Ligand-controlled polytypism of thick-shell CdSe/CdS nanocrystals

Polytypic metal chalcogenide nanocrystals

By engineering chemically identical but structurally distinct materials into intricate and sophisticated polytypic nanostructures, which often surpass their pure phase objects and even produce novel physical and chemical properties, exciting applications in the fields of photovoltaics, electronics and photocatalysis can be achieved. In recent decades, various methods have been developed for synthesizing a library of polytypic nanocrystals encompassing IV, III-V and II-VI polytypic semiconductors. The exceptional performances of polytypic metal chalcogenide nanocrystals have been observed, making them highly promising candidates for applications in photonics and electronics. However, achieving high-precision control over the morphology, composition, crystal structure, size, homojunctions, and periodicity of polytypic metal chalcogenide nanostructures remains a significant synthetic challenge. This review article offers a comprehensive overview of recent progress in the synthesis and control of polytypic metal chalcogenide nanocrystals using colloidal synthetic strategies. Starting from a concise introduction on the crystal structures of metal chalcogenides, the subsequent discussion delves into the colloidal synthesis of polytypic metal chalcogenide nanocrystals, followed by an in-depth exploration of the key factors governing polytypic structure construction. Subsequently, we provide comprehensive insights into the physical properties of polytypic metal chalcogenide nanocrystals, which exhibit strong correlations with their applications. Thereafter, we emphasize the significance of polytypic nanostructures in various applications, such as photovoltaics, photocatalysis, transistors, thermoelectrics, stress sensors, and the electrocatalytic hydrogen evolution. Finally, we present a summary of the recent advancements in this research field and provide insightful perspectives on the forthcoming challenges, opportunities, and future research directions.

Heavy-Metal-Free Colloidal Quantum Dots: Progress and Opportunities in Solar Technologies

Colloidal quantum dots (QDs) hold great promise as building blocks in solar technologies owing to their remarkable photostability and adjustable properties through the rationale involving size, atomic composition of core and shell, shapes, and surface states. However, most high-performing QDs in solar conversion contain hazardous metal elements, including Cd and Pb, posing significant environmental risks. Here, a comprehensive review of heavy-metal-free colloidal QDs for solar technologies, including photovoltaic (PV) devices, solar-to-chemical fuel conversion, and luminescent solar concentrators (LSCs), is presented. Emerging synthetic strategies to optimize the optical properties by tuning the energy band structure and manipulating charge dynamics within the QDs and at the QDs/charge acceptors interfaces, are analyzed. A comparative analysis of different synthetic methods is provided, structure-property relationships in these materials are discussed, and they are correlated with the performance of solar devices. This work is concluded with an outlook on challenges and opportunities for future work, including machine learning-based design, sustainable synthesis, and new surface/interface engineering.

Controlling Phase in Colloidal Synthesis

A fundamental precept of chemistry is that properties are manifestations of the elements present and their arrangement in space. Controlling the arrangement of atoms in nanocrystals is not well understood in nanocrystal synthesis, especially in the transition metal chalcogenides and pnictides, which have rich phase spaces. This Perspective will cover some of the recent advances and current challenges. The perspective includes introductions to challenges particular to chalcogenide and pnictide chemistry, the often-convoluted roles of bond dissociation energies and mechanisms by which precursors break down, using very organized methods to map the synthetic phase space, a discussion of polytype control, and challenges in characterization, especially for solving novel structures on the nanoscale and time-resolved studies.

Proton Shuttle-Assisted Surface Reconstruction toward Nonpolar Facets-Terminated Zinc-Blende CdSe/CdS Core/Shell Quantum Dots

Surface reconstruction can rearrange the surface atoms of a crystal without the need of growth processes and has the potential to synthesize crystals with novel morphologies and facets that cannot be obtained through regular synthesis. However, little is known about the molecular mechanisms of the surface reconstruction process. Here, utilizing surface reconstruction, we report the synthesis of nonpolar facets (110) facets)-terminated dodecahedral zinc-blende CdSe/CdS core/shell quantum dots. The morphology transformation is achieved by first fully exchanging the cadmium carboxylate ligand with oleylamine and then undergoing surface reconstruction. The surface reconstruction-induced morphology transformation is confirmed by transmission electron microscopy and absorption spectroscopy. Details of kinetic experiments and simulation results demonstrated that successful surface reconstruction must be assisted by a proton shuttle. Except for the first report on zinc-blende quantum dots terminated with (110) facets, the surface reconstruction aided by the proton shuttle offers valuable insights for devising methods to regulate the properties of nanocrystals.

