ZnAl2O4 decorated Al-doped ZnO tetrapodal 3D networks: microstructure, Raman and detailed temperature dependent photoluminescence analysis.

3D networks of Al-doped ZnO tetrapods decorated with ZnAl2O4 particles synthesised by the flame transport method were investigated in detail using optical techniques combined with morphological/structural characterisation. Low temperature photoluminescence (PL) measurements revealed spectra dominated by near band edge (NBE) recombination in the UV region, together with broad visible bands whose peak positions shift depending on the ZnO : Al mixing ratios. A close inspection of the NBE region evidences the effective doping of the ZnO structures with Al, as corroborated by the broadening and shift of its peak position towards the expected energy associated with the exciton bound to Al. Both temperature and excitation density-dependent PL results pointed to an overlap of multiple optical centres contributing to the broad visible band, with the peak position dependent on the Al content. While in the reference sample the wavelength of the green band remained unchanged with temperature, in the case of the composites, the deep level emission showed a blue shift with increasing temperature, likely due to distinct thermal quenching of the overlapping emitting centres. This assumption was further validated by the time-resolved PL data, which clearly exposed the presence of more than one optical centre in this spectral region. PL excitation analysis demonstrated that the luminescence features of the Al-doped ZnO/ZnAl2O4 composites revealed noticeable changes not only in deep level recombination, but also in the material's bandgap when compared with the ZnO reference sample. At room temperature, the ZnO reference sample exhibited free exciton resonance at ∼3.29 eV, whereas the peak position for the Al-doped ZnO/ZnAl2O4 samples occurred at ∼3.38 eV due to the Burstein-Moss shift, commonly observed in heavily doped semiconductors. Considering the energy shift observed and assuming a parabolic conduction band, a carrier concentration of ∼1.82 ×1019 cm-3 was estimated for the Al-doped ZnO/ZnAl2O4 samples.

Probing surface states in C60 decorated ZnO microwires: detailed photoluminescence and cathodoluminescence investigations.

ZnO microwires synthesised by the flame transport method and decorated with C60 clusters were studied in detail by photoluminescence (PL) and cathodoluminescence (CL) techniques. The optical investigations suggest that the enhanced near band edge recombination observed in the ZnO/C60 composites is attributed to the reduction of the ZnO band tail states in the presence of C60. Well-resolved free and bound excitons recombination, as well as 3.31 eV emission, are observed with increasing amount of C60 flooding when compared with the ZnO reference sample. Moreover, a shift of the broad visible emission to lower energies occurs with increasing C60 content. In fact, this band was found to be composed by two optical centres peaked in the green and orange/red spectral regions, presenting different lifetimes. The orange/red band exhibits faster lifetime decay, in addition to a more pronounced shift to lower energies, while the peak position of the green emission only shows a slight change. The overall redshift of the broad visible band is further enhanced by the change in the relative intensity of the mentioned optical centres, depending on the excitation intensity and on the C60 flooding. These results suggest the possibility of controlling/tuning the visible emission outcome by increasing the C60 amount on the ZnO surface due to the surface states present in the semiconductor. An adequate control of such phenomena may have quite beneficial implications when sensing applications are envisaged.

Hierarchical Aerographite 3D flexible networks hybridized by InP micro/nanostructures for strain sensor applications.

In the present work, we report on development of three-dimensional flexible architectures consisting of an extremely porous three-dimensional Aerographite (AG) backbone decorated by InP micro/nanocrystallites grown by a single step hydride vapor phase epitaxy process. The systematic investigation of the hybrid materials by scanning electron microscopy demonstrates a rather uniform spatial distribution of InP crystallites without agglomeration on the surface of Aerographite microtubular structures. X-ray diffraction, transmission electron microscopy and Raman scattering analysis demonstrate that InP crystallites grown on bare Aerographite are of zincblende structure, while a preliminary functionalization of the Aerographite backbone with Au nanodots promotes the formation of crystalline In2O3 nanowires as well as gold-indium oxide core-shell nanostructures. The electromechanical properties of the hybrid AG-InP composite material are shown to be better than those of previously reported bare AG and AG-GaN networks. Robustness, elastic behavior and excellent translation of the mechanical deformation to variations in electrical conductivity highlight the prospects of AG-InP applications in tactile/strain sensors and other device structures related to flexible electronics.