COLLOIDAL QUANTUM-DOTS FOR ELECTROLUMINESCENT DEVICES AND METHODS OF PREPARING SAME

FIELD OF THE DISCLOSURE

The present disclosure relates generally to the field of colloidal quantum-dot synthesis, and in particular to semiconducting quantum-dot nanocrystals for electroluminescent devices and methods of preparing same.

BACKGROUND

Colloidal quantum-dots (QDs) are semiconducting, nanoscale particles (also denoted “nanoparticles”) dispersing in solvents. Inorganic colloidal core/shell QDs have attracted tremendous interest over the past decades for applications in solid-state lighting, displays, energy harvesting devices, catalysis, photodetectors, and biomedical labeling. Colloidal QDs feature such properties as a narrow emission peak, optical stability, and tunable emission wavelengths (see references [1] to [4]). Recent advances in synthetic protocols have achieved near-unity photoluminescence quantum yields (PLQY s), wherein all photons absorbed by the QDs result in photoluminescent emissions from the QDs (see references [5] to [10]). To achieve these high-quality colloidal QD materials, reagents with tuned reactivity are carefully combined in solution at a specified rate, enabling the growth of epitaxial shells of semiconducting chalcogenides. The shells enable bandgap engineering to achieve charge balance in electroluminescent devices, as well as physical separation between emissive nanoparticle cores, which limits inter-particle energy transfer that ultimately lowers the PLQY at high colloidal concentrations (see references [3] and [11]). Low colloidal PLQYs and specifically low PLQYs observed in QD thin-films could occur due to surface defects arising from poor ligand coverage/binding to the chalcogenide surfaces of the nanocrystals (see references [12] and [13]), improper selection of shells, and suboptimal shell thickness, inducing “photoluminescence (PL) Winding” due to so-called non-radiative “Auger recombination” . Both suboptimal shelling and imperfect surface coverage can also adversely impact the charge transport mechanism, for example, in electroluminescent QD light- emitting diodes (QLEDs). In terms of color gamut and color purity in displays, extremely narrow emission of luminescent core/shell QDs makes QLEDs superior and basically irreplaceable to their main high-efficiency opponents, that is, organic light-emitting diodes (OLEDs) that typically exhibit a much broader spectral emission.

Despite advancements in nanoparticle synthesis, aerobic stability of solution-processed QLED devices has not been investigated properly to date. Indeed, instability of inorganic nanocrystalline luminescent core/shell QD layer and/or other organic/inorganic functional layers in device architectures during ambient-air fabrication can lead to low device efficiency and short operational lifetime. Aerobic-stable QLEDs will certainly open the gate for low-cost large-scale manufacturing of these devices for display and solid-state lighting, for example, in roll-to-roll (R2R) and inkjet printing processes.

Colloidal nanocrystalline QDs can be degraded by oxygen, water, thermal heating, and exposure to the ultraviolet (UV) light. Several groups have aimed to address the issue of surface defect driven instability of colloidal QDs, with strategies ranging from ultra-thick shell growth over small semiconducting nanoparticle cores, to methods of employing bulky organic ligands such as “dendron boxes” (see references [14] to [17]). However, simultaneously obtaining aerobic stability and excellent optoelectronic properties for electroluminescent devices could be very challenging. For instance, in the case of employing bulky ligands, even though closely-packed organic dendrons can provide excellent chemical (for example, suitable for biomedical labeling) and aerobic thin-film stability (for example, suitable for QLEDs or solar cell applications), due to their insulating nature, dendron-functionalized QDs largely lack the requirements for charge injection and charge transport, making them rather unsuitable for realizing super-efficient QLEDs.

In more recent works, the importance of ligand passivation on both the electrical and colloidal stability of photoluminescent nanomaterials has emerged as a key factor (see references [18] to [22]). In particular, the impact of ligand type and ligand attachment procedures has been shown to produce photoluminescent nanomaterials with either X-type, Z-type or crystal-bound passivation of the QD surfaces (see references [14] and [23]-[25]).

Therefore, there is always a desire for colloidal core/shell QD nanocrystals with sophisticated compositions as well as suitable surface ligands for improved aerobic stability and electroluminescent device performance. Preserving the optoelectronic properties such as high PLQY and excellent charge transport characteristics in colloidal QDs for QLED applications is of paramount importance.

SUMMARY

According to one aspect of this disclosure, there is provided a semiconducting quantum-dot (QD) nanoscale particle comprising: a QD core; an inner shell coated on the QD core; an intermediate shell coated on the inner shell; and an outer shell coated on the intermediate shell; each of the QD core, the inner shell, the intermediate shell, and the outer shell comprises one or more selected Group II and Group VI elements; the inner shell is different from the QD core; the intermediate shell is different from the inner shell, and comprises ZnSe _x Si- _x , where 0 < x < 1 ; and the outer shell is different from the intermediate shell.

In some embodiments, the inner shell, the intermediate shell, and the outer shell are configured to provide a lower step-height of energy change when electrons and holes travel inwardly from the outer shell towards the QD core during optical or electrical excitation.

In some embodiments, a bandgap of the QD core is smaller than the inner, intermediate, and outer shells and falls within the bandgap energy thereof.

In some embodiments, the inner, intermediate, and outer shells are configured for confining excitons within the QD core.

In some embodiments, x is selected from any one of 0.4 < x < 0.6 and x = 0.5.

In some embodiments, the QD core is a binary, ternary, or quaternary core comprising an alloy of the one or more selected Group II and Group VI elements.

In some embodiments, the QD core comprise CdS, CdSe, ZnS, ZnSe, ZnSeS, CdZnSe, CdZnS, CdZnSeS, and/or ZnTeSe.

In some embodiments, the inner shell comprises ZnSe.

In some embodiments, the outer shell comprises ZnS.

In some embodiments, the QD nanoscale particle is fabricated in an aerobic condition.

In some embodiments, the QD nanoscale particle has an in-solution photoluminescence quantum yield (PLQY) greater than or equal to 90% and a solid-state thin spin-coated film PLQY greater than or equal to 50%, measured in the aerobic condition.

In some embodiments, the QD nanoscale particle is configured for having an external quantum efficiency (EQE) of 5% to 20% in operation.

In some embodiments, the QD nanoscale particle is substantially free of EQE roll-off.

In some embodiments, the QD nanoscale particle is configured for having a photo luminescence (PL) peak greater than 500 nm with a maximum device brightness level greater than 300,000 cd/m ² .

In some embodiments, the QD nanoscale particle is configured for having a PL peak less than 500 nm with a maximum device brightness level of 50,000 cd/m ² .

In some embodiments, the QD core comprises alloyed CdZnSe, the inner shell comprises ZnSe, and the outer shell comprises ZnS; and the QD nanoscale particle has a PL peak at 482 nm and a full width at half maximum (FWHM) of the PL peak of about 21 nm.

In some embodiments, the QD nanoscale particle has a single PL peak for all emission colors and a FWHM of the PL peak of less than 30 nm. In some embodiments, the QD nanoscale particle has a single PL peak between 480 nm and 700 nm.

In some embodiments, the QD nanoscale particle has a single PL peak at 482 nm, 532 nm, 556 nm, 590 nm, 610 nm, or 622 nm.

In some embodiments, the QD nanoscale particle has a diameter selected from any one of 8 nanometers (nm) to 19 nm and 10 nm to 19 nm.

In some embodiments, at least one of the QD nanoscale particle and the QD core has a size distribution between 5% and 20%.

In some embodiments, the QD core has a diameter selected from any one of one nm to 8 nm and 3 nm to 7 nm.

In some embodiments, the inner shell has a thickness selected from any one of 2 crystal-lattice monolayers to 12 crystal-lattice monolayers and 8 crystal-lattice monolayers to 10 crystal-lattice monolayers.

In some embodiments, the inner shell has a thickness selected from any one of about one nm to 7 nm and about 4 nm to about 5 nm.

In some embodiments, the intermediate shell has a thickness selected from any one of one crystal-lattice monolayers to 6 crystal- lattice monolayers and one crystal- lattice monolayers to 2 crystal-lattice monolayers.

In some embodiments, the intermediate shell has a thickness selected from any one of 0.5 nm to 3 nm and 0.5 nm to one nm.

In some embodiments, the outer shell has a thickness selected from any one of one crystallattice monolayers to 6 crystal-lattice monolayers and 2 crystal-lattice monolayers to 4 monolayers.

In some embodiments, the outer shell has a thickness selected from any one of about 0.5 nm to about 3 nm and about one nm to about 2 nm.

In some embodiments, a total thickness of the inner, the intermediate, and the outer shell is 4 crystal-lattice monolayers to 24 crystal-lattice monolayers.

In some embodiments, a total thickness of the inner, the intermediate, and the outer shell is 2 nm to 13 nm.

In some embodiments, the QD nanoscale particle is passivated by crystal-bound thiol surface ligands.

According to one aspect of this disclosure, there is provided a plurality of above-described semiconducting QD nanoscale; the plurality of QD nanoscale particles are soluble in a solvent having potassium hydroxide (KOH) for suspending in the solvent in a dispersed manner. According to one aspect of this disclosure, there is provided a semiconducting QD nanoscale particle comprising: a QD core; an inner shell coated on the QD core; an alloy intermediate shell coated on the inner shell; and an outer shell coated on the intermediate shell; the outer shell comprises a crystal-bound thiol molecule coated surface for surface passivation.

In some embodiments, the total thickness of the inner, intermediate, and outer shells is 4 to 24 crystal-lattice monolayers.

According to one aspect of this disclosure, there is provided a colloidal QD material comprising a plurality of above-described semiconducting QD nanoscale particles.

According to one aspect of this disclosure, there is provided a method for preparing a plurality of semiconducting QD nanoscale particles, the method comprising: Step 1 : preparing QD cores to obtain a synthetic solution; and Step 2: forming a plurality of shells about the QD cores in the synthetic solution to obtain a solution of QD nanoscale particles from the synthetic solution, the plurality of shells comprising an inner shell on each core, an intermediate shell of ZnSe _x Si- _x (where 0 < x < 1) on each inner shell, and an outer shell in each intermediate shell.

