This work explored possibilities to obtain colloidal quantum dots (QDs) with ideal photoluminescence (PL) properties, i.e., monoexponential PL decay dynamics, unity PL quantum yield, ensemble PL spectrum identical to that at the single-dot level, single-dot PL nonblinking, and antibleaching. Using CdSe/CdS core/shell QDs as the model system, shell-epitaxy, ligand exchange, and shape conversion of the core/shell QDs were studied systematically to establish a strategy for reproducibly synthesizing QDs with the targeted properties. The key synthetic parameter during epitaxy was application of entropic ligands, i.e., mixed carboxylate ligands with different hydrocarbon chain length and/or structure. Well-controlled epitaxial shells with certain thickness (∼3–8 monolayers of the CdS shells) were found to be necessary to reach ideal photoluminescence properties, and the size of the core QDs was found to play a critical role in determining both photophysical and photochemical properties of the core/shell QDs. Effects of shape of the core QDs were unnoticeable, and shape of the core/shell QDs only affected photophysical properties quantitatively. Surface ligands, amines versus carboxylates, were important for photochemical properties (antiblinking and antibleaching) but barely affected photophysical properties as long as entropic ligands (mixed carboxylate ligands with distinguishable hydrocarbon chain lengths) were applied during epitaxy. Chemical environment (in polymer or in air), coupled with surface ligands, determined photochemical properties of the core/shell QDs with a given core size and shell thickness.

Colloidal quantum dots are promising optical and optoelectronic materials for various applications, whose performance is dominated by their excited-state properties. This article illustrates synthetic control of their excited states. Description of the excited states of quantum-dot emitters can be centered around exciton. We shall discuss that, different from conventional molecular emitters, ground-state structures of quantum dots are not necessarily correlated with their excited states. Synthetic control of exciton behavior heavily relies on convenient and affordable monitoring tools. For synthetic development of ideal optical and optoelectronic emitters, the key process is decay of band-edge excitons, which renders transient photoluminescence as important monitoring tool. On the basis of extensive synthetic developments in the past 20–30 years, synthetic control of exciton behavior implies surface engineering of quantum dots, including surface cation/anion stoichiometry, organic ligands, inorganic epitaxial shells, etc. For phosphors based on quantum dots doped with transition metal ions, concentration and location of the dopant ions within a nanocrystal lattice are found to be as important as control of the surface states in order to obtain bright dopant emission with monoexponential yet tunable photoluminescence decay dynamics.

Formation of CdSe nanocrystals with two-dimensional quantum confinement (CdSe 2D nanocrystals) was studied with preformed CdSe nanocrystals in the size range between 1.7 and 2.2 nm as seeds. Specifically, the 2D CdSe nanocrystals were encased with six {100} facets of the zinc-blende (face-center-cubic) structure, that is, 1.5 nm in thickness with quite large atomically flat {100} basal planes (∼8 nm width and X ≈ 45 nm length). Symmetry breaking between the thickness and lateral directions occurred in the early stage by rapid formation of single-dot intermediates with flat yet polar {100} basal planes and the desired thickness from the seeds through intraparticle ripening. Two single-dot intermediates fused together through their reactive side facets—mostly the nonpolar {110} ones—to form 2D embryos with the same thickness. Such oriented attachment continued selectively onto the reactive side facets of the 2D embryos. Simultaneously, intraparticle ripening occurred slowly on the side facets of the 2D nanocrystals, which converted unstable side facets gradually to four stable {100} ones. When ∼3 stable {100} side facets were developed, oriented attachment would continue on the remaining active one, which would result in the second symmetry breaking between two lateral directions. Cadmium acetate assisted both formation of single-dot intermediates and oriented attachment. Cadmium alkanoates with a long hydrocarbon chain selectively stabilized polar {100} facets on the nanocrystals including single-dot intermediates and shuttled insoluble acetate to the reactive surface of the nanocrystals.

A one-pot/three-step synthetic scheme was developed for phase-pure epitaxy of CdS shells on zinc-blende CdSe nanocrystals to yield shells with up to sixteen monolayers. The key parameters for the epitaxy were identified, including the core nanocrystal concentration, solvent type/composition, quality of the core nanocrystals, epitaxial growth temperature, type/concentration of ligands, and composition of the precursors. Most of these key parameters were not influential when the synthetic goal was thin-shell CdSe/CdS core/shell nanocrystals. The finalized synthetic scheme was reproducible at an almost quantitative level in terms of the crystal structure, shell thickness, and optical properties.

