Nanocrystal heterostructures for sub-bandgap emission

Abstract

Nanocrystal heterostructures having a core / shell structure that emit light at wavelengths longer than the bulk bandgap would typically allow for. In some disclosed embodiments, there are two shell layers, and the three materials are selected from sulfide, selenide, and telluride semiconductors. Also, a solar module having an encapsulation layer including type II nanocrystal heterostructures, exhibiting sub-bandgap emission are incorporated into the encapsulation layer for improved photovoltaic performance is disclosed.

Description

[0001] NANOCRYSTAL HETEROSTRUCTURES FOR SUB-BANDGAP EMISSION CROSS REFERENCE TO RELATED APPLICATION(S) This application claims priority to U. S. Application No. 63 / 725,992, filed November 27, 2024, the content of which is incorporated herein by reference in its entirety.

[0002] FIELD OF THE DISCLOSURE

[0003] The present invention is directed to nanocrystal heterostructures for sub-bandgap emission of light. Certain type II nanocrystal heterostructures of the present invention have been found to exhibit high tunability of their emission peak and can be suitable for down-conversion of visible and ultraviolet (UV) light into near infrared (NIR) light. In some embodiments these nanoparticles are incorporated into solar modules to improve power conversion efficiency.

[0004] BACKGROUND OF THE DISCLOSURE

[0005] In general, the bandgap of a semiconductor sets the energy / wavelength of photons that are emitted (luminescence) under typical excitation conditions. For example, with CdS, CdSe, and CdTe quantum dots, it is rare to achieve emission at wavelengths less than about 750 nm owing to their bulk band gaps of 2.42 eV (512 nm), 1.74 eV (713 nm), and 1.50 eV (827 nm), respectively. Furthermore, making nanocrystals (crystals characterized by at least one dimension on the scale of tens of nanometers or smaller) of any semiconductor will increase the bandgap (decrease the emission wavelength) when at least one dimension of the crystal structure is roughly smaller than the Bohr radius for a given semiconductor, which is about 7.5 nm for CdTe (diameter of about 15 nm), about 4.5 nm for CdSe, and about 2.5 nm for CdS. Therefore, emission at wavelengths shorter / small er than the bandgap are typically seen from QDs (due to quantum confinement). In other words, the emission from nanocrystals is typically blue shifted in emission wavelength from the bulk counterparts. There are many applications requiring photons at longer / greater wavelengths than these bulk values, including 850nm to 1000 nm or up to 1240 nm (1.0 eV). Moreover, nanosized particles bring many advantages including easier processability in liquids and polymers and minimal scattering of light due to their small size.

[0006] By forming an interface between any two of these or similar semiconductor materials at the nanoscale, radiative recombination can occur between an electron on one side of the interface and a hole on the other side, which results in sub-bandgap emission. Moreover, a nanoscale interface allows for a greater interface area to volume ratio in these kinds of nanoscale materials, sometimes called quantum dots (QDs). This effect has been observed previously, but with low photoluminescence quantum yields (PL QY, less than 50%) and broad emission linewidth (i.e., full width at half maximum (FWHM) of >200 nm in wavelength, or >300 meV in energy space), making them impractical for most applications. Moreover, previous attempts to produce type II heterostructures with these compounds has had limited peak emissions of typically shorter / less than about 850 nm. Although the present invention focuses on this family of II- VI semiconductors (CdS, CdSe, and CdTe), the concept is applicable to many other combinations of semiconductors at the nanoscale, including groups IV, IILV, I-III-VI, other II-VI, perovskite, and other semiconductors known in the art.

[0007] SUMMARY OF THE DISCLOSURE

[0008] Few materials can emit near-infrared (NIR) light efficiently and they often have broad photoluminescence FWHM. Some solar cells suffer from lower spectral efficiency at higher photon energies than in the NIR, and further some solar cells have narrow spectral responses in the NIR. Also, many solar modules suffer from ultraviolet (UV) or visible light degradation. Materials that efficiently down-convert visible and / or UV light to NIR light remain needed in various applications, especially in solar modules.

[0009] In one aspect of the present invention, synthesis of unique core / shell or core / shell / shell (CSS) nanocrystals from various compositions of semiconductor materials, e.g., CdS, CdSe and CdTe, is provided where the quantum dots have yielded surprisingly wide degrees of tunability in their peak emissions, with narrow FWHMs, and with high PL QYs greater than 60%.

[0010] In another aspect of the present invention, QDs having a core / shell or CSS structure are provided with each layer selected from a semiconductor, e.g. from CdS, CdSe and CdTe, where the first shell layer upon the core has a narrow thickness of less than 1 - 2 nm, where the quantum dots have surprisingly wide degrees of tunability in their peak emission, with a narrow FWHMs, and with high PL QYs greater than 60%.

[0011] In another aspect of the present invention, the quantum dots of the present invention are combined with photovoltaic modules for improving the light harvesting efficiency by down-conversion of visible photons into near-infrared (NIR) light that is more efficiently harvested by solar cells. This combination can improve the photocurrent, particularly for bifacial CdTe modules.

[0012] BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Fig. 1 (a) shows a core / shell / shell structure of the present invention.

[0014] Fig. 1 (b) illustrates a cross-section of the core / shell / shell structure shown in Fig. 1 (a).

[0015] Fig. 1(c) shows the narrow and intense photoluminescence emissions of several CSS quantum dots that can be achieved in the NIR region by varying three key structural parameters.

[0016] Fig. 1 (d) illustrates the core / shell / shell structure having different core sizes and shell thicknesses. Fig. 2 (a) shows TEM images of CdSe core (bottom), CdSe / CdTe core / shell (middle) and CdSe / CdTe / CdSe CSS (top) stages of the synthesis.

[0017] Fig. 2 (b) shows a high-resolution TEM image of CSS QDs with the FFT pattern of the outlined QD in the inset.

[0018] Fig. 2 (c) shows an inverse FFT image of the CSS QD outlined in b) with false color to identify the larger lattice region.

[0019] Fig. 2 (d) shows filtered inverse FFT image of the same QD with lattice spacings as indicated (scalebar is 2 nm).

[0020] Fig. 2 (e) shows high angle annular dark field STEM image of CSS QDs after energy dispersive x-ray spectroscopy (EDX) scan (scalebar is 5 nm).

[0021] Fig. 2 (f) shows corresponding EDX maps of Se and Te superimposed on each other (scalebar is 5 nm).

[0022] Fig. 2 (g) shows absorption and PL spectra of aliquots taken from each step of the CSS synthesis 9inset shows time-resolved PL of the final CSS QDs.

[0023] Fig. 3 (a) shows TEM images of the three different sizes of cores (left), after CdTe shell growth (middle), and after the final CSS QDs (right) (scale bars are 20 nm).

[0024] Fig. 3 (b) shows total diameter histograms of the final CSS QDs in Fig. 3(a) with indicated core diameters.

[0025] Fig. 3 (c) shows absorption and PL spectra of the CdSe core with indicated diameters (spectra are offset for clarity).

[0026] Fig. 3 (d) shows absorption and PL spectra of the final CSS QDs, offset according to their corresponding core spectra in Fig. 3(c). Fig. 3 (e) shows core radius dependence of the position (filled black squares) and the full-width-at-half-maximum (filled red circles) of the PL peak of the CSS QDs.

[0027] Fig. 3 (f) illustrates changes in PL QY as a function of the core radius of the CSS QDs.

[0028] Fig. 4(a) shows TEM images after CdTe shell growth (left) and final CdSe shell growth (right) stages. Nominal CdTe shell thicknesses in the number of monolayers (ML) are indicated (scalebars correspond to 50 nm).

[0029] Fig. 4(b) shows diameter histogram at the CdSe / CdTe core / shell stage with indicated nominal thickness in MLs.

[0030] Fig. 4(c) shows absorption and PL spectra of the initial core (bottom-most) and the final CSS QDs of different CdTe shell thickness as indicated.

[0031] Fig. 4(d) shows CdTe shell thickness (plotted as Te reagent amount added during synthesis) dependence of the position (filled black squares) and the linewidth (filled red circles) of the PL peak of CSS QDs.

[0032] Fig. 4(e) shows temporal dependence of PL intensity of CSS QDs with different CdTe shell thickness. The redline corresponds to “0” Te load case.

[0033] Fig. 4(f) shows PL lifetime (1 / e time - filled black squares) and PL QY (filled red circles) of CSS QDs as the Te load is increased. The data points at “0” Te load corresponds to CdSe-only QDs that were synthesized with the same batch of cores without the CdTe shell growth step for d) - f). Figs. 5(a) to 5(f) all show results for varying the final CdSe shell thickness.

[0034] Fig. 5(a) shows TEM images of CSS QDs at the CdSe / CdTe core / shell stage (bottom) and with increasing final CdSe shell thickness as indicated. The scale bars are 20 nm.

[0035] Fig. 5(b) shows diameter histograms of the core, CdSe / CdTe core / shell stage, and CSS QDs of increasing final CdSe shell thickness as indicated.

[0036] Fig. 5(c) shows absorption and PL spectra of the core QDs (bottom-most), core / shell QDs with 3 ML of CdTe (second from the bottom), and CSS QDs with increasing final CdSe shell thickness. The spectra are offset for clarity.

[0037] Fig. 5(d) shows the PL FWHM and centered peak energy of the CSS QD as the CdSe load is increased, showing both a redshift and narrowing of its emission.

[0038] Fig. 5(e) shows the intensity average PL lifetime and PL QY of the material which shows both an increase in lifetime and QY as more CdSe is deposited. Fig. 5(f) shows the intensity average PL lifetime and PL QY of the material which shows both an increase in lifetime and QY as more CdSe is deposited,

[0039] Fig. 6 show comparisons of structural parameter dependence of PL peak position with calculations. The experimental data points correspond to CSS QDs that nominally match the indicated structural parameters.

[0040] Fig. 6(a) shows core radius (rc) dependence at fixed CdTe (si) and CdSe (S2) shell thicknesses. Bottom panel shows the corresponding calculated electron and hole confinement energies. The inset is the hole energy expanded to show details.

[0041] Fig. 6(b) shows comparison of si dependence at fixed rcand total radius with electron and hole energies shown in the bottom panel.

[0042] Fig. 6(c) shows comparison of S2 dependence at fixed rcand si, with electron and hole energies shown in the bottom panel.

[0043] Fig. 7 shows the radiative recombination rate of CSS QDs versus calculated overlap integral. The radiative recombination rates are estimated from experimentally measured PL QY and lifetimes. Fig. 8 defines the dimensions and regions of CSS QD.

