Phase Transition Control in the Supercooled State for Robust Active

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In-Situ Phase Transition Control in the Supercooled State for Robust Active Glass Fiber Shichao Lv,†,‡ Maoqing Cao,§ Chaoyu Li,§ Jiang Li,§ Jianrong Qiu,∥ and Shifeng Zhou*,†,‡ †

State Key Laboratory of Luminescent Materials and Devices, School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China ‡ Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, Special Glass Fiber and Device Engineering Technology Research and Development Center of Guangdong Province, Guangzhou 510640, China § Key Laboratory of Transparent Opto-functional Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China ∥ College of Optical Science and Engineering, State Key Laboratory of Modern Optical Instrumentation, Zhejiang University, Hangzhou 310027, China ABSTRACT: The construction of a dopant-activated photonic composite is of great technological importance for various applications, including smart lighting, optical amplification, laser, and optical detection. The bonding arrangement around the introduced dopants largely determines the properties, yet it remains a daunting challenge to manipulate the local state of the matrix (i.e., phase) inside the transparent composite in a controllable manner. Here we demonstrate that the relaxation of the supercooled state enables in-situ phase transition control in glass. Benefiting from the unique local atom arrangement manner, the strategy offers the possibility for simultaneously tuning the chemical environment of the incorporated dopant and engineering the dopant−host interaction. This allows us to effectively activate the dopant with high efficiency (calculated as ∼100%) and profoundly enhance the dopant−host energy-exchange interaction. Our results highlight that the in-situ phase transition control in glass may provide new opportunities for fabrication of unusual photonic materials with intense broadband emission at ∼1100 nm and development of the robust optical detection unit with high compactness and broadband photon-harvesting capability (from X-ray to ultraviolet light). KEYWORDS: controllable luminescence, doping, phase transition, transparent composites

1. INTRODUCTION Phase transition is a fundamental physical process which is widely studied for improving the properties of various materials. An important example is the phase engineering in doped optical materials for enhancing the radiative transition probability of the incorporated luminescent centers. This technological process is of increasing importance as a pathway of improving the functionality and efficiency of a biological probe, a solar concentrator, light-emitting diodes, a fiber amplifier, and lasers.1−5 For example, phase control in fluorite nanocrystals for tuning the arrangement of the substitutional rare-earth dopants has resulted in the bright upconversion biological label.1 Reversible phase transition in the laser crystal has induced striking new spectral features of transition metal dopants including two-color luminescence and increased luminescence efficiency.3 Despite the enormous progress achieved on rational phase control in various single-phase systems, a long-standing barrier has been how to precisely tune the phase of the functional structures embedded in hybrid photonic composites while still maintaining their excellent optical transmission property. One underlying problem is © 2017 American Chemical Society

thought to be the difficulty in balancing the phase transition and crystal growth processes. The undesirable crystal growth under high-temperature sintering is dictated to a great extent by the increased probability of grain-boundary migration that reshapes the particle.6,7 This phenomenon has been frequently observed in classic high-temperature solid-state or sol−gel reactions featuring a low interfacial diffusion barrier (Elow) that dramatically promotes the interface junction and pore generation (Figure 1a). In sharp contrast, phase transition essentially depends on the local atom reorganization rather than the long-range diffusion process.8 Based on these findings, we recognize that phase transition and particle growth are not always contradictory elements. Therefore, it is possible to smoothly trigger phase transition without crystal growth through rationally introducing an external constraint with significantly enhanced interfacial diffusion barrier (Ehigh) Received: April 16, 2017 Accepted: May 31, 2017 Published: May 31, 2017 20664

DOI: 10.1021/acsami.7b05317 ACS Appl. Mater. Interfaces 2017, 9, 20664−20670

Research Article

ACS Applied Materials & Interfaces

Figure 1. Schematic representation showing the phase transition processes. (a) In the conventional sol−gel method, the low interfacial diffusion barrier may lead to undesirable particle growth during phase transition. (b) In the case of in-situ phase transition in the supercooled state, the enhanced interfacial diffusion barrier may help to prevent particle growth during relaxation.

