Project full title: " Nanowires for solid state lighting "
Project acronym: NWs4LIGHT
Grant agreement no: 280773
Deliverable D1.4:
“Report on QDs with tunable color and high quantum yield”
Responsible author: Dr Beata Kardynal, FZJ
Summary
The synthesis of the InP/ZnS nanocrystals with wavelengtnh in the range of green and red has
been developed in Forschungszetrum Jülich. The main findings are:
•
Colour tunability
Nanocrystals emitting at green and red spectral range and with quantm yield of at least
40% can be achieved.
•
InP core synthesis
Good quality InP core can be synthesized provided that there indium does not bind
with oxygen during the reaction.
•
ZnS shell formation
ZnS shell is only a few monolayers thick most likely due to the large lattice mismatch
with InP. Even such a thin layer passivates the nanocrystals. If single ZnS precursors
are used, the reaction is simple and performed at low temperature.
•
Integration with LED
Preliminary calculations suggest that colour conversion to achieve white light from
blue LED will require sub-mm thick layers of nanocrystals. Nanocrystals can be
embedded in polymers to form such plates but the transfer will have to optimized to
reduce current degradation of the nanocrystals.
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Project full title: " Nanowires for solid state lighting "
Project acronym: NWs4LIGHT
Grant agreement no: 280773
Colloidal nanocrystals, called also quantum dots to emphasize three dimensional
confinement of excitons, are considered as a possible colour conversion medium.
The
advantages include colour tunable with size and relatively easy synthesis at moderately low
temperatures. In this project we chose to test InP nanocrystals with ZnS shells to avoid toxic
cadmium.
Although InP colloidal nanocrystals synthesis based nanocrystals has been
demonstrated, the efficiency of the red light emitting quantum dots has been too low for
applications in solid state lighting.
1. Development of synthesis
Synthesis of colloidal nanocrystals is performed by a reaction of precursors of the
compounds or elements that will build the nanocrystal in a solvent at an elevated temperature,
which normally does not exceed 300°C. In order to control the reaction, organic molecules
and ligands are added to the reaction pot. The reaction is performed with argon flow above
the solution to prevent unwanted reactants (water vapour, oxygen etc.) from the gas phase
mixing with precursors.
We have tested two different routes to synthesis InP/ZnS nanocrystals: heat-up route and
hot-injection route, both schematically shown in figure 1. In the first route (figure 1a), all
precursors, for core and shell, are mixed at room temperature and then slowly heated to the
reactiontemperature. The core-shell structure of the nanocrystals is a result of different
reactivity of the precursors. In the second route, the reaction is initiated by injection of cold
precursors into a hot solution. As a result the core and shell reactions can be separated by
injecting the core precursors followed by the injection of the shell precursors. A parameter
space for the synthesis is rather large and includes temperature, time, type and concentration
of precursors and ligand, addition of amine to slow down the reaction. We have performed
comprehensive studies of these parameters, which we summarise below.
2
Project full title: " Nanowires for solid state lighting "
Project acronym: NWs4LIGHT
Grant agreement no: 280773
a)
STEP 1
STEP 2
b)
Figure 1 a) Schematic diagrams of a) the heat—up synthesis method where all precursors are heated up
together b) hot injection method where reaction is controlled by injection of precursors into hot solvent. Core
(left panel) and shell (right panel) are formed in two steps of reaction in hot injection method.
b)
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Reaction time:
1min
120min
2
1
Reaction time 120min
Photoluminescence
(arb. units)
Absorbance (arb. units)
a)
10-1
1
10-2
400
500
600
700
Wavelength (nm)
800
0
50
100
150
200
250
Time (ns)
Figure 2: a) Absorption and PL spectra of InP/ZnS nanocrystals formed using heat up method for different
amount of time b) photoluminescence decay curve obtained by illuminating the nanocrystals with a laser pulse
at time zero. A very fast initial decay precedes almost single-exponential decay.