[0044] Fig. 9a shows representative powder XRD of core, 1-3 ML CdTe, and the subsequent 5ML of CdSe final shell growth. The asterisks indicate the sample holder signal.

[0045] Fig. 9b shows high magnification HRTEM image of CdSe / CdTe heterostructures.

[0046] Fig. 9c shows the lattice spacing on the selected plane showing twice that of the (0002) spacing. Fig. 9d shows lattice strain for the (0002) face and the (1120) face as a function of deposited monolayers of CdTe on a CdSe core.

[0047] Fig. 10 shows comparison of the CSS QDs after growth to about 6.5 nm with respect to Te loading from Figs. 4(a) to 4 (g).

[0048] Fig. Ila shows the transient PL intensity decay measured at different wavelengths across the PL peak of a “2ML” CSS QD sample shown in Fig. 3(b).

[0049] Fig. 1 lb shows the PL lifetime calculated from 1 / e time decay.

[0050] Figs 12a-12c show simulated data electron hole and overlap from EMA calculations assuming infinite barrier potential and disregarding Coulomb perturbations, a) when rcis increased while si / s2 are constant, b) when Si is increased while the total size and rcis held constant, and c) when S2 in increased while rc / s2 are held constant. Figs. 13a-13b show additional simulated data electron and hole overlap from EMA calculations for a) equally increased rcand S2 in the large QD regime, and b) when only S2 is increased in the large QD regime.

[0051] Figs. 14a-14b shows the affect of temperature of an Se and Te precursor infusion on target peak positions and FWHM modulation.

[0052] Figs. 15a- 15c illustrate morphology of resulting quantum dots based on infusion temperature, and in the case of 15c, possible additional processing, such as annealing.

[0053] Fig. 16 illustrates PL changes caused by annealing, for example, which may enable a pathway to larger QDs across a large spectral range without morphological distortions that can negatively impact QY.

[0054] DEFINITIONS AND ABBREVIATIONS

[0055] Colloidal suspension: A mixture consisting of a disperse phase (the suspended particles) and a continuous phase (the liquid medium of suspension), wherein the mixture either does not settle, or would take a very long time to settle appreciably.

[0056] Dispersibility: The ability of QDs to form a colloidal suspension.

[0057] Emission spectrum: Those portions of the electromagnetic spectrum over which QDs (or a composition containing them) exhibit PL (in response to excitation by a light source) whose amplitude is at least 1% of the peak PL emission.

[0058] Nanoparticle: A nanoscale particle of a solid material. The nanoparticles disclosed herein are preferably crystalline and have a size of less than 500 nanometers in dimension. The nanoparticles disclosed herein may form a colloidal suspension. Embodiments of the disclosed nanoparticles may be of a single material or may include an inner core and one or more shells of differing materials. The nanoparticles may further include a plurality of ligands bound to the nanoparticle outer surface. Exemplary nanoparticles which may be utilized in the compositions, systems and methodologies described herein may comprise metals, metal oxides, metal chalcogenides, semiconductors, and insulators. Nanoparticles may be crystalline (i.e., nanocrystals), amorphous, or mixtures thereof.

[0059] Photoluminescence (PL): The emission of light (electromagnetic radiation, in the form of photons) after the absorption of light. It is one form of luminescence (light emission) and is initiated by photoexcitation (excitation by photons). Polar solvents: A polar solvent is any solvent containing an electric dipole. Exemplary polar solvents include acetone, ethanol, water, ethanol / water mixtures, isopropanol, isopropanol / water mixtures, methanol, methanol / water mixtures, dimethyl sulfoxide, diethyl sulfoxide, tetrahydrofuran, and tetrahydrofuran / water mixtures.

[0060] Polymers (as well as polar polymers): A large molecule, or macromolecule, composed of many repeating subunits. Polymers range from familiar synthetic plastics such as polystyrene or poly (methyl methacrylate) (PMMA), to natural biopolymers such as DNA and proteins that are fundamental to biological structure and function. Polymers, both natural and synthetic, are created via polymerization of many smaller molecules, e.g., monomers. Exemplary polymers include poly (methyl methacrylate) (PMMA), polystyrene, silicones, epoxy resins and the like.

[0061] Polar polymers: Polymers containing only carbon and hydrogen atoms are non-polar polymers. Polar polymers typically contain other atoms such as chlorine, fluorine, oxygen, nitrogen, and sulfur whereby the polymer will contain a permanent electric dipole called a polar polymer. Exemplary polar polymers include poly vinyl alcohol, an ethylene vinyl alcohol copolymer, polyvinyl acetate, polyurethane, polyolefin elastomer, ethylene vinyl acetate, an acrylic polymer, polyvinyl butyral, and polyamides, e g., nylons.

[0062] Quantum Dots: A nanoparticle that exhibits size dependent electronic and optical properties due to quantum confinement. The quantum dots disclosed herein preferably have at least one dimension less than about 50 nanometers. The disclosed quantum dots may be colloidal quantum dots. Some of the quantum dots which may be utilized in the compositions, systems and methodologies described herein are made from a binary semiconductor material having a formula MX where M is a metal and X is typically selected from sulfur, selenium, tellurium, nitrogen, phosphorus, arsenic, antimony, or mixtures thereof. Exemplary binary quantum dots which may be used in the compositions, systems and methodologies described herein include CdS, CdSe, CdTe, PbS, PbSe, PbTe, ZnS, ZnSe, ZnTe, InP, InAs, Cu2S, and In2S3. Other quantum dots which may be utilized in the compositions, systems and methodologies described herein are ternary, quaternary, and / or alloyed quantum dots including, but not limited to, ZnSSe, ZnSeTe, ZnSTe, CdSSe, CdSeTe, HgSSe, HgSeTe, HgSTe, ZnCdS, ZnCdSe, ZnCdTe, ZnHgS, ZnHgSe, ZnHgTe, CdHgS, CdHgSe, CdHgTe, ZnCdSSe, ZnCdSeTe, ZnHgSeTe, ZnHgSSe, CdHgSSe, CdHgSeTe, CuAlS2, CuAlSe2, CuFeSe2, CuFeSe2, CuInS2, CuAlSexS2-x, CuInS2, CuInSe2, CuInGaSe2, CuInZnS2, CuZnSnSe2, CuInSexS2-x, CuInZnSexS2-x, AgInS2, AgInSe2, AgInGaSexS2-x, and Agin SexS2-x where 0 < x < 2. Embodiments of the disclosed quantum dots may be of a single material or may include an inner core and at least one shell of differing materials. The at least one shell may be a thin shell or layer formed by any suitable method, such as cation exchange. The quantum dots further include a plurality of ligands bound to the quantum dot surface.

[0063] Ligand: A ligand is an ion or molecule that binds to another, usually larger, molecule. In general, a ligand bonds to a metal atom, which in the case of the present disclosure, is part of a quantum dot and / or nanoparticle. A ligand may be configured to bind to a particular receptor, interact with various types of matter in prescribed ways, and / or the like. Capping ligands are configured to stabilize the interface where nanoparticles, such as quantum dots, interact with their surrounding medium.

[0064] Solar module: An optical device that converts sunlight into electricity using photovoltaics (PVs) made from at least one semiconductor material. Often these are planar devices with one face facing the sun, but in some cases they are not planar. Solar modules typically include polymers and / or glass elements for encapsulation. Some kinds of solar modules are bifacial, wherein they can convert light from both sides of the device (the sun-facing side and the ground facing side). A solar module may sometimes be referred to as a photovoltaic module. Generally, PV cells in a solar module may be crystalline, semi-crystalline, or amorphous, and they are safely packaged in multiple protective layers including front cover, encapsulate, and back sheet. They may be thin films of less than about five microns or larger layers of greater than about 100 microns. The encapsulate layer is typically a polymeric material to protect against weather, corrosive environment, UV radiation, low mechanical stress, and low energy impacts. Most often polymeric encapsulate materials include ethylene vinyl acetate (EVA), polyurethanes, polyolefin elastomers (POE), or epoxies.

[0065] Solubility: When used in reference to QDs, the ability of QDs to form a clear colloidal suspension without haze caused by formation of aggregates.

[0066] Type II Heterostructures: The introduction of heterostructures, which is the combination of two different semiconductors, has not only allowed enhanced performance and stability (e.g., straddling type I band-offset core / shell heterostructures) but also led to additional desirable effects. In particular, staggered “type II” band-offset heterojunctions can impart spatial separation of electrons and holes with low wavefunction overlap. A type II band offset occurs when both the conduction and valence band levels of a semiconductor are both at higher energy levels than the conduction and valence bands respectively of the attached different semiconductor. This feature of type II QD heterostructures can lead to increased carrier lifetimes that can facilitate carrier extraction, suppression of Auger recombination, modulation of biexciton lifetimes, and Stokes-shifted emission, among several benefits. Core / shell and rod / rod / rod structures consisting of CdS / ZnSe or CdSe / CdTe heterojunctions have been the most frequently explored type II colloidal heterostructures.

[0067] The following explanations of terms and abbreviations are provided to better describe the present disclosure and to guide those of ordinary skill in the art in the practice of systems, methodologies and compositions disclosed herein.

[0068] As used herein, “comprising” means “including”, and the singular form “a” or “an” or “the” include plural references unless the context clearly indicates otherwise. Unless the context clearly indicates otherwise, the term “or” is inclusive, and thus refers to both a single element of stated alternative elements and a combination of two or more of those elements.

[0069] Unless explained otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one or ordinary skill in the art to which this disclosure relates. Suitable methods and compositions are described herein for the practice or testing of the systems, methodologies and compositions described herein. However, it is to be understood that other methods and materials similar, or equivalent to, those described herein may be used in the practice or testing of these systems, methodologies and compositions disclosed herein. Consequently, the systems, methodologies, compositions and examples disclosed herein are illustrative only and are not intended to be limiting. Other features of the present disclosure will be apparent to those skilled in the art from the following detailed description and the appended claims.

[0070] Unless otherwise indicated, all numbers expressing quantities of components, percentages, temperatures, times, and so forth as used in the specification or claims are to be understood as being modified by the term “about”. Unless otherwise indicated, non-numerical properties such as colloidal, continuous, crystalline, and so forth as used in the specification or claims are to be understood as being modified by the term “substantially”, meaning to a great extent or degree. Accordingly, unless otherwise indicated implicitly or explicitly, the numerical parameter and / or non-numerical properties set forth herein are approximations, and the optimal values of these properties and parameters may depend on the desired properties sought, the limits of detection under standard test conditions or methods, the limitations of the processing methods, and / or the nature of the property or parameter. When directly and explicitly distinguishing embodiments from disclosed prior art, the embodiment numbers are not approximations unless the word “about” is recited.

