FLUORESCENT MANGANESE-DOPED ZINC SULPHIDE NANOPARTICLES FOR
SPECTRAL SHIFTING
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Suranjan Sen , Pratibha Sharma , Chetan Singh Solanki , Rajdip Bandyopadhyaya
Department of Energy Science and Engineering, IIT Bombay, Powai, Mumbai- 400076, India
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Department of Chemical Engineering, IIT Bombay, Powai, Mumbai- 400076, India
ABSTRACT
Spectral shifting by luminescent nanoparticles is expected
to enhance solar cell performance by avoiding wastage of
high energy photons. Manganese-doped zinc sulphide
nanoparticles, with a PL emission peak around 600 nm,
were selected as a possible candidate for such a
luminescent down-shifting material. To investigate their
fluorescence properties, ZnS:Mn nanoparticles were
synthesized by a chemical route using thiourea to limit the
reaction rate and achieve controlled, uniform formation of
nanoparticles.
Incipient ZnS:Mn nanoparticles were
encapsulated in ZnS shells by employing the in-situ
thermal decomposition of pre-formed zinc-thiourea
complex. The resulting core-shell nanoparticles were
dispersed in a protective matrix composed of sodium
hexametaphosphate. TEM studies showed the formation
of spherical and distinct ZnS:Mn nanoparticles in the size
range 50-100 nm, while UV-Vis spectra clearly showed
quantum confinement effects.
INTRODUCTION
Present-day solar photovoltaic technology is based largely
on silicon solar cells, comprising roughly ~90% of the
global PV industry [1]. Despite the numerous advantages
of PV over other renewable energy options, cost remains
a major stumbling block- the power generation cost for PV
is approximately five times the cost associated with
conventional technologies [2].
In order to make PV accessible to a greater number of
people, it is important that the cost per watt generated be
reduced dramatically. One way of doing this is to ensure
better spectral utilisation, that is, absorbing as many
photons as possible from incident sunlight. The incident
solar spectrum at AM1.5 [3] shows significant intensities
beyond ~300 nm due to UV filtering by ozone layer,
reaching a maximum at ~530 nm. In a typical solar cell,
many of the incident photons end up being wasted through
thermalisation [4] when their energy exceeds the band gap
of silicon. The spectral response of a solar cell indicates
how well it performs under different incident wavelengths
of light, and is not uniform across the range of incident
wavelengths, with a maximum at ~900-1000 nm [5].
The problem of poor spectral response at high photon
energies can be overcome through the use of luminescent
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down-shifting [4]. The luminescent material coupled to a
solar cell may be composed of organic dyes or
nanoparticles, although nanoparticles have taken
precedence in recent years due to certain advantages.
These include the ability to control the range of absorption
and emission through adjustment of synthesis conditions,
as well as higher photo-stability after suitable surface
modification [6-9].
A survey of fluorescent nanoparticles reveals a variety of
materials, mostly II-VI semiconductors, each with different
synthesis routes- involving low and high temperature
conditions- and precursors- organometallic or ionic in
nature [10-21].
Of the nanoparticles surveyed,
manganese-doped zinc sulphide, or ZnS:Mn, was selected
over CdSe and CdS, PbS, ZnS and ZnSe. Unlike the
cadmium and lead-based nanoparticles, ZnS:Mn contains
relatively non-toxic zinc and manganese. It absorbs high
energy photons at wavelengths below ~340 nm and shows
emission close to 600 nm [19], corresponding to a
significant expected increase in solar cell spectral
response through fluorescent wavelength shifting.
The mechanism of fluorescence (see Fig. 1) in ZnS:Mn
involves a coupling of the ZnS host lattice energy levels
2+
with the Mn dopant levels. Excitation from the valence
band to the conduction band of ZnS [22] is followed by a
non-radiative energy transfer to defect levels (route 1 in
Fig. 1). Subsequently, blue emission occurs at ~420 nm
[23] and ~490 nm [24,25] due to relaxation from sulphur
and zinc vacancy levels respectively. An alternative route
in the presence of manganese involves non-radiative
energy transfer to the dopant levels (route 2 in Fig. 1),
subsequently producing orange emission at ~600 nm.
Figure 1 Excitation and relaxation routes for ZnS:Mnsolid lines show radiative excitation/relaxation while
dotted lines show non-radiative relaxation [26]
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EXPERIMENTAL DETAILS
ZnS:Mn nanoparticles were synthesized in a water-ethanol
solvent mixture using 0.01 moles Zn(CH3COO)2.2H2O,
0.06 moles CS(NH2)2 (final ratio of zinc acetate:
thiourea = 1:6), and 0.001 moles Mn(CH3COO)2.4H2O
(ratio of
Mn: Zn= 1: 10) as precursors (all from Merck
India). Initial hydrolysis of 0.03 moles thiourea was carried
out under mild heating in the presence of ammonia
solution to provide a pH~11 [27-28]. This hydrolysed
thiourea, containing sulphide ions, was added into an
2+
2+
ethanolic Zn -Mn
solution to form bare ZnS:Mn
nanoparticles. These incipient nanoparticles were added
2+
into a Zn -thiourea complex mixture [29] to achieve
encapsulation in a ZnS shell by in-situ thermal
decomposition of the complex.
This strategy for developing a shell around ZnS:Mn cores
was not found in surveyed literature- the conventional
technique for shell formation involves slow dropwise
addition of excess zinc and sulphur precursors into the
reaction medium in the presence of ZnS:Mn nanoparticles
[30]. The in-situ complex decomposition technique has
the advantage of avoiding regions of high reactant
concentration in the medium since complex decomposition
yields ZnS molecules more uniformly than dropwise
reactant addition.
