Title: ﾇﾐﾀﾇﾎﾊ ﾎﾔﾎﾐﾌﾋﾅﾍﾍﾟ ﾒﾅﾇ ﾄﾎﾏﾎﾂｲﾄｲ
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Nanocomposites and nanomaterials
Spectroscopy of Er^3+ ions in erbium-stabilized nanocrystalline zirconium 

R. Francini^1, A. Zolotovsky^2, Z. Zhuchenko^2, G. G. Tarasov^2

^1 Dipartimento di Fisica and Istituto Nazionale di Fisica della Materia, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, I-00133 Roma, Italy 
^2 Institute of Semiconductor Physics, National Academy of Sciences of Ukraine,
Prospect Nauki 45, 03028 Kiev, Ukraine  
E-mail: tarasov@isp.kiev.ua

Erbium doped zirconium nanocrystals are recognized to be a very promising material for numerous photonic applications. It combines the advantages of fast oxygen conductor with the rich energy level structure of trivalent erbium ions and the photoluminescence (PL) efficiency enhanced due to small dimensions of nanostructures. We report the near-infrared and visible up-conversion spectra of nanocrystalline (Er_2O_3)_x(ZrO_2)_1-x prepared by the sol-gel methods. Three Er_2O_3- stabilized ZrO_2 samples ((Er_2O_3)_x(ZrO_2)_1-x with composition x= 0.05, 0.10, and 0.10) have been grown by precipitation with ammonia solution of Ziconyl chloride octahydrate (98 % Aldrich) and erbium chloride hexahydrate (99.9 % Aldrich).  The resulting hydrous compounds were calcined in flowing air at 1000°C. The samples were characterized by XRD Rietveld analysis. The results of TEM, EDX and XRD analysis confirm coexistence of various phases: monoclinic/tetragonal, tetragonal/cubic and purely tetragonal. Thus one can expect difference in the PL and up-conversion PL spectra. The spectra are studied at various temperatures in order to detect the influence of phonon subsystem on the intra-J-manifold relaxation in the Er^3+ ions. 
It is shown that for the samples with 0.05≤ x≤ 0.10 the photoluminescence spectra of Er^3+ are defined by similar erbium environment in spite of different symmetry of the host ZrO_2 matrix. The nature of low temperature hot band revealed in the ^4I_13/2 → ^4I_15/2 spectrum  is interpreted in terms of the intra-manifold relaxation caused by the spatial confinement effects in erbium-stabilized nanocrystalline zirconium.  The phonon properties of the three low pressure (cubic, tetragonal, and especially monoclinic) phases of ZrO_2 are different and different are the related dielectric properties. It is shown that the confinement effects can also affect the spectroscopic properties of rare earth ions in nanocrystals due to cutting the low-energy phonon modes in the 20-40 nm crystals. As result the phonon assisted relaxation for electrons can be restricted leading to a rapid increase of the intensity of hot bands that originate from the upper crystal field levels in the ^4I_15/2 ground state of Er^3+ ions as temperature decreases below 8 K. These factors, together with many others (like pumping regime, excitation wavelength and power density, dopant concentration, temperature and radiative lifetimes of the Er^3+ excited states) result in the complex up-conversion emissions observed in nano crystallites doped with Er^3+. For theoretical analysis
the tetragonal phase was modeled by Zr_6O_12 particles, in which one of the Zr^4 ^+ ions is replaced by an Er^3 ^+ ion. The effect of an uncompensated charge was taken into account by introducing a compensation potential. The multi-electron molecular orbitals in the cluster-like approximation were used for calculation of the transition probabilities. The cubic and monoclinic phases were modeled by Zr_19O_38 particles, in which one of the Zr^4 ^+ ions was replaced by the Er^3 ^+ ion. The oxygen atoms around erbium were located in accordance with the parameters of the monoclinic phase obtained for the samples from the XRD analysis. For comparison with experiment, the probabilities of the corresponding phases with coefficients equal to the products of the concentration of the corresponding phase and the molar concentration of Er_2O_3 in this phase for each sample were added. We obtained good correlation between calculated and measured spectra for the ^4I_13 _/ _2 → ^4I_15 _/ _2 transition.