- Email: [email protected]
Energy gap studies of ZnS nanocrystallites
Materials Science in Semiconductor Processing 13 (2010) 214–216
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Energy gap studies of ZnS nanocrystallites S. Chellammal n, S. Sankar Department of Physics, College of Engineering, Anna University, Guindy, Chennai 600 025, Tamil Nadu, India
a r t i c l e in fo
abstract
Available online 15 December 2010
Zinc Sulphide nanocrystallites (ZnS) of average size 4 nm have been synthesized by the chemical precipitation method. The structural studies are carried out by the X-ray diffraction method. The hexagonal structure of the sample is confirmed by JCPDS. Using the impedance analysis method, the effect of grain interior and grain boundary regions on their conductivity has been studied for various temperatures. The activation energies and band gap values are calculated by the Arrhenius plot. & 2010 Published by Elsevier Ltd.
Keywords: Quasicrystals Quantum dots Semiconductor compounds II–VI semiconductors.
1. Introduction A systematic study of size effect on the impedance properties of semiconductor nanocrystallites is essential for understanding their technological applications [1–3]. ZnS is a direct band gap semi conducting material with a band gap 3.67 eV. Impedance studies were carried out for ZnS nanocrystallites and the results are presented in the paper. Powder sample of ZnS nanocrystallites was prepared for the present work using the chemical precipitation method and its impedance properties have been studied. X-ray characterization of the prepared nanocrystallites has also been carried out for the confirmation of structure and also to obtain the average size of the nanocrystallite. The effect of grain interior and grain boundary was studied with the help of an impedance plot. Nanocrystallites metal chaleogenides, when doped with impurities, exhibit very interesting electronic structure and transition probabilities [4–6] and also have many applications in the field of spintronics. Among the II–VI semiconductors, zinc sulphide is widely studied for its stability and technological applications [7,8]. ZnS nanoparticles and rare earth ions have a variety of applications in optical devices [9,10]. Recent literature shows several studies on the optical properties of ZnS nanoparticles. On the other hand, impedance studies
n
Corresponding author. Tel.: + 91 44 24811272; fax: + 91 44 22203160. E-mail address: [email protected] (S. Chellammal).
1369-8001/$ - see front matter & 2010 Published by Elsevier Ltd. doi:10.1016/j.mssp.2010.10.003
and electrical characterization of ZnS nanoparticles are very few. The present studies are, hence, dedicated towards the impedance analysis of ZnS nanoparticles. 2. Experimental design ZnS nanocrystallites have been synthesized by the method of chemical precipitation using the reactants Zinc Chloride and Sodium Sulphide with ethylene glycol as the capping agent at room temperature. The precipitation was carried out over a reflex time of 600 s for 100 ml of reflex volume of each of the reacting solutions. The precipitates were centrifuged and washed with de-ionized water repeatedly and then with methanol finally. Then the precipitates were dried in vacuum. Powder X-ray diffraction studies were carried out using ˚ Philip 2275/20 Mo-Ka radiation (wave length= 0.70930 A; X-ray diffractometer) to determine the structural phase of the sample. The average size of the sample was determined from the line broadening of the X-ray diffraction peaks corrected for instrumental broadening using Scherer’s equation [11]. A pellet of 8.23 mm diameter and 1.52 mm thickness prepared using the ZnS nanocrystalline powder was placed in between two platinum electrodes and complex impedance (Zn) measurements were carried out as a function of both frequency (from 1 Hz to 1 MHz) and temperature (from 543 to 833 K) using an Impedance/Grain-Phase Analyzer (SOLARTRON 1260) together with a dedicated
S. Chellammal, S. Sankar / Materials Science in Semiconductor Processing 13 (2010) 214–216
computer and software to acquire the impedance data. The whole setup of sample compartment inside the furnace was evacuated up to 1 10 5 Torr, in order to prevent oxidation of the sample during heating. The heating rate was maintained at 2 K/min. The temperature of the furnace was measured with a resolution of 71 K using a Eurotherm (818 P) PID temperature controller. The data were collected during both heating and cooling cycles. It was found that the data were consistent during both thermal cycles. The dielectric parameter such as complex conductivity (sn) was calculated from the raw data of Z0 and Z00 from the sample dimensions. 3. Results and analysis The XRD spectrum of the synthesized nanocrystalline ZnS powder sample is presented in Fig. 1. The average grain size is calculated from the XRD data using the Scherer formula [11] approximately equal to 4 nm. The indices of
Fig. 1. XRD pattern of the synthesized ZnS nanocrystallites.