Against general wisdom in crystallization, the nucleation of InP and III-V quantum dots (QDs) often dominates their growth. Systematic studies on InP QDs identified the key reason for this: the dense and tight alkanoate-ligand shell around each nanocrystal. Different strategies were explored to enable necessary ligand dynamics—i.e., ligands rapidly switching between being bonded to and detached from a nanocrystal upon thermal agitation—on nanocrystals to simultaneously retain colloidal stability and allow appreciable growth. Among all the surface-activation reagents tested, 2,4-diketones (such as acetylacetone) allowed the full growth of InP QDs with indium alkanoates and trimethylsilylphosphine as precursors. While small fatty acids (such as acetic acid) were partially active, common neutral ligands (such as fatty amines, organophosphines, and phosphine oxides) showed limited activation effects. The existing amine-based synthesis of InP QDs was activated by acetic acid formed in situ. Surface activation with common precursors enabled the growth of InP QDs with a distinguishable absorption peak between ~450 and 650 nm at mild temperatures (140–180 °C). Furthermore, surface activation was generally applicable for InAs and III-V based core/shell QDs.

CdSe–stearates nanocrystal–ligands complex as a whole possess strongly temperature- and size-dependent yet well-defined solubility in small organic solvents, which shows little solvent effects as long as the complexes remained intact. A quantitative thermodynamic model is developed to describe such solubility behavior, which differs fundamentally from conventional models for micron colloids. The model reveals that the conformation entropy of the n-alkanoate chain released in dissolution greatly stabilize the colloidal solution but the strong chain–chain interdigitation between adjacent particles in solid diminishes the solubility. These understandings result in “entropic ligands” (see full disclosure in another report (10.1021/acs.nanolett.6b00730)) as the universal means to battle processability challenges of colloidal nanocrystals.

Solution processability of nanocrystals coated with a stable monolayer of organic ligands (nanocrystal–ligands complexes) is the starting point for their applications, which is commonly measured by their solubility in media. A model described in the other report (10.1021/acs.nanolett.6b00737) reveals that instead of offering steric barrier between inorganic cores, it is the rotation/bending entropy of the C–C σ bonds within typical organic ligands that exponentially enhances solubility of the complexes in solution. Dramatic ligand chain-length effects on the solubility of CdSe-n-alkanoates complexes shall further reveal the power of the model. Subsequently, “entropic ligands” are introduced to maximize the intramolecular entropic effects, which increases solubility of various nanocrystals by 102–106. Entropic ligands can further offer means to greatly improve performance of nanocrystals-based electronic and optoelectronic devices.

Single-molecular spectroscopy reveals that photoluminescence (PL) of a single quantum dot blinks, randomly switching between bright and dim/dark states under constant photoexcitation, and quantum dots photobleach readily. These facts cast great doubts on potential applications of these promising emitters. After ∼20 years of efforts, synthesis of nonblinking quantum dots is still challenging, with nonblinking quantum dots only available in red-emitting window. Here we report synthesis of nonblinking quantum dots covering most part of the visible window using a new synthetic strategy, i.e., confining the excited-state wave functions of the core/shell quantum dots within the core quantum dot and its inner shells (≤ ∼5 monolayers). For the red-emitting ones, the new synthetic strategy yields nonblinking quantum dots with small sizes (∼8 nm in diameter) and improved nonblinking properties. These new nonblinking quantum dots are found to be antibleaching. Results further imply that the PL blinking and photobleaching of quantum dots are likely related to each other.

CdSe magic-size clusters with close-shell surface and fixed molecular formula are well-known in the size range between ∼1 and 3 nm. By applying high concentration of cadmium alkanoates as ligands, a conventional synthetic system for CdSe nanocrystals was tuned to discriminate completion from initiation of atomic flat facets. This resulted in ∼4–13 nm CdSe nanocrystals with hexahedral shape terminated with low-index facets, namely three (100), one (110), and two (111) facets. These low-symmetry (Cs group with single mirror plane) yet monodisperse hexahedra were found to be persistent not only in a broad size range but also under typical synthetic temperatures for growth of both CdSe and CdS. Atomic motion on the surface of the nanocrystals under enhanced ligand dynamics initiated intraparticle ripening without activating interparticle ripening, which converted the hexahedral nanocrystals to monodisperse spherical ones. This new synthetic strategy rendered optimal color purity of photoluminescence (PL) of the CdSe and CdSe/CdS core/shell nanocrystals, with the ensemble PL peak width comparable with that of a corresponding single dot.