It should be noted above that an excess of thiourea is
required relative to zinc acetate because thiourea is a slow
reactant and synthesis with equimolar ratios was
unsuccessful. Smaller amounts of the precursors than
mentioned above lead to formation of zinc oxide rather
than zinc sulphide in the presence of ammonia. Mild
heating and alkaline conditions are crucial to achieve
appreciable rates of thiourea hydrolysis, without which
product formation is not possible.
RESULTS
Optical characterization
UV-Visible spectroscopy (see Fig. 2) performed over the
absorption range 200-400 nm showed that the absorption
threshold of the nanoparticles shifted to lower wavelengths
with capping.
Figure 2 UV-Visible absorption spectra of uncapped
(curve 1) and capped (curve 2) nanoparticle samples
Band gap estimation of both samples was performed on
the basis of the following equation, where A is
absorbance, K is a constant, λ is the wavelength and λs is
the absorption threshold [33]:
2
1 1 
 A
  = K  − 
λ
 λ λs 
(1)
A portion of the core-shell ZnS:Mn nanoparticles prepared
by the above procedure were added drop wise into a
0.02M aqueous solution of sodium hexametaphosphate
(S.D. Fine Chemicals India), a capping agent, which
polymerized in-situ to form –P-O-P– linkages [31]. The
capping agent protects the nanoparticles from oxidation
and avoids agglomeration.
SHMP was chosen in
preference to other commonly used capping agents due to
its low cost and the reported enhancement in
photoluminescence intensity and quantum efficiency of
ZnS:Mn nanoparticles modified at the surface by
phosphate groups [32].
Fluorescence spectrometry (Perkin Elmer LS55) and UVVisible absorption spectroscopy (Perkin Elmer Lambda
35) were performed on the liquid phase suspensions of
both capped and uncapped nanoparticles. Transmission
electron microscopy of capped and uncapped samples
(PHILIPS CM200 TEM) was performed by casting a drop
of each sample onto a copper grid and letting it dry.
Figure 3 Determination of absorption threshold for
uncapped (curve 1) and capped (curve 2) samples
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From the plot of (A/λ) versus 1/λ, the threshold
wavelength was found by constructing a tangent to the
curve and locating its intersection with the 1/λ axis (see
Fig. 3). The absorption threshold was observed to lie at
303 nm for the uncapped sample, and at 265 nm for the
capped sample. These values show that a widening of
band gap from 4.10 eV to 4.69 eV occurred due to
capping (bulk ZnS has a band gap of 3.7 eV [18]). An
enhanced quantum confinement effect operates in the
presence of the capping agent. Thus the SHMP polymer
was effective at limiting particle growth in the reaction
medium.
Fluorescence spectroscopy (see Fig. 4) revealed the
occurrence of defects in the ZnS host lattice, which gave
rise to prominent emission peaks at 420 and 490 nm,
under excitation wavelengths of 250 nm and 350 nm
respectively. The lower energy emission corresponding to
manganese dopant level transition could only be observed
at the higher energy excitation of 250 nm and was
centered at ~650 nm.
Figure 5 TEM image of uncapped nanoparticles,
size~50-100 nm (scale bar 500 nm)
Figure 6 TEM image of capped sample, showing
network formation by SHMP (scale bar 500 nm)
DISCUSSION
Figure 4 Fluorescence spectra at excitation of 250 nm
(curve 1) and 350 nm (curve 2)
Electron microscopy
TEM images of uncapped samples showed a fairly uniform
distribution of well-formed, spherical nanoparticles (see
Fig. 5), size 50-100 nm, with no significant signs of
agglomeration, giving indirect evidence of formation of the
ZnS shell around the ZnS:Mn nanoparticles. When
capped with SHMP (see Fig. 6), the sample showed only
a cross-linked polymeric structure on TEM, the matrix
being too dense to allow penetration of the incident
electron beam. Thus nanoparticles were not directly
visible within the matrix, although it was inferred that they
were embedded within it since the same sample without
any capping agent clearly showed the presence of
nanoparticles (see Fig. 5). The use of a more dilute
capping agent solution would probably allow embedded
nanoparticles to be observed.
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The thiourea-derived ZnS:Mn nanoparticles appeared
bluish-purple under UV excitation, indicating that
manganese doping was incomplete. Dopant incorporation
into the host lattice seems to be slow, possibly limited by
the slow thiourea reactant- although the exact reason for
this is unclear at present. A faster reacting sulphur
precursor, such as sodium sulphide, may favour dopant
incorporation. Nonetheless, distinct and well-formed
nanoparticles were formed, as confirmed by TEM.
No real benefit of using the SHMP polymer was observed
in terms of fluorescence intensity when the spectral
intensities of capped and uncapped samples were
compared. TEM images of the capped sample showed a
dense, network-like structure (see Fig. 6). It is possible
that the dense structure resulted in attenuation of the
emitted light
from the nanoparticles, nullifying any
luminesence enhancement due to SHMP. Therefore, the
use of a lower polymer concentration is advisable, to
ensure that the polymer only forms a protective cap
around each nanoparticle rather than a network.
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ACKNOWLEDGEMENT
Acknowledgements to SAIF (IITB) for TEM, to Colloids
and
Nanomaterials
Lab
(IITB)
for
UV-Visible
characterization, and to Optical Spectroscopy Lab (IITB)
for fluorescence characterization, with thanks to all
instrument operators.
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nanoparticle films prepared by chemical deposition and
sol–gel methods”, Chemical Physics Letters, 288, 1998,
pp. 188–196
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