Article Cite This: Chem. Mater. 2017, 29, 9758-9766
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Controlled Synthesis, Evolution Mechanisms, and Luminescent Properties of ScFx:Ln (x = 2.76, 3) Nanocrystals
Juan Xie,†,¶ Xiaoji Xie,‡,¶ Chao Mi,§ Ziyu Gao,‡ Yue Pan,‡,⊥ Quli Fan,† Haiquan Su,⊥ Dayong Jin,*,§ Ling Huang,*,‡ and Wei Huang*,†,‡ †
Key Laboratory for Organic Electronics and Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, Nanjing 210023, China ‡ Key Laboratory of Flexible Electronics (KLOFE) and Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), Nanjing 211816, China § Institute for Biomedical Materials and Devices, Faculty of Science, University of Technology Sydney, Sydney, New South Wales 2007, Australia ⊥ School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, China S Supporting Information *
ABSTRACT: Kinetic or thermodynamic control has been employed to guide the selective synthesis of conventional organic compounds, and it should be a powerful tool as well for accessing unusual inorganic nanocrystals, particularly when a series of members with similar chemical compositions and phase structures exist. Indeed, a comprehensive mapping of the energy barrier distribution of each nanocrystal in a predefined reaction system will enable not only the precise synthesis of nanocrystals with expected sizes, morphologies, phase structures, and ultimately functionalities, but also disclosure of the evolution details of nanocrystals from one structure to another. Using ScFx:Ln (x = 2.76, 3) series as a proof-of-concept, we have successfully mapped out the energy barriers that correspond to each of the ScFx:Ln nanocrystals, unraveled suitable temperatures for each type of nanocrystal formation, recorded their phase transition procedures, and also discovered the relationships of the products at each reaction stage. To testify how this approach allows one to tailor the structure-related optical properties, different lanthanide-doped ScFx nanocrystals were synthesized and a wide-range of luminescence fine-tuning was achieved, which not only showcases high quality of the nanocrystals, but also provides more candidates for various luminescence applications, especially when single-particle upconversion emission is required.
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INTRODUCTION The functionalities of nanocrystals directly correlate with their sizes,1 morphologies,2−4 and especially phase structures.5−8 A typical example is that the surface plasmon properties of spherical Au nanocrystals vary as a function of their diameters9−11 and also as a function of their aspect ratios in the case of nanorods.12−14 Moreover, the chemical reactivity changes as well when face centered cubic phase structures of Au nanostructures are transformed into hexagonal close packed ones.15−17 Thus, it is fair to say that the ability to control nanoscale phase structure is central to being able to synthesize nanocrystals with different physical and chemical properties and especially desired functionalities.18−24 Among the myriad methods that are employed for controlled synthesis of target nanocrystals, surfactant molecules,25−27 reactants,25,28,29 pH,30,31 and concentrations32 are preferentially considered factors for manipulating final products in terms of size, morphology, and phase structures when there is minor or © 2017 American Chemical Society
no requirement on the reaction temperature. However, for nanocrystals whose growth is sensitive to reaction temperature, especially when multiple phase structures exist, proper temperature control will become a significant parameter that facilitates precise synthesis of specific nanocrystals.29,33 From thermodynamic point of view, continuous increasing the reaction temperature would assist the reactants to consecutively overpass upper level energy barriers that correspond to nanocrystals with certain chemical compositions and phase structures.21,33,34 Thus, successfully figuring out the locations of each energy barrier in a predefined reaction system will not only enable comprehensive unraveling of the relationships between the type of nanocrystals and reaction temperatures, but also it can guide the synthesis so that Received: August 23, 2017 Revised: November 2, 2017 Published: November 3, 2017 9758
DOI: 10.1021/acs.chemmater.7b03561 Chem. Mater. 2017, 29, 9758−9766
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Chemistry of Materials
Scheme 1. Schematic Presentation of Temperature Related Energy Map Distribution of Nanocrystals and Possible Reaction Pathwaysa
Stage I: tetragonal ScF2.76:Yb/Er nanocrystals are formed at ∼100 °C, which then assemble into tetragonal ScF2.76:Yb/Er nanorods. Stage II: orthorhombic ScF3:Yb/Er nanocrystals are generated at ∼160 °C and then assemble into ScF3:Yb/Er nanorods. Stage III: formation of cubic ScF3:Yb/Er nanocrystals via a dissolution and recrystallization process. a
constant, have drawn rapidly increased attention.26,37,38 Although Sc belongs to the rare earth family, it has no 4f electrons and behaves more like transition metals. Complicated enough, ScFx has two chemical compositions that fall into six different crystal structures, that is, one in tetragonal, one in rhombohedral, three in orthorhombic, and one in cubic.39,40 Harsh reaction conditions such as high temperature, high pressure, and month-long reaction were usually required to study bulk ScFx and their phase transitions.41,42 However, comprehensive investigations on the precise synthesis, phase transition details, and especially the fascinating upconversion luminescent properties of ScFx:Ln nanocrystal series are still empty.
