J Mater Sci: Mater Electron DOI 10.1007/s10854-016-5532-z
Influence of the spray distance to substrate on optical properties of chemically sprayed ZnS thin films Ahmad M. Al-Diabat1
•
Naser M. Ahmed1,2 • M. R. Hashim1,2 • Khaled M. Chahrour1
Received: 10 March 2016 / Accepted: 9 August 2016 Ó Springer Science+Business Media New York 2016
Abstract In this study, zinc sulfide (ZnS) nanostructures with different morphologies were effectively fabricated on glass substrates at 200 °C via spray pyrolysis. The Zn2? and S2- ions were sourced from aqueous solutions of zinc acetate and thiourea, respectively. The samples were deposited at different spray distances (15, 20, 25, 30 and 35 cm) in order to examine the effect of spray distance on the optical and structure properties of synthesized ZnS nanostructures. Following the deposition procedure, the films were characterized using X-ray diffraction, UV–Vis– NIR spectrometry, photoluminescence (PL) spectroscopy and field emission scanning electron microscopy. The crystallinity and morphology of the cubic ZnS films were found to vary with spray distance, where the average particle size appears to increase with the increasing spray distance. The highest absorption values were obtained for ZnS films crystallized at a spray distance of 30 cm. The PL analysis specified the presence of violet and green emissions, which are attributable to Zn and S vacancies. The band gap of the ZnS films was observed to decrease slightly from 3.82 to 3.30 eV with the increasing spray distance. The results indicate that the spraying distance affects the characteristics of ZnS nanostructures.
& Ahmad M. Al-Diabat
[email protected] 1
Nano-Optoelectronics Research and Technology Laboratory, School of Physics, Universiti Sains Malaysia, USM, 11800 Penang, Malaysia
2
Institute of Nano-Optoelectronics Research & Technology Laboratory (INOR), School of Physics, Universiti Sains Malaysia, USM, 11800 Penang, Malaysia
1 Introduction The excellent and unique characteristics of zinc sulfide (ZnS) such as its non-toxic nature and the high band gap of its cubic and hexagonal phases (*3.7 and 3.77 eV, respectively) make it a highly potential semiconducting material [1]. Thus, ZnS has been utilized as a source material for an array of devices that include electroluminescence (EL) and cathode luminescence (CL) devices in addition to photovoltaic (PV) devices, field-effect transistors (FET), gas sensors, biosensors and solar cells [2–6]. To optimally harness its distinctive qualities, a number of techniques have been proposed for the preparation of ZnS films, such as sol gel, pulsed laser deposition, hydrothermal, chemical vapor deposition, atomic layer deposition (ALD), radio frequency (RF), magnetron sputtering, and chemical bath deposition (CBD) [7–11]. However, choosing a technique to deposit or synthesize zinc sulfide or any related material requires the consideration for certain factors, such as the quality of the material being prepared, cost and ease of preparation. The aim of this research is to synthesize ZnS films at relatively low cost with the use of simple operating devices, while maintaining high film quality. Based on these considered factors, spray pyrolysis was selected as the technique with the highest potential for the successful deposition of ZnS films and other associated materials, given its simplicity and cost-effectiveness. Moreover, the films are expected to have enhanced homogeneity and high crystallinity which are devoid of further annealing after the deposition and treatment of a relatively large area of the substrate. To optimize the effectiveness of this selected technique, the current study analyses for the first time the consequence of the distance between the substrate and the nozzle on the stability phase, surface morphology, crystallization,
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topography and optical properties, using Zn2? and S2- ions which were sourced from aqueous solutions of zinc acetate and thiourea, respectively, and the air had functioned as a carrier gas.
2 Experimental The spray solution was prepared by mixing thiourea (CH4N2S) and zinc acetate dehydrate (Zn(CH3COOH)22H2O), which served as sources for S2- and Zn2? ions, respectively. The molar Zn/S ratio was fixed at 1:1 using deionized water as the solvent. The solution was sprayed directly onto a glass substrate at different spray distances of 35, 30, 25 and 20 cm under the temperature of 200 °C. Air was used as a carrier gas at a pressure of 3 bars. The chemical reaction is explained in the equation below: ZnðCH3 COOÞ2 þ CSðNH2 Þ2 þ 2H2 O ! ZnS # þ 2CH3 COOH " þ 2NH3 " þCO2 " The glass substrates were subsequently placed on the electric heater for no less than an hour in order to complete the crystalline growth process. The crystalline phase of ZnS films was characterized using X-ray diffraction (XRD:X-ray Philips X,Pert diffractometer equipped with ˚ ). The surface Cu-Ka radiation source (kCu = 1.5418 A morphology of the ZnS films was analyzed using the field emission scanning electron microscopy (FESEM: FEI NovaNano SEM 450). The Zn/S ratio was confirmed with electron dispersive spectroscopy (EDS). The optical properties were determined from the absorbance spectra recorded using Shimadzu UV–Vis 1800 double-beam UV– Vis spectrophotometer and photoluminescence spectroscopy using Jobin–Yvon HR 800 UV equipped with He– Cd laser at 325 nm excitation sources.