Covalent inorganic complexes enabled zinc blende to wurtzite phase changes in CdSe nanoplatelets

Phase changes in colloidal semiconductor nanocrystals (NCs) are essential in material design and device applications. However, the transition pathways have yet to be sufficiently studied, and a better understanding of the underlying mechanisms is needed. In this work, a complete ligand-assisted phase transition from zinc blende (ZB) to wurtzite (WZ) is observed in CdSe nanoplatelets (NPLs). By monitoring with in situ absorption spectra along with electrospray ionization mass spectrometry (ESI-MS), we demonstrated that the transition process is a ligand-assisted covalent inorganic complex (CIC)-mediated phase transition pathway, which involves three steps, ligand exchange on ZB CdSe NPLs (Step 1), dissolution of NPLs to form CICs (Step 2), and conversion of CdSe-CIC assemblies to WZ CdSe NPLs (Step 3). In particular, CICs can be directly anisotropically grown to WZ CdSe NPL without other intermediates, following pseudo-first-order kinetics (kobs = 9.17 × 10-5 s-1). Furthermore, we demonstrated that CICs are also present and play an essential role in the phase transition of ZnS NPLs from WZ to ZB structure. This study proposes a new crystal transformation pathway and elucidates a general phase-transition mechanism, facilitating precise functional nanomaterial design.

Role of carboxylates in the phase determination of metal sulfide nanoparticles

Techniques are well established for the control of nanoparticle shape and size in colloidal synthesis, but very little is understood about precursor interactions and their effects on the resultant crystalline phase. Here we show that oleate, a surface stabilizing ligand that is ubiquitous in nanocrystal synthesis, plays a large role in the mechanism of phase selection of various metal sulfide nanoparticles when thiourea is used as the sulfur source. Gas and solid-phase FTIR, 13C, and 1H NMR studies revealed that oleate and thiourea interact to produce oleamide which promotes the isomeric shift of thiourea into ammonium thiocyanate, a less reactive sulfur reagent. Because of these sulfur sequestering reactions, sulfur deficient and metastable nanoparticles are produced, a trend seen across four different metals: copper, iron, nickel, and cobalt. At low carboxylate concentrations, powder XRD indicated that the following phases formed: covellite (CuS); vaesite (NiS2); smythite (FeS1.3), greigite (FeS1.3), marcasite (FeS2) and pyrite (FeS2); and cattierite (CoS2). At high sodium oleate concentration, these phases formed: digenite (CuS0.55), nickel sulfide (NiS), pyrrhotite (FeS1.1), and jaipurite (CoS).

Recent advances in quantum dot-based fluorescence-linked immunosorbent assays

Fluorescence immunoassays have been given considerable attention among the quantitative detection methods in the clinical medicine and food safety testing fields. In particular, semiconductor quantum dots (QDs) have become ideal fluorescent probes for highly sensitive and multiplexed detection due to their unique photophysical properties, and the QD fluorescence-linked immunosorbent assay (FLISA) with high sensitivity, high accuracy, and high throughput has been greatly developed recently. In this manuscript, the advantages of applying QDs to FLISA platforms and some strategies for their application to in vitro diagnostics and food safety are discussed. Given the rapid development of this field, we classify these strategies based on the combination of QD types and detection targets, including traditional QDs or QD micro/nano-spheres-FLISA, and multiple FLISA platforms. In addition, some new sensors based on the QD-FLISA are introduced; this is one of the hot spots in this field. The current focus and future direction of QD-FLISA are also discussed, which provides important guidance for the further development of FLISA.

Synthesis of CdSe and CdSe/ZnS Quantum Dots with Tunable Crystal Structure and Photoluminescent Properties

Mastery over the structure of nanocrystals is a powerful tool for the control of their fluorescence properties and to broaden the range of their applications. In this work, the crystalline structure of CdSe can be tuned by the precursor concentration and the dosage of tributyl phosphine, which is verified by XRD, photoluminescence and UV-vis spectra, TEM observations, and time-correlated single photon counting (TCSPC) technology. Using a TBP-assisted thermal-cycling technique coupled with the single precursor method, core-shell QDs with different shell thicknesses were then prepared. The addition of TBP improves the isotropic growth of the shell, resulting in a high QY value, up to 91.4%, and a single-channel decay characteristic of CdSe/ZnS quantum dots. This work not only provides a facile synthesis route to precisely control the core-shell structures and fluorescence properties of CdSe nanocrystals but also builds a link between ligand chemistry and crystal growth theory.

General Bottom-Up Colloidal Synthesis of Nano-Monolayer Transition-Metal Dichalcogenides with High 1T'-Phase Purity

Phase engineering of nanomaterials provides a promising way to explore the phase-dependent physicochemical properties and various applications of nanomaterials. A general bottom-up synthesis method under mild conditions has always been challenging globally for the preparation of the semimetallic phase-transition-metal dichalcogenide (1T'-TMD) monolayers, which are pursued owing to their unique electrochemical property, unavailable in their semiconducting 2H phases. Here, we report the general scalable colloidal synthesis of nanosized 1T'-TMD monolayers, including 1T'-MoS₂, 1T'-MoSe₂, 1T'-WS₂, and 1T'-WSe₂, which are revealed to be of high phase purity. Moreover, the surfactant-reliant stacking-hinderable growth mechanism of 1T'-TMD nano-monolayers was unveiled through systematic experiments and theoretical calculations. As a proof-of-concept application, the 1T'-TMD nano-monolayers are used for electrocatalytic hydrogen production in an acidic medium. The 1T'-MoS₂ nano-monolayers possess abundant in-plane electrocatalytic active sites and high conductivity, coupled with the contribution of the lattice strain, thus exhibiting excellent performance. Importantly, the catalyst shows impressive endurability in electroactivity. Our developed general scalable strategy could pave the way to extend the synthesis of other broad metastable semimetallic-phase TMDs, which offer great potential to explore novel crystal phase-dependent properties with wide application development for catalysis and beyond.

