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Cobalt and iron co-doped ZnSe nanocrystals:Mid-IR luminescence at room temperature
Journal Pre-proof Cobalt and iron co-doped ZnSe nanocrystals:Mid-IR luminescence at room temperature Huawei Shi, Xiaoxia Cui, Xusheng Xiao, Yantao Xu, Chao Liu, Chaoqi Hou, Haitao Guo PII:
S0022-2313(19)32002-2
DOI:
https://doi.org/10.1016/j.jlumin.2020.117102
Reference:
LUMIN 117102
To appear in:
Journal of Luminescence
Received Date: 11 October 2019 Revised Date:
6 February 2020
Accepted Date: 6 February 2020
Please cite this article as: H. Shi, X. Cui, X. Xiao, Y. Xu, C. Liu, C. Hou, H. Guo, Cobalt and iron codoped ZnSe nanocrystals:Mid-IR luminescence at room temperature, Journal of Luminescence (2020), doi: https://doi.org/10.1016/j.jlumin.2020.117102. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Cobalt and iron co-doped ZnSe nanocrystals:Mid-IR luminescence at room temperature Huawei Shi,ab‡ Xiaoxia Cui,ab‡ Xusheng Xiao,ab Yantao Xu,ab Chao Liu,c Chaoqi Hou, ab a
and Haitao Guoabd*
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics
and precision Mechanics, Chinese Academy of Science (CAS), Xi’an, Shaanxi 710119, China b
Center of Materials Science and Optoelectronics Engineering, University of Chinese
Academy of Sciences (UCAS), Beijing 100049, China c
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of
Technology, Wuhan, Hubei 430070, China d
Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan,
Shanxi 030006, China ‡
These authors contributed equally to this work and can be regarded as co-first
authors *
Corresponding Author, E-mail address: [email protected]
Abstract: Fe:Co:ZnSe nanocrystals with different co-doping ratios of Fe2+/Co2+ ions were fabricated by hydrothermal synthesis. The facile method used in the present work avoids the mid-infrared quench effect induced by the organic molecular introduced in the past preparation process. These nanocrystals are spherical in shape and exhibit a cubic sphalerite structure with an average grain size of about 15 nm. Through the energy conversion between Co2+ and Fe2+ ions, mid-infrared fluorescences at 3.3 µm and 4.4 µm were detected under 1550 nm laser excitation at room temperature. The fluorescence band of 4.4 µm in the Fe:Co:ZnSe nanocrystals is thought to be an overlapping emission involving the peak centered at 4648 nm corresponding to the Co2+:4T1 (F) → 4T2 (F) energy transition and the peak centered at 4273 nm corresponding to Fe2+:5T2 (D) → 5E (D) energy transition. These high-quality optically active nanomaterials show potential applications in mid-infrared devices
such as composite fiber-amplifiers and lasers materials. Keywords: Mid-infrared; Transition metal ions; Energy transfer; Fe:Co:ZnSe nanocrystals; Photoluminescence 1. INTRODUCTION Mid-infrared (MIR) light sources in the range of 2-5 µm are of great interest in a variety of applications such as molecular spectroscopy, biological sensors, medical diagnostics, etc. [1-4]. Because of small crystal field splitting, ultrabroad wavelength tunability, large emission cross section, and low phonon energy, the transition metal ions (TM2+), including Cr2+, Co2+, Fe2+ and Ni2+ ions, doped wideband II-VI semiconductor crystals have been reported as efficient MIR fluorescence and laser materials [5-7]. In 1996, researchers from the Lawrence Livermore National Laboratory, USA, firstly reported the MIR lasing from TM2+-doped II-VI semiconductor crystals [8]. Since then, several research works on TM2+-doped II-VI chalcogenide crystals (i.e. ZnS, ZnSe, ZnTe, CdS, CdSe, and CdSeTe) have been performed, in which, the Fe2+ ions doped II-VI chalcogenide crystals laser has a wide tunable wavelength range and high optical efficiency [9-14]. Meanwhile, ZnSe has been recognized as an excellent MIR laser matrix material with a wide range of light transmission and good chemical stability [15]. In 1999, Adams et al. reported a 3.98-4.54 µm laser output from Fe:ZnSe single crystal at low temperature (15-180 K) under the Er:YAG laser pumping, and the maximum energy output was 12 µJ at 130 K [10]. Due to the strong multi-phonon quenching effect, the free-running broadband pump source cannot form an effective lasing at room temperature (290-300 K). Until 2005, the first room temperature gain-switched lasing of Fe:ZnSe crystal at 4.4 µm under excitation of 2.92 µm with a pulse duration of 5 ns was reported by J. Kernal et al. [16]. Currently, room temperature gain-switched lasing of Fe:ZnSe crystal becomes commonly. However, the preparation of high optical quality Fe:ZnSe crystal is still a difficult and time-consuming work, and the pump source is restricted to be 3 µm, which is an unusual one.
In view of the above problems, some studies related with the co-doping and chemical preparation were conducted. The fluorescence and lasing of Fe2+ ions in the bulk ZnSe or ZnS crystals have been achieved through codoping of Co2+ or Cr2+ ions by thermo-diffusion method [17-20]. However, the uneven distribution of doping ions in the crystals and the long preparation period limited its further application. Recently, Myoung et al. reported the 3.5-4.5 µm MIR emission at 35-200 K of Fe:ZnSe quantum dots that fabricated by micro emulsion hydrothermal synthesis [21]. The hydrothermal synthesis is easy and the distribution of Fe2+ ions in ZnSe matrix are homogenous. But the organic phase was introduced during the reaction, which severely quenched the MIR fluorescence. In this study, a series of Fe:Co:ZnSe nanocrystals with different Fe2+/Co2+ ions doping concentrations were synthesized via hydrothermal method without introduction of organic composition. A strong fluorescence emission centered at 4.4 µm in the Fe:Co:ZnSe nanocrystals was achieved at room temperature. The preparation of highly efficient MIR optical nanocrystals brings new opportunities for available hot-pressed ceramic materials [22], composite fibers lasers [23], and random powder lasers [24].
