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Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots
Materials Science in Semiconductor Processing ] (]]]]) ]]]–]]]
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Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots Elaheh K. Goharshadi a,b,n, Roya Mehrkhah a, Paul Nancarrow c a b c
Department of Chemistry, Ferdowsi University of Mashhad, Mashhad 91779, Iran Center of Nano Research, Ferdowsi University of Mashhad, Iran School of Chemistry and Chemical Engineering, Queen’s University Belfast, UK
a r t i c l e i n f o
Keywords: Zinc sulfide Nanoparticle Semiconductor compounds Photoluminescence
abstract A facile and rapid microwave irradiation method was developed to prepare ZnS nanoparticles (NPs) using a set of ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl) imide anion and different cations of 1-alkyl-3-methyl-imidazolium. The phases, structures, and optical absorption properties of the NPs were determined in depth with X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, UV–vis absorption spectroscopy (UV–vis), diffuse reflectance spectroscopy (DRS), and photoluminescence spectroscopy (PL). The average crystallite size of the ZnS NPs calculated from the XRD pattern was of the order of 2.8 nm which exhibits cubic zinc blende structure. The energy band gap measurements of NPs were carried out by UV and DRS. The results revealed that the ZnS NPs exhibit strong quantum confinement effect. The optical band gap energy increases significantly compared with those of the bulk ZnS. The refractive indices for different ZnS nanosamples and different concentrations of ZnS NPs for a typical sample suspended in deionized water were also measured. & 2012 Elsevier Ltd. All rights reserved.
1. Introduction Zinc sulfide, ZnS, is one of the important II–VI semiconductors. It has received great attention due to its unique properties and wide applications in flat-panel displays [1], sensors and lasers [2,3], thin film electroluminescent devices and infrared windows [1], lightemitting diodes [4], photodiodes [5], thin film solar cells [6], and photocatalytic degradation of organic pollutants such as dyes, halogenated derivatives, and p-nitrophenol in waste water treatment [7–13]. Zinc sulfide crystal usually exhibits a polymorphism of two phases with different stacking sequences of closepacked planes to each structure [14]: one is the cubic phase with a zinc blende structure (C-ZnS) and the other is the hexagonal phase with a wurtzite structure (H-ZnS).
n Corresponding author at: Department of Chemistry, Ferdowsi University of Mashhad, Mashhad 91779, Iran. Tel.: þ 98 511 8797022x308; fax: þ 98 511 8796416. E-mail address: [email protected] (E.K. Goharshadi).
At atmospheric pressure, C-ZnS is more stable at low temperatures and transforms to H-ZnS only at TZ1023 1C. Since the inherent crystal structures of ZnS have important effects on its physical and chemical properties, the controlled synthesis of ZnS is crucial for its practical applications. Various chemical routes towards achieving controlled ZnS NPs including hydrothermal method [15,16], chemical precipitation method [17], sol–gel method [18], ultrasonic irradiation [19,20], microemulsion-assisted solvothemal process [21], microwave [22], metal organic chemical vapor deposition template method [23], solvothermal technique [24], electrochemical deposition method [25], thermal evaporation process [26–30], gas-phase condensation [31], and gamma-irradiation technique [32,33] have been used. Among these preparation methods, microwave irradiation synthesis has many advantages such as very short reaction time, production of small inorganic particles with narrow particle size distribution, and high purity [34]. Ionic liquids have aroused increasing interest worldwide as green solvents due to their large liquid state range, favorable solvation behavior, low melting temperature,
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.09.012
Please cite this article as: E.K. Goharshadi, et al., Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots, Materials Science in Semiconductor Processing (2012), http ://dx.doi.org/10.1016/j.mssp.2012.09.012
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non-volatility, non-flammability, stability in air, high ionic conductivity, high thermal stability in a wide temperature range, relatively low viscosity, low vapor pressure, and selectivity in chemical reactions [35]. ILs are synthesized by combining bulky organic cations e.g. imidazolium or pyridinium with a wide variety of anions. The large range of organic cation and anion pairs makes it possible to design solvents with specific properties to suit specific applications such as separation, extraction processes [36–38], and synthesis of NPs [39–42]. In the present study, we employed the microwave method to prepare the ZnS NPs using a set of ILs based on the bis(trifluoromethylsulfonyl) imide anion and different cations of 1-alkyl-3-methyl-imidazolium. The structural features and optical properties of the ZnS NPs were determined in depth with XRD, TEM, Raman spectroscopy, UV–vis, PL, and DRS. Since the refractive index of NPs is the key input parameter for optical determination of their concentrations, the refractive indices for different ZnS nanosamples and different concentrations of ZnS NPs for a typical sample with different concentrations of ZnS NPs for a typical sample suspended in deionized water were measured.
