Size and shape controlled ZnTe nanocrystals with quantum confinement effect

Article abstract of DOI:10.1039/b008376n

A simple one-pot synthesis of size and shape controlled ZnTe nanocrystals using a monomeric molecular precursor, [Zn(TePh)₂][TMEDA], has been studied by varying the growth temperature or the templating surfactants.

Full text of DOI:10.1039/b008376n

Size and shape controlled ZnTe nanocrystals with quantum confinement effect
Young-wook Jun, Chang-Shik Choi and Jinwoo Cheon*
Department of Chemistry and School of Molecular Science-BK21, Korea Advanced Institute of Science and
Technology (KAIST), Taejon 305-701, Korea. E-mail: jcheon@kaist.ac.kr
Received (in Cambridge, UK) 17th October 2000, Accepted 16th November 2000
First published as an Advance Article on the web
A simple one-pot synthesis of size and shape controlled
ZnTe nanocrystals using a monomeric molecular precursor,
[Zn(TePh)2][TMEDA], has been studied by varying the
growth temperature or the templating surfactants.
the zinc center adopts a distorted tetrahedral geometry with a
large Te–Zn–Te angle of 118.29(6)° and a small N–Zn–N angle
of 84.1(4)° (Fig. 1).27
A one-pot synthesis of ZnTe nanocrystals was carried out by
the thermolysis of [Zn(TePh)2][TMEDA]. According to TGA,
it is observed that the thermolysis of [Zn(TePh)2][TMEDA]
begins with the dissociation of the TMEDA donor ligand, with
ZnTe and Ph2Te produced in the following thermolysis step at
higher temperatures.28 As similarly observed by Yamamoto and
Steigerwald in related work on alkyl- or phenyl-chalcogenolate
ligand systems,12,29 the thermolysis of its complexes cleanly
produces the desired nanocrystals.
In a typical synthesis of spherical ZnTe nanocrystals, upon
injection of the precursor (0.50 g, 0.88 mmol) dissolved in 5 ml
of trioctylphosphine into the hot dodecylamine solvent (8.17 g,
44.1 mmol), immediate formation of nanocrystals was observed
by the appearance of a pale yellow color. The crystal growth
temperature was kept at either 180 or 240 °C for 2 h and the
resulting solution was pale yellow in both cases. The solution
was treated with butanol and centrifuged to isolate the yellow
nanocrystals as a solid product.
Using the same conditions and procedures, the injection of
the precursor into the mixed surfactant solvent trioctylamine (20
ml)–dimethylhexylamine (5 ml) and leads to a shape change of
the nanocrystals from spherical to rod-like and after 2 h a gray
precipitate of ZnTe nanocrystals was obtained from the initially
pale yellow solution at 180 °C. It is believed that the
combination of the two different surfactants provides rod-like
micelles during the one-dimensional crystal growth process.30
The obtained spherical ZnTe nanocrystals are moderately
monodispersed and their sizes can be controlled by changing the
growth temperature. Relative to the position of the 548 nm (2.26
eV) absorption band edge of bulk ZnTe, blue shifts of 0.53 and
1.31 eV are found for the nanocrystals grown at 180 and 240 °C,
respectively with larger shifts being seen for samples grown at
higher growth temperature (Fig. 2A).31 Similar blue shifts are
also observed in the photoluminescence spectra: band maxima
are 451 and 377 nm, respectively, for samples grown at 180 and
In recent years, nanomaterials have drawn enormous interest
from the scientific community because of their special charac-
teristics that are different from the bulk such as quantum