2. EXPERIMENTAL SECTION 2.1 Materials Zinc chloride (ZnCl2, 98%), cobalt chloride hexahydrate (CoCl2· 6H2O, 99.99%), ferrous chloride (FeCl2, 99.99%), sodium borohydride (NaBH4, ≥96%), selenium (Se, 99.9999%), and ethanol (C2H6O, ≥99.7%) were used as raw materials. All of the above regents were commercially purchased and used without further purification. Deionized water (18.25 MΩ cm-1) was produced using a reagent water system (EasypureⅡ, Barnstead). 2.2 Hydrothermal Synthesis The Fe:Co:ZnSe nanocrystals were synthesized by hydrothermal method. Firstly, 15 mL sodium hydrogen selenide (NaHSe) aqueous solution was prepared by mixing 10 mmol (0.7896 g) selenium (Se) powder and 21 mmol (0.7944 g) sodium
borohydride (NaBH4) in 15 mL deionized water in N2 atmosphere, excess NaBH4 was used to prevent the oxidation of Se2- and Fe2+ ions during the reaction process. Zinc ions precursor with different concentrations were prepared by dissolving 9.4-9.67 mmol (1.2812-1.3180 g) ZnCl2 in 15 mL ethanol. Mixing solution with different
concentrations of Fe2+ ions and the fixed Co2+ ions were prepared by
dissolving 0.03-0.3 mmol (0.0038-0.0380 g) FeCl2 and 0.3 mmol (0.0714 g) CoCl2· 6H2O in 5 mL deionized water under N2 atmosphere. Next, the 15 mL ZnCl2 ethanol solution was added to the above Fe2+ and Co2+ aqueous solution, mixing them evenly to form a cationic mixture. Afterwards, the cationic mixture was added into 15 mL NaHSe aqueous solution, then the Fe:Co:ZnSe slurry was produced. After stirred for 10 min, all the mixture was transferred into a stainless-steel Teflon-lined autoclave. 45 mL ethanol was added into the autoclave to make sure the filling fraction of autoclave is 80%. Then, the autoclave was sealed and heat-treated at 180 °C for 12 h. After the autoclave was cooled to room temperature naturally, the precipitate was separated by centrifugation and washed subsequently with ethanol. Then, the products were dried at 60 °C under vacuum for 24 h. Finally, the products were grinded uniformly by a quartz mortar, and the Fe:Co:ZnSe nanocrystals was obtained.
2.3 Characterization The X-ray diffractometer (XRD) analysis was conducted to study the structural properties of as-synthesized freshly dried ZnSe and Fe:Co:ZnSe nanocrystals by a DX-2700 X-ray diffractmeter of Cu K-α1 radiation (λ = 1.5406 Å) with a scanning rate of 0.02 degree/s operated at 40 kV and 30 mA. The shapes and sizes of Fe:Co:ZnSe nanocrystals were measured using a field emission transmission electron microscope (FE-TEM, JEM-F200; JEOL, JPN). In addition, the elemental content and properties of the nanocrystals were investigated by energy-dispersive X-ray spectroscopy (EDS). The hydroxyl (-OH) content and absorption of other molecule in Fe:Co:ZnSe nanocrystals’ surface was identified by a Fourier transform IR (FTIR) spectrometer (Vertex 70v; Bruker, DE) in the range of 4000-1200 cm-1, KBr (99.999%, PIKE Tech) was used in the measurement as the Transmitting agent. Room
temperature absorption measurements in visible and near-IR spectral regions were performed with PerkinElmer Lambda 7505 and ball integral. For MIR photoluminescence (PL) spectra, a commercial continuous fiber laser (CSL-144610) with a wavelength of 1550 nm was used as an excitation source, the spectra were obtained with a fluorescence spectrometer (FSP920; Edinburgh, UK) equipped with a liquid nitrogen-cooled InSb detector (C4159-5671; Hamamatsu, JPN).
3. RESULTS AND DISCUSSION Fig. 1 shows the XRD patterns of Fe:Co:ZnSe nanocrystals with different doping concentrations. The peaks centered at 27.2°, 31.5º, 45.2°, 53.6°, 65.8º, 72.6º, 83.4º, and 89.8º correspond to (111), (200), (220), (311), (400), (331), (422), and (511) lattice planes of cubic zinc sphalerite structure, respectively, which are consistent with the values in the Joint Committee on Powder Diffraction Standards (JCPDS) Card (No. 88-2345). No impurity phases were observed, indicating a pure cubic sphalerite ZnSe phase was formed. Based on the Debye-Scherrer’s formula [25], d = Kλ/βcosθ the average sizes of Fe:Co: ZnSe nanocrystals were calculated to be 15.3, 15.1, 14.7, and 14.3 nm for the samples with Fe2+ ions concentration of 0.3, 0.6, 1, and 3 mol %, respectively. The average size decreases slowly with the increasing of Fe2+ ions concentration. This can be explained by the fact that the doping of Fe2+ ions hinders the growth of ZnSe grains, therefore, smaller nanocrystals were obtained with higher doping concentration under the same hydrothermal conditions [26].
Fig. 1. XRD patterns of 0.3-3 mol % Fe 3 mol % Co:ZnSe nanocrystals.
In Fig. 2, the elements of Fe, Co, Zn, Se, C, Cu, O, and Si were detected for 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals by EDS measurement. The strong C and Cu signals were detected due to the surfactant binding to the background of copper mesh and carbon film. The O and Si signals were detected due to the contaminant from air. The existence of Fe and Co elements confirmed the successful doping of Fe2+ and Co2+ ions into ZnSe matrix. The actual contents of Fe and Co elements in 0.6 mol % Fe 3 mol % Co:ZnSe samples are evaluated to be about 0.48 mol % and 1.12 mol %, respectively. Partial Fe2+ and Co2+ ions did not successfully replace Zn2+ ions in combination with Se2- for the chemical equilibrium, and the undoped Fe2+ and Co2+ ions were removed during washing and centrifuging processes.