[0071] DETAILED DESCRIPTION

[0072] The presently described invention relates to quantum dots having a core / shell structure where at least components are each selected from among CdS, CdSe, CdTe, alloys, or other semiconductors, in nanoparticle form, typically nanocrystalline form. The nanoparticles may be composed of amorphous, doped, or defective semiconductor materials, or they may be approximately single crystals. In the nanocrystalline form, the cores and any subsequent shell(s) may take on a faceted, non-spherical shape. As used herein, a quantum dot having a core / shell structure is a core quantum dot surrounded by a typically spherical and / or conformal shell. However, the shell may be non-uniform or only cover certain crystal facets, as is the case of a sub monolayer (ML) shell. As used herein, a core / shell / shell (CSS) structure is a core / shell structure that is at least partially further surrounded by another semiconductor shell. For example, certain embodiments provide quantum dots having a core / shell structure including a core quantum dot at least partially surrounded by a first shell. Certain embodiments provide quantum dots having a CSS structure including a core quantum dot surrounded by a first shell which, in turn, is surrounded, at least in part, by a second shell. Synthesis of these heterostructured quantum dots and a use case in solar modules is further described.

[0073] Relatively simple and widely accessible synthesis pathways to an array of tunable properties make colloidal QDs an ideal platform for discovering novel photophysics and developing nextgeneration photonics / electronics. In addition to the well-established wide spectral range of absorption and emission, many characteristics such as charge separation, polarized emission, and multi-exciton processes may be tuned with synthetic control over size and shape. The introduction of heterostructures has not only allowed enhanced performance and stability (e.g., straddling type I band-offset core / shell heterostructures) but also led to additional desirable effects. In particular, staggered type II band-offset heterojunctions can impart spatial separation of electrons and holes with low wavefunction overlap. This feature of type II QD heterostructures can lead to increased carrier lifetimes that can facilitate carrier extraction, suppression of Auger recombination, modulation of biexciton lifetimes, and massively Stokes-shifted emission, among several benefits. Core / shell and rod / rod / rod structures consisting of CdS / ZnSe or CdSe / CdTe heterojunctions are explored type II colloidal heterostructures. Although CdTe / CdSe core / shell QDs have shown promise with respect to deep red to near infrared (NIR) emission, achieving high photoluminescence quantum yields (PL QYs), narrow linewidths, and wide spectral tunability has been challenging. Therefore, the CdTe / CdSe core / shell system has fallen out of favor even for relatively straightforward applications such as light down-conversion.

[0074] Some of these difficulties may stem from starting with the CdTe core. While the inverse structure of CdSe / CdTe core / shell can exploit the exquisite control of size and size distribution established for CdSe QDs, terminating with CdTe shell often leads to stability issues with poor PL characteristics. Hence, most of these core / shell structures consist of CdTe cores. Known routes of synthesizing CdTe QDs lead to the zinc-blende structure with often limited maximum size and faceting, thereby limiting the range of hole confinement energies and potentially limiting CdSe shell growth. Although the bulk heterojunction emission energy can indeed be achieved by CdTe / CdSe core / shell heterostructures, it has been mostly in tetrapod-like structures which are grown from contiguous CdTe domains separated by stacking faults rather than growing concentrically. Such structures can lead to low PL QYs and broad emission line widths. The growth of a CdSe shell on the CdTe core is also challenging due to the large lattice Mismatch of around 7%. In thin film heterostructures, the critical thickness for dislocation formation is only 1.2 nm for CdSe growth on CdTe. That corresponds to only about 4 monolayers of CdSe. Indeed, most reports of CdTe / CdSe core / shell QDs have very thin CdSe shell (~0.5 nm or less) to achieve relatively narrow linewidths and reasonable PL QYs. A thin CdSe shell can also impose the electron wavefunction to spread more into the CdTe core, increasing electron-hole overlap. While the increased overlap may help PL QY, the thin shell will require a large CdTe core to achieve longer wavelength emission in the NIR. Such an approach will necessitate the use of a larger amount of Te, one of the rarest metals / metalloids in the earth’s crust. More fundamentally, the thin shell may limit the low energy end of the emission spectral range to the bulk CdTe band gap due to high electron confinement energy.

[0075] An alternative solution towards developing a highly tunable, narrow emission and high PL QY CdSe and CdTe based QD heterostructures may be through scaffolding CdTe on CdSe QDs then growing a final, passivating CdSe shell, i.e., a CdSe / CdTe / CdSe core / shell / shell (CSS) structure. Similar structures have been achieved for type I or quasi-type II band offset materials. One of the earliest examples has been CdS / HgS / CdS quantum dot quantum wells. More recently, the CdS / CdSe / CdS heterostructure, sometimes referred to as spherical quantum wells, has been shown to exhibit biexciton repulsion and suppressed Auger recombination while preserving high single exciton QY. A transition from type I to type II band offset through increasing Te composition in the first shell has also been shown in the ZnSe / ZnTeSe / ZnSe CSS system which has shown near unity QY and control over gain cross section. This approach of introducing a thin first shell, in some cases less than a single molecular layer, also has the desirable effects of alleviating lattice strain and minimizing the amount of Te used.

[0076] Here, we develop CdSe / CdTe / CdSe CSS QDs with a thin CdTe first shell layer (having a thickness of approximately 1 nm or less) and overall diameter of approximately 6.5 nm or less. We explore their synthesis and how their emissive properties can be tuned through the three structural parameters: 1) the size of the core CdSe QD, 2) the thickness of the first CdTe shell, and 3) the second CdSe shell thickness. The thin first CdTe shell / layer allows for narrow size distribution and therefore narrow linewidths to be maintained while allowing spectral tunability in the deep red / NIR and high PL QYs (e.g., greater than 50%). By synthesizing CSS QDs with systematically varying structure and comparing to effective mass approximation calculations, we also elucidate how each structural parameter affects spectral features, providing guidelines for optimizing optical characteristics for targeted applications.

[0077] One of the desirable features of type II band offset heterostructures is the effective band gap, and therefore the emission energy, that is smaller than the band gap of either component semiconductor alone. While achieving longer wavelength emission through type II core / shell QDs is well established, simultaneously imparting narrow linewidths and high PL QYs needed for many applications has been challenging. For an ensemble of QDs, inhomogeneous broadening due to a size distribution is usually the main cause of line broadening. For the CdTe / CdSe core / shell QDs, starting with CdTe QDs as the core limits size and size distribution and therefore the PL linewidth compared to the prototypical CdSe QDs. The large lattice mismatch between CdTe and CdSe and the resulting lattice strain can also limit achievable size and size uniformity and / or induce undesirable defects, leading to limited control over spectral tunability, PL QY, and linewidth. Our approach to minimize the undesirable size distribution and lattice strain effects exploits high quality wurtzite CdSe QDs as the starting point for sequential epitaxial growth of CdTe and CdSe shells to achieve the CSS structure. A highly monodispersed ensemble of CdSe QDs provides initial narrow emission linewidth. The epitaxial growths of a thin CdTe inner shell and a final CdSe shell are then followed, leading to a CSS structure (Figure la), where the electron and the hole wavefunctions are dictated by band structure and confinement imposed by the size of both CdSe and CdTe domains. This process allows a set of readily variable structural parameters, the core CdSe radius (rc), CdTe first shell thickness (57), and the CdSe second shell thickness (52), to be tuned to control emission wavelength, PL QY, and linewidth. Narrow and intense emission was achieved in the NIR region ranging in wavelength from 677 to 1057 nm (1.83 to 1.17 eV) as shown in Figure 1c by varying these 3 structural parameters.

[0078] The initial CdSe core provides a narrow starting size distribution and therefore facilitates maintaining narrow linewidth. Given the type II band offset between CdSe and CdTe, the core radius (rc) will be one of the main factors in determining the electron confinement energy. The thickness of the CdTe first shell (s ) and its distance from the center of the QD (determined by rc) can be expected to be the most significant in controlling the hole confinement energy. In addition to passivating hole trap states that can often exist on CdTe termination, the CdSe second shell thickness (52) can control the degree of electron delocalization, allowing a systematic variation in electron-hole overlap. With CdTe shell thickness approximately 1 nm or less, the hole confinement energy can be large. The large hole confinement energy can then lead to increased spread of the hole wavefunction into the CdSe regions, providing a complimentary means to control electronhole overlap, which will critically impact the efficiency of radiative recombination. We discuss the effects of each of the 3 structural parameters on the observed optical properties and compare the experimental results to calculations based on the effective mass approximation (EMA) model to provide insights on the observed dependences.

[0079] General Synthesis and Characterization of CSS QDs". The core CdSe QDs of 2.5 nm diameter are synthesized through a minor modification of a previously reported method (of Kalyuzhny and Murray). Larger cores are synthesized through solution homoepitaxy from these small CdSe QDs. After precipitation and redissolution, sequential growth of CdTe and CdSe shells are carried out. Transmission electron microscope (TEM) images of aliquots taken at each step of an example synthesis are shown in Figure 2a. An increase in the average size with only a modest increase in the size distribution can be seen with each step of the shell growth (average diameter of 2.5 nm for the core, 4.1±0.5 nm for the core / shell, and 6.7±0.6 nm for the final CSS QDs for the samples shown in Figure 2a). Powder X-ray diffraction (XRD) patterns after each step of the growth are shown in Supporting Figure 9a. Upon CdTe shell growth, XRD peaks downshift slightly to smaller angles as expected with a larger lattice material growth. Final CdSe shell growth leads to upshift of these peaks, corresponding to a decrease in the lattice constant.