Figure 2. (a) XRD patterns of the glass and composite samples obtained by relaxation of the supercooled state at various temperatures. (b) Raman spectra of the glass and composite samples obtained by relaxation of the supercooled state at various temperatures. (c) Heat-treatment temperaturedependent crystallite size for samples obtained by the sol−gel method and relaxation of the supercooled state.

at relatively high temperatures (>900 °C) can be well indexed to various tantalite crystalline phases. The precipitated phases in the sample heat-treated between 900 and 950 °C can be ascribed to the orthorhombic Gd3TaO7 (PDF 00-038-1409). With further increase in temperature, the phase transition is evident, and the pure monoclinic M′-type GdTaO4 (PDF 01072-2017) can be obtained at 975 °C. The Raman scattering spectra of the composite exhibit the fingerprint peaks of the tantalite crystalline phase corresponding to the Ta−O stretching modes (810 cm−1) and the deformation of the TaO4 group (276 cm−1) (Figure 2b), consistent with the XRD results.12,13 The crystallite size of various phases inside the composite was calculated, and the results are compared with the phase obtained through the sol−gel method (Figure 2c). Significantly, we observe no clear increase in the crystallite size (∼10 nm) during phase transition in the supercooled state, indicating the success in suppression of undesirable particle growth. In stark contrast, the phase transition in the sol−gel sample leads to a notable increase (∼3-fold) in the physical dimension of the crystalline phase, and the crystallite size of the final M′-type GdTaO4 product was estimated to be ∼60 nm. We further study the phase transition by using highresolution transmission electron microscopy (HRTEM) (Figure 3). The HRTEM image of the composite heat-treated at 900 °C (Figure 3a) exhibits lattice fringes of the (202) plane

(Figure 1b).9 Following this line of inquiry, we focus our attention on the supercooled state because this metastable state can not only provide sufficient potential energy for supporting relaxation but also hold a wide range of viscosities (103 Pa·s ∼ 1013.6 Pa·s) for controlling diffusion. We show herein that in-situ phase transition in glass is a facile strategy for generating a highly transparent composite with various optical functions. The proof-of-principle investigation of the tantalite-based material system demonstrates for the first time a robust and multifunctional glass fiber with broadband emitting feature and efficient energy-harvesting ability.

2. RESULTS AND DISCUSSION To validate our proposal, we chose tantalite as the target material because it can provide rich phases for various optical functions and has notorious difficulty in obtaining the corresponding transparent composite.10,11 In our experiment, a typical melting−quenching method was employed to fabricate tantalite glass, which was then heat-treated for relaxation above the glass transition temperature for reaching various supercooled states. The phase transition process was first examined by X-ray powder diffraction (XRD) (Figure 2a). The broad diffraction bands in as-made material and composite heattreated at low temperatures (6fold) of the emission intensity (Figure 4a). However, the Ni2+doped tantalite samples with the same phases synthesized by the sol−gel method do not show any emission (Figure 4a). We suspect that the absence of radiative transitions from the Ni2+ dopant could be ascribed to the activation failure in the sol−gel method. Indeed, the notable difference in valence state and chemical property between the dopant (Ni2+) and host ions (Gd3+ or Ta5+) can drive the occurrence of segregation, and in some cases, transition metal dopants may even aggregate at the grain boundary.18 On the contrary, the melting−quenching approach may fully break −Ni−O−Ni− bonding and freeze various metastable Ni-related structural units such as [NiO4]6− and [NiO5]8− in the supercooled state. As proved by the electronic absorption spectroscopy, the fingerprint absorption bands of distorted [NiO5]8− centers can be clearly identified in the melting−quenching sample (Figure 4b). Notably, [NiO5]8− units exhibit low crystal field stabilization energy (∼40 kJ/mol) 20667

