Introduction to electronic spectroscopy of lanthanide, properties, and their applications

The element shown in the bottom of the periodic table is classified as lanthanide series element. There are total 15 element occurred in this series including La 3+, Ce 3+, Pr 3+, Nd 3+, Pm 3+, Sm 3+, Eu 3+, Gd 3+, Tb 3+, Dy 3+, HO 3+, Er 3+, Tm 3+, Yb 3+, and Lu 3+. These elements are arranged on the basis of atomic number. From the long years, back lanthanide elements are used as dopant ions in many inorganic host lattices such as aluminates, borates, silicates, tungstate, fluoride, phosphate, and vanadates. In past few years, a lot of study had been carried out on the location of lanthanide level into band structure of several inorganic host compounds. This type of study is very much useful to predicting the exact location of lanthanide level and 4f-electron binding energies based on some theoretical model to calculate red shift, centroid shift, charge transfer, and chemical shift. These are studied on the basis of the observed spectroscopic data to draw the host referred binding energy level scheme in which the 4f n and 4f n−1 5d levels for all divalent and trivalent lanthanides ions are placed relative to the top of the V·B; however, in case of vacuum referred binding energy scheme, all the energy levels are relative to that of an electron at rest in vacuum. Therefore, such energy level scheme provided methods for the visualization of the relevant location of lanthanide levels in relative in the band structure of the inorganic host lattice. Also, we know that the application of lanthanide-doped inorganic host depends upon the observed photophysical properties. Among the 15 lanthanide element, Ce 3+, Pr 3+, Sm 3+, Eu 3+, Tb 3+, and Dy 3+ are potentially used in LEDs and display devices. However, the other elements such as Nd 3+, HO 3+, Er 3+, Tm 3+, and Yb 3+ show near visible-NIR emission spectra and exhibit the emission bands in visible to NIR range. Recently, upconversion luminescent nanoparticle is proposed for wastewater treatment. In past few years, researcher reported many types of lanthanide-doped luminescent host materials for lighting devices. But only few of them are commercially used. Currently, QD-based LEDs attract the attention of many researchers worldwide due to their high brightness and high quantum yield. Only few materials are reported for such types of LED applications. Hence, the search for new luminescent materials and QDs is not end in near future due to continuous innovation and development in exiting indoor and outdoor lighting technology. Recently, lanthanide-doped (Ce 3+, Pr 3+, Sm 3+, Yb 2+, Nd 3+, etc.) oxide materials show excellent luminescence properties in UV visible to NIR range and also applicable as an active layer on the front surface to improve conversion efficiency of Si solar cell. Among the different types of renewable and sustainable source of energy, solar energy is a prominent and a clean source for energy generation. Thus, PV solar cells are the nonpolluting energy source of the 21st century. And it is well known that Si solar cell is everywhere accepted for the PV application than the other PV devices like organic solar cell, perovskite solar cell, Tandem solar cell, and CdTe cell. It is most commercially used from few years. Now, researchers are focused on finding other alternatives for Si cell and also they are taking an effort on how to increase the efficiency of existing Si solar cell. Therefore, the luminescent materials with UC/DC phosphor mechanism were theoretically and practically proposed by many research groups to enhance the PV response of Si solar cell. Hence, the lanthanide ions play an important role in modifying the optical, electronic, electrical, and catalytic activity when it doped with some host lattices. In the following section, we have discussed the some important properties of lanthanides ions.