[0080] High-resolution TEM (HRTEM) of the first CdSe / CdTe core / shell also shows wurtzite character as shown in Supporting Figures 9b with some showing (11^0) reflections (inset) and some QDs showing (0001) lattice spacing of compressed wz-CdTe of 0.735nm in Figure 9c. Further HRTEM and energy dispersive x-ray spectroscopy (EDX) have also been carried out to verify the location of CdTe inner shell and the CSS structure. An example of HRTEM image of the CSS QDs along with a Fast Fourier transform (FFT) diffraction pattern of the top central outlined QD is shown in Figure 2b and its inset, respectively. Its inverse FFT image in Figure 2c with false color added identifies the location of the larger lattice CdTe (red) to be approximately at the center of the QD, consistent with the expected CSS structure. Figure 2d shows the filtered inverse FFT of the same QD with the indicated { 11^0} planes offset by an expected 60°. The spacing of (11^0) planes is 0.41 nm around the second shell (cyan) but exhibits a larger value of 0.43 nm near the core where CdTe is (red). This difference corresponds to about a 4% difference and is also corroborated by XRD angle downshift (Figure 9d). We believe this downshift is near isotropic (Figure 9d), although the relative strain downshift for the CdSe / CdTe structure is larger for the (0002) planes. High-resolution EDX elemental maps of Se and Te in Figure 2e further confirm the structure. Te (red) is centrally located with Se-rich shell surrounding it. The corresponding dark field image after the EDX map acquisition is shown in Figure 2f for comparison. We note that there are occasions where EDX maps show CdTe to be slightly off centered as is the case for one of the QDs (top row, second from the right) in Figure 2f.

[0081] The absorption and PL spectra at each growth step of the CSS QDs are shown in figure 2g. Upon CdTe shell growth, a redshift in both absorption edge and PL peak position is observed, along with a pronounced Stokes shift. The PL linewidth broadens, consistent with many reports of core / shell structures of CdSe / CdTe and CdTe / CdSe. The CdSe second shell growth leads to further redshift of the absorption and PL features. Surprisingly, the linewidth narrows significantly upon CdSe second shell growth, to the point where it becomes similar to that of the initial core CdSe QDs. The full-width-at-half-maximum (FWHM) progresses from 140 meV for the core CdSe to 170 meV after CdTe first shell growth and finally to 126 meV after the CdSe second shell growth. The corresponding peak position normalized linewidths (FWHM / peak position in eV) are 0.06 for the core, 0.1 for the core / shell, and 0.08 for the final CSS QDs. This linewidth narrowing upon CdSe second shell growth is accompanied by a drastic increase in PL QY to 88%. Time-resolved PL (TRPL) measurements reveal a single exponential decay with a lifetime of 59 ns for these CSS QDs (Figure 2g inset). The passivation of hole traps on the CdTe termination upon CdSe second shell growth can be expected to improve PL QY. However, only a few to -20% PL QY in CdTe / CdSe core / shell QDs has been reported for deep red / NIR emitting QDs and seldom at narrow linewidths. These core / shell QDs have also been limited to thin CdSe shells due to possible defects and reduced wavefunction overlap further deteriorating PL characteristics. As elaborated further later, the exceptionally high PL QY can be attributed to the thin CdTe layer and relatively small size regime of the CSS QDs developed here where the dimensions of the core and the shells can be used to optimize electron-hole overlap.

[0082] Effects of Core Size To investigate how the structural parameters of the CSS QDs modulate their absorption and emission characteristics, we first consider the effects of the core radius (rc). For this purpose, CSS QDs considered here have the CdTe first shell thickness (sf nominally constant at -0.6 nm, and the CdSe second shell thickness (52) is varied to maintain a fixed nominal total diameter of 6 nm. Figure 3a shows TEM images of 3 different size cores and their corresponding images after CdTe first shell and CdSe second shell growth steps. The range of core size examined here varies from 2.5 nm to 4.6 nm in average diameter. The size histogram of CSS QDs of 3 different rcare shown in Figure 3b, indicating similar total final diameter with slightly larger size distribution for the case with the largest rc. The absorption and PL spectra of the CdSe cores and the final CSS QDs are shown in Figures 3c and 3d, respectively. Despite the strong size-dependent absorption and PL peak positions of the bare core QDs, the absorption feature corresponding to CdSe band edge transition appears at -2 eV for all CSS QDs. This similarity in direct transition energy in CdSe arises from the overall size of the CSS QDs being kept nearly constant with CdSe making up the core and the final shell. However, the absorption tail in the red and the PL peak position both redshift with increasing core size. These spectral features are strongly affected by the hole confinement energy, which is largely dictated by the CdTe first shell. The electron confinement energy is determined by the CdSe regions (e.g., the core and the second shell) and, again, remains nearly constant with the nominally fixed CdTe shell thickness, si, and the total size. The hole energy is determined not only by si but also by the core radius, rc, which determines the volume of the CdTe shell at fixed si and as discussed later, the degree of the hole wavefunction penetration into the CdSe regions at high hole confinement energies.

[0083] The decreasing emission energy with increasing rcis accompanied by the narrowing of the linewidth (Figure 3e). The FWHMs of the final CSS QDs decrease from 0.22 to 0.16 eV with increasing starting core size. The PL peak energy normalized linewidth also shows a similar decreasing trend from 0.14 to 0.12. Given the TEM images and diameter histograms of the final CSS QDs in Figures 3a and 3b, differences in the size distribution are small and are unlikely to be the cause of this reduction in linewidth. In fact, the slight increase in the size distribution at the largest rccase seen in Figure 3b contradicts the notion that the overall size distribution directly determines the linewidth. This observed trend of decreasing FWHM with increasing rcis more likely caused by the decreasing energy level separation. As the degree of confinement is reduced, there is less energy difference across the range of sizes within the ensemble of QDs. The rcdependence of the CSS QDs’ PL QY also shows a decreasing trend with increasing rc(Figure 3f). As the core becomes larger and pushes the CdTe region to the second edge, the electron is more likely to be confined to the core and the hole to the outer perimeter. The decreasing PL QY may then be attributed to this reduction in electron-hole overlap. However, we also note that at fixed total size, the larger core cases have thinner CdSe shells, with the largest case potentially having a sub-population of incomplete shell, and therefore are likely to be more prone to surface-related non-radiative processes reducing PL QY. Note that this effect leads to an opposite trend compared to what has been observed in the CdTe / CdSe core / shell QDs where a thinner CdSe shell leads to higher PL QY because it allows the electron wavefunction to spread more into the core for better electron-hole overlap.

[0084] Effects of CdTe Shell Thickness: The thickness of the CdTe inner shell (si) can have several contributions to the PL characteristics of the CSS QDs. First and foremost, it will determine the degree of confinement of the hole, one of the key contributions to the spectral position of the PL peak. It can also affect the electron-hole overlap. To explore the effects of si, CSS QDs with the same starting core and the final size, approximately 2.5 nm and approximately 6.3 nm in diameter, respectively, with systematically varying si are examined. Figure 4a shows TEM images after the CdTe shell and CdSe second shell growth steps for 4 different si values. The amounts of Te reagent used for the CdTe shell growth correspond nominally to 1 to 4 monolayers (MLs) from bottom to top images, respectively. The CdSe / CdTe core / shell stage (the left column of TEM images labeled “CdTe / CdSe”) clearly shows increasing diameter with increasing amount of Te reagent. The images after the final CdSe shell (the right column of TEM images labeled “CSS”) exhibit nearly identical average size and size distribution, as intended. Figure 4b shows the size histograms of the QDs at the CdSe / CdTe core / shell stage (i.e., before the CdSe second shell growth). Both the size and size distribution of the CdSe / CdTe stage increase with increasing si from diameter of 3.5 ± 0.4 nm for si corresponding to 1 ML to 4.4 ± 0.7 nm for si corresponding to 4 MLs. Wide area STEM-EDX scans of these CSS QDs after the CdSe second shell growth show an increasing Te composition of 0.67, 1.43, 4.96, and 11.91% for the 1 to 4ML CdTe first shell samples with corresponding decrease in Se composition (Figure S2). These measured Te compositions also correlate closely to the expected Te composition based on the amount of Te reagent used (x-axis of Figure S2).

[0085] Figure 4c shows the absorption and PL spectra of the initial core CdSe (lowermost spectra) and the final CSS QDs of increasing si. The redshift of the PL peak position and the absorption features with si is expected due to decreasing degree of hole confinement and is clearly observed. Interestingly, PL line shape is asymmetric at 1ML, and a significant broadening occurs at 2ML. The dependence of PL peak position and FWHM on CdTe thickness is shown in Figure 4d. For the thinnest si, we suspect that many, if not most, QDs may have sub-monolayer of CdTe shell, leading to a significant population of species that may behave like Te-doped CdSe QDs rather than the CSS structure. The asymmetric line shape tailing in the red and broadened linewidth and increased Stokes shift compared to those expected for CdSe QDs are consistent with PL characteristics of reported Te-doped CdSe QDs. Very little absorption tail in the red compared to type II absorption is also consistent with this assignment.

[0086] The unusually broad PL of 2ML case is then likely arising from a combination of Te-doped CdSe and CSS structures. That is, the broad linewidth is due more to two distinct types of QDs rather than the size distribution. In fact, the size distribution at the CdSe / CdTe stage for the 2ML case is significantly narrower than that for the thicker CdTe cases of 3ML and 4ML as shown in Figure 5b. The reason for narrowing of PL width at larger si despite a broadened size distribution may then be due to a lesser degree of hole confinement that leads to confinement energy differences between QDs of varying thickness, si, in the ensemble being smaller. Beyond 2ML thickness, FWHM decreases with 4ML case exhibiting 154 meV width, comparable to 140 meV FWHM of single-composition CdSe QDs of similar size synthesized by seeded growth from the same small CdSe cores (plotted in Figure 4d as zero CdTe shell thickness).

[0087] In addition to PL peak position and FWHM, si has a significant effect on PL lifetime. TRPL measurements (Figure 4e) reveal an increasing PL lifetime with increasing si. Long PL lifetimes greater than 100 ns can be observed at larger si thicknesses (Figure 4f). We use 1 / e time when the PL intensity decays to 1 / e value of the maximum intensity when we refer to the PL lifetime throughout. For the case of 2 ML where an unusually broad PL FWHM is observed, TRPL measured at different emission wavelengths (Supporting Figure Ila) shows that shorter wavelength emission leads to faster decay whereas longer wavelength emission leads to slower exponential decay - again, consistent with the existence of a distribution of CdTe incorporation from Te-doped CdSe QDs to full CdTe shells within the CSS QD ensemble for the 2 ML case. Although the PL lifetime increases with further increasing si, PL QY shows a modest decrease, significantly less so than the rcdependence and maintains relatively high values (>30%) out to emission energy of 1.43 eV (870 nm).

