Mercury-telluride quantum dots and methods of manufacturing thereof

Abstract

A method of manufacturing HgTe quantum dots is described. The method comprises dissolving a mercury salt in a coordinating solvent to create a mercury solution; dissolving tellurium in a coordinating ligand compound to create a first tellurium solution; performing an initial injection of the first tellurium solution into the mercury solution to produce a synthesized HgTe QDs solution; when synthesis during the initial injection has completed, perform a subsequent continual injection of a second tellurium solution into the synthesized HgTe QDs solution; quenching the synthesized HgTe QDs solution with a solution of coordinating ligand; and separating the HgTe QDs from the rest of the quenched solution.

Description

MERCURY-TELLURIDE QUANTUM DOTS AND METHODS OF MANUFACTURING THEREOFCROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The current application claims the benefit of and priority under 35 U.S.C. § 119 (e) to U.S. Provisional Patent Application No. 63 / 727,525 entitled “Mercury-Telluride Quantum Dots and Methods of Manufacturing Thereof’ filed December 3, 2024. The disclosure of U.S. Provisional Patent Application No. 63 / 727,525 is hereby incorporated by reference in its entirety for all purposes.STATEMENT OF FEDERAL SUPPORT

[0002] This invention was made with government support under 1945572 awarded by the National Science Foundation. The government has certain rights in the invention.FIELD OF THE INVENTION

[0003] This application generally refers to systems for mercury-telluride quantum dots and methods of manufacturing thereof.BACKGROUND

[0004] Colloidal quantum dots (QDs) are nanometer sized crystals which display size dependent optical properties due to quantum confinement. Due to the semimetal / narrow bandgaps of bulk mercury chalcogenides and their concomitant large Bohr exciton radii (~40 nm), HgX (X=S, Se, Te) nanocrystals display extreme bandgap tunability, from 1.5 eV to 20 meV (830 nm to 62 urn) for HgTe. For this reason, HgX QDs are explored as low-cost solution processable alternatives for mid and long-wave infrared (>3micron) photodetection. However using HgTe QDs to explore short wave infrared (1 to 2 microns) imaging remains unexplored. There is great commercial interest in short wave infrared (SWIR) imaging due to the high spatiotemporal resolution imaging for applications such as defense, noninvasive biomedical imaging, mobile devices, machine vision, advanced drive assistance programs in cars and more. Most mercury chalcogenide QD research has focused on larger particles with optical bandgaps beyond the current detection rangeof commercial SWIR cameras (greater than 1600 nm). Considerably less is known about the synthetic routes and optical properties of small (<10nm) HgTe quantum dots, despite potential applications in near and shortwave infrared (NIR / SWIR) technologies such as SWIR imaging agents, photodetectors, light-emitting diodes and other optoelectronic applications.

[0005] HgTe nanocrystals with an excitonic absorbance feature at 830 nm (1.5 eV), emission spanning from 800 to 1400 nm and a photoluminescence quantum yield (PLQY) of 50% has been previously reported. A small HgTe cluster with an excitonic absorbance feature at 900 nm (1.37 meV) has been observed. Once isolated, the sample displayed a maximum absorbance of around 1100 nm (1.12 eV) and displayed a PLQY of 75%. Similarly, 2 to 3 monolayer HgTe / HgSe nanoplatelets have been synthesized via cation exchange from cadmium chalcogenide materials availing extreme confinement and optical bandgaps from 600 - 885 nm (1 .9-1 .4 eV) and quantum yields from 10% to 56%. However, despite high QYs, the fine-tuning of these nanomaterials in near infrared spectral windows is not demonstrated.SUMMARY OF THE INVENTION

[0006] The present disclosure provides systems and manufacturing methods for mercury-telluride quantum dots.

[0007] Some embodiments include a method of manufacturing HgTe quantum dots (QDs), the method comprising: dissolving a mercury salt in a coordinating solvent to create a mercury solution; dissolving tellurium in a coordinating ligand compound to create a first tellurium solution; performing an initial injection of the first tellurium solution into the mercury solution to produce a synthesized HgTe QDs solution; when synthesis during the initial injection has completed, perform a subsequent continual injection of a second tellurium solution into the synthesized HgTe QDs solution; quenching the synthesized HgTe QDs solution with a solution of coordinating ligand; and separating the HgTe QDs from the rest of the quenched solution.

[0008] In some embodiments, the mercury salt is mercury (II) acetate.

[0009] In some embodiments, the coordinating solvent comprises oleyamine.

[0010] In some embodiments, the tellurium comprises tellurium powder and the coordinating ligand compound is an organophosphine compound.

[0011] In some embodiments, the organophosphine compound comprises trioctylphosphine.

[0012] In some embodiments, the coordinating ligand comprises dodecanethiol.

[0013] In some embodiments, the second tellurium solution comprises tellurium dissolved in a coordinating ligand compound.

[0014] In some embodiments, the tellurium comprises tellurium powder and the coordinating ligand compound is an organophosphine compound.

[0015] In some embodiments, the organophosphine compound comprises triocytlphosphine.

[0016] In some embodiments, the coordinating ligand comprises dodecanethiol.

[0017] In some embodiments, the first tellurium solution has a different tellurium concentration than the second tellurium solution.

[0018] In some embodiments, the first tellurium solution has a higher tellurium content than the second tellurium solution.

[0019] Some embodiments further comprise performing absorption spectroscopy to monitor appearance of the synthesized HgTe QDs solution during the initial injection to determine when synthesis begins and finishes after the initial injection.

[0020] In some embodiments, quenching the synthesized HgTe QDs solution comprises adding a quenching solution into the HgTe QDs solution.

[0021] In some embodiments, the quenching solution comprises a dodecanethiol solution in tetrachloroethylene.

[0022] In some embodiments, separating the HgTe QDs comprises centrifuging the HgTe QDs out of the quenched solution.

[0023] Some embodiments further comprise combining the quenched solution with a solvent prior to separating the HgTe QDs.

[0024] In some embodiments, the solvent comprises methanol.

[0025] Some embodiments further comprise dissolving the washed HgTe in a dissolving solution.

[0026] In some embodiments, the dissolving solution is tetrachloroethylene.

[0027] Some embodiments further comprise washing the separated HgTe QDs in a solvent.

[0028] In some embodiments, the subsequent continuous injection comprises a slower injection rate than the initial injection with a longer duration of injection than the initial injection.

