# Surface Passivation and Photoluminescence of  Mn-Doped ZnS Nanocrystals

Authors: Ping Yang; Michael Bredol
Journal: Research Letters in Materials Science
 (2008)

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## Abstract

Enhanced photoluminescence (PL) is reported from Mn-doped ZnS nanocrystals (NCs) capped with ZnS (ZnS:Mn/ZnS core-shell NCs) and thioglycolic acid (TGA) (ZnS:Mn/ZnS core-shell NCs  dispersed in an alkaline TGA solution). The NCs were prepared using a reverse micelle route. Comparing with initial ZnS:Mn core NCs, the ZnS:Mn/ZnS core-shell NCs exhibit much stronger orange PL (~580 nm). This is presumably the result of effective passivation of quenching ZnS:Mn NCs surface states by a pure ZnS shell. As for TGA-capped ZnS:Mn/ZnS core-shell NCs, the parallel decrease of a defect-related emission of ZnS is associated with the formation of a shell surface layer of TGA-Zn complexes. In summary, the combination of ZnS shells with TGA ligands was demonstrated to yield ZnS:Mn NCs with narrow size distribution and intense PL.


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## Body

## 1. Introduction

Doped semiconductor
nanocrystals (NCs) have been studied extensively in the past few decades since
Bhargava [1] reported that ZnS:Mn2+ NCs exhibit high-luminescence
quantum efficiency in 1994. Mn-doped ZnS is widely recognized as an efficient
phosphor, especially, when it is used as a phosphor material for
electroluminescent displays [2]. Therefore, Mn-doped NCs have attracted much
attention in recent years [3–9]. Nanosized and
thus optically transparent emitters for electroluminescent displays made by
simple thick film technologies (e.g., printing) would offer exciting new
capabilities for industrial application of these nanomaterials in lamps or
displays, if present issues with efficiency and life time could be overcome.The photoluminescence (PL)
intensity of Mn-doped ZnS NCs is depending on the Mn2+ concentration
[10, 11]. In addition, especially in NCs the elimination of structural defects
plays an important role to enhance the luminescence of Mn2+, they
are known as luminescence quenching sites and play a role in photodegradation.
The surface states are likely to trap electrons and/or holes, inducing
nonradiative recombination. Thus more effective surface passivation methods
have been an
important objective and should lead to more efficient PL and
photostability. Bol and  Meijerink investigated, for example, the influence of Mn2+ concentration and a passivating polymer on the luminescence quantum efficiency
of ZnS:Mn NCs [5]. Yang and  Holloway reported on highly luminescent and photostable
yellow-emitting CdS:Mn/ZnS core-shell NCs [12].
They indicated that the intense
PL of the NCs is based on the effective passivation of CdS:Mn core surface
states by a ZnS shell. Kim et al. observed surface modification effects on the
PL properties of CdS and CdS:Mn NCs [13]. Furthermore, enhanced PL is also
observed when Mn-doped ZnS NCs are capped with an effective passivation layer,
such as ZnO [14] or SiO2 [15]. These inorganic capping agents
saturate the surface dangling bonds and substantially reduce the nonradiative
centers of the NCs. When dispersed in liquids, solvent molecules can act as
passivators as well. Cordero et al. [16]  suggested that the water molecules adsorbed on
CdSe NCs surface act to fill surface trap sites, leading to enhanced
luminescence.The reverse micelle
route is an established technique to prepare NCs with a narrow size
distribution. The NCs are generated inside the water pools of the micellar
system and thus size-selective NCs with narrow size distribution can be
obtained [17]. Additionally, it is quite easy to modify the surface of the NCs
during reverse micellar preparation with a shell material. Core/shell NCs
usually reveal high PL if the shell layer has a wider band gap than the core
NCs. For example, Santra et al. prepared highly water-dispersible,
multifunctional, CdS:Mn/ZnS core-shell NCs using a water-in-oil microemulsion
method [18]. In this article, we describe ZnS:Mn/ZnS core-shell NCs with narrow
size distribution via a reverse micelle route.Apart from surface passivation, another challenge is the
dispersibility of  NCs. Many organic
capping agents were already tried to reduce agglomeration of the NCs and
increase PL during preparation and dispersion. In this regard, thioglycolic
acid (TGA) is an effective capping agent to prepare CdTe [19, 20] and CdSe [21]
NCs because the thiole group in TGA can link effectively to the Cd2+ ion on the surface of the NCs. This leads to efficient surface passivation.
Thiole-capped CdTe colloidal solution has revealed high-PL quantum efficiency
(65%) using TGA as a capping agent [20]. Furthermore, the PL quantum efficiency
of these TGA-stabilized NCs reached up to 85% after illumination, as reported by
Gao’s group [19]. Therefore, we investigated the effect of the concentration
and pH of TGA solutions on the dispersion because TGA is stable in alkaline
solution [22].


