Direct visual evidence for chemical mechanisms of SERRS via charge transfer in Au20–pyrazine–Au20 junction

The essence of the chemical mechanism for surface‐enhanced resonance Raman scattering (SERRS) is the charge transfer (CT) between the metal and the molecule at the resonant electronic transition, which results in the mode‐selective enhancement in the SERRS spectrum. The site‐orientated CT can directly interpret the mode‐selective chemical enhancement in SERRS. However, it is a great challenge to intutively visualize the orientation and site of the CT. In this paper, for the pyrazine–Au₂ complex, a three‐dimensional (3D) cubic representation is built to provide direct visual evidence for chemical mechanisms of SERRS via CT from the Au₂ cluster to pyrazine at the resonant electronic transition. The relationship between the mode‐selective enhancements in SERRS and the site‐orientated CT was clearly revealed. The intracluster excitation (analog of plasmon excitation in large naonoparticles) was also visualized by the 3D cubic presentation, which provided the direct evidence of local electromagnetic field enhancement of SERRS. To study the quantum size effect and the coupling effect of the nanoparticles, the photoexcitation mechanisms of the Au₂₀–pyrazine complex and the Au₂₀–pyrazine–Au₂₀ junction were also investigated. The tunneling charge transfer from one Au₂₀ cluster to another Au₂₀ cluster outside the pyrazine in Au₂₀–pyrazine–Au₂₀ junction was also revealed visually. The calculated normalized extinction spectra of Au nanoparticles using the generalized Mie theory reveal that the resonance peak is red‐shifted due to the coupling between particles. Copyright © 2009 John Wiley & Sons, Ltd.     

On the road to high-resolution 3D molecular imaging

This contribution reviews the state-of-the-art in the domains of molecular imaging and depth profiling, the two methodological platforms required for 3D molecular imaging by secondary ion mass spectrometric (SIMS). Using molecular dynamics calculations, it also describes some of the mechanisms that make cluster projectiles such as C₆₀ so different for organic sample analysis. The discussion addresses issues that deserve proper attention on the way to 3D molecular imaging in SIMS, such as ultimate lateral resolution, limited molecular yields, chemical effects and damage, and highlights solutions currently in embryo in the many research teams concerned by 3D molecular imaging.     

The study of the structure and bioactivity of the B2O3 • Na2O • P2O5 system

In this study, we have investigated calcium and silicate‐free samples over a wide compositional range in the xB₂O₃·30 Na₂O·(70−x)P₂O₅ system, with 0 ≤ x ≤ 70 mol%, in order to determine the influence of the chemical composition on their structure and bioactive response in simulated body fluid. Information related to the chemical structures present in the network was obtained by means of Raman and infrared spectroscopy. For samples containing small amounts of P₂O₅, boron structures are preponderant. Upon increasing the phosphorus content, the samples' network is based on phosphate chains linked by boron groups through ‘P–O–B’ bridges. For high concentration of P₂O₅, the Q³ units form three‐dimensional network, whereas Q² units assist the chain formation. Regarding the in vitro assessment of bioactivity, the clear print of PO₄ asymmetric bending vibrations of apatite‐like layer in the 540–680 cm⁻¹ spectral domain, the scanning electron micrographs and energy dispersive x‐ray analysis spectra demonstrate that the studied borophosphate samples exhibit good bioactive response only for certain chemical compositions. More exactly, the highest bioactivity is obtained for 30% and 20% B₂O₃ (mol%) after 3 and 11 days of immersion, respectively. Therefore, the samples with 20–30 mol% boron content are valuable candidates that can be used as materials for tissue engineering applications. Copyright © 2013 John Wiley & Sons, Ltd.     

