254 IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 13, NO. 2, MARCH 2014

High Field Electroluminescence Characteristics ofPr3+-Doped ZnS/TiO2

Prabha Sana, Shammi Verma, M. M. Malik, and Meera Ramrakhiani

Abstract—This paper reports the fabrication, operation, andapplications of rare-earth (RE) ion praseodymium (Pr3+ )-dopedZnS/TiO2 core-shell nanoparticle-based thin-film electrolumines-cent (ACTFEL) devices. Pr3+ :ZnS/TiO2 ACTFEL devices showedwhite light emission due to impact excitation of the Pr3+ ions by hotcarriers followed by radiative RE relaxation under the high fieldof alternating current. The preparation of Pr3+ :ZnS/TiO2 core-shell nanoparticles was done by the colloidal synthesis method.To observe the effect of the TiO2 shell layer on electrolumines-cence (EL) brightness and properties of Pr3+ :ZnS, three types ofmolar ratios 1:7:7, 9:5:5, and 1:14:14 of Ti:Zn:S were taken inthe synthesis process, and to be employed for these three differentthin-film EL devices for high-field EL measurements. Each EL de-vice consists of glass substrate/ITO (indium-tin oxide)/TiO2 coatedPr3+ -doped ZnS nanocrystalline emission layer/Al (aluminum).The brightness characteristics of Pr3+ :ZnS/TiO2 TFELDs wereevaluated as a function of bias and frequency. Brightness–voltage(B–V) measurements at 600 Hz showed the maximum efficiency ofthe Pr3+ :ZnS/TiO2 EL device at molar ratios 1:14:14 of Ti:Zn:S.To observe the frequency-dependent characteristics, the EL devicewas tested at various frequencies. The measurements showed thedecrease in the threshold voltage of the EL device with increasedfrequencies. High-field voltage–current (V–I) characteristics alsoshowed the frequency-dependent onset voltage and current den-sity. It showed the minimum turn-on voltage and maximum currentdensity at a higher frequency of 1000 Hz.

Index Terms—Core-shell nanostructures, electroluminescence(EL) devices, photoluminescence, X-ray diffraction (XRD), zincsulphide.

I. INTRODUCTION

H IGH-FIELD electroluminescence (EL) devices haveproved to be an interesting concept for newer technolo-

gies, e.g., for the production of high-resolution, light-weight,compact flat panel displays for computer terminals or televi-sion screens. One of these technologies is the alternating cur-rent thin-film electroluminescence (ACTFEL) devices [1], [2],which shows promising performance in comparison with other

Manuscript received June 14, 2012; revised August 15, 2013; accepted Jan-uary 6, 2014. Date of publication January 9, 2014; date of current version March6, 2014. The review of this paper was arranged by Associate Editor Y.-H. Cho,

P. Sana and M. M. Malik are with the Department of Physics, Maulana AzadNational Institute of Technology, Bhopal 461051, India (e-mail: prabhasana@yahoo.co.in; manzar.malik@gmail.com).

S. Verma is with the Inter University Accelerator Centre, New Delhi 110067,India, (e-mail: shammiktl@gmail.com).

M. Ramrakhiani is with the Department of Physics, Rani Durgawati Univer-sity, Jabalpur 482001, India (e-mail: mramrakhiani@hotmail.com).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TNANO.2014.2298914

