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Qin X.,National University of Singapore | Liu X.,National University of Singapore | Huang W.,Nanjing University of Technology | Huang W.,Nanjing University of Posts and Telecommunications | And 2 more authors.
Chemical Reviews | Year: 2017

The synthesis of lanthanide-activated phosphors is pertinent to many emerging applications, ranging from high-resolution luminescence imaging to next-generation volumetric full-color display. In particular, the optical processes governed by the 4f-5d transitions of divalent and trivalent lanthanides have been the key to enabling precisely tuned color emission. The fundamental importance of lanthanide-activated phosphors for the physical and biomedical sciences has led to rapid development of novel synthetic methodologies and relevant tools that allow for probing the dynamics of energy transfer processes. Here, we review recent progress in developing methods for preparing lanthanide-activated phosphors, especially those featuring 4f-5d optical transitions. Particular attention will be devoted to two widely studied dopants, Ce3+ and Eu2+. The nature of the 4f-5d transition is examined by combining phenomenological theories with quantum mechanical calculations. An emphasis is placed on the correlation of host crystal structures with the 5d-4f luminescence characteristics of lanthanides, including quantum yield, emission color, decay rate, and thermal quenching behavior. Several parameters, namely Debye temperature and dielectric constant of the host crystal, geometrical structure of coordination polyhedron around the luminescent center, and the accurate energies of 4f and 5d levels, as well as the position of 4f and 5d levels relative to the valence and conduction bands of the hosts, are addressed as basic criteria for high-throughput computational design of lanthanide-activated phosphors. © 2017 American Chemical Society.


Wang Y.,National University of Singapore | Deng R.,National University of Singapore | Xie X.,Nanjing University of Technology | Huang L.,Nanjing University of Technology | And 3 more authors.
Nanoscale | Year: 2016

Optical tuning of lanthanide-doped upconversion nanoparticles has attracted considerable attention over the past decade because this development allows the advance of new frontiers in energy conversion, materials science, and biological imaging. Here we present a rational approach to manipulating the spectral profile and lifetime of lanthanide emission in upconversion nanoparticles by tailoring their nonlinear optical properties. We demonstrate that the incorporation of energy distributors, such as surface defects or an extra amount of dopants, into a rare-earth-based host lattice alters the decay behavior of excited sensitizers, thus markedly improving the emitters' sensitivity to excitation power. This work provides insight into mechanistic understanding of upconversion phenomena in nanoparticles and also enables exciting new opportunities of using these nanomaterials for photonic applications. © The Royal Society of Chemistry 2016.


Liu X.,National University of Singapore | Deng R.,National University of Singapore | Zhang Y.,National University of Singapore | Wang Y.,National University of Singapore | And 4 more authors.
Chemical Society Reviews | Year: 2015

Probing the nature of nanocrystalline materials such as the surface state, crystal structure, morphology, composition, optical and magnetic characteristics is a crucial step in understanding their chemical and physical performance and in exploring their potential applications. Upconversion nanocrystals have recently attracted remarkable interest due to their unique nonlinear optical properties capable of converting incident near-infrared photons to visible and even ultraviolet emissions. These optical nanomaterials also hold great promise for a broad range of applications spanning from biolabeling to optoelectronic devices. In this review, we overview the instrumentation techniques commonly utilized for the characterization of upconversion nanocrystals. A considerable emphasis is placed on the analytical tools for probing the optical properties of the luminescent nanocrystals. The advantages and limitations of each analytical technique are compared in an effort to provide a general guideline, allowing optimal conditions to be employed for the characterization of such nanocrystals. Parallel efforts are devoted to new strategies that utilize a combination of advanced emerging tools to characterize such nanosized phosphors. This journal is © The Royal Society of Chemistry.


