Robert I. R. Blyth

Organic single-layer white light-emitting diodes by exciplex emission from spin-coated blends of blue-emitting molecules

We report on white electroluminescence (EL) emission from a single-layer light-emitting diode based on a binary blend of organic soluble blue-emitting molecules, i.e., a diamine derivative and a substituted thiophene-1,1-dioxide. Weakly voltage-dependent white color, of coordinates (0.39, 0.40) according to the standard of the Commission Internationale de l’Eclairage, is obtained from the superposition of the blue emission from the donor and a low-energy peak due to a charge-transfer complex between the two molecules (exciplex). The EL spectrum is broader and more structured than the photoluminescence one: this could be due to the activation of exciplexes with different conformations as inferred from quantum-chemistry calculations.

Time-resolved x-ray excited optical luminescence from SnO2 nanoribbons : Direct evidence for the origin of the blue luminescence and the role of surface states

Time-resolved x-ray excited optical luminescence (XEOL) and x-ray absorption near edge structures have been employed to study the origin of the multicolor luminescence from SnO2 nanoribbons. The authors find that the yellow-green luminescence has a long lifetime while the blue luminescence a short one. The luminescence is attributed to the radiative decay of trapped electrons in oxygen vacancies just below the conduction band and electrons in the conduction band to intrinsic surface states in the band gap.

LaVo4: Eu Phosphor Films with Enhanced Eu Solubility

Eu doped rare-earth orthovanadates are known to be good red phosphor materials. In particular, LaVO{sub 4}:Eu is a promising candidate due to the low Eu-site point symmetry, and thus high dipole transition probability within Judd-Ofelt theory. However, the low solubility limit (< 3 mol %) of Eu in LaVO{sub 4} prevents its efficient use as a phosphor. We present optical evidence of enhanced Eu solubility as high as 10 mol % in LaVO{sub 4}:Eu thin films grown by pulsed laser deposition and postannealing. The photoluminescent intensity exceeded that of YVO{sub 4}:Eu thin films when excited below the host bandgap, indicating stronger direct emission of Eu in LaVO{sub 4}.

Calibration method at the N K-edge using interstitial nitrogen gas in solid-state nitrogen-containing inorganic compounds

The standard method of soft X-ray beamline calibration at the N K-edge uses the nu = 0 peak transition of gas-phase N(2). Interstitial N(2) gas trapped or formed within widely available solid-state ammonium- and amine-containing salts can be used for this purpose, bypassing gas-phase measurements. Evidence from non-nitrogen-containing compounds (KH(2)PO(4)) and from He-purged ammonium salts suggest that production of N(2) gas is through beam-induced decomposition. Compounds with nitrate or nitrite as anions produce coincident features and are not suitable for this calibration method.

The Origin and Dynamics of Soft X-Ray-Excited Optical Luminescence of ZnO

The distinct optical emission from ZnO materials, nanoneedles and microcrystallites synthesized with different sizes and morphologies by a flow deposition technique, is investigated with X-ray excited optical luminescence (XEOL) and time-resolved X-ray excited optical luminescence (TR-XEOL) from a synchrotron light source at the O K and Zn L(3,2) edges. The innovative use of XEOL, allowing site-specific chemical information and luminescence information at the same time, is fundamental to provide direct evidence for the different behaviour and the crucial role of bulk and surface defects in the origin of ZnO optical emission, including dynamics. XEOL from highly crystalline ZnO nanoneedles is characterized by a sharp band-gap emission (~380 nm) and a broad red luminescence (~680 nm) related to surface defects. Luminescence from ZnO microcrystallites is mostly dominated by green emission (~510 nm) associated with defects in the core. TR-XEOL experiments show considerably faster decay dynamics in nanoneedles compared to microcrystallites for both band-gap emission and visible luminescence. Herein we make a fundamental step forward correlating for the first time the interplay of size, crystallinity, morphology and excitation energy with luminescence from ZnO materials.

Nitrogen speciation in fine and coarse clay fractions of a Cryoboroll - new evidence from pyrolysis-mass spectrometry and nitrogen K-edge XANES

Soil clay fractions are usually enriched in nitrogen (N), but the chemical identity of this N is largely unknown. Therefore, we investigated organic N in fine and coarse clay of a clay-rich Cryoboroll by Curie-point pyrolysis-gas chromatography/mass spectrometry (Cp Py-GC/MS), Pyrolysis-field ionization mass spectroscopy (Py-FIMS) and synchrotron-based nitrogen K-edge X-ray absorption near edge structure (N-XANES) spectroscopy. The Cp Py-GC-MS revealed 30 structurally different N-containing compounds, such as substituted pyridines, pyrroles; pyrazines, pyrazoles, imidazoles, quinolines, side-chain N-containing benzenes, and single compounds of substituted benzotriazole, purine and indole. These accounted for about 10% of peak area in the Py-GC chromatograms. The Py-FIMS and N-XANES spectra indicated interlayer-NH4+ and revealed pyridinic and nitrilic N compounds, but disagreed in the proportions of pyrroles. All three complementary methods confirmed to different extents previous wet-chemical data on N-fra...

