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Effect of deposition temperature on the structural and optical properties of CdSe QDs thin films deposited by CBD method
Accepted Manuscript Title: Effect of deposition temperature on the structural and optical properties of CdSe QDs thin films deposited by CBD method Author: F. Laatar A. Harizi A. Smida M. Hassen H. Ezzaouia PII: DOI: Reference:
S0025-5408(16)30075-7 http://dx.doi.org/doi:10.1016/j.materresbull.2016.02.021 MRB 8667
To appear in:
MRB
Received date: Revised date: Accepted date:
17-11-2015 7-2-2016 14-2-2016
Please cite this article as: F.Laatar, A.Harizi, A.Smida, M.Hassen, H.Ezzaouia, Effect of deposition temperature on the structural and optical properties of CdSe QDs thin films deposited by CBD method, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2016.02.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of deposition temperature on the structural and optical properties of CdSe QDs thin films deposited by CBD method F. Laatara*, A. Harizib, A. Smidaa, M. Hassena,c, and H. Ezzaouiaa a
Photovoltaic Laboratory, Centre for Research and Technology Energy, Tourist Route
Soliman, BP 95, 2050 Hammam-Lif, Tunisia b
Photovoltaic and Semiconductor Materials Laboratory, Engineering Industrial Department,
ENIT, Tunis El Manar University, BP 37, Le Belvédère, 1002 Tunis, Tunisia c
Higher Institute of Applied Science and Technology of Sousse, City Taffala (Ibn Khaldun),
4003 Sousse Tunisia
E-mail adress: [email protected] (F. Laatar)
Graphical abstract
1
Highlights
Synthesis of CdSe QDs with L-Cysteine capping agent for applications in nanodevices.
The films of CdSe QDs present uniform and good dispersive particles at the surface.
Effect of bath temperature on the structural and optical properties of CdSe QDs thin films.
Investigation of the optical constants and dispersion parameters of CdSe QDs thin films.
Abstract Cadmium selenide quantum dots (CdSe QDs) thin films were deposited onto glass substrates by a chemical bath deposition (CBD) method at different temperatures from an aqueous solution containing L-Cysteine (L-Cys) as capping agent. The evolution of the surface morphology and elemental composition of the CdSe films were studied by AFM, SEM, and EDX analyses. Structural and optical properties of CdSe thin films were investigated by XRD, UV-Vis and PL spectroscopy. The dispersion behavior of the refractive index is described using the single oscillator Wemple-DiDomenico (W-D) model, and the physical dispersion parameters are calculated as a function of deposition temperature. The dispersive optical parameters such as average oscillator energy (Eo), dispersion energy (Ed), and static refractive index (no) were found to vary with the deposition temperature. Besides, the electrical free carrier susceptibility (χe) and the carrier concentration of the effective mass ratio (N/m*) were evaluated according to the Spitzer-Fan model. KEYWORDS: A. Thin films, B. Chemical synthesis, B. Optical properties, C. X-ray diffraction, D. dielectric properties.
1. Introduction Cadmium selenide (CdSe) is a solid material that belongs to the II-VI semiconductor group. CdSe has a direct-gap and a strong absorption coefficient in the visible spectral region. For these reasons, CdSe is widely used in photoconductivity and solar cells [1,2], electronic and optoelectronic applications [3], optical sensing [4], and gas sensing [5–7]. Different techniques were developed for the synthesis of CdSe thin films, such as electrodeposition, physical vapour deposition-electron beam evaporation (PVD-EBE) [8], vacuum evaporation [9], hot wall deposition [10], pyrolysis [11], chemical bath deposition (CBD) [12], sol-gel 2
process [13], and hydrothermal route [14]. The synthesis of films using the CBD method is easy and cheaper compared to other processes. Several complexing or stabilizing agent can be used for the preparation of CdSe nanoparticles such as L-Cysteine (L-Cys), triethanolamine (TEA) and nitrilotriacetic acid (NTA) [15–17]. We report here the preparation and the deposition of CdSe QDs thin films by the chemical bath deposition method using L-Cys as a stabilizing agent. The process is based on the dissolution of the CdSe/L-Cys powder at the appropriate temperature under vigorous stirring. This work was conducted to study the effect of the bath temperature on the optical properties of CdSe thin films prepared by the CBD method. Up to now, studies were mainly focused concentrated on the morphological, structural and some optical properties of the prepared thin films, but the effect of the bath temperature on the optical constants and the dispersion parameters has not been investigated in detail. For this reason, we have calculated and estimated some optical parameters such as the refractive index (n), the extinction coefficient (k), the oscillator energy (E0), and the dispersion energy (Ed), as a function of the bath temperature. In this study, we provide a precious contribution to the researchers who are interested in the optical properties of the CdSe nanomaterial as thin film. 2. Materials and methods 2.1. Chemicals Cadmium acetate dihydrate (Cd(OAc)2. 2H2O, ≥ 98%, Sigma Aldrich), selenium dioxide (SeO2, 99.999%, Sigma Aldrich),
L-cysteine (C3H7NO2S, ≥ 97%, Sigma Aldrich), and
sodium borohydride (NaBH4, ≥ 99%, Sinopharm Chemical Reagent) were used as received without additional purification. Ultra-pure water was used for the preparation of all aqueous solutions. Dilute aqueous solutions of sodium hydroxide were used to fix the pH at specific values. 2.2. Synthesis of L-cysteine functionalized CdSe QDs In a typical synthesis, firstly 8.56 mmol of Cd(OAc)2 and 13.8 mmol of L-Cys were dissolved in 100 mL of deionized water to obtain a clear solution. The pH of this solution was adjusted to 11.2 by dropwise addition of a 1 M aqueous solution of NaOH. An aqueous solution of Na2SeO3 (prepared by dissolving SeO2 into a NaOH solution) was added dropwise to the Cd(OAc)2/L-Cys mixture under vigorous stirring and the mixture was saturated with N2. Next, a NaBH4 aqueous solution was added to the previous mixture under vigorous stirring. 3
