Near-edge X-ray Fine Structure Spectroscopy Research Articles

Effect of bottom electrode on rectification performance in pyrene-terminated n-alkanethiolate

Rectification performance of pyrene-terminated alkanethiolate in large-area tunneling junction was explored on the effect of bottom-electrode identity (Ag, Au, Pt, Pd) as non-molecular factors. Rectification ratio (log|r–|) of pyrene-terminated junction significantly decreased in order of Ag (log|r–|= 2.2 ± 0.3) ≈ Pt (log|r–|= 2.2 ± 0.5) > Pd (log|r–|= 2.0 ± 0.5) > Au (log|r–|= 1.3 ± 0.6). Three hypotheses were considered for the origin of the bottom-electrode effect: (i) surface topography of bottom electrode, (ii) electronic structure, and (iii) supramolecular packing structure. Structural analyses on a molecular scale using atomic force microscopy, ultraviolet photoelectron spectroscopy, and near-edge X-ray fine structure spectroscopy revealed that the effect of bottom electrode on the supramolecular structure dominates the trend of rectification performance ruling out the other possibilities. These experimental results demonstrate that rectification performance in pyrene-based molecular diode is sensitive to the change in electrode's identity. Our work offers important insight about structure-rectification relationship in organic molecular diode

N-Alkanethiols Directly Grown on a Bare Si(111) Surface: From Disordered to Ordered Transition.

We observed the growth phase transition of n-alkanethiols (AT), CH3(CH2)n-1SH, n = 4-16, directly implanted on a bare Si(111) surface, forming an AT monolayer. These monolayers were characterized with static water-contact angle, high-resolution X-ray photoelectron spectroscopy, near-edge X-ray fine-structure spectroscopy, and grazing-angle reflection absorption Fourier-transform infrared spectroscopy. The integrated spectral results indicated that the implanted n-AT molecules formed a self-oriented and densely packed monolayer through formation of an S-Si bond. With the number of carbons in the alkyl chain at six or more, namely beginning at hexanethiol, the molecular monolayer began to develop an orientation-ordered structure, which is clearly shorter than that for AT monolayers on Au and Ag. This result implies that, with a stronger molecule-substrate interaction, an ordered molecular monolayer can form with a short chain.

Systematic Investigation of π-π Interactions in Near-Edge X-ray Fine Structure (NEXAFS) Spectroscopy of Paracyclophanes.

NEXAFS spectroscopy has potential for study of packing and order in organic materials but only if intermolecular effects are understood. This work studies how π-π interactions between adjacent unsaturated groups affect their NEXAFS spectra, with a broader goal of building a general understanding of the role of intermolecular effects in NEXAFS spectroscopy. These effects are examined using paracyclophane (PCP) molecules, in which the benzene-benzene separation distance is controlled by varying the length of the alkyl groups separating the benzene rings. NEXAFS spectroscopy and density functional theory (DFT) simulations are used to examine spectroscopic changes related to the strength of these π-π interactions. A characteristic red shift is observed as adjacent benzene rings get closer together. This shift is attributed to Coulombic and orbital interactions between the adjacent benzene rings, mediated through overlapping π/π* orbitals.

Adsorption and stability of malonic acid on rutile TiO2 (110), studied by near edge X-ray absorption fine structure and photoelectron spectroscopy

The adsorption of malonic acid on rutile TiO2 (110) has been studied using photoelectron spectroscopy and C K-edge, near edge X-ray fine structure spectroscopy (NEXAFS). Analysis of the O 1s and Ti 2p spectra suggest that the molecule adsorbs dissociatively in a doubly-bidentate adsorption geometry as malonate. The data are unable to distinguish between a chelating bonding mode with the backbone of the molecule lying along the [001] azimuth or a bridging geometry along the direction. Work carried out on a wiggler beamline suggests that the molecule is unstable under irradiation by high-flux synchrotron radiation from this type of insertion device.

Fe L-Edge X-ray Absorption Spectra of Fe(II) Polypyridyl Spin Crossover Complexes from Time-Dependent Density Functional Theory

L-edge near-edge X-ray fine structure spectroscopy (NEXAFS) has become a powerful tool to study the electronic structure and dynamics of metallo-organic and biological compounds in solution. Here, we present a series of density functional theory calculations of Fe L-edge NEXAFS for spin crossover (SCO) complexes within the time-dependent framework. Several key factors that control the L-edge excitations have been carefully examined using an Fe(II) polypyridyl complex [Fe(tren(py)3)](2+) (where tren(py)3 = tris(2-pyridylmethyliminoethyl)amine) as a model system. It is found that the electronic spectra of the low-spin (LS, singlet), intermediate-spin (IS, triplet), and high-spin (HS, quintet) states have distinct profiles. The relative energy positions, but not the spectral profiles, of different spin states are sensitive to the choice of the functionals. The inclusion of the vibronic coupling leads to almost no visible change in the resulting NEXAFS spectra because it is governed only by low-frequency modes of less than 500 cm(-1). With the help of the molecular dynamics sampling in acetonitrile at 300 K, our calculations reveal that the thermal motion can lead to a noticeable broadening of the spectra. The main peak position is strongly associated with the length of the Fe-N bond.