[0029] Some embodiments further comprise chilling the mercury solution to a constant temperature from 0°C to 20°.

[0030] Some embodiments further comprise chilling the mercury solution to a constant temperature from 0°C to 10°.

[0031] Some embodiments further comprise binding polyethylene glycol molecules to the HgTe QDs to form HgTe QD micelles; wherein the HgTe QD micelles are water soluble and biologically nontoxic.

[0032] In some embodiments, the HgTe QD micelles are configured as emitters in short wave infrared imaging.

[0033] Additional embodiments and features are set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the specification or may be learned by the practice of the disclosure. A further understanding of the nature and advantages of the present disclosure may be realized by reference to the remaining portions of the specification and the drawings, which forms a part of this disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0034] The description will be more fully understood with reference to the following figures, which are presented as embodiments of the invention and should not be construed as a complete recitation of the scope of the invention, wherein:

[0035] Fig. 1 A illustrates a method of cold injection synthesis that yields quantum dots with diameters from 1.6-2.6 nm with high photoluminescence quantum yields in accordance with some embodiments.

[0036] Fig. 1 B illustrates a method of continuous injection synthesis of HgTe quantum dots in accordance with some embodiments. This synthesis isolates quantum dots with diameters from 2-6 nm and photoluminescence from 1000-16000 nm andphotoluminescence quantum yields of about 80%. The continuous injection is employed after the cold injection. The cold injection reacts to completion, then the continuous injection is implemented in the same synthesis.

[0037] Fig. 1 C illustrates the combination of the cold injection and slow injection synthesis that is used to isolate QDs with diameters greater than 5 nm and photoluminescence out to 1600 nm in accordance with some embodiments.

[0038] Fig. 1 D illustrates high resolution transmission electron microscopy (HRTEM) images of spherical HgTe quantum dots made through continuous injection in accordance with some embodiments.

[0039] Fig. 1 E illustrates the high photoluminescent quantum yields (PLQY) obtained from QDs synthesized via the cold injection and continuous injection methods as a function of photoluminescence maxima in nanometers in accordance with some embodiments.

[0040] Fig. 2 illustrates a method of synthesizing HgTe quantum dots in accordance with embodiments.

[0041] Fig. 3A illustrates absorbance and photoluminescence spectra of solution samples isolated from a cold injection reaction in accordance with embodiments.

[0042] Fig. 3B illustrates absorbance and photoluminescence spectra of HgTe quantum dots in accordance with prior art.

[0043] Fig. 4A illustrates transmission electron microscopy images of quantum dots used for sizing analysis in accordance with embodiments.

[0044] Fig. 4B illustrates the corresponding sizing distribution histogram from transmission electron microscopy image in Fig. 4A in accordance with embodiments.

[0045] Fig. 4C illustrates powder X-ray diffraction pattern of a HgTe quantum dot sized to be 1.72 nm in accordance with embodiments.

[0046] Fig. 4D illustrates crystal maker graphics of a 1.7 nm zinc blende HgTe quantum dot (~87 atoms) and a 3 nm HgTe quantum dot (-400 atoms) in accordance with embodiments.

[0047] Fig. 5A illustrates the photoluminescence quantum yield plotted as a function of photoluminescence maxima in accordance with embodiments.

[0048] Fig. 5B illustrates capillary image of a HgTe quantum dots dissolved in various samples and concentration matched at the excitation wavelength of 890 nm in accordance with embodiments.

[0049] Fig. 5C illustrates photoluminescence and absorbance of HgTe quantum dot thin film and in solution dissolved in tetrachloroethylene in accordance with embodiments.

[0050] Fig. 6A illustrates photoluminescence microscopy image of thin film of quantum dots at room temperature obtained using wide-field fluorescence microscope in accordance with embodiments.

[0051] Fig. 6B illustrates photoluminescence intensity time-trace (red) of a single quantum dot and of the background in accordance with embodiments.

[0052] Fig. 6C illustrates the photoluminescence spectra of three single quantum dots and ensemble film in accordance with embodiments.

[0053] Fig.7A illustrates the method of using water soluble polyethylene glycol molecules to form water soluble suspensions of the HgTe QDs in accordance with some embodiments. The HgTe QDs are encapsulated by thiol-terminated polyethylene glycol derivates and yielded biologically nontoxic HgTe QD micelles. The encapsulation allows the retention of high PLQY in water (30%).

[0054] Fig.7B illustrates the HgTe QD micelles as contrast agents for in-vivo SWIR imaging of mice in accordance with some embodiments. The image depicts a live mouse imaged under an infrared camera, all light is coming from the direct emission of the QDs through the skin of the mouse.DETAILED DESCRIPTION

[0055] The disclosure includes a slow injection technique allowing for the isolation of spherical HgTe quantum dots (QDs) with quantum yields greater than or equal to about 80% over the wavelength region from 1000 to 1600 nm. The quantum yields are high for any material in this wavelength region, while displaying tunability over a large range of the short-wave infrared spectrum. The disclosed technology includes a multi-step HgTe reaction. Other previous reactions rely on hot temperatures and very reactive precursors to obtain shape control and monodispersity. In contrast, the disclosed synthesis does not rely on high temperatures or highly reactive precursors. The high quantum yields in thesolid state, air-stability, and spherical shape of the synthesized HgTe QDs may be integrated into infrared devices such as photodetectors, light-emitting diodes, field effect transistors, photovoltaics, sensitizers for photon upconversion, and direct fluorophores for biological imaging.

[0056] Bulk HgTe is a semimetal, which upon quantum confinement opens a bandgap that may be optically tuned from the visible to the long wave infrared. Such HgTe nanocrystals may be utilized in device applications due to this bandgap tunability and analogous surface chemistry to their cadmium chalcogenide counterparts. For this reason, HgX (e.g. X = Te, Se, S) materials may be in scalable midwave infrared detectors. HgTe quantum dots have not been extensively explored in the shortwave infrared compared to InAs or PbS despite occasional reports of high photoluminescent quantum yields (PLQY) for HgX materials (up to 75%) in this wavelength. Disclosed herein are a variety of methods of making HgTe QDs, utilizing mercury and chalcogen precursors.

