Nonlinear optical property and fluorescence quenching behavior of PVP capped ZnS nanoparticles co-doped with Mn2+ and Sm3+

Journal of Luminescence 166 (2015) 167–175

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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin

Nonlinear optical property and fluorescence quenching behavior of PVP capped ZnS nanoparticles co-doped with Mn2 þ and Sm3 þ S. Prasanth a, P. Irshad a, D. Rithesh Raj a, T.V. Vineeshkumar a, Reji Philip b, C. Sudarsanakumar a,n a b

School of Pure and Applied Physics, Mahatma Gandhi University, Kottayam, Kerala 686562, India Optics group, Raman Research Institute, C.V. Raman Avenue, Bangalore 560080, India

art ic l e i nf o

a b s t r a c t

Article history: Received 12 May 2014 Received in revised form 12 May 2015 Accepted 14 May 2015 Available online 27 May 2015

ZnS nanoparticles co-doped with different percentages of Mn2 þ and Sm3 þ were synthesized by the chemical co-precipitation method using polyvinylpyrrolidone (PVP) as capping agent. Cubic zinc blende phase of the samples was confirmed from X-ray diffraction. The strong interaction between PVP and ZnS nanoparticles was studied from Fourier Transform Infrared (FTIR) spectrum. The band gap values of ZnS and co-doped ZnS nanoparticles were calculated from UV‐Visible spectra. The photoluminescence spectra of pure ZnS nanoparticles showed an emission at 436 nm and when doped with Mn2 þ and Sm3 þ an extra peak with high intensity was observed at 596 nm. On increasing the mole percentage of dopants the intensity of the extra peak showed an enhancement until a certain concentration and then a reduction with further increase in concentration. The binding parameters were determined by Stern‐ Volmer relation. The nonlinear absorption coefficients of the doped and undoped samples were calculated using Z-scan technique. & 2015 Elsevier B.V. All rights reserved.

Keywords: PVP ZnS nanoparticles Fluorescent quenching Three photon absorption

1. Introduction Zinc sulfide is a II–VI compound semiconductor with direct and wide band gap ( 3.6 eV) [1]. It has traditionally shown remarkable versatility, novel fundamental properties and diverse applications such as field emitters, field effect transistors (FETs), p-type conductors, catalyzators, UV-light sensors, chemical sensors (including gas sensors), biosensors, nonlinear optic devices and nanogenerators [2]. Optically active luminescence centers doped nanocrystals create new opportunities for luminescent study [3] and the luminescent property of the ZnS nanoparticles are highly dependent on the doped ions [4]. In the case of nanoparticles a large number of surface defects are present, which act as a nonradiative pathway for the excited electrons and become unfavorable to the luminescent properties of the nanocrystals. In order to overcome this difficulty, one can use an organic or inorganic material to cap the surface of the nanoparticles [5,6]. ZnS nanostructural materials have been prepared using various physical and chemical methods with a view of their commercial or potential applications. The chemical precipitation method is generally used for the synthesis of II–VI semiconductor nanostructures and this n Corresponding author. Tel.: þ 91 481 2731043, þ91 9447141561 (mobile); fax: þ91 481 2730423. E-mail address: [email protected] (C. Sudarsanakumar).

http://dx.doi.org/10.1016/j.jlumin.2015.05.028 0022-2313/& 2015 Elsevier B.V. All rights reserved.

method has a number of advantages including easy processability at ambient conditions, possibility of doping different kinds of impurities with high doping concentrations even at room temperature, good control over the chemistry of doping and easiness of surface capping with a variety of different steps involved in the synthetic process of nanoparticles [7]. In the present work we synthesized ZnS nanoparticles capped with polyvinylpyrrolidone (PVP) by the chemical precipitation method and further doped with manganese and samarium at different concentrations (5, 10, and 15 wt%). Investigation on the optical non linearity and fluorescence quenching behavior of codoped ZnS nanoparticles were carried out.

