Ex Situ Photoelectron Emission Microscopy of Polycrystalline Bismuth and Antimony Telluride Surfaces Exposed to Ambient Oxidation

The surfaces of textured polycrystalline N-type bismuth telluride and P-type antimony telluride materials were investigated using ex situ photoelectron emission microscopy (PEEM). PEEM enabled imaging of the work function for different oxidation times due to exposure to air across sample surfaces. The spatially averaged work function was also tracked as a function of air exposure time. N-type bismuth telluride showed an increase in the work function around grain boundaries relative to grain interiors during the early stages of air exposure-driven oxidation. At longer time exposure to air, the surface became homogenous after a ∼5 nm-thick oxide formed. X-ray photoemission spectroscopy was used to correlate changes in PEEM imaging in real space and work function evolution to the progressive growth of an oxide layer. The observed work function contrast is consistent with the pinning of electronic surface states due to the defects at a grain boundary.

Determination of Hafnium Zirconium Oxide Interfacial Band Alignments Using Internal Photoemission Spectroscopy and X-ray Photoelectron Spectroscopy

Doped ferroelectric HfO₂ is highly promising for integration into complementary metal-oxide semiconductor (CMOS) technology for devices such as ferroelectric nonvolatile memory and low-power field-effect transistors (FETs). We report the direct measurement of the energy barriers between various metal electrodes (Pt, Au, Ta, TaN, Ti/Pt, Ni, Al) and hafnium zirconium oxide (Hf_0.58Zr_0.42O₂, HZO) using internal photoemission (IPE) spectroscopy. Results are compared with valence band offsets determined using the three-sample X-ray photoelectron spectroscopy (XPS) as well as the two-sample hard X-ray photoelectron spectroscopy (HAXPES) techniques. Both XPS and IPE indicate roughly the same dependence of the HZO barrier on metal work function with a slope of 0.8 ± 0.5. XPS and HAXPES-derived barrier heights are on average about 1.1 eV smaller than barrier heights determined by IPE, suggesting the presence of negative charge in the HZO.

Engineering the Microstructure and Morphology of Explosive Films via Control of Interfacial Energy

Physical vapor deposition of organic explosives enables growth of polycrystalline films with a unique microstructure and morphology compared to the bulk material. This study demonstrates the ability to control crystal orientation and porosity in pentaerythritol tetranitrate films by varying the interfacial energy between the substrate and the vapor-deposited explosive. Variation in density, porosity, surface roughness, and optical properties is achieved in the explosive film, with significant implications for initiation sensitivity and detonation performance of the explosive material. Various surface science techniques, including angle-resolved X-ray photoelectron spectroscopy and multiliquid contact angle analysis, are utilized to characterize interfacial characteristics between the substrate and explosive film. Optical microscopy and scanning electron microscopy of pentaerythritol tetranitrate surfaces and fracture cross sections illustrate the difference in morphology evolution and the microstructure achieved through surface energy modification. X-ray diffraction studies with the Tilt-A-Whirl three-dimensional pole figure rendering and texture analysis software suite reveal that high surface energy substrates result in a preferred (110) out-of-plane orientation of pentaerythritol tetranitrate crystallites and denser films. Low surface energy substrates create more randomly textured pentaerythritol tetranitrate and lead to nanoscale porosity and lower density films. This work furthers the scientific basis for interfacial engineering of polycrystalline organic explosive films through control of surface energy, enabling future study of dynamic and reactive detonative phenomena at the microscale. Results of this study also have potential applications to active pharmaceutical ingredients, stimuli-responsive polymer films, organic thin film transistors, and other areas.

Core hole processes in x-ray absorption and photoemission by resonant Auger-electron spectroscopy and first-principles theory

Electron-core hole interactions are critical for proper interpretation of core-level spectroscopies commonly used as analytical tools in materials science. Here we utilize resonant Auger-electron spectroscopy to uniquely identify exciton, shake, and charge-transfer processes that result from the sudden creation of the core hole in both x-ray-absorption and photoemission spectra. These effects are captured for the transition-metal compounds SrTiO3 and MoS2 by fully ab initio, combined real-time cumulant, and Bethe-Salpeter equation approaches to account for core hole dynamics and screening. Atomic charges and excited-state electron-density fluctuations reflect materials' solid-state electronic structure, loss of translational symmetry around the core hole, and breakdown of the sudden approximation. They also demonstrate competition between long- and short-range screening in a solid.

