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Color manipulation of intense multi-luminescence from Ca-ZnOS:Mn2+ by Mn2+ concentration effect Jun-Cheng Zhang, Li-Zhen Zhao, Yun-Ze Long, Hong-Di Zhang, Bin Sun, Wen-Peng Han, Xu Yan, and Xusheng Wang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b03570 • Publication Date (Web): 16 Oct 2015 Downloaded from http://pubs.acs.org on October 19, 2015
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Color manipulation of intense multi-luminescence from CaZnOS:Mn2+ by Mn2+ concentration effect Jun-Cheng Zhang,*,† Li-Zhen Zhao,†,‡ Yun-Ze Long,† Hong-Di Zhang,† Bin Sun,† Wen-Peng Han,† Xu Yan,† and Xusheng Wang§ †
College of Physics, Qingdao University, Qingdao 266071, P. R. China
‡
Growing Base for State Key Laboratory, Qingdao University, Qingdao, 266071, P. R. China
§
Functional Materials Research Laboratory, Tongji University, Shanghai 200092, P. R. China
Supporting Information ABSTRACT: Color manipulation of intense multi-luminescence from CaZnOS:Mn2+ has been realized by adjusting Mn2+ concentration. Not only the photoluminescence (PL) of Mn2+ emission from 4T1(4G) to 6A1(6S) shows a red shift from yellow to red with increasing Mn2+ concentration, which is in contrast to the fixed PL emission reported by Hintzen et al. (Chem. Mater., 2009), but also mechanoluminescence (ML) and cathodoluminescence (CL) have a similar variation. More attractively, the brightness of multi-luminescence is surprisingly intense for all the CaZnOS:Mn2+ with a large-scale Mn2+ doping (0.1-10 mol%). Based on the investigation of crystal field, various spectral results, and PL lifetimes, the red-shift mechanism of multi-luminescence reported here has been proposed to arise from the exchange interaction effect of Mn2+ pairs at higher concentrations. In addition to correcting the previous misunderstanding on the emission of CaZnOS:Mn2+, these findings extend the tunable emission window, opening up new opportunities in multifunctional applications of PL, ML and CL involving multi-color light sources, displays, and stress imaging, especially providing a novel resolution to design ML colors.
■ INTRODUCTION The well-known phosphors are such materials that can emit light when exposed to various sources including photons, electrons, and X-rays.1,2 During the past century, people have keenly experienced the great development of phosphor applications in light sources and displays, such as fluorescent lamps, light-emitting diodes (LED), cathode ray tubes (CRT), flat panel displays (FPD), and X-ray computed tomography (X-CT).2,3 Up to now, great efforts are always being made to exploit novel phosphors with excellent performance due to the urgent demand of practical applications, including the photoluminescent (PL) phosphors for next-generation light sources LED,4,5 as well as cathodoluminescent (CL) phosphors for full-color field emission displays (FED) as a type of promising FPD.6,7 In recent years, research on the mechanoluminescent (ML) phosphors, whose luminescence is especially excited by mechanical stimuli, has made dramatic advances.8-11 With the landmark exploitation of intense, durable and multi-color ML semiconductors ZnS:Mn2+/Cu+ series by Jeong et al.,12-14 the applications of ML in mechanicallydriven excitation sources,15,16 colorful displays,12,13 and white-light sources,14,17 as well as stress sensors and stress imaging,18-20 seem to be turning into reality. The applications of various phosphors in lightings and displays are strongly dependent on the emission wave-
length (i.e. color) and brightness. The abundant color points can be obtained in single-phase phosphors by the different color manipulation technologies, including various luminescent centers doping/codoping, energy transfer, and substitution of host ions.9,12-14,21-24 The red shift of emission induced by the concentration effect of luminescent centers seems to be also an alternative way to tune the emission color of PL and CL, in spite of the absence of related report on ML.25-27 However, the brightness is usually weakened too much to apply because of concentration quenching. In the present work, we successfully tuned the emission color of PL, CL and ML in CaZnOS:Mn2+ from yellow to red through varying Mn2+ concentration. More attractively, the brightness is extremely intense for a large-scale Mn2+ doping (0.1-10 mol%). CaZnOS, a direct-gap semiconductor composed of ZnS and CaO layers, was firstly discovered by Petrova et al.28 and then investigated by Clarke et al.29 It is an excellent host lattice for several luminescent centers, especially for Mn2+.30-32 The efficient PL from CaZnOS:Mn2+ has shown the great potential in applications such as light sources excited by ultraviolet(UV-), near ultraviolet- (NUV-) and blue-light LED.30 Our group also illustrated the outstanding ML from CaZnOS:Mn2+ in 2013.33 It can simultaneously sense and image different mechanical stimuli including ultrasonic vibration, impact, friction, and compression with an intense
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emission, indicating the practical prospect on stress sensors. However, there is no the related report on CL of CaZnOS:Mn2+, let alone to the attempt to tune the emission color. Our research on the color manipulation of intense multi-luminescence from CaZnOS:Mn2+ will further extend the related multifunctional applications assuredly.
were performed in the same FESEM attached a CL detector with an acceleration voltage of 5 kV.