Fig. 2. EDS spectrum of 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals; (insert) expanded view of the peaks of Fe and Co.
The morphologies of 0.6 mol % Fe 3 mol % Co:ZnSe nanocrystals were characterized by FE-TEM (see Fig. 3(a)). The as-prepared nanocrystals were nearly spherical in shape and distributed homogeneously, and the sizes were approximately 15 nm, consistent with the XRD result. The HR-TEM image of 0.6 mol % Fe 3 mol % Co: ZnSe is presented in Fig. 3(b). The crystalline interplanar spacing of (200) plane of samples is 0.292 nm, which is very similar to the standard value of cubic sphalerite ZnSe phase (0.284 nm). Fig. 3(c) shows the EDS spectra of 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals. The Co, Fe, Zn, and Se were evenly distributed, confirming the advantage of TM2+ distribution uniformity of hydrothermal method compared to that of thermal diffusion one. The Co and Fe elements don’t exist aggregation in the ZnSe matrix, which demonstrated the Fe:Co:ZnSe nanocrystals prepared in this work may achieve more intense luminescence because uneven concentration distribution of doped-ions will contribute to fluorescence quenching [27].
Fig. 3. (a) FE-TEM image of 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals. (b) HRTEM image of 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals. (c) EDS spectra and the element distribution of 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals.
Fig. 4(a) shows the absorption spectra of Fe:Co:ZnSe nanocrystals measured with an integrating sphere. The Co2+ and Fe2+ ions replace the positions of partial Zn2+ ions, and the scheme of energy-level diagram for Co2+ and Fe2+ ions in the tetrahedral ZnSe crystal field is shown in the inset of Fig 5(a). Compared with the absorption spectra of undoped ZnSe nanocrystals (Fig. 4(b)), an absorption band near 760 nm corresponding to the 4A2 (F) → 4T1 (P) energy transition of Co2+ ions was measured in both of the Fe:Co:ZnSe and Co:ZnSe nanocrystals’ absorption spectra. Another broader absorption band extending from 1200 nm to 2000 nm corresponding to the 4
A2 (F) → 4T1 (F) energy transition of Co2+ ions (Fig. 4(a)) were also measured.
Hence, a commercialized laser at 1550 nm was chosen as the excitation source in the following work. The absorption spectra of Co2+ ions for the Fe:Co:ZnSe nanocrystals are similar to those of bulk crystal at room temperature [19]. The excitonic absorption
band from 500 nm to 680 nm of Fe2+ ions (Fe2++hν→Fe1++hVB) was also shown in this scheme, confirming the existence of Fe2+ ions in the Fe:Co:ZnSe nanocrystals. Fig. 4(c) shows the absorption spectra after Kubelka-Munk treatment [28], and the value of band gap (Eg) for each nanocrystal is given in Fig. 4(d) through the intersection between the linear fit and photon energy axis [29]. The Eg value of blank ZnSe nanocrystals is 2.54 eV, and the value is 2.33 eV for the 3 mol % Co:ZnSe nanocrystals. The Eg value gradually decreases with the Fe2+ ions doped into Co:ZnSe nanocrystals, which is consistent with previous reported results [30]. For samples with higher TM2+ doping concentration, more crystal defects were induced, and the grain size was decreased, resulting in an increase of electron-hole pairs’number in the matrix semiconductor, thereby reducing the intrinsic gap band value.
Fig. 4. (a) Absorption spectra of 0.3-3 mol % Fe 3 mol % Co:ZnSe, 3 % mol Co:ZnSe nanocrystals. (b) Absorption spectra of ZnSe nanocrystals. (c) Kubelka-Munk transformed absorption spectra of nanocrystal samples. (d) Band gap (Eg) values.
Fig. 5. (a) Energy diagram of Co2+ and Fe2+ ions and its possible energy transfer channels. (b) Crystal structure scheme of Fe:Co:ZnSe nanocrystals.
The typical transmission spectra over 4000-1500 cm-1 range of 0.3-3 mol % Fe 3 mol % Co:ZnSe, 3 mol % Co:ZnSe, and pure ZnSe samples at room temperature are shown in Fig. 6. Compared to those of Co:ZnSe and ZnSe ones, the absorption peaks of Fe:Co:ZnSe nanocrystals located at 2920 cm-1 (~3.4 µm) were detected due to the absorption of Fe2+:5E (D) → 5T2 (D) for the Fe:Co:ZnSe nanocrystals. Stronger absorption can be observed in the spectra with the increasing of Fe2+ ions concentration. Usually, a large number of dangling bonds and impurities are present on the nanocrystals’ surface via hydrothermal process, which could suppress MIR fluorescence emission. With the doping of Fe2+ and Co2+ ions, more crystal defects were induced, resulting in the higher absorptions of -OH in Co:ZnSe and Fe:Co:ZnSe nanocrystals compared to that of pure ZnSe nanocrystals.
Fig. 6. FTIR spectra of 0.3-3 mol % Fe 3 mol % Co:ZnSe, 3 % mol Co:ZnSe, and pure ZnSe nanocrystals.
