Patent Application
20170271563
1. Technical Field
Embodiments of the present invention are in the field of quantum dots for light emitting diodes (LEDs) and other applications and more particularly, but not exclusively, to techniques for controlling the growth of quantum dot (QD) structures.
2. Background Art
Quantum dots having a high photoluminescence quantum yield (PLQY) may be applicable as down-converting materials in down-converting nanocomposites 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.
The various embodiments of the present invention are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which:
Embodiments described herein variously include techniques and/or mechanisms to provide a chemical substitute for water during the synthesis of quantum dot (or “QD”) structures—e.g., where the chemical substitute is to promote a controlled growth of nanocrystals. For some chemical reactions used to synthesize quantum dot structures, the presence of water which has been generated as a reaction byproduct (also referred to herein as “native water”) can promote later quantum dot growth. Native water may result from chemical processes which, for example, prepares a solution to be used in the formation of at least a portion of a quantum dot—e.g., an inner portion of a quantum dot (referred to herein as a “seed” or “core”) or an outer “shell” portion disposed around the inner portion.
Such native water can be inconsistently distributed over a reaction area and/or over time during quantum dot growth—e.g., due to at least some native water uncontrollably leaving a reaction chamber or condensing on the reaction chamber's interior. Inconsistent water distribution can result in variation in quantum dot growth, preventing or otherwise mitigating the controlled formation of quantum dots which are to have a certain desired size and/or shape (e.g., including a particular geometry, aspect ratio, etc.).
Furthermore, it may be difficult to remove native water in a consistent way which is repeatable from batch to batch. Due to its nature as a byproduct of the reaction chemistry (e.g., due to its low boiling point relative to reaction temperatures), the amount of native water available in a reactor cannot be easily adjusted to affect growth rates. As a result, the amount of native water available to drive quantum dot growth typically varies on a batch-to-batch basis as well, thus causing variation between different batches in the growth of quantum dot structures. For at least these reasons, some quantum dot features (e.g., including particle size, size distribution and/or shape) have, to date, been highly susceptible to variation in the presence, or absence, of native water. In turn, various operational characteristics of quantum dot structures—such as emission wavelength and quantum yield—may be impacted by the presence (or absence) of native water during fabrication processes.
In some embodiments, water is generated in a reaction chamber as a by-product of a chemical process to provide a solution used in quantum dot growth. Some or all such water may be removed from the chamber, where another chemical compound is then added to the chamber to facilitate growth of at least a portion of a quantum dot. The chemical compound may, for example, include a primary alcohol or a 1,2-diol. As used herein, “primary alcohol” refers to an alcohol which has a primary carbon atom and hydroxyl group (—OH) bonded thereto. The term “diol” refers herein to any of a variety of chemical compounds containing two hydroxyl groups. A “1,2-diol” may be any of a variety of diols which have two hydroxyl groups each bonded to a respective one of two carbon (C) atoms which adjoin each other in a chain or ring of carbon atoms.
Some embodiments described herein are not limited to a particular shape or size of a quantum dot. For example, a quantum dot formed by processing according to some embodiments may have any of a variety of shapes including, but not limited to, a sphere, rod, tetrapod, triangle, teardrop, sheet, etc. Alternatively or in addition, such a quantum dot may be formed of a single material or multiple materials—e.g., in a core/shell/optional shell/optional shell configuration or an alloyed composition.
The core and shell components of semiconductor structure 100—e.g., including their scale relative to each other, their locations relative to each other, etc.—are merely illustrative of some embodiments. Other embodiments may provide a semiconductor structure having more, fewer and/or differently configured structural components—e.g., wherein at least of the portion of such a semiconductor structure has disposed therein or thereon a residue of a chemical compound including a primary alcohol or a 1,2-diol.
