The influence of CdS quantum dots incorporation on the properties of CdO thin films

Eur. Phys. J. Appl. Phys. (2013) 64: 30303

DOI:

The influence of CdS quantum dots incorporation on the

properties of CdO thin films

DOI:10.1051/epjap/2013130418

PHYSICAL

JOURNAL

A

P

Regular Article

The influence of CdS quantum dots incorporation on the

properties of CdO thin films

Ayta¸c G¨ultekin1,a, Gamze Karanfil1, Faruk ¨Ozel2, Mahmut Ku¸s2, Rıdvan Say3, and Sava¸s S¨onmezo˘glu4

1

Department of Energy Systems Engineering, Faculty of Engineering, Karamano˘glu Mehmetbey University, 70100 Karaman, Turkey

2

Department of Chemical Engineering, Sel¸cuk University, Advanced Technology Research and Application Center, 42075 Konya, Turkey

3

Department of Chemistry, Faculty of Science, Anadolu University, 26470 Eski¸sehir, Turkey 4

Department of Materials Science and Engineering, Faculty of Engineering, Karamano˘glu Mehmetbey University, 70100 Karaman, Turkey

Received: 6 September 2013 / Received in final form: 30 October 2013 / Accepted: 6 November 2013 Published online: 9 December 2013 – c EDP Sciences 2013

Abstract. The aim of our work is to obtain nano-structured cadmium oxide (CdO) thin films by sol-gel spin

coating method and to investigate the effects of cadmium sulfide quantum dots (CdS QDs) doping on the structural modification and surface morphology evolution. X-ray diffraction (XRD) results show that the intensities of the peaks of the crystalline phase increase with the increase in CdS QDs concentrations. From scanning electron microscopy (SEM) images, the distinct variations in the morphology of the thin films were also observed. In addition, the evolution of surface morphology, roughness and granularity has been characterized by atomic force microscopy (AFM). Moreover, we have performed the optical characteristics of the thin films such as transparency, energy band gap and Urbach tail. The optical band gap of the thin films increases from 2.23 to 2.51 eV with the increase in CdS QDs concentrations due to the Moss–Burstein effect. The enhanced values of the transparency, energy band gap and crystallity indicate that addition of CdS QDs can be used to modify the optical, structural and morphological properties of CdO thin films.

1 Introduction

Recently a new wave of interest has risen on quantum dots (QDs) due to its inexhaustible industrial applica-tions. The reason for the wide acceptance of QDs can be found by its fascinating photophysical properties due to the its quantum confinement effect. Confinement ef-fects make QDs become promising materials possessing unique optical properties such as tunable photolumines-cence spectra with narrow bandwidth, high quantum yield, multiple electron-hole pair generation, very broad absorption spectrum, an outstanding photostability and fast response non-linear refractive index [1–3]. In addi-tion, nanometer-sized QDs colloids can be used to degrade water pollutants as efficient photocatalyst in environmen-tal technologies due to the large surface-to-volume ratio. Furthermore, due to the distinguished photoluminescence and electroluminescence properties, QDs have been ex-plored extensively for various applications, including imaging, detection, therapy, display, energy harvesting, and so on [2–8].

a

e-mail:aysari@yahoo.com

An example can be the use of cadmium sulfide (CdS) or cadmium tellurium sulfide (CdTeS) QDs with titanium dioxide (TiO2) nanowires: Medina-Gonzalez et al. recently

reported an efficiency increase of over 300% and 350% for CdS and CdTeS, respectively [9]. In optoelectronics, QDs are now being considered for a new generation of light emitting diodes (LEDs); these devices, if compared with the traditional ones used now, will supply more energy efficiency and produce brighter colors [10]. Biological ap-plications of the QDs can help to diagnose a disease, or to establish a possible contamination with dangerous bacter-ial strains [11–13]. QDs for these and similar applications are already commercially available.

