International Journal of Mechanical Engineering and Materials Sciences, 4(1) January-June 2011, pp. 25-31

Corresponding author: dnmaran@gmail.com, dnmaran@yahoo.com

Synthesis and characterization of Zinc Sulfide Nanoparticles using InertGas Condensation Technique

K. RAVICHANDRAN1 AND D. NEDUMARAN2

1Materials Science Centre, Department of Nuclear Physics, University of Madras, Maraimalai CampusGuindy, Chennai-600025, Tamilnadu, India

2Central Instrumentation & Service Laboratory, University of Madras, Maraimalai Campus, GuindyChennai-600025, Tamilnadu, India.

ABSTRACT

This study presents preparation and characterization ofZinc sulfide (ZnS) nanocrystalline (NPs) by the Inert GasCondensation (IGC) technique in helium atmosphere. Thenano-size ZnS powder was characterized using X-rayDiffraction (XRD), High Resolution Transmission ElectronMicroscopy (HRTEM) and Selected Area ElectronDiffraction (SAED) studies. HRTEM indicates thenanoparticles have the average particle size as 8 ± 0.65nm. The SAED and XRD analyses exhibited the hexagonalcrystal structure with dhkl = 3.13 Å, 1.91 Å, and 1.63Å.The Fourier Transform Infrared Spectra (FTIR) of thepolycrystalline ZnS powder is reported in conjunctionwith the powder XRD results to confirm the bonding ofZinc with Sulfur. Studies on Differential ScanningCalorimeter (DSC), Thermogravimetric Analysis (TGA)and Differential Thermal Analysis (DTA) providedadditional information relevant to the thermal behaviorof nano-ZnS. The temperature dependence of electricalconductivity (σ), dielectric constant (ε´) and dielectric loss(tanä) for nano-ZnS are also investigated over atemperature range of 490 ºC – 630 ºC and frequency rangeof 1 Hz – 1M Hz. Fluorescent lifetimes estimated at roomtemperature have been correlated with XRD results. Theactivation energies for electrical conduction in vacuumannealed (400 ºC for 1.50 hr) samples are found to be 1.51eV and 1.76 eV. The fluorescent lifetimes, t1 = 0.36 ns andt2 = 0.67 ns for as-prepared and for t = 0.83 ns for anannealed sample have been identified.Keywords: Nanocrystalline ZnS, HRTEM, Electricalconductivity, X-ray diffraction and Fluorescence lifetime.

1. INTRODUCTIONSemiconductor nanoparticles (NPs) have attractedconsiderable attention due to their quantumconfinement effect and large surface – to – volumeratio1-5 useful in many applications compared to thoseof bulk semiconductors. Significant research effortshave been made to study the optical and electronicproperties of these semiconductors (NPs). ZnS is oneof the important semiconductors belonging to the II– VI group, has an energy band gap6 (Eg), 3.7 eV at

room temperature. ZnS is being studied widelybecause of its potential applications in differenttechnological areas include, field effect transistors,transductors7, lasers8, sensors9, electro-opticmodulators, solid-state solar cell windows 10,optoelectronic devices11-12, photoconductors, light-emitting diodes13, electroluminescence14-15, infrared-windows16, conventional fluorescent lighting tocathode ray tubes, field emission displays, plasmadisplays, scintillation17 and pigment18. The propertiesrelated to such applications are strongly dependenton their structure and particle size. Therefore, it isimportant to control the particle size and structureduring synthesis. The synthesis of ZnS nanocrystalswere carried out by the inert gas condensationtechnique. This technique consists of evaporation ofthe material into an inert gas atmosphere whichundergoes inter-atomic collisions with inert gasatoms, so that the evaporated atoms lose kineticenergy and condense in the form of small crystals offew nanometer sizes. Then, the powder isaccumulated on the cold finger head as the loosepowder19-22. Although, ZnS is an importantsemiconductor nanocrystal, details of its electrical,dielectric and fluorescence life time properties are notstudied well. Hence, in this work, we describe thesynthesis of ZnS NPs and their detail characterizationwith XRD, HRTEM, SAED, TGA, DTA, DSC, electricalconductivity, dielectric properties and fluorescencelifetime measurements.