The normalized MIR fluorescence spectra of Fe:Co:ZnSe nanocrystals with different co-doping concentrations under 1550 nm excitation were shown in Fig. 7(a). Because the surface quality of the pressing samples such as roughness can influence the intensity of photoluminescence spectra, the spectra were normalized based on the emission of Co2+:4T2 (F) → 4A2 (F) energy transition considering that the designed concentrations of Co2+ ions are invariable in these samples. Energy transfer can be achieved due to the spectral overlapping of Co2+ ions emission, i.e. 4T2 (F) → 4A2 (F) and Fe2+ ions absorption, i.e. 5E (D) → 5T2 (D), and the excited state of Fe2+:5T2 (D) received the energy from Co2+:4T2 (F) level. These excitations may also be realized by Co2+:4T1 (F) → 4T2 (F) cross-relaxation processes. Therefore the incorporation of Co2+ and Fe2+ ions doped ZnSe nanocrystals provides a path to energy transfer from Co2+ to the Fe2+ ions. MIR fluorescences peaks of 3.4 and 4.7 µm were observed in the 3 mol % Co:ZnSe nanocrystals which are ascribed to the Co2+:4T2 (F) → 4A2 (F) and 4T1 (F) → 4T2 (F) energy transitions, respectively. Two new MIR fluorescence emission at peaks of 3.3 and 4.4 µm were observed in Fe:Co:ZnSe nanocrystals. The emission at around 3.3-3.4 µm in the Fe:Co:ZnSe nanocrystals is attributed to the 4T2 (F) → 4A2 (F) energy transition of Co2+ ions. Co-doping of Fe2+ ions in the matrix induces an obvious peak broadening and blue-shift (~90 nm) compared to that of Co:ZnSe nanocrystals. Furthermore, with the concentration of Fe2+ ions increasing, the effects of blue-shift become more severely. Because of the similar size for all the samples, the effect of nanocrystals’size on the luminescence spectra is negligible. The main reason is the change of the crystal field environment. In the case of the bulk Co:Fe:ZnSe polycrystals prepared by physically thermal diffusion method, the emission center of Co2+ ions stay consistent with that in the bulk Co:ZnSe polycrystals [19], and the crystal lattice changes a few by the doping ions. The differences of peak position and shape between the physically-prepared polycrystalline and present hydrothermally-prepared nanocrystals can be explained by the different crystal field environments around the TM2+ ions. The crystal structure
scheme of Fe:Co:ZnSe nanocrystals is shown in Fig. 5(b). These nanocrystals possess a cubic sphalerite structure, and the Co2+ and Fe2+ ions are in the center of tetrahedral. For the nanocrystals prepared by hydrothermal method, more severe crystal defects emerged with Fe2+ ions entering into the crystal lattice, and more severe energy level splitting happened with the change of crystal field environment, leading to the broadening and blue-shift of peaks [31,32]. For the emission at 4.4 µm, it is thought to be an overlapping emission of Co2+ and Fe2+ ions. Fig. 7(b) is the decomposed fluorescence peak at 4.4 µm for 3 mol % Fe 3 mol % Co:ZnSe nanocrystals, and the purple line (sum of decomposed peaks) is in good accordance with the dot one (the original spectra). The peak centered at 4648 nm is ascribed to the Co2+:4T1 (F) → 4T2 (F) energy transition, and the peak centered at 4273 nm is related with the Fe2+:5T2 (D) → 5E (D) energy transition due to the energy transfer from Co2+ to the Fe2+ ions. For the emission at 4.4 µm in the serial Fe:Co:ZnSe nanocrystals, the fluorescence intensity increases with the addition of Fe2+ ions initially, and then decreases with more Fe2+ ions doped into crystal lattice. 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals presents the strongest fluorescence at 4.4 µm in the present work. The effects of “concentration quenching” were happened in the 1 mol % Fe 3 mol % Co:ZnSe and 3 mol % Fe 3 mol % Co:ZnSe nanocrystal samples, and the quenching ratios are about 23.5 % and 41.8 % for them, respectively. The reason is the increasing of ions concentration leads to smaller ions spacing of the “active ions”, which further results in fluorescence quenching. Moreover, similar with that at around 3.3-3.4 µm, the peak shifting and broadening of Co2+ ions at 4648 nm were also found, resulting from the change of the crystal field environment.
Fig. 7. (a) Normalized fluorescence spectra of Fe:Co:ZnSe nanocrystals with different doping concentrations. (b) Decomposed fluorescence spectra of 3 mol % Fe 3 mol % Co:ZnSe nanocrystals and the fluorescence spectra of 3 mol % Co:ZnSe nanocrystals.
Conclusion A series of Fe:Co:ZnSe nanocrystals with spherical shape and cubic sphalerite structure were prepared by hydrothermal method. These nanocrystals are well dispersed with narrow size distribution of about 15 nm. Through the energy transfer between Co2+ and Fe2+ ions, two emissions of 3.3 µm and 4.4 µm were observed at room temperature under the 1550 nm laser pumping. Because of the change of the crystal field environment, an obvious peak broadening and blue-shift at 3.3 µm of Fe:Co:ZnSe nanocrystals compared to that of Co:ZnSe nanocrystals was found. The 4.4 µm emission is an overlapping emission of the Co2+:4T1 (F) → 4T2 (F) energy transition (4648 nm) and Fe2+:5T2 (D) → 5E (D) energy transition (4273 nm). The 0.6 mol % Fe 3 mol % Co:ZnSe exhibits the strongest fluorescence intensities at 4.4 µm in the serial co-doping samples. These nanocrystals have potential applications in the hot-pressed ceramic materials, composite fibers lasers, random powder lasers, and etc.
Acknowledgment The National Natural Science Foundation of China (Nos. 61935006l, 61475189), CAS Interdisciplinary Innovation Team project (JCTD-2018-19), and the Natural Science Basic Research Project in Shaanxi Province (2019JM-113, 2019JQ-236).
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1) A serial Fe:Co:ZnSe nanocrystals were synthesized via hydrothermal method without organic ligand. 2) The emission at 4273 nm was detected in the Fe:Co:ZnSe nanocrystals at room temperature. 3) The 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals exhibit the strongest fluorescence.