## 2. Experimental

All chemicals used were
of analytical grade or of  the highest purity available. Milli-Q water was used
as a solvent. Zinc acetate dihydrate (Zn(CH3COO)2·2H2O, 98+%), manganese acetate tetrahydrate (Mn(CH3COO)2·4H2O, 99+%), sodium sulfide nonahydrate (Na2S·9H2O, 98+%), polyoxyethylene(5)nonylether (Igepal CO-520, 99.9+%),
cyclohexane, and TGA were purchased from Sigma Aldrich,
St. Louis, USA.


### 2.1. Preparation of ZnS:Mn/ZnS Core-Shell NCs

Zn2+-, (Zn2+ + Mn2+)-, and S2--containing stock solutions
(0.5 M) were
prepared by dissolving Zn(CH3COO)2·2H2O, Mn(CH3COO)2·4H2O, and Na2S·9H2O in water. The molar ratio
of Mn to Zn was set to the desired value. Each solution was mixed with an
Igepal CO-520/cyclohexane stock solution, with the concentration of Igepal
CO-520 in the microemulsion set to 0.24 M.
The water-to-surfactant molar ratio W for the formation of the ZnS:Mn/ZnS core-shell NCs was 3.7. ZnS:Mn core NCs
were obtained by injecting first 5 mL S2--containing microemulsion
to the (Zn2+ + Mn2+)-containing microemulsion within 2
minutes, followed by further vigorous stirring for 15 minutes. Then 48 mL S2--containing
microemulsion was injected at a rate of 0.5 mL/min into above microemulsion
followed by stirring for 60 minutes.A ZnS shell layer was grown on the ZnS:Mn core NCs using 0.5 M solutions of Zn(CH3COO)2·2H2O and Na2S·9H2O prepared as described
above. Zn2+ or S2- ions aqueous solutions were mixed with
the Igepal CO-520/cyclohexane stock solution with the same W.
A S2--containing microemulsion was first injected at a rate of 0.5 mL/min into the ZnS:Mn NCs microemulsion and then stirred rapidly for 30 minutes.
A Zn2+-containing microemulsion was then injected at a very slow
rate of 0.2 mL/min and then stirred rapidly for 60 minutes. The unwanted
nucleation and growth of a separated ZnS phase was suppressed by maintaining a
slow addition rate of the Zn2+ microemulsion [23]. The molar ratio
of ZnS to ZnS:Mn in the microemulsion was 0.15–0.40 to investigate the effect of the
thickness of ZnS shell. Because anion
dangling bonds on the surface of NCs result in decreased luminescence [12, 19],
the amount of Zn2+ is in excess compared with that of S2- in the microemulsion. To provide optimal reproducibility, in each of the series,
the samples were prepared twice, always at the same time and from the same
stock solutions, keeping all reaction conditions the same.


### 2.2. Dispersion and Surface Passivation of the NCs

Anhydrous acetone was
first added to the microemulsion to precipitate the NCs. The NCs were then
centrifuged, washed thoroughly using anhydrous acetone, centrifuged again to
eliminate surfactant, then washed thoroughly by a 0.05 M Zn2+ solution to eliminate acetone and form a nearly S2--free
surface on the NCs. After this treatment, they were dispersed in an aqueous
solution of TGA with different TGA concentrations (0.01 ~ 0.05 M) and pH (7 ~ 10) adjusted using a 0.5 M NaOH solution.


