Lanthanide-doped KGd2F7 nanocrystals: controlled synthesis, optical

Subscriber access provided by Macquarie University

Article

Lanthanide-doped KGd2F7 nanocrystals: controlled synthesis, optical properties and spectroscopic identification of the optimum core/ shell architecture for highly enhanced upconverting luminescence Lihua Yu, Guowei Li, Yongsheng Liu, Feilong Jiang, and Maochun Hong Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b00040 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Crystal Growth & Design

Lanthanide-doped KGd2F7 nanocrystals: controlled synthesis, optical properties and spectroscopic identification of the optimum core/shell architecture for highly enhanced upconverting luminescence Lihua Yu,†,‡ Guowei Li,†,‡ Yongsheng Liu,*,† Feilong Jiang,† and Maochun Hong*,† †

CAS Key Laboratory of Design and Assembly of Functional Nanostructures, and

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (CAS), Fuzhou, Fujian 350002, China. ‡

Fujian Normal University, Fuzhou, Fujian, 350007, P.R. China.

KEYWORDS: KGd2F7, upconverting, core/shell, lanthanide ion, fluoride nanocrystals

ABSTRACT: Trivalent Lanthanide (Ln3+)-doped fluoride nanocrystals (NCs) in the category of ALn2F7 (A = Li, Na, and K) have recently emerged as an attractive alternative to the well-developed ALnF4-type fluorides owing to their unique crystallographic structures. In this paper, we reported for the first time the controlled synthesis of monodisperse Ln3+-doped monoclinic-phase KGd2F7 NCs via a facile

ACS Paragon Plus Environment

Crystal Growth & Design 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 32

thermal decomposition method. Characteristic downshifting and upconverting Luminescence (UCL) of Ln3+ ion ranging from the visible to near infrared spectral regions were readily tuned in the KGd2F7:Ln3+ NCs. Moreover, we have also fabricated a series of KGd2F7:Yb3+/Er3+ core/shell NCs with different architectures of inert KGd2F7 or active KGd2F7:Yb3+ shells, which thereby allowed us to carry out a comparative spectroscopic investigation of the optimum core/shell architecture for highly enhanced UCL of Ln3+. Specifically, the KGd2F7:Yb3+/Er3+ core/shell NCs with optimized thickness of inert KGd2F7 shell were found to outperform their counterparts with active KGd2F7:Yb3+ shell in terms of UCL intensity and lifetime. These results would unambiguously provide new fundamental insights into the rational design of complicated core/shell nanostructures with highly enhanced UCL of Ln3+, and would further push forward the development of novel luminescent nanomaterials based on Ln3+-doped KGd2F7 NCs for versatile applications.



INTRODUCTION Trivalent lanthanide (Ln3+) ions-doped inorganic nanocrystals (NCs) have shown

significant potential for a broad range of applications in biological imaging, sensing, anti-counterfeiting,

therapeutics,

full-colour

3D

display,

optogenetics

and

nanophotonics, because of their exceptional properties like sharp emission spectra, long excited-state lifetimes and high resistance to photobleaching.1-13 Although Ln3+doped inorganic NCs can now be made with precisely controlled compositions, dimensions, doping levels, morphologies as well as the well-designed core/shell architectures,14-23 the practical applications of Ln3+-doped NCs are still limited by their poor luminescence efficiency.24-26 In part, such deficiencies of Ln3+-doped inorganic NCs are primarily related to the notorious surface quenching effect and/or


Page 3 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the intrinsic limitation of the host matrices.27,28 To overcome these inherent limitations, many efforts have recently been devoted to developing practicable methods to improve the luminescence efficiency of Ln3+ ions when doped them in the lattices of diverse NCs, such as coating the optically active core NCs with an inert or active shell layer,29-31 manipulating the local crystal-field around the Ln3+ activator,32,33 and confining the excitation energy within a sublattice domain of tetrad Yb3+ cluster.34 Among these methods, growing inert and active shell layers around the optically active core-only NCs (namely, the core/inert shell and core/active shell strategies) are considered to be the most efficient approaches to alleviate the unwanted surface quenching effect.27,29 In these two methods, both the inert and active shell layers serve as an effective protection layer to spatially isolate the optically active core from the NC surface and the outermost environment. As a result, the notorious surface luminescence quenching through energy migration (or energy transfer) from the excited Ln3+ dopants to the surface detects or other surface quenchers can be effectively blocked, thereby resulting in a significantly increased luminescence of Ln3+ ion. Despite these significant advances, it has been a matter of much debate which way on earth is the optimal core/shell growth strategy between these two methods to eliminate the notorious surface luminescence quenching effect in Ln3+-doped luminescent NCs, largely due to the lack of comparative spectroscopic investigation of the core/inert shell and core/active shell NCs based on the same kind of host matrix. As an alternative route for the surface protection strategy, exploring the new host matrix also holds great promise in improving the luminescence efficiency of Ln3+doped NCs. Consequently, over the past decade, a large number of inorganic compounds ranging from oxides,35-39 phosphates,40 vanadates,41,42 and oxyfluorides43



Page 4 of 32

to fluorides44-48 have been explored and selected as host materials for the doping of Ln3+ ion, among which inorganic fluorides in the category of ALnF4 (A = Li, Na, or K, Ln = Y, Gd or Lu) were acknowledged to be the most efficient host materials to achieve the desirable downshifting luminescence (DSL) and upconverting luminescence (UCL) of various Ln3+ ions.1,49 In particular, ALn2F7-type fluorides, possessing unique crystallographic structures unlike those of the ALnF4 analogues, have recently emerged as a new kind of excellent fluoride host for Ln3+ doping, as exemplified

by

the

orthorhombic-phase

KYb2F7

and

KSc2F7

previously

reported.33,34,50 However, we would like to emphasize that, KGd2F7, one of the typical fluoride host crystallizing in the ALn2F7-type monoclinic-phase lattice,51,52 has been rarely reported on their controlled synthesis and optical properties especially in the form of Ln3+-doped NCs. It is believed that a comprehensive understanding of the optical properties of Ln3+ ion in the KGd2F7 NCs is of vital importance for developing novel luminescent NCs for diverse applications. In this work, we first report the controlled synthesis of monodisperse Ln3+-doped monoclinic-phase KGd2F7 (hereafter referred to as KGd2F7:Ln) NCs via a facile hightemperature thermal decomposition method. Characteristic UCL and DSL ranging from the visible to near infrared (NIR) spectral regions can be readily tuned in the KGd2F7:Ln NCs after doping with different Ln3+ emitters. The local crystal structure and site symmetry around Ln3+ dopant in the KGd2F7 NCs are investigated in detail based on the high-resolution photoluminescence (PL) excitation and emission spectra of KGd2F7:Eu at low temperature (10 K). Furthermore, by utilizing a layer-by-layer epitaxial growth protocol of thermal decomposition, we also fabricate a series of KGd2F7:Yb/Er@ KGd2F7 active core/inert shell and KGd2F7:Yb/Er@ KGd2F7:Yb active core/active shell NCs with experimentally optimized shell thickness and Yb3+


Page 5 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


doping content in the active shell layer, and then carry out a comparative spectroscopic investigation of the optimal core/shell architecture for achieving the highly enhanced UCL of Ln3+ ion in the optically active core-only NCs.



EXPERIMENTAL SECTION

Chemicals and materials. Ethanol, cyclohexane, acetone and were purchased from Sinopharm Chemical Reagent Co., China. Ln2O3 (Ln = Gd, Ce, Yb, Er, Tm, Ho, Eu and Tb), oleic acid (OA, >90%), 1-octadecene (ODE, >90%), oleylamine (OM, >90%) and CF3COOK were purchased from Sigma-Aldrich (China). Ln(CF3COO)3 was prepared as reported in the literature.53 All chemicals were used as received without purification unless otherwise noted. General procedure for the preparation of KGd2F7:Ln core-only NCs. The KGd2F7:Ln3+ (Ln = Er, Tm, Yb, Eu, Ce and Tb) core-only NCs were synthesized via a modified thermal decomposition method.15,54 Taking the synthesis of KGd2F7:Yb/Er (20/2 mol%) NCs for example, 0.5 mmol CF3COOK, 0.78 mmol Gd(CF3COO)3, 0.02 mmol Er(CF3COO)3 and 0.2 mmol Yb(CF3COO)3 were added to a 100 mL three-neck round-bottom flask containing 15 mL OA, 10 mL ODE and 5 mL OM at RT. The resulting mixture was then heated to 150 oC for 30 min under vigorous stirring in N2 atmosphere to form a transparent solution. Subsequently, the solution was heated to 315 oC with constant stirring for 60 min, and then cooled to RT naturally. The resulting KGd2F7:Yb/Er core-only NCs were precipitated by addition of acetone of 30 mL, collected via centrifugation, washed with ethanol and cyclohexane for several times, and finally re-dispersed in ethanol.