expected nanocrystals can be precisely and reliably obtained. Indeed, comprehensive analysis of existing energy barriers of certain reaction system has been widely employed to leverage the target materials during conventional organic synthesis and a series of compounds with fixed chemical compositions or crystal structures can then be precisely produced at according temperatures.35,36 However, there are very few reports on the precise synthesis, especially of inorganic nanocrystal series with high chemical and structural similarity where reaction energy barrier plays a key role in determining the ultimate structure and functionality of target materials. Herein, using ScFx:Ln (x = 2.76, 3) nanocrystal series as a typical example, the distribution of temperature related energy barriers that correspond to certain nanocrystals were successfully mapped out and guided by which, we have realized precise synthesis of tetragonal ScF2.76:Yb/Er, orthorhombic ScF3:Yb/Er, and cubic ScF3:Yb/Er nanocrystals with controllable morphologies and further disclosed how the nanocrystals gradually transform from one to another at relatively mild reaction conditions (Scheme 1). The wide-range (from blue, green to red and near-infrared), fine-tuning, and especially single-particle upconversion luminescence have proved their great potentials in biobarcoding, bioimaging, information storage, and anticounterfeiting. Equally important is that as a recently prevalent topic, the unique properties of Sc-based nanomaterials, such as different luminescence upconversion behavior from those of Y-based nanocrystals and outstanding negative thermal expansion
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EXPERIMENTAL SECTION
Methods. Synthesis of ScF3:Ln Upconversion Nanocrystals. In a typical experiment, ScCl3, YbCl3, and ACl3 (A = Er, Tm, and Ho), at varied molar ratios with a total lanthanide amount of 0.4 mmol, were added into a 50 mL three-necked flask containing oleic acid (10 mL) and 1-octadecene (10 mL). The resulting mixture was heated to 160 °C and kept for 10 min under vigorous stirring before cooling down to 70 °C. Subsequently, NH4F (1.2 mmol) was added to the flask and the reaction mixture was heated to 160 °C and kept for another 30 min. The reaction mixture was then heated to 300 °C in an argon atmosphere and maintained for 1.5 h before cooling down to room temperature. The product was precipitated by addition of ethanol, collected by centrifugation, washed with water and ethanol several times, and finally redispersed in cyclohexane. 9759
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Chemistry of Materials Monitoring the Nanocrystal Growth. The reaction mixture (2 mL) was extracted from the flask using a glass syringe (5 mL) at different time points and then injected into ethanol (6 mL) to quench the reaction. The products were collected by centrifugation, washed with ethanol, and redispersed in cyclohexane for further characterization. Characterization. Transmission electron microscopy (TEM) measurements were carried out on a Hitachi 7700 transmission electron microscope at an acceleration voltage of 100 kV. High resolution TEM (HRTEM) images and energy-dispersive X-ray spectra (EDS) were obtained on a Tecnai G2 F20 microscope. Powder X-ray diffraction (XRD) data were recorded on a Bruker D8advance X-ray diffractometer with CuKα radiation (λ = 1.5406 Å), keeping the operating voltage and current at 40 kV and 40 mA, respectively. Room temperature upconversion luminescence spectra were recorded on an Edinburgh F920 fluorescent spectrometer equipped with a diode laser (980 nm). Single-particle upconversion luminescence spectra were collected on a purpose-built stage-scanning confocal microscope. Briefly, a 976 nm single mode laser diode (Thorlabs, 300 mW, Butterfly FBG-Stabilized Laser) was used as the excitation source and the laser excitation was tightly focused onto the sample through a 100× objective lens (Olympus NA 1.4). Both luminescence intensity and lifetime were recorded by a Single Photon Counting Avalanche Diode (SPAD) detector (SPCM-AQRH-13-FC, PerkinElmer) through an epi-fluorescence optics setup and a multimode optical fiber that has a core size matching with system Airy disk for achieving confocal resolution.
providing multiple reaction routes toward precise nanocrystal synthesis. TEM images in Figure 1 depict the morphology variation details of ScF2.76:Yb/Er nanocrystals at different reaction times.
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RESULTS AND DISCUSSION Nanocrystal Synthesis and Evolution. Nanocrystals of ScFx:Ln were synthesized following a previous report where necessary revisions were adopted to ensure the highest-possible quality of final products.37 As illustrated in Scheme 1, the thermodynamically controlled evolution process of ScFx:Yb/Er (20/2 mol %) nanocrystals (abbreviated as ScFx:Yb/Er unless otherwise described) can be divided into three stages located at different reaction temperatures: (Stage I) 90−100 °C, formation of tetragonal-phase ScF2.76:Yb/Er nanorods, which are composed of individual ScF2.76:Yb/Er nanocrystals via oriented attachment; (Stage II) 100−150 °C, dissociation of tetragonal ScF2.76:Yb/Er nanorods and simultaneous formation of orthorhombic ScF3:Yb/Er nanocrystals, which further assemble into orthorhombic ScF3:Yb/Er nanorods at ∼160 °C; and (Stage III) formation of cubic ScF3:Yb/Er nanocrystals at temperature higher than 280 °C through a dissolution (of orthorhombic ScF3:Yb/Er nanorods) and recrystallization (of cubic ScF3:Yb/Er nanocrystals) process. The energy dispersive X-ray spectra Figure S1, (Supporting Information) and elemental analysis (Table S1) results have proved the chemical compositions of the as-synthesized nanocrystals. There are three critical temperatures at 100, 160, and 280 °C where tetragonal ScF2.76:Yb/Er nanorods, orthorhombic ScF3:Yb/Er nanorods, and cubic ScF3:Yb/Er nanocubes are formed, respectively. Further thermodynamic studies have shown that these three critical temperatures can actually be correlated with three energy barriers that need to be overpassed before the step-by-step transformations from tetragonal to orthorhombic and finally to cubic phases can be continuously proceeded. More importantly, this temperature related energy barrier map allows arbitrary manipulation of the crystal evolution process. In other words, except the continuous evolution from stage I to II and then III, this transformation can also occur separately from stage I to stage II, stage II to stage III, or directly from stage I to stage III (Scheme 1) as long as the corresponding energy barrier is conquered. Guided by this map, the products listed in each stage can be reliably obtained,
Figure 1. TEM images of tetragonal ScF2.76:Yb/Er nanocrystals and assembled nanorods. (a) Schematic oriented attachment process of tetragonal ScF2.76:Yb/Er nanocrystals into nanorods. (b−e) TEM images of ScF2.76:Yb/Er nanocrystals obtained after reacting at 100 °C for 90, 120, 180, and 240 min, respectively. (f−i) HRTEM images of the samples shown in panels b−e, respectively. The dot(s) inside the yellow circles represent each alignment moment of individual nanocrystals when assemble into nanorods.