Inhomogeneity of a highly efficient InGaN based blue LED studied by three‐dimensional atom probe tomography

The InGaN based multiple quantum well (MQW) structure in a commercially available white light emitting diode (LED) was studied by transmission electron microscopy (TEM) and three‐dimensional atom probe tomography (APT). The average In mole fraction by three‐dimensional (3D) APT was found to be about 18% in the InGaN well which is consistent with the secondary ion mass spectrometry (SIMS) analysis. The In distribution in the InGaN well layer was analyzed by the iso curve mapping of 3D APT and found to be non‐uniform in the InGaN active layer. In clustering or In rich regions in the range of 2–3 nm size were found, in contrast to recent reports. Our results thus indicate that In clustering is essential for high‐brightness InGaN based LEDs. We have also observed a discontinuity in the range of 50–100. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Synchrotron PEEM and ToF‐SIMS study of oxidized heterogeneous pentlandite,pyrrhotite and chalcopyrite

Synchrotron‐based photoemission electron microscopy (PEEM; probing the surface region) and time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS; probing the uppermost surface layer) have been used to image naturally heterogeneous samples containing chalcopyrite (CuFeS₂), pentlandite [(Ni,Fe)₉S₈] and monoclinic pyrrhotite (Fe₇S₈) both freshly polished and exposed to pH 9 KOH for 30 min. PEEM images constructed from the metal L₃ absorption edges were acquired for the freshly prepared and solution‐exposed mineral samples. These images were also used to produce near‐edge X‐ray absorption fine‐structure spectra from regions of the images, allowing the chemistry of the surface of each mineral to be interrogated, and the effect of solution exposure on the mineral surface chemistry to be determined. The PEEM results indicate that the iron in the monoclinic pyrrhotite oxidized preferentially and extensively, while the iron in the chalcopyrite and pentlandite underwent only mild oxidation. The ToF‐SIMS data gave a clearer picture of the changes happening in the uppermost surface layer, with oxidation products being observed on all three minerals, and significant polysulfide formation and copper activation being detected for pyrrhotite.     

Rapid,Microwave‐Assisted,and One‐Pot Synthesis of Magnetic Palladium–CoFe2O4–Graphene Composite Nanosheets and Their Applications as Recyclable Catalysts

Here, a microwave‐assisted approach has been demonstrated to rapidly prepare magnetic Pd–CoFe₂O₄–graphene (GE) composite nanosheets in ethylene glycol (EG) solvent. The generation of both Pd and CoFe₂O₄ nanoparticles is accompanied with the reduction process of graphene oxide (GO) by EG. The surface morphologies and chemical composition of the composite nanosheets are characterized by transmission electron microscopy (TEM), energy‐dispersive X‐ray spectrometer (EDS), powder X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR) measurements. The as‐prepared Pd–CoFe₂O₄–GE composite nanosheets exhibit a remarkable catalytic activity towards the reduction of 4‐nitrophenol by sodium borohydride (NaBH₄) at room temperature. The apparent kinetic rate constant (K _app) of this catalytic reaction could reach about 11.0 × 10^?3 s^?1. Moreover, the CoFe₂O₄ component exhibits a magnetic property, which could make the Pd–CoFe₂O₄–GE composite nanocatalysts separated from the suspension system. The catalytic conversion of the 4‐nitrophenol to 4‐aminophenol could reach 87.2% after four cycles. This work presents a simple, rapid, and versatile method to fabricate both metal and spinel‐type complex oxides on GE nanosheets, providing a new opportunity for their applications in the recyclable catalytic reaction.     

Bi<Stack><Subscript>3</Subscript><Superscript>+</Superscript></Stack> cluster primary ions in SIMS depth profiling of YBaCuO high-temperature superconductor films

SIMS depth profiling of YBa₂Cu₃O₇ high-temperature superconductor films was performed using a TOF.SIMS-5 instrument. A new approach was proposed to obtain additional information on the phase composition of YBa₂Cu₃O₇ films based on detection of cluster secondary ions.     

