Material and Ingenious Synthesis Strategy for Short-Wavelength

Oct 25, 2016 - Synopsis. An efficient short-wavelength infrared luminescent material (SWIR, 1.54 μm) is successfully synthesized by doping Er(III) io...
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Material and Ingenious Synthesis Strategy for Short-Wavelength Infrared Light-Emitting Device Jipeng Fu,†,‡,§ Su Zhang,*,† Ran Pang,† Yonglei Jia,†,‡ Wenzhi Sun,†,‡ Haifeng Li,†,‡ Lihong Jiang,† and Chengyu Li*,† †

State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China § Fraunhofer-Institute for Ceramical Technologies und Systeme IKTS, Dresden 01109, Gemmany S Supporting Information *

ABSTRACT: Infrared (IR) light-emitting materials have wide applications. However, variety and economical accesses to obtain IR materials and devices are still limited, because the IR-emitting materials are always suffering from two obstacles, namely, lower absorption and lower emission efficiency. In this work, using a modified high-temperature solid-state reaction an efficient shortwavelength IR luminescent material is successfully synthesized. On the basis of the excellent luminescent properties, a convenient IR light-emitting diode (LED) device is fabricated by combining the novel IR material with a commercial UV LED chip. Besides the anticipation that it may lead to a boost of the application of IR device in different fields, importantly, we also consider that the ingenious synthesis strategy may open a door for obtaining novel ions doped functional materials.

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the complicated preparation and lower stability.12 Furthermore, Nd- or Er-doped electroluminescent material will also achieve 1.02 and 1.5 μm emitting, but its efficiency is relatively low, and the operating temperatures need to be improved.13,14 Therefore, efficient and low-cost SWIR luminescent materials still need to be improved.

INTRODUCTION Infrared light-emitting materials continue to be of significant interest to scientists, due to their wide applications in biological imaging,1 optical fiber communication,2 silicon-based solar cell,3 laser crystal,4,5 military,6 and so forth. Especially, the material with emission around 1500 nm (short-wavelength infrared, SWIR) has a special fascination, not only because the wavelength is in the third low-loss window for optical fiber communication,7 but also according to the latest investigations it is in the second “tissue-transparent window” for in vivo optical imaging for disease screening and image-guided surgical interventions in biomedical field.8,9 Additionally, the 1500 nm SWIR is in the eye-safe wavelength range. Therefore, the SWIR material is attractive for military, laser rangefinder, night vision imaging system, and so on.10 Although the 1500 nm SWIR materials in many scientific fields have important applications, variety and economical accesses to obtain 1500 nm SWIR materials and devices are still limited. One of current 1500 nm SWIR acquisition is coming from the Er3+-doped laser glass and crystal. However, the preparation, maintenance, and operation of these materials and laser systems require a great amount of expenditure. Nowadays, benefiting from the development of semiconductor technology, using semiconductor materials to obtain SWIR light has made considerable progress. However, the IR emission band is always broad, which will range from tens to more than one hundred nanometers; for instance, InGaAsP.11 Besides, organic lightemitting diode (LED) is another alternative; to date, the emission can only be adjusted from 770 to 1200 nm as well as © XXXX American Chemical Society

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EXPERIMENTAL SECTION

Synthesis Method. The Er 3+ -doped SrO phosphor was synthesized by a modified high-temperature solid-state reaction, which was called as interlayer solid-state reaction (ISSR) method.1 At first, SrCO3 and Er2O3 (1 at%) were mixed well in an agate mortar for 30 min. Then, SrCO3 and SiO2 were mixed homogeneously in an agate mortar at molar ratio of 3:1. The mixed powder of SrCO3 and SiO2 plays a key role in this method. The starting material of Er3+doped SrO was put in the middle of the SrCO3 and SiO2 mixed powder as a sandwich structure in a corundum crucible with a lid. The crucible was placed in an electric furnace at 1530 °C for 240 min. If the samples were prepared by traditional solid-state method at 1530 °C, the product will melt, sublimate, and react with crucible. And if the synthesis temperature is lower than 1530 °C, the emission will be significantly decreased. After it was sintered, the products were stripped from the middle of the sandwich and grinded for further optical measurement. The Fabrication of the SWIR Emitting Device. The newly synthesized SrO:Er3+ powder was mixed with UV-curable epoxy resin at the weight ratio of 5:1. Then the mixture was painted on a commercial 380 nm UV chip cell. When turn on the UV chip, the resin Received: July 30, 2016

