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Here, we report on the growth, electronic, and surface properties of the electron-doped half-Heusler series Co1-xNixTiSb (001) grown by molecular beam epitaxy. High-quality epitaxial growth of thin films is achieved on InP (001) substrates using an InAlAs buffer layer for all nickel concentrations. The semiconductor to metal transition as a function of substitutional alloying was examined using electrical transport, Seebeck measurements, and angle-resolved photoemission spectroscopy (ARPES). Temperature-dependent electrical transport measurements of films with composition x ≤ 0.1 exhibit thermally activated behavior while x > 0.1 exhibit metallic behavior. Smooth, highly ordered film surfaces can be achieved following ex-situ transfer of the films and subsequent desorption of a sacrificial, protective antimony capping layer. Using this transfer technique, ARPES experiments were performed to investigate the effects of nickel alloying on the electronic band structure. An electron pocket is observed below the Fermi level at the bulk X point for compositions x > 0.1, in accordance with the crossover from semiconducting to metallic behavior observed in the transport measurements. [ABSTRACT FROM AUTHOR]

Organic-shell-free PbS nanoparticles have been produced in the size range relevant for quantum-dot solar cells (QDSCs) by a vapor aggregation method involving magnetron reactive sputtering. This method creates a beam of free 5–10 nm particles in a vacuum. The dimensions of the particles were estimated after their deposition on a substrate by imaging them using ex situ SEM and HRTEM electron microscopy. The particle structure and chemical composition could be deduced “on the fly”, prior to deposition, using X-ray photoelectron spectroscopy (XPS) with tunable synchrotron radiation. Our XPS results suggest that under certain conditions it is possible to fabricate particles with a semiconductor core and 1 to 2 monolayer shells of metallic lead. For this case the absolute energy of the highest occupied molecular orbital (HOMO) in PbS has been determined to be (5.0 ± 0.5) eV below the vacuum level. For such particles deposited on a substrate HRTEM has confirmed the XPS-based conclusions on the crystalline PbS structure of the semiconductor core. Absorption spectroscopy on the deposited film has given a value of ∼1 eV for the lowest exciton. Together with the valence XPS results this has allowed us to reconstruct the energy level scheme of the particles. The results obtained are discussed in the context of the properties of PbS QDSCs. [ABSTRACT FROM AUTHOR]

Vascularization of bioprinted constructs to ensure sufficient nutrient supply still remains to be a significant task in the tissue engineering community. In order to mimic functional tissue, it is necessary to be able to print vessels in various size scales, which places particularly high demands on the 3D printing technology and materials. In this preliminary study, we focused on the production of small hollow structures for the application in small functional units of living tissue. To fabricate hollow structures, the freeform reversible embedding of suspended hydrogels (FRESH) - method was utilized (Hinton et al.). A sodium alginate solution (5 % w/v) was used as a bioink. The scaffolds were fabricated with the Allevi 1 (Allevi Inc., PA, USA), a pneumatic extrusion-based bioprinter and plotted into a gelatine slurry serving as fugitive support. For first cell experiments, the bioink was loaded with immortalized mouse HL1-cells. A proof of concept could be shown since we were able to reliably create vessel-like structures with an inside diameter of 1.2 to 1.6 mm, a length of up to 8 mm and a wall thickness of 0.4 to 0.6 mm. In this study, the geometric requirements to print hollow structures for small functional tissues could be achieved. To expand the field of applications the resolution of the printing process has to be further improved. Moreover, the cell density should be increased to reach physiological cell numbers and extended with endothelial cells.

