Comparative electron diffraction study of the diamond nucleation layer on Ir(001)

The carbon layer formed during the bias enhanced nucleation (BEN) procedure on iridium has been studied by different electron diffraction techniques. In reflection high energy electron diffraction (RHEED) and low energy electron diffraction (LEED) the carbon nucleation layer does not give any indication of crystalline diamond even if the presence of domains proves successful nucleation. In contrast, X-ray photoelectron diffraction (XPD) shows a clear C 1s pattern when domains are present after BEN. The anisotropy in the Ir XPD patterns is reduced after BEN while the fine structure is essentially identical compared to a single crystal Ir film. The change in the Ir XPD patterns after BEN can be explained by the carbon layer on top of a crystallographically unmodified Ir film. The loss and change in the fine structure of the C 1s patterns as compared to a single crystal diamond film are discussed in terms of mosaicity and a defective structure of the ordered fraction within the carbon layer. The present results suggest that the real structure of the BEN layer is more complex than a pure composition of small but perfect diamond crystallites embedded in an amorphous matrix.     

Tuning light emission of PbS nanocrystals from infrared to visible range by cation exchange

Colloidal semiconductor nanocrystals, with intense and sharp-line emission between red and near-infrared spectral regions, are of great interest for optoelectronic and bio-imaging applications. The growth of an inorganic passivation layer on nanocrystal surfaces is a common strategy to improve their chemical and optical stability and their photoluminescence quantum yield. In particular, cation exchange is a suitable approach for shell growth at the expense of the nanocrystal core size. Here, the cation exchange process is used to promote the formation of a CdS passivation layer on the surface of very small PbS nanocrystals (2.3 nm in diameter), blue shifting their optical spectra and yielding luminescent and stable nanostructures emitting in the range of 700–850 nm. Structural, morphological and compositional investigation confirms the nanocrystal size contraction after the cation-exchange process, while the PbS rock-salt crystalline phase is retained. Absorption and photoluminescence spectroscopy demonstrate the growth of a passivation layer with a decrease of the PbS core size, as inferred by the blue-shift of the excitonic peaks. The surface passivation strongly increases the photoluminescence intensity and the excited state lifetime. In addition, the nanocrystals reveal increased stability against oxidation over time. Thanks to their absorption and emission spectral range and the slow recombination dynamics, such highly luminescent nano-objects can find interesting applications in sensitized photovoltaic cells and light-emitting devices.     

Evidence for the Band‐Edge Exciton of CuInS2 Nanocrystals Enables Record Efficient Large‐Area Luminescent Solar Concentrators

Ternary I‐III‐VI₂ nanocrystals (NCs), such as CuInS₂, are receiving attention as heavy‐metals‐free materials for solar cells, luminescent solar concentrators (LSCs), LEDs, and bio‐imaging. The origin of the optical properties of CuInS₂ NCs are however not fully understood. A recent theoretical model suggests that their characteristic Stokes‐shifted and long‐lived luminescence arises from the structure of the valence band (VB) and predicts distinctive optical behaviours in defect‐free NCs: the quadratic dependence of the radiative decay rate and the Stokes shift on the NC radius. If confirmed, this would have crucial implications for LSCs as the solar spectral coverage ensured by low‐bandgap NCs would be accompanied by increased re‐absorption losses. Here, by studying stoichiometric CuInS₂ NCs, it is revealed for the first time the spectroscopic signatures predicted for the free band‐edge exciton, thus supporting the VB‐structure model. At very low temperatures, the NCs also show dark‐state emission likely originating from enhanced electron‐hole spin interaction. The impact of the observed optical behaviours on LSCs is evaluated by Monte Carlo ray‐tracing simulations. Based on the emerging device design guidelines, optical‐grade large‐area (30×30 cm²) LSCs with optical power efficiency (OPE) as high as 6.8% are fabricated, corresponding to the highest value reported to date for large‐area devices.     

A Nyström method for a class of Fredholm integral equations on the real semiaxis

A class of Fredholm integral equations of the second kind, with respect to the exponential weight function \(w(x)=\exp (-(x^{-\alpha }+x^\beta ))\), \(\alpha >0\), \(\beta >1\), on \((0,+\infty )\), is considered. The kernel k(x, y) and the function g(x) in such kind of equations,

$$\begin{aligned} f(x)-\mu \int _0^{+\infty }k(x,y)f(y)w(y)\mathrm {d}y =g(x),\quad x\in (0,+\infty ), \end{aligned}$$

can grow exponentially with respect to their arguments, when they approach to \(0^+\) and/or \(+\infty \). We propose a simple and suitable Nyström-type method for solving these equations. The study of the stability and the convergence of this numerical method in based on our results on weighted polynomial approximation and “truncated” Gaussian rules, recently published in Mastroianni and Notarangelo (Acta Math Hung, 142:167–198, 2014), and Mastroianni, Milovanovi? and Notarangelo (IMA J Numer Anal 34:1654–1685, 2014) respectively. Moreover, we prove a priori error estimates and give some numerical examples. A comparison with other Nyström methods is also included.

