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Article
Cite This: ACS Appl. Nano Mater. 2019, 2, 1496−1504

www.acsanm.org

Bright and Uniform Green Light Emitting InP/ZnSe/ZnS Quantum
Dots for Wide Color Gamut Displays
Yongwook Kim,† Sujin Ham,‡ Hyosook Jang,† Ji Hyun Min,† Heejae Chung,† Junho Lee,†
Dongho Kim,*,‡ and Eunjoo Jang*,†
†

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Inorganic Material Lab, Samsung Advanced Institute of Technology, Samsung Electronics, 130 Samsung-ro, Yeongtong-gu,
Suwon-si, Gyeonggi-do 16678, Republic of Korea
‡
Department of Chemistry, Yonsei University, 50 Yonsei-ro, Seodaemun-gu, Seoul 03722, Republic of Korea
S
* Supporting Information

ABSTRACT: There is an urgent demand to improve the
efficiency and the color purity of the environment-friendly
quantum dots (QDs), which can be used in wide color gamut
(WCG) displays. In this study, we optimized the reaction
conditions for the InP core synthesis and the ZnSe/ZnS
multishell growth on the core. As a result, remarkable
improvements were achieved in the photoluminescence
quantum yield (PL QY, 95%) and the full width at halfmaximum (fwhm, 36 nm), with perfectly matched wavelength
(528 nm) for the green color in WCG displays. Injection of the
phosphorus precursor at a mild temperature during the InP core synthesis reduced the size distribution of the core to 12%, and
the shell growth performed at a high temperature significantly enhanced the crystallinity of the thick passivating layer. We also
investigated the photophysical properties, particularly the energy trap distributions and trap state emissions of the InP-based
QDs with different shell structures. The time-resolved and temperature-dependent PL spectra clearly indicated that the wellpassivated InP/ZnSe/ZnS QDs showed nearly trap-free emissions over a wide temperature range (77−297 K). Also, the onand off-time probability on single QD blinking and Auger ionization efficiencies also showed that these QDs were hardly
affected by the surface traps.
KEYWORDS: quantum dots, indium phosphide, multishell, photoluminescence, trapping rate, blinking suppression,
Auger ionization efficiency

■

By use of the magic-sized cluster (MSC, λabs = 386 nm) as a
starting material, the size distribution was considerably
reduced to 15% when the core size was >3 nm.12 Shell
passivation with wide-band-gap materials is also critical to
increase PL QY. Houbold et al. synthesized InP/ZnS core/
shell QDs that showed a 23% PL QY with thin ZnS (0.5 nm)
shell.13 Li and co-workers reported a heating process in one
pot for both core and shell growth, which produced core/shell
QDs with 50−70% of PL QY.14 In their report, the PL QY of
the core/shell structured QDs decreased as the ZnS shell
thickness increased beyond 1 nm because the ZnS shell had a
large lattice mismatch (7.7%) against the InP core. To alleviate
this mismatch, interlayers with an intermediate lattice constant
have been adopted. Kim and co-workers introduced the GaP
interlayer by cation exchange of In with Ga, and the resulting
InP/GaP/ZnS QDs showed 85% of PL QY.15 Lim et al.
prepared a gradient alloy ZnSeS shell of 1.9 nm thickness,
which resulted in the InP/ZnSeS QDs with 50% of PL QY.16

INTRODUCTION
Environmentally friendly InP-based QDs have risen in
popularity as an alternative to Cd-based materials in wide
color gamut (WCG) display applications.1−4 Because the
exemption from EU’s Restriction of Hazardous Substances
(RoHS) will expire in 2019,5 it is crucial to develop highquality InP-based QDs for commercial applications. However,
several disadvantages of InP have been also reported. Because
of the large Bohr radius (10.6 nm) and small band gap (1.27
eV) of InP compared with CdSe, the energy band gap of InPbased QDs changes more significantly according to their
size,4,6 so that their emission spectra are broader than those
from CdSe QDs with a similar size distribution. Numerous
studies have been performed to improve the PL QY as well as
the color purity of the InP QDs. The addition of Zn
carboxylates during7 or after8 the core growth showed a
significant effect on the size distribution, the wavelength
tunability, and the PL QY by surface passivation9 or forming
an InZnP alloy. To control the growth kinetics, less reactive P
precursors,10 such as tris(dimethylamino)phosphine, tris(triphenylsilyl)phosphine, or their mixture,11 have been
employed, but the size distribution was not yet satisfactory.
© 2019 American Chemical Society

