Pressure dependence of Mn2+ fluorescence in ZnS : Mn2+ nanoparticles

Journal of Luminescence 91 (2000) 139–145

Pressure dependence of Mn2+ fluorescence in ZnS : Mn2+ nanoparticles Wei Chena,c,*, Gohau Lib, Jan-Olle Malma, Yining Huangc, Reine Wallenberga, Hexiang Hanb, Zhaoping Wangb, Jan-Olov Bovina b

a Department of Inorganic Chemistry 2, Chemical Center, University of Lund P. O. Box 124, S-22100, Lund, Sweden National Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, P. O. Box 912, Beijing 100083, People’s Republic of China c Department of Chemistry, University of Western Ontario, London, ON N6A 5B7 Canada

Received 4 January 2000; received in revised form 13 April 2000; accepted 17 April 2000

Abstract The photoluminescence of Mn2+ in ZnS : Mn2+ nanoparticles with an average size of 4.5 nm has been measured under hydrostatic pressure from 0 to 6 GPa. The emission position is red-shifted at a rate of ÿ33.30.6 meV/ GPa, which is in good agreement with the calculated value of ÿ30.4 meV/GPa using the crystal field theory. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Nanoparticles; Pressure; Photoluminescence; Zinc sulfide; Manganese; Phonon coupling

1. Introduction In the past 20 years, nanostructured materials have been widely studied [1–5]. Dramatic changes in the physical properties of nanoparticles of semiconductor [1–3], metal [4] and magnetic [5] materials as a function of size have been observed. Some potential applications resulting from quantum size confinement have been suggested [1–5]. Doped semiconductor nanoparticles are particularly interesting because their electronic and optical properties are largely size dependent and may result in some important practical applica*Correspondence address. Present address: Nomadics, Inc. 1730 Cimarron Plaza, P. O. Box 2496, Stillwater, OK 74076, USA. E-mail address: [email protected] (W. Chen).

tions such as high-brightness displays [6 –17]. A typical such material is ZnS : Mn nanoparticles. Since the first report of Bhargava et al. [6], ZnS : Mn2+ nanoparticles have received much attention [7–17]. Recently, the preparation, physical properties and potential applications of doped ZnS nanoparticles have been reviewed [13]. Although the luminescence of ZnS : Mn nanoparticles has been extensively studied, the mechanisms for the efficiency enhancement and the lifetime shortening are still under debate. For example, recent studies [16,17] suggest that the lifetime shortening phenomenon reported by Bhargava et al. [6] is probably due to emission emanating from defects rather than from Mn2+ itself. In addition, it was reported that in CdS : Mn nanoparticles, the appearance of Mn2+ emission is not due to the particle size, but the synthesis

0022-2313/00/$ - see front matter # 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 0 ) 0 0 2 2 2 - 2

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route [14,15]. The interplay between quantum confinement and doped emitters in nanostructures still needs to be understood at a higher level. Variation of pressure has proven to be a powerful probe for understanding electronic and optical phenomena in condensed matters [18–22]. As an independent variable, pressure can be used to explore the nature of luminescence. This is because pressure decreases interatomic distance and therefore increases the overlap among adjacent electronic orbitals. Thus, pressure dependence of luminescence may provide some useful information on the electronic states of emitters, crystal field strength, energy transfer and the interactions between the luminescence centers and their hosts [18–23]. Further, since pressure has different effects on different luminescence centers and different recombination processes, the observed pressure behavior may help to distinguish different luminescence mechanisms [21–22]. The optical behavior of semiconductor nanoparticles under high external pressure has been investigated by several groups [23]. Alivisatos et al. observed that the pressure coefficients of the absorption edge in CdS [24] and CdSe [25] are close to those of the bulk. Schroeder et al. [23]. reported that the excitonic emission of CdS nanoparticles in glass shifted to higher energies with increasing pressure at a rate similar to that observed for band-to-band luminescence of corresponding bulk materials. The pressure dependence of luminescence of bulk ZnS : Mn was also examined [19,20,22]. However, to our knowledge, the effect of pressure on the luminescence behavior of ZnS : Mn nanoparticles has not yet received any attention. In this work, we report the pressure dependence of Mn2+ luminescence in ZnS : Mn nanoparticles.

