Enhancing Luminescence in Lanthanide‐Doped Upconversion Nanoparticles

The enthusiasm for research on lanthanide‐doped upconversion nanoparticles is driven by both a fundamental interest in the optical properties of lanthanides embedded in different host lattices and their promise for broad applications ranging from biological imaging to photodynamic therapy. Despite the considerable progress made in the past decade, the field of upconversion nanoparticles has been hindered by significant experimental challenges associated with low upconversion conversion efficiencies. Recent experimental and theoretical studies on upconversion nanoparticles have, however, led to the development of several effective approaches to enhancing upconversion luminescence, which could have profound implications for a range of applications. Herein we present the underlying principles of controlling energy transfer through lanthanide doping, overview the major advances and key challenging issues in improving upconversion luminescence, and consider the likely directions of future research in the field.     

Energy Migration Upconversion in Manganese(II)‐Doped Nanoparticles

We report the synthesis and characterization of cubic NaGdF₄:Yb/Tm@NaGdF₄:Mn core–shell structures. By taking advantage of energy transfer through Yb→Tm→Gd→Mn in these core–shell nanoparticles, we have realized upconversion emission of Mn²⁺ at room temperature in lanthanide tetrafluoride based host lattices. The upconverted Mn²⁺emission, enabled by trapping the excitation energy through a Gd³⁺ lattice, was validated by the observation of a decreased lifetime from 941 to 532 μs in the emission of Gd³⁺ at 310 nm (⁶P_7/2→⁸S_7/2). This multiphoton upconversion process can be further enhanced under pulsed laser excitation at high power densities. Both experimental and theoretical studies provide evidence for Mn²⁺ doping in the lanthanide‐based host lattice arising from the formation of F^? vacancies around Mn²⁺ ions to maintain charge neutrality in the shell layer.     

Controllable Red,Green, Blue (RGB) and Bright White Upconversion Luminescence of Lu2O3:Yb3+/Er3+/Tm3+ Nanocrystals through Single Laser Excitation at 980 nm

Light fantastic! Lu₂O₃:Yb³⁺/Er³⁺/Tm³⁺ nanocrystals with controllable red, green, blue (RGB) and bright white upconversion luminescence by a single laser excitation of 980 nm have been successfully synthesized (see picture). Due to abundant UC PL colors, it can potentially be used as fluorophores in the field of color displays, back light, UC lasers, photonics, and biomedicine.

     

Facile synthesis and multicolor luminescent properties of uniform Lu2O3:Ln (Ln=Eu3+, Tb3+, Yb3+/Er3+, Yb3+/Tm3+, and Yb3+/Ho3+) nanospheres

Multicolor Lu(2)O(3):Ln (Ln=Eu(3+), Tb(3+), Yb(3+)/Er(3+), Yb(3+)/Tm(3+), and Yb(3+)/Ho(3+)) nanocrystals (NCs) with uniform spherical morphology were prepared through a facile urea-assisted homogeneous precipitation method followed by a subsequent calcination process. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectrum (EDS), Fourier transformed infrared (FT-IR), thermogravimetric and differential thermal analysis (TG-DTA), and photoluminescence (PL) spectra as well as kinetic decays were employed to characterize these samples. The XRD results reveal that the as-prepared nanospheres can be well indexed to cubic Lu(2)O(3) phase with high purity. The SEM images show the obtained Lu(2)O(3):Ln samples consist of regular nanospheres with the mean diameter of 95 nm. And the possible formation mechanism is also proposed. Upon ultraviolet (UV) excitation, Lu(2)O(3):Ln (Ln=Eu(3+) and Tb(3+)) NCs exhibit bright red (Eu(3+), (5)D(0)→(7)F(2)), and green (Tb(3+), (5)D(4)→(7)F(5)) down-conversion (DC) emissions. Under 980 nm NIR irradiation, Lu(2)O(3):Ln (Ln=Yb(3+)/Er(3+), Yb(3+)/Tm(3+), and Yb(3+)/Ho(3+)) NCs display the typical up-conversion (UC) emissions of green (Er(3+), (4)S(3/2),(2)H(11/2)→(4)I(15/2)), blue (Tm(3+), (1)G(4)→(3)H(6)) and yellow-green (Ho(3+), (5)F(4), (5)S(2)→(5)I(8)), respectively.     