Received: December 31, 2018
Accepted: February 6, 2019
Published: February 6, 2019
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without Zn(OA)2 can create InP MSCs at only 110 °C with
the absorption peak at 386 nm.12 Therefore, it was inferred
that the In−P−Zn ligand complexes could retard the InP
nucleation. As the reaction temperature increased further to
240 °C,8 the In−P−Zn ligand complexes, which remained as
monomers at 170 °C, decomposed to cause uniform growth of
InP QDs. The synthetic scheme and absorption spectra of the
InP nanostructures at each growth step are shown in Figure
S1a,b. The average diameter of the InP core was controlled at 2
nm for the green emission, and the size distribution was very
narrow considering the core size (σ = 12%, Figure S1c). For
comparison, the InP QDs by injecting (TMS)3P at 280 °C
were also prepared. As shown in Figure S1d, the InP QDs
synthesized by injecting at 280 °C have broader half-width at
half-maximum of the 1s absorption peak (30 nm) than that
synthesized by injecting 150 °C (24 nm), which shows a larger
size distribution.
To maximize the surface passivation, we applied ZnSe/ZnS
multilayer for the shell structure. The ZnSe interlayer was used
to relieve the strain between the InP core and the ZnS shell.
When the energy band gap of the small InP core is close to that
of ZnSe (2.82 eV), the excitons of InP core can be delocalized
over the interfaces between InP and ZnSe. Thus, both the InP
core size and the ZnSe shell thickness could be optimized to
control the wavelength, especially for the green emission. The
ZnSe shell also has an effect on passivation the surface defect
of the InP core as well as a medium level of exciton
confinement. Also, since the ZnSe tends to grow more
epitaxially on InP core, it is advantageous to grow thick ZnSe
interlayer prior to the ZnS layer, which has a wide band gap
that confines excitons more tightly. Thus, the InP/ZnSe QDs
were prepared by growing a relatively thick (1.5 nm) ZnSe
shell on the InP core with a diameter of 2.0 nm, which resulted
in the red-shift of the emission spectra from 477 to 532 nm.
Next, a ZnS shell 0.5 nm in thickness was grown over the InP/
ZnSe QDs to form the InP/ZnSe/ZnS QDs shown in Scheme
1. We calculated the thickness and composition of each shell

Despite these efforts, however, the full width at half-maximum
(fwhm) of the InP-based QDs remain in the range 40−60 nm.
Recently, Ramasamy et al. reported a notable improvement of
the fwhm of InP/ZnSe/ZnS QDs to 38 nm (71% PL QY, λem
= 535 nm).17 They optimized the ratio of In/P (1.5/1) and P
injection temperature (room temperature) and applied the
successive ion layer adsorption and reaction (SILAR)
method18 for the shell coating to prepare QDs with 4.2 nm
diameter.
In this study, we produced very bright and uniform green
emitting InP/ZnSe/ZnS QDs by optimizing the reaction
parameters for the InP core synthesis and the growth of
uniform and thick ZnSe/ZnS shells. The injection temperature
of the P precursor was set to 150 °C to control the reaction
rate and form MSC as an intermediate, which enables uniform
InP cores. To prevent facet-selective anisotropic shell growth,
the shell coating was performed at high temperature (320 °C)
which results in highly crystalline and well passivating layer.
The resulting ZnSe and ZnS shells were grown to a thickness
of 1.5 and 0.5 nm, respectively, and the average diameter of
InP/ZnSe/ZnS QDs was close to 6.0 nm. The obtained InP/
ZnSe/ZnS QDs showed 95% PL QY at 528 nm peak emission
with fwhm of only 36 nm, which we believe to be the best
luminescent properties of InP-based QDs ever reported to
date. We also performed systematic comparisons in the
photophysical properties of the InP/ZnSe/ZnS, InP/ZnS,
and InP/ZnSe QDs through time-resolved and temperaturedependent PL measurements, blinking dynamics, and Auger
ionization efficiencies. Especially, the effects of the trap on PL
emission were directly compared through the decay associated
spectra (DAS), which has never been performed to understand
the properties of QDs before. The deep trap emission that
appeared at several tens of nanometers longer than the core
state emission could be defined as a surface emission. The InP/
ZnSe/ZnS QDs exhibited nearly trap-free emission over a wide
range of temperatures in accordance with the effective trap
passivation. The fast charge detrapping rate for the blinking
dynamics and the relatively lower Auger ionization efficiency
also support that the well-passivated InP/ZnSe/ZnS QDs are
barely affected by trap states, in comparison with InP/ZnS and
InP/ZnSe QDs.

Scheme 1. Synthesis of InP Core QDs and ZnSe/ZnS Shell
Coating

■

RESULTS AND DISCUSSION
Despite many efforts, the preparation of InP QDs with uniform
size and minimum defects is still challenging due to a poor
understanding of the structural and photophysical properties of
InP QDs. In particular, because of their small size (d < 3 nm),
it is more important to suppress the surface oxidation or defect
generation of the green emitting InP QDs. Previous studies on
the kinetics of InP QD formation4,19 showed that the different
reactivity between the active P and the stable In precursors was
one of the causes for the size distribution. During the InP
synthesis, unlike the LaMer type growth, it has been known
that the initial nucleation phase completely consumes the
highly reactive P precursor such as (TMS)3P, and further
growth takes place through the Ostwald ripening, which results
in a large size distribution. To solve this issue, we injected
(TMS)3P at 150 °C in the presence of both indium laurate
(In(LA)3) and zinc oleate (Zn(OA)2) precursors.12,20 At this
mild temperature the In−P−Zn ligand complexes21 were first
formed, and then they were converted to InP MSCs as the
temperature increased to 170 °C, showing a sharp absorption
peak at 370 nm. In comparison, the In−P ligand complexes