The molar ratio of Cd2+/L-Cys/Se2- was set as 1/2/0.5. The precursors were converted to CdSe QDs by refluxing the reaction mixture at 100°C for 3 h under N2. This conversion is accompanied by a change of the color of the solution to yellow. After cooling, the reaction mixture was concentrated down to 25 mL using an evaporator. Isopropanol was added to the CdSe nanoparticles dispersion and the mixture stirred for 1 h. CdSe QDs were recovered by filtration and dried under vacuum. In our synthesis, the L-Cys/Cd molar ratio equal to 2 was used to obtain a good coverage and a total saturation of the dangling bonds at the nanocrystals surface, leading to the inhibition of the non-radiative transitions. 2.3. Deposition of CdSe QDs onto glass substrate The glass substrates were successively cleaned in detergent solution, deionized water, and ethanol for 10 min in an ultrasonic bath. The CdSe thin films were prepared at different bath temperatures ranging from 30 to 80°C during 20 min. The as-deposited films were dried in air at 100°C during 10 min using a hot plate. 2.4. Characterization The crystallographic structure was carried out by X-ray diffraction technique (XRD) using a Bruker D 8 advance X-ray diffractometer with Cu Kα radiation (λCuKα = 1.5406 Å) operated at 40 kV and 40 mA. In order to correct/adjust the contribution from the instrumental effects to the experimental full-width at half-maximum (FWHM), βexp for each peak was obtained using the following relation [18,26,28]: 1
1 2 2 2 2 exp ins exp ins
(1)
where βins is the instrumental FWHM which was obtained using crystallized NaCl. The thickness of the deposited films at bath temperatures of 30, 45, 55, 70, and 80°C were measured as 818, 887, 913, 985, and 1104 nm by using profilometer Veeco Dektak 150. The microstructure of CdSe nanoparticles deposited on glass substrate at bath temperatures of 30°C and 80°C were characterized using a Scanning Electron Microscopy (SEM) ZEISS Ultra Plus FE-SEM that was equipped by EDX-SEM micrographs with an accelerating voltage of 2.5 kV. The elemental compositions of films were examined by Energy Dispersive X-ray (EDX) analysis. The surface morphology of the prepared CdSe thin films was investigated by using Atomic Force Microscopy (AFM) in tapping mode by a Nanoscope III (Veeco AFM head RTESP silicon pur) to a scanning area of 1 μm x 1 μm. The optical transmittance T(λ) 4
and reflectance R(λ) of the CdSe/L-Cys thin films were measured on a UV-vis-NIR Lambda 950 spectrophotometer, equipped with an integrating sphere, in the wavelength range 300– 850 nm. Photoluminescence measurements were recorded on a Perkin-Elmer LS 55 spectrometer using a Xenon light source with an excitation wavelength of 380 nm. 3. Results and discussion 3.1. Structural properties A survey of available literature shows that the CdSe thin films can be hexagonal (wurtzite) [19], cubic (zinc-sphalerite) [20] or of a mixed structure [JCPDS files No. 19-191 and 652891]. XRD was used to confirm the crystalline structure of CdSe thin films deposited on glass substrates at different bath temperatures. As shown in Fig. 1, CdSe films present the highest intensities reflections along the (111) plane and two weak reflections along the (220) and (311) planes at 25.49°, 42.49°, and 50.35°, respectively. These results indicate that the films are crystalline and composed of CdSe in cubic phase with preferred orientation along the (111) plane. In Table 1, the observed d-spacing and the identification of the diffraction peaks were compared using JCPDS data No. 65-2891 as reference and with powder diffraction files that confirm that the preferred orientation along (111) direction [15]. The lattice parameter (a) of the CdSe thin films was calculated for the cubic structure using the relation: a d hkl h 2 k 2 l 2
1/2
(2)
As the deposition temperature increases, the X-ray diffraction peaks are not shifted, indicating that the CdSe films have the same composition, regardless of the temperature used for the deposition. Fig. 1 shows also that the intensities of peaks increase with the deposition temperatures, further indicating that the crystallinity increased. XRD patterns can also be used to determine the nanoparticle size (D), the dislocation density (ρ), the micro-strain (ε), and the number of crystallites per unit area (N). The average particle sizes of films deposited at different bath temperatures were calculated from the highest peaks intensity using the DebyeScherrer’s equation [21]:
D
k cos
(3)
5
where D is the particle size of CdSe particles, λ is the X-ray wavelength (1.5406 Å), θ is Bragg diffraction angle, β the full-width at half-maximum (FWHM) of the peak, and k is a constant usually taken equal to 0.9. Fig. 2 shows the variation of the particle sizes versus deposition temperatures. Results obtained indicate that the crystallite sizes increased with the deposition temperature and that the largest crystallites were obtained at 80°C. The increase in the particle size is probably due to the coalescence of small nanoparticles during the heat treatment. The micro-strain (ε), was calculated by using the following relation [22]:
4 tan
(4)
The average dislocation density (δ) of synthesized CdSe thin films onto glass substrates was determined using the formula [23]:
1 D2
(5)
The calculated values of dislocation density (δ) and micro-strain (ε) are summarized in Table 2. The micro-strain decreases as a function of the deposition temperature, which leads to the decrease in the concentration of the lattice imperfections. This could also explain the increase in nanoparticles size. From these analyses, we evaluated in this study the key role of the bath temperature for the enhancement of crystal quality of thin films. Dislocation density decreases with increasing of bath temperature. It is well-known that the ρ indicates the quantity of defects in the structure. So, the decrease of defects in the volume of samples with the increase of bath temperature resulted the reduction in dislocations as well as the improvement of the films crystallinity. The number of particle per unit area can be calculated by using the following relation [24]:
N
t D3
(6)
where t is the CdSe thin film thickness. The calculated number of particle per unit area in the CdSe thin films are summarized in Table 2. The increase of bath temperature causes the decrease of the number of particles per unit area. This reduction in the number of crystallites indicates that the elaborated films at low bath temperature are composed of a large number of small particles, and that with the increase of the bath temperature, small particles probably coalescence to form the agglomerates.