General Expression for the Size-Dependent Optical Properties of Quantum Dots

While initial theories on quantum confinement in colloidal quantum dots (QDs) led to analytical band gap/size relations or sizing functions, numerical methods describe size quantization more accurately. However, because of the lack of reliable sizing functions, researchers fit experimental band gap/size data sets using models with redundant, physically meaningless parameters that break down upon extrapolation. Here, we propose a new sizing function based on a proportional correction for nonparabolic bands. Using known bulk parameters, we predict size quantization for groups IV, III-V, II-VI, and IV-VI and metal-halide perovskite semiconductors, including straightforward adaptations for negative-gap semiconductors and nonspherical QDs. Refinement with respect to experimental data is possible using the Bohr diameter as a fitting parameter, by which we show a statistically relevant difference in the band gap/size relation for wurtzite and zinc blende CdSe. The general sizing function proposed here unifies the QD size calibration and enables researchers to assess bulk semiconductor parameters and predict the size quantization in unexplored materials.

Alkyl selenol reactivity with common solvents and ligands: influences on phase control in nanocrystal synthesis

This study develops mechanistic understanding of the factors which control the phase in syntheses of copper selenide nanocrystals by investigating how the chemistry of the dodecylselenol reactant is altered by the ligand and solvent environment. ¹H NMR and ⁷⁷Se NMR were used to study how commonly used solvents (octadecene and dioctylether) and ligands (oleylamine, oleic acid, stearylamine, stearic acid and trioctyl phosphine) change the nature of the dodecylselenol reactant at 25 °C, 155 °C and 220 °C. Unsaturations were prone to selenol additons, carboxylates underwent selenoesterification, amines caused the release of H₂Se gas, and the phosphine formed phosphine selenide. Adventitious water caused oxidation to didodecyldiselenide. NMR studies were correlated with the phases that resulted in syntheses of nanocrystalline copper selenides, in which berzalianite, umangite or a metastable hexagonal phase were produced as identified by X-ray diffraction, depending on the ligand and solvent environemnts. Formation of the rare hexagonal Cu_2-xSe phase could be assigned to cases that included DD₂Se₂ as a reactive intermediate, or strong L-type ligation of amines which was dependant on alkyl chain length.

Quantum dots' size matters for balancing their quantity and quality in label materials to improve lateral flow immunoassay performance for C-reactive protein determination

Incorporating quantum dots (QDs) into dendritic mesoporous silica nanoparticles (DMSNs) for signal amplification of label materials represents an efficient strategy to improve the performance of lateral flow immunoassays (LFIAs). In this work, it is found that the CdSe/ZnS QD's size matters for balancing their loading amount and quantum yields (QYs) in the DMSNs-QDs based label materials and ultimately determining the performance of LFIA. The impacts of three CdSe/ZnS QDs with diameters of 9.1, 10.5 and 11.7 nm on CdSe/ZnS QDs incorporation and LFIA applications are studied. The increase of CdSe/ZnS QDs size from 9.1 to 11.7 nm results in a decrease in CdSe/ZnS QDs loading amount and an increase in QYs of incorporated CdSe/ZnS QDs. This trade-off leads to an optimized CdSe/ZnS QDs size of 10.5 nm, which exhibits the best LFIA performance due to the balanced QDs loading (2.26 g g^-1) and QY (57.1%). The 10.5 nm CdSe/ZnS QDs incorporated DMSNs-QDs for C-reactive protein (CRP) detection achieved a limit of detection of 5 pg mL^-1 (equivalent to 4.2 × 10^-14 M) with naked eye, which is lower than literature reports and commercial LFIA products. This study demonstrates that the CdSe/ZnS QD's size matters for improving the quality of DMSNs-QDs and their LFIA performance for CRP determination, providing new insights into the rational design of advanced label materials for improving LFIA performance.