As a reference,
In an embodiment, a portion of semiconductor structure 100—e.g., including at least a portion of nanocrystalline shell 104 and, in some embodiments, at least a portion of nanocrystalline core 102—has disposed therein or thereon a residual amount of a primary alcohol or a 1,2-diol (as illustrated by the shading of nanocrystalline shell 104). By way of illustration and not limitation, one or more nanocrystals of shell 104 may comprise unit cells, wherein an amount of the residual chemical compound (e.g., including a primary alcohol or a 1,2-diol) is at least 10 parts per million (ppm) of the nanocrystals' unit cells. The amount of residue disposed in or on shell 104 (or another such portion of a quantum dot) may, for example, be in a range of 10 ppm to 500 ppm (e.g., in a range of 30 ppm to 100 ppm, in some embodiments). In some embodiments, the residue may be a product of a reaction involving such a primary alcohol or 1,2-diol.
The following are attributes of a quantum dot that may be tuned for optimization, with reference to the parameters provided in
In accordance with some embodiments, a high PLQY quantum dot is based on a core/shell pairing using an anisotropic core. With reference to
A workable range of aspect ratio for an anisotropic nanocrystalline 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 nanocrystalline 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 nanocrystalline shell composed of a second, different, semiconductor material at least partially surrounding the anisotropic nanocrystalline core. In one such embodiment, the aspect ratio of the anisotropic nanocrystalline 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 nanocrystalline core may be substantially, but not perfectly, spherical. However, the nanocrystalline core may instead be faceted. In an embodiment, the anisotropic nanocrystalline core is disposed in an asymmetric orientation with respect to the nanocrystalline shell, as described in greater detail in the example below.
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 nanocrystalline core and nanocrystalline shell pairings, in an embodiment, the nanocrystalline shell completely surrounds the anisotropic nanocrystalline core. In an alternative embodiment, however, the nanocrystalline shell only partially surrounds the anisotropic nanocrystalline core, exposing a portion of the anisotropic nanocrystalline core, e.g., as in a tetrapod geometry or arrangement. In an embodiment, the nanocrystalline shell is an anisotropic nanocrystalline shell, such as a nano-rod, that surrounds the anisotropic nanocrystalline core at an interface between the anisotropic nanocrystalline shell and the anisotropic nanocrystalline core. The anisotropic nanocrystalline shell passivates or reduces trap states at the interface. The anisotropic nanocrystalline shell may also, or instead, deactivate trap states at the interface.
With reference again to the above described nanocrystalline core and nanocrystalline 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, Group III-V materials, Group IV-VI materials, Group materials, or Group II-IV-VI materials and, in one embodiment, are monocrystalline. 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). In an embodiment, the semiconductor structure further includes a nanocrystalline outer shell at least partially surrounding the nanocrystalline shell and, in one embodiment, the nanocrystalline outer shell completely surrounds the nanocrystalline shell. The nanocrystalline outer shell is composed of a third semiconductor material different from the first and second semiconductor materials. In a particular such embodiment, the first semiconductor material is cadmium selenide (CdSe), the second semiconductor material is cadmium sulfide (CdS), and the third semiconductor material is zinc sulfide (ZnS).
With reference again to the above described nanocrystalline core and nanocrystalline 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 nanocrystalline 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 nanocrystalline core parallel with the short axis of the nanocrystalline shell. In a specific such embodiment, the anisotropic nanocrystalline core has a diameter approximately in the range of 2-5 nanometers. In another embodiment, the anisotropic nanocrystalline core has a diameter approximately in the range of 2-5 nanometers. The thickness of the nanocrystalline shell on the anisotropic nanocrystalline core along a short axis of the nanocrystalline shell is approximately in the range of 1-5 nanometers of the second semiconductor material.
With reference again to the above described nanocrystalline core and nanocrystalline shell pairings, in an embodiment, the anisotropic nanocrystalline core and the nanocrystalline 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 nanocrystalline core. For example, in an embodiment, emission from the anisotropic nanocrystalline 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.
In an embodiment, a quantum dot based on the above described nanocrystalline core and nanocrystalline 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-conversion or up-shifting of light emitted from the LED. Thus, semiconductor structures according to some embodiments 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 color semiconductor structures may be used in devices of various embodiments. LED devices according to some embodiments 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.