The most common QDs are the binary semiconductor compounds consisting of II-VI elements, i.e., cadmium sul-fide (CdS), cadmium selenide (CdSe), cadmium telluride (CdTe) and zinc selenide (ZnSe), etc. Among the QDs mentioned above, CdS is a promising material for appli-cations in biology, solar cells, lasers and gas sensors. CdS which exhibit quantum size effects in electrical and optical properties with a band gap of 2.42 eV and high tunabil-ity [14,15]. Furthermore, CdS QDs allow oxide semicon-ductors to improve their optic, structural and electrical

The European Physical Journal Applied Physics

properties when embedded to the structure as a doping material [16–18].

The oxide semiconductor material, especially CdO shows very high electrical conductivity even without doping due to the existence of shallow donors caused by intrinsic interstitial cadmium atoms and oxygen vacan-cies [19]. Another advantage of the cheap CdO material is high transparency in the visible region of the solar spec-trum [20]. Beside, CdO is an n-type semiconductor with a rock-salt crystal structure, possesses a direct band gap of 2.2 eV [21,22]. It has a non-stoichiometric composition due to the presence of either interstitial cadmium or oxy-gen vacancies, which act as doubly charged donors [23].

As we know from the literature, no work has been pre-sented to date providing a detailed explanation for physi-cal parameters of CdS QDs doped CdO thin films. The aim of this study is to synthesize the nano-structured CdO thin films and CdS QDs by sol-gel method and Brust method, respectively and to investigate the effects of CdS QDs dop-ing on the optical, structural and surface morphology evo-lution of nano-structured CdO thin films.

2 Experimental procedure

In order to prepare CdS QDs, cadmium acetate dihydrate [Cd(OAc)2+2H2O] solution (0.01 M, 6 mL) was prepared

with ethanol [C2H6O]. Solution was stirred continuously

for 30 min in nitrogen ambient. Na2S (0.01 M, 6 mL) was

slowly added, stirred under nitrogen ambient for 30 min and then centrifuged to collect precipitate. It was washed in double distilled water and dried in air. The entire syn-thesis was carried out at room temperature, followed by redispersion in ethanol. A TEM (transmission electron mi-croscopy) image of CdS QDs using on a FEI TecnaiTM G2 Sprit transmission electron microscope 20–120 kV) is given in Figure1. The shape of these nanoparticles is close to spherical. They have been aggregated and formed nano-sized with average size about 43 nm.

In order to prepare the CdO solution, first, 1 mol cadmium acetate [Cd(CH3COO)2 + 2H2O] was added in

46 mol methanol solvent [CH3OH] and the obtained

solu-tion was kept in a magnetic stirrer for 1 h. Then, 0.2 mol glycerol [C3H8O3] and 0.5 mol triethylamine [C8H15N]

were added in the solution, and after, it was mixed in the magnetic stirrer for 1 h. Moreover, to obtain the CdS QDs doped CdO solution, CdS QDs was added into the undoped CdO solution at various concentrations (1%, 2%, 3%, 4%, 5%), and the solutions were subjected to the mag-netic stirrer for two additional hours. Finally, the pure and CdS QDs doped CdO solutions were aged at room tem-perature for one day before deposition. These films, i.e., pure, 1%, 2%, 3%, 4% and 5% CdS QDs doped CdO were labeled as C0, C1, C2, C3, C4 and C5, respectively.

Microscope glass slides were used as the substrates for thin films. Prior to deposition, the glass slides were se-quentially cleaned in an ultrasonic bath with acetone and ethanol. Finally they were rinsed with distilled water and dried. After the above treatment, spin coating process was used to deposited the solutions on the glass substrates.

Length: 43.62 nm

Length: 42.49 nm

Fig. 1. TEM image of CdS QDs.

The spinning process was performed using Holmarc spin coating unit and coating was done by rapidly deposit-ing ∼0.6 mL of solution onto a glass substrate spun at 6000 rpm for 30 s in air. This coating step was repeated five times to obtain as-deposited thin films. After each spinning process, samples were subjected to repeated an-nealing processes at a temperature of 300◦C for a 5-min period and finally were post-annealed at the temperature of 500 ◦C for 1 h in oven.

