Cathodoluminescence and photoluminescence of highly luminescent CdSe/ZnS
quantum dot composites
J. Rodriguez-Viejo, K. F. Jensen, H. Mattoussi, J. Michel, B. O. Dabbousi, and M. G. Bawendi
Citation: Applied Physics Letters 70, 2132 (1997); doi: 10.1063/1.119043
View online: http://dx.doi.org/10.1063/1.119043
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Cathodoluminescence and photoluminescence of highly luminescent
CdSe/ZnS quantum dot composites
J. Rodriguez-Viejo
Department of Chemical Engineering, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
K. F. Jensen
a)
Department of Chemical Engineering and Department of Materials Science and Engineering,
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
H. Mattoussi and J. Michel
Department of Materials Science and Engineering, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
B. O. Dabbousi and M. G. Bawendi
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
~Received 19 December 1996; accepted for publication 17 February 1997!
We report room-temperature cathodoluminescence and photoluminescence spectra originating from
ZnS overcoated CdSe nanocrystals, 33 and 42 Å in diameter, embedded in a ZnS matrix. The
thin-film quantum dot composites were synthesized by electrospray organometallic chemical vapor
deposition. Cathodoluminescence and photoluminescence are dominated by the sharp band-edge
emission characteristic of the initial nanocrystals. The emission wavelength can be tuned in a broad
window ~470–650 nm! by varying the size of the dots. The cathodoluminescence intensity depends
on the crystallinity of the ZnS matrix and the voltage and current density applied. © 1997
American Institute of Physics. @S0003-6951~97!01016-4#
Semiconductor nanocrystals smaller than the exciton
Bohr radius exhibit unique optical properties due to confine-
ment of the electron excitations.
1
The optical absorption and
emission can be tuned across the visible spectrum by chang-
ing the size of the nanocrystals, making these materials at-
tractive for applications in optoelectronics and nonlinear op-
tics. Macroscopic quantities of nearly monodisperse II–VI
semiconductor nanocrystals organically capped with trio-
ctylphosphines can be synthesized by solution techniques.
2
The quantum yield of CdSe quantum dots ~QDs!, as pre-
pared, is ;10%–15% with the yield typically being limited
by the existence of surface defects, which lead to nonradia-
tive paths for the electron–hole pair recombination. Solution
chemistry growth of a thin ZnS overlayer on the surface of
the CdSe particles passivates the defects and raises the quan-
tum yield to values between 40% and 50% for particles rang-
ing in size from 20 to 60 Å in diameter.
3,4
ZnSe films containing CdSe QDs have been previously
prepared by electrospray organometallic chemical vapor
deposition ~ES-OMCVD!.
5
In this method, the QDs are
transferred from a liquid dispersion into the gas phase by
electrospray,
6
and the transferred particles are subsequently
codeposited with a wide band-gap compound semiconductor
matrix grown by OMCVD. Since the luminescence wave-
length and properties are characteristic of the embedded
quantum dots, the wavelength can be tuned from ;470 to
;650 nm. The incorporation of nearly monodisperse lumi-
nescent quantum dots into wide band-gap semiconductor ma-
trices offers an alternative route for the fabrication of new
optoelectronic device structures.
7
The conductivity of the
matrix can be controlled by in situ doping during the
OMCVD process. These structures may offer significant ad-
vantages over polymer-based light emitting devices, which
require extensive synthetic procedures to change the emis-
sion wavelength, and have short lifetimes operating under
ambient atmosphere. Other approaches for in situ fabrication
of III–V QDs have been reported using selective area OM-
CVD, strain-induced self-organized growth of QDs, or mo-
lecular beam epitaxy.
8–10
These techniques provide a conve-
nient route for the synthesis of small-size nanocrystals.
However, control of the nucleation process is difficult and
the broad-size distributions result in inhomogeneous broad-
ening of the optical transitions.
Cathodoluminescence ~CL! and photoluminescence ~PL!
typically are used to explore the potential of these new com-
posites as phosphor materials in light emitting devices or
cathode ray tubes. In particular, CL provides information
about the excitation and deexcitation mechanism involved
during device operation, and about the optimum acceleration
range and current densities under which these materials can
be used. In this letter, we demonstrate cathodoluminescence
from ZnS overcoated CdSe quantum dots of 33 and 42 Å
diameter embedded in an inorganic matrix of ZnS grown by
OMCVD. This study aims at taking advantage of the size-
dependent properties of the quantum dots and the robust ZnS
matrix to generate CL emissions at various wavelengths.
