WO2011088159A1 - Dispositif optoélectronique contenant des nanocristaux colloïdaux émetteurs de grande taille - Google Patents

Dispositif optoélectronique contenant des nanocristaux colloïdaux émetteurs de grande taille Download PDF

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WO2011088159A1
WO2011088159A1 PCT/US2011/021037 US2011021037W WO2011088159A1 WO 2011088159 A1 WO2011088159 A1 WO 2011088159A1 US 2011021037 W US2011021037 W US 2011021037W WO 2011088159 A1 WO2011088159 A1 WO 2011088159A1
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nanocrystals
emitting
nanocrystal
core
semiconductor
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Xiaofan Ren
Keith Brian Kahen
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Eastman Kodak Company
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Priority claimed from US12/688,191 external-priority patent/US20110175030A1/en
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Publication of WO2011088159A1 publication Critical patent/WO2011088159A1/fr

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/04—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a quantum effect structure or superlattice, e.g. tunnel junction
    • H01L33/06—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies with a quantum effect structure or superlattice, e.g. tunnel junction within the light emitting region, e.g. quantum confinement structure or tunnel barrier
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K11/00—Luminescent, e.g. electroluminescent, chemiluminescent materials
    • C09K11/08—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials
    • C09K11/88—Luminescent, e.g. electroluminescent, chemiluminescent materials containing inorganic luminescent materials containing selenium, tellurium or unspecified chalcogen elements
    • C09K11/881—Chalcogenides
    • C09K11/883—Chalcogenides with zinc or cadmium
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/26—Materials of the light emitting region
    • H01L33/28—Materials of the light emitting region containing only elements of Group II and Group VI of the Periodic Table
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/02—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by the semiconductor bodies
    • H01L33/26—Materials of the light emitting region
    • H01L33/30—Materials of the light emitting region containing only elements of Group III and Group V of the Periodic Table
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/005—Processes
    • H01L33/0062—Processes for devices with an active region comprising only III-V compounds
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L33/00—Semiconductor devices having potential barriers specially adapted for light emission; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L33/005—Processes
    • H01L33/0083—Processes for devices with an active region comprising only II-VI compounds

Definitions

  • the present invention relates to devices using large-sized emitting
  • nanocrystals and particularly to large-sized core/shell nanocrystals with ternary ZnCdSe cores.
  • a quantum dot is a semiconductor whose excitons are confined in all three spatial dimensions. As a result, it has properties that are between those of bulk semiconductors and those of discrete molecules.
  • An immediate optical feature of colloidal quantum dots is their coloration. While the material which makes up a quantum dot defines its intrinsic energy signature, the quantum dots of the same material, but with different sizes, can emit light of different colors. The physical reason is the quantum confinement effect. Quantum confinement results from electrons and holes being squeezed into a dimension that approaches a critical quantum measurement, called the exciton Bohr radius. As with any crystalline semiconductors, a quantum dot's electronic wave functions extend over the crystal lattice. Similar to a molecule, a quantum dot has both a quantized energy spectrum and a quantized density of electronic states near the edge of the band gap.
  • Colloidal semiconductor quantum dots or colloidal nanocrystals
  • colloidal nanocrystals have been the focus of a lot of research. They are easier to manufacture in volume than self- assembled quantum dots.
  • Colloidal nanocrystals are synthesized from precursor compounds dissolved in solutions, much like traditional chemical processes. The synthesis is based on a three-component system composed of: precursors, organic surfactants, and solvents. When heating a reaction medium to a sufficiently high temperature, the precursors chemically transform into monomers. Once the monomers reach a high enough supersaturation level, the nanocrystal growth starts with a nucleation process followed by a growth process.
  • Colloidal nanocrystals can be used in biological applications since they are dispersed in a solvent. Additionally, the potential for low cost deposition processes makes colloidal nanocrystals attractive for light emitting devices, such as LEDs, as well as other electronic devices, such as, solar cells, lasers, and quantum computing (cryptography) devices.
