Luminescence from Lanthanide-Doped Nanoparticles


Introduction to Lanthanides

The lanthanide metals, or lanthanoids, are elements of atomic number 57 (Lanthanum) through 71 (Lutitium). They are also called the Rare Earth metals, and often include Yttrium, Y (atomic number 39) and Scandium, Sc (atomic number 21), because of their chemical similarity. You'll usually find the lanthanides and actinides in most periodic tables at the bottom. (I was once quite disconcerted to find the lanthanides and actinides missing from the periodic table in a chemistry classroom becasue the chart was too large to fit in the space above the blackboard.)

Where Do We Find Lanthanides?

In these different applications the lanthanide occurs in different forms, i.e., metal, ion, solid, or soluble form. For luminescent applications the lanthanide is usually an ion, with +3 being the most common ionic form for lanthanides (two common exceptions are the stable forms Ce4+ and Eu2+). Molycorp has produced a nice overview of lanthanides (and a separate volume on Cerium) that you can download as PDF files at:
http://www.molycorp.com/product_information/tech_data.asp

The purpose of this web page is to provide background on lanthanide ions as luminescent materials. As an example, the photos below show a 5-Euro note under different illumination sources. It is quite appropriate that the red color seen with ultraviolet (UV) excitation comes from europium ions!

  obverse reverse
under white light Euro front Euro back
under UV light Euro front UV Euro back UV
 Thanks to my good friend Andries Meijerink, Utrecht University, for the Euro note! (I will pay you back some day.)

Background on Luminescent Ions in Solids

Introduction

Solid materials containing optically active dopants, transition metal or lanthinide ions, find use in a wide variety of technological applications. Applications include lamp, display, and X-ray phosphors; optical-fiber amplifiers; solid-state lasers; and luminescence-based sensors. For an example the following discussion describes a transition metal ion.

Transparent Host Materials

A host is usually necessary to dilute the optically active ions and prevent rapid non-radiative processes from occuring, i.e., all of the energy being lost as heat. 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.


Coloration of Transparent Hosts

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 schematic is a 2-D representation of Cr3+ doped into alumina. For the two Cr3+ ions occuring in the 40 cation sites, we refer to this material as 2 mol-% Cr3+:Al2O3. Crystals of alumina are sapphire, and when containing Cr3+ they are rubys and red in color.

   Al  Al  Al  Al  Al  Al  Al  Al  Al  Al
    \ / \ / \ / \ / \ / \ / \ / \ / \ /
     O   O   O   O   O   O   O   O   O
    / \ / \ / \ / \ / \ / \ / \ / \ / \
   Al  Al  Cr  Al  Al  Al  Al  Al  Al  Al
    \ / \ / \ / \ / \ / \ / \ / \ / \ /
     O   O   O   O   O   O   O   O   O
    / \ / \ / \ / \ / \ / \ / \ / \ / \
   Al  Al  Al  Al  Al  Al  Al  Al  Cr  Al
    \ / \ / \ / \ / \ / \ / \ / \ / \ /
     O   O   O   O   O   O   O   O   O
    / \ / \ / \ / \ / \ / \ / \ / \ / \
   Al  Al  Al  Al  Al  Al  Al  Al  Al  Al

Optical Properties of Transition Metal Ions in Solids

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):


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.


Optical Properties of Lanthanide Ions in Solids

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 result of the shielding are narrow spectral lines. The following figure shows the energy levels of Nd and Eu. The y axis is in thousands of wavenumbers (cm-1).

The Dieke diagram shows the overall energy levels of the trivalent lanthanides.



Luminescent Nanoparticles

Introduction

I define luminescence as the emission of light from solid materials. It is closely related to atomic emission (light from gas-phase atoms), fluorescence (light from molecules), and phosphorescence (long-lived light emission).

Why Are Luminescent Nanoparticles of Interest?

under construction, sorry...

Background

Although much work has focused on size effects in semiconductor quantum dots, less is known about the effects on localized luminescent ions when the host material approaches nanometer dimensions. This project has brought together chemists and physicists to study nanostructured luminescent materials in a very detailed and synergistic manner. The detailed physical measurements have provided rapid feedback to modify the nanoparticle synthesis, and the availability of tailored samples has allowed the design of comprehensive spectroscopic experiments. This project is contributing to a detailed understanding of the structure and chemistry of nanoparticles and the radiative and nonradiative dynamics of luminescent ions in nanostructured materials.

Summary


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