2. Experiment ZnS : Mn2+ nanoparticles were typically prepared as following: a four-neck flask was charged with a solution containing 10 ml methacrylic acid and 400 ml ethanol (99.95%). The above solution was stirred under N2 for 2.5 h. A second solution containing 1.6 g of Na2S and 100 ml of ethanol and a third solution containing 5.8 g of

Zn(NO3)2  6(H2O), 0.26 g of Mn(NO3)2 and 100 ml of ethanol (Mn2+/Zn2+ molar ratio 5 : 95) were prepared and added to the first solution simultaneously via two different necks at the same rate. After the addition, the resulting solution was stirred constantly under N2 at 808C for 24 h and a transparent colloid of ZnS : Mn was formed. The pH value of the final solution is 2.4. This relatively low pH value is required to prevent the precipitation of unwanted Mn species from occurring [10]. The nanoparticles were separated from solution by centrifugation and dried in vacuum at room temperature. The identity and crystallinity of the particles were checked by powder X-ray diffraction. The average particle size was estimated by both high-resolution transmission electron microscopy (HRTEM) and XRD. The photoluminescence excitation (PLE) and emission (PL) spectra were recorded at room temperature using a SPEX Fluorolog fluorescence spectrophotometer. High-pressure photoluminescence spectra were recorded on a JY-HRD1 spectrometer equipped with a doublegrating monochromator. The blue (488 nm) line of a Ar+ laser was utilized as the excitation source. The sample, together with a small piece of ruby pressure calibrant, was placed in a 300 mm stainless-steel gasket, which was located between the parallel faces of two opposite diamonds of a diamond anvil cell (DAC). A mixture of methanol–ethanol (4 : 1) was used as pressure-transmitting fluid. The pressure in the DAC was determined by the well-known R1-line ruby fluorescence method. All measurements were carried out at room temperature. The XRD patterns were recorded on an INEL diffractometer using a monochromatized Cu Ka1 (l ¼ 1:54056 A˚) radiation with a-Si (a=0.543 nm) as an internal standard. HRTEM images of the particles were obtained on a JEM-4000EX electron microscope (400 kV) with a structural resolution of 0.16 nm.

3. Results and discussion Fig. 1 shows the XRD pattern of the ZnS : Mn particles which is characteristic of the zinc-blende

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Fig. 1. Powder X-ray diffraction pattern of ZnS : Mn nanoparticles.

modification of ZnS (sphalerite). The average size of the particles calculated from the Debye–Scherrer equation is around 4.5 nm [26]. Fig. 2 illustrates a typical HRTEM image, from which the size of the particles appears to be in the range of 4–5 nm. This is in agreement with the XRD result. In order to prove that Mn2+ ions were indeed doped in the ZnS nanoparticles, we carried out the mapping of Mn and S from the solution containing the particles. This experiment was performed by using a cryo-transmission electron microscope (the details will be described elsewhere [27]). In the particles we can see Mn, while in the polymers outside the particles, we cannot find Mn. The Smapping image is overlapped totally with that of Mn-mapping in the same section of the same sample [27]. This demonstrates clearly that Mn2+ ions were doped in the particles rather than in the capping polymers. In addition, the electron spin resonance (ESR) spectrum of this sample is similar to that for ZnS : Mn nanoparticles in literature [13]. This also supports that the Mn2+ ions were doped in the ZnS nanoparticles. Fig. 3 shows the photoluminescence excitation (PLE) and emission (PL) spectra. The emission of ZnS : Mn nanoparticels is at 591 nm (Fig. 3, right). This is consistent with the emission of Mn2+ in ZnS nanoparticles reported in literature [6 –17]. The emission is attributed to the 4 T1 26 A1 transition of Mn2+ [6]. It was reported [9] that in

Fig. 2. High resolution transmission electron microscopy (HRTEM) image of ZnS : Mn nanoparticles.

