DSpace at VNU: Emission characteristics of SPAN-80 activated ZnS nanocolloids

Journal of Luminescence 132 (2012) 2135–2142

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Journal of Luminescence
journal homepage: www.elsevier.com/locate/jlumin

Emission characteristics of SPAN-80 activated ZnS nanocolloids
Thu Huong Ngo a, Hong Van Bui a, Van Ben Pham a, Thi Hong Tran a, An Bang Ngac a, Nam Nhat Hoang b,n
a
b

Faculty of Physics, HUS, Vietnam National University, Hanoi, 334 Nguyen Trai, Thanh Xuan, Ha Noi, Viet Nam
Faculty of Technical Physics and Nanotechnology, UET, Vietnam National University, Hanoi, 144 Xuan Thuy, Cau Giay, Ha Noi, Viet Nam

a r t i c l e i n f o

abstract

Article history:
Received 9 August 2011
Received in revised form
5 March 2012
Accepted 26 March 2012
Available online 4 April 2012

Quantum surface effects (new emission bands, blueshifts, intensity enhancement) were observed in
SPAN-80 activated ZnS nanocolloids and explained in terms of time-dependent density functional
theory. The experimental evidences were demonstrated for both undoped and Cu, Mn-doped colloidal
phases. The photoluminescence spectra of these materials showed a new green band at 520 nm
(ZnS:Cu) and a yellow-orange band at 576 nm (ZnS:Mn) besides a blue band at 465 nm. All bands lie in

the visible region and are blueshifted, show sharp emissions with narrow widths and have
approximately 20-times stronger intensities in comparison with those of the bulk samples. The timeresolved luminescence spectra showed that the life-times of free electrons were 0.12 ms and 1.9 ms in
ZnS:Cu and ZnS:Mn correspondingly.
& 2012 Elsevier B.V. All rights reserved.

Keywords:
Photoluminescence
ZnS
TD-DFT
Nanocolloid

1. Introduction
For the importance of application in nanomedicine, the optical
nanocolloids continuously attract the attention of scientists
worldwide. The quantum effects arising from the surface modification of nanoparticles by surface active agents (surfactants)
play important roles in introducing new physics and observables
of colloidal nanostructures. Although ZnS as the known wide
band gap semiconductor (Eg E3.7 eV) (which found a variety of
application in color displays, diodes, cathode ray tubes, transparent windows etc. [1]) has been extensively studied, there was
almost no evidence for the colloidal ZnS directly activated by
SPAN-80 (alias sorbitan monooleate). Here we report the observation and the DFT-based explanation of new emission bands,
their blueshifts and intensity enhancement in the undoped and
Cu, Mn-doped ZnS nanocolloids synthesized by using the traditional solid-state reaction method and activated by SPAN-80 as
colloidal agent (SPAN-80 nanocolloids). For the first time we
introduced SPAN-80 as a novel capping agent that can be used
to stimulate about 20-times stronger photoemission with remarkable narrower full-width.
Usually, the enhancement of ZnS photoluminescent (PL) ability
might be achieved by using a capping agent [2] and by doping
suitable elements, particularly Cu and Mn [3,4]. Recently, the
focus was paid on the development of new colloidal systems

which contain light-emitting particles in strong quantum confinement regimes [5,6]. The water-based optical colloids promise

n

Corresponding author.
E-mail address: (N.N. Hoang).

0022-2313/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
/>
yet rich application in nanomedicine [7]. A wide usability of
colloidal systems (e.g. ZnS:Mn) may also be found in modern
inkjet pigments [8] or in light-emitting diodes (LEDs) which
showed a strong photoluminescence [9]. In visible region, the PL
spectra of pure ZnS consist of two wide bands at around 450 and
540 nm which correspond to Zn and S vacancies particularly.
When doped with Cu and Mn, these self-activated characteristic
bands disappeared, or diminished, and the new luminescent
bands appeared at around 520 and 580 nm. These two bands
are the typical imprints of Cu2 þ and Mn2 þ luminescent centers in
ZnS matrix [10–17]. In general, the photoluminescence of different ZnS colloidal systems (doped or undoped) may be quite
different because of number of effects which can influence the
photoemission process, e.g. the surface effects arising from
the interaction between nanoparticles and colloidal agents. The
available literature pointed to two important aspects that might
occur in colloidal systems (in contrast to the bulk samples): (i) the
possible blue shift of Mn2 þ , redshift of Zn2 þ , S2 À luminescence
and blueshift of absorption edge (which associates with the
widening of band gap) [14,16,18–22]; (ii) the enhancement of
PL intensity [11–13,20,23–27]. In some cases, the redshift of
absorption edge was observed instead, e.g. in Ref. [10] the ZnS:Mn