General procedure for the preparation of inert KGd2F7 (or active KGd2F7:Yb) shell precursor. In a typical procedure, 3 mmol CF3COOK and Gd(CF3COO)3 with a mole ratio of 1:2 were mixed with 30 mL OA, 20 mL OM and 10 mL ODE in a 100 mL three-neck round-bottom flask, followed by heated to 150 oC for 30 min under vigorous stirring in N2 atmosphere to form a transparent solution, and then cooled down to 100 oC naturally for the following injection. The synthesis of the active KGd2F7:Yb precursor was also done as described above except for partially replacing the Gd(CF3COO)3 with Yb(CF3COO)3. General procedure for the preparation of core/inert shell or (core/active shell) NCs. The synthesis of the core/shell structured KGd2F7 NCs was conducted via a modified successive layer-by-layer injection protocols of thermal decomposition as previously reported.15,54 In a typical procedure, 0.5 mmol CF3COOK and 1 mmol Ln(CF3COO)3 (Ln =Gd/Er/Yb (0.78/0.02/0.2 mmol)) were added to a 250 mL threeneck round-bottom flask containing 15 mL OA, 10 mL OM and 5 mL ODE, followed by heated to 150 oC for 30 min under vigorous stirring in N2 atmosphere to form a transparent solution. Subsequently, the resulting mixture was heated to 315 °C and maintained for 60 min to obtain ~10 nm KGd2F7:Yb/Er core-only NCs. After retrieving 1 mL of reaction mixture for TEM analysis, 15 mL of inert or active shell precursors were immediately injected into the reaction mixture and ripened at 315 °C for 30 min, followed by the same injection and ripening cycles. The thickness of the inert KGd2F7 or KGd2F7:Yb active shell layers can be readily tuned by adjusting the number of injection and ripening cycle. After cooling down to RT, the resulting core/inert shell or core/active shell NCs were precipitated by addition of acetone, collected by centrifugation, washed with ethanol and cyclohexane for several times, and finally re-dispersed in ethanol.


Page 6 of 32

Page 7 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Structural and optical characterization. Powder XRD measurements were performed on a powder diffractometer (MiniFlex2, Rigaku) with Cu Kα1 radiation (λ = 0.154187 nm) from 10° to 70° at a scanning rate of 5° min-1. Both the TEM and high-resolution TEM measurements were conducted on a transmission electron microscope (TECNAIG2F20) equipped with an energy dispersive X-ray spectroscope (EDS). PL emission, excitation spectra and PL decay curve were recorded on a spectrometer equipped with both continuous (450 W) xenon and pulsed flash lamps (FLS980, Edinburgh Instrument). UCL spectra were carried out upon 980 nm excitation provided by a continuous-wave semiconductor laser diode. UCL decays were measured with a customized ultraviolet to mid-infrared steady-state and phosphorescence lifetime spectrometer (FSP920-C, Edinburgh Instrument) equipped with a digital oscilloscope (TDS3052B, Tektronix) and a tunable mid-band optical parametric oscillator (OPO) pulse laser as the excitation source (410-2400 nm, 10 Hz, pulse width ≤ 5 ns, Vibrant 355II, OPO-TEK), and the effective UCL lifetime (τeff) was determined by:

τ

1 𝐼

𝐼 𝑡 𝑑𝑡

Where I0 and I(t) represent the maximum UCL intensity and intensity at time t after cut off of the excitation light, respectively. The absolute UCQYs for the samples were measured with a customized UCL spectroscopy system at RT upon a 980-nm diode laser excitation at power density of 50 W/cm2, and the UCL peaks from Er3+ ions in the spectral range of 400-750 nm were integrated for the UCQY determination. All the spectral data collected were corrected for the spectral response of both the spectrometer and the integrated sphere.





RESULTS AND DISCUSSION Controlled synthesis, structure and morphology of the KGd2F7:Ln NCs. The

KGd2F7 crystal has a monoclinic crystal structure with space group P12/M1 (a = 4.039 Å, b = 4.045 Å, c = 5.852 Å, β = 90.15°, Z =2), wherein Gd3+ and K+ cations randomly occupy sites 1a (0, 0, 0) and 1h (1/2, 1/2, 1/2) with a crystallographic site symmetry of C2h (Figure 1a and Table S1). Monodisperse Ln3+-doped KGd2F7 NCs (Ln = Eu, Ce/Tb, Yb/Er, Yb/Tm and Yb/Ho) were fabricated via a modified thermal decomposition method we previously reported15, 54 by using the oleic acid (OA), 1octadecene (ODE), oleylamine (OM) as surfactant and/or solvent. As shown in Figure 1b, all the X-ray powder diffraction (XRD) patterns for the as-synthesized KGd2F7:Ln NCs can be exclusively indexed as the pure monoclinic-phase KGd2F7 crystal (JCPDS No. 27-0387), indicating the formation of highly crystalline KGd2F7 NCs without any other impurities such as cubic-phase KGdF4 and KGd3F10. Representative transmission electron microscopy (TEM) images for the as-synthesized KGd2F7 NCs doped with Yb3+/Er3+, Yb3+/Tm3+, Eu3+, and Ce3+/Tb3+ ions demonstrate their nearly spherical shapes with average sizes of 10.9 ± 1.1, 10.8 ± 1.2, 8.8 ± 1.0, and 8.8 ± 0.9 nm, respectively (Figure 1c-f). Specifically, the category of Ln3+ dopants was found to have negligible impact on their size, morphology and crystal phase of the resulting KGd2F7:Ln NCs.55 The corresponding high-resolution TEM images clearly show the high crystallinity of the as-synthesized KGd2F7:Ln NCs we obtained (Figure 1g-j). The lattice fringes are very clear with an observed d-spacing of 0.33 nm, in good agreement with the lattice spacing of the (-220) plane of monoclinic-phase KGd2F7 (JCPDS No. 27-0387). Compositional analyses for the as-synthesized NCs by energy dispersive X-ray spectroscopy reveal the presence of the K, Gd and F elements of host matrix and elements of Ln3+ dopants (Figure S1), indicative of the successful doping


Page 8 of 32

Page 9 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of Ln3+ ion in the lattice of the KGd2F7 NCs. Such successful doping of Ln3+ ion in the KGd2F7 NCs is conceivable due to the facts that the chemical valences of Ln3+ ions are the same as that of host Gd3+ and the ionic radii of Ln3+ ions are close to that of Gd3+. To the best of our knowledge, monodisperse and uniform Ln3+-doped KGd2F7 NCs remains nearly untouched before this work.

