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Research ArticleInfluence of Bath Temperature and Deposition Time onTopographical and Optical Properties of Nanoparticles ZnS ThinFilms Synthesized by a Chemical Bath Deposition Method
Raghad Zein1 and Ibrahim Alghoraibi 1,2
1Physics Department, Damascus University, Baramkeh, Damascus, Syria2Faculty of Pharmacy, Department of Basic and Supporting Sciences, Arab International University, Damascus, Syria
Correspondence should be addressed to Ibrahim Alghoraibi; [email protected]
Received 29 June 2018; Revised 7 November 2018; Accepted 21 November 2018; Published 4 February 2019
Academic Editor: Nageh K. Allam
Copyright © 2019 Raghad Zein and Ibrahim Alghoraibi. This is an open access article distributed under the Creative CommonsAttribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original workis properly cited.
In this paper, zinc sulfide nanoparticle (ZnS-NP) thin films were deposited onto glass substrates by chemical bath deposition usingzinc sulfate as the cation precursor and thiourea as the anionic precursor. Different bath temperatures (65, 70, 75, and 80°C) anddifferent deposition times (20, 30, 40, and 50min) were selected to study the performance of ZnS thin films. Topographical andoptical characterizations of the films were studied using the atomic force microscope (AFM) and UV-Vis spectroscope. The bestZnS thin films were deposited at a bath temperature (70°C) and a deposition time (30min) with homogeneous distribution, highdensity, and small average diameter (106 nm). The energy gap (Eg) was found to be in the range of 4.05-3.97 eV for the ZnSfilms. Optical constants (refractive index, n, extinction coefficient, k, and dielectric constant, ε) of the films were obtainedin the wavelength range 300-500 nm by using spectrophotometric measurement. The dispersion of the refractive index isanalyzed by using a single oscillator model. The oscillator energy E0 and dispersion energy Ed were determined using theWemple-DiDomenico single oscillator model. Urbach’s energy increases from 0.907 eV to 2.422 eV with increasing of depositiontime. The calculated radius of nanoparticles using Brus equation was 1.9, 2.3, 2.45, and 2.51 nm at deposition times 20, 30, 40,and 50min, respectively.
1. Introduction
Zinc sulfide is one of the most important II-VI semiconduc-tor materials due to its distinct electrical and optical proper-ties. Zinc sulfide nanoparticles (ZnS-NPs) with wide bandgap(3.5-3.8 eV) have recently received intensive attention to beused in many applications [1] such as antimicrobial activity[2, 3], electroluminescence devices, photonic devices [4], fieldemission devices [5], sensors [6, 7], and applications in infra-red windows [8] and lasers [9, 10]. A variety of physical andchemical methods have been used to synthesize ZnS-NPssuch as thermal evaporation [11], sputtering [12], spraypyrolysis [13], chemical vapor deposition [14], molecularbeam epitaxy [15], pulsed laser deposition [16], microwave[17, 18], wet chemistry processes [19, 20], sonochemical
preparation [21], and green synthesis [22]. Among the vari-ous methods, chemical bath deposition (CBD) is the mostcommonly used because of its simplicity, low cost, and coat-ing large area of the semiconductor with good quality andhigh purity of deposited thin films. The main goal of thispresent work is to study the influence of bath temperatureand time of deposition on surface topography and opticalproperties of ZnS-NP thin films.
2. Experimental Materials and Methods
2.1. Reaction Mechanism. The fundamental CBD growthmechanism involves mass transport of reactants, adsorption,surface diffusion, reaction, nucleation, and growth. TheZnS-NP thin films can be prepared by decomposition of
HindawiJournal of NanomaterialsVolume 2019, Article ID 7541863, 13 pageshttps://doi.org/10.1155/2019/7541863
thiourea [(NH2)2CS] (S2- ion source) in an alkaline solution
containing a zinc sulfate (ZnSO4) (Zn2+ ion source) andammonia as a complexing agent which allow controllingZn2+ concentration. The deposition process is based on theslow release of Zn2+ and S2- ions into the solution, which thencondense on an ion-by-ion basis on the substrate that isproperly mounted in a reaction solution. The depositionof ZnS thin film occurs when the ionic product of Zn2+ andS2− ions exceeds the solubility product of ZnS. The reactionmechanism for the deposition of (ZnS) thin film can be givenas follows:
Zn2+ + 2Oh− → Zn OH 2
NH4+ + Oh− →NH3 + h2O
Zn OH 2 + 4NH3 → Zn NH3 42+ + 2OH−
CS NH2 2 + 2OH− → S2− + CN2h2 + 2h2O
1
Finally, the ZnS films are formed on glass substratesaccording to the relation:
Zn NH3 4+2 + S2− → ZnS s + 4NH3 2
Heating the chemical bath improves the chemical reac-tion and accelerates the rate deposition.
