A cluster growth route to quantum-confined CdS nanowires

Article abstract of DOI:10.1039/a901821b

Quantum-confined CdS nanowires with diameters around 4 nm and lengths ranging from 150 to 250 nm were grown for the first time from cadmium bis(diethyldithiocarbamate) [Cd(DDTC)₂]₂ by removal of the four thione groups with ethylenediamine (en) at 117°C for 2 min.

Full text of DOI:10.1039/a901821b

A cluster growth route to quantum-confined CdS nanowires
Ping Yan, Yi Xie,* Yitai Qian and Xianming Liu
Structure Research Laboratory, and Department of Chemistry, University of Science and Technology of China, Hefei,
Anhui, 230026, P.R. China. E-mail: yxie@ustc.edu.cn
Received (in Cambridge, UK) 8th March 1999, Accepted 29th April 1999
Quantum-confined CdS nanowires with diameters around
4 nm and lengths ranging from 150 to 250 nm were grown for
the first time from cadmium bis(diethyldithiocarbamate)
[Cd(DDTC)2]2 by removal of the four thione groups with
ethylenediamine (en) at 117 °C for 2 min.
Semiconductor crystallites that are small in comparison to the
bulk exciton diameter show quantum confinement effects,
which have been intensely studied, and a number of good
reviews have mirrored this development.1,2 Various methods
for the production of II–VI and III–V nanocrystals with narrow
size distributions in the desired size range of between 1 and 15
nm in diameter have been developed.3
Special attention has been paid to one-dimensional struc-
tures, such as nanotubes and nanowires owing to their
interesting properties.4 However the challenge of fabricating
such 1D structures is substantial because technologically useful
Fig. 1 XRD pattern for CdS nanowires.
quantum wires require lateral dimensions of < ca. 10 nm. Such
a precision level is a formidable technological challenge.5
error, fit that of bulk hexagonal CdS.10 The (002) diffraction
Recently very long CdTe nanowires6 were fabricated by the
peak, the second strongest peak in bulk hexagonal CdS, is
molecular-beam epitaxy (MBE) growth technique, however,
unusually narrow and strong, indicating a preferential growth
their diameters are much larger than 10 nm. Considering the
along the c axis in the product. Using the Scherrer formula11 on
the (hk0) peaks (100) (110) and (200), the average diameter of
the nanowires is estimated as 4.0 nm. This is confirmed by
small exciton diameter of CdS (6 nm), the preparation of
quantum confined CdS nanowires (of diameter < 6 nm) is even
more difficult. A promising route is deposition of semi-
transmission electron microscopy (TEM)‡ images (Fig. 2),
conductors into anodic aluminium oxide membrane filters or
which reveal straight-line shapes and uniform diameters of ca.
4 nm and lengths ranging from 150 to 250 nm. IR spectroscopy
nuclear track-etched polycarbonate membranes with small
pores; the smallest semiconductor nanowire obtained by this
was carried out to examine the purity of the product, and
method to date is 9 nm.7
indicated the absence of both DDTC and ethylenediamine,
Here, quantum confined CdS nanowires were successfully
suggesting a high degree of purity.
obtained for the first time from
a cluster precursor
Fig. 3(a) shows the absorption spectrum of a sample
dispersed in distilled water. The clear maximum at 455 nm is
assigned to the optical transition of the first excitonic state.2 The
average particle diameter, as calculated from the maximum of
the absorption curve (lm = 455 nm) is 3.8 nm2 in good
agreement with the TEM and XRD results. Since the onset of
the absorption spectrum generally represents the larger end of
Cd2(S2CNEt2)4, which can be regarded as an inorganic core
[Cd2S2] with four capping groups.8 Nucleophilic attack by
ethylenediamine at the thione carbon can lead to removal of the
capping groups. This process is shown in Scheme 1 and the
resulting organic product has been characterized.9
The inorganic Cd2S2 cores can then combine with each other
and under appropriate chemical and physical environment,
[Cd2S2] may preferentially grow along a unique direction and
result in a one-dimensional structure.
In a typical process, 1 g of [Cd(S2CNEt2)]2, prepared by
precipitation from a stoichiometric mixture of NaS2CN(C2H5)2
and CdCl2 in water, was dissolved in 30 ml ethylenediamine in
a flask, and then heated to reflux (117 °C), and maintained at
this temperature for 2 min. The yellowish precipitate obtained
was filtered off and washed with ethanol.
The X-ray diffraction (XRD)† pattern of the nanowires is
shown in Fig. 1. All the reflections, to within experimental
NH2
Et2N
NH2
C
S
S
NH2
S
Et2N
NH2
390 K
S
C
4
HN
NH + 4Et2NH + 2H2S + [Cd2S2]
NH2
NEt2
Cd
Cd
C
S
S
C
S
NH2
S
S
NH2
NEt2
NH2
Scheme 1
Fig. 2 TEM images of the nanowires (scale bar, 50 nm).
Chem. Commun., 1999, 1293–1294
1293