Figure 1. (a) Crystal structure of monoclinic-phase KGd2F7 crystal. (b) XRD patterns, (c-f) representative TEM and (g-j) high-resolution TEM images for the as-synthesized KGd2F7:Ln NCs doped with Yb3+/Er3+ (20/2 mol%), Yb3+/Tm3+ (20/1 mol%), Eu3+ (20 mol%) and Ce3+/Tb3+ (10/10 mol%) ions. Note that their corresponding size distribution histograms are inserted in the bottoms of c-f, and the scale bars for c-f and g-j are 50 nm and 5 nm, respectively. In addition to the core-only KGd2F7:Ln NCs aforementioned, we have also synthesized a series of core/shell structured KGd2F7:20%Yb/2%Er@ KGd2F7



core/inert shell or KGd2F7:20%Yb/2%Er@ KGd2F7:Yb core/active shell NCs with tunable shell thickness of inert KGd2F7 shell (Figure 2a) and active KGd2F7:Yb shell doped with (Figure 2b) fixed or (Figure 2c) gradient concentration of Yb3+ around the optically active KGd2F7:Yb/Er core-only NCs, with an attempt to identify the better core/shell architecture for achieving the highly enhanced UCL of Ln3+ in the KGd2F7:Yb/Er core-only NCs. After the successive layer-by-layer epitaxial shell growth, the average sizes for the core/shell structured NCs with different architectures were observed to apparently increase from 8-10 nm for the core-only NCs to 12-19 nm (Figure 2d-f). In particular, when compared to their corresponding core-only counterparts, the core/shell structured NCs exhibit essentially the same crystal phase and morphology apart from the increased NC size, indicating the successfully epitaxial shell growth. Lattice fringes corresponding to the (-220) plane of monoclinic KGd2F7 can be also observed in high-resolution TEM images (insets of Figure 2d-f), which clearly demonstrate the highly crystalline nature of the core/shell structured NCs we prepared regardless of their different thicknesses of the inert and active shells.

Figure 2. Schematic illustrations of the KGd2F7:Ln core/shell NCs with tunable thickness of (a) inert KGd2F7 shell and active KGd2F7:Yb shell doped with (b) fixed


Page 10 of 32

Page 11 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


or (c) gradient concentration of Yb3+. (d-f) Typical TEM and high-resolution TEM images for the as-synthesized core/shell NCs of different sizes (13.1 ± 1.5, 15.7 ± 1.4 and 19.1 ± 1.5nm) and shell thicknesses (r, 1.0, 2.4 and 4.1 nm), demonstrating the high crystallinity and monodispersity of the core/shell NCs we prepared. The scale bars in TEM and high-resolution TEM images are 20 and 5 nm, respectively. Spectroscopic site of Ln3+ in the KGd2F7:Ln NCs. It is well known that the optical properties of Ln3+-doped NCs depends critically on the local crystal structure and/or crystal-field (CF) surroundings around Ln3+ dopant.56 To probe the practical local structure around Ln3+ in the KGd2F7 NCs, we first measured PL excitation, emission spectra and PL decays of Eu3+ ion in the KGd2F7:20%Eu3+ NCs at room temperature (RT), considering the fact that the Eu3+ ion is a sensitive structural probe to investigate the coordination environment around the cations substituted in the crystalline lattice. As shown in Figure 3a, upon UV excitation from the ground state of 7F0 to the 5L6 state of Eu3+ at 393 nm, typical PL emission lines peaking at 578, 593.1, 612, 650 and 700 nm were detected at RT, which can be readily assigned to the de-excitations from the 5D0 to its lower states of 7F0, 7F1, 7F2, 7F3 and 7F4 of Eu3+, respectively. These characteristic PL emission lines of Eu3+ with partially resolved CF splitting and dominant emission peaks of 5D0→7F1,2 transitions show clearly that Eu3+ ion should experience a well-ordered CF surroundings in the lattice of KGd2F7:Eu NCs, which can be confirmed by the mono-exponential decay of the 5D0 state of Eu3+ with a long lifetime up to 3.4 ms (Figure 3b). From the viewpoint of spectroscopy, most of Eu3+ ions are very likely located at substitutional Gd3+ lattice sites in the KGd2F7 NCs.52



Figure 3. (a) PL excitation (left), emission (right) spectra and (b) PL decay curve of Eu3+ ion in the KGd2F7:20%Eu3+ NCs at RT. (c) High-resolution PL emission spectrum of Eu3+ in the KGd2F7:20%Eu3+ NCs at 10 K and (d) theoretically allowed transition lines of Eu3+ in a C2h site symmetry. (e) PL excitation spectra of Eu3+ in the KGd2F7:20%Eu3+ NCs at 10 K by monitoring the characteristic emission lines of the 5

D0→7F1 transition of Eu3+ at 583.6, 588.6, and 593.1 nm, and (f) the PL decay curve

for the PL emission line of Eu3+ at 593.1 nm. In order to validate this hypothesis, we further measured the high-resolution PL spectra of Eu3+ in the KGd2F7:Eu NCs at low temperature (10 K). Note that the PL emission and excitation spectra and PL decays were recorded at 10 K to avoid the thermal broadening of spectral bands commonly observed at RT. As expected, much sharper and better resolved emission lines originating from the transitions of 5

D0→7F0,1,2,3,4 were clearly detected upon direct excitation of Eu3+ at 393 nm at 10 K

(Figure 3c), which thereby enables a detailed assignment of the CF emission lines of


Page 12 of 32

Page 13 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Eu3+ in the KGd2F7:Eu NCs. According to the monoclinic-phase KGd2F7 crystal structure, Eu3+ ion substituting for Gd3+ should situate at a crystallographic site with symmetry of C2h in theory (Figure 3d). If the doped Eu3+ ion remains such an ideal C2h symmetry, the electric-dipole 5D0→7F0,2,4 emissions of Eu3+ will be strictly forbidden, and the number for the Eu3+ 5D0→7F1 emission lines of the magneticdipole nature should be 3 (Figure 3d). As a matter of fact, the integrated PL intensity for the 5D0→7F2 transition of Eu3+ with 5 well-resolved emission lines was found to be comparable to that of the 5D0→7F1 transition peaking at 593 nm (Figure 3c). These findings coupled with the presence of 5D0→7F0 transition at 578 nm in Figure 3c show clearly that the site symmetry of Eu3+ in the monoclinic-phase KGd2F7 NCs should be restricted to Cs, Cn, or Cnv (n = 1, 2, 3, 4, 6). On the basis of the branching rules (Figure S2) and transition selection rules of Eu3+ ion in the 32 point groups (Table S2),6 the highest site symmetry of Eu3+ after replacing Gd3+ in the KGd2F7 NCs, distorted from C2h, is degraded to be Cs or C2. That is to say, the real symmetry of spectroscopic site of Eu3+ in the KGd2F7 NCs differs drastically from that of theoretical crystallographic site of Gd3+ (C2h) after the successful doping in the lattice of KGd2F7 NCs. To make sure whether all these PL emission peaks come from identical Eu3+ site in the KGd2F7:Eu3+ NCs, high-resolution PL excitation spectra were also recorded at 10 K by monitoring the three peaks of the 5D0→7F1 transition at 583.6, 588.6 and 593.1 nm, respectively (Figure 3e). All of these PL excitation spectra were observed to be coincident well with each other, which suggests that these PL emission peaks we detected in the KGd2F7:Eu NCs are ascribed to Eu3+ occupying the same type of spectroscopic site. This can be further confirmed by our PL decay studies where the PL decay profile exhibits obviously a single-exponential nature, and the PL lifetime



of 5D0 of Eu3+ was determined to be 5.5 ms by using a single-exponential fitting (Figure 3f). Such single-exponential decay behaviour is expected due to the nearly homogeneous CF environment around Eu3+ in the single lattice site of the KGd2F7:Eu NCs. Particularly, we would like to emphasize that the PL emission lines of Eu3+ ascribed to the surface sites were hardly detectable in these KGd2F7:Ln NCs even by using the state-of-the-art spectroscopic equipments, which is totally different from our previous observations in Eu3+ doped other NCs like KYF4 and ZnO with apparent surface Eu3+ sites.57, 58 This result further confirms that all the observed PL emission lines in Figure 3c are originating from Eu3+ ion in the lattice site of the KGd2F7:Ln NCs. By using the same synthetic procedures, Ce3+/Tb3+, Yb3+/Er3+, Yb3+/Tm3+ and Yb3+/Ho3+ ions can be also introduced into the lattice of the KGd2F7 NCs, thereby producing intense and typical DSL and UCL spanning a broad spectral range from visible to NIR (Figure 4). Taking the KGd2F7:Ce/Tb NCs for example, the PL excitation spectrum of Tb3+ (Figure 4a, left) was found to be dominated by the broad excitation lines peaking at ~287 nm that corresponds to the 4f/5d transition of Ce3+, whereas the DSL spectrum displays typical emission lines assigned to the 5

