Enhanced visible luminescence and modification in morphological properties of cadmium oxide nanoparticles induced by annealing

Transcription

1 Journal of Experimental Nanoscience ISSN: (Print) (Online) Journal homepage: Enhanced visible luminescence and modification in morphological properties of cadmium oxide nanoparticles induced by annealing B. Goswami & A. Choudhury To cite this article: B. Goswami & A. Choudhury (2015) Enhanced visible luminescence and modification in morphological properties of cadmium oxide nanoparticles induced by annealing, Journal of Experimental Nanoscience, 10:12, , DOI: / To link to this article: Published online: 02 Sep Submit your article to this journal Article views: 204 View related articles View Crossmark data Citing articles: 5 View citing articles Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 26 December 2017, At: 01:33

2 Journal of Experimental Nanoscience, 2015 Vol. 10, No. 12, , Enhanced visible luminescence and modification in morphological properties of cadmium oxide nanoparticles induced by annealing B. Goswami and A. Choudhury* Nanoscience Research Laboratory, Department of Physics, Tezpur University, Napaam, Assam , India (Received 15 July 2013; final version received 8 June 2014) Cadmium oxide nanoparticles synthesised by a simple sol gel synthesis method showed luminescence properties in the visible region of the electromagnetic spectrum. Both green and blue emissions were observed in photoluminescence spectra. We have investigated luminescence properties by changing the synthesis conditions. An enhanced luminescence of CdO nanoparticles was realised when these particles were annealed at different temperatures. Cadmium interstitial vacancies and oxygen vacancies played an important role in luminescence properties. X-ray diffraction confirmed annealing-induced changes in morphological properties. A good correlation between all the experimental results was obtained. Optical properties were investigated by diffuse reflectance spectra and photoluminescence spectra. Structural properties were investigated by high-resolution transmission electron microscopy. Keywords: cadmium oxide; nanoparticle; luminescence 1. Introduction Synthesis of binary chalcogenides of group II VI semiconductors is a growing area of research nowadays because of their chemical, physical, optical and electrical properties.[1] These semiconductors, as zero-dimensional nanoparticles, are especially interesting due to their increased surface to volume ratio for which they gain size-dependent optical properties. Pure bulk cadmium oxide (CdO) is an n-type semiconductor with a direct band of 2.5 ev and an indirect band gap (IBG) of 1.98 ev. CdO is a group IIB VI compound semiconductor. It adopts face-centred-cubic rock salt structure where Cd is octahedrally coordinated to its neighbours. CdO has a wide range of applications like the manufacture of cadmium-coated baths and paint pigments.[2] It belongs to the big family of transparent conducting oxides (TCO), such as In 2 O 3, SnO 2, ZnO, etc. TCOs have enormous practical applications in devices in which a transparent contact is required, e.g. LEDs, solar cells, LCDs, etc. Most commonly used TCO in such applications is Sn-doped In 2 O 3. Since In is a very costly and scarce material, scientists are focusing their research on finding an alternative material.[1] CdO is an example of such an alternative material. CdO films are transparent in the visible region of the electromagnetic spectrum. CdO has many applications due to its low ohmic resistivity and high optical transparency.[3] However, so far its *Corresponding author. amarjc@rediffmail.com; ajc@tezu.ernet.in Ó 2014 Taylor & Francis