When the reaction system was heated to 100 °C, ∼5 nm tetragonal ScF2.76:Yb/Er nanocrystals started to appear at 90 min (Figure 1b), which gradually merged into nanorods at 120 min (Figure 1c) and then more nanorods were formed at 180 min (Figure 1d). At 240 min, nanorods became the only product, while nanocrystals completely disappeared (Figure 1e). The 0.27 nm crystal lattice constant of the (110) facet of tetragonal ScF2.76:Yb/Er nanocrystals seen in the four high resolution TEM (HRTEM) images (Figure 1f−i) corresponding to nanocrystals in Figure 1b, the mixture of nanocrystals and nanorods in Figure 1c and d, and nanorods in Figure 1e, respectively, have clearly indicated that all of the four products are composed of the same construction unit, that is, tetragonal ScF2.76:Yb/Er nanocrystals, although they appear in different morphologies. Combining the gradual variation of the products in Figure 1b−e, together with the crystal lattice details distilled from HRTEM images in Figure 1f−i, it is rational to deduce that with the reaction time prolonging, individual ScF2.76:Yb/Er nanocrystal (Figure 1f) starts to approach, align, and then attach to each other to form nanorod (Figure 1g−i). This also well explains why the amount of nanocrystals keeps decreasing while that of the nanorods keeps increasing (Figure 1c,d) until finally nanorods become the only product (Figure 1e). The 9760
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Chemistry of Materials yellow curves in Figure 1f−i depict the alignment status of nanocrystals at each growth stage, which should belong to a typical oriented attachment process as illustrated in Figure 1a.43,44 For example, Figure 1g and h clearly show the alignment moment of three and two nanocrystals, respectively. The almost identical powder X-ray diffraction (XRD) data, especially the peak at 14° marked by the red rectangle in Figure S2 SI (SI) directly proves that tetragonal phase structure of ScF2.76:Yb/Er is maintained in all of the four products during the 240 min-long reaction process. To further investigate the thermodynamic behaviors of ScF2.76:Yb/Er nanocrystals, six batches of the reaction mixtures were heated to 110, 130, and 150 °C for 1 and 2 h, respectively. As shown in TEM images in Figure S3 (SI), besides ScF2.76:Yb/ Er nanorods, small nanocrystals start to show up at 110 °C for 1 h, which are confirmed to be orthorhombic ScF3:Yb/Er by the XRD data (Figure S3g, SI). The amount of ScF3:Yb/Er nanocrystals keeps increasing, while that of the nanorods is simultaneously decreasing at elevated temperatures, that is, 130 °C in Figure S3b (SI) and 150 °C in Figure S3c (SI), indicating that 110 °C is the border temperature between the formation of tetragonal ScF2.76:Yb/Er nanorods and orthorhombic ScF3:Yb/ Er nanocrystals. Above this temperature, tetragonal ScF2.76:Yb/ Er nanorods transform into orthorhombic ScF3:Yb/Er nanocrystals (Figure S3a−c, SI) and the higher the temperature, the faster the transformation. The same trend was seen as well in the case of 2 h reaction time (Figure S3d−f, SI). The XRD peaks at 14° and 22° in Figure S3g (SI) prove that tetragonal ScF2.76:Yb/Er nanorods coexist with orthorhombic ScF3:Yb/Er nanocrystals, which is consistent with the HRTEM results in Figure S3a, b, and d (SI). Further comparison of Figure S3d−f and a−c (SI) suggests that the content of orthorhombic ScF3:Yb/Er nanocrystals at 2 h reaction time is obviously higher than those observed at 1 h, while the content of tetragonal ScF2.76:Yb/Er nanorods goes on an opposite trend. Moreover, the continuous increase of the intensity ratios of the XRD peaks at 22° to 14° in Figure S3h (SI) representing the ratio of the amount of orthorhombic ScF3:Yb/Er nanocrystals to that of tetragonal ScF2.76:Yb/Er nanorods can further confirm from a different point of view the gradual increase of the amount of ScF3:Yb/Er and simultaneous decrease of ScF2.76:Yb/Er at either elevated temperatures or prolonged reaction times, which agrees very well with the results seen in Figure S3a−f (SI). The above discussion states that both higher temperature and longer reaction time favor the transformation of tetragonal ScF2.76:Yb/ Er nanorods to orthorhombic ScF3:Yb/Er nanocrystals (i.e., from Stage I to Stage II, Scheme 1), suggesting a typical thermodynamic reaction mode within this temperature range. Although tetragonal ScF2.76:Yb/Er nanorods can coexist with orthorhombic ScF3:Yb/Er nanocrystals even when being heated for 2 h at 150 °C (Figure S3f, SI), TEM image in Figure 2b indicates that they disappear completely once the reaction temperature reaches 160 °C even for 5 min and meanwhile orthorhombic ScF3:Yb/Er nanocrystals become the only product. This suggests that 160 °C is a critical turnover energy barrier for the complete transformation of tetragonal ScF2.76:Yb/Er nanorods to orthorhombic ScF3:Yb/Er nanocrystals. More interestingly, nanorods appear again after 10 min reaction at 160 °C (inside the red circles in Figure 2c) but in orthorhombic rather than tetragonal structure seen in Figure 1, as proved by the XRD data in Figure S4 (SI). These nanorods gradually become dominant at the reaction time of 20 min where only small amount of orthorhombic ScF3:Yb/Er
Figure 2. Assembly process of orthorhombic ScF3:Yb/Er nanocrystals. (a) Schematic oriented attachment process of ScF3:Yb/Er nanorods. (b−e) TEM images of ScF3:Yb/Er nanocrystals obtained after reacting at 160 °C for 5, 10, 20, and 30 min, respectively. (f−i) HRTEM images of samples in panels b−e, respectively. The yellow circles depict the alignment moment of individual nanocrystals when assemble into nanorods.