  • FIG. 1 gives a schematic of a typical prior art LED device 105. All of the device layers are deposited on the substrate 100. Above the substrate is the p-contact layer 110, the p- transport layer 120, the intrinsic emitting layer 130, the n-transport layer 140, and the n-contact layer 150. The anode 160 makes ohmic contact with the p-contact layer 110, while the cathode 170 makes ohmic contact with the n-contact layer 150).
  • LEDs is to combine either blue, violet, or near UV LEDs with appropriate phosphors.
  • Replacing traditional optically pumped phosphors with quantum dot phosphors has many advantages, such as, greatly reduced scattering, ease of color tuning, improved color rendering index (CRI), lower cost deposition process, and broader wavelength spectrum for optical pumping.
  • quantum dot phosphors have not been introduced into the marketplace due to some major shortcomings; such as, poor temperature stability and insufficient (10-30%) quantum yields for phosphor films with high quantum dot packing densities.
  • appropriate filler e.g., polymers or epoxies
  • nanocrystals These nanocrystals have size-tunable luminescence emission spanning the entire visible spectrum.
  • a single light source can be used for simultaneous excitation of different-sized dots, and their emission wavelength can be continuously tuned by changing the particle size. Since they can also be conjugated to biomolecules, such as, proteins or nucleic acids, this
  • nanocrystals are more stable than smaller ones as smaller nanocrystals have a higher percentage of reactive surface atoms. Atoms on the surface are energetic less stable than those that are well-ordered and packed in the interior. Larger nanocrystals have much weaker interactions with foreign species, which would lead to less obvious influence on the overall electronic structure of the nanocrystals.
  • the largest Zn x Cdi_ x Se nanocrystal reported by Zhong et al. has a diameter about 7.5 nm and emits blue due to the substantial amount of Zn present in the structure. It is believed that this represents the largest II-IV nanocrystal (before shelling) emitting in the visible region that has ever been reported in the open literature.
  • nanocrystals In order for colloidal nanocrystals to find applications in both biological and electronic device areas, it is important that the nanocrystals have a narrow size distribution and high emission quantum efficiencies. This requirement can be relatively easily met by regular-sized ( ⁇ 5 nm) nanocrystals. However, many applications also demand nanocrystals that are not only robust but also insensitive to their surface chemistry and surface conditions. Furthermore, the availability of nanocrystals with improved temperature stability is crucial to make possible the widespread commercial usage of quantum dot phosphors for display and lighting applications. It has been demonstrated by our group and others that regular-sized ( ⁇ 5 nm) nanocrystals are often inadequate meeting all the requirements.
  • each emitting nanocrystal includes a core structure wherein the cores have aspect ratio less than 2: 1 and a diameter greater than 10 nanometers and a protective shell surrounding the core.
  • each emitting nanocrystal includes a core structure wherein the cores have aspect ratio less than 2: 1 and a diameter greater than 10 nanometers and a protective shell surrounding the core.
  • the colloidal ternary nanocrystals made in accordance with the present method exhibit the desirable properties of high crystallinity, narrow size distribution, high emission efficiency, ability to form polycrystalline films with less than 5 % by volume of organic material, high temperature stability, stable fluorescence after removal of organic passivating ligands, and robustness for high temperature anneals.
  • Another advantage of the present invention is that the large-sized emitting colloidal nanocrystals exhibiting these properties can be used to create advantaged quantum dot phosphors, medical and biological sensors, high efficiency LEDs and lasers.
  • FIG. 1 shows a side-view schematic of a prior art inorganic light emitting device
  • FIG. 2 shows a schematic of a colloidal semiconductor nanocrystal
  • FIGS. 3A-B shows a representation of Transmission Electron Microscopy
  • FIG. 4 shows the PL spectrum of the large-sized ZnCdSe nanocrystals
  • FIG. 5 shows a representation of a photograph of drop-casted films composed of the large-sized ZnCdSe nanocrystals under a UV lamp:
  • a typical colloidal inorganic nanocrystal 205 is shown in FIG. 2.
  • the inorganic nanocrystal 205 is composed of a semiconductor core 200, on whose surface is bonded organic ligands 210.
  • the organic ligands 210 give stability to the resulting colloidal dispersion (the inorganic nanocrystals 205 and an appropriate solvent). Even though the inorganic nanocrystal 205 shown in FIG.