Heat up method, schematically depicted in Figure 1a, has produced nanocrystals of
controlled size, as seen from well-defined excitonic absorption peak and relatively narrow
photoluminescence peak, shown for a representative batch of nanocrystals in figure 2. Data in
3
Project full title: " Nanowires for solid state lighting "
Project acronym: NWs4LIGHT
Grant agreement no: 280773
the figure is shown for the nanocrystals extracted from the synthesis pot after 1 min and 120
min, from the time when the final temperature of 290°C was reached. Peak wavelength of
emission for the two extractions has shifted from 527 nm for 1 min to 540 nm for 120 min
reaction, while the photoluminescene width remained the same. Longer reaction time resulted
in a reduction of defect states that are seen as an emission tail at wavelengths longer than 600
nm. Time resolved photoluminescence, which measures the decay of photoluminescence
following a pulse of light, confirmed that among bright nanocrystals non-radiative
recombination is weak. It manifests itself as a fast decay of photoluminescence at time zero,
in Figure 2b. Apart from the sharp initial decay, the decay is close to exponential, as expected
from good quality nanocrystals.
While the quality of the nanocrystals from heat up method is very good, it is not possible to
extend the emission wavelength beyond 600 nm. Hot injection method proved to be suitable
for extending the emission wavelength with the peak emission up to 700 nm as shown in
Figure 3. Number of parameters was found to result in larger nanocrystals’ sizes, such as
amount of fatty acid (which slow down the reaction) or number of injections of precursors.
a)
b)
Number of emitted
photons per NC (10-3 s-1)
Absorption (arb. units)
1.0
0.5
0.0
400
500
600
Wavelength (nm)
700
800
3
2
1
0
450
500
550
600
650
700
750
Wavelength (nm)
Figure 3: a) Absorption b) photoluminescence spectrum of nanocrystals synthesizes using hot injection
technique. Size control is achieved with the amount of myristic acid, which regulated the reaction speed.
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Project full title: " Nanowires for solid state lighting "
Project acronym: NWs4LIGHT
Grant agreement no: 280773
Photoluminescence
(arb. units)
1.0
with Zn
defect
emission
0.5
without Zn
0.0
500
600
700
XPS counts (arb. units)
b)
a)
InP
InP+Zn
1.0
0.5
0.0
528
800
530
532
534
536
O1s Energy (eV)
Wavelength (nm)
Figure 4: a) Photoluminescence b) Oxygen 1S XPS line of InP cores synthesized with and without Zn.
Photoluminescence of cores synthesized without Zn is dominated by defects. XPS O1S peak of InP cores
synthesized without Zn can be fitted with peaks found in InOx , peak of InP cores synthesized with Zn can be
fitted with peaks found in native oxide of InP.
Quantum yield of the emission for the nanocrystals shown in Figure 3 varied from 60% for
the shortest wavelength emission and decreasing to a few percent for the longest wavelength
emission. Strong decrease of efficiency in Figure 3b is accompanied with defect emission at
long wavelengths.
included
Following extensive structural and optical characterization, which
transmission
electron
microscopy
(TEM)
in
Ernst
Ruska
Center
in
Forschungszentrum Jülich, X-ray photoemission spectroscopy (XPS), X-ray absorption
spectroscopy (XAS), Fourier transform infrared spectroscopy (FTIR) we identified oxidation
of elemental indium in the core of the nanocrystal as the reason for non-radiative
recombination. Figure 4 compares photoluminescence of InP cores synthesized with and
without Zn present during core formation. The difference is striking, with the main emission
shifted from defects to excitons upon adding Zn.
XPS spectra of oxygen from these
nanocrystals have been fitted and correspond to oxides of elemental indium in the cores
synthesized without Zn and to native oxide of InP in the cores synthesized with Zn. Further
studies revealed that high quantum yield of red-emitting InP/ZnS nanocrystals can be
obtained when cores are synthesized with reactive precursors of Zn such as Zn-stearate or Znundercyclate and at optimum molar ratio of In:Zn (Figure 5).
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Project full title: " Nanowires for solid state lighting "
Project acronym: NWs4LIGHT
Grant agreement no: 280773
40
Efficiency (%)
30
20
10
0
0.1
1
10
Zn:In ratio
Figure 5: Efficiency of InP/ZnS nanocrystals with the photoluminescence peak around 630 nm, as a function of
Zn:In molar ratio during synthesis of the core.
