Materials Letters 62 (2008) 3533–3535
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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
ZnO controllable sized quantum dots produced by polyol method: An experimental and theoretical study Nikolaos Bouropoulos a, Ioannis Tsiaoussis b, Panagiotis Poulopoulos a, Panayotis Roditis a, Sotirios Baskoutas a,⁎ a b
Department of Materials Science, University of Patras, GR-26504, Patras, Greece Department of Physics, Aristotle University of Thessaloniki, Thessaloniki 54124, Greece
A R T I C L E
I N F O
Article history: Received 13 February 2008 Accepted 18 March 2008 Available online 25 March 2008 Keywords: Zinc oxide Semiconductors Nanocrystals Quantum dots Excitons
A B S T R A C T Zinc oxide quantum dots were prepared in diethyleneglycol using zinc acetate as a precursor compound and the particle size of the dried powders was determined using the X-ray diffraction method (XRD). It was found that quantum dot size ranges from 4 to 9 nm and is influenced by the initial zinc acetate concentration and the centrifugation speed. Theoretical predictions concerning quantum dot size by the potential morphing method showed good agreement with the measurements from XRD. © 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental details
ZnO is a II–VI semiconductor with a great area of applications in electronic and optoelectronic devices [1]. It is well known that in comparison to the other wide band gap semiconductors such as CdSe, CdS, CdTe, ZnO has a large exciton binding energy of 60 meV, which permits excitonic emission even at room temperature [2]. Zinc oxide nanostructures with different morphologies are typically produced using the solid-vapor phase thermal sublimation, hydrothermal methods, electrochemical deposition, molecular beam epitaxy, and soft chemical solution methods [3–5]. Controllable tuning of nanosized semiconductor materials band gap plays a crucial role in the applications of such materials in the optoelectronics. Band gap depends strongly on the particle size and, often, it increases as the size decreases, i.e. a blue shift of the band gap occurs. The theoretical investigation of this phenomenon includes several approximate methods [6]. Recently we have developed a new numerical method, which is based on a novel formulation of the Hartree–Fock approximation with the flexible potential morphing method (PMM) [6,7]. In this Letter we apply the PMM method as well as a semi empirical equation [8] in order to calculate the band gap of high quality ZnO quantum dots (QDs) which have been prepared via the polyol precipitation method.
The ZnO colloidal solutions were produced from zinc acetate dihydrate Zn(CH3COO)2.2H2O (Sigma-Aldrich) in diethylene glycol, DEG (Sigma-Aldrich) using a modified method as developed by other researchers [9,10]. Appropriate amounts of Zinc Acetate were suspended in 50 ml diethylene glycol in a 100 ml three-neck spherical flask in order to produce concentrations of 0.04, 0.06, 0.08 and 0.01 M. The mixture was heated at 190 °C using a heating maddle under magnetic stirring and kept at this temperature until the formation of a cloudy suspension. Subsequently, the suspension was separated in two parts. The first part was kept without further treatment and the second part was centrifuged at different conditions. After finishing the centrifugation procedure the pellet was withdrawn and the supernatant was kept for further measurements. In all cases after the final treatment, a few drops of the suspension were placed on the surface of a microscope glass slide and dried in air at 80 °C. The structural characterization was performed via X-ray diffraction (XRD) using a standard powder diffractometer (Siemens D8) with Nifiltered CuKa1 radiation (λ = 0.154059 nm). The average particle size was calculated using the Scherrer formula:
⁎ Corresponding author. Tel.: +30 2610 969349; fax: +30 2610 969368. E-mail address:
[email protected] (S. Baskoutas). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.03.044
d¼
kk FWHM2h cosh
ð1Þ
where d is the average crystallite size, k is the Scherrer constant taken equal to 0.9. λ is the wavelength of the X-ray radiation, β is the full
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Table 1 Experimental conditions, zinc acetate concentration (CZnAc) estimated particle size of the resulted dry products by XRD and the corresponding band gap energy obtained by the UV spectra Sample number 1 2 3 4 5 6 7 8 9
CZnAc M 0.10 0.10 0.10 0.08 0.06 0.04 0.04 0.04 0.04
Centrifugation rpm – 6000 13000 – – – 6000 9000 13000
Size XRD nm
Band gap (eV)
9.24 9.20 7.35 6.69 5.8 5.37 4.57 4.57 4.41
3.22 3.22 3.22 3.22 3.44 3.46 3.50 3.50 3.51
width at half-maximum (FWHM)) and θ is the diffraction angle. In our case, the (101) reflection peak of ZnO was used to calculate the average particle size according to the Scherrer formula. The ultraviolet (UV) spectra were recorded at room temperature in the transmission geometry with the help of a Perkin Elmer Λ-35 UV–visible spectrometer at the wavelength range 330–390 nm. Finally, images of the samples concerning the structural properties of nanoparticles were obtained by High Resolution Transmission
Fig. 2. HRTEM image and the corresponding electron diffraction pattern of the nanoparticles of sample 9 as listed in Table 1. Scale bar is equal to 2 nm.