Fig. 3. Photoluminescence excitation (right, lem ¼ 591 nm) and emission (left) spectra of ZnS : Mn nanoparticles obtained by excitation at 330 and 490 nm, respectively.

Mn2+-activated ZnS nanocrystals in which the Mn2+ ions are distributed outside the ZnS nanocrystals, the PL is totally different from that of Mn2+-doped ZnS nanocrystals in which the Mn2+ is incorporated within the nanocrystals. When the Mn2+ is incorporated within the nanocrystals, both the 435 nm blue emission of ZnS and the orange Mn2+ emission at 590 nm are observed. However, in the Mn2+-activated ZnS nanocrystals in which the Mn2+ ions are

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distributed outside the ZnS nanocrystals, no orange emission at 590 nm is observed; a new peak at 350 nm appears and the blue 435 nm emission of ZnS is quenched considerably and shifted to 390 nm. Comparison of these observations with our own suggests that in our material the Mn2+ ions are incorporated within the ZnS nanoparticles. Generally, in doped semiconductors, there are two channels for luminescence excitation. One is indirect excitation, i.e., excitation into the excited levels of the host, followed by an energy transfer from the host to the impurity ions to cause the luminescence. The other is direct excitation of the impurity ions. It is clear that the emission position of ZnS : Mn nanoparticles obtained by indirect at 330 nm and direct excitation at 490 nm is the same, but the emission from direct excitation at 490 nm is much weaker, indicating that luminescence from indirect excitation of the host is much more efficient. The successful observation of Mn2+ emission in ZnS nanoparticles from indirect excitation proves that energy transfer from ZnS to Mn2+ is taking place and this is another evidence showing that Mn2+ is located within the ZnS clusters as described by Kane et al. [8]. It was observed [28,29] that the exciton PLE band of nanoparticles is at the longer wavelength side of the exciton absorption (ABS) band. Because the photo-excitation rate of excitons is concentrated around the absorption edge where the absorption index is fairly low and the dispersion is small [30]. This indicates that the exciton PLE band is correspondent to the absorption edge of semiconductors. Similar to the ABS spectrum, the PLE spectrum of nanoparticles is also size-dependent [28,29]. The exciton PLE band of our particles is at 330 nm (Fig. 3(a)), which is longer in wavelength than that (280 nm) reported by Bhargava et al. because the average size (4.5 nm) of the particles in this paper is larger than their particles (3.0 nm) [6]. The exciton PLE band is 20 nm blue-shifted from the absorption edge of bulk ZnS (350 nm), indicating quantum size effect [1]. According to the tight-binding model [31], the absorption edge of a 4.5 nm ZnS particle is 325 nm. Our observation is reasonably consistent with this theoretical result.

The d–d transitions of Mn2+ ions are observed in the PLE spectrum and their assignments are labeled in the spectrum (middle, Fig. 3). In addition to the exciton and transitions of Mn2+, there are several bands appearing in the PLE between 200 and 300 nm. They are from the highlying excited states of Mn2+. Details about the assignment of these high-lying states is discussed in Ref. [27]. The PL spectra at selected pressures are shown in Fig. 4. This emission band of Mn2+ shifts to lower energies with increasing pressure, accompanied by a slight increase in the intensity and FWHM. The linear relationship between the emission energy and the pressure over the entire pressure range studied suggests that there is no pressure-induced phase transition in ZnS : Mn nanoparticles during the measurement. The pressure dependence of the peak position is shown in

Fig. 4. Photoluminescence spectra of ZnS : Mn nanoparticles under different pressures at excitation of 488 nm line of the Ar+ laser.