2.2. Deposition of ZnS Films. ZnS films were deposited oncommercial glass substrates (75mm × 25mm × 1mm) bychemical bath deposition technique and using a homemadedip-coating apparatus. The substrates were cleaned in etha-nol for 10min, followed by ultrasonically cleaning withdouble distilled water for another 10min, finally dried inthe air and kept in plastic dry boxes. In this work, we pre-pare chemical bath using 4.2ml of ammonia 25% which isadded slowly to 15ml of 0.1M ZnSO4, after stirring for sev-eral minutes the solution becomes colorless and homoge-neous; thereafter, 15ml of 0.179M thiourea solution wasadded under stirring. When the water bath temperature(Tb) arrive at an appropriate temperature (between 65 and80°C), the reaction solution was placed in 50ml beaker intothe water bath pot. The glass substrates were then immersedvertically inside the beaker and supported against the wall ofthe beaker for deposition time (td = 20, 30, 40, 50 min). Eachsample are removed from the beaker and cleaned manytimes with deionized water to remove the white, adherentpowder precipitate in the solution during the deposition.Topography of the surface, particle size, and surface rough-ness of the films are examined using the atomic force
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Figure 1: 2 × 2μm2 AFM images of topography of ZnS thin films at bath temperature Tb = 65°C after different time intervals relative to thebeginning of the film deposition: td = 20, 30, 40, 50 min.
2 Journal of Nanomaterials
microscope (AFM, Nanosurf easyScan 2, Switzerland), tap-ping mode in air at room temperature. The optical transmis-sion and absorption studies of the deposited ZnS thin filmswere carried out with a UV-Vis spectrophotometer (model:Varian Carry 5000).
3. Results and Discussion
3.1. Topographical Properties. In this study, we have usedAFM to get information on the surface relief and to deter-mine the influence of bath temperature and deposition
time on the quality of the as-deposited ZnS films. The AFMmicrograph of ZnS films deposited at bath temperatureTb = 65°C for different times of deposition (td = 20, 30,40, 50 min) is shown in Figure 1.
From AFM images, it is observed that unclear structuresformed at lower deposition times (20, 30, and 40min) so itcould not be able to measure their diameters. At td = 50min, small spherical ZnS-NPs appeared with the mean graindiameter of 120nm approximately.
In the case of high bath temperature Tb = 70°C (asshown in Figure 2), the deposited ZnS thin film has a goodquality, is uniform, and completely covered the entire sub-strate surface area.
At lower deposition time td = 20 min, spherical ZnS-NPswith different sizes and unequal distribution formed on thesurface of the film, while the particles carpet the substrateat deposition time 30min with high density and uniformspherical shape. The grains have relatively low and narrowsize distributions with the mean diameter 106nm, meanheight 15 nm, and mean roughness 11 nm. The particles cov-ered 65% of the surface. The small particles accumulate con-tinuously at time 40min and cover the entire surface of thesubstrate by 85%. At higher deposition time 50min, the par-ticles start to aggregate and made huge grains (disappearedinterval border). The average diameters, heights, and surface
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Figure 2: 2 × 2μm2 AFM images for ZnS thin films/glass at bath temperature Tb = 70°C for different deposition times: td = 20, 30, 40,50 min.
Table 1: Measured average ZnS nanoparticle diameters, heights,and surface roughness at bath temperature Tb = 70°C for differentdeposition times: 20, 30, 40, and 50min.
