RIEHLE, Frank, Stefan (Im Brϋnneleacker 1, Umkirch, 79224, DE)
KRUGER, Michael (Scheffelweg 8, Emmendingen, 79312, DE)
YUAN, Ying (Unterbaselweg 16, Weil a. Rhein, 79576, DE)
RIEHLE, Frank, Stefan (Im Brϋnneleacker 1, Umkirch, 79224, DE)
KRUGER, Michael (Scheffelweg 8, Emmendingen, 79312, DE)
| CLAIMS 1. Luminescent polymer comprising a polyamide host and luminescent quantum dots integrated therein. 2. Luminescent polymer according to claim 1 , wherein said polyamide host comprises polyamide 6 which is polymerized from 6-aminocaproic acid. 3. Luminescent polymer according to claim 1 or 2, wherein said quantum dots comprise CdSe nanocrystals. 4. Luminescent polymer according to one of the claims 1 to 3, wherein said quantum dots are provided with a protective ligand. 5. Luminescent polymer according to claim 4, wherein said protective ligand comprises hexadecylamine, HDA, and trioctylphosphine oxide, TOPO. 6. Luminescent polymer according to claim 4, wherein said protective ligand comprises hexadecanol, HDO. 7. Luminescent polymer according to one of the claims 1 to 6, being polymerized in a mould to have a molded three-dimensional form. 8. Method for fabricating a luminescent polymer, said method comprising the steps of: Synthesizing luminescent quantum dot nano crystals having a predetermined light emitting wavelength; Polymerizing a polyamide polymer from a mixture of said quantum dot nanocrystals and a precursor molecule. 9. Method according to claim 8, wherein said precursor molecule is a monomer comprising 6-aminocaproic acid. 10. Method according to claim 8 or 9, further comprising the step of providing a protective ligand sphere to said quantum dot nanocrystals. 11. Method according to claim 10, wherein said ligand comprises hexadecylamine, HDA, and trioctylphosphine-oxide, TOPO. Method according to claim 10, wherein said protective ligand comprises hexadecanol, HDO. Method according to one of the claims 10 to 12, after the polymerization step further comprising the step of mechanically removing an excess of ligand by phase separation. Method according to one of the claims 8 to 13, wherein the polymerization step is performed at a temperature in the range of 150 to 300 °C, preferably in the range of 220 to 250 °C. Method according to one of the claims 8 to 14, wherein the polymerization step is performed under a protecting gas, preferable under nitrogen saturated atmosphere. Method according to one of the claims 8 to 15, wherein the polymerization step is performed in a casting mould. Method according to one of the claims 8 to 16, wherein the step of synthesizing luminescent quantum dot nanocrystals comprises: Degassing a mixture of Cd stearate, hexadecylamine, HDA, and trioctylphosphine oxide, TOPO, at an elevated temperature; Injecting a solution of Se in trioctylphosphine, TOP, into said mixture; Taking aliquots after a predetermined reaction time, said reaction time being adapted for adjusting the light emitting wavelength of the quantum dots to a predetermined value. Lighting device with a wavelength transforming layer for modifying the wavelength of the emitted radiation, said wavelength transforming layer comprising a luminescent polymer according to one of the claims 1 to 7. Use of the luminescent polymer according to one of the claims 1 to 7 for an energy conversion in lighting systems for green houses. Use of the luminescent polymer according to one of the claims 1 to 7 as a light absorbing layer in a solar concentrator cell. Fluorescent standard for confocal imaging techniques, said standard being fabricated from a three-dimensional hybrid material comprising a luminescent polymer according to one of the claims 1 to 7. |
The present invention relates to luminescent polymers which have luminescent quantum dots integrated therein.
Hybrid materials based on nanoparticles and polymeric systems show an increasing potential for the development of new functional composite materials. Examples are the incorporation of carbon nanostructures like carbon nanotubes into polymers to achieve superior mechanical strength, as well as the development of bulk-heterojunction devices out of semiconducting quantum dots (QDs) and semiconducting polymers as new hybrid-materials for LED and photovoltaic applications. Advantages of fluorescent semiconducting nanocrystals compared to conventional organic dyes are their higher photostability, their narrow light emission and their broad absorption characteristic enabling e.g. efficient energy down-conversion.
