METHOD TO MODIFY THE SURFACE OF QUANTUM DOTS AND A METHOD TO PREPARE A DISPERSION OF SURFACE MODIFIED QUANTUM DOTS

Abstract

A method to modify the surface of III-V quantum dots. III-V quantum dots provided with X type ligands and L type ligands at their outer surface are dispersed in a solvent include an additional compound. The additional compound has an organic acid of formula RYH or an organic salt of formula RYH+Z− and has at least one acidic proton H+ having a pKa in water equal to or lower than 16. During dispersion the acidic proton H+ protonates at least part of the L type ligands and at least part of the X type ligands are replaced. Surface modified quantum dots, dispersions having such surface modified quantum dots and to the use of such dispersions are disclosed.

Claims

1.-15. (canceled)

16. A method to modify the surface of quantum dots, said method comprising the steps of: providing III-V quantum dots, said quantum dots having X type ligands and L type ligands at their outer surface, with X type ligands comprising ligands able to donate one electron to a quantum dot to which the X type ligand is bound and with L type ligands comprising ligands able to donate two electrons to a quantum dot to which the L type ligand is bound; dispersing said quantum dots in a solvent further comprising an additional compound to form modified quantum dots, said additional compound comprising an organic acid of formula RYH or an organic salt of formula RYH+Z−, said additional compound comprising at least one acidic proton H+ having a pKa in water equal or lower than 16 and said additional compound comprising a R-group comprising at least one carbon atom, with said organic acid of formula RYH being selected from the group consisting of carboxylic acids, sulfonic acids, thiols and phenols, with said organic salt of formula RYH+Z− comprising an organic ammonium ion RYH+ and a negatively charged counterion Z−; whereby during said dispersing said acidic proton H+ of said additional compound protonates at least part of said L type ligands to form protonated L type ligands and at least part of said X type ligands are replaced by deprotonated acid RY− of said organic acid of formula RYH or by counterion Z− of said organic salt of formula RYH+Z− and/or by deprotonated conjugated base RY of ion RYH+ of said organic salt of formula RYH+Z−

17. The method according to claim 16, wherein at least part of the protonated L type ligands is removed from the outer surface of said quantum dots, simultaneously with the replacement of said X type ligands.

18. The method according to claim 16, wherein said X type ligands are selected from the group consisting of halides, carboxylates and thiolates and/or said L type ligands are selected from the group consisting of amines, phosphines and thiols.

19. The method according to claim 16, wherein said X type ligand comprises a halide and/or said L type ligand comprises an amine or a phosphine.

20. The method according to claim 16, wherein a salt of said protonated L type ligand and said X type ligand is formed during said dispersing.

21. The method according to claim 16, wherein said solvent comprises an apolar solvent or a polar solvent.

22. The method according to claim 16, wherein said method further comprises the step of: separating said modified quantum dots.

23. Quantum dots obtainable by the method defined in claim 16.

24. The quantum dots according to claim 23, said quantum dots being provided with RY− ligands selected from the group consisting of thiolates and carboxylates.

25. A method to prepare a dispersion comprising surface modified quantum dots, said method comprising the steps of: providing a first solution comprising quantum dots in a first solvent, said first solvent being an apolar solvent, said quantum dots being provided with X type ligands and L type ligands at their outer surface, with X type ligands comprising ligands able to donate one electron to the quantum dot to which the X type ligand is bound and with L type ligands comprising ligands able to donate two electrons to the quantum dot to which the L type ligand is bound; providing a second solution comprising a second solvent, an additional compound and optionally at least one stabilizing agent, said second solvent being a polar solvent, said additional compound comprising an organic acid of formula RYH or an organic salt of formula RYH+Z−, said additional compound comprising at least one acidic proton H+ having a pKa in water equal or lower than 16 and comprising a R-group comprising at least one carbon atom, with said organic acid being selected from the group consisting of carboxylic acids, sulfonic acids, thiols and phenols, with said organic salt of formula RYH+Z− comprising an organic ammonium ion RYH+ and a negatively charged counterion Z−; mixing said first solution and said second solution to provide modified quantum dots, said modified quantum dots being provided with RY− ligands of said organic acid of formula RYH or with counterion Z− ligands of said organic salt of formula RYH+Z− and/or with deprotonated conjugated base RY ligands of ion RYH+ of said organic salt of formula RYH+Z−, and to extract said surface modified quantum dots from said first solvent of said first solution into said second solvent of said second solution; removing said first solvent.