Deposition time(min)
Diameter(nm)
Height(nm)
Surface roughness(nm)
20 86 13 7
30 106 15 11
40 120 19 13
50 127 25 14
3Journal of Nanomaterials
20 25
15
12
9
6
24
21
18
15
140
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80
30 35 40 45 50
20 25 30 35 40 45 50
20
Exp Grow1 fitExp Grow1 fitExp Grow1 fit
25 30 35Deposition time (min)
Ra (n
m)
h (n
m)
d (n
m)
40 45 50
Figure 3: Changes of average diameters (squares), heights (circles), and roughness (triangles) for synthesized ZnS thin films at bathtemperature 70°C in term of deposition times: 20, 30, 40, 50min.
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Figure 4: 2 × 2 μm2 AFM images for ZnS thin films/glass at bath temperature Tb = 75°C for different deposition times: td = 20, 30, 40, 50 min.
4 Journal of Nanomaterials
roughness of ZnS thin films deposited at bath temperature70°C for different deposition times (20, 30, 40, and 50min)were showed in Table 1.
Figure 3 illustrates graphic curves for changes of averagediameters (d), heights (h), and roughness (Ra) in term ofdeposition time for synthesized ZnS thin films at bath tem-perature 70°C. We notice increasing diameter by 32%, heightby 48%, and roughness by 50% with increasing depositiontime. This indicates that longer deposition time will mainlyaffect the growth of particles [23].
For ZnS thin films deposited at bath temperatures morethan 70°C, the particle size and growth rate obviouslyincrease as shown in Figures 4 and 5 for bath temperatures75°C and 80°C, respectively, for different deposition times(20, 30, 40, and 50min). At td = 30 min, the average grainsize was 187nm and 228nm for bath temperatures 75°Cand 80°C, respectively. The increase of particle size with atemperature of bath is due to increasing ions and particles’kinetic in the solution, which in turn leads to crystal growthaccording to Ostwald ripening.
Comparing AFM images at different immersing times,we find that deposition time 30min is the most suitable timeto form ZnS-NPs with a small diameter and uniform spheri-cal shapes.
In Figure 6, we represent the topography of ZnS-NPthin film deposited at 30min for different bath temperatures
(65, 70, 75, and 80°C). The AFM micrographs showedunclear nanostructures formed at a temperature less than70°C. At high bath temperatures (more than 70°C), nanopar-ticles began to aggregate each other and form huge grains,while it has homogeneous spherical shape, high density,and regular size distribution at the temperature 70°C.
Table 2 shows a comparison between average grain size(diameters, heights) and mean roughness surface for ZnSthin films deposited at td = 30 for different bath temperatures.
The reduction of bath temperature from 80°C to 70°Cleads to decrease of the average diameter by 53%, the averageheight by 75%, and the average roughness by 38%. Increasingof the particle size with temperature is related to increase ofparticle’s kinetic and consequently the rate of nucleationand subsequent growth of ZnS-NPs.
3.2. Optical Properties. UV-Vis transmittance spectra ofZnS-NPs formed at deposition time 30min for different reac-tion temperatures (65, 70, 75, and 80°C) are given in Figure 7.
The transmittance of these films decreased with theincreasing bath temperature; this result related to increaseof the kinetics of particles in the solution leads to new ZnSgrains which fill up the voids in the ZnS thin layer that tendsto have more thickness and less transmittance. In addition,the decreasing transmittance can be linked with agglomera-tion and increasing grain size.
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Figure 5: 2 × 2μm2 AFM images for ZnS thin films/glass at bath temperature Tb = 80°C for different deposition times: td = 20, 30, 40, 50 min.
5Journal of Nanomaterials
Figure 8 shows optical transmittance and reflectancespectra at the UV-Vis region of the as-deposited ZnS filmsfor different deposition times at 70°C.
This figure shows that the films are transparent in the vis-ible region. It can be seen that the reflectance values increaseslightly with increasing deposition times as follows: 1%, 1.5%,3.5%, and 7% for times 20, 30, 40, and 50min, respectively,whereas the transmittance decreases steadily as follows:99%, 98.4%, 94%, and 86.6%, respectively. That result isdue to the new deposited particles on the surface of thefilm and growth of grains by increasing the time whichleads to increasing film thickness measured by the opticalinterference fringe method as 70, 89, 118, and 160nm for20, 30, 40, and 50min deposition times, respectively. The
sharp absorption edge of the ZnS films which observedat about 290–325nm demonstrates a narrow grain sizedistribution as well as a low concentration of defects in
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Figure 6: 2 × 2 μm2 AFM images for ZnS thin films/glass at different bath temperatures: Tb = 65°C, 70°C, 75°C, 80°C for deposition time of30min.