The crystallographic phase and crystallite size of all thin films have been determined by X-ray diffractome-ter over the range 30◦–70◦. One-dimensional data were collected using a Bruker D8 advance diffractometer with a diffracted beam monochromator and CuK_α radiation. Moreover, the morphologic characterizations of the CdO thin films have been performed by Zeiss LS-10 scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. The transmittance spectra are also recorded from 350 to 1500 nm wavelength with a data interval of 1 nm using Shimadzu 3600 UV-vis-NIR spec-trophotometer. Furthermore, the thicknesses of all of the thin films were measured by Woollam Vase M-2000 model ellipsometry.

3 Results and discussion

The optical transmission spectrum of CdO thin films de-posited at various CdS QDs concentrations is shown in Figure2. It is seen that the transmission improved signif-icantly after CdS QDs doping. At the longer wavelengths, the transmittance of the CdO thin films was found to vary from about 75% to 90% depending on the CdS QDs concentrations in solution. Similar results caused by QDs

500 750 1000 1250 1500 0 10 20 30 40 50 60 70 80 90 100 C0 C1 C2 C3 C4 C5 Transmission (%) Wavelength (nm)

Fig. 2. Optical transmission spectra of CdO thin films doped

at various CdS QDs concentrations.

effect have been reported in the literature [24]. This effect of CdS QDs doping on the transmission of CdO thin films can be attributed to the structural and surface effects [25]. These effects such as better crystallinity, less surface irreg-ularity, less surface roughness (surface scattering reduces the specular transmission, which in turn depends on the grain size and shape) and less defect density can increase the transparency [25]. However, the maximum value of transmittance for all investigated samples is not lying in the visible region but at longer wavelengths.

The optical band gap of the films is resolved by apply-ing the Tauc’s relation in the high-absorbance region by using the following equation [26]:

(αhυ) = A (hυ − Eg)r, (1)

where A is a constant, hυ is the photon energy, Egis the optical band gap of the material and the exponent r = 1/2 stands for the allowed direct transitions, since it gives the best linear graph in the band edge region and α is the absorption coefficient obtained by:

α = 1 d  ln  1 T  , (2)

where d is the thickness of the films as shown in Table 1. The absorbance spectra of the samples are illustrated in Figure 3 and the variation of Eg with the % CdS QDs (pure, 1%, 2%, 3%, 4% and 5%) is shown graphically in the inset of Figure3. Eg values which can be obtained by

extrapolating the linear portion to the photon energy axis, are given in Table1. As shown in Figure3and Table1, the increase in the Eg values (from 2.23 to 2.51 eV) could be

attributed to the CdS QDs incorporated into the CdO lat-tice. On the other hand, with increase in the amount of im-purity added, excess carriers are supplied by the imim-purity leading to blue shift in the absorption spectrum [27–29]. The blue shift of the absorption edge can be interpreted

500 750 1000 1250 1500 0,0 0,2 0,4 0,6 0,8 1,0 2,0 2,2 2,4 2,6 2,8 (α hν ) 2 × 10 13 (eV/m) 2 hν (eV) C0 C1 C2 C3 C4 C5 Absorbance (A) Wavelength (nm)

Fig. 3. Optical absorbance spectra of CdO thin films doped at

various CdS QDs concentrations. The inset shows the variation of Eg with at% CdS QDs doping level.

Table 1. Some optical constants of CdO thin films at various

CdS QDs concentrations.

Samples d (nm) EgTauc (eV) Eu(eV) C0 201± 0.21 2.23 ± 0.006 0.656 ± 0.000019 C1 209± 0.21 2.25 ± 0.006 0.625 ± 0.000016 C2 227± 0.22 2.38 ± 0.007 0.556 ± 0.000047 C3 240± 0.20 2.42 ± 0.004 0.471 ± 0.000049 C4 261± 0.22 2.49 ± 0.007 0.404 ± 0.000025 C5 274± 0.16 2.51 ± 0.003 0.388 ± 0.000031

by the effect of band gap widening or the Moss-Burstein effect. According to the Moss-Burstein model, the absorp-tion edge of a degenerate n-type semiconductor like CdO is shifted towards higher energies by an amount propor-tional to the electron density in the conduction band. The effect occurs when the carrier concentration exceeds con-duction band edge density of states, which corresponds to degenerate doping in semiconductors.