confined electronic band structures,1,2 novel optical,3 catalytic,4
and electronic properties.5 Since novel properties in nanoscale
materials depend on their size and shape, one of the frontier
issues of nanochemistry research is morphology controlled
synthesis of nanocrystals.
While sulfur and selenium based II/VI nanocrystals such as
CdS and CdSe are widely explored,6–9 there have not been
many reports on the preparation of metal telluride nanocrystals.
Furthermore, while there are some examples of Cd and Hg
based tellurides,10–14 studies on ZnTe are very limited. ZnTe, an
attractive semiconductor with a direct gap of 2.26 eV (ca. 548
nm) in the green region of the electromagnetic spectrum, can be
a useful material in several applications such as green light-
emitting diodes, buffer layers for HgCdTe IR detectors, or as the
first unit in a tandem solar cell.15 Until now, only two reports on
the preparation of colloidal ZnTe nanocrystals exist; however
the resulting nanocrystals were either large (length 500–1200
nm, width 30–100 nm)16 or only their optical properties were
investigated without isolation or characterization of the nano-
crystals.17 In fact, there have not been any reports on isolated
ZnTe nanocrystals < 10 nm. When ZnTe particles are on the
order of 10 nm, quantum size effects appear and control of the
optical properties is possible by simply changing the particle
size. It is then feasible to have tunability of the opto-electronic
properties from the green to the UV region.
In this report, we describe a simple one-pot synthesis of
morphology controlled ZnTe nanocrystals using a single
molecular precursor, [Zn(TePh)2][TMEDA]. Upon thermol-
ysis, this precursor produces ZnTe nanocrystals which are either
spherical or of rod-like structure depending on the growth
conditions and the choice of stabilizing surfactants. Fur-
thermore, the size of the spherical ZnTe nanocrystals is
controlled by the growth temperature and quantum confinement
effects are observed. To the best of our knowledge, this paper
reports the first isolated and well characterized ZnTe nano-
crystals with quantum confined properties.
Even though metal chalcogenolato complexes have been
widely studied previously as single-source precursors to group
12 chalcogenide semiconductor materials,6,12,18 the develop-
ment of zinc tellurolate molecular chemistry is still limited.19–21
A pyridine adduct of a mesityltellurolate zinc complex prepared
by Bochmann et al.22 and the dimeric compound [Zn{TeSi-
(SiMe3)3}2]2 prepared by Bonasia and Arnold23 have been
known to generate ZnTe upon pyrolysis. In our study, we
synthesized a polymeric phenyltellurolate zinc complex using a
dealkylsilation process that was similarly employed to obtain a
organotellurolato cadmium compound.24 The reaction of dime-
thylzinc with 2 equiv. of PhTeSiMe3 gives the pale yellow
product Zn(TePh)2, which was then reacted with TMEDA to
give [Zn(TePh)2][TMEDA].25 X-Ray crystallographic studies
of [Zn(TePh)2][TMEDA]26 shows that it is monomeric and that
Fig. 1 ORTEP drawing of [Zn(TePh)2][TMEDA]. Selected bond distances
(Å) and angles (°): Te(1)–Zn(1) 2.5769(14), Te(2)–Zn(1) 2.5876(15),
Zn(1)–N(1) 2.126(9), Zn(1)–N(2) 2.145(9); Te(1)–Zn–Te(2) 118.29(6),
N(1)–Zn(1)–N(2) 84.1(4).
DOI: 10.1039/b008376n
Chem. Commun., 2001, 101–102
This journal is © The Royal Society of Chemistry 2001
101