[0057] A combination of a specific mercury precursor (e.g. mercury acetate), a coordinating solvent (e.g. oleylamine) and stabilizing ligands in the reaction (e.g. dodecanethiol) may provide small spherical HgTe QDs which may be isolated at low temperatures (e.g. ~0 °C).

[0058] Additionally, a standard tellurium source (e.g. TOPTe) may be used in the synthesis, avoiding the use of any highly reactive species. It has been discovered that this synthesis may generate small HgTe QDs (e.g. ~1.6 nm) with absorbance features less than 800 nm, extending their confinement to the near infrared.

[0059] Fig. 1A is a schematic of cold injection synthesis of HgTe QDs in accordance with an embodiment of the invention. The QDs may have a small diameter. For example, the QDs may have a diameter of less than 2 nm. The QDs may exhibit high photoluminescence quantum yields (PLQY). The graph on the right illustrates the PLQY which may be over 80% with photoluminescence (PL) maxima of individual samples ranging from 900-1180 nm. The small QDs may retain their optical properties upon exposure to air, in the solid state and in a variety of polar and nonpolar solvents, with short monoexponential lifetimes (~7 ns). This cold injection synthesis technique is described in detail below.

[0060] As illustrated, a tellurium solution 102 is injected into a mercury solution 104 which is placed in a cooling mechanism 106. The cooling mechanism 106 may be an ice bath or a cold-water bath or a cold oil bath. The cooling mechanism 106 produces a chilled mercury solution.

[0061] The tellurium solution 102 is prepared in a glovebox at room temperature (between 10 °C and 25 °C). A stock solution of 1.0 M TOPTe is prepared by dissolving tellurium powder (250 mg) in n-trioctyl phosphine (2 mL) inside the glovebox. The Te / TOP solution is allowed to stir overnight to form a clear yellow solution. The 1 .0 M stock solution is diluted to 0.50 M with tetrachloroethylene for the QD synthesis. The tellurium solution 102 contains 0.50 M TOPTe (e.g. 0.190 mL, 95 pmol) and dodecanethiol (e.g. 10 pL of a 20 mM DDT solution in TOE, 0.2 pmol).

[0062] The mercury solution 104 is prepared by mercury (II) acetate (e.g. 0.150 g, 471 pmol) dissolved in 5 mL of oleylamine (70%). Light heat is applied to the stirring solution until all the mercury salt has dissolved. Once the mercury has dissolved, the solution is allowed to cool to room temperature. The mercury solution 104 may be placed in a flask which is placed in an ice bath and cooled to approximately 6 °C.

[0063] The tellurium solution 102 is then injected into the reaction flask causing an immediate color change to brown. The reaction may be allowed to react in the ice bath (~5 minutes) then removed and allowed to stir at room temperature for the remainder of the reaction. Aliquots of the solution are removed to probe the reaction via absorbance measurements where the appearance and shift of an excitonic feature is monitored.

[0064] It has been discovered that this synthetic method faces a clear challenge in that it is optimized only for smaller HgTe QDs with PL at 1180 nm or below. Commercial short-wave infrared (SWIR) detectors typically span out to 1550 nm thus a large portion of the infrared region may not be sampled with these QDs. Without restriction to any particular theory, the increase in broadness of the PL peaks as a function of reaction time indicates that an Ostwald type of ripening regime may be responsible for the growth of these HgTe QDs. Ostwald ripening may cause the dissolution of smaller particles into larger particles at slow rates leading to increased polydispersity in the final samples. Once all the available tellurium has reacted, Ostwald ripening may dominate as the prevailing mechanism. By not allowing the tellurium source to deplete, the synthesis to isolate QDswhich emit past the bounds of traditional short-wave infrared detectors (e.g. 1550 nm) may be extended, while maintaining high quantum yields over 50%.

[0065] It has been discovered that supplying a constant source of tellurium provides HgTe QDs with PL maxima from 1000-1500 nm and high quantum yields of over 80%. Fig. 1 B illustrates a diagram of continuous injection synthesis of HgTe QDs in accordance with an embodiment of the invention. This synthesis isolates QDs with photoluminescence from 1000-15000 nm and high PLQY over 80%. In some examples, a syringe pump may be utilized to inject a slow supply of trioctylphosphine (TOPTe) over the course of hours. Any type of pressure regulator can be used to provide continuous supply of the telluride source. The synthesis begins with a fast injection at room temperature following similar synthetic conditions to the cold injection method where mercury acetate is dissolved into oleylamine as the solvent. After the fast injection reaction is complete, a slow injection of additional tellurium is performed. Within this synthetic regime, there may be less of an increase in the broadness of the PL peaks and the high quantum yields may be better maintained as a function of reaction time.

[0066] The quantum yield of the HgTe QDs may be over 80% past 1300 nm. Previous materials only have quantum yields at around 30% past 1300 nm. The quantum yield past 1300 nm may make this the brightest materials in the infrared region. Other materials which emit well in this region, such as InAs QDs or PbSe QDs, often rely on heavy ligand shelling through difficult synthetic methods such as colloidal-atomic layer deposition. The fabricated HgTe QDs may provide strong resistance to air exposure and bright solid-state photoluminescence without the need of shelling for stability.

[0067] Fig. 1 C illustrates a synthesis method that combines cold injection and slow injection. Such method can be used to isolate QDs with diameters from about 1.5 nm to greater than 5 nm and photoluminescence from about 900 nm to about 1600 nm. The combination method starts with cold injection and then continues with slow injection to isolate QDs of various sizes and various photoluminescence.

[0068] Fig. 1 D illustrates high resolution transmission electron microscopy (HR-TEM) images of spherical HgTe QDs made through continuous injections as discussed in connection with Fig. 1 B. Even for the larger samples that the synthesis produces (e.g. > 4 nm), spherical shapes are observed in cryogenic transmission electron microscopyimaging. The spherical shapes may provide advantageous performance when integrated into emissive devices as spherical particles provide enhanced carrier transport mobilities.

[0069] Fig. 1 E illustrates PLQY as a function of PL maxima. Fig. 1 E shows PLQY values for QDs with photoluminescence from about 900 nm to about 1400 nm. The various sizes QDs can be synthesized using the combination method shown in Fig. 1 C.

[0070] Fig. 2 is a flow chart of a process for producing HgTe QDs in accordance with an embodiment of the invention. Mercury salt is dissolved (202) in a coordinating solvent to create a mercury solution. The mercury salt may be mercury (II) acetate. The mercury salt may be 400 mg and 1 .25 mmol. The solvent may be oleyamine. The oleyamine may be 70% in concentration. The mercury salt may be dissolved 202 into the solvent under inert atmosphere.

