Patent Application
20160333268
Embodiments of the present invention are in the field of quantum dots and, in particular, irregular large volume semiconductor coatings for quantum dots (QDs).
Quantum dots having a high photoluminescence quantum yield (PLQY) may be applicable as down-converting materials in down-converting nano-composites used in solid state lighting applications. Down-converting materials are used to improve the performance, efficiency and color choice in lighting applications, particularly light emitting diodes (LEDs). In such applications, quantum dots absorb light of a particular first (available or selected) wavelength, usually blue, and then emit light at a second wavelength, usually red or green.
Embodiments of the present invention include irregular large volume semiconductor coatings for quantum dots (QDs).
In an embodiment, a semiconductor structure includes a quantum dot structure having an outermost surface. A crystalline semiconductor coating is disposed on and completely surrounds the outermost surface of the quantum dot structure. The crystalline semiconductor coating has an irregular outermost geometry.
In an embodiment, a semiconductor structure includes a nanocrystalline core of a first semiconductor material. A nanocrystalline shell of a second semiconductor material different from the first semiconductor material is disposed on and surrounds the nanocrystalline core. A crystalline semiconductor coating of a third semiconductor material different from the first and second semiconductor materials is disposed on and completely surrounds the nanocrystalline shell. The crystalline semiconductor coating has an irregular outermost geometry.
In another embodiment, a lighting apparatus includes a housing structure and a light emitting diode supported within the housing structure. The lighting apparatus also includes a light conversion layer disposed above the light emitting diode. The light conversion layer includes a plurality of quantum dots. Each quantum dot includes a nanocrystalline core of a first semiconductor material. A nanocrystalline shell of a second semiconductor material different from the first semiconductor material is disposed on and surrounds the nanocrystalline core. A crystalline semiconductor coating of a third semiconductor material different from the first and second semiconductor materials is disposed on and completely surrounds the nanocrystalline shell. The crystalline semiconductor coating has an irregular outermost geometry.
In another embodiment, a lighting apparatus includes a substrate and a light emitting diode disposed on the substrate. The lighting apparatus also includes a light conversion layer disposed above the light emitting diode. The light conversion layer includes a plurality of quantum dots. Each quantum dot includes a nanocrystalline core of a first semiconductor material. A nanocrystalline shell of a second semiconductor material different from the first semiconductor material is disposed on and surrounds the nanocrystalline core. A crystalline semiconductor coating of a third semiconductor material different from the first and second semiconductor materials is disposed on and completely surrounds the nanocrystalline shell. The crystalline semiconductor coating has an irregular outermost geometry.
Irregular large volume semiconductor coatings for quantum dots (QDs) and the resulting quantum dot materials are described herein. In the following description, numerous specific details are set forth, such as specific semiconductor coating materials, and quantum dot geometries and efficiencies, in order to provide a thorough understanding of embodiments of the present invention. It will be apparent to one skilled in the art that embodiments of the present invention may be practiced without these specific details. In other instances, well-known related apparatuses, such as the host of varieties of applicable light emitting diodes (LEDs), are not described in detail in order to not unnecessarily obscure embodiments of the present invention. Furthermore, it is to be understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.
One or more embodiments described herein are directed to semiconductor coatings for quantum dots. Certain embodiments may be directed to very stable high performance semiconductor quantum dot materials that include a seeded rod semiconductor structure (nanocrystalline core/shell pairing) coated with a thick layer of a third semiconductor material. The third semiconductor material coating has an appearance that is bumpy or spiky (irregular), although still crystalline. Such a configuration for a quantum dot structure allows for the addition of a relatively large amount of material without reducing the performance of the inner seeded rod structure.
To provide context, quantum dot structure such as quantum dot heterostructures (QDHs) may require protection in certain applications requiring higher, more reliable performance under the associated conditions of high incident intensity, high operating temperatures, and humidity. In accordance with an embodiment of the present invention, a QDH based on a nanocrystalline core of a first semiconductor material and a surrounding nanocrystalline shell of a second semiconductor material has a third semiconductor material coated there around. The third semiconductor material is applied as a coating that may provide an extra layer of protection for the quantum dot structure which results in a higher, more reliable performance under the conditions of high incident intensity, high operating temperatures, and humidity.