nanocrystals exist (Figure 2d), while TEM image in Figure 2e proves that the formation of ScF3:Yb/Er nanorods completes within 30 min. The 0.29 nm lattice constant observed in Figure 2f−i that corresponds to the (020) facet of samples shown in Figure 2b−e, respectively, suggesting that they are composed of the same construction unit, that is, orthorhombic ScF3:Yb/Er nanocrystals, no matter in the form of nanocrystals (Figure 2b), nanorods (Figure 2e), or their mixtures (Figure 2c,d). Same as the case of tetragonal ScF2.76:Yb/Er nanocrystals shown in Stage I (Scheme 1) and Figure 1f−i, the HRTEM images in Figure 2f−i also indicate that with the reaction time prolonging, individual orthorhombic ScF3:Yb/Er nanocrystals (Figure 2f) start to approach to each other, align their facets (Figure 2g,h), and finally attach linearly to form nanorods (Figure 2i) via oriented attachment process at 160 °C as depicted in Stage II (Scheme 1) and Figure 2a. The XRD data at different reaction times in Figure S4 (SI) further prove that orthorhombic phase structure is maintained in all of the four products shown in Figure 2b−e, respectively. The results of control experiments on orthorhombic ScF3:Yb/Er nanorods in Figure S5a−c (SI) together with the XRD data in Figure S5d (SI) have shown that these nanorods are very stable at 160 °C in terms of both morphology and phase structure even after being heated for as long as 3 h (Figure S5c, SI), implying very good thermal stability at this temperature. However, when the reaction temperature further increased to 200 °C, orthorhombic ScF3:Yb/Er nanorods can only remain stable for 30 min (Figure S6a,b, SI) before disassociating into shorter and shorter ones at the reaction times of 60 and 180 min (Figure S6c,d, SI), respectively. The continuous increasing of the amount of nanocrystals and simultaneous decreasing of nanorods again indicates the gradual disassociation of nanorods into individual nanocrystals during this heating process, which is exactly opposite to the oriented attachment process discussed in Figures 1 and 2. The XRD peaks at 22° in Figure S6e (SI) clearly prove that both nanorods and dissociated nanocrystals are composed of orthorhombic ScF3:Yb/Er nanocrystals. The coexistence of nanorods and nanocrystals of ScF3:Yb/Er even after being 9761
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Chemistry of Materials heated at 200 °C for 3 h (Figure S6d, SI) further suggests that the reaction will not proceed further (to Stage III, Scheme 1) if no more energy is provided (to overpass this reaction energy barrier). Indeed, when the reaction temperature rises to 280 °C, dissociation of orthorhombic ScF3:Yb/Er nanorods (Figure 3b)
above-discussed transformation has experienced a process where dissolution of orthorhombic ScF3:Yb/Er nanocrystals and recrystallization of cubic ScF3:Yb/Er nanocrystals happens simultaneously as depicted in Stage III (Scheme 1) and Figure 3a. Considering the fact that orthorhombic ScF3:Yb/Er nanocrystals transform to cubic ScF3:Yb/Er nanocrystals at 280 °C while no phase change can be seen at 200 °C and no nanorod dissociation happens at 160 °C even being heated for 180 min, there is obviously an energy barrier at 280 °C that prevents the formation of cubic ScF3:Yb/Er nanocrystals. Again, this should also belong to the characteristic thermodynamically controlled reaction process, which determines the final product. Alternatively, if presynthesized orthorhombic ScF3:Yb/Er nanorods (Figure S8a, SI) were used as reactant (i.e., from Stage II to Stage III, Scheme 1), they started to dissociate after being heated for 15 min at 300 °C (Figure S8b, SI) and in the meanwhile cubic ScF3 :Yb/Er nanocrystals appeared as indicated by the HRTEM image in the inset of Figure S8b (SI) and the characteristic 32° XRD peak in Figure S8e (SI). Then more and more ScF3:Yb/Er nanocubes with uniform size are obtained at 45 (Figure S8c, SI) and 75 min (Figure S8d, SI), respectively, which belong to a typical Ostwald ripening process.45−47 It is worth pointing out that the transformation proceeds so fast at 300 °C that the intermediate amorphous residues observed in Figure 3d and e were not seen here. The obviously improved signal-to-noise ratio of the XRD data at 75 min (Figure S8e, SI) further implies better crystallinity of the cubic ScF3:Yb/Er nanocrystals compared to those formed at lower temperature (280 °C, Figure 3h). To further explore more diversified reaction pathways for this transformation, the solution mixture containing all original reactants was directly heated to 300 °C (i.e., from Stage I to Stage III, Scheme 1) where both nanorods and nanocrystals appeared even at the reaction time of 0 min and the inset HRTEM image suggests the orthorhombic structure of ScF3:Yb/Er nanocrystals (Figure S9a,g, SI). Cubic ScF3:Yb/Er nanocrystals were then formed at 25 min (Figure S9c,g, SI) after an intermediate transition stage at 15 min (Figures S9b and S9g, SI). Starting from 35 min, more and more uniform ScF3:Yb/Er nanocubes were formed via an Ostwald ripening process (Figure S9d−f, SI), which agrees very well with the XRD peak variations at 26° and 32° (Figure S9g, SI), respectively. The characteristic 0.40 nm lattice constant of the (100) facet together with the Fourier transform diffraction patterns of the nanocubes in Figure S10 have further confirmed the cubic phase structure of ScF3:Yb/Er nanocrystals shown in Figure S9d. It is worthy to emphasize that the transformation proceeds so fast at high temperature that nanocrystals of tetragonal ScF2.76:Yb/Er shown in Stage I (Scheme 1) were not seen here. More importantly, the transformation proceeded apparently faster at 300 °C (35 min in Figure S9d and 45 min in Figure S8c, SI) than that at 280 °C (120 min in Figure 3f), while no nanocubes are formed at 200 °C even being heated for 180 min (Figure S6d, SI), which is rational since the system energy at 200 °C is not sufficient to overpass the reaction barrier located at 280 °C. The above discussion has proved a typical thermodynamically controlled transformation process of the continuous nanocrystal evolution from tetragonal ScF2.76:Yb/Er nanorods to orthorhombic ScF3:Yb/Er nanorods and then cubic ScF3:Yb/Er nanocubes, which could not happen only when certain temperature needed to overpass the energy barrier is
Figure 3. Phase transition of nanocrystals. (a) Schemed phase transition mechanism from orthorhombic ScF3:Yb/Er to cubic ScF3:Yb/Er nanocrystals. (b−g) TEM images of ScF3:Yb/Er nanocrystals obtained after reacting at 280 °C for 0, 15, 45, 60, 120, and 180 min, respectively. (h) XRD profiles of samples in panels b−g. The diffraction patterns at the top and bottom are the literature references of orthorhombic (JCPDS 44−1096) and cubic (JCPDS 46−1243) ScF3, respectively.