[0071] In some embodiments, the mercury solution may be heated until all mercury salt is dissolved. After heating, the mercury solution is allowed to cool to room temperature.

[0072] Tellurium is dissolved (204) in a solvent to produce a tellurium solution. The tellurium may be tellurium powder dissolved in an organophosphine compound. The organophosphine compound may include trioctlyphosphine (TOP). The tellurium powder dissolved in TOP may be referred to as Te-organophosphines (TOPTe). The organophosphine compound may include tributylphosphine (TBP), triphenylphosphine (TPP). The tellurium may be tellurium powder dissolved in one or more reactive leaving groups such as trimethylsilyl (TMS) and hexamethylphosphorous triamide (TDMAP). TOPTe may be less volatile than tellurium powder dissolved in reactive leaving groups.

[0073] The tellurium solution may include 1 M TOPTe (e.g. 0.300 mL, 0.30 mmol) and a coordinating ligand which may be dodecanethiol. The coordinating ligand may include 20 mM dodecanethiol (e.g. 40 pL, 0.80 pmol).

[0074] The entire tellurium solution is initially quickly injected (206) into a mercury solution to synthesize HgTe QDs in the mercury solution. The mercury solution may be at room temperature. In some embodiments, the mercury solution may be placed in a container which is placed in a cold bath and cooled. The mercury solution may be cooled to approximately 6 °C. In some embodiments, the mercury solution may be at atemperature of 0 °C to 20 °C. The mercury solution may react slower at lower temperatures.

[0075] It has been discovered that the combined mixture of tellurium solution and mercury solution may turn brown during injection. The brown solution corresponds to the reaction beginning as the QDs are brown. The mixture of tellurium solution and mercury solution may be monitored during the initial injection 206. Absorption spectroscopy may be used to monitor the appearance and shift of an excitonic feature from the QDs. In the absorbance spectra the QDs have a bump or peak attributed to their excitonic feature. The shift in the peak may be monitored to determine whether the reaction is complete. No further shift indicates the reaction is complete. The reaction may stop when all of the available tellurium has reacted.

[0076] After the reaction is determined to be complete, a tellurium solution is continually injected (208) into the HgTe QDs and mercury solution. The tellurium solution may include a tellurium complex (TOPTe), a coordinating ligand, and a solvent. The tellurium may include tellurium powder dissolved in TOP to form the complex TOPTe. The TOPTe may be 0.5 M TOPTe (e.g. 0.75 mL, 0.375 mmols). 1.0 M TOPTe may be diluted in a solvent which may be tetrachloroethlene (e.g. 0.75 mL) to form 0.50 M TOPTe. The coordinating ligand may be dodecanethiol. The dodecanethiol may be 60 pL, 6 pmol. For the continuous injection 208, a syringe pump loaded with the solution may be used to maintain an injection rate of 0.15 mL / H over the course of 5 hours. The continuous injection 208 may occur at room temperature and under inert environment. In some embodiments the continuous injection 208 may occur at a temperature from 0°C to 20°C. The injection rate of the continuous injection 208 may be lower than the injection rate of the initial injection 206. The tellurium solution initially injected 206 may have a different tellurium concentration than the tellurium solution continually injected 208. The tellurium solution initially injected 206 may have a higher tellurium concentration than the tellurium solution continually injected 208.

[0077] In some embodiments, the tellurium complex (TOPTe) may be substituted for an organophosphine compound such as tributylphosphine (TBP), triphenylphosphine (TPP). The tellurium complex (TOPTe) may also be substituted for one or more reactiveleaving groups such as trimethylsilyl (TMS) and hexamethylphosphorous triamide (TDMAP).

[0078] It has been discovered that the reaction may not occur without the initial fast injection. Without limitation to any particular theory, to initiate a QD nucleation reaction, a critical concentration of Te must be reached which may not be reached fast enough at the slow injection rate. The fast injection may nucleate high quality HgTe QDs. Just performing the continuous injection, the amount of Te injected may not be stoichiometric enough to nucleate a QD reaction and no or very few QDs may grow. Only using the initial injection may nucleate only very small QDs (e.g. emissions of less than 1000 nm). The second injection step that slowly adds small amounts of Te may grow the small QDs bigger. It is difficult to directly nucleate large particles (e.g. emission past 1200 nm) directly with a fast injection reaction. Therefore, it has been discovered that larger QDs may be produced that are still high quality by combining the two injections.

[0079] The HgTe QD reaction may be quenched (210) with a quenching ligand solution which may include a solution of excess of coordinating ligand such as dodecanethiol. The quenching ligand solution may include a 1 :1 ratio of a 4% volume solution of dodecanethiol in tetrachloroethylene.

[0080] HgTe QDs are separated (212) from the quenched solution. Separating 212 the HgTe QDs may include combining the quenched solution with a solvent and centrifuging the combined solution. The solvent may be methanol. The quenched solution to methanol ratio may be 2:1. The centrifuging may be at 12500 rpm for 2.5 minutes. The resulting HgTe QDs powder may be washed with 2 mL of methanol and either stored in a solid state or dissolved in 5 mL of tetrachloroethylene. The isolated HgTe QD solutions and powders may be stored in a freezer to maintain long term stability.EXEMPLARY EMBODIMENTS

[0081] Although specific embodiments of systems and apparatuses are discussed in the following sections, it will be understood that these embodiments are provided as exemplary and are not intended to be limiting.Example 1 : Ultrasmall HgTe quantum dots with near unity photoluminescent quantum yields in the near and shortwave infrared

[0082] Many embodiments provide a cold injection approach to directly synthesize a series of ultrasmall (< 2 nm) HgTe quantum dots with NIR / SWIR bandgaps. The sizes of these QDs are estimated through X-ray diffraction (XRD) and transmission electron microscopy (TEM) imaging to range from 1.72 to 2.39 nm in diameter. Interestingly the ultrasmall quantum dots retain a high PLQY (40-80%) in a variety of polar and nonpolar solvents as well as in thin films. Comparative imaging on a SWIR InGaAs camera demonstrates their brightness in comparison to reference dye I R-1061 . This further allows for the first single-particle photoluminescence imaging of HgTe QDs and the observation of photoluminescence intermittency (blinking) consistent with single quantum dot resolution. HgTe QDs can access extreme confinement regimes and motivate the use of small HgTe QDs as candidates for high efficiency emitters for NIR / SWIR device applications.