In an embodiment, a quantum dot structure is provided having an outermost smooth topography. A crystalline semiconductor coating is fabricated for the quantum dot structure and has an irregular outermost geometry. The irregular outermost geometry may have a topography that is lumpy, bumpy, spikey, or the like, i.e., that is not smooth. By foregoing a smooth outermost topography for the coating, a greater volume of material may be coated onto the underlying quantum dot. By enabling an increase in volume of a crystalline semiconductor coating for a quantum dot, the resulting structure may exhibit enhanced robustness against various conditions, as is described in association with
In an exemplary specific embodiment, the semiconductor material of the crystalline semiconductor coating is a crystalline coating of zinc sulfide (ZnS). A method of coating a quantum dot structure involves formation of a ZnS having an irregular distribution of between 0 and 4 added nanometers of material on a short axis of the quantum dot structure, and an irregular distribution of between 0 and 7 nanometers of material added on the long axis. Accordingly, in an embodiment, the crystalline semiconductor coating has a variable distribution of thickness along a single axis of the nanocrystalline shell. Although the distribution is irregular, in an embodiment, the entire foundational quantum dot structure is coated with the third crystalline semiconductor material. In one such embodiment, the coating provides an average of greater than approximately 3 nanometers of material in the width (coating the short axis) and an average of greater than approximately 5 nanometers of material in the length (coating the long axis). In an embodiment, reference to a zinc sulfide coating refers to a coating where the metal species of MS is predominantly zinc (Zn). However, secondary metals species, such as cadmium (Cd), may also be included.
In an exemplary synthesis, in accordance with an embodiment of the present invention, for operation (1) to a reaction flask (e.g., between 25 mL and 250 mL, but not excluding larger flask sizes), the following is added: (a) a magnetic stir bar, (b) solvent (trioctyl phosphine oxide (TOPO) is preferred, however, other high boiling solvents are also possible, such as octadecene), (c) a cadmium (Cd) precursor (cadmium oxide is preferred, but can instead or also include cadmium formate, cadmium acetate, cadmium nitrate, cadmium stearate, and other cadmium precursors; it is of note that dissociation temperatures of 280 degrees Celsius need only be achieved when cadmium oxide is used), (d) a zinc (Zn) precursor (preferably a highest purity zinc acetate dehydrate, 99.999% trace metals basis; however, this can instead or also include other zinc precursors at other purity levels, such as zinc acetate, zinc formate, zinc stearate, zinc oxide), (e) Ligand 1: a long-chain carboxylic acid (generally used is Oleic Acid, technical grade, 90%; however, other long chain carboxylic acids can be used), and (d) Ligand 2: and amine (generally used is octadecyl amine, but other long chain amines such as oleyl amine or alkyl chain of different lengths are also viable choices).
In operation (2) the reaction is equilibrated at approximately 120 degrees Celsius under flowing ultra-high purity (UHP) argon gas. The solvent will melt/liquefy and disperse other solids at approximately 60 degrees Celsius, so stirring is begun at this point and continued throughout the reaction. In one embodiment, stirring is performed at approximately 1200 RPM (i.e., to handle a large volume of material) but smaller scales can employed speeds as low as 800 RPM.
In operation (3), the mixture from (2) is degassed at approximately 120 degrees Celsius for a given time (e.g., approximately between 30 and 90 minutes, depending on the metal precursors involved).
In operation (4), the metal precursors are dissociated into metal-oleate forms. Temperatures approximately between 280 degrees Celsius and 310 degrees Celsius may be required if metal-oxide precursors are used which require dissociation to the metal-oleate form. However, this operation may be skipped if metal oxide precursors are not used, since equilibration at reaction temperature should facilitate formation of the Zn- and Cd-oleate forms).
In operation (5) the reaction mixture is equilibrated at a reaction temperature of approximately 235 degrees Celsius. Growth has been observed for some conditions as low as approximately 170 degrees Celsius and as high as approximately 270 degrees Celsius, but optimal conditional appear to be approximately 235 degrees Celsius.
