A solvothermal route to capped CdSe nanoparticles
Ujjal K. Gautam,a Michael Rajamathi,b Fiona Meldrum,c Peter Morgand and Ram Seshadria
a Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012 India.
E-mail: seshadri@sscu.iisc.ernet.in
b Department of Chemistry, St. Joseph’s College, Bangalore 560 025, India
c
Department of Chemistry, Queen Mary and Westfield College, University of London, Mile End Road, London, UK
E1 4NS. E-mail: f.c.meldrum@qmw.ac.uk
d Rockwell Science Center, 1049 Camino Dos Rios, Thousand Oaks, CA 91360, USA.
E-mail: pemorgan@rsc.rockwell.com
Received (in Cambridge, UK) 23rd November 2000, Accepted 28th February 2001
First published as an Advance Article on the web 14th March 2001
We present a convenient and safe one-pot route to capped 3
nm CdSe nanoparticles making use of common starting
materials and inexpensive, low-boiling solvents under sol-
vothermal conditions; H2Se required for the reaction is
generated in situ through the aromatization of tetralin by
Se.
wurtzite structure. Stacking 3 nm 3 3 nm slabs of CdSe along
the c axis in hexagonal and cubic (in different proportions)
arrangements permits simulations using the DIFFaX8 program,
which confirms a cubic zinc blende structure relatively free
from hexagonal defects. The Rietveld fit and cubic and
hexagonal DIFFaX simulations (using a correlation length of
about 3 nm in the c direction) are also displayed in Fig. 2(a).
The UV–VIS absorption spectrum in toluene [Fig. 2(b)]
shows absorption peaks at 514 and 412 nm, consistent with
quantum confinement. Bawendi, Steigerwald and Brus9 have
discussed the correlation of absorption spectra and particle
diameter for various nanoparticles. According to their correla-
tion, the particle size corresponding to the spectrum in Fig. 2(b)
is close to about 2.5 nm and not 3.0 nm as obtained here from
TEM. The discrepancy could arise from (i) the different crystal
structure obtained in the present case and (ii) the difficulty in
estimating sizes from the very small, low-contrast particles seen
in the TEM image. Toluene solutions of the nanoparticles also
show strong photoluminescence as seen from the emission
spectrum displayed in Fig. 2(b) as a dashed line.
In semiconductor nanocrystals of materials such as CdSe, the
happy union of bulk and molecular properties results in a rich
photophysics that is not only interesting in its own right, but also
lends itself to a number of applications.1 One such application
is the use of CdSe nanocrystals as fluorescent probes in
biological imaging.2 The key to all semiconductor nanoparticle
research is the ability to prepare stable (typically through
surface passivation – also referred to as capping) monodisperse
particles that have very few defects. A number of techniques
have been used to achieve this and have recently been reviewed
in this journal.3 Perhaps most widely used is the method
introduced by Bawendi and coworkers4 which involves the
reaction of an organocadmium precursor with an Se source in a
high temperature solvent such as trioctyl phosphine oxide
(TOPO), which also doubles as a capping agent.
We have been interested in the preparation of chalcogenide
nanoparticles from inexpensive starting materials of low
toxicity. Cadmium stearate is easy to prepare† and dissolves in
organic solvents such as toluene, in which it is known to react
with H2Se to yield chalcogenides. Since relatively high
temperatures are required to prepare defect-free, monodisperse
nanoparticles, we have carried out the synthesis in toluene using
stainless steel bombs. H2Se is not easily handled under these
conditions, so we have employed the aromatization of tetralin to
naphthalene by elemental Se in order to prepare H2Se in situ.
The addition of small amounts of dodecanethiol as a capping
agent results in 3 nm nanoparticles with a narrow size
distribution. The dark solution obtained from the reaction
precipitates solid products when the solvent polarity is in-
creased through the addition of propan-2-ol. The precipitated
solid easily redissolves in toluene forming bright orange
solutions.
The presence of the thiol capping was verified from C–H
stretches in the FTIR spectrum, as well as by thermogravimetry
in air. We observe a weight loss of 35% before 400 °C which,
if ascribed to the thiol cap, suggests that for 3 nm particles there
is one thiol molecule for every 22 Å2 of nanoparticle surface.
Our procedure is inspired by the simple method of Mitchell
and Morgan5 who prepared CuCr2Se4 in mineral oil at 330 °C
using stearate salts as the metal source and the aromatization of
sitosterol as the H2Se source. Qian and coworkers6 have
recently reported the preparation of 7 nm CdSe nanoparticles
under solvothermal conditions using ethylenediamine at
120 °C.
Fig. 1 displays a TEM image of the nanoparticles. While a
few large particles (ca. 10 nm) are occasionally seen, the vast
majority of the particles are smaller; with a mean diameter of
3.0 nm and a standard deviation of 0.16 nm (ca. 5% of the
mean). The powder XRD profile of the nanoparticles acquired
in transmission mode is displayed in Fig. 2(a). We find that the
profile is well fitted by the Rietveld7 method to the cubic (a =
6.01(1) Å) zinc blende structure rather than the usual hexagonal
Fig. 1 TEM image of a relatively dense arrangement of CdSe nanoparticles
showing a tendency to close packing in the plane (bar = 50 nm). The inset
shows a histogram of particle sizes.
DOI: 10.1039/b009394g
Chem. Commun., 2001, 629–630
This journal is © The Royal Society of Chemistry 2001
629
Gautam
