Red spectral shift and enhanced quantum efficiency in phonon-free photoluminescence from silicon nanocrystals

Crystalline silicon is the most important semiconductor material in the electronics industry. However, silicon has poor optical properties because of its indirect bandgap, which prevents the efficient emission and absorption of light. The energy structure of silicon can be manipulated through quantum confinement effects, and the excitonic emission from silicon nanocrystals increases in intensity and shifts to shorter wavelengths (a blueshift) as the size of the nanocrystals is reduced. Here we report experimental evidence for a short-lived visible band in the photoluminescence spectrum of silicon nanocrystals that increases in intensity and shifts to longer wavelengths (a redshift) with smaller nanocrystal sizes. This higher intensity indicates an increased quantum efficiency, which for 2.5-nm-diameter nanocrystals is enhanced by three orders of magnitude compared to bulk silicon. We assign this band to the radiative recombination of non-equilibrium electron-hole pairs in a process that does not involve phonons.     

Putting nanocrystals to work: from solutions to devices

The colloidal route to semiconductor nanocrystals is extremely flexible, with a high degree of control over size, size distribution, surface passivation and internal structure of the nanoparticles. Simple chemically controlled techniques can be used to assemble these particles into dense films or other microscopic structures, suitable for photonic devices. Working with semiconductors or semi-metals which in the bulk form have low or inverted bandgaps, and taking advantage of the blue shift in the quantum confinement regime, nanocrystals can readily be tuned to the infrared wavelengths of interest for telecommunications. Design flexibility is far greater than with conventional compound semiconductors or rare-earth-doped glasses. Preliminary results demonstrating optical gain from II-VI nanocrystal films at room temperature are reported.     

Mechanical-contact-induced transformation from the amorphous to the partially crystalline state in metallic glass

Sliding friction and wear experiments and electron microscopy and diffraction studies were conducted to examine the metallurgical microstructure of a metallic glass surface strained in sliding contact. Friction and wear experiments were conducted with aluminium oxide spheres 3.2 and 6.4 mm in diameter sliding, in reciprocating motion, on a metallic foil with a composition of Fe₆₇Co₁₈B₁₄Si₁ at a sliding velocity of 1.5 mm s^-1 (frictional heating is negligible) with a load of 2.5 N at room temperature and in a laboratory air atmosphere.The results of the investigation indicate that the amorphous alloy (metallic glass) can be crystallized during mechanical contact. Crystallites with a size range of 10–150 nm are produced on the wear surface of the amorphous alloy. A diffused honeycomb-shaped structure formed by dark gray bands is also produced during sliding. Considerable plastic flow occurs on an amorphous alloy surface with sliding and the flow film of the alloy transfers to the aluminium oxide pin surface. Multiple slip bands due to shear deformation are observed on the side of the wear track. Two distinct types of wear debris were observed as a result of sliding: an alloy wear debris and/or powdery and whiskery oxide debris. The wear rate of Fe₆₇Co₁₈B₁₄Si₁ was 5 × 10^-9 mm³ N^-1.     

Metal nitride cluster fullerenes: their current state and future prospects

The world of endohedral fullerenes was significantly enlarged over the past seven years by the cluster fullerenes, which contain structures such as the M(2)C(2) carbides and the M(3)N nitrides. While the carbide clusters are generated under the standard arc-burning conditions according to stabilization conditions, the nitride cluster fullerenes (NCFs) are formed by varying the composition of the cooling gas atmosphere in the arc-burning process. The special conditions for NCF synthesis is described in detail and the optimum conditions for the production of NCFs as the main product in fullerene syntheses are given. A general review of all NCFs reported to date consists of the structures, properties, and stability of the NCFs as well as the abundance of the NCFs in the fullerene soot. It is shown that all cages with even carbon atoms from C(68) to C(98) are available as endohedral nitride cluster structures (with the exception of C(72), C(74), and C(76)). Specifically, the NCFs form the largest number of structures that violate the isolated pentagon rule (IPR). Finally some practical applications of these cluster fullerenes are illustrated and an outlook is given, taking the superior stability of these endohedral fullerenes into account.     