Zhang Y.,National University of Singapore | Zhang L.,Macquarie University | Deng R.,National University of Singapore | Tian J.,Fudan University | And 5 more authors.
Journal of the American Chemical Society | Year: 2014

We report the synthesis of luminescent crystals based on hexagonal-phase NaYF4 upconversion microrods. The synthetic procedure involves an epitaxial end-on growth of upconversion nanocrystals comprising different lanthanide activators onto the NaYF4 microrods. This bottom-up method readily affords multicolor-banded crystals in gram quantity by varying the composition of the activators. Importantly, the end-on growth method using one-dimensional microrods as the template enables facile multicolor tuning in a single crystal, which is inaccessible in conventional upconversion nanoparticles. We demonstrate that these novel materials offer opportunities as optical barcodes for anticounterfeiting and multiplexed labeling applications. © 2014 American Chemical Society.


Yang W.,Institute of Materials Research and Engineering of Singapore | Li X.,National University of Singapore | Chi D.,Institute of Materials Research and Engineering of Singapore | Zhang H.,CAS Changchun Institute of Applied Chemistry | And 3 more authors.
Nanotechnology | Year: 2014

Photovoltaics and photocatalysis are two significant applications of clean and sustainable solar energy, albeit constrained by their inability to harvest the infrared spectrum of solar radiation. Lanthanide-doped materials are particularly promising in this regard, with tunable absorption in the infrared region and the ability to convert the long-wavelength excitation into shorter-wavelength light output through an upconversion process. In this review, we highlight the emerging applications of lanthanide-doped upconversion materials in the areas of photovoltaics and photocatalysis. We attempt to elucidate the fundamental physical principles that govern the energy conversion by the upconversion materials. In addition, we intend to draw attention to recent technologies in upconversion nanomaterials integrated with photovoltaic and photocatalytic devices. This review also provides a useful guide to materials synthesis and optoelectronic device fabrication based on lanthanide-doped upconversion materials. © 2014 IOP Publishing Ltd.


Zhou B.,Institute of Materials Research and Engineering of Singapore | Shi B.,Macquarie University | Jin D.,Macquarie University | Jin D.,University of Technology, Sydney | And 3 more authors.
Nature Nanotechnology | Year: 2015

Lanthanide-doped upconversion nanocrystals enable anti-Stokes emission with pump intensities several orders of magnitude lower than required by conventional nonlinear optical techniques. Their exceptional properties, namely large anti-Stokes shifts, sharp emission spectra and long excited-state lifetimes, have led to a diversity of applications. Here, we review upconversion nanocrystals from the perspective of fundamental concepts and examine the technical challenges in relation to emission colour tuning and luminescence enhancement. In particular, we highlight the advances in functionalization strategies that enable the broad utility of upconversion nanocrystals for multimodal imaging, cancer therapy, volumetric displays and photonics. © 2015 Macmillan Publishers Limited. All rights reserved.


Wang Q.,National University of Singapore | Wang Q.,NUS Suzhou Research Institute | Kam P.-Y.,National University of Singapore | Kam P.-Y.,NUS Suzhou Research Institute
Journal of Lightwave Technology | Year: 2016

We consider the design of the optimum detector for two-dimensional amplitude/phase modulated signals received in additive white Gaussian noise (AWGN) and a Gaussian distributed phase reference error (PRE) due to imperfect carrier phase estimation. We propose a novel approach of using the amplitude and phase information of the received signal, based on viewing the AWGN as an equivalent additive observation phase noise (AOPN) whose statistics is derived in our earlier work. This allows the AOPN to be combined with PRE, and the maximum a posterior probability (MAP)/maximum-likelihood (ML) detection scheme to be readily derived in amplitude-phase form. This amplitude-phase approach is simpler and more convenient than the conventional method of using the in-phase and quadrature components of signals in phase noise. For M-ary phase-shift keying, which only has one ring of signal points, the ML detector here turns out to be the same as the conventional minimum Euclidean distance (MED) detector which is derived without taking the PRE into account, and leads to angular bisector decision boundaries (DB). However, for constellations which have multiple rings, e.g., M-ary quadrature amplitude modulation (M-QAM) and amplitude phase-shift keying (M-APSK), the ML detector here is very computationally inefficient. Thus, simpler and closed-form approximations to the ML detector for equiprobable signals are given, which can be easily implemented online. All these simplified ML detectors are shown via simulations to perform almost the same as the exact one and perform much better than the MED detector. The approximate ML DB for both 8-star QAM and rotated 8-star QAM are illustrated as examples and shown to be not necessarily straight lines. As the variance of PRE or the signal-to-noise ratio (SNR) or both increases, the DB between two signal rings asymptotically becomes circular. This leads to an annular sector as the decision region for each signal point. For annular-sector decision regions, simple, accurate, and closed-form approximations to the symbol error probability (SEP) are obtained for both 8-star QAM and the rotated case, 16QAM and even general M-APSK. These expressions provide explicit insight into how the PRE variance affects the performance. Within a wide range of PRE variances, our SEP approximations agree very well with the Monte Carlo simulations for all SNR values of interest. For the special case of an M-APSK constellation which has the same number of points and the same phase values on each ring, such as 8-star QAM, one of the suboptimal ML detectors further simplifies to a structure that performs ring detection and phase detection separately, and the decision regions are always annular sectors. © 2016 IEEE.