layer based on the elemental compositions (by ICP-AES) and
theoretical calculation22 assuming perfect spherical QDs and
uniform shell growth (Table S1). Therefore, all values are
reported as the average for the collective structures and can be
different from the size measured by scanning transmission
electron microscopy (STEM). However, those are still helpful
to follow the progress of general particle evolution. The
absorption and emission spectra of the InP core and InP/ZnSe
and InP/ZnSe/ZnS QDs are depicted in Figure 1. The
absorbance increase below 450 nm indicates the degree of
ZnSe and ZnS shell growth. A decrease in the emission tail,
observed at slightly longer wavelengths to the peak, implies the
removal of the trap states. The resulting InP/ZnSe/ZnS QDs
showed absolute PL QY of 95.4 ± 2.1% and spectral
bandwidth of 36 nm at the 528 nm emission peak, which are
the highest PL QY and narrowest fwhm ever reported for green
emitting InP-based QDs. The absolute PL QY of QDs was
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Figure 1. Absorption (dotted lines) and emission (solid lines) spectra
of InP core (black), InP/ZnSe (dark cyan), and InP/ZnSe/ZnS QDs
(green).

measured using a quantum efficiency measurement system
with an integrating sphere by 10 times repetition. To cover the
color space of Digital Cinema Index (DCI) or National
Television System Committee (NTSC) standard, a narrow
fwhm of the green spectrum is crucial. Our green InP/ZnSe/
ZnS QDs can achieve 100% of color reproducibility to DCI-P3
standard due to simultaneous improvement of fwhm and PL
QY at perfectly matched emission wavelength. The LED
display utilizing QDs as color converting materials has an
advantage covering wider color space than any other displays,
such as the organic light emitting diode and the LED with
narrow band phosphors or yellow absorbing dyes. Because the
high quality red emitting QDs are also significantly important
for wide color gamut, our group have been working to improve
the fwhm of red InP QDs, previously known as about 45
nm.4,17
Although the shell growth at high temperature has benefits
for high crystallinity and uniform shell growth by overcoming
the thermodynamic facet preferences, most of previous studies
had performed shell growth below 300 °C to prevent
heterogeneous nucleation.4 However, we could increase the
shell growth temperature up to 320 °C, which is slightly lower
than boiling temperature of the solvent, trioctylamine, by
simultaneously controlling the injection steps and speeds of the
Se and S precursors. As shown in Figure S2, no small particle
could be produced by the heterogeneous nucleation. The
accelerated shell growth at the high temperature significantly
improved the crystallinity and uniformity of the core/shell
QDs. The high-resolution scanning transmission electron
microscopy (HR-STEM) images of InP/ZnSe and InP/
ZnSe/ZnS QDs in Figure 2a,b show highly crystalline
nanocrystals. The STEM energy-dispersive X-ray spectroscopy
(EDS) mapping images of the InP/ZnSe/ZnS QDs also show
InP core well passivated with ZnSe/ZnS multishell (Figure
S3). The nanoparticle in the high-angle angular dark-field
(HAADF) image is well overlaid with patterns of each atom:
In, P, Zn, Se, and S. The In atoms are localized around the
center of the particle, whereas the Zn atoms are delocalized
over the particle. The Se and S atoms compose inner and outer
shell, respectively. The P atoms distributing throughout the
particles rather than inside core is caused by the
trioctylphosphine used for chalcogen precursors. The X-ray
diffraction (XRD) patterns (Figure 2c) for the InP/ZnSe/ZnS
QDs during the shell growth also show evidently the epitaxial
growth of ZnSe and ZnSe/ZnS shell on zinc-blende InP core
by the peak shifting to higher angle and narrowing without

Figure 2. HR-STEM images of InP/ZnSe QD (a) and InP/ZnSe/
ZnS QD (b). (c) Powder XRD patterns of InP core (black), InP/
ZnSe (dark cyan), and InP/ZnSe/ZnS (green) QDs. The vertical bars
represent the diffraction patterns for bulk zinc-blende InP (gray) and
ZnS (green).