The absorption coefficient near the band edge shows an exponential dependence on photon energy [30]. The dependence of the optical absorption coefficient with pho-ton energy may arise from electronic transitions between localized states. The longer band tail observed in Figure4

can be caused by the variation in pore size and shape of the crystallites. The extrapolation of linear part of the curve where the transition from localized valence band states to extended conduction band states occurs. This curve will express the Urbach energy. The Urbach edge is determined by the degree of disorder (charged impuri-ties), structural defects such as broken or dangling bonds,

The European Physical Journal Applied Physics 2,0 2,2 2,4 2,6 2,8 13 14 15 16 17 ln ( α )

Photon energy (eV) C0 C1 C2 C3 C4 C5

Fig. 4. The variety of Urbach energies of the thin films with

CdS QDs content.

vacancies, non-bridging atoms or chain ends in the con-sidered semiconductor materials [31]. In other words, the Urbach energy represents a quantitative characteristic of static disorder on the material and reflects the band tail extent of the density of electron states. The slope of the linear dependence of ln α on photon energy follows the exponential relation [32,33]: α = α0exp  hυ Eu  , (3)

where α0is a constant and Euis the Urbach energy which

corresponds to the width of the band tail. The estimated values of Eufrom the Figure 4decreases with increase in

CdS QDs concentration as shown in Table 1. The width of the Urbach tail has decreased in the high concentration CdS QDs doped CdO thin films, which may be due to the strength of interband optical transitions (dispersion energy) and the impurity levels introduced by doping in the band structure [34,35].

3.2 Structural analysis

The XRD pattern of CdO thin films doped at different concentrations of CdS QDs are given in Figure 5. The presence of multiple diffraction peaks of (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) in the diffraction patterns re-vealed that all the films were polycrystalline with the cu-bic crystal structure (JCPDS-Card No. 03-065-2908). All of these results can be seen in Figure5and they are in ac-cordance with the literature [36,37]. Furthermore, it also confirms the synthesized structures was free of impuri-ties as it does not contain any characteristics XRD peaks other than CdO peaks. The amount of CdS QDs in the films might be too small to generate visible peaks for CdS. It is important to note that with increasing dopant concentration, a narrowing for all peaks was observed.

30 35 40 45 50 55 60 65 70 C0 C1 C2 C3 C4 C5 25 30 35 40 D (nm) Samples (111) (200) (220) (222) (311) C0 C1 C2 C3 C4 C5 Intensity (a.u.) 2θ (degree)

Fig. 5. X-ray diffraction pattern of CdO thin films at various

CdS QDs concentrations. In the inset of figure, the change of crystallite sizes of the (1 1 1) peaks according to the CdS QDs concentrations.

The particle size of each sample was calculated from the peak width at full width at half-maximum (FWHM) of a peak using the Scherrer equation for the (1 1 1) plane [38]:

D = 0.9λ

β cos θ, (4)

where D is the crystallite size (nm), λ is the wavelength of CuKα radiation (nm), θ is the Bragg angle and β is FWHM of diffraction peak. Furthermore, the crystallite sizes have been estimated from the FWHM of the (1 1 1) peaks. Crystallite size versus dopant concentrations graph is plotted in the inset of Figure5. It is clearly seen that the crystallite size increases from 27 to 40 nm with increase in CdS QDs concentrations. XRD measurements confirm that crystallinity of CdO thin films have been effectively enhanced by the incorporation of CdS QDs.