Different approaches for the incorporation of semiconducting quantum dots into non photoactive polymers have been reported by several groups to realize down conversion LEDs for pure and mixed colors emission including the generation of white light and for the generation of UV- shielding nanocomposite films using ZnO/PMMA hybrids. In addition, quantum dots have been incorporated in thin films out of ZnS and solvent based polymers with potential for flat panel displays as well as in polymerizable ionic liquid matrices. One major challenge is to avoid aggregation processes and luminescence quenching while incorporating the QDs into a threedimensional matrix. This was for example achieved by using additional protecting trioctylphosphinoxid (TOPO) ligand to prevent the attack of CdSe/ZnS quantum dots incorporated into polylaurymethacrylate matrices using a radical polymerization reaction. Alternatively, a silica shell was used as protective shell for the incorporation of InP/ZnS nanoparticles into silicone. In both cases the photoluminescence of the nanoparticles was maintained in the hybrid material. So far 50% quantum yield is the best of our knowledge. A quantum dot (QD) in the terminology of the present application signifies a semiconductor whose excitons are confined in all three spatial dimensions. As a result, they have properties that are between those of bulk semiconductors and those of discrete molecules. Potential application fields can be found in electronic components such as transistors, solar cells, LEDs and diode lasers as well as agents for medical imaging. Quantum dots are semiconductors whose conducting characteristics are closely related to the size and shape of the individual crystal. Generally, the smaller the size of the crystal, the larger the band gap, the greater the difference in energy between the highest valence band and the lowest conduction band l becomes. Therefore, more energy is needed to excite the dots and concurrently, more energy is released when the crystal returns to its resting state. For example, in fluorescent dye applications, this equates to higher frequencies of light emitted after excitation of the dot as the crystal size grows smaller, resulting in a colour shift from red to blue in the light emitted. The main advantage in using quantum dots can be seen in a very precise control over the optical properties of the materials, due to the high level of control which can be achieved over the size of the crystals produced.
There are several ways to confine excitons in semiconductors resulting in different methods to produce quantum dots. One of the most promising techniques is the colloidal synthesis. Colloidal semiconductor nanocrystals are synthesized from precursive compounds dissolved in solution and there are colloidal methods to produce many different semiconductors. Typical QDs are made from binary alloys such as cadmium selenide, cadmium sulphide, indium arsenide, and indium phosphide. QDs may also be made from ternary alloys such as cadmium, selenide sulphide. These quantum dots can contain as few hundred to a few thousands of atoms. This corresponds to about 2 to 10 nanometres. Large batches of quantum dots may be synthesized via colloidal synthesis. Due to the scalability and the convenience of bench-top conditions, colloidal synthetic methods are most promising for commercial applications and are furthermore acknowledged to represent the least toxic of the different forms of synthesis.
Important technical applications for quantum dots are firstly light conversion layers. Such colour converting luminescent materials may, for instance, be used as coating material for white light emitting diodes (LED). By converting blue portions of the radiation into red portions, the adaptation to the solar spectrum can be optimized and the so-called "cold" white light can be avoided for illumination. When using quantum dots as a converting coating of UV and blue LEDs, all desired colours can be generated with a high accuracy and with minimized conversion losses. In particular, for novel displays such materials are required. Furthermore, illumination systems can be coated in order to generate radiation with a predetermined desired wavelength, for instance, by adapting the light emission to emit radiation which corresponds to the absorption characteristics, for instance, of chlorophyll. This may be used for directly enhancing the generation rate of bio-fuels when modifying the illumination of plants and algae. Luminescent materials which emit light at well defined wavelengths can also be used as reference standards for three-dimensional microscopy. Known luminescence conversion layers suffer from the disadvantage that their quantum efficiencies are mostly far below 50%. Furthermore, conventional QD synthesis mostly implies a process comprising several steps because usually one or more protective layers have to be applied to a core material. This procedure is connected with higher material losses due to purification steps and side reactions.
Conventional standards for three-dimensional fluorescence microscopy suffer from the problem that the fluorescent dye bleaches in the course of times. Accordingly, a bleaching factor has to be taken into account by performing respective corrections. In particular, the z-axis calibration for evaluating three-dimensional images is often inaccurate and too complicated. Consequently, there exists a need for luminescent polymers comprising luminescent quantum dots which can be fabricated in a particularly simple way, have a high quantum yield and furthermore do not suffer from bleaching to a high extent.