DOI: 10.1021/acsami.7b05317 ACS Appl. Mater. Interfaces 2017, 9, 20664−20670

Research Article

ACS Applied Materials & Interfaces methods. To demonstrate the potential of the proposed strategy, Eu3+ ions with the same doping level were introduced into the glass and composite samples, and their optical properties were studied. To avoid the scattering effect on the spectral feature, all of the samples were carefully ground into the powders with the same particle size (∼10 μm). Figure 5a shows the representative excitation spectra of samples by monitoring the typical 5D0 → 7F2 transition of Eu3+ at 611 nm.23 A set of excitation bands at ∼200 and 313 nm can be observed, which are originated from the charge transfer transition of the TaO4 group and 8S2/7 → 6P7/2 transition of Gd3+, respectively.23,24 Significantly, the in-situ phase transition from orthorhombic Gd3TaO7 to monoclinic M′-type GdTaO4 considerably increases the charge transfer excitation intensity of the TaO4 group (∼6-fold) and 8S2/7 → 6P7/2 transition probability of Gd3+ (∼5.5-fold). The results provide direct evidence for the highly efficient energy transfer from the GdTaO4 host to Eu3+. In contrast, the crystallization of Gd3TaO7 only leads to a slight change of the charge transfer excitation of the TaO4 group and 8S2/7 → 6P7/2 transition of Gd3+, indicating the limited impact of the pure crystallization of glass on the energy exchange process. It can also be confirmed by a direct comparison of the emission brightness of the samples under ultraviolet ray irradiation (Figure 5b, c). To shed more light on the energy transfer process, the radiative decay curve and luminescence spectra of the Gd3+ donor at the ultraviolet region (313 nm) was studied (Figure 5d and the inset of Figure 5d). Decay curves exhibit a rapid decrease (from 19.83 to 1.93 μs) during the phase transition, firmly demonstrating the existence of an extra energy-releasing pathway from Gd3+ to Eu3+. Moreover, the decrease of Gd3+ emission intensity is another evidence. Based on the decay dynamics of Gd3+ (8S2/7 → 6P7/2 transition), the energy transfer efficiency (ηE) can be calculated. It is defined as the ratio of donors (Gd3+ ions) that are depopulated by energy transfer to acceptor (Eu3+ ions) over the total number of donors being excited, represented as ηE = 1 − (τ /τS)

Figure 6. (a) Schematic representation showing the working principle of the broadband detector. (b) Photograph of the composite fiber fabricated by the fiber-drawing method. (c) Optical microscope image of the detection unit under natural light (left) and ultraviolet light (right). (d) X-ray excited luminescence spectra of glass and composite samples. The insets show the fiber array for drawing (left) and the constructed prototype of detector (right). (e) The line profile of luminescence recorded along the dotted line shown in (c).

can provide a tiny pixel upon ultraviolet or X-ray excitation. We successfully fabricated composite fiber by the fiber-drawing method and further assembled them into a prototype of detector (Figures 6b, c and the inset of Figure 6d). As shown in Figure 6c−e, pixels exhibit bright luminescence under excitation with ultraviolet or X-ray excitation. The boundary size of the pixel was estimated to be 150 μm, which is mainly limited by the size of the fiber. The newly built detector provides notable advantages, including broadband response ability, high compactness, easy fabrication, and robust mechanical properties. Taking into account the rapid development in the submicrometer fiber-drawing techniques and impressive features of our composite fiber such as the high density (∼5 g/cm3),28 further improvement in the compactness of the functional component can be expected. Furthermore, the constructed detector with excellent light-guiding properties may effectively suppress the optical crosstalk (Figures 6c,e), which has long been recognized as a significant issue in highresolution imaging.29,30

(1)

where ηE is the energy transfer efficiency from Gd ions to Eu3+ ions; τ is the lifetime of Gd3+ ions in composite samples obtained by relaxation of the supercooled state at various temperatures; and τS is the lifetime of Gd3+ ions in glass without doping.25,26 The energy transfer efficiency in composite embedded with M′-type GdTaO4 was estimated to be 90.74%, which is about 18 times higher compared with the sample before phase transition (4.87%). The above results strongly indicate the prominent role of in-situ phase transition on the energy transfer process. Furthermore, it can be anticipated that the existence of a dual energy transfer channel (Gd3+ and TaO4 groups) may further strengthen the light-harvesting capability (Figure 5d). The availability of superior high-energy photon-harvesting capability and interesting broadband emission from the composite prompted us to design and fabricate novel photonic components. Especially, high content of Ta and Gd elements (∼80 wt %) of the composite with typical absorption at 50.2 keV (Gd element) and 67.4 keV (Ta element) may further extend its harvesting ability from ultraviolet to X-ray wavebands,27 offering unique opportunities for the development of a broadband detection platform. The basic design is shown in Figure 6a: the detector is composed of regularly arranged composite fibers, in which each fiber works independently and 3+

