US20150191648A1 - Surface-Modified Nanoparticles - Google Patents

Surface-Modified Nanoparticles Download PDF

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US20150191648A1
US20150191648A1 US14/579,339 US201414579339A US2015191648A1 US 20150191648 A1 US20150191648 A1 US 20150191648A1 US 201414579339 A US201414579339 A US 201414579339A US 2015191648 A1 US2015191648 A1 US 2015191648A1
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nanoparticle
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Cong-Duan Vo
Hao Pang
Imad Naasani
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Samsung Electronics Co Ltd
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Nanoco Technologies Ltd
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Definitions

  • the present invention generally relates to nanoparticles. More particularly, it relates to methods for modifying the external surface of semiconductor nanoparticles.
  • QDs quantum dots
  • nanoparticles Semiconductor nanoparticles are gaining interest due to their potential in commercial applications as diverse as biological labeling, solar cells, catalysis, biological imaging, and light-emitting diodes.
  • the first is the large surface-to-volume ratio: as a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. This leads to the surface properties playing an important role in the overall properties of the material.
  • the second factor is that, for many materials (including semiconductor nanoparticles), the electronic properties of the material change with particle size.
  • the band gap typically becomes gradually larger as the size of the nanoparticle decreases. This effect is a consequence of the confinement of an “electron in a box,” giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than a continuous band as observed in the corresponding bulk semiconductor material.
  • Semiconductor nanoparticles tend to exhibit a narrow bandwidth emission that is dependent upon the particle size and composition of the nanoparticle material.
  • the first excitonic transition increases in energy with decreasing particle diameter.
  • core nanoparticles Semiconductor nanoparticles of a single semiconductor material, referred to herein as “core nanoparticles,” along with an outer organic passivating layer, tend to have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface that can lead to non-radiative electron-hole recombinations.
  • One method to eliminate defects and dangling bonds on the inorganic surface of the nanoparticle is to grow a second inorganic material (typically having a wider band-gap and small lattice mismatch to that of the core material) on the surface of the core particle to produce a “core-shell” particle.
  • Core-shell particles separate carriers confined in the core from surface states that would otherwise act as non-radiative, recombination centers.
  • Another approach is to prepare a core-multi shell structure where the “electron-hole” pair is completely confined to a single shell layer consisting of a few monolayers of a specific material such as a quantum dot-quantum well structure.
  • the core is typically a wide bandgap material, followed by a thin shell of narrower bandgap material, and capped with a further wide-bandgap layer.
  • An example is CdS/HgS/CdS grown using substitution of Hg for Cd on the surface of the core nanocrystal to deposit just a few monolayers of HgS that is then overgrown by monolayers of CdS.
  • the resulting structures exhibit clear confinement of photo-excited carriers in the HgS layer.
  • II-VI materials for example, ZnS, ZnSe, CdS, CdSe, and CdTe, as well as core-shell and core-multi shell structures incorporating these materials.
  • cadmium and other restricted heavy metals used in conventional QDs are highly toxic elements and are of major concern in commercial applications.
  • the inherent toxicity of cadmium-containing QDs prevents their use in applications involving animals or humans.
  • recent studies suggest that QDs made of a cadmium chalcogenide semiconductor material can be cytotoxic in a biological environment unless protected.
  • oxidation or chemical attack through a variety of pathways can lead to the formation of cadmium ions on the QD surface that can be released into the surrounding environment.
  • surface coatings such as ZnS can significantly reduce the toxicity, it may not completely eliminate it because QDs can be retained in cells or accumulated in the body for a long period of time, during which their coatings may undergo some form of degradation that exposes the cadmium-rich core.
  • the toxicity affects not only the progress of biological applications but also other applications including optoelectronic and communication because heavy metal-based materials are widespread in many commercial products including household appliances such as IT and telecommunication equipment, lighting equipment, electrical and electronic tools, toys, leisure and sports equipment. Legislation to restrict or ban certain heavy metals in commercial products has been already passed in many jurisdictions throughout the world.