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where d ¼ dðE=BÞ=dðDq =BÞ. For Mn2+ ion its value is ÿ10 [32]. The parameters Dq , B and C may be estimated from the following equations [32,33]: 22B þ 7C ¼4A2 ð4F Þ;

ð2Þ

10B þ 5C ¼4A1 ; 4Eð4GÞ

ð3Þ

17B þ 5C ¼4Eð4DÞ

ð4Þ

100D2q ÿ ð14B þ 5C ÿ E2 Þð22B þ 7C ÿ E2 Þ ¼

12B2 ðE2 ÿ 22B ÿ 7CÞ ; ð13B þ 5C ÿ E2 Þ

ð5Þ

100D2q ÿ ð10B þ 7C ÿ E3 Þð10B þ 5C ÿ E3 Þ ¼

Fig. 5. Pressure dependence of the Mn2+ fluorescence in ZnS : Mn nanoparticles. The solid line is the least-squares fit to the data.

Fig. 5. The pressure coefficient obtained from Mn2+ fluorescence in ZnS : Mn nanoparticles is ÿ33.30.6 meV/GPa. The pressure coefficients of Mn2+ emission in bulk ZnS : Mn reported are ÿ31 meV/GPa [19]. Compared to these values in bulk ZnS doped with Mn2+, the pressure coefficient of the Mn2+ fluorescence in ZnS : Mn nanoparticles is just a little larger in value than that in the bulk. The pressure dependence of Mn2+ emission in ZnS : Mn can be calculated by using crystal field theory [19]. The pressure-induced change in the energy (E) of the Mn2+ emission band of ZnS : Mn is a function of the crystal field strength Dq and the Racah parameter B only [18,19], assuming that the Racah parameter ratio C/B is independent of pressure [19], dðDq Þ dE 1 dB ¼d ; þ ðE ÿ Dq dÞ dP B dP dP

ð1Þ

36B2 ðE3 ÿ 10B ÿ 5CÞ ; ð19B þ 7C ÿ E3 Þ

ð6Þ

where E2=E(4T2)ÿE(6A1) and E3=E(4T1)ÿ E(6A1). From the excitation spectrum (middle, Fig. 3), we obtained the values of E1 ; E2 and E3 are equal to 2.69, 2.52 and 2.37 eV, respectively. Using these values, we calculated B; C, and Dq to be 551, 3246 and 600 cmÿ1, respectively. For bulk ZnS : Mn, dB=dP, dðDq Þ=dP are ÿ3.48 and 18.2 cmÿ1/GPa, respectively [34]. From Eq. (1), we obtain dE=dP  ÿ230.6 cmÿ1/GPa= ÿ30.4 meV/GPa, which is consistent with the experimental value. This result demonstrates that the pressure behavior of Mn2+ emission in ZnS nanoparticles can be explained by crystal field theory. The fact that the dB=dP and dðDq Þ=dP used in the calculation are the bulk values suggests that dB=dP or dðDq Þ=dP does not vary much in bulk and in nanostructured ZnS : Mn. From the pressure behavior of optical properties, useful information on the interactions between the relevant electronic states of the center and its surrounding lattice ions can be obtained. The absolute value of the pressure coefficient can reflect the strength of the interaction between the emitter and the host or the strength of the crystal field effect on the emitter [21]. Larger pressure coefficient of an emitter normally indicates a stronger interaction between the emitter and the host. Due to size effect, the overlap of the electron and hole wavefunctions and the exciton binding