  • nanocrystals can be synthesized to have shapes ranging from quantum rods and wires, to tetrapods and other multiple connected nanocrystals.
  • a shell is often grown outside the semiconductor core with a semiconducting material having an energy bandgap being higher than that of the core. This bandgap engineering not only moves the exciton-generation zone further away from the surface where quenching can take place, but ensures carrier confinement to the core region. It is well known in the art that in order to reduce the deleterious effects of surface states on the optical and electrical properties of nanocrystals, it is
  • CdSe- based nanocrystals can be used to generate red, green, and blue light, and the quantum size effects dictate the length scale of the nanocrystals.
  • a simple way to increase the size of the nanocrystal while maintaining the emission color is to grow a very thick shell outside of the CdSe core. Formation of the shell is conducted by slowly adding molecular precursors into the core solution at high temperatures in coordination solvents. It is well-known that lattice mismatch between core and shell materials leads to accumulation of interface strain. Such strain eventually would be released through the formation of misfit dislocations that degrade optical properties. Therefore, nanocrystals with thick shells usually have unsatisfactory optical properties.
  • Another way to increase the size of the nanocrystal while maintaining the emission color is to add some Zn to the CdSe in order to increase the bandgap of the semiconductor material and localize the exciton states.
  • the resulting material is the ternary alloy ZnCdSe.
  • the alloy content is either homogenously distributed throughout the nanocrystal or it can have some radial dependence.
  • ternary semiconductor alloys are created by adding, at the start of the synthesis, appropriate ratios of cations (e.g., ZnCdSe) or anions (CdSeTe) into the synthesis reaction mixture (R. Bailey et al, JACS 125, 7100 (2003)). This procedure would normally result in an alloy homogenously distributed throughout the nanocrystal volume.
  • the synthetic scheme would be to initially create a CdSe core, shell it with ZnSe, and then perform an appropriate anneal (K. Kahen et al. WO 2009058172 Al; K. Kahen et al.
  • the diffusion profile would be such that the maximum Zn concentration in the nanocrystal would occur at the surface, while in the core center the Zn content would be much lower (ZnCdSe, but with a high Cd/Zn ratio).
  • the surface region of the annealed nanoparticle would show the strongest random alloy attributes, with the core region behaving mainly as crystalline CdSe.
  • e-h pairs created in the core CdSe-like region would not only get localized by the increasing energy gap of the ZnCdSe surface region, but also by carrier localization generated by the band of random alloy surrounding the core region of the nanocrystal.
  • the size of the ZnCdSe nanocrystals prepared with the methods discussed above is typically 3-5 nm. Larger ZnCdSe nanocrystals, with sizes reaching 7.5 nm, were reported by Zhong et al (J. Am. Chem. Soc, 125, 8589 (2003); J. Phys. Chem. B, 108, 15552 (2004)).
  • the typical solvent/ligand system used to prepare these ZnCdSe nanocrystals includes TOPO, or HDA (or ODA), and about 1-5 wt% fatty acid. The small amount of fatty acid reacts with CdO in situ to form Cd- fatty acid salt that acts as the real Cd precursor for the nanocrystal synthesis.
  • This solvent/ligand system is ideal for synthesizing ZnCdSe nanocrystals with small to medium sizes (less than 8 nm), as has been demonstrated.
  • new solvent/ligand system and synthetic method are needed; and some clues may be drawn in the synthesis of large non-emissive CdSe nanocrystals.
  • CdSe nanocrystals as large as 11.5 nm has been prepared by injecting a mixture of dimethylcadmium and TOPSe into a hot solution of TOPO and TOP [C. B. Murray, et al, JACS, 115, 8706 (1993)].
  • CdMe 2 the extremely toxic cadmium precursor
  • Peng, et al. reported rice-shaped CdSe nanocrystals with a size of up to 30 nm along the long axis and 8-10 nm along the short axis [Z. A. Peng et al, JACS, 124, 3343 (2002)]. These nanocrystals are formed by using a less reactive cadmium precursor, cadmium phosphonic acid complexes, in the presence of large excess of starting materials, and with regular replenish of the monomer concentration. The same experiment was tried out in our lab, and the formation of rice-shaped CdSe nanocrystals was observed, even though the sizes were not as large as reported. The as-formed nanocrystals could be isolated and purified.