Semiconductor structures according to some embodiments may be advantageously used in biological imaging in, e.g., one or more of the following environments: fluorescence resonance energy transfer (FRET) analysis, gene technology, fluorescent labeling of cellular proteins, cell tracking, pathogen and toxin detection, in vivo animal imaging or tumor biology investigation. Accordingly, some embodiments contemplate probes having quantum dots described herein.
Semiconductor structures according to some embodiments may be advantageously used in photovoltaic cells in layers where high PLQY is important. Accordingly, some embodiments contemplate photovoltaic devices using quantum dots described herein.
There are various synthetic approaches for fabricating CdSe quantum dots. For example, in an embodiment, under an inert atmosphere (e.g., ultra high purity (UHP) argon), cadmium oxide (CdO) is dissociated in the presence of surfactant (e.g., octadecylphosphonic acid (ODPA)) and solvent (e.g., trioctylphopshine oxide (TOPO); trioctylphosphine (TOP)) at high temperatures (e.g., 350-380 degrees Celsius). Resulting Cd2+ cations are exposed by rapid injection to solvated selenium anions (Se2−), resulting in a nucleation event forming small CdSe seeds. The seeds continue to grow, feeding off of the remaining Cd2+ and Se2− available in solution, with the resulting quantum dots being stabilized by surface interactions with the surfactant in solution (ODPA). The aspect ratio of the CdSe seeds is typically between 1 and 2, as dictated by the ratio of the ODPA to the Cd concentration in solution. The quality and final size of these cores is affected by several variables such as, but not limited to, reaction time, temperature, reagent concentration, surfactant concentration, moisture content in the reaction, or mixing rate. The reaction is targeted for a narrow size distribution of CdSe seeds (assessed by transmission electron microscopy (TEM)), typically a slightly cylindrical seed shape (also assessed by TEM) and CdSe seeds exhibiting solution stability over time (assessed by PLQY and scattering in solution).
For the cadmium sulfide (CdS) shell growth on the CdSe seeds, or nanocrystalline cores, under an inert atmosphere (e.g. UHP argon), cadmium oxide (CdO) is dissociated in the presence of surfactants (e.g., ODPA and hexylphosphonic acid (HPA)) and solvent (e.g. TOPO and/or TOP) at high temperatures (e.g., 350-380 degrees Celsius). The resulting Cd2+ cations in solution are exposed by rapid injection to solvated sulfur anions (S2−) and CdSe cores. Immediate growth of the CdS shell around the CdSe core occurs. The use of both a short chain and long chain phosphonic acid promotes enhanced growth rate at along the c-axis of the structure, and slower growth along the a-axis, resulting in a rod-shaped core/shell nanomaterial.
CdSe/CdS core-shell quantum dots have been shown in the literature to exhibit respectable quantum yields (e.g., 70-75%). However, the persistence of surface trap states (which decrease overall photoluminescent quantum yield) in these systems arises from a variety of factors such as, but not limited to, strain at the core-shell interface, high aspect ratios (ratio of rod length to rod width of the core/shell pairing) which lead to larger quantum dot surface area requiring passivation, or poor surface stabilization of the shell.
In order to address the above synthetic limitations on the quality of quantum dots formed under conventional synthetic procedures, in an embodiment, a multi-faceted approach is used to mitigate or eliminate sources of surface trap states in quantum dot materials. For example, lower reaction temperatures during the core/shell pairing growth yields slower growth at the CdSe—CdS interface, giving each material sufficient time to orient into the lowest-strain positions. Aspect ratios are controlled by changing the relative ratios of surfactants in solution as well as by controlling temperature. Increasing an ODPA/HPA ratio in reaction slows the rapid growth at the ends of the core/shell pairings by replacing the facile HPA surfactant with the more obstructive ODPA surfactant. In addition, lowered reaction temperatures are also used to contribute to slowed growth at the ends of the core/shell pairings. By controlling these variables, the aspect ratio of the core/shell pairing is optimized for quantum yield. In one such embodiment, following determination of optimal surfactant ratios, overall surfactant concentrations are adjusted to locate a PLQY maximum while maintaining long-term stability of the fabricated quantum dots in solution. Furthermore, in an embodiment, aspect ratios of the seed or core (e.g., as opposed to the seed/shell pairing) are limited to a range between, but not including 1.0 and 2.0 in order to provide an appropriate geometry for high quality shell growth thereon.