D4→7F6,5,4,3 transitions of Tb3+ when indirectly excited at 287 nm (Figure 4a, right),

suggesting that the Tb3+ emission can be achieved via an energy transfer process from the Ce3+ to Tb3+. In this energy transfer process, Ce3+ ion acts as an effective lightharvesting antenna to absorb UV excitation light and then transfer the excitation energy to the adjacent Tb3+ ions, resulting in an overall green emission output of Tb3+ as presented in the inset of Figure 4a. Apart from the typical DSL of Tb3+, multicoloured UCL can be also obtained from these Ln3+-doped KGd2F7 NCs. Upon 980nm diode laser excitation with a power density of 50 W/cm2, the KGd2F7:


Page 14 of 32

Page 15 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20%Yb/2%Er NCs exhibit characteristic sharp emission lines resulting from the 2H11/2, 4

S3/2→4I15/2 and 4F9/2→4I15/2 transitions of Er3+ (Figure 4b). These emission bands of

Er3+ lead to an overall green colour output that is visible to the naked eye (Figure 4b, insert). By contrast, the KGd2F7:20%Yb/1%Tm NCs feature a blue coloured UCL resulting from the 1G4→3H6, 1G4→3F4, and 3H4→3H6 transitions of Tm3+ (Figure 4c), while the KGd2F7:20%Yb/5%Ho NCs display an orange coloured UCL due to 5

F3→5I8, 5F4→5I8, and 5F5→5I8 transitions of Ho3+ (Figure 4d). Taken together, these

results suggest strongly that the KGd2F7 crystal is a new type of promising host material for Ln3+ doping.

Figure 4. (a) PL excitation (left) and emission (right) spectra of Tb3+ in the KGd2F7:Ce3+/Tb3+ NCs at RT. (b-d) UCL spectra for the KGd2F7 NCs doped with Yb3+/Er3+, Yb3+/Tm3+ and Yb3+/Ho3+ couples upon a 980-nm diode laser excitation with a power density of 50 W/cm2, and the insets show their corresponding UCL photographs when dispersed in cyclohexane solution.



Identifying the optimum shell thickness in the KGd2F7:Yb/Er@ KGd2F7 core/inert shell NCs. Even though multicolour UCL of Ln3+ can be easily achieved after doping the typical UCL couples of Yb3+/Er3+, Yb3+/Tm3+ and Yb3+/Ho3+ into the lattices of KGd2F7 NCs, these small-sized core-only NCs (~10 nm) usually suffer from severe surface luminescence quenching effect owing to their large surface areato-volume ratios. Therefore, the absolute upconversion quantum yield (UCQY), defined as the ratio of the number of emitted photons to the number of the absorbed photons, was determined to be less than 0.01% (a value that is very close to the detection limit of our UCQY measurement system59) for all the core-only NCs (Table S3), upon a 980-nm diode laser excitation at a power density of 50 W/cm2. To greatly improve UCL efficiency of these core-only NCs, a series of KGd2F7:Yb/Er@ KGd2F7 core/inert shell NCs with different shell thicknesses were prepared via a successive epitaxial growth of inert KGd2F7 shell around the optically active KGd2F7:Yb/Er coreonly NCs (Figure 5a). After the layer-by-layer epitaxial growth of inert KGd2F7 shell, the average sizes for the core/inert shell NCs were found to gradually increase from 10.9 ± 1.1 nm for the core-only NCs to 12.7 ± 1.7, 15.7 ± 1.4, 18.4 ± 1.8, and 22.0 ± 1.6 nm, with the corresponding inert shell thicknesses of about 0.9, 2.4, 3.7 and 5.6 nm, respectively (Figure 5b-f). Notably, the overall UCL intensities for these core/inert shell NCs were observed to increase first and then decrease when increasing the thickness of inert shell from 0 to 5.6 nm (Figure 5g, h), with the highest achieved in the KGd2F7:Yb/Er@ KGd2F7 core/inert shell NCs possessing a shell thickness of ~3.7 nm. This can be firmly evidenced by the evolution of UCL brightness of Er3+ in the digital photographs of the core/inert shell NCs when dispersed them in cyclohexane solution (Figure 5h). The initial enhancement in the overall UCL intensity of these KGd2F7:Yb/Er@ KGd2F7 core/inert shell NCs is primarily ascribed


Page 16 of 32

Page 17 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to the suppressed surface quenching effect after inert shell growth, while the subsequent decrease may be caused by the reduced overall Yb3+/Er3+ doping content owing to the excessive growth of inert KGd2F7 shell (> 3.7 nm) that will somewhat degrade the absorption capability of 980-nm excitation light. In this regard, the optimum thickness of KGd2F7 inert shell grown around the optically active KGd2F7:Yb/Er core-only NCs is estimated to be ~3.7 nm to minimize the unwanted surface luminescence quenching effect.

Figure 5. (a) Schematic illustration of the formation of the KGd2F7:20%Yb/2%Er @KGd2F7 core/inert shell NCs with a tunable thickness of inert KGd2F7 shell from 0 to ~5.6 nm, and (b-f) their corresponding TEM images and size distribution histograms (scale bar is 20 nm). (g) Typical UCL spectra, (h) integrated UCL intensities and photographs, and (i) UCL lifetimes of Er3+ in the KGd2F7:Yb/Er @KGd2F7 core/inert shell NCs as a function of the thickness of the inert KGd2F7 shell.



Since both the suppressed surface quenching and the reduced overall doping content of Yb3+ (or Er3+) ion will prolong the lifetime of excited state of Er3+, we then examined the UCL decays of the 4S3/2 (542 nm) and 4F9/2 (656 nm) states of Er3+ upon 980-nm excitation for the core-only and core/inert shell NCs. As shown in Figures 5i and S3, the UCL lifetimes for the 4S3/2 and 4F9/2 states of Er3+ were determined to significantly increase from 0.03 and 0.04 ms for the core-only NCs to 0.17 and 0.28 ms for the core/inert shell NCs with a shell thickness of about 5.6 nm. Such significantly prolonged UCL lifetimes of Er3+ in the core/inert shell NCs are primarily associated with the decreased energy migration probability to the NC surface defects or other quenchers, because the epitaxial inert shell growth around the KGd2F7:Yb/Er core-only NCs can effectively separate them in space from the UCL emitter of Er3+ located at the core region. As such, multiphonon relaxation processes of Er3+ ion mediated by those unwanted surface defects or quenchers can be reduced, which thereby results in the enhanced UCL and prolonged lifetime of Er3+. Determining the optimum Yb3+ doping content in the active shell of the KGd2F7:Yb/Er@KGd2F7:Yb core/shell NCs. Besides the inert KGd2F7 shell growth, epitaxial growth of an active shell (namely, Yb3+-doped KGd2F7 shell) around the optically active core-only NCs is another commonly used strategy to enhance UCL of Ln3+. To examine the optimum Yb3+ doping content in the active KGd2F7:Yb shell, we synthesized a series of KGd2F7:20%Yb/2%Er@ KGd2F7:x%Yb core/active shell NCs with nearly identical shell thickness of ~1.2 nm by varying the doping concentration of Yb3+ ion (x) from 5 to 20 mol% (Figure 6a). Although the doping concentration of Yb3+ ion in the active shell was observed to have no distinct impact on the NC size, morphology and dispersity (Figure 6b-e), the UCL intensities of these core/active shell NCs are strongly affected (Figure 6f, g). For instance, the integrated


Page 18 of 32

Page 19 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


UCL intensity for the KGd2F7:20%Yb/2%Er@ KGd2F7:10%Yb core/active shell NCs was determined to be the highest, which was about 1.2, 1.6 and 3.8 times stronger than those of the KGd2F7:20%Yb/2%Er@ KGd2F7:Yb core/active shell NCs doped with 5 mol%, 10 mol%, 15 mol% and 20 mol% Yb3+ in the active shell (Figure 6g). By correlating the integrated UCL intensity with the Yb3+ doping concentration, the optimum Yb3+ doping concentration in the active shell was determined to be ~10 mol%, as supported by the brightness of UCL photographs of these core/active shell NCs under 980-nm laser irradiation wherein the UCL of the KGd2F7:20%Yb/2%Er@ KGd2F7:10%Yb core/active shell NCs is the brightest (Figure 6g, inset).