changing on the patterns. The peaks shifting were occurred by
coating shell materials which have smaller lattice constant on
the InP core. The peaks narrowing, which is related to growth
of the crystal size, also could be an evidence for epitaxial
growth on InP core while the crystalline structure was
maintained as zinc-blende.
To study the effects of the passivating shell materials as well
as the structures on the photophysical properties, InP/ZnS
QDs were also prepared by growing ZnS shell with 1.0 nm in
thickness on the same InP core as a model system. The
absorption and emission spectra, HR-STEM images, and XRD
patterns are shown in Figure S4. The absorption and PL
spectra of the InP/ZnS QDs (λem = 505 nm, fwhm 50 nm, PL
QY 46%) showed a smaller red shift after shell coating and
slightly higher efficiency than the InP/ZnSe QDs (λem = 532
nm, fwhm 39 nm, PL QY 34%) because of the better exciton
confinement effect of the ZnS shell. The large lattice mismatch
between InP and ZnS induces inhomogeneous broadening of
surface trap levels,23 which results in broader fwhm of InP/ZnS
than that of InP/ZnSe.
When the emission spectra of InP/ZnSe, InP/ZnS, and InP/
ZnSe/ZnS are fitted with Gaussian functions at the front line,
we observed a tail on the lower energy side of each spectrum
(Figure S5). Supposedly, tails at longer wavelengths are due to
the trap states. To identify the influence of defects on the QD
decay dynamics, we measured PL lifetimes using the spectrally
resolved time-correlated single-photon counting (TCSPC)
technique across the whole QD emission spectra. In Figure S6,
the PL decay profiles, dependent on the probe wavelength,
were measured at 10 nm spectral intervals for all QDs. The PL
decay curves were fitted with triexponential functions: a fast
component (τ1 = 3−4 ns) attributed to the intrinsic charge
transfer processes between the neutral (core) and surface trap
(charged) states, an intermediate component (τ2 = 30−40 ns)
due to the intrinsic recombination of initially populated neutral
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Figure 3. Time-resolved emission spectra (a−c) and their corresponding decay associated spectra (d−f) of InP/ZnS (a, d), InP/ZnSe (b, e), and
InP/ZnSe/ZnS (c, f) QDs in toluene obtained by excitation at 430 nm.

states, and a long-lived component (τ3 > 100 ns) from the trap
state emissions.24−29 On the basis of the PL decay curves,
which were analyzed by global fitting to obtain the kinetic time
constants and respective amplitudes, we reconstructed the
time-resolved PL spectra (TRPS) and the corresponding DAS
that are shown in Figure 3. In the TRPS (Figure 3a−c), the
red-shifted peak maximum represents the emission at longer
wavelengths in the PL spectra, which is attributable to the
long-lived component. This assignment is further supported by
the different relative peak amplitudes of the τ1, τ2, and τ3
components in the DAS (Figure 3d−f). We found that the
contribution of the trap states to the total PL spectra of InP/
ZnSe/ZnS was only 13%, while those of InP/ZnS and InP/
ZnSe were 37% and 41%, respectively. The relative
contributions from the τ2 component in the total PL spectra
corresponded with their respective PL QYs because the τ2
component originates from the radiative recombination
process of excitons. Consequently, our findings provide
evidence that effective passivation occurred for the InP/
ZnSe/ZnS QDs. Moreover, these spectrally resolved PL decay
curves clearly prove that the long-wavelength tails in the
steady-state PL spectra are closely associated with the trap
state emission. Because previous studies confirmed that surface
traps affect the deep traps as well as shallow traps,24,30,31 we
expected to observe the same tendency for the deep trap state
emission. In this study, we defined the deep trap state emission
as surface emission that appeared at several tens of nanometers
longer than the core state emission, rather than a trap within
the band gap.
To understand the deep trap state emission mechanism,
which depends on the shell passivation of the QDs, we
analyzed the steady-state PL spectra over a wide temperature
range (77−297 K, Figure 4 and Figure S7). The temperature
dependence of the thermal population is an important factor
that causes charge transfers between the neutral and trap
states.24 All core/shell QDs displayed no low-energy trap
emission at room temperature, in contrast with the InP cores
that showed strong trap state emission at longer wavelengths.
Notably, for the InP/ZnSe/ZnS QDs, we observed no trap
state emission over a wide range of temperatures, which
suggests that the surface passivation by the ZnSe/ZnS
multishell encapsulation of QDs successfully reduced defects,
leading to enhanced PL QY. Both the InP/ZnSe and InP/ZnS
QDs showed increased low-energy trap emission as the
temperature decreased. These properties are quite similar to
the CdSe-based QDs.30−33 For comparison, we plotted the
relative intensities of the two emission bands in Figure 4b,

Figure 4. (a) Two-dimensional PL spectra (wavelength vs temperature) of the InP/ZnS, InP/ZnSe, and InP/ZnSe/ZnS QDs from 77
to 297 K. The core PL appears at higher energies, whereas trap
emission is broader and red-shifted. (b) Temperature dependence of
the relative intensities of two-band emission for the InP/ZnS (blue
circles), InP/ZnSe (dark cyan triangles), and InP/ZnSe/ZnS QDs
(green squares).