into orthorhombic ScF3:Yb/Er nanocrystals starts at 15 min (Figure 3c), which is obviously faster than that at 200 °C (Figure S6) where the contribution from increased temperature remains the only possibility. Amorphous residues with almost no XRD peaks (Figure 3e,h) were obtained at the reaction time of 45 and 60 min, suggesting the gradual diminish of orthorhombic ScF3:Yb/Er nanocrystals (Figure 3c−e). Meanwhile, the new peak at 32 °C in the samples obtained at 120 min proves the formation of cubic ScF3:Yb/Er nanocrystals (Figure 3f,h). HRTEM images in Figure S7a and b (SI) further confirm that both nanorods (inside red rectangle) and nanocubes (inside red square) observed in Figure 3f are actually cubic ScF3:Yb/Er nanocrystals, which further become the only product with well-shaped nanocubes at 180 min (Figure 3g). The amorphous residues in Figure 3d and e should be the intermediates of dissolved orthorhombic ScF3:Yb/Er nanocrystals but before the formation of cubic ones (Figure S7c, SI). The results in Figures 3 and S7 have proved that the 9762
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Figure 4. Optical properties of nanocrystals. (a) Upconversion emission spectra of cubic ScF3:Yb/Er nanocrystals, orthorhombic ScF3:Yb/Er nanorods, and tetragonal ScF2.76:Yb/Er nanorods. Upconversion emission spectra of cubic nanocrystals of (b) ScF3:Yb/Er, (c) ScF3:Yb/Ho, and (d) ScF3:Yb/Tm at varying doping concentrations. (e) Digital photos of samples in panels a−d.
are at the medium level, and tetragonal nanocrystals give the weakest. Although such comparison was arbitrarily made by keeping other measuring conditions identical, such as the solution concentrations of the nanocrystals used, the laser power density, and the instrument parameters, it is straightforward and helpful to filter out the most suitable one for ultimate applications, for example, bioimaging. The green (545 nm) and red (657 nm) emission originates from the characteristic radiative transition from 2H11/2, 4S3/2, and 4F9/2 of Er3+ to its 4I15/2 level, respectively, as depicted in the upconversion diagram in Figure S11a (SI). The green to red ratio (G/R) also decreases from 4.10 in cubic ScF3:Yb/Er nanocubes to 2.73 in orthorhombic ScF3:Yb/Er nanorods and 1.80 in tetragonal ScF2.76:Yb/Er nanorods (Figure S12, SI). We
satisfied, and then the reaction advances toward the next stages, that is, 160 °C for orthorhombic ScF3:Yb/Er and 280 °C for cubic ScF3:Yb/Er nanocrystals. Once the energy barrier is conquered, higher temperature can markedly accelerate the reaction rates, as proved by the results in Figure S6 (SI) at 200 °C and Figures S8 and S9 (SI) at 300 °C. Optical Properties of Lanthanide-Doped ScFx:Ln Nanocrystals. In terms of the structure-related optical properties, contrary to NaScF4:Yb/Er (20/2 mol %) nanocrystals that generate strong red upconversion emission under 980 nm laser excitation,37 ScF x:Yb/Er nanocrystals at tetragonal, orthorhombic, and cubic phases, all produce strong green upconversion emission (Figure 4a) where cubic nanocrystals show the strongest luminescence, orthorhombic ones 9763
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Figure 5. Single-particle upconversion luminescence spectra. (a−c) Emission intensities (in arbitrary unit) and lifetimes (in microsecond) of randomly picked 7 ScF3:Yb/Tm (30/8 mol %) nanocrystals measured at blue, red, and near-infrared emission regions, respectively. (d) Visible− near-infrared upconversion emission spectra of ScF3:Yb/Tm (30/8 mol %) nanocrystals under 976 nm laser excitation through a purpose-built confocal microscope.
energy and alters the upconversion emission pathway (Figure S11b, SI). The luminescence spectra in Figure 4c and d indicate that except the strong green (Figure 4c2) and blue (Figure 4d2) upconversion emission generated by ScF3:Yb/Ho (30/2, mol %) and ScF3:Yb/Tm (30/0.5, mol %), respectively, variation of the codoping concentration of Yb3+/Ho3+ and Yb3+/Tm3+ (Figure S1 and Table S1) can further create a rich library of upconversion emission codes with predefined luminescence intensities that may potentially be used for luminescent codebased gene or disease multiplexing, especially when combined with the results in Figure 4b. Figure 4e lists the optical images of the samples discussed above, confirming the wide-range upconversion emission capability and fine-tunability of cubic ScF3 nanocrystals, which is rarely seen in a single-host material among previous upconversion studies. To further prove the high quality of the host material, ∼20 nm cubic ScF3:Yb/Tm (30/8, mol %) nanocrystals were used for single-particle upconversion luminescence study (Figure 5).48 Under a purposely built confocal microscope,49 the unique optical properties of each individual nanocrystals were comprehensively characterized. The strong upconversion emission from three typical wavelengths of Tm3+, at 450− 500, 645−655, and 775−825 nm were clearly imaged using three narrow bandpass filters (Figure 5d). The rationally strong
think that nanocrystals synthesized at higher temperatures shall have better crystallinity and less defects, which results in upconversion emission intensity sequence as cubic > orthorhombic > tetragonal (Figure 4a). Further fine-tuning the codoping concentration of Yb3+/Er3+ using cubic ScF3 nanocrystals as host material has resulted in a wide range of upconversion emissions that vary from almost pure green (Yb3+/Er3+, 20/2 mol %) with G/R at 4.10 (Figure 4b1), to yellow composed of the mixture of green and red upconversion emission (Yb3+/Er3+, 20/1 mol % and Yb3+/Er3+, 20/0.5 mol %) with G/R at 1.30 (Figure 4b2) and 0.75 (Figure 4b3), respectively, and finally to almost pure red (Yb3+/Er3+, 10/0.5 mol %) with G/R at 0.18 (Figure 4b4). This means that proper codoping of Yb3+/Er3+ into cubic ScF3 nanocrystals can produce ∼23-times G/R ratio variation (Figure S12, SI). More intriguingly, the emission routes varies with dopant concentrations, providing an ideal platform for upconversion mechanism studies. For example, ScF3:Yb/Er (20/2 mol %) exhibits three-photon upconversion process for both green and red emission (Figure S13a, SI), while the red emission of ScF3:Yb/Er (10/0.5 mol %) becomes a two-photon process (Figure S13b, SI). This should be due to the increased Yb3+/ Er3+ concentration initiates back energy transfer from excited Er3+ to nearby Yb3+ or Er3+, which redistributes the excitation 9764
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Chemistry of Materials
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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21401103, 21371095), National Basic Research Program of China (973 Program, 2015CB932200), Natural Science Foundation of Jiangsu Province (BM2012010, BE2015699, BL2014075), Qing Lan Project, and Synergetic Innovation Center for Organic Electronics and Information Displays.