Solvatochromism as an efficient tool to study N,N‐dimethylamino‐ and cyano‐substituted π‐conjugated molecules with an intramolecular charge‐transfer absorption

A representative data set has been gained by the measurement of the electronic absorption spectra of 12 systematically selected push–pull systems with an intramolecular charge‐transfer (CT) absorption and the general structure D–π–A (D = donor, A = acceptor) featuring electron‐withdrawing CN groups, electron‐donating N(CH₃)₂ groups, and various π‐conjugated backbones in 32 solvents with different polarities. The longest‐wavelength absorption maxima λ_max and the corresponding wavenumbers $\tilde {v}_{{\rm max}} $ were evaluated from the UV/Vis spectra measured in 32 well‐selected solvents. The D–π–A push–pull systems were further characterized by quantum‐chemical quantities and simple structural parameters. Structure–solvatochromism relationships were evaluated by multidimensional statistic methods. Whereas solvent polarizability and solvent cavity size proved to be the most important factors affecting the position of λ_max, the solvent polarity was less important. The most important characteristics of organic CT compounds are the energy of the LUMO, the permanent dipole moment, the COSMO (COnductor‐like Screening MOdel) area, the COSMO volume, the number, and ratio of N,N‐dimethylamino and cyano groups, and eventually the number of triple bonds (π‐linkers). A relation between the first‐order polarizability α, the longest‐wavelength absorption maxima λ_max, and the structural features has also been found. The higher‐order polarizabilities β and γ are not related to the observed solvatochromism. Copyright © 2010 John Wiley & Sons, Ltd.     

Observation of π Berry phase in quantum oscillations of three‐dimensional Fermi surface in topological insulator Bi2Se3

We report the quantum transport studies on Bi₂Se₃ single crystal with bulk carrier concentration of ～10¹⁹ cm^–3. The Bi₂Se₃ crystal exhibits metallic character, and at low temperatures, the field dependence of resistivity shows clear Shubnikov–de Haas (SdH) oscillations above 6 T. The analysis of these oscillations through Lifshitz–Kosevich theory reveals a non‐trivial π Berry phase coming from three‐dimensional (3D) Fermi surface, which is a strong signature of Dirac fermions with three‐dimensional dispersion. The large Dingle temperature and non zero slope of Williamson–Hall plot suggest the presence of enhanced local strain field in our system which possibly transforms the regions of topological insulator to 3D Dirac fermion metal state. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Effect of microheterogeneity of CF3CH2OH–H2O mixed solvent on reactions of carbocations from 1,2,2,3‐tetramethyl‐1,2‐dihydroquinoline

Carbocations are key intermediates in many important organic reactions. The remarkable effect of the solvent composition on the kinetic parameters of the carbocation decay and product composition was found in the photolysis of 1,2,2,3‐tetramethyl‐1,2‐dihydroquinoline ( 1 ) in 2,2,2‐trifluoroethanol (TFE)–H₂O mixtures. The rate constant of the intermediate carbocation decay has a maximum, and the activation energy is minimal in the TFE–H₂O mixture 3 : 7 (v/v). In the steady‐state photolysis, products of oligomerization of 1 with n up to 8 and their adducts with TFE and H₂O were identified at this solvent composition. The results were rationalized in terms of TFE clustering in aqueous mixtures, with the maximum of cluster formation at 30 vol % TFE. The clusters form a pseudo‐phase, in which the molecules of 1 are concentrated and the carbocations are generated. TFE, H₂O and 1 compete in the combination reaction with the photogenerated carbocation to afford the products. This effect was not observed for 1,2,2,4‐tetramethyl‐1,2‐dihydroquinoline ( 2 ), the isomer of 1 , due to steric hindrance at C(4) carbon atom of the heterocycle, the active site of the intermediate carbocation, which makes impossible for the carbocation from 2 to react further with 2 . Thus, the kinetic parameters and the product composition in the photolysis of 1 in TFE–H₂O mixtures reflect the changes in the microstructure of the binary solvent. Copyright © 2013 John Wiley & Sons, Ltd.     

