Fast transformation of a rare-earth doped luminescent sub-microcrystal via plasmonic nanoislands

Ting Kong a, Chengyun Zhang a, Xuetao Gan b, Fajun Xiao b, Jinping Li a, Zhengkun Fu a, Zhenglong Zhang *a and Hairong Zheng *a
aSchool of Physics and Information Technology, Shaanxi Normal University, 710119, Xi’an, China. E-mail: zlzhang@snnu.edu.cn; hrzheng@snnu.edu.cn
bMOE Key Laboratory of Material Physics and Chemistry Under Extraordinary Conditions and Shaanxi Key Laboratory of Optical Information Technology, School of Science, Northwestern Polytechnical University, Xi’an, 710072, China

Received 6th January 2020 , Accepted 13th February 2020

First published on 14th February 2020


An efficient and fast transformation scheme for the matrix crystal of rare-earth doped luminescent micro-nanomaterials is developed by using plasmonic gold/silver nanoislands. The transformation is realized through an oxidation reaction from a polycrystalline sub-microcrystal to a single crystal, accompanied by the optimization of the crystal structure and a significant increase in luminescence. The crystal transformation can be achieved in tens of milliseconds, and the rate is controlled not only by the laser illumination power and wavelength, but also by the size and nanogap of nanoislands. Particularly, single crystal transformation is also achieved even at very low temperature, which provides a new way to obtain single crystal materials in a harsh environment. Moreover, the crystal transformation efficiency of the gold plasmonic islands is very stable in air over at least three months. This plasmon driven crystal transformation rapidly provides highly crystalline nanomaterials, which breaks the dependence of high temperature, long period and high energy consumption in the traditional annealing treatment.


Introduction

As a series of important luminescent materials, rare-earth (RE) elements and their related derivative compounds have received significant attention due to their narrow emission bands,1–4 long lifetimes,5 multiple fluorescence colors,6,7 and electrochemical properties.8,9 These unique properties endow RE-based nanomaterials with great potential for applications in various fields such as displays, bioimaging, light-emitting sources, and temperature sensors.10–13 The luminescence performance of doped RE3+ ions is strongly dependent on the crystal-field strength and site symmetry of a host matrix. Therefore, nanomaterials with high crystallinity, especially a single crystal, can be used as an ideal host matrix that provides minimal optical scattering and an optimal crystal-field environment.14–16 To optimize the crystallinity of materials, relatively high temperature, long time periods and high energy consumption are usually required in the traditional annealing treatment.17–19 Meanwhile, it is hard to control the crystal size and morphology, and the aggregation of nanoparticles (NPs) at high temperature severely restricts optical performance and applications.20,21

Benefiting from the surface plasmon resonance of noble metals with strong optical absorption, a new technology was presented to rapidly obtain single crystal nanomaterials in our previous work.22,23 The plasmonic catalysis of adsorbed gold (Au) NPs was employed to realize a crystal transformation from polycrystalline materials to single crystals, which can solve the bottleneck problems of conventional crystal transformation.20 Although a convenient and efficient scheme was provided for the rapid achievement of single crystal nanomaterials, there are several practical problems that need to be solved urgently. The effective adsorption of Au NPs on the polycrystalline surface has brought a new challenge to the fabrication of composite micro-/nano structures, which limits its applications in many nanomaterials. Since the Au NPs are randomly adsorbed on the polycrystalline surface, it is hard to achieve the quantitative analysis, and residual Au NPs will inevitably affect the purity and applications of the crystal materials, especially for luminescence applications. Therefore, it is important to develop a more universal method that not only effectively utilizes the thermal and catalytic effects of Au NPs, but also avoids the disadvantages of traditional transformation methods.