In some embodiments, the method further comprises: Step 3: applying thiol ligands to the solution of QD nanoscale particles to form crystal-bound thiol molecule coated surfaces of the outer shells of the QD nanoscale particles for obtaining surface-passivated QD nanoscale particles.

In some embodiments, the method further comprises: Step 4: purifying the surface-passivated QD nanoscale particles.

In some embodiments, the thiol ligand comprises a mixture of oleic acid, trialkylphosphine, and 1 -Octanethiol or 1 -Dodecane thiol.

In some embodiments, the ligands comprise a mixture of 95% to 97% oleic acid, 1% to 3% trialkylphosphine, and 1% to 3% 1 -Octanethiol or 1 -Dodecane thiol.

In some embodiments, the thiol ligand comprises an alkyl moiety of 6 to 12 carbon-chain length or aromatic moiety.

In some embodiments, the thiol ligand comprises 1 -Octanethiol, 1 -Dodecanethiol, 1- Hexanthiol, 2-Ethylhexane- 1 -thiol, 2-Phenylethanethiol, and/or p-Toluenethiol.

In some embodiments, the Step 1 comprises: reacting a first precursor comprising one or more Group II elements with a second precursor comprising one or more Group VIA elements to produce the QD cores.

In some embodiments, the one or more Group II elements comprise Zn and/or Cd, and the one or more Group VIA elements comprise S, Se, and/or Te. In some embodiments, the Step 2 comprises: coating the alloyed Group II-VI QD cores with the inner shell, the intermediate shell, and the outer shell via controlled reaction of a third precursor comprising one or more Group II elements with a fourth precursor containing one or more Group VI elements at the surfaces of the QD cores.

In some embodiments, the Step 2 comprises: adjusting a thickness of the intermediate shell to adjust the energy bandgap and minimizing the crystalline strain between the ZnSe inner shell and the ZnS outer shell.

In some embodiments, a temperature range for said Step 1 and/or Step 2 is between 240 °C and 310 °C.

In some embodiments, the Step 3 comprises: adding the thiol ligands to the solution of QD nanoscale particles at a temperature selected from 160 °C to 260 °C and 210 °C to 260 °C.

In some embodiments, the Step 1 comprises: (i) forming a first solution using a first amount of cadmium oxide, a second amount of zinc acetate, a third amount of oleic acid, and a fourth amount of octadecene or paraffin oil; (ii) degassing the first solution under a first pressure at a first temperature for a first time period, and then heating the degassed first solution to a second temperature under inert atmosphere to remove water and acetic acid vapors; (iii) preparing a first precursor via dissolution of a fifth amount of a first component in a sixth amount of a second component under inert conditions; (iv) injecting a seventh amount of the first precursor into the degassed first solution; and (v) annealing the solution obtained at step (iv) at a third temperature for a second time period to obtain the synthetic solution.

In some embodiments, the Step 1 further comprises: determining the sixth amount of the second component based on a size of the QD cores to be prepared.

In some embodiments, the Step 1 further comprises: determining the sixth amount of the second component based on an emission color to be obtained.

In some embodiments, the first amount is 0.33 millimoles (mmol) to 0.5 mmol or 0.0424 gram (g) to 0.0642 g; the second amount is 6 mmol to 12 mmol or 1.1010 g to 2.2018 g; the third amount is 13 milliliters (ml); the fourth amount is 9 ml; the first pressure is 0.1 Torr; the first temperature is 110 °C; the first time period is two hours; the second temperature is 300 °C; the fifth amount of the first component is one mmol or 0.079 g of elemental Se; the sixth amount is 3.0 ml to 4 ml; the seventh amount is 1.0 ml to 2.0 ml of two molarities (M) of the first precursor; the third temperature is 300 °C; and the second time period is 5 minutes to 20 minutes.

In some embodiments, the QD nanoscale particles have a PL emission maximum between 600 nm and 700 nm. In some embodiments, the first amount is 0.1 mmol to 0.4 mmol or 0.0128 g to 0.0514 g, the second amount is 0.9 mmol to 0.6 mmol or 0.165 g to 0.110 g, and a total quantity of the cadmium oxide and the zinc acetate is one mmol; the third amount is 1.5 ml; the fourth amount is 5 ml; the first pressure is 0.1 Torr; the first temperature is 110 °C; the first time period is one hour; the second temperature is 280 °C; the fifth amount of the first component is 6 mmol or 0.4738g of elemental Se; the sixth amount is 3.0 ml or 6.0 ml; the seventh amount is 0.5 ml to one ml of two molarities (M) of the first precursor or 1.8 ml to 2.5 ml of one M of the first precursor; the third temperature is 260 °C to 280 °C; and the second time period is 5 minutes to 15 minutes.

In some embodiments, the prepared QD material has a PL emission maximum between 500 nm and 610 nm.

In some embodiments, the method further comprises: if the first amount is zero, preparing a second precursor, and after a third time period from step (iv), injecting into the solution obtained at step (iv) an eighth amount of the second precursor; said preparing the second precursor comprises: preparing a second-precursor solution using a ninth amount of cadmium oxide, a tenth amount of oleic acid, and an eleventh amount of octadecene or paraffin oil, and degassing the second-precursor solution under a second pressure at a fourth temperature for a fourth time period, and then heating the degassed second-precursor solution to a fifth temperature under inert atmosphere and then cool to a sixth temperature to remove water and acetic acid vapors and obtain the second precursor.

In some embodiments, said injecting into the solution obtained at step (iv) the eighth amount of the second precursor comprises: injecting into the solution obtained at step (iv) the eighth amount of the second precursor with a twelfth amount of the second component.

In some embodiments, the first amount is zero to 0.3 mmol or 0 to 0.0385 g, the second amount is one mmol to 0.7 mmol or 0.184 g to 0.128 g, and a total quantity of the cadmium oxide and the zinc acetate is one mmol; the third amount is 1.5 ml; the fourth amount is 5ml; the first pressure is 0. 1 Torr; the first temperature is 120 °C; the first time period is one hour; the second temperature is 260 °C to 280 °C; the fifth amount of the first component is 6 mmol or 0.4738 g of elemental Se; the sixth amount is 3.0 ml or 6.0 ml; the seventh amount is 0.5 to one ml of 2 M of the first precursor, or 1.8 to 2.5 ml of one M of the first precursor; the third temperature is 260 °C to 280 °C; if the first amount is non-zero, the second time period is between 5 minute to 15 minutes; and, if the first amount is zero, the second time period is between 15 minute to 30 minutes, the eighth amount is 0.1 ml to 0.6 ml, the third time period is zero minute to two minutes, the ninth amount is one mmol or 0. 1284 g, the tenth amount is one ml, the eleventh amount is one ml, the second pressure 0.1 Torr, the fourth temperature is 120 °C, the fourth time period is one hour, the fifth temperature is 260 °C, and the sixth temperature is 50 °C.

In some embodiments, the twelfth amount is 0.5 ml to one ml.

In some embodiments, the prepared QD material has a PL emission maximum between 480 nm and 530 nm.

In some embodiments, the Step 2 comprises: preparing a Se precursor; preparing a S precursor; preparing a Se-S precursor by mixing a first molar ratio of the Se and S precursors under inert condition; and after said preparing cores, growing the plurality of shells layer-by-layer by sequentially injecting the Se, Se-S, and S precursors to the synthetic solution to obtain a solution of the QD nanoscale particles.

In some embodiments, the Step 2 further comprises: preparing a zinc precursor; and injecting the zinc precursor to the synthetic solution after the S precursor is injected thereinto.

In some embodiments, said preparing the zinc precursor comprises: preparing a zinc-precursor solution using an thirteenth amount of zinc acetate, a fourteenth amount of oleic acid, and a fifteenth amount of octadecene or paraffin oil; degassing the zinc-precursor solution under a third pressure at a seventh temperature for a fifth time period to remove water and acetic acid vapors; and cooling the degassed zinc-precursor solution to a eighth temperature under inert conditions to obtain the zinc precursor.

In some embodiments, said thirteenth amount is 10 mmol or 1.8350 g; the fourteenth amount is 10 ml; the fifteenth amount is 10 ml; the third pressure is 0.1 Torr; the seventh temperature is 120 °C; the fifth time period is one hour; and the eighth temperature is 100 °C.

In some embodiments, said preparing the Se precursor comprises: preparing the Se precursor using an sixteenth amount of elemental Se, a seventeenth amount of the second component, and an eighteenth amount of octadecene or paraffin oil under inert conditions, and continuous stirring to obtain the Se precursor.

In some embodiments, the sixteenth amount is 6 mmol or 0.4738 g; the seventeenth amount is 3.0 ml; and the eighteenth amount is 3 ml to 9 ml.

In some embodiments, said preparing the S precursor comprises: preparing the S precursor using a nineteenth amount of elemental S, a twentieth amount of phosphine-derivative solvent, and a twenty- first amount of octadecene or paraffin oil under inert conditions, and continuous stirring at a ninth temperature to obtain the S precursor. In some embodiments, the nineteenth amount is 6 mmol or 0.1924g; the twentieth amount is selected from any one of 3.0 ml and 4.0 ml; the twenty-first amount is 3 ml to 9 ml; and the ninth temperature is 70 °C to 110 °C.

In some embodiments, the first molar ratio is 1 : 1.

In some embodiments, said growing the plurality of shells layer-by-layer comprises: (a) processing the synthetic solution by adding thereinto a twenty-second amount of the Se precursor while maintaining a tenth temperature to form a first shell layer of ZnSe; (b) processing the synthetic solution by injecting thereinto a twenty-third amount of the Se-S precursor while maintaining an eleventh temperature to form a second shell layer of ZnSeS; (c) processing the synthetic solution by injecting thereinto a twenty- fourth amount of the S precursor while maintaining a twelfth temperature to form a third shell layer of ZnS; and (d) cooling the processed synthetic solution to a thirteenth temperature to obtain the solution of QD nanoscale particles.