6
3.2. CdSe thin films morphology 3.2.1. SEM and EDX analyses Fig. 3(a) and (b) show the Scanning Electron Microscopy (SEM) micrographs of the deposited CdSe QDs thin films at deposition temperatures of 30°C and 80°C, respectively. From these figures, we observe that QDs sizes were relatively uniform with a spherical arrangement. The average diameters of CdSe QDs in these films were about ~2 nm and ~4 nm, respectively, which is in good agreement with XRD results. Energy dispersive X-ray analysis (EDX) was used to determine the elements composition of the synthesized CdSe thin films at deposition temperature of 30°C and 80°C. Fig. 4(a) and (b) show the EDX patterns of thin films deposited at 30°C and 80°C, respectively, and the relative elemental composition are depicted in the corresponding tables. The results of these analyses confirms that the composition of the prepared films consists of cadmium and selenium with a molar ratio of ∼1:1, indicating that these elements are in a nearstoichiometric ratio. A small compositional variation is observed between the deposited CdSe thin films at 30 and 80°C, which can be caused by the rearrangement of atoms. 3.2.2. AFM analysis AFM was used to explore the external morphology of CdSe thin films surface prepared by the CBD method on glass substrate at different bath temperatures. In order to evaluate the QDs size as well as the surface roughness of films, an area of 1 μm x 1 μm was scanned in tapping mode. The Root-Mean-Square roughness (RMS) and the nanocrystallite sizes were calculated using standard software (WS × M 5.0 software) [25–28]. The diameter of a nanoparticle was defined as the half of its height. Fig. 5 illustrates the bi-dimensional (2D) and three-dimensional (3D) AFM images of CdSe thin films deposited on glass substrates at 30, 45, 55, 70 and 80°C. These images show that the bath temperature has an influence on the surface morphology of CdSe films. It can be seen that with the increase of the bath temperature, the size of nanoparticles increases and consequently voids decrease. As regards the deposited film at low bath temperature (30°C), the development process of particles is carried out to form a discontinuous film. The difficulty is engendered by the low bath temperature, which results in a high percentage of voids and to smaller CdSe QDs compared to films prepared at high temperature (80°C). The deposited thin film at 55°C is 7
characterized by an homogeneous particles structure and separated by the grains boundaries. This uniformity on the film surface attests for a deposition method efficient for decreasing the density of voids in the CdSe thin films with increasing bath temperature. In Fig. 5(a), it is shown that the deposited films are compact and dense. The particles grown have a spherical morphology and are homogeneously distributed over the substrate surface. The prepared CdSe QDs by CBD at higher temperature (80°C) reveal a roughly inhomogeneous surface with a random grain dispersion. However, the CdSe QDs show an increase in size as the bath temperature increases to 80°C. The films prepared at higher bath temperature indicate that the smaller nanocrystals are accumulated to form grains or agglomerates. Finally, we can conclude that an increase in the deposition temperature improves the nanoparticle size and the film crystallinity. The instrumental scan of the AFM microscopy clearly shows that the particle size of CdSe QDs increases from ~2.5 to ~4 nm and the film roughness increases from 0.93 to 1.53 nm (Table 3). The value of nanoparticle size obtained by AFM were confirmed by SEM analysis and XRD patterns using the Debye-Scherrer formula. 3.3. Optical properties 3.3.1. Optical absorption and optical energy gap of CdSe QDs thin film Fig. 6(a) and (b) show the transmittance (T) and the reflectance (R) spectra of CdSe thin films deposited at different bath temperatures in the wavelength range 300–850 nm. All the transmittance spectra show that the absorption edge is between 500 nm and 564 nm and the average transmission in the wavelength range 500–850 nm is between 38 % and 46 %. As described in Fig. 6(a), the film transmittance decreases with the increase in bath temperature from 30°C up to 80°C. This originates from the reduction of voids concentration and from the increase of film thickness. The variations of the absorbance of CdSe thin films depending on the deposition temperature, in the wavelength range 350–850 nm are shown in Fig. 7. The optical absorption increases with increasing the deposition temperature. This variation is due to the increase in the CdSe QDs size and to the decrease in defects. The energy band gap of CdSe films was calculated by extrapolating the straight line-portion of ( h )2 vs. (h ) to ( h )2 = 0 [29], as shown in Fig. 8 and Table 3. Fig. 8 shows the band-gap dependence of CdSe thin films vs the bath temperature. It can be seen that the bandgap energy decreases from 2.37 to 2.25 eV as the bath temperature increases from 30 to 80°C. The increase of the deposition temperature has an effect on various parameters such as particle size, presence of impurities, percentage of 8
voids, structural parameters, layer structure, carrier concentration, and lattice strain. Each change of one of these parameters influences the bandgap energies values. However, we showed that the nanoparticles size, lattice parameters, and the micro-strain have a direct effect on the films. Hence, we consider that the observed decrease in energy band gap with increasing the deposition temperature is due to the increase in QDs size and film thickness and therefore a decrease in lattice strain (Table 2 and 3). Therefore, the deposited thin films show a red-shift in their absorbance spectra toward lower energies, because the size of CdSe QDs increases with the increase of the deposition temperature. The redshift of the optical band gap with the increase in the nanoparticles size have been similarly reported by various groups for chemically synthesized CdSe and ZnSe nanoparticles [30–33]. 3.3.2. Dispersive optical parameters of CdSe QDs thin film The values of the optical constants, refractive index (n) and extinction coefficient (k) were calculated from the reflectance R using the following relation [34,35]:
4R 1 R 2 n k 2 (R 1) 1 R
(7)
where k is related to the absorption coeffcient (α) and the wavelength (λ) by k / 4 . The variation of refractive index n and extinction coefficient k as a function of the wavelength are shown in Fig. 9(a) and (b), respectively. From these figures, we can clearly see the increase in both of n and k values when increasing the bath temperature. The refractive index dispersion of the studied CdSe QDs films can be analyzed by the Wemple-DiDomenico (W-D) single effective-oscillator model [36,37]:
n 2 h 1
Ed E 0
E 02 h
2
(8)
where h is the photon energy, E 0 is the energy of the effective dispersion oscillator and considered as an average energy band gap, and E d is the dispersion energy which is linked to the average strength of the inter-band optical transitions. As shown in Fig. 10, the values of the dispersion parameters E 0 and E d can be obtained by plotting (n 2 1)2 vs. (h )2 and adjusting the experimental data to straight line identified by Eq. (8). The oscillator parameters E 0 and E d for CdSe thin films are directly determined from the slope (E 0 E d )1 and the intercept with the vertical axis (E 0 / E d ) , such as shown
9
in Fig. 10. The values of E 0 and E d parameters change with increasing the bath temperature and are given in Table 4. The values of the dispersion parameter E 0 decrease with increasing the deposition temperature. This result confirms that the optical band gap of the CdSe thin films decreases with increasing bath temperature. The oscillator energy E 0 is indicative of an average gap of the material. It is related approximately to the lowest direct band gap E g by E 0 1.5 E Wg D [38].