existing technologies such as cathode ray tube, plasma, and liq-uid crystal displays. Most importantly, ac EL devices are moreattractive because of the deterioration in performance of dc ELdevices with time. To fabricate ACTFEL devices, wide bandgapsemiconductor is required as an active layer (phosphor). Zincsulfide (ZnS) has wide bandgap (3.7 eV) and shows most ef-ficient and stable luminescence by excitation in an alternatingfield, and a reasonable lifetime for the EL panels. ZnS-basedEL occurs at very high electric fields, typically 1 MV/cm underdc condition and two MV/cm under ac condition. A potentialadvantage of ZnS arises at nanoscale due to the special confine-ment of photoexcited electrons and holes. And these confinedcharges modify the optical, electrical, and other related proper-ties of ZnS [3]–[6]. Recently, transition metal ion- and rare-earth(RE) ion-doped semiconducting nanocrystals (NCs) also haveattracted considerable attention due to their interesting proper-ties, such as high luminescence quantum efficiency, short radia-tive lifetime [7], size-dependent emission color tunability [8],multicolor EL, etc. These materials are considered to be lu-minophors. Research into RE-doped ZnS has been concentratedon the search for efficient red, green, and blue phosphor. The ELof RE ions such as Ce-, Tb-, Eu-, Pr-, Tm-doped different hostmaterials has been intensively investigated [9]–[12]. The REelement praseodymium (Pr) has partially filled the inner shell(4f), which is shielded from its surroundings by completely filledouter (5s and 5p) orbitals. Due to this shielding, the intra-4f-shelltransitions result in very sharp optical emissions spectra [13]. Prions are often used as white-light-emitting luminescent centersin ZnS host layers [14]. In RE-doped compounds, the 4f lev-els lie even deeper inside the bandgap of the host matrix [15],and therefore RE-doped EL devices show greater stability inapplied fields. However, one of the main drawbacks of ZnStype of phosphors to be used in EL devices is its instabilityagainst moisture and environmental conditions during their use.Coating the phosphor particles with thin dielectric layers ofinert/dielectric materials like SiO2 , Al2O3 , or other oxides is afrequently applied technique to enhance the stability of sulphidephosphors [16], [17]. The dielectric layer affects the phosphor–dielectric interface that determines the interface states that playa large role in the production of the current necessary for lightgeneration. During device operation at high electric fields, thedielectric requires high breakdown voltage and expected to be asdefect free as possible. The dielectric should also prevent chargeleakage into the phosphor layer and have good adhesion withphosphor and the contacts. Therefore, our emphasis was to pre-pare Pr3+ -doped ZnS/TiO2 core-shell nano-phosphors, wherea TiO2 (high dielectric strength, εr = 60) thin protective layeron the ZnS phosphor nanoparticles makes it resistant against

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SANA et al.: HIGH FIELD ELECTROLUMINESCENCE CHARACTERISTICS OF PR3 + -DOPED ZNS/TIO2 255

moisture and help in fast charge transfer process or re-duce leakage current. This study reported the fabrication ofpraseodymium (Pr3+ )-doped ZnS/TiO2 core-shell nanoparti-cles by a solution-based synthesis method which is the mosteffective technique for complete encapsulation of the core sur-face. To observe the effect of the TiO2 capping layer on ZnSEL properties, the samples were prepared at three different con-centrations of TiO2 . The ACTFEL devices were fabricated andtheir V–B and V–I characteristics were tested at various ac fre-quencies, i.e., 600 to 1000 Hz. To analyze the EL characteristics,photoluminescence (PL) spectra, the X-ray diffraction (XRD)pattern, and the TEM pattern were also used to investigate thehigh-field transport and impact excitation of Pr3+ luminescentimpurities in ZnS with the goal of understanding mechanismsof present core-shell nanoparticles-based ACTFEL devices.

II. EXPERIMENTAL DETAILS

A. Preparation of the Material

Pr3+ -doped zinc sulphide/titanium dioxide (ZnS:TiO2) core-shell nanoparticles were prepared from a solution-based col-loidal synthesis method. In order to obtain the desired composi-tions, stoichiometric amounts of 0.02625 mole Zn(CH3COO)2(zinc acetate) were dissolved in 50 ml of deionized water andkept in three necked round-bottom (RB) flasks.

The solution of 0.035 at% of Pr6O11 in 20 ml C2H5OH wasmixed with the previously prepared Zn(CH3COO)2 solution (ina RB vessel). The RB vessel was kept under constant stirringat room temperature for 3 h at a pressure of 10−3 Torr. After-ward 0.02625 mole Na2S was used to prepare 50 ml aqueoussolution (in deionized water) and slowly injected inside the RBvessel. Simultaneously, the Ti precursor was prepared by mixing1.42 ml Ti{OCH9CH3)2}4 (titanium isopropoxide) with 20 ml2-propanol in a beaker and heated up for 1 h at 100 ◦C underconstant stirring. Furthermore, the Ti solution was immediatelyinjected into the RB vessel after the Na2S injection for desiredcomposition of Ti:Zn:S of molar ratio 1:7:7. The reaction vesselwas kept at 80 ◦C for 1 h under constant stirring. The final prod-uct was milky white transparent colloidal solution. The preparedsample was centrifuged at 5000 r/min and washed several timeswith deionized water; later the precipitates were collected anddried. Similar experiments were performed to prepare Pr-dopedZnS/TiO2 core-shell nanoparticles for other two compositionratios of Ti:Zn:S at molar ratios of 9:5:5 and 1:14:14.