Atomic Identification of Interfaces in Individual Core@shell Quantum Dots

CdSe@CdS Core@shell quantum dots (QDs) have been widely studied in recent years, due to their architecture which allows to tailor properties by controlling structure and composition. However, since CdSe and CdS have the same crystal structure, same cations, and similar lattice parameters, it is very challenging to image the interface. Herein, high-resolution transmission electron microscopy, high-angle annular dark-field imaging, and energy-dispersive X-ray spectroscopy elemental mapping are combined to characterize the core@shell structure and identify the interface in the CdSe@CdS QDs with different CdS shell thicknesses. By examining changes in lattice spacing in an individual CdSe@CdS quantum dot, the atomic core@shell interface is identified. For thin-shelled QDs, an ideal coherent interface forms between core and shell due to the small lattice mismatch, and the lattice spacing remains unchanged at the core and shell regions. For thick-shelled QDs, the lattice spacing is different at the core and shell regions, while the heterostructured interface is still coherent and cannot be clearly imaged. As the shell thickness further increases, a sharp core@shell interface appears. The results define an approach to characterize the heterostructure of two materials with the same crystalline structure and cations.

Effect of Surface Ligands in Perovskite Nanocrystals: Extending in and Reaching out

The rediscovery of the halide perovskite class of compounds and, in particular, the organic and inorganic lead halide perovskite (LHP) materials and lead-free derivatives has reached remarkable landmarks in numerous applications. First among these is the field of photovoltaics, which is at the core of today’s environmental sustainability efforts. Indeed, these efforts have born fruit, reaching to date a remarkable power conversion efficiency of 25.2% for a double-cation Cs, FA lead halide thin film device. Other applications include light and particle detectors as well as lighting. However, chemical and thermal degradation issues prevent perovskite-based devices and particularly photovoltaic modules from reaching the market. The soft ionic nature of LHPs makes these materials susceptible to delicate changes in the chemical environment. Therefore, control over their interface properties plays a critical role in maintaining their stability. Here we focus on LHP nanocrystals, where surface termination by ligands determines not only the stability of the material but also the crystallographic phase and crystal habit. A surface analysis of nanocrystal interfaces revealed the involvement of Brønsted type acid–base equilibrium in the modification of the ligand moieties present, which in turn can invoke dissolution and recrystallization into the more favorable phase in terms of minimization of the surface energy. A large library of surface ligands has already been developed showing both good chemical stability and good electronic surface passivation, resulting in near-unity emission quantum yields for some materials, particularly CsPbBr₃. However, most of those ligands have a large organic tail hampering charge carrier transport and extraction in nanocrystal-based solid films.

The unique perovskite structure that allows ligand substitution in the surface A (cation) sites and the soft ionic nature is expected to allow the accommodation of large dipoles across the perovskite crystal. This was shown to facilitate electron transfer across a molecular linked single-particle junction, creating a large built-in field across the junction nanodomains. This strategy could be useful for implementing LHP NCs in a p–n junction photovoltaic configuration as well as for a variety of electronic devices. A better understanding of the surface propeties of LHP nanocrystals will also enable better control of their growth on surfaces and in confined volumes, such as those afforded by metal–organic frameworks, zeolites, or chemically patterened surfaces such as anodic alumina, which have already been shown to significantly alter the properties of in-situ-grown LHP materials.

Near-infrared heavy-metal-free SnSe/ZnSe quantum dots for efficient photoelectrochemical hydrogen generation

Solar-driven photoelectrochemical (PEC) hydrogen production is one of the most effective strategies for solar-to-hydrogen energy conversion. Among various types of semiconductors used for PEC anodes, colloidal quantum dots (QDs) have been widely used as new and promising absorbers for PEC and other optoelectronic devices. However, currently, most efficient optoelectronic devices contain toxic Pb/Cd elements or non-earth-abundant elements (In/Ag). It is still a challenge to produce Pb/Cd-free QDs without using any toxic and non-earth-abundant elements. Here, we synthesized SnSe QDs via a diffusion-controlled hot injection approach and further stabilized the as-prepared SnSe QDs via a cation exchange reaction. The as-synthesized Zn-stabilized SnSe QDs (SnSe/ZnSe) have an orthorhombic crystal structure with indirect bandgaps ranging from 1 to 1.37 eV. Zn stabilization can significantly decrease the number of QD surface metallic Sn bonds, thereby decreasing the number of recombination centers of defects/traps. As a proof-of-concept, SnSe/ZnSe QDs are used as light absorbers for PEC hydrogen production, leading to a saturated photocurrent density of 7 mA cm^-2, which is comparable to best values reported for PEC devices based on toxic-metal-free QDs. Our results indicate that Zn-stabilized SnSe QDs have great potential for use in emerging optoelectronic devices.

Technology progress on quantum dot light-emitting diodes for next-generation displays

Quantum dot light-emitting diodes (QD-LEDs) are widely recognised as great alternatives to organic light-emitting diodes (OLEDs) due to their enhanced performances. This focus article surveys the current progress on the state-of-the-art QD-LED technology including material synthesis, device optimization and innovative fabrication processes. A discussion on the material synthesis of core nanocrystals, shell layers and surface-binding ligands is presented for high photoluminescence quantum yield (PLQY) quantum dots (QDs) using heavy-metal free materials. The operational principles of several types of QD-LED device architectures are also covered, and the recent evolution of device engineering technologies is investigated. By exploring the fabrication process for pixel-patterning of QD-LEDs on an active-matrix backplane for full-colour display applications, we anticipate further improvement in device performance for the commercialisation of next-generation displays.

