Abstract ZnS nanoparticles with different sizes are synthesized using a reverse micelles method in the presence of Tb 3+ ions. The photoluminescence, excitation and absorption spectra of ZnS/Tb 3+ NPs are studied to elucidate remarkable energy transfer from both ZnS host and the surfactant, i.e., AOT, to Tb 3+ ions. When Tb 3+ ions are introduced merely on the outside of ZnS nanoparticles in reverse micelles, obvious energy transfer from ZnS to the Tb 3+ ions is also observed, indicating the important role of spatial confinement on the performance of energy transfer between host and luminescent centers which are even not doped into the host lattices

Jiahua Zhang

Li Chen

Shaozhe Lu

Xiaojun Wang

Xinguang Ren

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Chemical Physics Letters 409 (2005) 144–148On the energy transfer from nanocrystalline ZnS to Tb3+
ions confined in reverse micelles
Li Chen a,c, Jiahua Zhang a,*, Shaozhe Lu a, Xinguang Ren a, Xiaojun Wang a,b,*
a Key Laboratory of Excited State Processes, Changchun Institute of Optics, Fine Mechanics and Physics,
Chinese Academy of Sciences, 16 Eastern South-Lake Road, Changchun 130031, PR China
b Department of Physics, Georgia Southern University, Statesboro, GA 30460, USA
c Graduate School of the Chinese Academy of Sciences, Changchun, Peoples Republic of China
Received 20 December 2004; in final form 11 April 2005
Available online 31 May 2005Abstract
ZnS nanoparticles with different sizes are synthesized using a reverse micelles method in the presence of Tb3+ ions. The photo-
luminescence, excitation and absorption spectra of ZnS/Tb3+ NPs are studied to elucidate remarkable energy transfer from both
ZnS host and the surfactant, i.e., AOT, to Tb3+ ions. When Tb3+ ions are introduced merely on the outside of ZnS nanoparticles
in reverse micelles, obvious energy transfer from ZnS to the Tb3+ ions is also observed, indicating the important role of spatial con-
finement on the performance of energy transfer between host and luminescent centers which are even not doped into the host
lattices.
 2005 Elsevier B.V. All rights reserved.1. Introduction
The photoluminescence (PL) properties of nanocrys-
talline II–IV semiconductors doped with 3d transition
metal and rare earth (RE) ions have attracted much
attention in both basic and applied research field during
this decade [1–15]. RE ions-doped nanocrystalline
semiconductors is expected to form a new luminescent
materials with widely application such as on the opto-
electronic devices because of the special sharp lines from
the intra-shell 4f transitions of RE ions [14]. In addition,
it was reported that RE ions especially Tb3+/Eu3+ could
be possibly used as sensitive luminescent probes in bio-
molecular system [16–18], and nanometer-sized semi-
conductors such as CdSe/ZnS were investigated
intensively in respect of biological applications [19,20].
Promising widely application of such materials can be0009-2614/$ - see front matter  2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.04.106
* Corresponding authors. Fax: +86 431 6176317.
E-mail addresses: samsonciomp2000@yahoo.com (L. Chen),
zjiahua@public.cc.jl.cn (J. Zhang).anticipated if the combination of the RE ions and semi-
conductor nanoparticles (NPs) is achieved.
Compared to the easiness of doping Mn2+ into ZnS
host in nanometer regime [1,11,21,22], trivalent ions of
rare earth have much difficulty to be incorporated into
the ZnS NPs although it has been reported that RE ions
can be doped into the bulk semiconductors as efficient
activators, producing luminescence via 4fn  4fn transi-
tions [23–25]. A few papers, to the best of our knowl-
edge, reported on the synthesis and luminescence
properties of RE3+-doped ZnS NPs, however, rare of
them exhibits adequate evidence of energy transfer be-
tween ZnS host and RE3+ ions activators. This Letter
is focused on the energy transfer between ZnS NPs
and RE ions.
Although the surface of NPs generally plays a nega-
tive role on luminescence, we found that just on the sur-
face of NPs, remarkable energy transfer occurs between
ZnS host and Tb3+ ions. For the materials synthesized
by conventional methods such as precipitation method,
