One-step synthesis of size tuned zinc selenide quantum dots via a temperature controlled molecular precursor approach

Article abstract of DOI:10.1039/b002983l

One-step size-controlled synthesis of ZnSe quantum dots is studied and the obtained QDs are luminescent with the emission wavelength varying over a wide range (up to 100 nm) depending on the particle size; the single- molecular precursor is an air-stable

Full text of DOI:10.1039/b002983l

One-step synthesis of size tuned zinc selenide quantum dots via a temperature
controlled molecular precursor approach
Young-wook Jun, Ja-Eung Koo 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) 13th April 2000, Accepted 30th May 2000
Published on the Web 20th June 2000
One-step size-controlled synthesis of ZnSe quantum dots is
studied and the obtained QDs are luminescent with the
emission wavelength varying over a wide range (up to 100
nm) depending on the particle size; the single-molecular
precursor is an air-stable bis(phenylselenolato)zinc
N,N,NA,NA-tetramethylethylenediamine (TMEDA) complex,
which effectively affords different sizes of ZnSe QDs
depending on growth temperatures.
Nanomaterials are of great interest owing to the novel optical,
electronic and catalytic properties that arise from the quantum
size effects and large surface areas that are characteristic of
these species. In particular, quantum dots (QDs) of semi-
conducting materials have received special attention because
their electronic band gaps can be tuned, thus varying their
N(1) 2.120(9), Zn(1)–N(2) 2.147(9); Se(1)–Zn–Se(2) 125.54(6), N(1)–
Fig. 1 ORTEP drawing of Zn(SePh)2(TMEDA). Selected bond distances
(Å) and angles (°): Se(1)–Zn(1) 2.398(2), Se(2)–Zn(1) 2.399(2), Zn(1)–
optical response from the IR to the UV depending on the size of
the dots.1–5 The tunability of the band gap makes QDs useful for
many applications such as light emitting diodes5 and as
ultrasmall luminescent tags for biological studies.6
Zn(1)–N(2) 86.3(3).
One-step size controlled synthesis of QDs was carried out by
the thermolysis of Zn(SePh)2(TMEDA) (eqn. 1).17,18
Bulk materials that absorb or emit in the blue to near-UV
regions of the electromagnetic spectrum are being extensively
studied owing to their potential uses in optical sensors and
lasers.7,8 The best known bulk semiconductors with band gaps
in this region are GaN (Egap = 3.4 eV) and ZnSe (Egap = ca. 2.7
eV). Synthetic studies on QDs of these two materials with
decent electronic band tunings, however, have been very
limited.9,10 For isolated colloidal ZnSe QDs, the use of a
diselenocarbamate single-source precursor and a dual source
precursor system are two successful demonstrations.10
One of the important current issues of nanomaterials research
is the controlled synthesis of QDs. In many cases, however,
successful nanostructured materials syntheses1,11 with desired
size and shape criteria, require multi-step processes and thus
developing a simple and controlled method of synthesizing QDs
is of particular interest.
Here we present a one-step synthesis of ZnSe QDs the sizes
of which can be precisely tuned by simple temperature control.
We demonstrate that these are luminescent in the blue region
and that the emission wavelength varies over a very wide range
(up to 100 nm) depending on the particle size. The key to this
synthesis is the proper choice of a molecular precursor which
has good solubility, adequate thermal stability, and simple
ligand elimination processes that produce desired materials.
Notably, our obtained QDs have a high monodispersity without
size selective precipitation.
[Zn(SePh)2][TMEDA]æhææeatÆZnSe + TMEDA + Ph2Se
(1)
In a typical QD synthesis, Zn(SePh)2(TMEDA) (0.5 g, 0.101
mmol) was dissolved in trioctylphosphine (10 ml) and the
resulting solution injected into hot trioctylphosphine oxide
(3.92 g, 10.1 mmol). The latter solution is kept at one of four
different temperatures: 320, 340, 367 or 385 °C. After 1 h, the
resulting yellow solution was cooled to 80 °C and treated with
an excess of methanol to generate a yellow flocculate, which is
separated by centrifugation and washed with methanol. The
resulting pale yellow powder was readily redispersed in toluene.
No further size selection is carried out.
The QDs obtained are highly monodispersed with sizes that
depend on the growth temperature. Relative to the position of
the 460 nm (2.7 eV) absorption band edge for bulk ZnSe, the
absorption band edges of the ZnSe QDs are blue-shifted. Larger
shifts are seen for higher growth temperatures. Thus, the
absorption band shifts are 0.95, 0.37, 0.29 and 0.17 eV for QDs
grown at 385, 367, 340 and 320 °C, respectively. Similar blue
shifts are also observed in the photoluminescence spectra: the
band maxima are 387 (3.20 eV), 429 (2.89 eV), 443 (2.80 eV)
and 451 nm (2.75 eV) for samples grown at 385, 367, 340 and
320 °C, respectively (Fig. 2). These results show that smaller
quantum dots are obtained at higher growth temperatures where
more nucleation sites are present and relatively less ZnSe
material is available for each nucleus during growth kinetics.
High resolution transmission electron micrographs show that
the ZnSe QDs are roughly spherical and that the particles within
a single sample have relatively uniform sizes (Fig. 3). The
average sizes of the ZnSe QDs are 2.7 ± 0.2, 4.0 ± 0.35 nm, 4.4
± 0.35 and 4.9 ± 0.29 nm for samples grown at 385, 367, 340,
320 °C, respectively (Fig. 2C). The ZnSe particles are in the
cubic phase, as determined by X-ray diffraction and selected
area electron diffraction.18 Energy dispersive X-ray emission
analysis of the QDs confirms that the particles have a 1 1 Zn Se
stoichiometry.
The compound bis(phenylselenolato)zinc, Zn(SePh)2,12 is
polymeric and poorly soluble in organic solvents, and is thus not
particularly convenient as a precursor for the synthesis of QDs.
We observe, however, that the polymer reacts with N,N,NA,NA-
tetramethylethylenediamine (TMEDA) to form a soluble, air-
stable, monomeric adduct of stoichiometry Zn(SePh)2-
(TMEDA) (Fig. 1).13–15 As similarly observed by Yamamoto
and Steigerwald in related work on alkyl- or phenyl-chalcoge-
nolate ligand systems, this ligand is more thermally stable than
other bulky selenolate ligands such as SeSi(SiMe3)3, and
thermolysis of its complexes produces clean QDs with reduced
contamination caused by undesired ligand fragmentation
processes.16
The results above demonstrate that monodispersed ZnSe QDs
can be prepared by means of a simple and convenient one-pot
DOI: 10.1039/b002983l
Chem. Commun., 2000, 1243–1244
This journal is © The Royal Society of Chemistry 2000
1243

