CHAPTER IV SYNTHESIS AND CHARACTERIZATION OF METAL IONS DOPED ZINC SELENIDE NANOPARTICLES

Transcription

1 132 CHAPTER IV SYNTHESIS AND CHARACTERIZATION OF METAL IONS DOPED ZINC SELENIDE NANOPARTICLES 4.1 Introduction Introducing impurity atoms into a semiconductor host leads to an increase in the free-carrier concentration. In n-type semiconductors, free electrons in the conduction band are majority charge carriers, whereas holes in the valence band are majority carriers in p-type semiconductors. Shallow donors and acceptors have a much smaller ionization energy compared to the bandgap energy of the host semiconductor; hence, they can readily contribute to electrical conductivity. The optical properties of impurity-doped nanocrystals have attracted much attention. Since from both electronic states and electromagnetic fields are modified, optical properties of impurities may change drastically in nanostructures. Since the first report of ZnS:Mn 2+ nanoparticles, several studies on doped metal chalcogenide quantum dots have appeared, including new preparation methods, luminescence properties and potential applications. For doped nanoparticles, the most fundamentally interesting results are the luminescence enhancement and the lifetime

2 133 shortening of the Mn 2+ emission with decreasing size. The first synthesis of ZnS: Mn 2+ quantum dots carried out by Bhargava et al., was made in toluene with diethyl salt precursors and methacrylic acid as surfactant [144]. They obtained particles with two emission peaks (265 and 584 nm) and 18% quantum yield at room temperature. Since then a lot of syntheses both in water and organic solvents have been carried out with changing surfactants such as hexametaphosphate, thioglycerol, polyethylenoxide and hydroxypropyl cellulose. ZnS:Mn 2+ were synthesized in block copolymer nano reactors and zeolites and with different amounts of Mn 2+ to investigate the influence of the dopant concentration on the fluorescence properties. Qi Xiao et al., [145] synthesized Mn doped ZnS quantum dots and thioglycolic acid was used as a stabilizing agent. The effects of Mn 2+ concentration on photoluminescence of Mn:ZnS quantum dots have been investigated. The nonlinear properties of the Mn doped ZnSe quantum dots have been studied by Deepak More et al [146]. Mariya Hardzei had successfully prepared the Mn 2+ doped ZnSe quantum dots and studied the effect of ph on the luminescence property [147]. The synthetic challenges of doping and the intentional introduction of impurities into a semiconducting material is a common approach for tuning the electronic, optical, mechanical and magnetic properties of the materials [ ]. Since, it had provided fertile grounds for the investigation of the basic chemistries of homogeneous nucleation and crystal growth in the presence of impurities.

3 134 In this chapter, ZnSe quantum dots have been synthesized by doping transition metal manganese (Mn 2+ ) and the alkaline rare earth metal magnesium (Mg 2+ ). Since, Mn 2+ and Mg 2+ doped nanocrystals have been extensively investigated for use in various applications other than biomedical labeling, such as displays, sensors and lasers [152, 153]. These dopants could be potential candidates for fluorescent labeling agents especially in biology. The aim of the present work is to study the effect of these dopants on the optical and structural properties of ZnSe quantum dots and to achieve the size confinement and monodispersity. N-Methylaniline was used as a surface passivating organic ligand. 4.2 Synthesis and Characterization of Manganese doped Zinc Selenide quantum dots Much effort has been made to realize Mn 2+ doping in II VI semiconductors in order to produce new materials for various applications ranging from solar cell to spintronics [154, 155]. Mn 2+ acts as a paramagnetic centre (s=5/2) which substitutes for the group II cation in the semiconductor lattice. This interaction between the semiconductor and the Mn 2+, results in a new class of materials with interesting magnetic and optical properties which have not been observed in the bulk. ZnSe quantum dots doped with Mn 2+ ions are intensively studied due to its unique optical and magneto optical properties [ ]. ZnSe: Mn 2+ quantum dots may be used as nontoxic fluorescent markers in biomedicine or sensors.

4 Synthesis All the reagents were of analytical grade and used without further purification. 0.2 M solution of zinc acetate monohydrate [(CH 3 COO 2 )Zn H 2 O)] was prepared in 20 ml of distilled water and 0.02 M manganese sulphate (MnSO 4 ) was added to the solution. After that, 20 ml of solution of 0.2 M sodium selenite (Na 2 SeO 3 5H 2 O) solution was added. Finally, 0.2 ml NMA was added drop wise to the same solution as a capping agent. It is stirred vigorously for 36 h using magnetic stirrer. The resultant product was dried at 180 C for 10 h. The synthesized product has been characterized by XRD, UV-Vis, PL, FTIR and TEM Optical studies UV- visible absorption spectrum of N - Methylaniline passivated ZnSe: Mn 2+ is shown in Figure 4.1(a). It exhibits absorption edge at 300 nm which is blue shifted from that of the bulk ZnSe whose absorption edge is located at 460 nm [159]. The band gap of Mn 2+ doped ZnSe are derived based on the well-established equation [eq.(1)]. (αhν) (4.2eV) hν Figure 4.1 (a) (a) UV - Visible absorption spectrum and (b) bandgap plot of NMA passivated ZnSe:Mn 2+ Quantum dots (b)

5 136 ( hν E ) A g 2 1 α = (1) hν where α, E g and A are the absorption coefficient, band gap and constant respectively. By extrapolating the linear region in the plots of ( αhν ) 2 versus h ν (Figure 4.1. (b)), the band gap value is estimated as 4.2 ev. But the band gap of the bulk ZnSe is 2.7 ev. This blue shift indicates the quantum confinement of the particles. The radius of the nanocrystals R can be calculated using the following equation [eq.(2)] π h e E = Eg + smallerterms R me mh ε R (2) where E is the energy of the first excited state, E g = 2.7 ev is the band gap energy of bulk ZnSe, m e * and m h * are the effective masses of the electron and hole in ZnSe (m e = 0.15m o, m h = 0.66m o, where m o = 9.11x10-28 gm, the free electron mass)[152] respectively, є = 9.2, is the semiconductor dielectric constant, ћ = 6.58 x ev is the reduced Plank constant and e = 1.6 x C is the electron charge. The size of the particle is calculated as 2.7 nm which is almost equal to the value observed in the TEM. Photoluminescence spectrum of N-Methylaniline passivated ZnSe: Mn 2+ quantum dots is shown in Figure 4.2. The excitation wavelength used was at 250 nm and the emission peaks were found at different wavelengths such as 376 nm, 400 nm, 422 nm, 505 nm, 609 nm. The sharp band edge

6 137 emission is centered at 376 nm, the peak centered at 400 nm and 422 nm are due to the trap state emission. The other peak centered at 505 nm is usually defect related and may originate from the self activated centers and the broadened peak observed at 609 nm is due to the deep level formed by the incorporation of Mn 2+ in the ZnSe quantum dots [160, 161]. Similar emission was observed by Deepak More, the strong blue emission observed at 425 nm corresponds to band edge emission and the weak band emission around 585 nm is attributed to trap state [146]. Mariye Hardezi reported that, the emission at 480 nm belongs to ZnSe excitionic radiative recombination, while the other band at 590 nm was due to the radiative recombination through d-d levels of Mn 2+ ion incorporated to ZnSe matrix [147]. In general, when Mn 2+ is placed substitutionally on a cation sites in a II VI semiconductor host lattice, degenearices in its internal electronic structure are lifted by the crystal field. This produces localized levels ( 4 T 1 and 6 A 1 ) that are within the energy gap of the nanocrystals. In the present work, the emission band centered at 609 nm is attributed to ( 4 T 1 and 6 A 1 ) transition within the 3d shell of Mn 2+. When Mn 2+ ions were incorporated into the ZnSe lattice and substituted for host cation sites, the mixing between the s p electrons of the host ZnSe and the d electrons of Mn 2+ occurred and made the forbidden transition of 4 T 1 and 6 A 1 partially allowed, resulting in the characteristic emission of Mn 2+.

7 138 8x10 5 7x x10 5 Intensity 5x10 5 4x10 5 3x x x Wavelength (nm) Figure 4.2 Photoluminescence spectrum of NMA passivated ZnSe:Mn 2+ Quantum dots Structural analysis (311) (331) Intensity (cps) (220) (400) (440) (511) (531) θ (degree) Figure 4.3 XRD pattern of NMA passivated ZnSe:Mn 2+ Quantum dots

8 139 XRD pattern of N-Methylaniline passivated ZnSe:Mn 2+ quantum dots is shown in Figure 4.3. All the reflection peaks can be indexed to cubic system with the lattice constant of (a = b = c = 5.65Å), which is in good agreement with the reported data (JCPDS card no: ). No other peaks related to impurities were detected Morphological study Figure 4.4 (a) shows the TEM micrograph of N-Methylaniline passivated ZnSe: Mn 2+ quantum dots. The dispersed quantum dots can be clearly seen, size of the quantum dots are in the range of 2 5 nm and the size distribution was fairly narrow as illustrated by particle size histograms shown in Figure 4.4 (b) and most of the quantum dots are in the size of 2 nm. High resolution transmission electron microscopy images are shown in Figure 4.4 (c). Higher magnification images showed that the size of the particles is about 2 5 nm. Also it indicates that the quantum dots are crystalline. Figure 4.4 (d) is a typical selected area electron diffraction (SAED) pattern of the ZnSe: Mn 2+ Quantum dots. These pattern spots can be readily indexed as the (311), (400), (331), (440), (511), (531) planes for the cubic structure of ZnSe.

9 Number of particles Particle size (nm) (a) (b) (c) (d) Figure 4.4 (a) TEM image, (b) Histogram of particle size distribution, (c) HRTEM images and (d) SAED image of N- Methylaniline passivated ZnSe:Mn 2+ Quantum dots Similarly, Obiwataobi S. Oluwafemi et al has successfully synthesized Hexadecylamine capped Mn doped wurtzite CdSe naonoparticles with an average size of about 4.75 nm [162]. R. Shankar et al., had synthesized Mn 2+ doped ZnS nanoparticles with the size of about 2 3 nm by chemical precipitation method [163]. JunanLiu et al., reported that Mn 2+ doped ZnSe quantum dots by nucleation doping method with the diameter of about 4 nm [164].

10 Functional properties Figure 4.5 EDAX spectrum of NMA passivated ZnSe:Mn 2+ Quantum dots 100 Transmittance (%) Wavenumber (cm -1 ) Figure 4.6 FTIR spectrum of NMA passivated ZnSe:Mn 2+ Quantum dots

11 142 Figure 4.5 describes the EDAX spectrum of Mn 2+ doped ZnSe nanoparticles. It indicates that the nanoparticles contain Zn, Se and Mn. The atomic percentage of Zn : Se : Mn is : : This matches the stoichiometric ratio quite well. It is also noted that no impurity in the nanoparticles was observed within the detection limit of EDAX. FTIR spectrum of Mn 2+ doped ZnSe is shown in Figure 4.6. KBr pellet technique was employed for the sample preparation. A KBr pellet is prepared by grinding the solid ample with solid potassium bromide and applying pressure to the dry mixture. KBr is chosen because it is transparent to the infrared radiation. This FTIR analysis is undertaken in order to establish the chemisorbed N-Methylaniline on the surface of nanoparticles. This analysis could provide the evidence that N-Methylaniline act as a ligand to control the size of the nanoparticles. In the FTIR spectrum the peak observed at 3403cm -1, 1579 cm -1 and 1025 cm -1 are attributed due to N-H and C-N stretching of N-Methylaniline. This clearly confirms that N-Methylaniline cover the surface of ZnSe: Mn 2+ nanoparticles. 4.3 Synthesis and Characterization of Magnesium doped Zinc Selenide quantum dots It is well known that the electronic structure of a given semiconductor is largely affected by doping effects and particle size. In the case of transition metal doping, the localized d levels would be introduced in the band gap, which can decrease the photo threshold energy of semiconductors but also serve as the recombination centers for photo induced charge carriers [165].

12 143 Alternatively, alkaline earth metal ions have no localized d levels, which can be taken as the candidate dopants for simplifying the correlation between structures and photo catalytic properties. Venkatachalem et al., [166, 167] found that doping of TiO 2 nanoparticles with Mg 2+ produces higher photo catalytic activities than those of undoped TiO 2 nanoparticles. Nevertheless, the entry of alkaline metal ions into the TiO 2 lattice also results in the creation of significant lattice defects because of the charge compensation and the ionic radius mismatch between Mg 2+ and Ti 4+ which may put huge uncertainties to the origin of photo activites. Mg 2+ has an ionic radius of 0.57Å, which is very close to that 0.60 Å for Zn 2+ in tetrahedral coordination [168]. It is noteworthy that the doping of Mg 2+ is not reported in ZnSe, rather many reports had been documented with ZnO. Since Mg 2+ is expected to be straight forward for generating nanocrystals with tunable optoelectronic properties which are promising for use in solution processable devices [169]. In the present work, Mg doped ZnSe nanoparticles were synthesized using N-Methylaniline as an organic ligand by chemical method and the functional characteristics have been investigated Synthesis All the chemicals were of analytical grade and used for the synthesis as such. In a typical synthesis, 0.2 M solution of zinc acetate

13 144 monohydrate [(CH 3 COO 2 ) Zn.H 2 O)] was dissolved in 20ml of distilled water under magnetic stirring. then, 0.02 M magnesium sulphate (MgSO 4 ) was added to the solution and subsequently 20 ml solution of 0.2 M Sodium selenite (Na 2 SeO 3.5H 2 O) was added. Finally, 0.2 ml NMA was added drop wise to the same solution as a capping agent. The mixture was stirred vigorously for 36 hours using magnetic stirrer. The role of NMA is to stabilize the particles against aggregation. The resultant product was dried at 180 C for 10 hours. Optical and Morphological properties of the synthesized product were studied by using UV-Vis spectrum, PL studies, XRD, TEM analysis, FTIR and EDAX Optical studies Optical absorption properties of Mg 2+ doped ZnSe quantum dots were measured at room temperature. Figure 4.7 (a) shows an intense band to band absorption in the UV region. In the Figure 4.7 (b) the absorption edge is observed at 350 nm and the bandgap value is found to be 3.56 ev. The observation of the blue shift in the band gap implies that Mg 2+ was successfully incorporated into cubic ZnSe lattices. Studies on Mg 2+ doped ZnO thin films with wurtzite structure indicate that the band gap of the film can be continuously tuned from 3.3 ev to 4.0 ev by adjusting Mg 2+ content [170].

14 145 Figure 4.7 (a) UV - Visible absorption spectrum of NMA passivated ZnSe:Mg 2+ Quantum dots Figure 4.7 (b) Bandgap plot of NMA passivated ZnSe:Mn 2+ Quantum dots

15 146 Figure 4.8 Photoluminescence spectrum of NMA passivated ZnSe:Mg 2+ Quantum dots Photoluminescence spectra of Mg 2+ doped ZnSe nanoparticles is shown in Figure 4.8. Photoluminescence emission peaks were obtained for the excitation wavelength of 255 nm. The near band edge emission peak was observed at 400 nm and the emission peak was broadened. The significant blue shift was observed in the nanoparticles due to the quantum confinement. The difference between the absorption onset and emission peak was around 50 nm. This is due to the deep trap state emission of the smaller size particles. Also the defects could be attributed to the vacancy of selenium in the nanoparticles. The trap level emission was observed at 645 nm and is due to impurity levels in ZnSe. According to the self-purification theory proposed

16 147 by Gustavo et al., [171] most of the impurity atoms are located on or near the surface of the semiconductor nanoparticles. The existence of surface states acts as trap centers for excitons. It is significant that impurities are actually embedded inside the ZnSe for the PL enhancement. Based on the above fact, the surface passivation of N-Methylaniline make an effect on the enhanced PL intensity of the Mg 2+ doped ZnSe. The above factors can be explained by the following steps: (i) the first added Zn 2+ ions were used for nucleation. Since the Mg 2+ ions were mostly located on or near the surface, the surface states probably trapped electrons and this lead to the non - radiative recombination, resulting in the reduction of quantum yields. (ii) For the passivation layer of epitaxial growth, additional Zn 2+ ions reacted with the N-Methylaniline in the reaction solution, which contributes to the enhanced PL intensity. Y.S.Wang et al., [172] synthesized Mg 2+ doped zinc oxide nanocrystals and investigated the optical properties.the photoluminescence spectra of Mg 2+ doped ZnO nanocrystals consists of band edge and defect states emission bands. The defect band is believed to be due to deep traps and becomes dominant at high levels of doping. PL excitation spectra, measured with detection at a wavelength, where the deep trap emission is maximum, reveal a linear increase in the peak energy with the increasing Mg 2+ doping levels upto 10%. The dependence of the optical band gap on Mg 2+ concentration derived from absorption and the PL spectra correspond well with each other. Tae Hyun Kim et al., [173] fabricated Mg 2+ doped ZnO thin

17 148 flims by laser ablation method and studied the PL spectra for different Mg 2+ concentrations. The peak of near band edge emission (NBE) shifted to blue with increase in the content of Mg 2+ while the intensity of defect emission decreased dramatically. On the other hand, the red emission which is due to the defects in the films increased substantially with Mg 2+ content. From the PL spectrum (Figure 4.8) a weak emission is observed at 645 nm. The concentration of the dopant Mg 2+ is less in ratio, as observed in the Table. 4.1, resulting low intensity defect level emission Structural studies Figure 4.9 XRD pattern of NMA passivated ZnSe:Mg 2+ Quantum dots Figure 4.9 shows the XRD pattern of Mg doped ZnSe nanoparticles. The peaks are readily indexed to the cubic phase of ZnSe. It is well matched with standard JCPDS card no Peaks related to magnesium are not

18 149 observed in the pattern indicating that magnesium atoms are not incorporated into the cubic lattice of ZnSe. Magnesium ions may be located at interstitial site or vacancies of zinc or selenium. No other peak related to impurities is observed Morphological Studies TEM images of Mg doped ZnSe nanoparticles shown in Figure The images clearly depict the formation of nanoparticles. The average size of the nanoparticles is 15 nm and some of the particles were in the shape of triangle and hexagonal. The formation of different shapes in the same sample is due to the influence of dopant ion and capping agent. The lattice fringes are clearly seen in the HRTEM image and it clearly depicts the formation of single crystalline Mg 2+ doped ZnSe nanoparticles. SAED pattern is shown in the Figure Spot patterns were indexed to cubic structure of the ZnSe. Figure 4.10 TEM images of NMA passivated ZnSe:Mg 2+ Quantum dots

19 150 Figure 4.11 HRTEM and SAED pattern of NMA passivated ZnSe:Mg 2+ Quantum dots. Y. S. Wang et al., [172] of Mg 2+ doped ZnO nanocrystals and TEM analysis reveals that the synthesized particles were of sphereical in nature with size in the range of 9 12 nm. YeFeng Yang et al., [169] synthesized Mg 2+ doped ZnO nanocrystals. 5% of Mg 2+ dopants on ZnO results a tetrapod structures. The tetra pods were 3.6 nm in diameter Functional characteristics and elemental analysis FTIR spectrum of Mg 2+ doped ZnSe nanoparticles as shown in Figure Functional group vibrations confirm the formation of N-Methylaniline passivated Mg 2+ doped ZnSe nanoparticles. The vibrations at 3428 cm -1 and 1655 cm -1 were corresponds to the N-H stretching of chemisorbed N-Methylaniline molecule. In addition, the vibration at 1032 cm -1 corresponds to the C-N stretching, it clearly evident the surface passivation of N-Methylaniline during the formation of ZnSe nanocrystals.

20 Transmittance (%) Wavenumber ( cm -1 ) Figure 4.12 FTIR spectrum of NMA passivated ZnSe:Mg 2+ Quantum dots. The EDAX spectrum of Mg doped ZnSe nanoparticles shown in Figure Strong peaks were observed for the energy levels of Zn, Se and Mg. It is apparently evident the incorporation of Mg into the ZnSe. Moreover, the atomic percentage of Mg is less than 2% indicating that the Mg ion is not sufficient to replace Zn ion and it may be located as an impurity in the lattice of ZnSe. No other signals were detected in EDAX spectrum. Figure 4.13 EDAX spectrum of NMA passivated ZnSe:Mg 2+ Quantum dots.

21 152 Table 4.1 Elemental analysis obtained by EDAX Element Line Weight % Atom % Formula Mg K Mg Zn K Zn Se K Se Total Conclusions In summary, transition metal Mn 2+ and alkaline rare earth metal Mg 2+ doped ZnSe quantum dots have been synthesized by wet chemical route. N-Methylaniline was used as a surface passivating ligand to obtain size confined ZnSe quantum dots. The synthesized metal ions doped ZnSe quantum dots have been characterized by UV Visible absorption, photoluminescence, XRD, TEM, FTIR and EDAX analyses. Strong quantum confinement effect have been observed in the both samples of Mn 2+ and Mg 2+ doped ZnSe quantum dots with the band gap of 4.2 ev and 3.56 ev, respectively. Structural studies show cubic ZnSe formation. Surface morphology of nanoparticle is clearly seen in the TEM studies. Presence of N-Methylaniline in the synthesized ZnSe is clearly evidenced from the functional peaks of FTIR spectrum. The existence of metal ion with their atomic percentage is observed from EDAX analysis.