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DOI: 10.1021/acs.inorgchem.6b01849 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

crystal structure of SrO. The doped Er3+ does not significantly influence the XRD patterns. The novel SWIR material SrO:Er3+ can overcome the two challenges to some degree. For the first challenge, although f−f transition is parity-forbidden, in a solid crystal field the opposite parity of 4fn-1n′l′ configuration will mix into the 4fn configuration due to odd items. Hence, the parity-forbidden transition is released. Judd−Ofelt (J-O) theory is one of the most successful theories to estimate the intensity of the electricdipole transitions of rare-earth ions. According to the J-O theory, f−f transition strength is proportional to the square of the reduced matrix elements and the oscillator strength parameters Ωt (t = 2, 4, 6). The oscillator strength parameters are closely related to crystal symmetry properties of the matrix and the coordination environment parameters. Therefore, a suitable host will lead to a higher luminescent efficiency of f−f transitions. According to the following results, SrO may be an excellent SWIR host material, which may simultaneously offer a suitable crystal lattice for higher radiative transition and lower nonradiative transitions probability of rare-earth ions, and therefore results in an enhanced luminescent efficiency. For the second challenge, improving the absorption efficiency of a luminescent or optical material is a great challenge. Using sensitization and energy transfer is the most favored way. In Lns organic complexes, a suitable organic chain will sensitize the emission through energy absorption and transfer (called antenna effect).19 Organic complexes, however, have intrinsic limitations, such as easier quenching and lower physicochemical stability. These shortcomings are particularly unsatisfactory for SWIR Lns because of the relatively small energy gaps between different stark levels of Lns in the IR range.20 Comparatively, Lns-activated inorganic solid materials have unique advantages of their own. Herein, in SrO:Er3+, the emission of erbium ions can be sensitized by the host. On the basis of all these outstanding advantages mentioned above, an excellent novel SWIR device is fabricated by combining the material with a 1 W 380 nm UV LED chip. (The UV chip emission spectra is shown in Figure S2.) Figure 2 shows the photograph, the emission spectrum, and the schematic diagram of the device. It is shown that the 1538 nm SWIR emission is dominated in the emission spectrum, which is due to the transition from 4I13/2 to ground state 4I15/2 (Figure 2b). In the following sections, we will introduce the luminescent properties of the novel SWIR material and the device in details from two aspects: emission and absorption. First of all, we investigate and analyze the emission spectrum of SrO:Er3+. The emission spectrum of the SWIR device at IR range is shown in Figure 2b,c. The steady-state photoluminescent (SSPL) emission spectrum of the SrO:Er3+ phosphor is shown in Figure 3. It is known that the emission intensity depends on the possibility for the f−f transition. Actually, for a certain Lns-activated solid material, the line strength is associated with the host. Accordingly, the line strength becomes an intrinsic characteristic of the materials to some degree. The J-O theory is the most useful theory in estimating the forced electric-dipole transitions of rare-earth ions. According to the J-O model, the line strength for an electric-dipole transition between an initial manifold |(S, L)J⟩ and a final manifold |(S′, L′)J′⟩ is obtained by

can be cured by the UV light from the underneath chip. After that the UV light of the chip will be absorbed by the novel SWIR material SrO:Er3+, and only the characteristic emission of it can be detected. Additionally, the SWIR material can be preserved by the epoxy resin from environment damage. This device was kept well after one year in normal room condition, and the emission intensity was not much influenced, indicating a considerable physicochemical stability of the material. Characterization. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) characterizations were performed with a JEOL 3000F microscope (Japan) operating at 300 kV. The structure of the sintered samples was identified by powder X-ray diffraction (XRD) analysis (Bruker AXS D8), with graphitemonochromatized Cu Kα radiation (λ = 0.154 05 nm) operating at 40 kV and 40 mA. The measurements of photoluminescence (PL) and photoluminescence excitation (PLE) spectra were performed using a Hitachi F7000 spectrometer equipped with a 150 W xenon lamp under a working voltage of 700 V. The excitation and emission slits were set, respectively, at 1.0 and 2.5 nm. The luminescence decay curve and IR spectrum were measured at Edinburgh Analytical Instruments FLS980. The quantum efficiency yields were analyzed with a PL quantumefficiency measurement system (C9920−02, Hamamatsu Photonics, Shizuoka, Japan) by a 150 W xenon lamp. The diffuse-reflectance spectra (DRS) were obtained by a UV−visible spectro-photometer (Hitachi U4100) using BaSO4 as a reference. Fourier transform infrared (FT-IR) spectrometry was measured by using a Thermo Nicolet Nexus (Washington, USA) 670 spectro-photometer. The IR map was recorded by putting a night vision ATN NVM14−3 before a normal camera with a 800 nm filter. All of the measurements were performed at room temperature.