Interfacial structure and electrical properties of LaAlO3 gate dielectric films on Si by metalorganic chemical vapor deposition

LaAlO₃ (LAO) gate dielectric films were deposited on Si substrates by low-pressure metalorganic chemical vapor deposition. The interfacial structure and composition distribution were investigated by high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), secondary-ion mass spectroscopy (SIMS), and Auger-electron spectroscopy (AES). HRTEM confirms that there exists an interfacial layer between LAO and Si in most samples. AES, SIMS, and XPS analyses indicate that the interfacial layer is compositionally graded La–Al silicate and the Al element is severely deficient close to the Si surface. Electrical properties of LAO films were evaluated. No evident difference in electrical properties between samples with and without native SiO₂ layers was observed. The electrical properties are discussed in terms of LAO growth mechanisms, in relation to the interfacial structure. PACS 73.40.Qv; 81.15.Gh; 77.55.+f; 68.35.-p     

Wide band gap and p‐type conductive Cu–Nb–O films

Cu–Nb–O films with a thickness of ca. 150 nm were prepared on borosilicate glass substrates using CuNbO₃ ceramic target at substrate temperature of 500 °C by pulsed laser deposition. The X‐ray diffraction patterns showed that the Cu–Nb–O films were amorphous or an aggregation of fine crystals. The post‐annealed film at 300 °C in N₂ gas showed 80% transmission in visible light (band gap = 2.6 eV) and high p‐type conductivity of 21 S cm^–1. The Cu–Nb–O film with a thickness of 100 nm, fabricated from the target with a composition of Cu/Nb = 0.9, showed the highest p‐type conductivity of 116 S cm^–1. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Co9S8 Nanoparticles Incorporated in Hierarchically Porous 3D Few‐Layer Graphene‐Like Carbon with S,N‐Doping as Superior Electrocatalyst for Oxygen Reduction Reaction

Electrochemical oxygen reduction reaction (ORR), using nonprecious metal catalysts, has attracted great attention due to the importance in renewable energy technologies, such as fuel cells and metal–air batteries. A simple and scalable synthetic route is demonstrated for the preparation of a novel 3D hybrid nanocatalyst consisting of Co₉S₈ nanoparticles which are incorporated in N,S‐doped carbon (N, S–C) with rational structure design. In particular, the hybrid catalyst is prepared by direct pyrolysis and calcination of a gel mixture of Mg,Co nitrate‐thiourea‐glycine under Ar atmosphere, with subsequent HCl washing. The properties of obtained hybrid catalyst are quite dependent on calcination temperature and added glycine amount. Under a molar ratio of Co5‐Mg15‐tu10‐gl45 and a calcination temperature of 900 °C, Co₉S₈ nanoparticles are embedded in a well‐developed carbon matrix which shows a porous 3D few‐layer graphene‐like N, S–C with open and hierarchical micro–meso–macro pore structure. Because of the synergistic effect between Co₉S₈ nanoparticles and well‐developed carbon support, the composite exhibits high ORR activity close to that of commercial Pt/C catalyst. More importantly, the composite displays superior long‐term stability and good tolerance against methanol. The strategy developed here provides a novel and efficient approach to prepare a cost‐effective and highly active ORR electrocatalyst.     

0D–2D and 1D–2D Semiconductor Hybrids Composed of All Inorganic Perovskite Nanocrystals and Single‐Layer Graphene with Improved Light Harvesting

In this study, inorganic cesium lead iodide (CsPbI₃) perovskite nanoparticles (PNPs) and perovskite nanowires (PNWs) with single‐layer graphene (SLG) are combined to obtain 0D–2D PNP–SLG and 1D–2D PNW–SLG hybrids with improved light harvesting. Time‐resolved single‐nanostructure photoluminescence studies of PNPs, PNWs, and related hybrids reveal (i) quasi‐two‐state photoluminescence blinking in PNPs, (ii) highly polarized photoluminescence emitted by PNWs and (iii) efficient interfacial electron transfer between perovskite nanostructures and SLG in both PNP–SLG and PNW–SLG hybrids. Doping of poorly absorbing, highly conductive SLG with perovskite nanocrystals and nanowires provides a simple, yet efficient path to obtain hybrids with increased light‐harvesting properties for potential utilization in the next‐generation photodetectors and photovoltaic devices, including polarization sensitive photodetectors.     