26. The method according to claim 25, wherein said R-group comprises at least one additional functional group capable of participating in hydrogen bonding and/or said R-group comprises from 1 to 20 carbon atoms.

27. The method according to claim 25, wherein said stabilizing agent comprises an amine, a monofunctional or bifunctional amine having a chain of short chain of 1 to 5 carbon atoms.

28. The method according to claim 25, wherein said first solvent comprises an alkane and/or said second solvent is selected from N,N′-dimethylformamide, formamide and dimethylsulfoxide.

29. A dispersion comprising quantum dots as defined in claim 24.

30. Use of a dispersion as defined in claim 29 to deposit a film.

31. A dispersion comprising quantum dots obtainable by the method defined in claim 26.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0093] The present invention will be discussed in more detail below, with reference to the attached drawings, in which:

[0094] FIG. 1 is a schematic illustration of the ligand exchange and removal of X type ligands and L type ligands from the surface of quantum dots to prepare modified quantum dots according to the present invention;

[0095] FIG. 2 is a schematic illustration of the preparation of a dispersion using an extraction process according to the present invention;

[0096] FIG. 3 is a schematic illustration of the ligand exchange reaction that ensues during the extraction of quantum dots from an apolar phase to a polar phase according to a method according to the present invention;

[0097] FIG. 4 shows the 1D H-NMR spectrum of purified In(As,P) quantum dots in toluene-d8 stabilized by an oleylamine ligand shell;

[0098] FIG. 5 shows the H-NMR spectra during the progressive addition of 1, 2.5 and 4 equivalents of UDA to a solution of OlAm-capped In(As,P) quantum dots;

[0099] FIG. 6 shows the overlaid H-NMR spectra of the purified InAs quantum dots and the InAs quantum dots after UDA addition and 1 cycle of precipitation/redispersion;

[0100] FIG. 7 shows a quantitative Rutherford backscattering spectrometry spectrum for In(As,P) quantum dots;

[0101] FIG. 8 shows the overlaid x-ray photoelectron spectroscopy spectra before and after the addition of UDA and consequent purification of the quantum dots.

DESCRIPTION OF EMBODIMENTS

[0102] The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings are only schematic and are non-limiting. The size of some of the elements in the drawing may be exaggerated and not drawn on scale for illustrative purposes. The dimensions and the relative dimensions do not correspond to actual reductions to practice of the invention.

[0103] When referring to the endpoints of a range, the endpoints values of the range are included.

[0104] When describing the invention, the terms used are construed in accordance with the following definitions, unless indicated otherwise.

[0105] The term ‘and/or’ when listing two or more items, means that any one of the listed items can by employed by itself or that any combination of two or more of the listed items can be employed.

[0106] The term “ligand” refers to any molecule or ion capable of interacting, either weakly or strongly, with a quantum dot for example through covalent interaction, ionic interaction, Van der Waals interaction or by any other molecule interaction with the outer surface of the quantum dot. The term ligands include L type ligands as well as X type ligands.

[0107] The term “surface termination” refers to the ligands present at the surface of a quantum dot and include all types of ligands as for example L type ligands and X type ligands.

[0108] The term “modified quantum dots” or “surface modified quantum dots” refers to quantum dots provided with the exchanged ligands. For the purpose of this invention, the terms modified quantum dots and surface modified quantum dots are interchangeable.

[0109] “2D-NOESY” refers to 2D Nuclear Overhauser Effect Spectroscopy. “2D-DOSY” or “2D-DOSY NMR” refers to 2D Diffusion Ordered Spectroscopy. In 2D NOESY spectra, the presence of so-called negative nuclear overhauser effect (nOe) cross peaks indicates interaction of a compound with the quantum dot surface. A 2D-DOSY NMR spectrum of a dispersion of quantum dots yields the diffusion coefficient of quantum dots. For tightly bound ligands, this diffusion coefficient matches the diffusion coefficient of a single quantum dot—the ligand diffuses together with the quantum dot—while dynamic ligands exhibit a diffusion coefficient that is the population average of the diffusion coefficient of the ligand in the bound and free state.

[0110] “XPS” refers to X-ray Photoelectron Spectroscopy.

[0111] “RBS” refers to Rutherford Backscattering Spectrosopy.