3 Results and discussion 3.1 Structural analysis XRD patterns of ZnS films deposited at different spray distances (15, 20, 25, 30 and 35 cm) are depicted in Fig. 1. A highly crystalline product was obtained when the deposition temperature was gradually increased to 200 °C at the spray distance of 30 cm. For comparative analysis, the films were deposited with the same temperature but with different spray distances. The film deposited at 15 cm exhibited relatively less crystallinity compared to the films deposited at farther spray distances. The films prepared at spray rates of 20, 25, 30 and 35 ml min-1 were characterized by a multi well-defined peak of high intensity, which is indexed as (111), (220), (311) reflection of the
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Fig. 1 XRD patterns of ZnS deposited at different spray distance
cubic (zinc blende) ZnS phase. This is in conformity with JCPDS card No. 03-065-4576. In addition, the intensity of peaks was observed to increase with the increasing spray distance from 15 to 30 cm, which possibly improved crystallinity by maintaining the preferred growth along the (111), (220), (311) cubic plane. The intensity of peak subsequently decreased at the spray distance of 35 cm. The lattice parameter and broadening values (characteristically associated with crystallite size) are detailed in Table 1. It can be inferred that the as-synthesized ZnS films exhibit a strong preferred orientation along the (111) direction. The average crystallite size (D) is calculated using Scherer formula as (1) shown below [12]: D¼
0:9k b cos h
ð1Þ
where h denotes the Bragg angle, b ignifies the full-width at half-maximum (FWHM) measured in radian, and k symbolizes the wavelength of the X-ray radiation used. The details of the calculated average crystallite size are outlined in Table 1. A slight increase is observed for the crystallite sizes with an increase in the spray distance from 15 to 30 cm, and afterwards it declined from 6.9 to 26.6 nm at
Table 1 Structural (lattice parameter) and microstructural parameters (crystallite size and microstrain) obtained from X-ray diffraction patterns of ZnS thin films deposited at different spray distance a (A)
aao
6.9
5.255
-.033
29.0672
8.2
5.321
-.0154
28.7484
10.5
5.379
-.0046
30
28.6122
26.6
5.4038
-.00003
35
28.9519
13.9
5.342
-.0115
Sample
Distance
2h (°)
1
15
29.4423
2
20
3
25
4 5
D (nm)
ao
J Mater Sci: Mater Electron
the spray distance of 35 cm. The lattice parameter (a) of cubic zinc blend type structure can be calculated based on the underlying formula (2) [13]:
demonstrated that all the films’ polycrystalline naturally has a cubic structure and the preferred orientation was along the (111) plane for all films [14, 15].