The real parts (1 ) of dielectric constants of the CdSe thin films were determined by the following relation:
e2 N 2 1 n k 2 c m 2
2
(9)
where is the high-frequency dielectric constant, λ is the wavelength, N / m is the ratio of the carrier concentration (N) to the effective mass (m*), e is the electronic charge, and c is the velocity of light. The plot of (n2–k2) versus λ2 for CdSe thin films is shown in Fig. 11. The values of and N / m have been estimated by extrapolating the straight linear part of this dependence to zero wavelength. From Table 4, it is clear that the value of ε∞ increases with increasing bath temperature. This demeanor can be attributed to the increase in the free carrier concentration. The contribution from the free carrier electric susceptibility ( e ) to the real dielectric constant is discussed using the Spitzer-Fan model by [39]:
e2 N 2 2 4e c m
(10)
Fig. 12 shows (4e ) versus 2 . e increases in magnitude with the wavelength and becomes sufficiently great to reduce the refractive index and the dielectric constant in the near infrared region. The values of the optical parameters extrapolated from the WempleDiDomenico and Spitzer–Fan models are listed in Table 4. 3.3.3. Photoluminescence (PL) spectra Fig. 13 shows the photoluminescence (PL) spectra of CdSe QDs thin films prepared by the CBD method. Results obtained show that after excitation at 380 nm, two PL emission bands can be observed for all samples. The light absorption of this excitation consists of PL band centered between 546.2 and 552.35 nm and PL sub-band centered between 618.63 and 632.57 nm, corresponding to green and orange region, respectively (see Fig. 13). The observed 10
intense PL bands at around 548 nm may originate from the presence of the electron-hole recombination phenomena, trapped at defect sites [40]. The appearance of the bands at 540 nm and 560 nm in the PL spectra of CdSe thin films deposited via spin-coating method has already be reported by Chaure et al. [40]. These results are slightly different from those described by R.S. Singh et al. who observed an intense band in the green region (531 nm) and a less intense one in the blue region (485 nm) [41]. The emission of the PL sub-band centred at 618.63 nm and 632.57 nm could be related to defects, impurities, or trapping of CdSe QDs in the deep states [42,43]. The comparison between the position of the emission PL sub-bands and that of CdSe bulk to 730 nm [44] shows a blueshift which originates from quantum confinement. The increase in the PL intensity and the widening of emission bands are due to the increase in the deposition temperature, because this increase improves the crystallinity of CdSe QDs. Weak changes of the emission band position with bath temperature are observed, indicating the lattice remains the same. Deep states in crystalline nanomaterials are mainly associated with stoichiometric defects, dangling bonds or external atoms such as oxygen [45]. 4. Conclusion CdSe thin films were successfully deposited onto glass substrates using the CBD method at different bath temperatures. AFM micrographs revealed that the synthesized films have a nanocrystalline structure and clearly showed that the size of CdSe QDs is of ca. 4 nm for the deposition temperature of 80°C, which is in agreement with SEM and X-ray diffraction analysis. XRD indicated that the deposited thin films are crystalline with a cubic structure and with the (111) preferred orientation. The decrease of the microstrain and the dislocation density of CdSe thin films with the increase of bath temperature revealed the improvement of the films crystallinity. The bandgap energies (Eg in the range 2.25–2.37 eV) were determined from optical results. PL spectra show a blue shift compared to bulk CdSe, which originates from the quantum confinement of nanoparticles. The intensity of PL spectra increases with increasing the bath temperature, which is attributed to the radiative recombination of the luminescent centers. The optical absorption edge, refractive index and the dielectric constant of the films were found to be influenced by the bath temperature. The refractive index dispersion of the films obeyed to the single oscillator model. The results show that the bath temperature influences the optical bandgaps and the optical parameters Ed, E0, ε∞, and N/m* of CdSe thin films. Finally, the deposited crystalline CdSe thin films are suitable for many optical devices, such as solar cells and gas sensors. 11
Acknowledgements The authors would like to acknowledge Prof. Raphaël Schneider (LRGP, Université de Lorraine) for helpful comments and corrections during the revision of this manuscript.
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[41] R.S. Singh, S. Bhushan, A.K. Singh, S.R. Deo, J Dig., Newspaper of Nanomaterials and Biostructures 6 (2011) 403–412. [42] M. Sharma, A.B. Sharma, N. Mishra, R.K. Pandey, Mater. Res. Bull. 46 (2011) 453–459. [43] C.X. Shan, Z. Liu, S.K. Hark, Phys. Rev. B 74 (2006) 153402. [44] W.Z. Wang, Y. Geng, P. Yan, F.Y. Liu, Y. Xie, Y.T. Qian, J. Am. Chem. Soc. 121 (1999) 4062–4063. [45] K. Sharma, Alaa S. Al-Kabbi, G.S.S. Saini, S.K. Tripathi, Mater. Res. Bull. 47 (2012) 1400–1406.
14
Figures and tables captions Fig. 1. X-ray diffraction patterns of CdSe thin films deposited at different bath temperatures. Fig. 2. Nanoparticle size and FWHM of CdSe thin films. Fig. 3. SEM image of the CdSe thin films prepared at (a) 30°C and (b) 80°C. Fig. 4. EDX pattern of the CdSe thin films prepared at (a) 30°C and (b) 80°C. Fig. 5. 2D and 3D AFM images of CdSe thin films deposited at different bath temperatures: (a) 30°C, (b) 45°C, (c) 55°C, (d) 70°C, (e) 80°C). Fig. 6. Transmittance spectra of CdSe thin films deposited at different bath temperatures. The inset shows the reflectance spectra. Fig. 7. Absorbance spectra of the CdSe thin films deposited at different bath temperatures. Fig. 8. Plot of (αhν)2 versus (hν) of CdSe thin films deposited at 30°C and 80°C. Fig. 9. The variation of (a) refractive index and (b) extinction coefficient with wavelength for the CdSe thin films. Fig. 10. Plots of (n2 − 1)−1 vs. (hν)2 of the CdSe thin films. Fig. 11. Plots of (n2 − k2) vs. λ2 for CdSe thin films prepared with different bath temperatures. Fig. 12. Plots of −4πχe vs. λ2 of CdSe thin films deposited at different bath temperatures. Fig. 13. PL spectra of CdSe thin films prepared with different bath temperatures.
15
Fig. 1.
16
Fig. 2.
17
Fig. 3.
18
Fig. 4.
Element
Weight%
Atomic%
SeL
50.28
58.53
CdL
49.72
41.47
Total
100.00
Element
Weight%
Atomic%
SeL
51.33
59.85
CdL
48.67
39.16
Total
100.00
19
Fig. 5.
20
21
Fig. 6.
22
Fig. 7.
23
Fig. 8.
24
Fig. 9.
25
Fig. 10.
26
Fig. 11.
27
Fig. 12.
28
Fig. 13.
29
Table 1. Experimental XRD data of CdSe thin films of different bath temperature.
