Color-tunable up-conversion emission in Y2O3:Yb3+, Er3+ nanoparticles prepared by polymer complex solution method
© Lojpur et al.; licensee Springer. 2013
Received: 10 February 2013
Accepted: 11 March 2013
Published: 22 March 2013
Powders of Y2O3 co-doped with Yb3+ and Er3+ composed of well-crystallized nanoparticles (30 to 50 nm in diameter) with no adsorbed ligand species on their surface are prepared by polymer complex solution method. These powders exhibit up-conversion emission upon 978-nm excitation with a color that can be tuned from green to red by changing the Yb3+/Er3+ concentration ratio. The mechanism underlying up-conversion color changes is presented along with material structural and optical properties.
42.70.-a, 78.55.Hx, 78.60.-b
Up-conversion materials have the ability to convert lower energy near-infrared radiations into higher energy visible radiations. These materials have gained considerable attention because of their use in a wide range of important applications, from solid compact laser devices operating in the visible region and infrared quantum counter detectors to three-dimensional displays, temperature sensors, solar cells, anti-counterfeiting, and biological fluorescence labels and probes [1–6]. Further efforts in development of methods for preparation of up-conversion (UC) materials are therefore justified with aims of enhancing their UC efficiency and reducing production costs. In addition, methods for UC nanoparticle (UCNP) synthesis are of particular interest for use in two-photon bio-imaging, sensitive luminescent bio-labels, and GaAs-coated highly efficient light-emitting diodes .
Lanthanide-based UC materials and UCNPs are of special interest due to unique spectroscopic properties of rare-earth ions like sharp intra-4f electronic transitions and existence of abundant, long-living electronic excited states at various energies that facilitate electron promotion to high-energy states . In principal, lanthanide-based UC materials and UCNPs consist of three components: a host matrix, a sensitizer, and an activator dopant. The choice of the host lattice determines the distance between the dopant ions, their relative spatial position, their coordination numbers, and the type of anions surrounding the dopant. The properties of the host lattice and its interaction with the dopant ions therefore have a strong influence on the UC process . It has been shown that UC emission efficiency depends strongly on host phonon energy, where in low-phonon-energy hosts, multi-phonon relaxation processes are depressed and efficiency-enhanced . Because of their excellent chemical stability, broad transparency range, and good thermal conductivity, rare-earth sesquioxides are well-suited host materials . Their phonon energy (ca. 560 cm−1) is higher compared to the most UC-efficient fluoride materials (ca. 350 cm−1), but lower compared to other host types (phosphates, vanadates, molybdates, titanates, zirconates, silicates, etc.). In addition, easy doping can be achieved with RE ions because of similarity in ionic radius and charge. For sensitizer dopant, Yb3+ is the most common choice for excitation around 980 nm, where a variety of inexpensive optical sources exists. This ion has a simple energy level structure with two levels and a larger absorption cross section compared to other trivalent rare-earth ions. The energy separation of Yb3+2F7/2 ground state and 2F5/2 excited state match-up well the transitions of an activator dopant ion, which has easy charge transfer between its excited state and activator states. For visible emission, Er3+, Tm3+, Ho3+, and Pr3+ are commonly used as activator dopants [12–16]. UC emission of different colors can be obtained in a material with different activators and their combinations. Er3+-doped materials emit green and red light, Tm3+ blue, Ho3+ green, and Pr3+ red.
In recent times, a lot of effort is directed towards UC color tuning to obtain a material with characteristic emission usually by combining two or more activator ions  or by utilizing electron–electron and electron–phonon interactions in existing one-activator systems [18, 19]. In this research we showed that color tuning from green to red can be achieved in Yb3+/Er3+ UCNP systems on account of changes of Yb3+ sensitizer concentration. For this purpose we prepared Y2O3 NPs, the most well-known rare-earth sesquioxide host, co-doped with different Yb3+/Er3+ ratios. Nanosized phosphors offer a number of potential advantages over traditional, micro-scale ones in optical properties, such as high-resolution images and high luminescence efficiency [20, 21]. However, Vetrone et al. showed that CO32− and OH− species are frequently adsorbed on the surface of sesquioxide nanoparticles . Their high vibrational energies (about 1,500 and 3,350 cm−1 for CO32− and OH−, respectively) decrease the UC efficiency through multi-phonon relaxations. For this reason we applied polymer complex solution (PCS) synthesis  since we found earlier that the PCS method provides sesquioxides with low surface area and defects and no adsorbed species on the surface [24–26].