2. EXPERIMENTAL DETAILSThe experimental setup developed for the gas phasecondensation to synthesis nano structured materialsconsists of an ultra high vacuum chamber operatingat a base pressure in the range of < 5 x 10-7 torr.Evaporation of ZnS sample was carried out on a Moboat at 1100 °C in a He atmosphere (1 torr). Thematerial was collected on a cold finger headmaintained at liquid nitrogen temperature. Theproducts were characterized using an X-ray

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diffractometer (XRD, Seifert 3000P) with Cu - Káradiation (ë = 1.5406 Å). The XRD patterns wererecorded in steps of 0.02° with 1 second counting timein each step and the 2θ scanning range of 10 – 80°.HRTEM images and SAED patterns were capturedusing a JEOL – JEM 3010 transmission electronmicroscope operating at an accelerating voltage of 300kV and has resolution of 0.14 nm. The samples weredispersed in ethanol by sonication and drop castedon a carbon-coated copper grid. HRTEM and SAEDdata were analyzed using the Gatan DigitalMicrograph software and the particle size wasdetermined by measuring lattice planes on HRTEMimages using multiple fields of the sample. Thesewere employed to evaluate the grain size andstructure of nanocrystalline ZnS. FTIR spectra weretaken on MIR8300 spectrometer in the frequencyrange 500 cm-1 – 4000 cm-1, to identify oxidation, ifany, and formation of nanocrystalline ZnS. Theprepared ZnS NPs were mixed with KBr powder andpressed into a pellet for FTIR measurement.Background correction was made using a pure KBrpellet as reference. Thermal properties were measuredon a Perkin - Elmer Thermogravimetry Analyser(TGA 7) and Differential Scanning Calorimetry(DSC7). A Differential Thermal Analyser DTA 1700model was used at a heating rate of 10 ºC/min inNitrogen atmosphere over the range of 50 - 850ºC, 50- 650ºC, and 500 - 1200ºC with Al2O3 as reference.Impedance spectroscopy was carried out on aSolartran 1260 model for conductivity study.Photoluminescence spectra were obtained usingFluromax-2 Spectroflurometer using a xenon lamp asan excitation source at room temperature. Theexcitation wavelength and filter wavelength used inthis study are 280 nm and 250 nm, respectively. Thefluorescence lifetime was recorded using a picosecond Laser and time correlated single photoncounting (TCSPC) setup. In the pico second Lasersetup, the samples were excited at 595 nm with LEDand pulse converter (IBH, Model TB-01) of 1.4 nsFWHM. The luminescence decay curves werecollected at front face geometry at the magic angle of54.7° which was dispersed in a monochromator (f/3aperture), counted by Micro Channel Plate (R 3809UMCP-PMT, HAMAMATSU) and processed throughConstant Fraction Discriminator (CFD), time toamplitude converter (TAC) and Multi ChannelAnalyzer (Oxford MCA). The fluorescence lifetime ofthe sample was further analyzed using IBH dataanalysis software.

3. RESULTSThe phase identification was carried out with the helpof standard JCPDS data base (File No. 75 – 1547).

Figure 1(A) shows the XRD patterns of the as-prepared ZnS, which identify a mixed partialamorphous and crystalline phase of ZnS. Figure 1(B)shows the XRD patterns of ZnS NPs annealed at 400°C in vacuum (10-5 torr) for 1.50 h. Three majordiffraction intensity peaks from planes (1 1 2) [2θ =28.41°, d = 3.13 Å ], (1 1 0) [2q = 47.35 °, d = 1.91] and(1 1 2) [2θ = 56.16º, d = 1.63 Å] are identified and theycould be indexed to the hexagonal structure withlattice parameters a = b = 3.821 Å , and c = 6.247 Å. Allother high angle peaks have submerged in thebackground due to the large line broadening whichattributes to the small size of the ZnS nanocrystals.The average size of ZnS has been calculated using theScherrer relation given in equation (1)