nanocolloids capped with polyphosphates of sodium showed the
redshift of absorption edge from 250 to 285 nm (the emission
lines were observed at 424 (Zn) and 592 nm (Mn)). In Ref. [11] the
polyacrylic acid coated nanocolloids showed the enhanced PL but
the possible shifts were not clearly resolved. In many cases, only
one effect was observed (e.g. Ref. [13]: enhanced PL in trioctylphosphine oxide capped ZnS:Mn; Ref. [21]: a large blueshift and
extension of band gap up to 4.6 eV in ZnS:Mn capped with
sodium hexametaphosphate) but there were also cases where


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T.H. Ngo et al. / Journal of Luminescence 132 (2012) 2135–2142

both PL shift and intensity enhancement were reported (Refs.
[13,20] and [16]: Zn and Mn emissions shifted to 438 and 578 nm
with tunable intensities in mercaptoacetic acid activated
ZnS:Mn). In addition, several colloidal systems may show the
shift of spectral lines due to doping content, such as the 3-mercaptopropionic acid (MPA) capped ZnS:Cu nanocolloids demonstrated the redshift of blue band from 440 to 487 nm according to
the increase of Cu content [18]. The ZnS:Cu,Cl/ZnS core-shell
nanocrystals [24] showed both enhancement of PL intensity and
redshifts (roughly 0.2 eV) of the blue bands centered around
2.5 eV (496 nm). This work also showed that the equal redshift,
for both undoped and Cu-doped samples, might be caused by the
increased thickness of the shell monolayer. Directly or indirectly,
the known experimental studies pointed out the importance of
surface modification on emission characteristics. This modification may be achieved by various means which include UV irradiation (blueshift of Mn2 þ luminescence at 2.15 eV, i.e. 577 nm, with
PL efficiency enhanced by 35% due to surface passivation [20]) or
g-irradiation (mechanoluminescence appeared with high intensity [23]) or core/shell modification (21 times enhanced PL in
ZnCdS:Mn/ZnS core–shell [26]) or activation by surfactants (color

tuning of emission bands in the water-based colloids ZnS:Cu [27]
and ZnS:Mn [16] activated by mercaptoacetic acid). At microscopic level, the effect of surface modification on emission
characteristics is still less understood as there were only limited
number of theoretical studies available [28,29]. For the surfactant-activated surfaces the computational difficulty arises from
large number of surfactant molecules need to be included to
simulate the optical change. In this paper, we report the spectral
characteristics of Cu and Mn-doped ZnS nanocrystallites emulsified by using the surfactant SPAN-80 and discuss the observed
spectral changes in terms of Time-Dependent (TD) Density Functional Theory (DFT). We assert that the surface activation by
SPAN-80 can lead to both blueshift and enhancement of PL
intensity. Mechanically, the blueshift and widening of band gap
may associate with growing Coulomb repulsion between filled
and unfilled states and the enhancement of emission with denser
valence band density of states. We show that both these effects
happen when the surfactant molecules SPAN-80 are attached to
the surface of ZnS nanoclusters. The SPAN-80 and SPAN-20
(sorbitan laurate) have been used as the surfactants to produce
the triethylamine (TEA) capped ZnS nanoparticles [2] where
SPAN-20 related samples showed the higher luminescence property, but both were not involved as direct capping agents.

The bright transparent liquid appeared at the top of the containing cuvette and was extracted. The 5% SPAN-80 (weight basis)
acetone solution was dropped in slowly so that the total surfactant concentration reached about 1%. The colloidal thin films were
prepared by spin-coating on silica substrate. The liquids themselves were used for optical characterization. The PL spectra were
recorded at 300 K using excitation wavelengths of 325 nm from
He–Cd laser and 632.8 nm from He–Ne laser on the Microspec
2300i spectrometer. The time-resolved PL spectra were recorded
at 300 K using N2 excitation laser (wavelength 337 nm).