upconversion luminescence intensities (the number of I in Figure 5a−c, SI) with reasonably small variation of randomly picked seven nanocrystals within a 4.8 μm × 4.8 μm scanning area (Figure S14) suggest the high quality and uniformity of cubic ScF3 nanocrystals as host materials. Remarkably, since the lifetime values are highly dependent on the distance between sensitizers (Yb3+) and activators (Tm3+),50 the very narrow variation in the luminescence lifetimes of each nanocrystal (the number of τ in Figure 5a−c) at three emission bands clearly suggests an even distribution of the doped Tm3+ inside the cubic ScF3 crystal lattice. Compared with previous results on 40 nm NaYF4:Yb/Tm nanocrystals,49 this work confirms the strategy for controlled synthesis of smaller ScF3 nanocrystals extremely successful to carry the optical barcodes for high-throughput multiplexing applications.
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CONCLUSION Guided by the temperature related energy barrier distribution map of the ScFx:Yb/Er (x = 2.76, 3) nanocrystal system, tetragonal ScF2.76:Yb/Er nanorods, orthorhombic ScF3:Yb/Er nanorods, cubic ScF3:Yb/Er nanocubes, and even the intermediate products shown in Scheme 1 can be precisely synthesized, and the transformation details were also successfully unravelled at each reaction step. Experimental results have shown that cubic ScF3:Yb/Er nanocrystals are thermodynamically stable while both tetragonal ScF2.76:Yb/Er and orthorhombic ScF3:Yb/Er ones are metastable. Comprehensive understanding of the evolution process of ScFx:Yb/Er nanocrystals is not only crucial to further complement the knowledge of Sc-related materials and chemistry with designed functionalities, but also it will greatly encourage the selective synthesis and further stimulate new applications of ScFx:Ln nanocrystals especially when single-particle upconversion luminescence is desired. We also believe that the principles disclosed here shall be instructive for precise synthesis and phase transition studies of other types of nanocrystal series with high similarity in chemical composition or phase structure where multiple reaction energy barriers exist. ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b03561. Experimental and synthesis details and supporting figures (PDF)
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REFERENCES
(1) Xu, J.; Zhou, S.; Tu, D.; Zheng, W.; Huang, P.; Li, R.; Chen, Z.; Huang, M.; Chen, X. Sub-5 nm Lanthanide-Doped Lutetium Oxyfluoride Nanoprobes for Ultrasensitive Detection of Prostate Specific Antigen. Chem. Sci. 2016, 7, 2572−2578. (2) Personick, M. L.; Mirkin, C. A. Making Sense of the Mayhem behind Shape Control in the Synthesis of Gold Nanoparticles. J. Am. Chem. Soc. 2013, 135, 18238−18247. (3) Padmos, J. D.; Personick, M. L.; Tang, Q.; Duchesne, P. N.; Jiang, D. E.; Mirkin, C. A.; Zhang, P. The Surface Structure of Silver-Coated Gold Nanocrystals and its Influence on Shape Control. Nat. Commun. 2015, 6, 7664. (4) Zhang, Y.; Huang, L.; Liu, X. Unraveling Epitaxial Habits in the NaLnF4 System for Color Multiplexing at the Single-Particle Level. Angew. Chem., Int. Ed. 2016, 55, 5718−5722. (5) Huang, X.; Li, S.; Huang, Y.; Wu, S.; Zhou, X.; Li, S.; Gan, C. L.; Boey, F.; Mirkin, C. A.; Zhang, H. Synthesis of Hexagonal ClosePacked Gold Nanostructures. Nat. Commun. 2011, 2, 292. (6) Wang, F.; Han, Y.; Lim, C. S.; Lu, Y.; Wang, J.; Xu, J.; Chen, H.; Zhang, C.; Hong, M.; Liu, X. Simultaneous Phase and Size Control of Upconversion Nanocrystals Through Lanthanide Doping. Nature 2010, 463, 1061−1065. (7) Liu, X.; Deng, R.; Zhang, Y.; Wang, Y.; Chang, H.; Huang, L.; Liu, X. Probing the Nature of Upconversion Nanocrystals: Instrumentation Matters. Chem. Soc. Rev. 2015, 44, 1479−1508. (8) Ananias, D.; Paz, F. A.; Yufit, D. S.; Carlos, L. D.; Rocha, J. Photoluminescent Thermometer Based on a Phase-Transition Lanthanide Silicate with Unusual Structural Disorder. J. Am. Chem. Soc. 2015, 137, 3051−3058. (9) Zhou, M.; Zeng, C.; Chen, Y.; Zhao, S.; Sfeir, M. Y.; Zhu, M.; Jin, R. Evolution from the Plasmon to Exciton State in Ligand-Protected Atomically Precise Gold Nanoparticles. Nat. Commun. 2016, 7, 13240. (10) Zhang, L.; Lei, D.; Smith, J. M.; Zhang, M.; Tong, H.; Zhang, X.; Lu, Z.; Liu, J.; Alivisatos, A. P.; Ren, G. Three-Dimensional Structural Dynamics and Fluctuations of DNA-Nanogold Conjugates by Individual-Particle Electron Tomography. Nat. Commun. 