In some embodiments, the tenth, eleventh, twelfth, and thirteenth temperatures are between 260 °C and 310 °C; the twenty-second amount is 3.0 ml to 9.0 ml; the twenty-third amount is 1.0 ml to 3.0 ml; and the twenty-fourth amount is 3.0 ml to 9.0 ml.

In some embodiments, the second component comprises a phosphine-derivative solvent.

In some embodiments, the second component comprises alkylphosphine.

In some embodiments, the second component comprises trioctylphosphine, tributylphosphine, or diphenylphosphine.

In some embodiments, the Step 3 comprises: injecting a twenty-fifth amount of the thiol ligands into the solution of QD nanoscale particles at an fourteenth temperature; annealing the thiol- molecule-added solution at a fifteenth temperature for a sixth time period under inert conditions; and cooling the annealed solution to a sixteenth temperature under inert conditions to obtain a solution of crystal-bound thiol-ligand-passivated QD nanoscale particles.

In some embodiments, the twenty- fifth amount is 5 mmol to 10 mmol; the fourteenth temperature is 220 °C to 240 °C; the fifteenth temperature is 210 °C to 230 °C; the sixth time period is 20 minutes to 45 minutes; and the sixteenth temperature is 100 °C.

In some embodiments, the Step 3 comprises: injecting a twenty-sixth amount of thiol ligands into the solution of QD nanoscale particles at a seventeenth temperature.

In some embodiments, the twenty-sixth amount is .5 ml to 2.0 ml; and the seventeenth temperature is below 260 °C.

In some embodiments, the Step 3 comprises: forming the thiol ligands using an twentyseventh amount of two or more thiol mixtures or thiol-functionalized aryl alkyl-based ligands. In some embodiments, the twenty-seventh amount is 5 mmol to 10 mmol.

In some embodiments, the Step 4 comprises: adding an equivalent-to-reaction-mixture volume of toluene to the solution of surface-passivated QD nanoscale particles to a first total volume to obtain a QD-toluene dispersion; precipitating the QD-toluene dispersion with reagent alcohol; centrifuging the precipitated QD-toluene dispersion at a first revolutions-per-minute (rpm) for a seventh time period while maintaining the precipitated QD-toluene dispersion at a eighteenth temperature to form an upper solution and a QD precipitate; dispersing the QD precipitate in hexanes or chloroform; precipitating the dispersed QD precipitate with reagent alcohol or acetonitrile; centrifuging the precipitated QD precipitate for a first number of cycles; and dispersing the centrifuged QD precipitate in a non-polar organic solvent.

In some embodiments, the first total volume is 80 ml; the first rpm is 7000-8000 rpm; the seventh time period is 2 minutes to 5 minutes; the eighteenth temperature is above 40 °C; and the first number of cycles are 3 cycles to 10 cycles.

In some embodiments, the Step 4 comprises: dispersing the solution of surface-passivated QD nanoscale particles in a non-polar organic solvent to obtain a QD dispersion; and precipitating the QD dispersion with a twenty-eighth amount of low-polar organic solvent.

In some embodiments, the twenty-eighth amount is 100 ml.

In some embodiments, the low-polar organic solvent comprises acetone, ethyl-methylacetate, or methanol.

According to one aspect of this disclosure, there is provided a method of testing a plurality of QD nanoscale particles for alkaline stability, the method comprising: (A) dissolving a twenty-eighth amount of the QD nanoscale particles in a twenty-ninth amount of regular or deuterated chloroform to obtain a testing QD solution; (B) applying alkaline treatment in chloroform to the testing QD solution; and (C) conducting proton nuclear magnetic resonance (1H-NMR) spectroscopy of the alkaline-treated QD solution.

In some embodiments, the twenty-eighth amount is 20 mg; and wherein the twenty-ninth amount is one ml.

In some embodiments, step (B) comprises: (B) adding to the testing QD solution and agitating therein a thirtieth amount of one M potassium hydroxide (KOH) dissolved in regular or deuterated methanol to obtain a KOH-treated QD solution.

In some embodiments, the thirtieth amount is 10 microliters.

In some embodiments, the testing method further comprises: conducting 1H-NMR spectroscopy of the testing QD solution. According to some aspects of this disclosure, there is provided a QD light-emitting diode (QLED) comprising: a QD layer comprising a plurality of above-described semiconducting QD nanoscale particles.

According to some aspects of this disclosure, there is provided a QLED comprising: a QD layer comprising a plurality of the semiconducting QD nanoscale particles prepared using abovedescribed method.

In some embodiments, the above-described QLED further comprises: an indium-tin- oxide (ITO) layer coupled to a transparent or semi-transparent substrate; a layer poly(3,4- ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) coupled to the ITO layer; a layer of Poly(9,9-dioctylfluorene-alt-N-(4-sec-butylphenyl)-diphenyla mine) (TFB) coupled to the PEDOT :PSS layer and the QD layer; a layer Mg-doped zinc oxide (MgZnO) coupled to the QD layer; and a silver layer coupled to the layer of MgZnO.

In some embodiments, the above-described QLED has at least one of: a current efficiency of at least 17.4 cd/A; a power efficiency of at least 14.8 Im/W; a brightness level of at least 235,000 cd/m ² under a driving voltage of 12 V; and a tum-on voltage lower than 2.1 V.

In some embodiments, the EQE of the above-described QLED is at least 10.0%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram showing the structure of a bandgap-engineered semiconducting quantum dot (QD) nanoscale particle (denoted “nanoparticle”) having a core and three shells (denoted “core/triple-shell”), according to some embodiments of this disclosure;

FIG. IB is a schematic diagram showing the bandgaps of the core and shells of the core/triple- shell QD nanoparticles having the structure shown in FIG. 1A;

FIG. 2A is a dark field scanning transmission electron microscopy image of red-emitting core/triple-shell QD nanoparticles having the structure shown in FIG. 1 A;

FIGs. 2B to 2E show the corresponding energy dispersive x-ray (EDX) spectra maps for the elements of the core and shells in exemplary red-emitting core/triple-shell QD nanoparticles having the structure shown in FIG. 1A, demonstrating the discrete Cd-doped ZnSe cores and subsequent ZnSe, ZnSe _x Si- _x and ZnS shells, wherein

FIG. 2B shows the EDX spectra map of sulfur (S) (K electron shell excitation),

FIG. 2C shows the EDX spectra map of zinc (Zn) (K electron shell excitation),

FIG. 2D shows the EDX spectra map of selenium (Se) (L electron shell excitation), and FIG. 2E shows the EDX spectra map of cadmium (Cd) (L electron shell excitation);

FIG. 2F shows EDX elemental profile of Cd, Se, Zn, and Se in the same CdZnSe/ZnSe/ZnSe _x Si- _x /ZnS core/triple-shell QDs array;

FIGs. 3A to 3F show the UV-Vis and PL spectra for exemplary core/triple-shell semiconducting QDs having the structure shown in FIG. 1A and with varying core sizes (increasing from FIG. 3A to 3F), demonstrating tunable emission over the entire visible spectral range, wherein

FIG. 3A shows the UV-Vis and PL spectra for green QDs with PL peak at 482 nm,

FIG. 3B shows the UV-Vis and PL spectra for green QDs with PL peak at 532 nm,

FIG. 3C shows the UV-Vis and PL spectra for green QDs with PL peak at 556 nm,

FIG. 3D shows the UV-Vis and PL spectra for yellow QDs with PL peak at 590 nm,

FIG. 3E shows the UV-Vis and PL spectra for amber QDs with PL peak at 610 nm, and

FIG. 3F shows the UV-Vis and PL spectra for red QDs with PL peak at 622 nm;

FIGs. 4A to 4E show the thiol molecules of varying structures that may be utilized as airstable coatings for core/triple-shell QD nanoparticles having the structure shown in FIG. 1A with crystal-bound thiol-passivation to the metal cation in the outer ZnS shell, according to some embodiments of this disclosure;

FIG. 5A is a schematic diagram showing crystal-bound and organic thiol ligands at the surfaces of core/triple-shell QD nanoparticles having the structure shown in FIG. 1A, according to some embodiments of this disclosure;

FIG. 5B is a schematic diagram showing the surface-bound thiol QD surface passivation;

FIG. 6 is a flowchart showing the steps of preparing core/triple-shell QDs having the structure shown in FIG. 1 A, according to some embodiments of this disclosure;

FIGs. 7A to 7F show the evaluation and comparison of the crystal-bound thiol-passivated core/triple-shell QDs having the structure shown in FIG. 1A and prepared by the method described herein and a prior-art QD material, wherein

FIG. 7A shows the comparison of the solution of the prior-art QDs with ambient light and the solution of the core/triple-shell QDs with ambient light,

FIG. 7B shows the comparison of the solution of the prior-art QDs under UV light and the solution of the core/triple-shell QDs under UV light,

FIG. 7C shows the comparison of the solution of the prior-art QDs after addition of potassium hydroxide (KOH) and the solution of the core/triple-shell QDs after addition of KOH, FIGs. 7D and 7E show the proton nuclear magnetic resonance (1H-NMR) spectra of the core/triple-shell QDs with 1 -Octanethiol ligand in chloroform (0.7 ml) after addition of 0.01 mmol KOH and after addition of 0.001 mmol KOH, respectively, and

FIG. 7F shows the spectrum of the core/triple-shell QDs with 1 -Octanethiol ligand in chloroform (0.7 ml) before addition of KOH;

FIG. 8A is a schematic diagram showing the structure of a QLED device with core/triple-shell QD nanocrystals having the structure shown in FIG. 1A;

FIG. 8B is a photograph showing an exemplary QLED device with the structure shown in FIG. 8A;

FIGs. 8C to 8H show the electrical characteristics and efficiency parameters of the QLED device shown in FIG. 8B, including current density vs. voltage (FIG. 8C), luminance vs. voltage (FIG. 8D), power efficiency vs. voltage (FIG. 8E), current efficiency vs. current density (FIG. 8F), external quantum efficiency (EQE) vs. current density (FIG. 8G), and power efficiency vs. luminance (FIG. 8H);

FIG. 9 shows the test results of tuning the emission color by changing the trioctylphosphine (TOP) concentration in a core synthesis step of fabricating the core/triple-shell QDs;

FIGs. 10A to 10F show comparisons of the electrical characteristics and efficiency parameters of the QLED devices fabricated with red-emitting triple-shell QDs disclosed herein and commercially available red-emitting double-shell QDs, both functionalized with 1 -Octanethiol surface ligands, including the comparisons of the red-emitting triple-shell QDs and the red-emitting double-shell QDs in terms of current density vs. voltage (FIG. 10A), luminance vs. voltage (FIG. 10B), power efficiency vs. voltage (FIG. 10C), current efficiency vs. current density (FIG. 10D), EQE vs. current density (FIG. 10E), and power efficiency vs. luminance (FIG. 10F).