3 Journal of Experimental Nanoscience 901 application has been confined to making films. Exploring structural and optical properties of CdO nanostructures is a new promising field for researchers. 2. Synthesis of cadmium oxide nanostructures CdO nanoparticles were prepared by a simple and cost-effective method.[3] Cadmium acetate 6.6 g (0.5 M) was dissolved in 100 ml of distilled water. Ammonia solution was then added to the above preparation with constant stirring until ph became 8. A white precipitate was obtained which was kept for 12 hours to settle and then centrifuged and washed three times with distilled water. The washed precipitate was then dried at 80 Candground. The resulting white powder was then calcined at 400, 600 and 800 Ctogetbrownpowder. 3. Study of structural properties Images of synthesised nanoparticles were taken by a high-resolution transmission electron microscope (HR-TEM/JEM-2100, 200 KV, JEOL). The size of a single CdO nanoparticle synthesised at 400 C is around 25 nm, as shown in the high-resolution image in Figure 1(a). Crystal planes can be seen through HR-TEM. Selective area electron diffraction (SAED, Figure 1(c)) was also taken to confirm the crystalline nature. The presence of cadmium and oxygen is confirmed by energy dispersive X-ray (EDX) spectra taken by the JEOL, JSM-6390LV, INCAx-sight EDX detector, as shown in Figure 1(b). The atomic percentage of oxygen present is and that of cadmium The unproportional ratio between cadmium and oxygen found from EDX spectra may be due to the defects present in the system which is discussed later. SAED shown in Figure 1(c) confirms that the particles are nanocrystalline. Figure 1. (a) HR-TEM, (b) EDX and (c) SAED of CdO nanoparticles.

4 902 B. Goswami and A. Choudhury 4. Study of optical properties Optical properties were studied using ultraviolet visible (UV-Vis) absorption spectroscopy and photoluminescence spectroscopy (PLS). UV-Vis absorption spectra were recorded in diffuse reflectance mode in the range nm by a Shimadzu 2450 UV- Vis spectrophotometer using BaSO 4 as reference. The diffuse reflectance data were obtained as relative percentage reflectance to the non-absorbing material BaSO 4 which can optically diffuse light. PLS was performed by Perkin Elmer LS Diffuse reflectance spectroscopy The Kubelka Munk function F(R) [4 6] can be derived from diffuse reflectance spectra. It is given by FðRÞ ¼ð1 RÞ 2 =2R (1) where R is the reflectance in percentage. For a semiconductor sample, an [F(R)hn] n vs. hn (in ev) plot gives Tauc s plot. For a direct band gap (DBG) semiconductor, n D 2, and for an IBG semiconductor, n D 1/2. Here, h is Planck s constant and n is the frequency of incident light. Fitting the linear portion of Tauc s plot and extrapolating it to the energy axis gives the band gap. Figure 2 shows a comparison of diffuse reflectance spectra of the samples. Figure 3(a) and 3(b) shows Tauc s plot of all three samples derived from using the Kubelka Munk function F(R). Figure 3(a) shows Tauc s plot for DBG CdO nanoparticles and Figure 3(b) shows that for IBG CdO nanoparticles. Instead of ideal direct band-edge Figure 2. Comparison of DRS of CdO nanoparticles annealed at three different temperatures.

5 Journal of Experimental Nanoscience 903 Figure 3. (a) Tauc s plot for the determination of the DBG of CdO nanoparticles. (b) Tauc s plot for the determination of the IBG of CdO nanoparticles. absorption we often find an exponential tail called the Urbach tail. The Urbach tail obeys the following relationship: 1 ðeþ ¼ 1 g expðe E g Þ=E o (2) where E o is the characteristic width of the absorption edge called the Urbach parameter and E g is the optical band gap. The Urbach parameter corresponds to the width of the tail of localised states within the band gap. Defects originating from disorder in the crystal, for example, doping or other structural disorders, lead to a tail in valence and conduction bands. This tail is called the Urbach tail. At low temperature, dopant impurities as well as other structural imperfections introduce lattice disorder. So, Urbach energy is used to characterise the degree of disorderliness.[7] We have calculated Urbach energy using relation (2). Figure 4 shows the Figure 4. ln [F(R)] vs. incident photon energy hn for CdO nanoparticles annealed at three different temperatures.