λ=β θ0.89cos

D (1)

where D is an average particle size in nm, β is the fullwidth at half maximum (FWHM) of X – ray reflectionexpressed in radians and θ is the position of thediffraction peaks in the diffractogram. The averagecrystalline size of the nanocrystalline ZnS is found tobe 8 nm. Figure 1(C) is the X-ray diffractionpattern of the bulk ZnS for comparison, which showsvery sharp peaks with average crystalline size as55 nm.

Figure 1: X-Ray Diffraction Pattern of Nanocrystalline ZnSSamples (A) as-prepared, (B) annealed in vacuum at

400 ºC for 1.5 h and (C) Bulk ZnS

Synthesis and Characterization of Zinc Sulfide Nanoparticles using Inert Gas... 27

Figure 2(a) shows the TEM image of annealed ZnSNPs which exhibit better crystallinity and have anaverage size as 8nm in agreement with thosecalculated results from the XRD patterns. The SAEDpattern containing three continuous rings countingfrom the center of the 1 st, 2nd, and 3 rd ringscorresponding to the (0 0 2), (1 1 0) and (1 1 2) planes,respectively of the hexagonal structure ofnanocrystalline ZnS that is consistent with the XRDresults too. The HRTEM image demonstrates the wellresolved diffraction fringes of the particle lattices(d = 0.31 Å, 0.19 Å, 0.16 Å) as shown in Figure 2(b).Figure 2(c) depicts distinct lattice fringes withinterplanar spacing d = 3.13 Å. The spacing betweenthe parallel fringes is about 0.31 nm, correspondingto that of (0 0 2) planes of ZnS hexagonal structure.Table 1. shows the comparison of interplanar spacingdhkl and average grain size obtained from XRD andHRTEM studies.

Table 1Comparison of Interplanar dhkl Spacing and

Average Grain Size Obtained from XRD andHRTEM Studies

2 in d (Å) Diffracting Average graindegree FWHM plane (h k l) size(nm)

XAD SAD XRD HRTEM

28.415 1. 266 3.1375 3.1377 (0 0 2) 8.00 8.0147.359 1. 812 1.9100 1.9135 (1 1 0)56.156 0. 903 1.6355 1. 6368 (1 1 2)

Figure 2(a): Typical TEM - Image of Nanocrystalline ZnSAnnealed in Vacuum at 400 ºC for 1.50 h Sample

Figure 2(b): High-resolution TEM - SAED of NanocrystallineZnS Sample shows Diffraction Rings Near d = 3.1 Å, 1.9Å and

1.6Å, Indicative of a Hexagonal Structure

Figure 2(c): HRTEM Lattice Fringes Image of NanocrystallineZnS and Confirming a Crystalline Phase

The FTIR spectra were recorded for the as-prepared and annealed nanocrystalline ZnS sample.Figure 3 shows FT-IR spectrum of the as-preparedsample with a broad absorption band (moistureabsorption) centered at 3436.90 cm-1 which exhibitsthe band O-H stretching vibrations and the presenceof water (from atmospheric absorbed water) gives (H-O-H) bending vibration at 1623.9 cm-1. The strongband at 618.6 cm-1 is due to ZnS (metal sulfide). The

28 International Journal of Mechanical Engineering and Materials Sciences

annealed sample of FT-IR spectrum confirms that thepeak uncondensed OH group vibration which(appears at 3436.90 cm-1) is absent in the spectrum.Hence, there might be complete absence of OHgrouping. This is a clear evidence for the presence ofpure nanocrystalline ZnS (618 cm-1).