Photoluminescence
(arb. units)
1
10-1
10-2
0
50
100
150
200
Time (ns)
Figure 6: photoluminescence decay curve obtained by illuminating the nanocrystals with a laser pulse at time
zero. A very fast initial decay precedes almost single-exponential decay.
Following optimization of InP core synthesis, we looked at the formation of the shell.
Standard method involves hot injection of individual precursors of Zn and S into a solution of
InP cores. The synthesis requires elevated temperature and some of the by-products are
oxidizing complexes which may degrade the cores. To overcome these limitations we turned
into using single precursors of ZnS.
Four different precursors were tested: ZnS-dimethyl,
ZnS-diethyl, ZnS-dibutyl and ZnS-dibenzyl. Each of the precursors resulted in a passivation
of the nanocrystals even at the reaction temperature of 90°C however ZnS-dibenzyl used
between 90°C and 170°C produced nanocrystals with highest efficiency. Effectiveness of the
shell can be seen in figure 7. Photoluminescence spectrum of the InP core, shown in figure 7a
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Project full title: " Nanowires for solid state lighting "
Project acronym: NWs4LIGHT
Grant agreement no: 280773
shows a small emission wing from the defects at long wavelength. This wing is greatly
reduced when the shell reaction is performed for 30 min and then 60 min. Photoluminescence
excitation spectroscopy provides more sensitive probe of the defect state. We used it to
monitor the photoluminescence from defects at 690 nm, while varying the excitation
wavelength. When the excitation wavelength matches the absorption on excitonic states in
the InP core (around 500 nm) a clear enhancement of photoluminescence from the defects is
observed. This means that excitons are transferred from the cores to the defect states. This
feature is greatly reduced after the shell is formed, showing that ZnS formation prevents such
energy transfer.
One more feature is visible in figure 7a: the emission wavelength of the nanocrystals shifts
to shorter wavelength with the shell reaction time. It is caused by etching of the cores during
the shell reaction and the continuous shift with the reaction time up to 60 min is only possible
if the shell is very thin and dynamic during this reaction.
Indeed, using all structural
characterization techniques available to us (TEM, XPS and XAS) we found that the shell
thickness is at most a few monolayers even when the shell reaction time is extended to 120
min. Since the same is the case for the standard synthesis method, we speculate that this is an
intrinsic problem caused by a lattice mismatch of 9% between InP and ZnS.
b) 1.0
a)
Photoluminescence
(arb. units)
PLE Counts (arb. units)
core
shell 30 min
shell 60 min
1.0
0.5
0.0
500
600
700
Wavelength (nm)
800
core only
shell 30 min
shell 60 min
0.5
0.0
400
500
600
Excitation wavelength (nm)
Figure 7: a) Photoluminescence of InP cores (clack line) and InP/ZnS nanocrystals with shells synthesized for
30min (red line) and 60 min (blue line) with zinc at 170 °C. b) Photoluminescence excitation curves measured
at 690 nm, when the excitation light wavelength was varied between 400 nm and 690 nm.
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Project full title: " Nanowires for solid state lighting "
Project acronym: NWs4LIGHT
Grant agreement no: 280773
Since there are no materials with a suitable bandgap and lattice constant for shells to InP
nanocrystals, we are planning in future to study intermediate layers (such as ZnSe) which
have smaller bandgap but better matching lattice constant, which would promote a growth of
ZnS. If parallels can be drawn from Cd-based nanocrystals, thicker ZnS shell should be
helpful in reducing Auger recombination and of course better protection from environment in
applications in solid state lighting. The latter can be also achieved if a suitable polymer
material can be used to embed the nanocrystals in it.