3. Results and discussion
Fig. 1 shows the XRD patterns (together with the SEM images
in the inset) of the bulk ZnS:Mn (a), ZnS:Cu (b) and the TEM image

of undoped colloidal ZnS sample ((c), inset). These samples
possess hexagonal wurtzite structure with main diffraction planes
(010), (002), (011), (012), (110), (013), and (112). The diffraction
patterns did not change in whole doping range (for Mn,
x¼0C1.2 Â 10 À 4 g/g and for Cu, x ¼0C3.5 Â 10 À 4 g/g). The average crystallite size as calculated by using the Scherrer formula
gave a value of about 10 nm. Note that the reported Bohr’s radius
for ZnS is about 10 nm. Therefore, the samples corresponded to
the strong quantum confinement regimes. The average particle
size determined from the SEM images was about 3 mm which
corresponds to the size of polycrystalline pieces. The average
particle size obtained via TEM image for colloidal system was
visibly smaller $ 35 nm. The emulsification process discussed in
the previous section appeared to have selected only smaller
particles and pasted them into the colloidal matrix. Using the
particle size analyzer (Horiba, LA950) we have obtained the size
distribution of colloidal systems before emulsification as shown
in Fig. 2(a). To our experience there was a continuous distribution
of size from nanometer to micrometer in every particle system,
including the commercial powders of micron size. So a small
portion of particles with nanometer size always existed and the
problem was to how to efficiently collect them. It appeared from
Fig. 2(a) that we have obtained a liquid containing only particles
of selected size, mainly below 50 nm with peaking distribution at
35 nm and narrow distribution of size from 20 to 40 nm. It should
be noted that, the total luminescent power of a polycrystalline
system is averaged from all particles therefore the distribution of
size is one of the important factors to determine the widening of

2. Experimental
Unlike in other works, where the surfactants were directly

involved in the synthesis (usually by wet chemical route), here we
utilized the classical solid-state reaction method to prepare the
doped and undoped ZnS polycrystallites, the surfactant was
involved only in emulsifying process. The ZnS, ZnS:Cu, and
ZnS:Mn bulk samples were first prepared in argon from the
starting ZnO, CuS powder (Merck), MnS powder and MnCl2, CuCl2
beads (Sigma-Aldrich). All chemicals were of purity grade greater
than 99.9%. The annealing temperature and time varied from
700 1C to 1250 1C and from some hours to 36 h. The structures of
the final bulk materials were investigated by X-ray diffraction
(XRD) method on the D8-Advance Brucker diffractometer using
˚ 2y from 101 to 701). The scanning
CuKa radiation (la ¼1.5056 A,
electron microscopy (SEM) study of surfaces was carried out on
JEOL5410-VL microscope. The bulk samples were then ground
again in methanol and the resulting powder was dried at 120 1C
for 2 h. The powder was then dissolved in water using ultrasound and left span at 3500 rpm in a centrifuge for 30 min.

Fig. 1. XRD patterns and SEM images of the bulk ZnS:Mn with x¼ 8 Â 10 À 5 g/g (a),
ZnS:Cu with x¼ 1 Â 10 À 4 g/g (b) and TEM image of undoped colloidal ZnS (c).


T.H. Ngo et al. / Journal of Luminescence 132 (2012) 2135–2142

2137

and acetone; (4) peak V results from a typical C ¼O stretching
mode occurred in both SPAN-80 and acetone; (5) peak VI shows a
typical O–H resonance usually assigned to the in-plane vibration
of water. The features below 1500 cm À 1 correspond mainly to

C–C, C–O vibration modes etc. If H(2) atoms of SPAN-80 were
involved in binding, then the CH2 stretching modes should vary,
but indeed we did not observe any variation in the positions of
peaks III and IV from that of pure SPAN-80. If O(1) atoms of SPAN80 were involved in binding (to Zn centers) then the corresponding C¼O stretching mode of SPAN-80 (peak V) should vary, which
was also not observed. Instead we recognized the small downshifts of both O–H (peak I) and C–O (peak IX) vibration modes.
Therefore we believed that SPAN-80 was attached to ZnS
nanoparticle surface via its H(1) hydrophilic end.
The binding of a surfactant to the particle surface has certain
stabilization effect on the total energy of the particle-surfactant
system. In Table 1 we give the total energy gain DE defined as

DE ¼ EðZnS=SPANÀ80Þ-½EðZnS clusterÞ þEðSPANÀ80ފ

Fig. 2. Size distribution for the bulk and colloidal samples (a) and FTIR spectra of
pure SPAN-80 and colloidal ZnS with their PL fingerprints in the inset (b).