[0083] In many embodiments, the synthesis process uses low temperatures (<10 °C) to suitably slow the reaction and access early time kinetics of these nanoclusters. Lower temperatures necessitate the use of mercury (II) acetate as the precursor salt, which has higher solubility in oleylamine in contrast to other common mercury halide salt precursors. Several embodiments add a small amount of 1 -dodecanethiol (DDT) into the tellurium source solution. The S-Hg bond of dodecanethiol is known to be strong which enables strong surface passivation and decreased aggregation in the nucleation phase of HgTe QD growth.

[0084] During the synthesis, mercury (II) acetate is dissolved into excess oleylamine and held in a water ice bath. A solution containing tellurium dissolved in trioctylphosphine (TOPTe) and 1-dodecanthiol (DDT) is injected into the cooled reaction mixture, upon which the solution turns brown. Aliquots of the reaction are extracted and probed using UV-Vis spectroscopy over 1 hour. The reaction is quenched with a solution of excess of dodecanethiol diluted in TCE resulting in well suspended quantum dots capped with DDT. Then centrifuge the product and redissolve the QDs in TCE for optical characterization. The rate of reaction, size and optical properties of the synthesized HgTe QDs can be tuned based on the temperature of the Hg / oleylamine solution in the ice bath beforeTOPTe injection, or the reaction length before quenching. Fig. 3A shows the spectra of QD samples synthesized under the same reaction conditions (6 °C injection) and differ only in reaction time. After approximately one hour, no further shifts in the absorbance spectrum are observed suggesting that the tellurium source is consumed. Therefore, from nucleation, the excitonic absorbance feature can be tuned around 250 nm. The surface passivation and atomic percentage of the QDs is analyzed using energy dispersive spectroscopy (EDS). The EDS confirms the HgTe identify of the QDs and that they are passivated with DDT.

[0085] Fig. 3A shows absorbance and PL spectra of solution samples isolated from a standard cold injection reaction performed at 6 °C. All spectra are recorded in TCE. Absorbance spectra are denoted in dashed lines while PL is shown in solid. The reaction length of each sample is denoted. And the gaussian fit used to extract the full-width at half maximum (FWHM) is shown over the photoluminescence spectra. The Amax and FWHM of each PL spectra is denoted.

[0086] When the reaction is quenched immediately after the TOPTe injection, a strong excitonic feature is observed in the absorbance spectrum at 715 nm (1.73 eV) (Fig. 3A). This represents the lowest wavelength and smallest particles produced via this synthetic method. Some embodiments compare absorbance features of small HgTe QDs as seen in Fig. 3B, demonstrating the difference in the band gaps. The reported HgTe QDs have absorbance features at 900 nm or below, which still exhibit red shifted absorbance features (830 nm, 900 nm) and PL maxima (1050 nm, 1200 nm) than the smallest sample in accordance with embodiments with absorbance at 715 nm and PL maxima at 915 nm. (Figs. 3A and 3B).

[0087] Fig. 3B shows absorbance and PL spectra of HgTe QDs most similar in size and photophysical properties from literature. The Amax and FWHM of each PL spectra is denoted. (See, e.g., Rogach, A. I.; et al., Colloidally Prepared CdHgTe and HgTe Quantum Dots with Strong Near-Infrared Luminescence, physica status solidi (b) 2001 , 224 (1 ), 153-158; Prado, Y.; et al., Seeded Growth of HgTe Nanocrystals for Shape Control and Their Use in Narrow Infrared Electroluminescence. Chem. Mater. 2021 , 33 (6), 2054-2061 ; the disclosures of which are incorporated by reference.)

[0088] Throughout the course of one reaction, the PL maxima can be tuned ~200 nm. A fully cooled reaction flask will produce QDs with PL maxima ranging from 915-1100 nm (Fig. 3A). When the reaction flask is not allowed to fully cool the reaction can be extended to isolate QDs with PL maxima up to ~1200 nm. The QD samples show relatively narrow FWHM of their photoluminescence spectra ranging from 100 to 154 nm (130-140 meV) for the smallest to largest QDs sampled which is consistent. In a standard reaction, the FWHM broadens (100 to 134 nm) as a function of reaction time (Fig. 3A). The increase in the FWHM as the reaction progresses over long reaction times provides an early suggestion that an Ostwald ripening mechanism may be responsible for the growth of the QDs. Similar FWHM behavior and reaction kinetics were observed in a single injection synthesis of InAs QDs, attributed to an Ostwald ripening mechanism of growth.

[0089] To estimate the size of the small HgTe QDs, powder X-ray diffraction (PXRD) measurements and transmission electron microscopy (TEM) imaging can be used. TEM sizing analysis is performed on the smallest and largest samples from the synthesis and are found to be 1.65 nm and 2.29 nm (± 0.30 nm) respectively (Fig. 4A). Sizing distributions collected from TEM analysis are plotted and the histograms are fit to a gaussian function (Fig. 4B). The sizing distributions have a standard deviation for the smallest QD sample (1.65 nm) of 0.28 nm and 0.36 nm for the larger QDs (2.29 nm) imaged. PXRD is performed on five zinc-blende HgTe QDs and the Scherrer equation is used to estimate particle diameter. The (111 ) reflection is fit to a gaussian equation to extract the FWHM for the Scherrer equation (Fig. 4C). The analyzed samples are found to range in size from 1.72 to 2.29 nm (± 0.30 nm). The PXRD results are corroborated with TEM imaging (Figure 2b and 2c). TEM sizing analysis is performed on the smallest and largest samples from the synthesis and are found to be 1 .64 nm and 2.40 nm (± 0.30 nm) respectively (Fig. 4C). The samples follow a consistent trend when the absorbance feature is plotted as a function of particle diameter, as shown in the sizing curve presented (Fig. 4D). The sizing curve focusses on HgTe QDs with interband transitions of 10 microns or less (or under 20 nm in diameter). Current fits for the HgTe QD sizing curve prove to be outdated and breakdown as the synthetic scaffold of sizes continues to expand (Fig. 4D). The data collected from PXRD is included and plotted in the inset (Fig. 4D). The calculated sizes align well with the trend. For context, 2.7 nm QDs contain ~400 atoms,whereas 1.7 nm samples contain ~90 atoms (Fig. 4E). The small size of the - 90 atom QDs are similar in scale to the size clusters isolated in Cd chalcogenide syntheses.