Tolman's Electronic Parameter of the Ligand Predicts Phase in the Cation Exchange to CuFeS2Nanoparticles

The metastable and thermodynamically favored phases of CuFeS₂ are shown to be alternatively synthesized during partial cation exchange of hexagonal Cu₂S using various phosphorus-containing ligands. Transmission electron microscopy and energy dispersive spectroscopy mapping confirm the retention of the particle morphology and the approximate CuFeS₂ stoichiometry. Powder X-ray diffraction patterns and refinements indicate that the resulting phase mixtures of metastable wurtzite-like CuFeS₂ versus tetragonal chalcopyrite are correlated with the Tolman electronic parameter of the tertiary phosphorus-based ligand used during the cation exchange. Strong L-type donors lead to the chalcopyrite phase and weak donors to the wurtzite-like phase. To our knowledge, this is the first demonstration of phase control in nanoparticle synthesis using solely L-type donors.

Synthesis of vulcanite (CuTe) and metastable Cu1.5Te nanocrystals using a dialkyl ditelluride precursor

This study demonstrates that a dialkyl ditelluride reagent can produce metastable and difficult-to-achieve metal telluride phases in nanocrystal syntheses. Using didodecyl ditelluride and without the need for phosphine precursors, nanocubes of the pseudo-cubic phase (Cu1.5Te) were synthesized at the moderate temperature of 135 °C. At the higher temperature of 155 °C, 2-D nanosheets of vulcanite (CuTe) resulted, a nanomaterial in a phase that has not been previously achieved through thermal decomposition methods. Materials were characterized with TEM, powder XRD and UV-Vis-NIR absorbance spectroscopy.

Visualizing Heterogeneity of Monodisperse CdSe Nanocrystals by Their Assembly into Three-Dimensional Supercrystals

We show that the self-assembly of monodisperse CdSe nanocrystals synthesized at lower temperature (∼310 °C) into three-dimensional supercrystals results in the formation of separate regions within the supercrystals that display photoluminescence at two distinctly different wavelengths. Specifically, the central portions of the supercrystals display photoluminescence and absorption in the orange region of the spectrum, around 585 nm, compared to the 575 nm photoluminescence maximum for the nanocrystals dispersed in toluene. Distinct domains on the surfaces and edges of the supercrystals, by contrast, display photoluminescence and absorption in the green region of the spectrum, around 570 nm. We attribute the different-colored domains to two subpopulations of NCs in the monodisperse ensemble: the nanocrystals in the "orange" regions are chemically stable, whereas the nanocrystals in the "green" regions are partially oxidized. The susceptibility of the "green" nanocrystals to oxidation indicates a lower coverage of capping molecules on these nanocrystals. We propose that the two subpopulations correspond to nanocrystals with different surfaces that we attribute to the polytypism of CdSe.

Phase-Controlled Growth of CuInS2 Shells to Realize Colloidal CuInSe2/CuInS2 Core/Shell Nanostructures

Synthetic routes to deposit CuInS₂ (CIS) shells with either a cubic chalcopyrite (CP) or a hexagonal wurtzite (WZ) phase on trigonal pyramidal-shaped CuInSe₂ (CISe) core nanocrystals (NCs) with a cubic CP crystal structure have been developed and governed by tuning the amount of the sulfur precursor tert-dodecanethiol. During the synthesis of CP-CIS/CP-CISe core/shell NCs, the CP-CIS shell initially starts to grow epitaxially in a uniform way, while the further addition of the CIS precursor induces islandlike growth, and finally a branched CIS shell is formed. In a stark contrast, when a WZ-CIS shell is deposited, it initially grows on a portion of each of the facets of the trigonal pyramidal-shaped CISe cores to form a monolayer, which then continues to increase in thickness and forms a multilayered WZ-CIS shell. Both CP-CISe/CP-CIS core/shell NCs and CP-CISe/WZ-CISe core/shell NCs exhibit rather low photoluminescence quantum yields (<10%), even with a smaller-sized CISe core, which calls for further refinements of the shell growth methods. Synthetic methods for the growth of CIS shells as described here allow for direct deposition of cadmium-free ternary compounds as shell materials and provide important insights into the different modes of growth of heterostructured NCs, ranging from epitaxial to island- and branched-like, as well to the facet-specific multilayer deposition.

Tailoring the Heterostructure of Colloidal Quantum Dots for Ratiometric Optical Nanothermometry

Colloidal quantum dots (QDs) are a fascinating class of semiconducting nanocrystals, thanks to their optical properties tunable through size and composition, and simple synthesis methods. Recently, colloidal double-emission QDs have been successfully applied as competitive optical temperature sensors, since they exhibit structure-tunable double emission, temperature-dependent photoluminescence, high quantum yield, and excellent photostability. Until now, QDs have been used as nanothermometers for in vivo biological thermal imaging, and thermal mapping in complex environments at the sub-microscale to nanoscale range. In this Review, recent progress for QD-based nanothermometers is highlighted and perspectives for future work are described.