[0088] Effects of CdSe Second Shell Thickness: The emission characteristics are also tunable by varying the thickness of the CdSe second shell ( z), which can help to extend the electron wavefunction across the full radius of the CSS QDs. It can also help to redshift the final PL position to as red as 1.44 eV even for very small CdSe cores (2.5 nm diameter) with thin (~1 nm) CdTe shell. Both the electron confinement energy and the electron-hole overlap will be strongly affected by S2. To examine its effects on the optical properties, aliquots were taken at different stages of CdSe second shell growth step from the same CSS QD synthesis (i.e., from the same CdSe / CdTe core / shell batch with 2.5 nm core and nominally 3MLs of CdTe shell). The systematic increase in the overall diameter can be seen in the series of TEM images (Figure 5a) and the size histogram (Figure 5b). The size distribution exhibits a modest increase upon initial CdTe growth on the CdSe cores but remains similar during the final CdSe second shell growth stage.

[0089] The absorption and PL spectra of the core, CdSe / CdTe first, and the CSS QDs with increasing CdSe second shell thickness are shown in Figure 5c. The absorption tail in the red shows gradual redshift with increasing S2. This redshift can be expected since the increasing CdSe second shell thickness leads to larger net volume of CdSe which will reduce the electron confinement energy. The PL peak position also redshifts and is accompanied by decreasing linewidth (Figure 5d). Since the actual size distribution is not significantly changing (Figure 5b), we again attribute this line narrowing to smaller differences between QDs with respect to their energy level spacing as the average size becomes larger. The results of TRPL measurements (Figure 5e) indicate that the PL lifetime increases slightly with increasing S2, and this trend is shown in Figure 5f along with PL QY, which does not show an obvious dependence on S2. While PL QY can be highly sensitive to a variety of factors, including extrinsic ones that might vary from sample to sample, it remains > 30% for all cases.

[0090] Comparison to Calculations’. To gain further insights into the structural effects on the PL properties of CSS QDs, we now compare our experimental results to calculations. The effective mass approximation used here follows prior reports and assumes spherical symmetry of the CSS QDs and that the wavefunctions go to zero at the surface of the QD. Imposing the boundary conditions that the wavefunctions and their effective-mass-normalized derivatives are continuous at the interfaces, the confinement energies (calculated independently for the electron and the hole), effective band gap (Eg, the sum of bulk type II band gap of CdSe / CdTe plus the lowest electron and hole confinement energies), coefficients for the radial wavefunctions, and the electron-hole overlap integrals were numerically calculated. Details of the calculations, including the parameters used, are given in the Supporting Information.

[0091] Figure 6 shows the rcdependence of the calculated energies with fixed si (0.6 nm) and S2 (0.8 nm). In the top panel of Figure 6a, the calculated Eg (offset by a constant Eo) is compared to the measured PL peak positions of CSS QD samples with nominally corresponding dimensions. We note that the infinite potential at the surface leads to overestimation of confinement energies and therefore of Eg, especially at the small sizes considered here. Neglecting Coulomb energy and comparing Egto PL peak position, which can have significant Stokes shift even in the singlecomposition QD, will lead to further overestimation. While all these factors will depend on the QD size, the size range studied here is quite small, and we believe the constant offset (Eo) provides a simple and sufficient means to understand the trends observed. Indeed, the rcdependence of PL peak position in Figure 6a is well captured by the calculated values of Eg - Eo. Figure 6a bottom panel shows the confinement energies of the electron and the hole, revealing that the observed dependence on rcarises mainly from the electron confinement energy.

[0092] Interestingly, the hole confinement energy initially decreases with rcbut exhibits a small rise at larger rc(Figure 14a bottom panel inset). The former behavior is expected as the CdTe shell is pushed out further with rcat fixed thickness, but the latter effect is somewhat unexpected. In this thin CdTe shell thickness regime, the large hole confinement energy leads to a significant spread of the hole wavefunction into the core as seen in the radial distribution functions shown for three different rcvalues in Figure 12a. As rcincreases, hole wavefunction spreads more into the core CdSe region. The higher potential for the hole in the core can then increase its energy.

[0093] The comparison of the electron and hole radial probability distribution for different rcin Figure 12a also reveals that increasing rccan lead to reduced overlap between the two oppositely charged carriers. The key effect is that electron probability increases in the core while decreasing in the CdSe second shell. The electron being localized more in the core, while its confinement energy being reduced by increasing core size, is accompanied by the hole wavefunction that shifts out with increasing rc. This increasing degree of charge separation may then help to explain the decreasing PL QY with rcobserved in Figure 3f.

[0094] The CdTe shell thickness dependence is shown in Figure 6b, where si is varied while rcand S2 are held constant. Once again, the calculated Eg, offset by Eoto account for the overestimate of the calculation due to the factors described above, predicts the redshift of the PL peak position with si very well. Although the dependence of Egon si (Figure 6b top panel) appears similar to its dependence on rc(Figure 6a top panel), the reason behind it is quite different. In this case, it is the hole confinement energy rather than the electron confinement energy that dominates the behavior. As expected, increasing; leads to larger CdTe volume and lowers the hole energy. CdSe volume, on the other hand, decreases, leading to an increase in the electron confinement energy. The radial probability distribution of electrons and holes shown in Figure 12b shows that, in the case of extremely thin CdTe shell, the hole wavefunction essentially spreads fully across the CSS QD, leading to a very high electron-hole overlap. It also reveals that there is a lesser degree of charge separation across the CdTe thickness range examined (Figure 12b) compared to the rcvariation case (Figure 12a), which may help to explain the smaller decrease in the PL QY with increasing si than with increasing rc.

[0095] The dependence on the CdSe second shell thickness also reveals several interesting effects. The calculated Eg, offset by Eo, agrees with the measured PL peak positions (Figure 6c top panel). While the decreasing Egbeing dominated by the decreasing electron confinement energy is expected since only the CdSe volume is increasing, there is some noticeable decreasing trend in the hole confinement energy especially at the very small S2 values despite rcand si being held constant (Figure 6c bottom panel). This effect arises largely from the infinite potential imposed at the surface of the CSS QD and manifests as a sharper drop in the hole probability distribution near the surface at the thinnest CdSe second shell case, leading to a more asymmetric hole distribution (Figure 12c bottom panel). As the CdSe second shell becomes thicker, this artifact of the infinite potential at the surface is alleviated. Nevertheless, the net effect on Egis minimal since electron confinement plays the main role in determining the dependence of Egon S2. An important aspect to note here is that, as S increases to thickness similar to rc, the spread of the electron wavefunction across the CdTe layer to the CdSe second shell maintains or improves the electron-hole overlap (Figure 12c). The PL QY remaining roughly similar with increasing si in Figure 5f is consistent with this observation.

[0096] Finally, to verify the importance of electron-hole overlap on the observed PL properties, we compare radiative recombination rate (kr) to the overlap integral. Kr is estimated from measured PL lifetime (To) and PL QY as r = PL QY / TOKrfor CSS QDs versus calculated electron-hole overlap integral is shown in Figure 7. The outlying data points (open red squares) are CSS QDs with approximately monolayer of CdSe second shell thickness. These CSS QDs are prone to significant deviations such as incomplete final shells and more dominant surface non-radiative recombination processes that lead to the nonconforming behavior. Despite different structural parameters, rest of the CSS QDs exhibit linear dependence of kron the overlap integral.

[0097] These results immediately point to engineering CSS structures such that electron-hole overlap can be maximized for enhancing PL (in the absence of perfectly passivated surface). In the core / shell structure, either the core or the shell must be kept at very small sizes to achieve large overlap. However, this restriction severely limits the band gap energies that can be accessed. With the CSS structure, on the other hand, there are several different pathways to achieve desired emission wavelengths while achieving large electron-hole overlap. Extended calculations on rcand si dependences of Egand overlap integral are shown in Figure 13, where the CdSe regions (core and the second shell) are kept at the same size (i.e., rc= S2). Maintaining similar rcand S2 values may be the most straightforward way to spread the electron wavefunction across the CdTe region, which can in turn lead to large overlap integrals. One approach to do so and to extend the emission wavelength range is to increase rcand S2 equally (Figure 13a). This approach relies on modulation of the electron confinement energy and can achieve a significant range of Egwhile maintaining large overlap integral. However, it does require fairly large rc(~3 nm) if small si thickness and low emission energy are desired. An overall diameter >10 nm may be needed for NIR purposes, which may not be the most desirable and practical with respect to synthesis. Slightly increasing si thickness to 2 nm while maintaining similar but small rcand S2 sizes (1.3 nm for the cases shown in Figure 13b) can allow similar range of Eg and very large overlap integral within routinely achievable II-VI QD sizes. Even at our thin (~1 nm) CdTe thickness and small maximum overall diameter (~6.5 nm), we have been able to achieve CSS QDs with emission wavelength at 900 nm (1.38 eV) and PL QY of 53% following this principle.

[0098] Example Applications-. These quantum dots can be used in various applications. A particularly useful application is to incorporate core / shell and / or CSS quantum dots by blending them into the encapsulation layer of certain solar modules. Blending of these CSS quantum dots into the encapsulation layer of solar modules, it can be possible to achieve a higher degree of performance from solar modules, especially showing tunability in their peak emission, with a narrow FWHMs, and with high PL QYs greater than 60%.

[0099] Unique features of the QDs of the present invention include various combinations of cores and shells, including, CdSe core / CdTe shell / CdSe second shell, CdTe core / CdSe shell / CdTe second shell, CdS core / CdSe shell / CdTe second shell, and various other combinations. Ultrathin shells, perhaps below a single monolayer may start to look more like interfacial atoms or dopants by some descriptions, and controlling these compositions is key to the narrow FWHM and high PL QY. Further, the present invention involves combining QDs of this kind into solar modules, particularly for UV or visible to NIR conversion. The final devices could be silicon substrates, organic photovoltaics, perovskite, or CdTe (cadtel) based solar modules, typically by compounding these QDs into the encapsulation layer.

[0100] One approach towards developing a highly tunable, narrow emission and high PL QY CdSe and CdTe based QD heterostructures may be through scaffolding CdTe on CdSe QDs then growing a final, passivating CdSe shell, i.e., a CdSe / CdTe / CdSe core / shell / shell (CSS) structure.

[0101] The total thickness / diameter of the CSS structures can vary typically from about 4 nm to about 12 nm, most usually from about 6 nm to about 7.5 nm. Control of the layer thicknesses allows for narrow size distribution and therefore narrow linewidths to be maintained while allowing spectral tunability in the deep red / NIR and high PL QYs.