6 904 B. Goswami and A. Choudhury Table 1. Comparison between DBG, IBG and Urbach energy. Sample name DBG energy, E DBG (ev) IBG energy, E IBG (ev) Urbach energy (ev) Urbach energy plot for all the samples. Calculated values of Urbach energy and band gaps are summarised in Table 1. CdO nanoparticles were annealed at three different temperatures. It is seen that the DBG increases with increase in annealing temperature, while the IBG decreases. Urbach energy also decreases with increase in annealing temperature. We have calculated particle sizes of the samples by using Eva software. Particles sizes are increasing with increase in annealing temperature. With increase in particle size, the DBG of the samples should decrease, but here we have an opposite case. Increase in DBG implies that there must be decrease in defect in the system. Since Urbach energy decreases, it confirms that defects in the system decrease with increase in annealing temperature Photoluminescence spectroscopy Photoluminescence (PL) spectra are shown in Figure 5(a) and 5(b). Figure 5(a) shows the PLS of CdO nanoparticles annealed at 400 C and Figure 5(b) gives the comparison of CdO nanoparticles annealed at three different temperatures. From PL spectra (Figure 5(a)) of CdO nanoparticles annealed at 400 C, we have found the green emitting peak is actually composed of two peaks. The deconvoluted green peak is shown in the inset. PL spectra of CdO nanoparticles resemble that of ZnO nanoparticles. PL spectra of both compounds have blue and green emissions which are results of emissions from interstitial vacancies and oxygen vacancies.[8] In a crystal, a certain number of defects are always present, because entropy is increased by the presence of disorder in the structure.[9] Oxygen vacancies create deep levels and Cd interstitials are shallow donors in CdO Figure 5. (a) Photoluminescence spectroscopy of CdO nanoparticles. (b) Comparison of photoluminescence spectra of CdO nanoparticles annealed at three different temperatures.

7 Journal of Experimental Nanoscience 905 nanoparticles. Cd interstitials are Frenkel-type defects. These defects can move outward.[9] We consider green and blue peaks of the 400, 600 and 800 C samples as B 1, B 2, B 3 and G 1, G 2, G 3, respectively. Then, the intensity ratios of the peaks are G 1 /G 2 D 0.54, G 1 /G 3 D 1.15, B 1 /B 2 D 0.44 and B 1 /B 3 D As seen in Figure 5(a), the green peak as stated above consists of two peaks. This may be ascribed to the emission from two different levels of oxygen vacancy defect states. From Figure 5(b), we found that annealing slightly redshifted the emission peak positions for the 600 and 800 C samples. Redshifting can be ascribed to increase in particle size. The intensity of the blue emitting peak of the 600 C sample was increased to a greater extent. But it was slightly decreased for 800 C sample than that of 400 C. Intensity of blue emission peak of 600 C sample increased than that of 400 C. This is because annealing at 600 C increases the number of Cd-interstitial vacancies by providing enough ionisation energy.[8] At this temperature, Cd-interstitial vacancies, which are Frenkel-type defects, get enough thermal energy to diffuse through the crystal and go to the surface which in turn increases the blue peak intensity. However, these defects are unstable in comparison to oxygen vacancy defects.[8] At high temperature, i.e. at 800 C, oxygen vacancies predominate. It may be possible that at 800 C, the cadmium of Cd-interstitials gets oxidised.[8] Hence, the intensity of the green peak of the samples increased, while blue peak decreased than that of the 400 C sample. At this temperature, possibility of electrons to jump down to the oxygen vacancy levels increases as now there are lesser number of Cd-interstitial defects within band gap. 5. Study of morphological properties Morphological properties were studied by X-ray diffraction (XRD). The XRD pattern was collected using a Bruker D8 focus AXS X-ray diffractometer with Cu K 1 radiation (λ D A ). Monteponite CdO with a cubic structure was matched with the XRD data of our samples (JCPDF #050640). XRD studies have been done to find the crystal structure, as shown in Figure 6. To determine the crystallite size, researchers earlier used the Scherrer formula which is given by D ¼ 0:9λ=b cos u (3) where b D FWHM (full width at half-maximum), λ D 1.54 A,2u D peak position and D D crystallite size. It is always difficult to separate the size and strain broadening present in XRD peaks. The Williamson Hall (W H) method is used to separate these two effects using the following equation [10]: b cos u ¼ðkλ=DÞþh sin u (4) where h is the strain, the value of D represents the size of the crystallites and the constant k typically remains close to 1. When we plot b cos u vs. sin u, as shown in Figure 7, wegeta straight line with slope h, which in turn gives the strain. We can also find the crystallite size using this method. The crystallite size often matches the grain size, but it may be different from the particle size in the nano range. At 400 C, from HR-TEM we found that the particle sizewas25nmbutthecrystallitesizefromthew Hplotwasfoundtobe24nm.