DETAILED DESCRIPTION

OVERVIEW

Embodiments disclosed herein relate to colloidal quantum-dot (QD) materials with improved aerobic stability and electroluminescent properties for electrically driven devices such as QD lightemitting diodes (QLEDs) and possibly QD field-effect transistors (QD FETs), preparation methods thereof, and applications thereof. The QD materials disclosed herein may be in the form of nanocrystals and show colloidal stability in solution and aerobic stability in solid-state thin-films, while simultaneously maintaining the charge transport properties in QLEDs, which is not easily achieved with prior-art core/shell QDs. Herein, “colloidal stability” means long shelf-life with no observable change in the optical properties of QDs in solution form, and “aerobic stability” means the stability of QD thin-films during the device preparation in open air, even under relatively high humidity levels. The exemplary spin-coated QLED devices fabricated under an ambient air condition (that is, in an aerobic condition) exhibit extremely high brightness levels and negligible (for example, substantially zero) EQE roll-offs.

As used herein, “QDs”, “QD nanocrystals”, “QD materials”, “QD nanoparticles”, “QD nanoscale particles”, and “semiconducting QD nanoparticles” maybe used interchangeably, which are familiar to one of skill in the art. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention pertains.

The terms “QDs in solution form” and “QD solution” refer to a solution having a solvent with a plurality of QD nanoscale particles wherein the QD nanoscale particles are suspending in the solvent in a dispersed manner such that the solution exhibits a liquefied form without precipitation of the QD nanoscale particles. Accordingly, the QD nanoscale particles that may form such a solution (such as the QD nanoscale particles described here) may be considered soluble in the solvent.

According to one aspect of this disclosure, the QD materials disclosed herein are prepared as nanoscale particle (nanoparticle) structures having an alloyed QD core coated with a thick, three-layer shell (also denoted “triple-shell” hereinafter) of selected chalcogenide composition for bandgap. As those skilled in the art understand, a “bandgap” of a material is the minimum energy required to excite an electron in a bound state into a free state suitable for conduction, and “bandgap engineering” is a process of controlling or altering the bandgap of a material.

In some embodiments, the QD materials disclosed herein comprise semiconducting QD nanoparticles having II- VI alloy QD cores (that is, alloy of selected Group II and Group VI elements) and three II- VI shells (that is, one or more selected Group II and Group VI elements) coated on the II- VI alloy QD cores.

According to one aspect of this disclosure, the core/triple-shell QD materials disclosed herein are prepared with high-temperature annealing of thiol ligands to produce crystal-bound thiol passivation of the nanocrystal surfaces. The use of bandgap-engineered triple-shell configuration together with the crystal-bound thiol-passivation in the core/triple-shell QDs enables the materials to have high solution and especially high thin- film PLQY s (with no additional post treatment), superior optoelectronic performances in QLED devices even under a relatively high-humidity aerobic condition, and improved stability. In some embodiments, a method is used for optimizing current bandgap engineering for good exciton confinement, whereby the method comprises:

Step 1 : A first precursor comprising one or more Group II elements (that is, Group IIB elements in the periodic table; for example, zinc (Zn), cadmium (Cd)) is reacted with a second precursor containing one or more Group VIA elements (for example, sulfur (S), selenium (Se), tellurium (Te) to produce alloyed II- VI nanoparticles as the alloyed QD cores.

Step 2: A solution of QD nanoscale particles is obtained after the alloy QD core is subsequently coated with ZnSe, ZnSe _x Si- _x (where 0 < x < 1), and ZnS shells via controlled reaction of a third precursor comprising one or more Group II elements with a fourth precursor containing one or more Group VI elements at the surfaces of the alloy II-VI QD cores. Specially, the thickness (for example, in terms of the number of monolayers) of the ZnSe _x Si- _x intermediate shell (denoted “Shell 2”, that is, the shell intermediate the inner and outer shells) may be adjusted to adjust the energy bandgap and minimize the crystalline strain between the ZnSe inner shell (denoted “Shell 1”) and the ZnS outer shell (denoted “Shell 3”). Thus, the ZnSe _x Si- _x intermediate shell plays a pivotal role in achieving high PLQY s and tailoring the optoelectronic characteristics (that is, charge transport) in a way to obtain superior device performances. For example, with extensive research efforts, applicant has found that, in some embodiments, it is preferable to choose 0.4 < x < 0.6 for ZnSe _x Si- _x for improved device performance, and in some other embodiments, it is preferable to choose x = 0.5 for further improved device performance which has been confirmed experimentally.

In some embodiments, the method further comprises:

Step 3 : Thiol molecules or thiol ligands are added at high temperatures to the zinc-rich reactant solution obtained at Step 2 (having one Group II element of zinc) to produce crystal-bound thiol surface passivation for further improving the stability.

In some embodiments, the alloy II-VI QD cores comprises CdS, CdSe, ZnS, ZnSe, ZnSeS, CdZnSe, CdZnS, CdZnSeS, and/or ZnTeSe compositions.

In some embodiments, the alloy II-VI QD cores have a diameter of one (1) nanometer (nm) to 8 nm (tuned based on composition and desired PL peak maxima) and a size distribution between 5% to 20%, based on electron microscopy analyses.

In some embodiments, the reaction temperature range for above-described Steps 1 (synthesis of alloyed II-VI QD core) and 2 (three layered II-II-VI alloyed shell growth over QD cores) is preferably between 240 °C and 310 °C; and preferably in Step 3 (crystal-bound thiol ligand passivation) between 210 °C and 260 °C. In some embodiments, the reaction is preferably solvated in a high boiling point apolar solvent such as octadecene, paraffin oil or similar.

In some embodiments, a precursor for Group II elements of Step 1 and Step 2 is preferably Zn-oleate or Cd-oleate, which are produced through inert reaction of dry oleic acid (90%, technical grade) with Group II oxides or carbon acid salts such as acetates, acetylacetonates or similar, at a temperature in the range of 120 °C and 260 °C.

In some embodiments, a precursor for Group VI elements of Step 1 and Step 2 is preferably Se, S, or Te -alkyl- or arylphosphine, produced through reaction of alkylphosphines (for example, trioctylphosphine, tributylphosphine, or diphenylphosphine) with elemental Se, Te, or S at room temperatures under inert conditions with agitation. Furthermore, the sulfur precursor is preferentially produced through mixing precursor of alkylthiols and S-alkylphosphine, prepared by addition of alkylthiols (for example, 1 -Octanethiol, 1 -Dodecanethiol, or the like) to the alkylphosphine solutions after dissolution under inert conditions at room temperature.

In some embodiments, the QD cores are covered with three consecutive discrete shells of Group II-VI compounds or Group II-II-VI alloy compounds, where the bandgap of the bulk material of each subsequent shell material is larger than the bandgap of the previous one(s).

According to X-ray diffraction (XRD) analysis, the crystalline structure of QDs in some embodiments is determined to be as “zinc-blende”, favorable for appropriate epitaxial growth of the shells atop with minimized intermediate crystalline mismatch, leading to excellent optoelectronic properties.

In some embodiments, QDs of different emission colors may have different overall core diameters varying between 3 nm to 7 nm. It is also known in the art that, for the same emission peak maximum (i.e., the same bandgap), ternary CdZnSe cores may be larger in size and thus more stable than traditional CdSe binary cores.

Moreover, in some embodiments, the inner Shell 1 comprises binary ZnSe compounds with a thickness of 2 to 12 crystal-lattice monolayers (equivalent to approximately one (1) nm to 7 nm) and epitaxially covering the QD core. In some other embodiments, the inner Shell 1 may preferably have a thickness of 8 to 10 crystal- lattice monolayers (equivalent to approximately 4 nm to 5 nm).

In some embodiments, the intermediate Shell 2 comprises ternary ZnSe _x Si- _x compounds, where 0 < x < 1, with a thickness of one (1) to 6 crystal-lattice monolayers (equivalent to approximately 0.5 nm to 3 nm) and epitaxially covering the QD core and Shell 1. In some other embodiments, the intermediate Shell 2 may preferably have a thickness of one (1) to 2 crystal-lattice monolayers (equivalent to approximately 0.5 nm to one (1) nm). In some embodiments, the outer Shell 3 comprises inorganic ZnS compounds, with a thickness of one (1) to 6 crystal- lattice monolayers (equivalent to approximately 0.5 to 3 nm) epitaxially covering the QD core, Shell 1, and Shell 2. In some other embodiments, the outer Shell 3 may preferably have a thickness of 2 to 4 crystal-lattice monolayers (equivalent to approximately one (1) nm to 2 nm).

Thus, in some embodiments, the total thickness of the inner, intermediate, and outer shells is 4 to 24 crystal-lattice monolayers (equivalent to approximately 2 nm to 13 nm).