3. CONCLUSION Our work demonstrates that the relaxation of the supercooled state offers unique opportunities for tuning the local atom arrangement in the composite without loss of its high optical transmission property. The strategy allows for rationally controlling the chemical environment of the incorporated dopant and engineering the dopant−host interaction. The ability to effectively activate the dopants and establishment of the efficient energy-harvesting pathways may enable us not only to discover new gain materials with unprecedented optical 20668

DOI: 10.1021/acsami.7b05317 ACS Appl. Mater. Interfaces 2017, 9, 20664−20670

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properties such as broadband luminescence in the high-energy region of the infrared wavebands (Figure 4) but also to develop novel devices such as a composite-derived compact detector with broadband response feature (Figure 6). Furthermore, we note that the proposed strategy can be potentially applied to engineer a wide range of metastable condensed materials such as supercooled alloy, molecular liquids, and polymer.

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4. EXPERIMENTAL PROCEDURES Material Synthesis. The transparent glass and composite samples were prepared according to a classic glass forming method at a typical glass system of Li2O−Al2O3−SiO2−Gd2O3−Ta2O5. Raw materials (20 g) were melted in an alumina crucible at a temperature of 1600 °C for 30 min. The homogenized melt was cast into a slab on a preheated brass plate to form the precursor glass. The obtained sample was annealed at the glass transition temperature and then heat-treated above the glass transition temperature to obtain highly transparent composites. Samples were prepared by the sol−gel method using GdCl3·6H2O, TaCl5, and NiCl2·6H2O as precursors. Stoichiometric amounts of GdCl3·6H2O and NiCl2·6H2O were fully dissolved in ethanol, and the prescribed amount of TaCl5 was dissolved into a solution of HCl (20%). These prepared solutions were mixed together at the molar ratio of Gd3+:Ta5+:Ni2+ = 0.5:0.5:0.005. The mixture was stirred and refluxed at 70 °C for about 4 h to ensure complete reaction and evaporated at 100 °C. The dried powders were heat-treated at various temperatures with a heating rate of 2 °C/min and sintered for 3 h in air atmosphere. Material Characterization. The crystalline structure of glass and composite samples was examined by X-ray diffraction using Cu/Kα radiation. The Raman spectra of the glass and composite samples were recorded on a Renishaw InVia spectrometer with a 532 nm laser source. Microstructures were analyzed by transmission electron microscopy which was performed using a JEOL 2010F (scanning) transmission electron microscope. The excitation and emission spectra and luminescence decay curves were recorded on a FLS920 fluorescence spectrophotometer (Edinburgh Instrument Ltd., U.K.). The absorption spectra were measured by a JASCO FP-6500 doublebeam spectrophotometer. The X-ray excited luminescence spectra were measured on a homemade spectrophotometer with an X-ray tube of W anode operating at 30 kV and 15 mA.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Shifeng Zhou: 0000-0003-4609-763X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant 11474102), the National Science Fund for Excellent Young Scholars of China (Grant 51622206), Guangdong Natural Science Funds for Distinguished Young Scholars (Grant S2013050014549), the Tip-Top Scientific and Technological Innovative Youth Talents of Guangdong Special Support Program (Grant 2015TQ01C362), the Science and Technology Project of Guangdong (Grant 2015B090926010), Fundamental Research Funds for the Central University (Grant x2clD2174800), and Open Fund of State Key Laboratory of Information Photonics and Optical Communications (Beijing University of Posts and Telecommunications), P. R. China (IPOC2016B003) 20669

DOI: 10.1021/acsami.7b05317 ACS Appl. Mater. Interfaces 2017, 9, 20664−20670

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DOI: 10.1021/acsami.7b05317 ACS Appl. Mater. Interfaces 2017, 9, 20664−20670