which clearly demonstrated that the relative trap emission
intensities were highly dependent on the temperature. The
relatively more prominent deep trap emission of InP/ZnSe
QDs is thought to be a consequence of the surface defects. The
ZnSe surfaces are prone to oxidation, and as a result the
formation of selenium oxide phases was indicated by X-ray
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reliably described by the two states. First, the InP/ZnSe/ZnS
QDs exhibited significantly longer on-time compared with
other core/shell QDs: the on-time fractions of InP/ZnS, InP/
ZnSe, and InP/ZnSe/ZnS were 34%, 26%, and 76%,
respectively, which corresponded to their respective PL QYs.
The on-time fractions are not directly matched with the PL QY
due to the difference in the measurement conditions, a liquid
state dissolved in toluene for PL QY, and a solid state
embedded in polystyrene. In these three QDs, effective
passivation by shell encapsulation is one possible cause for
the distinctive blinking behaviors due to their similar PL QY
tendencies. The overall QD blinking behaviors, including the
trapping/detrapping rates, reflect the differences in the trap
densities and energetic distributions.
Generally, the blinking behaviors can be explained by a traprelated mechanism.42,44−50 Especially in the charging model,
charge carrier photoionization to the trap states generates a
charged QD which remains in a nonfluorescent state due to
fast nonradiative energy transfer such as the Auger
recombination.44−47 To compare the trapping/detrapping
rate constants, the probability density distributions of the onand off-times, expressed as P(αon) and P(αoff), were generated
for QDs with different structures, as shown in Figures S11a and
11b, respectively. To capture the representative features and
eliminate the artifacts induced by a particular QD, each plot
was generated with at least 100 single QDs. More details about
the probability density plots can be found in Figure S11. The
probability was weighted with the average time for the nearestneighboring event and was plotted on a log−log scale. The
power law distributions of the on- and off-time probability
densities demonstrate that the charge transfer rates are widely
distributed between the neutral and charged states. These
behaviors are a consequence of energetic diffusion in the trap
states, i.e., change in the density and position of charges
trapped in the QDs.42,45 The trapping (αon) and detrapping
rates (αoff) of both InP/ZnSe and InP/ZnS QDs were higher
than those of the InP/ZnSe/ZnS QDs, suggesting that the
imperfect passivation of single-shelled QDs leads to larger
fluctuations between the on and off states. This also renders
the trap state energies more sensitive to changes in the external
environment.42,45 Besides, a larger coefficient of the probability
densities indicates faster charge trapping or detrapping
processes if compared within the same time window.50
Therefore, large αon and αoff values represent rapid charge
trapping and detrapping processes, respectively. Notably, the
αon values were smaller than the αoff values for all three
samples. Schematics for the energetics between the charged
and neutral states with regard to their relative transition rates
are shown in Figure S11c. These processes are based on the
activation energy barriers (ΔE1 and ΔE2) between the neutral
and charged QD parabolic potential curves. The energetic
positions of the neutral and charged states depend on the αon
and αoff values, and these two states are located at the same
energy level when αon = αoff. When αoff > αon, the energy level
of the charged QD is higher than that of the neutral state
because a larger αoff value indicates that the detrapping process
is faster than the trapping process. In addition, the fact that the
InP/ZnSe/ZnS QD exhibited a larger difference between the
αon and αoff (0.17) compared with InP/ZnS (0.10) and InP/
ZnSe (0.08) suggested that the charged states in InP/ZnSe/
ZnS QDs were located at much higher energy levels.
Consequently, the InP/ZnSe/ZnS QDs are less affected by

photoelectron spectroscopy (XPS). On the other hand, ZnSterminated QDs showed almost no surface oxide as shown in
Figure S8. As the temperature increased, the PL spectra
became red-shifted and broader for all QDs in accordance with
previous studies34−36 (Figure S9). When the temperature
increases, the atoms vibrate more strongly, increasing the
interatomic spacing. The interactions between the lattice
phonons and free carriers (electrons and holes) also affect the
band gap to a smaller extent.34,37,38 In other words, a small free
energy difference along the classical bath coordinate promotes
temperature-dependent surface PL, whereas large coupling
between quantum (phonon) modes induces broadening and
red shifts.31 This is schematically presented in Figure S10.
Furthermore, we evaluated the energetic trap densities
through the blinking dynamics by comparing the blinking
behaviors with the trapping/detrapping rates of each QD using
single-dot confocal PL microscopy. Investigating the charge
trapping process has been one of the most popular methods to
explore the effects of trap states on the QDs. Charge trapping
dynamics prevents radiative recombination and is therefore a
limiting factor for practical commercialization.39−42 Thus, it is
essential to understand charge trapping dynamics to produce
QDs with superior optical properties. Charge trapping impedes
continuous PL emission, and it is noted as blinking or PL
intermittency in single-dot PL measurements.42 Typical PL
trajectories and intensity histograms are shown in Figure 5 by

Figure 5. Representative blinking dynamics for individual QDs: InP/
ZnS, InP/ZnSe, and InP/ZnSe/ZnS. Each row presents the PL
intensity trace (left) and the intensity histogram (right) of a single
QD. The horizontal gray line is the PL intensity threshold, and the
numerical value at the top right of each histogram indicates the ontime fraction in percentage. The PL intensity of the off-state was the
same as the background. The PL intensity was measured by
integrating over 5000 excitation pulses (20 ms).

counting the number of photons emitted in 20 ms time
intervals. The emissions from all isolated dots present a
random pattern of switching between bright (“on”) and dark
(“off”) states. The intensity histograms showed a bimodal
distribution for all samples. Compared to the previous
reports,29,43 all our samples showed stable on-intensity with
very little fluctuation. Also, since the on- and off-states were
clearly distinguished from each other, the dynamics can be
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indicated that the long-wavelength tail emission from the
surface trap state was nearly eliminated in the InP/ZnSe/ZnS
QDs, while the InP/ZnSe and the InP/ZnS QDs showed
relatively higher surface trap emissions. Furthermore, InP/
ZnSe/ZnS QDs showed no shallow or deep trap emission over
a wide temperature range (77−297 K), indicating that the
ZnSe/ZnS multishell structure successfully passivated defect
structures. From the single QD blinking dynamics, the larger
difference between αoff and αon values (0.17) and the lower
Auger ionization efficiencies (3.8%) of InP/ZnSe/ZnS QDs
also suggested that the charged states of these QDs are located
at much higher energy levels, which are scarcely affected by the
trap state. These photophysical studies evidently supported the
superior optical properties of these well-passivated InP/ZnSe/
ZnS QDs.

