Deliverable D1.4: Report on QDs with tunable color and high quantum yield. Summary

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1 Deliverable D.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.

2 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.. 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. In the first route (figure a), 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

3 a) STEP STEP 2 b) Figure 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. a) 3 2 Absorbance (arb. units) Reaction time: min 20min b) (arb. units) Reaction time 20min 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 a, 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

4 the figure is shown for the nanocrystals extracted from the synthesis pot after min and 20 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 min to 540 nm for 20 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) Absorption (arb. units) b) Number of emitted photons per NC (0-3 s - ) 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. 4

5 a) b) (arb. units) with Zn without Zn defect emission XPS counts (arb. units) InP InP+Zn Os Energy (ev) Figure 4: a) b) Oxygen S XPS line of InP cores synthesized with and without Zn. of cores synthesized without Zn is dominated by defects. XPS OS peak of InP cores synthesized without Zn can be fitted with peaks found in InO x, 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. Following extensive structural and optical characterization, which included 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). 5

6 40 Efficiency (%) 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. (arb. units) 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 70 C produced nanocrystals with highest efficiency. Effectiveness of the shell can be seen in figure 7. spectrum of the InP core, shown in figure 7a 6

7 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. 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 20 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. a) (arb. units) core shell 30 min shell 60 min b) PLE Counts (arb. units) core only shell 30 min shell 60 min Excitation wavelength (nm) Figure 7: a) of InP cores (clack line) and InP/ZnS nanocrystals with shells synthesized for 30min (red line) and 60 min (blue line) with zinc at 70 C. b) excitation curves measured at 690 nm, when the excitation light wavelength was varied between 400 nm and 690 nm. 7

8 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 00 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 00%, still more than 00 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 8

9 elastomer and drop casting the mixture on a plate. Such prepared mixture is then cured in vacuum at 250 C for 3 hours. 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. (arb units) PMMA PC DC (arb. units) 0 - NC solution PMMA PC DC-OE Time (ns) Figure 8: a) Photoluminesnce spectra b) time resolved photoluminescence at a peak wavelength for nanocrystals in PMMA, PC and DC-OE

10 700 PL peak position PMMA 660 PC DC-OE Week no. Figure 9: change of the peak position of the photoluminescence for nanocrystals in PMMA, PC and DC-OE 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 00 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. 0