B. Thin-Film EL Cell Fabrication and Measurement

To study the EL properties of Pr3+ :ZnS/TiO2 nanocrystals,thin-film devices were prepared by a triplet layer structure,namely a nanocrystal emission thin layer of thickness 0.5 mm ofPr3+ : ZnS/TiO2 sandwiched between two electrodes. The bot-tom conductor was ITO, indium tin oxide, an alloy of 90 wt%In2O3 and 10 wt% SnO3 . This bottom electrode is transparentfor generated light to come out, so that it could be measured.Aluminum was used as the second electrode on top. A thin micasheet having a small window of 2 mm × 2 mm was fixed overthe emissive layer of Pr3+ :ZnS/TiO2 nanoparticles. For the EL

Fig. 1. Setup for electroluminescence brightness measurement.

study, ac voltages of various frequencies were provided to theconducting glass plate and aluminum electrode using EL powersupply M-EHT-100. An AFG wide-band high-voltage amplifierwas used for supplying high voltage. Voltage versus brightnesscharacteristics of EL devices were measured by a photomulti-plier tube (PMT) which was connected to a picoammeter. TheRCA-931 PMT was operated at 750 V. For EL studies, the out-going voltage function generator was set at 0.1 V. The outputwas fed to the input of the EL power supply wideband amplifier(WBA). The EL cell was connected to ac power supply (WBA)and placed at the slit of the PMT. The output of the PMT wasgiven to the picoammeter. The setup for EL studies is shownin Fig. 1. AC voltages of different frequencies 600, 800, and1000 Hz were applied, and EL brightness at different voltageswere measured at each frequency. The corresponding currentwas recorded by a microammeter, which is connected in serieswith the EL cell. A particular frequency was set in the audiogenerator and voltage applied to the EL cell was increased andcorresponding current and brightness were recorded.

III. RESULTS AND DISCUSSION

A. Structural Analysis

The structural analysis of Pr3+ :ZnS/TiO2 of three differentcomposition ratios was examined using the XRD (using a D8Advance Bruker X-ray diffractometer) pattern, shown in Fig. 2.The XRD pattern was more intense for the composition ratioTi:Zn:S of 1:14:14 of Pr3+ :ZnS/TiO2 NCs, shown in Fig. 2(c).For this sample, the reflection planes (111) and (311) at 2θ val-ues 28.67◦ and 56.3◦ were indexed as the lattice planes of a ZnScubic zinc blended (JCPDS File No. 77-2100) structure witha = 5.414 A. On the other hand, the reflection planes (111) and(112) at 2θ values 31.65◦ and 45.42◦ were indexed as the lat-tice planes of a TiO2 orthorhombic structure (JCPDS 84-1750)of primitive lattice with a = 4.531 A. The XRD patterns ofPr3+ -doped ZnS/TiO2 core-shell nanocrystals at compositionmolar ratios 1:7:7 and 9:5:5 were also analyzed, emphasizedto cubic zinc-blended ZnS and orthorhombic TiO2 structuresshown in Fig. 2(a) and (b). Although at a molar ratio 9:5:5 ofPr3+ : ZnS/TiO2 , the ZnS reflection plane was less intense and

256 IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 13, NO. 2, MARCH 2014

Fig. 2. XRD spectra of Pr3+ : ZnS/TiO2 at three molar ratio (a) 9:5:5,(b) 1:7:7, and (c) 1:14:14, respectively.

not observed well, while the TiO2 orthorhombic structure withintense reflection planes (111) and (112) was observed. The av-erage grain sizes of the prepared nanocrystals were estimated bythe Debye–Scherrer formula, approximately 94 nm, 22 nm, and19 nm at three molar ratios 1:7:7, 9:5:5, and 1:14:14, respec-tively. The XRD pattern of Pr3+ -doped ZnS/TiO2 core-shellnanocrystals at a composition ratio 1:14:14 revealed the goodcrystalline phase of ZnS and TiO2 both with the least particlesize of 19 nm, in comparison with two other XRD patterns. ATEM study in Fig. 3 showed the formation of core-shell struc-tures with size of ZnS as core ranging from 5 to 20 nm, whileTiO2 shell thickness varied from 3 to 10 nm. It was observed thatthe small ZnS nanoparticles (dark region) were uniformly coated