Another important objective of this study is to correct the misunderstanding of the fixed PL emission from CaZnOS:Mn2+. The previous reports from Hintzen et al.30 showed that all the PL emission of CaZn1-xMnxOS (0 ≤ x ≤ 0.2) located at 614 nm, without any change with varying Mn2+ doping concentration, which is in contrast to our observation on the change of PL color (yellow→orange→ red).
PL Measurements. Photoluminescence excitation (PLE) and PL spectra were recorded by a fluorescence spectrometer (F-4600, Hitachi) with a scan speed of 60 nm min-1. Photographs of PL were recorded by a Nikon D3 camera with a Nikkor 50 mm f1.2 lense. PL lifetimes were recorded using a spectrophotometer (FLS920, Edinburgh Instruments Ltd.).
Hence, herein, we firstly display the novel color manipulation of intense PL, ML and CL from CaZnOS:Mn2+ by Mn2+ concentration effect. Subsequently, the changes of luminescent spectra, absorption edge, and band gap have been investigated. The origin of multi-luminescent color manipulation has also been explained according to the investigation of crystal filed, quenching concentration, and Mn2+ PL lifetimes. Finally, possibilities for the fixed PL emission reported previously are discussed. ■ EXPERIMENTAL SECTION Synthesis. Polycrystalline phosphors with the composition CaZn1-xMnxOS (0 ≤ x ≤ 0.1) were synthesized by the solid-state reaction among CaO obtained from the thermal decomposition of CaCO3 (Kojundo Chemical Lab, 99.99%), ZnS (Kojundo Chemical Lab, 99.99%) and MnCO3 (Kojundo Chemical Lab, 99.99%). A stoichiometric mixture was thoroughly ground, pelletized, and subsequently sintered in an alumina crucible at 1100 °C for 3 h in a horizontal tube furnace under argon atmosphere. The prepared product was pulverized and screened through a 20 μm sieve for the measurements of X-ray diffraction, diffuse reflectance, PL, PL lifetimes, and thermoluminescence (ThL), as well as for the preparation of ML samples. Powder X-ray Diffraction. X-ray diffraction (XRD) measurements were carried out by X-ray powder diffractometer (D8 Advance, Bruker AXS GmbH) using Cu Kα (λ = 1.54056 Å) radiation operated at 40 kV and 40 mA in the 2θ range from 10o to 90o with a step size of 0.02o (2θ s-1) for the normal scan, while 2θ from 31o to 32o with a step size of 0.005o (2θ s-1) for the refined scan. Morphology, composition and CL measurements. The surface morphologies, energy dispersive X-ray spectroscopy (EDS) and elemental mapping of sintered samples were examined by field emission scanning electron microscopy (FESEM, JSM-7001F, JEOL Ltd.). FESEM observation of pellets showed that the size of the CaZn1xMnxOS (0 ≤ x ≤ 0.1) grains was approximately 1-3 μm, without a noticeable change with increasing Mn2+ concentration, and these grains tightly bound to each other [Figure S1, Supporting Information]. The CL measurements
Diffuse reflectance spectra. The diffuse reflectance spectra were measured using a UV/vis/NIR spectrophotometer (V570, Jasco Co.) with a scan speed of 120 nm min-1.