2. Experimental Starting materials for the synthesis of ZnS nanoparticles were zinc nitrate hexahydrate (Merck), sodium sulfide (Merck) and polyvinylpyrrolidone (CDH). All the reactants were 99.9% pure and used without further purification. PVP was used as the dispersant that adsorbs the single colloidal particle to form a molecular folium to prevent the particle from coalescing. Aqueous solutions of 0.5 M zinc nitrate, 0.5 M sodium sulfide and 10 wt% of PVP were prepared in deionized (DI) water. Into this PVP, equal amounts of zinc nitrate and sodium sulfide were added under vigorous stirring at 70 °C for two hours. A white precipitate

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of ZnS-PVP was formed, then washed and filtered with DI water and isopropyl alcohol several times to remove the excess organic residues. The collected sample was dried and stored in the desiccator for further characterization. For doping ZnS nanoparticles with manganese and samarium, we selected manganese (II) chloride tetrahydrate (CDH) and samarium nitrate (CDH). Equal amounts of each manganese and samarium were dissolved in DI water separately to form 5, 10, 15, 17, and 20 wt% solutions and were later added to 10 wt% PVPþ 0.5 M Zn(NO3)3 and then sodium sulfide was added to this solution and stirred vigorously at 70 °C for two hours. A white precipitate of ZnS-PVP/Mn,Sm was formed. The sample was then washed and filtered with DI water and isopropyl alcohol several times to remove the excess organic residues.

3. Results and discussion 3.1. X-ray diffractometry To study the crystalline nature of the samples and the particle size, X-ray diffraction study was performed (PANalytical X-ray diffractometer) with Cu Kα radiation (λ ¼1.54 Å). The diffraction pattern of the samples (ZnS doped at 5, 10, and 15 wt%) are shown in Fig. 1. The three diffraction peaks corresponding to (1 1 1), (2 2 0), and (3 1 1) planes confirm the zinc blende crystal structure of the particle (JCPDS 800020). In all the XRD patterns, broadening of the peaks indicates the nanocrystalline nature of the samples. No additional peak due to impurity phases was observed in the XRD patterns of the doped samples, indicating that the dopant concentration has no effect on the crystalline phase. The average crystalline size of the samples were estimated according to the Debye–Scherrer formula D¼0.9λ/β wcos θ, where λ is the wavelength of X-ray, β is the full width at half maximum of the diffraction peaks in radians. Based on these, the average crystalline sizes estimated are given in Table 1. 3.2. Transmission Electron Microscopy (TEM) The transmission electron microscopic images of ZnS doped with 10% Mn2 þ and Sm3 þ are recorded using TEM (JEOL JEM 2100) and are shown in Fig. 2. It shows the presence of isolated and spherical grains made of ZnS–Mn2 þ , Sm3 þ nanoparticles coated by polymer (PVP) covers, with an average particle size of

Fig. 1. XRD pattern of ZnS nanoparticles and ZnS nanoparticles co-doped with Mn and Sm.

Table 1 Crystalline sizes calculated from (1 1 1) plane of XRD data. Sample

2θ (deg) d Spacing (Å)

hkl

FWHM (β) (deg)

Particle size (nm)

ZnS ZnS–Mn,Sm (5%) ZnS–Mn,Sm (10%) ZnS–Mn,Sm (15%)

29.080 28.969

3.071 3.082

(1 1 1) 0.945 (1 1 1) 1.102

9 7

29.305

3.045

(1 1 1) 2.304

4

29.252

3.051

(1 1 1) 1.728

5

4 nm and are consistent with the particle size calculated from XRD. The reason for the appearance of aggregates of nanoparticles is due to the static attraction of their surface groups [8]. 3.3. FTIR spectroscopy FTIR spectroscopy was carried out to understand the formation of ZnS-PVP nanoparticles. FTIR spectrum for PVP, PVP-ZnS and PVP-ZnS/Mn,Sm (5%) nanoparticles were recorded on a Shimadzu (model: IR Prestige 21, with ZnSe ATR crystals) spectrometer and are shown in Figs. 3 and 4. In pure PVP, the C ¼O stretch band is observed at 1658 cm  1 and it is red shifted to 1642 cm  1 in ZnS-PVP nanocomposites, indicating a strong interaction between ZnS nanoparticles and C¼ O of PVP [9]. The other bands observed in the FTIR spectrum are listed in Table 2 [10]. It is inferred from Fig. 4 that by the addition of dopants the bands are shifted to lower wavelength side this indicating the presence of dopants (Mn,Sm) in the host matrix. 3.4. UV‐Vis‐NIR Absorption spectroscopy The absorption spectra of ZnS-PVP were recorded using Schimadzu 2401 UV‐Vis Spectrophotometer in the wavelength range of 200–800 nm and are shown in Fig. 5. The band gap energy of the ZnS nanoparticles can be evaluated from the UV‐Vis spectra by using the relation