     

A note on Hermite–Fejér interpolation at Laguerre zeros

In order to approximate functions defined on the real semiaxis, we introduce a new operator of Hermite–Fejér-type based on Laguerre zeros and prove its convergence in weighted uniform metric.     

Preparation of 4-inch Ir/YSZ/Si(001) substrates for the large-area deposition of single-crystal diamond

Diamond/Ir/YSZ/Si(001) is currently the most promising multilayer structure for the future realisation of large-area diamond single crystals. A decisive key is the preparation of the iridium layers on silicon. It is shown in this work that high quality iridium films with mosaic spread below 0.2° can be grown on oxide buffer layers with a mosaic spread higher than 1°. An averaging process during the coalescence of the iridium islands provides a plausible mechanism for this phenomenon. The oxide buffer and the iridium overlayers can be grown homogeneously on 4-inch wafers in a similar quality as for 1 × 1 cm² samples. Bias enhanced nucleation followed by 40 h growth on the large-area Ir/YSZ/Si(001) wafers yields diamond films with a mosaicity of 0.16° (tilt) and 0.34° (twist). For a further increase of the area of heteroepitaxial diamond nucleation the homogeneity of the plasma discharge has to be improved.     

Transmission electron microscopy study of the very early stages of diamond growth on iridium

The diamond nuclei generated during bias enhanced nucleation (BEN) on iridium were not detected so far by high resolution transmission electron microscopy (HRTEM). The aim of the present work was to investigate their earliest appearance after BEN by applying very short growth steps, ranging from 5 s to 1 min. On all the samples with growth step crystalline diamond could be identified unequivocally by HRTEM and reflection high energy electron diffraction (RHEED). After 5 s the former nuclei have evolved into crystallites of 2 nm thickness and about 10 nm width. At that time the carbon precursor phase which appears amorphous in HRTEM and which was formed by the ion bombardment has largely disappeared. After 10 s no residues are left, which proves that the 1–2 nm-thick amorphous carbon layer is only stable under biasing conditions. The rapid etching of the precursor phase and the simultaneous slow increase in volume of the tiny diamond crystals results in a minimum in carbon coverage several seconds after termination of the BEN process.     

Selective Capture of Anti-N-glucosylated NTHi Adhesin Peptide Antibodies by a Multivalent Dextran Conjugate

Tentacle-like polymers decorated with several copies of peptide antigens can be interesting tools for increasing the ability to capture circulating antibodies in patient sera, using cooperative effects for stronger avidity. We previously showed that antibodies from multiple sclerosis (MS) patient sera preferentially recognize hyperglucosylated adhesin protein HMW1ct of non-typeable Haemophilus influenzae (NTHi). We selected the C-terminal HMW1ct(1347–1354) minimal epitope and prepared the diglucosylated analogue Ac-KAN(Glc)VTLN(Glc)TTG-K(N₃)-NH₂ to graft a 40 kDa dextran scaffold modified with glycidyl-propargyl moieties to perform a copper catalyzed alkyne-azide coupling reaction (CuAAC). Quantitative NMR measurements allowed the characterization of the peptide loading (19.5 %) on the multivalent dextran conjugate. This novel polymeric structure displayed optimal capturing properties of both IgG and, more interestingly, IgM antibodies in MS sera. Specific antibodies from a representative MS serum, were successfully depleted using a Sepharose resin bearing the new glucosylated multivalent conjugate, as confirmed by ELISA. These results may offer a promising proof-of-concept for the selective purification of high affinity autoantibodies from sera of autoimmune patients, in general, and of specific high affinity antibodies against a minimally glcosylated epitope Asn(Glc) from sera of multiple sclerosis (MS) patients, in particular.     

Extending the Colloidal Transition Metal Dichalcogenide Library to ReS2 Nanosheets for Application in Gas Sensing and Electrocatalysis

Among the large family of transition metal dichalcogenides, recently ReS₂ has stood out due to its nearly layer‐independent optoelectronic and physicochemical properties related to its 1T distorted octahedral structure. This structure leads to strong in‐plane anisotropy, and the presence of active sites at its surface makes ReS₂ interesting for gas sensing and catalysts applications. However, current fabrication methods use chemical or physical vapor deposition (CVD or PVD) processes that are costly, time‐consuming and complex, therefore limiting its large‐scale production and exploitation. To address this issue, a colloidal synthesis approach is developed, which allows the production of ReS₂ at temperatures below 360 °C and with reaction times shorter than 2h. By combining the solution‐based synthesis with surface functionalization strategies, the feasibility of colloidal ReS₂ nanosheet films for sensing different gases is demonstrated with highly competitive performance in comparison with devices built with CVD‐grown ReS₂ and MoS₂. In addition, the integration of the ReS₂ nanosheet films in assemblies together with carbon nanotubes allows to fabricate electrodes for electrocatalysis for H₂ production in both acid and alkaline conditions. Results from proof‐of‐principle devices show an electrocatalytic overpotential competitive with devices based on ReS₂ produced by CVD, and even with MoS₂, WS₂, and MoSe₂ electrocatalysts.