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energy increase in nanoparticles [1–3]. This increase will influence the exciton recombination rate, emission efficiency and energy. Thus, the luminescence pressure coefficient should be different in nanoparticles and in bulk solids. However, this estimated variance has not been observed yet. This is probably due to the surface effect, because when electrons or holes are trapped at the surface defect sites or surface states, the overlap decreases [3]. Due to the competition between the surface and the confinement, the overall properties, like the carrier relaxation time [35], are only weakly dependent on size. This is also likely the same reason for the weakly size dependence of luminescence pressure coefficient in nanoparticles. In doped nanoparticles, the dopant emission energy is mainly determined by the crystal field and the electron–phonon coupling [27]. We have observed the shift of Mn2+ emission to higher energies as the particles are smaller in size [27]. The predominant factor for the shift is the phonon coupling, whose strength is size-dependent and is determined by both the size confinement and the surface modification of the nanoparticles. The crystal field strength is decreased with the decreasing of the particle size [27]. However, the change in the crystal field has little contribution to the emission shift of Mn2+ in ZnS : Mn nanoparticles. In this case, we would like to suggest that the pressure dependence of the Mn2+ emission energy is mainly related to the electron–phonon interaction. In the 4.5-nm-sized ZnS : Mn nanoparticles the surface is even a little more significant than the confinement in determining the electron–phonon interaction [27]. The electron–phonon coupling strength (the Huang–Rhys parameter), measured from the Stokes shift, is 3.18, which is larger than that in bulk ZnS : Mn (3.09) [27]. This indicates that the electron–phonon interaction in this nanoparticle sample is a little stronger than that in bulk ZnS : Mn. This suggests that the change of the phonon coupling strength under pressure is a little faster than that in bulk ZnS : Mn, because a stronger interaction (electron–phonon coupling) will cause a larger pressure coefficient as described in Ref. [21]. As a consequence, the pressure shift of the Mn2+ emission energy or the luminescence pressure coefficient of the nanoparticle (ÿ33.3 meV/

GPa) is a little larger than that of bulk ZnS : Mn (ÿ31 meV/GPa). A large enhancement in the emission of Mn2+ and a lifetime shortening from ms in bulk ZnS : Mn to ns in ZnS : Mn nanoparticles have been reported [6]. This is attributed to the enhancement in the coupling of the sp electrons of the ZnS host to the d electrons of Mn2+ in nanostructures [6,13]. This enhancement should be observed in the pressure– luminescence measurement. However, the pressure behavior of the Mn2+ luminescence in ZnS : Mn nanoparticles of 4.5 nm suggests that the strength of the interaction between sp electrons of the host and the d electrons of Mn2+ in the nanoparticles under study is almost the same as that in bulk ZnS : Mn. Little enhancement is observed in the coupling of the sp electrons of the ZnS host to the d electrons of the Mn impurity in the 4.5 nm sized ZnS : Mn2+. This is probably due to the fact that the quantum confinement in the particles is too weak. Because the average size of the nanoparticles is 0.1 nm larger than the exciton Bohr diameter (4.4 nm [36]). According to quantum theory, the quantum confinement in this size region is very weak [37]. Thus the enhancement in the coupling of the sp electrons of the ZnS host to the d electrons of the Mn impurity is very small and is therefore not easy to be observed in the pressure–luminescence measurement.

4. Summary We have measured the PL of the Mn-doped ZnS nanoparticles under hydrostatic pressure at room temperature. The emission position shifts to the low energy at a rate of dE=dP¼ ÿ33:3  0:6 meV=GPa, which is in good agreement with the calculated result of ÿ30.4 meV/GPa using crystal field theory. The pressure coefficient of the Mn2+ related fluorescence in ZnS nanoparticles is just a little higher in value than that in the bulk material. This is attributed to the enhancement in the electron–phonon interaction in the nanoparticles. No evidence is found in our pressure–luminescence measurement for the enhancement in the interaction between the sp

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electrons of the host and the d electrons of Mn2+ in the nanoparticles under study.

Acknowledgements The authors would like to thank the Swedish Natural Science Research Council (NFR), Foundation For Strategic Research (SSF) of Sweden and the National Natural Science Foundation of China for financial supports. W.C. is grateful to the exchange program between China and Sweden and a fellowship from the Centre for Chemical Physics, The University of Western Ontario.

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