In this study, a self-assembled array of plasmonic Au nanoislands (NIs) is used to achieve a fast transformation of RE3+ doped sub-microcrystals. With the assistance of plasmonic thermal and catalytic effects of AuNIs, polycrystalline NaYF4 can quickly transform to single crystal Y2O3, which is an excellent luminescent host matrix for RE3+ ions due to its high physical and chemical stability. It is found that the crystal transformation is activated by taking advantage of the thermal effect generated from the localized surface plasmon resonance (LSPR) of plasmonic AuNIs, leading to a high lattice temperature of the crystal during transformation. The single crystal transformation is also realized even at very low temperature, which reveals a more powerful aspect of plasmonic AuNIs. The transformation rate can be easily controlled by the laser power and wavelength, as well as by the size and nanogap of the AuNIs. In addition, the separated AuNIs prevent Au from attaching to the nanocrystal product, which extends the crystal transformation to other materials.

Experimental section

Sample preparation

All reagents, such as Ln(NO3)3 (Ln = Y, Eu) (99.9%) and NaF (98%), and solvents were purchased from Sigma-Aldrich Chemicals Co and used as received. Deionized water was used throughout the experiments, and the wet-chemical method was employed for the synthesis of polycrystalline NaYF4:Eu3+. An aqueous solution of 25 mM Y(NO3)3, 0.25 mM Eu(NO3)3 and 8.75 mM NaF was heated to 75 °C and vigorously stirred for 2 hours. A white precipitated product was collected by centrifugation and washed with water and ethanol. Gold/silver (Au/Ag) NI films were prepared by evaporating 5 nm gold/silver in a high vacuum onto precleaned glass and silicon substrates, followed by annealing at 200–600 °C for 30 s under an argon atmosphere. The polycrystalline NaYF4 particles were then deposited on the Au/Ag NI films.

General material characterization

For characterizing sample morphologies, scanning electron microscopy (SEM) images were obtained with a FEI-Nova NanoSEM 450 at 10 kV, and atomic force microscopy (AFM) images were obtained with Bruker-JPK NanoWizard Ultras. X-ray diffraction (XRD) was performed with a Rigaku D/Max2550VB +/PC diffractometer at a scanning rate of 7° min−1 with graphite monochromatic Cu Kα (40 kV, 40 mA) radiation (λ = 0.154 06 nm). Extinction spectra were acquired with a PerkinElmer Lambda 950 spectrometer. Nanoscale thin foil was cut along the vertical direction with a focused ion beam (FIB, FEI Helios G4 CX) for in situ transmission electron microscopy (TEM) imaging, and verification tests of the structural information were conducted using TEM with energy-dispersive X-ray (EDX) spectroscopy, selected area electron diffraction (SAED) and high angle annular dark-field (HAADF) imaging. The HAADF, SAED and EDX images were obtained with a FEI Titan cubed Themis G2 300 microscope operated at 300 kV and equipped with a probe aberration corrector and a monochromator. In situ laser irradiation and luminescence spectra measurements were conducted with a LabRam HR Evolution Raman system with a 100× (NA = 0.9) objective. A 532 nm continuous laser was employed as the laser irradiation and excitation source for plasmon driven crystal transformation. To avoid any transformations during acquisition of luminescence spectra, very low laser power was used to obtain the luminescence emission.