Table 2: Measured average ZnS nanoparticles’ diameter, height,and surface roughness at different bath temperatures (70, 75, and80°C).
Temperature(°C)
Diameter(nm)
Height(nm)
Roughness(nm)
70 106 15 11
75 187 43 17
80 228 61 18
350 375 400 425 450 475 500
90
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100
93.7%
97.5%98.5%
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smitt
ance
(%)
Wavelength (nm)
65ºC70ºC
75ºC80ºC
99%
Figure 7: Transmittance spectra of ZnS thin film at deposition timetd = 30 min and different bath temperatures Tb = 65, 70, 75, 80°C.
6 Journal of Nanomaterials
the films, and it clearly shifts to higher wavelengths forthicker films. This absorption edge shift is associated witha decrease in the energy bandgap of the larger ZnS nano-particles. Figure 9 shows the decreasing optical transmit-tance at the visible region with increasing film thickness(increasing immersion time).
The low reflectance and high transmittance at the visibleregion emphasize the importance of using ZnS thin films forsolar cell applications.
In order to determine optical bandgap, the Tauc relation-ship is used as follows [24]:
ahv = A hv − Egn, 3
where a is absorption coefficient a = −107 ln T /t , A isa constant, h is Planck’s constant, v is photon frequency,Eg is optical bandgap, and n is 1/2 for allowed direct semi-conductor bandgap. The bandgap energy of ZnS thin filmwas estimated by plotting ahv 2 against hv as in Figure 10;
the linear nature of the plot indicates that ZnS is a directbandgap material. The optical bandgap of the films can beevaluated from extrapolating the linear portion to the hvaxis. Eg was determined to be 4.05, 4.01, 3.99, and 3.97 eVfor the ZnS films deposited at different times 20, 30, 40, and50min, respectively, which closely agree with the valuesreported for ZnS thin films obtained by CBD [25, 26].
In Figure11, we have reported the variation of bandgapenergy as a function of diameters of deposited ZnS-NPs. Itis noticed the increment of the energy gap with decreasinggrain size; this is due to the quantum size effect whereas theenergy levels are confined to potential wells of small dimen-sion. The distance between energy levels increases as thecrystal size becomes smaller [27].
For many amorphous and crystalline semiconductors,an exponential dependence of absorption coefficient α <104 cm−1 may take Urbach’s empirical formula [28]: a v =a0 exp hv/EU , where a0 is a constant and EU (Urbachenergy) is an energy characterizing the degree of disorderintroduced from defects and grain boundaries; also, it is inter-preted as the width of the tail of localized states associated withthe amorphous states in forbidden band. Figure 12 representsthe logarithm of absorption coefficient as function of thephoton energy at different deposition times 20, 30, 40, and50min. The value of EU is calculated from the inverse slopeof the linear part of curves and also listed in Table 3.
It has been found that Urbach’s energy increases withtime of deposition. This is probably due to increasing struc-tural disorders and defects in prepared films.
The variation of bandgap energy (Eg) and Urbach energy(EU) of ZnS thin films with deposition times is shown inFigure 13. The Eg values of ZnS films decrease with increas-ing deposition time which is due to the agglomeration ofthe ZnS-NPs. The minimum value of EU obtained at 20minindicates a very weak absorption tail due to minimizeddefects and impurities which improves the transparencyand optical conductivity of the film coated at that time.