In another aspect, an additional or alternative strategy for improving the interface between CdSe and CdS includes, in an embodiment, chemically treating the surface of the CdSe cores prior to reaction. CdSe cores are stabilized by long chain surfactants (ODPA) prior to introduction into the CdS growth conditions. Reactive ligand exchange can be used to replace the ODPA surfactants with ligands which are easier to remove (e.g., primary or secondary amines), facilitating improved reaction between the CdSe core and the CdS growth reagents.
In addition to the above factors affecting PLQY in solution, self-absorption may negatively affect PLQY when these materials are cast into films. This phenomenon may occur when CdSe cores re-absorb light emitted by other quantum dots. In one embodiment, the thickness of the CdS shells around the same CdSe cores is increased in order to increase the amount of light absorbed per core/shell pairing, while keeping the particle concentration the same or lower in films including the quantum dot structures. The addition of more Cd and S to the shell formation reaction leads to more shell growth, while an optimal surfactant ratio allows targeting of a desired aspect ratio and solubility of the core/shell pairing.
Accordingly, in an embodiment, an overall method of fabricating a semiconductor structure, such as the above described quantum dot structures, includes forming an anisotropic nanocrystalline core from a first semiconductor material. A nanocrystalline shell is formed from a second, different, semiconductor material to at least partially surround the anisotropic nanocrystalline core. In one such embodiment, the anisotropic nanocrystalline core has an aspect ratio between, but not including, 1.0 and 2.0, as described above.
With reference to the above described general method for fabricating a nanocrystalline core and nanocrystalline shell pairing, in an embodiment, prior to forming the nanocrystalline shell, the anisotropic nanocrystalline core is stabilized in solution with a surfactant. In one such embodiment, the surfactant is octadecylphosphonic acid (ODPA). In another such embodiment, the surfactant acts as a ligand for the anisotropic nanocrystalline core. In that embodiment, the method further includes, prior to forming the nanocrystalline shell, replacing the surfactant ligand with a second ligand, the second ligand more labile than the surfactant ligand. In a specific such embodiment, the second ligand is one such as, but not limited to, a primary amine or a secondary amine.
With reference again to the above described general method for fabricating a nanocrystalline core and nanocrystalline shell pairing, in an embodiment, forming the nanocrystalline shell includes forming the second semiconductor material in the presence of a mixture of surfactants. In one such embodiment, the mixture of surfactants includes a mixture of octadecylphosphonic acid (ODPA) and hexylphosphonic acid (HPA). In a specific such embodiment, forming the nanocrystalline shell includes tuning the aspect ratio of the nanocrystalline shell by tuning the ratio of ODPA versus HPA. Forming the second semiconductor material in the presence of the mixture of surfactants may also, or instead, include using a solvent such as, but not limited to, trioctylphosphine oxide (TOPO) and trioctylphosphine (TOP).
With reference again to the above described general method for fabricating a nanocrystalline core and nanocrystalline shell pairing, in an embodiment, forming the anisotropic nanocrystalline core includes forming at a temperature approximately in the range of 350-380 degrees Celsius. In an embodiment, forming the anisotropic nanocrystalline core includes forming a cadmium selenide (CdSe) nanocrystal from cadmium oxide (CdO) and selenium (Se) in the presence of a surfactant at a temperature approximately in the range of 300-400 degrees Celsius. The reaction is arrested prior to completion. In one such embodiment, forming the nanocrystalline shell includes forming a cadmium sulfide (CdS) nanocrystalline layer on the CdSe nanocrystal from cadmium oxide (CdO) and sulfur (S) at a temperature approximately in the range of 120-380 degrees Celsius. That reaction is also arrested prior to completion.
