Synthesis and Luminescence Properties of Core/Shell ZnS:Mn/ZnO Nanoparticles
Daixun Jiang
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Lixin Cao
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Wei Liu
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Ge Su
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Hua Qu
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Yuanguang Sun
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Bohua Dong
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D. Jiang L. Cao (&) W. Liu G. Su H. Qu Y. Sun B. Dong Institute of Materials Science and Engineering, Ocean University of China
, Qingdao 266100,
People's Republic of China
In this paper the influence of ZnO shell thickness on the luminescence properties of Mn-doped ZnS nanoparticles is studied. Transmission electron microscopy (TEM) images showed that the average diameter of ZnS:Mn nanoparticles is around 14 nm. The formation of ZnO shells on the surface of ZnS:Mn nanoparticles was confirmed by X-ray diffraction (XRD) patterns, high-resolution TEM (HRTEM) images, and X-ray photoelectron spectroscopy (XPS) measurements. A strong increase followed by a gradual decline was observed in the room temperature photoluminescence (PL) spectra with the thickening of the ZnO shell. The photoluminescence excitation (PLE) spectra exhibited a blue shift in ZnO-coated ZnS:Mn nanoparticles compared with the uncoated ones. It is shown that the PL enhancement and the blue shift of optimum excitation wavelength are led by the ZnO-induced surface passivation and compressive stress on the ZnS:Mn cores.
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In recent years, a great deal of attention has been devoted
to doped semiconductor nanomaterials mainly due to their
unique luminescence properties arising from quantum
confinement effects. The doping ions act as recombination
centers for the excited electronhole pairs and play an
influential role in determining optical properties, resulting
in strong luminescence. Among the semiconductor
nanomaterials, zinc sulfide is particularly suitable for use as
luminescent host materials for a variety of dopants because
of its wide band gap energy at room temperature. There
have been numerous reports about the structural and
luminescence properties of doped ZnS nanocrystals, such
as ZnS nanocrystals doped with manganese [14], copper
[57], silver [8], samarium [9], europium [10, 11], and
terbium [12]. It has been extensively studied for a variety
of commercial devices, such as electro-optic modulators,
photoconductors, optical coatings, field effect transistors,
infrared windows, electroluminescent display [1317].
Unfortunately, serious drawbacks exist in these
nanomaterials, such as their instability under high temperature
treatment [18], degradation during the operation and
dispersal into vacuum, which will contaminate the field
emitter and thus hinder the electron emission [19], and high
surface-to-volume ratio because of small particle size,
which will result in low luminescence efficiency. In order
to conquer such serious shortcomings, accordingly, core/
shell structural nanomaterials have been developed and
have shown dramatically enhanced properties. Enhanced
luminescence and/or stability has been observed in
ZnS:Mn/SiO2 nanoparticles [20], ZnS:Mn/ZnS
nanocrystals [21], ZnS:Mn/Zn(OH)2 nanoparticles [22], ZnS:Mn/
ZnO nanocrystals [23], and CdSe/ZnO nanoparticles [24].
In addition, the ZnO particles on the ZnS:Ag particulates
have demonstrated a high effectiveness to prevent the
degradation of ZnS:Ag particulates from electron
bombardment [25]. Compared with the uncoated nanocrystals,
the photoluminescence of the nanocrystals with a core/shell
structure is enhanced. This is usually interpreted as being
due to the surface passivation that inhibits the nonradiative
recombination, thus improving the photoluminescence
properties. At the same time, a higher stability will be
obtained, coming from the protection effect of the
surrounding matrix [26].
ZnO-coated ZnS:Mn nanoparticles have been prepared
by different groups [23, 27]; however, the ZnO shells
reported in these literatures are of rather poor crystallinity,
which is indicated by their X-ray diffraction (XRD) results.
Usually, the ZnO shell with higher crystallinity will
produce stress on the ZnS:Mn because of the lattice mismatch,
and bring efficient luminescent transitions inducing
enhanced luminescence [28]. In this paper, a convenient
route to prepare higher crystallinity core/shell structural
ZnS:Mn/ZnO nanoparticles is reported, with the emphasis
on the improved luminescence properties brought by the
ZnO capping on the ZnS:Mn nanoparticles. The shell
material, ZnO, is a wide band gap semiconductor with
excellent chemical and thermal stability. Therefore, surface
modification of ZnS:Mn nanoparticles by ZnO shell is
expected to have a passivating effect on the surface states
of ZnS:Mn, which would result in enhanced luminescence.
All the reactants and solvents used in this work were of
analytical grade and used without any further purification.
Synthesis of ZnS:Mn Cores
Typically, 0.01 mol zinc acetate [Zn(CH3COO)2 2H2O],
0.005 mol thioacetamide [CH3CSNH2], and 5 9 10-5 mol
manganese acetate [Mn(CH3COO)2 4H2O] were put into
a Teflon-lined stainless steel autoclave of 72 mL capacity,
and then the autoclave was filled with a mixture solvent of
ethylenediamine and deionized water (in 1:1 volu (...truncated)