1 h2 þ k 2 þ l 2 ¼ 2 a2 dhkl
3.2 Morphological observations
ð2Þ
where, dhkl is the spacing (distance) between the crystal planes, corresponding to Miller indices, h, k, and l. It can be deduced from Table 1 that the calculated lattice parameter marginally increases with the increasing spray distance from 15 to 30 cm, and it subsequently decreased at 35 cm. This phenomenon can be attributed to the enhanced chemical uniformity (stoichiometry) of the whole deposited ZnS films. Previous findings are in tandem with past studies on the preparation of zinc sulfide using the spraying method. Where Ali A. Yousif and K. Ben Bacha were equipped with films sulfide at different levels of thickness and different substrates respectively, The XRD results
The surface topography of the ZnS films synthesized onto glass substrates at different spray distances is shown in Fig. 2. The surface of the films is depicted as a smooth covering over the entire substrate. Furthermore, the formed particles are uniformly distributed and fall within the nanoscale range, although the average particle size appears to increase with the increase in the spray distance to 30 cm, signifying that the crystallization (grain growth) process improves with the increasing spray distance. Figure 3 shows the ratio Zn:S:O of ZnS films at varying spray distances. Evidently, in increased distances the oxygen in the film would increase too and the percentage of
Fig. 2 FESEM images of ZnS grown at various spray distance
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J Mater Sci: Mater Electron Fig. 3 Zn, S and O atomic ratio of ZnS films grown at various spray distance
sulfur would decrease. As has been reported, this can be explained, by the positive relationship between the amount of oxygen and the spray distances: the further the spray distances, the more likely ZnS film interacts with oxygen (from substrate) forming Zn(S,O), O that will replace S and thus, its content is reduced. 3.3 UV–Vis absorption analysis Figure 4a shows the optical absorption of the deposited ZnS films. It is discernible from the UV–Vis spectra that the absorption of deposited ZnS film increases with the increasing spray distance from 15 up to 30 cm. This is possibly a result of improved crystallinity confirmed by the increase in the crystallite size. Figure 4a shows the optical absorption spectra of the deposited ZnS films. The ZnS film deposited at 15 cm exhibited the minimum absorption, which increases with the increasing spray distance up to 30 cm. This might be attributed to improved crystallinity resulting from increased crystallite size. Tauc’s relation (3) was applied to calculate the band gap (Eg) of the as-deposited ZnS films. The value of m is taken as because ZnS is generally known as a direct band gap material m ahm ¼ A hm Eg ð3Þ where A is a constant which varies depending on the kind of material, a symbolizes the absorption coefficient, hv denotes the energy of the incident photon, and the exponent m signifies the type of transition. The energy band gap is determined via the extrapolation of the linear region of the curve towards X-axis (hm), where Eg represents the intercept, as shown in Fig. 4b. It is observed in Table 2 that the value of the band gap energy Eg decreases from 3.8 to 3.5 eV with the increasing spray distance from 15 to 30 cm. The rationale behind the observed inverse relationship between the energy gap values and the spray distances can be elucidated as follows: the increasing spray distance
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Fig. 4 a UV–Vis–NIR absorption characteristics of ZnS deposited at different spray distance. b Calculation of optical band gap from the UV–Vis–NIR absorption spectra at various spray distance
Table 2 Band gap of ZnS as function of ZnS films deposition at different spray distance Spray distance (cm)
15
20
25
30
35
Band gap(ev)
3.82
36.8
3.5
3.5
3.3
causes the ZnS solution to interact with oxygen prior to its deposition on the substrate, resulting in the formation of Zn(S,O) solution where O diffuses into the ZnS host lattice. This phenomenon will lead to a decrease in the band gap energy values. 3.4 Photoluminescence study Figure 5 shows the photoluminescence (PL) spectra of ZnS film synthesized at spray distance of 30 cm. In the PL spectrum, a band edge or band to band transition peak could not be discovered, because of the dominance of
J Mater Sci: Mater Electron
Intensity (a.u.)
increase with the increasing spray distance, given that a highly crystalline product was obtained when the deposition temperature gradually attained 200 °C at the spray distance of 30 cm. The PL measurement indicates that the Zn and S vacancies were formed. The UV–visible spectra showed that the energy gap decreased from 3.82 to 3.3 eV with increasing spray distance, possibly a result of the formation of Zn(S,O) solid solution and point defects.
350
400
450
500
550
600
650
wavelength (nm)
Fig. 5 Photoluminescence measurements for a film deposited at spray distance 30 cm
defect in a specific crystalline peaks system. Two distinctive peaks are observed at 402 and 527 nm (3.1 and 2.35 eV), consistent with some earlier studies recorded in the literature. Kumar et al. attributed the intense peak at *410 nm to the optical activation of S vacancies while the peak positioned at *520 nm is possibly due to the formation of Zn vacancies [16, 17]. These results indicate that the films can be used as excellent visible light detectors. It can also be deduced that the green light provides adequate energy to activate electron excitation from zinc vacancies [16]. Previous results are also in resonance with those obtained by Xin Zeng, where the synthesis of zinc sulfide films by chemical spraying is found to have been affected by the energy gap when the spray rate is changed owing to the presence of oxygen, a PL study indicated that both sulfur and zinc vacancies existed in the film, further causing violet and green emissions [16].
4 Conclusion ZnS thin films were effectively deposited on glass substrates at low temperature (200 °C) at different spray distances through the adoption of the chemical spray pyrolysis technique. The X-ray diffraction analysis confirms the formation of three peaks of (111), (220) and (311) at different spray distances. The crystallite size was observed to
Acknowledgments The authors gratefully acknowledge the financial support by the University Sains Malaysia fellowship and School of Physics, under Grant Nos. 1001/PFIZIK/811175 and 304/PFIZIK/ 6312076.
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