ML Characterization. To evaluate ML properties under the different mechanical stimuli, two kinds of ML samples were prepared. For the ML measurements under friction and ultrasonic vibration, the composite films with a thickness of ~500 μm were prepared by mixing the screened CaZnOS:Mn2+ powders and a transparent epoxy resin (SpeciFix, Struers GmbH) at a weight ratio of 1:1. For the ML measurements under compression, the circular composite disks with a diameter of 25 mm and a thickness of 15 mm were prepared by mixing the above powders and resin at a weight of 1:9. Friction induced ML (tribo-ML) was measured under the two modes, i.e. manual friction with a metal tip and mechanical friction applied by an inhouse made friction machine with a transparent resin rod of 5 mm diameter. Ultrasonic waves were generated using a 20 MHz single element piezoelectric transducer for medical application (NDK Corp.) and an ultrasonic pulser-receiver (Panametrics-NDT 5072PR, Olympus Corp.). Compression was produced with a universal testing machine (WDW-20, Jadarason Co. Ltd.). The photographs and videos of ML were obtained using the same camera for PL photographs. The ML spectra were recorded with a photon multi-channel analyzer system (QE65000, Ocean Optics). The ML intensity was measured using a photoncounting system that consists of a photomultiplier tube (H10682-01, Hamamatsu Photonics K.K.) and a photon counter (C8855-01, Hamamatsu Photonics K.K.). Before the ML characterization, the specimens were irradiated with 254 nm UV light for 1 min. The ML intensity measurements were repeated five times to reduce error. All of the relative standard deviations were less than 5%. The mean values were used for comparison of ML intensity. ThL Test. ThL curves were measured using a fluorescence spectrometer (F-7000, Hitachi) connected with an in-house made temperature control unit. All the specimens were irradiated with 254 nm UV light for 1 min before the ThL test. All measurements except ThL were performed at room temperature. ■ RESULTS AND DISCUSSION
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Crystal Structure and Substitution Site for Mn2+. Figure 1a shows the powder XRD patterns of CaZn1xMnxOS (0 ≤ x ≤ 0.1) and the standard PDF card of CaZnOS for comparison. It reveals that the synthesized phosphors are the single phase of CaZnOS. None of impu31.2 31.4 31.6 31.8 32.0
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fixed red emission with peak center at ~614 nm, regardless of Mn2+ doping concentration.30 However, the opposite phenomenon has been surprisingly observed in our research. Figure 2 illuminates the evident color manipulation of PL from CaZnOS:Mn2+ by Mn2+ concentration effect. The undoped CaZnOS phosphor appears a white daylight color in body. The color of Mn2+ doped CaZnOS phosphors changes from white to pink with increasing Mn2+ concentration from 0 to 10 mol% [Figure 2a]. When these phosphors are irradiated by 254 nm UV light, except the undoped CaZnOS without luminescence, they emit lights with different colors. The color of lights varies from yellow through orange to red with increasing Mn2+ concentration [Figure 2b]. The PL brightness for all concentrations is highly intense for naked eyes. The PL spectra of CaZnOS:Mn2+ are composed of two emission bands, a weak band centered at 530 nm and a strong band centered in the vicinity of 615 nm. The change of luminescent color from dark yellow to light yellow in the phosphors with x value from 0.001 to 0.005 originates from the decrease of weak emission band [Figure 2c]. When the x value increases to 0.02, the weak emission band disappears [Figure 2d]. The PL color change from light yellow through orange to red when varying x value from 0.005 to 0.1 has been attributed to the red shift (~8 nm) of the strong emission peak [Figure 2e].
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rity lines have been detected even in the case of 10 mol% Mn2+ doping. Figure 1b demonstrates the refined shift of the strongest diffraction peak in the 2θ range from 31o to 32o. Both two peaks shift to the lower-angle side with increasing Mn2+ concentration, indicating the lattice expansion induced by the larg1.0 (c) x=0 x=0.004 er size Mn2+ ions (CN4, 0.66 Å) substitux=0.001 2+ 0.8 tion for Zn ions (CN4, 0.60 Å). It is x=0.002 x=0.003 known that the lattice shrinkage will oc0.6 x=0.004 cur and the diffraction peak will shift to x=0.005 2+ x=0.01 higher-angle side if the larger size Ca 0.4 ions (CN6, 1.00 Å) are substituted by the 0.2 smaller size Mn2+ ions (CN6, HS 0.83 Å 34 and LS 0.67 Å). However, the peak shift 0.0 is very tiny due to the slight size differ450 500 550 600 650 700 750 800 Wavelength (nm) 1.0 (d) ence between Mn2+ and Zn2+. The largest (e) red shift x=0.005 peak shift from CaZnOS to 0.8 ~ 8 nm x=0.01 o x=0.02 1.000 CaZn0.9Mn0.1OS is less than 0.05 . Cax=0.03 0.6 ZnOS has an unusual entirely nonx=0.04 x=0.06 centrosymmetric structure with space 0.4 x=0.08 0.975 x=0.1 group P63mc (Figure S2, Supporting In0.2 formation). It is composed of isotypic 0.0 puckered hexagonal ZnS and CaO layers 0.950 500 550 600 650 700 750 800 605 610 615 620 625 arranged so that [ZnOS3] tetrahedra are Wavelength (nm) Wavelength (nm) all aligned in parallel, resulting in a polar structure and piezoelectric behavior.28.29 2+ Figure 2. Color manipulation of PL from CaZnOS:Mn by Mn concentration efAccordingly, the introducing of lumines- fect. Optical images of CaZn Mn OS (x = 0, 0.001, 0.002, 0.003, 0.004, 0.005, 0.01, 1-x x cent centers Mn2+ ions into the host lat- 0.02, 0.03, 0.04, 0.06, 0.08, and 0.1): (a) under daylight lamp lighting and (b) under tice of CaZnOS provides the possibility 254 nm UV lamp excitation. (c) Normalized PL spectra of CaZn1-xMnxOS (x = 0.001, for PL, ML and CL. 0.002, 0.003, 0.004, 0.005, and 0.01) under 282 nm excitation source. Inset shows 0
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Color Manipulation of PL, ML and PL spectra (λex = 282 nm) of CaZn1-xMnxOS (x = 0 and 0.004). (d) Normalized PL CL. Hintzen et al. reported that the Mn2+- spectra of CaZn1-xMnxOS (x = 0.005, 0.01, 0.02, 0.03, 0.04, 0.06, 0.08, and 0.1) under activated CaZnOS phosphors showed a 282-302 nm excitation source. (e) Enlarged PL spectra, showing the red shift.
