Effects of doping on the surface energies of nanocrystals and evidence from studies at high pressure

Article abstract of DOI:10.1016/j.ssc.2011.02.004

Zn_1-XMn_XS (X=0.85% and 1.26%) nanoparticles have been synthesized using a specially designed equipment and we have studied the influence of doping Mn²⁺ on the surface energy of ZnS. The high pressure behaviors of ZnS nanocrystals with different dopant contents have been investigated using angle-dispersive synchrotron X-ray powder diffraction up to 45.1 GPa. Theoretical calculations show that doping with Mn²⁺ increases the surface energy of the nanocrystals. The theoretical result has been further corroborated by our experimental observation of an increase in the phase transition pressure of Mn²⁺ doped ZnS nanocrystals in diamond-anvil-cell studies.

Full text of DOI:10.1016/j.ssc.2011.02.004

Solid State Communications 151 (2011) 607–609
Contents lists available at ScienceDirect
Solid State Communications
journal homepage: www.elsevier.com/locate/ssc
Effects of doping on the surface energies of nanocrystals and evidence from
studies at high pressure
∗
∗
Zhaocun Zong, Yanmei Ma, Tingting Hu, Guangliang Cui, Qiliang Cui , Mingzhe Zhang , Guangtian Zou
State Key Laboratory of Superhard Materials, Jilin University, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 September 2010
Received in revised form
Zn1 Mn S (X = 0.85% and 1.26%) nanoparticles have been synthesized using a specially designed
−X
X
2+
equipment and we have studied the influence of doping Mn on the surface energy of ZnS. The high
pressure behaviors of ZnS nanocrystals with different dopant contents have been investigated using
angle-dispersive synchrotron X-ray powder diffraction up to 45.1 GPa. Theoretical calculations show that
2
5 January 2011
Accepted 7 February 2011
by X.C. Shen
doping with Mn2 increases the surface energy of the nanocrystals. The theoretical result has been further
+
2
+
corroborated by our experimental observation of an increase in the phase transition pressure of Mn
doped ZnS nanocrystals in diamond-anvil-cell studies.
Available online 17 February 2011
©
2011 Elsevier Ltd. All rights reserved.
Keywords:
A. Semiconductors
A. Surfaces and interfaces
B. Chemical synthesis
E. High pressure
1. Introduction
2. The experiment
Dilute magnetic semiconductor Zn1−xMnxS nanoparticles were
synthesized through the reaction between the solution and H2S
at the gas–liquid interface, using new equipment, as seen in
Fig. 1(a). H2S gas was generated by dropping HCl into a Na2S
aqueous solution. The method enabled avoidance of the side
effects caused by other impurities [11]. The synthesis procedure
can be described as follows. The solution was prepared from the
Nanometer-scale semiconductor crystallites have been exten-
sively studied in order to explore their unique properties and po-
tential applications [1,2]. Much effort has been focused on II–VI
semiconductor nanocrystals [3–7], where doping is fundamental
to controlling the properties [8]. The doped ions are mainly dis-
tributed on the surfaces of the nanocrystals [9]. So an interest-
ing question is that of what surface-related properties would be
brought about by the doped ions on the surface. The surface and
interfacial energies are the most important physical attributes of
nanomaterials because the ratio of surface to volume will increase
with crystal size decreasing. The surface and interface energies
greatly affect the physical properties of nanomaterials. An anoma-
lous behavior of nanomaterials as regards the surface energy often
induces many novel nanosize effects, which open up many possi-
bilities for technological applications [10]. However, so far the re-
search on nanomaterial surface energy has been mainly restricted
to theoretical studies. It is essential to develop new experimental
tools and methods for understanding nanomaterial surface energy.
Here, we investigate the effects of doping on the surface energies
of nanocrystals and use high pressure methods to prove the change
in surface energy.
analytical reagent Zn(CH3COO)
2
·2H2O and Millipore water. The
ion concentration of Zn2 in the solution was 2 × 10 M and
+
−3
2+
the ion concentration of Mn
in the solution was adjusted by
using Mn(CH3COO)
2
·4H2O. The concentration of the surfactant
hexadecyl trimethyl and ammonium bromide in the solution was
−
5
9
× 10 M. The mixed solution was first heated to 75 °C using
microwaves, and then it was dripped and spread out on the wetted
convex surface. Nitrogen carried the H2S gas into the growth
chamber and this reacted with the cations on the thin gas–liquid
interface. The synthesized nanoparticles were then collected into
the receiving chamber, filled with disodium ethylene diamine in
tetraacetate solution. The reaction process could be controlled by
adjusting the concentration of H2S gas and the dripping rate of the
solution. The nanoparticles were rinsed with Millipore water and
with ethanol at least three times.
For high pressure experiments, the synthesis sample was
loaded into a hole of 90 µm diameter in a compressible
gasket of stainless steel. Small ruby chips were placed into
the sample chamber for pressure measurements using the
∗
Corresponding author. Tel.: +86 431 85166089; fax: +86 431 85166089.
E-mail addresses: cql@jlu.edu.cn (Q. Cui), zhangmz@jlu.edu.cn (M. Zhang).
0
038-1098/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2011.02.004