emission band. Here by introducing a suitable surfactant SPAN-80
we were able to provide a selection of narrow size segment, hence
we might expect the colloidal systems to show the narrower
emission widths.
Before going into the details of optical characteristics, we
briefly summarize the structure and properties of colloidal agent.
The SPAN-80 (Chemical Abstract Service (CAS) registry no. 133843-8) is a food additive with formula C24H44O6 and molecular
weight 428.61 g/mol. Its PL fingerprint is shown in the inset of
Fig. 2(b). The obtained PL of the solvent was relatively weak in
comparison with that of ZnS nanoparticles, and could be well
subtracted as the background without complication. SPAN 80,
commonly used water-in-oil emulsifier, has two functional ends,
one hydrophilic (H(1), see the structure given in Fig. 2(b)) for
polar and one hydrophobic (H(2), same figure) for non-polar

solvent. SPAN-80 can be attached to the surface of ZnS nanoparticles by either hydrogen H(1) or H(2) (to S center) or oxygen O(1)
(to Zn center). The binding of SPAN-80 can be analyzed using FTIR
data given in Fig. 2(b). As seen, the difference between colloidal
system and pure SPAN-80 may be addressed as follows. (1) peak I
being assigned to O–H vibration in water and SPAN-80 (O–H(1))
shows a small down-shift which argues for the weakening of O–H
bonding due to binding of H(1) to nanoparticles; (2) peak II may
be assigned to the C–H stretching modes of acetone present in the
solvent; (3) peaks III, IV correspond to the asymmetric and
symmetric modes of CH2 stretching vibration in both SPAN-80

ð1Þ

where E is the ground state energy for optimized geometry of the
system given in parenthesis. DE was calculated for three cases of
attachment H(1), H(2) and O(1). The calculation was based on DFT
approach using gradient corrected functional of Perdew–Burke–
Ernzerhof [30] (i.e. GGA/PBE functional) and double numeric
wave function basis set with polarized and diffuse functions
added (DNP 3.5 set in DMol3 [31]). It appeared from the data
given that in the agreement with FTIR analysis presented above
the most suitable attachment point was hydrophilic H(1), therefore we computed the electronic structure on the basis of
H(1) attachment.
In Fig. 3 we show the PL spectra of ZnS:Cu colloids where the
comparison with that of the bulk is given in the inset. As seen, at
the doping concentration x ¼4 Â 10 À 5 g/g there appeared a new
green band at around 520 (or 533) nm except a blue one at 465
(or 476) nm. Both are characterized by the emission–
recombination of electrons located in the conduction band down
to the acceptor levels (465 or 476 nm) and levels of Cu2 þ trapping

centers (520 or 533 nm) in the band gap of ZnS (Fig. 3, inset). The
intensity of the 520 nm band was much greater than that of
the blue band. As the Cu concentration increased, the intensity of
the blue band decreased while the intensity of the green band
increased and reached maximum at x¼3.5 Â 10 À 4 g/g. This suggested that the Cu2 þ ions might be substituted into the Zn sites.
However, at the higher concentration of Cu, the intensity of the
green band decreased (curve (h), Fig. 3, inset). This behavior was
probably caused by the re-absorption by the Cu2 þ ions themselves. From Fig. 3 it is worth to note the differences between the
emission of colloidal and bulk samples. At first, all emission bands
of colloids were blueshifted (11–13 nm) in comparison with that
of the bulks and there was above 10-times stronger emission (at
the same doping concentration) from the colloidal samples. The
emissions from colloidal samples had also narrower band width;
the FWHM (full-width at half maximum) values were about half
of that for the bulk samples (i.e. 50 in contrast to 100 nm).
Second, in the colloidal samples with Cu concentrations above
Table 1
Total energy gain DE (eV) of emulsified ZnS for various ZnxSx clusters as obtained
from the stable optimal geometries of binary ZnS/SPAN-80 system.
Cluster

Zn12S12
Zn24S24
Zn36S36
Zn48S48

DE/H(1)

DE/H(2)
Hydrophobic end

DE/O(1)

Hydrophilic end
À 1.82
À 2.38
À 2.49
À 2.50

À 1.50
À 1.57
À 0.19
À 0.16

À 0.82
À 0.28
À 1.60
À 1.50


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T.H. Ngo et al. / Journal of Luminescence 132 (2012) 2135–2142

Fig. 3. The PL spectra of colloidal ZnS:Cu as excited by 325 nm He–Cd laser at
300 K. The inset shows the spectra for the bulk samples (upper) and the energy
diagram of possible transitions corresponding to the blue, green, yellow and red
bands (lower).