Applying the confinement effects, particle size could becalculated using the first theoretical calculation for semi-conductor nanoparticles given by Brus [29] and Chukwuo-cha et al. [30] and based on “effective mass approximation”(EMA). In this approximation, an exciton is considered tobe confined to a spherical volume of the crystallite andthe mass of electron and hole is replaced with effectivemasses (me and mh). The calculations provide an analyticalexpression for how the electronic bandgap of the semicon-ductor nanocrystal (Eg qd ) is modified relative to that ofthe bulk semiconductor (Ebulk). In other words, the energyof quantum confinement (ΔE = Eg qd − Ebulk) is directlyrelated to the nanocrystal radius (R) with Brus equation:
Eg nano = Ebulk +h2
8R21m∗
e+ 1m∗
h−
1 786e24πε0εrR
, 4
where R is the radius of the nanoparticle, Eg nano is calcu-lated from the excitonic absorption peak of the nanoparti-cle, EZnS bulk is bulk semiconductor bandgap (3.65 eV),
300 350 400 450 500
0
20
40
60
80
100
0
20
40
60
80
100
Tran
smitt
ance
(%)
Wavelength (nm)20 min30 min
40 min50 min
Refle
ctan
ce (%
)Figure 8: Optical transmittance and reflectance spectra of ZnS thinfilm at bath temperature Tb = 70°C for different deposition timestd = 20, 30, 40, 50 min.
30 40 50 60 70 80 90
86
88
90
92
94
96
98
100
Tran
smita
nce (
%)
Thickness (nm)
Figure 9: Optical transmittance vs. thickness of ZnS thin films atdifferent deposition times.
7Journal of Nanomaterials
me ∗ is the effective mass of the electron (me ∗ ZnS =0 34mo), mh ∗ is the effective mass of the hole (mh ∗ZnS = 0 23mo), e is the charge of the electron, and ε is
the dielectric constant of the material, for ZnS = 8 76 and ε0vacuum permittivity constant. The calculated radius forZnS nanoparticles using Brus equation was 1.9, 2.3, 2.45,and 2.51 nm at different deposition times 20, 30, 40, and50min, respectively (less than Bohr radius 2.67 nm). Quan-tum confinement effect in the nanosemiconductor of ZnShas been studied using the Brus equation. The first termin the left-hand side of the equation represents the bandgapenergy of bulk materials, which is characteristic of thematerial. The second additive term of the equation repre-sents the additional energy due to quantum confinementhaving R−2 dependence on the bandgap energy. It canindeed be thought of as the infinite square-well contribu-tion to the bandgap. The third subtractive term stands forthe columbic interaction energy exciton having R−1 depen-dence (often neglected due to the high dielectric constant ofsemiconductor material).
The graphs of ground state confinement energy againstsize (radius) for zinc sulfide nanoparticles in Figure 14 showthe dependence of confinement on the size of quantum dots.The result shows that ground state confinement energy is
2.5 3.0 3.5 4.0
0.0
0.2
0.4
0.6
0.8
t = 20 min
Photon energy hv (eV)
(ahv
)2 ×
1013
(eV
/cm
)2
(a)
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0.04
0.08
0.12t = 30 min
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(ahv
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(eV
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(ahv
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0.00
0.04
0.08
0.12
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t = 50 min
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(ahv
)2 ×
1013
(eV
/cm
)2
0.00
0.04
0.08
0.12
(d)
Figure 10: Plot of (ahv)2 vs. photon energy hv for ZnS thin films deposited at different deposition times: 20, 30, 40, and 50min at bathtemperature of 70°C.
80 90 100 110 120 1303.96
3.98
4.00
4.02
4.04
4.06
Ener
gy g
ap (e
V)
Diameter (nm)
Figure 11: Variation of ZnS thin film bandgap with particlediameters at fixed bath temperature Tb = 70°C.
8 Journal of Nanomaterials
inversely proportional to the size (radius). Thus, as oneincreases the radius (size), the confinement energy decreases,but never reaches zero. That is, the lowest possible energy forthe quantum dot sample is not zero. Confinement beginswhen the radius of the quantum dot sample is comparableor of the order of the exciton Bohr radius.