The ZnS overcoated CdSe quantum dots were prepared
as described elsewhere.
3–5
The quantum yield of the dots in
hexane was around 45%. The trioctylphosphine ~TOP!/
trioctylphosphine oxide ~TOPO! surface cap was exchanged
by pyridine ~lowering the quantum yield to ;10%–15%!
and dispersed in anhydrous pyridine under nitrogen and
mixed with acetonitrile ~1:2! to achieve stable operation of
the electrospray. The CdSe/ZnS quantum dots were incorpo-
rated by ES-OMCVD, carried out in a tubular up-flow
a!
Electronic mail: kfjensen@mit.edu
2132 Appl. Phys. Lett. 70 (16), 21 April 1997 0003-6951/97/70(16)/2132/3/$10.00 © 1997 American Institute of Physics
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OMCVD reactor equipped with an external resistive heater.
The transferred QDs were combined with the matrix precur-
sors, hydrogen sulfide ~25
m
mol/min!, DEZn or DMZn ~2.5
m
mol/min!, and the carrier gas H
2
in the mixing zone at the
reactor inlet. The growth temperature range from 100 to 250
°C and the total pressure was maintained at 600 Torr. The
composites were grown on glass substrates with film thick-
nesses ranging from 0.5 to 1
m
m. A thin ~;0.1
m
m! ZnS
layer was grown before the thin-film composite, and the
composite was covered by a similar ZnS film to ensure com-
plete coverage of the quantum dots.
PL measurements were carried out at room temperature
in a Spex Fluorolog-2 spectrometer with front-face collec-
tion. CL experiments were conducted at room temperature in
a scanning electron microscope ~SEM! equipped with an Ox-
ford Mono-CL spectrometer. The electron-beam energy and
current ranged between 1 and 40 kV and 1 and 160 nA,
respectively.
Figure 1 shows room-temperature absorption, photolu-
minescence, and cathodoluminescence of thin-film compos-
ites containing CdSe/ZnS QDs of 33 ~a! and 42 Å ~b! diam-
eter. Quantum confinement shifts the peak of the smaller
crystallites to higher energies and shows that CL and PL near
band-edge spectra originate from the same state of the
nanocrystals and not from the polycrystalline ZnS matrix.
The band-edge PL spectra have narrow bandwidths ~30–40
nm! and are redshifted from the absorption maximum. The
QDs incorporated inside the thin films show a redshift of the
PL emission when compared to the same nanocrystals in
solution. This is caused by aggregation of the nanocrystals
inside the inorganic matrix during the ES-OMCVD process,
5
and energy transfer to the larger particles in the agglomer-
ate.
11
The PL quantum yield ~QY! has been obtained by
comparing the PL signal of the thin-film composites to
rhodamine 640 in methanol. A background correction to sub-
tract the contribution from the ZnS matrix in the absorption
spectra has been done for each of the films. Quantum yields
between 10% and 15% were measured, although the rough-
ness of the films ~2000–4000 Å! makes it difficult to obtain
an accurate number due to the scattering of the light. These
values agree with the PL quantum yield of the ZnS over-
coated CdSe quantum dots dispersed in pyridine. The high
PL QY values obtained mean that when illuminated with a
low power UV lamp, the thin films glow at the wavelength
characteristic of the quantum dots at ambient light. The
room-temperature CL is nearly identical to the PL, with a
small redshift ~;6nm!and wider linewidths ~40–50 nm!.A
similar spectral shift has already been observed in the elec-
troluminescence of quantum-dot/polymer composites,
7
and
in the luminescence of quantum dots under high electric
fields,
12
which can be attributed to a quantum-confined Stark
effect.
Figure 2 shows CL spectra from films grown at 100 and
250 °C and the corresponding x-ray diffraction patterns. The
difference in CL emission shows that the crystallinity of the
inorganic host matrix has a significant effect on the quality
of the cathodoluminescence spectrum. ZnS thin films grown
by ES-OMCVD at temperatures around 100 °C are poorly
crystallized, showing broad x-ray diffraction features and the
presence of a hexagonal phase. When the growth tempera-
ture is increased to 250 °C, the structure is predominantly
zinc blende and the x-ray diffraction spectrum shows well-
defined peaks. The amorphous contribution to the x-ray dif-
fraction profile in spectra a and b of the inset of Fig. 2
originates mainly from the glass substrate due to the small
thickness of the ZnS films ~;0.5–1
m
m!. We attempted to
increase the CL intensity by improving the crystallinity of
the ZnS thin-film composites grown at low temperatures by
annealing the samples at ;300 °C. However, these tempera-
tures are high enough to alloy the CdSe dots with the sur-
rounding ZnS matrix, decreasing the photoluminescence
quantum yield, but still too low to modify the crystallinity of
the ZnS matrix.