6
08
Z. Zong et al. / Solid State Communications 151 (2011) 607–609
a
b
c
d
Fig. 1. (a) A schematic diagram showing the central configuration of the synthesis system for Zn1−xMnxS nanoparticles. (b) The representative morphology of Zn1−xMnxS
2
+
nanoparticles. (c) The X-ray powder diffraction patterns for three synthesized samples corresponding to solutions containing different Mn ion concentrations. (d) The
high resolution TEM image of the Zn1−xMnxS nanoparticles. The inset gives the electron diffraction powder pattern of the sample.
quasihydrostatic ruby pressure scale. The pressure-transmitting
medium was a methanol–ethanol (4:1) mixture. We confirmed
that quasihydrostatic conditions were maintained throughout
the experiments. The precision of our pressure measurements
was estimated to be around 0.05 GPa. The high pressure X-ray
diffraction patterns and spectra were measured in situ up to 43.5
and 45.1 GPa, respectively, for nanocrystals with different Mn2
+
doping levels by using an angle-dispersive X-ray diffraction source
(
λ = 0.386556 Å) at beam line X17B3 of the National Synchrotron
Light Source at Brookhaven. The Bragg diffraction rings were
recorded with an imaging plate detector.
3. Results and discussion
The morphology of the nanoparticles was characterized by
using transmission electron microscopy (TEM). The nanoparticles
were assembled according to size, with diameters of about
Fig. 2. (a) ZB structure model with surface atoms. (b) ZB structure model with one
surface Zn atom replaced by one Mn atom in a (311) lattice plane.
5
0–150 nm, as seen in Fig. 1(b). Three synthesized samples
2+
corresponding to solutions with different Mn ion concentrations
were characterized by using X-ray powder diffraction, as seen in
Fig. 1(c). The result shows that the structures of the samples were
the same. The three peaks correspond to the (111), (220), and (311)
planes of the zinc-blende (ZB) structure. The broadened diffraction
peaks were caused by the small sizes of the nanocrystals.
The nanocrystal size was estimated at about 4 nm using the
Debye–Scherrer formula. By the refinement of the X-ray powder
the doped contents for the samples were 0.85% and 1.26%, which
correspond to mole percentages of 2% and 30% Mn2 ion content
+
solutions respectively.
To understand how the doped ions influence the surface
energy of the nanocrystals, we calculated the total energies of the
doped and undoped structures, and compared the total energies
between the two. The calculations were performed by means of
the pseudopotential plane wave density of the functional method
implemented in the CASTEP code treatment within the Materials
Studio package.
diffraction with different Mn2 doping contents, the peaks for
+
(
220), (111) and (311) all shift toward the small angle direction.
Among these shifts, there was a relatively major shift toward the
small angle direction in the (311) lattice planes when the Mn2
+
2+
The X-ray powder diffraction results for different Mn doping
2+
ions in the solutions reached a 30% mole fraction. This could be
content nanocrystals indicated that the locations of the Mn were
mainly in (311) lattice planes. We established two ZB structure
models with surface atoms; one was the pure ZB structure, and
the other had one Mn atom replacing a Zn atom in the (311) lattice
plane, as seen in Fig. 2.
2+
due to the fact that the radius of the Mn ion is larger than that of
2
+
Zn and caused the crystal lattice to expand. The doped contents
of the three samples were analyzed by using energy-dispersive X-
ray spectroscopy. The result shows that the mole percentages of

Z. Zong et al. / Solid State Communications 151 (2011) 607–609
609
Fig. 3. X-ray diffraction patterns collected at various pressures for different Mn2 doped ZnS samples. The peaks derived from the gasket are distinguished from reflections
+
2+
of Zn1−xMnxS by using ‘‘G’’ markings. (a) X-ray diffraction patterns for the sample doped with 0.85% mole per cent of Mn content. The right inset shows X-ray diffraction
2+
patterns collected at 17.7 GPa pressures for the sample doped with 0.85% mole per cent of Mn content. (b) X-ray diffraction patterns for the sample doped with 1.26% mole
2+
2+
per cent of Mn content. The right inset shows X-ray diffraction patterns collected at 18.3 GPa pressures for the sample doped with 1.26% mole per cent of Mn content.
The calculated results show that total energy of undoped ZB
structure is −23398.755 eV and the total energy of the doped ZB
structure with one Mn atom replacing a Zn atom in the (311) lattice
plane is −22317.960 eV. The energy difference between the two
be used as an important tool for probing the surface energies of
nanocrystals.
4. Conclusions
2+
structures comes from the doping of the Mn ion on the surface.
2
+
The doping of Mn leads to an increase of the total energy which
is just the increase in the surface energy.
In conclusion, we have synthesized Zn1−xMnxS nanoparticles
2+
using new equipment and studied the influence of doped Mn
Fig. 3 shows in situ XRD experiments on Mn2 doped ZnS
samples in a diamond anvil cell (DAC) at room temperature at
pressures up to 43.5 and 45.1 GPa. The diffraction patterns of
samples at ambient pressure can be indexed to a cubic structure
with space group F43m(NO.216). With increase of the pressure,
the diffraction lines shift toward higher angles and the ZB peak
intensity weakens. For samples doped with 0.85% mole per cent
+
on the ZnS surface energy. Theoretical calculations show that
2+
the doping with Mn
increases the surface energy of the
nanocrystals. The theoretical result was further corroborated by
our experimental observation of an increase in the phase transition
2+
pressure of Mn doped ZnS nanocrystals in diamond-anvil-cell
studies.
2
+
of Mn content, a new peak starts to emerge at approximately
7.7 GPa, as shown in the Fig. 3(a) inset on the right, suggesting
Acknowledgements
1
that a transition had occurred at this pressure. According to
the indexing result, this indicates that the new diffraction peak
corresponds to the (200) line of rock-salt structure. We consider
this new structure to be the rock-salt (RS) phase [12]. With the
pressure increasing to 18.9 GPa, the RS (111), (220), (400) and
This work was funded by the National Science Foundation
of China, Nos 90923032, 20873052 and 11074089. It was also
supported by the Ministry of Science and Technology of China, No.
2
011CB808204.
(
420) peaks begin to appear, and the ZB (220) and (311) peaks
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