energy transfer from the host to the RE3+ is very difficult
L. Chen et al. / Chemical Physics Letters 409 (2005) 144–148 145to be observed because the RE ions are freely distributed
in the solution. Therefore, we tried to synthesize the ZnS
NPs in the presence of Tb3+ using a reverse micelles
method in which the RE3+ ions are confined in a limited
space around the ZnS particles, thus energy transfer
from ZnS to RE3+ can probably be expected. Prepara-
tion of the NPs in reversed micelles has been intensively
investigated [21,22,26,27] and the variation of particle
size can be obtained conveniently by changing the value
of x0 = [H2O]/[surfactant], the molar ratio of water-to-
surfactant. The anionic surfactant sodium bis(2-ethyl-
hexyl) sulfosuccinate (known as aerosol-OT or AOT)
is a popular surfactant used to form a thermodynami-
cally stable system (reverse micelles solution) when dis-
solved in organic solvents. Typically, the radius of the
water pool (Rw) follows the relationship: Rw (Å) = 1.5
x0 [26].
In this contribution, we report the synthesis of ZnS/
Tb3+ NPs in different sizes in the presence of Tb3+, as
well as Mn2+-doped ZnS NPs prepared using a reverse
micelles (water/AOT/isooctane) method for comparison.
In addition, we observed the energy transfer from the
ZnS host and AOT to trivalent terbium ions, respec-
tively, which are most likely on the surface of nanocrys-
talline ZnS. The steady-state spectra of both NPs and
the surfactant AOT are investigated for better under-
standing the optical properties of the materials and con-
trolling the preparation processes, which strongly affect
the applications of these semiconducting NPs.250 300 350 400 450 500 550 600 650
B
PLE PL5
D
4-
7 F
4
5 D
4-
7 F
5
5 D
4-
7 F
6
ZnS/Tb3+
A
PLE PLZnS:Mn 2+
Wavelength(nm)
In
te
ns
ity
(a
.u
.)
Fig. 1. PL (right, kex = 290 nm) and PLE (left) spectra of ZnS:Mn
2+
(A) and ZnS/Tb3+ (B) NPs prepared in water/AOT/isooctane reverse
micelles (kem = 580 nm for ZnS:Mn
2+ and kem = 542 nm ZnS/Tb
3+ NPs
in the PLE spectra measurements, a: x0 = 10, b: x0 = 14.4, c: x0 = 20).2. Experimental
All materials were of analytical grade and were used
directly without further purification. Surfactant sodium
bis(2-ethylhexyl) sulfosuccinate (AOT) was a white
pastelike solid purchased from Fluka. An aqueous solu-
tion of 0.1 M zinc chloride with appropriate amount of
reactants were made by dissolving ZnCl2 and TbCl3 Æ
6H2O into 50 ml deionized water and keeping the molar
ratio of Zn2+/Tb3+ to 10/1. Stock solutions of 0.1 M
AOT in isooctane and aqueous solution of 0.1 M Na2-
S Æ 9H2O were prepared, respectively. Reverse micelle
solutions containing Zn2+/Tb3+ were obtained by adding
a mount of stock solution containing Zn2+/Tb3+ to
50 ml stock 0.1 M AOT solution, yielding x0 values of
10, 14.4 and 20, respectively. Meanwhile, reverse micelle
solutions containing S2 were prepared by adding the
corresponding amount of the stock Na2S Æ 9H2O aque-
ous solution to 50 ml as-prepared AOT solutions while
keeping the corresponding values of x0 unchanged.
Equal amounts (50 ml) of reverse micelles solutions with
Zn2+/Tb3+ or S2, respectively were then mixed and stir-
red simultaneously with a magnetic stirrer at room tem-
perature. The sample described as Tb3+/AOT was
prepared by adding a mount of Tb3+ aqueous solutionto 0.1 M AOT stock solution (50 ml) for spectral mea-
surements. For the preparation of ZnS:Mn2+ nanoparti-
cles, the Tb3+ reactant was replaced by Mn2+ from a
stock solution containing ZnCl2 and MnCl2 Æ 4H2O
(manganese chloride tetrahydrate) in which the molar
ratio of Zn2+/Mn2+ was 100/1, and the following steps
were similar to those described above. The steady-state
PL and photoluminescence excitation (PLE) spectra
were carried out at room temperature using a Hitachi
F-4500 Spectra-fluorometer. Absorption measurements
were performed on a Perkin–Elmer model Lambda 9
UV–visible spectrometer, where a micellar solution
was used as a reference.3. Results and discussion
Fig. 1A and B shows the PL and PLE spectra of
ZnS:Mn2+ and ZnS/Tb3+ NPs in water/AOT/isooctane
microemulsion, respectively. In the case of ZnS:Mn2+
