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Design of Lanthanide-doped Colloidal Nanoparticles: Applications as Phosphors, Sensors and Photocatalysts Debashrita Sarkar, Sagar Ganguli, Tuhin Samanta, and Venkataramanan Mahalingam Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01593 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018
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Design of Lanthanide-doped Colloidal Nanocrystals: Applications as Phosphors, Sensors and Photocatalysts Debashrita Sarkar, Sagar Ganguli, Tuhin Samanta and Venkataramanan Mahalingam* Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur, 741246, West Bengal, India Abstract The unique optical characteristics of lanthanides (Ln3+) like high colour purity, long excited state lifetimes, less perturbation of excited states by the crystal field environment and ability towards easy spectral conversion of wavelengths through upconversion and downconversion processes have caught the research attention of many scientists in the recent past. To broaden the scope of using these properties, it is important to make suitable Ln3+doped materials, particularly in colloidal forms. In this feature article, we discuss the different synthetic strategies to make Ln3+-doped nanoparticles in colloidal forms, particularly on ways of functionalizing hydrophobic surfaces to hydrophilic ones for enhancing their dispersibility and luminescence in aqueous medium. We have enumerated the various strategies and sensitizers utilized to increase the luminescence of the nanoparticles. Further, the use of these colloidal nanoparticle systems in sensing application by appropriate selection of capping ligands has been discussed. In addition, we have shown how the energy transfer efficiency from Ce3+ to Ln3+ ions can be utilized for detection of toxic metal ions and small molecules. Finally, we discuss examples where the spectral conversion ability of these materials has been used in photocatalysis and solar cell applications. Keywords:
Lanthanides, nanocrystals, sensitization, colloidal, phosphors, sensing,
photocatalysis, luminescence 1. Introduction Lanthanide luminescence Luminescence has become an inevitable tool for understanding many physical, chemical and biological processes. The broader scope of luminescence in various applications led to the design and synthesis of several luminescent materials, such as quantum dots, metal clusters, transition metal and lanthanide (Ln3+)-doped materials, etc. Among these, Ln3+1
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doped materials are particularly interesting due to their unique optical characteristics, which originate from intra 4f→4f transitions. Though, these transitions are forbidden according to Laporte rule, Ln3+ luminescence is still observed. This is attributed to admixing of states and relaxation in symmetry, which increases the probability of transition.1 The transitions related to Ln3+ ions possess unique characteristics like narrow bandwidth leading to high colour purity of emission, longer excited state luminescence lifetimes typically in µs to ms range, and large Stokes shift. Most importantly, due to the shielding effect of outer 5s and 5d orbitals, the 4f→4f transitions are largely unaffected by the ligand field around them resulting into unique unperturbed characteristic peak for each individual ion. In addition, the luminescence from the Ln3+ ions can span over a wide spectral window, say from ultraviolet (UV) to near-infrared (NIR) region (Figure 1). This is possible due to the presence of multiple energy levels that arise from splitting owing to spin-orbit coupling. Although emissions from multiple energy levels are feasible, the selection rules limit the observation to only few transitions with reasonable strength.2 The characteristic sharp luminescence signals from Ln3+ ions are broadly divided into two processes, Stokes and anti-Stokes shifted emissions. Stokes shift is the conversion of high energy photons into lower energy photons. This process has further classified into two categories namely downshifting and downconversion. In energy terms, downshifting is the shift of the emission energy towards lower energy (higher wavelength) compared to the excitation energy and is shown by most Ln3+ ions like Ce3+, Tb3+, Eu3+, etc. Downconversion is the process of converting one high energy photon into two or more lower energy photons (usually displayed by Pr3+, Eu3+ and Tb3+with the support of Gd3+).3 Typically, UV energy photons are converted into two photons of visible energy. This property of the Ln3+ ions is useful in the conversion of harmful radiation such as X-rays or gamma rays into visible radiation. The second category called anti–Stokes emission is unique on its own. It is the process of converting two or more lower energy photons into higher energy ones. This type of emission, also termed as upconversion are typically observed from Ln3+ like Er3+, Tm3+, Ho3+ in presence of Yb3+, which acts as a sensitizer. Such emission is often utilised for imaging purpose in biomedical applications. Figure 2 shows the schematic illustration differentiating the luminescence originating from the above mentioned three different processes.4 The above features are possible due to the long excited state lifetimes as well as the presence of multiple energy levels with a suitable energy gap between them. If the 2
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distance between the energy levels is very close, the occurrence of non-radiative relaxation will be dominant over excited state phenomenon (i.e. energy transfer). Despite several salient features, the luminescence from Ln3+ ions is prone to concentration quenching, i.e., sequential transfer of excited energy between closely spaced identical Ln3+ ions, resulting in reduced luminescence quantum yield due to increase in the probability of non-radiative relaxation processes. This demands the Ln3+ ions to be diluted in suitable host matrices by replacing the cations of the host materials. To minimize lattice defects, the host matrices are chosen such that the atomic radii of the cations are similar to that of the Ln3+. Hosts for the Lanthanide Ions Use of suitable hosts with low phonon energy is indispensable for the observation of strong luminescence from the Ln3+-doped materials and their usage as phosphors. Some of the well-studied hosts for the Ln3+ ions are oxides, fluorides, vanadates, molybdates, and phosphates. Due to the similarity in ionic radii as well as optically silent nature, yttrium based hosts dominate the literature work. Among different host matrices, both oxides and fluorides have been extensively studied. Oxide materials like garnets (Y3Al5O12, Y3AGa5O12, Lu3Al5O12, etc.), sesquioxides (Y2O3, Gd2O3, La2O3, etc.) are generally prepared via solid state methods. They are well known phosphors and widely employed in solid state applications. Garnet materials are quite interesting hosts for the Ln3+ ions as they possess different crystal sites for the Ln3+ ions to reside. This also has strong influence on the luminescence properties. For example, Ce3+-doped yttrium aluminium garnet (YAG) is a vital component for the development of commercial white light emitting diodes. Strong white light emission has been reported from Tm3+/Yb3+/Er3+-doped Lu3Ga5O12 via low energy excitation.5 Similar to garnets, sesquioxides are very suitable and well-researched hosts for Ln3+ ions. In fact, both Tb3+ and Eu3+ doped Y2O3 are well known green and red phosphors and are extensively used in cathode ray tubes (CRT).6 In fact, Vetrone and co-workers have prepared Ln3+-doped Y2O3 with quite small crystallite sizes (1000 ºC
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Y3Sc2Ga3−xAlxO12:Ce3+
Solid state at T>1000 ºC
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Li2SrSiO4:Eu2+
Combustion at T