  • Peng's group investigated different kinds of safe, common, and low-cost organic compounds to be used as coordinating solvents/ligands for the synthesis of high quality II- VI nanocrystals [L. Qu, et al, Nano Letters, 1, 333 (2001)]. Their work shows that among all of the solvent/ligand system tested, fatty acids are excellent candidates for synthesizing relatively large-sized CdSe
  • a solvent system of 50 wt% of fatty acid and 50 wt% of TOPO yields CdSe nanocrystals in a very broad size range from about 2 nm to 25 nm.
  • the shape of the CdSe nanocrystals with a diameter up to 25 nm can be purposely controlled to dot- shape.
  • the ability of fatty acids to enable synthesis of large nanocrystals is believed to come about from the fast growth rates of nanocrystals in this solvent system. This reaction was successfully reproduced and large-sized CdSe nanocrystals were indeed obtained.
  • each emitting nanocrystal includes a core structure that has a diameter greater than 10 nm and a aspect ratio less than 2: 1.
  • Ai and An are independently selected from an element from the subgroup of IIB of the periodic table, when B represents an element of the main group of VI of the periodic table;
  • Ai and An are independently selected from an element from the main group of III of the periodic table, when B represents an element of the main group of V of the periodic table;
  • an outer shell is grown on the ternary core with a semiconducting material having an energy bandgap being higher than that of the ternary core. Since shelling with III-V compounds remains problematic, it is preferred that the shell material also is composed of II-VI semiconducting material, with either a binary or a ternary alloy composition. Examples are ZnS, ZnSe, ZnSeS, ZnSeTe, or ZnSeS. It is well-known that large lattice mismatch between core and shell materials leads to accumulation of interface strain. Such strain eventually can be released through the formation of misfit dislocations that degrade optical properties. Therefore, it is advantageous that the shell material is chosen such that the difference between the crystal lattice values of the shell and ternary core materials is small.
  • the preferred temperature range for Tl and T2 is between 250° C to 400° C, and more preferably between 290° C to 360° C. It is preferable that T2 is equal to or lower than Tl . It should be noted that if a solvent with lower boiling point is used, the inventive process disclosed here can also be carried out at lower temperatures, as long as the desired nanocrystals are obtained.
  • the column II element comprised herein is preferably independently selected from the group consisting of Zn, Cd and Hg.
  • the column VI element comprised herein is preferably independently selected from the group consisting of S, Se and Te.
  • Preferred embodiments are nanocrystals having the composition Zn x Cdi_ x Se, Zn x Cdi_ x S, Zn x Cdi_ x Te, or Hg x Cdi_ x Se.
  • the column III element comprised herein is preferably independently selected from the group consisting of Al, Ga and In.
  • the column V element comprised herein is preferably independently selected from the group consisting of P, As and Sb.
  • Preferred embodiments are nanocrystals having the composition Al x Ini_ x P, Ga x Ini_ x P, Al x Ini_ x As, or Ga x Ini_ x As.
  • the index x has a value of
  • x can have a value between about 0.2 or about 0.3 to about 0.8 or about 0.9.
  • the cation precursor used for synthesizing the ternary core is a group II or group III material selected from a group of Cd, Zn, Hg, Al Ga, In, and more preferable that the group II or group III cation precursor is a chemical compound selected from a group including Cd(Me) 2 , CdO, CdC0 3 , Cd(Ac) 2 , CdCl 2 , Cd(N0 3 ) 2 , CdS0 4 , ZnO, ZnC0 3 , Zn(Ac) 2 , Zn(Et) 2 , Hg 2 0, HgC0 3 , Hg(Ac) 2 , In(Ac) 2 , Ga(Me) 3 , Ga(acac) 3 , InCl 3 or Al(Me) 3 .
  • the anion precursor used in the synthesis is a material selected from a group consisting of sulfide(S), selenium (Se), tellurium (Te), phosphorus (P), arsenic (As), and antimony (Sb).