This object is solved by the subject matter of the independent claims. Advantageous embodiments are the subject matter of the dependent claims. According to the present invention, the luminescent polymer is formed from a polyamide host structure with luminescent quantum dots integrated therein.
Polyamides or nylons are a versatile family of thermoplastics that have broad range properties ranging from relative flexibility to significant stiffness, strength and toughness. Major properties such as resistance to chemicals, toughness, thermal stability and well established processability are key considerations that made polyamides suitable for a broad range of applications during the last few decades.
It could be shown by the inventors of the present application that by integrating the luminescent quantum dots into such a polyamide host, the initial quantum yield can be maintained. A hybrid material according to the present invention exhibits quantum yields of more than 65%, thus offering a high potential for applications in direct light and energy conversion.
Furthermore, a luminescent polymer according to the present invention suffers from bleaching effects much less than conventional luminescent polymers. Moreover, due to the mechanical characteristics of the cross linked polyamide host, three-dimensional structures can be formed that are particularly suitable for being used as a standard in three-dimensional luminescent microscopy. Polyamides comprise a wide range of materials, depending on the monomers employed. Commonly used products are designated as nylon 6; 6,6; 6,12; 1 1 and 12 with the nomenclature designing the number of carbon atoms that separate the repeating amide group. Nylon 6 and nylon 6,6 continue to be the most widespread types among polyamide commercial products. Two basic reactions are used to synthesize polyamide engineering polymers: firstly the polymerization of a dibasic acid and a diamine or, secondly, the polymerization of an amino acid or lactam. Although all commonly used polyamides can be employed according to the principles of the present invention, it could be shown that particularly advantageous characteristics can be achieved by fabricating the polyamide host as a 6 polyamide polymerized from 6 amino caproic acid.
According to an advantageous embodiment, the quantum dots comprise CdSe nanocrystals. However, also other QD materials, such as CdS, CdTe and ternary alloys out of e.g. CdZnSe and CdZnS can be used. Preferably, the quantum dot nanocrystal will be prepared by a colloidal synthesis method as mentioned above. These core semiconductor materials allow for optimized size distribution, surface quality, and colour tuning in the visible spectrum. CdZnS can be fine tuned across the entire blue region CdZnSe nanocrystals can provide narrow band emission wavelengths from 500 to 550 nanometres and CdSe is used to make the most efficient and narrow band emission in the yellow red part of the visible spectrum (550 to 650 nanometres). Each semiconductor material is chosen specifically to address the wavelength region of interest to optimize the physical size of the QD material, which is important to achieve good size distributions, high stability and efficiency as well as ease of processability.
In certain embodiments, quantum dots can include a ternary semiconductor alloy. The use of a ternary semiconductor alloy can also permit use of the ratio of cadmium to zinc in addition to the physical size of the QD nanocrystal in order to tune the colour of the emission. According to the present invention, the quantum dots are provided with a protective ligand such as hexadecylamine (HDA), and Trioctylphosphineoxide (TOPO). Alternatively, also hexadecanol (HDO) can be used as the protective ligand. The advantage of providing such a ligand can be seen in preventing aggregation processes and luminescence quenching while incorporating the QDs into a three-dimensional matrix. Furthermore, TOPO prevents a disintegration of the cadmium stearate in the presence of higher temperatures.
The advantageous characteristics of the luminescent polymer according to the present invention may most effectively be used when applying it to a lighting device as a wavelength transforming layer, using it for energy conversion in lighting systems for greenhouses or using it as a light absorbing layer in a solar concentrator cell.
Furthermore, a fluorescent standard for confocal imaging techniques can be provided in an advantageous way, said standard being fabricated from a three-dimensional hybrid material comprising a luminescent polymer according to the present invention.
In contrast to existing standards for calibrating three-dimensional fluorescent microscopes, standards fabricated from the luminescent polymer according to the present invention exhibit significant less bleaching and therefore allow for a much more accurate calibration in z-direction. Consequently, a high three-dimensional resolution of such images can be achieved. For the application as a fluorescence reference standard, the quantum dot polymer hybrid material can be cast into a desired three-dimensional form and polymerized in situ in a mold.