Figure 6. (a) Schematic illustration of the KGd2F7:20%Yb/2%Er@ KGd2F7:x%Yb core/active shell NCs with varied Yb3+ doping concentration (x) from 5 to 20%, and their corresponding (b-e) TEM images and size distribution histograms (scale bar is 50 nm). (f) Typical UCL spectra, (g) integrated UCL intensities and photographs, (h) and UCL lifetimes of Er3+ in the KGd2F7:Yb/Er@ KGd2F7:Yb core/active shell NCs as a function of the Yb3+ doping concentration in the active shell.



We attribute the change in the UCL intensity of the KGd2F7:20%Yb/2%Er@ KGd2F7:x%Yb core/active shell NCs as a function of Yb3+ doping concentration to the synchronic effect of absorption and depletion of 980-nm excitation light. Although an elevated Yb3+ doping concentration in the active shell can promote the absorption of 980-nm excitation light, it can also result in the increased energy migration (or depletion) from the excited Yb3+ (or Er3+) to the surface defects or other quenchers as the interionic distance of Ln3+ ions shortens. Such increased energy migration probability can be further validated by our UCL decay studies (Figures 6h and S4). A shortening in the Ln3+ UCL lifetime is considered to be an explicit and convincing indicator for the increased energy migration probability, which can increase the nonradiative rate of the excited state of Ln3+ ion and thus shortened UCL lifetime. As anticipated, both the UCL lifetimes of the 4S3/2 and 4F9/2 states of Er3+ in the KGd2F7:Yb/Er@ KGd2F7:Yb core/active shell NCs gradually reduce with increasing the Yb3+ doping concentration from 5 to 20 mol% in the active shell (Figure 6h). Furthermore, it is worthy of pointing out that all the UCL lifetimes for the 4S3/2 and 4

F9/2 states of Er3+ in the KGd2F7:Yb/Er@ KGd2F7:Yb core/active shell NCs were

found to be much shorter than those in the KGd2F7:Yb/Er@ KGd2F7 core/inert shell NCs with almost identical shell thickness (Figure S4), which thereby provides another solid evidence to turn out the increased energy migration from the excited Ln3+ dopant to the surface defects or other quenchers after the doping of Yb3+ in the active shell. To further probe the origin underlying this reduced UCL lifetime of Er3+ after the Yb3+ doping in the active shell, we then synthesized two sets of core/active shell NCs with different architectures as indicated in Figure 7a-d, through a layer-by-layer epitaxial growth of active KGd2F7:Yb shell doped with fixed (10 mol%) and gradient


Page 20 of 32

Page 21 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(10 and 5 mol%) concentration of Yb3+ around the KGd2F7:20%Yb/2%Er core-only NCs,

namely,

KGd2F7:20%Yb/2%Er@

KGd2F7:10%Yb@

KGd2F7:10%Yb@

KGd2F7:10%Yb (Figure 7a, denoted as C/S-Af-1), and KGd2F7:20%Yb/2%Er@ KGd2F7:10%Yb@ KGd2F7:5%Yb@ KGd2F7 (Figure 7b, denoted as C/S-Ag-1). For comparison, KGd2F7:20%Yb/2%Er@ KGd2F7:10%Yb/2%Er@ KGd2F7:5%Yb/2%Er @KGd2F7 (Figure 7c, denoted as C/S-Ag-2) and KGd2F7:20%Yb/2%Er@ KGd2F7:10%Yb/2%Er@ KGd2F7:10%Yb/2%Er@ KGd2F7:10%Yb/2%Er (Figure 7d, denoted as C/S-Af-2) core/active shell NCs with 2 mol% Er3+ ion simultaneously doped into the active shells were also prepared by using the similar synthesis procedure. Despite of their almost identical NC sizes, shapes and distributions (Figure 7e-h), these as-synthesized core/active shell NCs were found to be indeed different from each other in term of the integrated UCL intensity and lifetime of Er3+. As compared in Figure 7i, the integrated UCL intensity for the C/S-Ag-1 with gradient Yb3+ doping concentration in the active shells was slightly enhanced by a factor of ~2.8 when compared with their C/S-Af-1 counterparts with fixed Yb3+ doping concentration of 10 mol%, accompanied by the significantly increased UCL lifetimes of the 4S3/2 and 4F9/2 states of Er3+ from 0.06 and 0.13 ms to 0.12 and 0.18 ms (Figure 7j), respectively. Moreover, after doping additional Er3+ emitter (2 mol%) into the active shell of the C/S-Af-1 and C/S-Ag-1 NCs (i.e., C/S-Af-2 and C/S-Ag-2), both the integrated UCL intensity and lifetime of Er3+ for the C/S-Af-2 and C/S-Ag-2 were observed to decrease apparently relative to their respective counterparts (Figure 7i, j). These results solidly confirm that the high Yb3+ or Er3+ dopant concentration in the active shell do facilitate the energy migration from the excited Ln3+ to surface defects and other quenchers, thereby leading to the shortened UCL lifetime of Er3+ located at the optically active core region.



Figure 7. Schematic illustrations of the designed architectures for the (a) C/S-Af-1, (b) C/S-Ag-1, (c) C/S-Ag-2 and (d) C/S-Af-2 core/active shell NCs, and their corresponding (e-h) TEM images and size distribution histograms (scale bar is 20 nm). Comparison of their (i) UCL spectra and (j) lifetimes of the 4S3/2 (542 nm) and 4F9/2 (656 nm) states of Er3+ upon a 980-nm laser excitation. Spectroscopic investigation of the optimum core/shell architecture for highly enhanced UCL of Ln3+. After the acquisition of these optimized experimental parameters including the thickness of inert shell and the doping concentration of Yb3+ in the active shell, we then systematically investigated the UCL properties of the C/SAf-1, C/S-Ag-1 and the KGd2F7:20%Yb/2%Er@ KGd2F7@ KGd2F7@ KGd2F7 (denoted as C/S-I) with almost identical thickness of inert KGd2F7 and active KGd2F7:Yb shells (Figures 8a-c and S5). Figure 8d shows the typical UCL spectra for the C/S-Af-1, C/S-Ag-1 and C/S-I NCs measured under identical experimental


Page 22 of 32

Page 23 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conditions upon a 980-nm diode laser excitation with a power density of 50 W/cm2. Regardless of their different core/shell architectures, all the UCL spectra were observed to be in good agreement with each other in terms of line positions, shapes and relative intensities, which clearly suggests that all the Er3+ emitters experience nearly the same CF environment in these three types of core/shell NCs. However, the overall integrated UCL intensity for the C/S-I with the optimized inert shell of pure KGd2F7 (~3.7 nm) was determined to be the highest, which is about 4.5 and 1.8 times stronger than those of C/S-Af-1 and C/S-Ag-1 NCs with fixed (10 mol%) and gradient Yb3+ in the active shells (Figure 8d). Likewise, the absolute UCQYs were observed to increase markedly from 0.01% for the C/S-Af-1 and 0.11% for the C/S-Ag-1 to 0.28% for the C/S-I upon 980-nm laser excitation at a power density of 50 W/cm2 (Figure 8e). Since all the optically active Er3+ emitters in these three types of core/shell NCs are located at the core regions that can be spatially isolated from the surface defects or other surface quenchers by the inert and active shells, such enhancement in the UCL intensity and UCQY for the C/S-I NCs relative to those of the C/S-Af-1 and C/S-Ag-1 clearly demonstrates the overwhelming advantages of the inert shell growth over the active shell around the core-only NCs in producing intense UCL. As anticipated, both the UCL lifetimes for the 4S3/2 and 4F9/2 states of Er3+ in the C/S-I NCs were measured to be much longer than those in their C/S-Af and C/S-Ag counterparts (Figure 8f, g). Taken together, these results unambiguously provide convincing spectroscopic evidences to turn out that the growth of inert shell is the most efficient way to maximize the UCL efficiency of Ln3+-doped core/shell NCs.