  • the anion precursor is selected from a group including bis(trimethylsilyl)sulfide, tri-n- alkylphosphine sulfide, hydrogen sulfide, tri-n-alkenylphosphine sulfide, alkylamino sulfide, alkenylamino sulfide, tri-n-alkylphosphine selenide, alkenylaminoe selenide, tri-n-alkylamino selenide, tri-n-alkenylphosphine selenide, tri-n-alkylphosphine telluride, alkenylaminoe telluride, tri-n-alkylamino telluride, tri-n-alkenylphosphine telluride, Tris(trimethylsilyl)phosphine, or bis(trimethylsilyl)arsenide.
  • the element form of the anion can also be used, such as S, Se, Te, P and
  • the coordination ligands that can be used are alkyl phosphine; alkyl phosphine oxide, alkyl phosphite; alkyl phosphate; alkyl amine; alkyl phosphonic acid; or fatty acid.
  • the alkyl chain of the coordination ligand is preferably a hydrocarbon chain of length greater than 4 carbon atoms and less than 30 carbon atoms, which can be saturated, unsaturated, oligomeric in nature. It can also have aromatic groups in its structure.
  • Suitable coordination ligands and ligand mixture include but are not limited to trioctylphosphine, tributylphosphine,
  • the coordination ligands can be used by diluting the coordination ligands using at least one non-coordinating or weakly coordinating solvent selected from a group including but not limited to 1-nonadcene, 1 -octadecene, cis-2-methyl-7-octadecene, 1-heptadecene, 1-pentadecene, 1-tetradecenedioctylether, dodecyl ether, hexadecyl ether, esters and the like.
  • the non-coordinating or weakly coordinating solvent such as 1- octadecene, esters, ethers, or the combinations thereof, can be used alone without the more strongly coordinating ligands.
  • the weight percentage of fatty acids in the total ligand/solvent mixture is preferably no less than 30%, and more preferably no less than 40%.
  • the emitting nanocrystals 205 it is then necessary to create a layer composed of them in order to apply these nanocrystals in devices.
  • three low cost techniques for forming nanocrystal films are depositing the colloidal dispersion of the nanocrystals by drop casting, spin coating and inkjetting.
  • Common solvents for drop casting or spin coating colloidal nanocrystals are a 9: 1 mixture of hexane:octane [C. B. Murray et al., Annu. Rev. Mater. Sci. 30, 545 (2000)].
  • the organic ligands 210 need to be chosen such that the colloidal nanocrystals 205 are soluble in non-polar solvents. As such, organic ligands with hydrocarbon-based tails are good choices, such as, the alkylamines. Using well- known procedures in the art, the ligands coming from the growth procedure
  • ligand exchange can be used to attach an organic ligand 210 (to the nanoparticles 205) whose tail is soluble in polar solvents; pyridine is an example of a suitable ligand.
  • the organic ligands 210 attached to the colloidal nanocrystals 205 evaporate as a result of annealing the films in an inert atmosphere or under vacuum.
  • the organic ligands 210 can be made to evaporate from the film during an annealing process [C. B. Murray et al., Annu. Rev. Mater. Sci.
  • anneal temperature is below 220° C, or in multiple steps where sequentially each step has a greater temperature than the prior step. Consequently, for films formed by drop casting or spin coating with non-polar solvents, shorter chained primary amines, such as, hexylamine are preferred; for films formed with polar solvents, pyridine is a preferred ligand.
  • a particularly suitable material is any material selected from a group including pyridine, pyridine derivatives, mercapto-alkyl acid, mercapto- alkenyl acid, mercapto-alkyl amine, mercapto-alkenyl amine, mercapto-alkyl alcohol, mercapto-alkenyl alcohol, dihydrolipolic acid, alkylamino acid, alkenyl amino acid, aminoalkylcarboic acid, hydroxyalkylcarboic acid or hydroxyalkenylcarboic acid, but it is not limited to these materials as is well known in the art.
  • the solvents used for making the dispersion of the nanocrystals functionalized with low-boiling-point coordinating ligands include, but are not limited to, toluene, hexane, heptane, octane, ethanol, methanol, propanol, pyridine, pyridine derivatives, or combinations thereof.