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two emission bands, and show the consistent spectral decrease (0.001 ≤ x ≤ 0.01) and red shift (0.005 ≤ x ≤ 0.1) with those of PL spectra. It is to note that compared with spectral shift (~8 nm) of PL, the larger shift (~12 nm) of tribo-ML spectra is not induced by an instrument effect. We ascribe it to the spectral widening derived from the rough ML spectra because of the discrete tribo-ML. In addition to PL and ML, there are similar spectral change and red shift for intense CL in CaZnOS:Mn2+ with varying Mn2+ concentration [Figure S3, Supporting Information]. The consistent results of multi-luminescence suggest that the color manipulation of PL, ML and CL originates from the same physical mechanism.
On the other hand, the tribo-ML color varies from yellow to red with increasing Mn2+ concentration, similar with the color change of PL. Figure 4a displays that the tribo-ML chromaticity points locate between yellow and red regions. As the Mn2+ concentration is increased, the CIE coordinates (x, y) vary systematically from (0.564, 0.425) of x = 0.001 to (0.640, 0.347) of x = 0.1 [Figure 4a, inset], confirming the noticeable red shift of ML. The tribo-ML spectra of CaZnOS:Mn2+ also consist of
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The color manipulation of ML from CaZnOS:Mn2+ can also be realized by Mn2+ concentration effect. In order to exhibit the ML color directly and distinctly, the ML films were hand-scribed gently by a metal tip to and fro, and therefore the photographs were recorded by a camera with an exposure time of 5 s. At the same time, the ML spectra were also measured by a spectrometer to observe the spectral change. As shown in Figure 3, the undoped sample displays none of ML, nevertheless all the doped samples (0.001 ≤ x ≤ 0.1) show the intense tribo-ML, even for the samples with very low Mn2+ concentrations (x ≤ 0.002) and those with very high concentrations (x ≥ 0.08). It should be specially pointed out that the intense tribo-ML from CaZnOS:Mn2+ with a large-scale Mn2+ doping can even be directly observed by 1.0 (c) x=0.005 naked eyes under the daylight lamp lightx=0.01 0.8 ing [Video S1, Supporting Information], x=0.02 x=0.03 0.6 besides in dark. It indicates the promising x=0.04 x=0.06 prospect in applications of mechanically0.4 x=0.08 driven displays and light sources. x=0.1
Shift of Absorption Edge and Variation of Band Gap. In addition to the spectral changes of multiluminescence, the absorption edges and band gaps of CaZn1-xMnxOS illuminate the successive variation with increasing Mn2+ concentration. Figure 5a shows the diffuse reflection spectra of undoped and Mn2+ doped CaZnOS. All the phosphors have a strong drop in reflection near the 300 nm UV region, corresponding to the valenceto-conduction absorption of host lattice. This absorption edge is slightly shifted to the longer wavelength with increasing Mn2+ concentration. Furthermore, three absorp-
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Figure 4. Color manipulation of tribo-ML from CaZnOS:Mn by Mn concentration effect. (a) The CIE coordinate (x, y) values obtained from tribo-ML spectra of CaZn1-xMnxOS (x = 0, 0.001, 0.002, 0.003, 0.004, 0.005, 0.01, 0.02, 0.03, 0.04, 0.06, 0.08, and 0.1) excited by manual friction. (b) Normalized tribo-ML spectra of CaZn1-xMnxOS (x = 0.001, 0.002, 0.003, 0.004, 0.005, and 0.01). Inset shows tribo-ML spectra of CaZn1-xMnxOS (x = 0 and 0.003). (c) Normalized tribo-ML spectra of CaZn1-xMnxOS (x = 0.005, 0.01, 0.02, 0.03, 0.04, 0.06, 0.08, and 0.1). (d) Enlarged tribo-ML spectra, showing the red shift.