  • III-V and IV-VI materials such as GaN, GaP, GaAs, InP, and InAs. Due to their increased covalent nature, III-V and IV-VI highly crystalline semiconductor nanoparticles are more difficult to prepare and much longer annealing times are usually required.
  • III-VI and IV-VI materials being prepared in a similar manner to that used for the II-VI materials. Methods for synthesizing core and core-shell nanoparticles are disclosed, for example, in U.S. Pat. Nos. 6,379,635, 7,803,423, 7,588,828, 7,867,556, and 7,867,557. The contents of each which are hereby incorporated by reference, in their entireties.
  • nanoparticles require that the semiconductor nanoparticle be compatible with a particular medium.
  • some biological applications such as fluorescence labeling, in vivo imaging and therapeutics require that the nanoparticles be compatible with an aqueous environment.
  • organic medium such as aromatic compounds, alcohols, esters, or ketones.
  • ink formulations containing semiconductor nanoparticles dispersed in an organic dispersant have been proposed for use in fabricating thin films of semiconductor materials for photovoltaic (PV) devices.
  • LEDs are becoming increasingly important in, for example, automobile lighting, traffic signals, general area lighting, and liquid crystal display (LCD) backlighting and display screens.
  • Nanoparticle-based light-emitting devices have been made by embedding semiconductor nanoparticles in an optically clear (or sufficiently transparent) LED encapsulation medium, typically a silicone or an acrylate, which is then placed on top of a solid-state LED.
  • an optically clear (or sufficiently transparent) LED encapsulation medium typically a silicone or an acrylate
  • semiconductor nanoparticles potentially has significant advantages over the use of more conventional phosphors.
  • semiconductor nanoparticles provide the ability to alter the emission spectrum of an LED-based illumination device.
  • Semiconductor nanoparticles also have strong absorption properties and low scattering when the nanoparticles are well dispersed in a medium.
  • the nanoparticles may be incorporated into an LED encapsulating material. It is important that the nanoparticles be well dispersed in the encapsulating material to prevent loss of quantum efficiency. Methods developed to date are problematic because the nanoparticles tend to agglomerate when formulated into conventional LED encapsulant materials, thereby reducing the optical performance of the nanoparticles.
  • the capping or passivating of particles not only prevents particle agglomeration from occurring, the capping ligand also protects the particle from its surrounding chemical environment and provides electronic stabilization (passivation) to the particles, in the case of core material.
  • the capping ligand is usually a Lewis base bound to surface metal atoms of the outermost inorganic layer of the particle. The nature of the capping ligand largely determines the compatibility of the nanoparticle with a particular medium.
  • These capping ligand are usually hydrophobic (for example, alkyl thiols, fatty acids, alkyl phosphines, alkyl phosphine oxides, and the like).
  • the nanoparticles are typically dispersed in hydrophobic solvents, such as toluene, following synthesis and isolation of the nanoparticles. Such capped nanoparticles are typically not dispersible in more polar media.
  • QDs For many commercial applications of QDs it is desirable to incorporate the QDs in an encapsulating material, such as an LED encapsulant or a polymer. In such situations it is important that the QDs remain as fully mono-dispersed as possible and without significant loss of quantum efficiency.
  • QDs can agglomerate when formulated into encapsulant matrices, reducing the optical performance of the quantum dots.
  • oxygen can migrate through the encapsulant to the surfaces of the quantum dots, which can lead to photo-oxidation and, as a result, a drop in quantum yield (QY).
  • QDs are generally more compatible with hydrophobic resins, such as acrylates, compared to more hydrophilic resins, such as epoxies.
  • polymer films made of QDs dispersed in acrylates tend to have higher initial quantum yields (QYs) than QD films using hydrophilic resins such as epoxy resins.
  • QYs initial quantum yields
  • films of hydrophobic resins, such as acrylates tend to be permeable to oxygen, while epoxy resin polymers and similar hydrophilic polymers tend to be better at excluding oxygen.
  • the QY of QDs in hydrophobic polymers can decrease precipitously over time due to oxidation.