three peaks observed at 2y = 12.195, 20.633 and 24.435 for the ZnS sample has been identified as (1 0 0), (1 1 0), (2 0 0) and (2 1 1) of the hexagonal structural phase of ZnS using the corresponding JCPDS data (PDF No.: 011280 [16], version 2.3V). We have estimated the average size (d) of our present nanocrystallites using the Scherrer formula d ¼ 0:9K l=o cos y where the factor K =4/3 accounts for the quasi-spherical geometry of the nanocrystals, l is the wavelength of X-rays used, o is the width on a 2y-scale and y is the scattering angle of X-rays. In general, three semicircular arcs can be expected in the impedance spectra in the high, intermediate and low frequency regions. They are correlated to grain and grain boundary regions of the sample and electrode–electrode interface effects, respectively. Fig. 2 shows the impedance spectra in the low temperature region of ZnS sample. In this region, only a single semicircular arc is obtained since the relaxation times of grain and grain boundary regions become equal, and all of them merge due to the disorder of grain boundaries. In this region low conduction occurs in the sample. As temperature increases, the high frequency part of the semicircle separates from the low frequency part of the semicircle, which implies the presence of two distinct relaxation processes of grain interior and grain boundary [12]. It has also been observed that the conduction increases with increase in temperature. The conductivity properties of the sample at different temperatures reveal the grain contribution mechanism. In the high temperature region, conduction may be attributed to the ions across the grain boundaries and low temperature conduction may be attributed to that of grains as illustrated in Fig. 3. The activation energies as calculated from Fig. 3 are 1.71 and 3.64 eV for ZnS nanocrystallites. The activation energy increases at very high temperature owing to the larger grain
-1.4x107
-100 -80 -60 -40 -20 0 20 150
-1.2x107 -1.0x107
Z''
-8.0x106
598 K 623 k 648 K 673 K
873 K
200
250
300
350
-6.0x106 -4.0x106 -2.0x106 0.0 0.0
215
5.0x106
1.0x107 Z'
Fig. 2. Impedance spectra for ZnS nanocrystallites.
1.5x107
216
S. Chellammal, S. Sankar / Materials Science in Semiconductor Processing 13 (2010) 214–216
8
ZnS
7
log R ohm
6 Eg = 4.05 eV
5 4 3 2 1 0.0012
0.0014 1/T K-1
0.0016
0.0018
Fig. 4. Arrhenius plot for ZnS nanocrystallites (calculation of energy gap). Fig. 3. Arrhenius plot for ZnS nanocrystallites (calculation of activation energy).
interface. Increase in conductivity with increase in temperature implies decrease in defect density, from which it is clear that the increase in activation energy with respect to temperature reflects the blocking nature of grain boundaries. Increase in conductivity with increase in temperature implies decrease in defect density [13]. The prepared ZnS nanoparticles may contain open volume defects including vacancies. On heating, considerable decrease in the concentration of these defects can be expected, which in turn might give rise to an enhancement in the crystalline or more ordered phase of the sample. The phase transition indicated by the slope in the Arrhenius plot may be attributed to such a transition from a defective phase to a more ordered phase. The electrical resistance of ZnS nanocrystalline sample is measured as a function of temperature in the range of 543–833 K using an Impedance/Grain-Phase Analyzer (SOLARTRON 1260). Fig. 4 shows a plot of variation of resistance as log R versus 1/T. Resistance decreases linearly with increasing temperature, indicating semi-conducting behavior. From Fig. 4, we can calculate the energy gap Eg ¼ slope 2 KB It is found that the energy gap for the ZnS nanocrystallites is 4.05 eV; but for bulk ZnS the energy gap is said to be 3.67 eV. In earlier studies [14] the energy gap value for ZnS nanocrystalline (size 11 nm) was 3.9 eV. So the energy gap increases with decrease in size due to quantum confinement of the nanoparticles. In optical studies [15] ZnS quantum dots had a diameter less than 4 nm and a band gap of about 4.2 eV by the optical method. So the optical method is not similar to the electrical method.