2. Experimental section 2.1. Materials All ILs, namely 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C2mim][NTf2]), 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl) imide ([C4mim][NTf2]), 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C6mim][NTf2]), and 1-octyl3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C8mim][NTf2]) used in this work were synthesized according to the literature [43]. All ionic liquids were analyzed by NMR, Karl-Fischer titration for water content, and ion chromatography for chloride content. In all cases, the water mass fraction was found to be less than 0.001 and chloride mass fraction was less than 5 10 6. All other chemicals used were of analytical grade and used as received without further purification. 2.2. Experimental In synthesis of ZnS NPs, 2.4 g zinc acetate (Zn(CH3COO)2 2H2O) was dissolved in 100 ml deionized water 2 ml ionic liquid and 0.8205 g thioacetamide (TAA) was added. The mixture was kept in ambient environment with a stirring rate of 500 rpm for 20 min. Then, the reaction was carried out in a domestic microwave oven. The microwave oven (1000 W) followed a working cycle of 10 s on and 5 s off (30% power) and a white precipitate started to appear. The reactions occurring during microwave irradiation are as follows [44]: CH3–CS–NH2 þH2O-CH3(NH2)C(OH)–SH
(1)
CH3(NH2)C(OH)–SHþH2O-CH3(NH2)C(OH)2 þH2S
(2)
Table 1 Characteristics of the ZnS NPs from XRD patterns. Sample ILs
D311 (nm)a
2y
Lattice
1 2 3 4 5
3.13 2.61 2.87 2.88 2.47
57.48 58.78 57.68 57.16 57.75
5.316 5.340 5.299 5.342 5.300
Without ILs [C2mim][NTF2] [C4mim][NTF2] [C6mim][NTF2] [C8mim][NTF2] a
Cell volume ˚ parameter (A) (A˚ 3) 150.229 152.273 148.792 152.444 148.777
Average crystalite size calculated for (311) plane.
CH3(NH2)C(OH)2-CH3(NH)2COþH2O
(3)
Zn(CH3COO)2 2H2OþH2S-ZnS þ2CH3COOH
(4)
At first, TAA is hydrolyzed and an intermediate and hydrogen sulfide is produced (reactions (1) and (2)). The intermediate loses water and produces acetamide (CH3(NH2)CO). Zinc acetate reacts with hydrogen sulfide and zinc sulfide is produced. After cooling to room temperature, the resulting precipitate, zinc sulfide, was centrifuged (15 min with 12,000 rpm) and washed with ethanol and deionized water several times to remove excess ILs and any possible ionic remnants. At last, the products were dried in a vacuum oven at 60 1C for 10 hand white ZnS NPs were obtained. In this work, four ILs based on the bis(trifluoromethylsulfonyl) imide anion and different cations of 1-alkyl-3-methyl-imidazolium were used (Table 1). A similar procedure was used for the fabrication of ZnS NPs in the absence of ILs. 2.3. Characterization The powder phases were determined by means of a Bruker/ D8 Advanced diffractometer in the 2y range from 201 to 801, by step of 0.041, with graphite monochromatic ˚ The formation of nanocrysCu Ka radiation (l ¼1.541 A). tallites and identification of phases were done by TEM using LEO 912 AB instrument and the electron beam accelerating voltage was 120 kV. The FT-Raman spectra in the region 200–3200 cm 1 were recorded employing a 1801 back-scattering geometry and a Bomem MB-154 Fourier transform Raman spectrometer operating at the 1064 nm excitation line of a Nd:YAG laser. It was equipped with a ZnSe beam splitter and a TE cooled InGaAs detector. Rayleigh filtration was afforded by a set of two holographic technology filters. The spectra were accumulated for 1500 scans with a resolution of 2 cm 1. The laser power at the sample was 500 MW. The PL spectra of the prepared nanocrystals dispersed in methanol were measured using a Shimadzu RF-1501 Spectrofluorophotometer at room temperature with a Xe lamp as the excitation light source. The refractive indices of the ZnS nanofluids were measured by CARL Zeiss refractometer. The nanofluids with a required volume concentration were
Please cite this article as: E.K. Goharshadi, et al., Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots, Materials Science in Semiconductor Processing (2012), http ://dx.doi.org/10.1016/j.mssp.2012.09.012
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prepared by dispersing a specified amount of nanoparticles in base fluid of water by applying ultrasonic processor (Sonicator 4000) for 30 min. Ultrasonic waves were emitted from a titanium horn which was directly immersed in the solution. The frequency of ultrasound wave is 20 kHz. The total acoustic power injected into the solution was found to be 700 W. 3. Results and discussion Fig. 1 shows the XRD patterns of ZnS NPs. Three diffraction peaks correspond to the (1 1 1), (2 2 0), and (3 1 1) planes of the cubic crystalline ZnS (JCPDS card no. 05-0566). The significant broadening of the diffraction peaks is ascribed to the very small crystallite size. The strong and sharp diffraction peaks indicate the good crystallization of the samples. No additional peaks in the XRD were observed revealing the high purity of the prepared ZnS NPs. The crystallite size of ZnS NPs, D, can be calculated using the Scherrer’s equation: kl bhkl cosyhkl (311)
(111)
5000
ð1Þ
(220)
Dhkl ¼
e
Intensity (a.u.)
4000
d
3000
c
2000
b 1000
a 0 20
40
60
80
2θ (degree) Fig. 1. The XRD pattern of ZnS NPs for: (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4 and (e) sample 5.