JCPDS (65-2891)
30 °C
45 °C
55 °C
70 °C
80 °C
Peaks
2θ
d
2θ
d
2θ
d
2θ
d
2θ
d
2θ
d
(hkl)
(°)
(Å)
(°)
(Å)
(°)
(Å)
(°)
(Å)
(°)
(Å)
(°)
(Å)
(111)
25.48
(220)
42.214 2.1390 42.49 2.1258 42.39 2.1305 42.35 2.1325 42.49 2.1258 42.47 2.1267
(311)
49.956 1.8241 50.35 1.8108 50.31 1.8121 50.33 1.8115 50.54 1.8044 50.38 1.8098
3.4929 25.49 3.4916 25.37 3.5078 25.51 3.4889 25.61 3.4755 25.56 3.4822
30
Table 2. Bath temperature Crystallite size
Lattice parameter
°C
a Å
D nm
Observed
Strain
Dislocation density Number of crystallites
103 1016 lines / m2
per unit area
1018
JCPDS
30
2.33
6.0001
68.29
18.42
64.66
45
2.47
6.0012
63.02
16.39
58.86
55
2.79
6.0027
55.32
12.84
42.04
70
3.48
6.0057
43.77
8.26
23.37
80
3.62
6.0116
38.95
7.63
23.27
6.050
31
Table 3. Bath temperature
UV-Vis (Tauc)
AFM mesurements
°C
Gap energy, Eg (eV)
Roughness, RMS
Crystallite size (nm)
(nm) 30
2.37
0.93
2.5
45
2.34
1.05
2.83
55
2.31
1.18
3.2
70
2.28
1.32
3.6
80
2.25
1.53
4
32
Table 4. Bath Temperature
E
E
(eV )
(eV )
30
3.40
6.33
2.86
45
3.45
8.32
55
3.08
70 80
C
0
d
w
s
E gTauc
E Wg D
N /m*
(eV )
10
3.86
2.37
2.26
3.23
5- 44
3.41
4.69
2.34
2.3
8.15
7- 48
8.79
3.85
5.91
2.31
2.05
3.65
9- 75
3.39
14.43
5.25
7.56
2.28
2.26
12.24
12- 90
3.47
19.60
6.64
9.43
2.25
2.31
16.71
17-120
48
cm
3
e
(eV )
(103 )
33
S0025-5408(16)30075-7 http://dx.doi.org/doi:10.1016/j.materresbull.2016.02.021 MRB 8667
To appear in:
MRB
Received date: Revised date: Accepted date:
17-11-2015 7-2-2016 14-2-2016
Please cite this article as: F.Laatar, A.Harizi, A.Smida, M.Hassen, H.Ezzaouia, Effect of deposition temperature on the structural and optical properties of CdSe QDs thin films deposited by CBD method, Materials Research Bulletin http://dx.doi.org/10.1016/j.materresbull.2016.02.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of deposition temperature on the structural and optical properties of CdSe QDs thin films deposited by CBD method F. Laatara*, A. Harizib, A. Smidaa, M. Hassena,c, and H. Ezzaouiaa a
Photovoltaic Laboratory, Centre for Research and Technology Energy, Tourist Route
Soliman, BP 95, 2050 Hammam-Lif, Tunisia b
Photovoltaic and Semiconductor Materials Laboratory, Engineering Industrial Department,
ENIT, Tunis El Manar University, BP 37, Le Belvédère, 1002 Tunis, Tunisia c
Higher Institute of Applied Science and Technology of Sousse, City Taffala (Ibn Khaldun),
4003 Sousse Tunisia
E-mail adress: [email protected] (F. Laatar)
Graphical abstract
1
Highlights
Synthesis of CdSe QDs with L-Cysteine capping agent for applications in nanodevices.
The films of CdSe QDs present uniform and good dispersive particles at the surface.
Effect of bath temperature on the structural and optical properties of CdSe QDs thin films.
Investigation of the optical constants and dispersion parameters of CdSe QDs thin films.
Abstract Cadmium selenide quantum dots (CdSe QDs) thin films were deposited onto glass substrates by a chemical bath deposition (CBD) method at different temperatures from an aqueous solution containing L-Cysteine (L-Cys) as capping agent. The evolution of the surface morphology and elemental composition of the CdSe films were studied by AFM, SEM, and EDX analyses. Structural and optical properties of CdSe thin films were investigated by XRD, UV-Vis and PL spectroscopy. The dispersion behavior of the refractive index is described using the single oscillator Wemple-DiDomenico (W-D) model, and the physical dispersion parameters are calculated as a function of deposition temperature. The dispersive optical parameters such as average oscillator energy (Eo), dispersion energy (Ed), and static refractive index (no) were found to vary with the deposition temperature. Besides, the electrical free carrier susceptibility (χe) and the carrier concentration of the effective mass ratio (N/m*) were evaluated according to the Spitzer-Fan model. KEYWORDS: A. Thin films, B. Chemical synthesis, B. Optical properties, C. X-ray diffraction, D. dielectric properties.
1. Introduction Cadmium selenide (CdSe) is a solid material that belongs to the II-VI semiconductor group. CdSe has a direct-gap and a strong absorption coefficient in the visible spectral region. For these reasons, CdSe is widely used in photoconductivity and solar cells [1,2], electronic and optoelectronic applications [3], optical sensing [4], and gas sensing [5–7]. Different techniques were developed for the synthesis of CdSe thin films, such as electrodeposition, physical vapour deposition-electron beam evaporation (PVD-EBE) [8], vacuum evaporation [9], hot wall deposition [10], pyrolysis [11], chemical bath deposition (CBD) [12], sol-gel 2
process [13], and hydrothermal route [14]. The synthesis of films using the CBD method is easy and cheaper compared to other processes. Several complexing or stabilizing agent can be used for the preparation of CdSe nanoparticles such as L-Cysteine (L-Cys), triethanolamine (TEA) and nitrilotriacetic acid (NTA) [15–17]. We report here the preparation and the deposition of CdSe QDs thin films by the chemical bath deposition method using L-Cys as a stabilizing agent. The process is based on the dissolution of the CdSe/L-Cys powder at the appropriate temperature under vigorous stirring. This work was conducted to study the effect of the bath temperature on the optical properties of CdSe thin films prepared by the CBD method. Up to now, studies were mainly focused concentrated on the morphological, structural and some optical properties of the prepared thin films, but the effect of the bath temperature on the optical constants and the dispersion parameters has not been investigated in detail. For this reason, we have calculated and estimated some optical parameters such as the refractive index (n), the extinction coefficient (k), the oscillator energy (E0), and the dispersion energy (Ed), as a function of the bath temperature. In this study, we provide a precious contribution to the researchers who are interested in the optical properties of the CdSe nanomaterial as thin film. 2. Materials and methods 2.1. Chemicals Cadmium acetate dihydrate (Cd(OAc)2. 2H2O, ≥ 98%, Sigma Aldrich), selenium dioxide (SeO2, 99.999%, Sigma Aldrich),
L-cysteine (C3H7NO2S, ≥ 97%, Sigma Aldrich), and
sodium borohydride (NaBH4, ≥ 99%, Sinopharm Chemical Reagent) were used as received without additional purification. Ultra-pure water was used for the preparation of all aqueous solutions. Dilute aqueous solutions of sodium hydroxide were used to fix the pH at specific values. 2.2. Synthesis of L-cysteine functionalized CdSe QDs In a typical synthesis, firstly 8.56 mmol of Cd(OAc)2 and 13.8 mmol of L-Cys were dissolved in 100 mL of deionized water to obtain a clear solution. The pH of this solution was adjusted to 11.2 by dropwise addition of a 1 M aqueous solution of NaOH. An aqueous solution of Na2SeO3 (prepared by dissolving SeO2 into a NaOH solution) was added dropwise to the Cd(OAc)2/L-Cys mixture under vigorous stirring and the mixture was saturated with N2. Next, a NaBH4 aqueous solution was added to the previous mixture under vigorous stirring. 3