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tion bands located at 426, 488 and 572 nm arise and then increase gradually with increasing Mn2+ concentration. These bands originate from the absorption of Mn2+, and also explain the color change of CaZn1-xMnxOS in body [Figure 2a]. 100
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Figure 5. (a) Diffuse reflection spectra of CaZn1-xMnxOS (x = 0, 0.005, 0.02, 0.04, 0.08, and 0.1). (b) Variation of band gap (Eg) with Mn2+ concentration (x = 0, 0.001, 0.002, 0.003, 0.004, 0.005, 0.01, 0.02, 0.03, 0.04, 0.06, 0.08, and 0.1). Inset shows the curve of (αhν)2 vs. hν derived from diffuse reflection spectral data for CaZnOS. In order to better localize the optical band gaps, the absorption spectra were calculated from reflectance spectra by using the Kubelka-Munk function: F(R) = α/S = (1R)2/2R, where R, α, and S are the reflection, the absorption, and the scattering coefficient, respectively.35 According to the Tauc relation of direct-gap semiconductor, the absorption coefficient α is exhibited as (αhν)2 = A(hν-Eg), where hν is the incident photon energy, A is a constant that relates to the effective masses associated with the valence and conduction bands, and Eg is the optical band gap of direct-gap semiconductor.36 The values of band gap Eg can be estimated by extrapolating the linear region of (αhν)2 to the abscissa (hν). According to this method, the
band gap of undoped CaZnOS was calculated to be 4.16 eV [Figure 5b, inset], which is little larger than the previously reported values, i.e. 3.71 eV and 4.0 eV.29,30 Figure 5b describes the trend of Eg in CaZn1-xMnxOS. It decreases smoothly from 3.92 eV of x = 0.001 to 3.73 eV of x = 0.1. The similar phenomenon has also been reported in ZnS.37 The narrowed band gap in this research is likely due to the Mn2+ related trap levels introduced into host lattice CaZnOS.20,33,37 One kind of trap levels is the donor level formed by Mn2+ substitution for Zn2+. There is an electronegativity difference between Mn (1.55) and Zn (1.65) in spite of equivalent substitution. Accordingly, the boundhole states Mno are created, and then electrons are trapped by Mno to form donor levels of isoelectronic traps near the conduction band.38 Moreover, the acceptor levels near the valence band are likely to be introduced by Zn vacancies which are ubiquitous in metal doped Zn-VI semiconductors.39 These trap levels play an important role in producing ML,33 besides narrowing band gap. Concentration Quenching of PL, ML and CL. The PLE spectra of CaZnOS:Mn2+ consist of two parts, i.e. the excitation band of host lattice (200-350 nm) and that of Mn2+ (350-550 nm) [Figure 6], which are consistent with the diffuse reflectance spectra. The former is much stronger than the latter, indicating the efficient energy transfer from host lattice to Mn2+. The excitation peak of host lattice shifts to the longer-wavelength region from 280 nm (x = 0.001) to 304 nm (x = 0.1) with increasing Mn2+ concentration. These results are consistent with the shift of absorption edges and band gaps, as well as the reports from Hintzen et al.30 For the PL spectra of CaZnOS:Mn2+ with lower Mn2+ concentrations (x < 0.02), in addition to the strong 4T1(4G) →6A1(6S) emission from Mn2+, there is a weak emission band located at 530 nm [Figure 2c and Figure S4, Supporting Information], which has not been reported in CaZnOS:Mn2+-related literature. In the work of Hintzen et al., the emission wavelength of CaZnOS:Mn2+ was recorded only from 550 nm, thus this band could not be observed. Because of no emission in undoped CaZnOS [Figure 2c, inset], this band is most likely due to the defect centers formed by Mn2+ doping. Interestingly, it is depressed with increasing Mn2+ concentration until the complete disappearance at x = 0.02. Further studies are needed to investigate the detailed reason. When the Mn2+ concentration is no less than 2 mol% (x ≥ 0.02), there is only one strong emission from Mn2+. The location of this emission does not change with the excitation wavelength (not matter by host excitation or Mn2+ direct excitation) for the sample with a given Mn2+ concentration [Figure S5, Supporting Information]. Figure 7 shows the dependences of PL intensity of CaZn1-xMnxOS on Mn2+ concentration. The maximum intensity is reached at a lower concentration with x = 0.004 for host lattice excitation, while at a higher concentration with x = 0.04 for all the Mn2+ direct excitations (λex = 395, 434, 496, and 516 nm). Both are attributed to the
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6 Figure 7. Relative PL intensity of 4T1(4G)→ A1(6S) emission 2+ as a function of Mn concentration in CaZn1-xMnxOS (x = 0, 0.001, 0.002, 0.003, 0.004, 0.005, 0.01, 0.02, 0.03, 0.04, 0.06, 0.08, and 0.1).