Figure 8. TEM images, size distribution histograms and HRTEM images for the (a) C/S-Af-1, (b) C/S-Ag-1 and (c) C/S-I core/shell NCs (the scale bars are 20 and 5 nm, respectively). The average sizes for the core/shell NCs with different architectures were determined to be 18.8 ± 1.4, 18.8 ± 1.8, and 18.4 ± 1.4 nm, with the corresponding inert or active shell thickness of about 3.7, 3.7 and 3.5 nm, respectively. Comparisons of (d) UCL spectra, (e) UCQY, and (f) UCL decay curves and (g) the determined lifetimes of the 4S3/2 (542 nm) and 4F9/2 (656 nm) states of Er3+ upon 980nm laser excitation for the C/S-Af-1, C/S-Ag-1, and C/S-I core/shell NCs.



CONCLUSION

In summary, we have reported a new class of Ln3+-doped fluoride NCs based on the monoclinic-phase KGd2F7. The optical properties including steady-state PL and


Page 24 of 32

Page 25 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


excited state dynamics of KGd2F7 NCs doped with the typical DSL and UCL Ln3+ emitters were comprehensively surveyed. High-resolution PL spectroscopy by utilizing Eu3+ ion as a sensitive structural probe revealed that the doped Ln3+ ions occupy only one lattice site with symmetry distorted from theoretical C2h to Cs or C2 in KGd2F7 NCs. Through a successive layer-by-layer epitaxial growth approach, we have also fabricated a series of KGd2F7:Ln core/shell NCs with different architectures of inert and active shells, which thereby allowed us to carry out a comparative spectroscopic investigation of the optimum core/shell design strategy for achieving the most efficient UCL of Ln3+ doped in the core-only NCs. By using the typical UCL couple of Yb3+/Er3+ as a model, we unravelled the correlation between the integrated UCL intensity (or UCQY) and the shell thickness of inert KGd2F7 shell and the Yb3+ doping concentration in the active KGd2F7:Yb shell around the KGd2F7:Yb3+/Er3+ core-only NCs. Specifically, the KGd2F7:Ln core/shell NCs with optimized thickness of inert KGd2F7 shell were found to outperform their counterparts with fixed or gradient Yb3+ doping concentration in the active shells in terms of the UCL intensity and lifetime, thereby providing direct evidence to support our finding that the epitaxial inert shell growth around the optically active core-only NCs is the optimal core/shell design strategy to spatially isolate the core from the deleterious surface defects or other quenchers and thus minimize the unwanted surface quenching effect. These results would unambiguously provide new fundamental insights into the rational design of complex core/shell nanostructures for achieving highly efficient UCL of Ln3+, and would further push forward the applications of Ln3+-doped KGd2F7 nanomaterials in diverse fields such as biological sensing and imaging that remain nearly untouched.



ASSOCIATED CONTENT



Supporting Information

The Supporting Information is available free of charge on the ACS Publications website at DOI: ******.

Figure S1. EDS analyses of as-synthesized KGd2F7:Ln3+; Figure S2. Branching rules of the 32 point groups; Figure S3, 4. UCL decay curves and lifetimes of Er3+ in the KGd2F7 core-only and core/shell KGd2F7 NCs; Figure S5. TEM images, size distribution histograms and HRTEM images of the different core/shell architectures of C/S-Af-1, C/S-Ag-1 and C/S-I NCs. Table S1. Crystallographic data of the monoclinic-phase KGd2F7 structure; Table S2. Theoretically allowed transition lines of 5D0-7FJ of Eu3+ ions at 32 crystallographic point groups; Table S3. Comparison of some experimental parameters for KGd2F7:Yb/Er core-only and different core/shell architecture.



AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected].

*E-mail: [email protected].

Notes


Page 26 of 32

Page 27 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The authors declare no competing ﬁnancial interest.



ACKNOWLEDGMENT

This work is supported by the Strategic Priority Research Program of CAS (XDB20000000 and XDA09030102), the NSFC (Nos. 21390392, 21473205, 21871256, and 21731006), Youth Innovation Promotion Association of CAS, and the Natural Science Foundation of Fujian Province (No. 2017J01038).



REFERENCES

(1) Zhou, B.; Shi, B. Y.; Jin, D. Y.; Liu, X. G. Controlling upconversion nanocrystals for emerging applications. Nat. Nanotechnol. 2015, 10, 924-936. (2) Chen, S.; Weitemier, A. Z.; Zeng, X.; He, L. M.; Wang, X. Y.; Tao, Y. Q.; Huang A. J. Y.; Hashimotodani, Y.; Kano, M.; Iwasaki, H.; Parajuli, L. K.; Okabe, S.; Teh, D. B. L.; All, A. H.; Tsutsui-Kimura, I.; Tanaka, K. F.; Liu X. G.; McHugh, T. J. Near-infrared deep brain stimulation via upconversion nanoparticle-mediated optogenetics. Science 2018, 359, 679-683. (3) Liu, Y. J.; Lu, Y. Q.; Yang, X. S.; Zheng, X. L.; Wen, S. H.; Wang, F.; Vidal, X.; Zhao, J. B.; Liu, D. M.; Zhou, Z. G.; Ma, C. S.; Zhou, J. J.; Xi, J. A.; Piper, P.; Jin, D. Y. Amplified stimulated emission in upconversion nanoparticles for super-resolution nanoscopy. Nature 2017, 543, 229-234. (4) Lu, Y. Q.; Zhao, J. B.; Zhang, R.; Liu, Y. J.; Liu, D. M.; Goldys, E. M.; Yang, X. S.; Xi, P.; Sunna, A.; Lu, J.; Shi, Y.; Leif, R. C.; Huo, Y. J.; Shen, J.; Piper, J. A.; Robinson J. P.; Jin, D. Y. Tunable lifetime multiplexing using luminescent nanocrystals. Nat. Photonics. 2014, 8, 33-37. (5) Liu, Y. S.; Tu, D. T.; Zhu, H. M.; Ma, E.; Chen, X. Y. Lanthanide-doped luminescent nano-bioprobes: from fundamentals to biodetection. Nanoscale 2013, 5, 1369-1384. (6) Liu, Y. S.; Tu, D. T.; Zhu, H. M.; Chen, X. Y. Lanthanide-doped luminescent nanoprobes: controlled synthesis, optical spectroscopy, and bioapplications. Chem. Soc. Rev. 2013, 42, 6924-6958. (7) Zhu, X. J.; Feng, W.; Chang, J.; Tan, Y. W.; Li, J. C.; Chen, M.; Sun, Y.; Li, F. Y. Temperature-feedback upconversion nanocomposite for accurate photothermal therapy at facile temperature. Nat. Commun. 2016, 7, 10437 (8) Zhou, J.; Liu, Q.; Feng, W.; Sun, Y.; Li, F. Y. Upconversion Luminescent Materials: Advances and Applications. Chem. Rev. 2015, 115, 395-465. (9) Fan, W. P.; Bu, W. B.; Shi, J. L. On The Latest Three-Stage Development of Nanomedicines based on Upconversion Nanoparticles. Adv. Mater. 2016, 28, 3987-4011.