The molar ratio of Cd2+/L-Cys/Se2- was set as 1/2/0.5. The precursors were converted to CdSe QDs by refluxing the reaction mixture at 100°C for 3 h under N2. This conversion is accompanied by a change of the color of the solution to yellow. After cooling, the reaction mixture was concentrated down to 25 mL using an evaporator. Isopropanol was added to the CdSe nanoparticles dispersion and the mixture stirred for 1 h. CdSe QDs were recovered by filtration and dried under vacuum. In our synthesis, the L-Cys/Cd molar ratio equal to 2 was used to obtain a good coverage and a total saturation of the dangling bonds at the nanocrystals surface, leading to the inhibition of the non-radiative transitions. 2.3. Deposition of CdSe QDs onto glass substrate The glass substrates were successively cleaned in detergent solution, deionized water, and ethanol for 10 min in an ultrasonic bath. The CdSe thin films were prepared at different bath temperatures ranging from 30 to 80°C during 20 min. The as-deposited films were dried in air at 100°C during 10 min using a hot plate. 2.4. Characterization The crystallographic structure was carried out by X-ray diffraction technique (XRD) using a Bruker D 8 advance X-ray diffractometer with Cu Kα radiation (λCuKα = 1.5406 Å) operated at 40 kV and 40 mA. In order to correct/adjust the contribution from the instrumental effects to the experimental full-width at half-maximum (FWHM), βexp for each peak was obtained using the following relation [18,26,28]: 1
1 2 2 2 2 exp ins exp ins
(1)
where βins is the instrumental FWHM which was obtained using crystallized NaCl. The thickness of the deposited films at bath temperatures of 30, 45, 55, 70, and 80°C were measured as 818, 887, 913, 985, and 1104 nm by using profilometer Veeco Dektak 150. The microstructure of CdSe nanoparticles deposited on glass substrate at bath temperatures of 30°C and 80°C were characterized using a Scanning Electron Microscopy (SEM) ZEISS Ultra Plus FE-SEM that was equipped by EDX-SEM micrographs with an accelerating voltage of 2.5 kV. The elemental compositions of films were examined by Energy Dispersive X-ray (EDX) analysis. The surface morphology of the prepared CdSe thin films was investigated by using Atomic Force Microscopy (AFM) in tapping mode by a Nanoscope III (Veeco AFM head RTESP silicon pur) to a scanning area of 1 μm x 1 μm. The optical transmittance T(λ) 4
and reflectance R(λ) of the CdSe/L-Cys thin films were measured on a UV-vis-NIR Lambda 950 spectrophotometer, equipped with an integrating sphere, in the wavelength range 300– 850 nm. Photoluminescence measurements were recorded on a Perkin-Elmer LS 55 spectrometer using a Xenon light source with an excitation wavelength of 380 nm. 3. Results and discussion 3.1. Structural properties A survey of available literature shows that the CdSe thin films can be hexagonal (wurtzite) [19], cubic (zinc-sphalerite) [20] or of a mixed structure [JCPDS files No. 19-191 and 652891]. XRD was used to confirm the crystalline structure of CdSe thin films deposited on glass substrates at different bath temperatures. As shown in Fig. 1, CdSe films present the highest intensities reflections along the (111) plane and two weak reflections along the (220) and (311) planes at 25.49°, 42.49°, and 50.35°, respectively. These results indicate that the films are crystalline and composed of CdSe in cubic phase with preferred orientation along the (111) plane. In Table 1, the observed d-spacing and the identification of the diffraction peaks were compared using JCPDS data No. 65-2891 as reference and with powder diffraction files that confirm that the preferred orientation along (111) direction [15]. The lattice parameter (a) of the CdSe thin films was calculated for the cubic structure using the relation: a d hkl h 2 k 2 l 2
1/2
(2)
As the deposition temperature increases, the X-ray diffraction peaks are not shifted, indicating that the CdSe films have the same composition, regardless of the temperature used for the deposition. Fig. 1 shows also that the intensities of peaks increase with the deposition temperatures, further indicating that the crystallinity increased. XRD patterns can also be used to determine the nanoparticle size (D), the dislocation density (ρ), the micro-strain (ε), and the number of crystallites per unit area (N). The average particle sizes of films deposited at different bath temperatures were calculated from the highest peaks intensity using the DebyeScherrer’s equation [21]:
D
k cos
(3)
5
where D is the particle size of CdSe particles, λ is the X-ray wavelength (1.5406 Å), θ is Bragg diffraction angle, β the full-width at half-maximum (FWHM) of the peak, and k is a constant usually taken equal to 0.9. Fig. 2 shows the variation of the particle sizes versus deposition temperatures. Results obtained indicate that the crystallite sizes increased with the deposition temperature and that the largest crystallites were obtained at 80°C. The increase in the particle size is probably due to the coalescence of small nanoparticles during the heat treatment. The micro-strain (ε), was calculated by using the following relation [22]:
4 tan
(4)
The average dislocation density (δ) of synthesized CdSe thin films onto glass substrates was determined using the formula [23]:
1 D2
(5)
The calculated values of dislocation density (δ) and micro-strain (ε) are summarized in Table 2. The micro-strain decreases as a function of the deposition temperature, which leads to the decrease in the concentration of the lattice imperfections. This could also explain the increase in nanoparticles size. From these analyses, we evaluated in this study the key role of the bath temperature for the enhancement of crystal quality of thin films. Dislocation density decreases with increasing of bath temperature. It is well-known that the ρ indicates the quantity of defects in the structure. So, the decrease of defects in the volume of samples with the increase of bath temperature resulted the reduction in dislocations as well as the improvement of the films crystallinity. The number of particle per unit area can be calculated by using the following relation [24]:
N
t D3
(6)
where t is the CdSe thin film thickness. The calculated number of particle per unit area in the CdSe thin films are summarized in Table 2. The increase of bath temperature causes the decrease of the number of particles per unit area. This reduction in the number of crystallites indicates that the elaborated films at low bath temperature are composed of a large number of small particles, and that with the increase of the bath temperature, small particles probably coalescence to form the agglomerates.