The aspect ratio of the fabricated semiconductor structures may be controlled by one of several methods. For example, ligand exchange may be used to change the surfactants and/or ligands and alter the growth kinetics of the shell and thus the aspect ratio. Changing the core concentration during core/shell growth may also be exploited. An increase in core concentration and/or decrease concentration of surfactants results in lower aspect ratio core/shell pairings. Increasing the concentration of a shell material such as S for CdS will increase the rate of growth on the ends of the core/shell pairings, leading to longer, higher aspect ratio core/shell pairings.
As mentioned above, in one embodiment, nanocrystalline cores undergo a reactive ligand exchange which replaces core surfactants with ligands that are easier to remove (e.g., primary or secondary amines), facilitating better reaction between the CdSe core and the CdS growth reagents. In one embodiment, cores used herein have ligands bound or associated therewith. Attachment may be by dative bonding, Van der Waals forces, covalent bonding, ionic bonding or other force or bond, and combinations thereof. Ligands used with the cores may include one or more functional groups to bind to the surface of the nanocrystals. In a specific such embodiment, the ligands have a functional group with an affinity for a hydrophobic solvent.
In an embodiment, lower reaction temperatures during shell growth yields slower growth at the core/shell interface. While not wishing to be bound by any particular theory or principle it is believed that this method allows both core and shell seed crystals time to orient into their lowest-strain positions during growth. Growth at the ends of the core/shell pairing structure is facile and is primarily governed by the concentration of available precursors (e.g., for a shell of CdS this is Cd, S:TOP). Growth at the sides of the core/shell pairings is more strongly affected by the stabilizing ligands on the surface of the core/shell pairing. Ligands may exist in equilibrium between the reaction solution and the surface of the core/shell pairing structure. Lower reaction temperatures may tilt this equilibrium towards more ligands being on the surface, rendering it more difficult for growth precursors to access this surface. Hence, growth in the width direction is hindered by lower temperature, leading to higher aspect ratio core/shell pairings.
In general consideration of the above described semiconductor or quantum dot structures and methods of fabricating such semiconductor or quantum dot structures, in an embodiment, quantum dots are fabricated to have an absorbance in the blue or ultra-violet (V) regime, with an emission in the visible (e.g., red, orange, yellow, green, blue, indigo and violet, but particularly red and green) regime. The above described quantum dots may advantageously have a high PLQY with limited self-absorption, possess a narrow size distribution for cores, provide core stability over time (e.g., as assessed by PLQY and scattering in solution), and exhibit no major product loss during purification steps. Quantum dots fabricated according one or more of the above embodiments may have a decoupled absorption and emission regime, where the absorption is controlled by the shell and the emission is controlled by the core. In one such embodiment, the diameter of the core correlates with emission color, e.g., a core diameter progressing from 3-5.5 nanometers correlates approximately to a green→yellow→red emission progression.