Results and discussion

A schematic of in situ crystal transformation from polycrystalline to a single crystal with the assistance of AuNIs under laser irradiation is shown in Fig. 1(a). The AuNIs are prepared by evaporating gold and annealing at 300 °C, forming a gold nanoparticle array on a glass substrate. As shown in the AFM image in Fig. 1(b), the average size of the nanoparticles is about 15 nm and the average roughness value is 3.5 nm. The SEM image in Fig. 1(c) shows uniform 500 nm flower-like sub-microparticles of Eu3+ doped NaYF4 synthesized by the co-precipitation procedure.24 More detailed sample morphology information is shown in Fig. S1 (ESI). The polycrystalline NaYF4 particles are dispersedly distributed on the AuNI surface prior to laser irradiation, and the luminescence spectrum of doped Eu3+ is used to monitor thecrystal transformation of the matrix materials. The in situ luminescence spectra of a single sub-microparticle on the AuNIs before and after laser irradiation are shown in Fig. 1(d). Due to the poor crystallinity of initial polycrystalline NaYF4, the wide bands with weak luminescence intensities centered at 590 nm, 615 nm and 700 nm are observed, which are related to the transitions from 5D0 to 7F1, 7F2 and 7F4, respectively (Fig. S2, ESI). It is found that the single flower-like NaYF4 sub-microparticle can be in situ transformed to a smooth sphere-like particle with 25 mW laser irradiation for 150 ms. In particular, a sharp band with strong luminescence intensity at 610 nm can be observed, which indicates that an optimized nanocrystal with better crystallinity and stability is produced after the laser irradiation.
image file: d0tc00060d-f1.tif
Fig. 1 (a) Schematic of fast transformation of a RE3+ doped luminescent crystal with plasmonic AuNIs; (b) AFM image of the AuNIs film; (c) SEM image of polycrystalline NaYF4:Eu3+ nanoparticles; and (d) in situ luminescence spectra of an Eu3+-doped single sub-microparticle before and after laser irradiation (25 mW for 150 ms), and inserted SEM images show initial and transformed sub-microparticles, respectively; the scale bar is 200 nm.

To examine the structural properties of the optimized product, a thin foil cut using a FIB is in situ characterized by using EDX, SAED and high-resolution TEM. As shown in Fig. 2(b), the EDX elemental mappings of the thin foil shown in Fig. 2(a) indicate the existence of only Y, O and Eu elements; Na and F elements present in the initial NaYF4 sub-microparticles are not present in the product. Furthermore, the result of the SAED pattern (Fig. 2c) confirms a single crystal Y2O3 product with a cubic Ia[3 with combining macron] structure and lattice parameter a = 10.604 Å. More detailed results for the crystal structure are shown in Fig. S3 (ESI). The high-resolution TEM image in Fig. 2(d) fits well with the projection of the Y2O3 structure along the [011] direction, and the interplanar crystal spacings of 2.67 Å and 3.09 Å belong to (400) and (222) crystal planes, respectively. Therefore, it is clear that a fast crystal transformation from a polycrystal NaYF4 to a single crystal Y2O3 is realized with the assistance of plasmonic AuNIs, resulting in a remarkable improvement of luminescence emission.


image file: d0tc00060d-f2.tif
Fig. 2 (a) TEM image and (b) EDX elemental mappings of a cut thin foil from the optimized nanoparticle; (c) SAED pattern of the thin foil marked with a dashed blue circle in (a) taken along the [011] zone axes; and (d) high-resolution TEM image of the nanocrystal taken along the [011] zone axis.

By monitoring the Eu3+ luminescence spectra, the dynamic process of crystal transformation is in situ investigated by precisely controlling the irradiation time. As shown in Fig. 3(a), NaYF4 transforms to Y5O4F7 and YOF after irradiation for around 50 ms and 120 ms, respectively; the final Y2O3 crystal is formed at around 150 ms. It is found that the luminescence intensity is strongly improved with the increased irradiation time, indicating that the crystal has been optimized dramatically. Previously, when using adsorbed AuNPs, the first step of the transformation from NaYF4 to Y5O4F7 was finished in only 1 ms, which is due to the excellent catalytic effects of abundant AuNPs at the beginning. Although the first step is much faster for AuNPs, the second and third steps become slower because of the aggregation and ejectment of AuNPs. Here, the transformation rate is not very fast at the beginning, but the AuNIs can provide a more stable and efficient platform for plasmon driven crystal transformation.


image file: d0tc00060d-f3.tif
Fig. 3 (a) Dynamic process of plasmon driven in situ single crystal transformation via using AuNI films at 25 mW laser power; (b) laser power dependent transformation time; (c) luminescence spectra of the sub-microcrystal with laser irradiation at 532 nm for 1 s and 633 nm for 30 min (5 mW), respectively; and (d) transformation time in a low temperature environment with a laser power of 20 mW at 532 nm.