To obtain accurate and reproducible results from fluorescent imaging applications, it is generally essential to maximize the performance of the optical system. Careful calibration and instrumentation adjustment are required for high precision imaging of fluorescent probes, particularly in multi-colour applications that involve multiple exposures, repetitive scans or three- dimensional sectioning. With the fluorescence microscopy reference standard according to the present invention, the alignment sensitivity and stability of confocal laser scanning microscopes can be examined and the optical sectioning thickness (Z resolution) in three-dimensional imagining applications can be confirmed in a particularly simple and effective way. The accompanying drawings are incorporated into and form a part of the specification to illustrate several embodiments of the present invention. These drawings together with a description serve to explain the principles of the invention. The drawings are merely for the purpose of illustrating the preferred and alternative examples of how the invention can be made and used and are not to be construed to limiting the invention to only the illustrated and described embodiments. Furthermore, several aspects of the described embodiments may form individually or in different combinations solutions according to the present invention. Further features and advantages will become apparent from the following more particular description of the various embodiments of the invention, as illustrated in the accompanying drawings in which like references refer to like elements, and wherein: Figure 1 shows a schematic diagram of the fabrication of a luminescent polymer comprising a polyamide host and luminescent quantum dots integrated therein; Figure 2 shows the absorption spectra of differently sized cadmium selenide quantum dots;
Figure 3 shows the photo-luminescence intensity of differently sized quantum dots;
Figure 4 shows a laser scanning microscopical investigation of a thin film with a photo- luminescent wavelength of 566 nanometres; Figure 5 shows a laser scanning microscopical investigation of a thin film with a photo- luminescent wavelength of 589 nanometres;
Figure 6 shows the laser scanning microscopical investigation of a thin film with a photo- luminescent wavelength of 630 nanometres;
Figure 7 shows a schematic flow diagram of the fabrication process according to the present invention;
Figure 8 shows the bleaching characteristics of CdSe QD polymer films under different laser power exposure conditions.
Figure 1 shows the in situ polymerization of a CdSe quantum dot nylon hybrid material according to the present invention. The material exhibits quantum yields of more than 65% with a high potential for applications in direct light and energy conversion. The present invention provides an easy, reproducible one pot in situ polymerization to obtain a quantum dot polymer hybrid material out of prefabricated cadmium selenide quantum dots and nylon with photo-luminescent quantum yields exceeding 60%.
The CdSe quantum dots were synthesized at 300°C from Cd stearate and trioctylphosphine selenide (TOPSe) in an organic matrix consisting of trioctylphosphine oxide and hexadecylamine using a hot injection method as described in the article Y. Yuan et al., J. Nanosci. Nanotech., 2010, Vol.10, p.6014-6045.
In the following, the detailed steps of an advantageous improved method for synthesizing colloidal stable cadmium selenide nanocrystals with high fluorescence quantum efficiencies, in particular, a method for fabricating core CdSe quantum dots in a hexadecylamine/ trioctylphosphineoxide matrix is described. Fabricating core CdSe quantum dots in a hexadecanolamine/trioctylphosphine matrix
1. Preparation of Cd and Se precursors a) 1 mmol of red brown cadmium oxide, CdO, and 4 mmol colourless stearic acid HSA , and a catalytic amount of succinic acid are heated for 5 to 60 minutes under inert gas to 200°C. The reaction that leads to the forming of cadmium stearate, Cd(SA) 2 is finished when a clear uncoloured solution is yielded. Cd(SA) 2 is solid at room temperature and can be used for fabricating cadmium selenide nanocrystals without further preparation steps. A slight excess of HSA is necessary for completely transforming CdO and for yielding the colourless solution. Equation 1 shows the chemical reaction: CdO + 2 HSA Cd(SA) 2 + H 2 0 (200 °C) (1) b) 1 mmol black selenium, Se, is dissolved in 1 ml colourless trioctylphosphine, TOP, under inert gas at 200°C (6 to 24 hours). Trioctylphosphine selenide, TOPSe is generated as a reaction product. The reaction is finished if a clear colourless solution is yielded. The 1 molar Se precursor solution can be stored at room temperature under air exclusion and can be used after a systematic aging process. This systematic aging of the Se precursor solution under air exposure is an essential step of the synthesis of strongly fluorescent CdSe nanocrystals. The forming of trioctylphosphine selenide is described by the following reaction equation:
Se + TOP - TOPSe (200 °C) (2)
2. Synthesis of CdSe nanocrystals The reaction comprises hexadecylamine (HDA), and trioctylphosphine oxide( TOPO) ,in a molar ratio of 6 : 4. The matrix at the same time serves as a solvent and as a ligand for forming the CdSe nanocrystals in a temperature region between 100°C and 300°C. Furthermore, TOPO prevents the disintegration of cadmium stearate at high temperatures, such as 300°C. The molar rate of Cd to Se is 1 : 1 , whereas the molar rate of Cd and Se, respectively, to the matrix is 1 : 100. All following synthesis steps are performed under inert gas atmosphere, for instance, nitrogen.