An isotope effect in electrolytic hydrogen absorption of some transition metals studied by ERDA and SIMS techniques

The depth profiles of protium and deuterium which were charged electrolytically, were measured by elastic recoil detection analysis (ERDA) and secondary ion mass spectrometry (SIMS) techniques in order to study the isotope effect in hydrogen absorption of Ti, Zr, Nb, Ni and Pd. The absolute loading ratios of H(D)/metal were calculated from the ERDA spectra and the depth profiles of SIMS were compared with the results of the ERDA. The isotope absorption ratios are estimated to be (D/H)_Ti=0.43, (D/H)_Zr=0.53, (D/H)_Nb=0.17 and (D/H)_Pd=0.10. The content in Ni is below the detection limit. The mass balance equations based on the transport–absorption model, were applied to analysis of the experimental results. This model reveals that the isotope absorption ratios for the Nb and Pd cases are governed mainly by the flux of hydrogen ions flowing to the surface of the metal electrode. However, the competition between the absorption–conversion process and the recombination process mainly determine the isotope ratio for the Ti and Zr cases.     

Ultrahigh‐Density Plasmonic‐Nanoparticle‐Sensitized Semiconductor Photocatalysts Profit from Cooperative Light Harvesting and Charge Separation Processes: Experiments,Simulations, and Multifunctional Plasmonics

Here, a controlled synthesis of remarkable 3D photocatalysts is presented that is composed of ultrahigh‐density unaggregated plasmonic Au nanoparticles (AuNPs) chemically bound to vertically aligned ZnO nanorod arrays (ZNA) through bifunctional molecular linkers. Experimental probes and electromagnetic simulations of electron transfer and localized plasmonic coupling processes are exploited to gain insight into the underlying light‐irradiation‐induced interactions in the 3D ZNA–AuNPs photocatalysts. Highly dense AuNPs on ZNA surfaces act as sinks for the storage of UV‐generated electrons, which promote the separation of charge carriers and create numerous photocatalytic reaction centers. Furthermore, 3D finite‐difference time domain simulation indicates that significant visible light confinement and enhancement around the ZNA–AuNPs interfacial plasmon “hot spots” contribute to efficient conversion of light energy to electron‐hole pairs. Significantly, in comparison with the bare ZNA, the 10‐nm‐sized AuNPs‐decorated ZNA exhibits 10.6‐fold enhanced photoreaction rate in the entire UV–vis region. Moreover, various novel hybrid structures based on the plasmonic AuNPs and diverse nanostructures (films, powdered nanorods, mesoporous, and nanotubes) or functional materials (multiferroic BiFeO₃, CuInGaSe₂ absorber layers, and photoactive TiO₂) are successfully constructed using the present synthesis methodology. It may stimulate the progress in materials science toward the synthesis of multifunctional plasmonic heterostructures or devices.     

Solvent effects on chemical processes: new solvents designed on the basis of the molecular–microscopic properties of (molecular solvent + 1,3‐dialkylimidazolium) binary mixtures

The purpose of this work was to analyze the microscopic feature of binary solvent systems formed by a molecular solvent (acetonitrile or dimethylformamide or methanol) and an ionic liquid (IL) cosolvent [1‐(1‐butyl)‐3‐methylimidazolium tetrafluoroborate or 1‐(1‐butyl)‐3‐methylimidazolium hexafluorophosphate]. The empirical solvatochromic solvent parameters E_T(30), π*, α, and β were determined from the solvatochromic shifts of adequate indicators. The behavior of the solvent systems was analyzed according to their deviation from ideality. The study focused on the identification of solvent mixtures with relevant solvating properties in order to select mixed solvents with particular characteristics. The comparison of the molecular–microscopic solvent parameters corresponding to the selected binary mixtures with both ILs considered at similar mixed‐solvent composition revealed that the difference is centered on the basic character of them. A kinetic study of a nucleophilic aromatic substitution reaction between 1‐fluoro‐2,4‐dinitrobenzene (FDNB) and 1‐butylamine (BU) developed in (acetonitrile or dimethylformamide + IL) solvent mixtures is presented in order to investigate and compare the solvent effects on a chemical process. For the explored reactive systems the solvation behavior is dominated by both the dipolarity/polarizability and the basicity of the media, contributing these solvent properties to accelerating the chemical process. Copyright © 2007 John Wiley & Sons, Ltd.     

