Local phonon imaging of AlN nanostructures with nanoscale spatial resolution.

We demonstrate local phonon analysis of single AlN nanocrystals by two complementary imaging spectroscopic techniques: tip-enhanced Raman scattering (TERS) and nano-Fourier transform infrared (nano-FTIR) spectroscopy. Strong surface optical (SO) phonon modes appear in the TERS spectra with their intensities revealing a weak polarization dependence. The local electric field enhancement stemming from the plasmon mode of the TERS tip modifies the phonon response of the sample, making the SO mode dominate over other phonon modes. The TERS imaging allows the spatial localization of the SO mode to be visualized. We were able to probe the angle anisotropy on the SO phonon modes in AlN nanocrystals with nanoscale spatial resolution. The excitation geometry and the local nanostructure surface profile determine the frequency position of SO modes in nano-FTIR spectra. An analytical calculation explains the behaviour of SO mode frequencies vs. tip position with respect to the sample.

Crystal Structure and Predominant Defects in CdS Quantum Dots Fabricated by the Langmuir-Blodgett Method.

The crystal structure and shape of the CdS quantum dots (QDs) obtained by the Langmuir-Blodgett method were studied by transmission electron microscopy, extended X-ray absorption fine structure spectroscopy (EXAFS), and ultraviolet spectroscopy. X-ray photoelectron spectroscopy (XPS) and stationary photoluminescence spectroscopy (PL) methods were used to determine the dominant surface defects. Initially synthesized QDs within the Langmuir-Blodgett film of fatty behenic acid have a cubic structure and oblate spheroid shape, while free-standing QDs obtained after the matrix evaporation have a wurtzite structure and sphere-like shape. QDs within the matrix demonstrate a wide PL band centered at 2.3 eV corresponding to defect-assisted radiative recombination; after the matrix annealing and passivation of the QD surface in an ammonia atmosphere, the PL spectrum demonstrates a high-intensity band-edge peak together with a low-intensity defect-assisted shoulder. It was established that sulfur (VS) vacancies are the dominating defects. A model of simultaneous band-edge and defect-assisted recombination through the VS level was proposed.

Luminescence line shapes of band to deep centre and donor-acceptor transitions in AlN.

Energy structure and electron coupling with local lattice vibrations have been investigated for deep centres in AlN using hybrid functional density functional theory. Local phonon energies and Huang-Rhys parameters have been calculated for defects and defect complexes containing most common unintentional impurities of carbon, oxygen and silicon, and for intrinsic vacancies, nitrogen split-interstitial defect, and complexes of Al and N vacancies in AlN. Luminescence line shapes of band to deep centre transitions in AlN have been calculated in dependence on temperature for most abundant defects in AlN. Donor-acceptor luminescence line shapes for shallow donor to deep acceptor and deep donor to deep acceptor transitions have been considered theoretically. Configuration diagrams of oxygen and silicon DX centres have been calculated by density functional theory with hybrid functional, and peak energies of optical transitions of an electron from the DX-centres to deep acceptors have been estimated. Possible assignments of the experimental luminescence bands in AlN based on the calculations have been discussed.