Huawei Shi: Conceptualization, Methodology, Validation, Data curation, Writing-Original Draft. Xiaoxia Cui: Conceptualization, Methodology, Validation, Writing - Original Draft. Xusheng Xiao: Visualization, Investigation. Yantao Xu: Validation. Chao Liu: Resources. Chaoqi Hou: Supervision. Haitao Guo: Conceptualization, Methodology, Writing - Reviewing and Editing, Supervision.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
S0022-2313(19)32002-2
DOI:
https://doi.org/10.1016/j.jlumin.2020.117102
Reference:
LUMIN 117102
To appear in:
Journal of Luminescence
Received Date: 11 October 2019 Revised Date:
6 February 2020
Accepted Date: 6 February 2020
Please cite this article as: H. Shi, X. Cui, X. Xiao, Y. Xu, C. Liu, C. Hou, H. Guo, Cobalt and iron codoped ZnSe nanocrystals:Mid-IR luminescence at room temperature, Journal of Luminescence (2020), doi: https://doi.org/10.1016/j.jlumin.2020.117102. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.
Cobalt and iron co-doped ZnSe nanocrystals:Mid-IR luminescence at room temperature Huawei Shi,ab‡ Xiaoxia Cui,ab‡ Xusheng Xiao,ab Yantao Xu,ab Chao Liu,c Chaoqi Hou, ab a
and Haitao Guoabd*
State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics
and precision Mechanics, Chinese Academy of Science (CAS), Xi’an, Shaanxi 710119, China b
Center of Materials Science and Optoelectronics Engineering, University of Chinese
Academy of Sciences (UCAS), Beijing 100049, China c
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of
Technology, Wuhan, Hubei 430070, China d
Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan,
Shanxi 030006, China ‡
These authors contributed equally to this work and can be regarded as co-first
authors *
Corresponding Author, E-mail address: [email protected]
Abstract: Fe:Co:ZnSe nanocrystals with different co-doping ratios of Fe2+/Co2+ ions were fabricated by hydrothermal synthesis. The facile method used in the present work avoids the mid-infrared quench effect induced by the organic molecular introduced in the past preparation process. These nanocrystals are spherical in shape and exhibit a cubic sphalerite structure with an average grain size of about 15 nm. Through the energy conversion between Co2+ and Fe2+ ions, mid-infrared fluorescences at 3.3 µm and 4.4 µm were detected under 1550 nm laser excitation at room temperature. The fluorescence band of 4.4 µm in the Fe:Co:ZnSe nanocrystals is thought to be an overlapping emission involving the peak centered at 4648 nm corresponding to the Co2+:4T1 (F) → 4T2 (F) energy transition and the peak centered at 4273 nm corresponding to Fe2+:5T2 (D) → 5E (D) energy transition. These high-quality optically active nanomaterials show potential applications in mid-infrared devices
such as composite fiber-amplifiers and lasers materials. Keywords: Mid-infrared; Transition metal ions; Energy transfer; Fe:Co:ZnSe nanocrystals; Photoluminescence 1. INTRODUCTION Mid-infrared (MIR) light sources in the range of 2-5 µm are of great interest in a variety of applications such as molecular spectroscopy, biological sensors, medical diagnostics, etc. [1-4]. Because of small crystal field splitting, ultrabroad wavelength tunability, large emission cross section, and low phonon energy, the transition metal ions (TM2+), including Cr2+, Co2+, Fe2+ and Ni2+ ions, doped wideband II-VI semiconductor crystals have been reported as efficient MIR fluorescence and laser materials [5-7]. In 1996, researchers from the Lawrence Livermore National Laboratory, USA, firstly reported the MIR lasing from TM2+-doped II-VI semiconductor crystals [8]. Since then, several research works on TM2+-doped II-VI chalcogenide crystals (i.e. ZnS, ZnSe, ZnTe, CdS, CdSe, and CdSeTe) have been performed, in which, the Fe2+ ions doped II-VI chalcogenide crystals laser has a wide tunable wavelength range and high optical efficiency [9-14]. Meanwhile, ZnSe has been recognized as an excellent MIR laser matrix material with a wide range of light transmission and good chemical stability [15]. In 1999, Adams et al. reported a 3.98-4.54 µm laser output from Fe:ZnSe single crystal at low temperature (15-180 K) under the Er:YAG laser pumping, and the maximum energy output was 12 µJ at 130 K [10]. Due to the strong multi-phonon quenching effect, the free-running broadband pump source cannot form an effective lasing at room temperature (290-300 K). Until 2005, the first room temperature gain-switched lasing of Fe:ZnSe crystal at 4.4 µm under excitation of 2.92 µm with a pulse duration of 5 ns was reported by J. Kernal et al. [16]. Currently, room temperature gain-switched lasing of Fe:ZnSe crystal becomes commonly. However, the preparation of high optical quality Fe:ZnSe crystal is still a difficult and time-consuming work, and the pump source is restricted to be 3 µm, which is an unusual one.
In view of the above problems, some studies related with the co-doping and chemical preparation were conducted. The fluorescence and lasing of Fe2+ ions in the bulk ZnSe or ZnS crystals have been achieved through codoping of Co2+ or Cr2+ ions by thermo-diffusion method [17-20]. However, the uneven distribution of doping ions in the crystals and the long preparation period limited its further application. Recently, Myoung et al. reported the 3.5-4.5 µm MIR emission at 35-200 K of Fe:ZnSe quantum dots that fabricated by micro emulsion hydrothermal synthesis [21]. The hydrothermal synthesis is easy and the distribution of Fe2+ ions in ZnSe matrix are homogenous. But the organic phase was introduced during the reaction, which severely quenched the MIR fluorescence. In this study, a series of Fe:Co:ZnSe nanocrystals with different Fe2+/Co2+ ions doping concentrations were synthesized via hydrothermal method without introduction of organic composition. A strong fluorescence emission centered at 4.4 µm in the Fe:Co:ZnSe nanocrystals was achieved at room temperature. The preparation of highly efficient MIR optical nanocrystals brings new opportunities for available hot-pressed ceramic materials [22], composite fibers lasers [23], and random powder lasers [24].