Polymer complex solution method is a modified combustion method where instead of classical fuel (urea, glycine, carbohydrazide) an organic water-soluble polymer (in our case polyethylene glycol (PEG)) is used. The utility of this polymeric approach comes from the coordination of metal cations on the polymer chains during gelation process, resulting in very low cation mobility. Polymer precursor works both as a chelating agent and as an organic fuel to provide combustion heat for the calcination process. In this way PCS provides mixing of constituting elements at the atomic level and allows homogeneous control of very small dopant concentration. The first step in the PCS method is preparation of an aqueous solution containing metal salts and PEG. In the second step, removal of the excess water forces polymer species into closer proximity, converting the system into a resin-like gel. Upon ignition, an oxide powder is obtained, while considerable resin mass is lost as the polymer matrix is burned away.
Using this procedure, three Y2O3 samples doped with 0.5 at.% of Er3+ and 1, 2.5, and 5 at.% of Yb3+ ions were synthesized. In brief, appropriate stoichiometric quantities of yttrium oxide (Y2O3), erbium oxide (Er2O3), and ytterbium oxide (Yb2O3) (all Alfa Aesar, 99.9%, Ward Hill, MA, USA) were mixed and dissolved in hot nitric acid. In the obtained solutions, PEG ( = 200, Alfa Aesar) was added in 1:1 mass ratio. The formed metal-PEG solution was stirred at 80°C, resulting in a metal-PEG solid complex which was further fired at 800°C in air. The powders were additionally annealed at 800°C for 2 h in order to decompose the residual PEG and nitrite ions and to obtain pure crystal phase.
Crystal structures of samples are checked by X-ray diffraction (XRD) measurements. Measurements are performed on a Rigaku SmartLab system (Shibuya-ku, Japan) operating with Cu Kα1,2 radiation at 30 mA and 40 kV, in the 2θ range from 15° to 100° (using continuous scan of 0.7°/s). Transmission electron microscopy (TEM) is conducted using a JEOL-JEM 2100 instrument (Akishima-shi, Japan) equipped with LaB6 cathode and operated at 200 kV. The up-conversion luminescence emissions and decays are measured upon excitation with 978-nm radiation (OPO EKSPLA NT 342, 5.2-ns pulse, Vilnius, Lithuania) on a Horiba Jobin-Yvon Model FHR1000 spectrofluorometer system (Kyoto, Japan) equipped with an ICCD Jobin-Yvon 3771 detector. For measurements of up-conversion emission intensity dependence on excitation power, a continuous-wave laser is used (980-nm radiation).
Results and discussion
The presence of nitrate, water, and carbon species on nanoparticle surfaces is checked by Fourier transform infrared (FT-IR) spectroscopy. Only Y-O stretching vibrations of the host lattice at 560 cm−1 are noted (see Additional file 1: Figure S1 for the FT-IR spectrum of Y1.97Yb0.02Er0.01O3 sample). This is favorable for efficient emission since the high phonon energy of species adsorbed on the surface of nanoparticles may enhance significantly nonradiative de-excitation [13, 22].
Emission decay times for Y 2 O 3 :Yb 3+ , Er 3+ nanoparticles upon 978-nm excitation
Green emission lifetime (ms)
Red emission lifetime (ms)
In conclusion, yttrium oxide powders doped with Er3+ ions and co-doped with different concentrations of Yb3+ ions are successfully prepared using polymer complex solution method. This simple and fast synthesis method provides powders consisting of well-crystallized nanoparticles (30 to 50 nm in diameter) with no adsorbed species on their surface. The powders exhibit up-conversion emission upon 978-nm excitation, with a color that can be tuned from green to red by changing the Yb3+/Er3+ concentration ratio. This effect can be achieved in nanostructured hosts where electron–phonon interaction is altered compared to the bulk material.
The authors would like to acknowledge the support from the Ministry of Education, Science and Technological Development of the Republic of Serbia (grant no. 45020).
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