With reference to the above described embodiments concerning semiconductor structures, such as quantum dots, and methods of fabricating such structures, the concept of a crystal defect, or mitigation thereof, may be implicated. For example, a crystal defect may form in, or be precluded from forming in, a nanocrystalline core or in a nanocrystalline shell, at an interface of the core/shell pairing, or at the surface of the core or shell. In an embodiment, a crystal defect is a departure from crystal symmetry caused by one or more of free surfaces, disorders, impurities, vacancies and interstitials, dislocations, lattice vibrations, or grain boundaries. Such a departure may be referred to as a structural defect or lattice defect. Reference to an exciton is to a mobile concentration of energy in a crystal formed by an excited electron and an associated hole. An exciton peak is defined as the peak in an absorption spectrum correlating to the minimum energy for a ground state electron to cross the band gap. The core/shell quantum dot absorption spectrum appears as a series of overlapping peaks that get larger at shorter wavelengths. Because of their discrete electron energy levels, each peak corresponds to an energy transition between discrete electron-hole (exciton) energy levels. The quantum dots do not absorb light that has a wavelength longer than that of the first exciton peak, also referred to as the absorption onset. The wavelength of the first exciton peak, and all subsequent peaks, is a function of the composition and size of the quantum dot. An absorbance ratio is absorbance of the core/shell nanocrystal at 400 nm divided by the absorbance of the core/shell nanocrystal at the first exciton peak. Photoluminescence quantum yield (PLQY) is defined as the ratio of the number of photons emitted to the number of photons absorbed. Core/shell pairing described herein may have a Type 1 band alignment, e.g., the core band gap is nested within the band gap of the shell. Emission wavelength may be determined by controlling the size and shape of the core nanocrystal, which controls the band gap of the core. Emission wavelength may also be engineered by controlling the size and shape of the shell. In an embodiment, the amount/volume of shell material is much greater than that of the core material. Consequently, the absorption onset wavelength is mainly controlled by the shell band gap. Core/shell quantum dots in accordance with some embodiments have an electron-hole pair generated in the shell which is then funneled into the core, resulting in recombination and emission from the core quantum dot. Preferably emission is substantially from the core of the quantum dot.
Method 200 may comprise, at 210, performing a first reaction to dissolve a precursor of a semiconductor material, wherein a by-product of the first reaction includes water. By way of illustration and not limitation, the first reaction performed in 210 may dissolve a cadmium precursor cadmium oxide (CdO) to create Cd2+ ions, where water is generated as a by-product of such CdO dissolution. In other embodiments, method 200 further comprises, at 220, removing from a product of the first reaction some or all of the water which the first reaction created as a by-product. Such native water may, for example, be removed at 220 by an application of a vacuum to a reaction chamber in which first reaction takes place.
Method 200 may further comprise, at 230, combining a chemical compound with the product of the first reaction after the water is removed at 220, wherein the chemical compound is a primary alcohol or a 1,2-diol. A molecule of the 1,2-diol may, for example, include six or more carbon atoms which are arranged in a chain or a ring. By way of illustration and not limitation, the 1,2-diol may be 1,2-hexanediol or 1,2-dodecanediol. In some embodiments, a primary alcohol of the chemical compound includes 1-octadecanol. In some embodiments, method 200 further includes, at 240, performing a second reaction with the chemical compound to form a portion of a quantum dot. The second reaction may include a nanocrystalline growth that, for example, is to form an outer portion of nanocrystalline core 102 or some or all of nanocrystalline shell 104. The chemical compound may be added before or during such nanocrystalline growth, in various embodiments. Use of the chemical compound may, for example, facilitate a controlled tuning of anisotropic II-VI rods seeded by II-VI cores—e.g., wherein the aspect ratio of a quantum dot can be varied between 1.1:1 to 12:1 based on the chemical compound used, the concentration thereof, the duration of the reaction and/or the like.
The second reaction at 220 may be performed to variously grow nanocrystalline structures of one or more quantum dots. Such nanocrystalline structures may, for example, include any of a variety of II-VI semiconductor compounds, III-V semiconductor compounds, IV-IV semiconductor compounds and semiconductor compounds, including any of various alloys of such compounds. By way of illustration and not limitation, a quantum dot synthesized according to some embodiments may include a nanocrystal of one of zinc oxide (ZnO), zinc sulfide (ZnS), zinc selenide (ZnSe), zinc telluride (ZnTe), cadmium oxide (CdO), cadmium sulfide (CdS), cadmium selenide (CdSe), cadmium telluride (CdTe), mercury sulfide (HgS), mercury selenide (HgSe), mercury telluride (HgTe), mercury oxide (HgO), aluminum nitride (AlN), aluminum phosphide (AlP), aluminum arsenide (AlAs), aluminium antimonide (AlSb), gallium nitride (GaN), gallium phosphide (GaP), gallium arsenide (GaAs), gallium antimonide (GaSb), gallium selenide (GaSe), indium nitride (InN), indium phosphide (InP), indium arsenide (InAs), indium antimonide (InSb), thallium nitride (TlN), thallium phosphide (TlP), thallium arsenide (TlAs), thallium antimonide (TlSb), lead sulfide (PbS), lead selenide (PbSe), lead telluride (PbTe), magnesium oxide (MgO), magnesium sulfide (MgS), magnesium selenide (MgSe), or alloys thereof or mixtures thereof.