3
where Dhkl is the crystalite size perpendicular to the normal line of (hkl) plane, k is a constant (0.9), bhkl is the full width at half maximum of the (hkl) diffraction peak, yhkl is the Bragg angle of (hkl) peak, and l is the wavelength of X-ray. The peak position and the FWHM were obtained by fitting the measured peaks with two Gaussian curves in order to find the true peak position and width corresponding to monochromatic Cu Ka radiation. The characteristics of the ZnS NPs from the XRD patterns were summarized in Table 1. Since the systematic error in the lattice parameter, a, decreases as the Bragg angle increases, the values of the average crystallite size and the lattice parameter were calculated for (311) plane. The average crystallite size decreases when ILs were used because of the adsorption of ILs on the surface of ZnS NPs and thereby prevent further growth. In fact, ILs work as both a solvent to absorb microwave and capping agent. The lattice constants of ZnS nanosamples derived from the XRD data exhibit a lattice contraction with respect to the value reported for bulk cubic ZnS (a ¼5.41 A˚ [45]). The lattice contraction occurs because of higher surface to volume ratio. This lattice contraction shows the existence of a surface stress. The surface atoms form a large fraction of the material and hence the particles will be in a strained condition due to the extra surface energy they possess. This may cause a contraction of the lattice without drastic change in the crystal structure. The morphology of the products was characterized by TEM. Typical TEM images of samples 1 and 3 are shown in Fig. 2. All the particles display uniform cubic morphology with average size of about 6.8 nm for sample 1 and 6.0 nm for sample 3 which is in an agreement with the results deduced from XRD. The average particle size decreases when ILs were used because of the adsorption of ILs on the surface of ZnS NPs. Fig. 3 shows the Raman spectra (200–400 cm 1) of the samples 1 and 3. The peak at 258.88 cm 1 can be assigned to transverse optical (TO) phonons of ZnS NPs. The peak at 354 cm 1 can be assigned to longitudinal optical (LO) phonon frequency [46]. According to the
Fig. 2. The TEM images of ZnS NPs for: (a) sample 1 and (b) sample 3.
Please cite this article as: E.K. Goharshadi, et al., Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots, Materials Science in Semiconductor Processing (2012), http ://dx.doi.org/10.1016/j.mssp.2012.09.012
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Fig. 3. The Raman spectrum of ZnS NPs for sample 1. Inset: sample 3.
Table 2 Raman frequency shifts for TO and LO peaks and the change in the lattice constant.
Sample 1 Sample 3
DoTO (cm 1)
DaTO (nm)
DoLO (cm 1)
DaLO (nm)
17.12 12.00
0.0087 0.0061
5.92 8.00
0.0024 0.0032
result of phonon combinations corresponding to different critical points, the peak found at 218 cm 1 is assigned to [TO–TA]x [47,48]. The position of Raman peak varies with the change of the interatomic force which is characterized by the change of bond length, as well as the change of the lattice spacing. The relationship between the Raman shift and the change of the lattice parameter can be expressed by the following equation [49]:
Do ¼
3goo Da ao
ð2Þ
where oo is Raman frequency of bulk ZnS. The values of Raman frequency of TO and LO phonons frequencies for bulk cubic ZnS crystal are at 276 and 351 cm 1, respectively [50]. ao is the bulk ZnS lattice constant, Da is the change in lattice constant, and g is the Gru€neisen constant with a value of 1.28 for ZnS [51]. Table 2 shows the Raman frequency shifts, Do, for TO (DoTO) and LO (DoLO) peaks and the corresponding change in the lattice constant (DaTO and DoLO) for samples 1 and 3. The lattice constants of ZnS nanosamples derived from the XRD data exhibit a lattice contraction with respect to the value reported for bulk cubic ZnS (a ¼5.41 A˚ [45]). This lattice contraction leads to a shift of the phonon frequencies with decreasing of the particle size. The lattice contraction indicates the occurrence of surface optimization or reconstruction during the growth of NPs. Table 3 represents the optical properties of ZnS NPs and Fig. 4 shows a typical UV absorption spectrum. The
Table 3 Optical properties of ZnS NPs. Sample
lmax,
1 2 3 4 5
280 204 199 249 210 a b c
UV
(nm)
Eg, UV (eV)a
dUV (nm)b
lmax,
3.92 6.12 6.23 3.95 5.21
4.80 1.43 1.64 2.26 2.21
352 350 349 350 348
DRS
Eg, DRS (eV)c 3.52 3.54 3.55 3.54 3.56
Calculated from UV–vis spectra. Calculated using Eq. (4). Calculated from DRS spectra.
electronic properties of NPs changes significantly as the dimensions of the particles become comparable or less than the Bohr radius of the exciton which can be observed as a blue-shift in the maximum wavelength of absorption peaks (lmax, UV). As Table 3 shows a blue-shift occurs compared with the maximum absorption peak of bulk ZnS (345 nm). This shift of the absorption edges to shorter wavelengths is explained due to the quantum confinement of ZnS NPs [52]. In fact, the blue-shift is induced in the ZnS NPs with respect to bulk because of surfacestress-driven contraction. The magnitude of the blueshifts increases when ILs were used because of decreasing the particle size of ZnS NPs. The optical energy band gap, Eg was calculated from the UV–vis spectra by using the Tauc relation [53]: ðahuÞn ¼ BðhuEg Þ
ð3Þ
where hu is the photon energy, a is the absorption coefficient, B is a constant relative to the material, and n is either 2 for a direct transition or 1/2 for an indirect transition. The energy intercept of a plot of (ahu)n versus hu yields Eg. Since ZnS is a direct band gap semiconductor, the value of n is 2 (see inset of Fig. 4). The values of direct
Please cite this article as: E.K. Goharshadi, et al., Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots, Materials Science in Semiconductor Processing (2012), http ://dx.doi.org/10.1016/j.mssp.2012.09.012
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band gap energy (lmax,UV) of the samples were summarized in Table 3. Since the size-related band gap shift of semiconductor nanocrystals can be quantified, it is possible to calculate an optical particle size with the band gap shift measured from absorption spectra. The relation between the particle size and effective band gap of a nanomaterial can be given by the effective mass approximation model of Brus [54] as ! ph 1 1 1:8e2 Eg,n ¼ Eg,b þ 2 ð4Þ þ n n eR 2R me mh where Eg, b is the bulk band gap (3.54 eV), R is the particle radius, mne and mnh are the effective masses of the electron and hole, respectively where mne ¼0.42 mo, mnh ¼0.61 mo. mo is the mass of a free electron, h is Plank constant, and e is the bulk optical dielectric constant which is 8.76. As Table 3 shows the band gap increases with decreasing size (dUV) due to the quantum confinement of ZnS NPs. The diffuse reflectance spectra of ZnS NPs are shown in Fig. 5. Table 3 shows the band gap (Eg,DRS) and the maximum wavelength (lmax,DRS) calculated based on DRS. A considerable blue-shift in the absorbing band edge is observed for ZnS NPs which could be attributed to the quantum size effect of semiconductors. According to quantum size effect, an increase in the band-gap with the decrease in particle dimensions could be predicted [55]. As Table 3 shows the band gaps calculated based on DRS are not very sensitive to the particles size in contrast with those of calculated using UV spectra. In fact, for crystallite sizes smaller than 10 mm, it has been reported that the DRS systematically underestimate the band gap with an error which increases with decreasing crystallite size [56]. Photoluminescent spectrum is an effective tool to evaluate the defects and optical properties of ZnS NPs as
Absorbance (a.u.)