[0102] The size / diameter of the initial core typically varies from about 2.4 nm to about 4.5 nm, most preferably from about 2.4 nm to about 3 nm. The first shell layer is typically thin and ranges from about 0.6 nm to about 1.2 nm. This first shell layer may sometimes be as thin as a few monolayers, thus only about 1 nm in thickness. The second shell layer may typically be thicker than the first shell layer and can usually range in thickness from about 0.6 nm to about 4 Nm, most usually from about 0.9 nm to 1.8 nm.

[0103] According to an aspect of the present disclosure, quantum dots are provided. In an example embodiment, a quantum dot includes a core of a first semiconductor material, a first shell layer of a second semiconductor material upon the core, and a second shell layer upon the first shell layer and the core. The second shell layer is of a semiconductor material characterized as a different material than the first shell layer, wherein the first shell layer is characterized as having a type II band offset from either the core or the second shell semiconductor material or both, and the quantum dot is characterized as having a total size of 30 nm or less in diameter.

[0104] In an example embodiment, the quantum dot is further characterized as having peak spectral emissions in the range of from about 600 nm to about 1240 nm, or from about 800 nm to 900 nm, with narrow FWHMs of about less than about 200 meV, or less than 100 nm, and photoluminescence quantum yields of at least 50%, and up to about 90%.

[0105] In an example embodiment, the core of a first semiconductor material is selected from the group consisting of CdS, CdSe, and CdTe, said first shell layer of a different semiconductor material upon the core is selected from the group consisting of CdS, CdSe, and CdTe, and said second shell layer upon the first shell layer wherein the second shell layer is of a different semiconductor material than the first shell layer and is selected from the group consisting of CdS, CdSe, and CdTe.

[0106] In an example embodiment, the first shell layer is characterized as having a thickness of less than 2 nanometers or in some cases less than a monolayer of material. In an example embodiment, the first shell layer is characterized as having a thickness of less than 2 nanometers or in some cases less than a monolayer of material and the quantum dot is further characterized as having peak spectral emissions in the range of from about 600 nm to about 1240 nm, or from about 800 nm to 900 nm, with narrow FWHMs of about less than about 200 meV, or less than 100 nm, and photoluminescence quantum yields of at least 50%, and up to about 90%. In an example embodiment, the core is CdSe, the first shell layer is CdTe and the second shell layer is CdSe.

[0107] In an example embodiment, the core is CdSe, the first shell layer is CdTe and the second shell layer is CdSe and the quantum dot is further characterized as having peak spectral emissions in the range of from about 600 nm to about 1240 nm, or from about 800 nm to 900 nm, with narrow FWHMs of about less than about 200 meV, or less than 100 nm, and photoluminescence quantum yields of at least 50%, and up to about 90%.

[0108] In an example embodiment, the core is CdS, the first shell layer is CdTe and the second shell layer is CdSe.

[0109] In an example embodiment, the core is CdSe, the first shell layer is CdTe and the second shell layer is CdS.

[0110] In an example embodiment, the core is CdTe, the first shell layer is CdSe and the second shell layer is CdS.

[0111] In an example embodiment, the quantum dot further includes a ZnS shell

[0112] In an example embodiment, the quantum dot further comprises organic ligands.

[0113] In an example embodiment, the emission of light includes photons with less energy than the band gap, including any quantum confinement or strain effects, of any individual constituent materials, whether a core or a shell. In an example embodiment, at least one of said semiconductor materials is an alloyed composition composed of combinations of sulfide, selenide, and telluride, either as a composition gradient or homogeneous composition.

[0114] According to another aspect of the present di sclosure, a solar module is provided. In an example embodiment, a solar module includes a polymeric encapsulation layer around the solar module including a polymer selected from the group consisting of ethylene vinyl acetate, polyolefin elastomers, polyurethane, and epoxy and quantum dots comprising a core of a first semiconductor material and a shell layer of a second semiconductor material upon the core wherein the shell layer is of a different material than the core, wherein the shell layer is characterized as having a type II band offset from the core semiconductor material in the bulk form.

[0115] In an example embodiment, the quantum dots are further characterized as having peak spectral emissions in the range of from about 600 nm to about 1240 nm, or from about 800 nm to 900 nm, with narrow FWHMs of about less than about 200 meV, or less than 100 nm, and photoluminescence quantum yields of at least. 50%, and up to about 90%

[0116] In an example embodiment, the first semiconductor material is selected from the group consisting of CdS, CdSe, CdTe, and alloys thereof, and the second semiconductor material is a different semiconductor material than the first semiconductor material, and the second semiconductor material is selected from the group consisting of CdS, CdSe, CdTe, and alloys thereof.

[0117] In an example embodiment, the first semiconductor material is selected from the group consisting of CdS, CdSe, CdTe, and alloys thereof, and the second semiconductor material is a different semiconductor material than the first semiconductor material, and the second semiconductor material is selected from the group consisting of CdS, CdSe, CdTe, and alloys thereof and, the quantum dots are further characterized as having peak spectral emissions in the range of from about 600 nm to about 1240 nm, or from about 800 nm to 900 nm, with narrow FWHMs of about less than about 200 meV, or less than 100 nm, and photoluminescence quantum yields of at least 50%, and up to about 90%. In an example embodiment, the core is CdSe and the shell layer is CdTe.

[0118] In an example embodiment, the core is CdSe and the shell layer is CdTe and the quantum dots are further characterized as having peak spectral emissions in the range of from about 600 nm to about 1240 nm, or from about 800 nm to 900 nm, with narrow FWHMs of about less than about 200 meV, or less than 100 nm, and photoluminescence quantum yields of at. least 50%, and up to about 90%.

[0119] In an example embodiment, the solar cell includes semiconductors selected from the group consisting of CdS, CdSe, CdTe, Silicon, CuInS₂., and CuInSe₂.

[0120] In an example embodiment the quantum dots further comprise a second shell of a second shell material that is different from the second semiconductor material.

[0121] In an example embodiment, the second shell material is selected from the group consisting of CdS, CdSe, and CdTe.

[0122] According to another aspect, a bifacial solar module is provided In an example embodiment, a bifacial solar module includes fluorescent quantum dots that comprise a core of a first semiconductor material and a shell layer of a second semiconductor material upon the core wherein the shell layer is of a different material than the core, wherein the shell layer is characterized as having a type II band offset from the core semiconductor material

[0123] In an example embodiment, the quantum dots are further characterized as having peak spectral emissions in the range of from about 600 nm to about 1240 nm, or from about 800 nm to 900 nm, with narrow FWHMs of about less than about 200 meV, or less than 100 nm, and photoluminescence quantum yields of at least 50%, and up to about 90%.

[0124] In an example embodiment, the quantum dots are comprised of a core of a material selected from the group consisting of CdS, CdSe, and CdTe, and a shell layer upon the core wherein the first shell layer is of a different material than the core, and the shell layer is selected from the group consisting of CdS, CdSe, and CdTe. According to another aspect, a method of synthesizing quantum dots having a core / shell / shell structure with the core is provided. In an example embodiment, each shell is formed of a material selected from the group consisting of CdS, CdSe, and CdTe.

[0125] In various embodiments, a method of making a quantum dot includes synthesizing a first semiconductor nanocrystal with a diameter of 2 to 6 nm, adding first precursors to grow a shell of a second semiconductor material around the first semiconductor material with a thickness of 0.5 to 4 monolayers, and adding second precursors to grow a shell of a third semiconductor material around the first and second semiconductor material with a thickness of 0.5 to 4 monolayers. The first and said second semiconductors form a type II band offset and the second and third semiconductors also form a type II band offset.

[0126] In some embodiments, the first semiconductor and third semiconductor are the same composition For example, the first semiconductor may be selected from the group consi sting of CdS and CdSe, the second semiconductor may be CdTe, and, the third semiconductor may be selected from the group consisting of CdS and CdSe. In another example, the first semiconductor may be CdTe, the second semiconductor may be selected from the group consisting of CdS and CdSe, and, the third semiconductor may be CdTe.

[0127] In some embodiments, the method further includes causing growth of a fourth semiconductor grown as an additional shell on top of the third semiconductor that forms a type I band offset with the third semiconductor. For example, third precursors may be added to grow the fourth semiconductor as an additional shell around the third semiconductor. The total nanocrystal diameter is less than 20 nm, in an example embodiment.

[0128] In some embodiments, addition of the first, second, and / or third precursors is performed slowly As used herein, adding a precursor slowly indicates that a fraction of the total precursor amount to be added is added at a time (e.g., at a time scale a few seconds to a minute) rather the adding the total precursor amount to be added in one or two injection events For example, addition of anionic precursors of the first second, and / or third precursors may be performed over a period of at least

[0129] 1 five minutes In some embodiments, the addition of the anionic precursors of the first second, and / or third precursors may be performed over a period of at least five minutes such that the addition of the anionic precursors is substantially uniform over time (e.g., on a minute by minute or 30 second by 30 second time scale)

[0130] In some embodiments, an amount of first, second, and / or third precursors added for the second, third semiconductor, and / or fourth semiconductor is determined by using the lattice constant of the respective semiconductor and the corresponding volume increase for a single mono-layer of the respective semiconductor and estimating the number of first semiconductor nanocrystals in the mixture from the concentration calculated based at least in part on an optical density of the mixture.

[0131] In some embodiments, a chalcogenide and / or an amine are added and the mixture is heated to a temperature above 200 °C. In various embodiments the chalcogenide is selected from the group consisting of sulfur, selenium, and tellurium.

[0132] EXAMPLES

[0133] The following examples are non-limiting and are merely intended to further illustrate the compositions, systems and methodologies described herein.

[0134] Materials: CdO (99%), trioctylphosphine (TOP, 97%), trioctylphosphine oxide (TOPO, 99%), oleic acid (OA, 90% technical grade), 1 -octadecene (ODE, 99%), paraffin oil, oleylamine (Oam, 70% technical grade), diphenylphosphine (DPP, 98%), Te (99.99%), acetonitrile (99%, Anhydrous) were obtained from Sigma Aldrich. N-octadecylphosphonic acid (ODPA, 98%) was obtained from SEQENS. Se (shot pellets, 99.999% trace metals grade) was obtained from Lesker. Ethanol (ACS Reagent Grade), methanol (ACS Reagent Grade), toluene (ACS Reagent Grade) were obtained from Fischer Scientific. All chemicals were used as received.

[0135] EXAMPLE 1

[0136] Synthesis of Small CdSe Core QDs: Small CdSe QDs (~2.5 nm diameter) were synthesized by modifying a previously described method by Owen et al., J. Am. Chem. Soc., 2010, 132(51), pp.