  • nanoparticles that are compatible with polymers that are effective at excluding oxygen, such as epoxides while maintaining the integrity and photo-physical properties of the nanoparticle.
  • the present invention is directed to overcoming, or at least reducing, the effects of one or more of the problems discussed above.
  • the surfaces of the nanoparticles may be modified with amphiphilic macromolecules, for example, amphiphilic copolymers.
  • the surface modification renders the QDs more compatible with oxygen-excluding matrices, such as epoxy resins, polyurethane resins, polyester resins or any hydrophilic inorganic/organic hybrid resins such as (meth)acrylate-functionalized, polyhedral, oligomeric, silsesquioxane (POSS).
  • oxygen-excluding matrices such as epoxy resins, polyurethane resins, polyester resins or any hydrophilic inorganic/organic hybrid resins such as (meth)acrylate-functionalized, polyhedral, oligomeric, silsesquioxane (POSS).
  • FIG. 1 shows emission spectra of unmodified QDs (A) and PEG 2000 -PPS 10 -treated QDs (B).
  • nanoparticle that is rendered compatible with oxygen-excluding matrices, such as epoxy resins.
  • oxygen-excluding matrices such as epoxy resins.
  • quantum dot, QD, nanoparticle, and nanocrystal are used interchangeably herein to mean nanoparticles such as those described in the Background section, above.
  • the instant disclosure is not limited to any particular type of nanoparticle.
  • Nanoparticles of metal oxides for example, iron oxides, magnetic nanoparticles, titanium oxides, zinc oxide, zirconium oxide, aluminum oxide
  • gold nanoparticles and silver nanoparticles can be all treated and surface-modified using the methods described herein.
  • the nanoparticle may include a semiconductor material, preferably a luminescent semiconductor material.
  • the semiconductor material may incorporate ions from any one or more of Groups 2 to 16 of the periodic table, and may include binary, ternary and quaternary materials, that is, materials incorporating two, three or four different ions respectively.
  • the nanoparticle may incorporate a semiconductor material, such as, but not limited to, CdS, CdSe, CdTe, ZnS, ZnSe, ZnTe, InP, InAs, InSb, AlP, AlS, AlAs, AlSb, GaN, GaP, GaAs, GaSb, PbS, PbSe, Si, Ge and combinations thereof.
  • nanoparticles may have diameters of less than around 100 nm, less than around 50 nm, less than around 20 nm, less than around 15 nm and/or may be in the range of around 2 to 10 nm in diameter.
  • Nanoparticles that include a single semiconductor material may have relatively low quantum efficiencies because of non-radiative electron-hole recombination that occurs at defects and dangling bonds at the surface of the nanoparticles.
  • the nanoparticle cores may be at least partially coated with one or more layers (also referred to herein as “shells”) of a material different than that of the core, for example a different semiconductor material than that of the “core.”
  • the material included in the (or each) shell may incorporate ions from any one or more of Groups 2 to 16 of the periodic table.
  • each shell may be formed of a different material.
  • the core is formed from one of the materials specified above and the shell includes a semiconductor material of larger band-gap energy and similar lattice dimensions as the core material.
  • Exemplary shell materials include, but are not limited to, ZnS, ZnO, MgS, MgSe, MgTe and GaN.
  • An exemplary multi-shell nanoparticle is InP/ZnS/ZnO. The confinement of charge carriers within the core and away from surface states provides nanoparticles of greater stability and higher quantum yield.
  • an advantage of the disclosed methods is that these methods can be used to modify the surface of cadmium-free nanoparticles, that is, nanoparticles comprising materials that do not contain cadmium. It has been found that it is particularly difficult to modify the surface of cadmium-free nanoparticles. Cadmium-free nanoparticles readily degrade when prior art methods, such as prior art ligand exchange methods, are used to modify the surface of such cadmium-free nanoparticles. For example, attempts to modify the surface of cadmium-free nanoparticles have been observed to cause a significant decrease in the luminescence quantum yield (QY) of such nanoparticles.