In operation (6), a rapid injection of a mixture of (a) QDH (CdSe/CdS core/shell nanorods), and (b) 7.4 wt % sulfur: tri-octyl phosphate (TOP) stock solution into the mixture of (5) is performed.
In operation (7), coating growth is allowed to occur for approximately 4 hours and 45 minutes (with continued stirring, UHP argon flow, and a temperature maintained at the reaction temperature of operation (5).
In operation (8), after the growth period, slow cooling of the mixture to room temperature is performed with continued stirring and continued UHP argon flow.
In operation (9), at a temperature less than approximately 80 degrees Celsius, the mixture is exposed to air and solvent (e.g., toluene or cyclohexane) is injected. The mixture is allowed to recover while stirring under argon until the reaction solution has a temperature below approximately 25 degrees Celsius.
In operation (10), two precipitation/centrifugation cycles are performed using iso-propyl alcohol (IPA) and methanol (MeOH) as anti-solvents, and cyclohexane as a solvent to purify the materials. For larger scale reactions, centrifugation speeds are capped at approximately 4200 RPM based on the rotor capability. For smaller scales, centrifuge is performed at speeds up to approximately 9000 RPM (e.g., max capability of a particular rotor).
In operation (11), the final solid product dissolved is in an organic solvent (e.g., cyclohexane is preferred; other solvents are possible, such as hexane, toluene).
With reference to operations (1)-(11), in a particular embodiment, the following ratios are used during the semiconductor coating process: (a) Zn:Cd at a ratio of approximately 37:1, (b) Ligand 1:Ligand 2 at a ratio of approximately 10:1, (c) total ligand:total metal at a ratio of approximately 2.25:1, and (d) metal:sulfur at a ratio of approximately 1.26:1.
The resulting coated quantum dots having the high volume crystalline ZnS coating thereon have irregular outermost geometries, which can be described as bumpy, lumpy, spiky, etc. Thus, in an embodiment, an irregular outermost geometry of a crystalline semiconductor coating can be described as having has a topography such as lumpy, bumpy or spikey.
In accordance with an embodiment of the present invention, the addition of the above described ZnS crystalline semiconductor coating to a quantum dot provides an extra layer of protection which results in a higher, more reliable performance under conditions of high incident intensity, high operating temperatures, and humidity. As an example of rigorous temperature conditions,
As an example of rigorous humidity conditions,
As an example of rigorous on-LED conditions,
Referring again to
As described above, hetero-structure-based quantum dots may have an irregular semiconductor coating formed thereon. It is to be appreciated that the resulting structure having a third semiconductor layer thereon may also be referred to as a quantum dot heterostructure (QDH). For instances when description is intended to exclude the outer irregular coating, a hetero-structures may be referred to as a nano-crystalline core and nano-crystalline shell pairing. The nano-crystalline core and nano-crystalline shell pairing may have specific geometries suitable for performance optimization. In an example, several factors may be intertwined for establishing an optimized geometry for a quantum dot having a nano-crystalline core and nano-crystalline shell pairing. As a reference,
Referring to
In addition to material composition, the following are attributes of a quantum dot that may be tuned for optimization, with reference to the parameters provided in
The length (aSHELL) of the shell is tunable to provide longer radiative decay times as well as increased light absorption. The overall aspect ratio of the structure 700 (e.g., the greater of aSHELL/bSHELL and aSHELL/cSHELL) may be tuned to directly impact PLQY. Meanwhile, overall surface/volume ratio for 700 may be kept relatively smaller to provide lower surface defects, provide higher photoluminescence, and limit self-absorption. Referring again to
In accordance with an embodiment of the present invention, a high PLQY quantum dot is based on a core/shell pairing using an anisotropic core. With reference again to
A workable range of aspect ratio for an anisotropic nano-crystalline core for a quantum dot may be selected for maximization of PLQY. For example, a core essentially isotropic may not provide advantages for increasing PLQY, while a core with too great an aspect ratio (e.g., 2 or greater) may present challenges synthetically and geometrically when forming a surrounding shell. Furthermore, embedding the core in a shell composed of a material different than the core may also be used enhance PLQY of a resulting quantum dot.