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RESULTS AND DISCUSSION Lanthanide ions (Lns) are spectroscopically fantastic and magic. Lns activated optical and luminescent materials have attracted considerable attention.15,16 Because of the abundant f−f transitions of Lns, the emissions can exhibit a large characteristic peak at IR range. Nevertheless, Lns-activated SWIR materials are still faced with two main challenges. One is the relative low transition probability originated from the forbidden nature of the 4f−4f transitions, which will result in a low luminescent intensity in some cases.5,17,18 The second one is the lower absorption efficiency of Lns materials. In this article, a new Lns-activated SWIR material, Er3+-activated SrO phosphor, is successfully synthesized based on a novel and ingenious method. Figure 1 shows the XRD patterns and the TEM images of SrO: Er3+. (The SEM images of the sample are shown in Figure S1.) The XRD proves the fine f m3̅m cubic

Figure 1. (a) X-ray diffraction pattern of SrO:0.01Er3+ and the standard pattern of SrO (Powder Diffraction File No. 48−1477). (b) TEM image of an individual SrO macro-particle. (c) HRTEM images of an edge area in (b). B

DOI: 10.1021/acs.inorgchem.6b01849 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Therefore, the radiative transition rate Arad from the initial state J(4I13/2) to the terminal state J(4I15/2) can be calculated using eq 25 A rad =

64π 4e 2 −3

3h(2J + 1)λ

n(n2 + 22) ed S [(S , L)J ; (S′, L′)J ′] 9 (2)

where λ̅ is the mean wavelength of the emission transition. The matrix elements ⟨∥U(t)∥⟩ depend only on angular momentum of the Er3+ states and are independent of the host.5 The J-O parameters Ω2,4,6, however, exhibit the influence of the host on the transition probabilities, since they contain the crystal-field parameters, interconfigurational radial integrals, and the interaction between the Lns and the local environment. In this article, what we are interested in is the 4I13/2 to 4I15/2 transition of Er3+ for the SWIR emission. Fortunately, the radiative transition probabilities of 4I13/2→4I15/2 depend mainly on Ω6, because the reduced matrix elements ⟨∥U(2)∥⟩ and ⟨∥U(4)∥⟩ of this transition are relatively small.5,21 Further, Ω6 is strongly affected by the change of the radial integrals ⟨4f|rs|nl⟩. Namely, Ω6 is sensitive to the change of the electron density of the 4f and the 5d orbitals. Hence, Ω6 will increase with a decrease of 6s electron density, because the 6s electrons are assumed to shield the 5d orbital or to repulse the 5d electron and to diminish the existing probability of 5d electrons.21 For SrO, which is a typical ion crystal, the s electron of the cation is completely transferred to oxygen. Therefore, it is reasonable to qualitatively accept that the Ω6 for SrO:Er3+ is relatively high, and it is easy to understand that why the SWIR emission of Er3+ in SrO is intensive. Besides, the quantum efficiency (η) of radiative decay of 4 I13/2→4I15/2 transition is also strongly associated with the nonradiative rate (Anonrad), since η is given by η = τm/τR and 1/

Figure 2. (a) The schematic diagram of the SWIR device. (b) The emission spectrum of the SWIR device. (c) Fine structure of the SWIR transition from 4I13/2 to 4I15/2 due to the splitting of the ground state 4 I15/2 of Er3+. (d) The photograph of the device. (e) The IR image captured by a night vision device with 800 nm filter.

S ed[(S , L)J ; (S′, L′)J ′] =

∑

Ω t |⟨(S , L)J ||U (t )||(S′ , L′)J ′⟩|2 (1)

t = 2,4,6

where Ω2,4,6 are J-O intensity parameters; three ⟨∥U ∥⟩ are the reduced matrix elements of the unit tensor operators. (t)

Figure 3. (a) The SSPL emission spectra of SrO:Er3+ in visible and IR range. (b) The SSPL excitation spectrum of SrO:Er3+. (c) FT-IR spectrum of SrO:Er3+. (d) Decay curve of the 1538 nm emission of Er3+ under excitation at 377 nm. C