Fig. 4. The UV–vis spectrum of sample 1dispersed in ethanol. Plots of (ahu)2 vs. photon energy (inset).
5
e
4
d
3
c
2
b
1
a 250
300
350 400 450 Wavelength (nm)
500
550
Fig. 5. Diffuse reflectance spectra for: (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4 and (e) sample 5.
a photonic material [57]. Also, photoluminescence spectrum is sensitive to synthetic conditions, size, and shape of NPs. Fig. 6 shows the PL spectra of ZnS NPs recorded with an excitation wavelength of 290 nm at room temperature. Broadening of the emission peak could be attributed to both size distribution and increase in the surface states owing to the increase in surface to volume ratio for ZnS NPs. Also, the results of PL spectra are consistent with those of XRD which indicate that the particles with better crystallinity have higher PL intensities than those of poor crystallinity. It is thought that nanosamples with higher crystallinity have low concentration of defects which act as sites for nonradiative recombination of electron–hole pairs. As a result, the emission intensities enhances. The refractive indices for different ZnS nanosamples and different concentrations of sample 3 suspended in
Please cite this article as: E.K. Goharshadi, et al., Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots, Materials Science in Semiconductor Processing (2012), http ://dx.doi.org/10.1016/j.mssp.2012.09.012
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Intensity (a.u.)
5000 4000
e
3000
d c
2000
b
1000
a
0 350
400 450 Wavelength (nm)
500
Fig. 6. The PL spectra for: (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4 and (e) sample 5. The excitation wavelength was 290 nm.
Table 4 Refractive indexes of different ZnS nanosamples at 298 K. Sample
D311 (nm)
Refractive index
1 2 3 4 5
3.13 2.61 2.87 2.88 2.47
1.36 1.37 1.37 1.37 1.37
1. One of the most important features in this work is that ZnS NPs were prepared without using any surfactant. 2. X-ray diffraction and Raman measurements confirm that there exists lattice contraction in the cubic ZnS NPs. The lattice contraction indicates the occurrence of surface optimization or reconstruction during the growth. 3. The optical band gap energy of ZnS NPs was calculated using two methods, namely, UV–vis spectroscopy and diffuse reflectance spectroscopy. 4. The band gap of ZnS nanosamples estimated from the absorption peaks is much larger than that of bulk ZnS due to the quantum confinement effect. 5. The photoluminescence intensity increases with increasing the particles crystallinity and decreasing particles size. 6. The refractive index increases with decreasing the size of ZnS NPs and increasing the values of ZnS nanofluid concentration.
Acknowledgment The authors wishes to thank Professor Sayyed Faramarz Tayyari for taking the Raman spectra, Mrs. Somayeh Laleh for taking the FTIR spectra, and Mrs. Roksana Pesian for taking TEM images. The authors also express their gratitude to Ferdowsi University of Mashhad for support of this project (P349).
1.3614 1.3612 Refractive Index (a.u.)
In summary, the present work contains the following important features:
1.3610 References
1.3608
1.3604
[1] [2] [3] [4]
1.3602
[5]
1.3606
1.3600 0.2
[6]
0.4
0.6 0.8 1.0 1.2 Concentration (mg/ml)
1.4
[7] [8]
Fig. 7. The refractive indices for different concentrations of ZnS NPs for sample 3 suspended in deionized water.
[9] [10]
deionized water using ultrasound waves at 25 1C were measured and the results were shown in Table 4 and Fig. 7. The results show the values of refractive index increases with decreasing the size of ZnS NPs and increases with ZnS nanofluid concentration. 4. Conclusions
[11] [12] [13] [14] [15] [16]
A novel method for the synthesis of ZnS quantum dots using a microwave irradiation technique in the presence of ILs was described. The method is convenient, straightforward, rapid, and efficient.