[0090] Fig. 4A shows TEM images of QDs used for sizing analysis. The left image shows the smallest size sampled (1.65 nm) and the largest size (2.29 nm). Fig. 4B shows the corresponding sizing distribution histogram from TEM image. The histogram for the smallest QD sampled (1 .65 nm) is on the left where the largest QD sampled is shown on the right (2.29 nm). Both histograms are fit to a gaussian function shown in solid line. Fig. 4C shows PXRD pattern of a HgTe QD sized to be 1.72 nm via Scherrer analysis. Diagnostic zinc-blende reflections are labeled. The simulated HgTe zinc-blende pattern is included. Fig. 4D shows crystal maker graphics of a 1 .7 nm zinc blende HgTe QD (-87 atoms) and a 3 nm HgTe QD (-400 atoms).

[0091] The small HgTe QDs are colloidally stable and display bright photoluminescence. PLQY are evaluated using both an integrating sphere (absolute) and via comparison to known standards (relative). With little optimization, PLQYs are above 80% for all samples, and near unity for the NIR QDs (Fig. 5A) in TCE. Furthermore, excited-state photoluminescence decay measurements are performed on representative HgTe QD samples. Upon 405 nm pulsed excitation, the QDs show a near monoexponential lifetime of -7 ns. The primary lifetime of 7 ns is consistent with the lifetimes observed for the excitonic transitions in HgTe QDs (2 ns) and highly confined 2 to 3 monolayer HgTe nanoplatelets (7-50 ns). The photoluminescence of HgTe QDs in a variety of organic solvents such as hexanes, acetone, dichloromethane and ethanol is investigated. A stock HgTe QD solution is diluted to have the same optical density at the excitation source of 890 nm in each of the solvents tested. To compare the brightness of the photoluminescence, the samples are uniformly excited with 890 nm light and imaged with a SWIR camera alongside one another. A reference dye I R-1061 with a known PLQY is also included in capillary tube included in the image for comparison. The QD photoluminescence is the brightest and most stable in hexanes, followed by acetone, DCM and ethanol (Fig. 5B). To demonstrate the relative brightness of the sample in each solvent, the pixel intensity across the image is plotted (Fig. 5B). Compared to the bright HgTe QD samples, the dye has very little pixel intensity which is consistent with the low QY of IR-1061 of 5.0 x 10'3. The trend in relative brightness is corroborated throughquantum yield measurements in each of the tested solvents. The sample has the highest PLQY in TCE (90%) calculated with IR-1061 as the reference dye. The PLQY results for each studied solvent corroborated the trend observed in the SWIR images. The QY remains the highest in hexanes (83%), followed by acetone (66%), DCM (56%) and found to be the lowest in ethanol (36%). Although these samples are highly emissive in polar solvents such as ethanol, the photostability of the QDs decreases over hours in such solvents.

[0092] Fig. 5A and Fig. 1 E show the PLQY plotted as a function of photoluminescence maxima. Reference dye IR-106140 and IR-14080 is used to measure the PLQY. Fig. 5B shows capillary image of a HgTe QDs dissolved in various samples and concentration matched at the excitation wavelength of 890 nm. The samples are uniformly excited and imaged alongside one another to compare relative brightness in each solvent. A reference dye IR-1061 (QY= 5 x 10-3) is included for qualitative comparison. A rectangular ROI is drawn over the relevant area of the raw image and used to crop images to the displayed size. The average signal intensity is plotted as a function of horizontal distance and shown below. Camera settings are as follows: gain = 1 ET = 0.1 ms. Fig. 5C shows photoluminescence and absorbance of HgTe QD sample thin film and in solution dissolved in TCE. The solution and film sample are concentration matched to the excitation wavelength of 890 nm. The raw photoluminescence intensity of the thin film sample (blue) and solution state (green) sample are plotted along with the absorption from each sample.

[0093] The small HgTe QDs retain their photoluminescence properties in the solid state. The solid-state photoluminescence is investigated in thin films. A representative QD sample is concentrated to match the optical density of a drop cast thin film (Fig. 5C). The representative sample QD sample used in the comparative experiment has a relatively small stokes shift in solution (~50 nm). The stokes shift can vary between samples likely due to a larger distribution of sizes in some samples. For example, a sample of similar PL wavelengths shown in Fig. 3A has a larger stokes shift (~100 nm) in comparison to the representative sample from Fig. 5C (~50 nm). The relative photoluminescence intensity and spectra of the representative sample in film and solution are compared. The film retains the majority of the photoluminescence intensity that thesolution sample has. In thin film, a red shift in the photoluminescence maximum (from 1050 nm to 1100 nm) and broadening of the FWHM (by 40 nm) are observed (Fig. 5C). The red shift in photoluminescence observed in the solid state may be due to energy transfer occurring, film inhomogeneity, or reabsorption. To estimate the relative quantum yield of the film, the ratio of the optical density and integrated PL intensity in solution and film are compared. The solution QD sample in TCE is found to have a PLQY of -80% through absolute and relative methods.

[0094] By comparing the ratio of the absorbance O.D. and integrated photoluminescence spectra of the QDs in film and solution, the PLQY of the film is estimated to be ~60%. The photoluminescence of solid HgTe QDs is imaged on a SWIR InGaS camera. The HgTe powders display bright emission on the SWIR camera which is significant as the PL of QDs can commonly quench in the solid state. However, the QD samples can retain strong solid-state photoluminescence and -60% quantum yields in thin films.

[0095] The photostability of the QDs in thin films allows the study of the photoluminescence spectra and intermittency under a fluorescence microscope with single particle resolution. A home built wide-field photoluminescence microscope is utilized to study the single particle photoluminescence of the ultrasmall HgTe QDs. The films of QDs used for single particle imaging are prepared by drop casting a diluted solution of QDs in hexanes onto a glass coverslip at a small incline. The film is excited by a 405 nm continuous wave diode laser with an intensity of 200 W / cm2. The photoluminescence of the film is recorded over time using an EMCCD camera. It is noteworthy that the quantum efficiency of the camera is <20% in the spectral region of interest (-950 nm). The low camera efficiency in this region makes it challenging to record any photoluminescence, however the high PLQY and brightness enables imaging of the QDs at single particle resolution.