Figure 3(a) shows the TGA curve of the as-prepared sample (A) and the sample (B) annealed at400 °C for 1.5 h in vacuum. The TGA graph showsthat the as-prepared sample loses weight from 50 °Cto 220 °C as a result of the evaporation of adsorbedwater vapor. However, such low temperature weightloss is not observed up to 500 °C with the annealedsample. The steady weight loss at 540 °C is assignedto the starting temperature of the decomposition ofzinc sulfide. Comparing the decompositiontemperatures of the as-prepared and the annealedsamples of ZnS nanoparticles, it is observed that theannealed sample possesses a decompositiontemperature (650°C) higher than those of the as-prepared sample (622.50°C). Percentagedecomposition does not match with stoichiometricvalues of Sulfur (32%) and hence, it is presumed thatthe decomposition is due to the formation of non-stoichiometric products. Figure 3 (b) shows the DSCcurves for the as-prepared sample (A) and annealedsample (B) at 400 °C for 1.5 h in vacuum anneal. Thespectrum indicates the decomposition of zincsulphide sample above 540 °C. The product is not onlythe zinc metal but also non-stoichiometric zincsulphide. It is also confirmed by a series of exothermicpeaks between 940 oC and 1004 oC in the DTA tracesshown in Fig. 3(c).

The complex impedance plots of annealedsamples measured in the temperature range of 490°C – 630 °C are shown in Figs. 4 (a) and (b). It shouldbe noted from these plots that only one depressedsemicircle exists whose center is slightly shifted belowthe real axis. This behaviour is commonly consideredto be an indication of a non-Debye relaxation process.At low temperatures and at low frequencies, the curvedeviates from a single semicircle. The measuringtemperatures are given in the right corner of Figs. 4(a)and (b). The impedance plots are fitted with complexnon-linear least-squares (CNLLS) fit to obtain theresistance of the sample. With the geometry of thesamples, these resistance values have been convertedinto conductivity using the relation given inequation (2)

σ = =ρ1 l

RA (2)Figure 3(a): TGA Curves for the Nanocrystalline ZnS Sample

(A) as-prepared and (B) annealed in vacuum at400 ºC for 1.5 h

Figure 3(b): DSC Curves for the Nanocrystalline ZnS Sample(A) as-prepared and (B) annealed in vacuum at

400 ºC for 1.5 h

Figure 3(c): DTA Curves for the Nanocrystalline ZnS SampleAnnealed in Vacuum at 400 ºC for 1.5 h

Synthesis and Characterization of Zinc Sulfide Nanoparticles using Inert Gas... 29

where is the conductivity, = Resistivity, l =thickness of the pellet, R is the resistance of the sampleand A is the area of cross section.

Figure 4(c) shows the Arrhenius plot ofnanocrystalline ZnS NP sample. The temperaturedependence of the activation energy is expressedusing the equation (3)

− σ =σ 0 exp aT

EkT (3)

where σo is the pre-exponential factor withdimensions of Ω-1 cm-1 K, Ea is the activation energyfor d.c conductivity and k is the Boltzman constant.The conductivity increases in the temperature rangeof 490 ºC – 630 ºC (Resistance also graduallydecreases). The plot (Conductivity vs temperature)in Fig. 4(c) shows two activation energy valuesobtained for the nanocrystalline ZnS in the lowertemperature region between 490 ºC and 570 ºC andin the higher temperature region between 590 ºC and630 ºC which corresponds to the activation energies,Ea = 1.76 eV and Ea = 1.51 eV, respectively. The reasonfor the first activation energy (Ea = 1.76 eV) is thatZnS being a semiconducting material, the band gapis (Eg = 3.70 eV) large and the threshold energyrequired to initialize the conduction process is high.Further the lowering of the activation energy after570 ºC indicates the reduction in the forbidden gapleading to high electrical conductivity.