Unprecedented surface stabilized InP quantum dots with bidentate ligands

For InP-based QDs, the current technology does not outperform CdSe-based QDs in many respects, one of which is stability. The optical stability of QDs is closely related to their surface properties, so QDs often use organic ligands for surface protection. These organic ligands are dynamically attached and detached on the QD surface; during detachment, their surfaces are easily damaged and oxidized, thereby deteriorating their optical characteristics. Therefore, we have synthesized a ligand 1,2-hexadecanedithiol with a bidentate form, inducing one ligand to bind to the QD surface strongly through the chelate effect, as a good way to improve the stability of the QDs; thus, the PL stability of the green-light-emitting InP-based QDs was greatly increased. To confirm the existence of the dithiol ligand, we used thermogravimetric analysis/simultaneous thermal analysis-mass spectroscopy (TGA/STA-MS). After that, we applied the ligand to blue-light-emitting ZnSe QDs and red-light-emitting InP QDs, and for those two types of QD we also confirmed that the stability was increased. Additionally, we tested dithiol exchanged QDs at a high temperature of 150 °C, and the increase of stability was effective even in a high temperature condition.

Precursor reaction kinetics control compositional grading and size of CdSe1-x S x nanocrystal heterostructures

We report a method to control the composition and microstructure of CdSe1-x S x nanocrystals by the simultaneous injection of sulfide and selenide precursors into a solution of cadmium oleate and oleic acid at 240 °C. Pairs of substituted thio- and selenoureas were selected from a library of compounds with conversion reaction reactivity exponents (k E) spanning 1.3 × 10-5 s-1 to 2.0 × 10-1 s-1. Depending on the relative reactivity (k Se/k S), core/shell and alloyed architectures were obtained. Growth of a thick outer CdS shell using a syringe pump method provides gram quantities of brightly photoluminescent quantum dots (PLQY = 67 to 90%) in a single reaction vessel. Kinetics simulations predict that relative precursor reactivity ratios of less than 10 result in alloyed compositions, while larger reactivity differences lead to abrupt interfaces. CdSe1-x S x alloys (k Se/k S = 2.4) display two longitudinal optical phonon modes with composition dependent frequencies characteristic of the alloy microstructure. When one precursor is more reactive than the other, its conversion reactivity and mole fraction control the number of nuclei, the final nanocrystal size at full conversion, and the elemental composition. The utility of controlled reactivity for adjusting alloy microstructure is discussed.

Organic-to-water dispersible Mn:ZnS–ZnS doped core–shell quantum dots: synthesis, characterization and their application towards optical bioimaging and a turn-off fluorosensor

Dot-in-dot core/shell Mn:ZnS/ZnS QDs as a good fluorescent agent for bioimaging and a turn-off fluorescent probe for detection of heavy metal ions.

Colloidal branched CdSe/CdS 'nanospiders' with 2D/1D heterostructure

In this paper we report the synthesis of colloidal CdSe/CdS core-shell heteronanoplatelets with epitaxially grown wurtzite (WZ) 1D CdS branches or legs by using cadmium diethyldithiocarbamate as a single-source precursor. The growth of WZ branches was achieved by exploiting zinc blende-wurtzite polytypism of cadmium chalcogenides induced by oleylamine. Synthesized 'nanospiders' exhibit enhanced absorption in the UV-blue region and narrow and relatively intense red photoluminescence depending on the amount of CdS in the heteronanostructure.

Ultrafast optical properties of type-II CdZnS/ZnSe core-shell quantum dots

In this study, the ultrafast optical properties of type-II CdZnS/ZnSe core-shell quantum dots were investigated using the Z-scan and transient absorption technique with femtosecond pulses. With 800-nm wavelength excitation, the CdZnS/ZnSe quantum dots exhibited two-photon absorption, and the two-photon absorption cross section was obtained as about 3.37 × 10⁶ GM. In addition, the transfer time of electrons and the recombination lifetime of a single exciton were obtained. For the photoluminescence of the CdZnS/ZnSe quantum dots at temperatures from 80 to 280 K, the peak position redshifted by 60 meV, width broadened by 3 meV, and intensity decreased by a factor of four.

Direct optical lithography of functional inorganic nanomaterials

Photolithography is an important manufacturing process that relies on using photoresists, typically polymer formulations, that change solubility when illuminated with ultraviolet light. Here, we introduce a general chemical approach for photoresist-free, direct optical lithography of functional inorganic nanomaterials. The patterned materials can be metals, semiconductors, oxides, magnetic, or rare earth compositions. No organic impurities are present in the patterned layers, which helps achieve good electronic and optical properties. The conductivity, carrier mobility, dielectric, and luminescence properties of optically patterned layers are on par with the properties of state-of-the-art solution-processed materials. The ability to directly pattern all-inorganic layers by using a light exposure dose comparable with that of organic photoresists provides an alternate route for thin-film device manufacturing.