Intrinsic Properties of Individual Inorganic Silicon-Electrolyte Interphase Constituents

Because of the complexity, high reactivity, and continuous evolution of the silicon-electrolyte interphase (SiEI), "individual" constituents of the SiEI were investigated to understand their physical, electrochemical, and mechanical properties. For the analysis of these intrinsic properties, known SiEI components (i.e., SiO₂, Li₂Si₂O₅, Li₂SiO₃, Li₃SiO_x, Li₂O, and LiF) were selected and prepared as amorphous thin films. The chemical composition, purity, morphology, roughness, and thickness of prepared samples were characterized using a variety of analytical techniques. On the basis of subsequent analysis, LiF shows the lowest ionic conductivity and relatively weak, brittle mechanical properties, while lithium silicates demonstrate higher ionic conductivities and greater mechanical hardness. This research establishes a framework for identifying components critical for stabilization of the SiEI, thus enabling rational design of new electrolyte additives and functional binders for the development of next-generation advanced Li-ion batteries utilizing Si anodes.

Cubic SnGe nanoalloys: beyond thermodynamic composition limit

Tin-germanium alloys are increasingly of interest as optoelectronic and thermoelectric materials as well as materials for Li/Na ion battery electrodes. However, the lattice incompatibility of bulk Sn and Ge makes creating such alloys challenging. By exploiting the unique strain tolerance of nanosized crystals, we have developed a facile synthetic method for homogeneous SnxGe1-x alloy nanocrystals with composition varying from essentially pure Ge to 95% Sn while still maintaining the cubic structure.

Adsorption and fusion of hybrid lipid/polymer vesicles onto 2D and 3D surfaces

We investigated the formation of hybrid lipid/polymer (1,2-dioleoyl-sn-glycero-3-phosphocholine and poly(ethylene oxide-b-butadiene); DOPC/EO22Bd37) films onto planar silica surfaces. Using laser scanning confocal microscopy, atomic force microscopy, and quartz crystal microbalance analysis, we monitored the adsorption and fusion of hybrid lipid/polymer vesicles onto planar borosilicate glass cleaned via chemical etching or RF/air plasma treatment. In addition we used cryo-electron microscopy to characterize film formation on mesoporous silica nanoparticles. As the polymer content in the vesicles increased, the resulting hybrid lipid/polymer films on borosilicate glass - cleaned by chemical etching or plasma treatment - were more heterogeneous, indicating a large number of adsorbed vesicles rather than continuous bilayer films at higher polymer loadings. The observed lateral fluidity of both DOPC and hybrid lipid/polymer films also decreased substantially with increasing polymer fraction and was found to be relatively insensitive to changes in pH. Films prepared from vesicles with higher polymer loadings were completely immobile. We also found that polymer vesicles did not interact with clean plasma-treated glass surfaces, which may be due to elevated OH and Si-OH on plasma-treated surfaces. Conformal hybrid lipid/polymer coatings consistent with bilayers could be formed on mesoporous silica nanoparticles and imaged via cryo-electron microscopy. These results expand the library of biocompatible materials that can be used for coating silica-based materials and nanoparticles.

Insights into the spontaneous formation of hybrid PdO x /PEDOT films: electroless deposition and oxygen reduction activity

Hybrid palladium oxide/poly(3,4-ethylenedioxythiophene) (PdO_x/PEDOT) films were prepared through a spontaneous reaction between aqueous PdCl₄²⁻ ions and a nanostructured film of electropolymerized PEDOT. Spectroscopic and electrochemical characterization indicate the presence of mixed-valence Pd species as-deposited (19 ± 7 at% Pd⁰, 64 ± 3 at% Pd²⁺, and 18 ± 4 at% Pd⁴⁺ by X-ray photoelectron spectroscopy) and the formation of stable, electrochemically reversible Pd⁰/α-PdO_x active species in alkaline electrolyte and furthermore in the presence of oxygen. The elucidation of the Pd speciation as-deposited and in solution provides insight into the mechanism of electroless deposition in neutral aqueous conditions and the electrocatalytically active species during oxygen reduction in alkaline electrolyte. The PdO_x/PEDOT film catalyses 4e⁻ oxygen reduction (n = 3.97) in alkaline electrolyte at low overpotential (0.98 V vs. RHE, onset potential), with mass- and surface area-based specific activities competitive with, or superior to, commercial 20% Pt/C and state-of-the-art Pd- and PEDOT-based nanostructured catalysts. The high activity of the nanostructured hybrid PdO_x/PEDOT film is attributed to effective dispersion of accessible, stable Pd active sites in the PEDOT matrix.

Hybrid PdO_x/PEDOT films efficiently catalyse the direct 4e⁻ oxygen reduction reaction in alkaline electrolyte.

Simple, benign, aqueous-based amination of polycarbonate surfaces

Polycarbonate is a desirable material for many applications due to its favorable mechanical and optical properties. Here, we report a simple, safe, environmentally friendly aqueous method that uses diamines to functionalize a polycarbonate surface with amino groups. The use of water as the solvent for the functionalization ensures that solvent induced swelling does not affect the optical or mechanical properties of the polycarbonate. We characterize the efficacy of the surface amination using X-ray photo spectroscopy, Fourier transform infrared spectroscopy (FT-IR), atomic force microscopy (AFM), and contact angle measurements. Furthermore, we demonstrate the ability of this facile method to serve as a foundation upon which other functionalities may be attached, including antifouling coatings and oriented membrane proteins.