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Please cite this article as: E.K. Goharshadi, et al., Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots, Materials Science in Semiconductor Processing (2012), http ://dx.doi.org/10.1016/j.mssp.2012.09.012
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots Elaheh K. Goharshadi a,b,n, Roya Mehrkhah a, Paul Nancarrow c a b c
Department of Chemistry, Ferdowsi University of Mashhad, Mashhad 91779, Iran Center of Nano Research, Ferdowsi University of Mashhad, Iran School of Chemistry and Chemical Engineering, Queen’s University Belfast, UK
a r t i c l e i n f o
Keywords: Zinc sulfide Nanoparticle Semiconductor compounds Photoluminescence
abstract A facile and rapid microwave irradiation method was developed to prepare ZnS nanoparticles (NPs) using a set of ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl) imide anion and different cations of 1-alkyl-3-methyl-imidazolium. The phases, structures, and optical absorption properties of the NPs were determined in depth with X-ray powder diffraction (XRD), transmission electron microscopy (TEM), Raman spectroscopy, UV–vis absorption spectroscopy (UV–vis), diffuse reflectance spectroscopy (DRS), and photoluminescence spectroscopy (PL). The average crystallite size of the ZnS NPs calculated from the XRD pattern was of the order of 2.8 nm which exhibits cubic zinc blende structure. The energy band gap measurements of NPs were carried out by UV and DRS. The results revealed that the ZnS NPs exhibit strong quantum confinement effect. The optical band gap energy increases significantly compared with those of the bulk ZnS. The refractive indices for different ZnS nanosamples and different concentrations of ZnS NPs for a typical sample suspended in deionized water were also measured. & 2012 Elsevier Ltd. All rights reserved.
1. Introduction Zinc sulfide, ZnS, is one of the important II–VI semiconductors. It has received great attention due to its unique properties and wide applications in flat-panel displays [1], sensors and lasers [2,3], thin film electroluminescent devices and infrared windows [1], lightemitting diodes [4], photodiodes [5], thin film solar cells [6], and photocatalytic degradation of organic pollutants such as dyes, halogenated derivatives, and p-nitrophenol in waste water treatment [7–13]. Zinc sulfide crystal usually exhibits a polymorphism of two phases with different stacking sequences of closepacked planes to each structure [14]: one is the cubic phase with a zinc blende structure (C-ZnS) and the other is the hexagonal phase with a wurtzite structure (H-ZnS).
n Corresponding author at: Department of Chemistry, Ferdowsi University of Mashhad, Mashhad 91779, Iran. Tel.: þ 98 511 8797022x308; fax: þ 98 511 8796416. E-mail address: [email protected] (E.K. Goharshadi).
At atmospheric pressure, C-ZnS is more stable at low temperatures and transforms to H-ZnS only at TZ1023 1C. Since the inherent crystal structures of ZnS have important effects on its physical and chemical properties, the controlled synthesis of ZnS is crucial for its practical applications. Various chemical routes towards achieving controlled ZnS NPs including hydrothermal method [15,16], chemical precipitation method [17], sol–gel method [18], ultrasonic irradiation [19,20], microemulsion-assisted solvothemal process [21], microwave [22], metal organic chemical vapor deposition template method [23], solvothermal technique [24], electrochemical deposition method [25], thermal evaporation process [26–30], gas-phase condensation [31], and gamma-irradiation technique [32,33] have been used. Among these preparation methods, microwave irradiation synthesis has many advantages such as very short reaction time, production of small inorganic particles with narrow particle size distribution, and high purity [34]. Ionic liquids have aroused increasing interest worldwide as green solvents due to their large liquid state range, favorable solvation behavior, low melting temperature,
1369-8001/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.mssp.2012.09.012
Please cite this article as: E.K. Goharshadi, et al., Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots, Materials Science in Semiconductor Processing (2012), http ://dx.doi.org/10.1016/j.mssp.2012.09.012
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non-volatility, non-flammability, stability in air, high ionic conductivity, high thermal stability in a wide temperature range, relatively low viscosity, low vapor pressure, and selectivity in chemical reactions [35]. ILs are synthesized by combining bulky organic cations e.g. imidazolium or pyridinium with a wide variety of anions. The large range of organic cation and anion pairs makes it possible to design solvents with specific properties to suit specific applications such as separation, extraction processes [36–38], and synthesis of NPs [39–42]. In the present study, we employed the microwave method to prepare the ZnS NPs using a set of ILs based on the bis(trifluoromethylsulfonyl) imide anion and different cations of 1-alkyl-3-methyl-imidazolium. The structural features and optical properties of the ZnS NPs were determined in depth with XRD, TEM, Raman spectroscopy, UV–vis, PL, and DRS. Since the refractive index of NPs is the key input parameter for optical determination of their concentrations, the refractive indices for different ZnS nanosamples and different concentrations of ZnS NPs for a typical sample with different concentrations of ZnS NPs for a typical sample suspended in deionized water were measured.