8 906 B. Goswami and A. Choudhury Figure 6. XRD pattern of CdO nanoparticles annealed at different temperatures. The lattice parameters of CdO samples are calculated using the Nelson Riley parameter,[11] NRF ¼ 1=2½cos 2 u=sin u þ cos 2 u=uš (5) where u is the Bragg angle. By extrapolating the lines to NRF D 0, the true lattice parameter is obtained, as shown in Figure 8. Comparison between the lattice constant, crystallite size, interplanar distance and strain of the nanoparticles is shown in Table 2 below. At 600 C, the crystallite size increased. At 800 C, the crystallite size again slightly decreased than that at 600 C. Figure 7. W H plot of all three CdO nanoparticle samples annealed at different temperatures.

9 Journal of Experimental Nanoscience 907 Figure 8. Lattice constant vs. NRF of CdO nanoparticles. Table 2. Comparison between lattice constant, interplanar spacing, crystallite size, strain and particle size. Sample name Lattice constant (A ) Interplanar spacing, d (nm) Crystallite size (nm) (W H method) Strain from W H plot ( 10 3 ) Electrical studies The I V measurements were done using a Keithley electrometer automatic system. Pellets were made from all the three samples and conductivity measurements were taken after making contact with silver paste. The I V curve shows a linear relationship, showing the contact to be ohmic. The electrical conductivity of the samples is measured using the following relation: V ¼ l=a ðdi=dvþ (6) where l is the thickness of the sample and A is the area of the sample. The slope of the graph (Figure 9) gives (di/dv).[12] Conductivity of all the samples is given in Table 3. Conductivity of the CdO nanoparticles for the samples gradually increases with increase in annealing temperature, as shown in Figure 9. This type of behaviour is expected from an n-type intrinsic semiconductor. In electrical conductivity, electrons trapped in oxygen vacancies also contribute. As the temperature of the samples was increased, trapped electrons got sufficient thermal energy, then moved to the conduction band and became free electrons. 7. Hall-effect measurements The Lorentz force law is given by F ¼ qðe þ v BÞ (7) Equation (7) states that a force (F) will be exerted when a magnetic field (B) intersects a moving charge (q) with velocity v, as shown in Figure 10. Therefore, if a current (I) is run

10 908 B. Goswami and A. Choudhury Figure 9. Current vs. voltage plot of CdO nanoparticles. Table 3. Conductivity of the samples. Sample name Conductivity (V 1 cm 1 ) Figure 10. Schematic diagram of the Hall effect.