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concentration quenching effect. The former one occurs due to the energy transfer from host lattice to defects induced by Mn2+ doping. The latter one is attributed to the energy transfer from one Mn2+ to another and eventually to defects. The critical distance among Mn2+ ions at the quenching concentration Rc can be estimated by the equation Rc ≈ 2[3V/(4πXcN)]1/3,
(1)
Where V is the unit cell volume, Xc the quenching concentration of activator ion, and N the number of total activator sites per unit cell. For the CaZnOS:Mn2+ system, V = 139.388 Å3, and Xc = 0.04, and N = 2, the Rc is determined to be ~14.9 Å. The uniform distribution of Mn2+ in the host lattice has been confirmed by the FESEM-CL image [Figure S3a, Supporting Information] and elemental distribution mapping [Figure S6, Supporting Information]. No irregular Mn2+-cluster has been observed even when the Mn concentration is high (x = 0.1). The EDS data support the existence of Mn, but Mn was not detected at lower concentration (x < 0.04) [Figure S7, Supporting Information].
Relative CL int. (a.u.)
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Mn composition x Figure 8. Relative MLs (including Tribo-ML, CompressML, and Ultrasonic-ML) and CL intensities of 4 6 T1(4G)→ A1(6S) emission as a function of Mn2+ concentration in CaZn1-xMnxOS (x = 0, 0.001, 0.002, 0.003, 0.004, 0.005, 0.01, 0.02, 0.03, 0.04, 0.06, 0.08, and 0.1). Tribo-ML: mechanical friction excited ML, load of 10 N, rotate speed of 150 round min-1; Compress-ML: compression excited ML, compressive load of 0-1000 N, deformation rate of 3 mm min-1; Ultrasonic-ML: ultrasonic vibration excited ML, ultrasonic frequency of 20 MHz, ultrasonic output power of 2.94 mW.
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Figure 8 exhibits the dependence of MLs (including Tribo-ML, Compress-ML, and Ultrasonic-ML) and CL intensities of CaZn1-xMnxOS on Mn2+ concentration. The maximum intensity for all three types of MLs is reached at x = 0.003, while that for CL at x = 0.004. All these emission quenching curves show a similar trend with that of PL under the host lattice excitation. It suggests that the intense ML and CL in CaZnOS:Mn2+ occur through an effective energy transfer process from host to Mn2+. Different from most phosphors, CaZnOS:Mn2+ with largescale Mn2+ doping shows intense multi-luminescence, even for those with higher Mn2+ concentration which far exceeds the critical value [Figure 2b and Figure 3]. This attractive finding as well as color manipulation would assuredly facilitate the practical multifunctional applications in the fields of full-color displays, lightings and stress imaging. Moreover, the small but really existed difference between quenching concentration of ML and that of PL and CL will be discussed elsewhere. Mechanism of Color Manipulation. The emission of Mn2+ is strongly dependent on the crystal filed of host lattice due to the strong coupling of 3d5 electrons and lattice vibration. It can vary from green (weak crystal field) to orange/red (strong crystal field). The aforementioned decreases of lattice parameter and energy band indeed indicate the change of crystal field with increasing Mn2+ concentration. However, the lattice expansion occurs in this research as Mn2+ is larger than Zn2+. The increase of lattice parameters and cell volume leads to a decrease in the crystal filed strength around Mn2+. According to the Tanabe-Sugano diagram for Mn2+,41 a blue shift of 4T1→6A1 transition is expected on decreasing crystal filed strength, in contradiction to our observed red shift. Therefore, the crystal filed effect should be ruled out. Considering the results of concentration quenching, we ascribed the red shift of multi-luminescence to the formation of Mn2+ pairs at higher concentrations. It is known that based on the exchange interaction effect, Mn2+ pairs can reduce the energy difference between ground and first excited states, causing that the emission of Mn2+ pairs is at lower energy than that of single ions, i.e. emission red shift.42-44 PL lifetime measurements presented below provide the related evidence. Figure 9a shows the PL decay curves of 4T1(4G)→6A1(6S) emission for Mn2+ in CaZnOS. At lower Mn2+ concentrations, the emission behavior shows a single exponential decay since there is only one type of luminescent center, i.e. isolated Mn2+. At higher Mn2+ concentrations, the decay becomes nonexponential and faster. For simplicity, lifetimes in this work were determined by fitting the initial single exponential region. The Mn2+ concentrationdependent lifetimes of 4T1(4G)→6A1(6S) emission are plotted in Figure 9b. The lifetimes with increasing Mn2+ concentration decrease from 2.124 ms for x = 0.001 to 1.304 ms for x = 0.08. The trend displays a relatively flat decrease at both lower Mn2+ concentrations (x ≤ 0.01) and higher con-
centration (x ≥ 0.04), while it shows a steep decrease in the range of 0.01 ≤ x ≤ 0.04. Considering the quenching concentration of 4 mol% Mn2+ (x = 0.04) [Figure 7], the contribution from concentration quenching to the steep shortening of lifetimes should be excluded in the concentration range of x = 0.01 to 0.04. Therefore, the decrease of lifetimes is attributed to the formation of Mn2+ pairs. Due to the exchange interaction of Mn2+ pairs, the spin selection rule is partially lifted from spin-forbidden to spinallowed, leading to the shorten lifetime.44 The mixing of emission from Mn2+ pairs with a shorter lifetime and that from isolated Mn2+ with a longer lifetime also explains the fact that nonexponential decay under higher Mn2+ concentrations. According to the variation of lifetimes, the formation of Mn2+ pairs begins in about x = 0 .01. The critical distance is estimated by equation (1) to be ~23.7 Å.