Fine-tuning the crystal structure of CdSe quantum dots by varying the dynamic characteristics of primary alkylamine ligands

Primary alkylamines are generally used as ligands for the synthesis of colloidal II–VI group quantum dots (QDs).

Colloidal Dual-Diameter and Core-Position-Controlled Core/Shell Cadmium Chalcogenide Nanorods

To capitalize on shape- and structure-dependent properties of semiconductor nanorods (NRs), high-precision control and exquisite design of their growth are desired. Cadmium chalcogenide (CdE; E = S or Se) NRs are the most studied class of such, whose growth exhibits axial anisotropy, i.e., different growth rates along the opposite directions of {0001} planes. However, the mechanism behind asymmetric axial growth of NRs remains unclear because of the difficulty in instant analysis of growth surfaces. Here, we design colloidal dual-diameter semiconductor NRs (DDNRs) under the quantum confinement regime, which have two sections along the long axis with different diameters. The segmentation of the DDNRs allows rigorous assessment of the kinetics of NR growth at a molecular level. The reactivity of a terminal facet passivated by an organic ligand is governed by monomer diffusivity through the surface ligand monolayer. Therefore, the growth rate in two polar directions can be finely tuned by controlling the strength of ligand-ligand attraction at end surfaces. Building on these findings, we report the synthesis of single-diameter CdSe/CdS core/shell NRs with CdSe cores of controllable position, which reveals a strong structure-optical polarization relationship. The understanding of the NR growth mechanism with controllable anisotropy will serve as a cornerstone for the exquisite design of more complex anisotropic nanostructures.

Synthesis of Reabsorption-Suppressed Type-II/Type-I ZnSe/CdS/ZnS Core/Shell Quantum Dots and Their Application for Immunosorbent Assay

We report a phosphine-free one-pot method to synthesize ZnSe/CdS/ZnS core-shell quantum dots (QDs) with composite type-II/type-I structures and consequent reabsorption suppression properties. The as-synthesized QDs possess high efficient red emission (with quantum yield of 82%) and high optical stability. Compared to type-I QDs, the ZnSe/CdS/ZnS QDs show larger Stokes shift and lower reabsorption which can reduce the emission loss and improve the level of fluorescence output. The ZnSe/CdS/ZnS QDs are used as fluorescent labels to exploit their application in fluorescence-linked immunosorbent assay (FLISA) for the first time in the detection of C-reactive protein (CRP) with a limit of detection (LOD) of 0.85 ng/mL, which is more sensitive than that of CdSe/ZnS type-I QDs based FLISA (1.00 ng/mL). The results indicate that the ZnSe/CdS/ZnS type-II/type-I QDs may be good candidates for applications in biomedical information detection.

Quantifying engineered nanomaterial toxicity: comparison of common cytotoxicity and gene expression measurements

BACKGROUND

When evaluating the toxicity of engineered nanomaterials (ENMS) it is important to use multiple bioassays based on different mechanisms of action. In this regard we evaluated the use of gene expression and common cytotoxicity measurements using as test materials, two selected nanoparticles with known differences in toxicity, 5 nm mercaptoundecanoic acid (MUA)-capped InP and CdSe quantum dots (QDs). We tested the effects of these QDs at concentrations ranging from 0.5 to 160 µg/mL on cultured normal human bronchial epithelial (NHBE) cells using four common cytotoxicity assays: the dichlorofluorescein assay for reactive oxygen species (ROS), the lactate dehydrogenase assay for membrane viability (LDH), the mitochondrial dehydrogenase assay for mitochondrial function, and the Comet assay for DNA strand breaks.

RESULTS

The cytotoxicity assays showed similar trends when exposed to nanoparticles for 24 h at 80 µg/mL with a threefold increase in ROS with exposure to CdSe QDs compared to an insignificant change in ROS levels after exposure to InP QDs, a twofold increase in the LDH necrosis assay in NHBE cells with exposure to CdSe QDs compared to a 50% decrease for InP QDs, a 60% decrease in the mitochondrial function assay upon exposure to CdSe QDs compared to a minimal increase in the case of InP and significant DNA strand breaks after exposure to CdSe QDs compared to no significant DNA strand breaks with InP. High-throughput quantitative real-time polymerase chain reaction (qRT-PCR) data for cells exposed for 6 h at a concentration of 80 µg/mL were consistent with the cytotoxicity assays showing major differences in DNA damage, DNA repair and mitochondrial function gene regulatory responses to the CdSe and InP QDs. The BRCA2, CYP1A1, CYP1B1, CDK1, SFN and VEGFA genes were observed to be upregulated specifically from increased CdSe exposure and suggests their possible utility as biomarkers for toxicity.

CONCLUSIONS

This study can serve as a model for comparing traditional cytotoxicity assays and gene expression measurements and to determine candidate biomarkers for assessing the biocompatibility of ENMs.