intermediate inorganic [Cd2S2] core. Ethylenediamine has been
extensively used as a template in solvothermal processes. For
example, Li et al.14 reported that elemental reaction of Cd and
S in ethylenediamine resulted in CdS nanorods with diameters
of 25–40 nm14 although no detailed mechanism was pro-
posed.
In summary, quantum-confined CdS nanowires with diame-
ters of ca. 4 nm and lengths ranging from 150 to 250 nm have
been grown for the first time from [Cd(DDTC)2]2, by removal
of the four thione groups with ethylenediamine (en) at 117 °C
for 2 min. Further studies may extend the method for the
preparation of other quantum-confined nanowires.
Financial support from the Chinese National Foundation of
Natural Science Research through the Outstanding Youth
Science Fund and Huo Yingdong Foundation for Young
Teachers is gratefully acknowledged. This work is also
supported by the Climbing Plan from the State Science and
Technology Commission of China.
Notes and references
Fig. 3 (a) Absorption spectrum of a CdS sample dispersed in H2O. (b)
Excitation (solid line) and photoluminescence (dashed line) spectra of CdS
nanowires. The spectra were taken in reflection geometry. The excitation
wavelength for the emission spectrum was 370 nm, and the monitoring
wavelength for the excitation spectrum was 440 nm.
† XRD patterns were obtained on a Japan Rigaku D/Max gA rotation anode
X-ray diffractometer with Ni-filtered Cu-Ka radiation (l = 1.54178 Å).
‡ TEM measurements were made on a Hitachi H-800 transmission electron
microscope with an accelerating voltage of 200 kV.
1 A. P. Alivisatos, Science, 1996, 271, 933.
the size distribution,12 the position of the absorption edge (le =
515 nm) shows that the lengths of the nanowires (150–250 nm)
are much larger than the exciton diameter of CdS (6 nm). The
large difference between sizes calculated from le and lm, which
can be related to the width of the particle size distribution, is
characteristic for nanowires. The clear appearance of a blue-
shift of the absorption peak relative to bulk CdS indicates that
the CdS nanowires are quantum-confined. The excitation
spectrum (monitoring wavelength at 440 nm) shows absorption
bands at 290 and 370 nm [Fig. 3(b)]. Under photoluminescent
excitation at 370 nm, the nanowires emit blue light at 440 and
460 nm [Fig. 3(b)] with a 55 nm blue shift relative to bulk CdS.
These features are close to the optical absorption edge of CdS,
suggesting a near band-edge emission.
When [Cd(DDTC)2]2 is thermolyzed at 250 °C for 30 min in
trioctylphosphine oxide (TOPO), the morphology for most of
the CdS nanocrystallites was reported to be thin plates.13 In our
experiments, other nucleophiles such as pyridine and diethyla-
mine were also tested to remove the capping groups, however,
no wire-like products were obtained. These results show that
ethylenediamine plays a critical role in the formation of wire
structures and probably serves as a director for the growth of the
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Chem. Commun., 1999, 1293–1294