauxite heated at 300 °C in acidified groundwater has the lowest direct costs ($6.86 per person per year) and material-intensity.

Monochromatic Photocathodes from Graphene-Stabilized Diamondoids

The monochromatic photoemission from diamondoid monolayers provides a new strategy to create electron sources with low energy dispersion and enables compact electron guns with high brightness and low beam emittance for aberration-free imaging, lithography, and accelerators. However, these potential applications are hindered by degradation of diamondoid monolayers under photon irradiation and electron bombardment. Here, we report a graphene-protected diamondoid monolayer photocathode with 4-fold enhancement of stability compared to the bare diamondoid counterpart. The single-layer graphene overcoating preserves the monochromaticity of the photoelectrons, showing 12.5 meV ful width at half-maximum distribution of kinetic energy. Importantly, the graphene coating effectively suppresses desorption of the diamondoid monolayer, enhancing its thermal stability by at least 100 K. Furthermore, by comparing the decay rate at different photon energies, we identify electron bombardment as the principle decay pathway for diamondoids under graphene protection. This provides a generic approach for stabilizing volatile species on photocathode surfaces, which could greatly improve performance of electron emitters.

Hybrid metal-organic chalcogenide nanowires with electrically conductive inorganic core through diamondoid-directed assembly

Controlling inorganic structure and dimensionality through structure-directing agents is a versatile approach for new materials synthesis that has been used extensively for metal-organic frameworks and coordination polymers. However, the lack of 'solid' inorganic cores requires charge transport through single-atom chains and/or organic groups, limiting their electronic properties. Here, we report that strongly interacting diamondoid structure-directing agents guide the growth of hybrid metal-organic chalcogenide nanowires with solid inorganic cores having three-atom cross-sections, representing the smallest possible nanowires. The strong van der Waals attraction between diamondoids overcomes steric repulsion leading to a cis configuration at the active growth front, enabling face-on addition of precursors for nanowire elongation. These nanowires have band-like electronic properties, low effective carrier masses and three orders-of-magnitude conductivity modulation by hole doping. This discovery highlights a previously unexplored regime of structure-directing agents compared with traditional surfactant, block copolymer or metal-organic framework linkers.

Defect-Tolerant Aligned Dipoles within Two-Dimensional Plastic Lattices

Carboranethiol molecules self-assemble into upright molecular monolayers on Au{111} with aligned dipoles in two dimensions. The positions and offsets of each molecule's geometric apex and local dipole moment are measured and correlated with sub-Ångström precision. Juxtaposing simultaneously acquired images, we observe monodirectional offsets between the molecular apexes and dipole extrema. We determine dipole orientations using efficient new image analysis techniques and find aligned dipoles to be highly defect tolerant, crossing molecular domain boundaries and substrate step edges. The alignment observed, consistent with Monte Carlo simulations, forms through favorable intermolecular dipole-dipole interactions.

Molecular flux dependence of chemical patterning by microcontact printing

We address the importance of the dynamic molecular ink concentration at a polymer stamp/substrate interface during microcontact displacement or insertion printing. We demonstrate that by controlling molecular flux, we can influence both the molecular-scale order and the rate of molecular exchange of self-assembled monolayers (SAMs) on gold surfaces. Surface depletion of molecular ink at a polymer stamp/substrate interface is driven predominantly by diffusion into the stamp interior; depletion occurs briefly at the substrate by SAM formation, but diffusion of molecules into the bulk of the stamp dominates over practical experimental time scales. As contact time is increased, the interface concentration varies significantly due to diffusion, affecting the quality and coverage of printed films. Controlling interfacial concentration improves printed film reproducibility and the fractional coverage of multicomponent films can be controlled to within a few percent. We first briefly review the important aspects of molecular ink diffusion at a stamp interface and how it relates to experimental duration. We then describe two examples that illustrate control over ink transfer during experiments: the role of contact time on monolayer reproducibility and molecular order, and the fine control of fractional monolayer coverage for the displacement printing of 1-adamantanethiolate SAMs by 1-dodecanethiol.