Evolution of nonlinear optical properties: from gold atomic clusters to plasmonic nanocrystals

Atomic clusters of metals are an emerging class of extremely interesting materials occupying the intermediate size regime between atoms and nanoparticles. Here we report the nonlinear optical (NLO) characteristics of ultrasmall, atomically precise clusters of gold, which are smaller than the critical size for electronic energy quantization (～2 nm). Our studies reveal remarkable features of the distinct evolution of the optical nonlinearity as the clusters progress in size from the nonplasmonic regime to the plasmonic regime. We ascertain that the smallest atomic clusters do not show saturable absorption at the surface plasmon wavelength of larger gold nanocrystals (>2 nm). Consequently, the third-order optical nonlinearity in these ultrasmall gold clusters exhibits a significantly lower threshold for optical power limiting. This limiting efficiency, which is superior to that of plasmonic nanocrystals, is highly beneficial for optical limiting applications.     

Generalized spectral decomposition: a theory and practice to spectral reconstruction

The generalized spectral decomposition (GSD) theorem is introduced, and the generalized fundamental stimulus and metameric black are analyzed to show how they convey the valuable features in terms of color information. The suggestion would be considered as the generalization of Cohen and Kappauf's matrix R theory and its later application in parameric correction by Fairman. The GSD theorem provides a modular model whose arguments can be elaborately set up for high-performance spectral recovery. It is also shown that the suggested methods for spectral decomposition and/or spectral reconstruction proposed by different researchers could be considered as special cases of GSD.     

Melting and superheating of crystalline solids: From bulk to nanocrystals

Melting of solids is a common phenomenon in nature. It is also one of the most important phase transformations in materials science and engineering. In recent years, extensive experimental and theoretical investigations in conjunction with computer simulations on melting of solids, with various geometries ranging from bulk forms to nanometer-sized clusters, have greatly enhanced our understanding of the nature of melting. This paper reviewed the up-to-date research results on this classical and cutting-edge topic. Emphasis was made on melting and superheating of nano-sized particles and thin films including thermodynamic and kinetic analyses of the size effect and the interfacial structure effect on melting processes.     

The different roles of proximity in multiple informal network relationships: evidence from the cluster of high technology applied to cultural goods in Tuscany

The role of proximity in innovation and inter-organisational networks has received increasing attention over the past decade. Despite the rich literature, most contributions principally focus on only one type of relationship and mainly consider formal relationships such as R&;D partnerships, EU projects or patents. The aim of the study is to investigate the role of various forms of proximity in multiple informal inter-organisational relationships. The article focuses on two research questions: (i) How do the various forms of proximity influence the formation of different informal relationships? and (ii) What is the impact of social ties on relationships for innovation? ERGM is applied to measure the different impact of the proximities on three network relationships operating in the cluster of High Technology applied to Cultural Goods. The results underline the heterogeneous impact of various forms of proximity on the different relationships and the strong impact of social ties on innovation.     

Temperature-dependent near-infrared spectra of bovine serum albumin in aqueous solutions: spectral analysis by principal component analysis and evolving factor analysis

Fourier transform near-infrared (FT-NIR) spectra have been measured for bovine serum albumin (BSA) in an aqueous solution (pH 6.8) with a concentration of 5.0 wt% over a temperature range of 45-85 degrees C. Not only conventional spectral analysis methods, such as second-derivative spectra and difference spectra, but also chemometrics, such as principal component analysis (PCA) and evolving factor analysis (EFA), have been employed to analyze the temperature-dependent NIR spectra in the 7500-5500 and 4900-4200 cm-1 regions of the BSA aqueous solution. Intensity changes of bands in the 7200-6600 cm-1 and 4650-4500 cm-1 regions in the difference spectra indicate variations of the hydration and secondary structure of BSA in the aqueous solution, respectively. The plot of a band intensity at 7080 cm-1 in the different spectra shows a clear turning point at 63 degrees C, revealing that a significant change in the hydration occurs at about 63 degrees C. The forward and backward eigenvalues (EVs) from EFA suggest that marked changes in the hydration and secondary structure of BSA take place in the temperature ranges of 61-65 degrees C and 59-63 degrees C, respectively. In addition, the temperature of 71 degrees C marked in the EFA plots may correspond to the onset temperature of increase in the intermolecular beta-sheet structure.     

Size effects on concrete fracture energy: dimensional transition from order to disorder

The nominal fracture energy of concrete structures is constant for relatively large structures, whereas it increases with size for relatively small structures. If the energy dissipation space is modeled as a monofractal domain, with a non-integer dimension comprised between 2 and 3, a unique slope in the bilogarithmic fracture energy versus size diagram is found, as was stated in a previous paper [1]. On the other hand, when the scale range extends over more than one order of magnitude, a continuous transition from slope+1/2 to zero slope may appear, according to the hypothesis of multifractality of the fracture surface [1]. This means that, at small scales, a Brownian microscopic disorder is prevalent whereas, at large scales, the effect of disorder vanishes, yielding a macroscopical homogeneous behavior. The dimensional transition from disorder to order may be synthesized by a Multifractal Scaling Law (MFSL) valid for toughness, in perfect correspondence with the MFSL valid for strength, which has been described in a previous paper [2]. The MFSL for fracture energy is applied, as a bestfitting method, to relevant experimental results in the literature, allowing for the extrapolation of fracture energy values valid for real-sized structures.     

