Lattice Strain Relaxation and Compositional Control in As-Rich GaAsP/(100)GaAs Heterostructures Grown by MOVPE.

The fabrication of high-efficiency GaAsP-based solar cells on GaAs wafers requires addressing structural issues arising from the materials lattice mismatch. We report on tensile strain relaxation and composition control of MOVPE-grown As-rich GaAs1-xPx/(100)GaAs heterostructures studied by double-crystal X-ray diffraction and field emission scanning electron microscopy. Thin (80-150 nm) GaAs1-xPx epilayers appear partially relaxed (within 1-12% of the initial misfit) through a network of misfit dislocations along the sample [011] and [011-] in plane directions. Values of the residual lattice strain as a function of epilayer thickness were compared with predictions from the equilibrium (Matthews-Blakeslee) and energy balance models. It is shown that the epilayers relax at a slower rate than expected based on the equilibrium model, an effect ascribed to the existence of an energy barrier to the nucleation of new dislocations. The study of GaAs1-xPx composition as a function of the V-group precursors ratio in the vapor during growth allowed for the determination of the As/P anion segregation coefficient. The latter agrees with values reported in the literature for P-rich alloys grown using the same precursor combination. P-incorporation into nearly pseudomorphic heterostructures turns out to be kinetically activated, with an activation energy EA = 1.41 ± 0.04 eV over the entire alloy compositional range.

In situ growth of well-dispersed CdS nanocrystals in semiconducting polymers.

A straight synthetic route to fabricate hybrid nanocomposite films of well-dispersed CdS nanocrystals (NCs) in poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) is reported. A soluble cadmium complex [Cd(SBz)2]2·MI, obtained by incorporating a Lewis base (1-methylimidazole, MI) on the cadmium bis(benzyl)thiol, is used as starting reagent in an in situ thermolytic process. CdS NCs with spherical shape nucleate and grow well below 200°C in a relatively short time (30 min). Photoluminescence spectroscopy measurements performed on CdS/MEH-PPV nanocomposites show that CdS photoluminescence peaks are totally quenched inside MEH-PPV, if compared to CdS/PMMA nanocomposites, as expected due to overlapping of the polymer absorption and CdS emission spectra. The CdS NCs are well-dispersed in size and homogeneously distributed within MEH-PPV matrix as proved by transmission electron microscopy. Nanocomposites with different precursor/polymer weight ratios were prepared in the range from 1:4 to 4:1. Highly dense materials, without NCs clustering, were obtained for a weight/weight ratio of 2:3 between precursor and polymer, making these nanocomposites particularly suitable for optoelectronic and solar energy conversion applications.

CdS-polymer nanocomposites and light-emitting fibers by in situ electron-beam synthesis and lithography.

A straightforward, electron-beam induced synthesis and patterning approach to the in situ generation of CdS nanocrystals in nanocomposite films and light-emitting electrospun nanofibers is used. Smartly combining room-temperature nanoimprinting, electrospinning, and electron-beam decomposition of nanocrystal precursors and subsequent nucleation of nanoparticles in a polymer matrix allows exploitation of the most favorable flow conditions of organics to produce various nanocomposite nanostructures.

Light-emitting nanocomposite CdS-polymer electrospun fibres via in situ nanoparticle generation.

We report on the simple, in situ generation of CdS nanocrystals inside electrospun polymer fibres by thermal decomposition of a cadmium thiolate precursor, leading to nanocomposite light-emitting fibres. The modifications induced in the precursor by the thermal decomposition are investigated by a morphological, structural and spectroscopic analysis of the resulting nanocomposite fibres. This approach allows us to overcome nanofabrication difficulties related to disfavoured micro- or nanofluidic molecular flow as given by the direct incorporation of particles in the electrospinning solution. This method therefore enables the synthesis of luminescent, CdS-based composite fibres with emission peaked in the visible range, suitable as building blocks for nanophotonic devices based on light-emitting nanomaterials.

Mono- and bi-functional arenethiols as surfactants for gold nanoparticles: synthesis and characterization.