Covalent attachment of diamondoid phosphonic acid dichlorides to tungsten oxide surfaces

Diamondoids (nanometer-sized diamond-like hydrocarbons) are a novel class of carbon nanomaterials that exhibit negative electron affinity (NEA) and strong electron-phonon scattering. Surface-bound diamondoid monolayers exhibit monochromatic photoemission, a unique property that makes them ideal electron sources for electron-beam lithography and high-resolution electron microscopy. However, these applications are limited by the stability of the chemical bonding of diamondoids on surfaces. Here we demonstrate the stable covalent attachment of diamantane phosphonic dichloride on tungsten/tungsten oxide surfaces. X-ray photoelectron spectroscopy (XPS) and Fourier-transform infrared (FTIR) spectroscopy revealed that diamondoid-functionalized tungsten oxide films were stable up to 300-350 °C, a substantial improvement over conventional diamondoid thiolate monolayers on gold, which dissociate at 100-200 °C. Extreme ultraviolet (EUV) light stimulated photoemission from these diamondoid phosphonate monolayers exhibited a characteristic monochromatic NEA peak with 0.2 eV full width at half-maximum (fwhm) at room temperature, showing that the unique monochromatization property of diamondoids remained intact after attachment. Our results demonstrate that phosphonic dichloride functionality is a promising approach for forming stable diamondoid monolayers for elevated temperature and high-current applications such as electron emission and coatings in micro/nano electromechanical systems (MEMS/NEMS).

Photoresponsive molecules in well-defined nanoscale environments

Stimuli-responsive molecules are key building blocks of functional molecular materials and devices. These molecules can operate in a range of environments. A molecule's local environment will dictate its conformation, reactivity, and function; by controlling the local environment we can ultimately develop interfaces of individual molecules with the macroscopic environment. By isolating molecules in well-defined environments, we are able to obtain both accurate measurements and precise control. We exploit defect sites in self-assembled monolayers (SAMs) to direct the functional molecules into precise locations, providing a basis for the measurements and engineering of functional molecular systems. The structure and functional moieties of the SAM can be tuned to control not only the intermolecular interactions but also molecule-substrate interactions, resulting in extraction or control of desired molecular functions. Herein, we report our progress toward the assembly and measurements of photoresponsive molecules and their precise assemblies in SAM matrices.

Surface defects on plate-shaped silver nanoparticles contribute to its hazard potential in a fish gill cell line and zebrafish embryos

We investigated and compared nanosize Ag spheres, plates, and wires in a fish gill epithelial cell line (RT-W1) and in zebrafish embryos to understand the mechanism of toxicity of an engineered nanomaterial raising considerable environmental concern. While most of the Ag nanoparticles induced N-acetyl cysteine sensitive oxidative stress effects in RT-W1, Ag nanoplates were considerably more toxic than other particle shapes. Interestingly, while Ag ion shedding and bioavailability failed to comprehensively explain the high toxicity of the nanoplates, cellular injury required direct particle contact, resulting in cell membrane lysis in RT-W1 as well as red blood cells (RBC). Ag nanoplates were also considerably more toxic in zebrafish embryos in spite of their lesser ability to shed Ag into the exposure medium. To elucidate the "surface reactivity" of Ag nanoplates, high-resolution transmission electron microscopy was performed and demonstrated a high level of crystal defects (stacking faults and point defects) on the nanoplate surfaces. Surface coating with cysteine was used to passivate the surface defects and demonstrated a reduction of toxicity in RT-W1 cells, RBC, and zebrafish embryos. This study demonstrates the important role of crystal defects in contributing to Ag nanoparticle toxicity in addition to the established roles of Ag ion shedding by Ag nanoparticles. The excellent correlation between the in vitro and in vivo toxicological assessment illustrates the utility of using a fish cell line in parallel with zebrafish embryos to perform a predictive environmental toxicological paradigm.

High-fidelity chemical patterning on oxide-free germanium

Oxide-free germanium can be chemically patterned directly with self-assembled monolayers of n-alkanethiols via submerged microcontact printing. Native germanium dioxide is water soluble; immersion activates the germanium surface for self-assembly by stripping the oxide. Water additionally provides an effective diffusion barrier that prevents undesired ink transport. Patterns are stable with respect to molecular exchange by carboxyl-functionalized thiols.

Directing substrate morphology via self-assembly: ligand-mediated scission of gallium-indium microspheres to the nanoscale

We have developed a facile method for the construction of liquid-phase eutectic gallium-indium (EGaIn) alloy nanoparticles. Particle formation is directed by molecular self-assembly and assisted by sonication. As the bulk liquid alloy is ultrasonically dispersed, fast thiolate self-assembly at the EGaIn interface protects the material against oxidation. The choice of self-assembled monolayer ligand directs the ultimate size reduction in the material; strongly interacting molecules induce surface strain and assist particle cleavage to the nanoscale. Transmission electron microscopy images and diffraction analyses reveal that the nanoscale particles are in an amorphous or liquid phase, with no observed faceting. The particles exhibit strong absorption in the ultraviolet (∼200 nm), consistent with the gallium surface plasmon resonance, but dependent on the nature of the particle ligand shell.