In order to produce an optimal synthesis product, HDA is heated for 30 minutes to about 300°C. HDA forms carbonate together with C0 2 from the air which is an inferior ligand for cadmium selenide nanocrystals. By a well defined heating, the C0 2 is removed again. For the actual reaction, 0.1 mmol solid Cd(SA) 2 is dissolved in 4 mmol TOPO under an elevated temperature of about 100°C and is then added to 6 mmol HDA at 300°C. Subsequently, the aged Se precursor solution is swiftly added to the uncoloured solution. An instantaneous colour change to red indicates the forming of CdSe nanocrystals. A sharp distinction between nucleation and growth can be observed, leading to the forming of homogenous nanocrystals. This observation is directly connected with the aging process of the Se precursor. In order to structure the ligands at the surface of the homogenous nanocrystals, an annealing step for enhancing the fluorescence quantum efficiency is performed by heating the nanocrystals for about 2 hours to 300°C. During this step, defects are cured by annealing the crystal, while surface damaging processes such as, for instance, the Ostwald ripening, are suppressed due to the HDA ligands firmly bound to the surface of the nanocrystals.
In a next step, the reaction mixture is slowly cooled until the solidification point is reached in order to promote performing of a protective HDA ligand sphere of up to 100 nanometres around the nanocrystals. Thus, the solubility and stability of the nanocrystals is enhanced and therefore an important criterion for a successful integration of nanocrystals into polymers is fulfilled.
Generally, for forming a protective ligand sphere, three conditions have to be fulfilled: a) presence of a sharp separation between nucleation and growth or forming of homogenous nanocrystals, which can be reached, for instance, by a well defined aging of the Se precursor under air atmosphere, b) the pre-treatment of the HDA ligand at 300°C in order to get rid of interfering C0 2 which allows annealing the HDA ligand at the crystal surface at high temperatures in order to promote healing of detrimental defects, and c) a slow cooling which allows the forming of a protective ligand sphere at the now highly organized crystal surface.
Fabricating core CdSe quantum dots with hexadecanol ligand system
Another advantageous approach for synthesizing colloidal stable CdSe nanocrystals is the use of aliphatic alcohols such as hexadecanol (R-OH) as a novel ligand system. This alcoholic ligand system leads to CdSe core quantum dots having medium quantum efficiency. This ligand can be used for luminescent CdS, CdSe and CdTe quantum dots. The alcoholic ligand system is very cost efficient and may be an advantageous ligand for all applications where not the optimization of the quantum efficiency but of the fluorescence stability under illumination is important. For instance, when using the cadmium selenide quantum dot polymer hybrid material according to the present invention as fluorescence calibration standard for laser scanning microscopy, nanocrystals with hexadecanol as a ligand have proved a higher light stable than cadmium selenide quantum dots with an HDA/TOP as ligands.
An example for synthesizing hexadecanol covered cadmium selenide nanocrystals is described in the following.
