Optical lanthanide ions

Let us now consider MMCT for the case in which the donating ion is a lanthanide ion with a partly filled 4/ shell M(/")M(d°)CT. The trivalent lanthanide ions with a low fourth ionization potential are Ce, Pr ", Tb ". Their optical absorption spectra show usually allowed 4f-5d transitions in the ultraviolet part of the spectrum [6, 35]. These are considered as MC transitions, although they will undoubtedly have a certain CT character due to the higher admixture of ligand orbitals into the d orbitals. In combination with M(d°) ions these M(/") ions show MMCT transitions. An early example has been given by Paul [36] for Ce(III)-Ti(IV) MMCT in borosilicate glasses. The absorption maximum was at about 30000 cm ... 

In view of this the divalent lanthanide ions are expected to show intense optical absorption in the whole visible region in compounds containing M(d°) ions. Since these compounds are not easy to prepare, our earlier warning is relevant. Only results on well-prepared and characterized compounds may be... 

The other notable feature of the optical spectroscopy of the lanthanide ions is that their absorption bands generally have very low extinction coefficients, because f-f transitions are... 

Stevens formalism turned out to be very powerful, and works easily as long as only the ground 2S+1Lj multiplet ofthe lanthanide ion is considered. As such, it has been widely used in studies on EPR properties of lanthanide-based inorganic systems [6, 22], while it is not well suited for optical spectroscopy. Indeed, when starting to include excited multiplets the Stevens formalism becomes much too involved. This is the reason why a more general formalism, developed by Wybourne [3], is of widespread use in optical studies - naturally dealing with excited multiplets - and... 

The lanthanides have electrons in partly filled 4/orbitals. Many lanthanides show colors due to electron transitions involving the 4/orbitals. However, there is a considerable difference between the lanthanides and the 3d transition-metal ions. The 4/ electrons in the lanthanides are well shielded beneath an outer electron configuration, (5.v2 5p6 6s2) and are little influenced by the crystal surroundings. Hence the important optical and magnetic properties attributed to the 4/ electrons on any particular lanthanide ion are rather unvarying and do not depend significantly upon the host structure. Moreover, the energy levels are sharper than those of transition-metal ions and the spectra resemble those of free ions. 

The rare earth (RE) ions most commonly used for applications as phosphors, lasers, and amplifiers are the so-called lanthanide ions. Lanthanide ions are formed by ionization of a nnmber of atoms located in periodic table after lanthanum from the cerium atom (atomic number 58), which has an onter electronic configuration 5s 5p 5d 4f 6s, to the ytterbium atom (atomic number 70), with an outer electronic configuration 5s 5p 4f " 6s. These atoms are nsnally incorporated in crystals as divalent or trivalent cations. In trivalent ions 5d, 6s, and some 4f electrons are removed and so (RE) + ions deal with transitions between electronic energy sublevels of the 4f" electroiuc configuration. Divalent lanthanide ions contain one more f electron (for instance, the Eu + ion has the same electronic configuration as the Gd + ion, the next element in the periodic table) but, at variance with trivalent ions, they tand use to show f d interconfigurational optical transitions. This aspect leads to quite different spectroscopic properties between divalent and trivalent ions, and so we will discuss them separately. 

Trivalent lanthanide ions have an outer electronic configuration 5s 5p 4f", where n varies from 1 (Ce +) to 13 (Yb +) and indicates the number of electrons in the unfilled 4f shell. The 4f" electrons are, in fact, the valence electrons that are responsible for the optical transitions. 

Lanthanides activated luminescent materials are widely used for solid-state lasers, luminescent lamps, flat displays, optical fiber communication systems, and other photonic devices. It is because of the unique solid-state electronic properties that enable lanthanide ions in solids to emit photons efficiently in visible and near IR region. Due to the pioneer work by Dieke, Judd, Wyboume, and others in theoretical and experimental studies of the... 

Although optical spectra of lanthanide-doped insulating nanociystals embedded in amorphous matrices are very similar to the free-standing nanocrystal counterparts, their excited state dynamics behaves very differently from that in simple nanocrystals. Some distinct dynamic properties have recently been found for nanocrystals embedded in polymers or glasses. Simple models for the interaction between lanthanide ions and the matrices were also proposed. However, further studies are needed in order to quantitatively understand the observed size-dependence and dynamic mechanisms. 

Apart from Eu3+ and Tb3+, few studies have been reported on optical properties of lanthanide ions doped in ZnS nanociystals. Bol et al. (2002) attempted to incorporate Er3"1" in ZnS nanociystal by ion implantation. They annealed the sample at a temperature up to 800 °C to restore the crystal structure around Er3"1", but no Er3"1" luminescence was observed. Schmidt et al. (1998) employed a new synthesis strategy to incorporate up to 20 at% Er3"1" into ZnS (1.5-2 nm) cluster solutions which were stabilized by (aminopropyl)triethoxysilane (AMEO). Ethanolic AMEO-stabilized Er ZnS clusters in solutions fluoresce 200 times stronger at 1540 nm than that of ethanolic AMEO-Er complexes. This is explained by the very low phonon energies in ZnS QDs, and indicates that Er3+ ions are trapped inside chalcogenide clusters. However the exact position of Er3+ in ZnS clusters remains unknown. Further spectroscopic and structural analyses are required in order to obtain more detailed information. 

In the above sections, our attention was primarily focused on the structural and optical properties of lanthanide doped in nanoparticles such as spherical QDs. Lanthanides doped in some other novel low-dimensional nanostructures including core-shell, one-dimensional (ID) nanowires and nanotubes, two-dimensional (2D) nanofilms, hollow nanospheres, 2D nanosheets and nanodisks have also attracted extensive attention. It is expected that their unique structures could result in unusual mechanical, electronic, optical and magnetic properties. So far few papers have been reported for lanthanide ions other than Eu3+ in these materials. Much attention is focused on the optical properties of Eu3+ ions in view of their very good spectroscopic properties. 

The optical properties of lanthanide ions in some ID nanomaterials may behave differently from those of isotropic nanoparticles and bulk materials. Nanowires (NWs) of EuiLaPCU phosphors with a diameter of 10-20 nm and a length of several hundred nanometers were... 

Initially, interest for NIR emission of lanthanide ions stemmed from the development of optical libers, lasers and amplifiers for telecommunication (Kido and Okamoto, 2002 Kuriki et al., 2002) and there are a wealth of theoretical and technical papers published in this area. Up-conversion processes have also been the subject of intense investigations (Auzel, 2004). These two areas of research and development mostly deal with purely inorganic compounds or, more recently, with luminescent polymers they will not be covered in this chapter, with the exception of the latter, which will be partly described. 

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