3.3 Morphological analysis

Surface morphology of pure and CdS QDs doped CdO thin films was investigated by examining the SEM results at 20 k× magnification (scale; 200 nm). The microstruc-ture of the films was influenced by the doping of CdS QDs as shown in Figure 6. It can be seen that all samples ad-here well to the substrate without any cracks and all of them (especially C1, C2 and C5) have a quite homoge-neous surface morphology. These images also proved that when CdS QDs concentration reaches up to 2%, an ag-glomeration has been started. In other words, nano-sized particles have combined with each other and formed a structure with sharp grain boundaries (C3, C4 and C5). Furthermore, increase in grain size of CdO films with

Fig. 6. Surface morphology of pure and CdS QDs doped CdO thin films at 20 k× magnification (scale; 200 nm).

increasing concentrations of CdS QDs are clearly seen in the micrographs. Thus the SEM features accordance with the XRD results that the particle size increases when dop-ing concentration increases.

In addition, the evolution of surface morphology, roughness and granularity have been characterized by AFM. Figure 7 shows the two-dimensional AFM images of the thin films at different CdS QD doping. All thin films exhibit a smooth surface with uniform grains. Al-though having the same granularity when doped, the origi-nal nanograins transform into spherical particles with

different dimensions. Also, average grain size of these thin films (pure, 1%, 2%, 3%, 4% and 5%) were about 47, 73, 91, 142, 295 and 313 nm, respectively. Grain size increases with increase in doping which are shown the similar prop-erties as SEM images. This may be due to the bigger clus-ters formed by the coalescence of two or more grains, and increasing of roughness. The root mean square (rms) is the most widely used parameter to characterize surface roughness. The rms roughness of these thin films (pure, 1%, 2%, 3%, 4% and 5%) were about 1.02, 1.52, 2.61, 3.26, 5.23 and 5.47 nm, respectively.

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Fig. 7. 2-D AFM images of pure and CdS QDs doped CdO thin films at 5 µm × 5 µm.

4 Conclusions

The influences of CdS QDs doping concentrations on structural, optical and morphological properties have been investigated. The obtained band gap value of pure CdO (2.23 eV) increases and reaches a maximum value

of 2.51 eV for 5% CdS QDs doping. This increase can be attributed to the Moss–Burstein effect. It is clear that CdS QDs are sufficiently effective for CdO doping and leads to a better crystallization. Also, the thin films consisted of spherical particles having submicron diameters and had important change upon doping. The surface roughness

(rms) of the thin films increases from 1.02 to 5.47 nm, and at the same time, average grain size as well grows up from about 47–313 nm with increase of the doping. The improved optical, structural and morphological properties achieved by CdS QDs incorporation here are compara-ble to pure CdO thin films, suggesting that the CdS QDs doped CdO thin films have potential applications for the future of nanotechnology.

References

1. R.D. Schaller, V.I. Klimov, Phys. Rev. Lett. 92, 186601 (2004)

2. Y.J. Choi, Y.J. Kim, J.W. Lee, Y. Lee, Y.B. Lim, H.W. Chung, J. Nanosci. Nanotechnol. 12, 2160 (2012)

3. K.O. Jung, J. Mater. Chem. 20, 8433 (2010)

4. L. Etgar, W. Zhang, S. Gabriel, S.G. Hickey, M.K. Nazeeruddin, A. Eychm¨uller, B. Liu, M. Gr¨atzel, Adv. Mater. 24, 2202 (2012)

5. Z.B. Li, W. Cai, X. Chen, J. Nanosci. Nanotechnol. 7, 2567 (2007)

6. R. Rossetti, J.L. Ellison, J.M. Gibson, L.E. Brus, J. Chem. Phys. 80, 4464 (1984)

7. B. Mognetti, A. Barberis, S. Marino, F.D. Carlo, V. Lysenko, O. Marty, A. G´elo¨en, J. Nanosci. Nanotechnol.

10, 7971 (2010)