Electronic traps at the inorganic–organic interface of colloidal quantum dots (QDs) are detrimental to their luminescent properties. Several types of interface traps were identified for single-crystalline CdSe/CdS core/shell QDs, which were all found to be extrinsic to either the core/shell structure or their optical performance. The electron traps—presumably excess or unpassivated Cd surface sites—are shallow ones and could be readily isolated from the electron wave function of the excitons with more than ∼2 monolayers of CdS shell. There were two identifiable deep hole traps within the bandgap of the QDs, i.e., the surface adsorbed H2S and unpassivated surface S sites. The surface adsorbed H2S could be removed by either degassing processes or photochemical decomposition of H2S without damaging the QDs. The unpassivated surface S sites could be removed by surface treatment with cadmium carboxylates. Understanding of the surface traps enabled establishment of new phosphine-free synthetic schemes for either single-precursor or successive-ion-layer-adsorption-and-reaction approach, which yielded CdSe/CdS core/shell QDs with near-unity photoluminescence quantum yield and monoexponential photoluminescence decay dynamics with 2–10 monolayers of CdS shell.

Understanding photoluminescence (PL) intermittency of single quantum dots (QDs) (intensity blinking by randomly switching between distinguishable brightness states under continuous excitation) has been a long-standing fundamental challenge and potential roadblock for their applications. Here we introduce a new analysis method for single-molecule spectroscopy that treats the blinking as photochemical/chemical processes (switching between neutral/bright and charged/dim states). It uncovers the channels for charging (bright to dim) and discharging (dim to bright) involved in PL blinking of single CdSe/CdS core/shell QDs. Both charging and discharging of the single CdSe/CdS core/shell QD possess a photochemical channel (∼10–5 to 10–6 events/photon) that linearly depends on excitation in both single- and multi-exciton regime. These two linear channels coupled to a spontaneous discharging channel (∼2 events/s) to dictate the QDs from nonblinking to gradually blinking under increasing excitation. For high-quality CdSe/CdS core/shell QDs, Auger ionization of multiexciton for both charging and discharging is negligible.

Transition metal doped semiconductor nanocrystals (d-dots) possess fundamentally different emission properties upon photo- or electroexcitation, which render them as unique emitters for special applications. However, in comparison with intrinsic semiconductor nanocrystals, the potential of d-dots has been barely realized, because many of their unique emission properties mostly rely on precise control of their photoluminescence (PL) decay dynamics. Results in this work revealed that it would be possible to obtain bright d-dots with nearly single-exponential PL decay dynamics. By tuning the number of Mn2+ ions per dot from ∼500 to 20 in Mn2+ doped ZnSe nanocrystals (Mn:ZnSe d-dots), the single-exponential PL decay lifetime was continuously tuned from ∼50 to 1000 μs. A synthetic scheme was further developed for uniform and epitaxial growth of thick ZnS shell, ∼7 monolayers. The resulting Mn:ZnSe/ZnS core/shell d-dots were found to be essential for necessary environmental durability of the PL properties, both steady-state and transient ones, for the d-dot emitters. These characteristics combined with intense absorption and high PL quantum yields (70 ± 5%) enabled greatly simplified schemes for various applications of PL lifetime multiplexing using Mn:ZnSe/ZnS core/shell d-dots.

Discrete nature of thickness and flat basal planes of two-dimensional (2D) nanostructures display unique diffraction features. Their origin was uncovered by a new analysis method of powder X-ray diffraction, which reveals thickness and lattice orientation of the 2D nanostructures. Results indicate necessity of adoption of a different unit cell from the corresponding bulk crystal with the same internal atomic packing. For CdSe 2D nanostructures with zinc blende atomic packing, pseudotetragonal lattices are adequate, instead of face-centered cubic.

A systematic and reproducible method was developed to study the decay dynamics of an exciton, a photogenerated electron–hole pair, in semiconductor nanocrystals in solution. Results revealed that the excitons in plain core CdSe nanocrystals in either zinc-blende or wurtzite or mixed lattice structures could be reproducibly prepared to decay radiatively in unity quantum yield and in single channel. The single-channel lifetime was found to increase monotonically by increasing size of the CdSe nanocrystals, with zinc-blende ones increasing in a relatively slow pace. Surface inorganic stoichiometry was found to be a sensitive parameter to affect the exciton decay dynamics for all crystal structures with different sizes. Excess Se (Cd) sites on the surface were found to induce short (long) lifetime channels for the excitons. Both types of traps reduced the quantum yield of the radiative decay of the excitons, and the hole traps associated with Se sites were nearly not emissive. With optimal surface inorganic stoichiometry, primary amines were identified as “ideal” organic ligands for CdSe core nanocrystals to achieve unity radiative decay of excitons in single channel in comparison to other types of neutral ligands commonly applied in the field.