In some embodiments, cation exchange may occur at the temperature range preferred for Steps 1 and 2 such that the Group II composition of the QD cores and shells varies with time and produces a gradient change (that is, gradual and slight variation over time) in composition/bandgap over the shell thickness. With such slight variation of the ratio between the elements in each shell composition, the shell bandgap gradually changes so that better crystalline matching between the adjacent core/Shell 1, Shell 1/Shell 2, and Shell 2/Shell 3 may be achieved, which relates to both physical change in the material compositions and consequently the change in their conduction and valance band energy levels.

In some embodiments, core/triple-shell QDs of different emission colors have different diameters varying between 8 nm and 19 nm and a size distribution of 5% to 20%, based on electron microscopy analyses. In some other embodiments, the diameter range of core/triple-shell QDs of different emission colors is between 10 nm and 19 nm. Due to the large nanoparticle sizes, the present disclosure mainly focuses on producing the emission colors between 480 nm and 700 nm PL peak maxima.

As an example, according to US Patent Publication No. 2021/0054274 Al entitled “II-II-VI Alloy Quantum Dot, Preparation Method Therefor and Application Thereof’, published on February 25, 2021, in the case of core/triple-shell QDs (CdZnSe core coated with an inner shell of ZnSe, an intermediate shell of CdZnS, and an outer shell of ZnS), the alloyed CdZnSe core size can be designed such that blue to cyan emission can be obtained from 420 nm to 485 nm with solution PLQYs of greater than 90%. Therefore, compared to the compositions proposed in the present disclosure, the alloyed QDs reported in US 2021/0054274 Al are smaller in size. However, the full width at half maximum (FWHM) varies between 19 nm to 48 nm as the PL peak shifts from 420 nm to 485 nm with increasing the nanoparticle size. Such a color-dependent and broad FWHM is undesirable for high-resolution displays.

In contrary to the QDs disclosed in US 2021/0054274 Al, the core/triple-shell QDs disclosed herein are prepared using a different synthetic method. As a result, the PL spectra of the core/triple- shell QDs disclosed herein have a substantively constant FWHM less than 30 nm with a single emission maximum (that is a single PL peak) for all emission colors. It is also important to note that the FWHM for alloyed CdZnSe/ZnSe/ZnSe _x Si- _x /ZnS QDs with cyan emission (PL peak at 482 nm) disclosed in the present disclosure is only 21 nm, which is less than half of the FWHM of the priorart alloyed CdZnSe/ZnSe/CdZnS/ZnS QDs with cyan emission (PL peak at 485 nm) disclosed in US 2021/0054274 Al, thereby providing an important technological advantage for high-resolution display applications compared to that disclosed in US 2021/0054274 Al.

In some embodiments, the core/triple-shell QDs are passivated by crystal-bound thiol surface ligands.

In some embodiments, thiol ligands are added to the core/triple-shell QD reaction mixture in Step 3, preferentially at temperatures in the range of 160 °C to 260 °C, for surface passivation.

In some embodiments, the reaction solution is preferably cation-rich (for example, excess of Zn-oleate) at the time of thiol-ligand addition.

In some embodiments, the thiol ligands preferably comprise an alkyl moiety of 6 to 12 carbon- chain length or aromatic moiety.

In some embodiments, for example, the thiol ligands comprise 1 -Octanethiol, 1- Dodecanethiol, 1 -Hexanthiol, 2-Ethylhexane- 1 -thiol, 2-Phenylethanethiol, p-Toluenethiol, and/or the like.

In some embodiments, a combination of two or three different functional thiol ligands may be used for better surface passivation, leading to higher PLQYs with minimized PL blinking, improved stability and solubility, and/or the like. Particularly, the mixed ligand approach may also improve the thin- film PLQY by providing better physical separation between the closely-packed QD nanoparticles.

In some embodiments, the thiol ligands are preferably crystal-bound with Shell 3 of the core/triple-shell QDs.

In some embodiments, crystal-bound thiol ligands are preferentially shown to produce ultrastable core/triple-shell QDs in solution and thin- film, for PL emission and with stability to corrosive or oxidizing conditions.

In some embodiments, the thiol-passivated core/triple-shell QDs are purified from excess ligands and unreacted precursors at elevated temperatures of 50 °C to 100 °C via repetitive precipitation/solubilization procedures using toluene or hexane (as the solvent) with reagent alcohol (as the anti-solvent, consisting of mixture of ethanol and additives like methanol and isopropyl alcohol) and chloro form/ acetonitrile solvent/anti-solvent combinations. In some embodiments, purified thiol-passivated core/triple-shell QDs contain less than 10% of residual oleic acid or phosphine surface ligands.

In some embodiments, a mixed ligand approach may help to further minimize the residual oleic acid or phosphonic surface ligands.

In some embodiments, stable dispersions, preferentially without sediment formation over a time period of six (6) months to one (1) year, may be prepared in various organic solvents, including alkanes (C6-C12 carbon chain length), aromatic solvents (for example, toluene and/or the like), and chlorinated solvents (for example, chloroform, dichloromethane, and/or the like).

In some embodiments, PL emission of the core/triple-shell QDs may be tuned from 480 nm to 700 nm, via tuning of QD core sizes.

In some embodiments, core sizes of the core/triple-shell QDs (and thus the emissions thereof) may change during the above-described shell growth procedure of Step 2, due to the cation exchange mechanism described above.

In some embodiments, the core/triple-shell QDs has an in-solution PLQY greater than or equal to 90% and a solid-state thin spin-coated film PLQY greater than or equal to 50% even without any post treatment (for example, chlorination), measured in an aerobic condition.

In some embodiments, the core size and consequently the emission color may be finely tuned by varying the concentration of alkylphosphines (for example, trioctylphosphine (TOP), tributylphosphine, or diphenylphosphine) or oleic acid in the core synthesis Step 1 , while keeping all the precursor amounts and other synthesis parameters (such as temperature, time, and/or the like) fixed. Of course, this is advantageous for facile tuning the emission color without the need for any other complicated optimizations.

In some embodiments, the photoluminescent core/triple-shell QDs (which are generally nanomaterials) may be used for fabricating QLEDs with a multi-layer component structure for producing efficient and stable luminescence in aerobic preparation conditions.

In some embodiments, the QLED device comprises (from top to bottom) silver cathode, Mg- doped zinc oxide nanomaterial, the above-described photoluminescent core/triple-shell QD nanomaterials, Poly(9,9-dioctylfluorene-alt-N-(4-sec-butylphenyl)-diphenyla mine) (TFB), poly(3,4- ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), and indium-tin-oxide (ITO) coated glass substrate.

In some embodiments, the QLED devices are prepared in open-air conditions via spin coating of each layer from a solution or stable dispersion on top of ITO-coated substrates, before depositing silver via thermal deposition method. In some embodiments, the QLED devices are covered with a transparent substrate and sealed with epoxy and glass coverslips under open-air conditions.

In some embodiments, the QLED devices fabricated and operating in an aerobic condition (and even under high humidity levels) may achieve high EQEs of 5% to 20% for all emission colors, with a maximum device brightness level greater than 300,000 cd/m ² (achieved for the core-triple/shell QDs emitting with PL peaks greater than 500 nm). In some embodiments, the core-triple/shell QDs (operating in an aerobic condition) may provide PL emission in blue-color range (for example, with a PL peak less than 500 nm) with a maximum device brightness level up to 50,000 cd/m ² (such as from 30,000 cd/m ² to 50,000 cd/m ² in some embodiments, or from 40,000 cd/m ² to 50,000 cd/m ² in some other embodiments). Of course, the core-triple/shell QDs disclosed herein may also operate under inert conditions with above-described measurements, but the efficiency and brightness values will be higher.

The systems and methods disclosed herein address the aforementioned needs in the QLED technology by providing efficient II-II-VI core/triple-shell (thick-shell) QD compositions with narrow emission linewidths, exhibiting high colloidal (solution) and thin-film PLQYs even without any additional post treatment. This leads to excellent optoelectronic properties in solution-processed QLEDs under ambient-air conditions. In some embodiments, the entire synthesis processes may be carried out in a Schlenk-line system but without the need for using any nitrogen- filled glove-box, which is advantageous for large-scale industrial manufacturing. As those skilled in the art understand, a Schlenk line system is a vacuum gas manifold system commonly used in chemistry. The Schlenk line system generally uses a vacuum pump to introduce a purified inert gas such as nitrogen to a container for safely manipulating moisture-sensitive and air-sensitive compounds. A glove-box is a sealed container with gloves extending thereinto in a sealed manner to allow a user to manipulate the objects in the container.

The exemplary spin-coated QLED devices fabricated under ambient conditions exhibit extremely high brightness levels and negligible (for example, substantially zero) external quantum efficiency (EQE) “roll-off’ (or “droop”), which would pave the way for their applications in future electrically driven, for example, QD displays as well as in indoor and outdoor solid-state lighting systems.

STRUCTURE OF QD NANOPARTICLES

FIG. 1 shows the structure of a luminescent QD nanoparticle 100 according to some embodiments of this disclosure. As shown, the QD nanoparticle 100 comprises an inorganic semiconducting QD core 102 and three inorganic semiconducting shells 104, 106, and 108 with organic thiol ligand surface passivation to the outermost shell 108.

In these embodiments, the QD core 102 may be a binary, ternary, or quaternary core (an alloy of two, three or four elements) comprising Group II-VI elements such as CdS, CdSe, ZnS, ZnSe, ZnSeS, CdZnSe, CdZnS, ZnCdSeS, ZnTeSe, and/or the like (that is, an alloy of these two, three or four Group II-VI elements). The QD core 102 is covered with three consecutive discrete shells 104 to 108 of Group II-VI elements, where the bandgap of the bulk material of an outer shell 106 or 108 is larger than that the shell 104, 106 inner thereto. The three shells layers 104 to 108 comprise different materials.