  • the resulting film comprises the large-sized semiconductor nanocrystals having an aspect ratio less than 2:1 and diameter greater than 10 nm, wherein the film has less than 10%, and more preferably less than 5%, by volume of organic materials.
  • the diameter of the semiconductor nanocrystals comprising the film can also be greater than 12 nm, or even 14 nm.
  • the nanocrystals comprising the film and device have less than 5% by surface area functionalized with organic ligands.
  • the nanocrystals comprising the film and device are substantially spherical in shape.
  • colloidal semiconductor nanocrystals have been the subject of intensive experimental and theoretic study because of their emission phenomena associated with quantum confinement.
  • the large-sized semiconductor nanocrystals disclosed in the present invention emit strongly in the visible region, with a light emission efficiency no less than 30%, and in most cases, no less than 40%.
  • the size distribution of the as-prepared nanocrystals is very narrow, with a FWHM in the range of 20-35 nm, and in most cases, 20-25 nm.
  • These highly luminescent, stable nanocrystals are potential ideal nano-emitters for light-emitting devices, quantum information devices, solar cells, or semiconductor lasers in optoelectronic applications. They are also very promising biological labels.
  • An application for incorporating emitting nanocrystals in light emitting devices is to employ them as emissive phosphors that are optically pumped by a higher energy (the wavelength of the pump source is shorter than the average emission wavelength) light source.
  • the light source can be an LED (either organic or inorganic), a laser, a compact fluorescent lamp, or any other incoherent light source that is well known in the art.
  • the phosphors can be used to produce white light, convert higher energy light into a specific visible wavelength band (for example, produce green light), or any other desired wavelength conversion (including producing infrared light) as is well known in the art.
  • the excellent thermal and environmental stability, and the robustness of these large-sized emitting nanocrystals can also be demonstrated by the thin film studies.
  • the as-prepared colloidal nanocrystals was ligand-exchanged with the low-boiling point pyridine and drop-casted from a EtOH/PrOH solution on a glass substrate to form a smooth and pinhole-free film.
  • the film showed intense emission under the UV light.
  • the film was then sealed in a glass tube under the inert atmosphere and subjected to a 160° C anneal under vacuum for 30-45 min. At the end of the process, all the pyridine ligand was expected to evaporate off, and the nanocrystals were no longer passivated by organic ligands.
  • Typical II-IV nanocrystals going through such treatment would lose majority of the emission, or become completely non-emissive. In contrast, no detectable emission intensity loss was observed for the film made with the large-sized nanocrystals prepared with the method disclosed in this invention.
  • the same film was put back to the tube-oven and annealed at 270° C for 15-25 min under nitrogen. After cooling down, the film still emitted fairly bight (see Example section). This experiment demonstrated the exceptional thermal and environmental stability, and robustness of these large-sized emitting nanocrystals.
  • each emitting nanocrystal includes a core structure wherein the core has a aspect ratio less than 2: 1 and a diameter greater than 10 nanometers and a protective shell surrounding the core.
  • the device can be an optoelectronic device, and the optoelectronic device can be a display backlight, multicolor display, full color display, monochrome display or lighting device.
  • the nanocrystal layer in the device can be formed by co-depositing small ( ⁇ 2 nm), conductive inorganic nanoparticles along with the large-sized emitting nanocrystals 205 to form the inorganic light emitting layer.
  • a subsequent inert gas (Ar or N 2 ) anneal step can be used to sinter the smaller inorganic nanoparticles amongst themselves and onto the surface of the large-sized emitting nanocrystals 205. Sintering the inorganic nanocrystals results in the creation of a continuous, conductive semiconductor matrix. Through the sintering process, this matrix is also connected to the large-sized emitting nanocrystals 205 and forms a polycrystalline inorganic light emitting layer.
  • a conductive path is created from the edges of the inorganic light emitting layer, through the semiconductor matrix and to each large-sized emitting nanocrystals 205, where electrons and holes recombine in the large-sized emitting nanocrystals 205. It should also be noted that encasing the large-sized emitting nanocrystals 205 in the conductive semiconductor matrix has the added benefit that it protects the nanocrystals environmentally from the effects of both oxygen and moisture.