We develop a super-saturation technique to extend the previously established doping density limit for ultra-high vacuum monolayer doping of silicon with phosphorus. Through an optimized sequence of PH3 dosing and annealing of the silicon surface, we demonstrate a 2D free carrier density of ns=(3.6±0.1)×1014 cm-2, ~50% higher than previously reported values. We perform extensive characterization of the dopant layer resistivity, including room temperature depth-dependent in situ four point probe measurements. The dopant layers remain conductive at less than 1 nm from the sample surface and importantly, surpass the semiconductor industry target for ultra-shallow junction scaling of <900...-1 at a depth of 7 nm. [ABSTRACT FROM AUTHOR]

We present angle-resolved photoemission spectroscopy measurements of the surface states on in-situ grown (111) oriented films of Pb1-xSnxSe, a three-dimensional topological crystalline insulator. We observe surface states with Dirac-like dispersion at ̄Γ and ̄M in the surface Brillouin zone, supporting recent theoretical predictions for this family of materials. We study the parallel dispersion isotropy and Dirac-point binding energy of the surface states, and perform tight-binding calculations to support our findings. The relative simplicity of the growth technique is encouraging, and suggests a clear path for future investigations into the role of strain, vicinality, and alternative surface orientations in (Pb,Sn)Se solid solutions. [ABSTRACT FROM AUTHOR]

Abstract: Thin films of CaF2 co-doped with low concentrations of Eu and Sm ions were grown by pulsed laser deposition (PLD) using a KrF (λ=248nm) as the ablation source. To the best of our knowledge, the work presented here is the first report of rare-earth-doped CaF2 films grown by PLD with this source. Combined laser excitation-emission spectroscopy was used to map out electronic transitions of Eu3+ with 7F0→5D1 excitation and the 5D0→7F1 emission. At the low concentrations used here the crystal field center of cubic symmetry is dominant in the films that are same for laser targets. However, charge compensated centers are present in the bulk crystal precursor. The removal of the charge compensated centers in the films and the target is likely caused by the target preparation where high pressure and temperature were applied. [Copyright &y& Elsevier]

The deep understanding of cancer and tumor genesis, as well as the development of new therapy strategies still remains one of the emerging challenges in modern medicine. To meet these challenges it seems to be absolutely necessary to overcome the drawbacks of the established 2D in vitro models. Especially the missing microenvironment of the tumor, which means the absence of stroma and immune cells, results in a missing cell-cell and cell-stroma interaction as well as disrupted functional communication pathways. Modern 3D culture systems and 3D printing or rather bioprinting technologies attempt to solve this issue and aim to closely mimic natural tumor microenvironment. In this preliminary work we are going to present the first steps of establishing an artificial 3D tumor model utilising a low cost 3D printer. Therefore the printer had been modified with an open-source syringe pump to become a functional bioprinter using viscosity modulated alginate hydrogel. In the first attempts L929 mouse fibroblasts, which are an integral component of natural stroma, had been incorporated into the hydrogel matrix and printed into scaffolds. Subsequent to the printing process the scaffolds got ionically crosslinked with a 5% w/v aqueous solution of CaCl2 to become mechanically stable. After three days of cultivation viability testing had been performed by utilising FDG staining and PET CT to obtain a volumetric viability measurement. The viability imaging showed vital cells homogeneously distributed in the scaffold and therefore stands as an evidence for a working low cost bioprinting process and a successful first step for the development of an artificial 3D tumor model.