unchanged. The yellow-orange band was characterized by the

radiation–recombination of electrons in 3d5 shell of Mn2 þ ions:
(4G)4T1-(6S)6A1. At the Mn2 þ concentration over 8 Â 10 À 5 g/g,
the absorption caused the transfer of energy of excited state into
the thermal energy and not to the emission energy [6,7]. The
spectra for the colloidal samples showed even narrower FWHM in
comparison with that of the Cu-doped ZnS. The FWHM for most
cases holds within 30–40 nm and this was valid also for the
undoped sample. The growth of intensity caused by colloidal
agent was about 20-times larger, that is on the relative scale,
twice more intensive than for the Cu-doped case. For all cases,
doped and undoped, we have also observed the blue-shifts
(DlmaxC6–11 nm) of PL maxima for the colloids. The shifts were
not depending on doping but on the use of surfactant.
The time-resolved PL spectra for the blue band of the
Cu-doped bulk sample x¼8 Â 10 À 5 g/g (curve d in the inset of
Fig. 3) and for the yellow-orange band of the Mn-doped colloid
x¼4 Â 10 À 5 g/g (curve c in Fig. 4) are shown in Fig. 5. Due to the
suppression of 476 nm emission in the Cu-doped colloids, the
time-resolved PL spectra obtained with colloidal samples for this
band were very weak. As seen in Fig. 5 when the time-delay
increased from 30 to 85 ns, the intensity of the 476 nm emission
band decreased about 2.6 times but its position remained
unchanged. The lifetime of free electrons at conduction band, as
deduced from the luminescent extinguishing curve of the 476 nm
band, was about 0.12 ms. Similarly for the Mn-doped cases, when
the time-delay increased from 80 ms to 1000 ms, the intensity of
the yellow-orange band gradually decreased but its position
remained constant. The fluorescent lifetime of this band was
determined to be 1.9 ms at 300 K. This result was similar to the
ones obtained by other authors for the bulk ZnS:Mn [32–34]. The

prolonged decay time in ZnS:Mn was ascribed to the parity (and
hence spin forbiddance) of 4T1-to-6A1 transition of Mn2 þ ions. It is
worthwhile noting that there was a dependence of lifetime on
nanocrystallite size. For comparison we have fabricated the
nanocrystallites ZnS:Mn by co-precipitation technique, which
produced the nanocrystallites of size approximately 5 nm, and
we have obtained the lifetime $0.8 ms. The difference of lifetime
between bulk and polycrystalline material may be ascribed to the
surface defects. As the energy of excited states is transferred
easily to the defects at the surface than to the ones inside the
bulk, the lifetime of emission is correspondingly shorter in the
nanocrystallites than in the bulk materials. It is clear from the

Fig. 4. The PL spectra of colloidal ZnS:Mn as excited by 325 nm He–Cd laser at
300 K. The inset shows the spectra for the bulk samples.

8 Â 10 À 5 g/g, the intensity of the 520 nm peak totally dominated
over the blue band and the PL spectra showed only one peak. The
mechanism for such behavior might be both the size effect and
the lower inter-particle re-absorption in colloidal samples due to
low particle concentration in polymeric matrix. The colloidal
samples visibly consisted of smaller particles of narrow size
distribution and this homogeneity caused the enhancement of
emission ability of these samples. The decisive answer requires
the theoretical evaluation yet.
Fig. 4 shows the PL spectra of the ZnS:Mn samples with the Mn
concentration varying within x¼0–1.2 Â 10 À 4 g/g. The inset also
shows the spectra of the bulk samples for comparison. Besides the
green and blue bands, a yellow-orange band appeared at around
576 nm. As the Mn concentration increased, the intensity of the

green and blue bands diminished but the intensity and width of
the yellow-orange band increased gradually and reached the
maximum at x¼8 Â 10 À 5 g/g. The position of this band remained

Fig. 5. Time-resolved PL spectra of 576 nm band of a colloidal sample ZnS:Mn
(x ¼4 Â 10 À 5 g/g) as excited by 337 nm laser at 300 K. The inset shows the timeresolved PL spectra of 476 nm band of a bulk ZnS:Cu (x ¼ 8 Â 10 À 5 g/g), also
excited by 337 nm laser at 300 K.


T.H. Ngo et al. / Journal of Luminescence 132 (2012) 2135–2142

spectra given in Fig. 5 that the Mn2 þ activated emission bands
have only millisecond component. In the PL spectra of Mn-doped
ZnS (Fig. 4), we might also observe that the blue band (476 and
465 nm in the bulk and colloid respectively) was weak in the bulk
samples and totally disappeared in the colloids.
To search for a possible description of observed phenomena at
microscopic level, we studied the electronic structure of colloidal
ZnS using time-dependent density functional theory. The DFT was
usually the first choice due to high speed and accuracy at
moderate computational cost. At first, the cell parameters (a, c)
of the hexagonal wurtzite structure of ZnS were optimized using
local density approximation (LDA) functional (the GGA/PBE functional was also used for comparison). As known (i.e. for the
oxides), the optimized cells offered by LDA were usually smaller
and the cells obtained via the gradient corrected functionals were
larger than the experimental cells. It also happened in many cases
that the LDA results were closer to the experimental data than the
ones of improved GGA versions. From the data listed in Table 2 we
may also reveal that the LDA optimized cell matched better with
the data given in both Ref. [35] and JCPDS no. 36-1450 [36] than