  • an inorganic light emitting device including a plurality of independently controlled light emitting elements, wherein at least one light emitting element comprises: a first patterned electrode; a second electrode opposed to the first electrode; and a polycrystalline inorganic light emitting layer comprising emitting semiconductor nanocrystals (the emitting semiconductor nanocrystals could be embedded within a semiconductor matrix formed between the electrodes), wherein the emitting semiconductor nanocrystal has a core structure wherein the core has a aspect ratio less than 2: 1 and a diameter greater than 10 nanometers and a protective shell surrounding the core.
  • Such an inorganic light emitting device can be used as a display backlight, multicolor display, full color display, monochrome display or lighting device.
  • the emissive semiconductor material used in a device is type II-VI or III-V semiconductor material having a ternary composition.
  • the column II element comprised herein is preferably independently selected from the group consisting of Zn, Cd and Hg.
  • the column VI element comprised herein is preferably independently selected from the group consisting of S, Se and Te. Thus, all ternary combinations of these elements are within the scope of the invention.
  • Preferred embodiments are nanocrystals having the composition Zn x Cdi_ x Se, Zn x Cdi_ x S, Zn x Cdi_ x Te, Hg x Cdi_ x Se, ZnSe x Si_ x , ZnSe x Tei_ x , CdSe x Si_ x , or CdSe x Tei_ x .
  • the column III element comprised herein is preferably independently selected from the group consisting of Al, Ga and In.
  • the column V element comprised herein is preferably independently selected from the group consisting of P, As and Sb.
  • Preferred embodiments are nanocrystals having the composition Al x Ini_ x P, Ga x Ini_ x P, Al x Ini_ x As, Ga x Ini_ x As, or InP x Asi_ x .
  • the semiconductor material is type III-V semiconductor material selected from but not limited to Al x Ini_ x P, Ga x Ini_ x P, Al x Ini_ x As, or Ga x Ini_ x As.
  • the semiconductor material is type II-VI semiconductor material selected from but not limited to Zn x Cdi_ x Se, Zn x Cdi_ x S, Zn x Cdi_ x Te, Hg x Cdi_ x Se, ZnSe x Si_ x , or ZnSe x Tei_ x .
  • the index x has a value of 0.001 ⁇ x ⁇ 0.999, preferably of 0.0K x ⁇ 0.99, or more preferred of 0.05 ⁇ x ⁇ 0.95 or 0.1 ⁇ x ⁇ 0.9. In even more preferred embodiments, x can have a value between about 0.2 or about 0.3 to about 0.8 or about 0.9.
  • the emitting semiconductor material used in a device has less than 5% by area of the surface functionalized with organic ligands. Removal of the majority of the insulting organic ligands facilitates both charge transporting through the polycrystalline inorganic light emitting layer comprising the large-sized emitting semiconductor nanocrystals, and direct charge recombination in the emitting nanocrystals.
  • the diameter of the semiconductor nanocrystals can also be greater than 12 nm, or even 14 nm, with an aspect ratio less than 2: 1.
  • the large-sized emitting nanocrystals are substantially spherical in shape.
  • the large-sized semiconductor nanocrystals emit strongly in the visible region, with a light emission efficiency no less than 30%, and more preferably, no less than 40%.
  • the polycrystalline inorganic light emitting layer comprising emitting large-sized semiconductor nanocrystals is formed by a mixture of large-sized nanocrystals having different compositions, or a mixture of large-sized and small-sized emitting nanocrystals having the same, or different compositions.
  • the cadmium precursor is cadmium acetate
  • the zinc precursor is Zn(Et) 2
  • the selenium precursor is TOPSe.
  • the coordinating solvent for the growth is a mixture of trioctylphosphine oxide (TOPO), trioctylphosphine (TOP) and stearic acid.
  • TOPO and TOP are degassed at 190° C for 60 minutes prior to their usage.
  • 0.046 g (0.2 mmol) cadmium acetate and 3 g stearic acid were added into a three-neck flask.