2. Towards integration of NCs with NW-LEDs
In order to generate white light of correlated colour temperature of 4000 K from blue LED
emitting at 455 nm and InP/ZnS nanocrystals of 560 nm and 680 nm, both with 100 nm full
width half maximum, as much as 33.3% of total power flux has to originate from 560 nm NCs
and 55.6% from 680 nm NCs. Such large fluxes from the nanocrystals in combination with
small absorption probability of light by the nanocrystals mean that even if the nanocrystals are
distributed on the grid of 20 nm and their quantum yield is 100%, still more than 100 µm
thick layer of nanocrystals is needed. Considering the height of the nanowires of the order of
a few µm, it is clear that the only method of integrating such number of nanocrystals as a
colour conversion medium is as with conventional phosphors.
In the preliminary work toward fabricating such colour conversion plates, we explored
three different polymers that could serve as matrices for the nanocrystals. These were
polymethylmethacrylate (PMMA), polycarbonate (PC) and DC-OE6630. The last material is
an silicon based elastomer that is developed by Dow Corning specifically for encapsulation of
LEDs. Samples based on PMMA or PC were prepared by mixing the nanocrystals (dissolved
in a small quantity in toluene) with the solution of PMMA or PC in toluene. The two mixed
phases were stirred with a magnetic stirrer for a few hours after which the mixture was
dispensed on a clean glass plate. The mixture was either spin coated or drop casted on the
plate with only the second method producing smooth, thick enough layers. Such prepared
samples were dried at room temperature in air overnight before testing. Sample preparation
based on DC-OE6630 also starts from mixing the nanocrystals’ solution with the pre-mixed
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Project full title: " Nanowires for solid state lighting "
Project acronym: NWs4LIGHT
Grant agreement no: 280773
elastomer and drop casting the mixture on a plate. Such prepared mixture is then cured in
vacuum at 250 °C for 3 hours. Photoluminescence spectra of all three types of the materials
are shown in figure 8a. The spectra are very similar to each other; there is no difference in the
amount of photoluminescence at long wavelength, the FWHM of the spectra is 95nm. Time
resolved photoluminescence shows that transfer of the nanocrystals from the solution to the
matrix did however lead to increased non-radiative recombination and this effect will have to
be quantified and eliminated in future.
The samples were studied over a period of up to nine weeks while they were stored in the
laboratory, mostly dark but not protected from the ambient. There was no clear change of the
photoluminescence of the samples (figure 9) although the peak wavelength was jumping
randomly by a few nanometers.
These initial results are encouraging but more work is needed to understand the nature of
degradation of the nanocrystals, prevent it. Long term stability of the photoluminescence
peak position and measurements of the quantum yield of the nanocrystals in the matrices will
also need to be performed.
1
PMMA
PC
DC
Photoluminescence
(arb. units)
Photoluminescence
(arb units)
1.0
0.5
NC solution
PMMA
PC
DC-OE6630
10-1
10-2
0.0
500
600
700
Wavelength (nm)
800
0
100
200
Time (ns)
Figure 8: a) Photoluminesnce spectra b) time resolved photoluminescence at a peak wavelength for
nanocrystals in PMMA, PC and DC-OE6630.
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Project full title: " Nanowires for solid state lighting "
Project acronym: NWs4LIGHT
Grant agreement no: 280773
PL peak position
700
690
680
670
PMMA
PC
DC-OE6630
660
650
0
2
4
6
8
10
Week no.
Figure 9: change of the peak position of the photoluminescence for nanocrystals in PMMA, PC and DC-OE6630.
3. Conclusions
A hot injection method of synthezing of colloidal InP/ZnS nanorystals is suitable for
fabrication of nanocrystals emitting at a wide range of wavelengths.
Although the
nanocrystals have broad spectra (up to 100 nm), the quantum yield of 40% at 630nm emission
wavelengths is possible. Critical parameters for high performance nanocrystals is presence of
zinc precursor during core formation and low temperature synthesis of the ZnS shell. It is
expected that further progress in the quality of the nanocrystals can be achieved by
introducing intermediate layer beween the InP core and ZnS shell.
Preliminary attempts to convert blue LED into white light involved mixing of hte
nanocrystals with polymers for both mechanical stability and prtection from environment.
Such materials seem stable over time of weeks but the transfer of nanocrystals from the
original solution to the polymer results in their degradation. Further studies are needed to
improve the process.
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