Accordingly, in an embodiment, a semiconductor structure includes an anisotropic nano-crystalline core composed of a first semiconductor material and having an aspect ratio between, but not including, 1.0 and 2.0. The semiconductor structure also includes a nano-crystalline shell composed of a second, different, semiconductor material at least partially surrounding the anisotropic nano-crystalline core. In one such embodiment, the aspect ratio of the anisotropic nano-crystalline core is approximately in the range of 1.01-1.2 and, in a particular embodiment, is approximately in the range of 1.1-1.2. In the case of rounded edges, then, the nano-crystalline core may be substantially, but not perfectly, spherical. However, the nano-crystalline core may instead be faceted. In an embodiment, the anisotropic nano-crystalline core is disposed in an asymmetric orientation with respect to the nano-crystalline shell, as described in greater detail in the example below. In other embodiments, however, the anisotropic nano-crystalline core is disposed in an on-axis (centered) with respect to the nano-crystalline shell.
Another consideration for maximization of PLQY in a quantum dot structure is to provide an asymmetric orientation of the core within a surrounding shell. For example, referring again to
With reference to the above described nano-crystalline core and nano-crystalline shell pairings, in an embodiment, the nano-crystalline shell completely surrounds the anisotropic nano-crystalline core. In an alternative embodiment, however, the nano-crystalline shell only partially surrounds the anisotropic nano-crystalline core, exposing a portion of the anisotropic nano-crystalline core, e.g., as in a tetrapod geometry or arrangement. In an embodiment, the nano-crystalline shell is an anisotropic nano-crystalline shell, such as a nano-rod, that surrounds the anisotropic nano-crystalline core at an interface between the anisotropic nano-crystalline shell and the anisotropic nano-crystalline core. The anisotropic nano-crystalline shell passivates or reduces trap states at the interface. The anisotropic nano-crystalline shell may also, or instead, deactivate trap states at the interface.
With reference again to the above described nano-crystalline core and nano-crystalline shell pairings, in an embodiment, the first and second semiconductor materials (core and shell, respectively) are each materials such as, but not limited to, Group II-VI materials (where the group II species could include species from Groups II (e.g., magnesium) or XII of the periodic table), Group III-V materials, Group IV-VI materials, Group I-III-VI materials, or Group II-IV-VI materials and, in one embodiment, are mono-crystalline. In one such embodiment, the first and second semiconductor materials are both Group II-VI materials, the first semiconductor material is cadmium selenide (CdSe), and the second semiconductor material is one such as, but not limited to, cadmium sulfide (CdS), zinc sulfide (ZnS), or zinc selenide (ZnSe).
With reference again to the above described nano-crystalline core and nano-crystalline shell pairings, in an embodiment, the semiconductor structure (i.e., the core/shell pairing in total) has an aspect ratio approximately in the range of 1.5-10 and, 3-6 in a particular embodiment. In an embodiment, the nano-crystalline shell has a long axis and a short axis. The long axis has a length approximately in the range of 5-40 nanometers. The short axis has a length approximately in the range of 1-5 nanometers greater than a diameter of the anisotropic nano-crystalline core parallel with the short axis of the nano-crystalline shell. In a specific such embodiment, the anisotropic nano-crystalline core has a diameter approximately in the range of 2-5 nanometers. The thickness of the nano-crystalline shell on the anisotropic nano-crystalline core along a short axis of the nano-crystalline shell is approximately in the range of 1-5 nanometers of the second semiconductor material.
With reference again to the above described nano-crystalline core and nano-crystalline shell pairings, in an embodiment, the anisotropic nano-crystalline core and the nano-crystalline shell form a quantum dot. In one such embodiment, the quantum dot has a photoluminescence quantum yield (PLQY) of at least 90%. Emission from the quantum dot may be mostly, or entirely, from the nano-crystalline core. For example, in an embodiment, emission from the anisotropic nano-crystalline core is at least approximately 75% of the total emission from the quantum dot. An absorption spectrum and an emission spectrum of the quantum dot may be essentially non-overlapping. For example, in an embodiment, an absorbance ratio of the quantum dot based on absorbance at 400 nanometers versus absorbance at an exciton peak for the quantum dot is approximately in the range of 5-35.