[0137] 18206-18213. Briefly, 128.8 mg (1 mmol) CdO, 0.56 g ODPA, and 6 g TOPO were loaded into a 25-ml round bottom flask. The reaction mixture was degassed under vacuum at 150 °C for 30 min, then heated to 380 °C under Ar. After the reaction mixture became a clear solution, 3 ml of TOP was injected and heated to 385 °C. Meanwhile, 150 ml ofDPP and 1.5 ml of 2 M Se in TOP were mixed in a glovebox, loaded into a syringe, brought out of the glovebox, and immediately injected into the reaction mixture at 385 °C. After ~10 s, the reaction mixture was cooled to 130 °C within 1 min with external air jet. The product was purified by centrifugation at 3800 rpm for 5 min after adding equal volumes of toluene and ethanol. The supernatant was removed, and the remaining solid was redissolved in hexanes and centrifuged for an additional 5 min at 3800 rpm. To the supernatant, ethanol was added to precipitate the QDs and centrifugation at 3800 rpm for 5 min was carried out. The QDs were then redissolved in hexanes and the precipitation / centrifugation cycle was repeated once more. The final resulting pellet was redissolved in a measured volume of hexanes and the concentration was determined from optical density at the first absorption peak using previously reported extinction coefficients. The small CdSe QDs in hexanes were then precipitated once more, centrifuged, dried under vacuum, and redissolved in 6 ml TOP in the glovebox.

[0138] EXAMPLE 2

[0139] Synthesis of Larger CdSe QDs-. A stock solution of 0.5 M Cd-OA was prepared by degassing a mixture 20 mmol of CdO, 20 ml of OA, and 20 ml of ODE at 120 °C for 30 min under vacuum, heating to 250°C under Ar, and stirring at 250°C for 60 min. After cooling, the Cd-OA stock solution was stored in a glovebox. To successively enlarge the CdSe QDs, 2.4 ml of Cd-OA stock solution was diluted with 4.8 ml OA, 4.8 ml ODE, and 6 ml of TOP in the glovebox. The desired quantity of the resulting 0.067 M Cd-OA was then mixed with an equimolar quantity of 2 M Se in TOP and kept in the glovebox until needed. Meanwhile, 4 ml l-ODE was added to a 25-ml round bottom flask and degassed at 120 °C under vacuum for 30 min, back filled with Ar, and heated. Once the temperature reached 300 °C, a desired quantity of CdSe seed QDs dissolved in TOP was then swiftly injected. The slow injection of solution containing TOPSe and Cd-OA immediately followed and completed over a 30-min period. After all the reagents were added the reaction mixture was cooled to room temperature by air jet and purified by dissolving in equal volume of toluene, precipitating with excess ethanol, and centrifuging. The resulting pellet was successively redissolved in chloroform, precipitated with equal volumes of methanol and acetonitrile, and centrifuged before being redissolved into TOP.

[0140] EXAMPLE 3

[0141] Synthesis of a QD of the structure CdSe / CdTe / CdSe is as follows.

[0142] In a 25-ml round bottom flask, 42.3 mg (0.33 mmol) CdO, 3.1 ml OA, 2 ml Oam, and 2 ml ODE were loaded and degassed under vacuum for 60 min at 110 °C before being heated to 250 °C under Ar. Once a clear solution of Cd-OA was obtained, the reaction mixture was cooled to 230 °C and 60 nmol of seed CdSe QDs in TOP were injected into the flask. Then, 0.025 M TOPTe (0.5 mmol Te dissolved in 20 ml of TOP at 100 °C) was injected dropwise into the reaction mixture over 1 h to grow CdTe shells on the CdSe cores. The amount of TOPTe solution added varied depending on the thickness of the CdTe shell desired and the starting core size. The amount of TOPTe needed for each ML of CdTe was determined by approximating each ML as a spherical shell with an increase in radius of 0.32 nm, determining the number of unit cells of CdTe needed for the corresponding volume increase, and estimating the number of CdSe seed QDs in the reaction mixture from the concentration calculated by optical density. For the CdSe second shell, the reaction continued in the same reaction flask that the CdSe / CdTe core / shell QDs were synthesized in. The reaction mixture was immediately heated to 250 °C after CdTe shell growth, and 0.1 M TOPSe was injected dropwise over 40 min. The amount of TOPSe needed for each ML of CdSe second shell growth was determined in the same manner as described for CdTe shells but using the unit cell size of CdSe and an increase in radius of 0.3 nm. The reaction mixture was then allowed to stir for an additional 10 min, cooled to room temperature under Ar, and diluted with toluene. The resulting product was either stored in reaction mixture or purified as was performed on the enlarged CdSe QDs.

[0143] Characterization of the prepared QDs was as follow: Unless otherwise noted, UV-Vis absorption spectra were measured in a 3 ml quartz cuvette (1 cm pathlength) in an Agilent HP 8453 spectrometer with 100 ml of raw reaction mixture diluted to 3 ml with toluene. Static PL spectra and relative PL QYs were measured on a HORIBA Nanolog Fluorescence Spectrometer. Final samples were also assayed with absolute PL QY via a 400 nm Ti: Sapphire laser routed into an integrating sphere incident on an Edison monochromator and was used as a reference for relative PL QY for other QD samples. TRPL was conducted using the HORIBA Nanolog Spectrofluorometer with a 405 nm pulsed laser at 1600 ns period. High resolution bright-field transmission electron microscopy and selected area electron diffraction were carried out on a JEOL 2100F TEM operating at 200 kV. STEM-HAADF and STEM-EDS were obtained on a FEI Talos F200X TEM operating at 200 kV with a 20 keV dispersion. Powder x-ray diffraction was measured on a Bruker D8 Diffractometer with a knife edge on a stainless steel XRD holder with purified and drop-cast samples prepared in an N2-filled glovebox.

[0144] EXAMPLE 4

[0145] Synthesis of a QD of the structure CdTe / CdSe / CdTe is similar to example 3, where the chalcogen precursors are varied (sulfur, selenium, and tellurium).

[0146] EXAMPLE 5

[0147] Synthesis of a QD of the structure CdS / CdSe / CdTe is similar to example 3, where the chalcogen precursors are varied (sulfur, selenium, and tellurium).

[0148] EXAMPLE 6

[0149] Alloyed semiconductors offer further tunability of bandgap and band alignment. For example, alloys of CdSe and CdTe can have a smaller bandgap than CdSe and CdTe owing to bandgap bowing. The minimal bandgap composition is roughly CdSeo.sTeo7 (30% selenide, 70% telluride) according to literature, which has a bulk band gap of around 1.40eV, or 0. leV smaller than CdTe (https: / / doi.org / 10.1080 / 14686996.2018.1497403). The bulk ‘type II band gap’ for CdSe / CdTe (energy separation between CdSe conduction band and CdTe valence band) experimentally seems to be around 1.1-1.2 eV, which puts the lower limit of the alloyed type II bandgap to around 1.0-1.1 eV, or 1240 nm at the reddest. This implies that with a CdSe / CdSeo Teo.? heterostructure, the emission wavelength could be as red as about 1240 nm.

[0150] EXAMPLE 7 A solar module including hetrostructured nanocrystals that emit sub-bandgap emission is made by incorporating the nanoparticles into the encapsulation layer or layers by blending them into a polymer. In some cases these are the same QDs as those prepared in example 3. In some embodiments the nanoparticles emit light with a peak emission at around 830 nm to 900 nm or out to beyond 1000 nm. The presence of these nanoparticles improves the power conversion efficiency of the solar module, which may further include a polymeric encapsulation layer around the solar module including a polymer selected from the group consisting of ethylene vinyl acetate (EVA), polyurethanes, polyolefin elastomers (POE), and epoxies, and QDs comprising a core of a first semiconductor material and a shell layer of a second semiconductor material upon the core wherein the shell layer is of a different material than the core, wherein the shell layer is characterized as having a type II band offset from the core semiconductor material.

[0151] The solar module may contain QDs further characterized as having peak spectral emissions in the range of from about 600 nm to about 1100 nm, or from about 800 nm to 900 nm, with narrow FWHMs of about less than about 200 meV, or less than 100 nm, and photoluminescence quantum yields of at least 50%, and up to about 90%. In some cases the QDs are comprised of a core of a material selected from the group consisting of CdS, CdSe, and CdTe, and at least one shell layer upon the core wherein the shell layer is of a different material than the core, and the shell layer is selected from the group consisting of CdS, CdSe, and CdTe. The QDs may have a core that is CdSe and a shell layer that is CdTe, for example. The solar module would typically include solar cells made from semiconductors selected from the group consisting of CdS, CdSe, CdTe, Silicon, CuInS2, and CuInSe2. The solar module may be a bifacial module further including QDs on the rear side (not facing the sun) comprising a core of a first semiconductor material and a shell layer of a second semiconductor material upon the core wherein the shell layer is of a different material than the core, wherein the shell layer is characterized as having a type II band offset from the core semiconductor material.

[0152] In some embodiments, the solar module may contain QDs having a core / shell structure forming a type II band offset, where the shell comprises a different semiconductor material from the core material. In some embodiments, the solar module may contain QDs having a CSS structure where the first shell comprises a material (e.g., semiconductor material) that is different from the core material and from the second shell material. The core material and the second shell material may be the same or different. In some embodiments, each of the core material, first shell material, and second shell material are selected from the group consisting of CdS, CdSe, CdTe, and alloys thereof.

[0153] EXAMPLE 8:

[0154] An example of a synthesis of a QD with 135-145meV (approximately 75-85 nm) FWHM of a target emission of 850 nm QD is as follows:

[0155] In a 25-ml round bottom flask, CdO, OA, OAm, l-ODE were loaded and degassed under vacuum for 60 min at 110 °C before being heated to Temperature 1 under Ar. Once a clear solution of Cd-OA was obtained, the reaction mixture was then cooled to Temperature 1 and a quantity of seed CdSe QDs as synthesized in manner like Example 2 in TOP were injected into the flask. Then TOPTe was added to grow a 2.5 ML CdTe shell on the CdSe cores. The quantity of TOPTe precursor was chosen by approximating each ML as a spherical shell with an increase in radius of 0.32 nm (lattice constant) and determining the number of unit cells of CdTe needed for the corresponding volume increase and estimating the number of CdSe seed QDs in the reaction mixture as determined by the optical density of the first exciton peak. After precursor addition was complete, the vessel was immediately heated to Temperature 2 after CdTe shell growth, and a quantity of TOPSe was added to grow 3 ML of CdSe. The concentration was pre-determined in a similar manner to the TOPTe precursor but with an increase in radius of 0.3 nm per ML and the unit cell value of CdSe instead of CdTe. The reactor was then cooled to room temperature. The QDs were isolated in powder form for further use by the addition of excess ethanol and centrifugation and redissolution in toluene twice.