Pt/Fe3O4 core/shell triangular nanoprisms were synthesized using seed-mediated heteroepitaxy. Their well-defined shape, facets, and ordered-assembly allowed detailed analysis of mechanism of the heteroepitaxy. At the Pt–Fe3O4 interface, existence of both lattice and chemical mismatch resulted in facet-selective epitaxy along ⟨111⟩ directions of two lattices. X-ray absorption fine structure measurements demonstrated that the Pt seed nanocrystals were composed of an iron-rich Pt–Fe metallic thin layer sandwiched between the Pt core and a Fe–O outer-surface. The Fe–O outer-surface of the seed nanocrystals presumably offered epitaxial sites for the following deposition of the Fe3O4 shell. Each tip and side of a triangular nanoprism respectively possessed a groove and a ridge, and a (111) plane parallel to the basal planes linked all grooves and ridges. This interesting (111) plane approximately bisected the triangle nanoprisms and located near the Pt-seed. The outer surface of the hybrid nanocrystals was also found to be facet-selective, that is, solely {111} facets of Fe3O4 lattice. These polar {111} facets allowed the surface to be only occupied with high-density iron ions, and thus offered best surface coordination for the electron donating ligands in the solution.Keywords: core/shell; facet-selectivity; heteroepitaxy; interface; nanoprism ; surface;

For the most studied nanocrystal system in the literature, experimental results in this paper revealed that formation of either zinc blende or wurtzite CdSe nanocrystals was dominated by the ligand–surface interaction, instead of the interior structure difference. This conclusion was considered to be reasonable, given the very small energy difference between wurtzite and zinc blende CdSe (only 1.4 meV per CdSe unit and about 1000 times smaller than the energy of a single cadmium–ligand bond). Cadmium carboxylate ligands in the form of cadmium fatty acid salts promoted formation of the zinc blende structure. Conversely, cadmium phosphonate ligands with a long hydrocarbon chain favored the formation of the wurtzite structure. The effects of either cadmium carboxylate or cadmium phosphonate ligands were found to play a determining role during both nucleation and growth. Different from our expectation, fatty amine was found to be only a secondary factor for crystal structure determination. With an appropriate choice of capping ligands, it was possible to achieve precise control of the crystal structure of the CdSe nanocrystals in both nucleation and growth for either the zinc blende or wurtzite structure.

Here we report the first series of phase-pure zinc-blende CdSe/CdS core/shell quantum dots (QDs) with reproducibly controlled shell thickness (4–16 monolayers), which are nonblinking (≥95% ‘on’ time) in single-exciton regime for the entire series. These unique QDs possess well-controlled yet simple excited-state decay dynamics at both single-dot and ensemble levels, extremely small nonblinking volume threshold, if any, and unique ‘on’ and ‘off’ probability statistics. The outstanding optical properties of the QDs at the single-dot level were found to be correlated well with their ensemble properties. These small and bright nonblinking QDs offer promising technical application prospect in both single-dot and ensemble levels. The consistent and reproducible experimental results shed new light on the mechanisms of blinking of QDs.

Nearly monodisperse zinc-blende CdSe/CdS core/shell nanocrystals were synthesized by epitaxial growth of 1–6 monolayers of CdS shell onto presynthesized zinc-blende CdSe core nanocrystals in one pot. To retain the zinc-blende structure, the reaction temperature was lowered to the 100–140 °C range by using cadmium diethyldithiocarbamate as a single-source precursor and primary amine as activation reagents for the precursor. Although the wurtzite counterparts grown under the same conditions showed optical properties similar to those reported in the literature, zinc-blende CdSe/CdS core/shell nanocrystals demonstrated surprisingly different optical properties, with ensemble single-exponential photoluminescence decay, significant decrease of photoluminescence peak width by the shell growth, and comparatively high photoluminescence quantum yields. The lifetime for the single-exponential ensemble photoluminescence decay of zinc-blende CdSe/CdS core/shell nanocrystals with 3–4 monolayers of CdS shell was reproducibly found to be approximately 16.5 ± 1.0 ns.