[0112] FIG. 1 shows the different steps of the ligand exchange and removal of X type ligands and L type ligands from the surface of quantum dots according to the present invention to prepare modified quantum dots. Tetrahedral shaped InAs quantum dots are dispersed in toluene. The In-rich (indicated with blobs in FIG. 1) quantum dots are provided with chloride ligands (X type ligands) and oleylamine ligands (L type ligands). Upon adding compound of formula RYH (for example a carboxylic acid, a thiol or water), an acid-base reaction ensues after which the oleylamine ligands and the chloride ligands are displaced, an alkylammonium chloride salt desorbs and the incoming RY.sup.− ligands coordinate the surface of the quantum dots (step i)). The thus obtained quantum dots provided with the RY.sup.− ligands are referred to as the surface modified quantum dots. The properties of such surface modified quantum dots depend amongst others upon the length and the polarity of the RY.sup.− ligands. Consequently, the properties of the surface modified quantum dots can be influenced by the choice of the RY.sup.− ligands.

[0113] To fully displace the salt from the surface, the method may comprise the additional step of making the solution more polar, for example by adding a polar solvent such as ethanol (EtOH), to precipitate the surface modified quantum dots (step ii)).

[0114] Subsequently, the supernatant comprising the formed salt is discarded and the surface modified quantum dots are redispersed in an apolar solvent, for example in toluene (step iii)).

[0115] FIG. 2 shows the different steps to prepare a dispersion comprising quantum dots according to the present invention. The method uses an extraction process. InAs quantum dots provided with chloride ligands (X type ligands) and oleylamine (OlAm) ligands (L type ligands) are dispersed in an apolar solvent, for example in an alkane such as octane, to form a first solution.

[0116] A second solution comprising a compound of formula RYH in an polar solvent, for example N,N-dimethylformamide (DMF), is prepared. The compound of formula RYH has an acidic proton. The RY-group of the compound of formula RYH further comprises a functional group capable of participating in hydrogen bonding, for example a —OH, —SH or —NH group. The RY-group comprises preferably and/or said R-group comprises from 1 to 20 carbon atoms. The compound of formula RYH comprises for example 1,2-ethanedithiol, 3-mercaptoproponic acid or thioglycerol.

[0117] Preferably, the second solution further comprises a stabilizing agent, for example a mono- or bifunctional amine, such as butyl amine, ethylene diamine and ethanolamine.

[0118] The first solution is placed on top of the second solution, forming a biphasic system. The first and second solution are shaked thoroughly. Ligand exchange occurs at the interphase of the first and second solution and the surface of the InAs quantum dots becomes decorated by the 2 ligands added to the second solution.

[0119] The small polar ligands RY.sup.− that are now present on the surface of the quantum dots render the InAs quantum dots dispersible in the polar solvent (DMF) and the quantum dots extracted from the apolar phase (alkane) to the polar phase (DMF).

[0120] After discarding the alkane phase modified quantum dots with the polar ligands in the polar solvent (DMF) are left. This dispersion is for example suitable as ink. The dispersion can be deposited to form a layer of quantum dots, for example by spin-coating and results in a uniform layer of InAs quantum dots having short organic ligands.

[0121] It is clear that the modified quantum dots can be redispersed in another solvent. Therefore the quantum dots in the polar solvent can be precipitated for example by adding an apolar solvent such as toluene. Subsequently, the precipitated quantum dots can be redispersed in a solvent of choice depending on the deposition technique to deposit a layer of quantum dots or depending on the application of the layer of quantum dots. The dispersion comprising quantum dots is for example deposited by means of spincoating to form a layer of InAs quantum dots with new ligands RY.sup.− on the surface of the quantum dots.

[0122] FIG. 3 is a schematic illustration of the ligand exchange reaction that ensues during the extraction of the InAs quantum dots from an apolar solvent (for example an alkane such as hexane) to a polar phase (for example DMF). The bifunctional, polar nature of the ligands in DMF renders the underlying quantum dots soluble in polar solvents.

Example: Synthesis and Characterization of InAs Quantum Dots

Materials

[0123] Toluene (anh. 99.8%) and ethanol (anh.) were purchased from VWR and purified via the freeze-pump-thaw method before bringing into a N.sub.2 glovebox.

[0124] Oleylamine (OlAm) (>96% prim. amine, 80-90% C18 content) was purchased from Acros Organics, dried over CaH.sub.2 and consequently vacuum distilled.

[0125] Tris(diethylamino)phosphine (97%), 10-undecenoic acid (98%) were purchased from Sigma-Aldrich.

[0126] Tris-dimethylaminoarsine (99%) was ordered from Strem chemicals.

[0127] Toluene-d8 was purchased from Eurisotop.

