Facile synthesis and optical properties of ultrathin Cu-doped ZnSe nanorods

Article abstract of DOI:10.1039/c3ce41493k

Successful doping of anisotropic semiconductor nanocrystals with impurities offers an effective pathway to manipulate their physical properties and enhance the application performances. However, such doping into anisotropic nanocrystals is seldom reported because it needs simultaneous controls in the crystal growth for a specific shape and composition engineering for transition-metal doping. Here, ultrathin Cu-doped ZnSe nanorods are synthesized by a growth-doping process. The doped nanorods are characterized by XRD and TEM techniques to reveal their crystal structure. Their optical properties are described in terms of UV-Vis absorption spectra and PL spectra. The tunable emission from 480 to 520 nm evidences the successful doping of Cu into ZnSe nanorods. The effects of the reaction time, the reaction temperature and the surface ligand on the optical properties are discussed in detail. After purification, the doped nanorods with a Cu/Zn ratio of 1% give a quantum yield of 7%. This emission could be retained for weeks in air, which is important for its future applications in many fields.

Full text of DOI:10.1039/c3ce41493k

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Facile synthesis and optical properties of ultrathin
Cu-doped ZnSe nanorods
Cite this: CrystEngComm, 2013, 15,
0495
1
a
b
a
c
c
a
Shufang Kou, Tingting Yao, Xiaofeng Xu, Rui Zhu, Qing Zhao and Jian Yang*
Successful doping of anisotropic semiconductor nanocrystals with impurities offers an effective pathway to
manipulate their physical properties and enhance the application performances. However, such doping into
anisotropic nanocrystals is seldom reported because it needs simultaneous controls in the crystal growth for a
specific shape and composition engineering for transition-metal doping. Here, ultrathin Cu-doped ZnSe nanorods
are synthesized by a growth–doping process. The doped nanorods are characterized by XRD and TEM techniques
to reveal their crystal structure. Their optical properties are described in terms of UV-Vis absorption spectra and PL
spectra. The tunable emission from 480 to 520 nm evidences the successful doping of Cu into ZnSe nanorods. The
effects of the reaction time, the reaction temperature and the surface ligand on the optical properties are
discussed in detail. After purification, the doped nanorods with a Cu/Zn ratio of 1% give a quantum yield of 7%.
This emission could be retained for weeks in air, which is important for its future applications in many fields.
Received 28th July 2013,
Accepted 3rd October 2013
DOI: 10.1039/c3ce41493k
www.rsc.org/crystengcomm
1
2
Introduction
(TOP)/tri-n-octylphosphine oxide (TOPO) but the usage of
organometallics like diethylzinc makes the synthesis toxic and
expensive. Then, Peng and Pradhan developed a growth–
Semiconductor nanocrystals doped with transition metal
ions have been an emerging class of functional materials
with great application potential in many fields, such as
biological imaging, photovoltaics, light-emitting-diodes, and
doping strategy for Cu-doped ZnSe nanocrystals with “green”
and safe reactants, like carboxylates or oxides.
to further improve the quantum yield and the stability of the
emission, surface coating of another semiconductor with a wide
band gap has been demonstrated to be a necessary step.
Since the optical properties of semiconductor nanocrystals
are highly dependent on their size, shape and structure,
the doping of transition metal ions into anisotropic nano-
crystals attracts our attention. However, it is difficult to
achieve the component modulation by dopants and simulta-
neously keep the anisotropic shape. As for one-dimensional
Cu-doped ZnSe nano-crystals, there are only two reports to
One of them presented the synthesis of Cu-doped
ZnSe nanorods by the thermal annealing of a precursor
obtained by a solvothermal method. The other reported a
gold-catalyzed vapor transport method for the Cu-doped ZnSe
nanowires. All these one-dimensional nanostructures have
diameters in the range of 50–200 nm and lengths up to
several micrometers. These sizes are too large to exhibit
the obvious quantum size effect that is an important charac-
teristic of semiconductor nanocrystals. Furthermore, the
optical properties of these nanorods/nanowires have not
been studied.
In this paper, ultrathin Cu-doped ZnSe nanorods with
diameters of only a few nanometers have been synthesized
under mild experiment conditions. The ultrathin nanorods
are characterized by XRD patterns, TEM and HRTEM images,
and UV-Vis absorption spectra. The successful doping of Cu
13,14
In order
1
–5
so on.
Their superior performances could be associated
with the unique changes of the optical, magnetic and
electronic properties caused by the dopants, even in a small
amount. For example, as far as the optical properties are
concerned, the doped nanocrystals could exhibit a minimum
15,16
17–19
6
7–9
self-absorption, a long excited-state lifetime,
a large
20
1
0,11
Stokes shift and a good thermal stability.
make them an excellent alternative to the conventional semi-
conductor nanocrystals for optical devices.
Compared with Mn-doped ZnSe nanocrystals, Cu-doped
nanocrystals are studied to a lesser extent, because the huge
difference between Cu
the doping of Cu ions into ZnSe nanocrystals. However,
their unique emission in the visible spectrum makes
them important in some applications.
Cu-doped ZnSe nanocrystals could be achieved by a similar
approach to that of CdSe nanocrystals in tri-n-octylphosphine
All the features
21,22
date.
x
Se and ZnSe crystal structures impedes
21
22
7
,8
The synthesis of
a
Key Laboratory of Colloid and Interface Chemistry, Ministry of Education,
School of Chemistry and Chemical Engineering, Shandong University, Jinan,
2
50100, PR China. E-mail: yangjian@sdu.edu.cn; Tel: +86 531 88364489
b
State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy,
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian
16023, PR China
1
c
State Key Laboratory for Mesoscopic Physics and Department of Physics,
Electron Microscopy Laboratory, Peking University, Beijing, PR China
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ions into the ZnSe nanorods has been evidenced by PL spec-
tra and ICP-AES analysis. The effects of the reaction tempera-
ture and reaction time on the product are also discussed in
terms of their optical properties. To the best of our know-
ledge, it is the first report of the optical properties of
Cu-doped ZnSe quantum nanorods.
microscope (TEM) images were achieved by a transmission
electron microscope of JEM-1011 at an accelerating voltage of
100 kV. High-resolution TEM (HRTEM) images and selected
area electron diffraction (SAED) patterns were recorded on a
JEOL 2010 microscope. Ultraviolet-visible (UV-Vis) absorption
spectra were acquired in the range of 300–700 nm at room
temperature by a Shimadzu UV-2450 spectrophotometer.
Experimental
Materials
Photoluminescence (PL) spectra were obtained from a
Thermo fluorometer (Lumina 222-263000) at room tempe-
rature. The quantum yield (QY) of the doped nano-crystals
was determined with quinine sulfate in 0.1 M H SO solution
Zinc acetate dihydrate (ZnAc ·2H O, 99%), methanol (99.5%),
hexane (99.5%) and acetone (99.5%) were ordered from
Sinopharm Chemical Reagent Co. Ltd. Selenium powders
2
2
2
4
as a reference. The Cu/Zn ratio in the doped nanocrystals was
determined by inductively coupled plasma atomic emission
spectroscopy (ICP-AES, Thermo IRIS Intrepid IIXSP).
(
(
(
Se, 99.5%); oleylamine (OAm, 80–90%) and quinine sulfate
99%) were purchased from ACROS organics. Stearic acid
SA) and CuAc (99%) came from Shanghai Chemical Reagent
2
Results and discussion
Factory. Tetramethylammonium hydroxide (TMAH, 97%) was
bought from Aldrich. 1-Octadecene (ODE, 90%) and tributyl
phosphine (TBP, 95%) were obtained from Aladdin Chemical
Reagent Co. Ltd. All these reagents were used without any
further purification.