the ones of GGA/PBE and GGA/PW91 [29]. Therefore based on this
cell we fine-tuned the other settings such as basis set, wave
function size (cut-off), smearing value, density mixing, mixing
schemes, core-electron treatment model etc. to achieve the
consistent approximation of band gap [10,21,22,37–39] and
optical data which includes reflectivity, transmittance [38,40],
absorption [10,15,18,20,21,24,38,40,41] and core-level emission
[20,41]. Our result of band gap 2.9 eV by GGA/RPBE functional,
although still smaller than the experimental gaps, was far better
approximation than the ones given in Ref. [29] (2.15 eV at best).
The theoretical PL bands were then determined within the frame
of TD-DFT [42,43]. Table 2 compares the obtained results with the
experimental data.
As seen in Fig. 6(a), the main emission bands related to the
relaxation of excited states to Zn 3d and S 3p levels in the
undoped ZnS were identified at 320, 400, 445, 540 (S), 590 (Zn),
877 nm. The bands should be equally assigned to both Zn and S

except the one at 540 and 590 nm which should solely belong to S
and Zn emissions respectively. The features around 445 and
540 nm were frequently observed in ZnS photoemission but both
might also broaden towards 410 and 510 nm when the impurities, such as Cu (see the curve related to Cu 4s emission), were
present. For the ZnS:Cu emission characteristics, the main features could be identified at 348 (S 3p), 410 (Zn, S), 510 (Cu 3d),
540 (Zn, S), 877 (Zn, S), 958 (Cu 3d), 2195 nm (Cu 3d, S) and some
other in ultraviolet and infrared region. The emission relating to
4s electrons of Cu might be seen at 410 nm but it should be much
weaker than the 510 nm emission originating from Cu 3d electrons. The Cu specific emissions might also be seen at longer
wavelength, e.g. at 958 and 2195 nm. These bands should be
absent in the pure ZnS. There was a clear suppression of the
445 nm band and of an ultraviolet band at 320 nm due to doping.

There was also no Cu specific emission at 320 nm; instead, the
weak emission might be expected at 348 nm due to the relaxation
to S 3p levels. Another effect caused by Cu doping was the clear
blueshift of the yellow band at 590 nm (in the undoped ZnS) to
the green region at 510–540 nm.
The partial emissions of atomic centers for the Mn-doped ZnS
are shown in Fig. 6(b). The Mn specific bands in visible region
might be recognized at 410 (Mn), 450, 495–506, 590, 746 (Mn)
and 1450 nm (Mn). In the ultraviolet region the bands were
identified at 330 (Mn) and 370 nm (Mn). The 370, 410 and
746 nm bands were characteristic for the relaxation to Mn 3d
levels whereas the band at 450 nm resulted from the transitions
to Mn 4s levels. The small emission related to Zn centers could
also be seen at 360 nm but the 370 and 746 nm emissions were
associated only with Mn centers. The 450 nm band which were
also seen in the PL of undoped and Cu-doped ZnS should be
originated from Zn and S centers too. Besides a strong emission
usually observed at 590 nm, the features at around 500 nm might
also be expected for ZnS:Mn. In comparison with the PL of the
undoped ZnS, there was a clear suppression of features at 400,
and 877 nm and new emission bands at 360, 495 and 1450 nm
were observed. A redshift might be expected for 590 nm emission

Table 2
This work
3.88, 6.37 (GGA/PBE)
˚
Cell constants a, c (A)
3.77, 6.20 (LDA/num. basis set)
3.79, 6.34 (LDA/plane wave)

Band gap (eV)
2.6 (GGA/PBE)
2.9 (GGA/RPBE)
2.8 (LDAþ U)

PL (nm)

a

Other works

3.7943(2), 6.2679(5) [35];
3.820, 6.257 [36];
3.85, 6.29 [29] (GGA/PW91)
3.56–3.79 [37] (and references therein);
3.51, 3.84 [38];
3.88 [22]a; 3.70 [39];
4.36–4.77 [10]b;
4.10–4.69 [21]c
GGA/RPBE (periodic structure):
Zn: 421 [46]; 425 [22]a;
Zn, S: 400, 445; S: 540 (ZnS)
424 [10]b; 427 [47]d
Zn, S: 410, 540; Cu: 510 (ZnS:Cu)
S: 438 [16]g; 440 [48];
Zn, S, Mn: 450, 500, 590; Mn: 410 (ZnS:Mn) 450 [41,46]; 440–487 [18]e;
GGA/RPBE (colloidal clusters):
460 [47]d; 470 [12]h
Zn, S: 425, 500 (ZnS)
Cu: 520 [49]; 510 [46];