Synthesis and Characterization

[0128] In(As,P) quantum dots were synthesized via a wet chemical route. In a 25 mL three-neck flask, InCl.sub.3 and OlAm are degassed and consequently heated to the reaction temperature. Tris(dimethylamino)arsine is injected at a 1:1 ratio to In, after which Tris(diethylamino)phosphine (TDEAP) is injected in a 3:1:1 ratio to reduce the arsine and produce In(As,P) quantum dots. The dual reactivity of TDEAP leads to some P alloying. All steps of the synthesis including reaction, purification and characterisation were performed under nitrogen atmosphere to avoid oxidation.

[0129] Transmission electron micrographs confirmed that the synthesized quantum dots have a tetrahedral shape.

[0130] H-NMR spectroscopy was used to characterize the ligands stabilizing the InAs quantum dots. FIG. 4 shows the 1D .sup.1H-NMR spectrum of purified InAs quantum dots in toluene-d8 with the resonances annotated according to the OlAm molecule in the inset of FIG. 4.

[0131] .sup.1H-NMR was also used to probe ligand exchange reactions between organic ligands. The ligand exchange of OlAm with Undec-10-enoic acid (UDA) was probed by stepwise additions of excess UDA. The terminal unsaturation allows for easy identification in .sup.1H-NMR. In FIG. 5 the .sup.1H-NMR spectra are shown during the progressive addition of 1, 2.5 and 4 equivalents of UDA to a solution of OlAm-capped InAs quantum dots. Besides the alkene resonance 5 from OlAm in the initial trace and the residual toluene around 7 ppm, addition of 1 equivalent of UDA yields 3 extra peaks. One at 5.20, at 6.05 and 7.75 ppm, corresponding to the terminal olefinic CH.sub.2 U.sub.1 and CH U.sub.2 in UDA, as well as a protonated oleylammonium species OlNH.sup.+.sub.3 respectively.

[0132] Notably, the olefinic resonances of UDA contain both a very broad and sharp contribution already at 1 equivalent added. The broad resonances for the protons U1 and U2 indicate the surface ligation of the UDA molecule to the quantum dots, whereas simultaneously an OlNH.sub.3.sup.+ salt is formed. Further addition of carboxylic acid does not induce visible changes in the OlNH.sup.+.sub.3 peak at 7.75 ppm, but rather yields another peak first at 10.2 ppm which evolves to 11.5 ppm after 4 equivalents of UDA. Broad peaks above 10 ppm are typical of carboxylic acids.

[0133] .sup.1H-NMR was also used to characterize the exchanged and purified InAs quantum dots in FIG. 6. The formed OlNH.sub.3.sup.+ salt at 7.75 ppm is removed during a cycle of precipitation/redispersion as can be noted by its absence in FIG. 6. The broad resonances for U1 and U2 of the UDA molecule persist throughout purification, indicating ligation of UDA to the quantum dots. The reduced intensity of the OlAm signal 5 represents the fraction of the OlAm that is removed during the ligand exchange in the form of a salt.

[0134] To identify the presence of chloride surface ligands (X type ligands), techniques complementary to the .sup.1H-NMR analysis are required. FIG. 7 shows the Rutherford backscattering spectrometry spectrum of the above described In(As,P) quantum dots. Rutherford backscattering spectroscopy is a quantitative technique allowing to determine the composition. According to the Rutherford backscattering spectrometry technique, the composition was determined to be equal to InAs.sub.0.76P.sub.0.15Cl.sub.0.26, proving that the quantum dots are In-rich and accumulate a large chloride content. Together with the .sup.1H-NMR data presented (see above), it can be concluded that the surface termination of the quantum dots comprise L type ligands (OlAm) and X type ligands (Cl-ions).

[0135] In order to confirm that the identified X-type ligands are on the surface and are displaced in parallel with the L-type OlAm, x-ray photoelectron spectroscopy (XPS) was used to monitor the relative Cl content before and after the UDA titration. In FIG. 8 the region of the XPS-spectrum around 200 keV is presented and shows a clear drop in the Cl/As ratio, which arises from the loss of Cl-ions in the form of OlNH.sub.3Cl. The relative reduction in the OlAm resonance from H-NMR and the drop in the Cl-peak in XPS allows for an estimation of the initial Cl:OlAm content on the quantum dot, which in this case yields 1.45:1.