(

αhν = A hν − E g

)n

where, α is the absorption coefficient, hυ is the incident photon energy, A is a constant and Eg is the band gap energy of the material. The exponent n depends on the type of the transition. Here, the transitions are direct so we take n ¼1/2. The band gap energy is calculated by extrapolating the linear portion of the (αhν)2 vs hν graph on the hν axis to α ¼0 as shown in Fig. 6. Calculated band gap values of doped and undoped ZnSPVP nanoparticles are listed in Table 3. The band gap value of undoped ZnS nanoparticles (4.3 eV) was higher than that of the bulk ZnS (3.6 eV). It is found that on increasing the percentage of dopants the band gap value decreases. The reduction in the band gap is due to the dopant related levels introduced in the host levels. The reduction in the band gap energy due to the introduction of defects in a material can be established by Urbach energy [11]. The equation for the Urbach energy is given by α = αe E / Eu where α is the absorption coefficient , E is the photon energy and Eu is the Urbach energy. The Urbach energy is calculated by plotting ln α vs E. The Urbach energy of each sample is shown in Fig. 7. The Urbach energy values of pure ZnS and 5, 10, 15 wt% of Mn and Sm co-doped ZnS were 0.776, 1.303, 1.533, and 1.956 eV respectively. The increase in Urbach energy is due to the impurity levels introduced by doping in the band structure of ZnS nanoparticles [12].

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169

Fig. 2. TEM micrographs of PVP capped ZnS nanoparticles doped with 10% of Mn and Sm.

Fig. 4. FTIR spectra of (a) ZnS-PVP and (b) ZnS/Mn,Sm (5%)-PVP. Fig. 3. FTIR spectra of PVP.

3.5. Photoluminescence spectroscopy The photoluminescence (PL) spectra of the doped and undoped ZnS-PVP nanoparticles were recorded using Fluoromax-4 spectrophotometer at an excitation wavelength of 350 nm. The photoluminescence spectrum of ZnS nanoparticles shows an emission at 436 nm (Fig. 8). The photoluminescence spectrum of Mn2 þ (5 wt% ) doped ZnS nanoparticles (Fig. 9) at an excitation wavelength of 350 nm shows two emission peaks, one at 436 nm and the other at 583 nm. The peak observed at 583 nm is attributed to the Mn emission in the ZnS host lattice, similar yellow emission was reported [13]. This emission is due to the transition from 4T1 to 6A1 level of Mn2 þ ions in ZnS host. Fig. 10 is the emission spectra of ZnS co-doped with Mn2 þ and Sm3 þ (5, 10, 15, 17, and 20 wt%) at an excitation wavelength of 350 nm showing high intensity peak at 596 nm. It is observed that

Table 2 FTIR assignment of ZnS-PVP and ZnS/Mn,Sm-PVP. ZnS-PVP

Band assignment

3221 cm  1 1642 cm  1 1534 cm  1 1290 cm  1 1123 cm  1

O–H stretching C ¼ O stretch C–H bending C–N stretching C–O–H stretching

incorporation of Sm into Mn doped ZnS, the emission wavelength at 583 nm is red shifted to 593 nm. This red shift is attributed to the energy transfer from Mn2 þ ions to Sm3 þ . Since Sm ions are not excited directly by the exciting radiation there is no back transfer energy from Sm to Mn ions. The energy levels of Sm3 þ ions are closer to that of Mn2 þ ions and are shown in Fig. 11. Mn2 þ

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ions transfer energy to Sm3 þ ions and an orange emission occurs from Sm3 þ ions through 4G5/2-6H7/2 transition [14]. Fluorescence lifetime measurements were carried out with an excitation wavelength of 390 nm using a picosecond diode

(Spectra-NanoLED source S-390). The effects of various concentrations of dopants (Mn2 þ , Sm3 þ ) on the lifetime of ZnS-PVP are listed in Table 4. The luminescence decay curves are shown in Fig. 12. The increase in dopant ion concentration causes a change in the luminescence decay of the host, ZnS. From this the non-radiative energy transfer to the dopant ion is evident with ZnS acting as donor of energy and the dopants (Mn and Sm) acting as acceptors [15]. Also, on increasing the mole percentage of dopants the peak intensity at 596 nm increases and beyond a particular percentage (10 wt%) quenching behavior is observed (Fig. 10). It has been considered that the concentration quenching is caused by the transfer of recombination energy of excitons to the induced defect levels. On increasing the concentration of dopants the number of energy transfer process between the initially absorbing impurity ions to another identical ions increases through non-radiative Table 3 Bandgap values of doped and undoped ZnS-PVP. Sample

Bandgap (eV)

ZnS ZnS–Mn,Sm (5%) ZnS–Mn,Sm (10%) ZnS–Mn,Sm (15%)

4.3 3.4 3.2 3.0

Fig. 5. Absorption spectra of doped and undoped ZnS-PVP.

