Multipulse electrochemical/chemical synthesis of CdS/S core/shell nanocrystals exhibiting ultranarrow photoluminescence emission lines

Article abstract of DOI:10.1021/jp991141u

Ensembles of sulfur capped, cadmium sulfide nanocrystals (CdS/S NCs) which exhibit photoluminescence (PL) emission line widths of 15-35 meV at 20 K have been obtained. These CdS/S NCs were synthesized using a new variant of the electrochemical/chemical (E/C) method: Cadmium (Cd°) NCs were first electrodeposited from an aqueous Cd²⁺ plating solution using a train of 8-10 ms plating pulses separated by ≈1.0 s mixing segments at the open circuit potential. These Cd° nanoparticles were then oxidized to Cd-(OH)₂, and CdS/S NCs were obtained by exposure of Cd(OH)₂ nanoparticles to H₂S at 300°C. Ensembles of 200 000-400 000 CdS/S NCs prepared using this multipulse technique exhibited PL emission lines narrower than 35 meV. CdS/S NCs were also synthesized using the same E/C method except that a single Cd° plating pulse was employed, and these CdS/S NCs exhibited PL emission line widths of 125-180 meV. TEM analysis of Cd(OH)₂ precursor particles confirms that the narrower lines obtained using multipulse Cd° electrodeposition result from improved size monodispersity of the CdS core, which is attributed to the diffusional decoupling of Cd° nanoparticles during growth.

Full text of DOI:10.1021/jp991141u

5750
J. Phys. Chem. B 1999, 103, 5750-5753
“
Multipulse” Electrochemical/Chemical Synthesis of CdS/S Core/Shell Nanocrystals
Exhibiting Ultranarrow Photoluminescence Emission Lines
†
S. Gorer and R. M. Penner*
Institute for Surface and Interface Science and Department of Chemistry, UniVersity of California, IrVine,
IrVine, California 92679-2025
ReceiVed: April 5, 1999; In Final Form: May 21, 1999
Ensembles of sulfur capped, cadmium sulfide nanocrystals (CdS/S NCs) which exhibit photoluminescence
(
PL) emission line widths of 15-35 meV at 20 K have been obtained. These CdS/S NCs were synthesized
o
using a new variant of the electrochemical/chemical (E/C) method: Cadmium (Cd ) NCs were first
electrodeposited from an aqueous Cd plating solution using a train of 8-10 ms plating pulses separated by
2
+
o
≈
1.0 s “mixing” segments at the open circuit potential. These Cd nanoparticles were then oxidized to Cd-
(
OH) , and CdS/S NCs were obtained by exposure of Cd(OH) nanoparticles to H S at 300 °C. Ensembles
2
2
2
of 200 000-400 000 CdS/S NCs prepared using this “multipulse” technique exhibited PL emission lines
narrower than 35 meV. CdS/S NCs were also synthesized using the same E/C method except that a single
o
Cd plating pulse was employed, and these CdS/S NCs exhibited PL emission line widths of 125-180 meV.
TEM analysis of Cd(OH)
2
precursor particles confirms that the narrower lines obtained using multipulse Cdo
electrodeposition result from improved size monodispersity of the CdS core, which is attributed to the diffusional
o
decoupling of Cd nanoparticles during growth.
I. Introduction
SCHEME 1. The Electrochemical/Chemical (E/C)
Synthesis of CdS/S Core/Shell Nanocrystals
Most of what is currently known regarding the size-dependent
electronic properties of semiconductor nanocrystals has been
learned by investigating suspensions of semiconductor nano-
crystals which are narrowly dispersed in size (refs 1-5 are
relevant review articles). These suspensions are usually obtained
using variants of a technique pioneered in the 50s by Victor
6
LaMer in which colloid particles are nucleated in a temporally
discrete fashion, and then grown at diffusion control to the
7
desired final mean diameter. This technique leads to very
narrow particle size distributions because, as demonstrated by
8
Reiss in 1954, small particles grow faster than larger particles.