Fig. 1 shows the XRD pattern of the product, which could be
indexed as the zinc-blended phase of ZnSe (JCPDS card
no. 37-1463). No diffraction peaks related to Cu impurities
are observed in the figure. The results are the same as the
2
3
undoped ZnSe nanorods, indicating that the introduction
of a small amount of Cu into the ZnSe nanorods (~1.1%)
would not affect their crystal structure. This conclusion
is also supported by the lattice constant calculation. The
calculated lattice constant of the product is a = 5.668 Å, very
close to that of ZnSe in the zinc-blended phase (a = 5.669 Å).
Similar results are also reported for many doped semiconductor
Synthesis of copper stearate
1
0 mmol of SA was dissolved in 20 mL of methanol and then
the solution was heated to 50 °C to get a clear solution.
After that, 10 mmol of TMAH in methanol was added to this
solution. 20 min later, 5 mmol of CuAc
introduced into the solution dropwise. As a result,
blue precipitate appeared in the solution immediately. The
precipitate was collected by centrifugation and was dried in
vacuum for the later use.
2
in methanol was
a
5
,24,25
nanocrysals.
The shape and size of the product are investigated by TEM
microscopy. Fig. 2a shows a typical TEM image of the
2
3
ultrathin ZnSe nanorods prepared by a reported method.
The shape and size of the ultrathin nanorods are in good
agreement with this report. Fig. 2b presents a TEM image
of the product after the Cu-doping process. Both the one-
dimensional structural feature and the ultrathin diameter of
the ZnSe nanorods are maintained in the product. The
nanorods exhibit diameters of ~3 nm and lengths of up to
tens of nanometers, which would result in an obvious
quantum-size effect in the optical properties. The clear lattice
fringes in the HRTEM images confirm the crystal nature of
the nanorods, as illustrated in Fig. 2c and d. The lattice
Synthesis of Cu-doped ZnSe nanorods
2
3
ZnSe nanorods were prepared based on a modified report.
Then, the obtained ZnSe nanorods, 5% of CuSt to ZnSe in
molar ratio, and 5 mL of ODE were added into a clean three-
necked round-bottomed flask. The flask was connected to a
Schlenk line and flushed with nitrogen three times to remove
oxygen and moisture. After that, the solution was gradually
2
−
1
heated to 100 °C at a rate of 2 °C min . After reacting at
00 °C for 100 min, the solution was further heated to 180 °C
immediately. Appropriate amounts of Se powder in TBP and
ZnAc in a mixture of OAm/ODE were injected into the
1
2
solution for the shell growth. After 30 min, the solution was
cooled to room temperature and was mixed with acetone.
The product was collected by centrifugation and re-dispersed
in hexane for characterization. The actual atomic ratio of
Cu/Zn in these ZnSe nanorods is around 1.1%.
Characterization
X-ray diffraction (XRD) patterns were obtained from an X-ray
powder diffractometer (Bruker D8 Advance, German) with
monochromatic Cu Kα as a radiation source. The working
voltage of the diffractometer and the according current were
kept at 40 kV and 40 mA, respectively. Transmission electron
Fig. 1 XRD pattern of the Cu-doped ZnSe nanorods at 1.1%. The lines at the
bottom present the standard XRD diffraction peaks of bulk ZnSe in a cubic phase
(JCPDS card no. 37-1463).
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absorption spectra, there is a remarkable change in the PL
spectra. The emission band of the ZnSe nanorods moves
from 430 to 520 nm after the doping process. The emission
band at 520 nm is the characteristic feature of Cu-doped
1
3,14,24
ZnSe,
which is assigned to the recombination of
excited electrons from the conduction band of the host mate-
2
4,26–28
rial (ZnSe) and holes from the d-orbital of copper ions.
This result directly confirms the successful doping of Cu into
ZnSe. The quantum yield of the emission is approximately
1
9%, lower than those of the Cu-doped ZnSe nanocrystals
2
9
15
with excess S atoms on the surface or with a ZnS shell.
This could be attributed to the structure defects in/on the
nanorods, which is also documented for the emission of the
2
3
ultrathin ZnSe nanorods. After several more rounds of
washing and centrifugation, the quantum yield goes down
to 3–7%, comparable to that of the Cu-doped spherical
7
,8,25
nanocrystals.
This result could be explained by the
removal of the surface ligands on the nanorods during the
repeated washing.
The formation process of the Cu-doped nanorods follows
a typical growth–doping strategy well-demonstrated in the
1
3,14,24
cases of the spherical nanocrystals.
In brief, the host
nanorods are synthesized by the reaction of Zn carboxylates
and Se powders in oleylamine. Then, these nanorods react
with copper stearate at 100 °C to cause Cu ions to adsorb
Fig. 2 (a) TEM images of ZnSe nanorods, and (b) TEM images, (c, d) HRTEM
2
+
images and (e) electron diffraction pattern of the Cu-doped ZnSe nanorods at 1.1%.
onto the surface or diffuse into the lattices of the host
2
+
nanocrystals. In order to ensure the Cu ions are well-
encapsulated, another thin layer of ZnSe is grown on the
doped nanorods (Scheme 1). The growth–doping strategy
(Scheme 1) maintains the shape of nanocrystals and realizes
the doping simultaneously.
Fig. 4 shows the temporal evolution of the UV-Vis absorp-
tion spectra and the PL spectra of the solution at 180 °C,
which offers a possible pathway to tune the optical properties
distances between neighboring fringes further suggest the
growth direction of the nanorods along the [111] axis of
the zinc blended phase. The crystal nature of the nanorods is
also verified by the SAED pattern, as shown in Fig. 2e. The
diffuse diffraction rings are in line with the zinc-blended
phase of ZnSe, which is again consistent with the result from
the XRD pattern.
The UV-Vis absorption spectra and PL spectra of the ZnSe
nanorods before and after the Cu-doping process are shown
in Fig. 3. It is noted that the absorption onset of the ZnSe
nanorods is redshifted by about 10 nm after the doping pro-
cess. This could be attributed to the size increase of the
nanorods during the reaction. Compared with the UV-Vis
Scheme 1 Schematic illustration of the formation of the ultrathin doped nanorods.
Fig. 3 UV-Vis absorption spectra (solid line) and PL spectra (dashed line) of the
ZnSe nanorods (red) and the Cu-doped ZnSe nanorods at 1.1% (blue). The inset
shows the digital images of the doped nanocrystals obtained without or with the
irradiation of a hand-held lamp.
Fig. 4 Temporal evolutions of (a) UV-Vis absorption spectra and (b) PL spectra of
the reaction solution at 180 °C for different reaction times.
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of the doped nanorods. When the reaction temperature
reaches 180 °C, the Cu-related emission centred at 483 nm
could be easily observed in the PL spectra. This result indi-
cates the doping of Cu into ZnSe could be achieved at a low
temperature. As the reaction at 180 °C proceeds, the absorp-
tion onset of the nanorods is slowly redshifted (~11 nm),
indicative of the size growth (Fig. 4a). Meanwhile, the emis-
sion shifts from 483 to 520 nm and increases in its intensity
(Fig. 4b). The former could be attributed to the band-gap
narrowing of ZnSe due to the particle growth. The latter
might be caused by the better coverage of the shell growth of
ZnSe on the doped nanorods, which is supposed to passivate
the surface and avoid the dissipation of Cu ions into the
solution again.
Fig. 6 The dependences of (a) the actual molar ratio of Cu/Zn and (b) the
quantum yield (QY) on the nominal ratio of Cu/Zn.