Micro‐Raman characterization of laser‐induced local thermo‐oxidation of thin chromium films

Micro‐Raman spectroscopy was applied to the characterization of the chemical composition and topography of protective oxide layers formed under atmospheric conditions on the surface of thin chromium films. Strips of the layers were produced by local thermal heating using focused sub‐picosecond pulsed laser radiation. It is shown that a CrO₂ layer is initially formed on the chromium surface at low light exposures. Increasing the exposure results in the transformation of the CrO₂ layer to Cr₂O₃. The influence of the etching conditions on the composition and thickness of the oxide layers is investigated. The topography of the CrO₂ and Cr₂O₃ oxide layers in transverse sections of the strips is demonstrated by the Raman mapping. Copyright © 2011 John Wiley & Sons, Ltd.     

Three‐Dimensional Multilayer Assemblies of MoS2/Reduced Graphene Oxide for High‐Performance Lithium Ion Batteries

Three‐dimensional (3D) multilayer molybdenum disulfide (MoS₂)/reduced graphene oxide (RGO) nanocomposites are prepared by a solution‐processed self‐assembly based on the interaction using different sizes of MoS₂ and GO nanosheets followed by in situ chemical reduction. 3D multilayer assemblies with MoS₂ wrapped by large RGO nanosheets and good interface are observed by transmission electron microscopy. The interaction of Na⁺ ions with oxygen‐containing groups of GO is also investigated. The measurement of lithium ion batteries (LIBs) shows that MoS₂/RGO anode nanocomposite with a weight ratio of MoS₂ to GO of 3:1 exhibits an excellent rate performance of 750 mAh g^?1 at 3 A g^?1 outperforming many previous studies and a high reversible capacity up to ≈1180 mAh g^?1 after 80 cycles at 100 mA g^?1. Good rate performance and high capacity of MoS₂/RGO with 3D unique layered‐structures are attributed to the combined effects of continuous conductive networks of RGO, good interface facilitating charge transfer, and strong RGO sheets preventing the volume expansion. Results indicate that 3D multilayer MoS₂/RGO prepared by a facile solution‐processed assembly can be developed to be an excellent nanoarchitecture for high‐performance LIBs.     

Yolk–Shell‐Structured Cu/Fe@γ‐Fe2O3 Nanoparticles Loaded Graphitic Porous Carbon for the Oxygen Reduction Reaction

Core–shell Cu/γ‐Fe₂O₃@C and yolk–shell‐structured Cu/Fe@γ‐Fe₂O₃@C particles are prepared by a facile synthesis method using copper oxide as template particles, resorcinol‐formaldehyde as the carbon precursor, and iron nitrate solution as the iron source via pyrolysis. With increasing carbonization temperature and time, solid γ‐Fe₂O₃ cores are formed and then transformed into Fe@γ‐Fe₂O₃ yolk–shell‐structured particles via Ostwald ripening under nitrogen gas flow. The composition variations are studied, and the formation mechanism is proposed for the generation of the hollow and yolk–shell‐structured metal and metal oxides. Moreover, highly graphitic carbons can be obtained by etching the metal and metal oxide nanoparticles through an acid treatment. The electrocatalytic activity for oxygen reduction reaction is investigated on Cu/γ‐Fe₂O₃@C, Cu/Fe@γ‐Fe₂O₃@C, and graphitic carbons, indicating comparable or even superior performance to other Fe‐based nanocatalysts.