4. Conclusion Zinc Sulphide nanocrystallites of size 4 nm have been synthesized using the chemical precipitation method. X-ray studies reveal hexagonal structure of the sample. Impedance analyses have yielded the activation energy for grain as 1.71 eV (low temperature conduction) and for grain boundary as 3.64 eV (high temperature conduction). Further, the energy gap studies have yielded 4.05 eV for the prepared nanoparticles. This value is greater than the value (3.67 eV) of bulk ZnS sample, which is due to the quantum confinement effect of nanoparticles. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Chen S, Liu W. Langmuir 1999;15:8100–4. Nanda J, Sapra S, Sarma DD. Chem Mater 2000;12:1018–24. Chestnoy N, Hull R, Brus LE. J Chem Phys 1986;85:2237–42. Norris DJ, Yao N, Charnock FT, Kennedy TA. Nano Lett 2001;1:3–7. Chen W, Malm JO, Zwiller V, Huang Y, Liu S, Wailenberg R. Phys Rev B 2000;61:11021–4. Soo YL, Ming ZH, Huang SW, Kao YH, Bhargava RN, Gallugher D, et al. Phys Rev B 1994;50:7602–97. Geng BY, Zhang LD, Wang GZ, Xie T, Zhang YG, Meng GW. Appl Phys Lett 2004;84:2157–9. Bhattacharjee B, Ganguli D, Iakoubovesku K, Stesmans A, Chaudhuri S. Bull Mater Sci 2002;25:175–80. Khosravi AA, Kundu M, Jatwa L, Deshpande SK, Bhagwat UA, Sastry M, et al. Appl Phys Lett 1995;67:2702–4. Lee S, Song D, Kim D, Lee J, Kim S, Park JY, et al. Mater Lett 2004;58: 342–6. Cullity BD. Elements of X-ray diffraction. 2nd ed.. Addison-Wesley; 1978 pp. 99–106. Biju V, Abdul Khadar M. J Mater Sci Eng A 2001;304–306:814–9. Ross Macdonald J, editor. New York: John Wiley; 1987. Singh PK. Prog Cryst Growth Charact Mater 2002;44:175–82. Wageh S, Zhao Su Ling, Xu Xu-Rong. J Cryst Growth 2003;255:332–7. Fuler ML. Philos Mag 1929;8:658.
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Energy gap studies of ZnS nanocrystallites S. Chellammal n, S. Sankar Department of Physics, College of Engineering, Anna University, Guindy, Chennai 600 025, Tamil Nadu, India
a r t i c l e in fo
abstract
Available online 15 December 2010
Zinc Sulphide nanocrystallites (ZnS) of average size 4 nm have been synthesized by the chemical precipitation method. The structural studies are carried out by the X-ray diffraction method. The hexagonal structure of the sample is confirmed by JCPDS. Using the impedance analysis method, the effect of grain interior and grain boundary regions on their conductivity has been studied for various temperatures. The activation energies and band gap values are calculated by the Arrhenius plot. & 2010 Published by Elsevier Ltd.
Keywords: Quasicrystals Quantum dots Semiconductor compounds II–VI semiconductors.
1. Introduction A systematic study of size effect on the impedance properties of semiconductor nanocrystallites is essential for understanding their technological applications [1–3]. ZnS is a direct band gap semi conducting material with a band gap 3.67 eV. Impedance studies were carried out for ZnS nanocrystallites and the results are presented in the paper. Powder sample of ZnS nanocrystallites was prepared for the present work using the chemical precipitation method and its impedance properties have been studied. X-ray characterization of the prepared nanocrystallites has also been carried out for the confirmation of structure and also to obtain the average size of the nanocrystallite. The effect of grain interior and grain boundary was studied with the help of an impedance plot. Nanocrystallites metal chaleogenides, when doped with impurities, exhibit very interesting electronic structure and transition probabilities [4–6] and also have many applications in the field of spintronics. Among the II–VI semiconductors, zinc sulphide is widely studied for its stability and technological applications [7,8]. ZnS nanoparticles and rare earth ions have a variety of applications in optical devices [9,10]. Recent literature shows several studies on the optical properties of ZnS nanoparticles. On the other hand, impedance studies
n
Corresponding author. Tel.: + 91 44 24811272; fax: + 91 44 22203160. E-mail address: [email protected] (S. Chellammal).