(a)
Normallized intensity (a.u.)
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1 0.1 0.01 1 0.1 0.01 1 0.1 0.01 1 0.1 0.01 1 0.1 0.01 1 0.1 0.01 1 0.1 0.01 1 0.1 0.01 1 0.1 0.01
x=0.001 2.124±0.009 ms x=0.003 2.119±0.008 ms x=0.005 2.115±0.007 ms x=0.01 2.103±0.007 ms x=0.02 1.847±0.006 ms x=0.03 1.667±0.007 ms x=0.04 1.399±0.009 ms x=0.06 1.311±0.013 ms x=0.08 1.304±0.018 ms 0
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Figure 9. (a) PL decay curves of CaZn1-xMnxOS (x = 0.001, 0.003, 0.005, 0.01, 0.02, 0.03, 0.04, 0.06, and 0.08) after the strongest host excitation (282-302 nm), monitoring
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6 T1(4G)→ A1(6S) emission of Mn2+ (609-618 nm). (b) Curve 2+ of Mn concentration-dependent lifetimes.
On the basis of above discussion, the schematic diagram of multi-luminescence processes in CaZnOS:Mn2+ is illustrated in Figure 10. The Mn2+ ions under the various luminescence processes are firstly excited by the energy transferred from host lattice, although there are different mechanisms of interaction between host lattice and various excitation modes. For PL process, under UV irradiation, the host lattice transfers the absorbed energy efficiently to Mn2+, resulting in the excitation of Mn2+.30 For ML process, once the mechanical stimuli is applied, the trapped electrons are detrapped and released to the conduction band due to the produced piezoelectric field. A non-radiative recombination occurs between detrapped electrons and holes, followed by the energy transfer to Mn2+.33 For CL process, the electron-hole pairs are possibly formed when the host lattice is irradiated by electron beam. The energy released by the recombination of electron-hole pairs is transferred to Mn2+. Another excited path could be that the lower-energy secondary electrons created by cathode-ray irradiation on host lattice excite Mn2+ directly.45 After Mn2+ is excited by different sources, if at lower Mn2+ concentrations, the excited isolated-Mn2+ deexcites to give rise the yellow emission of 4T1(4G)→ 6 A1(6S). While at higher concentrations, Mn2+ pairs are formed. Due to the exchange interaction of Mn2+ pairs, red shift of 4T1(4G)→6A1(6S) emission occurs. With the further increase of Mn2+ concentration, the number of such interacting pairs has increased, leading to concentration quenching.