Synthesis of 10 nm β‐NaYF4:Yb,Er/NaYF4 Core/Shell Upconversion Nanocrystals with 5 nm Particle Cores

A new method is presented for preparing gram amounts of very small core/shell upconversion nanocrystals without additional codoping of the particles. First, ca. 5 nm β‐NaYF₄:Yb,Er core particles are formed by the reaction of sodium oleate, rare‐earth oleate, and ammonium fluoride, thereby making use of the fact that a high ratio of sodium to rare‐earth ions promotes the nucleation of a large number of β‐phase seeds. Thereafter, a 2 nm thick NaYF₄ shell is formed by using 3–4 nm particles of α‐NaYF₄ as a single‐source precursor for the β‐phase shell material. In contrast to the core particles, however, these α‐phase particles are prepared with a low ratio of sodium to rare‐earth ions, which efficiently suppresses an undesired nucleation of β‐NaYF₄ particles during shell growth.     

Synthesis and Upconversion Luminescence in LaF3:Yb3+, Ho3+, GdF3: Yb3+, Tm3+ and YF3:Yb3+, Er3+ obtained from Sulfide Precursors

Rare earth fluorides are mainly obtained from aqueous solutions of oxygen‐containing precursors. Probably, this method is simple and efficient, however, oxygen may partially be retained in the fluoride structure. We offer an alternative method: obtaining fluorides and solid solutions based on them from an oxygen‐free precursor. As starting materials, we choose sulfides of rare‐earth elements and solid solutions based on them. The fluorination is carried out by exposure to hydrofluoric acid of various concentrations. The transmission electron microscopy images revealed the different morphologies of the products, which depend on the concentration of the fluorinating component (HF) and the host element. The solid solution particle size varied from 30–35 nm in the case of GdF₃:Yb³⁺, Tm³⁺ (4 % HF) to larger structures with dimensions exceeding 200 nm, such as that for LaF₃:Yb³⁺, Ho³⁺ (40 % HF). The thermal characteristics, such as the temperatures of the transitions and melting and enthalpies, were determined for the solid solutions and simple fluorides. Applicability of the materials obtained as biological luminescent markers was tested on the example of upconversion luminescence, and good upconversion properties were detected.     

不同形貌Er,Yb共掺六方NaYF4的合成及上转换发光特性研究

利用水热法成功合成了不同形貌的稀土掺杂六方NaY0.95Yb0.03Er0.02F4,包括柱状、粒状、片状、管状等.通过XRD,SEM,TEM对合成样品的物相结构及晶粒形态进行了表征,探讨络合剂EDTA用量;表面活性剂CTAB,P123,十二烷基苯磺酸钠;热溶剂水、乙二醇、聚乙二醇对晶体生长方向的影响,并对不同形态样品进行上转换发光性能测试,分析晶粒形态对上转换发光强度与寿命的影响,结果显示晶粒越小发光强度越强,相当粒径的管状样品的发光强度比粒状的强,不同晶粒形态上转换的主要能量传递模式也不相同.研究结果可以指导我们可控合成适应实际应用需求的晶粒形态及优良上转换发光性能的材料.     

Highly Luminescent Colloidal Eu3+‐Doped KZnF3 Nanoparticles for the Selective and Sensitive Detection of CuII Ions

This article describes a green synthetic approach to prepare water dispersible perovskite‐type Eu³⁺‐doped KZnF₃ nanoparticles, carried out using environmentally friendly microwave irradiation at low temperature (85 °C) with water as a solvent. Incorporation of Eu³⁺ ions into the KZnF₃ matrix is confirmed by strong red emission upon ultraviolet (UV) excitation of the nanoparticles. The nanoparticles are coated with poly(acrylic acid) (PAA), which enhances the dispersibility of the nanoparticles in hydrophilic solvents. The strong red emission from Eu³⁺ ions is selectively quenched upon addition of Cu^II ions, thus making the nanoparticles a potential Cu^II sensing material. This sensing ability is highly reversible by the addition of ethylenediaminetetraacetic acid (EDTA), with recovery of almost 90 % of the luminescence. If the nanoparticles are strongly attached to a positively charged surface, dipping the surface in a Cu^II solution leads to the quenching of Eu³⁺ luminescence, which can be recovered after dipping in an EDTA solution. This process can be repeated for more than five cycles with only a slight decrease in the sensing ability. In addition to sensing, the strong luminescence from Eu³⁺‐doped KZnF₃ nanoparticles could be used as a tool for bioimaging.     

Highly Intensified Upconversion Luminescence of Ca2+‐doped Yb/Er:NaGdF4 Nanocrystals Prepared by a Solvothermal Route

Upon introducing Ca²⁺ dopants into the grain lattices by substituting Gd³⁺ ions, irregular Yb/Er:NaGdF₄ nanocrystals prepared through a simple solvothermal route convert into highly uniform nanorods. Meanwhile, their upconversion luminescence intensifies by about 200 times, probably due to a modification of the crystal structure of NaGdF₄ and an improvement in the crystallinity of the nanophase.     