(10) Lee, J.; Yoo, B.; Lee, H.; Cha, G. D.; Lee, H. S.; Cho, Y.; Kim, S. Y.; Seo, H.; Lee, W.; Son, D.; Kang, M.; Kim, H. M.; Park, Y. I.; Hyeon, T.; Kim, D. H. Ultra-Wideband MultiDye-Sensitized Upconverting Nanoparticles for Information Security Application. Adv. Mater. 2017, 29, 1603169. (11) Gai, S. L.; Li, C. X.; Yang, P. P.; Lin, J. Recent Progress in Rare Earth Micro/Nanocrystals: Soft Chemical Synthesis, Luminescent Properties, and Biomedical Applications. Chem. Rev. 2014, 114, 2343-2389. (12) You, W. W.; Tu, D. T.; Zheng, W.; Huang, P.; Chen, X. Y. Lanthanide-doped disordered crystals: Site symmetry and optical properties. J. Lumin. 2018, 201, 255-264. (13) Tang, Z. M.; Liu, Y. Y.; He, M. Y.; Bu, W. B. Chemodynamic Therapy: Tumour Microenvironment‐Mediated Fenton and Fenton‐like Reactions. Angew. Chem. Int. Ed. 2019, 58, 946-956. (14) Fu, H. H.; Peng, P. F.; Li, R. F.; Liu, C. P.; Liu, Y. S.; Jiang, F. L.; Hong, M. C.; Chen, X. Y. A general strategy for tailoring upconversion luminescence in lanthanide-doped inorganic nanocrystals through local structure engineering. Nanoscale 2018, 10, 9353-9359. (15) Zhuo, Z.; Liu, Y. S.; Liu, D. J.; Huang, P.; Jiang, F. L.; Chen, X. Y.; Hong, M. C. Manipulating energy transfer in lanthanide-doped single nanoparticles for highly enhanced upconverting luminescence. Chem. Sci. 2017, 8, 5050-5056. (16) Fischer, S.; Swabeck, J. K.; Alivisatos, A. P. Controlled Isotropic and Anisotropic Shell Growth in β-NaLnF4 Nanocrystals Induced by Precursor Injection Rate. J. Am. Chem. Soc. 2017, 139, 12325-12332. (17) Fischer, S.; Bronstein, N. D.; Swabeck, J. K.; Chan, E. M.; Alivisatos, A. P. Precise Tuning of Surface Quenching for Luminescence Enhancement in Core-Shell LanthanideDoped Nanocrystals. Nano Lett. 2016, 16, 7241-7247. (18) Liu, Y. S.; D. Tu, T.; Zhu, H. M.; Li, R. F.; Luo, W. Q.; Chen, X. Y. A Strategy to Achieve Efficient Dual-Mode Luminescence of Eu3+ in Lanthanides Doped Multifunctional NaGdF4 Nanocrystals. Adv. Mater. 2010, 22, 3266-3271. (19) Shen, B.; S. Cheng, M.; Gu, Y. Y.; Ni, D. R.; Gao, Y. L.; Su, Q. Q.; Feng, W.; Li, F. Y. Revisiting the optimized doping ratio in core/shell nanostructured upconversion particles. Nanoscale 2017, 9, 1964-1971. (20) Xing, H. Y.; Zhang, S. J.; Bu, W. B.; Zheng, X. P.; Wang, L. J.; Xiao, Q. F.; Ni, D. L.; Zhang, J. M.; Zhou, L. P.; Peng, W. J.; Zhao, K. L.; Hua, Y. Q.; Shi, J. L. Ultrasmall NaGdF4 Nanodots for Efficient MR Angiography and Atherosclerotic Plaque Imaging. Adv. Mater. 2014, 26, 3867-3872. (21) Liu, Y. S.; Zhou, S. Y.; Zhuo, Z.; Li, R. F.; Chen, Z.; Hong, M. C.; Chen, X. Y. In vitro upconverting/downshifting luminescent detection of tumor markers based on Eu3+-activated core-shell-shell lanthanide nanoprobes. Chem. Sci. 2016, 7, 5013-5019. (22) Chen, B.; Kong, W.; Liu, Y.; Lu, Y. H.; Li, M. Y.; Qiao, X. S.; Fan, X. P.; Wang, F. Crystalline Hollow Microrods for Site-Selective Enhancement of Nonlinear Photoluminescence. Angew. Chem., Int. Ed. 2017, 56, 10383-10387. (23) Li, X. M.; Guo, Z. Z.; Zhao, T. C.; Lu, Y.; Zhou, L.; Zhao, D. Y.; Zhang, F. Filtration Shell Mediated Power Density Independent Orthogonal Excitations-Emissions Upconversion Luminescence. Angew. Chem., Int. Ed. 2016, 128, 2510-2515. (24) Ma, C. S.; Xu, X. X.; Wang, F.; Zhou, Z. G.; Liu, D. M.; Zhao, J. B.; Guan, M.; Lang, C. I.; Jin, D. Y. Optimal Sensitizer Concentration in Single Upconversion Nanocrystals. Nano Lett. 2017, 17, 2858-2864.


Page 28 of 32

Page 29 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Johnson, N. J. J.; He, S.; Diao, S.; Chan, E. M.; Dai, H. J.; Almutairi, A. Direct Evidence for Coupled Surface and Concentration Quenching Dynamics in Lanthanide-Doped Nanocrystals. J. Am. Chem. Soc. 2017, 139, 3275-3282. (26) Rabouw, F. T.; Prins, P. T.; Villanueva-Delgado, P.; Castelijns, M.; Geitenbeek R. G.; Meijerink, A. Quenching Pathways in NaYF4:Er3+,Yb3+ Upconversion Nanocrystals. Acs Nano. 2018, 12, 4812-4823. (27) Wang, F.; Wang, J. A.; Liu, X. G. Direct Evidence of a Surface Quenching Effect on Size-Dependent Luminescence of Upconversion Nanoparticles. Angew. Chem., Int. Ed. 2010, 49, 7456-7460. (28) Chen, Q. S.; Xie, X. J.; Huang, B. L.; Liang, L. L.; Han, S. Y.; Yi, Z. G.; Wang, Y.; Li, Y.; Fan, D. Y.; Huang, L.; Liu, X. G. Confining Excitation Energy in Er3+-Sensitized Upconversion Nanocrystals through Tm3+-Mediated Transient Energy Trapping. Angew. Chem., Int. Ed. 2017, 129, 7713-7717. (29) Vetrone, F.; Naccache, R.; Mahalingam, V.; Morgan, C. G.; Capobianco, J. A. The Active-Core/Active-Shell Approach: A Strategy to Enhance the Upconversion Luminescence in Lanthanide-Doped Nanoparticles. Adv. Funct. Mater. 2009, 19, 2924-2929. (30) Homann, C.; Krukewitt, L.; Frenzel, F.; Grauel, B.; Wurth, C.; Resch-Genger, U.; Haase, M. NaYF4:Yb,Er/NaYF4 Core/Shell Nanocrystals with High Upconversion Luminescence Quantum Yield. Angew. Chem., Int. Ed. 2018, 57, 8765-8769. (31) Zhang, F.; Che, R. C.; Li, X. M.; Yao, C.; Yang, J. P.; Shen, D. K.; Hu, P.; Li, W.; Zhao, D. Y. Direct Imaging the Upconversion Nanocrystal Core/Shell Structure at the Subnanometer Level: Shell Thickness Dependence in Upconverting Optical Properties. Nano Lett. 2012, 12, 2852-2858. (32) Dong, H.; Sun, L. D.; Wang, Y. F.; Ke, J.; Si, R.; Xiao, J. W.; Lyu, G. M.; Shi, S.; Yan, C. H. Efficient Tailoring of Upconversion Selectivity by Engineering Local Structure of Lanthanides in NaxREF3+x Nanocrystals. J. Am. Chem. Soc. 2015, 137, 6569-6576. (33) Wang, Y. B.; Wei, T.; Cheng, X. W.; Ma, H.; Pan, Y.; Xie, J.; Su, H. Q.; Xie, X. J.; Huang, L.; Huang, W. Insights into Li+-induced morphology evolution and upconversion luminescence enhancement of KSc2F7:Yb/Er nanocrystals. J. Mater. Chem. C 2017, 5, 35033508 (34) Wang, J.; Deng, R. R.; MacDonald, M. A.; Chen, B. L.; Yuan, J. K.; Wang, F.; Chi, D. Z.; Hor, T. S. A.; Zhang, P.; Liu, G. K.; Han, Y.; Liu, X. Enhancing multiphoton upconversion through energy clustering at sublattice level. Nat. Mater. 2013, 13, 157-162. (35) Liu, Y. S.; Zhou, S. Y.; Tu, D. T.; Chen, Z.; Huang, M. D.; Zhu, H. M.; Ma, E.; Chen, X. Y. Amine-Functionalized Lanthanide-Doped Zirconia Nanoparticles: Optical Spectroscopy, Time-Resolved Fluorescence Resonance Energy Transfer Biodetection, and Targeted Imaging. J. Am. Chem. Soc. 2012, 134, 15083-15090. (36) Pan, Y.; Xie, X. J.; Huang, Q. W.; Gao, C.; Wang, Y. B.; Wang, L. X.; Yang, B. X.; Su, H. Q.; Huang, L.; Huang, W. Inherently Eu2+/Eu3+ Codoped Sc2O3 Nanoparticles as HighPerformance Nanothermometers. Adv. Mater. 2018, 30, 1705256. (37) Zhao, B. Z.; Xie, X. J.; Xu, S. L.; Pan, Y.; Yang, B. X.; Guo, S. H.; Wei, T.; Su, H. Q.; Wang, H. B.; Chen, X. Q.; Dravid, V. P.; Huang, L.; Huang, W. From ScOOH to Sc2O3: Phase Control, Luminescent Properties, and Applications. Adv. Mater. 2016, 28, 6665-6671. (38) Paik, T.; Gordon, T. R.; Prantner, A. M.; Yun, H.; Murray, C. B. Designing Tripodal and Triangular Gadolinium Oxide Nanoplates and Self-Assembled Nanofibrils as Potential Multimodal Bioimaging Probes. Acs Nano 2013, 7, 2850-2859.