  • the flask was placed on a Schlenk line and vacuum was applied.
  • the mixture went clear after heating at 100° C for 5 - 10 minutes.
  • After cooling down, the flask was transferred into the box, and 1.1 ml TOPO was added.
  • the mixture was degassed at 100° C for 30 minutes. After switching to argon
  • the flask contents were taken up to 350° C, and 1 ml TOPSe solution in TOP prepared by dissolving 0.7896 g (10 mmol) Se in 10 ml Top in the dry box was added into the solvent mixture by quick injection from a syringe, followed by the injection of a ZnEt 2 solution in TOP (Zn:Cd ratio varies from 2: 1 to 8:1). After the injection, the reaction mixture was stirred at 300 °C for 1 hour. The reaction was stopped by removing the heating source.
  • the final step in the process was shelling of the ZnCdSe ternary cores.
  • a three-neck reaction flask loaded with 500 ⁇ as-prepared crude ZnCdSe cores, 3 ml TOPO and 2 ml HDA was heated to 190° C.
  • the solution of ZnEt 2 (1 M, 0.8 ml) and TOPSe (1M, 1.2 ml) in 2 ml TOP was slowly added dropwise under vigorous stirring. After the addition the temperature was lowered to 180° C and the solution was left to stir for another hour to form Zn x Cdi_ x Se/ZnSe nanocrystals.
  • TEM analysis revealed the formation of Zn x Cdi_ x Se/ZnSe nanocrystals in a size range from 10 nm to 15 nm (Fig. 3A-B). Photoluminescence measurement showed that the large-sized emitting nanocrystals have emission quantum yield up to 50% and FWHM as narrow as 21 nm (Fig. 4).
  • dispersions were created with alcohols as the solvents. More specifically, ⁇ 1 ml of the crude solution was added to 3 ml of toluene, and 10 ml of methanol in a centrifuge tube. After centrifuging for a few minutes, the supernatant became clear. It was decanted off and 3-4 ml of pyridine was added. The plug quickly dissolved in the pyridine to produce a clear solution. The solution was heated at 80° C under continuous stirring for 24 hours in order to exchange the TOPO/stearic acid/TOP organic ligands 210 for pyridine organic ligands 210.

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Abstract

La présente invention concerne un dispositif utilisant une couche contenant des nanocristaux semi-conducteurs émetteurs, chaque nanocristal émetteur comportant une structure de noyau et les noyaux ayant un rapport largeur/longueur inférieur à 2:1 et un diamètre supérieur à 10 nanomètres et une enveloppe de protection entourant le noyau.
PCT/US2011/021037 2010-01-15 2011-01-13 Dispositif optoélectronique contenant des nanocristaux colloïdaux émetteurs de grande taille WO2011088159A1 (fr)

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US12/688,212 US20110175054A1 (en) 2010-01-15 2010-01-15 Device containing large-sized emitting colloidal nanocrystals
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US12/688,191 US20110175030A1 (en) 2010-01-15 2010-01-15 Preparing large-sized emitting colloidal nanocrystals
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CN114316950A (zh) * 2022-01-12 2022-04-12 广东粤港澳大湾区国家纳米科技创新研究院 通过前体胶囊制备量子点材料的方法和量子点材料、量子点组合物以及量子点器件

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Cited By (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
EP3058598A1 (fr) * 2013-10-17 2016-08-24 Nanophotonica, Inc. Point quantique destiné à émettre de la lumière et son procédé de synthèse
EP3058598A4 (fr) * 2013-10-17 2017-03-29 Nanophotonica, Inc. Point quantique destiné à émettre de la lumière et son procédé de synthèse
US9887318B2 (en) 2013-10-17 2018-02-06 Nanophotonica Quantum dot for emitting light and method for synthesizing same
US9780256B2 (en) 2013-12-12 2017-10-03 Nanophotonica Method and structure of promoting positive efficiency aging and stabilization of quantum dot light-emitting diode
CN114316950A (zh) * 2022-01-12 2022-04-12 广东粤港澳大湾区国家纳米科技创新研究院 通过前体胶囊制备量子点材料的方法和量子点材料、量子点组合物以及量子点器件

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