50 μΙ of the TOPSe precursor (produced as described above) are filled with 1.21 g hexadecanol (HDO) and 40 mg cadmium laureate under inert gas atmosphere into a sealable microwave synthesis glass container. During a pre-heating step (80°C), cadmium laureate is solved within 3 minutes under fast stirring and then the reaction mixture is heated by means of 300 W microwave power to 200°C. Depending on the reaction time, cadmium selenide core quantum dots can be synthesized with different sizes. In contrast to known fabrication methods, the CdSe quantum dots according to the present invention are fabricated without an additional anorganic protection layer such as ZnS or CdS (which is also called core shell materials). These shell materials have a larger band gap compared to cadmium selenide. As shown in Fig. 1 , the synthesis of the CdSe quantum dot polymer hybrid material according to the present invention is done as a simple "single pot reaction" without any pre-purification of the QD synthesis product. The best results were achieved by performing the in situ polymerization under inert gas atmosphere, for instance, nitrogen. It could be shown that the in situ polymerization of a polyamide was able to maintain the superior fluorescence of the quantum dot also within the polymer material. Other known polymers yielded hybrid materials with significantly deteriorated fluorescence efficiency. In particular, a polyamide host material fabricated as described by the following reaction equation has been proved to be particularly efficient. m HOOC(CH 2 )niCOOH + m H 2 N(CH 2 )n 2 NH 2 -
HOOC(CH 2 )[-OC(CH 2 ) n1 CONH-(CH 2 ) n2 -NH] (m . ir (CH 2 ) n NH 2 (3) wherein n 1 ,2=1 - 12 (tested) By modifying the cooling of the QD hybrid material out, the transparency of thicker films can be endured. Furthermore, the use of HDA or HDO as ligand ensures in combination with the nylon post material that the fluorescence keeps a stable value even when being irradiated with a laser for a longer time. Figure 3 shows that by stopping the reaction for fabricating the quantum dots, green 301 , yellow 302, orange 303 and red 304 emitting quantum dots were obtained by stopping the reaction after 3 seconds, 15 seconds, 60 seconds and 300 seconds, respectively. This led to differently sized quantum dots. In particular, it could be shown that green emitting QDs had a diameter of
3.18±0.56 nm, yellow emitting QDs had a diameter of 3.50±0.46 nm, orange emitting QDs had a diameter of 4.06±0.43 nm, and red emitting QDs had a diameter of 5,57±0.71 nm. The corresponding absorption spectra are shown in Figure 2. Here, reference numeral 201 represents the curve for the green quantum dots, reference numeral 202 the yellow, 203 the orange, and 204 the red quantum dots.
The polymerization of 6 amino caproic acid monomers in the presence of as-prepared cadmium selenide quantum dots may advantageously be performed at 220 to 250°C in a straightforward process under ambient conditions or under nitrogen atmosphere. All four differently sized quantum dots can be incorporated into the nylon polymer and laser scanning microscopical, LSM imaging of the hybrid materials revealed the homogenous distribution of the yellow, orange and the red emitting quantum dots in the polymer while keeping their original emission colour as can be seen from Figures 4 to 6. The green emitting quantum dots, however, showed a red shift due to the further growth during the polymerization reaction (not shown in the Figure). The best long term stability and quantum yields of 65% were found for the red emitting quantum dot polymer hybrid material, which exhibited only little loss compared to the original red emitting quantum dots which had a quantum efficiency of 72%. The flow diagram of Fig. 7 illustrates a method for fabricating the hybrid QD material according to the present invention. In particular, Cd-stearate was prepared from 32.1 mg CdO (0.25 mmol) and 248.9 mg stearic acid (0.875 mmol) at 200 °C under nitrogen atmosphere. The reaction was stopped when a colourless solution appeared. TOPO (99%) and stearic acid (≥97%) were purchased from Sigma Aldrich, HDA (99%) was obtained from Fluka and CdO (99.998%) from Alfa Aesar. Se powder (99.999%) and TOP (97%) were obtained from ABCR (Karlsruhe, Germany). 6-Aminocaproic acid (≥99%) was purchased from Sigma Aldrich. For synthesis of CdSe quantum dots with defined size, typically a mixture of 277 mg Cd-stearat (0.25 mmol), and 3.866 g trioctylphosphine oxide (TOPO) (10 mmol) was added to preheated 3.622 g hexadecylamine (HDA) (15 mmol) was then degassed at 100 °C. 0.25 ml of a 1 M solution of Se in TOP was swiftly injected at 300 °C and the colourless solution became yellow within 1 s and turned to red after about 5 s. Aliquots were taken after different reaction times and dissolved in chloroform reaching an absorbance of about 0.5. The reaction was monitored by UV-VIS spectroscopy by taking subsequently time aliquots. For TEM investigation the QDs were precipitated in a mixture of chloroform and methanol by centrifugation at 5000 rcf for 5 min at room-temperature and re-dissolved in chloroform. No size selective methods were applied additionally.