The extinction coefficient could be calculated using therelation: k = αλ/4π. Figure 15 shows the variation of extinc-tion coefficient as a function of wavelength; it shows a
2.4 2.6 2.8 3.0 3.2 3.4 3.6
8.0
8.2
8.4
8.6
8.8
9.0
9.2
ln (�훼
)
Photon energy h�휈 (eV)
EU = 0.91 eV
t = 20 min
(a)
ln (�훼
)
2.4 2.6 2.8 3.0 3.2 3.4 3.6Photon energy h�휈 (eV)
EU = 1.13 eV
8.2
8.4
8.6
8.8
9.0
9.2 t = 30 min
(b)
ln (�훼
)
Photon energy h�휈 (eV)
EU = 1.53 eV
2.45 2.50 2.55 2.60 2.65 2.709.05
9.10
9.15
9.20
9.25
t = 40 min
(c)
ln (�훼
)
Photon energy h�휈 (eV)
EU = 2.42 eV
2.4 2.5 2.6 2.7 2.8 2.9 3.0 3.18.95
9.00
9.05
9.10
9.15
9.20
9.25
t = 50 min
(d)
Figure 12: Logarithm of absorption coefficient as function of the photon energy at different deposition times: 20, 30, 40, and 50min.
Table 3: Urbach’s energy and bandgap for ZnS films deposited atdifferent times: 20, 30, 40, and 50min.
Deposition time (min) Energy gap (eV) Urbach’s energy (eV)
20 4.05 0.91
30 4.01 1.13
40 3.99 1.53
50 3.97 2.42
20 25 30 35 40 45 503.96
3.98
4.00
4.02
4.04
4.06
Deposition time (min)
Ener
gy g
ap (e
V)
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2.4
2.6U
rbac
h′s e
nerg
y (e
V)
Figure 13: Variation of energy gap and Urbach’s energy withdeposition time of ZnS films.
9Journal of Nanomaterials
sharp increase at the ultraviolet region that is due to highabsorbance of incident photons near the bandgap.
The refractive index (n) is the ratio of the velocity of lightin a vacuum to its velocity in a specified medium. The valueof the refractive index was calculated from the equation:
n = 1 + R1 − R
2− k2 + 1
1/2
+ 1 + R1 − R
, 5
where (R) is the reflectivity and k is extinction coefficient.The variation of refractive index vs. wavelength is shown in
Figure 16, which shows that the maximum value of (n) is~2.6 for all films at the same wavelength. Also, it shows thatthe films become more transparent in the visible region. Therefractive index dispersion data were evaluated according tothe single effective oscillator model proposed by Wempleand DiDomenico [31] and Chiad et al. [32]. It is well knownfrom dispersion theory that, in the region of low absorption,the index of refraction n is given in a single oscillator modelby the expression:
n2 − 1 = E0EdE20 − hv 2 , 6
where Ed and E0 are single oscillator constants, E0 is theenergy of the effective dispersion oscillator, and Ed is theso-called dispersion energy, which measures the intensityof the interband optical transitions. The oscillator energyE0 is an average of the optical bandgap and can be relatedto the optical bandgap Eg in close approximation E0 ≈ 2 Eg.Figure 17 shows the plot of n2 − 1 −1 vs. hv 2 for the filmsprepared at different deposited times 20, 30, 40, and 50min.In all cases, linear dependence was observed. The valuesof E0 and Ed can then be calculated from the slope E0Ed
−1
of the straight line and the intercept on the vertical axis(E0/Ed). It was found that Ed varies between 1.23 and8.76 eV, while E0 varies from 4.06 to 5.7 eV for the differ-ent deposition times. Furthermore, the values of the staticrefractive index (n0) can be calculated by extrapolating theWemple-DiDomenico dispersion equation to E→ 0. Thecalculated values of n0 are 1.14, 1.18, 1.34, and 1.59 for thefilms deposited at different times 20, 30, 40, and 50min,respectively. The obtained values are given in Table 4.
1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.60.4
0.5
0.6
0.7
0.8
Con
finem
ent e
nerg
y (e
V)
Quantum dot radius (nm)
Figure 14: Variation of ground state confinement energy withquantum dot radius for ZnS thin films deposited at differenttimes: 20, 30, 40, and 50min at fixed bath temperature Tb = 70°C.
300 350 400 450 5000.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Extin
ctio
n co
effici
ent (k)
Wavelength (nm)
20 min30 min
40 min50 min
Figure 15: Extinction coefficient as function to wavelength ofincident light for ZnS thin films at different deposition times: 20,30, 40, and 50min.