Zhou B.,Institute of Materials Research and Engineering of Singapore | Yang W.,Institute of Materials Research and Engineering of Singapore | Han S.,National University of Singapore | Sun Q.,National University of Singapore | And 2 more authors.
Advanced Materials | Year: 2015

A strategy of interfacial energy transfer upconversion is demonstrated through the use of a terbium (Tb3+) dopant as energy donor or energy migrator in core-shell-structured nanocrystals. This mechanistic investigation presents a new pathway for photon upconversion, and, more importantly, contributes to the better control of energy transfer at the nanometer length scale. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Peng J.,Agency for Science, Technology and Research Singapore | Xu W.,National University of Singapore | Teoh C.L.,Agency for Science, Technology and Research Singapore | Han S.,National University of Singapore | And 10 more authors.
Journal of the American Chemical Society | Year: 2015

Development of highly sensitive and selective sensing systems of divalent zinc ion (Zn2+) in organisms has been a growing interest in the past decades owing to its pivotal role in cellular metabolism, apoptosis, and neurotransmission. Herein, we report the rational design and synthesis of a Zn2+ fluorescent-based probe by assembling lanthanide-doped upconversion nanoparticles (UCNPs) with chromophores. Specifically, upconversion luminescence (UCL) can be effectively quenched by the chromophores on the surface of nanoparticles via a fluorescence resonant energy transfer (FRET) process and subsequently recovered upon the addition of Zn2+, thus allowing for quantitative monitoring of Zn2+. Importantly, the sensing system enables detection of Zn2+ in real biological samples. We demonstrate that this chromophore-UCNP nanosystem is capable of implementing an efficient in vitro and in vivo detection of Zn2+ in mouse brain slice with Alzheimers disease and zebrafish, respectively. © 2015 American Chemical Society.


Wang F.,Institute of Materials Research and Engineering of Singapore | Wang F.,City University of Hong Kong | Deng R.,City University of Hong Kong | Liu X.,Institute of Materials Research and Engineering of Singapore | And 2 more authors.
Nature Protocols | Year: 2014

Sodium gadolinium fluoride (NaGdF4) is an ideal host material for the incorporation of luminescent lanthanide ions because of its high photochemical stability, low vibrational energy and its ability to mediate energy exchanges between the lanthanide dopants. This protocol describes the detailed experimental procedure for synthesizing core-shell NaGdF4 nanoparticles that incorporate lanthanide ions into different layers for efficiently converting a single-wavelength, near-IR excitation into a tunable visible emission. These nanoparticles can then be used as luminescent probes in biological samples, in 3D displays, in solar energy conversion and in photodynamic therapy. The NaGdF4 nanoparticles are grown through co-precipitation in a binary solvent mixture of oleic acid and 1-octadecene. Doping by lanthanides with controlled compositions and concentrations can be achieved concomitantly with particle growth. The lanthanide-doped NaGdF4 nanoparticles then serve as seed crystals for subsequent epitaxial growth of shell layers comprising different lanthanide dopants. The entire procedure for the preparation and isolation of the core-shell nanoparticles comprising two epitaxial shell layers requires ∼15 h for completion. © 2014 Nature America, Inc. All rights reserved.

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