the trap states compared with the other single-shelled QDs,
which explains their exceptionally high PL QYs.
The Auger ionization efficiencies for all QDs are consistent
with the charge transfer process schematically described in
Figure S11c. The Auger ionization efficiency is significantly
affected by the trapping probability for QD. The trapping
probability depends on the trap densities at higher energy
states, since the annihilation energy of exciton contributes to
the ejection of other charge carriers into adjacent trapping
positions. With regard to this, the differences in blinking rates
are described by trap densities that are at least 1Eg (which is as
high as the band gap energy) above the edge of the lowest
conduction band. Previous studies observed that the
exponential truncation (τfalloff) in the on-time probability
plots originated from the Auger ionization processes.25,46,51 As
shown in Table 1, we calculated the probabilities of creating a

■

Table 1. Calculated Probabilities of Creating n Excitons in
Both Single Pulse (with n = 1 (SX) and 2 (BX)) and
Multiexciton (MX, n ≥ 2) as Well as the Auger Ionization
Efficiencies
τfalloff
(s)
InP/ZnS
InP/ZnSe
InP/ZnSe/
ZnS

PSX
(×10−2)

PBX
(×10−4)

PMXa
(×10−4)

Pionize
(×10−4)

Pionize/
PMX

6.9
2.2
7.3

1.5
2.0
1.9

1.1
2.1
1.9

1.1
2.1
1.9

13.7
21.4
7.2

EXPERIMENTAL METHODS

Materials. Indium acetate (In(OAc)3, 99.99%), lauric acid (LA,
98%), zinc acetate (99.99%), oleic acid (OA, 90%), sulfur (99.98%),
selenium (99.99%), 1-octadecene (ODE, 90%), trioctylamine (TOA,
95%), and toluene (anhydrous 99.8%) were purchased from SigmaAldrich. Tris(trimethylsilyl)phosphine (TMS3P, 98%) and trioctylphosphine (TOP, 90%) were purchased from Strem. Acetone (HPLC
grade) was purchased from Samchun. All chemicals were used as
purchased.
Synthesis of InP Core. InP core was prepared by quickly
injecting a phosphine precursor to a solution containing indium and
zinc precursors. Briefly, zinc acetate (1.2 mmol) and OA (2.4 mmol)
were mixed in 10 mL of ODE. The mixture was evacuated at 120 °C
for 1 h, then refilled with N2, and cooled to RT. After addition of
indium acetate (0.6 mmol) and LA (1.8 mmol), the reaction mixture
was evacuated again at 120 °C for 1 h, refilled with N2, and heated to
150 °C. TMS3P (0.4 mmol) dissolved in TOP (1 mL) was quickly
injected into the reaction mixture, which was then heated to 240 °C
for successive growth. The reaction was monitored by measuring the
absorption spectrum of the aliquot and quenched by rapid cooling
below 200 °C. The resulting InP core was precipitated and washed
with acetone.
Preparation of InP/ZnSe/ZnS Core/Shell/Shell Quantum
Dots (QDs). A ZnSe shell was grown over InP core by reacting
Zn(OA)2 and Se/TOP. The Zn(OA)2 solution was prepared by
dissolving zinc acetate (2.4 mmol) and oleic acid (4.8 mmol) in 10
mL of TOA at room temperature, degassing at 120 °C under vacuum,
and refilling with N2 gas. The reaction mixture was heated to 180 °C.
The InP core dissolved in toluene was injected into the solution.
Then Se/TOP (0.4 M, up to a total amount of 1.8 mL) was added
stepwise by raising the temperature to 320 °C. The shell growth was
done by reacting at 320 °C for 1 h. Successively, the ZnS shell was
grown in the same reaction flask. A S/TOP solution (1.0 M, 0.8 mL)
was injected to the above reaction solution at 320 °C and kept at that
temperature for 40 min. After cooling to 280 °C, another 1.7 mL of
1.0 M S/TOP was injected and maintained for 1 h.
Preparation of InP/ZnS Core/Shell QDs. The ZnS shell was
grown over InP core using a similar method as described above. The
Zn(OA)2 solution was prepared in the same way, but with different
amounts of zinc acetate (2.0 mmol) and OA (4.0 mmol). At 220 °C,
InP core dissolved in toluene and 2 mL of 1.0 M S/TOP were
injected to the Zn(OA)2 solution. The shell growth was done by
reacting at 280 °C for 1 h.
Preparation of InP/ZnSe Core/Shell QDs. The InP/ZnSe QDs
were prepared by quenching the InP/ZnSe/ZnS reaction before S/
TOP injection. All core/shell QDs were purified before characterization to remove the reaction solvent and excess precursors.
Ensemble Spectroscopy. Steady-state absorption spectra were
recorded by using a UV/vis spectrometer (Cary 5000, Varian).
Steady-state photoluminescence (PL) of QDs in toluene was
measured by using a fluorescence spectrophotometer (F7000,
Hitachi). The absolute PL QY were measured using a quantum