SCHEME 2. Two Voltage Pulse Programs Employed
for the Electrodeposition of Cadmium Nanoparticles in
This Work
We have recently shown that this same strategy (i.e.,
instantaneous nucleation and diffusion controlled growth) does
not lead to good size monodispersity for the growth of colloid
9,10
particles on surfaces from solution-phase precursors. Brown-
ian dynamics simulations11 demonstrate that the growth of
nearest neighbors on the surface are diffusionally coupled, and
the growth law for every particle is therefore based on the
number and proximity of its neighbors, not simply on the
diameter of the particle as in colloid suspensions. Ngo and
Williams1 have predicted that reduction or elimination of
this diffusional coupling leads immediately to improved par-
ticle size monodispersity for particles which nucleate instanta-
neously.
2
In this letter, we describe a technique for dramatically
improving the size monodispersity for cadmium sulfide/sulfur
core/shell nanocrystals (henceforth CdS/S NCs) prepared on
graphite surfaces using the electrochemical/chemical (E/C)
a train of 8-12 ms deposition voltage pulses separated by 1-5
13-17
method
shown in Scheme 1. Improved size monodispersity
s “mixing” segments during which growth is suspended (Scheme
was achieved by depositing cadmium precursor particles using
2
2
b) instead of a single, longer deposition pulse (e.g., Scheme
a). The effect of these shorter deposition pulses and mixing
*
Direct correspondence to rmpenner@uci.edu.
Present address: Symyx Technologies, 3100 Central Expressway, Santa
†
segments is to diffusionally decouple the growth of densely
nucleated regions on the graphite surface from sparsely nucle-
Clara, California 95051.
1
0.1021/jp991141u CCC: $18.00 © 1999 American Chemical Society
Published on Web 06/24/1999

Letters
J. Phys. Chem. B, Vol. 103, No. 28, 1999 5751
ated regions.11 Improvements in the size monodispersity of
cadmium NCs translate into a narrowing of the PL emission
lines for CdS/S NCs synthesized via the method of Scheme 1
using the multipulse approach, as well as increased spatial
homogeneity of the mean size on the graphite surface. CdS/S
NCs synthesized from cadmium NCs deposited using a single
plating pulse exhibit photoluminescence (PL) emission line
1
3,14,16
widths of 125-180 meV,
whereas ensembles of ≈300 000
Cd/S NCs nanocrystals synthesized using the “multipulse”
approach exhibit PL emission line widths of 15-35 meV.
II. Experimental Methods
2
Figure 1. Histograms of Cd(OH) NCs prepared by oxidizing metallic
cadmium nanoparticles. The cadmium precursor particles were elec-
trodeposited at -350 mV vs E°′Cdo/Cd2+ using either of two plating
programs: (a) A single 100 ms deposition pulse or (b) A train of ten,
CdS/S core/shell NCs were synthesized using the E/C
14
procedure described previously. Briefly, the following three
step procedure was employed: (1) Cdo NCs which were
narrowly dispersed in size were electrochemically deposited
from an aqueous 1.0 mM CdF2, 0.1 M NaF (pH ≈ 6.2) plating
solution onto a freshly cleaved graphite basal plane surface (the
1
0 ms plating pulses separated by ∼1s.
brought about by growth pulses having a duration, τ )
2
r /2DCd2+. For the plating pulse duration employed here of 10
o
details of the Cd plating procedure are discussed in greater
2
+
-6
2
ms, and the diffusion coefficient for Cd of 2.4 × 10 cm
, r ≈ 2 µm.