  • QY luminescence quantum yield
  • Examples of cadmium free nanoparticles include nanoparticles comprising semiconductor materials, e.g., ZnS, ZnSe, ZnTe, InP, InAs, InSb, AlP, AlS, AlAs, AlSb, GaN, GaP, GaAs, GaSb, PbS, PbSe, Si, Ge, and particularly, nanoparticles comprising cores of one of these materials and one or more shells of another of these materials.
  • semiconductor materials e.g., ZnS, ZnSe, ZnTe
  • InP InAs, InSb, AlP, AlS, AlAs, AlSb, GaN, GaP, GaAs, GaSb, PbS, PbSe, Si, Ge, and particularly, nanoparticles comprising cores of one of these materials and one or more shells of another of these materials.
  • the nanoparticles are at least partially coated with a surface binding ligand such as myristic acid, hexadecylamine and/or trioctylphosphineoxide.
  • a surface binding ligand such as myristic acid, hexadecylamine and/or trioctylphosphineoxide.
  • Such ligands are typically derived from the solvent in which the core and/or shelling procedures were carried out.
  • ligands of this type can increase the stability of the nanoparticles in non-polar media, provide electronic stabilization, and/or negate undesirable nanoparticle agglomeration, as mentioned previously, such ligands typically prevent the nanoparticles from stably dispersing or dissolving in more polar media, such as epoxy resins.
  • amphiphilic macromolecules i.e., macromolecules having a polar portion and a non-polar portion
  • amphiphilic macromolecules are block co-polymers having a first block with an affinity for the QD surface and a second block that is more polar than the first block and/or has affinity for epoxide resin.
  • the first block is non-polar.
  • the first block may incorporate one or more atoms having affinity for the QD surface.
  • the first block incorporates at least three atoms having affinity for the QD surface.
  • the first block incorporates two or more monomers, each monomer including at least one atom having affinity for the QD surface.
  • the second block comprises a polyalkylene oxide.
  • the second block includes polyethylene oxide (PEG).
  • a suitable block copolymer is a polyalkylene glycol-poly(alkylene sulfide) block copolymer such as polyethylene glycol-b-poly(propylene sulfide) (PEG-PPS):
  • PEG-PPS polyethylene glycol-b-poly(propylene sulfide)
  • n and m are integers.
  • the values of n and m may be selected to optimize the interaction of the copolymer with the QD surface and with the matrix.
  • n and m are independently from about 5 to about 500, about 5 to 100, or about 5 to about 50.
  • n is 45 and m is 10.
  • PEG-PPS block copolymers can be synthesized using a procedure described in Wang et al., Polymer, 2009, 50, 2863, the entire contents of which are hereby incorporated by reference. Briefly, thiolate-terminated propylene sulfide oligomer is reacted with PEG chain bearing a thiol reactive 2-bromoacetate terminal group.
  • RAFT-CTA reversible addition fragmentation chain transfer agent
  • PEG-CTA polyethylene glycol macro RAFT CTA
  • n for the PEG-CTA macromolecule can be selected to provide an optimum balance between compatibility with an epoxy matrix and steric hindrance with molecules on the QD surface.
  • PEG-CTA macromolecule as illustrated above, can be synthesized as described in C. D. Vo et al., J. Polym. Sci. Part A Polym. Chem., 2010, 48, 2032, the entire contents of which are incorporated herein by reference. Briefly, oligo-ethylene glycol methyl ether is reacted with the RAFT CTA in the presence of a coupling agent such as N,N′-dicyclohexyl carbodiimide (DCCI), as shown here:
  • DCCI N,N′-dicyclohexyl carbodiimide
  • RAFT-CTA The synthesis of the RAFT-CTA is described in Lai, J. T. et al., Macromolecules, 2002, 35, 6754, the entire contents of which are incorporated herein by reference.
  • the QD surface can be modified with the amphiphilic macromolecules by exposing the QD to a solution of the macromolecule.
  • a toluene solution of QD can be added to a toluene solution of the copolymer and the combined solution can be stirred for a time sufficient to allow surface modification to occur.
  • the surface modification is performed in an inert atmosphere, such as under nitrogen.