According to the present invention, an in-situ synthesis of the quantum dot-nylon hybrid material was performed using the unpurified QDs. Different amounts of as prepared CdSe QDs (40 mg, 80 mg and 200 mg, the concentration of the QDs in solution is estimated to be about 0.25~0.3 wt %) were obtained from their growth solution without applying any purification steps. The QDs capped with a monolayer of TOPO/HDA ligands were re-dispersed in 2g of the nylon monomes 6-Aminocaproic acid (Company Sigma-Aldrich) Then, the mixture was heated to 220~250 °C with and without N 2 protection. Product synthesized under N 2 protection are superior and allow repeatable heating cycles without significant fluorescence quantum yield degradation. First, the NCs powder melted and turned from solid to liquid. Later, the nylon monomer also became liquid and the appearance of air bubbles demonstrated the successful polymerization of nylon monomers due to condensation. A clear QD-polymer solution was obtained after 2~5 min heating at 220~250 °C. After solidification of the polymer hybrid material excess of ligands phase separated from the hybrid material and could be removed easily.
According to advantageous embodiment, the quantum dots hybrid material can be processed while kept in liquid phase above 150°C and a multitude of different forms and shapes can be generated. The transparency of the resulting product can be increased by fast cooling of the liquid phase.
Fig. 8 shows experimental details on the bleaching behaviour of the inventive hybrid material.
In particular, Fig. 8 is a graph showing bleaching experiments of CdSe QD polymer films (containing HDA capped QDs) under different laser power exposure conditions: After some time a constant intensity stays showing a stable fluorescent situation. After stopping the laser exposure the fluorescent intensity raises to the initial value (reversibility). The initial change in intensity after laser irradiation is depending on the laser power, the QD loading of the film and on the type of ligand shell around the QDs. The experiments have been performed at 2 Photon excitation conditions at an excitation wavelength of 780 nm. Commercially available QDs with the same emission wavelength of 630nm with an inorganic ZnS protective shell do not show a stable fluorescence under similar experimental conditions.
In summary, a novel in-situ synthesis approach leading to highly luminescent CdSe quantum dot-nylon hybrid materials is demonstrated. This materials exhibit quantum yields of more than 65% with a high potential for applications in direct light and energy conversion.
According to the present invention, an easy and reproducible one pot in-situ polymerization process is presented, in order to obtain quantum dot polymer hybrid materials out of CdSe quantum dots and nylon with photoluminescent quantum yields exceeding 60%. The CdSe quantum dots were synthesized at 300 °C from Cd-stearate and trioctylphosphine-selenide (TOPSe) in an organic matrix consisting out of trioctylphosphine-oxide and hexadecylamine using a hot injection method. Green, yellow, orange and red emitting quantum dots were obtained by stopping the reaction after 3s, 15s, 60s and 300s respectively leading to differently sized quantum dots
The polymerization of 6-Aminocaproic acid monomers in the presence of as-prepared CdSe quantum dots was performed at 220-250°C in a straightforward process under nitrogen atmosphere or ambient conditions. In principle all four differently sized quantum dots could be incorporated into the nylon polymer.
Laser scanning microscopical (LSM) imaging of the hybrid materials revealed the homogenious distribution of the yellow, the orange and the red emitting quantum dots in the polymer while keeping their original emission color. In contrast the green emitting quantum dots showed a significant redshift due to further growth during the polymerization reaction. The best longterm stability and quantum yields of 65% were found for the red emitting quantum dot-polymer hybrid material, which exhibited only a little loss compared to the original red-emitting quantum dots (QE 72% ). The quantum dot hybrid material can be processed under N 2 protection to prevent photooxidation while kept in liquid phase above 150°C and different forms and shapes are available.
The transparency of the resulting product can be increased by fast cooling of the liquid phase. The red fluorescent quantum dot-polymer hybrid was further used as cover material for conventional low-cost blue LEDs to demonstrate the successful energy down conversion capability. The so collected energy is converted to light emitting at longer wavelength with a narrow bandwith. The polymer hybrid materials according to the present invention have high potential as light absorbing layers in solar concentrator cells, as phosphors for detectors or screens, or as energy converting layers for biofuel production. Since plants are using only 1 % of the energy of the sun spectrum for the photosnthesis their growth rate can be dramatically enhanced by focussing the energy towards the two absorption maxima of Chlorophyll a and b (400-500 nm and 600-700 nm, respectively). In the latter case the emission wavelength of our red emitting quantum dot polymer hybrid material matches exactly to the absorption maxima of chlorophyll b. This makes them ideal candidates for energy down conversion of UV, blue green, yellow or orange light into red light which then could be utilized for enhancing plant growth, and might have impact for future biofuel production application such as the improved growth of algaes as alternative biofuel material which is not in competition with the food production chain.
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