1369-8001/$ - see front matter & 2010 Published by Elsevier Ltd. doi:10.1016/j.mssp.2010.10.003
and electrical characterization of ZnS nanoparticles are very few. The present studies are, hence, dedicated towards the impedance analysis of ZnS nanoparticles. 2. Experimental design ZnS nanocrystallites have been synthesized by the method of chemical precipitation using the reactants Zinc Chloride and Sodium Sulphide with ethylene glycol as the capping agent at room temperature. The precipitation was carried out over a reflex time of 600 s for 100 ml of reflex volume of each of the reacting solutions. The precipitates were centrifuged and washed with de-ionized water repeatedly and then with methanol finally. Then the precipitates were dried in vacuum. Powder X-ray diffraction studies were carried out using ˚ Philip 2275/20 Mo-Ka radiation (wave length= 0.70930 A; X-ray diffractometer) to determine the structural phase of the sample. The average size of the sample was determined from the line broadening of the X-ray diffraction peaks corrected for instrumental broadening using Scherer’s equation [11]. A pellet of 8.23 mm diameter and 1.52 mm thickness prepared using the ZnS nanocrystalline powder was placed in between two platinum electrodes and complex impedance (Zn) measurements were carried out as a function of both frequency (from 1 Hz to 1 MHz) and temperature (from 543 to 833 K) using an Impedance/Grain-Phase Analyzer (SOLARTRON 1260) together with a dedicated
S. Chellammal, S. Sankar / Materials Science in Semiconductor Processing 13 (2010) 214–216
computer and software to acquire the impedance data. The whole setup of sample compartment inside the furnace was evacuated up to 1 10 5 Torr, in order to prevent oxidation of the sample during heating. The heating rate was maintained at 2 K/min. The temperature of the furnace was measured with a resolution of 71 K using a Eurotherm (818 P) PID temperature controller. The data were collected during both heating and cooling cycles. It was found that the data were consistent during both thermal cycles. The dielectric parameter such as complex conductivity (sn) was calculated from the raw data of Z0 and Z00 from the sample dimensions. 3. Results and analysis The XRD spectrum of the synthesized nanocrystalline ZnS powder sample is presented in Fig. 1. The average grain size is calculated from the XRD data using the Scherer formula [11] approximately equal to 4 nm. The indices of
Fig. 1. XRD pattern of the synthesized ZnS nanocrystallites.
three peaks observed at 2y = 12.195, 20.633 and 24.435 for the ZnS sample has been identified as (1 0 0), (1 1 0), (2 0 0) and (2 1 1) of the hexagonal structural phase of ZnS using the corresponding JCPDS data (PDF No.: 011280 [16], version 2.3V). We have estimated the average size (d) of our present nanocrystallites using the Scherrer formula d ¼ 0:9K l=o cos y where the factor K =4/3 accounts for the quasi-spherical geometry of the nanocrystals, l is the wavelength of X-rays used, o is the width on a 2y-scale and y is the scattering angle of X-rays. In general, three semicircular arcs can be expected in the impedance spectra in the high, intermediate and low frequency regions. They are correlated to grain and grain boundary regions of the sample and electrode–electrode interface effects, respectively. Fig. 2 shows the impedance spectra in the low temperature region of ZnS sample. In this region, only a single semicircular arc is obtained since the relaxation times of grain and grain boundary regions become equal, and all of them merge due to the disorder of grain boundaries. In this region low conduction occurs in the sample. As temperature increases, the high frequency part of the semicircle separates from the low frequency part of the semicircle, which implies the presence of two distinct relaxation processes of grain interior and grain boundary [12]. It has also been observed that the conduction increases with increase in temperature. The conductivity properties of the sample at different temperatures reveal the grain contribution mechanism. In the high temperature region, conduction may be attributed to the ions across the grain boundaries and low temperature conduction may be attributed to that of grains as illustrated in Fig. 3. The activation energies as calculated from Fig. 3 are 1.71 and 3.64 eV for ZnS nanocrystallites. The activation energy increases at very high temperature owing to the larger grain
-1.4x107
-100 -80 -60 -40 -20 0 20 150
-1.2x107 -1.0x107
Z''
-8.0x106
598 K 623 k 648 K 673 K
873 K
200
250
300
350
-6.0x106 -4.0x106 -2.0x106 0.0 0.0
215
5.0x106
1.0x107 Z'
Fig. 2. Impedance spectra for ZnS nanocrystallites.