For example, in one embodiment, the inner shell 104 (also denoted “shell 1”) comprises ZnSe, the intermediate shell 106 (also denoted “shell 2”) comprises ZnSe _x Si- _x , where 0 < x < 1, and the outer shell (also denoted “shell 3”) comprises ZnS. The resulting QD nanoparticles (or simply denoted “QDs”) have Type 1 characteristics, that is, the bandgap of the QD cores 102 is smaller in energy level compared to the shells 104 to 108, thereby producing a gradient increase in bandgap energy between the first and third shells 104 and 108 as shown in FIG. IB. The use of ZnSe _x Si- _x between ZnSe and ZnS results in a lower step-height of energy change from inner to outer (see FIG. IB), as electron and hole wave-functions are confined effectively within the QD core 102, yielding a high solution/thin film PLQY s and good charge balance in electroluminescent QLED devices.

FIG. 2A shows a dark field scanning transmission electron microscopy image of red-emitting core/triple-shell QDs 100 and FIGs. 2B to 2E shows the corresponding energy dispersive X-ray (EDX) spectra maps for the elements sulfur (S) (FIG. 2B, K electron shell excitation), zinc (Zn) (FIG. 2C, K electron shell excitation), selenium (Se) (FIG. 2D, L electron shell excitation), and cadmium (Cd) (FIG. 2E, L electron shell excitation) of the core and/or shells 102 to 108 in the II-VI semiconductorbased QD nanostructure 100, demonstrating the discrete Cd-doped ZnSe QD cores and subsequent ZnSe, ZnSe _x Si- _x and ZnS shells. FIG. 2F shows elemental (Cd, Se, Zn, S, from bottom to top) composition profile of scanning line 110 in FIG. 2A in the same CdZnSe/ZnSe/ZnSe _x Si- _x /ZnS core/triple-shell QDs array.

QDs 100 with a Type 1 bandgap structure achieve high PLQYs and good electroluminescent device efficiencies, due to “effective localized confinement” of excitons within QD cores 102 of the core/triple-shell QDs 100 (caused by the composition of the core and the triple-shells, and the Type I bandgap structure). Additionally, binding of the hole-accepting thiol surface functional ligands to the QD nanocrystals may induce hole-trapping states with prolonged lifetimes thereby promoting hole transfer efficiency in QD-based systems, which is beneficial in QLED devices for example. Additionally, the combination of thick triple-shell structure and good surface coverage provides good aerobic stability because it protects the core from extrinsic degradations, mostly due to oxygen and moisture diffusion. Note that an exciton is a quasi-particle consisting of an oppositely-charged electron-hole (e-h) pair bound to each other by the Coulomb force and that light is emitted through radiative e-h recombination within the QDs. Exciton localization to the QD cores 102 helps to mitigate non-radiative exciton recombination and ultimately achieve good aerobic stability of PL properties such as thin- film PLQY.

FIGs. 3A to 3F show UV-Vis and PL spectra for exemplary triple-shell semiconductor QDs with different core diameters, demonstrating tunable emission over the visible spectra.

In some embodiments, the final core/triple-shell QD nanostructures 100 have a diameter in the range between 8 nm and 19 nm, a size distribution of 5% to 20%, and a substantively constant FWHM of the PL peak of less than 30 nm with a single emission maximum. The QDs 100 has an insolution PLQY greater than or equal to 90% and a solid-state thin spin-coated film PLQY greater than or equal to 50% from a thin film of the prepared QDs 100, measured in an aerobic condition. Especially, as shown in the following, achieving such high thin film PLQY s with excellent charge transporting properties guarantees a high QLED device performance. In some other embodiments, the diameter range of core/triple-shell QDs of different emission colors is between 10 nm and 19 nm.

In some embodiments, QD surface passivation is achieved via direct addition of thiol ligands to a cation-rich synthetic solution (for example, excess of Zn-oleate) at temperatures of 160 °C to 260 °C, producing crystal-bound thiol molecule coated surfaces. In some embodiments, the temperature range of this step is between 210 °C and 260 °C. FIGs. 4A to 4E show the thiol molecules of varying structures that may be utilized as air-stable coatings for QD nanoparticles 100 with crystalbound thiol passivation to the metal cation in the outer ZnS shell 108. FIG. 5 A is a schematic diagram showing crystal-bound and organic thiol ligands at the surfaces of the core/triple-shell QDs (that is, crystal-bound thiol QD surface passivation), where “S” represents sulfur atom, “M” represents metal atom, and “R” represents organic molecule residue. As a comparison, FIG. 5B shows the surfacebound thiol QD surface passivation, which is weaker bonded to the QD surface.

PREPARATION OF the CORE/TRIPLE-SHELL QD NANOCRYSTALS

In some embodiments, the QD material 100 disclosed herein may be prepared by the process 200 shown in FIG. 6.

Specifically, the QD cores 102 are first synthesized (step 202). Then, the triple shells 104 to 108 are formed on the QD cores 102 (Step 204) and ligand exchange is conducted (step 206). The surface-passivated QDs 100 are then purified (step 208). The details of steps 202 to 208 are now described.

STEP 202: SYNTHESIZING QD CORES

The detail of synthesizing QD cores 102 depends on the material of the QD cores 102 and the maximum emission to be tuned thereto.

EXAMPLE Cl :

In some embodiments, synthesis of Zn _x Cdi- _x Se (where 0 < x < 1) alloyed QD cores 102 with PL emission maximum between 600 nm and 700 nm comprises:

(i) Weigh 0.33 mmol to 0.5 mmol (0.0424 g to 0.0642 g) of cadmium oxide and 6 mmol to 12 mmol (1.1010 g to 2.2018 g) of zinc acetate and place in a 100 ml 3 -neck round bottom flask with 13 ml oleic acid and 9 ml octadecene or paraffin oil to form the first solution.

(ii) Remove water and acetic acid vapors via degassing the first solution under 0.1 Torr pressure at 1 10 °C for 2 hours, and then anneal to 300 °C under inert atmosphere.

(iii) Prepare Se precursor via dissolution of one (1) mmol (0.0790 g) elemental Se in 3.0 ml phosphine-derivative solvent such as alkylphosphines (for example, trioctylphosphine, tributylphosphine, or diphenylphosphine) under inert conditions.

(iv) Inject 1.0 ml to 2.0 ml of two (2) molarities (M) of the Se precursor solution obtained at step (iii) into the degassed first solution and anneal at 300 °C for 5 to 20 minutes to obtain the corecontaining synthetic solution.

EXAMPLE C2:

In some embodiments, synthesis of Zn _x Cdi- _x Se (where 0 < x < 1) alloy QD cores 102 with photo luminescence emission maximum between a wavelength range of 500 nm and 610 nm comprises:

(i) Weigh 0.1 mmol to 0.4 mmol (0.0128 g to 0.0514 g) of cadmium oxide and 0.9 mmol to 0.6 mmol (0.165 g to 0.110 g) of zinc acetate to form a total quantity of one (1) mmol of an initial precursor and place in a 100 ml 3 -neck round bottom flask with 1.5 ml oleic acid and 5 ml octadecene or paraffin oil to form the first solution.

(ii) Remove water and acetic acid vapors via degassing the first solution under 0.1 Torr pressure at 1 10 °C for one (1) hour, and then heat to 280 °C under inert atmosphere.

(iii) Prepare Se precursor via dissolution of 6 mmol (0.4738 g) of elemental Se in 3.0 ml or 6.0 ml phosphine-derivative solvent such as alkylphosphines (for example, trioctylphosphine, tributylphosphine, or diphenylphosphine) under inert conditions. (iv) Inject 0.5 ml to one (1) ml of two (2) molarities (M) Se precursor solution or 1.8 ml to 2.5 ml of one (1) M Se precursor solution into the degassed first solution and anneal at 260 °C to 280 °C for 5 minutes to 15 minutes to obtain the core-containing synthetic solution.

As described above, in some embodiments, the core size and consequently the emission color may be finely tuned by varying the concentration of alkylphosphines (for example, trioctylphosphine (TOP), tributylphosphine, or diphenylphosphine) or oleic acid in the core synthesis Step 1, while keeping all the precursor amounts and other synthesis parameters (such as temperature, time, and/or the like) fixed. Of course, this is advantageous for facile tuning the emission color without the need for any other complicated optimizations. For example, FIG. 9 shows the test results of tuning the emission color by changing the TOP concentration. As can be seen, the use of 2.0 ml TOP gives rise to a PL peak at 598 nm with a FWHM of 26 nm, the use of 1.0 ml TOP gives rise to a PL peak at 570 nm with a FWHM of 26 nm, and the use of 0.5 ml TOP gives rise to a PL peak at 545 nm with a FWHM of 23 nm.

EXAMPLE C3:

In some embodiments, synthesis of Zn _x Cdi- _x Se (where 0 < x < 1) alloy QD cores 102 with PL emission maximum between 480 nm and 530 nm comprises:

(i) Weigh 0 to 0.3 mmol (0 to 0.0385 g) of cadmium oxide and one (1) mmol to 0.7 mmol (0.184 g to 0.128 g) of zinc acetate to form a total quantity of one (1) mmol of an initial precursor (which may not have cadmium if the cadmium oxide is selected as zero(0)) and place in a 100 ml 3- neck round bottom flask with 1.5 ml oleic acid and 5 ml octadecene or paraffin oil to form the first solution.

(ii) Remove water and acetic acid vapors via degassing the first solution under 0.1 Torr pressure at 120 °C for one (1) hour, and then anneal at 260 °C to 280 °C under inert atmosphere.

(iii) Prepare precursors including:

(iii-1) Prepare Se precursor via dissolution of 6 mmol (0.4738 g) of elemental Se in 3.0 ml or 6.0 ml phosphine-derivative solvent such as alkylphosphines (for example, trioctylphosphine, tributylphosphine, or diphenylphosphine) under inert conditions.

