Quantum dot nanoparticles having enhanced stability and luminescence efficiency

Abstract

Certain dithio-compounds have been found to be superior capping ligands for quantum dot (QD) nanoparticles. Example dithio-ligands include dithiocarbamate ligands. These strongly binding ligands are capable of coordinating to both positive and negative atoms on the surface of the nanoparticle. The ligands are bi-dentate and thus their approach to the QD surface is not as sterically hindered as is the approach of mono-dentate ligands. These ligands can therefore completely saturate the QD surface.

Claims

1. A method for preparing quantum dot (QD) nanoparticles comprising a plurality of dithiocarbamate capping ligands, the method comprising: reacting a core nanoparticle with shell precursors in the presence of capping ligands to grow at least one semiconductor shell on the core, to form a core-shell nanoparticle having the capping ligands bound to an outermost surface thereof; and exchanging at least a portion of the capping ligands with the dithiocarbamate capping ligands, wherein the core is an InP or an InPZnS alloy-based core, and the shell precursors comprise zinc and sulfur or selenium.

2. The method recited in claim 1 further comprising etching the core prior to reacting the core with the shell precursors.

3. The method recited in claim 2 wherein hydrofluoric acid is used to etch the cores.

4. The method recited in claim 1 wherein the capping ligands are oleylamine.

5. The method recited in claim 1 wherein the capping ligands are myristic acid.

6. A method for preparing core-shell nanoparticles comprising a plurality of first dithiocarbonate capping ligands, the method comprising: reacting a core nanoparticle with shell precursors in the presence of capping ligands to grow at least one semiconductor shell on the core, to form a core-shell nanoparticle having the capping ligands bound to an outermost surface thereof; and exchanging at least a portion of the capping ligands with dithiocarbonate capping ligands, wherein the core is an InP or an InPZnS alloy-based core, and the shell precursors comprise zinc and sulfur or selenium.

7. The method recited in claim 6 further comprising etching the core prior to reacting the core with the shell precursors.

8. The method recited in claim 7 wherein hydrofluoric acid is used to etch the cores.

9. The method recited in claim 6 wherein the capping ligands are oleylamine.

10. The method recited in claim 6 wherein the capping ligands are myristic acid.

11. The method recited in claim 6, wherein the dithiocarbonate capping ligands used in the exchanging step are of the structure: ##STR00006## wherein R and R are selected from alkyl groups, aryl groups, functionalized alkyl group, functionalized aryl groups, and amphiphilic groups.

12. The method recited in claim 6, wherein the dithiocarbonate capping ligands used in the exchanging step are zinc ethylxanthate capping ligands.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

(1) FIG. 1 is a flow diagram of a process according to the invention for preparing QDs with dithiocarbamate ligands.

DETAILED DESCRIPTION OF THE INVENTION

(2) The dithiocarbamate ligands described herein can be used as capping ligands for generally any type of semiconductor QD nanoparticle. Particularly useful QD nanoparticles are prepared as described in U.S. Pat. Nos. 7,588,828, 7,803,423, 7,985,446, and 8,062,703 (referred to herein collectively as the seeding patents), the entire contents of which are hereby incorporated by reference. FIG. 1 illustrates such a method of preparing QD nanoparticles. Briefly, the method involves reacting core material precursors in the presence of a molecular cluster compound, which acts as a seed for crystal growth (101). The seeding patents describe a number of precursor materials and molecular cluster compounds. As an example, suitable precursor compounds for an InP-based core must provide a source of indium and a source of phosphorus. For example, the indium source may be indium myristate and the phosphorus source may be tris(trimethylsilyl)phosphine. It will be appreciated that other indium and phosphorus sources may be used.

(3) The core-forming reaction may be conducted in the presence of a molecular seeding compound. Suitable molecular seeding compounds are described at length in the seeding patents referenced above. One example of a suitable molecular seeding compound is the zinc sulfide-based molecular seeding compound described in U.S. Pat. No. 8,062,703. The core precursor compounds and the molecular seeding compound are heated in a solvent under conditions described in the seeding patents and U.S. Patent Publication No. 2010/0068522, the entire contents of which are hereby incorporated by reference. Generally, a non-electron donating solvent is used for the reaction. One example of an appropriate solvent is a heat transfer fluid such as THERMINOL (SOLUTIA INC., ST. LOUIS MISSOURI 63141).