2. EXPERIMENTAL SECTION 2.1 Materials Zinc chloride (ZnCl2, 98%), cobalt chloride hexahydrate (CoCl2· 6H2O, 99.99%), ferrous chloride (FeCl2, 99.99%), sodium borohydride (NaBH4, ≥96%), selenium (Se, 99.9999%), and ethanol (C2H6O, ≥99.7%) were used as raw materials. All of the above regents were commercially purchased and used without further purification. Deionized water (18.25 MΩ cm-1) was produced using a reagent water system (EasypureⅡ, Barnstead). 2.2 Hydrothermal Synthesis The Fe:Co:ZnSe nanocrystals were synthesized by hydrothermal method. Firstly, 15 mL sodium hydrogen selenide (NaHSe) aqueous solution was prepared by mixing 10 mmol (0.7896 g) selenium (Se) powder and 21 mmol (0.7944 g) sodium
borohydride (NaBH4) in 15 mL deionized water in N2 atmosphere, excess NaBH4 was used to prevent the oxidation of Se2- and Fe2+ ions during the reaction process. Zinc ions precursor with different concentrations were prepared by dissolving 9.4-9.67 mmol (1.2812-1.3180 g) ZnCl2 in 15 mL ethanol. Mixing solution with different
concentrations of Fe2+ ions and the fixed Co2+ ions were prepared by
dissolving 0.03-0.3 mmol (0.0038-0.0380 g) FeCl2 and 0.3 mmol (0.0714 g) CoCl2· 6H2O in 5 mL deionized water under N2 atmosphere. Next, the 15 mL ZnCl2 ethanol solution was added to the above Fe2+ and Co2+ aqueous solution, mixing them evenly to form a cationic mixture. Afterwards, the cationic mixture was added into 15 mL NaHSe aqueous solution, then the Fe:Co:ZnSe slurry was produced. After stirred for 10 min, all the mixture was transferred into a stainless-steel Teflon-lined autoclave. 45 mL ethanol was added into the autoclave to make sure the filling fraction of autoclave is 80%. Then, the autoclave was sealed and heat-treated at 180 °C for 12 h. After the autoclave was cooled to room temperature naturally, the precipitate was separated by centrifugation and washed subsequently with ethanol. Then, the products were dried at 60 °C under vacuum for 24 h. Finally, the products were grinded uniformly by a quartz mortar, and the Fe:Co:ZnSe nanocrystals was obtained.
2.3 Characterization The X-ray diffractometer (XRD) analysis was conducted to study the structural properties of as-synthesized freshly dried ZnSe and Fe:Co:ZnSe nanocrystals by a DX-2700 X-ray diffractmeter of Cu K-α1 radiation (λ = 1.5406 Å) with a scanning rate of 0.02 degree/s operated at 40 kV and 30 mA. The shapes and sizes of Fe:Co:ZnSe nanocrystals were measured using a field emission transmission electron microscope (FE-TEM, JEM-F200; JEOL, JPN). In addition, the elemental content and properties of the nanocrystals were investigated by energy-dispersive X-ray spectroscopy (EDS). The hydroxyl (-OH) content and absorption of other molecule in Fe:Co:ZnSe nanocrystals’ surface was identified by a Fourier transform IR (FTIR) spectrometer (Vertex 70v; Bruker, DE) in the range of 4000-1200 cm-1, KBr (99.999%, PIKE Tech) was used in the measurement as the Transmitting agent. Room
temperature absorption measurements in visible and near-IR spectral regions were performed with PerkinElmer Lambda 7505 and ball integral. For MIR photoluminescence (PL) spectra, a commercial continuous fiber laser (CSL-144610) with a wavelength of 1550 nm was used as an excitation source, the spectra were obtained with a fluorescence spectrometer (FSP920; Edinburgh, UK) equipped with a liquid nitrogen-cooled InSb detector (C4159-5671; Hamamatsu, JPN).
3. RESULTS AND DISCUSSION Fig. 1 shows the XRD patterns of Fe:Co:ZnSe nanocrystals with different doping concentrations. The peaks centered at 27.2°, 31.5º, 45.2°, 53.6°, 65.8º, 72.6º, 83.4º, and 89.8º correspond to (111), (200), (220), (311), (400), (331), (422), and (511) lattice planes of cubic zinc sphalerite structure, respectively, which are consistent with the values in the Joint Committee on Powder Diffraction Standards (JCPDS) Card (No. 88-2345). No impurity phases were observed, indicating a pure cubic sphalerite ZnSe phase was formed. Based on the Debye-Scherrer’s formula [25], d = Kλ/βcosθ the average sizes of Fe:Co: ZnSe nanocrystals were calculated to be 15.3, 15.1, 14.7, and 14.3 nm for the samples with Fe2+ ions concentration of 0.3, 0.6, 1, and 3 mol %, respectively. The average size decreases slowly with the increasing of Fe2+ ions concentration. This can be explained by the fact that the doping of Fe2+ ions hinders the growth of ZnSe grains, therefore, smaller nanocrystals were obtained with higher doping concentration under the same hydrothermal conditions [26].
Fig. 1. XRD patterns of 0.3-3 mol % Fe 3 mol % Co:ZnSe nanocrystals.
In Fig. 2, the elements of Fe, Co, Zn, Se, C, Cu, O, and Si were detected for 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals by EDS measurement. The strong C and Cu signals were detected due to the surfactant binding to the background of copper mesh and carbon film. The O and Si signals were detected due to the contaminant from air. The existence of Fe and Co elements confirmed the successful doping of Fe2+ and Co2+ ions into ZnSe matrix. The actual contents of Fe and Co elements in 0.6 mol % Fe 3 mol % Co:ZnSe samples are evaluated to be about 0.48 mol % and 1.12 mol %, respectively. Partial Fe2+ and Co2+ ions did not successfully replace Zn2+ ions in combination with Se2- for the chemical equilibrium, and the undoped Fe2+ and Co2+ ions were removed during washing and centrifuging processes.
Fig. 2. EDS spectrum of 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals; (insert) expanded view of the peaks of Fe and Co.
The morphologies of 0.6 mol % Fe 3 mol % Co:ZnSe nanocrystals were characterized by FE-TEM (see Fig. 3(a)). The as-prepared nanocrystals were nearly spherical in shape and distributed homogeneously, and the sizes were approximately 15 nm, consistent with the XRD result. The HR-TEM image of 0.6 mol % Fe 3 mol % Co: ZnSe is presented in Fig. 3(b). The crystalline interplanar spacing of (200) plane of samples is 0.292 nm, which is very similar to the standard value of cubic sphalerite ZnSe phase (0.284 nm). Fig. 3(c) shows the EDS spectra of 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals. The Co, Fe, Zn, and Se were evenly distributed, confirming the advantage of TM2+ distribution uniformity of hydrothermal method compared to that of thermal diffusion one. The Co and Fe elements don’t exist aggregation in the ZnSe matrix, which demonstrated the Fe:Co:ZnSe nanocrystals prepared in this work may achieve more intense luminescence because uneven concentration distribution of doped-ions will contribute to fluorescence quenching [27].
Fig. 3. (a) FE-TEM image of 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals. (b) HRTEM image of 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals. (c) EDS spectra and the element distribution of 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals.
Fig. 4(a) shows the absorption spectra of Fe:Co:ZnSe nanocrystals measured with an integrating sphere. The Co2+ and Fe2+ ions replace the positions of partial Zn2+ ions, and the scheme of energy-level diagram for Co2+ and Fe2+ ions in the tetrahedral ZnSe crystal field is shown in the inset of Fig 5(a). Compared with the absorption spectra of undoped ZnSe nanocrystals (Fig. 4(b)), an absorption band near 760 nm corresponding to the 4A2 (F) → 4T1 (P) energy transition of Co2+ ions was measured in both of the Fe:Co:ZnSe and Co:ZnSe nanocrystals’ absorption spectra. Another broader absorption band extending from 1200 nm to 2000 nm corresponding to the 4
A2 (F) → 4T1 (F) energy transition of Co2+ ions (Fig. 4(a)) were also measured.
Hence, a commercialized laser at 1550 nm was chosen as the excitation source in the following work. The absorption spectra of Co2+ ions for the Fe:Co:ZnSe nanocrystals are similar to those of bulk crystal at room temperature [19]. The excitonic absorption
band from 500 nm to 680 nm of Fe2+ ions (Fe2++hν→Fe1++hVB) was also shown in this scheme, confirming the existence of Fe2+ ions in the Fe:Co:ZnSe nanocrystals. Fig. 4(c) shows the absorption spectra after Kubelka-Munk treatment [28], and the value of band gap (Eg) for each nanocrystal is given in Fig. 4(d) through the intersection between the linear fit and photon energy axis [29]. The Eg value of blank ZnSe nanocrystals is 2.54 eV, and the value is 2.33 eV for the 3 mol % Co:ZnSe nanocrystals. The Eg value gradually decreases with the Fe2+ ions doped into Co:ZnSe nanocrystals, which is consistent with previous reported results [30]. For samples with higher TM2+ doping concentration, more crystal defects were induced, and the grain size was decreased, resulting in an increase of electron-hole pairs’number in the matrix semiconductor, thereby reducing the intrinsic gap band value.
Fig. 4. (a) Absorption spectra of 0.3-3 mol % Fe 3 mol % Co:ZnSe, 3 % mol Co:ZnSe nanocrystals. (b) Absorption spectra of ZnSe nanocrystals. (c) Kubelka-Munk transformed absorption spectra of nanocrystal samples. (d) Band gap (Eg) values.
Fig. 5. (a) Energy diagram of Co2+ and Fe2+ ions and its possible energy transfer channels. (b) Crystal structure scheme of Fe:Co:ZnSe nanocrystals.
The typical transmission spectra over 4000-1500 cm-1 range of 0.3-3 mol % Fe 3 mol % Co:ZnSe, 3 mol % Co:ZnSe, and pure ZnSe samples at room temperature are shown in Fig. 6. Compared to those of Co:ZnSe and ZnSe ones, the absorption peaks of Fe:Co:ZnSe nanocrystals located at 2920 cm-1 (~3.4 µm) were detected due to the absorption of Fe2+:5E (D) → 5T2 (D) for the Fe:Co:ZnSe nanocrystals. Stronger absorption can be observed in the spectra with the increasing of Fe2+ ions concentration. Usually, a large number of dangling bonds and impurities are present on the nanocrystals’ surface via hydrothermal process, which could suppress MIR fluorescence emission. With the doping of Fe2+ and Co2+ ions, more crystal defects were induced, resulting in the higher absorptions of -OH in Co:ZnSe and Fe:Co:ZnSe nanocrystals compared to that of pure ZnSe nanocrystals.
Fig. 6. FTIR spectra of 0.3-3 mol % Fe 3 mol % Co:ZnSe, 3 % mol Co:ZnSe, and pure ZnSe nanocrystals.
The normalized MIR fluorescence spectra of Fe:Co:ZnSe nanocrystals with different co-doping concentrations under 1550 nm excitation were shown in Fig. 7(a). Because the surface quality of the pressing samples such as roughness can influence the intensity of photoluminescence spectra, the spectra were normalized based on the emission of Co2+:4T2 (F) → 4A2 (F) energy transition considering that the designed concentrations of Co2+ ions are invariable in these samples. Energy transfer can be achieved due to the spectral overlapping of Co2+ ions emission, i.e. 4T2 (F) → 4A2 (F) and Fe2+ ions absorption, i.e. 5E (D) → 5T2 (D), and the excited state of Fe2+:5T2 (D) received the energy from Co2+:4T2 (F) level. These excitations may also be realized by Co2+:4T1 (F) → 4T2 (F) cross-relaxation processes. Therefore the incorporation of Co2+ and Fe2+ ions doped ZnSe nanocrystals provides a path to energy transfer from Co2+ to the Fe2+ ions. MIR fluorescences peaks of 3.4 and 4.7 µm were observed in the 3 mol % Co:ZnSe nanocrystals which are ascribed to the Co2+:4T2 (F) → 4A2 (F) and 4T1 (F) → 4T2 (F) energy transitions, respectively. Two new MIR fluorescence emission at peaks of 3.3 and 4.4 µm were observed in Fe:Co:ZnSe nanocrystals. The emission at around 3.3-3.4 µm in the Fe:Co:ZnSe nanocrystals is attributed to the 4T2 (F) → 4A2 (F) energy transition of Co2+ ions. Co-doping of Fe2+ ions in the matrix induces an obvious peak broadening and blue-shift (~90 nm) compared to that of Co:ZnSe nanocrystals. Furthermore, with the concentration of Fe2+ ions increasing, the effects of blue-shift become more severely. Because of the similar size for all the samples, the effect of nanocrystals’size on the luminescence spectra is negligible. The main reason is the change of the crystal field environment. In the case of the bulk Co:Fe:ZnSe polycrystals prepared by physically thermal diffusion method, the emission center of Co2+ ions stay consistent with that in the bulk Co:ZnSe polycrystals [19], and the crystal lattice changes a few by the doping ions. The differences of peak position and shape between the physically-prepared polycrystalline and present hydrothermally-prepared nanocrystals can be explained by the different crystal field environments around the TM2+ ions. The crystal structure
scheme of Fe:Co:ZnSe nanocrystals is shown in Fig. 5(b). These nanocrystals possess a cubic sphalerite structure, and the Co2+ and Fe2+ ions are in the center of tetrahedral. For the nanocrystals prepared by hydrothermal method, more severe crystal defects emerged with Fe2+ ions entering into the crystal lattice, and more severe energy level splitting happened with the change of crystal field environment, leading to the broadening and blue-shift of peaks [31,32]. For the emission at 4.4 µm, it is thought to be an overlapping emission of Co2+ and Fe2+ ions. Fig. 7(b) is the decomposed fluorescence peak at 4.4 µm for 3 mol % Fe 3 mol % Co:ZnSe nanocrystals, and the purple line (sum of decomposed peaks) is in good accordance with the dot one (the original spectra). The peak centered at 4648 nm is ascribed to the Co2+:4T1 (F) → 4T2 (F) energy transition, and the peak centered at 4273 nm is related with the Fe2+:5T2 (D) → 5E (D) energy transition due to the energy transfer from Co2+ to the Fe2+ ions. For the emission at 4.4 µm in the serial Fe:Co:ZnSe nanocrystals, the fluorescence intensity increases with the addition of Fe2+ ions initially, and then decreases with more Fe2+ ions doped into crystal lattice. 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals presents the strongest fluorescence at 4.4 µm in the present work. The effects of “concentration quenching” were happened in the 1 mol % Fe 3 mol % Co:ZnSe and 3 mol % Fe 3 mol % Co:ZnSe nanocrystal samples, and the quenching ratios are about 23.5 % and 41.8 % for them, respectively. The reason is the increasing of ions concentration leads to smaller ions spacing of the “active ions”, which further results in fluorescence quenching. Moreover, similar with that at around 3.3-3.4 µm, the peak shifting and broadening of Co2+ ions at 4648 nm were also found, resulting from the change of the crystal field environment.
Fig. 7. (a) Normalized fluorescence spectra of Fe:Co:ZnSe nanocrystals with different doping concentrations. (b) Decomposed fluorescence spectra of 3 mol % Fe 3 mol % Co:ZnSe nanocrystals and the fluorescence spectra of 3 mol % Co:ZnSe nanocrystals.
Conclusion A series of Fe:Co:ZnSe nanocrystals with spherical shape and cubic sphalerite structure were prepared by hydrothermal method. These nanocrystals are well dispersed with narrow size distribution of about 15 nm. Through the energy transfer between Co2+ and Fe2+ ions, two emissions of 3.3 µm and 4.4 µm were observed at room temperature under the 1550 nm laser pumping. Because of the change of the crystal field environment, an obvious peak broadening and blue-shift at 3.3 µm of Fe:Co:ZnSe nanocrystals compared to that of Co:ZnSe nanocrystals was found. The 4.4 µm emission is an overlapping emission of the Co2+:4T1 (F) → 4T2 (F) energy transition (4648 nm) and Fe2+:5T2 (D) → 5E (D) energy transition (4273 nm). The 0.6 mol % Fe 3 mol % Co:ZnSe exhibits the strongest fluorescence intensities at 4.4 µm in the serial co-doping samples. These nanocrystals have potential applications in the hot-pressed ceramic materials, composite fibers lasers, random powder lasers, and etc.
Acknowledgment The National Natural Science Foundation of China (Nos. 61935006l, 61475189), CAS Interdisciplinary Innovation Team project (JCTD-2018-19), and the Natural Science Basic Research Project in Shaanxi Province (2019JM-113, 2019JQ-236).
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1) A serial Fe:Co:ZnSe nanocrystals were synthesized via hydrothermal method without organic ligand. 2) The emission at 4273 nm was detected in the Fe:Co:ZnSe nanocrystals at room temperature. 3) The 0.6 mol % Fe 3 mol % Co: ZnSe nanocrystals exhibit the strongest fluorescence.
Huawei Shi: Conceptualization, Methodology, Validation, Data curation, Writing-Original Draft. Xiaoxia Cui: Conceptualization, Methodology, Validation, Writing - Original Draft. Xusheng Xiao: Visualization, Investigation. Yantao Xu: Validation. Chao Liu: Resources. Chaoqi Hou: Supervision. Haitao Guo: Conceptualization, Methodology, Writing - Reviewing and Editing, Supervision.
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: