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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:Mn2+

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

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shortening of the Mn2+ emission with decreasing size. The first synthesis of

ZnS: Mn2+ 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:Mn2+ were synthesized in block copolymer

nano reactors and zeolites and with different amounts of Mn2+ 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 Mn2+ 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

Mn2+ 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

[148 – 151]. Since, it had provided fertile grounds for the investigation of the

basic chemistries of homogeneous nucleation and crystal growth in the

presence of impurities.

134

In this chapter, ZnSe quantum dots have been synthesized by

doping transition metal manganese (Mn2+) and the alkaline rare earth metal

magnesium (Mg2+). Since, Mn2+ and Mg2+ 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 Mn2+ doping in II – VI

semiconductors in order to produce new materials for various applications

ranging from solar cell to spintronics [154, 155]. Mn2+ 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 Mn2+, 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 Mn2+ ions

are intensively studied due to its unique optical and magneto – optical

properties [156 – 158]. ZnSe: Mn2+ quantum dots may be used as nontoxic

fluorescent markers in biomedicine or sensors.

135

4.2.1 Synthesis

All the reagents were of analytical grade and used without further

purification. 0.2 M solution of zinc acetate monohydrate

[(CH3COO2)Zn·H2O)] was prepared in 20 ml of distilled water and 0.02 M

manganese sulphate (MnSO4) was added to the solution. After that, 20 ml of

solution of 0.2 M sodium selenite (Na2SeO3·5H2O) 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.

4.2.2 Optical studies

UV- visible absorption spectrum of N - Methylaniline passivated

ZnSe: Mn2+ 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 Mn2+ doped ZnSe are derived based

on the well-established equation [eq.(1)].

1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.00.00.51.01.52.02.53.03.54.04.55.05.56.0

(αhν

)2

hν

(4.2eV)

(a) (b)

Figure 4.1 (a) UV - Visible absorption spectrum and (b) bandgap plot of NMA passivated ZnSe:Mn2+ Quantum dots

136

( )ν

να

hEhA g 2

1−

= (1)

where α , Eg and A are the absorption coefficient, band gap and constant

respectively. By extrapolating the linear region in the plots of versus 2)( ναh

νh (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)].

2 2 2

2

1 1 1.82g

e h

eE E smallertermsR m m R

πε

⎡ ⎤= + + − +⎢ ⎥

⎣ ⎦

h (2)

where E is the energy of the first excited state, Eg = 2.7 eV is the band gap

energy of bulk ZnSe, me* and mh* are the effective masses of the electron

and hole in ZnSe (me = 0.15mo, mh = 0.66mo, where mo = 9.11x10-28gm, the

free electron mass)[152] respectively, є = 9.2, is the semiconductor dielectric

constant, ћ = 6.58 x 10-16 eV is the reduced Plank constant and e = 1.6 x

10-19 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:

Mn2+ 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

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 Mn2+ 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 Mn2+ ion incorporated to ZnSe

matrix [147]. In general, when Mn2+ 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 (4T1 and 6A1) that are within the energy gap of the nanocrystals.

In the present work, the emission band centered at 609 nm is

attributed to (4T1 and 6A1) transition within the 3d shell of Mn2+. When

Mn2+ 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 Mn2+ occurred and made the forbidden transition of 4T1 and 6A1

partially allowed, resulting in the characteristic emission of Mn2+.

138

250 300 350 400 450 500 550 600 650 7000

1x105

2x105

3x105

4x105

5x105

6x105

7x105

8x105

Inte

nsity

Wavelength (nm)

376 400

422 505

609

Figure 4.2 Photoluminescence spectrum of NMA passivated

ZnSe:Mn2+ Quantum dots

4.2.3 Structural analysis

20 30 40 50 600

100

200

300

400

500

600

Inte

nsity

(cps

)

2θ (degree)

(311

)

(400

)

(331

)

(511

)(440

)

(531

)

(220

)

Figure 4.3 XRD pattern of NMA passivated ZnSe:Mn2+ Quantum dots

139

XRD pattern of N-Methylaniline passivated ZnSe:Mn2+ 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: 010690). No other peaks

related to impurities were detected.

4.2.4 Morphological study

Figure 4.4 (a) shows the TEM micrograph of N-Methylaniline

passivated ZnSe: Mn2+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: Mn2+ Quantum dots. These pattern spots can be

readily indexed as the (311), (400), (331) , (440) , (511) , (531) planes for the

cubic structure of ZnSe.

140

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.50

5

10

15

20

25

30

Num

ber o

f par

ticle

s

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:Mn2+ 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 Mn2+ doped ZnS nanoparticles with the size of about 2 – 3 nm by

chemical precipitation method [163]. JunanLiu et al., reported that Mn2+

doped ZnSe quantum dots by nucleation doping method with the diameter of

about 4 nm [164].

141

4.2.5 Functional properties

Figure 4.5 EDAX spectrum of NMA passivated ZnSe:Mn2+ Quantum

dots

4000 3500 3000 2500 2000 1500 1000 500

0

20

40

60

80

100

Tran

smit

tan

ce (

%)

Wavenumber (cm-1)

1579

1025

3403

Figure 4.6 FTIR spectrum of NMA passivated ZnSe:Mn2+ Quantum dots

142

Figure 4.5 describes the EDAX spectrum of Mn2+ doped ZnSe

nanoparticles. It indicates that the nanoparticles contain Zn, Se and Mn. The

atomic percentage of Zn : Se : Mn is 55.19 : 43.12 : 1.69. 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 Mn2+ 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-1and 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: Mn2+ 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].