(4) It may be desirable to monitor core growth 102 (e.g., via an optical property of the QD core) during the synthesis of the core. For example, the absorbance spectrum may be monitored as the QD core grows and the reaction may be stopped when the core reaches the proper size to yield the desired absorbance and/or emission spectrum. Once the desirable optical value is obtained and the reaction stopped, the cores can be isolated 103, for example, by filtration. It may be desirable to add a non-solvent to the reaction mixture to induce precipitation of the cores. Once the cores are isolated, they may be reacted with shell precursors 104 to grow one or more semiconductor shells on the cores. It may be desirable to pretreat the cores, for example, by etching a small amount of the material from the core, prior to reacting the core with the shell precursors. An acid, such as hydrofluoric acid may be used to etch the core.

(5) The shell precursors react to form a shell of semiconductor material on the QD cores. If a non-coordinating solvent is used during the shelling reaction it may be necessary to add a coordinating ligand such as myristic acid during the shelling reaction to stabilize the shelling reaction. If a coordinating solvent such as TOP/TOPO is used, then solvent molecules themselves may act as ligands to stabilize the shelling reaction. In either case, the outermost semiconductor surface of the QDs is bound to capping ligands.

(6) The next step 105 is to exchange at least a portion of the capping ligands with the disclosed dithio-ligands. The dithio ligands have the general structure (1):

(7) ##STR00001##

(8) Examples of dithio-ligands include dithiocarbamate ligands. Examples of suitable first dithiocarbamate capping ligands are represented by structures (2) and (3): (2) (3)

(9) ##STR00002##

(10) Other suitable dithio-ligands include alkyldithiocarbonate (i.e., xanthate) ligands, having the general structure (4):

(11) ##STR00003##

(12) The R groups (that is, R and R) may be any functional groups, and are generally hydrocarbons. Examples include alkyl or aryl groups. One example is ethyl groups. According to some embodiments, the R groups may contain functional groups with the ability to tailor the hydrophilicity of the QDs, for example to render the QDs more soluble in hydrophilic environments. An example of such an R group is a C12 hydrocarbon chain with a carboxylic acid functionality on the C12 position. Once suitably coordinated to the surface of the dots, the carboxylic acid can be deprotonated with a base and the dots transferred to hydrophilic media. Alternatively, the R groups may be an amphiphilic group such as a PEG (polyethyleneglycol).

(13) A first dithio-capping ligand may be exchanged to bind to the electropositive atoms on the surface. For example, the first dithio-capping ligand may bind to zinc on the surface of a ZnS or ZnSe surface. The first dithio-capping ligand compounds (1)-(4) may be provided as a salt of sodium or potassium. The dithio-ligands may be provided as a powder or in a solvent such as THERMINOL.

(14) Another example of a suitable dithio-ligand is represented by structure (5):

(15) ##STR00004##

(16) In structure (5), R is as defined above and R is typically an alkyl group. Compounds of structure (5) may exist in both monomeric and dimeric forms. When R is an ethyl group, the monomeric form is a major species. Compounds according to structure (5) have great potential for coordinating to surface sulfide and/or selenium ions, thereby satisfying the preferred coordination of the metal (Zn) atom.

(17) Zinc dithiocarbamates, such as illustrated in structure (5) above, have been used as single source precursors for ZnS nanoparticles. Those compounds are viable candidates for single source precursors for depositing ZnS shells upon InP core nanoparticles. However, zinc dithiocarbamates require high temperatures to make them decompose. Such high temperatures negatively impact the performance of the InP core nanoparticle. The decomposition temperature is significantly lower if the zinc dithiocarbamates are provided in the presence of amines. However, addition of amines to InP nanoparticles has a particularly drastic quenching effect on radiative efficiency.