The rate of the above crystal transformation can be easily controlled by the power of laser irradiation. The required irradiation time for the transformation to single crystal Y2O3 depends on the laser power, as shown in Fig. 3(b). As the laser power increases from 6 mW to 25 mW, the crystal transformation time decreases from around 800 ms to 150 ms. When the irradiation wavelength is changed from 532 nm to 633 nm, no crystal transformation can be observed even after irradiation for 30 min at the same power (Fig. 3c). Since a wavelength of 633 nm is far away from the plasmon resonance of AuNIs (Fig. S4, ESI), the photothermal effect is much relatively lower and it is hard to achieve crystal transformation even after irradiation for 30 min with a power of 5 mW. Moreover, the energy of the hot electron produced by the relaxation of the plasmon excited at 633 nm is lower than that excited at 532 nm, which makes it unable to assist the crystal transformation. Particularly, the plasmon driven crystal transformation is also realized at very low temperature. As shown in Fig. 3(d), the laser irradiation time increases from 40 s to 1000 s with decreasing temperature from 0 °C to −90 °C. Due to the powerful aspect of the localized thermal and catalytic effects, plasmonic AuNIs can still be used to achieve single crystal transformation at low temperature, which allows us to get single crystal materials in a way that the traditional methods cannot reach. In addition, to identify the role of AuNIs in plasmon driven crystal transformation, the polycrystalline NaYF4 without AuNIs is irradiated on the clean surface of glass and silicon substrates. As shown in Fig. S5 (ESI), there are no changes in the luminescence spectra for either substrate following 25 mW of 532 nm laser irradiation for 30 min, which indicates that the plasmonic AuNIs play an important role in crystal transformation. Therefore, the plasmon driven crystal transformation is realized by taking the LSPR of plasmonic AuNIs under resonance excitation. With laser irradiation onto AuNIs, the LSPR can be excited and then decayed into electron–hole pairs through Landau damping.25 The hot carriers quickly transfer energy to the lattice via elastic electron–electron scattering and electron–phonon coupling, and a high lattice temperature occurs over several picoseconds.26,27 The heat generated from the AuNIs dissipates into the NaYF4 sub-microparticles by phonon–phonon interactions, leading to temperature equilibration in a few nanoseconds, and then a crystal transformation is activated when enough heating is conveyed to the lattice.23 Except for the plasmonic thermal effect, hot carriers induced by plasmon decay can transfer to the antibonding O–O state of O2, thus generating a superoxide anion (O2) with strong oxidation, which can promote the oxidization reaction of NaYF4.28,29

Because the LSPR of the AuNIs is dependent on the geometry properties (size and gap) of the Au NPs,30 it provides an easy way to control the crystal transformation by using a series of plasmonic AuNIs prepared at different annealing temperatures. As shown in Fig. 4, AuNIs after annealing at temperatures ranging over 200–600 °C (II–VI) are selected to study the effect of plasmon driven crystal transformation. Under white-light illumination, the brown-green color of the smooth Au film (I) is seen in Fig. 4(a), this is due to interband absorption of the thin smooth Au film deposited on the glass substrate. No obvious changes in the luminescence spectrum can be observed even after 25 mW laser irradiation for 30 min, which indicates that the smooth Au film has no plasmonic catalysis effect (Fig. S6, ESI). With increasing annealing temperature over 200–600 °C, the color changes to red gradually because of LSPR absorption in the AuNIs. The average particle sizes are around 10 nm, 15 nm, 20 nm, 35 nm and 40 nm for II–VI NIs, respectively, and the corresponding nanogaps between the AuNPs is increased (Fig. 4b). Under 532 nm (25 mW) laser irradiation, the transformation rate is first increased and then decreased for the II–VI NIs, and the fastest transformation can be completed in around 150 ms for the III NIs (Fig. 4c). To understand the above results, the LSPR extinction positions for II–VI NIs are shown in Fig. 4(d). The extinction peaks of AuNIs corresponding to III–VI NIs are almost maintained at 540 nm, which are closely matched to the laser irradiation wavelength of 532 nm. Besides, with an increase in the size and nanogap, the absorption cross section of AuNIs weakens gradually, resulting in a degraded crystal transformation rate by using the III–VI NIs. Therefore, the plasmon driven crystal transformation can be controlled by the laser irradiation power and wavelength, as well as by the size and nanogap of the plasmonic NIs. The AuNIs annealed at 300 °C (III) is the optimal platform for plasmon driven crystal transformation.