The effect of OAm used for the shell growth on the optical
properties of the doped nanorods has been illustrated in
always far below the nominal ones over the tested range,
indicating that only a small amount of Cu ions are incor-
porated into the host nanorods. Different from this result,
there is not a monotonic relationship between the quantum
yield and the nominal Cu/Zn ratio. The quantum yield of
the doped nanorods reaches the maximum at the nominal
ratio of Cu/Zn at 1.0%. Too many Cu ions might induce the
Fig. 5a. As the molar ratio of ZnAc : OAm increases to 1 : 2, an
2
emission at 430 nm is observed in the PL spectra, indicating
the heterogeneous nucleation of ZnSe in the solution. This
result indicates less Zn and Se for shell growth on the doped
nanorods, resulting in the low intensity and the blue-shift of
the Cu-doped emission. Both of them are confirmed in the
comparison with the PL spectra of the nanorods obtained
formation of Cu2 Se, as indicated from the color of the
−x
product. The emission of the doped nanorods could be
retained for weeks even in air, which paves the way for its
potential applications.
2
with a molar ratio of ZnAc : OAm at 1 : 1.2. Fig. 5b shows the
effect of the reaction temperature for the shell growth on the
optical properties of the doped nanorods. As this temperature
reaches 240 °C, the doped nanorods display obvious redshifts
both in the absorption spectra and in the PL spectra, which
could be attributed to the band-gap narrowing caused by the
particle growth. Meanwhile, the intensity of the dopant-
related emission decreases but that of the band-gap emission
increases. The results could be attributed to the hetero-
geneous nucleation of ZnSe at a high temperature and
Conclusions
In conclusion, ultrathin Cu-doped ZnSe nanorods are synthe-
sized by the adsorption of Cu ions on the surface of ultrathin
ZnSe nanorods, followed by shell growth. The XRD pattern
and the TEM images indicate that this doping does not affect
the crystal structure and unique morphology of ZnSe. The
good encapsulation of Cu inside the ultrathin ZnSe nanorods
has been confirmed by its characteristic emission over 480 to
2
+
the release of Cu ions into the solution due to the self-
6
,13
purification effect.
The latter is supported by the forma-
520 nm. The as-obtained doped nanorods in the solution
tion of Cu2−xSe (JCPDS card no. 06-0680) in the solution.
Fig. 6 shows the dependence of the actual molar ratio of
Cu/Zn and the quantum yield on the nominal ratio of Cu/Zn
in the reaction. It is noticed that the actual ratio of Cu/Zn in
the doped nanorods increases with the nominal content of
Cu ions in the solution. But the actual Cu/Zn ratios are
could show a quantum yield as high as 19%. But it drops
to 7% after purification, which could be attributed to
the removal of the surface ligands during the purification
process. The emission could be retained for weeks in air,
which opens a door for its applications in biological imaging,
sensitive detection and optoelectronic devices.
Acknowledgements
This work was supported by Natural Science Foundation
of China (no. 21071055, 51172076), New Century Excellent
Talents in University (NCET-10-0369), New Faculty Start-up
funding in Shandong University and Independent Innovation
Foundations of Shandong University (2012ZD007), Shandong
Provincial Natural Science Foundation for Distinguished
Young Scholar (JQ201205).
References
Fig. 5 UV-Vis absorption spectra (solid line) and PL spectra (dash line) of the
1
N. Pradhan, D. M. Battaglia, Y. C. Liu and X. G. Peng,
Nano Lett., 2007, 7, 312–317.
Cu-doped ZnSe nanorods prepared by (a) different molar ratios of ZnAc
b) different annealing temperatures.
2
/OAm or
(
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Paper
2
3
4
5
6
7
8
R. Thakar, Y. C. Chen and P. T. Snee, Nano Lett., 2007, 7,
429–3432.
V. Wood, J. E. Halpert, M. J. Panzer, M. G. Bawendi and
V. Bulovic, Nano Lett., 2009, 9, 2367–2371.
D. R. Gamelin, R. Beaulac, L. Schneider, P. I. Archer and
G. Bacher, Science, 2009, 325, 973–976.
S. K. Panda, S. G. Hickey, H. V. Demir and A. Eychmuller,
Angew. Chem., Int. Ed., 2011, 50, 4432–4436.