[0136] In a second experiment the same quantum dots as described above, i.e. In(As,P) quantum dots with oleylamine and chloride ligands, are titrated by up to 30 equivalents of hexadecanethiol (HdSH). The .sup.1H-NMR spectrum before addition of HdSH shows the characteristic resonances of oleylamine tightly bound to In(As,P).

[0137] After addition of the first equivalents of HdSH, the presence of ammonium protons is identified in the .sup.1H-NMR, demonstrating that HdSH protonates surface bound oleylamine. After addition of 30 equivalents of HdSH, the quantum dots are purified by precipitation with EtOH and redispersion in toluene-ds.

[0138] From the quantitative .sup.1H-NMR spectra of the discarded supernatant and the redispersed quantum dots one can conclude that oleylamine has been removed from the surface as an oleylammonium salt, whereas the total ligand density has remained constant, suggesting the binding of titrated HdSH as the conjugate base HdS.sup.−.

[0139] Analysis of the supernatant with a spectrophotometric test for chloride ions demonstrates the desorption of chloride.

[0140] In summary, by adding HdSH to In(As,P) quantum dots provided with oleylamine and chloride ligands, the surface bound oleylamine ligands are protonated, the deprotonated thiolate HdS.sup.− binds to the surface of the quantum dots and the chlorides are replaced from the surface of the quantum dots.

[0141] In a further experiment, In(As,P) quantum dots are synthesized according to the same method as in the first example mentioned above, with the notable difference that InCl.sub.3 is substituted by InBr.sub.3.

[0142] In a 1D .sup.1H-NMR experiment, the organic ligand of the synthesized quantum dots is identified to be tightly bound oleylamine. From X-ray Fluorescence experiments, it is identified that bromide is present on the purified quantum dots, similar to the chloride ligands identified by x-ray photoelectron spectroscopy above. The slight change in synthesis method thus also yields a hybrid surface chemistry, where bromide acts as X type ligand and oleylamine acts as the L type ligand.

[0143] The .sup.1H-NMR spectrum is recorded again after addition of 1 equivalent UDA. The .sup.1H-NMR spectrum shows the appearance of the same alkylammonium salt at 8 ppm, confirming that also in this case the ligand exchange is facilitated by the protonation of the surface bound L-type ligand.

[0144] The results suggest the same chemistry where L type ligands (olyelamine) and X type ligands (bromide) are displaced via the protonation of oleylamine and the binding of the deprotonated carboxylic acid.

[0145] A further third example illustrates the two phase exchange with mercapto-1,2-propanediol and n-butylamine in dimethylformamide (DMF).

[0146] First, purified In(As,P) quantum dots with oleylamine and chloride ligands are dispersed in n-octane.

[0147] A second solution is prepared by dissolving 50 microliters of 3-mercapto-1,2-propanediol and 10 microliters of n-butylamine in 3 mL dimethylformamide (DMF).

[0148] The dispersion of quantum dots in octane is placed on top of the DMF solution via pipetting. The two solutions form a 2-phase system due to the difference in polarity between n-octane and DMF. As octane has a lower density, it forms the top-most phase, with DMF resting underneath.

[0149] After shaking the vial, the black color of the In(As,P) quantum dots is transferred to the lower DMF phase, proving that the quantum dots are extracted to the DMF phase. Careful separation of the two phases via pipetting allows for a careful analysis of their respective contents.

[0150] Quantitative .sup.1H-NMR spectroscopy of the octane phase shows that the oleylamine, initially present as L type ligand, remained in the octane phase after the quantum dots are extracted to DMF. .sup.1H-NMR spectroscopy of the DMF phase shows broad resonances characteristic of surface bound 3-mercapto-1,2-propanediol and n-butylamine.

[0151] 2D DOSY spectroscopy confirms that the small polar ligands are bound to the quantum dots, as they feature a diffusion coefficient that corresponds to the quantum dot diameter.

[0152] Absorption spectroscopy of the DMF phase demonstrates that the quantum dots are quantitatively extracted.

[0153] The binding of the small polar ligand 3-mercapto-1,2-propanediol extracts the quantum dots to the DMF phase, whereas the oleylamine remains in the octane phase. The excess of stabilizing agent butylamine compared to oleylamine acts here to soak up the proton, which explains the absence of oleylammonium in the octane phase.

[0154] The dispersion in DMF is purified by inducing precipitation with toluene.

[0155] After decantation and redispersion in DMF, a conductive thin film is produced by spincoating the ligand exchanged QD dispersion from DMF onto a substrate equipped with metallic contacts to measure the electrical properties of the film.