• Identification of structure-determining parameters and damage behavior. • Analytical description of the mechanical compressive response. • Influence of cubic unit cell parameters on SN-curves were established. • Failure in high cycle fatigue regime at stress levels above yield stresses. • Ductile damage of single struts are detected before layer collapses using micro CT. The topology-property relationship of porous structures, especially tested under quasi-static and cyclic loadings, is an essential factor for the design and safe application of porous structures in the medical context. Within the scope of this paper, the mechanical properties and damage behavior of additively manufactured porous structures were investigated in quasi-static and fatigue compression tests taking different designs of porous structures with cubic unit cells into account. The influence of structure-determining parameters on the quasi-static and fatigue properties were highlighted. For example, the mechanical properties were analytically described as a function of the porosity and the strut thickness. A global layer-by-layer damage behavior was observed both under quasi-static and fatigue lading. Moreover, a ductile local damage behavior was identified using the digital image correlation technique and the micro computed tomography. Under fatigue compression loading cubic porous structures mostly fail in the high cycle fatigue regime at stress levels above their compressive offset stresses, which is relevant for a damage-tolerant design. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) with different gate length and widths were irradiated with [sup 60]Co γ-rays to doses up to 600 Mrad. Little measurable change in dc performance of the devices was observed for doses lower than 300 Mrad. At the maximum dose employed, the forward gate current was significantly decreased, with an accompanying increase in reverse breakdown voltage. This is consistent with a decrease in effective carrier density in the channel as a result of the introduction of deep electron trapping states. The threshold voltage shifted to more negative voltages as a result of the irradiation, while the magnitude of the drain–source current was relatively unaffected. This is consistent with a strong increase of trap density in the material. The magnitude of the decrease in transconductance of the AlGaN/GaN HEMTs is roughly comparable to the decrease in dc current gain observed in InGaP/GaAs heterojunction bipolar transistors irradiated under similar conditions. © 2002 American Institute of Physics. [ABSTRACT FROM AUTHOR]

AlGaN/GaN high electron mobility transistors (HEMTs) with a range of gate lengths (0.8–1.2 μm) and widths (100–200 μm) were exposed to 40 MeV protons at fluences of 5×10[sup 9] or 5×10[sup 10] cm[sup -2]. The drain–source currents in the devices decreased by 15%–20% at the higher fluence, while the extrinsic transconductance decreased by ∼30% under the same conditions. Based on the increases in the reverse breakdown voltage and the channel resistance, the main degradation mechanism is believed to be creation of deep trap states in the band gap which remove electrons from the channel. The maximum frequency of oscillation, f[sub MAX], also decreased as a result of the proton-induced damage, with a change of -20% at the shorter gate widths and -50% at the largest widths. The reverse recovery switching time was essentially unaffected by the irradiation, remaining at ∼1.6×10[sup -8] s. Postradiation annealing at 800 °C was successful in restoring the dc and rf performance parameters to >=90% of their original values. The AlGaN/GaN HEMTs are much more robust than their AlGaAs/GaAs counterparts to displacement damage and appear well-suited to radiation environment applications. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The topological crystalline insulator tin telluride is known to host superconductivity when doped with indium (Sn1-xInxTe), and for low indium content (x=0.04) it is known that the topological surface states are preserved. Here we present the growth, characterization, and angle resolved photoemission spectroscopy analysis of samples with much heavier In doping (up to x≈0.4), a regime where the superconducting temperature is increased nearly fourfold. We demonstrate that despite strong p-type doping, Dirac-like surface states persist. [ABSTRACT FROM AUTHOR]

Abstract We examine nickel silicide as a viable ohmic contact metallization for low-temperature, low-magnetic-field transport measurements of atomic-scale devices in silicon. In particular, we compare a nickel silicide metallization with aluminium, a common ohmic contact for silicon devices. Nickel silicide can be formed at the low temperatures (

Athree-dimensional (3D) Dirac semimetal is a novel state of quantum matter which has recently attracted much attention as an apparent 3D version of graphene. In this paper, we report results on the electronic structure of the 3D Dirac semimetal Na3Bi at a surface that reveals its nontrivial ground state. Our studies reveal that the two 3D Dirac cones go through a topological change in the constant energy contour as a function of the binding energy, featuring a Lifshitz point, which is missing in a strict 3D analog of graphene. Our results identify an example of a band saddle-point singularity in 3D Dirac materials. This is in contrast to its two-dimensional analogs such as graphene and the Dirac surface states of a topological insulator. The observation of multiple Dirac nodes in Na3Bi connecting via a Lifshitz point along its crystalline rotational axis away from the Kramers point serves as a decisive signature for the symmetry-protected nature of the Dirac semimetal's topological bulk ground state. [ABSTRACT FROM AUTHOR]