Solid materials containing optically active metal ions, transition or lanthinide metals, find uses in a wide variety of technological applications. Optical applications include lamp, display, and X-ray phosphors, optical-fiber amplifiers, solid-state lasers, and luminescence-based sensors.
A host is usually necessary to dilute the optically active ions and prevent rapid non-radiative processes from occuring. Hosts are usually insulators, but semiconductors can also serve as hosts for optically active ions, as long as the luminescent excited state does not overlap with the conduction band leading to quenching. The constituents of ionic solids have closed-shell electronic configurations that result in a large gap between the ground state and the next higher energy level. The large band gap of ionic solids makes them electrical insulators and optically transparent.
Example: Al2O3
Al3+ - 1s22s22p6
O2- - 1s22s22p6
Absorption spectrum of Al2O3:
The visible region of the spectrum occurs from approximately 1.8 to 3.2 eV.
Transparent host materials can be colored by doping optically active metal ions into the host materials, or by creating optically-active lattice defects.
The following is a 2-D representation of 10 mol-% Eu3+:Y2O2S, which is a red phosphor in cathode-ray tubes (televisions and computer monitors). The Eu3+ ions randomly replace Y3+ ions in the lattice.
Y Y Eu Y Y Y Y Y Y
\ / \ / \ / \ / \ / \ / \ / \ /
O O S O O S O O
/ \ / \ / \ / \ / \ / \ / \ / \
Y Y Y Y Y Eu Y Y Y
\ / \ / \ / \ / \ / \ / \ / \ /
O S O O S O O S
/ \ / \ / \ / \ / \ / \ / \ / \
Eu Y Y Y Y Y Y Y Y
\ / \ / \ / \ / \ / \ / \ / \ /
S O O S O O S O
Some other phosphor examples are ZnS:Ag (blue) and ZnS:Cu,Al (yellow-green). Two common solid-state lasers are Nd3+:Y3Al5O8 (Nd:YAG, wavelength = 1.06 µm) and Ti3+:Al2O3 (Ti-sapphire, wavelength range = 700-900 nm) The optical properties of transition-metal and lanthanide metal ions are quite different and are discussed below.
Optically active lattice defects are called color centers or F-centers (F for farbe, the German word for color). Color centers are usually created by exposure of a solid material to ultraviolet, X-ray, or gamma-ray radiation.
The electronic configuration of trivalent lanthanide (a.k.a. rare-earth) ions are:
1s22s22p63s23p63d104s24p64d105s25p65d04fn
where n varies from 0 to 14 for the series:
La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu atomic #: 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 (subtract 3 for the number of electrons in the trivalent ion)
The outer 5s and 5p electrons shield the 4f electrons from large perturbations due to the lattice, i.e., bonding. Therefore, the overall energy level structure does not change very much for a given lanthanide ion in different hosts. The Hamiltonian can be written as follows:
Because of the shielding, Hc may be neglected leaving a free-ion Hamiltonian. The Dieke diagram shows the overall energy levels of the trivalent lanthanides.
The spectra of transition-metal ions in solids depend on perturbation of d electrons by the host lattice.
See the Tanabe-Sugano Diagrams taken from:
Interpretation of the spectra of first-row transition metal complexes (textbook problems) by R. J. Lancashire.
Absorption spectrum of Cr3+-doped Al2O3 (ruby):
Solid-state materials are useful in different forms for different applications. The most common forms are single crystals, polycrystalline powders, films, and glasses. The following list shows a variety of synthetic methods to prepare different forms of solid materials.
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