16 G. C. Xing, W. Ji, Y. G. Zheng and J. Y. Ying, Opt. Express,
2008, 16, 5710–5715.
17 L. S. Li, N. Pradhan, Y. J. Wang and X. G. Peng, Nano Lett.,
2004, 4, 2261–2264.
18 P. D. Cozzoli, L. Manna, M. L. Curri, S. Kudera, C. Giannini,
M. Striccoli and A. Agostiano, Chem. Mater., 2005, 17, 1296–1306.
19 M. D. Regulacio and M. Y. Han, Acc. Chem. Res., 2010, 43,
621–630.
20 T. T. Yao, S. F. Kou, Y. H. Sun, Q. Zhao and J. Yang,
CrystEngComm, 2012, 14, 8440–8445.
21 B. J. Xi, D. C. Xu, S. L. Xiong, C. M. Wang, X. M. Feng,
H. Y. Zhou and Y. T. Qian, J. Phys. Chem. C, 2008, 112,
5333–5338.
22 J. W. L. Yim, D. Chen, G. F. Brown and J. Q. Wu, Nano Res.,
2010, 2, 931–937.
23 T. T. Yao, Q. Zhao, Z. P. Qiao, F. Peng, H. J. Wang, H. Yu,
C. Chi and J. Yang, Chem.–Eur. J., 2011, 17, 8663–8670.
24 N. Pradhan, D. Goorskey, J. Thessing and X. G. Peng, J. Am.
Chem. Soc., 2005, 127, 17586–17587.
25 J. S. Han, H. Zhang, Y. Tang, Y. Liu, X. Yao and B. Yang,
J. Phys. Chem. C, 2009, 113, 7503–7510.
3
D. J. Norris, A. L. Efros and S. C. Erwin, Science, 2008, 319,
1
776–1779.
R. Viswanatha, S. Brovelli, A. Pandey, S. A. Crooker and
V. I. Klimov, Nano Lett., 2011, 11, 4753–4758.
S. Gul, J. K. Cooper, C. Corrado, B. Vollbrecht, F. Bridges,
J. H. Guo and J. Z. Zhang, J. Phys. Chem. C, 2011, 115,
2
0864–20875.
H. Y. Chen, T. Y. Chen and D. H. Son, J. Phys. Chem. C,
010, 114, 4418–4423.
0 B. B. Srivastava, S. Jana and N. Pradhan, J. Am. Chem. Soc.,
011, 133, 1007–1015.
1 N. Pradhan and X. G. Peng, J. Am. Chem. Soc., 2007, 129,
339–3347.
9
2
1
1
1
1
2
3
2 C. B. Murray, D. J. Norris and M. G. Bawendi, J. Am. Chem.
Soc., 1993, 115, 8706–8715.
3 D. A. Chen, R. Viswanatha, G. L. Ong, R. G. Xie,
M. Balasubramaninan and X. G. Peng, J. Am. Chem. Soc.,
26 G. B. Stringfellow and R. H. Bube, Phys. Rev., 1968, 171,
903–915.
27 J. F. Suyver, T. van der Beek, S. F. Wuister, J. J. Kelly and
A. Meijerink, Appl. Phys. Lett., 2001, 79, 4222–4224.
28 R. G. Xie and X. G. Peng, J. Am. Chem. Soc., 2009, 131,
10645–10651.
29 S. Jana, B. B. Srivastava, S. Acharya, P. K. Santra, N. R. Jana,
D. D. Sarma and N. Pradhan, Chem. Commun., 2010, 46,
2853–2855.
2
009, 131, 9333–9339.
1
4 N. S. Karan, D. D. Sarma, R. M. Kadam and N. Pradhan,
J. Phys. Chem. Lett., 2010, 1, 2863–2866.
1
5 H. B. Shen, H. Z. Wang, X. M. Li, J. Z. Niu, H. Wang,
X. Chen and L. S. Li, Dalton Trans., 2009, 10534–10540.
This journal is © The Royal Society of Chemistry 2013
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