1.5x107
216
S. Chellammal, S. Sankar / Materials Science in Semiconductor Processing 13 (2010) 214–216
8
ZnS
7
log R ohm
6 Eg = 4.05 eV
5 4 3 2 1 0.0012
0.0014 1/T K-1
0.0016
0.0018
Fig. 4. Arrhenius plot for ZnS nanocrystallites (calculation of energy gap). Fig. 3. Arrhenius plot for ZnS nanocrystallites (calculation of activation energy).
interface. Increase in conductivity with increase in temperature implies decrease in defect density, from which it is clear that the increase in activation energy with respect to temperature reflects the blocking nature of grain boundaries. Increase in conductivity with increase in temperature implies decrease in defect density [13]. The prepared ZnS nanoparticles may contain open volume defects including vacancies. On heating, considerable decrease in the concentration of these defects can be expected, which in turn might give rise to an enhancement in the crystalline or more ordered phase of the sample. The phase transition indicated by the slope in the Arrhenius plot may be attributed to such a transition from a defective phase to a more ordered phase. The electrical resistance of ZnS nanocrystalline sample is measured as a function of temperature in the range of 543–833 K using an Impedance/Grain-Phase Analyzer (SOLARTRON 1260). Fig. 4 shows a plot of variation of resistance as log R versus 1/T. Resistance decreases linearly with increasing temperature, indicating semi-conducting behavior. From Fig. 4, we can calculate the energy gap Eg ¼ slope 2 KB It is found that the energy gap for the ZnS nanocrystallites is 4.05 eV; but for bulk ZnS the energy gap is said to be 3.67 eV. In earlier studies [14] the energy gap value for ZnS nanocrystalline (size 11 nm) was 3.9 eV. So the energy gap increases with decrease in size due to quantum confinement of the nanoparticles. In optical studies [15] ZnS quantum dots had a diameter less than 4 nm and a band gap of about 4.2 eV by the optical method. So the optical method is not similar to the electrical method.
4. Conclusion Zinc Sulphide nanocrystallites of size 4 nm have been synthesized using the chemical precipitation method. X-ray studies reveal hexagonal structure of the sample. Impedance analyses have yielded the activation energy for grain as 1.71 eV (low temperature conduction) and for grain boundary as 3.64 eV (high temperature conduction). Further, the energy gap studies have yielded 4.05 eV for the prepared nanoparticles. This value is greater than the value (3.67 eV) of bulk ZnS sample, which is due to the quantum confinement effect of nanoparticles. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
Chen S, Liu W. Langmuir 1999;15:8100–4. Nanda J, Sapra S, Sarma DD. Chem Mater 2000;12:1018–24. Chestnoy N, Hull R, Brus LE. J Chem Phys 1986;85:2237–42. Norris DJ, Yao N, Charnock FT, Kennedy TA. Nano Lett 2001;1:3–7. Chen W, Malm JO, Zwiller V, Huang Y, Liu S, Wailenberg R. Phys Rev B 2000;61:11021–4. Soo YL, Ming ZH, Huang SW, Kao YH, Bhargava RN, Gallugher D, et al. Phys Rev B 1994;50:7602–97. Geng BY, Zhang LD, Wang GZ, Xie T, Zhang YG, Meng GW. Appl Phys Lett 2004;84:2157–9. Bhattacharjee B, Ganguli D, Iakoubovesku K, Stesmans A, Chaudhuri S. Bull Mater Sci 2002;25:175–80. Khosravi AA, Kundu M, Jatwa L, Deshpande SK, Bhagwat UA, Sastry M, et al. Appl Phys Lett 1995;67:2702–4. Lee S, Song D, Kim D, Lee J, Kim S, Park JY, et al. Mater Lett 2004;58: 342–6. Cullity BD. Elements of X-ray diffraction. 2nd ed.. Addison-Wesley; 1978 pp. 99–106. Biju V, Abdul Khadar M. J Mater Sci Eng A 2001;304–306:814–9. Ross Macdonald J, editor. New York: John Wiley; 1987. Singh PK. Prog Cryst Growth Charact Mater 2002;44:175–82. Wageh S, Zhao Su Ling, Xu Xu-Rong. J Cryst Growth 2003;255:332–7. Fuler ML. Philos Mag 1929;8:658.