DOI: 10.1021/acs.inorgchem.6b01849 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry τm = Arad + Anonrad, where τm is the measured lifetime of excited levels. τR is the calculated radiative lifetime, namely, the reciprocal of Arad. For Er3+-doped optical materials, Anonrad is generally given by Anonrad = WMPR + WET + WOH +·...,22,23 where WMPR, WET, and WOH are the multiphonon relaxation rate, energy transfer rate between active ions, and the energy transfer rate between active ions and hydroxyl groups, respectively. Obviously, the nonradiative decay will reduce the radiative intensity as well as the emission efficiency. Among these nonradiative relaxation processes, multiphonon relaxation is a principal process.23 The WMPR rate for 4f levels of lanthanide ions is described as18,24,25 WMPR = W0e−αΔE / h ω

In the above discussion, we elucidate the strong emission intensity of the novel SWIR material SrO:Er3+. The higher spontaneous emission rate and the lower nonradiative relaxation rate are demonstrated. However, if one expects higher EQE for Lns-doped materials, higher absorption efficiency is essential. Because of the intrinsic characteristic of the f−f transitions, their narrow absorption lines cannot always match well with excitation source. Having broad absorption band is a great advantage. In the following content of the article, the host-sensitive process in the novel SWIR material is revealed. The excitation spectra are obtained by monitoring the maximum emission of the Er3+ ions at 1538 and 559 nm (shown in Figure 3b). A broad excitation band ranging from 250 to 450 nm is observed, which is named H band. Besides, some sharp excitation peaks are also observed in the excitation spectrum, which correspond to the f−f transitions of Er3+ ions: 4 I15/2→4G11/2 (380 nm), 4I15/2→2H11/2 (520 nm), and 4 I15/2→4F7/2 (490 nm). Similarly, a broad absorption band from 250 to 450 nm and a series of narrow peaks of f−f transitions of Er3+ are also revealed in the diffuse reflection spectrum of SrO:Er3+ shown in line 1 of Figure 4. This broad absorption band is attributed to the SrO host.

(3)

where the W0 is the relaxation rate when ΔE = 0, and α is parameter correlated with electron−phonon coupling strength of host. ΔE is the energy gap between 4I13/2 and 4I15/2 level of Er3+. Since ΔE for 4I13/2−4I15/2 transition of Er3+ almost keeps constant (ΔE = 6500 cm−1) in different materials, the WMPR value in different material is mainly attributed to the phonon energy of host matrix. The lower the phonon energy of a host, the more the number of phonons needed for bridging the energy gap, and consequently the smaller the nonradiative decay rate is. Considering that the phonon energy for commercial IR tellurite, germanate, silicate, and phosphate glasses are 750, 900, 1100, and 1300 cm−1, respectively,22 the lower phonon energy around 400−600 cm−1 for SrO indicates a lower MPR rate.26 For WET and WOH, it is reasonable to speculate that they are relatively low. The emission intensity as a function of the doping content is shown in Figure S3. This and our previous work indicated that energy transfer quenching was observed when the doped concentration is larger than 2.0 mol %.27 For the case of 1 mol % Er3+ in SrO, the energy transfer rate WET is assumed to be small. Figure 2c shows the FT-IR spectrum of SrO:Er3+ from 500 to 4000 cm−1. The vibrational signals of −O−H, −C−H, and −N−H groups are not found in the figure. This result indicates that SrO:Er3+ has a lower WOH. And it also suggests that the SrO:Er3+ phosphor synthesized by our method suffered less from the environmental damage.27 The experimental lifetime for the 1538 nm transition is measured by excited at 377 nm (shown in Figure 3d). The decay curves can be well-fitted with the first-order exponential equation, I = a exp(−t/τ), where I is the luminescence intensity at time t, a is constant, and τ is defined luminescent lifetime. The lifetime measurement for the 4I13/2 excited state of the SrO:Er3+ is ∼11.57 μs (the screen capture of the operation interface of the spectrometer is given in Figure S4). This value is significantly smaller than the reported value, or even it is abnormal. It is known that the lifetime of transition from 4I13/2 to 4I15/2 is in a time scale of milliseconds, ∼1−12 ms.14,22 Although the temperature and codoped ions will decrease the lifetime, a scale of microsecond lifetime is somewhat shocking. This value is lower by 3 orders of magnitude. Considering the lower Anonrad, higher Arad is the only reasonable explanation. The smooth emission line and the fine structure of the Stark sublevels exhibited in the emission spectroscopy (Figure 2b,c) are all indicating that the SWIR emission is considerably strong.28 However, unfortunately, we could not give numerical values of the J-O parameters for the polycrystalline form of the material. In view of the unique SWIR property and the deficiency of the spectral studies of SrO:Er 3+ , more experimental and calculated works are still needed.

Figure 4. (a) The diffuse reflection spectrum of SrO:Er3+ (line 1); the emission spectrum of the SWIR device at visible range using a NUV 380 nm chip (1 W) combined with the SrO:Er3+ phosphor (line 2); the emission spectrum of undoped SrO, excitation λ is 350 nm (line 3). (b) Schematic energy diagram for the emission mechanism.

Host emission is generally observed in alkaline earth oxide, such as MgO, CaO, SrO, and BaO.29 When monitoring the 1538 nm SWIR emission of Er3+, the emergence of the absorption band of host indicates an energy transfer from host to Er3+ or, namely, the SWIR emission of Er3+ can be sensitized by host. Similar phenomenon is reported by Xiao et al. that the D

DOI: 10.1021/acs.inorgchem.6b01849 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 1.54 μm emission of Er3+ ions can be enhanced by the host emission of ZnO.30 A nonradiative energy transfer from host to Er3+ is introduced. Attractively, it is found that the intensity of the emission of SrO host is relatively strong. An undoped SrO (UD SrO) polycrystalline powder is synthesized as a contrast. A bright green emission is obtained. The emission spectrum of UD SrO is shown in line 3 of Figure 4. The quantum efficiency of UD SrO is measured of 10%. (The PL excitation and emission spectra for UD SrO are shown in Figure S5. More information about the luminescent properties of UD SrO can be found in our previous work.27) Interestingly, the wavelength range of the host emission is opportunely overlapped with the absorption of the 4I15/2→2H11/2 transition of Er3+. Compared with the diffuse reflection, the characteristic absorption of f−f transition of Er3+ can be clearly seen in the emission line of the SWIR device at visible range (Figure 4, line 2), where the peaks are assigned to 4 I 15/2 → 2 H 11/2 (520 nm), 4 I 15/2 → 4 F 7/2 (490 nm), and 4 I15/2→4S3/2 (550 nm). These phenomena can be explained as a radiative energy transfer process. Furthermore, Figure 2a shows that the ratio between the 2 I13/2→4I15/2 transition and 2I11/2→4I15/2 transition is enhanced in the emission spectrum of the device compared with the SSPL emission spectra of the phosphor. This phenomenon may also be attributed to the energy transferred from host, which may cause the 2H9/2→4I13/2 transition. As a result, the emission of 1538 nm assigning to 2I13/2→4I15/2 is pumped. A sketch of the transitions of SrO:Er3+ is depicted in Figure 4b to illustrate the possible mechanism of emission and energy-transfer process.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (S.Z.) *E-mail: [email protected]. (C.L.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21401184 and 51402288), the Hong Kong, Macao and Taiwan Science and Technology Cooperation Special Project of the Ministry of S c i e n c e a n d T ec h n o l o g y o f C h i n a ( G r a n t N o . 2014DFT10310), and the Fund for Creative Research Groups, China (Grant No. 21221061). The authors are grateful to Profs. S. Zhang, G. Hong, and Y. Yu for their assistance in IR measurements and discussion.

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CONCLUSION In conclusion, a convenient short-wavelength IR light-emitting device is fabricated based on a novel Er3+-doped SrO phosphor. The 1.54 μm SWIR emission is generated from the 4 I13/2→4I15/2 transition of Er3+. Because of the unique chemical and crystalline characteristic of the SrO host, a high spontaneous emission rate and a low nonradiative rate is deduced. Meanwhile, a strong host sensitization process is observed. The broad absorption band of SrO host is overlapped with the emission range of the UV-LED chip. Therefore, the SWIR emission of Er3+ will be further improved due to the energy transfer. Importantly, it should be pointed out that the excellent SWIR emission property is closely related to the synthesis method, which will significantly increase the reaction temperature and endow the SrO with a considerable host emission. It is because of the ingenious synthesis method and the inherent properties of SrO that the novel SWIR emitting material is obtained. We anticipate a wide application of the material in photoelectric, biomedical imaging, military, and other related fields, and we consider that the synthesis method may be an ingenious and convenient strategy to obtain other novel Lns-doped functional materials. However, the calculated and experimental investigations are still deficient, which will be of our continuous focus.

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SEM images of Lns-doped SrO, emission spectrum of the 380 nm LED chip, plot of emission intensity as a function of doping content, screen capture of operation interface of spectrometer, PL excitation and emission spectra of UD SrO (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01849. E

DOI: 10.1021/acs.inorgchem.6b01849 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

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DOI: 10.1021/acs.inorgchem.6b01849 Inorg. Chem. XXXX, XXX, XXX−XXX