We have recently shown that Cd(OH)2 NCs prepared by
detail in the next paragraph); (2) Cdo NCs spontaneously
-
1 14
s
o
oxidized in the plating solution at open circuit (large Cd NCs
1
4
having radii greater than 30 Å were incompletely oxidized);
3) the graphite surface was removed from the electrochemical
cell, rinsed with Nanopure water, and transferred to a quartz
tube furnace in which they were heated in flowing H2S at 300
electrodeposition can be converted on a particle-by-particle basis
to CdS nanoparticles which are encapsulated in a sulfur or
polysulfide shell (as shown in Scheme 1). Conversion to CdS/S
core/shell NCs was effected by exposure of Cd(OH)2 NCs to
(
°
C and atmospheric pressure for 10 min. As described in detail
14
H2S(g) at 300 °C for several minutes. Because every Cd(OH)2
NC on the surface is converted into a CdS/S core/shell NC,14
improvements in the size monodispersity of the Cd(OH)2
precursor particles should be immediately transferable to CdS/S
NCs in step 3 of the E/C synthesis scheme. It has not so far
been possible to resolve the CdS core of the CdS/S core/shell
NCs by TEM so the size dispersion of the CdS core cannot be
14
previously, this procedure yielded CdS/S core/shell NCs
having a wurtzite core 17-50 Å in radius and a sulfur shell up
to 30 Å in thickness. The size and monodispersity of the CdS
core of these CdS/S NCs were determined by the corresponding
o
properties of the Cd(OH)2 (or Cd ) NCs which were deposited
in step 1.
We make a comparison here between CdS/S NCs synthesized
1
4
directly determined in this way.
o
from Cd precursor particles deposited in a single 100 ms plating
However, improvements to the size monodispersity of the
CdS core should be immediately apparent from PL spectra of
these CdS/S NCs since the emission line width is inhomoge-
neously broadened by particle size dispersion. Based on this
fact, the expectation is that improved size monodispersity for
the CdS core will translate into a reduced PL emission line
width. This hypothesis is supported by the data shown in Figure
pulse (as shown schematically in Scheme 2a) and CdS/S NCs
o
synthesized from Cd nanocrystals deposited using a train of
1
2
0 millisecond pulses separated by a second or more (Scheme
b). In both cases, the deposition potential was -350 mV vs
E ′Cdo/Cd2+. When a train of 10 pulses was employed, the
o
“
mixing” potential (the potential applied between plating pulses)
was near the rest potential for the surface and little or no current
was detected. Photoluminescence spectra were acquired using
2
. In Figure 2a, the PL spectrum for CdS/S NCs synthesized
o
using a single, 100 ms Cd deposition pulse (spectrum “sp”)
are compared with “multipulse” CdS/S NCs prepared using a
14
the previously described spectrometer and procedures. Particles
were sized using transmission electron microscopy (TEM), also
as previously described.14
o
train of 10 × 10 ms Cd deposition pulses (spectrum “x”). The
PL emission lines in both spectra “x” and “sp” are blue-shifted
from the CdS single crystal (spectrum “sp”) by ≈130 meV
indicating that the mean diameter of the CdS cores are ≈42
III. Results and Discussion
1
9
Cadmium nanoparticles were electrodeposited on graphite
using either of two deposition waveforms: a single 100 ms
plating pulse (i.e., Scheme 2a) or a train of 8 to 10 ms plating
Å. The line widths of these two spectra, however, are very
different: The full width at half-maximum of the single pulse
sample is 125 meV, whereas the line width for the sample
prepared by multipulse deposition is 18 meV. A line width of
18 meV approaches the 15 meV line width which has been
pulses separated by ≈1.0 s mixing intervals at the rest potential
o
(
Scheme 2b). Cd NCs were then permitted to oxidize at open
circuit to Cd(OH)2, and NCs of this intermediate were sized
using TEM.1 Populations of Cd(OH)2 nanoparticles synthesized
using multiple plating pulses exhibited a relative standard
deviation of the diameter, RSDdiameter, of 15 to 20%, whereas
reported in two previous spectroscopic studies of single CdS
8
20,21
nanocrystallites,
but in Figure 2a, emission from a 200 µm
diameter area of the surface encompassing 200,000-400,000
CdS/S NCs was probed.