Fig. 3. TEM image of Pr3+ doped ZnS/TiO2 core shell nanoparticles at amolar ratio of Ti:Zn:S of 1:14:14.

with a thin shell layer of TiO2 (lighter region) to form core shellstructures (region “a” in Fig. 3). The larger ZnS nanoparticlesform a core shell structure with a thick nonuniform shell of TiO2(region “b” in Fig. 3). Therefore, TEM revealed the variation inthe size distribution of nanoparticles.

B. Photo Luminescence Emission Spectra Analysis

Room temperature photoluminescence (PL) emission spec-tra of Pr3+ :ZnS/TiO2 nanocrystals at three molar compositionratios of Ti:Zn:S were studied at the excitation wavelength of250 nm (4.9 eV). This excitation wavelength was blue shiftedfrom the excitation wavelength of 332 nm (3.73 eV) correspond-ing to the bandgap energy of bulk ZnS host. This blue shift inZnS nanocrystals was a reflection of the bandgap increase ow-ing to the quantum confinement effect. The characteristic bandsin the PL spectra of samples with emission wavelengths at 379,484, 525, and 665 nm were observed, as shown in Fig. 4(a).The PL emission wavelength at 379 nm contributed to near-band-edge emission originated from the recombination of freeexcitons of the ZnS nanocrystalline size regime [19]. The lu-minescence peaks at 484, 525, and 665 nm were correspondingtransition energy levels 3P0–3H4 ,

3P0–3H5 and 3P0–3F2 , respec-tively, due to Pr3+ ions in the ZnS host matrix. These emissionsoccurred through energy transfer from the excited state of theZnS host lattice to the electrons of Pr3+ ions. The schematic di-agram of energy transfer and deep trap level transitions of Pr3+

ions in the ZnS host matrix is shown in Fig. 4(b). It revealed asharp emission peak at 665 nm due to Pr3+ ions from the deep-trap energy levels transition 3P0–3F2 . This sharp peak was due tothe 4f–4f transition (forbidden transition) of trivalent Pr3+ lan-thanide ions in the ZnS host lattice. Fig. 4(a) revealed the similaremission spectrum for other two molar ratios 7:5:5 and 1:7:7 forPr3+ : ZnS/TiO2 nanostructures. At a molar ratio 1:14:14, thehighest emission intensity at 665 nm due to the 3P0–3F2 transi-tion of Pr3+ ions in the ZnS host and lowest emission intensityat 379 nm due to the band edge transition ZnS host were clearlyobserved. This indicated the higher emission intensity due tothe impurity level transition in ZnS, also revealed the complete

SANA et al.: HIGH FIELD ELECTROLUMINESCENCE CHARACTERISTICS OF PR3 + -DOPED ZNS/TIO2 257

Fig. 4. (a) Photoluminescence emission spectra of samples at excitation wave-length 250 nm at three molar ratios (a) 1:7:7, (b) 9:5:5, and (c) 1:14:14 respec-tively. (b) Schematic representation of deep trap level transition of Pr3+ ions inthe ZnS host matrix.

encapsulation of the ZnS nanocrystal surfaces by TiO2 at thismolar ratio and provided maximum stability.

C. Electric Process and Light Emission Mechanism

The mechanism of EL process in the Pr3+ :ZnS/TiO2 EL de-vice is based on excitation of Pr3+ luminescence centers bymajority charge carriers. These charge carriers accelerated un-der the action of a strong electric field of the order of 1 to 2MV/cm. The underlying mechanism of high-field EL chargecarriers are 1) electron injection into the conduction band ofthe semiconductor, 2) subsequent acceleration of the electron inthe high electric field to optical energies, 3) impact excitationof the luminescence center, 4) impact ionization, and 5) re-capture of electron [20]. The schematic presentation of electricprocess and light emission mechanism of the ACTFEL deviceis shown in Fig. 5. During device operation, electron injectionoccurs from the TiO2–ZnS interface into ZnS. When enoughvoltage is applied to breakdown ZnS in the device and thresh-old is reached, the electrons trap in interface states of TiO2 /ZnS.

Fig. 5. Energy band diagram illustrating the five primary physical processesresponsible for Pr3+ :ZnS/TiO2 ACTFEL device operation.

These electrons can tunnel into the conduction band of ZnS [21].Tunneling of electrons from the insulator–phosphor interface tothe conduction band of ZnS is very sensitive to the field strength.Electrons trapped in energy levels in the bandgap at the interfacecan tunnel through the energy barrier. The barrier height at theinterface of TiO2 and ZnS is 1.2 eV. The width of this barrier isinversely proportional to the electric field in the phosphor layer.The high electric field in the ZnS layer accelerates the electrons,so that they reach to ballistic energies and travel across the ZnS.These energetic electrons may excite the ZnS host or nonlumi-nescent centers which then transfer energy to the luminescentdopant Pr3+ , or the electrons may directly strike the luminescentcenters causing impact excitation or impact ionization [22]. Inthe process of impact excitation, the excited ZnS host ions returnto the ground state by transferring its energy to the Pr3+ lumines-cent centers. Radiative de-excitation occurs through photon gen-eration with the photon energy matching the energy level tran-sition of the electron in the Pr3+ ion in the ZnS host, known asEL emission [23], [24]. In this process, the electron-luminescentcenter interaction probability is related to the luminescent cen-ter radiative cross-section. When an electron in the ground stateof the luminescent center is excited to a higher electronic statelocalized on the activator ion core, de-excitation from the ex-cited state may occur either radiatively or nonradiatively. Ra-diative relaxation paths result in the emission of a photon whosewavelength corresponds to the difference in energy between theexcited and lower energy states of luminescent center. On theother hand, in the process of impact ionization, due to the higherbreakdown field, the electrons populate in the conduction bandof ZnS. Although it has been stated that impact excitation is amore efficient process than impact ionization. In the last processafter the collision of energetic electrons with the luminescencecentre Pr3+ , the electrons are again accelerated and the pro-cess continues. Because once an electron travels across the ZnSfrom either the interface or impact ionization process, it will be

258 IEEE TRANSACTIONS ON NANOTECHNOLOGY, VOL. 13, NO. 2, MARCH 2014

Fig. 6. (a) B–V characteristic curve of Pr3+ : ZnS/TiO2 core shell nanopar-ticles at frequency 600 Hz for Ti:Zn:S of (a) 1:7:7, (b) 9:5:5, and (c) 1:14:14respectively; and (b) B–V characteristic curve at frequencies 600, 800, and1000 Hz for Ti:Zn:S of 1:14:14.

captured by the interface states on the other side of the ZnS.And for the next voltage pulse, the polarities of the elec-trodes are switched and the process starts again in the oppositedirection.

D. EL Characteristics

V–B characteristics of Pr3+ :ZnS/TiO2-based three EL deviceswere studied at ac frequency 600 Hz, as shown in Fig. 6(a).As above a certain threshold voltage of the order of 300 V,charge transfers in a semiconductor layer and EL starts, andabove 500 V saturation region starts and an increase in volt-age beyond this saturation region leads to destructive breakdown in the sample. Therefore, all the measurements were takenwithin the range of 300 to 550 V. V–B characteristics revealedthe turn-on threshold voltages 450, 440, and 290 V for threeEl devices for composition ratios Ti:Zn:S of 1:7:7, 9:5:5, and