Direct solar spectral irradiance and transmittance measurements from 350 to 2500 nm

A radiometrically stable, commercially available spectroradiometer was used in conjunction with a simple, custom-designed telescope to make spectrally continuous measurements of solar spectral transmittance and directly transmitted solar spectral irradiance. The wavelength range of the instrument is 350-2500 nm and the resolution is 3-11.7 nm. Laboratory radiometric calibrations show the instrument to be stable to better than 1.0% over a nine-month period. The instrument and telescope are highly portable, can be set up in a matter of minutes, and can be operated by one person. A method of absolute radiometric calibration that can be tied to published top-of-the-atmosphere (TOA) solar spectra in valid Langley channels as well as regions of strong molecular absorption is also presented. High-altitude Langley plot calibration experiments indicate that this technique is limited ultimately by the current uncertainties in the TOA solar spectra, approximately 2-3%. Example comparisons of measured and modtran-modeled direct solar irradiance show that the model can be parameterized to agree with measurements over the large majority of the wavelength range to the 3% level for the two example cases shown. Side-by-side comparisons with a filter-based solar radiometer are in excellent agreement, with a mean absolute difference of tau = 0.0036 for eight overlapping wavelengths over three experiment days.     

Size effect in quasibrittle fracture: derivation of the energetic Size Effect Law from equivalent LEFM and asymptotic analysis

Within the framework of Ba?ant’s theory, the size effect on the ultimate fracture properties of notched structures is studied from an energy based asymptotic analysis in which the resistance curve behavior is considered through an analytic expression. The scaling on the relative crack length at peak load as well as the size effect on the corresponding resistance to crack growth are investigated. It is shown that, for intermediate structure sizes, the relative crack length at peak load decreases with respect to the structure size while the resistance grows. These scalings lead to a size effect on nominal strength which is in agreement with Ba?ant’s Size Effect Law (SEL) for small and large structures sizes, but for intermediate sizes, an additional asymptotic regime occurs instead of a simple crossover regime as expected in SEL. The slope and the extent in size of the additional asymptotic regime depend only on the exponent characterizing the curvature of the R-curve. The comparison between the resulting size effect and Ba?ant’s SEL shows that SEL provides an approximate size effect which is in agreement with the expected asymptotic behaviors, with the exception of the extreme cases corresponding to very strong and slight R-curve’s curvatures. On this basis, the safety design of large structures is discussed.     

Visible tunable lighting system based on polymer composites embedding ZnO and metallic clusters: from colloids to thin films

The development of phosphor devices free of heavy metal or rare earth elements is an important issue for environmental reasons and energy efficiency. Different mixtures of ZnO nanocrystals with Cs₂Mo₆I₈(OOC₂F₅)₆ cluster compound (CMIF) dispersed into polyvinylpyrrolidone matrix have been prepared by very simple and low cost solution chemistry. The resulting solutions have been used to fabricate highly transparent and luminescent films by dip coating free of heavy metal or rare earth elements. The luminescence properties of solution and dip-coated films were investigated. The luminescence of such a system is strongly dependent on the ratios between ZnO and CMIF amounts, the excitation wavelength and the nature of the system. By varying these two parameters (ratio and wavelength), a large variety of colors, from blue to red as well as white, can be achieved. In addition, differences in the luminescence properties have been observed between solutions and thin films as well as changes of CMIF emission band maximum wavelength. This may suggest some possible interactions between the different luminophore centers, such as energy transfer or ligands exchange on the Mo₆ clusters.     