6
3.2. CdSe thin films morphology 3.2.1. SEM and EDX analyses Fig. 3(a) and (b) show the Scanning Electron Microscopy (SEM) micrographs of the deposited CdSe QDs thin films at deposition temperatures of 30°C and 80°C, respectively. From these figures, we observe that QDs sizes were relatively uniform with a spherical arrangement. The average diameters of CdSe QDs in these films were about ~2 nm and ~4 nm, respectively, which is in good agreement with XRD results. Energy dispersive X-ray analysis (EDX) was used to determine the elements composition of the synthesized CdSe thin films at deposition temperature of 30°C and 80°C. Fig. 4(a) and (b) show the EDX patterns of thin films deposited at 30°C and 80°C, respectively, and the relative elemental composition are depicted in the corresponding tables. The results of these analyses confirms that the composition of the prepared films consists of cadmium and selenium with a molar ratio of ∼1:1, indicating that these elements are in a nearstoichiometric ratio. A small compositional variation is observed between the deposited CdSe thin films at 30 and 80°C, which can be caused by the rearrangement of atoms. 3.2.2. AFM analysis AFM was used to explore the external morphology of CdSe thin films surface prepared by the CBD method on glass substrate at different bath temperatures. In order to evaluate the QDs size as well as the surface roughness of films, an area of 1 μm x 1 μm was scanned in tapping mode. The Root-Mean-Square roughness (RMS) and the nanocrystallite sizes were calculated using standard software (WS × M 5.0 software) [25–28]. The diameter of a nanoparticle was defined as the half of its height. Fig. 5 illustrates the bi-dimensional (2D) and three-dimensional (3D) AFM images of CdSe thin films deposited on glass substrates at 30, 45, 55, 70 and 80°C. These images show that the bath temperature has an influence on the surface morphology of CdSe films. It can be seen that with the increase of the bath temperature, the size of nanoparticles increases and consequently voids decrease. As regards the deposited film at low bath temperature (30°C), the development process of particles is carried out to form a discontinuous film. The difficulty is engendered by the low bath temperature, which results in a high percentage of voids and to smaller CdSe QDs compared to films prepared at high temperature (80°C). The deposited thin film at 55°C is 7
characterized by an homogeneous particles structure and separated by the grains boundaries. This uniformity on the film surface attests for a deposition method efficient for decreasing the density of voids in the CdSe thin films with increasing bath temperature. In Fig. 5(a), it is shown that the deposited films are compact and dense. The particles grown have a spherical morphology and are homogeneously distributed over the substrate surface. The prepared CdSe QDs by CBD at higher temperature (80°C) reveal a roughly inhomogeneous surface with a random grain dispersion. However, the CdSe QDs show an increase in size as the bath temperature increases to 80°C. The films prepared at higher bath temperature indicate that the smaller nanocrystals are accumulated to form grains or agglomerates. Finally, we can conclude that an increase in the deposition temperature improves the nanoparticle size and the film crystallinity. The instrumental scan of the AFM microscopy clearly shows that the particle size of CdSe QDs increases from ~2.5 to ~4 nm and the film roughness increases from 0.93 to 1.53 nm (Table 3). The value of nanoparticle size obtained by AFM were confirmed by SEM analysis and XRD patterns using the Debye-Scherrer formula. 3.3. Optical properties 3.3.1. Optical absorption and optical energy gap of CdSe QDs thin film Fig. 6(a) and (b) show the transmittance (T) and the reflectance (R) spectra of CdSe thin films deposited at different bath temperatures in the wavelength range 300–850 nm. All the transmittance spectra show that the absorption edge is between 500 nm and 564 nm and the average transmission in the wavelength range 500–850 nm is between 38 % and 46 %. As described in Fig. 6(a), the film transmittance decreases with the increase in bath temperature from 30°C up to 80°C. This originates from the reduction of voids concentration and from the increase of film thickness. The variations of the absorbance of CdSe thin films depending on the deposition temperature, in the wavelength range 350–850 nm are shown in Fig. 7. The optical absorption increases with increasing the deposition temperature. This variation is due to the increase in the CdSe QDs size and to the decrease in defects. The energy band gap of CdSe films was calculated by extrapolating the straight line-portion of ( h )2 vs. (h ) to ( h )2 = 0 [29], as shown in Fig. 8 and Table 3. Fig. 8 shows the band-gap dependence of CdSe thin films vs the bath temperature. It can be seen that the bandgap energy decreases from 2.37 to 2.25 eV as the bath temperature increases from 30 to 80°C. The increase of the deposition temperature has an effect on various parameters such as particle size, presence of impurities, percentage of 8
voids, structural parameters, layer structure, carrier concentration, and lattice strain. Each change of one of these parameters influences the bandgap energies values. However, we showed that the nanoparticles size, lattice parameters, and the micro-strain have a direct effect on the films. Hence, we consider that the observed decrease in energy band gap with increasing the deposition temperature is due to the increase in QDs size and film thickness and therefore a decrease in lattice strain (Table 2 and 3). Therefore, the deposited thin films show a red-shift in their absorbance spectra toward lower energies, because the size of CdSe QDs increases with the increase of the deposition temperature. The redshift of the optical band gap with the increase in the nanoparticles size have been similarly reported by various groups for chemically synthesized CdSe and ZnSe nanoparticles [30–33]. 3.3.2. Dispersive optical parameters of CdSe QDs thin film The values of the optical constants, refractive index (n) and extinction coefficient (k) were calculated from the reflectance R using the following relation [34,35]:
4R 1 R 2 n k 2 (R 1) 1 R
(7)
where k is related to the absorption coeffcient (α) and the wavelength (λ) by k / 4 . The variation of refractive index n and extinction coefficient k as a function of the wavelength are shown in Fig. 9(a) and (b), respectively. From these figures, we can clearly see the increase in both of n and k values when increasing the bath temperature. The refractive index dispersion of the studied CdSe QDs films can be analyzed by the Wemple-DiDomenico (W-D) single effective-oscillator model [36,37]:
n 2 h 1
Ed E 0
E 02 h
2
(8)
where h is the photon energy, E 0 is the energy of the effective dispersion oscillator and considered as an average energy band gap, and E d is the dispersion energy which is linked to the average strength of the inter-band optical transitions. As shown in Fig. 10, the values of the dispersion parameters E 0 and E d can be obtained by plotting (n 2 1)2 vs. (h )2 and adjusting the experimental data to straight line identified by Eq. (8). The oscillator parameters E 0 and E d for CdSe thin films are directly determined from the slope (E 0 E d )1 and the intercept with the vertical axis (E 0 / E d ) , such as shown
9
in Fig. 10. The values of E 0 and E d parameters change with increasing the bath temperature and are given in Table 4. The values of the dispersion parameter E 0 decrease with increasing the deposition temperature. This result confirms that the optical band gap of the CdSe thin films decreases with increasing bath temperature. The oscillator energy E 0 is indicative of an average gap of the material. It is related approximately to the lowest direct band gap E g by E 0 1.5 E Wg D [38].
