Rare earth metals are a specific class of elements in the periodic table that comprise of the lanthanide metal series along with scandium and yttrium (see Figure 1). Incidentally, it has been found that these elements are not as rare as formerly believed, yet they are still referred to as such. These materials can be observed through a useful technique called cathodoluminescence.
Figure 1. Part of the periodic table corresponding to the rare-earth metals. Below a SEM image of YAG:Ce3+ particles is shown together with an RGB CL image which was constructed from three RGB color filtered CL intensity maps as measured with a PMT (t = 10 μs, HV = 10 kV, I = 35 pA). Sample courtesy of Professor Xia.
It is possible to dopecrystalline host materials like sapphire (Al2O3), silica (SiO2), yttrium oxide (Y2O3), yttrium aluminum garnet (YAG), yttrium vanadate (YVO), or gadolinium oxide (Gd2O3) with rare-earth metal ions, which impart them special optical properties. In lanthanide-doped materials, most of the optical transitions are intra-4f transitions. The 4f electron shell is protected from the environment, for example, phonon coupling and crystal field effects, by the filled 5p and 5s shells. Consequently, these transitions are relatively insensitive to the host and slightly analogous to free-ion emission. In addition, they usually have small linewidths and correspondingly extended lifetimes up to the millisecond range.
If different lanthanide ions (mostly trivalent ions) are used, a broad range of such intense transitions can be engineered throughout the infrared and visible spectral region . Therefore, these materials are used for many different applications. In fact, they can be specifically used for making efficient visible and IR (fiber) lasers.
Erbium-doped materials form the basis for the signal amplifiers in optical fibers applied for global Internet communication , and therefore they are particularly relevant. Moreover, rare-earth doped materials are abundantly used as phosphors in lightings like fluorescent tubes and LEDs and in display technology like smartphones and flat panels. They are also highly suitable as scintillators that transform high-energy radiation (X-rays, fast electrons) into visible light, which can easily be detected with traditional detectors like radiation detectors, CT scanners, and electron microscopes.
Cathodoluminescence imaging is an extremely valuable method used for analyzing rare-earth doped materials. Through the electron beam, local excitation of rare-earth transitions can be achieved effectively because the transitions can be directly driven through the (wide) band gap of the host matrices. This facilitates spectroscopic studies on materials in nanostructured form or (nano) powder at the pertinent length scales.
In the case of scintillator applications, the electron beam can simulate the energetic radiation to which the materials are generally exposed. It can then test the radiation hardness, spectral output, and efficiency . Certain geological minerals like zircon are also heavily doped with rare earth ions, which have a major impact on the CL response. This is a new, exciting application area . Due to their radiation hardness and efficient light emission, rare-earth doped nanoparticles can be applied as CL markers in biological sections.
Figure 1 shows an RGB CL map on YAG:Ce3+ microcrystals, obtained with the help of a filter wheel and a photomultiplier tube. Listed in the caption are beam current (I), dwell time (t), and acceleration voltage (HV). The typical yellow emission from Ce3+ can be clearly seen including some contamination that locally blocks the excitation/emission of CL. Ce3+ has a radiative lifetime of just <500 ns and often gives bright CL emission, thus enabling rapid mapping.
As stated before, there are certain rare-earth complexes that have considerably longer lifetimes up to the millisecond range, in which case it becomes important to limit the scanning speed to prevent blurring and comet tail-like features in the CL maps. In order to inspect the spectral content in more detail, hyperspectral CL scanning can also be performed.
The result of such a measurement on a single CL crystal is shown in Figure 2. The intense broad yellow emission band (see spectrum in Figure 2) correlates well with the blue emission from GaN-based LEDs. This is the reason why YAG:Ce3+ is one of the most extensively used phosphor materials. Examples of CL spectra measured on Gd2O3:Eu3+ and SiO2:Er3+ were also shown. The Gd2O3:Eu3+ spectrum was determined with a 1200 lines/mm high-resolution grating so as to correctly resolve the narrow-band Eu transitions in the spectrum.
Figure 2. Top: SEM and corresponding false color RGB image derived from a hyperspectral CL scan on a single YAG:Ce3+ particle where each pixel corresponds to a CL spectrum (t = 20 ms, HV = 10 kV, I = 35 pA). Bottom: CL spectra measured on YAG:Ce3+ (taken from hyperspectral dataset above), Gd2O3:Eu3+ spectrum (t = 2 s, HV =10 kV, I = 42 pA), and SiO2:Er3+ (t = 2 s, HV = 30 kV, I =300 pA). Europium sample courtesy of Dr Jens Adam (Leibnitz-Institut für Neue Materialien). Data taken from Ref.. Erbium sample courtesy of Prof. Albert Polman (AMOLF).
The transitions, thus observed, relate to 5D0 to 7F1,2 intra-4f transitions . Likewise, a 600 lines/mm grating was employed for the SiO2:Er3+ spectrum, wherein the well-known intra-4f transition from 4I13/2 to 4I15/2 applied in telecommunication technology is clearly seen.
 A. J. Kenyon, Recent developments in rare-earth doped materials for optoelectronics, Prog. Quant. Electron. 26, 225 (2002).
 J. Adam et al. Light Emission Intensities of Luminescent Y2O3:Eu and Gd2O3:Eu Particles of Various Sizes, Nanomaterials. 7, 26 (2017).
 K. Keevend et al. Tb3+-doped LaF3 nanocrystals for correlative cathodoluminescence electron microscopy imaging with nanometric resolution in focus ion beam-sectioned biological samples, Nanoscale, DOI: 10.1039/c6nr09187c(2017).
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