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sion position has no change with increasing Mn2+ concentration. Furthermore, Ref. 30 did not show optical images of CaZnOS:Mn2+ under UV or blue light excitation, and the emission positions were only observed in luminescent spectra. It is known that in the measurement of luminescent spectra, the position could be blurred by an overquick scan speed, such as 400 nm min-1 of Ref. 30. Accordingly, a slow scan speed of 60 nm min-1 has been used to acquire the accurate position in this research. ■ CONCLUSIONS Color manipulation from yellow to red of multiluminescence including PL, ML and CL, has been realized in CaZnOS:Mn2+ through varying Mn2+ concentration. The PL and ML brightness for all large-scale Mn2+ doping (0.110 mol%) is surprisingly intense for naked eyes even when the concentration quenching occurs at higher Mn2+ concentrations. The spectral results indicate that the color change of multi-luminescence originates from the red shift of Mn2+ 4T1(4G)→6A1(6S) emission with increasing Mn2+ concentration. The absorption edges of CaZnOS:Mn2+ show a similar red shift, thus the calculated values of band gap gradually decrease with increasing Mn2+ concentration. This behavior is ascribed to the introduction of trap levels induced by Mn2+ doping. The red shift of Mn2+ emission has been explained by the exchange interaction of Mn2+ pairs, supported by the shorten lifetimes of 4T1(4G)→6A1(6S) emission. However, the crystal field effect has been ruled out according to the XRD investigation. Our findings not only correct the misunderstanding on the fixed PL emission of CaZnOS:Mn2+, but also open a door for exploiting the PL, ML and CL multifunctional applications in multi-color light sources, displays, and stress imaging. Especially, color manipulation by adjusting the doping concentration of luminescent centers provides a novel resolution to design various ML colors in single host lattice, attractive from an applied point of view. ■ ASSOCIATED CONTENT Supporting Information
Figure 10. Schematic illustration of the UV, mechanical stimuli and cathode-ray excited luminescence processes in CaZnOS:Mn2+. However, it is still wondered why CaZnOS:Mn2+ reported by Hintzen et al. showed a fixed emission position, regardless of Mn2+ concentration. Referring to the report of Ref. 30 that a large amount of secondary phases were formed for the preparation of Mn2+ doped CaZnOS samples, the explanation, which we feel is more likely, is that the neighbour Mn2+ ions are insulated by so many impurities, and the formation of Mn2+ pairs is obstructed. The emission is only from the isolated Mn2+, hence the emis-
The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional information pertaining to FESEM, crystal structure, color manipulation of CL, PL spectra, elemental distribution mappings, and EDS spectra (PDF) Video on intense tribo-ML from CaZnOS:Mn2+ (AVI) ■ AUTHOR INFORMATION Corresponding Author
*(J. C. Zhang) Email:
[email protected] 8
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Chemistry of Materials or
[email protected].
Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (11404181,21203105, 51373082 and 51572195), the Shandong Provincial Natural Science Foundation, China (ZR2013EMQ003 and ZR2014EMM010), the Program of Science and Technology in Qingdao City (13-14-195-jch), the Natural Science Foundation of Shandong Province for Distinguished Young Scholars (JQ201103), the Taishan Scholars Program of Shandong Province (ts20120528), and the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province. ■ REFERENCES (1) Blasse, G.; Grabmaier B. C. Luminescent Materials; Sprinder-Verlag: Berlin, 1994. (2) Shionoya S.; Yen W. M. Phosphor Handbook; CRC Press: New York, 1999. (3) Ronda C. Luminescence: from Theory to Applications; Wiley-VCH: Weinheim, 2008. (4) Pimputkar S.; Speck J. S.; DenBaars S. P.; Nakamura S. Prospects for LED lighting. Nat. Photonics 2009, 3, 180. (5) Lin C. C.; Liu R. S. Advances in Phosphors for Lightemitting Diodes. J. Phys. Chem. Lett. 2011, 2, 1268. (6) Talin A. A.; Dean K. A.; Jaskie J. E. Field emission displays: a critical review. Solid State Electron. 2001, 45, 963. (7) Xu N. S.; Huq S. E. Novel cold cathode materials and applications. Mat. Sci. Eng. R. 2005, 48, 47. (8) Chandra B. P. Mechanoluminescence. In Luminescence of Solids; Vij D. R., Ed.; Plenum: New York, 1988; pp 361-389. (9) Xu C. N.; Watanabe T.; Akiyama M.; Zheng X. G. Direct view of stress distribution in solid by mechanoluminescence. Appl. Phys. Lett. 1999, 74, 2414. (10) Xu C. N. Coatings. In Encyclopedia of Smart Materials; Schwartz M., Ed.; Wiley: New York, 2002; pp 190-201. (11) Peng D.; Chen B.; Wang F. Recent advances in doped mechanoluminescent phosphors. ChemPlusChem 2015, 80, 1209. (12) Jeong S. M.; Song S.; Lee S. K.; Choi B. Mechanically driven light-generator with high durability. Appl. Phys. Lett. 2013, 102, 051110. (13) Jeong S. M.; Song S.; Lee S. K.; Ha N. Y. Color manipulation of mechanoluminescence from stress-activated composite films. Adv. Mater. 2013, 25, 6194. (14) Jeong S. M.; Song S.; Joo K. I.; Kim J.; Hwang S. H.; Jeong J.; Kim H. Bright, wind-driven white mechanoluminescence from zinc sulphide microparticles embedded in a polydimethylsiloxane elastomer. Energy Environ. Sci. 2014, 7, 3338. (15) Terasaki N.; Zhang H.; Yamada H.; Xu C. N. Mechanoluminescent light source for a fluorescent probe molecule. Chem. Commun. 2011, 47, 8034. (16) Terasaki N.; Yamada H.; Xu C. N. Ultrasonic wave induced mechanoluminescence and its application for photocatalysis as ubiquitous light source. Catal. Today 2013, 201, 203. (17) Chen L.; Wong M. C.; Bai G.; Jie W.; Hao J. White and green light emissions of flexible polymer composites under electric field and multiple strains. Nano Energy 2014, 14, 372.
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