[0096] The thin films used for single-particle imaging are well dispersed with bright QDs as shown in the photoluminescent images (Fig. 6A). The photoluminescence intensity traces of 50 individual QDs are studied. All the single QDs sampled display photoluminescence intermittency (blinking), where discrete periods of no photoluminescence intensity (or off states) are observed (Fig. 6B). It is important to notethat at a 405 nm excitation the thin films exhibit a lower quantum yield (by ~20%) which likely increases the amount of blinking observed. The blinking behavior is similar to that observed universally for other types of nanocrystals. Blinking in nanocrystalline systems is often attributed to a range of possible phenomena, including Auger recombination in charged QDs and nonradiative channels related to traps or hot-carrier trapping. Power law analysis is performed on a plot of the on / off times of the single QDs samples. The on / off times show similar power law distributions consistent with a distribution of possible traps.

[0097] Despite observed blinking, the QDs are photostable under 405 nm excitation for longer than 6 minutes. The photoluminescence spectra of 10 single HgTe QDs are analyzed and compared to that of the ensemble film (Fig. 6C). All single QD spectra analyzed have a FWHM narrower than the FWHM of the ensemble film (Fig. 6C). The single QD spectra shows a FWHM ranging from 103 to 125 meV whereas this ensemble spectrum has a FWHM of 150 meV. Since the single QD FWHM values are narrower than the ensemble this indicates some amount of inhomogeneous broadening in the ensemble spectra and room for improvement in the synthesis.

[0098] Fig. 6A shows PL microscopy image of thin film of QDs at room temperature obtained using wide-field fluorescence microscope. Fig. 6B shows PL intensity time-trace (red) of a single QD and of the background (black). The background trace is recorded from an area on the film where no particles existed. The right panel shows the frequency of the PL intensity trace of QD. Fig. 6C shows the PL spectra of three single QDs and ensemble film. The detector efficiency is plotted over the PL traces to show the low efficiency PL region of interest.

[0099] Fig. 7A illustrates a method of forming water soluble HgTe QDs. The thiol terminated PEG ligands are bonded to the surface of the QDs to form the water-soluble HgTe QD micelles. These micelles are biologically nontoxic and emissive in water. They can be used as imaging contrast agents. Any thiol terminated polyethylene glycol can be used to encapsulate the QDs and form a biologically non-toxic micelle. The thiol terminated polyethylene glycol ligands are reacted with the QDs in a solution of chloroform in a molar excess of the polyethylene glycol. The chloroform is evaporated,and the resulting QD micelle powder is redissolved in water or a buffer then syringe filtered through a 0.22-micron PTFE syringe filter.

[0100] Fig. 7B illustrates an image of live mouse during in-vivo SWIR imaging where the HgTe QD micelles are used as contrast agents intravenously. The QD micelle absorbance and photoluminescence spectra is shown, along with the filter and camera settings. The light observed is SWIR photoluminescence directly from the HgTe QD micelles. The bright photoluminescence through skin allowed for 100 frame per second imaging and heart-rate resolution.

[0101] There are several nanocrystal systems that have optical transitions in the near and shortwave infrared. Lead chalcogenides can be quantum confined to emit from 1.5 eV to 0.3 eV (830 nm to 4000 nm) and have reported quantum yields in solution ranging from 3-90 % strongly depending on the optical bandgap but are limited by the bulk bandgaps (e.g PbTe bandgap of 0.32 eV). InAs and InSb also display size tunable SWIR bandgaps, ranging from 1.7 eV to 0.3 eV (730 nm to 4000 nm) with quantum yields of 2- 82%, again variable and depending on shelling procedures. However, HgTe appears to have some unique properties. First, HgTe QDs display high quantum yields which are air stable without a core-shell procedure unlike the aforementioned indium and lead materials which require shells to obtain high PLQY. Second, HgTe QDs appear to show rather short radiative lifetimes (2-10 ns) consistent with other ll-VI materials, but considerably shorter than Pb chalcogenides (> 1 ps). Third, HgTe QDs can be synthesized in air, and appear to be resistant against oxidative damage, unlike lead and indium base nanomaterials. Finally, as HgTe materials have no bulk bandgap, they can be quantum confined for long wave infrared (8-12 pm) to NIR / SWIR bandgaps.

[0102] Many embodiments provide a tunable low temperature synthesis for the direct isolation of a variety of HgTe quantum dots under 2 nm in size. Using a cold-injection synthesis, the process can produce a size series that maintains high PLQY and solid- state photoluminescence. The smallest of these samples represent clusters of roughly 90 atoms or less. Due to the excellent photoluminescence properties, the first single-particle photoluminescence imaging study on HgTe quantum dots is also performed. Overall, the presented cold injection synthesis provides an interesting platform to investigate the initial stages of HgTe nucleation and the smallest bounds of confinement for bulk HgTe. Theefficient PLQY (>80%) and bright solid state photoluminescence of the presented QDs motivates the use of small (>3 nm) HgTe QDs as emitters in NIR / SWIR devices.

[0103] Both mercury acetate and mercury chloride can form toxic fumes upon heating, therefore by utilizing low temperatures the synthesis reduces the risk of forming excess fumes. Additionally low temperatures may mitigate the formation of dangerous organomercury products similar to cadmium chalcogenides where organocadmium byproducts are suggested as short-lived intermediates during high temperature cadmium chalcogenide QD syntheses.

[0104] Mercury (II) acetate (99%), ntrioctly phosphine (TOP), tetrachloroethylene (TCE), n-hexanes, acetone, ethanol, 2-propanol, dichloromethane (DCM), methanol, Oleylamine (70%), tellurium powder (99%), 1 -dodecanethiol (DDT), and Mercury(ll) acetate (98%) and Infrared dyes IR-140 and IR-1061 are used.

[0105] TOPTe is prepared in a glovebox at room temperature. A stock solution of 1 .0 M TOPTe is prepared by dissolving tellurium powder (250 mg) in n-trioctyl phosphine (2 mL) inside the glovebox. The Te / TOP solution is allowed to stir overnight to form a clear yellow solution. The 1.0 M stock solution is diluted to 0.50 M with tetrachloroethylene for the QD synthesis.