Zn, S: 425; Cu: 522 (ZnS:Cu)
525 [12]g
Zn, S: 472; Mn: 575 (ZnS:Mn)
Mn: 577 [20]i; 578 [16]g; 560 [45]
Experimental data (bulk):
580 [13]f; 580 [14];
476, 533 (Cu); 476, 582 (Mn)
590 [15,17]; 592 [10]b
Experimental data (colloid):
465, 520 (Cu); 465, 576 (Mn)

ZnS/polyvinyl alcohol (PVA).
ZnS:Mn passivated by sodium hexametaphosphate (SHMP).
c
ZnS:Mn with core-shell structure/SHMP.
d
ZnS:Sn/thiourea.
e
ZnS:Cu,Cl/3-mercaptopropionic acid (MPA).
f
ZnS:Mn/trioctylphosphine oxide (TOPO).
g
ZnS:Mn/mercaptoacetic acid.
h
ZnS:Cu capped with TOPO and SHMP.
i
ZnS:Mn passivated by UV-irradiation.
b

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T.H. Ngo et al. / Journal of Luminescence 132 (2012) 2135–2142

Fig. 6. Theoretical PL bands of the Cu-doped (a) and Mn-doped (b) ZnS as obtained on the basis of a periodic structure model given in the upper-left part of the figure. The
blueshifts of spectral lines caused by COSMO effect (c) (the inset shows the relative shifts (%) for individual emission bands) and the theoretical emissions as obtained on
the basis of TD-DFT simulation using Zn9S9 model cluster activated by 6 SPAN-80 molecules (d); the inset shows the blueshift of valence band DOS due to surfactant
attachment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article).

which should broaden towards 628 nm (Zn center emission). The
most apparent difference from the Cu doped case was probably
the persistence of a band at 450 nm and the disappearance of the
one at 540 nm. The emission associated with S centers at this
wavelength totally diminished and the one associated with Zn
centers seemed to be blueshifted to around 500 nm.
To clarify the effect of surfactant binding, we first estimated
the influence of solvent on emission characteristics of ZnS
nanoparticles using the conductor-like screening model (COSMO)
[44]. Within COSMO, each solvent is represented by a dielectric
constant e and its influence on solute follows from the polarization effect that solvent as continuous medium expresses on the
solute. The effect is similar to putting a solute within a cavity of
Coulomb force field. The deviation of screening effect estimated
by COSMO from the exact solvation for strong polar solvents
(such as water) is smaller than 1% and for the non-polar solvents
(such as SPAN-80 with e E2) is less than 10%. Several solvents
with different dielectric constants ranging from 1 (vacuum) to 80
(water) were tested on a small cluster Zn9S9 consisted of three
hexagon layers Zn3S3. Overall, a large blueshift of emission

maxima according to the increase of dielectric constant e of the
solvent (Fig. 6(c)) was observed. The relative shifts varied also
upon emission bands, i.e. it reached maximum around 16% for the

ultraviolet band (320 nm), 21% for the violet band (410 nm)
and 32% for the green band (570 nm). For SPAN-80 (e E2 as of
common oils) the relative blueshifts might be expected at 7–13%
(or 20–80 nm, i.e. on the energy scale roughly 0.25 eV). However,
the error of the above estimation for oils was known to be as large
as the estimated shifts.
The effect of solvent in creation of screening charges on the
surface of solute nanoparticles may be more accurately evaluated
at low level theory using DFT. In general, we could expect that the
binding of the hydrophilic end H(1) of SPAN-80 to S centers of ZnS
nanoparticles would deplete electrons from H(1) centers and raise
the S 3p orbital occupation level. This should introduce a stronger
Coulomb repulsion between occupied states (hybridization
between Zn 4s and S 3p) and unoccupied states, which in turn
would push the occupied states a bit lower below Fermi level.
Therefore, we would consequently observe a blueshift of the total
DOS. This scenario might be verified using the large clusters of
size up to Zn36S36 (plus 5 SPAN-80 molecules, that is 442 atoms in
total) but smaller clusters were adequate to express the relative
shifts. The DOS-s were usually much easier to compute than the
PL which required TD-DFT calculation on the geometry optimized
by evaluating the excited states using single excitation
(CI-Singles). From the results obtained (inset, Fig. 6(d)), we