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240 °C (Fig. 2B). These results suggest that smaller nano-
crystals are produced at higher growth temperatures where more
nucleation sites exist and relatively less available ZnTe material
is present for each nucleus during the growth process.
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Fig. 2 Optical spectra of ZnTe nanocrystals grown at (a) 240 and (b) 180 °C;
(A) UV–VIS absorption spectra, (B) Photoluminescence spectra.
High resolution transmission electron micrographs
(HRTEM) show that the spherical ZnTe nanocrystals have
average sizes of 4.2 (±1.1) and 5.4 (±0.9) nm for samples grown
at 240 and 180 °C, respectively (Fig. 3A). Powder X-ray
diffractometry (XRD) and selected area diffractometry (SAED)
reveal patterns corresponding to (111), (220) and (311) of the
cubic phase of ZnTe nanocrystals (Fig. 3C, D). This result is
similar to that of TOPO-capped spherical ZnSe nanocrystals
described in a previous report.31 TEM of the rod-like ZnTe
nanocrystals show that the diameters of the rod-like nano-
crystals are quite uniform (ca. 25 nm) with lengths of several
hundred nanometers (200–700 nm) giving an aspect ratio from
8 to 30 (Fig. 3B). The rod-like ZnTe nanocrystals are also cubic
phase as confirmed by XRD and SAED analysis.
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25 TMEDA (2.56 g, 22.0 mmol) was slowly added to Zn(TePh)2 (6.97 g,
14.7 mmol) in toluene (100 ml) and the reaction mixture was stirred for
24 h. After pyridine (10 ml) and heptane (50 ml) were added, insolubles
were filtered off and the filtrate was concentrated and recrystallized at
224 °C to give colorless needle-shaped crystals. (6.02 g, 72.2%), mp
122–124 °C, Anal. Calc. for C18H26N2Te2Zn: C, 36.6; H, 4.40; N, 4.74;
Te, 43.2; Zn, 11.1. Found: C, 36.6; H, 4.50; N, 4.70; Zn, 11.0%.
dH(CDCl3, 25 °C): 7.78 (d, 4H), 7.04 (t, 2H), 6.89 (t, 4H), 2.62 (s, 4H),
2.48 (s, 12H).
26 Crystal data: C18H26N2Te2Zn, Mr = 590.98, monoclinic, space group
P21/n, a = 8.888(1), b = 17.866(3), c = 14.016(2) Å, b = 103.10(1)°,
U = 2167.8(5) Å3, Z = 4, Dc = 1.811 g cm23, F(000) = 1128, m(Mo-
Ka) = 3.772 mm, R1 = 0.0616, wR2 = 0.1659. CCDC 182/1854. See
http://www.rsc.org/suppdata/cc/b0/b008376n/ for crystallographic files
in .cif form.
27 The Te–Zn–Te angles are slightly smaller than those reported for related
compounds with larger ligands such as [Zn(mesityl)2(py)2] and
[Zn{TeSi(SiMe3)3}2(py)2], (126.9 and 131.9°, respectively). The steric
repulsions between these bulkier ligands are responsible for the larger
angles as compared to compact phenyl ligands. This trend is clearly
shown by the gradual increase of angle from 118.29 to 126.9 to 131.9°
as the ligand size increases from phenyl to mesityl to sitel. The Zn–Te
and Zn–N bond lengths are 2.5822(65) and 2.136(5) Å which are almost
identical to those of [Zn{TeSi(SiMe3)3}2(py)2] and [Zn(mesityl)2(py)2]
and similar to those seen for other zinc complexes with organochalco-
gen and amine ligands.19–21
28 In TGA, we observed that the dissociation of TMEDA and the
generation of Ph2Te and ZnTe occur at 158 and 178 °C, respectively.
29 K. Osakada and T. Yamamoto, J. Chem. Soc., Chem. Commun., 1987,
1117.
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Dedieu, Langmuir, 1998, 14, 7359; Y. D. Li, H. W. Liao, Y. Ding, Y. T.
Qian, L. Yang and G. E. Zhou, Chem. Mater., 1998, 10, 2301; C. C.
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31 Y. Jun, J. Koo and J. Cheon, Chem. Commun., 2000, 1243.
Fig. 3 (A) HRTEM analysis of spherical ZnTe nanocrystals grown at
240 °C, (B) TEM analysis of rod-like ZnTe nanocrystals, (C) powder X-ray
diffraction and (D) selected area diffraction patterns of 4.2 nm ZnTe
nanocrystals.
In conclusion, the results here constitute a simple and
convenient one-pot synthesis of morphology controlled ZnTe
nanocrystals using
a
monomeric molecular precursor,
[Zn(TePh)2][TMEDA]. By varying the growth temperature or
the choice of the templating surfactants, the size and shape of
the nanocrystals are controllable and quantum size effects are
observed. We believe that this strategy can be extended to the
facile synthesis of nanocrystals of other materials.
This work was supported by the Tera Level Nanodevices
National Program of KISTEP. We thank KBSI for the TEM
analyses and Professor S. J. Kim and Dr Y.-M. Kim of Ewha
Womans’ University for X-ray crystallographic analysis of the
sample.
Notes and references
1 N. Chestnoy, R. Hull and L. E. Brus, J. Chem. Phys., 1986, 85, 2237.
2 L. E. Brus, J. Chem. Phys., 1984, 80, 4403.
102
Chem. Commun., 2001, 101–102