Directional Study of the Optical Properties of Tb3+‐ and Eu3+‐Doped Nanoparticles Embedded in Silica Photonic Crystals

The effects of the stop band (SB) in colloidal photonic crystals composed of silica spheres containing Eu³⁺‐ and Tb³⁺‐doped yttria nanoparticles are analysed. Reflection and transmission spectra indicate movement of the stop band, due to the 111 series of planes, towards shorter wavelengths with increasing angle of observation. The profile of the emission spectra is modified by the presence of the SB depending on the angle of measurement. Such a modification is more effective for a narrow emission band and it is thus more evident in the case of Tb³⁺ than Eu³⁺. An angular effect is also observed in the lifetime, which presents two maxima and one minimum. In the case of Tb³⁺ the maxima are at observation angles of 35 and 50°, and the minimum at 45°. We attribute this behaviour to penetration of the excitation beam at 475 nm modulated by the stop band. The ions excited in this way emit from different depths in the crystal, and therefore their lifetime will be affected differently by the same stop band, depending on the thickness of the crystal that must be crossed. Eu³⁺ shows a similar but less pronounced effect for two reasons: first, the main stop band (due to the 111 planes) is not effective at the excitation wavelength of 392 nm; second, the broadness of the Eu³⁺ emission is comparable to the width of the SB, and a decrease in the transition rate at the wavelength of the SB maximum is compensated by an increase at the sides of the SB.     

Strong Single‐Band Blue Emission from Colloidal Ce3+/Tm3+‐Doped NaYF4 Nanocrystals for Light‐Emitting Applications

An intense single‐band blue emission at λ=450 nm is observed from Tm³⁺ ions through Ce³⁺ sensitization, for the first time, in colloidal Ce³⁺/Tm³⁺‐doped NaYF₄ nanocrystals. The intense Tm³⁺ emission through broad‐band excitation is advantageous for developing luminescent nanocomposites because they can be easily incorporated into polymers. The composites can easily be coated over UV light‐emitting diodes (LEDs) to develop phosphor‐based blue LEDs.     

Infrared‐to‐Visible Light Conversion in Er3+–Yb3+:Lu3Ga5O12 Nanogarnets

Er³⁺–Yb³⁺ co‐doped Lu₃Ga₅O₁₂ nanogarnets were prepared and characterized; their structural and luminescence properties were determined as a function of the Yb³⁺ concentration. The morphology of the nanogarnets was studied by HRTEM. Under 488 nm excitation, the nanogarnets emit green, red, and near‐infrared light. The decay curves for the (⁴S_3/2, ²H_11/2) and ⁴F_9/2 levels of the Er³⁺ions exhibit a non‐exponential nature under resonant laser excitation and their effective lifetimes are found to decrease with an increase in the Yb³⁺ concentration from 1.0 to 10.0 mol %. The non‐exponential decay curves are well fitted to the Inokuti–Hirayama model for S=8, indicating that the mechanism of interaction for energy transfer between the optically active ions is of dipole–quadrupole type. Upon 976 nm laser excitation, an intense green upconverted emission is clearly observed by the naked eyes. A significant enhancement of the red‐to‐green intensity ratio of Er³⁺ ions was observed with an increase in Yb³⁺ concentration. The power dependence and the dynamics of the upconverted emission confirm the existence of two‐photon upconversion processes for the green and red emissions.     

Direct Synthesis of Hexagonal NaGdF4 Nanocrystals from a Single‐Source Precursor: Upconverting NaGdF4:Yb3+,Tm3+ and Its Composites with TiO2 for Near‐IR‐Driven Photocatalysis

A novel single‐source precursor NaGd(TFA)₄(diglyme) (TFA=trifluoroacetate) was synthesized, characterized thoroughly, and used to obtain the hexagonal phase of NaGdF₄ nanoparticles as an efficient matrix for lanthanide‐doped upconverting nanocrystals (NCs) that convert near‐infrared radiation into shorter‐wavelength UV/visible light. These NCs were then used to prepare well‐characterized TiO₂@NaGdF₄:Yb³⁺,Tm³⁺ nanocomposites to extend the absorption range of the TiO₂ photocatalyst from the UV to the IR region. While the visible/near IR part of the photoluminescent spectra remains almost unaffected by the presence of TiO₂, the UV part is strongly quenched due to the absorption of TiO₂ above its gap at approximately 380 nm by energy transfer or FRET. Preliminary results on the photocatalytic activity of the above obtained nanocomposites are presented.     