(39) Macedo, A. G.; Ferreira, R. A. S.; Ananias, D.; Reis, M. S.; Amaral, V. S.; Carlos, L. D.; Rocha, J. Effects of Phonon Confinement on Anomalous Thermalization, Energy Transfer, and Upconversion in Ln3+-Doped Gd2O3 Nanotubes. Adv. Funct. Mater. 2010, 20, 624-634. (40) Debasu, M. L.; Ananias, D.; Rocha, J.; Malta, O. L.; Carlos, L. D. Energy-transfer from Gd(III) to Tb(III) in (Gd,Yb,Tb)PO4 Nanocrystals. Phys. Chem. Chem. Phys. 2013, 15, 15565-15571. (41) Shen, J.; Sun, L. D.; Zhu, J. D.; Wei, L. H.; Sun, H. F.; Yan, C. H. Biocompatible Bright YVO4:Eu Nanoparticles as Versatile Optical Bioprobes. Adv. Funct. Mater. 2010, 20, 37083714. (42) Wang, F.; Xue, X. J.; Liu, X. G. Multicolor Tuning of (Ln,P)-Doped YVO4 Nanoparticles by Single-Wavelength Excitation. Angew. Chem., Int. Ed. 2008, 47, 906-909. (43) Xu, J.; Zhou, S. Y.; Tu, D. T.; Zheng, W.; Huang, P.; Li, R. F.; Chen, Z.; Huang, M. D.; Chen, X. Y. Sub-5 nm lanthanide-doped lutetium oxyfluoride nanoprobes for ultrasensitive detection of prostate specific antigen. Chem. Sci. 2016, 7, 2572-2578. (44) Stouwdam, J. W.; Van Veggel, F. C. J. M. Near-infrared Emission of Redispersible Er3+, Nd3+, and Ho3+ Doped LaF3 Nanoparticles. Nano Lett. 2002, 2, 733-737. (45) Heer, S.; Kompe, K.; Gudel, H. U.; Haase, M. Highly Efficient Multicolour Upconversion Emission in Transparent Colloids of Lanthanide-Doped NaYF4 Nanocrystals. Adv. Mater. 2004, 16, 2102-2105. (46) Lei, P. P.; An, R.; Yao, S.; Wang, Q. S.; Dong, L. L.; Xu, X.; Du, K. M.; Feng, J.; Zhang, H. J. Ultrafast Synthesis of Novel Hexagonal Phase NaBiF4 Upconversion Nanoparticles at Room Temperature. Adv. Mater. 2017, 29, 1700505. (47) Wang, Z.; Feng, J.; Song, S. Y.; Sun, Z. Q.; Yao, S.; Ge, X.; Pang, M.; Zhang, H. J. Pure and intense orange upconversion luminescence of Eu3+ from the sensitization of Yb3+-Mn2+ dimer in NaY(Lu)F4 nanocrystals. J. Mater. Chem. C 2014, 2, 9004-9011. (48) Zheng, W.; Zhou, S. Y.; Chen, Z.; Huang, P.; Liu, Y. S.; Tu, D. T.; Zhu, H. M.; Li, R. F.; Huang, M. D.; Chen, X. Y. Sub-10 nm Lanthanide-Doped CaF2 Nanoprobes for TimeResolved Luminescent Biodetection. Angew. Chem., Int. Ed. 2013, 52, 6671-6676. (49) Ju, Q.; Tu, D. T.; Liu, Y. S.; Li, R. F.; Zhu, H. M.; Chen, J. C.; Chen, Z.; Huang M. D.; Chen, X. Y. Amine-Functionalized Lanthanide-Doped KGdF4 Nanocrystals as Potential Optical/Magnetic Multimodal Bioprobes. J. Am. Chem. Soc. 2012, 134, 1323-1330. (50) Ding, Y. J.; Teng, X.; Zhu, H.; Wang, L. L.; Pei, W. B.; Zhu, J. J.; Huang, L.; Huang, W. Orthorhombic KSc2F7:Yb/Er nanorods: controlled synthesis and strong red upconversion emission. Nanoscale 2013, 5, 11928-11932. (51) Lisiecki, R. VUV and UV-vis optical study on KGd2F7 luminescent host doped with terbium and co-doped with europium. J. Lumin. 2013, 143, 293-297. (52) Pierrard, A.; Gredin, P.; Dupont, N.; De Kozak, A.; Viana, B.; Aschehoug, P.; Vivien, D. Luminescence study of Eu3+: KGd2F7. Solid State Sci. 1999, 1, 149-161. (53) Roberts, J. E. Lanthanum and Neodymium Salts of Trifluoroacetic Acid. J. Am. Chem. Soc. 1961, 83, 1087-1088. (54) Huang, P.; Zheng, W.; Zhou, S. Y.; Tu, D. T.; Chen, Z.; Zhu, H. M.; Li, R. F.; Ma, E.; Huang, M. D.; Chen, X. Y. Lanthanide-Doped LiLuF4 Upconversion Nanoprobes for the Detection of Disease Biomarkers. Angew. Chem., Int. Ed. 2014, 53, 1252-1257. (55) Wang, F.; Han, Y.; Lim, C. S.; Lu, Y. H.; Wang, J.; Xu, J.; Chen, H. Y.; Zhang, C.; Hong, M. H.; Liu, X. G. Simultaneous phase and size control of upconversion nanocrystals through lanthanide doping. Nature 2010, 463, 1061-1065.


Page 30 of 32

Page 31 of 32 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(56) Tu, D. T.; Liu, Y. S.; Zhu, H. M.; Li, R. F.; Liu, L. Q.; Chen, X. Y. Breakdown of Crystallographic Site Symmetry in Lanthanide-Doped NaYF4 Crystals. Angew. Chem., Int. Ed. 2013, 52, 1128-1133. (57) Wang, Y. H.; Liu, Y. S.; Xiao, Q. B.; Zhu, H. M.; Li, R. F.; Chen, X. Y. Eu3+ doped KYF4 nanocrystals: synthesis, electronic structure, and optical properties. Nanoscale 2011, 3, 3164-3169. (58) Liu, Y. S.; Luo, W. Q.; Li, R. F.; Liu, G. K.; Antonio, M. R.; Chen, X. Y. Optical Spectroscopy of Eu3+ Doped ZnO Nanocrystals. J. Phys. Chem. C 2008, 112, 686-694. (59) Zheng, W.; Huang, P.; Tu, D. T.; Ma, E.; Zhu, H. M.; Chen, X. Y. Lanthanide-doped upconversion nano-bioprobes: electronic structures, optical properties, and biodetection. Chem. Soc. Rev. 2015, 44, 1379-1415.



For Table of Contents Use Only Lanthanide-doped KGd2F7 nanocrystals: controlled synthesis, optical properties and spectroscopic identification of the optimum core/shell architecture for highly enhanced upconverting luminescence Lihua Yu,†,‡ Guowei Li,†,‡ Yongsheng Liu,*,† Feilong Jiang,† and Maochun Hong*,†

We first report the controlled synthesis and optical properties of monodisperse lanthanide-doped KGd2F7 nanocrystals, and then carry out a comparative spectroscopic investigation of the optimum core/shell architecture for highly enhanced upconverting luminescence.


Page 32 of 32

Lanthanide-doped KGd2F7 nanocrystals: controlled synthesis, optical

Recommend Documents