Widely tunable GaAs bandgap via strain engineering in core/shell nanowires with large lattice mismatch.

The realisation of photonic devices for different energy ranges demands materials with different bandgaps, sometimes even within the same device. The optimal solution in terms of integration, device performance and device economics would be a simple material system with widely tunable bandgap and compatible with the mainstream silicon technology. Here, we show that gallium arsenide nanowires grown epitaxially on silicon substrates exhibit a sizeable reduction of their bandgap by up to 40% when overgrown with lattice-mismatched indium gallium arsenide or indium aluminium arsenide shells. Specifically, we demonstrate that the gallium arsenide core sustains unusually large tensile strain with hydrostatic character and its magnitude can be engineered via the composition and the thickness of the shell. The resulted bandgap reduction renders gallium arsenide nanowires suitable for photonic devices across the near-infrared range, including telecom photonics at 1.3 and potentially 1.55 µm, with the additional possibility of monolithic integration in silicon-CMOS chips.

Preparation of non-oxidized Ge quantum dot lattices in amorphous Al2O3, Si3N4 and SiC matrices.

The preparation of non-oxidized Ge quantum dot (QD) lattices embedded in Al2O3, Si3N4, SiC matrices by self-assembled growth was studied. The materials were produced by magnetron sputtering deposition, using different substrate temperatures. The deposition regimes leading to the self-assembled growth type and the formation of three-dimensionally ordered Ge QD lattices in different matrices were investigated and determined. The oxidation of the Ge QDs in different matrices was monitored and the best conditions for the production of non-oxidized Ge QDs were found. The optical properties of the Ge QD lattices in different matrices show a strong dependence on the Ge oxidation and the matrix type.

Percolated Si:SiO2 Nanocomposites: Oven- vs. Millisecond Laser-Induced Crystallization of SiOx Thin Films.

Three-dimensional nanocomposite networks consisting of percolated Si nanowires in a SiO2 matrix, Si:SiO2, were studied. The structures were obtained by reactive ion beam sputter deposition of SiOx (x ≈ 0.6) thin films at 450 ∘C and subsequent crystallization using conventional oven, as well as millisecond line focus laser treatment. Rutherford backscattering spectrometry, Raman spectroscopy, X-ray diffraction, cross-sectional and energy-filtered transmission electron microscopy were applied for sample characterization. While oven treatment resulted in a mean Si wire diameter of 10 nm and a crystallinity of 72% within the Si volume, almost single-domain Si structures of 30 nm in diameter and almost free of amorphous Si were obtained by millisecond laser application. The structural differences are attributed to the different crystallization processes: conventional oven tempering proceeds via solid state and millisecond laser application via liquid phase crystallization of Si. The five orders of magnitude larger diffusion constant in the liquid phase is responsible for the three-times larger Si nanostructure diameter. In conclusion, laser treatment offers not only significantly shorter process times, but moreover, a superior structural order of nano-Si compared to conventional heating.

Rapid Synthesis of Sub-10†nm Hexagonal NaYF4-Based Upconverting Nanoparticles using Therminol®â€ 66.

We report a simple one-pot method for the rapid preparation of sub-10 nm pure hexagonal (ß-phase) NaYF4-based upconverting nanoparticles (UCNPs). Using Therminol®â€…66 as a co-solvent, monodisperse UCNPs could be obtained in unusually short reaction times. By varying the reaction time and reaction temperature, it was possible to control precisely the particle size and crystalline phase of the UCNPs. The upconversion (UC) luminescence properties of the nanocrystals were tuned by varying the concentrations of the dopants (Nd3+ and Yb3+ sensitizer ions and Er3+ activator ions). The size and phase-purity of the as-synthesized core and core-shell nanocrystals were assessed by using complementary transmission electron microscopy, dynamic light scattering, X-ray diffraction, and small-angle X-ray scattering studies. In-depth photophysical evaluation of the UCNPs was pursued by using steady-state and time-resolved luminescence spectroscopy. An enhancement in the UC intensity was observed if the nanocrystals, doped with optimized concentrations of lanthanide sensitizer/activator ions, were further coated with an inert/active shell. This was attributed to the suppression of surface-related luminescence quenching effects.

Faceted nanostructure arrays with extreme regularity by self-assembly of vacancies.

Semiconductor quantum dots and wires are important building blocks for future electronic and optoelectronic devices. The common way of producing semiconductor nanostructures is by molecular beam epitaxy (MBE). In this additive growth process atoms are deposited onto crystalline surfaces and self-assemble into 3D structures. Here we present a subtractive process, in which surface vacancies are created by ion impacts. On terraces of crystalline surfaces their nucleation forms depressions which coarsen and finally lead to a self-organized 3D morphology. It is shown that this kind of spontaneous pattern formation is inherent to the ion induced erosion process on crystalline surfaces and is analogous to 3D growth by MBE. However, novel facets are found due to slightly different energetics and kinetics of ad-atoms and surface vacancies, especially at Ehrlich-Schwoebel step-edge barriers. Depending on the crystal orientation, three-fold, four-fold, six-fold symmetry, as well as extremely regular periodic nanogrooves can be produced on different orientations of group IV (Si, Ge) and III-V (GaAs, InAs) semiconductors.

Solution processable organic field-effect transistors utilizing an alpha,alpha'-dihexylpentathiophene-based swivel cruciform.

A pentathiophene-based swivel cruciform, which allows rotation between the cruciform arms, was synthesized. Homogeneous microcrystalline films were processed from solution, and field-effect transistors utilizing this dimer gave hole mobilities up to 0.012 cm2/V.s.