Figure 4 (d) and Fig. 4(e) show the frequencydependence of the dielectric constant ε´, dielectricloss tanä for the selected temperatures. Thefrequency dependent behaviour of dielectricconstant as well as dielectric loss, tanδ can be furtherexplained as follows: The dielectric constant anddielectric loss in the nanocrystalline ZnS is attributedto four types of polarization - Interfacial, dipolar,atomic and electronic. At lower frequency, all thefour types of polarization contributed. The gradualdecrease in the dielectric constant and dielectric losswith frequency is primarily due to the interfacial anddipolar polarizations. Furthermore, in the higherfrequency range, the dielectric constant would besaturated because the electronic exchange cannotfollow the a.c field beyond a certain criticalfrequency.

The excitation and emission spectra ofnanocrystalline ZnS sample has been observedusing a photoluminescence spectrometer with aXenon lamp as an excitation source. A sharp excitationpeak was observed at λex = 375 nm for nanocrystallineZnS. The emission peak was observed at peak λem =440 nm.

Figure 4(a): Complex Impedance Plot of NanocrystallineZnS at Different Temperatures

Figure 4(b): Complex Impedance Plot of NanocrystallineZnS at Different Temperatures

Figure 4(c): Arrhenius Plot of Nanocrystalline ZnS Sample

Figure 4(d): The Frequency Dependance of DielectricConstant of Nanocrystalline ZnS Sample Annealed in

Vacuum at 400 ºC for 1.5 h

30 International Journal of Mechanical Engineering and Materials Sciences

Figure 4(e): The Frequency Dependance of Dielectric LossFactor of Nanocrystalline ZnS Sample Annealed in

Vacuum at 400 ºC for 1.5 h

Figure 5 shows the fluorescence lifetime of the as–prepared and annealed nanocrystalline ZnS. Thefluorescence lifetime measurements were carried outusing time correlated single photon countingtechnique. Here, excitation was carried out at 375 nmand emission was observed at 440 nm. Fluorescencedecay time curve can be well fitted using a bi-exponential function given as:

− τ − τ= +1 2/ /1 2( ) t tI t A e A e (4)

Two exponential decay times were obtained atτ1 = 0.36 ns and τ2 = 0.67 ns with relative amplitudes

of 67.14%, and 32.86%, respectively for the as-prepared ZnS. This may be due to the existence oftwo different phases (i.e amorphous and crystallinephases). Such a small fluorescence decay time innanoparticles has been attributed to a large overlapof electron–hole wave function due to quantumconfinement in ZnS. The single exponential decaytime obtained (τ = 0.83 ns with relative amplitude100%) for nanocrystalline ZnS sample annealed at 400°C in 1.50 h vacuum could be due to conversion ofamorphous phase to well crystalline phase as seenfrom the XRD analysis and agree well with theobservation from fluorescence life timemeasurements.

4. CONCLUSIONIn the present work, we have demonstrated the gasphase synthesis method which can be useful toprepare small nanocrystalline Zinc sulfide. Thenanostructure phase formation of the ZnS wasconfirmed by XRD and HRTEM. Thermal propertiesof the as-prepared and the annealed nanocrystallineZnS samples using TGA, DSC and DTA reveal thepartial amount of sulfur decomposition above 540 ºC.Impedance Spectroscopy studies confirmed, twoactivation energies for the annealed ZnS NPs in thelower (490 ºC - 570 ºC) and higher (590 ºC - 630 ºC)temperature regions corresponding to activationenergies of Ea = 1.76 eV and Ea = 1.51 eV, respectively.Photoluminescence lifetime measurements providedtwo lifetimes (0.179 ns and 1.07 ns) for the as-preparedsample and a single lifetime (0.83 ns) for the annealedsample.

ACKNOWLEDGEMENTThe authors would like to thank Prof. Dr. S. N. Sahu,Nanostructure, Cluster & Nano-Bio system Lab., Institute ofPhysics, Bhubaneswar, India and Prof. Dr P. R. Subramanian,Professor. & Head (Retd.), Department of Nuclear Physics,University of Madras for his helpful discussions andencouragement.

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Synthesis and Characterization of Zinc Sulfide Nanoparticles using Inert Gas... 31

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