Referring again to
In an embodiment, as described above, the crystalline semiconductor coating 799 is irregular in form. That is, the crystalline semiconductor coating 799 may be described as lumpy, bumpy, spiky or the like, referring to the non-smooth (rough) exterior of the crystalline semiconductor coating 799 (versus, say, a smooth rod structure). The irregular aspect of the coating crystalline semiconductor coating 799 leaves a variable thickness of the third semiconductor material along any one of the axis of the nano-crystalline shell 704. For example, in one embodiment, although variable, the thickness of the semiconductor coating 799 formed along the long surface (along the a-shell axis) is greater than approximately 3 nanometers on average. Meanwhile, the thickness of the semiconductor coating 799 formed along the short surface (along the b-shell axis) at the ends of the nano-crystalline shell 704 is greater than approximately 5 nanometers on average.
It is also to be appreciated that, although already a variable thickness coating, the nano-crystalline shell 704 may be formed with or without alignment to a global center of the crystalline semiconductor coating 799. In one embodiment, then, the nano-crystalline shell 704 is formed centered to a global center of the crystalline semiconductor coating 799. In another embodiment, the nano-crystalline shell 704 is formed off-set from a global center of the crystalline semiconductor coating 799.
In an embodiment, a quantum dot based on the above described nano-crystalline core and nano-crystalline shell pairings is a down-converting quantum dot. However, in an alternative embodiment, the quantum dot is an up-shifting quantum dot. In either case, a lighting apparatus may include a light emitting diode and a plurality of quantum dots such as those described above. The quantum dots may be applied proximal to the LED and provide down-shifting or up-shifting of light emitted from the LED. Thus, semiconductor structures according to the present invention may be advantageously used in solid state lighting. The visible spectrum includes light of different colors having wavelengths between about 380 nm and about 780 nm that are visible to the human eye. An LED will emit a UV or blue light which is down-converted (or up-shifted) by semiconductor structures described herein. Any suitable ratio of emission color from the semiconductor structures may be used in devices of the present invention. LED devices according to embodiments of the present invention may have incorporated therein sufficient quantity of semiconductor structures (e.g., quantum dots) described herein capable of down-converting any available blue light to red, green, yellow, orange, blue, indigo, violet or other color. These structures may also be used to downconvert or upconvert lower energy light (green, yellow, etc) from LED devices, as long as the excitation light produces emission from the structures.
Referring to
The above described insulator coating may be formed to encapsulate a quantum dot using a reverse micelle process. For example,
In another aspect, a polymer matrix composition is applied to a lighting device to provide a layer having a dispersion of semiconductor structures therein for inclusion in the lighting device. In one embodiment, the dispersion of semiconductor structures is a dispersion of quantum dots such as those described above in association with
In a first exemplary embodiment, a method of applying a light-conversion layer to a surface of a light-emitting diode (LED) includes first, separately, forming a polymer matrix from a mixture of quantum dots. The resulting polymer matrix includes a dispersion of the quantum dots therein and is then applied to the surface of the LED. In one such embodiment, applying the polymer matrix to the surface of the LED involves using a technique such as, but not limited to, spraying, dip-coating, spin-coating, or drop-casting. The polymer matrix can be cured with ultra-violet (UV) light exposure or heating, in one embodiment. It is to be appreciated that the polymer matrix having the dispersion of quantum dots therein can be applied to discrete LED devices or, in another embodiment, prior to dicing the LEDs from a wafer having a plurality of LED dies. In the latter case, application of the polymer matrix or matrix can involve uniform distribution across the wafer prior to dicing the wafer.
With respect to illustrating the above concepts in a resulting device configuration,
Different approaches may be used to provide a quantum dot layer in a lighting device. In an example,
Although described herein as applicable for on-chip applications, polymer matrix compositions may also be used as remote layers. In an example,
In another example,
In another example,
In additional examples,
Thus, irregular large volume semiconductor coatings for quantum dots (QDs) and the resulting quantum dot materials have been disclosed.
This application claims the benefit of U.S. Provisional Application No. 61/928,577, filed Jan. 17, 2014, the entire contents of which are hereby incorporated by reference herein.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US15/11720 | 1/16/2015 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61928577 | Jan 2014 | US |