Method 200 may result in a portion of the quantum dot having disposed therein or thereon a residual amount of the chemical compound which is provided at 230 (or a residual amount of a product formed by a reaction involving the chemical compound)—e.g., wherein an amount of the residue is at least 10 ppm, relative to unit cells of the portion. In some embodiments, any residual amount of the chemical compound that may be disposed in one portion (e.g., a core) of a quantum dot may, for example, be less than the residual amount of the chemical compound which is in or on a second portion (e.g., a shell structure).
A chemical substitute for water may be used, for example, to promote growth of binary or alloyed cadmium sulfide (CdS) anisotropic quantum rods, seeded by binary or alloyed cadmium selenide (CdSe), or cores. In an illustrative scenario according to one embodiment, cadmium sulfide (CdS) shell structures may be variously grown on cadmium selenide (CdSe) seeds (or other such nanocrystalline cores) under an inert atmosphere such as ultra-high purity argon. In such an embodiment, cadmium oxide (CdO) may be dissociated in the presence of a surfactant, such as octadecylphosphonic acid or hexylphosphonic acid, and a solvent such as trioctylphosphine oxide or trioctylphosphine, at a high temperatures (e.g., 300° C.-380° C.). The product of such a reaction may subsequently be exposed to a vacuum to remove the evolved native water. A replacement compound is then introduced, and resulting Cd2+ cations in solution are exposed by rapid injection to solvated sulfur anions (S2−) and cadmium selenide (CdSe) cores. A growth of cadmium sulfide (CdS) shells around the cadmium selenide (CdSe) core may then occur. The use of both a short chain and long chain phosphonic acid may promote an enhanced growth rate at along a c-axis of the QD structure, and a slower growth rate along an a-axis, resulting in a rod-shaped core/shell nanomaterial.
A desired final size and/or shape of quantum dots may be facilitated by a native water substitute which is selectively utilized according to implementation-specific details. For example, use of different amounts of octadecanol (for example) during the growth of the cadmium sulfide (CdS) may results in shapes ranging from a long rod to a shorter, wider bullet-shape (as shown, for example, in
One or more aspects of QD growth (e.g., QD shape, QD growth rate) may depend on the particular chemical compound which is to substitute native water, a concentration of the chemical compound, when the chemical compound is introduced, a duration of reaction with the chemical compound, etc. For example, use of a substitute chemical compound may allow the controlled tuning of anisotropic II-VI rods seeded by II-VI cores, where the final particle aspect ratio can be varied between 1.1:1 to 12:1. Addition of a diol or a primary alcohol may allow QD morphologies to be controlled—e.g., where QDs are selectively formed to have any in a range of shapes including rods, nail-like structures, triangles and/or the like.
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,
Some or all quantum dots of lighting device 800 may have variously disposed therein or thereon residual chemical compound (e.g., including a primary alcohol or a 1,2-diol) which, for example, is an indicator of processing such as that of method 200.
Referring to
In another example,
Referring to
In another example,
In additional examples,
Referring to
Thus, quantum dots (QDs) and methods for efficient fabrication thereof have been disclosed.
This application claims the benefit of U.S. Provisional Application No. 62/308,765, filed Mar. 15, 2016, the entire contents of which are hereby incorporated by reference herein.
| Number | Date | Country | |
|---|---|---|---|
| 62308765 | Mar 2016 | US |