Intense Upconversion Luminescence of CaSc2O4:Ho3+/Yb3+ from Large Absorption Cross Section and Energy‐Transfer Rate of Yb3+

Concentration‐optimized CaSc₂O₄:0.2 % Ho³⁺/10 % Yb³⁺ shows stronger upconversion luminescence (UCL) than a typical concentration‐optimized upconverting phosphor Y₂O₃:0.2 % Ho³⁺/10 % Yb³⁺ upon excitation with a 980 nm laser diode pump. The ⁵F₄+⁵S₂→⁵I₈ green UCL around 545 nm and ⁵F₅→⁵I₈ red UCL around 660 nm of Ho³⁺ are enhanced by factors of 2.6 and 1.6, respectively. On analyzing the emission spectra and decay curves of Yb³⁺: ²F_5/2→²F_7/2 and Ho³⁺: ⁵I₆→⁵I₈, respectively, in the two hosts, we reveal that Yb³⁺ in CaSc₂O₄ exhibits a larger absorption cross section at 980 nm and subsequent larger Yb³⁺: ²F_5/2→Ho³⁺: ⁵I₆ energy‐transfer coefficient (8.55×10^?17 cm³ s^?1) compared to that (4.63×10^?17 cm³ s^?1) in Y₂O₃, indicating that CaSc₂O₄:Ho³⁺/Yb³⁺ is an excellent oxide upconverting material for achieving intense UCL.     

Facile Synthesis of Lanthanide (Ce,Eu, Tb,Ce/Tb,Yb/Er,Yb/Ho,and Yb/Tm)‐Doped LnF3 and LnOF Porous Sub‐Microspheres with Multicolor Emissions

Monodisperse YF₃ and YOF porous sub‐microspheres were synthesized by using a novel sacrificing template method with amorphous Y(OH)CO₃ ⋅ x H₂O as the precursors and the template. It was found that the size and shape were well maintained, and the condensed precursor was transformed into uniform porous structures after fluoridation. By fine‐tuning the feed of the fluorine source, the final product could be converted from YF₃ to YOF. A possible growth mechanism is proposed for the uniform porous YF₃ structure and the porous yolk–shell‐like YOF structure. The luminescence properties showed that the as‐synthesized YF₃:Ln³⁺ (Ln=Eu, Tb, Ce, Ce/Tb, Yb/Er, Yb/Ho, and Yb/Tm) products exhibited strong multicolor emissions, which included down‐/upconversion and energy‐transfer processes. Additionally, YOX (X=Cl and Br) could be obtained if a different halogen source was used during calcination. However, the spheres were almost completely destroyed. Our novel synthetic route can also be extended to other lanthanide fluorides (REF₃, RE=Gd, Lu), which may open a facile way to fabricate novel porous nanostructures.     

Activation of SO2 with [(η5‐C5Me5)2Ln(THF)2] (Ln=Eu,Yb) Leading to Dithionite and Sulfinate Complexes

The reaction of decamethylytterbocene [(η⁵‐C₅Me₅)₂Yb(THF)₂] with SO₂ at low temperature gave two new compounds, namely, the Yb^III dithionite/sulfinate complex [{(η⁵‐C₅Me₅)₂Yb(μ₃,1κ²O^1,3,2κ³O^2,2′,4‐S₂O₄)}₂{(η⁵‐C₅Me₅)Yb(μ,1κO,2κO′‐C₅Me₅SO₂)}₂] ( 1 ) and the Yb^III dithionite complex [{(η⁵‐C₅Me₅)₂Yb}₂(μ,1κ²O^1,3,2κ²O^2,4‐S₂O₄)] ( 2 ). After extraction of 1 , the mixture was heated to give the dinuclear tetrasulfinate complex [{(η⁵‐C₅Me₅)Yb}₂(μ,κO,κO’‐C₅Me₅SO₂)₄] ( 3 a ). In contrast, from the reaction of [(η⁵‐C₅Me₅)₂Eu(THF)₂] with SO₂ only the tetrasulfinate complex [{(η⁵‐C₅Me₅)Eu}₂(μ,κO,κO’‐C₅Me₅SO₂)₄] ( 3 b ) was isolated. Two major reaction pathways were observed: 1) reductive coupling of two SO₂ molecules to form the dithionite anion S₂O₄^2?; and 2) nucleophilic attack of one metallocene C₅Me₅ ligand on the sulfur atom of SO₂. The compounds presented are the first dithionite and sulfinate complexes of the f‐elements.