2. Experimental section 2.1. Materials All ILs, namely 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C2mim][NTf2]), 1-butyl-3methylimidazolium bis(trifluoromethylsulfonyl) imide ([C4mim][NTf2]), 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C6mim][NTf2]), and 1-octyl3-methylimidazolium bis(trifluoromethylsulfonyl) imide ([C8mim][NTf2]) used in this work were synthesized according to the literature [43]. All ionic liquids were analyzed by NMR, Karl-Fischer titration for water content, and ion chromatography for chloride content. In all cases, the water mass fraction was found to be less than 0.001 and chloride mass fraction was less than 5 10 6. All other chemicals used were of analytical grade and used as received without further purification. 2.2. Experimental In synthesis of ZnS NPs, 2.4 g zinc acetate (Zn(CH3COO)2 2H2O) was dissolved in 100 ml deionized water 2 ml ionic liquid and 0.8205 g thioacetamide (TAA) was added. The mixture was kept in ambient environment with a stirring rate of 500 rpm for 20 min. Then, the reaction was carried out in a domestic microwave oven. The microwave oven (1000 W) followed a working cycle of 10 s on and 5 s off (30% power) and a white precipitate started to appear. The reactions occurring during microwave irradiation are as follows [44]: CH3–CS–NH2 þH2O-CH3(NH2)C(OH)–SH
(1)
CH3(NH2)C(OH)–SHþH2O-CH3(NH2)C(OH)2 þH2S
(2)
Table 1 Characteristics of the ZnS NPs from XRD patterns. Sample ILs
D311 (nm)a
2y
Lattice
1 2 3 4 5
3.13 2.61 2.87 2.88 2.47
57.48 58.78 57.68 57.16 57.75
5.316 5.340 5.299 5.342 5.300
Without ILs [C2mim][NTF2] [C4mim][NTF2] [C6mim][NTF2] [C8mim][NTF2] a
Cell volume ˚ parameter (A) (A˚ 3) 150.229 152.273 148.792 152.444 148.777
Average crystalite size calculated for (311) plane.
CH3(NH2)C(OH)2-CH3(NH)2COþH2O
(3)
Zn(CH3COO)2 2H2OþH2S-ZnS þ2CH3COOH
(4)
At first, TAA is hydrolyzed and an intermediate and hydrogen sulfide is produced (reactions (1) and (2)). The intermediate loses water and produces acetamide (CH3(NH2)CO). Zinc acetate reacts with hydrogen sulfide and zinc sulfide is produced. After cooling to room temperature, the resulting precipitate, zinc sulfide, was centrifuged (15 min with 12,000 rpm) and washed with ethanol and deionized water several times to remove excess ILs and any possible ionic remnants. At last, the products were dried in a vacuum oven at 60 1C for 10 hand white ZnS NPs were obtained. In this work, four ILs based on the bis(trifluoromethylsulfonyl) imide anion and different cations of 1-alkyl-3-methyl-imidazolium were used (Table 1). A similar procedure was used for the fabrication of ZnS NPs in the absence of ILs. 2.3. Characterization The powder phases were determined by means of a Bruker/ D8 Advanced diffractometer in the 2y range from 201 to 801, by step of 0.041, with graphite monochromatic ˚ The formation of nanocrysCu Ka radiation (l ¼1.541 A). tallites and identification of phases were done by TEM using LEO 912 AB instrument and the electron beam accelerating voltage was 120 kV. The FT-Raman spectra in the region 200–3200 cm 1 were recorded employing a 1801 back-scattering geometry and a Bomem MB-154 Fourier transform Raman spectrometer operating at the 1064 nm excitation line of a Nd:YAG laser. It was equipped with a ZnSe beam splitter and a TE cooled InGaAs detector. Rayleigh filtration was afforded by a set of two holographic technology filters. The spectra were accumulated for 1500 scans with a resolution of 2 cm 1. The laser power at the sample was 500 MW. The PL spectra of the prepared nanocrystals dispersed in methanol were measured using a Shimadzu RF-1501 Spectrofluorophotometer at room temperature with a Xe lamp as the excitation light source. The refractive indices of the ZnS nanofluids were measured by CARL Zeiss refractometer. The nanofluids with a required volume concentration were
Please cite this article as: E.K. Goharshadi, et al., Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots, Materials Science in Semiconductor Processing (2012), http ://dx.doi.org/10.1016/j.mssp.2012.09.012
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prepared by dispersing a specified amount of nanoparticles in base fluid of water by applying ultrasonic processor (Sonicator 4000) for 30 min. Ultrasonic waves were emitted from a titanium horn which was directly immersed in the solution. The frequency of ultrasound wave is 20 kHz. The total acoustic power injected into the solution was found to be 700 W. 3. Results and discussion Fig. 1 shows the XRD patterns of ZnS NPs. Three diffraction peaks correspond to the (1 1 1), (2 2 0), and (3 1 1) planes of the cubic crystalline ZnS (JCPDS card no. 05-0566). The significant broadening of the diffraction peaks is ascribed to the very small crystallite size. The strong and sharp diffraction peaks indicate the good crystallization of the samples. No additional peaks in the XRD were observed revealing the high purity of the prepared ZnS NPs. The crystallite size of ZnS NPs, D, can be calculated using the Scherrer’s equation: kl bhkl cosyhkl (311)
(111)
5000
ð1Þ
(220)
Dhkl ¼
e
Intensity (a.u.)
4000
d
3000
c
2000
b 1000
a 0 20
40
60
80
2θ (degree) Fig. 1. The XRD pattern of ZnS NPs for: (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4 and (e) sample 5.