1:14:14 respectively. An exponential increase in brightness wasobserved along with the increase in voltage. This was due tothe increase in transfer of charge carriers to the EL layer alongwith voltage [25]. The comparative analysis of the V–B curveemphasized the lower turn-on threshold voltage with highestbrightness for the Pr3+ :ZnS/TiO2 EL device at a compositionratio of Ti:Zn:S of 1:14:14. It might have these reasons: 1) ob-tained smaller nanoparticles have increased oscillator strength,which improves the electron–hole radiative recombination andenhances the EL [26], 2) high crystallinity of Pr3+ :ZnS/TiO2is effectively promoting the charge transfer in the ZnS activelayer [27], [28], and 3) the uniform coating of the thin dielectricTiO2 layer on ZnS nanocrystals improves the electron injec-tion, resulting in higher electron mobility [29]. These reasonsmight have contributed to the significant decrease in the turn-onvoltage thereby increasing the overall performance of the de-vice, particularly in the value luminescence of about 46 cd/cm2 .The PL emission spectrum confirmed the higher emission in-tensity, caused by uniform thin coating of TiO2 on ZnS. There-fore, it might be concluded that at a ratio Ti:Zn:S of 1:14:14Pr3+ :ZnS/TiO2 nanocrystals have good response to material re-liability and device performance. The frequency-dependent V–Bcharacteristics of Pr3+ :ZnS/TiO2 nanocrystals of the EL deviceat the composition ratio 1:14:14 were studied at three frequen-cies 600, 800, and 1000 Hz, as shown in Fig. 6(b). It revealedthat, as applied frequencies increased, the light emission startsat de-trapping of the EL centres with the increase in frequency,although brightness increased exponentially with the increasedapplied voltage. The luminescence of an ACTFEL device isvery sensitive to the drive frequency. The emission intensityincreases linearly in most of the cases, as the drive frequencyincreases. As the frequency increases, there are more pulses pergiven time yielding more excitation of the luminescent centersand higher brightness.

E. V–I Characteristics

V–I characteristics of Pr3+ :ZnS/TiO2 of the nanocrystallineEL device of the composition ratio Ti:Zn:S of 1:14:14 werestudied at three frequencies 600, 800, and 1000 Hz, shownin Fig. 7. It revealed the increased current with increased ap-plied voltage, and also the value of onset threshold voltage wasdecreased with the increase in applied frequency. At the maxi-mum applied frequency of 1000 Hz, the turn-on threshold volt-age was minimized and having highest current density of value0.074 mA/cm2 . The overall concept of electric process and lightemission mechanism in the present Pr3+ :ZnS/TiO2 core-shellnanostructures-based EL device was investigated through V–Band V–I characteristics. It is emphasized that the TiO2 shelllayer not only encapsulates the surface of ZnS nanoparticlesbut also provides the dielectric layer for electron transport. Thebasic mechanism of high field EL has been discussed in briefin Section III-C, in sequence: generation of charge carriers,acceleration of these carriers, and their inelastic collisions toexcite luminescence centres. The spatial separation of the se-quential steps in adjacent layers (insulator/active layer/insulator)in an EL device was proposed by Williams [30], evident for

SANA et al.: HIGH FIELD ELECTROLUMINESCENCE CHARACTERISTICS OF PR3 + -DOPED ZNS/TIO2 259

Fig. 7. V–I characteristic curve and of Pr3+ : ZnS/TiO2 core shell nanoparti-cles at three frequencies 600, 800, and 1000 Hz for composition ratio of Ti:Zn:Sof 1:14:14.

separation of the acceleration and collision excitation in the ad-jacent layer of the EL device by non-ohmic characteristics. TheV–I characteristics of the Pr3+ : ZnS/TiO2 EL device showednon-ohmic characteristics. It revealed that the acceleration ofcharge carriers and collision excitation processes were spatiallyseparated.

IV. CONCLUSION

This paper reports the utilization of Pr3+ -doped ZnS/TiO2core-shell nanoparticles for ACTFEL device aspects and showsthe dependence of electrical and optical properties on the coreto shell ratio of ZnS/TiO2 . Pure colors are produced by the innershell RE transition in the ZnS host and provides a very robustsemiconductor host which can withstand the high fields neededto provide hot carriers for impact excitation of the RE ions inACTFELDs. The V–B measurements of three Pr3+ :ZnS/TiO2EL devices were performed at 600 Hz, and higher bright-ness with lower threshold was achieved for the EL device ofTi:Zn:S ratio of 1:14:14. The frequency-dependent V–B char-acteristics of this EL device show minimum onset voltage withhigher brightness at frequency 1000 Hz. Also, V–I characteris-tics showed non-ohmic behavior, and revealed the separation ofthe acceleration and collision excitation events in the EL pro-cess. The versatility and robust nature of RE-doped ZnS/TiO2core shell nanoparticle technology holds great promise for op-toelectronic and photonic applications.

ACKNOWLEDGMENT

The authors would like to thank Rani Durgawati University(RDVV), Jabalpur, for providing EL facility. The authors alsowould like to thank S. Sahare, RDVV, Jabalpur, for her help andsupport in this work.

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Prabha Sana was born in Sohagpur (Dist. Hoshanga-bad), Madhya Pradesh, India in 1982. She obtainedthe B.Sc. and M.Sc. degrees in physics from Barkat-ullah University, Madhya Pradesh, India, in 2003 and2005, respectively. Her major fields of study are con-densed matter physics and novel materials, She re-ceived the Ph.D. degree in 2013 from the Departmentof Physics, Maulana Azad National Institute of Tech-nology, Bhopal, on the topic entitled “Investigationson swift heavy ion irradiated ZnS colloidal nanocrys-tals for photo devices.”

She is working at MANIT Bhopal, as a principle investigator on her re-search project “In situ measurements of Quantum Dots based heterojunctionPhoto Devices under Swift Heavy Ions Irradiation” sanctioned by Inter Uni-versity Accelerator Centre, New Delhi, India. Her research interests includesemiconductor nanomaterials, luminescence, heterojunctions particularly forfield emission display devices, photo detectors and inorganic solar cells, andthe study of high energy radiation (swift heavy ions)-induced modifications onthese nanomaterials based devices.

Shammi Verma was born in Kaithal, Haryana, Indiain 1987. He received the B.Sc. and M.Sc. degrees inphysics from Kurukshetra University, Kurukshetra,Haryana. His major field of study was condensedmatter physics and semiconductor physics. He is cur-rently working toward the Ph.D. degree from Jawa-harlal Nehru University, New Delhi, India.

He is currently working as a Senior Research Fel-low at Inter University Accelerator Centre (IUAC),New Delhi, India. His research interests include fab-rication of devices such as metal–semiconductor het-

erojunctions, particularly, Schottky barrier diodes, solar cells, magnetic tunneljunctions (MTJs) etc. through lithography techniques and their electrical trans-port and defect-related (DLTS) measurements under radiation environments(swift heavy ion and low energy radiations).

Mr. Verma is a member of Semiconductor Society of India (SSI; membershipnumber: 201103634) and Indian Physics Association, India.

M. M. Malik was born in Bhopal, India, in 1965. Hereceived the B.Sc. degree from Barkatullah Univer-sity, Bhopal, India, in 1985, and received the M.Sc.degree in physics in 1988, from the same university,and the Ph.D. degree in 1992 by Jamia Millia Islamia(Central University) New Delhi, India, on the topicentitled “Electrical & Structural Studies of Chalco-genide Glasses.”

He was awarded Senior Research Fellowship byCSIR, New Delhi, in 1991. He visited ICTP TriesteItaly in 1994 for research work. He joined Maulana

Azad National Institute of Technology (MANIT) Bhopal, in July 1994 and cur-rently he is a Professor and Head of the Department of Physics, MANIT.

Dr. Malik is member of various professional societies which includes LifeMember ISTE and Indian Chapter of ICTP Trieste, Italy. He is presently guid-ing students on the research areas of amorphous semiconductors, luminescence,nano materials, electrets and green synthesis.

Meera Ramrakhiani received the M.Sc. and Ph.D.degrees from the University of Jabalpur (now RaniDurgavati University), Jabalpur, India,

She is currently a Professor and Head of the De-partemnt of Physics and Electronics, Rani Durga-vati University Jabalpur, and has 35 years of teach-ing experience at under graduate and post-graduatelevel. Fifteen students have completed their Ph.D.under her supervision and many more are working inthe field of nanomaterials, luminescence, and photo-voltaic solar cells. She has participated in about 50

seminars/symposia etc. and presented has work at national/International level.She has visited Italy, Hungary, Singapore, USA and China for the research work.About 300 published papers are there to her credit. She has successfully carriedout two research project funded by MPCST.

Dr. Ramrakhiani has received Vijaya Shree Award by India InternationalFriendship Society in 1997, The 20th Century Award of Achievement by Inter-national biographical Center, Cambridge, England in 1998, Women of the Year2005 Jeweler Issued by American Biographical Institute, Inc.