Stable gold nanoparticles stabilized by different mono and bi-functional arenethiols, namely, benzylthiol and 1,4-benzenedimethanethiol, have been prepared by using a modified Brust's two-phase synthesis. The size, shape, and crystalline structure of the gold nanoparticles have been determined by high-resolution electron microscopy and full-pattern X-ray powder diffraction analyses. Nanocrystals diameters have been tuned in the range 2 ÷ 9 nm by a proper variation of Au/S molar ratio. The chemical composition of gold nanoparticles and their interaction with thiols have been investigated by X-ray photoelectron spectroscopy. In particular, the formation of networks has been observed with interconnected gold nanoparticles containing 1,4-benzenedimethanethiol as ligand.

The effect of polymer matrices in the in-situ CdS formation under UV irradiation of precursor-polymer films.

Polymer films embedding cadmium thiolate precursors have been irradiated with ultraviolet laser pulses resulting in the formation of cadmium sulfide crystalline nanoparticles through a macroscopically non-destructive procedure for the host matrix. Controlling the number of the incident laser pulses, the gradual increase of the size of the nanoparticles is accomplished, and consequently the progressive change of the emission characteristics of the formed nanocomposites. The X-ray diffraction and transmission electron microscopy measurements were used for the full characterization of the nanoparticles. This study compares two polymer matrices, poly(methyl methacrylate) and a cyclic olefin copolymer, and reveals the importance of each one for the emission characteristics of the formed cadmium sulfide nanocrystals. It is found that the poly(methyl methacrylate) matrix contributes to the increase of the trap states on the surface of the formed nanocrystals, causing the broadening of their emission. On the other hand the cadmium sulfide nanoparticles, grown into the cyclic olefin copolymer matrix, exhibit narrower emission spectra.

Synthesis and Microstructural Investigations of Organometallic Pd(II) Thiol-Gold Nanoparticles Hybrids.

In this work the synthesis and characterization of gold nanoparticles functionalized by a novel thiol-organometallic complex containing Pd(II) centers is presented. Pd(II) thiol, trans, trans-[dithiolate-dibis(tributylphosphine)dipalladium(II)-4,4'-diethynylbiphenyl] was synthesized and linked to Au nanoparticles by the chemical reduction of a metal salt precursor. The new hybrid made of organometallic Pd(II) thiol-gold nanoparticles, shows through a single S bridge a direct link between Pd(II) and Au nanoparticles. The size-control of the Au nanoparticles (diameter range 2-10 nm) was achieved by choosing the suitable AuCl(4) (-)/thiol molar ratio. The size, strain, shape, and crystalline structure of these functionalized nanoparticles were determined by a full-pattern X-ray powder diffraction analysis, high-resolution TEM, and X-ray photoelectron spectroscopy. Photoluminescence spectroscopy measurements of the hybrid system show emission peaks at 418 and 440 nm. The hybrid was exposed to gaseous NO(x) with the aim to evaluate the suitability for applications in sensor devices; XPS measurements permitted to ascertain and investigate the hybrid -gas interaction.

Controlled nucleation and growth of CdS nanoparticles in a polymer matrix.

In-situ synchrotron X-ray diffraction (XRD) was used to monitor the thermal decomposition (thermolysis) of Cd thiolates precursors embedded in a polymer matrix and the nucleation of CdS nanoparticles. A thiolate precursor/polymer solid foil was heated to 300 degrees C in the X-ray diffraction setup of beamline W1.1 at Hasylab, and the diffraction curves were each recorded at 10 degrees C. At temperatures above 240 degrees C, the precursor decomposition is complete and CdS nanoparticles grow within the polymer matrix forming a nanocomposite with interesting optical properties. The nanoparticle structural properties (size and crystal structure) depend on the annealing temperature. Transmission electron microscopy (TEM) and photoluminescence (PL) analyses were used to characterize the nanoparticles. A possible mechanism driving the structural transformation of the precursor is inferred from the diffraction features arising at the different temperatures.