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 TiO2

nanoparticles with Mg2+ produces higher photo catalytic activities than those

of undoped TiO2 nanoparticles. Nevertheless, the entry of alkaline metal ions

into the TiO2 lattice also results in the creation of significant lattice defects

because of the charge compensation and the ionic radius mismatch between

Mg2+ and Ti4+ which may put huge uncertainties to the origin of photo

activites. Mg2+ has an ionic radius of 0.57Å, which is very close to that 0.60 Å

for Zn2+ in tetrahedral coordination [168]. It is noteworthy that the doping of

Mg2+ is not reported in ZnSe, rather many reports had been documented with

ZnO. Since Mg2+ 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.

4.3.1 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

144

monohydrate [(CH3COO2) Zn.H2O)] was dissolved in 20ml of distilled water

under magnetic stirring. then, 0.02 M magnesium sulphate (MgSO4) was

added to the solution and subsequently 20 ml solution of 0.2 M Sodium

selenite (Na2SeO3.5H2O) 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.

4.3.2 Optical studies

Optical absorption properties of Mg2+ 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 Mg2+ was

successfully incorporated into cubic ZnSe lattices. Studies on Mg2+ 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 Mg2+ content

[170].

145

Figure 4.7 (a) UV - Visible absorption spectrum of NMA passivated

ZnSe:Mg2+ Quantum dots

Figure 4.7 (b) Bandgap plot of NMA passivated ZnSe:Mn2+

Quantum dots

146

Figure 4.8 Photoluminescence spectrum of NMA passivated

ZnSe:Mg2+ Quantum dots

Photoluminescence spectra of Mg2+ 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

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 Mg2+ doped ZnSe. The above factors can be explained by the

following steps: (i) the first added Zn2+ ions were used for nucleation. Since

the Mg2+ 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 Zn2+ ions reacted with the N-Methylaniline in the

reaction solution, which contributes to the enhanced PL intensity.

Y.S.Wang et al., [172] synthesized Mg2+ doped zinc oxide

nanocrystals and investigated the optical properties.The photoluminescence

spectra of Mg2+ 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 Mg2+ doping

levels upto 10%. The dependence of the optical band gap on Mg2+

concentration derived from absorption and the PL spectra correspond well

with each other. Tae Hyun Kim et al., [173] fabricated Mg2+ doped ZnO thin

148

flims by laser ablation method and studied the PL spectra for different Mg2+

concentrations. The peak of near band edge emission (NBE) shifted to blue

with increase in the content of Mg2+ 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 Mg2+ content.

From the PL spectrum (Figure 4.8) a weak emission is observed at

645 nm. The concentration of the dopant Mg2+ is less in ratio, as observed in

the Table. 4.1, resulting low intensity defect level emission.

4.3.3 Structural studies

Figure 4.9 XRD pattern of NMA passivated ZnSe:Mg2+ 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 88-2345. Peaks related to magnesium are not

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.

4.3.4 Morphological Studies

TEM images of Mg doped ZnSe nanoparticles shown in Figure

4.10. 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 Mg2+ doped ZnSe nanoparticles. SAED pattern

is shown in the Figure 4.11. Spot patterns were indexed to cubic structure of

the ZnSe.

Figure 4.10 TEM images of NMA passivated ZnSe:Mg2+ Quantum dots

150

Figure 4.11 HRTEM and SAED pattern of NMA passivated

ZnSe:Mg2+ Quantum dots.

Y. S. Wang et al., [172] of Mg2+ 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

Mg2+ doped ZnO nanocrystals. 5% of Mg2+ dopants on ZnO results a tetrapod

structures. The tetra pods were 3.6 nm in diameter.

4.3.5 Functional characteristics and elemental analysis

FTIR spectrum of Mg2+ doped ZnSe nanoparticles as shown in

Figure 4.12. Functional group vibrations confirm the formation of

N-Methylaniline passivated Mg2+ 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.

151

4 0 0 0 3 5 0 0 3 0 0 0 2 5 0 0 2 0 0 0 1 5 0 0 1 0 0 0 5 0 00

2 0

4 0

6 0

8 0

1 0 0

1655

Tran

smitt

ance

(%)

W a v e n u m b e r ( c m -1 )

3428

1032

Figure 4.12 FTIR spectrum of NMA passivated ZnSe:Mg2+ Quantum

dots.

The EDAX spectrum of Mg doped ZnSe nanoparticles shown in

Figure 4.13. 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:Mg2+ Quantum

dots.

152

Table 4.1 – Elemental analysis obtained by EDAX

Element Line Weight % Atom % Formula

Mg K 1.18 1.56 Mg

Zn K 37.99 42.33 Zn

Se K 60.83 56.11 Se

Total 100.00 100.00

4.4 Conclusions

In summary, transition metal Mn2+ and alkaline rare earth metal

Mg2+ 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 Mn2+ and Mg2+

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.