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Fig. 2 Optical spectra and size distribution of ZnSe QDs grown at (a) 385
(b) 367 (c) 340 (d) 320 °C. (A) UV–VIS absorption spectra, (B)
photoluminescence spectra and (C) size distribution.
12 Zn(SePh)2 was prepared according to a modification of literature
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1997, 31, 19). Selenophenol (2.14 g, 13.54 mmol) was slowly added to
Zn[N(SiMe3)2]2 (2.61 g, 6.76 mmol) in toluene (50 ml) for 30 min. A
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temp. for 12 h, after which heptane (10 ml) was added. The precipitate
was collected by filtration, washed with heptane (5 ml), and dried in
vacuo, to give a white powder (2.32 g, 90%).
13 TMEDA (1.07 g, 9.17 mmol) was added to a suspension of Zn(SePh)2
(1.81 g, 6.11 mmol) in toluene (50 ml). The mixture was stirred for 24
h to give a pale yellow solution, which was filtered, treated with pyridine
(5 ml) and heptane (30 ml), and cooled to 224 °C to give colorless
crystals (1.765 g, 59%), mp 137–138 °C. Anal. Calc. for
C
18H26N2Se2Zn: C, 43.8; H, 5.27; N, 5.67. Found: C, 43.1; H, 5.34; N,
5.37%. H(CDCl3, 25 °C): 7.90 (t, 2H, JHH 7 Hz, o-CH), 6.86 (m, 3H,
m-CH and p-CH), 2.67 (s, 2H, NCH2), 2.48 (s, 6H, NMe2).
14 An X-ray crystallographic study of Zn(SePh)2(TMEDA) (Fig. 1) shows
that the zinc center adopts a distorted tetrahedral geometry with a large
Se–Zn–Se angle of 125.55° and a small N–Zn–N angle of 86.33°. The
Zn–Se and Zn–N bond lengths are 2.398 and 2.134 Å, respectively. The
bond lengths and angles are similar to those seen for other zinc
complexes with organochalcogen and amine ligands.15 Crystal data:
C22H16N2Se2Zn, Mr = 531.69, monoclinic, space group P21/c, a =
11.202(2), b = 12.7326(14), c = 15.125(3) Å, = 107.050(18)°, U =
2062.5(6) Å3, Z = 4, Dc = 1.340 g cm23, F(000) = 1040, (Mo-Ka)
= 47.2 cm21, R1 = 0.0623, wR2 = 0.1634. CCDC 182/1665. See
http://www.rsc.org/suppdata/cc/b0/b002983l/ for crystallographic files
in .cif format.
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16 Molecular precursor strategies with chalcogenolate ligands have been
similarly used to prepare other II/VI semiconducting materials. See: (a)
J. G. Brennan, T. Siegrist, P. J. Carrol, S. M. Stuczynski, L. E. Brus and
M. L. Steigerwald, J. Am. Chem. Soc., 1989, 111, 4141; (b) J. G.
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17 Upon themolysis of Zn(SePh)2(TMEDA), the generation of Ph2Se was
identified by 1H NMR spectroscopy.
Fig. 3 HRTEM images of ZnSe QDs grown at 340 °C.
synthesis from an air-stable monomeric molecular precursor.
The growth of QDs follows a simple ligand elimination reaction
and by varying the growth temperature it is possible to control
their size. The results constitute a good demonstration of
controlling the size of semiconductor QDs with band gaps in the
blue region of the electromagnetic spectrum. We believe that
this strategy can be extended to facile size controlled synthesis
of QDs of other materials that, at present, are difficult or
complicated to prepare.
This work was supported by the KOSEF (1999-1-122-001-5)
and we thank KBSI and KRISS for the TEM analyses.
Notes and references
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18 X-Ray diffraction spectra show three broad peaks at 2 = 27.41 (111),
48.04 (200) and 68.76° (311), similar to the cubic phase observed in
CdSe.4 Interestingly, the synthesis of hexagonal ZnSe QDs has
previously been reported.10a
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Chem. Commun., 2000, 1243–1244