Electron Microscopy (HRTEM, JEOL 2011, 200 kV). Additional structural analysis was carried out from the corresponding Selected Area Electron Diffraction Patterns (SAED). 3. Results and discussion The experimental conditions and the calculated band gap from the UV spectra are listed in Table 1. From Table 1 we observe that using the DEG precipitation method ZnO quantum dots were obtained. This technique as earlier reported is an excellent and very attractive method to obtain ZnO quantum dots. The main advantages are focused on the simplicity of the method, the lower operation temperature in comparison with other methods, and the activity of polyol to inhibit particle aggregation [11,12]. We have to notice also that the resulting zinc oxide colloids produced from zinc acetate concentrations of 0.04 and 0.04 M were stable for several weeks while in the case that they settle down as happens with the higher concentration they can redispersed by ultrasonication. XRD patterns of ZnO samples prepared using four different zinc acetate concentrations as listed in Table 1 without further treatment by centrifugation are shown in Fig. 1a. The diffraction peaks can be indexed to the wurtzite type ZnO (JCPDS card no: 80-0074). The estimated average size from the Sheerer formula is also shown on the Fig. 1a. It is clear that the particles are in nanometer size and by increasing zinc acetate concentrations size increases. The dependence of particle size with zinc acetate concentration was also clearly demonstrated by previous studies. In our case the
Fig. 1. X-ray diffraction patterns A) ZnO nanoparticle samples prepared at different initial zinc acetate concentrations. B) dried powder obtained from sample with CZnAc = 0.04 M without centrifugation (0 rpm) and after centrifugation at 6000, 9000 and 13,000 rpm. The patterns have vertically shifted for clarity.
Fig. 3. UV absorption spectra of ZnO QDs after precipitation and after centrifugation at 0, 6000, 9000 and 13,000 rpm respectively.
N. Bouropoulos et al. / Materials Letters 62 (2008) 3533–3535
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band gap energy equal to 5 eV). From these data we have obtained results concerning the particle size, which are depicted in Fig. 4. Furthermore, for comparison reasons, we have used also another approach to estimate the QDs mean particle size which was described by Viswanatha et al [8] and is based on the following semiempirical equation: DEg ¼ 100 18:1d2 þ 41:4d 0:8
1
ð2Þ
were ΔΕg is the shift in the band gap expressed in eV and d is the diameter of the nanocrystal, in nm. The estimated results using the above formula from experiments 5–9 as listed in Table 1 are plotted in the Fig. 4. As we can see the results we have obtained with PMM are in a better agreement with experimental ones obtained using Sheerer formula, in comparison with the theoretical results we have obtained with the semi empirical equation. Previous studies using the PMM method on other kind of semiconductor quantum dots have proven the validity of the PMM method [6].
4. Conclusions
Fig. 4. Mean size of ZnO quantum dots as determined from the XRD patterns and by theoretical calculations using the PMM theory and the semi empirical equation (SEMP).