2016, 7, 11083. (11) Li, Z.; Foley, J. J. t.; Peng, S.; Sun, C. J.; Ren, Y.; Wiederrecht, G. P.; Gray, S. K.; Sun, Y. Reversible Modulation of Surface Plasmons in Gold Nanoparticles Enabled by Surface Redox Chemistry. Angew. Chem., Int. Ed. 2015, 54, 8948−8951. (12) Kim, F.; Song, J. H.; Yang, P. Photochemical Synthesis of Gold Nanorods. J. Am. Chem. Soc. 2002, 124, 14316−14317. (13) Yu, Y.; Cui, F.; Sun, J.; Yang, P. Atomic Structure of Ultrathin Gold Nanowires. Nano Lett. 2016, 16, 3078−3084. (14) Wang, M.; Gao, C.; He, L.; Lu, Q.; Zhang, J.; Tang, C.; Zorba, S.; Yin, Y. Magnetic Tuning of Plasmonic Excitation of Gold Nanorods. J. Am. Chem. Soc. 2013, 135, 15302−15305. (15) Fan, Z.; Zhang, X.; Yang, J.; Wu, X. J.; Liu, Z.; Huang, W.; Zhang, H. Synthesis of 4H/fcc-Au@Metal Sulfide Core-Shell Nanoribbons. J. Am. Chem. Soc. 2015, 137, 10910−10913. (16) Fan, Z.; Zhu, Y.; Huang, X.; Han, Y.; Wang, Q.; Liu, Q.; Huang, Y.; Gan, C. L.; Zhang, H. Synthesis of Ultrathin Face-Centered-Cubic Au@Pt and Au@Pd Core−Shell Nanoplates from Hexagonal-ClosePacked Au Square Sheets. Angew. Chem., Int. Ed. 2015, 54, 5672−5676. (17) Fan, Z.; Bosman, M.; Huang, X.; Huang, D.; Yu, Y.; Ong, K. P.; Akimov, Y. A.; Wu, L.; Li, B.; Wu, J.; Huang, Y.; Liu, Q.; Png, C. E.; Gan, C. L.; Yang, P.; Zhang, H. Stabilization of 4H Hexagonal Phase in Gold Nanoribbons. Nat. Commun. 2015, 6, 7684.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Xiaoji Xie: 0000-0002-4830-1246 Quli Fan: 0000-0002-9387-0165 Haiquan Su: 0000-0003-2164-3219 Ling Huang: 0000-0003-1244-3522 Author Contributions ¶
These authors contributed equally.
Notes
The authors declare no competing financial interest. 9765
DOI: 10.1021/acs.chemmater.7b03561 Chem. Mater. 2017, 29, 9758−9766
Article
Chemistry of Materials
crystals: Crystal Structure Evolution and Multicolor Tuning. J. Am. Chem. Soc. 2012, 134, 8340−8343. (38) Hu, L.; Chen, J.; Sanson, A.; Wu, H.; Rodriguez, C. G.; Olivi, L.; Ren, Y.; Fan, L.; Deng, J.; Xing, X. New Insights into the Negative Thermal Expansion: Direct Experimental Evidence for the “GuitarString” Effect in Cubic ScF3. J. Am. Chem. Soc. 2016, 138, 8320−8323. (39) Melnikov, P.; Komissarova, L. N. New form of Scandium Fluoride. J. Phys. Chem. Solids 2006, 67, 1899−1900. (40) Melnikov, P.; Nalin, M.; Messaddeq, Y. Scandium Fluorides. J. Alloys Compd. 1997, 262, 296−298. (41) Aleksandrov, K. S.; Voronov, N. V.; Vtyurin, A. N.; Krylov, A. S.; Molokeev, M. S.; Oreshonkov, A. S.; Goryainov, S. V.; Likhacheva, A. Y.; Ancharov, A. I. Structure and Lattice Dynamics of the HighPressure Phase in the ScF3 Crystal. Phys. Solid State 2011, 53, 564− 569. (42) Aleksandrov, K. S.; Voronov, V. N.; Vtyurin, A. N.; Krylov, A. S.; Molokeev, M. S.; Pavlovskii, M. S.; Goryainov, S. V.; Likhacheva, A. Y.; Ancharov, A. I. Pressure-Induced Phase Transitions in ScF3 Crystal: Raman Spectra and Lattice Dynamics. Phys. Solid State 2009, 51, 810. (43) Yu, B.-B.; Zhang, X.; Jiang, Y.; Liu, J.; Gu, L.; Hu, J.-S.; Wan, L.J. Solvent-Induced Oriented Attachment Growth of Air-Stable Phase Pure Pyrite FeS2 Nanocrystals. J. Am. Chem. Soc. 2015, 137, 2211− 2214. (44) Penn, R. L.; Banfield, J. F. Oriented Attachment and Growth, Twinning, Polytypism, and Formation of Metastable Phases: Insights from Nanocrystalline TiO2. Am. Mineral. 1998, 83, 1077−1082. (45) Naduviledathu Raj, A.; Rinkel, T.; Haase, M. Ostwald Ripening, Particle Size Focusing, and Decomposition of Sub-10 nm NaREF4 (RE = La, Ce, Pr, Nd) Nanocrystals. Chem. Mater. 2014, 26, 5689−5694. (46) Voorhees, P. W. The Theory of Ostwald Ripening. J. Stat. Phys. 1985, 38, 231−252. (47) Johnson, N. J.; Korinek, A.; Dong, C.; van Veggel, F. C. SelfFocusing by Ostwald Ripening: A Strategy for Layer-by-Layer Epitaxial Growth on Upconverting Nanocrystals. J. Am. Chem. Soc. 2012, 134, 11068−11071. (48) Zhao, J.; Jin, D.; Schartner, E. P.; Lu, Y.; Liu, Y.; Zvyagin, A. V.; Zhang, L.; Dawes, J. M.; Xi, P.; Piper, J. A.; Goldys, E. M.; Monro, T. M. Single-Nanocrystal Sensitivity Achieved by Enhanced Upconversion Luminescence. Nat. Nanotechnol. 2013, 8, 729−734. (49) Ma, C.; Xu, X.; Wang, F.; Zhou, Z.; Wen, S.; Liu, D.; Fang, J.; Lang, C. I.; Jin, D. Probing the Interior Crystal Quality in the Development of More Efficient and Smaller Upconversion Nanoparticles. J. Phys. Chem. Lett. 2016, 7, 3252−3258. (50) Lu, Y.; Zhao, J.; Zhang, R.; Liu, Y.; Liu, D.; Goldys, E. M.; Yang, X.; Xi, P.; Sunna, A.; Lu, J.; Shi, Y.; Leif, R. C.; Huo, Y.; Shen, J.; Piper, J. A.; Robinson, J. P.; Jin, D. Tunable Lifetime Multiplexing using Luminescent Nanocrystals. Nat. Photonics 2013, 8, 32−36.