(18) The Applicants have discovered that zinc dithio compounds, such as zinc complexes of structures (2), (3), and (4), and structure (5) may be used as single-source shelling precursors (i.e., single source precursors providing elements of a ZnS shell upon a core nanocrystal) if the zinc dithiocarbamates are pre-coordinated to an amine. Another suitable shelling precursor is amine-coordinated zinc alkylthiophosphate, i.e., amine-coordinated compounds of dithio-ligands having the following structure (6):

(19) ##STR00005##

(20) Pre-coordinating an amine with the zinc dithiocarbamate allows the amine and the zinc dithiocarbamate to be introduced into the solution of nanoparticle cores during shelling at a 1:1 ratio of amine to zinc dithiocarbamate. Thus, no free amine is available to participate in quenching reactions with the core surface. The decomposition temperature of the zinc dithiocarbamate is suitably lowered by the amine, but without the adverse quenching effects observed when free amine is present. Examples of suitable amines include amines having long hydrocarbon constituents. A particularly suitable amine is oleylamine.

EXAMPLES

Example 1

(21) The quenching effect of free amine was demonstrated by shelling InP alloy-based cores in the presence and in the absence of oleylamine. InPZnS alloy-based cores were prepared essentially as described in U.S. Pat. No. 7,558,828. Two samples of InP alloy-based cores were each suspended in THERMINOL. Zinc acetate (0.862 g) was added to each mixture and the mixtures were heated to 230 C. for 2 hours. Dodecanethiol (1.69 mL) was added to both mixtures and oleylamine (0.2 mL) was added to one of the mixtures. Both mixtures were allowed to react for a further 1 hours.

(22) The quantum dots shelled in the absence of oleylamine exhibited photoluminescence at 644 nm having a full width at half maximum (FWHM) of 94 nm and a quantum yield (QY) of 75%. The quantum dots shelled in the presence of oleylamine exhibited photoluminescence at 644 nm having a FWHM of 90 nm and a quantum yield (QY) of 62%. The lower QY of the sample shelled in the presence of amine illustrates the quenching effect of the amine.

Example 2

(23) InPZnS alloy-based cores were prepared as generally described in U.S. Pat. No. 7,588,828. The cores were suspended in THERMINOL (40 mL), to which was also added zinc acetate (5.76 g), myristic acid (1.5 g), zinc stearate (11.32 g). The mixture was heated at 180 C. and then zinc diethyldithiocarbamate (2.295 g) was added and left for 20 minutes. High turbidity was observed and the photoluminescence of the reaction mixture remained very low, suggesting low reactivity/solubility at that temperature. The mixture was heated to 230 C. and left for 3 hrs and gradually the powder dissolved but the QY remains very low throughout.

Example 3

(24) InPZnS alloy-based cores were shelled under the same conditions described in Example 2, except that oleylamine (5 mL) was added to the mixture following the addition of diethyldithiocarbamate at 180 C. The mixture was heated for an additional 20 minutes at 180 C., yielding quantum dots having a photoluminescence peak at 523 nm with a FWHM of 53 nm and a QY of 73%. The difference between Examples 2 and 3 suggest that amine is critical to shelling.

Example 4

(25) InPZnS alloy-based cores prepared as generally described in U.S. Pat. No. 7,588,828 were shelled as described in Example 3, except that the zinc diethyldithiocarbamate was first complexed with oleylamine by premixing the two together under nitrogen using a water bath set to 50 C. prior to adding to the shelling reaction. The shelling reaction yielded quantum dots having luminescence QY of 75%. Similar reactions yielded quantum dots having luminescence QY as high as 85%.

Example 5

(26) InPZnS alloy-based cores prepared as generally described in U.S. Pat. No. 7,588,828 were shelled as described in Example 4, except that zinc ethlyxanthate was used in the place of zinc diethyldithiocarbamate. InPZnS alloy-based cores (500 mg) myristic acid (1.5 g), zinc acetate (2.4 g), and zinc stearate (5.3 g) were stirred under vacuum in THERMINOL at 100 C. for 1 hour and then heated to 215 C. before cooling to 140 C. A premixed solution of zinc ethylxanthate (0.97 g) and oleylamine 1.05 (mL) in THERMINOL (5 mL) was added to the core solution. The solution was stirred for 1 hr then at 180 C. for one hour. Octanol (2.4 mL) was added and the solution was stirred an additional 30 minutes. The reaction yielded quantum dots having luminescence QY of 85%.

(27) Although particular embodiments of the present invention have been shown and described, they are not intended to limit what this patent covers. One skilled in the art will understand that various changes and modifications may be made without departing from the scope of the present invention as literally and equivalently covered by the following claims.