### 2.3. Characterization

Emission and excitation
spectra were recorded with a Shimadzu RF-5301PC fluorescence spectrometer.
Powder X-ray diffraction (XRD) was taken with a Philips diffractometer. The
size distribution of the samples was measured by dynamic light scattering (DLS,
Malvern high-performance particle sizer).


## 2.1. Preparation of ZnS:Mn/ZnS Core-Shell NCs

Zn2+-, (Zn2+ + Mn2+)-, and S2--containing stock solutions
(0.5 M) were
prepared by dissolving Zn(CH3COO)2·2H2O, Mn(CH3COO)2·4H2O, and Na2S·9H2O in water. The molar ratio
of Mn to Zn was set to the desired value. Each solution was mixed with an
Igepal CO-520/cyclohexane stock solution, with the concentration of Igepal
CO-520 in the microemulsion set to 0.24 M.
The water-to-surfactant molar ratio W for the formation of the ZnS:Mn/ZnS core-shell NCs was 3.7. ZnS:Mn core NCs
were obtained by injecting first 5 mL S2--containing microemulsion
to the (Zn2+ + Mn2+)-containing microemulsion within 2
minutes, followed by further vigorous stirring for 15 minutes. Then 48 mL S2--containing
microemulsion was injected at a rate of 0.5 mL/min into above microemulsion
followed by stirring for 60 minutes.A ZnS shell layer was grown on the ZnS:Mn core NCs using 0.5 M solutions of Zn(CH3COO)2·2H2O and Na2S·9H2O prepared as described
above. Zn2+ or S2- ions aqueous solutions were mixed with
the Igepal CO-520/cyclohexane stock solution with the same W.
A S2--containing microemulsion was first injected at a rate of 0.5 mL/min into the ZnS:Mn NCs microemulsion and then stirred rapidly for 30 minutes.
A Zn2+-containing microemulsion was then injected at a very slow
rate of 0.2 mL/min and then stirred rapidly for 60 minutes. The unwanted
nucleation and growth of a separated ZnS phase was suppressed by maintaining a
slow addition rate of the Zn2+ microemulsion [23]. The molar ratio
of ZnS to ZnS:Mn in the microemulsion was 0.15–0.40 to investigate the effect of the
thickness of ZnS shell. Because anion
dangling bonds on the surface of NCs result in decreased luminescence [12, 19],
the amount of Zn2+ is in excess compared with that of S2- in the microemulsion. To provide optimal reproducibility, in each of the series,
the samples were prepared twice, always at the same time and from the same
stock solutions, keeping all reaction conditions the same.


## 2.2. Dispersion and Surface Passivation of the NCs

Anhydrous acetone was
first added to the microemulsion to precipitate the NCs. The NCs were then
centrifuged, washed thoroughly using anhydrous acetone, centrifuged again to
eliminate surfactant, then washed thoroughly by a 0.05 M Zn2+ solution to eliminate acetone and form a nearly S2--free
surface on the NCs. After this treatment, they were dispersed in an aqueous
solution of TGA with different TGA concentrations (0.01 ~ 0.05 M) and pH (7 ~ 10) adjusted using a 0.5 M NaOH solution.


## 2.3. Characterization

Emission and excitation
spectra were recorded with a Shimadzu RF-5301PC fluorescence spectrometer.
Powder X-ray diffraction (XRD) was taken with a Philips diffractometer. The
size distribution of the samples was measured by dynamic light scattering (DLS,
Malvern high-performance particle sizer).