12.8
10.2
3.8

a

PMX is the probability of forming a multiexciton state (MX) under
the given excitation conditions. Simple Poisson statistics was used to
demonstrate that the formation of biexciton or higher excitonic
species is a highly probable event on the millisecond time scale. Most
of the parameters were calculated based on ref 52.

total of n excitons (n = 1 and 2) and multiexcitons (MX, n ≥
2) in a single pulse and the associated Auger ionization
efficiencies as described by Peterson et al.52 The Auger
ionization probabilities (Pionize) of InP/ZnSe and InP/ZnS
QDs are larger than that of InP/ZnSe/ZnS. The same trend
was observed for the Auger ionization efficiency (Pionize/PMX),
suggesting that the InP/ZnSe/ZnS QDs have lower trap
densities than the core/shell QDs at higher energy states.
The photostability of the InP/ZnSe/ZnS QDs dissolved in
toluene (4 μM) were also examined by continuously
irradiating at a power of 30 μW/cm2 and wavelength of 458
nm under ambient conditions for 60 h. The relative PL
intensities do not show any change larger than 2%; in addition,
no spectral shift was observed as shown in Figure S12. The
shelf lifetime under nitrogen conditions is much longer than a
year, which is confirmed by no absolute PL QY changes one
year after synthesizing the QDs.

■

CONCLUSION
We have reduced the size distribution of InP cores by
regulating the reactivity of In and P precursors. Moreover, the
highly crystalline and well-passivated ZnSe/ZnS shell coating
performed at high temperature (320 °C) remarkably enhanced
the PL QY of QDs. The resulting InP/ZnSe/ZnS QDs showed
the highest quantum efficiency (95%) and the narrowest
emission width (36 nm) for green emitting InP-based QDs
ever reported. To study the photophysical properties that
depend on the shell compositions and structures, InP/ZnSe
and InP/ZnS QDs were also prepared using the same InP
cores. A comparison using TRPS and DAS analyses clearly
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Article

ACS Applied Nano Materials

structures for long on-time durations.42,52,53 In our study, the offtime (blinking recovery) kinetics were not governed by a single rate
constant for all samples because the probability densities were
distributed over a wide range of durations. Indeed, the blinking
recovery kinetics were fitted to a power law equation, P(τoff) ∝ τ−αoff.
Conversely, the on-time kinetics followed a truncated power law
behavior, P(τon) ∝ τ−αone−τ/τfalloff, which shows a “bent” curvature for
long τon, as previously reported by Wang et al. In log−log plots, the
power law coefficient is the slope of the linear fit, and the falloff time
(τfalloff) indicates the start of exponential truncation.50,53,54
Note that the probability P of creating multiple excitons within a
given bin time should be considered to understand the on-time
kinetics using Auger ionization processes. Thus, the probabilities of
creating n excitons in a single pulse (with n = 1, 2) and multiexciton
(MX, n ≥ 2) were calculated in the same manner (eqs 1 and 2)
according to the work by Peterson et al.25,52,55−57