Cellulose nanocrystals and self-assembled nanostructures from cotton, rice straw and grape skin: a source perspective

Cellulose nanocrystals (CNCs) have been derived by sulfuric acid hydrolysis (64–65 wt% H₂SO₄, 10 mL/g cellulose, 45 °C) of pure cellulose isolated from cotton, rice straw and grape skin, producing relatively consistent products in 60, 45 and 30 min, respectively, and generally reflecting the extent of crystallinity and crystallite sizes of these cellulose sources. CNCs in nanorod forms are observed from all three cellulose sources and, in the case of cotton and grape skin, in the presence of more dominant forms of nanoparticles. Cotton CNCs are <10-nm-wide nanorods at up to 40 aspect ratios, whereas rice straw CNCs are flat ribbon cross-sectional shaped in 10:2:1–44:5:1 length/width/thickness ratios, and those from grape skin are abundant nanoparticles but fewer nanorods, all of very different nanoscale dimensions. Freezing (?196 °C) and freeze-drying (?50 °C) of dilute CNC suspensions induce self-assembling of these CNC populations into yet further distinctly different morphologies. Self-assembled cotton CNCs are loosely organized nanorods and nanospheres, whereas grape skin CNCs are mainly nanospheres of 5-nm-sized nanoparticles clusters around nanorod cores. Uniquely, rice straw CNCs assembled anisotropically into ultra-thin non-porous fibers. These source-linked unique CNC geometries and the ability of CNCs to self-assemble into different morphologies present wide ranging dimensions of these renewable cellulose nanomaterial building blocks from by-products of the world largest fiber, cereal and fruit crops.     

Pd<Subscript>3</Subscript> cluster catalysis: Compelling evidence from in operando spectroscopic,kinetic, and density functional theory studies

Identification of metal cluster catalysis is a topic that is being investigated since a long time. Here, we report a Pd3 metal cluster catalytic reaction investigated by means of operando studies. We discovered that atomically defined tri-nuclear palladium (Pd₃) is a surprisingly active catalyst for the cycloisomerization of 2-phenylethynylaniline. Operando ¹H NMR spectroscopy and X-ray extended absorption fine structure (EXAFS) measurements have indicated that the structural integrity of such a catalyst remains intact throughout the reaction, which has also been confirmed by an ex situ X-ray photoelectron spectroscopy (XPS) study and catalyst recycling experiments. Kinetic data derived from operando IR spectroscopy measurements have shown that Pd₃ is the active catalytic species. Density functional theory calculations have revealed a reaction pathway consistent with the kinetic data, further supported by NMR titration and X-ray crystal structure studies. Overall, the present study presents a clear example of metal cluster catalysis.

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Synthesis and catalytic properties of metal nanoparticles: Size, shape, support, composition, and oxidation state effects

Exciting new opportunities are emerging in the field of catalysis based on nanotechnology approaches. A new understanding and mastery of catalysis could have broad societal impacts, since about 80% of the processes in the chemical industry depend on catalysts to work efficiently. Efforts in surface science have led to the discovery of new heterogeneous catalysts, however, until recently the only way to develop new or improved catalysts was by empirical testing in trial-and-error experiments. This time-consuming and costly procedure is now rapidly being replaced by rational design methods that utilize fundamental knowledge of catalysts at the nanoscale. The advent of nanoscience and nanotechnology is providing the ability to create controlled structures and geometries to investigate and optimize a broad range of catalytic processes. As a result, researchers are obtaining fundamental insight into key features that influence the activity, selectivity, and lifetime of nanocatalysts. This review article examines several new findings as well as current challenges in the field of nanoparticle based catalysis, including the role played by the particle structure and morphology (size and shape), its chemical composition and oxidation state, and the effect of the cluster support.     

Productivity and mobility in academic research: evidence from mathematicians

Using an exhaustive database on academic publications in mathematics all over the world, we study the patterns of productivity by mathematicians over the period 1984–2006. We uncover some surprising facts, such as the weakness of age related decline in productivity and the relative symmetry of international movements, rejecting the presumption of a massive “brain drain” towards the US. We also analyze the determinants of success by top US departments. In conformity with recent studies in other fields, we find that selection effects are much stronger than local interaction effects: the best departments are most successful in hiring the most promising mathematicians, but not necessarily at stimulating positive externalities among them. Finally we analyze the impact of career choices by mathematicians: mobility almost always pays, but early specialization does not.     

Index determination of opaque and semitransparent metallic films: application to light absorbers

Index-determination methods based on reflectance and transmittance measurements are developed for both opaque and semitransparent metallic films. Results are given concerning chromium and nickel layers manufactured by electron-beam deposition. To take account of the evolution of the optical constants versus layer thickness, an inhomogeneous layer model is used, which permits us to obtain a good agreement between measurements and calculations. Results are applied to the design and manufacture of light absorbers for which accurate index knowledge is required. Measured absorption is higher than 0.999 on both broadband and monochromatic components.