The real parts (1 ) of dielectric constants of the CdSe thin films were determined by the following relation:
e2 N 2 1 n k 2 c m 2
2
(9)
where is the high-frequency dielectric constant, λ is the wavelength, N / m is the ratio of the carrier concentration (N) to the effective mass (m*), e is the electronic charge, and c is the velocity of light. The plot of (n2–k2) versus λ2 for CdSe thin films is shown in Fig. 11. The values of and N / m have been estimated by extrapolating the straight linear part of this dependence to zero wavelength. From Table 4, it is clear that the value of ε∞ increases with increasing bath temperature. This demeanor can be attributed to the increase in the free carrier concentration. The contribution from the free carrier electric susceptibility ( e ) to the real dielectric constant is discussed using the Spitzer-Fan model by [39]:
e2 N 2 2 4e c m
(10)
Fig. 12 shows (4e ) versus 2 . e increases in magnitude with the wavelength and becomes sufficiently great to reduce the refractive index and the dielectric constant in the near infrared region. The values of the optical parameters extrapolated from the WempleDiDomenico and Spitzer–Fan models are listed in Table 4. 3.3.3. Photoluminescence (PL) spectra Fig. 13 shows the photoluminescence (PL) spectra of CdSe QDs thin films prepared by the CBD method. Results obtained show that after excitation at 380 nm, two PL emission bands can be observed for all samples. The light absorption of this excitation consists of PL band centered between 546.2 and 552.35 nm and PL sub-band centered between 618.63 and 632.57 nm, corresponding to green and orange region, respectively (see Fig. 13). The observed 10
intense PL bands at around 548 nm may originate from the presence of the electron-hole recombination phenomena, trapped at defect sites [40]. The appearance of the bands at 540 nm and 560 nm in the PL spectra of CdSe thin films deposited via spin-coating method has already be reported by Chaure et al. [40]. These results are slightly different from those described by R.S. Singh et al. who observed an intense band in the green region (531 nm) and a less intense one in the blue region (485 nm) [41]. The emission of the PL sub-band centred at 618.63 nm and 632.57 nm could be related to defects, impurities, or trapping of CdSe QDs in the deep states [42,43]. The comparison between the position of the emission PL sub-bands and that of CdSe bulk to 730 nm [44] shows a blueshift which originates from quantum confinement. The increase in the PL intensity and the widening of emission bands are due to the increase in the deposition temperature, because this increase improves the crystallinity of CdSe QDs. Weak changes of the emission band position with bath temperature are observed, indicating the lattice remains the same. Deep states in crystalline nanomaterials are mainly associated with stoichiometric defects, dangling bonds or external atoms such as oxygen [45]. 4. Conclusion CdSe thin films were successfully deposited onto glass substrates using the CBD method at different bath temperatures. AFM micrographs revealed that the synthesized films have a nanocrystalline structure and clearly showed that the size of CdSe QDs is of ca. 4 nm for the deposition temperature of 80°C, which is in agreement with SEM and X-ray diffraction analysis. XRD indicated that the deposited thin films are crystalline with a cubic structure and with the (111) preferred orientation. The decrease of the microstrain and the dislocation density of CdSe thin films with the increase of bath temperature revealed the improvement of the films crystallinity. The bandgap energies (Eg in the range 2.25–2.37 eV) were determined from optical results. PL spectra show a blue shift compared to bulk CdSe, which originates from the quantum confinement of nanoparticles. The intensity of PL spectra increases with increasing the bath temperature, which is attributed to the radiative recombination of the luminescent centers. The optical absorption edge, refractive index and the dielectric constant of the films were found to be influenced by the bath temperature. The refractive index dispersion of the films obeyed to the single oscillator model. The results show that the bath temperature influences the optical bandgaps and the optical parameters Ed, E0, ε∞, and N/m* of CdSe thin films. Finally, the deposited crystalline CdSe thin films are suitable for many optical devices, such as solar cells and gas sensors. 11
Acknowledgements The authors would like to acknowledge Prof. Raphaël Schneider (LRGP, Université de Lorraine) for helpful comments and corrections during the revision of this manuscript.
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Figures and tables captions Fig. 1. X-ray diffraction patterns of CdSe thin films deposited at different bath temperatures. Fig. 2. Nanoparticle size and FWHM of CdSe thin films. Fig. 3. SEM image of the CdSe thin films prepared at (a) 30°C and (b) 80°C. Fig. 4. EDX pattern of the CdSe thin films prepared at (a) 30°C and (b) 80°C. Fig. 5. 2D and 3D AFM images of CdSe thin films deposited at different bath temperatures: (a) 30°C, (b) 45°C, (c) 55°C, (d) 70°C, (e) 80°C). Fig. 6. Transmittance spectra of CdSe thin films deposited at different bath temperatures. The inset shows the reflectance spectra. Fig. 7. Absorbance spectra of the CdSe thin films deposited at different bath temperatures. Fig. 8. Plot of (αhν)2 versus (hν) of CdSe thin films deposited at 30°C and 80°C. Fig. 9. The variation of (a) refractive index and (b) extinction coefficient with wavelength for the CdSe thin films. Fig. 10. Plots of (n2 − 1)−1 vs. (hν)2 of the CdSe thin films. Fig. 11. Plots of (n2 − k2) vs. λ2 for CdSe thin films prepared with different bath temperatures. Fig. 12. Plots of −4πχe vs. λ2 of CdSe thin films deposited at different bath temperatures. Fig. 13. PL spectra of CdSe thin films prepared with different bath temperatures.
15
Fig. 1.
16
Fig. 2.
17
Fig. 3.
18
Fig. 4.
Element
Weight%
Atomic%
SeL
50.28
58.53
CdL
49.72
41.47
Total
100.00
Element
Weight%
Atomic%
SeL
51.33
59.85
CdL
48.67
39.16
Total
100.00
19
Fig. 5.
20
21
Fig. 6.
22
Fig. 7.
23
Fig. 8.
24
Fig. 9.
25
Fig. 10.
26
Fig. 11.
27
Fig. 12.
28
Fig. 13.
29
Table 1. Experimental XRD data of CdSe thin films of different bath temperature.
JCPDS (65-2891)
30 °C
45 °C
55 °C
70 °C
80 °C
Peaks
2θ
d
2θ
d
2θ
d
2θ
d
2θ
d
2θ
d
(hkl)
(°)
(Å)
(°)
(Å)
(°)
(Å)
(°)
(Å)
(°)
(Å)
(°)
(Å)
(111)
25.48
(220)
42.214 2.1390 42.49 2.1258 42.39 2.1305 42.35 2.1325 42.49 2.1258 42.47 2.1267
(311)
49.956 1.8241 50.35 1.8108 50.31 1.8121 50.33 1.8115 50.54 1.8044 50.38 1.8098
3.4929 25.49 3.4916 25.37 3.5078 25.51 3.4889 25.61 3.4755 25.56 3.4822
30
Table 2. Bath temperature Crystallite size
Lattice parameter
°C
a Å
D nm
Observed
Strain
Dislocation density Number of crystallites
103 1016 lines / m2
per unit area
1018
JCPDS
30
2.33
6.0001
68.29
18.42
64.66
45
2.47
6.0012
63.02
16.39
58.86
55
2.79
6.0027
55.32
12.84
42.04
70
3.48
6.0057
43.77
8.26
23.37
80
3.62
6.0116
38.95
7.63
23.27
6.050
31
Table 3. Bath temperature
UV-Vis (Tauc)
AFM mesurements
°C
Gap energy, Eg (eV)
Roughness, RMS
Crystallite size (nm)
(nm) 30
2.37
0.93
2.5
45
2.34
1.05
2.83
55
2.31
1.18
3.2
70
2.28
1.32
3.6
80
2.25
1.53
4
32
Table 4. Bath Temperature
E
E
(eV )
(eV )
30
3.40
6.33
2.86
45
3.45
8.32
55
3.08
70 80
C
0
d
w
s
E gTauc
E Wg D
N /m*
(eV )
10
3.86
2.37
2.26
3.23
5- 44
3.41
4.69
2.34
2.3
8.15
7- 48
8.79
3.85
5.91
2.31
2.05
3.65
9- 75
3.39
14.43
5.25
7.56
2.28
2.26
12.24
12- 90
3.47
19.60
6.64
9.43
2.25
2.31
16.71
17-120
48
cm
3
e
(eV )
(103 )
33