300 350 400 450 5001.0
1.5
2.0
2.5
Wavelength (nm)
20 min30 min
40 min50 min
Refr
activ
e ind
ex (n
)Figure 16: The variation of refractive index as function towavelength of incident light for ZnS thin films at differentdeposition times: 20, 30, 40, and 50min.
10 Journal of Nanomaterials
The real and imaginary parts of dielectric constant weredetermined using relation:
ε = εr + εi = n − ik 2,εr = n2 − k2,εi = 2nk,
7
where εr is the real part and is the normal dielectric constantand εi is the imaginary part and confirms the free carrier con-tribution to the absorption. The variation of εr and εi versusincident photon energy (hv) is shown in Figures 18 and 19.The variation of εr and εi with the increase of the wavelengthof the incident radiation is due to the change of reflectanceand absorbance.
The behavior of εr is similar to that of the refractive indexbecause of the smaller value of k2 compared with n2, while εimainly depends on the k value, which is related to the varia-tion of absorption coefficient. εi represents the absorption ofradiation by free carriers. The figures revealed that the realpart is higher than the imaginary part for all samples.
The optical conductivity was calculated using the follow-ing relation: σ = αnc/4π, where c is the speed of light.Figure 20 shows optical conductivity for different times ofdeposition. It was observed that the optical conductivity
decreases as the time increases to 50min. The increased opti-cal conductivity at a low wavelength (ultraviolet region) isdue to the high absorbance of ZnS thin films in that region.
4. Conclusion
The nanoparticle ZnS thin films have been successfullysynthesized with different deposition times and differentbath temperatures. The topographical and optical studieswere carried out using the atomic force microscope andUV-visible spectroscope. The particle size and surface rough-ness, as well as thickness values, were increased withincreasing deposition time and bath temperature. How-ever, ZnS thin films prepared at td = 30 min and Tb = 70°Cshowed homogeneous nanoparticles with high density, less
Table 4: The values of energy gap Eg, single oscillator energy (E0),dispersion energy (Ed), and refractive index (n0) for the ZnS filmsdeposited at different times: 20, 30, 40, and 50min.
Deposition times (min) Eg (eV) E0 (eV) Ed (eV) n0
20 4.05 4.06 (≈Eg) 1.23 1.14
30 4.01 5.53 (=1.37 Eg) 2.25 1.18
40 3.99 5.6 (=1.4 Eg) 4.48 1.34
50 3.97 5.7 (=1.44 Eg) 8.76 1.59
2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.80
1
2
3
4
5
6
7
20 min30 min
40 min50 min
Real
die
lect
ric co
nsta
nt
Photon energy (eV)
Figure 18: εr vs. hv for ZnS thin films at different deposition times:20, 30, 40, and 50min.
20 min30 min
40 min50 min
Imag
inar
y di
elec
tric
cons
tant
2.4 2.6 2.8 3.0 3.2 3.4 3.6 3.80.0
0.5
1.0
1.5
Photon energy (eV)
Figure 19: εi vs. hv for ZnS thin films at different deposition times:20, 30, 40, and 50min.
5 6 7 8 9 10 11 12 13 140.0
0.5
1.0
1.5
2.0
2.5
3.0
20 min30 min
40 min50 min
(n2 −
1)−
1
(h�휈)2
Figure 17: Plots of n2 − 1 vs. hv 2 for ZnS thin films at differenttd: 20, 30, 40, and 50min.
11Journal of Nanomaterials
agglomeration, high transparent, and low reflectance in thevisible region which could be recommended for antireflec-tion coating and solar cells. The optical bandgap energywas decreased from 4.05 to 3.97 eV upon increasing thedeposition time, while Urbach’ energy was increased from0.907 eV to 2.422 eV. The quantum confinement effect innanoparticles ZnS has been studied using the Brus equation.It is noticed that the ground state confinement energy isinversely proportional to the size (radius) of ZnS-NPs.
Data Availability
The data used to support the findings of this study are avail-able from the corresponding author upon request.
Conflicts of Interest
The authors declare that there are no conflicts of interestregarding the publication of the present work.
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0
4
8
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ical
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uctiv
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12 Journal of Nanomaterials
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