(iii-2) If the initial precursor does not comprise cadmium, prepare cadmium precursor comprising:

(iii-2a) Weigh one (1) mmol (0. 1284 g) of cadmium oxide and place in 100 ml 3-neck round bottom flask with one (1) ml oleic acid and one (1) ml octadecene or paraffin oil to prepare a Cd-precursor solution; (iii-2b) Remove water and acetic acid vapors from the Cd-precursor solution via degassing the second-precursor solution under 0.1 Torr pressure at 120 °C for one (1) hour, and then heat to 260 °C under inert atmosphere and finally cool to 50 °C to obtain the cadmium precursor.

(iv) Inject 0.5 to one (1) ml of two (2) M Se precursor solution or 1.8 to 2.5 ml of one (1) M Se precursor solution into the degassed first solution and anneal at 260 °C to 280 °C for 5 minutes to 15 minutes to obtain the core- containing synthetic solution. Alternatively, if the first precursor does not comprise cadmium, inject 0.5 to one (1) ml of two (2) M Se precursor solution or 1.8 to 2.5 ml of one (1) M Se precursor solution into the degassed first solution and, after zero (0) minute to two (2) minutes, inject into the solution 0.1 ml to 0.6 ml of cadmium precursor with or without additional 0.5 ml to one (1) ml phosphine-derivative solvent such as alkylphosphines (for example, trioctylphosphine, tributylphosphine, or diphenylphosphine), and then anneal at 260 °C to 280 °C for 15 minutes to 30 minutes to obtain a synthetic solution.

STEP 204: FORMATION OF TRIPLE SHELLS ON QD CORES

EXAMPLE SI :

In some embodiments, the formation of triple shells 104 to 108 on Zn _x Cdi- _x Se alloy QD cores 102 with the PL peak in the range of 480 nm to 700 nm comprises (for example, the QD cores 102 in Example Cl, C2 or C3):

(1) Preparation of zinc precursor only for QD cores 102 in Example C2 and C3 comprises:

(la) Weigh 10 mmol (1.8350 g) of zinc acetate and place in a 100 ml 3-neck round bottom flask with 10 ml oleic acid and 10 ml octadecene or paraffin oil to form the zinc-precursor solution;

(lb) Remove water and acetic acid vapors from the zinc-precursor solution via degassing the zinc-precursor solution under 0.1 Torr pressure at 120 °C for one (1) hour;

(lc) Cool the degassed zinc-precursor solution to 100 °C under inert conditions.

(2) Preparation of Se precursor comprises:

Dissolution of 6 mmol (0.4738 g) elemental Se in 3.0 ml phosphine-derivative solvent such as alkylphosphines (for example, trioctylphosphine, tributylphosphine, or diphenylphosphine) and add 3 ml to 9 ml octadecene or paraffin oil under inert condition and continuous stirring to obtain a clear transparent solution.

(3) Preparation of S precursor comprises:

Dissolution of 6 mmol (0. 1924g) elemental S powder in 3.0 ml or 4.0 ml phosphine-derivative solvent such as alkylphosphines (for example, trioctylphosphine, tributylphosphine, or diphenylphosphine) and add 3 ml to 9 ml octadecene or paraffin oil under inert condition and continuous stirring at an elevated temperature in the range of 70 °C to 110 °C to obtain a clear transparent solution.

(4) Preparation of Se-S precursor comprises:

Mixing the 1 : 1 molar ratio of Se and S precursors under inert condition.

(5) After final annealing step of Zn _x Cdi- _x Se alloy QD core 102, layer-by-layer shell growth is performed by sequentially injecting the Se, Se-S, and S precursors (prepared at steps (2) to (4)) into the synthetic solution at 260 °C to 310 °C, and then injecting zinc precursor (prepared at step (1)) via incremental injection of one (1) ml volume such that an excess of at least one (1) mmol (with respect to Group VI element concentration in solution) is maintained throughout the shell growth reaction. More specifically, this step includes:

(5a) Firstly, processing the synthetic solution by injecting thereinto the 3.0 ml to 9.0 ml Se precursor while the temperature is maintained at 260 °C to 310 °C to form the first shell layer of ZnSe.

(5b) Secondly, processing the synthetic solution by injecting thereinto a mixture of the Se and S precursors (that is, 1.0 ml to 3.0 ml Se-S precursor) while the temperature is maintained at 260 °C to 310 °C to form the second shell layer of ZnSeS.

(5c) Thirdly, processing the synthetic solution by injecting thereinto the 3.0 ml to 9.0 ml S precursor while the temperature is maintained at 260 °C to 310 °C to form the third shell layer of ZnS.

(5 d) Finally, the processed synthetic solution is cooled to 200 °C to 260 °C while maintaining a suitable excess of Zn to S therein, to obtain a solution of core/triple-shell QDs 100 for ligand exchange at step 206.

In this example, the ligands present at the surface of the nanoparticles without ligand exchange are a mixture of oleic acid (95% to 97%), trialkylphosphine (1% to 3%), and 1-Octanethiol or 1- Dodecane thiol (1% to 3%).

EXAMPLE S2:

In some embodiments, the formation of triple shells 104 to 108 on Zn _x Cdi- _x Se alloy QD cores 102 with the PL peak between 600 nm and 700 nm (for example, the QD cores 102 in Example Cl) is similar to Example SI except that in this example, Step (1) of preparation of zinc precursor is not used.

STEP 206: LIGAND EXCHANGE

EXAMPLE LI :

In some embodiments, thiol ligand exchange of the core/triple-shell QDs 100 comprises: at 220 °C to 240 °C, inject 5 mmol to 10 mmol of the thiol ligand into the solution of core/triple-shell QDs 100 (obtained at step 204) and anneal at 210 °C to 230 °C for 20 minutes to 45 minutes under inert conditions, and then cool to 100 °C under inert conditions to obtain a solution of crystal-bound thiol ligand passivated core/triple-shell QD nanoparticles 100 for purification.

EXAMPLE L2:

In some embodiments, thiol ligand exchange of the core/triple-shell QDs 100 comprises:

Injection of 0.5 ml to 2.0 ml thiol ligands into the gradient cooling down solution of the reaction mixture (that is, the solution of core/triple-shell QDs 10) at temperature below 260 °C. EXAMPLE L3:

In some embodiments, thiol ligand exchange of the core/triple-shell QDs 100 comprises:

Injection of equivalent amount (5 mmol to 10 mmol) of two or more different thiol mixtures, or thiol-functionalized aryl alkyl-based ligands.

STEP 208: PURIFICATION

EXAMPLE Pl :

In some embodiments, purification of the solution of crystal-bound thiol ligand passivated core/triple-shell QD nanoparticles 100 comprises:

(I) An equivalent-to-reaction-mixture volume of toluene is added directly to the solution of crystal-bound thiol ligand passivated core/triple-shell QD nanoparticles 100 to a total volume of preferably 80 ml to obtain a QD-toluene dispersion.

(II) The QD-toluene dispersion is precipitated with 100 ml of reagent alcohol.

(III) Centrifugation of the precipitated QD-toluene dispersion at preferably 7000-8000 revolutions per minute (rpm) for 2 minutes to 5 minutes, while solution temperature is maintained above 40 °C, which enables separation of the QD product 100 from the majority of excess reagents in solution.

(IV) The upper solution is discarded, and the QD precipitate is dispersed in hexanes or chloroform, precipitated with reagent alcohol or acetonitrile, and separated via centrifugation for an additional 3 cycles to 10 cycles.

(V) The final QD product is dispersed in the non-polar organic solvents of choice for further application.

EXAMPLE P2:

In some embodiments, purification of the solution of crystal-bound thiol ligand passivated core/triple-shell QD nanoparticles 100 comprises:

(I) Dispersing the solution of crystal-bound thiol ligand passivated core/triple-shell QD nanoparticles 100 in a non-polar organic solvent, and then the QD dispersion in non-polar solvent is precipitated with 100 ml of low-polar organic solvent (acetone, ethyl-methylacetate, methanol). TESTING AND TEST RESULTS

Testing of crystal-bound thiol passivation for alkaline stability may be conducted on purified QDs 100. In an example, testing of crystal-bound thiol passivation of the QDs 100 for alkaline stability includes:

(A) In a glass vial or NMR tube, preferably 20 mg of the QD material 100 is dissolved in one (1) ml of regular or deuterated chloroform.

(B) After dispersion, preferably 10 micro liters of one (1) M potassium hydroxide (KOH) dissolved in regular or deuterated methanol is added to the vial or tube and agitated by hand. QDs 100 feature crystal-bound thiol passivation aggregate, but preferentially stay dispersed in solution.

(C) Proton nuclear magnetic resonance (1H-NMR) spectroscopy of the QDs before and after KOH treatment may be conducted to demonstrate persistent attachment of ligand to the QD surfaces for crystal-bound thiol-passivated QDs.

To evaluate the impact of crystal-bound passivation on particle stability, the prepared QDs were subjected to a harsh alkaline treatment (such as KOH treatment) in chloroform and tested for ligand desorption.

FIGs. 7A to 7C show the evaluation and comparison of the crystal-bound thiol-passivated core/triple-shell QDs 100 prepared by the method described herein and a prior-art QD material. Prior to addition of KOH, solution transparency and QD PLQY is comparable between the prior-art QDs and the core/triple-shell QD material, both in chloroform solvent. FIG. 7A shows the comparison of the solution of the prior-art QDs 242 with ambient light and the solution of the core/triple-shell QDs 100 with ambient light. FIG. 7B shows the comparison of the solution of the prior-art QDs 242 under UV light and the solution of the core/triple-shell QDs 100 under UV light.

As shown in FIG. 7C, after addition of KOH (0.04 mmol in 20 microliters (pl) methanol), the prior-art QDs 242, featuring amine-based ligand attachment, instantly destabilize and crash out of solution. By comparison, the crystal-bound thiol-passivated QDs 100 exhibit exceptional stability after addition of KOH, with only mild aggregation of nanoparticles observed in solution as a shift in color/transparency.

The crystal-bound thiol-passivated core/triple-shell QDs 100 prepared by the method described herein are evaluated using NMR spectroscopy. FIGs. 7D to 7F show the 1H-NMR spectra of the thiol-passivated core/triple-shell QDs 100 before and after addition of KOH.