Electrodeposited NixCo3−xO4 nanostructured films as bifunctional oxygen electrocatalysts

Nanostructured Ni_xCo_3−xO₄ films serve as effective electrocatalysts for both the oxygen reduction reaction and oxygen evolution reaction in alkaline electrolyte.

Controlled nucleation and growth of pillared paddlewheel framework nanostacks onto chemically modified surfaces

The nucleation and growth of metal-organic frameworks onto functional surfaces stands to facilitate the utility of these supramolecular crystalline materials across a wide range of applications. Here, we demonstrate the solvothermal nucleation and growth of a pillared paddlewheel porphyrin framework 5 (PPF-5) onto semiconductor surfaces modified with carboxylic acids. Using versatile diazonium and catechol chemistries to modify silicon and titania surface chemistries, we show that solvothermally grown PPF-5 selectively nucleates and grows as stacked crystalline sheets with preferential (001), (111), and (110) crystallographic orientations. Furthermore, variations in the synthesis temperature produce modified stack morphologies that correlate with changes in the surface-nucleated PPF-5 photoluminescence.

Lithographically defined three-dimensional graphene structures

A simple and facile method to fabricate 3D graphene architectures is presented. Pyrolyzed photoresist films (PPF) can easily be patterned into a variety of 2D and 3D structures. We demonstrate how prestructured PPF can be chemically converted into hollow, interconnected 3D multilayered graphene structures having pore sizes around 500 nm. Electrodes formed from these structures exhibit excellent electrochemical properties including high surface area and steady-state mass transport profiles due to a unique combination of 3D pore structure and the intrinsic advantages of electron transport in graphene, which makes this material a promising candidate for microbattery and sensing applications.

Large area mosaic films of graphene-titania: self-assembly at the liquid-air interface and photo-responsive behavior

Photo-responsive graphene-titania composite nanofilms were formed via evaporative induced self-assembly at the air-liquid interface from the UV-photo-reduction of titania-graphene oxide colloidal solutions.

Nanostructured ruthenium oxide electrodes via high-temperature molecular templating for use in electrochemical capacitors

Ruthenium oxide is a model pseudocapacitive materials exhibiting good electronic and protonic conduction and has been shown to achieve very high gravimetric capacitances. However, the capacitance of thermally prepared ruthenium oxide is generally low because of low protonic conductivity resulting from dehydration of the oxide upon annealing. High-temperature processing, however also produces the electrically conducting ruthenium oxide rutile phase, which is of great interest for electrochemical capacitors. Here, unusual electrochemical characteristics were obtained for thermally prepared ruthenium oxide when fabricated in the presence of alkyl-thiols at high temperature. The performance characteristics have been attributed to enhanced multifunctional properties of the material resulting from the novel processing. The processing method relies on a simple, solution-based strategy that utilizes a sacrificial organic template to sterically direct hierarchical architecture formation in electro-active ruthenium oxide. Thin films of the templated RuO(2) exhibit energy storage characteristics comparable to hydrous ruthenium oxide materials formed under dramatically different conditions. Extensive materials characterization has revealed that these property enhancements are associated with the retention of molecular-sized metal oxide clusters, high hydroxyl concentrations, and formation of hierarchical porosity in the ruthenium oxide thin films.

Metalloporphyrin assemblies on pyridine-functionalized titanium dioxide

Porphyrin adsorption on TiO2 nanoparticles has been achieved for multiple porphyrins, and in mixed porphyrin assemblies, via axial ligation to surface-bound pyridine anchored by either para carboxylic or phosphonic functionalizations. Homogenous assemblies were prepared and characterized, while mixed metalloporphyrin assemblies were demonstrated by controlling the concentration ratios of respective porphyrins in the modifying solution. Evaluation of the assemblies using spectroscopic techniques and electrochemistry confirms high porphyrin retention, while exhibiting their surface bound optical and electrochemical properties. A thorough study is discussed where several metalloporphyrins have been evaluated (Ru(CO)OEP, Ru(CO)TPP, and ZnTPP) for relative comparisons and relationships to pyridyl axial binding strengths. The systematic study evaluates multiple background cases using either H2TPP, TiO2 modification with benzoic acid, or unmodified TiO2 to confirm the high affinity of Ru and Zn porphyrins for surface-anchored pyridyl sites. The simple method of step-by-step coordinative anchoring of porphyrins to TiO2 using small, commercially available molecules is highly adaptable for use in dye-sensitized solar cells (DSSC) where intimate contact between the absorbing dye and the semiconductor is required. DSSC devices with novel mixed porphyrin assemblies were shown to give higher power performance than DSSCs utilizing sensitization with only one type of porphyrin.