11 Journal of Experimental Nanoscience 909 Figure 11. Hall voltage vs. magnetic field of all three CdO nanoparticle samples. through the sample (palette) and a magnetic field is generated perpendicular to the current, then a force will be exerted on electrons moving through the material.[13] Measurements were taken by a standard laboratory setup where contacts were provided by silver paint. This will generate a potential perpendicular to both the current and the field, known as the Hall voltage (V H ),[14] defined by V H ¼ IB=dne (8) where d is the thickness of the strip, n is the density of charge carriers and e is the charge of an electron. Given this, n is calculated by The Hall coefficient is given by n ¼ IB=deV H (9) R H ¼ 1=nq (10) The slope of the graph of the Hall voltage versus magnetic field gives the Hall coefficient from which we can calculate charge concentration. Hall voltage vs. magnetic field graphs are shown in Figure 11. The Hall coefficient and charge concentration of the samples are given in Table 4 below. N-type electrical conductivity in CdO is due to Cd-interstitial and oxygen vacancies. With gradual increase in temperature, conductivity increases due to increase in the number of available free electrons in the conduction band. Hence, the charge density also increases as we have found in Hall-effect measurements. Table 4. Hall coefficient and charge density of the samples. Sample name Hall coefficient Charge density (m 3 )

12 910 B. Goswami and A. Choudhury 8. Conclusion We have synthesised CdO nanoparticles by a simple sol gel method. We increased the annealing temperature from 400 to 800 C. With the rise in temperature, i.e. 600 C, we have found that due to increase in Cd-interstitial vacancies on the surface, there is an enhancement in blue PL emission. Upon heating upto 800 C, the blue emission which is due to Cd-interstitial vacancy first increased then quenched. Blue emission which is considered to be due to oxygen vacancy defects continues to increase as with the increase in temperature, interstitial cadmium which moved to the surface gets oxidised, creating more oxygen vacancies. With the increase in annealing temperature, the crystallite size changes and is highest for 600 C and lowest for 400 C. As thermal energy is provided by increase in annealing temperature, electrons trapped by oxygen vacancies get excited to the conduction band and become free electrons. Hence, conductivity increases with increase in temperature and so also charge density. Acknowledgements Authors of this paper are highly thankful to DST Nanomission and SAIF, NEHU, Shillong. References [1] King PDC, Veal TD. Conductivity in transparent oxide semiconductors. J Phys. 2011;23: [2] Manickathai K, Viswanathan SK, Alagar M. Synthesis and characterisation of CdS and CdO nanoparticles. Indian J Pure Appl Phys. 2008;46: [3] Lanje AS, Nighthoujam RS, Sharma SJ, Pode RB. Luminescence and electrical resistivity properties of cadmium oxide nanoparticles. Indian J Pure Appl Phys. 2011;49: [4] Kubelka P. New contributions to the optics of intensely light-scattering materials. Part I. J Opt Soc Am. 1948;38: [5] Yang L, Kruse B, Revised Kubelka Munk theory. I. Theory and application. J Opt Soc Am. 2004;21: [6] Yang L, Miklavcic S, Revised Kubelka Munk theory. III. A general theory of light propagation in scattering and absorptive media. J Opt Soc Am. 2005;22: [7] Sadao Adachi. Properties of Group-IV, III-V and II-VI semiconductors. West Sussex: John Wiley & Sons; [8] Zeng H, Duan G, Li Y, Yang S, Xu X, Cai W. Blue luminescence of ZnO nanoparticles based on non-equilibrium processes: defect origins and emission controls. Adv Funct Mater. 2010;20: [9] Kittel Charles. Introduction to solid state physics. 7th ed. New Delhi: Wiley; [10] Nogi K, Hosokawa M, Naito M, Yokoyama T. Nanoparticle technology handbook. 2nd ed. Kidlington: Elsevier; [11] Nelson JB, Riley DP. An experimental investigation of extrapolation methods in the derivation of accurate unit-cell dimensions of crystals. Proc Phys Soc. 1945;57:160. [12] Jbaier DS. Physical properties of CdO thin films prepared by spray pyrolysis technique. Eng Technol J. 2013;31: [13] Bhosale CH, Kambale AV, Kokate AV, Rajpure KY. Structural, optical and electrical properties of chemically sprayed CdO thin films. Mater Sci Eng B. 2005;122: [14] Hall EH. On a new action of the magnet on electric currents. Amer J Math. 1879;2: