Brighter glow in ZnS nanocrystals with polarized light

Superlattices and Microstructures 43 (2008) 132–140 www.elsevier.com/locate/superlattices

Brighter glow in ZnS nanocrystals with polarized light Santa Chawla ∗ , N. Karar, Harish Chander National Physical Laboratory, Dr. K.S. Krishnan Road, New Delhi 110012, India Received 1 June 2007; received in revised form 24 July 2007; accepted 31 July 2007 Available online 14 September 2007

Abstract No known reports exist on luminescence enhancement under polarized light excitation. In this study, ZnS nanocrystals have been observed to produce brighter luminescence when excited by polarized light. ZnS:Mn bulk and nanocrystals have shown fivefold to tenfold increase in photoluminescence (PL) intensity when excited with linearly polarized light at 305 nm and 340 nm. Luminescence enhancement to a lesser degree was observed with linearly polarized light excitation for ZnS:Cu, Al and ZnS:Ag, Al nanocrystals. The observations suggest emission intensity dependence on the degree of anisotropy, which could be correlated mainly with the symmetry of the luminescence center and also to a lesser extent with nanoparticle shape asymmetry. c 2007 Elsevier Ltd. All rights reserved.

Keywords: Optical properties of nanocrystals; Photoluminescence; Polarized light; Luminescence enhancement

1. Introduction ZnS in both bulk and nanocrystalline forms has been used as a phosphor very effectively in display devices and the quest for getting brighter output in the visible range by doping with different impurities has been an important field of research [1,2]. Doped nanocrystals of ZnS have been a topic of intensive research for their color tunability and better luminescence efficiency leading to a wide variety of applications as light emitters in the UV and visible. In the present work, a novel way to obtain a brighter luminescence glow has been found, mainly by excitation of ZnS nanocrystals with polarized light. PL studies are usually done under unpolarized excitation. Increase in the luminescence intensity and change in emission characteristics by excitation with

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E-mail address: [email protected] (S. Chawla). c 2007 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2007.07.034

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polarized radiation, to the best of our knowledge, for common phosphors and their nanocrystals (NC), have not been reported so far. The present work analyzes the effect of polarized light excitation on the PL of cubic bulk ZnS:Mn and doped nanocrystals (DNC) of ZnS:Mn, ZnS:Cu, Al and ZnS:Ag, Al. Past work in this area involved analysis of polarization of the emitted light for cubic ZnS single crystal [3,4], porous Si nanocrystals [5], Si nanowire [6], Cd chalcogenides [7], ZnO and InP nanowires [8,9]. Polarization of photoluminescence may also indicate symmetry of luminescence centers and relaxation of excited carriers in phosphors and the angular distribution of nanowire orientation [3,9]. 2. Experiments Nanocrystalline ZnS as free-standing powder was prepared by the wet chemical synthesis route at room temperature with zinc acetate and sodium sulfide as the precursor solutions and acetate solution of the dopant, e.g., Mn [10]. The crystal structure and crystallite size of the ZnS nanocrystals was determined by means of powder x-ray diffraction using a Brucker –AXS D8 Advance Diffractometer. The morphology and crystalline shape was studied by scanning electron microscopy (SEM, LEO 440 system). Room temperature PL measurements on the powder samples were made using a Perkin-Elmer LS55 Luminescence spectrometer with a Xe lamp source. Polarizers could be inserted at both the excitation and emission sides between the sample and the respective monochromators. Polaroid film type polarizers, supplied by Perkin-Elmer, were used. The film material was adequately transparent from 300 nm to the near IR range and the polarizers were usable between 300–750 nm and produced 100% linearly polarized light without any other polarization component, in that range. The luminescence showed a negligible vertically polarized (vp) component under unpolarized (up) or horizontally polarized (hp) excitation. The time resolved decay of the luminescence was measured with an Edinburgh Instruments FLSP920 lifetime spectrometer with a µs pulse xenon lamp or ns pulse arc lamp. 3. Results X-ray diffraction patterns of ZnS nanocrystals (Fig. 1) showed cubic zincblende structure with (111) as the most prominent peak. The broadness of the observed XRD peaks indicates small crystallite size. The average crystallite size estimated from the Scherrer formula is about 2–4 nm. SEM micrographs (Fig. 2) of the doped ZnS nanocrystals reveal that some ZnS nanocrystals are elongated in shape. There may be agglomeration of particles as no capping was done. ZnS bulk and nanocrystals were excited with polarized and unpolarized light at two wavelengths, 340 nm (3.64 eV) and 305 nm (4.08 eV). The PL emission spectra are shown for ZnS:Mn nanocrystal and bulk ZnS:Mn excited at 340 nm [Fig. 3(a) and (b) respectively], excited at 305 nm [Fig. 4(a) and (b) respectively], ZnS:Cu, Al and ZnS:Ag, Al nanocrystals all excited at 305 nm [Fig. 5(a) and (b) respectively]. The nomenclature indicates the state of polarization of the exciting light and emitted light; e.g., ‘hp–up’ signifies horizontally polarized excitation and detection of unpolarized emission. Under exactly the same experimental conditions, the PL output of ZnS:Mn was more with 340 nm excitation compared to 305 nm excitation for the 595 nm peak; the PLE at 595 nm showed a strong peak at 340 nm. Excitation by linearly polarized light (hp) produced brighter luminescence for all ZnS phosphors. For ZnS:Mn, the prominent PL peak at 595 nm (orange) was related to the internal electronic transitions of Mn2+ and a smaller peak at 430 nm (blue) was related to what was termed as self-activated (SA) luminescence [1]. However, when ZnS:Mn nanocrystals were excited with

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Fig. 1. XRD patterns of (a) ZnS:Mn and (b) ZnS:Cu, Al nanocrystals.

Fig. 2. SEM micrographs of (a) ZnS:Mn and (b) ZnS:Cu, Al nanocrystals.

polarized light (hp) at 340 nm [Fig. 3(a)] and 305 nm [Fig. 4(a)], the effect was a striking enhancement of the blue peak by 10.5 times compared to that for unpolarized (up) excitation and

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Fig. 3. (Color online) PL spectra of (a) ZnS:Mn nanocrystal, (b) ZnS:Mn bulk phosphor excited at 340 nm. The legend indicates the polarization state of the exciting light and emitted light; e.g., hp–up signifies excitation by horizontally polarized (hp) light and detection of unpolarized (up) emission.

the blue peak became stronger than the orange peak. PL spectra of ZnS:Cu, Al and ZnS:Ag, Al could be deconvoluted into two peaks and the blue SA luminescence showed more luminescence amplification (3.5 times) under ‘hp’ excitation compared to the blue/green peak corresponding to Ag/Cu emission (2 times). The results for all ZnS phosphors are listed in Table 1. For all the samples, there is invariably a blue shift with linearly polarized ‘hp’ excitation compared to unpolarized ‘up’ excitation up to 20 nm for the blue peak, whereas very little peak shift (±3 nm)) for ‘hp’ excitation for the orange peak was observed. This indicated that the emission process for the blue peak under ‘hp’ excitation involved states having larger energy separation than that under ‘up’ excitation and hence interplay of selection rules amongst states might be playing a role.

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Fig. 4. (Color online) PL spectra of (a) ZnS:Mn nanophosphor, (b) ZnS:Mn bulk phosphor excited at 305 nm. Note that the blue emission becomes stronger than the Mn related orange emission for nanocrystal ZnS:Mn under hp excitation.

Time resolved decay of the luminescence was recorded for ZnS:Mn nanocrystals at each peak emission wavelength 430 nm and 595 nm for ‘up’ excitation at 305 nm and 340 nm, respectively. The decay curves [Fig. 6] clearly indicated that while the 430 nm component almost completely decayed in 70 µs, the 595 nm component did so in 11 ms. For the decay curves as shown in Fig. 6, a biexponential fit of the blue emission (430 nm) decay gave decay times of 2.8 µs (13%) and 19 µs (87%), whereas the initial part of the orange emission (595 nm) showed decay times of 1 µs (8%) and 12 µs (92%). The decay time indicates the time taken from the beginning of the decay to a level of about 37% of the original PL intensity. Time resolved decay measurement for ZnS:Mn on the 20 ms scale gave a lifetime of the longer decaying component as 1.6 ms which matches well with the reported value for cubic ZnS:Mn and the long lifetime was supposed to

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Fig. 5. (Color online) PL spectra of (a) ZnS:Cu, Al, (b) ZnS:Ag, Al nanocrystal excited at 305 nm. Horizontally polarized (hp) excitation enhances the blue luminescence emission considerably.

be due to the spin and parity forbidden transition [1,11]. Though decay times in nanoseconds were reported [2], our ZnS:Mn nanophosphors showed no exponential decay in the nanosecond region, for both peak emission wavelengths. 4. Discussion The study revealed two main effects for ZnS nanocrystals: (i) luminescence (PL) enhancement occurred under polarized excitation and (ii) luminescence enhancement of certain PL peaks depended critically upon the wavelength and polarization of the exciting light.

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Table 1 PL data for ZnS nanocrystal and bulk crystal under different polarized excitations Phosphor

Ex. λ (nm)

Ex. pol.

Em. pol.

Peak 1 λ (nm)

Peak 1 int. (a.u.)

305

up hp hp

up up hp

458.5 429.5 432.0

48.6 131.4 42.6

2.7

340

up hp hp

up up hp

455.0 447.5 444.0

72.6 121.0 41.0

1.7

305

up hp hp

up up hp

469.0 444.0 443.5

32.5 99.1 36.0

3.1

340

up hp hp

up up hp

458.0 447.5 450.0

16.0 44.2 15.7

2.8

305

up hp hp

up up hp

436.0 423.0 425.5

4.2 43.6 14.0

340

up hp hp

up up hp

434.5 421.0 426.0

305

up hp hp

up up hp

443.0 423.0 426.5

340

up hp hp

up up hp

441.0 420.0 425.5

ZnS:Ag, Al nanocrystal

ZnS:Cu, Al nanocrystal

ZnS:Mn nanocrystal

ZnS:Mn bulk

Peak 2 λ (nm)

Peak 2 int. (a.u.)

10.5

594.5 595.5 599.0

15.8 28.2 9.5

1.8

10.4

594.0 593.0 597.0

23.6 41.2 15.2

1.7

6.2 31.8 10.7

5.1

580.5 578.0 583.5

27.2 35.3 12.5

1.3

2.2 5.5 1.6

2.5

580.5 583.5 584.0

43.6 48.6 17.0

1.1

0.92 9.6 3.0

Peak 1 LE

Peak 2 LE

Luminescence enhancement (LE) = PL (hp exc.)/PL (up exc.), emission detected unpolarized.

In the present case, comparing the data, linearly polarized ‘hp’ excitation produced enhancement of PL but the emission was partly polarized in the plane of incidence, as the vertically polarized PL was negligible. The luminescence enhancement was maximum for the blue emission. Emitted intensity parallel to the incident polarization Ik was about 30–40% of the total emission, which was also in the plane of incidence. For ZnS phosphors, PL is reported to be produced by three mechanisms: blue self-activated (SA) luminescence due to donor–acceptor pair (DAP) recombination between a shallow donor and V2+ Zn acceptor at the nearest neighbour site; green/blue–green luminescence of ZnS:Cu, Al and ZnS:Ag, Al due to DAP recombination between the AlZn donor and CuZn /AgZn acceptor in various DA pairs having different intrapair distances; orange luminescence due to internal electronic transitions of the dopant, e.g., the Mn2+ 4 T1 –6 A1 transition [1–4]. Whereas the SA center has a characteristic symmetry, ZnS:Cu, Al and ZnS:Ag, Al in the zincblende structure have no such characteristic symmetry. Moreover, orientation of the center responsible for luminescence and charge transfer to the center as governed by selection rules would also be deciding factors for the PL output and its polarization state. The polarization dependence of PL is usually related to the anisotropy of either the crystalline structure or the luminescence center, or shape anisotropy as in the case of nanowires. XRD

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Fig. 6. (Color online) Time resolved decay of ZnS:Mn nanocrystal under ‘up’ excitation. The legend indicates the excitation wavelength and emission wavelength at which decay measurement was done. It can be seen that decay is much faster for blue (430 nm) luminescence compared to Mn related orange (595 nm) emission.