[0106] Synthetic procedure of the “cold” injection HgTe QDs. The reaction is performed under ambient atmosphere. Mercury (II) acetate (0.150 g, 471 pmol) is dissolved in 5 mL of oleylamine (70%). Light heat is applied to the stirring solution until all the mercury salt has dissolved. Once the mercury has dissolved, the solution is allowed to cool to room temperature. The flask is placed in an ice bath and cooled to approximately 6 °C. The tellurium source solution is then injected into the reaction flask causing an immediate color change to brown. The tellurium source solution contains 0.50 M TOPTe (0.190 mL, 95 pmol) and dodecanethiol (10 pL of a 20 mM DDT solution in TCE, 0.2 pmol). The reaction is allowed to react in the ice bath (~5 minutes) then removed and allowed to stir at room temperature for the remainder of the reaction. Aliquots of the solution are removed to probe the reaction via absorbance measurements where the appearance and shift of an excitonic feature is monitored. If the reaction if fully cooled in the ice bath (~6 °C), an excitonic feature at 715 nm is observed almost immediately after injection. The observed absorbance features shift continuously for approximately onehour. The size and optical properties of the QD can be tuned by the reaction length before quenching. The reaction is quenched with a 20 mM DDT solution in TCE (4 mL, 80 pmol). The quenched product is then isolated through centrifuging in ~3 mL aliquots with a 1 :1 ratio of 2-propanol at 12500 rpm for 3:00 minutes. The isolated powder is washed once with 2 mL of 2-propanol and dissolved into 5 mL of TCE. The isolated product is stored overnight in the freezer to increase stability. If the samples are left out at room temperature for more than 24 hours, ripening is observed along with a decrease in PLQY.

[0107] Photophysical Characterization. All samples used for optical characterization are concentrated at an optical density (O.D.) of 0.10 or less. Absorption spectra are recorded with a UV-Vis spectrophotometer. Photoluminescence spectra are measured with a spectrometer equipped with liquid nitrogen cooled InGaAs photodiode and Si photomultiplier tube detectors. Absolute PLQY measurements are recorded with a petite integrating sphere in the spectrometer. Infrared emitting dyes IR-140 (Qf = 0.167) and IR- 1061 (Qf = 5.0 x 10’3) are used as references for the relative quantum yield measurements. Photoluminescent lifetime measurements are performed on a home-built optical set up, using short wave sensing superconducting nanowire single photon detectors (SNSPDs) and a time-lagged single photon counting module. For lifetime measurements a 405 nm laser at a 1 MHz repetition rate is used.DOCTRINE OF EQUIVALENTS

[0108] While the above description contains many specific embodiments of the invention, these should not be construed as limitations on the scope of the invention, but rather as an example of one embodiment thereof. It is therefore to be understood that the present invention may be practiced in ways other than specifically described, without departing from the scope and spirit of the present invention. Thus, embodiments of the present invention should be considered in all respects as illustrative and not restrictive. Accordingly, the scope of the invention should be determined not by the embodiments illustrated, but by the appended claims and their equivalents.

Claims

WHAT IS CLAIMED IS:1 . A method of manufacturing HgTe quantum dots (QDs), the method comprising: dissolving a mercury salt in a coordinating solvent to create a mercury solution; dissolving tellurium in a coordinating ligand compound to create a first tellurium solution; performing an initial injection of the first tellurium solution into the mercury solution to produce a synthesized HgTe QDs solution; when synthesis during the initial injection has completed, perform a subsequent continual injection of a second tellurium solution into the synthesized HgTe QDs solution; quenching the synthesized HgTe QDs solution with a solution of coordinating ligand; and separating the HgTe QDs from the rest of the quenched solution.

2. The method of claim 1 , wherein the mercury salt is mercury (II) acetate.

3. The method of claim 1 , wherein the coordinating solvent comprises oleyamine.

4. The method of claim 1 , wherein the tellurium comprises tellurium powder and the coordinating ligand compound is an organophosphine compound.

5. The method of claim 4, wherein the organophosphine compound comprises trioctylphosphine.

6. The method of claim 1 , wherein the coordinating ligand comprises dodecanethiol.

7. The method of claim 1 , wherein the second tellurium solution comprises tellurium dissolved in a coordinating ligand compound.

8. The method of claim 7, wherein the tellurium comprises tellurium powder and the coordinating ligand compound is an organophosphine compound.

9. The method of claim 8, wherein the organophosphine compound comprises triocytlphosphine.

10. The method of claim 7, wherein the coordinating ligand comprises dodecanethiol.

11. The method of claim 1 , wherein the first tellurium solution has a different tellurium concentration than the second tellurium solution.

12. The method of claim 11 , wherein the first tellurium solution has a higher tellurium content than the second tellurium solution.

13. The method of claim 1 , further comprising performing absorption spectroscopy to monitor appearance of the synthesized HgTe QDs solution during the initial injection to determine when synthesis begins and finishes after the initial injection.

14. The method of claim 1 , wherein quenching the synthesized HgTe QDs solution comprises adding a quenching solution into the HgTe QDs solution.

15. The method of claim 14, wherein the quenching solution comprises a dodecanethiol solution in tetrachloroethylene.

16. The method of claim 1 , wherein separating the HgTe QDs comprises centrifuging the HgTe QDs out of the quenched solution.

17. The method of claim 16, further comprising combining the quenched solution with a solvent prior to separating the HgTe QDs.

18. The method of claim 17, wherein the solvent comprises methanol.

19. The method of claim 17, further comprising dissolving the washed HgTe in adissolving solution.

20. The method of claim 19, wherein the dissolving solution is tetrachloroethylene.

21. The method of claim 1 , further comprising washing the separated HgTe QDs in a solvent.

22. The method of claim 1 , wherein the subsequent continuous injection comprises a slower injection rate than the initial injection with a longer duration of injection than the initial injection.

23. The method of claim 1 , further comprising chilling the mercury solution to a constant temperature from 0°C to 20°.

24. The method of claim 23, further comprising chilling the mercury solution to a constant temperature from 0°C to 10°.

25. The method of claim 1 , further comprising binding polyethylene glycol molecules to the HgTe QDs to form HgTe QD micelles; wherein the HgTe QD micelles are water soluble and biologically nontoxic.

26. The method of claim 25, wherein the HgTe QD micelles are configured as emitters in short wave infrared imaging.

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