image file: d0tc00060d-f4.tif
Fig. 4 (a) Optical and (b) AFM images of the smooth Au film (corresponding to I) and AuNIs with annealing at 200 °C, 300 °C, 400 °C, 500 °C and 600 °C (corresponding to II–VI), respectively; (c) crystal transformation rates with II–VI NIs; and (d) wavelengths of the LSPR extinction peak for AuNIs corresponding to II–VI NIs.

The plasmon driven crystal transformation can also be achieved by using AgNIs annealed at 350 °C. As shown in the AFM image in Fig. 5(a), the average Ag nanoparticle size is about 15 nm and the average roughness value is 3.8 nm. The LSPR peak is located at 520 nm, which matches well with the irradiation wavelength of 532 nm (Fig. 5b). By analyzing in situ luminescence spectra of a single sub-microparticle before and after irradiation, the crystal transformation is finished in less than 50 ms with the laser irradiation at 532 nm for 25 mW (Fig. 5c), which is much faster than that using AuNIs. In addition, a comparison of the transformation rate stabilities using AuNIs and AgNIs is shown in Fig. 5(d). The transformation time is around 0.05–0.8 s for fresh AgNIs, and increases to around 0.8–2 s and 17–23 s when they are stored in air for 10 and 20 days, respectively. Due to the easy oxidation of silver in air, the LSPR properties of AgNIs are unstable, and the plasmonic catalysis effect decreases gradually. Meanwhile, the transformation time of AuNIs is extremely stable at around 150 ms (±10 ms) over at least three months in air. Although a little slower crystal transformation is achieved by using AuNIs, a much more stable plasmonic efficiency is maintained for a much longer time. Hence it prefers to realize crystal transformation with the assistance of AuNIs.


image file: d0tc00060d-f5.tif
Fig. 5 (a) AFM images and (b) normalized extinction spectrum of AgNIs after annealing at 350 °C. A magnified AFM image of AgNIs is in the upper right corner of (a); (c) luminescence spectra of a single sub-microparticle before and after 532 nm laser irradiation (25 mW for 50 ms); and (d) the stability of transformation time for AuNIs and AgNIs.

Conclusions

Self-assembled plasmonic NIs are designed to realize a fast crystal transformation from polycrystalline NaYF4 to single crystal Y2O3. Higher purity and crystallinity single crystals are obtained in around 50 ms and 150 ms for Ag and Au NIs, respectively. The crystal transformation is activated by taking advantage of the thermal effect generated from LSPR of plasmonic AuNIs, leading to a high lattice temperature of the crystal during transformation. Importantly, the plasmon driven crystal transformation is also achieved even at very low temperature, which is impossible with traditional approaches. The rate of crystal transformation is not only dependent on the laser power and wavelength, but also on the size and nanogap of plasmonic NIs. Although a little slower crystal transformation is achieved for AuNIs, a much more stable plasmonic efficiency is maintained over three months in air. A simple and effective system of AuNIs is designed to achieve fast crystal transformation, which can overcome the limitations of conventional methods and extend the applications of RE-based luminescent materials to a much broader field.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No. 11574190 and 11504224), the Natural Science Foundation of Shaanxi Province (Grant No. 2019JQ-142 and 2019JM-441), and the Fundamental Research Funds for Central Universities (Grant No. GK201701008, GK201903013, 2018TS067 and 310201911fz049).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/d0tc00060d

This journal is © The Royal Society of Chemistry 2020