Dynamic double lattice of 1-adamantaneselenolate self-assembled monolayers on Au{111}

We report a complex, dynamic double lattice for 1-adamantaneselenolate monolayers on Au{111}. Two lattices coexist, revealing two different binding modes for selenols on gold: molecules at bridge sites have lower conductance than molecules at three-fold hollow sites. The monolayer is dynamic, with molecules switching reversibly between the two site-dependent conductance states. Monolayer dynamics enable adsorbed molecules to reorganize according to the underlying gold electronic structure over long distances, which facilitates emergence of the self-organized rows of dimers. The low-conductance molecules assume a (7 × 7) all-bridge configuration, similar to the analogous 1-adamantanethiolate monolayers on Au{111}. The high-conductance molecules self-organize upon mild annealing into distinctive rows of dimers with long-range order, described by a (6√5 × 6√5)R15° unit cell.

Creating favorable geometries for directing organic photoreactions in alkanethiolate monolayers

The products of photoreactions of conjugated organic molecules may be allowed by selection rules but not observed in solution reactions because of unfavorable reaction geometries. We have used defect sites in self-assembled alkanethiolate monolayers on gold surfaces to direct geometrically unfavorable photochemical reactions between individual organic molecules. High conductivity and stochastic switching of anthracene-terminated phenylethynylthiolates within alkanethiolate monolayers, as well as in situ photochemical transformations, have been observed and distinguished with the scanning tunneling microscope (STM). Ultraviolet light absorbed during imaging increases the apparent heights of excited molecules in STM images, a direct manifestation of probing electronically excited states.

Self-assembled monolayers of 2-Adamantanethiol on Au{111}: control of structure and displacement

We have investigated the formation of 2-adamantanethiolate self-assembled monolayers on Au{111} and their displacement by n-dodecanethiol, using scanning tunneling microscopy, X-ray photoelectron spectroscopy, and infrared reflection absorption spectroscopy. Well-ordered 2-adamantanethiolate monolayers undergo rapid and significant molecular exchange upon exposure to n-dodecanethiol solutions, but their structures and intermolecular interactions template the growth of n-dodecanethiolate domains. Annealing 2-adamantanethiolate monolayers at 78 degrees C decreases the density of vacancy islands, while increasing the overall order and the average domain sizes in the films. This results in slower displacement by n-dodecanethiol molecules, as compared to unannealed monolayers. The secondary sulfur position on the adamantyl cage influences the lattice structure and exchange of 2-adamantanethiolate monolayers by alkanethiols.

Self-assembly of carboranethiol isomers on Au111: intermolecular interactions determined by molecular dipole orientations

Self-assembled monolayer (SAM) structures and properties are dominated by two interactions: those between the substrate and adsorbate and those between the adsorbates themselves. We have fabricated self-assembled monolayers of m-1-carboranethiol (M1) and m-9-carboranethiol (M9) on Au[111]. The two isomers are nearly identical geometrically, but calculated molecular dipole moments show a sizable difference at 1.06 and 4.08 D for M1 and M9 in the gas phase, respectively. These molecules provide an opportunity to investigate the effect of different dipole moments within SAMs without altering the geometry of the assembly. Pure and co-deposited SAMs of these molecules were studied by scanning tunneling microscopy (STM). The molecules are indistinguishable in STM images, and the hexagonally close-packed adlayer structures were found to have ((square root of 19) x (square root of 19))R23.4 degrees unit cells. Both SAMs display rotational domains without the protruding or depressed features in STM images associated with domain boundaries in other SAM systems. Differing orientations of molecular dipole moments influence SAM properties, including the stability of the SAM and the coverage of the carboranethiolate in competitive binding conditions. These properties were investigated by dynamic contact angle goniometry, Kelvin probe force microscopy, and grazing incidence Fourier transform infrared spectroscopy.

Scanning electron microscopy of nanoscale chemical patterns

A series of nanoscale chemical patterning methods based on soft and hybrid nanolithographies have been characterized using scanning electron microscopy with corroborating evidence from scanning tunneling microscopy and lateral force microscopy. We demonstrate and discuss the unique advantages of the scanning electron microscope as an analytical tool to image chemical patterns of molecules highly diluted within a host self-assembled monolayer and to distinguish regions of differential mass coverage in patterned self-assembled monolayers. We show that the relative contrast of self-assembled monolayer patterns in scanning electron micrographs depends on the operating primary electron beam voltage, monolayer composition, and monolayer order, suggesting that secondary electron emission and scattering can be used to elucidate chemical patterns.