o
Cd(OH)2 NCs synthesized using a single Cd deposition pulse
As shown in Figure 2b, the radius of the CdS core can be
o
have RSDdiameter values in the range from 30 to 50%. Cd(OH)2
particle size histograms for two representative experiments are
compared in Figure 1. The nucleation densities on these two
adjusted by increasing or decreasing the Cd plating pulse
duration, while maintaining the decoupling time at 1 s. For
example, the CdS/S NCs probed in spectrum “w” were
9
-2
o
surfaces were approximately equal: 2 × 10 cm . The
improvement in size monodispersity for the multipulse-grown
Cd(OH)2 can be attributed to decoupling of the growth between
regions on the surface separated by a distance, r, which is
synthesized using 10 × 8 ms Cd plating pulses. This spectrum
exhibits a 150 meV blue shift (characteristic of a core radius of
39 Å) and a line width of just 15 meV. Increasing the pulse
duration to 12 ms yields the CdS/S NC sample probed in

5752 J. Phys. Chem. B, Vol. 103, No. 28, 1999
Letters
Figure 3. Photoluminescence spectra of CdS/S NCs on the graphite
(0001) surface at 20 K using hνex ) 3.53 eV involving the use of two
different aperatures (200 µm and 70 µm as indicated) for the acquisition
of photoluminescence from the surface. (a) Spectra for the CdS/S NCs
of spectrum “sp” in Figure 2a. (b) Spectra for the CdS/S NCs of
spectrum “w” in Figure 2b.
NCs synthesized using the multipulse technique. In Figure 3b,
for example, PL spectra from 70 µm regions of the graphite
surface possess a line width of 15-18 meV which is comparable
to the width seen for PL spectra of the “parent” 200 µm diameter
region. The energy of maximum emission is identical to that of
the parent spectrum.
As already noted, the PL emission line width seen for CdS/S
nanocrystals synthesized using the multipulse technique ap-
proaches the line width observed for single CdS NCs in two
previous studies: Guyot-Sionnest and co-workers21 observed
PL line widths of 15 meV for CdS NCs having a diameter of
5
5 Å using two-photon excitation at 10 K, and Eychm u¨ ller and
2
0
co-workers measured PL line widths of 15 meV for 70 Å
diameter CdS NCs at the same temperature (20 K) and excitation
power densities (220 nW cm ) employed here. PL emission
Figure 2. Photoluminescence spectra of CdS/S NCs on the graphite
-2
(
0001) surface at 20 K using hνex ) 3.53 eV. (a) Comparison of CdS/S
o
lines for multipulse CdS/S NCs (Figure 2b) did not exhibit
phonon loss peaks which were also absent in the spectra of
NCs prepared from Cd NCs deposited using a single, 100 ms plating
pulse (spectrum “sp”), multipulse CdS/S NCs deposited using a
o
21
sequence of 10 × 10 ms Cd plating pulses (spectrum “x”), and a
Guyot-Sionnest but which were observed in the spectra of
2
0
macroscopic (0001) oriented CdS single crystal (spectrum “sc”). (b)
Comparison of three CdS/S samples prepared using the multipulse
Eychm u¨ ller at moderate illumination intensities.
o
method with Cd plating programs of 10 × 8 ms (spectrum “w”), 10
IV. Summary
×
10 ms (spectrum “x”), and 10 × 12 ms (spectrum “y”).
We report a new E/C procedure for synthesizing CdS/S core/
shell NCs involving the use of a train of 10 short 8-10 ms
voltage pulses delineated by 1 s “mixing” periods to synthesize
spectrum “y” having a blue shift of 58 meV (corresponding to
a CdS core radius of 61 Å) and a line width of 35 meV.
It is readily possible to demonstrate that spectrum “sp” in
Figure 2a is broadened as a consequence of particle size
inhomogeneities. In Figure 3a, spectrum “sp” is again shown
at the top together with three PL spectra which were acquired
at three different 70 µm diameter areas located inside this larger
o
Cd precursor particles. Upon oxidation of these “multipulse”
NCs to Cd(OH)2, improved size monodispersity is evident in
histograms of the Cd(OH)2 precursor nanoparticles as compared
with NCs synthesized using a single cadmium plating pulse of
100 ms. CdS/S core/shell NCs prepared from such Cd(OH)2
precursors exhibit PL emission line widths of 15-35 meV.