Interface reconstruction in V-oxide heterostructures determined by x-ray absorption spectroscopy

We present an x-ray absorption study of the dependence of the V oxidation state on the thickness of LaVO3 (LVO) and capping LaAlO3 (LAO) layers in the multilayer structure of LVO sandwiched between LAO. We found that the change of the valence of V as a function of LAO layer thickness can be qualitatively explained by a transition between electronically reconstructed interfaces and a chemical reconstruction. The change as a function of LVO layer thickness is complicated by the presence of a considerable amount of V4+ in the bulk of the thicker LVO layers.

Chapter 9 – Nitrogen Compounds in Dissolved and Solid Environmental Samples

The chapter describes the state of the art in Nitrogen K-edge X-ray absorption near-edge structure (N-XANES) spectroscopy applied to environmental problems. Technical details of beamline optimization are given as well as advice for sample preparation and spectral evaluation. The thorough overview of reference compounds includes mineral N-compounds, amino acids, amides, nitriles, and a wide range of heterocyclic compounds. We show new evidence of radiation damage on N-XANES spectra of proteinaceous N-reference compounds and soil-related materials, and propose approaches to minimize it. Calculated N-XANES spectra using density functional theory showed excellent agreement with measurements that resolved pyridinic and pyrrolic N in the same molecule. Using environmental samples, we demonstrate that measurements of soil solutions indicate a large contribution of mineral N (NO3−, NH4+). In particle-size fractions and bulk soil samples, differences in the proportions of various N functions may originate from clay-organic matter interactions, rhizosphere processes, and soil management practices. In conclusion, N-XANES is a valuable component in the multimethod approach toward the study of inorganic and organic N in environmental samples.

The effect of the surface of SnO2 nanoribbons on their luminescence using x-ray absorption and luminescence spectroscopy

X-ray excited optical luminescence (XEOL) and x-ray absorption near-edge structure in total electron, x-ray fluorescence, and photoluminescence yields at Sn M5,4-, O K-, and Sn K-edges have been used to study the luminescence from SnO2 nanoribbons. The effect of the surface on the luminescence from SnO2 nanoribbons was studied by preferential excitation of the ions in the near-surface region and at the normal lattice positions, respectively. No noticeable change of luminescence from SnO2 nanoribbons was observed if the Sn ions in the near-surface region were excited selectively, while the luminescence intensity changes markedly when Sn or O ions at the normal lattice positions were excited across the corresponding edges. Based on the experimental results, we show that the luminescence from SnO2 nanoribbons is dominated by energy transfer from the excitation of the whole SnO2 lattice to the surface states. Surface site specificity is not observable due to its low concentration and weak absorption coefficient although the surface plays an important role in the emission as a luminescence center. The energy transfer and site specificity of the XEOL or the lack of the site specificity from a single-phase sample is discussed.

Zinc Porphyrin‐Driven Assembly of Gold Nanofingers

Nanofingers of gold covered by porphyrins are prepared by a combination of atomic manipulation and surface self-organization. A submonolayer of zinc(II) 5,10,15,20-tetrakis(4-tert-butylphenyl)-porphyrin (ZnTBPP) axially ligated to a self-assembled monolayer of 4-aminothiophenol (4-ATP) on Au(111) is prepared and studied using a combination of ultrahigh vacuum techniques. Under the electric field produced by the STM tip, the relatively weakly bound Au surface atoms along the discommensuration lines become mobile due to the strong bond to 4-ATP, while the tendency of the porphyrins towards self-assembly result in a collective motion of gold clusters. The clusters diffuse onto the surface following well-defined pathways along the [112] direction and then reach the step edges where they assembled, thus forming nanofingers. First-principles density functional theory calculations demonstrate the reduction of the binding energies between the surface gold clusters and the substrate induced by adsorption of thiols. Scanning tunneling microscopy images show assemblies across three adjacent discommensuration lines of the Au(111)-(22 x square root 3) reconstruction, which collectively diffuse along these lines to form islands nucleated at step edges.