Fabrication of Lithographically Defined Chemically Patterned Polymer Brushes and Mats

Chemically patterned surfaces comprised of polymer mats and brushes of well-defined chemistry were fabricated at the length scale of 10 nm. A key concept is the integration of new materials, cross-linked polymer mats, with traditional lithographic processing. Resist was patterned on top of cross-linked polystyrene mats. After etching, regions of the remaining mat with dimensions ranging from 10 to 35 nm were separated by interspatial openings to the underlying substrate. End-grafted polymer brushes, in this case hydroxyl-terminated poly(2-vinylpyridine) or polystyrene−poly(methyl methacrylate) random copolymer, were grafted into the exposed, interspatial regions from films spin-coated over the patterned mat. Both block copolymer wetting studies, with polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA), and near-edge X-ray fine structure spectroscopy showed that with sufficient cross-linking the polymer mat chemistry was unaffected by the subsequent grafting of the polymer brush. The precise definition...

Wavelet analysis of soil variation at nanometre- to micrometre-scales: an example of organic carbon content in a micro-aggregate

This paper demonstrates the potential of wavelet analysis to investigate fine-scale spatial variation in soil without statistical assumptions that are generally implausible. We analysed the optical densities of different forms of carbon which were measured at intervals of 50 nm along a 16-µm transect on a soil micro-aggregate using near-edge X-ray fine-structure spectroscopy (NEXAFS). We found different patterns of scale-dependent variation between the carbon forms, which could be represented by pair-wise wavelet correlations at the different scales, and by principal components analysis of all the correlations at each scale. These results represent only one small soil micro-aggregate and are not presented as general findings about soil carbon, but they do indicate that fine-scale variation of soil carbon can be complex in ways that the wavelet analysis can accommodate but alternative spatial statistics such as variograms cannot. Among the patterns of variation that the analysis could identify were scale-dependent correlations of the different forms of carbon. In some cases, positive correlations were found at coarser scales and negative at the finest scales, suggesting a multi-scale pattern in which contrasting forms of carbon are deposited in common clumps but at finer scales either one or the other form dominates. Aromatic and carboxylic carbon varied jointly in this way. Other forms of carbon, such as carboxylic and aliphatic carbon, were strongly correlated at the finest scales but not the coarser scales. We found evidence for changes in the variance and correlation of forms of carbon along the transect, indicating that the spatial distribution of carbon at these fine scales may be very complex in ways that are inconsistent with the assumptions of geostatistics. This quantitative analysis of the spatial patterns of different soil components at micro-scales offers a basis for formulating and testing specific hypotheses on replicated samples.

Room temperature ferromagnetism in chemically synthesized ZnO rods

We report structural and magnetic properties of pure ZnO rods using x-ray diffraction (XRD), magnetization hysteresis ( M−H) loop and near edge x-ray fine structure spectroscopy (NEXAFS) study at O K edge. Sample of ZnO was prepared by co-precipitation method. XRD and selective area electron diffraction measurements infer that ZnO rods exhibit a single phase polycrystalline nature with wurtzite lattice. Field emission transmission electron microscopy, field emission scanning electron microscopy micrographs infers that ZnO have rod type microstructures with dimension 200 nm in diameter and 550 nm in length. M–H loop studies performed at room temperature display room temperature ferromagnetism in ZnO rods. NEXAFS study reflects absence of the oxygen vacancies in pure ZnO rods.

Imperfect surface order and functionalization in vertical carbon nanotube arrays probed by near edge X-ray absorption fine structure spectroscopy (NEXAFS)

Probing surface order as well as the degree of structural modification in carbon nanotube systems is of fundamental importance for incorporation of these materials into practical functional devices. The current study pertains to the analysis of the surface order of vertically-aligned single-walled and multi-walled carbon nanotube arrays of varying length and composition by means of near-edge X-ray fine structure spectroscopy (NEXAFS). Both NEXAFS and scanning electron microscopy (SEM) studies concluded that the nanotubes in these samples were oriented vertically to the plane of the surface. However, NEXAFS polarization analysis provided a more quantitative and nuanced description of the surface structure, indicative of far less localized surface order, an observation partially attributed to misalignment and bending of the tubes. Moreover, it was demonstrated by NEXAFS that the surface order of the arrays was imperfect and relatively independent of the height of the nanotube arrays. In addition, we have shown that NEXAFS can be used to correlate the extent of chemical functionalization and oxygenation with disruption of the electronic and physical structure of nanotubes embedded in array motifs.

Direct Correlation of Organic Semiconductor Film Structure to Field‐Effect Mobility

Near-edge X-ray fine structure spectroscopy is used to measure simultaneous chemical conversion, molecular ordering, and defect formation in soluble oligothiophene precursor films. Film structure is correlated to OFET performance. Molecular orientation is determined by evaluating antibonding orbital overlap with the polarized electric field vector of incident soft X-rays (see Figure and cover). Upon conversion, the molecules become vertically oriented, allowing π overlap in the plane of hole transport.