8. X. Wang, X.L. Ma, X. Feng, Y.F. Zheng, J. Nanosci. Nanotechnol. 10, 7812 (2010)

9. Y. Medina-Gonzalez, W.Z. Xu, B. Chen, N. Farhanghi, P.A. Charpentier, Nanotechnol. 22, 065603 (2011) 10. X. Wang, X. Yan, W. Li, K. Sun, Adv. Mater. 24, 2742

(2012)

11. W.M. Leevy, T.N. Lambert, J.R. Johnson, J. Morris, B.D. Smith, Chem. Commun. 20, 2331 (2008)

12. H. Zhu, U. Sikora, A. ¨Ozcan, Analyst 137, 2541 (2012) 13. Q. Wang, T. Fang, P. Liu, X. Min, X. Li, J. Coll. Interf.

Sci. 363, 476 (2011)

14. M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95, 69 (1995)

15. S. Gorer, G.J. Hodes, Phys. Chem. B 98, 5338 (1994) 16. B. Irene, L.V. Teresa, G.J. Roberto, Photochem.

Photo-biol. Chem. 30, 47 (2011)

17. H. Kangqiang, C. Li, D. Jieguan, X. Jianwen, J. Nano-mater. 1, 2012 (2012)

18. B. Giuseppe, A.M. Maria, G. Lisa, C. Mario, P.P. Pier, C. Roberto, P. Tommaso, Biomaterials 31, 6555 (2010) 19. R. Haul, D. Just, J. Appl. Phys. 33, 487 (1962)

20. M. Ristic, S. Popovic, S. Music, Mater. Lett. 58, 2494 (2004)

21. M. Ortega, G. Santana, A. Morales, Solid State Electron.

44, 1765 (2000)

22. S. Akın, G. Karanfil, A. G¨ultekin, S. S¨onmezo˘glu, J. Alloys Compd. 579, 272 (2013)

23. T.P. Gujar, V.R. Shinde, W.Y. Kim, K.D. Jung, C.D. Lokhande, O.S. Joo, Appl. Surf. Sci. 254, 3813 (2008)

24. X. Wang, D. Li, Y. Guo, X. Wang, Y. Du, R. Sun, Opt. Mater. 34, 646 (2012)

25. F. ¨Oz¨utok, K. Ert¨urk, V. Bilgin, Acta. Phys. Pol. A 121, 221 (2012)

26. J. Tauc, Mater. Res. Bull. 5, 721 (1970)

27. E.V. Yutaka, F. Zhang, X.T. Jin, M. Taketoshi, M. Akira, J. Phys. Chem. B 109, 24441 (2005)

28. G.Z. Shen, J.H. Cho, J.K. Yoo, G.C. Yi, C. Lee, J. Phys. Chem. B 109, 5491 (2005)

29. R.T. Zaera, I. Mora, J. Bisquert, J. Phys. Chem. C 112, 16318 (2008)

30. F. Urbach, Phys. Rev. 92, 1324 (1953)

31. N.F. Mott, E.A. Davis, Electronic processes in non-crystalline materials (Clarendon Press, Oxford, 1979)

32. P. Nagels, M.H. Brodsky, Electronic transport in amorphous semiconductors, amorphous semiconductors

(Springer-Verlag, New York, 1979)

33. M. Pal, Y. Tsujigami, A. Yoshikado, H. Sakata, Phys. Status Solidi A 182, 727 (2000)

34. S.M. Wasim, G. Marin, C. Rincon, G.J. Sanchez, Appl. Phys. 84, 5823 (1998)

35. S.M. Wasim, G. Marin, C. Rincon, G.J. Sanchez, A.E. Mora, J. Appl. Phys. 83, 3318 (1998)

36. T.P. Gujar, V.R. Shinde, W.Y. Kim, K.D. Jung, C.D. Lokhande, O.S. Joo, Appl. Surf. Sci. 254, 3813 (2008)

37. J.S. Cruz, G.T. Delgado, R.C. Perez, S.J. Sandoval, Thin Solid Films 493, 83 (2005)

38. B.D. Cullity, Elements of X-ray diffraction