NPs, there are two emission bands in the PL spectrum
under the excitation of 290 nm UV light. One is the blue
emission band peaking at around 400 nm and the other
the orange emission band peaking at 580 nm. It is well
known that the blue one is defect-related emission of
ZnS, called self-activation luminescence [28], and the
orange emission band is originated from 4T1–
6A1 transi-
tion of Mn2+ inside ZnS NPs. The PLE spectra (moni-
toring at 580 nm) of the ZnS:Mn2+ NPs clearly present
146 L. Chen et al. / Chemical Physics Letters 409 (2005) 144–148a band edge excitation peak of ZnS NPs, which shifts
from 307 to 296 nm when reducing particle sizes by
decreasing the values of x0 from 20 to 10 due to the
quantum confinement effect. Hence, one can point out
that there is energy transfer from the ZnS NPs to the
Mn2+ impurities, which gives rise to the orange
emission.
In comparison with the spectra of ZnS:Mn2+ NPs,
the PL spectrum of ZnS/Tb3+ under the excitation of
290 nm UV light also presents the broad blue emission
band at around 400 nm and a group of sharp lines due
to the 5D4–
7FJ (J = 6, 5, 4, 3) transitions of Tb
3+, in
which the 5D4–
7F5 emission at 542 nm is the strongest.
The PL excitation spectra of ZnS/Tb3+ NPs with various
sizes monitored at 542 nm were collected as shown in
Fig. 1B. There appear two peaks in each PLE spectrum
of the NPs for different x0 values. One located around
260 nm is related to AOT, which will be discussed later.
The other located around 300 nm is corresponding to
the band edge excitation of ZnS NPs in comparison with
the PLE spectra of ZnS:Mn2+ NPs shown in Fig. 1A. In
addition, the absorption edge from ZnS/Tb3+ NPs in re-
verse micelles shifts towards the shorter wavelength side
with decreasing x0 values as presented in Fig. 2, show-
ing the same trend as the PLE peaks for ZnS/Tb3+
NPs with reducing sizes in Fig. 1B due to the quantum
confinement effect, and similar reports can be seen in
[29] on analyzing the sizes variation of ZnS:Mn2+ NPs
modified by organic surfactants. Based on the blue shift
of absorption edges from 310 to 290 nm, the sizes of
ZnS/Tb3+ NPs can be estimated to be from 3.0 to
4.5 nm within the framework of effective mass approxi-
mation theory [30]. The onsets of absorption (deter-
mined by the linear extrapolation of the steep part of
the UV absorption curves towards the base line)260 280 300 320 340 360 380
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
ω
0
= 20
ω
0
= 14.4
ω
0
= 10
A
bs
or
ba
nc
e(
a.
u.
)
Wavelength(nm)
Fig. 2. Absorption spectra of ZnS/Tb3+ NPs taken on various values
of x0 in reversed micelles.[31,32] are in conformity with the PL excitation peaks
shown in Fig. 1B, which is further indicating that the
absorption edges in the range of 290–310 nm are origi-
nated from the band edge excitation from ZnS host.
Fig. 3 is the PL spectra of ZnS NPs in the presence of
different concentrations of Tb3+ under the excitation
of 290 nm. One can observe the decrease of the ZnS blue
band following the enhancement of the Tb3+ emission
lines with increasing Tb3+ concentrations, indicating
the performance of energy transfer from the ZnS NPs
to Tb3+.
The observation of energy transfer, however, cannot
be an evidence of Tb3+ being inside the ZnS lattices.
Our further experiment indicated that energy transfer
from ZnS NPs to Tb3+ ions located on the surface of
the NPs can effectively occur in a sample prepared by
means of mixing two kind of reverse micelles: one only
contains ZnS NPs and the other only Tb3+ with the
same concentrations and values of x0 as in the case of
preparing ZnS/Tb3+ NPs. The mixture sample exhibits
nearly the same PL and PLE spectra as those of ZnS/
Tb3+ NPs (see Fig. 4). Therefore, it can be concluded
that an energy transfer between the ZnS NPs and
Tb3+ can occur across the ZnS/Tb3+ NPs interface in
water/AOT/isooctane microemulsions.
The PLE peak at 260 nm in ZnS/Tb3+ NPs presented
in Fig. 1B was also observed in the ethanol [11] or the
aqueous system previously [33]. To determine the origin
of the 260 nm PLE peak in ZnS/Tb3+ NPs, PL
(kex = 290 nm) and PLE spectra of AOT and Tb
3+/
AOT in isooctane were measured and presented in