Fig. 6. Tauc plot of ZnS and Co-doped ZnS nanoparticles.

S. Prasanth et al. / Journal of Luminescence 166 (2015) 167–175

171

Fig. 7. Urbach energy of ZnS and co-doped ZnS nanoparticles. 3000000

ZnS-Mn 2500000

Intensity

2000000

1500000

1000000

500000

350

400

450

500

550

600

650

Wavelength(nm) Fig. 9. PL spectra of Mn (5 wt%) doped ZnS-PVP. Fig. 8. PL spectra of ZnS-PVP.

transition. After a number of such processes, a transfer to a quenching site (defect) may become involved, thus further decreasing the radiative process [16,17]. The fluorescence quenching generally occurs by two mechanisms, dynamic and static, depending on the way of interaction between the acceptor and the donor [18,19]. The fluorescence quenching data at different quencher concentrations were analyzed by the well-known Stern–Volmer equation [20–22].

F0 = 1 + K sv [Q ] = 1 + k q τ0 [Q ] F F0 and F are the fluorescent intensities observed in the absence and the presence of quencher. K sv is the Stern–Volmer quenching constant and [Q] is the concentration of quencher, k q is the

quenching rate constant, τ0 is the average lifetime of the molecule without any quencher. The Stern–Volmer plot of F0/F vs [Q] is presented in Fig. 13. The slope of the plot after linear regression yields the Stern–Volmer constant. The value of K sv , and k q are given in Table 5. 3.6. Nonlinear optical characterization The open-aperture Z-scan [23] experiment was used to measure the nonlinear optical properties of the samples suspended in 2-propanol. Here a laser beam of wavelength 532 nm is used for sample excitation, and its propagation direction is considered as the z-axis. The beam is focused using a convex lens of focal length 18 cm, and the focal point is taken as z ¼0. The beam has maximum energy density at the focus, which symmetrically reduces toward either side of it on the z-axis. In the experiment, the

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sample is placed in the beam at different positions with respect to the focus (i.e., scanned along the z-axis), and the corresponding transmissions are measured. The position transmission curve thus obtained is known as the open-aperture Z-scan curve. From this curve, the nonlinear absorption coefficient of the sample can be calculated. The sample suspensions were prepared such that all of them had the same linear transmission of 85% at 532 nm. Of the two pyro electric energy probes (Rj7620, Laser Probe Inc.) used, one monitored the input energy, while the other monitored the energy transmitted through the sample. The frequency-doubled output (532 nm) of a Q-switched Nd:YAG laser (Minilite,Continuum Inc.) was used for exciting the samples. The pulse width of the laser pulses was about 5 ns. The laser pulse energy used for the experiments was 100 mJ. Fig. 14 shows the open aperture Z-scan transmittance of the samples and is found that the experimental data fits accurately with transmission equation of three photon absorption (3PA) processes. Open circle represents the experimental data points and solid curves represent the theoretical fit. The nonlinear transmission equation for 3PA process is given by the equation [24]

(1 − R ) exp ( − αL) ∫ 2

T=

√ π p0

ln

(

(

) ) dt

1 ± p02 exp − t 2

(1)

Here T is the transmission of the sample, R is the Fresnel reflection coefficient at the sample–air interface, α is the linear

absorption coefficient, and L is the sample length. p0 is given by

p

−1/2 ⎡ 2 ⎤ 0 = ⎣⎢2γ (1 − R)2I02 L eff ⎦⎥

(2)

where γ is the three-photon absorption coefficient and I0 is the incident intensity of the laser beam. Leff is given by the equation

L eff =

[1 − exp (−2αL ) ] 2α

(3)