## 3. Results

The XRD pattern of the ZnS:Mn/ZnS core-shell NCs revealed zinc blende
structure. Because individual Mn impurities are most easily incorporated in NCs
with zinc-blende structure by adsorbing on their (001) facet [4], ZnS NCs with
zinc-blende structure are most desirable to enhance Mn2+ emission.
The mean size of the core/shell-NCs dispersed in TGA is ~11 nm, according to
DLS.Figure1(a) shows photographs of the ZnS:Mn core and ZnS:Mn/ZnS
core-shell NCs in microemulsion upon 366 nm UV and room light illumination. Figure
1(b) shows typical emission and excitation spectra of the ZnS:Mn core and
ZnS:Mn/ZnS core-shell NCs in microemulsion. While the ZnS:Mn/ZnS core-shell NCs
exhibit significant Mn2+ emission, the ZnS:Mn NCs show negligible Mn2+ emission. To investigate the influence of Mn2+ on the PL of the
ZnS:Mn NCs, we observed the PL spectra of the ZnS:Mn NCs with different Mn2+ ratio (1–8%). Highest PL intensity was found with a molar fraction of 4–5%; no change on position of excitation and
emission peaks was observed. The enhanced emission of the ZnS:Mn/ZnS core-shell
NCs strongly depends on the thickness of the ZnS shell. Efficient passivation
will take place
when the
molar ratio
of ZnS/ZnS:Mn in the precursor
microemulsion is 0.25–0.30 (inset in
Figure 1(b)).(a) Photographs of ZnS:Mn core NCs (1) and ZnS:Mn/ZnS
core-shell NCs (2), after preparation in microemulsion upon 366 nm UV (left) and
day light (right) illumination. (b) Excitation and PL spectra of ZnS:Mn and
ZnS:Mn/ZnS NCs in microemulsion. Inset: PL intensity of ZnS:Mn/ZnS core-shell
NCs at 580 nm and 430 nm versus molar ratio of ZnS/ZnS:Mn.
(a)(b)Figure2 shows the
excitation and PL spectra of the ZnS:Mn/ZnS core-shell NCs dispersed in an
alkaline TGA solution with different TGA concentrations and pH. The sharp
excitation spectrum reveals that the NCs exhibit a widened, but well-defined
band gap which means a narrow size distribution. Comparing with their initial
spectra in microemulsion, the defect-related emission was decreased and Mn2+ emission was increased. The PL intensity of the NCs dispersed in an alkaline
TGA solution depends strongly on the pH and concentration of the TGA solutions
(inset in Figure 2). When the concentration of TGA (pH 9.1) is higher than
0.035 M, the PL intensity of Mn2+ decreased and defect-related emission was increased slightly. Presumably, this
is caused by a deterioration of the surface due to a partial dissolution by the
TGA under these conditions.Figure 2
Excitation and PL spectra of ZnS:Mn/ZnS core-shell
NCs dispersed in a TGA solution (0.023 M,
pH 9.0). Inset: PL intensity of ZnS:Mn/ZnS core-shell NCs at 580 nm and 430 nm
against concentration of TGA with different pH. Initial excitation and emission
spectra of ZnS:Mn/ZnS core-shell NCs in microemulsion are shown for comparison.Figure3(a) shows photographs of ZnS:Mn/ZnS core-shell NCs
dispersed in water and in TGA solution upon 366 nm UV and room light illumination.
Figure 3(b) shows the pH dependence on dispersing the NCs into TGA solutions.
The NCs in this case showed no agglomeration and a transparent solution was
obtained. At the experimental TGA concentration (0.035 M, the NCs cannot be monodispersed completely when pH is less than
7.8, and the Mn2+ emission of the NCs in a TGA solution decreases
when pH is higher than 8.5. The optimal compromise for monodispersed NCs with
high PL was demonstrated to be a concentration of TGA of 0.015–0.03 M and a pH in the range
8–9.(a) Photographs of
ZnS:Mn/ZnS core-shell NCs dispersed in water (1) and a TGA solution (2) upon
366 nm UV (left) and day light (right) illumination. (b) Excitation and PL spectra
of ZnS:Mn/ZnS core-shell NCs dispersed in a TGA solution (0.035 M, pH 8.5). Inset: PL intensity of
ZnS:Mn/ZnS core-shell NCs at 580 nm and 430 nm against pH of TGA solutions
(0.035 M). Excitation and emission
spectra of ZnS:Mn/ZnS core-shell NCs dispersed in an aqueous solution with same
NCs concentration are shown for comparison.
(a)(b)