efficiency measurement system with integrated sphere (QE2100,
Otsuka) under 10 times repetition. For temperature-dependent
measurement, a temperature-controlled liquid nitrogen cryostat
(Oxford Instruments, Optistat DN) was used. Temperatures were
maintained to within ±0.05 K, and the system was allowed to
equilibrate for 30 min before measurement.
Structural Analysis. High-resolution scanning transmission
electron microscopy (HR-STEM) images were obtained with a
Titan ChemiSTEM system operated at 200 keV. Inductively coupled
plasma atomic emission spectroscopy (ICP-AES) analysis was
performed with a Shimadzu ICPS-8100 sequential spectrometer. Xray diffraction (XRD) patterns were taken with a Bruker D8 Advance
instrument using a Cu Kα X-ray source (λ = 1.5418 Å). X-ray
photoelectron spectroscopy (XPS) analysis was performed using a
Quantera II of PHI Ulvac with 45° of takeoff angle.
Time-Resolved PL Measurements. A time-correlated singlephoton-counting (TCSPC) system was used for measuring the
spontaneous fluorescence decay. The excitation light source was a
mode-locked Ti:sapphire laser (Spectra-Physics, MaiTai BB) which
provides ultrashort pulse (center wavelength of 800 nm with 80 fs at
full width at half-maximum, fwhm) with high repetition rate (80
MHz). This high repetition rate was reduced to 800 kHz by using
homemade pulse picker. The pulse-picked output was frequency
doubled by a 1 mm thick BBO crystal (type I, θ = 29.2°, EKSMA).
The fluorescence was collected by a microchannel plate photomultiplier (MCP-PMT, Hamamatsu, R3809U-51) with a thermoelectric cooler (Hamamatsu, C4878) connected to a TCSPC board
(Becker; Hickel SPC-130). The overall instrumental response
function was about 25 ps (fwhm). A vertically polarized pump
pulse by a Glan-laser polarizer was irradiated upon samples, and a
sheet polarizer set at an angle complementary to the magic angle
(54.7°) was placed in the fluorescence collection path to obtain
polarization-independent fluorescence decays.
Single-Molecule Confocal Microscopy. Samples were prepared
by spin-coating QD solutions on rigorously cleaned quartz coverslips
at 2000 rpm for 60 s. The QD solutions were prepared with
chloroform containing 20 mg mL−1 polystyrene (Aldrich, average
MW = 44000). The confocal microscope (TE2000-U, Nikon) was
equipped with a sample scanning stage at RT. To excite the samples, a
circularly polarized light from a picosecond pulsed diode laser (LDHD-C-450, Picoquent, 1 MHz repetition rate, prepared using a Berek
compensator (5540, New Focus)) was passed through a laser line
filter (FF01-450/10-25, Semrock) and collimating lens, and then it
was focused on the sample via an oil immersion objective (Plan Fluor,
1.3 NA, 100×, Nikon) with a power density corresponding to an
average number of excitons per pulse ⟨Nx⟩ = 0.1. Fluorescent signals
were passed through a dichroic mirror (Z458rdc, Chroma
Technology), spectrally filtered using a notch filter (HNPF-450.01.0, Kaiser Optical Systems) and a band-pass filter (LP02-473RU-25,
Semrock), and then split by using a nonpolarizing 50:50 beam splitter.
Half of the fluorescence was dispersed via a spectrograph (SpectraPro
2150i, Princeton Instruments) and projected onto an EMCCD
camera (PL PROEM:512B EMCCD, Princeton Instruments). The
other half was detected by an avalanche photodiode (APD) module
(SPCM-AQR-16-FC, EG&G). The fluorescent signal detected by the
APD was registered by a TCSPC unit (SPC 830, Becker; Hickl). The
TCSPC was operated in first-in-first-out regime, in which the arrival
time after the beginning of acquisition and the time lag with respect to
the excitation pulse were stored for each detected photon. The fwhm
of the overall instrumental response function was approximately 500−
600 ps. The data were processed by using a BIFL data analyzer
software (Scientific Software Technologies Center) to obtain
fluorescence intensity trajectories and the time-resolved fluorescence
decays.
Probability Density Plots and Auger Ionization Efficiencies.
Probability distributions for the on- and off-times were calculated for
100 individual QDs to exclude any artifacts introduced by unexpected
events. According to previously reported probability density plots, the
probability densities for off-time events consistently followed a power
law distribution, whereas those for on-time showed “bending”

P⟨Nx⟩(n) = e−⟨Nx⟩⟨Nx⟩n /n!

(1)

for the probability of the average number of created exciton on ⟨Nx⟩
with n as an integer and
PMX = 1 − e−⟨Nx⟩ − ⟨Nx⟩e−⟨Nx⟩

(2)

for the probability of the multiexciton on ⟨Nx⟩, PMX.
The average number of generated excitons ⟨Nx⟩ was calculated by
multiplying the absorption cross section (σ, cm−2) at the excitation
wavelength and the intensity of laser (j, photons/cm2·pulse): ⟨Nx⟩ =
jσ. In our previous study, the Auger ionization efficiency influences
the falloff time of on-time distribution, since Pfalloff is the probability of
forming the multiexciton state (PMX) and the ionization probability
(Pionize) as shown in eq 3.
P falloff =

ΔTrep
τ falloff

= PMXPionize

(3)

where Pionize = Pfalloff/PMX (Auger ionization probability, Pionize) and
ΔTrep is the repetition rate of the pulsed laser.

■

ASSOCIATED CONTENT

* Supporting Information
S

The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsanm.8b02063.
Elemental compositions of QDs, STEM images of QDs,
HR-STEM image and XRD patterns of InP/ZnS QDs,
steady-state emission spectra, wavelength-dependent
time-resolved PL decay curves, temperature-dependent
PL spectra, high-resolution XPS spectra, schematics of
core and surface emission for the QDs, and on- and offtime probability density plots (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [e-mail].
*E-mail: [e-mail].
ORCID

Yongwook Kim: 0000-0002-9270-5993
Sujin Ham: 0000-0002-7950-2745
Heejae Chung: 0000-0001-5436-3753
Junho Lee: 0000-0001-6723-1469
Dongho Kim: 0000-0001-8668-2644
Eunjoo Jang: 0000-0003-2573-0176
Author Contributions

The synthesis of quantum dots and structural analysis were
performed by Y.K., H.J., and J.H.M. The HR-STEM images
were obtained by J.L. The photophysical properties of
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ACS Appl. Nano Mater. 2019, 2, 1496−1504

Article

ACS Applied Nano Materials

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quantum dots were studied by S.H. and H.C. The research was
designed, planned, and coordinated by D.K. and E.J. This
article was written in consultation with all authors. Y.K and
S.H contributed equally to this work.
Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS
This work at Yonsei University was financially supported by
the Samsung Advanced Institute of Technology (SAIT) under
Project IO170214-04232-01.

■

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