Surface Geometric and Electronic Structure of BaFe2As2(001)

BaFe2As2 exhibits properties characteristic of the parent compounds of the newly discovered iron (Fe)-based high-TC superconductors. By combining the real space imaging of scanning tunneling microscopy/spectroscopy (STM/S) with momentum space quantitative Low Energy Electron Diffraction (LEED) we have identified the surface plane of cleaved BaFe2As2 crystals as the As terminated Fe-As layer - the plane where superconductivity occurs. LEED and STM/S data on the BaFe2As2(001) surface indicate an ordered arsenic (As) - terminated metallic surface without reconstruction or lattice distortion. It is surprising that the STM images the different Fe-As orbitals associated with the orthorhombic structure, not the As atoms in the surface plane.Comment: 12 pages, 4 figure

In Situ and Real Time Characterization of Spontaneous Grafting of Aryldiazonium Salts at Carbon Surfaces

Aryldiazonium cations are widely used to covalently functionalize carbon substrates that display a wide range of composition, from 100% sp² such as graphite or graphene to 100% sp³ such as diamond and nanodiamond. In this work we investigated the effect that changes in carbon composition have on aryldiazonium adsorption rates and surface reaction mechanism. Quartz crystal microbalance (QCM) was used to investigate the rates of adsorption in situ and in real time at two amorphous carbon substrates, one with high sp² content (a-C) and one with high sp³ content (a-C:H). A reversible Langmuir adsorption model was found to satisfactorily describe adsorption at a-C:H, yielding an adsorption rate coefficient <i>k</i>_a = 3.1 M^–1 s^–1 and a free energy of adsorption Δ<i>G</i>_a = −20.1 kJ mol^–1. This model, on the other hand, could not be applied for the interpretation of adsorption curves at a-C. Using electrochemical methods and X-ray photoelectron spectroscopy (XPS), we found that adlayers formed at a-C:H and a-C surfaces differ considerably in composition; in particular, a-C surfaces were found to display higher rates of dediazoniation with respect to a-C:H surfaces. Our findings are interpreted and discussed in the context of current proposed mechanisms for aryldiazonium reactions at surfaces that consist of an adsorption/desorption step followed by a chemisorption via dediazoniation step. Our observations are consistent with proposed mechanisms and strongly suggest that differences in carbon composition result in differences in the relative magnitude of adsorption and chemisorptions rate coefficients

Spontaneous Grafting of Nitrophenyl Groups on Amorphous Carbon Thin Films: A Structure–Reactivity Investigation

Amorphous carbon materials find numerous applications in diverse areas ranging from implantable biodevices to electronics and catalysis. The spontaneous grafting of aryldiazonium salts is an important strategy for the modification of these materials, and it is widely used to display a range of functionalities or to provide anchoring groups for further functionalization. We have investigated the spontaneous attachment of 4-nitrobenzenediazonium salts from aqueous solutions onto amorphous carbon materials that differ in their sp² content, with the aim of understanding to what extent bulk composition affects rates and yields of aryldiazonium adsorption at the carbon/solution interface. Amorphous carbons were deposited in the form of thin films via reactive magnetron sputtering and were characterized using a combination of Raman, infrared, UV–vis, and X-ray photoelectron spectroscopy to determine their sp² content. Attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR) was used to monitor in situ and in real time the aryldiazonium adsorption process at the carbon/solution interface. These measurements demonstrate that rates and yields of adsorption for the same aryldiazonium salt increase nonlinearly vs sp² concentration. Studies of aryldiazonium salt grafting as a function of time carried out ex situ via cyclic voltammetry showed that the amorphous carbon film with highest sp² content displays significantly lower grafting yields than glassy carbon, a material with 100% sp² content. Intercalation experiments using 4-nitrobenzylamine suggest that the difference in relative density of graphitic edge planes exposed at the carbon surface is in excellent agreement with the observed relative grafting yields. We discuss the implications of these results for the development of structure/reactivity relationships that can be leveraged for understanding the surface chemistry of disordered carbon materials

Heterogeneous Charge Transfer at the Amorphous Carbon/Solution Interface: Effect on the Spontaneous Attachment of Aryldiazonium Salts

The chemisorption of aryldiazonium salts is one of the most versatile reactions for the modification of carbon surfaces; in this work we investigated the spontaneous chemisorption of aryldiazonium salts at amorphous carbons of differing graphitic content in order to relate surface reactivity to the valence electronic properties of aryldiazonium cations and carbon surfaces. Two structural isomers that differ by their redox potential were chosen for our studies: 4-nitronaphthalenediazonium tetrafluoroborate (4NND) and 5-nitronaphthalenediazonium tetrafluoroborate (5NND). The adsorption of 4NND and 5NND was studied in situ via attenuated total internal reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and ex situ via electrochemistry on two types of graphitic amorphous carbons (a-C), containing 80% and 100% trigonally bonded carbon centers. These two forms of carbon were characterized via electrochemical impedance spectroscopy (EIS), and the more graphitic surface was found to display a heterogeneous charge transfer rate constant 2 orders of magnitude larger than the less graphitic surface. This was consistent with ultraviolet photoelectron spectroscopy (UPS) results showing that the density of occupied states near the Fermi level is higher for the more graphitic substrate. In situ and ex situ studies of adsorption rates show that, on the less graphitic a-C surface, differences in adsorption rate could be explained based on the reduction potentials of the two aryldiazonium cations. However, on the more graphitic surface, we observed no difference in adsorption rates or yields between the two isomers, thus suggesting that spontaneous electron transfer is not rate determining at these surfaces. Gerischer–Marcus theory was used in order to explain the differences in charge transfer rates between the two carbons and to interpret observed differences in aryldiazonium adsorption rates at these substrates. Finally, our results are discussed in light of the current proposed mechanism of aryldiazonium chemisorption

Carbohydrate Coatings via Aryldiazonium Chemistry for Surface Biomimicry

Carbohydrates are extremely important biomolecules and their immobilization onto solid surfaces is of interest for the development of new biomimetic materials and of new methods for understanding processes in glycobiology. We have developed an efficient surface modification methodology for the functionalization of a range of materials with biologically active carbohydrates based on aryldiazonium chemistry. We describe the synthesis and characterization of carbohydrate reagents, which were subsequently employed for the one-step, solution-based modification of carbon, metals, and alloys with monosaccharides. We used a combination of spectroscopic and nanogravimetric methods to characterize the structure of the carbohydrate layers; we report an average surface coverage of 7.8 × 10^–10 mol cm^–2 under our experimental conditions. Concanavalin A, a mannose-binding lectin, and Peanut Agglutinin, a galactose-binding lectin, were found to bind from solution to their respective monosaccharide binding partners immobilized at the surface. This result suggests that the spontaneous chemisorption of aryldiazonium monosaccharide precursors leads to the formation of monosaccharide layers that retain the biological recognition specificity of the parent carbohydrate molecule. Finally, we carried out measurements using fluorescently labeled Bovine Serum Albumin (BSA) and found that these carbohydrate coatings reduce unspecific adsorption of this protein at carbon surfaces. These results suggest that aryldiazonium-derived carbohydrate coatings may offer a promising strategy for preventing undesirable protein accumulation onto surfaces