## 4. Discussion

The ZnS:Mn NCs exhibit a violet/blue emission and an orange
emission upon 315 nm excitation. The violet/blue emission is associated with a
defect-related emission of the ZnS host, and orange emission is attributed to
theT14→A16 transition of the Mn2+ ion. Compared with Mn-doped ZnS NCs prepared by standard methods [5]
(coprecipitation in aqueous solution at 80 ~ 100∘C), the defect-related
emission is stronger from the as-prepared ZnS:Mn NCs because the preparation
was carried out at room temperature. Therefore, Mn2+ PL of
unpassivated ZnS:Mn NCs is weak. The intensity of defect-related emission
depends strongly on the ratio of Mn2+ in solution. As soon as Mn2+ is incorporated in ZnS NCs, the defect-related emission decreases and Mn2+
emission comes up because the energy transfer between ZnS host and Mn2+ impurity is very efficient.An inorganic shell passivation on the NCs not only leads to a
significant reduction of surface-related states, but also leads to the
confinement of charge carriers in the core region due to band offset potentials
(the very thin shell should have a band gap even larger than the slightly
larger core), resulting in more efficient and photostable luminescence. After
passivating by a ZnS shell, the emission from Mn2+ (580 nm) was
dramatically enhanced. The formation of the ZnS shell was carried out with a
slow injecting rate in order to achieve a crystallinity as high as possible in
the shell. However, the defect-related emission is still observed after
passivation. This result differs from the report by Yang and Holloway on
ZnS-passivated CdS:Mn NCs [12], where no defect-related emission was observed
after passivation. The difference between the behavior of CdS:Mn and ZnS:Mn NCs
originates most probably from the different band gap differences of core and
shell and thus a stronger tendency to confinement in CdS:Mn NCs.In general, TGA serves well as stabilizer to prepare CdTe [20]
and CdSe [21] NCs. CdTe NCs prepared this way possess negative surface charge
because of the surface carboxylic groups in TGA and exhibit efficient
luminescence for months and even years being stored under air in the dark at
room temperature. The ZnS:Mn/ZnS core-shell NCs have similar surface states as
compared to CdTe NCs. The ZnS:Mn/ZnS core-shell NCs possess a surface rich in
zinc and nearly free of S2- ions because the NCs were washed with a
Zn2+ solution before dispersion. The Zn2+ ions on the
surface of the NCs can be coordinated with the mercapto groups in TGA during
dispersion. The carboxyl group of TGA then is presented on the outer surface of
the NCs in an alkaline solution. Depending on pH, carboxyl groups tend to
dissociate to form (-COO-) [22]. Therefore, the surface of
TGA-capped NCs has negative charge, preventing the NCs from agglomeration. For
TGA-capped emitting NCs, a shell of thiolate complexes is probably formed at
the surface of the NCs, passivating dangling bonds, but without possibility of
energy transfer to it. This is a situation in analogy to, for example, a ZnS
shell on CdSe [21]. Therefore, a strong enhancement of PL was observed from
TGA-capped emitting NCs.


## 5. Conclusions

The preparation of ZnS:Mn and ZnS:Mn/ZnS core-shell NCs via a reverse
micelle route has been described. A ZnS shell leads to efficient passivation of
the ZnS:Mn core when the molar ratio of ZnS/ZnS:Mn in the precursor
microemulsion is 0.25–0.30. The ZnS:Mn/ZnS core-shell NCs
reveal  zinc-blende structure and narrow
size distribution (mean size 11 nm). The PL and dispersion of the ZnS:Mn/ZnS
core-shell NCs in an alkaline TGA solution depends strongly on the
concentration of TGA and pH. A monodispersed colloidal solution of the NCs with
high PL is obtained when the concentration of TGA is 0.015–0.03 M and pH is 8–9.



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*Source: 506065-2008-04-09.xml*