More specifically, FIGs. 7D and 7E show the spectra of the core/triple-shell QDs 100 with 1- Octanethiol ligand in chloroform (0.7 ml) after addition of 0.01 mmol KOH and after addition of 0.001 mmol KOH, respectively, and FIG. 7F shows the spectrum of the core/triple-shell QDs 100 with 1 -Octanethiol ligand in chloroform (0.7 ml) before addition of KOH. Clearly, FIGs. 7D to 7F demonstrate the particles colloidal stability and crystal-bound thiol passivation. In particular, upon addition of KOH to the chloroform dispersions no loss of ligands to solution is observed (this would be observed as a change in ratio of the -CH2- to -CH3- resonances at about 0.85 parts per million (ppm) and 1.25 ppm, respectively). Note that the peak arising at about 1.2 ppm in the upper plot of FIG. 7D is a result of water contamination in the KOH/methanol solution.

As observed, crystal-bound passivation enables the core/triple-shell QDs 100 to remain colloidally dispersed after KOH treatment, while prior-art QDs 242 with amine functionalities were found to immediately fall out of solution after exposure to the same concentration of KOH. Moreover, NMR analysis of the crystal-bound thiol-passivated QDs 100 before and after exposure to base showed no observable loss in bound ligands to the surface of the QDs 100, attributable to crystalbinding of thiol ligands to the nanocrystal surfaces (see references [25] and [26]).

Photo luminescent properties of the core/triple-shell QDs 100 under an aerobic condition are tested. Absolute PLQY s of the core/triple-shell QDs 100 are measured via the absolute method with an integrating sphere instrumentation in an aerobic condition. Exposure to air results in no observable losses in PLQY, with PLQY values equal to or greater than 90% preferably achieved by the core/triple-shell QDs 100 with PL maxima in the range of 480 nm to 700 nm. Moreover, the FWHM values of the emission peaks are less than 30 nm, with resulting electroluminescent devices having high color purity.

FABRICATION OF QLED DEVICES USING THE CORE/TRIPLE-SHELL QDS

In some embodiments, the core/triple-shell QDs 100 may be used for fabrication of QLED devices under an aerobic condition (that is, an ambient air condition) via spin coat deposition of each layer from a solution or stable dispersion on top of indium-tin-oxide (ITO) coated glass substrates (wherein ITO acts as the anode and the substrate may alternative be any other suitable transparent or semi-transparent substrate).

FIG. 8A shows the structure of a QLED device 300. With reference to FIG. 8A, the ITO- coated glass substrates 302 are first cleaned before use by sequential ultra-sonication in detergent and de-ionized water, acetone, and isopropanol, each for 10 minutes to 15 minutes. The ITO substrates 302 are further pretreated under UV-Ozone for 10 minutes to 15 minutes.

Room-temperature solutions of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) 304 are filtered and spun-cast at a speed of 5000 rpm, followed by annealing at 120 °C in air for 30 minutes. Hole-transport material solutions 306, such as Poly(9,9-dioctylfluorene-alt-N-(4-sec- butylphenyl)-diphenylamine) (TFB) solutions, are prepared in p-xylene (8 milligram per milliliter (mg mL' ¹ )), filtered and spun-cast at a speed of 3000 rpm and then annealed at 110 °C in air for 20 minutes.

After cooling the ITO/PEDOT:PSS/TFB (that is, the layers 302, 304, and 306) coated substrates to room temperature, a QD dispersion in n-octane solvent (5 mg mL ^-1 to 25 mg mL ^-1 ) (which comprises, for example, the photoluminescent core/triple-shell QD nanocrystals 100 prepared as described above) is filtered and spun-cast at a speed of 3000 rpm, then annealed at 80 °C in air for 30 minutes to form the QD layer 308.

Electron transport material (ZngoMgioO-PVP, ZMO-PVP) is dispersed in 1 -butanol at a concentration of 10 mg mL ^-1 to 30 mg mL ^-1 , filtered and spun-cast at a speed of 3000 rpm then annealed at 80 °C in air for 30 minutes to form the Mg-doped zinc oxide (MgZnO) layer 310.

To complete the devices, 130 nm of silver (0.5 Angstrom per second (A s" ¹ ) to one (1) A s" ¹ ) is thermally evaporated on top of the ZMO:PVP layer under 1 x 10" ⁷ Torr using a shadow mask to form the cathode layer 312.

Finally, the QLED device 300 is sealed under a glass coverslip (not shown) with epoxy prior to electroluminescent testing.

In this example, the QLED device 300 is fabricated using the core/triple-shell QD materials 100 disclosed herein and comprises a stack of six layers coated on the glass substrate with total thickness of about one (1) micron to 2 microns. FIG. 8B is a photograph showing an exemplary QLED device (4 pixels) with 4 mm ² pixel area prepared with the core/triple-shell photoluminescent QD nanomaterials 100 disclosed herein, after epoxy encapsulation with attainable maximum brightness levels exceeding 400,000 cd/m ² . In this example, the QLED device 300 is a monochromatic device with an electroluminescence maximum at 593 nm (corresponding to a PL maximum of 590 nm) with a FWHM of < 25 nm (see FIG. 3D). The electrical characteristics and efficiency parameters of the QLED device 300 in an aerobic condition are shown in FIGs. 8C to 81, which demonstrate the excellent aerobic stability of the electroluminescent QLED device 300.

FIGs. 8C to 8H respectively show the plots of current density vs. voltage (FIG. 8C), luminance vs. voltage (FIG. 8D), power efficiency vs. voltage (FIG. 8E), current efficiency vs. current density (FIG. 8F), EQE vs. current density (FIG. 8G), and power efficiency vs. luminance (FIG. 8H).

As shown in FIG. 8H, The QLED device 300 exhibits relatively droop-free electroluminescence behavior, observed as stable EQE with increasing the current. A high device EQE (the ratio of photons emitted by the QLED to electrons injected into the device) of 11% with a negligible (for example, substantially zero) droop is obtained. A maximum brightness level of greater than 420,000 candelas per square meter (cd/m ² ) (under a driving voltage of 12 V) with a tum-on voltage of 2.1 V to 2.2 V is also achieved. These performance values are indeed unprecedented for QLEDs fabricated under an aerobic condition.

In some embodiments, the QLED devices prepared with the core/triple-shell QDs 100 disclosed herein preferably have a tum-on voltage of less than or equal to 3.0 V.

In some embodiments, the exemplary QLED devices shown in FIGs. 8A to 8H prepared with the core/triple-shell QDs 100 disclosed herein preferably have 5% to 15% EQE, with less than or equal to 5% droop in the EQE over the current densities ranges of up to 500 milliamps per centimeter squared (mA/ cm ² ). The EQE droop remains extremely low even at higher current densities.

In some embodiments, the QLED devices prepared with the core/triple-shell QDs 100 disclosed herein preferably have a current efficiency greater than or equal to 30 candelas per Amp (cd/A).

In some embodiments, the QLED devices prepared with the core/triple-shell QDs 100 disclosed herein preferably have luminance greater than or equal to 100,000 cd/m ² with power efficiency greater than or equal to 20 lumens per watt (Im/W) for all emission colors thereof except for the blue color (the brightness of which may be up to 50,000 cd/m ² in some embodiments). QLED DEVICE EFFICIENCY COMPARISON WITH COMMERCIAL QDS

In order to compare the QLED device efficiency under ambient air conditions, as an example, the red-emitting triple-shell CdZnSe/ZnSe/ZnSeS/ZnS QDs (with a nanoparticle size of 15.1 nm giving rise to a PL peak at 622 nm as shown in FIG. 3F) functionalized with 1 -Octanethiol surface ligands are compared with a similar commercially available red-emitting double-shell CdZnSe/CdZnS/ZnS (with a nanoparticle size of 13 nm to 14 nm giving rise to a PL peak at 625 nm) functionalized with the same 1 -Octanethiol ligands. The solution and thin-film PLQYs of the tripleshell QDs and the commercial QDs were respectively recorded to be greater than 90% and greater than 50% for both QDs, also making them suitable emitters for the sake of comparison in this exemplary case study. As shown in FIGs. 10A to 10F of the QLEDs fabricated with the triple-shell QDs, the EQE, current efficiency, power efficiency, brightness level, and tum-on voltage are 10.0%, 17.4 cd/A, 14.8 Im/W, 235,000 cd/m ² (under a driving voltage of 12 V), and 2.1 V, respectively, which exhibit significant improvements compared to the commercial double-shell QDs wherein these values are 6.6%, 8.6 cd/A, 5.7 Im/W, 145,000 cd/m ² (under a driving voltage of 12 V), and 2.1 V, respectively. These experimental demonstrations clearly confirm that the proposed triple-shell configuration together with optimized surface-defect passivation provided in the present invention can lead to excellent aerobic stability, good charge transport properties, high PLQY, and high efficiency in solution-processed QLED devices. In some embodiments, the QLEDs fabricated with the triple-shell QDs may provide higher EQE, higher current efficiency, higher power efficiency, higher brightness level, and lower tum-on voltage than the values described above.

In above embodiments, the QD nanoscale particles and QLED devices are fabricated in an aerobic condition without using, for example, any nitrogen-filled glove-box. The above-described testing results show that the QD nanoscale particles and QLED devices disclosed herein provide improved performance than prior-art QD nanoscale particles and QLED devices although the priorart QD nanoscale particles and QLED devices may need to be fabricated in a nonaerobic condition such as nitrogen- filled glove-boxes.

However, those skilled in the art will appreciate that, in some embodiments, the QD nanoscale particles and QLED devices disclosed herein may alternatively be fabricated in a nonaerobic condition such as in nitrogen-filled glove-boxes.

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Although embodiments have been described above with reference to the accompanying drawings, those of skill in the art will appreciate that variations and modifications may be made without departing from the scope thereof as defined by the appended claims.