Nanocomposites and nanomaterials
Optical properties of rare-earth ions in inorganic nanocrystals 
R. Francini1, A. Zolotovsky2, Z. Zhuchenko2, G. G. Tarasov2
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: zhuchenko@isp.kiev.ua

Recently inorganic nanocrystals activated by rare-earth ions (RE3+) are in focus of research due to their potential applications as laser materials, up-converting materials, luminophores, as well as biomedical materials. Small sizes of particles result in their high sensitivity and selectivity for external radiation mainly due to quantum effects that are a result of the increasing ratio of surface to volume atoms in low-dimensional systems. Transition to quantum effects allows the design and fabrication of nanocomponents with new functionalities and characteristics for the improvement of existing materials. Thus fluorescence imaging in the second near infrared window (NIR-II) is widely used in cancer diagnosis and treatment due to its high spatial resolution and deep tissue penetration depths. In this sense rare-earth doped nanoparticles are promising candidates for NIR-II biomedical imaging due to their low toxicity, high photostability, deep tissue penetration, and tunable pharmacokinetic behavior. Despite these successful gains, challenges still remain in the bioimaging applications of NIR-II rare-earth nanoparticles. One of them is the limitation of the emission centers. Indeed, they are still mainly Nd3+ and Er3+ that greatly limits the development and application of near-infrared probe types. In spite of this properties even of these centers are not yet completely studied. 
In this report   peculiarities of the near-infrared and visible up-conversion spectra of nanocrystalline (Er2O3)x(ZrO2)1-x prepared by the sol-gel methods have been studied. It is shown that the photoluminescence spectra of Er3+ are defined by similar erbium environment in spite of different symmetry of the host ZrO2 matrix. It is shown in Fig.1

 Fig.1 Room temperature 4I13/2  4I15/2 spectra of nanocrystalline (Er2O3)x(ZrO2)1-x for the samples with different crystalline symmetries and different nanosizes.

 The spectra are fairly similar. We calculated energy levels for the 4I13/2 and 4I15/2 manifolds and explain the peculiarities of intra- and inter manifold relaxation for different temperatures.