The three photon absorption coefficient (γ) is obtained from the theoretical fitting of the Z-scan curve. The Z-scan curve gives the transmittance of the sample as a function of position. The values of γ for different samples are given in Table 6. From Table 6 it is clear that the value of three photon absorption coefficient (γ) is highest for ZnS co-doped with 10 wt% of Mn2 þ and Sm3 þ . This enhancement of nonlinear absorption is due to an increase in the number of energy transfer process between the initially absorbing impurity ions, to another identical ion through non-radiative transition which is confirmed by photoluminescence study as described above. In semiconductor nanomaterial direct 3PA is not possible at the intensities available from 532 nm laser pulses. Therefore a more probable mechanism here seems to be two photon absorption (2PA) induced excited state absorption, which is a sequential absorption process common in semiconductor materials. In addition, surface defects and surface dangling bonds may also contribute. The three photon absorption process leading to near excitonic excitation to the band edge in ZnS nanoparticles have been reported [25–27]. Chattopadhyay et al. [28] reported that the three photon absorption coefficient for Mn2 þ doped ZnS quantum dots was of the order of 10  24 m3/W2. The value for the effective three photon absorption coefficient for CdS nanowire is found to be in the order of 10  22 m3/W2 [26] and for other materials such as Bi12SiO20 (BSO) and Bi12GeO20 (BGO) the value of 3PA coefficient was of the Table 4 Fluorescence lifetime of doped and undoped ZnS nanoparticles.

Fig. 10. PL spectra of Mn,Sm doped (5, 10, 15, 17, and 20%) ZnS-PVP.

‹τ›a (s)

χ2

Sample

τ1

ZnS ZnS–Mn, Sm (5%) ZnS–Mn, Sm (10%) ZnS–Mn, Sm (15%)

3.72E  11 3.376 2.60E  10  0.2734

4.52E  09 1.54E–03 4.63E  10 0.3692

2.73E  10 1.892 6.08E  10 1.671

1.63E  10 8.41E–02

1.33E 09 2.76E–02

1.01E  09 1.22

a

3.64E  10

a1

τ2

a2

 301547 3.64E  10 3.02E þ 05 6.33E 10 0.986

‹τ› ¼ a1τ1 þ a2τ2, the magnitude of χ2denotes the goodness of the fit.

Fig. 11. Band diagram showing emission in ZnS co-doped with Mn and Sm.

S. Prasanth et al. / Journal of Luminescence 166 (2015) 167–175

173

Fig. 12. Fluorescence lifetime decay of (a) ZnS, (b) ZnS Mn,Sm 5%, (c) ZnS Mn,Sm 10%, and (d) ZnS Mn,Sm 15%.

order of 10  26 m3/W2 [29]. For Bi doped ZnO and Na doped ZnO nanoparticles the effective three photon absorption coefficient was found to be in the order of 10  24 m3/W2 [28,30]. From these values it is clear that the three photon absorption coefficient for pure ZnS and ZnS co-doped with Mn2 þ and Sm3 þ is higher than the reported values. 3.7. Conclusion

Fig. 13. Stern–Volmer plot.

Table 5 Binding parameters of ZnS nanoparticles co-doped with Mn and Sm.

K sv (M  1)

kq (M  1 S  1)

6.9019

6.833  109

Cubic ZnS nanoparticles co-doped with Mn2 þ and Sm3 þ , having average grain size of 6 nm, have been successfully synthesized by the chemical precipitation method using PVP as capping agent. FTIR spectra revealed the presence of PVP molecules on the surface of ZnS nanoparticles. The absorption edges of co-doped ZnS nanoparticles were red shifted with respect to the undoped ZnS nanoparticles. The band gap values of doped and undoped ZnS nanoparticles were determined from UV‐Vis spectroscopy. Undoped ZnS showed blue emission while ZnS co-doped with Mn and Sm showed orange emission. A rapid luminescence quenching with increasing dopant concentration was observed. The binding parameters were determined by Stern‐Volmer relation. Nonlinear optical transmission studies at 532 nm using 5 ns laser pulse showed the existence of three photon induced excited state absorption at this wavelength and the absorption coefficients were

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Fig. 14. Open aperture Z-scan of (a) ZnS, (b) ZnS Mn,Sm 5%, (c)ZnS Mn,Sm 10% and (d) ZnS Mn,Sm 15%.

Table 6 3PA coefficients of ZnS-PVP and Mn,Sm doped ZnS-PVP. Sample name ZnS ZnS Mn,Sm 5% ZnS Mn,Sm 10% ZnS Mn,Sm 15%

3PA coefficient (m3/w2)  22

2  10 3  10  22 13  10  22 4  10  22

W0 (μm)

Energy (μJ)

Pulse width (ns)

Linear transmission (%)

18 18 18 18

100 100 100 100

5 5 5 5

85 85 85 85

in the order of 10  22 m3/W2, indicating the possibility of potential applications in fabricating optical limiting devices.

Acknowledgment We thank the Department of Science and Technology, Government of India DST-PURSE (SR/S9/Z-23/2010)/22(C.G)) for financial support (Purse Programme) and one of the authors DRR is thankful to UGC-BSR for RFSMS fellowship.

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