[0156] EXAMPLE 9:

[0157] An example of a synthesis of a QD with 100-120meV (approximately 35-45 nm) FWHM with a target emission of 680 nm is as follows: Two stock solutions of Cd-OA and TOPTe or Cd-OA and TOPSe were prepared in advance by the heating of CdO, OA, 1-ODE under vacuum for 1 h followed by heating the solution to above 200 C under Ar flow. After the solution is cooled under Ar flow, where a quantity of 2M TOPSe or IM TOPTe as prepared previously by the dissolution of Se in TOP or Te in TOP, respectively. The precursors quantity added is such that cations are in excess to the anions. Additional TOP was added to improve solubility. The precursor was kept under inert atmosphere at room temperature.

[0158] In a round bottom flask, 1-ODE was loaded into and degassed at 110 C for 30 mins. The solution was then heated under Ar flow to Temperature 3, where a quantity of a CdSe core solution as synthesized in Example 2 at a diameter of 2-3 nm in TOP was added swiftly. Immediately after core addition, the Cd-OA and TOPTe stock solution was dropwise injected equivalent to 2 ML of CdTe. The quantity of Cd-OA and TOPTe precursor was pre-determined by approximating each ML as a spherical shell with an increase in radius of 0.32 nm and determining the number of unit cells of CdTe needed for the corresponding volume increase and estimating the number of CdSe seed QDs in the reaction mixture as determined by the optical density of the first exciton peak. Immediately after core addition, the Cd-OA and TOPSe stock solution was dropwise injected over to grow an alloy of CdSexTe1-xequivalent to 2 ML of CdSe. Immediately after the addition of Cd-OA and TOPSe, the flask was cooled to room temperature. The QDs were isolated in powder form for further use by the addition of excess ethanol and centrifugation and redissolution in toluene twice.

[0159] EXAMPLE 10:

[0160] An example of a synthesis of a QD with 140-220meV (approximately 115 - 175 nm) FWHM with a target emission of 1050 nm is as follows:

[0161] In a 25-ml round bottom flask, CdO, OA, OAm, l-ODE were loaded and degassed under vacuum for 60 min at 110 °C before being heated to 250 °C under Ar. Once a clear solution of Cd-OA was obtained, the reaction mixture was then cooled to Temperature 1 and a quantity of seed CdSe QDs as synthesized in Example 2 at a diameter of 4-5 nm in TOP were injected into the flask. Then TOPTe was added to grow a 3 ML CdTe shell on the CdSe cores. The quantity of TOPTe precursor was pre-determined by approximating each ML as a spherical shell with an increase in radius of 0.32 nm and determining the number of unit cells of CdTe needed for the corresponding volume increase and estimating the number of CdSe seed QDs in the reaction mixture as determined by the optical density of the first exciton peak. After precursor addition was complete, the vessel was immediately heated to Temperature 2 after CdTe shell growth, and a quantity of TOPSe was added to grow 3 ML of CdSe. The concentration was pre-determined in a similar manner to the TOPTe precursor but with an increase in radius of 0.3 nm per ML and the unit cell volume of CdSe instead of CdTe. The reactor was then cooled to room temperature. The QDs were isolated in powder form for further use by the addition of excess ethanol and centrifugation and redissolution in toluene twice.

[0162] EXAMPLE 11:

[0163] Synthesis of a QD towards the target wavelength and target linewidth as controlled by the temperature of precursor addition is possible between 150 - 300 °C can be conducted in a manner similar to Example 3 with modifications to the temperature of precursor addition. An example synthesis which targets a peak wavelength of 840 - 850nm target linewidth of 130meV by the variation of synthetic conditions. Figure 14a shows the minor variation across target peak positions with temperature of Se and Te precursor infusion while Figure 14b shows the FWHM modulation with respect to temperature of the Se and Te precursor infusion. Temperature changes lead to wavelength changes between 750 and 900 nm.

[0164] EXAMPLE 12:

[0165] Morphological change to improve QY and wavelength reproducibility to transform tetrapodal CdSe / CdTe or CdTe / CdTe / CdSe was performed as follows:

[0166] In a round bottom flask, CdO, OA, OAm, 1-ODE were loaded and degassed under vacuum for 60 min at 110C before being heated to Temperature 1 under Ar. Once a clear solution of Cd-OA was obtained, the reaction mixture was then cooled to Temperature 1 and a quantity of seed CdSe QDs as synthesized in Example 2 at a diameter of 2-3 nm in TOP were injected into the flask. Then TOPTe was added to grow a 2.5 ML CdTe shell on the CdSe cores. The quantity of TOPTe precursor was pre-determined by approximating each ML as a spherical shell with an increase in radius of 0.32 nm and determining the number of unit cells of CdTe needed for the corresponding volume increase and estimating the number of CdSe seed QDs in the reaction mixture as determined by the optical density of the first exciton peak. After precursor addition was complete, the vessel was immediately cooled to Temperature 3, and a quantity of TOPSe was added to grow 3 ML of CdSe. The concentration was pre-determined in a similar manner to the TOPTe precursor but with an increase in radius of 0.3 nm per ML and the unit cell volume of CdSe instead of CdTe. Then, the temperature was rapidly increased back to Temperature 2, up to 300 C. The reactor was then cooled to room temperature. The QDs were isolated in powder form for further use by the addition of excess ethanol and centrifugation and redissolution in toluene twice. Figure 15a, shows infusion of precursor at low temperatures showing tetrapodal growth, Figure 15b anneals said tetrapods at elevated temperatures to grow a homogeneous shell and Figure 15c shows another cycle of infusion at low temperature and high temperature annealing. The process leads to PL changes shown in Figure 16 and enables a pathway to larger QDs across a large spectral range without morphological distortions that can negatively impact QY.

[0167] Although the present invention has been described with reference to specific details, it is not intended that such details should be regarded as limitations upon the scope of the invention. Additional information regarding certain embodiments is provided in the attached appendix. Various modifications, substitutions, combinations, and ranges of parameters may be made or utilized in the compositions, and methodologies described herein.

Claims

CLAIMSThat which is claimed:Claim 1: A quantum dot comprising a core of a first semiconductor material, a first shell layer of a second semiconductor material upon the core, and -a second shell layer upon the first shell layer and the core wherein the second shell layer is of a semiconductor material characterized as a different material than the first shell layer, wherein the first shell layer is characterized as having a type II band offset from either the core or the second shell semiconductor material or both, and the quantum dot is characterized as having a total size of 30 nm or less in diameter.Claim 2. The quantum dot of claim 1 further characterized as having peak spectral emissions in the range of from about 600 nm to about 1240 nm, or from about 800 nm to 900 nm, with narrow FWHMs of about less than about 200 meV, or less than 100 nm, and photoluminescence quantum yields of at least 50%, and up to about 90%.Claim 3: The quantum dot of claim 1 wherein said core of a first semiconductor material is selected from the group consisting of CdS, CdSe, and CdTe, said first shell layer of a different semiconductor material upon the core is selected from the group consisting of CdS, CdSe, and CdTe, and said second shell layer upon the first shell layer wherein the second shell layer is of a different semiconductor material than the first shell layer and is selected from the group consisting ofCdS, CdSe, and CdTe.Claim 4: The quantum dot of claim 1 wherein the first shell layer is characterized as having a thickness of less than 2 nanometers or in some cases less than a monolayer of material.Claim 5: The quantum dot of claim 1 wherein the core is CdSe, the first shell layer is CdTe and the second shell layer is CdSe.Claim 6: The quantum dot of claim 1 wherein the core is CdS, the first shell layer is CdTe and the second shell layer is CdSe.Claim 7: The quantum dot of claim 1 wherein the core is CdSe, the first shell layer is CdTe and the second shell layer is CdS.Claim 8: The quantum dot of claim 1 wherein the core is CdTe, the first shell layer is CdSe and the second shell layer is CdS.Claim 9: The quantum dot of claim 1 wherein said quantum dot further comprises a ZnS shell.Claim 10: The quantum dot of claim 1 wherein said quantum dot further comprises organic ligandsClaim 11: The quantum dot of claim 1 wherein the emission of light includes photons with less energy than the band gap, including any quantum confinement or strain effects, of any individual constituent materials, whether a core or a shell.Claim 12: The quantum dot of claim 1 wherein at least one of said semiconductor materials is an alloyed composition composed of combinations of sulfide, selenide, and telluride, either as a composition gradient or homogeneous composition.Claim 13: A method of making a quantum dot, comprising:synthesizing a first semiconductor nanocrystal with a diameter of 2 to 6 nm,adding precursors to grow a shell of a second semiconductor material around said first semiconductor material with a thickness of 0.5 to 4 monolayers, andadding precursors to grow a shell of a third semiconductor material around said first semiconductor material with a thickness of 0.5 to 4 monolayers, and wherein,said first and said second semiconductors form a type II band offset, and wherein,said second and said third semiconductors also form a type II band offset, and wherein a total diameter of the quantum dot is less than 20 nm.Claim 14. The method of claim 13 wherein said first semiconductor and said third semiconductor are the same composition.Claim 15. The method of claim 13 wherein:said first semiconductor is selected from the group consisting of CdS and CdSe, said second semiconductor is CdTe, and,said third semiconductor is selected from the group consisting of CdS and CdSe.Claim 16. The method of claim 13 wherein:said first semiconductor is CdTe,said second semiconductor is selected from the group consisting of CdS and CdSe, and, said third semiconductor is CdTe.Claim 17. The method of claim 13 further comprising:causing growth of a fourth semiconductor grown as an additional shell on top of the third semiconductor that forms a type I band offset with the third semiconductorClaim 18. The method of claim 13 further comprising:slow addition of the anionic precursors over a period of at least five minutes.Claim 19 The method of claim 13 further comprising:adding an amount of precursors for said second or said third semiconductor that was determined by using the lattice constant of said semiconductor and the corresponding volume increase for a single mono-layer, and,estimating the number of first semiconductor nanocry stals in the mixture from the concentration calculated by optical density.Claim 20. The method of claim 13 further comprising:adding a chalcogenide selected from the group consisting of sulfur, selenium, and tellurium, and,adding an amine, and,heating to a temperature above 200 °C.

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