T.H. Ngo et al. / Journal of Luminescence 132 (2012) 2135–2142

deduced that the blueshift of DOS near the energy segment of
observed emissions (2.2–2.4 eV) due to the surfactant attachment
was about 0.13 eV. These values are half of that provided by
COSMO. As seen, the shift depended on energy; a smaller
(negative) energy induced larger shifts. Therefore, the binding of
surfactant affected mostly the valence band. The analysis of
molecular orbitals (MO) showed that the valence MOs extended
also below the surface layers and were located not only at the
surface. The analysis of charge population showed a small
increase of negative charge (0.11e À ) upon S centers and of
positive charge upon H(1) atoms of surfactant molecules. The
charge polarization between solute and surfactant reproduced the
COSMO effect but the shifts obtained were visibly smaller. It is
important to mention the difference between mechanisms of
charge polarization in DFT and COSMO. In COSMO, there was a
vacuum layer between solute and surfactant, but in DFT, there
was a bonding exchange between solute and surfactant. While
DFT estimates the shifts directly from the modification of valence
band MOs, COSMO obtains the final shifts by investigating the
reaction of solute putting inside a cavity of additional Coulomb
force field.
Another important result from the evaluation of DOS was that,
the larger the clusters were, the denser and larger DOS-s
appeared. The smaller clusters usually showed a comb-like
structure DOS due to a limited number of available states.
Therefore, the narrowing of PL band-widths in the spectra of
colloidal samples due to the higher homogeneity in nanoparticle
size could be understood.
Finally, Fig. 6(d) shows the theoretical PL of undoped and

doped ZnS nanocolloids as modeled on the basis of Zn9S9 cluster
activated by 6 SPAN-80 molecules using the TD-DFT approach.
The main features (in visible region) were identified at 412,
571 nm (undoped bulk); 425, 500 nm (undoped colloid); 425,
522 nm (Cu-doped colloid); and 472, 575 nm (Mn-doped colloid).
Recall that the experimental data were recorded at 476 (undoped
bulk), 465 (undoped colloid), 520 (colloidal ZnS:Cu), and 576 nm
(colloidal ZnS:Mn). As seen, the agreement was excellent for the
doped colloids: the displacements Dl between experimental and
calculated data were less than 2 nm. Unfortunately, the 465 nm
band was not resolved for the undoped case when Zn9S9 was used
as a model cluster. A clear blueshift of spectral lines due to
surfactant attachment was also observed but they were all below
10 nm, therefore were not as large as predicted by COSMO. The
individual excitations that contributed more than 95% of total
oscillation strength (intensity) of each PL band were as follows:
(1) 575 nm emission: the excitations from the levels just below
the Highest Occupied Molecular Orbital (HOMO), i.e. HOMO-1
and HOMO-2 to the Lowest Unoccupied Molecular Orbital
(LUMO); (2) 522 nm emission: HOMO-2 to LUMO þ1 and
HOMO-3 to LUMO; (3) 472 nm emission: HOMO-4 and HOMO-5
to LUMO.

4. Conclusion
By doping Cu2 þ (3d9) and Mn2 þ (3d5) into the bulk ZnS and
subsequently activating by using the SPAN-80 as surfactant we
have successfully prepared the colloidal materials which emitted
light at different wavelengths, blueshifted and with stronger
intensities in comparison with those of the original bulk samples.
The calculation using the time-dependent density functional

theory showed that the green emission at 520 nm and the
yellow-orange emission centered at 576 nm can be characterized
by the transitions of electrons in 3d shell of Cu2 þ and Mn2 þ
cations (4T1–6A1) particularly. The blue band at 465 nm was a
result of transition of delocalized (bonding) electron between Zn

2141

and S. All observed emissions were about 6–13 nm blueshifted
due to surfactant attachment. The explanation of these shifts
using the conductor-like screening model gave, however, the
values larger than 20 nm. The results obtained from TD-DFT
calculation was half of this value and agreed in excellence with
experimental data. The DFT demonstrated two quantum effects
when the surfactant molecule SPAN-80 was attached to the nanocluster of doped or undoped ZnS: (1) an increase in the number of
allowed quantum states of the system induced denser valence
DOS which in turn raised the overall oscillation strengths, thus
the intensity of allowed optical transitions, (2) an increase in the
valence band occupation induced an increase in Coulomb repulsion between occupied and unoccupied states which forced the
blueshift of optical transitions in a scale compatible with the
energy shift.

Acknowledgment
One of the authors (NTH) would like to thank the financial
support from the National Foundation for Science and Technology
Development of Vietnam (NAFOSTED), project code 103.02.73.09.
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