3
where Dhkl is the crystalite size perpendicular to the normal line of (hkl) plane, k is a constant (0.9), bhkl is the full width at half maximum of the (hkl) diffraction peak, yhkl is the Bragg angle of (hkl) peak, and l is the wavelength of X-ray. The peak position and the FWHM were obtained by fitting the measured peaks with two Gaussian curves in order to find the true peak position and width corresponding to monochromatic Cu Ka radiation. The characteristics of the ZnS NPs from the XRD patterns were summarized in Table 1. Since the systematic error in the lattice parameter, a, decreases as the Bragg angle increases, the values of the average crystallite size and the lattice parameter were calculated for (311) plane. The average crystallite size decreases when ILs were used because of the adsorption of ILs on the surface of ZnS NPs and thereby prevent further growth. In fact, ILs work as both a solvent to absorb microwave and capping agent. The lattice constants of ZnS nanosamples derived from the XRD data exhibit a lattice contraction with respect to the value reported for bulk cubic ZnS (a ¼5.41 A˚ [45]). The lattice contraction occurs because of higher surface to volume ratio. This lattice contraction shows the existence of a surface stress. The surface atoms form a large fraction of the material and hence the particles will be in a strained condition due to the extra surface energy they possess. This may cause a contraction of the lattice without drastic change in the crystal structure. The morphology of the products was characterized by TEM. Typical TEM images of samples 1 and 3 are shown in Fig. 2. All the particles display uniform cubic morphology with average size of about 6.8 nm for sample 1 and 6.0 nm for sample 3 which is in an agreement with the results deduced from XRD. The average particle size decreases when ILs were used because of the adsorption of ILs on the surface of ZnS NPs. Fig. 3 shows the Raman spectra (200–400 cm 1) of the samples 1 and 3. The peak at 258.88 cm 1 can be assigned to transverse optical (TO) phonons of ZnS NPs. The peak at 354 cm 1 can be assigned to longitudinal optical (LO) phonon frequency [46]. According to the
Fig. 2. The TEM images of ZnS NPs for: (a) sample 1 and (b) sample 3.
Please cite this article as: E.K. Goharshadi, et al., Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots, Materials Science in Semiconductor Processing (2012), http ://dx.doi.org/10.1016/j.mssp.2012.09.012
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Fig. 3. The Raman spectrum of ZnS NPs for sample 1. Inset: sample 3.
Table 2 Raman frequency shifts for TO and LO peaks and the change in the lattice constant.
Sample 1 Sample 3
DoTO (cm 1)
DaTO (nm)
DoLO (cm 1)
DaLO (nm)
17.12 12.00
0.0087 0.0061
5.92 8.00
0.0024 0.0032
result of phonon combinations corresponding to different critical points, the peak found at 218 cm 1 is assigned to [TO–TA]x [47,48]. The position of Raman peak varies with the change of the interatomic force which is characterized by the change of bond length, as well as the change of the lattice spacing. The relationship between the Raman shift and the change of the lattice parameter can be expressed by the following equation [49]:
Do ¼
3goo Da ao
ð2Þ
where oo is Raman frequency of bulk ZnS. The values of Raman frequency of TO and LO phonons frequencies for bulk cubic ZnS crystal are at 276 and 351 cm 1, respectively [50]. ao is the bulk ZnS lattice constant, Da is the change in lattice constant, and g is the Gru€neisen constant with a value of 1.28 for ZnS [51]. Table 2 shows the Raman frequency shifts, Do, for TO (DoTO) and LO (DoLO) peaks and the corresponding change in the lattice constant (DaTO and DoLO) for samples 1 and 3. The lattice constants of ZnS nanosamples derived from the XRD data exhibit a lattice contraction with respect to the value reported for bulk cubic ZnS (a ¼5.41 A˚ [45]). This lattice contraction leads to a shift of the phonon frequencies with decreasing of the particle size. The lattice contraction indicates the occurrence of surface optimization or reconstruction during the growth of NPs. Table 3 represents the optical properties of ZnS NPs and Fig. 4 shows a typical UV absorption spectrum. The
Table 3 Optical properties of ZnS NPs. Sample
lmax,
1 2 3 4 5
280 204 199 249 210 a b c
UV
(nm)
Eg, UV (eV)a
dUV (nm)b
lmax,
3.92 6.12 6.23 3.95 5.21
4.80 1.43 1.64 2.26 2.21
352 350 349 350 348
DRS
Eg, DRS (eV)c 3.52 3.54 3.55 3.54 3.56
Calculated from UV–vis spectra. Calculated using Eq. (4). Calculated from DRS spectra.
electronic properties of NPs changes significantly as the dimensions of the particles become comparable or less than the Bohr radius of the exciton which can be observed as a blue-shift in the maximum wavelength of absorption peaks (lmax, UV). As Table 3 shows a blue-shift occurs compared with the maximum absorption peak of bulk ZnS (345 nm). This shift of the absorption edges to shorter wavelengths is explained due to the quantum confinement of ZnS NPs [52]. In fact, the blue-shift is induced in the ZnS NPs with respect to bulk because of surfacestress-driven contraction. The magnitude of the blueshifts increases when ILs were used because of decreasing the particle size of ZnS NPs. The optical energy band gap, Eg was calculated from the UV–vis spectra by using the Tauc relation [53]: ðahuÞn ¼ BðhuEg Þ
ð3Þ
where hu is the photon energy, a is the absorption coefficient, B is a constant relative to the material, and n is either 2 for a direct transition or 1/2 for an indirect transition. The energy intercept of a plot of (ahu)n versus hu yields Eg. Since ZnS is a direct band gap semiconductor, the value of n is 2 (see inset of Fig. 4). The values of direct
Please cite this article as: E.K. Goharshadi, et al., Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots, Materials Science in Semiconductor Processing (2012), http ://dx.doi.org/10.1016/j.mssp.2012.09.012
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band gap energy (lmax,UV) of the samples were summarized in Table 3. Since the size-related band gap shift of semiconductor nanocrystals can be quantified, it is possible to calculate an optical particle size with the band gap shift measured from absorption spectra. The relation between the particle size and effective band gap of a nanomaterial can be given by the effective mass approximation model of Brus [54] as ! ph 1 1 1:8e2 Eg,n ¼ Eg,b þ 2 ð4Þ þ n n eR 2R me mh where Eg, b is the bulk band gap (3.54 eV), R is the particle radius, mne and mnh are the effective masses of the electron and hole, respectively where mne ¼0.42 mo, mnh ¼0.61 mo. mo is the mass of a free electron, h is Plank constant, and e is the bulk optical dielectric constant which is 8.76. As Table 3 shows the band gap increases with decreasing size (dUV) due to the quantum confinement of ZnS NPs. The diffuse reflectance spectra of ZnS NPs are shown in Fig. 5. Table 3 shows the band gap (Eg,DRS) and the maximum wavelength (lmax,DRS) calculated based on DRS. A considerable blue-shift in the absorbing band edge is observed for ZnS NPs which could be attributed to the quantum size effect of semiconductors. According to quantum size effect, an increase in the band-gap with the decrease in particle dimensions could be predicted [55]. As Table 3 shows the band gaps calculated based on DRS are not very sensitive to the particles size in contrast with those of calculated using UV spectra. In fact, for crystallite sizes smaller than 10 mm, it has been reported that the DRS systematically underestimate the band gap with an error which increases with decreasing crystallite size [56]. Photoluminescent spectrum is an effective tool to evaluate the defects and optical properties of ZnS NPs as
Absorbance (a.u.)