Figure 3 shows the influence of the electron-beam en-
ergy and current flow on the CL of the thin-film composites.
FIG. 1. Absorption ~dashed lines!, photoluminescence, and cathodolumi-
nescene ~dotted lines! spectra for thin-film quantum-dot composites contain-
ing 33 Å ~a! and 42 Å ~b! diameter ZnS overcoated CdSe nanocrystals. The
PL is strongly redshifted from the absorption spectrum due to extensive
agglomeration of the quantum dots inside the thin films, leading to energy
transfer to the larger dots. Sample ~b! has an optical density almost three
times larger than ~a! and a broader size distribution, which leads to the
larger redshift. Cathodoluminescence spectra were recorded at 30 kV, 20
nA. PL excitation wavelength was at 480 nm.
FIG. 2. Dependence of the CL intensity on the growth temperature, ~a! 100
°C and ~b! 250 °C, of the thin-film quantum-dot composites. CL was per-
formed at 30 kV, 40 nA. ~Inset! x-ray diffraction profiles, obtained with Cu
K
a
radiation, of both samples showing the difference in crystallinity. The
zinc blende and wurtzite diffractions of ZnS are labeled . and s, respec-
tively.
2133Appl. Phys. Lett., Vol. 70, No. 16, 21 April 1997 Rodriguez-Viejo
et al.
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The increase in CL signal with both acceleration voltage and
current density is related to the generation of secondary elec-
trons inside the ZnS matrix. The formation of the electron/
hole ~e/h! pairs is not uniform,
13
the majority of the excitons
being generated near the surface within a relatively small
radius. The saturation of CL with voltage could be explained
in terms of the small thickness ~0.5–1
m
m! of the films.
Increasing the voltage shifts the maximum efficiency of the
e/h pair generation towards the substrate, leading to a satu-
ration of the CL intensity. The saturation is not due to charg-
ing, since SEM images confirm that charging effects are ab-
sent for these samples, and similar results are obtained on
thin-film composites deposited on Si~100! substrates. With
increasing current density, light emission also saturates.
Electron-beam irradiation at high voltages or current
densities produced a decrease of the CL intensity over time.
This decay is faster in the poorly crystallized samples grown
at 100 °C than in the ones grown at 250 °C. Glassy films
formed by casting the quantum dots from solution on a Si
wafer show faster decay times, proving the importance of the
ZnS matrix in protecting the quantum dots. Annealing of an
irradiated sample at 100 °C for 3 h produced a partial recov-
ery of the CL signal. By analogy to the optical darkening
effect of semiconductor-doped glasses when exposed to laser
irradiation,
14
the decrease of the CL intensity over time may
be attributed to electron ionization of the quantum dots, fol-
lowed by trapping of the ejected electrons at deep traps in-
side the ZnS matrix.
We have reported the CL and PL of 33 and 42 A
˚
diam
ZnS overcoated CdSe nanocrystals embedded in polycrystal-
line ZnS matrices. The ZnS matrix provides surface passiva-
tion and significantly reduces the CL quenching of the quan-
tum dots. CL and PL of the CdSe/ZnS thin-film composites
originate from the same state in the nanocrystals. Therefore,
the whole visible spectrum, from red through green to blue,
can be realized by changing the size of the ZnS overcoated
CdSe nanocrystals. The results obtained in the CL experi-
ments show the potential of quantum dot composites in in-
organic optoelectronic devices.
Two of the authors, J.R.-V. and B.O.D., thank the Di-
reccio General de Recerca from Catalonia and the Saudi
Aramco, respectively, for fellowships. One of the authors,
M.G.B., thanks the David and Lucille Packard Foundation,
the Sloan Foundation, and the W. M. Keck Foundation for
support. This research was funded in part by NSF Grant No.
DMR-91-57491 and benefited from the use of MRSEC
shared facilities supported by NSF Grant No. DMR-94-
00034.
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FIG. 3. Room-temperature cathodoluminescence spectra of a thin-film com-
posite containing 33 Å ZnS overcoated CdSe nanocrystals at different ap-
plied voltages ~a! and currents ~b!. ~Inset! plot of the integrated intensities vs
voltage ~a! and current ~b!.
2134 Appl. Phys. Lett., Vol. 70, No. 16, 21 April 1997 Rodriguez-Viejo
et al.
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