These PL emission line widths are much narrower than the PL
emission lines observed for ensembles of CdS nanocrystals
prepared using a single cadmium plating pulse, and are narrower
than the PL emission lines seen in any previous study of this
material to our knowledge. In fact, the line widths reported here
approach the PL line width seen for a single CdS nanocrystallite
2
00 µm diameter region. Two effects caused by particle size
inhomogeneity are observed in these three spectra: First, the
emission line widths for the 70 µm diameter regions are
narrowed from 125 to ≈70 meV, and, second, the energy of
maximum emission varies for different 70 µm regions from 2.57
to 2.63 eV. On the basis of the predictions of the strong
confinement model for CdS, we conclude that the mean radii
of CdS/S NCs in these three 70 µm diameter regions differ by
as much as 9-10 Å. These two “fingerprints” of size polydis-
persity are absent, or nearly so, at surfaces covered with CdS/S
2
0,21
in prior work.
The use of a train of short voltage pulses instead of a single
longer pulse to synthesize cadmium NCs precursor particles is

Letters
J. Phys. Chem. B, Vol. 103, No. 28, 1999 5753
(5) Brus, L. J. Phys. Chem. Sol. 1998, 59, 459.
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(
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1
1
which are randomly nucleated on the surface. Since neighbor-
ing Cdo nanoparticles are separated by few hundreds of
angstroms on average, a 10 ms pulse is much too long to
diffusionally decouple these nanoparticles. The beneficial effect
of these longer plating pulses may derive from the fact that the
particle nucleation density fluctuations over distances of microns
on the surface since in recent Brownian dynamics simulations
of this nanoparticle growth process, the mean particle diameter
is inversely correlated with the local nucleation density. The
PL microprobe data presented here indicate that these longer
range fluctuations in the particle diameter are absent for samples
of CdS/S core/shell NCs prepared using the multipulse tech-
nique.
1
1
(
(
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ing these particles on 400 Å thick graphite flakes supported on a TEM grid
as previously described (ref 14).
(19) On the basis of the predictions of the Coulomb corrected strong
confinement equation
(
11
(
(
(
On the basis of this analysis, further improvements in the
particle size monodispersity should be obtainable using this
approach simply by further reducing the pulse duration and
increasing the number of pulses. Implementation of this mul-
tipulse strategy should permit similar improvements to be
1
(
(
demonstrated for metal nanoparticles prepared by electrodepo-
sition,9
,10,22
and for nanocrystals of other semiconductors
1
4,15
prepared using the E/C synthetic method.
2
2
2
p π
1
1
1.786e
ꢀR
∆
E )
+
-
- 0.248E*Ry
2
[
]
Acknowledgment. The financial support of this work by
the National Science Foundation (#DMR-9876479) and the
Petroleum Research Fund of the American Chemical Society
me mh
2
R
the following materials parameters for CdS were employed for the strong
confinement calculation: e ) 5.4, m /m ) 0.21, m /m ) 0.80, where m
e
o
h
o
o
-
31
)
9.11 × 10
kg, Eg (20 K) ) 2.56 eV, E*Ryd ) 29 meV. Reference:
(#33751-AC5) is gratefully acknowledged. Donations of highly
Sze, S. M. The Physics of Semiconductor DeVices, 2nd ed.; John Wiley &
oriented pyrolytic graphite from Dr. Art Moore of Advanced
Ceramics are also very much appreciated.
Sons: New York, 1969.
(
20) Tittel, J.; G o¨ hde, W.; Koberling, F.; Basch e´ , T.; Kornowski, A.;
Weller, H.; Eychm u¨ ller, A. J. Phys. Chem. B 1997, 101, 3013.
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(
(
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(
22) These optical size-selection methods have not been applied to
investigations of CdS NCs to our knowledge.
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(
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(
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