Fig. 4. The UV–vis spectrum of sample 1dispersed in ethanol. Plots of (ahu)2 vs. photon energy (inset).
5
e
4
d
3
c
2
b
1
a 250
300
350 400 450 Wavelength (nm)
500
550
Fig. 5. Diffuse reflectance spectra for: (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4 and (e) sample 5.
a photonic material [57]. Also, photoluminescence spectrum is sensitive to synthetic conditions, size, and shape of NPs. Fig. 6 shows the PL spectra of ZnS NPs recorded with an excitation wavelength of 290 nm at room temperature. Broadening of the emission peak could be attributed to both size distribution and increase in the surface states owing to the increase in surface to volume ratio for ZnS NPs. Also, the results of PL spectra are consistent with those of XRD which indicate that the particles with better crystallinity have higher PL intensities than those of poor crystallinity. It is thought that nanosamples with higher crystallinity have low concentration of defects which act as sites for nonradiative recombination of electron–hole pairs. As a result, the emission intensities enhances. The refractive indices for different ZnS nanosamples and different concentrations of sample 3 suspended in
Please cite this article as: E.K. Goharshadi, et al., Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots, Materials Science in Semiconductor Processing (2012), http ://dx.doi.org/10.1016/j.mssp.2012.09.012
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Intensity (a.u.)
5000 4000
e
3000
d c
2000
b
1000
a
0 350
400 450 Wavelength (nm)
500
Fig. 6. The PL spectra for: (a) sample 1, (b) sample 2, (c) sample 3, (d) sample 4 and (e) sample 5. The excitation wavelength was 290 nm.
Table 4 Refractive indexes of different ZnS nanosamples at 298 K. Sample
D311 (nm)
Refractive index
1 2 3 4 5
3.13 2.61 2.87 2.88 2.47
1.36 1.37 1.37 1.37 1.37
1. One of the most important features in this work is that ZnS NPs were prepared without using any surfactant. 2. X-ray diffraction and Raman measurements confirm that there exists lattice contraction in the cubic ZnS NPs. The lattice contraction indicates the occurrence of surface optimization or reconstruction during the growth. 3. The optical band gap energy of ZnS NPs was calculated using two methods, namely, UV–vis spectroscopy and diffuse reflectance spectroscopy. 4. The band gap of ZnS nanosamples estimated from the absorption peaks is much larger than that of bulk ZnS due to the quantum confinement effect. 5. The photoluminescence intensity increases with increasing the particles crystallinity and decreasing particles size. 6. The refractive index increases with decreasing the size of ZnS NPs and increasing the values of ZnS nanofluid concentration.
Acknowledgment The authors wishes to thank Professor Sayyed Faramarz Tayyari for taking the Raman spectra, Mrs. Somayeh Laleh for taking the FTIR spectra, and Mrs. Roksana Pesian for taking TEM images. The authors also express their gratitude to Ferdowsi University of Mashhad for support of this project (P349).
1.3614 1.3612 Refractive Index (a.u.)
In summary, the present work contains the following important features:
1.3610 References
1.3608
1.3604
[1] [2] [3] [4]
1.3602
[5]
1.3606
1.3600 0.2
[6]
0.4
0.6 0.8 1.0 1.2 Concentration (mg/ml)
1.4
[7] [8]
Fig. 7. The refractive indices for different concentrations of ZnS NPs for sample 3 suspended in deionized water.
[9] [10]
deionized water using ultrasound waves at 25 1C were measured and the results were shown in Table 4 and Fig. 7. The results show the values of refractive index increases with decreasing the size of ZnS NPs and increases with ZnS nanofluid concentration. 4. Conclusions
[11] [12] [13] [14] [15] [16]
A novel method for the synthesis of ZnS quantum dots using a microwave irradiation technique in the presence of ILs was described. The method is convenient, straightforward, rapid, and efficient.
[17] [18]
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Please cite this article as: E.K. Goharshadi, et al., Synthesis, characterization, and measurement of structural, optical, and phtotoluminescent properties of zinc sulfide quantum dots, Materials Science in Semiconductor Processing (2012), http ://dx.doi.org/10.1016/j.mssp.2012.09.012