Size/shape-controlled colloidal CdSe quantum disks with zinc-blende (cubic) crystal structure were synthesized using air-stable and generic starting materials. The colloidal CdSe quantum disks were approximately square, and their lateral dimensions were varied between 20 and 100 nm with the thickness controlled between 1 and 3 nm, which resulted in sharp and blue-shifted UV−vis and PL peaks due to one-dimensional quantum confinement. The quantum disks were grown with either ⟨001⟩ or ⟨111⟩ direction, polar directions in the single crystalline disks, as the short axis, and both basal planes were terminated with Cd ions. These surface Cd ions were passivated with negatively charged fatty acid ligands to neutralize the net positive charges caused by the excess monolayer of Cd ions. The coordination of the Cd ions and carboxylate groups further enabled the close-packing monolayer of fatty acid ligands on each basal plane. The close packing of the hydrocarbon chains of fatty acids dictated the up temperature limit for synthesis of the colloidal quantum disks, and the low temperature limit was found to be related to the reactivity of the starting materials. Overall, a high Cd to Se precursor ratio, negative-charged fatty acid ligands with a long hydrocarbon chain, and a proper temperature range (approximately between 140 and 250 °C) were found to be needed for successful synthesis of the colloidal CdSe quantum disks.

Formation of CdS nanocrystals in the classic approach (with octadecene (ODE) as the solvent and elemental sulfur and cadmium carboxylate as the precursors) was found to be kinetically dependent on reduction of elemental sulfur by ODE, which possessed a critical temperature (∼180 °C). After elemental sulfur was activated by ODE, the formation reaction of CdS followed closely. 2-tetradecylthiophene from the activation of S by ODE and fatty acids from the formation reaction of CdS were found to be the only soluble side products. The overall reaction stoichiometry further suggested that oxidation of each ODE molecule generated two molecules of H2S, which in turn reacted with two molecules of cadmium carboxylate molecules to yield two CdS molecular units and four molecules of fatty acids. In comparison to alkanes, octadecene was found to be substantially more active as a reductant for elemental sulfur. To the best of our knowledge, this is the first example of quantitative correlation between chemical reactions and formation of high-quality nanocrystals under synthetic conditions. To demonstrate the importance of such discovery, we designed two independent and simplified synthetic approaches for synthesis of CdS nanocrystals. One approach with its reaction temperature at the critical temperature of S activation (180 °C) used the same reactant composition as the classic approach but without any hot injection. The other approach performed at an ordinary laboratory temperature (≤100 °C) and in a common organic solvent (toluene) was achieved by addition of fatty amine as activation reagent of elemental sulfur.

Colloidal nanocrystals with “artificial” composition and electron band structure promise to expand the fields of nanomaterials and inorganic chemistry. Despite their promise as functional materials, the fundamental science associated with the synthesis, characterization, and properties of colloidal nanocrystals is still in its infancy and deserves systematic study. Furthermore, such studies are important for our basic understanding of crystallization, surface science, and solid state chemistry. “Band gap and composition engineering on a nanocrystal” (BCEN) refers to the synthesis of a colloidal nanocrystal with composition and/or electron energy band structure that are not found in natural bulk crystals. The BCEN nanostructure shown in the Figure includes a magnetic domain for the separation and recycling of the complex nanostructure, a photoactivated catalytic center, and an additional chemical catalytic center. A thin but porous film (such as a silicate) might be coated onto the nanocrystal, both to provide chemical stability and to isolate the reaction processes from the bulk solution. This example is a catalytic complex analogous to an enzyme that facilitates two sequential reactions in a microenvironment different from bulk solution. The synthesis of colloidal nanocrystals has advanced by a quantum leap in the past two decades. The field now seems ready to extend colloidal nanocrystal synthesis into the BCEN regime. Although BCEN is a very new branch of synthetic chemistry, this Account describes advances in related synthetic and characterization techniques that can serve as a useful starting point for this new area of investigation. To put these ideas into context, this Account compares this new field with organic synthesis, the most developed branch in synthetic chemistry. The structural and functional diversity of organic compounds results from extending design and synthesis beyond the construction of natural organic compounds. If this idea also holds true for inorganic nanocrystals, “artificial” BCEN nanocrystals will most likely outperform the inorganic nanocrystals with naturally occurring structure and composition. If the importance of artificial molecules is a positive lesson from organic synthesis, the practical disadvantage of organic chemistry is that purification can prove much more time consuming than the reaction itself. To get around this problem, colloidal nanocrystal chemists can attempt to avoid these potential purification challenges in the early stages of synthetic method development.