  • a mixture of QDs and an excess of amphiphilic macromolecules can be stirred at about 20° C. for several hours under nitrogen allowing surface modification to occur.
  • matrix components such as epoxy resin components
  • matrix components such as epoxy resin components
  • matrix components such as epoxy resin components
  • Any method of film preparation can be used. Exemplary methods of preparing films include drop coating, spin coating, and doctor blading. The films can be cured by conventional methods known in the art.
  • the embodiments disclosed herein can be further understood with reference to the following representative examples.
  • the examples illustrate that amphiphilic macromolecules as surface modifiers for QDs improve the dispersion and quantum yield of the QDs in epoxy resin.
  • the strategy can be extended to other macromolecules and block copolymers, for example, poly(ethylene glycol)-b-poly(glycidyl acrylate)-CTA (PEG-PGA-CTA) whose PEG and PGA are highly compatible with epoxy.
  • PEG-CTA and PEG-PPS is simple under mild conditions. It is possible to synthesize these polymers in large scale from commercially available chemicals (see CDVo et al. J. Polym. Sci. Part A Polym.
  • the surface of QDs can be modified using PEG-CTA via simple mixing of the two components without the need to use multi-step reactions under hash reaction conditions, which can lead to lower quantum yield. The method is therefore easy to scale up.
  • the PEG-CTA contains RAFT chain transfer agent (see CDVo et al. Macromolecules 2007, 40, 7119 and CDVo et al. J. Polym. Sci. Part A Polym. Chem. 2010, 48, 2032), which can protect the QDs from free radicals while PEG-PPS containing polypropylene sulfide can protect QD from oxidation thanks to its oxidative responsiveness.
  • PEG-PPS was synthesized as described in Wang et al., Polymer, 2009, 50, 2863, referenced above and incorporated herein by reference.
  • Red QDs (Red CFQD® heavy metal-free quantum dots available from Nanoco Technologies, Ltd. Manchester, U.K.) were modified with PEG 2000 -CTA and with PEG 2000 -PPS 10 . Mixtures of the above amphiphilic macromolecules and quantum dots with a weight ratio of about 1.25/1 in toluene were stirred at 20° C. overnight under nitrogen allowing surface modification to occur.
  • Epoxy films were prepared incorporating samples of each of the modified QDs and unmodified QDs. Typically, films were prepared by first mixing the toluene-removed modified QDs or unmodified QDs with a mixture of epoxy 135 A and epoxy 135 B (1:1, w/w), then depositing 80 microliters of the resultant resin into a small glass plate (19 mm ⁇ 14 mm ⁇ 0.5 mm) and finally curing on an 80° C. heating plate under N 2 overnight. Films incorporating unmodified QDs display macroscopic aggregation of the QDs, while films of the PEG 2000 -CTA-modified and the PEG 2000 -PPS 10 -modified show that the QDs remain well dispersed within the epoxy film.
  • Optical microscopy images of the red QDs in epoxy films at 50 ⁇ and 200 ⁇ magnification using pristine QDs, PEG550-CTA-treated QD, PEG 2000 -CTA-treated QD, and PEG 2000 -PPS 10 -treated QD show, in higher magnification, that QDs modified with amphiphilic macromolecules aggregate less in epoxy resin, compared to unmodified QDs.
  • FIG. 1 shows the emission spectra of A) unmodified, and B) PEG 2000 -PPS 10 -modified QDs in epoxy films recorded using a LabsphereTM integrating sphere.
  • the ratio of the emission/excitation peak area of the PEG-PPS treated film is higher than that of the unmodified film.
  • EQEs/LED absorbance of 25%/29% and 24/71% were determined for the unmodified and PEG-PPS-modified QD epoxy films respectively. It is clear that LED absorbance of the modified films is higher than that of the unmodified film, indicating better dispersion of the QDs in the modified film as shown in optical microscope images.
  • Table 1 shows the quantum yield of the unmodified and modified QDs (in epoxy), as measured using a Hamamatsu device.
  • Table 1 The data summarized in Table 1 demonstrates the improvement in optical properties of QDs upon modification of the QD surface, as described herein.

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