patterns for ZnS nanocrystals and bulk crystal synthesized for the present study exhibited cubic zincblende structure. For cubic ZnS, as the maximum polarization dependence of the luminescence was observed for the blue SA center, the anisotropy appears to be due to the symmetry of the luminescence center. The ZnS4 tetrahedra have unique orientation along the h111i direction, which is also the growth direction. The Zn and S ions of opposite polarity are visualized as forming a network of permanent dipole moments. Such a luminescence center has been described for ZnS:Cl single crystals for explaining the polarization dependent luminescence properties [3,4] of blue self-activated luminescence. The center has characteristic symmetry, which is lower than the symmetry of the host lattice. Such a center was proposed as a Zn2+ vacancy with an associated charge compensating donor center at the nearest neighbour site [1], e.g., Al+ coactivator in ZnS:Cu, Al or ZnS:Ag, Al or a shallow donor level for ZnS:Mn. Polarization of luminescence related to the SA center in ZnS:Al crystals has been explained by considering dipole transitions depending upon the symmetry of the center [1]. In our study, the XRD showed a strong (111) peak suggesting that many crystallites were oriented along the (111) axis. For PL measurements, the powder sample was kept in a holder, sandwiched tightly between a back metal plate and a front fused silica cover plate. Most of the grains at the top surface which are responsible for PL emission lie aligned against the cover plate with their h111i direction along the plate, i.e., in the X Z plane. In the experimental geometry, the sample holder is fixed vertically and the excitation beam coming out of a horizontal slit traveling on a horizontal plane falls on

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the sample. In the ‘hp’ configuration, the polarization direction is along the Y -axis, X Y being the horizontal plane. Thus the ‘hp’ polarization is perpendicular to the sample surface plane (cover plate). Hence most of the ZnS grains which lie along the surface of the plate would find their h111i direction perpendicular to the ‘hp’ polarization and the SA center would get excited and produce enhanced luminescence. For ‘up’ excitation, there would be some light whose electrical vector lies in the X Y plane at various angles to the sample surface plane (cover plate) and thus could excite grains lying with their h111i directions at different angles to the cover plate. The number of such grains would be less and emission caused by ‘up’ excitation is less. The doped nanocrystals under study have size below the exciton Bohr diameter (5 nm) [2] and luminescence enhancement by polarized light excitation in nanocrystals was observed to be more for bulk crystals. In elongated cubic NCs of size at the quantum confinement limit, carrier confinement has to be considered. In such a case, due to splitting of the ground state of holes into two doublets [7], band edge optical transition occurs with polarized light. Selection rules for the transition from the hole doublet states to the first conduction band state are the same for the bulk and NCs. After excitation, free charges would migrate to other defect states before producing luminescence by recombination. In such a case the original memory of the preferred excitation by polarized light would be entirely lost and the emitted luminescence would be partly polarized, which in the present case is observed to be in the horizontal plane. In addition to the characteristic symmetry of the luminescence center, for elongated NCs optical anisotropy may be more effective due to asymmetric dimensions, as SEM suggested elongated shapes of some of the nanocrystals under study. Hence in a macroscopically isotropic ensemble of NCs, anisotropic optical properties of NCs could be revealed under linearly polarized excitation. 5. Conclusions The results suggest that the observed polarization dependent luminescence enhancement for ZnS nanocrystals could arise mainly due to preferential excitation of particular defect centers of characteristic symmetry by polarized light and also nanocrystal shape asymmetry to a lesser extent. Stronger luminescence enhancement for self-activated luminescence compared to Mn impurity emission suggests that the symmetry of the luminescence center plays an important role. Whereas the intrinsic defects responsible for self-activated luminescence are associated with the tetrahedral sites, Mn ions emitting at 595 nm may be in the octahedral sites [11]. The above results suggest that brighter glow, as well as preferential enhancement of blue luminescence in ZnS, is possible with polarized excitation. Therefore, polarized excitation could be a practical tool for luminescence enhancement in display devices employing ZnS nanocrystals. Such nanocrystals may also be effectively used as a polarization sensitive photodetector. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

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