estimated size is larger and this can be attributed to the different heating temperature and the different dwelling time at heating temperature [10]. Fig. 1b shows the diffraction patterns from dry powder obtained from sample with CZnAc = 0.04 M without centrifugation (0 rpm) and after centrifugation at 6000, 9000 and 13,000 rpm. The corresponding particle sizes are also shown in Fig. 1b. We observe that the mean size decreases from 5.37 nm for the untreated suspension to 4.41 nm for the sample subjected to a centrifugation operation at 13000 rpm. Similar effects are observed for the sample with CZnAc = 0.1 M as shown in Table 1 where the size decreases from 9.24 to 7.35 nm. High Resolution Electron Microscopy image and the corresponding electron diffraction pattern of sample 9 is shown in Fig. 2. HRTEM study of the sample revealed the existence of large number of nanoparticles, where the size is found to be 4 to 6 nm. This is in good agreement with the results obtained from XRD data. In one case the d-spacing of the (0002) plane is indicated by parallel lines. The corresponding electron diffraction pattern shows the existence of the hexagonal ZnO phase, with some characteristic d-spacing of the (10 0), (10 1), (10 2) and (11 0) planes. In Fig. 3, the UV absorption spectra of three ZnO samples deposited on glass substrates for zinc acetate concentration of 0.04 M and centrifugation at 0, 6000, 9000 and 13000 rpm are shown. It can be seen that the absorption edge is shifted from 358 to 352.6 nm by changing the centrifuge conditions. Analytical results of all experiments using different zinc acetate concentrations and centrifuge parameters are shown in Table 1. From Table 1 it is clear the band gap energy depends on the particle size due to the quantum size effect. Next, the particle size was also estimated theoretically exploiting the UV absorption edge data. In particular, the Potential Morphing Method (PMM) [6,7] and the semi empirical formula have been applied [8]. As regards the PMM [6] we have used the following material parameters for ZnO: m⁎e = 0.24m0, m⁎h = 0.45m0, ε = 3.7 and Ve0 =Vh0(rh) = 0.08Eg(matrix)=400 meV [6] (assuming that the matrix is the diethylene glycol which has
In conclusion, ZnO quantum dots were prepared by the polyol method. The dried products were characterized using XRD and it was found that their mean size ranges from 4.41 to 9.24 nm and is depended on the initial zinc acetate concentration and on the further treatment of the colloids with centrifugation. Finally, the mean size as determined using the PMM theory is in good agreement with the results derived from the XRD patterns. Acknowledgments The XRD work was performed at the facilities of The Institute of Chemical Engineering and High Temperature Chemical Processes (ICEHT/ FORTH), Patras, Greece. The help of Dr V. Nikolakis and Dr. V. Drakopoulos to aquire the XRD spectra from the ZnO films is fully acknowledged. References [1] Z.L. Wang, Materials Today 10 (2007) 20–28. [2] T. Makino, C.H. Chia, N.T. Tuan, H.D. Sun, Y. Segawa, M. Kawasaki, A. Ohtomo, K. Tamura, H. Koinuma, Appl. Phys. Lett. 77 (2000) 975–977. [3] L. Schmidt-Mende, J.L. MacManus-Driscoll, Materials Today 10 (2007) 40–48. [4] S. Baskoutas, P. Giabouranis, S.N. Yannopoulos, V. Dracopoulos, L. Toth, A. Chrissanthopoulos, N. Bouropoulos, Thin Solid Films 515 (2007) 8461–8464. [5] A. Chrissanthopoulos, S. Baskoutas, N. Bouropoulos, V. Dracopoulos, D. Tasis, S.N. Yannopoulos, Thin Solid Films 515 (2007) 8524–8528. [6] S. Baskoutas, A.F. Terzis, J. Appl. Phys. 99 (2006) 013708 (the codes we are using are free available at http://www.matersci.upatras.gr/PMM/). [7] M. Rieth, W. Schommers, S. Baskoutas, Int. J. Mod. Phys. B 16 (2002) 4081–4092. [8] R. Viswanatha, S. Sapra, B. Satpati, P.V. Satyam, B.N. Devb, D.D. Sarma, J. Mater. Chem. 14 (2004) 661–668. [9] K.F. Lin, H.M. Cheng, H.C. Hsu, W.F. Hsieh, Appl. Phys. Lett. 88 (2006) 263117. [10] K.F. Lin, H.M. Cheng, H.C. Hsu, L.J. Lin, W.F. Hsieh, Chem. Phys. Lett. 409 (2005) 208–211. [11] C. Feldmann, Solid State Sci. 7 (2005) 868–873. [12] C. Feldmann, Adv. Funct. Mater. 13 (2003) 101–107.