Fig. 5. The emission spectrum of AOT has a band peak-
ing at around 380 nm and the PLE spectrum monitoring
at this band shows a broad PLE band peaking at 310 nm
with a shoulder at 260 nm. The PLE peak at 260 nm of
AOT becomes stronger and dominant in the PLE350 400 450 500 550 600 650
0
9
18
27
λ
ex
=290nm
 0
2.4×104 mol/L
9.6×104 mol/L
In
te
ns
ity
(a
.u
.)
Wavelength(nm)
[Tb3+]
Fig. 3. PL spectra of ZnS NPs with various concentrations of terbium
ions which is designated as [Tb3+]. (Tb3+-free ZnS NPs – solid line;
2.4 · 104 mol/L – dashed line; 9.6 · 104 mol/L – dotted line;
kex = 290 nm).
250 300 350 400 450 500 550 600 650
0
10
20
30
40
50
PLE PL
In
te
ns
ity
(a
.u
.)
Wavelength(nm)
Fig. 4. PL (kex = 305 nm) and PLE (kem = 542 nm) spectra of a
mixture of two reverse micelles which one is only containing of ZnS
NPs and the other only Tb3+, respectively (solid lines), and those of
ZnS/Tb3+ NPs in reverse micelles (dotted lines).
250 300 350 400 450 500 550 600 650
0
5
10
15
20
25
30
35
40
5 D
4-
7 F
3
5 D
4-
7 F
4
5 D
4-
7 F
5
5 D
4-
7 F
6
PLPLE
Wavelength(nm)
In
te
ns
ity
(a
.u
.)
Fig. 5. PL (kex = 290 nm) and PLE spectra of AOT (solid lines,
monitored at 380 nm) and Tb3+/AOT (dotted lines, monitored at
542 nm) in water/AOT/isooctane reverse micelles.
L. Chen et al. / Chemical Physics Letters 409 (2005) 144–148 147spectrum of Tb3+/AOT complex, as shown in Fig. 5,
indicating the dominant contribution of the 260 nm
excitation center to the Tb3+ emission lines in compari-
son with the other excitation center at 310 nm of AOT.
Accordingly, the PLE peak at 260 nm in ZnS/Tb3+ NPs,
as shown in Fig. 1B, is reasonably attributed to the exci-
tation of AOT. Furthermore, it is worth noting that with
decreasing the sizes of ZnS/Tb3+ NPs, the position of the
260 nm PLE peak keeps unchanged whereas the band
edge excitation peak of ZnS NPs shifts to the higher en-
ergy side due to the quantum size effects as shown in Fig.
1B. Upon 290 nm excitation Tb3+/AOT gives character-
istic emission lines for terbium 5D4–
7FJ transitions, as
shown in the PL spectrum in Fig. 5, whereas the absorp-
tion due to Tb3+ ion itself is negligible. Thus we can ar-gue that there exists energy transfer between AOT and
Tb3+ ions, and the band around 260 nm can be assigned
to the transitions connected with the carbonyl group of
AOT molecule through the intra-molecular energy
transfer mechanism (IMET) [34].
In the PLE spectra of ZnS/Tb3+ NPs (see Fig. 1B), we
can also observe that the 260 nm PLE peak grows rela-
tively to the band edge excitation peak of ZnS NPs as
the particles size decreases. This behavior can be inter-
preted as the result of the increase of surface/volume
ratio with decreasing NPs sizes, because the number of
AOT molecule is proportional to the surface area of
the nanosized water pool, and the ZnS molecular num-
ber is proportional to the volume of the water pool.4. Conclusion
ZnS NPs with different diameters have been synthe-
sized successfully using a reverse micelle method in the
presence of Tb3+. Both ZnS NPs and AOT molecule
can transfer excitation energy to Tb3+ ions to emit sharp
lines of 5D4–
7FJ transitions. When Tb
3+ ions are intro-
duced merely on the outside of ZnS NPs in reverse mi-
celles, obvious energy transfer from ZnS NPs to the
Tb3+ ions is also observed, indicating the important role
of spatial confinement on the performance of energy
transfer between host and luminescent centers which
are even not doped into the host lattices. Reverse micelle
is a good confinement environment for obtaining effec-
tive energy transfer between host and doped centers in
nanometer scaled system.Acknowledgments
This work is supported by the National Natural Sci-
ence Foundation of China under Grant Nos. 50172047,
90201010, 10274083 and One Hundred Talents Project
of Chinese Academy of Sciences.References
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On the energy transfer from nanocrystalline ZnS to Tb 3+ ions confined in reverse micelles

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On the energy transfer from nanocrystalline ZnS to Tb 3+ ions confined in reverse micelles

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