(18) 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. (19) Habas, S. E.; Lee, H.; Radmilovic, V.; Somorjai, G. A.; Yang, P. Shaping Binary Metal Nanocrystals Through Epitaxial Seeded Growth. Nat. Mater. 2007, 6, 692−697. (20) Yin, Y.; Talapin, D. The Chemistry of Functional Nanomaterials. Chem. Soc. Rev. 2013, 42, 2484−2487. (21) Yin, Y.; Alivisatos, A. P. Faceting of Nanocrystals during Chemical Transformation: From Solid Silver Spheres to Hollow Gold Octahedra. Nature 2005, 437, 664−670. (22) Skrabalak, S. E.; Au, L.; Li, X.; Xia, Y. Facile Synthesis of Ag Nanocubes and Au Nanocages. Nat. Protoc. 2007, 2, 2182−2190. (23) Yavuz, M. S.; Cheng, Y.; Chen, J.; Cobley, C. M.; Zhang, Q.; Rycenga, M.; Xie, J.; Kim, C.; Song, K. H.; Schwartz, A. G.; Wang, L. V.; Xia, Y. Gold Nanocages Covered by Smart Polymers for Controlled Release with Near-Infrared Light. Nat. Mater. 2009, 8, 935−939. (24) Tsang, M. K.; Bai, G.; Hao, J. Stimuli Responsive Upconversion Luminescence Nanomaterials and Films for Various Applications. Chem. Soc. Rev. 2015, 44, 1585−1607. (25) Boyer, J. C.; Vetrone, F.; Cuccia, L. A.; Capobianco, J. A. Synthesis of Colloidal Upconverting NaYF4 Nanocrystals Doped with Er3+, Yb3+ and Tm3+, Yb3+ via Thermal Decomposition of Lanthanide Trifluoroacetate Precursors. J. Am. Chem. Soc. 2006, 128, 7444−7445. (26) Zhao, B.; Xie, X.; Xu, S.; Pan, Y.; Yang, B.; Guo, S.; Wei, T.; Su, H.; Wang, H.; Chen, X.; Dravid, V. P.; Huang, L.; Huang, W. From ScOOH to Sc2O3: Phase Control, Luminescent Properties, and Applications. Adv. Mater. 2016, 28, 6665−6671. (27) Liu, D.; Xu, X.; Du, Y.; Qin, X.; Zhang, Y.; Ma, C.; Wen, S.; Ren, W.; Goldys, E. M.; Piper, J. A.; Dou, S.; Liu, X.; Jin, D. ThreeDimensional Controlled Growth of Monodisperse sub-50 nm Heterogeneous Nanocrystals. Nat. Commun. 2016, 7, 10254. (28) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. A general strategy for nanocrystal synthesis. Nature 2005, 437, 121−124. (29) Sun, X.; Zhang, Y.-W.; Du, Y.-P.; Yan, Z.-G.; Si, R.; You, L.-P.; Yan, C.-H. From Trifluoroacetate Complex Precursors to Monodisperse Rare-Earth Fluoride and Oxyfluoride Nanocrystals with Diverse Shapes through Controlled Fluorination in Solution Phase. Chem. - Eur. J. 2007, 13, 2320−2332. (30) Li, C.; Yang, J.; Quan, Z.; Yang, P.; Kong, D.; Lin, J. Different Microstructures of α-NaYF4 Fabricated by Hydrothermal Process: Effects of pH Values and Fluoride Sources. Chem. Mater. 2007, 19, 4933−4942. (31) Djalali, R.; Chen, Y. F.; Matsui, H. Au Nanocrystal Growth on Nanotubes Controlled by Conformations and Charges of Sequenced Peptide Templates. J. Am. Chem. Soc. 2003, 125, 5873−5879. (32) Williamson, C. B.; Nevers, D. R.; Hanrath, T.; Robinson, R. D. Prodigious Effects of Concentration Intensification on Nanoparticle Synthesis: A High-Quality, Scalable Approach. J. Am. Chem. Soc. 2015, 137, 15843−15851. (33) Mai, H.-X.; Zhang, Y.-W.; Si, R.; Yan, Z.-G.; Sun, L.-D.; You, L.P.; Yan, C.-H. High-Quality Sodium Rare-Earth Fluoride Nanocrystals: Controlled Synthesis and Optical Properties. J. Am. Chem. Soc. 2006, 128, 6426−6436. (34) Chen, Y.; Kim, M.; Lian, G.; Johnson, M. B.; Peng, X. G. Side Reactions in Controlling the Quality, Yield, and Stability of High Quality Colloidal Nanocrystals. J. Am. Chem. Soc. 2005, 127, 13331− 1337. (35) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.; Stoddart, J. F. Dynamic Covalent Chemistry. Angew. Chem., Int. Ed. 2002, 41, 898−952. (36) Zhang, K.; Zhu, X.; Jia, F.; Auyeung, E.; Mirkin, C. A. Temperature-Activated Nucleic Acid Nanostructures. J. Am. Chem. Soc. 2013, 135, 14102−14105. (37) Teng, X.; Zhu, Y.; Wei, W.; Wang, S.; Huang, J.; Naccache, R.; Hu, W.; Tok, A. I.; Han, Y.; Zhang, Q.; Fan, Q.; Huang, W.; Capobianco, J. A.; Huang, L. Lanthanide-Doped NaxScF3+x Nano9766
DOI: 10.1021/acs.chemmater.7b03561 Chem. Mater. 2017, 29, 9758−9766