Optical determination of crystal phase in semiconductor nanocrystals

Optical, electronic and structural properties of nanocrystals fundamentally derive from crystal phase. This is especially important for polymorphic II-VI, III-V and I-III-VI₂ semiconductor materials such as cadmium selenide, which exist as two stable phases, cubic and hexagonal, each with distinct properties. However, standard crystallographic characterization through diffraction yields ambiguous phase signatures when nanocrystals are small or polytypic. Moreover, diffraction methods are low-throughput, incompatible with solution samples and require large sample quantities. Here we report the identification of unambiguous optical signatures of cubic and hexagonal phases in II-VI nanocrystals using absorption spectroscopy and first-principles electronic-structure theory. High-energy spectral features allow rapid identification of phase, even in small nanocrystals (∼2 nm), and may help predict polytypic nanocrystals from differential phase contributions. These theoretical and experimental insights provide simple and accurate optical crystallographic analysis for liquid-dispersed nanomaterials, to improve the precision of nanocrystal engineering and improve our understanding of nanocrystal reactions.

Probing Homogeneous Line Broadening in CdSe Nanocrystals Using Multidimensional Electronic Spectroscopy

The finite spectral line width of an ensemble of CdSe nanocrystals arises from size and shape inhomogeneity and the single-nanocrystal spectrum itself. This line width directly limits the performance of nanocrystal-based devices, yet most optical measurements cannot resolve the underlying contributions. We use two-dimensional electronic spectroscopy (2D ES) to measure the line width of the band-edge exciton of CdSe nanocrystals as a function of radii and surface chemistry. We find that the homogeneous width decreases for increasing nanocrystal radius and that surface chemistry plays a critical role in controlling this line width. To explore the hypothesis that unpassivated trap states serve to broaden the homogeneous line width and to explain its size-dependence, we use 3D ES to identify the spectral signatures of exciton-phonon coupling to optical and acoustic phonons. We find enhanced coupling to optical phonon modes for nanocrystals that lack electron-passivating ligands, suggesting that localized surface charges enhance exciton-phonon coupling via the Fröhlich interaction. Lastly, the data reveal that spectral diffusion contributes negligibly to the homogeneous line width on subnanosecond time scales.

Chemical epitaxy of CdSe on GaAs

Chemical epitaxy of CdSe thin films on GaAs(100) and GaAs(111) substrates.

Quantum Dot Surface Engineering: Toward Inert Fluorophores with Compact Size and Bright, Stable Emission

The surfaces of colloidal nanocrystals are complex interfaces between solid crystals, coordinating ligands, and liquid solutions. For fluorescent quantum dots, the properties of the surface vastly influence the efficiency of light emission, stability, and physical interactions, and thus determine their sensitivity and specificity when they are used to detect and image biological molecules. But after more than 30 years of study, the surfaces of quantum dots remain poorly understood and continue to be an important subject of both experimental and theoretical research. In this article, we review the physics and chemistry of quantum dot surfaces and describe approaches to engineer optimal fluorescent probes for applications in biomolecular imaging and sensing. We describe the structure and electronic properties of crystalline facets, the chemistry of ligand coordination, and the impact of ligands on optical properties. We further describe recent advances in compact coatings that have significantly improved their properties by providing small hydrodynamic size, high stability and fluorescence efficiency, and minimal nonspecific interactions with cells and biological molecules. While major progress has been made in both basic and applied research, many questions remain in the chemistry and physics of quantum dot surfaces that have hindered key breakthroughs to fully optimize their properties.

To Battle Surface Traps on CdSe/CdS Core/Shell Nanocrystals: Shell Isolation versus Surface Treatment

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.

Piezoelectric Control of the Exciton Wave Function in Colloidal CdSe/CdS Nanocrystals

Using multiband k·p calculations, we show that strain-engineered piezoelectricity is a powerful tool to modulate the electron-hole spatial separation in a wide class of wurtzite CdSe/CdS nanocrystals. The inherent anisotropy of the hexagonal crystal structure leads to anisotropic strain and, consequently, to a pronounced piezoelectric field along the c axis, which can be amplified or quenched through a proper design of the core-shell structure. The use of large cores and thick shells promotes a gradual departure from quantum confined nanocrystals to a regime dominated by piezoelectric confinement. This allows excitons to evolve from the usual type-I and quasi-type-II behavior to a type-II behavior in dot-in-dots, dot-in-rods, rod-in-rods, and dot-in-plates. Piezoelectric fields explain experimental observations for giant-shell nanocrystals, whose time-resolved photoluminescence reveals long exciton lifetimes for large cores, contrary to the expectations of standard quantum confinement models. They also explain the large differences in exciton lifetimes reported for different classes of CdSe/CdS nanocrystals.

A Two-Step Synthetic Strategy toward Monodisperse Colloidal CdSe and CdSe/CdS Core/Shell Nanocrystals

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.

Quantum Yield Heterogeneity among Single Nonblinking Quantum Dots Revealed by Atomic Structure-Quantum Optics Correlation

Physical variations in colloidal nanostructures give rise to heterogeneity in expressed optical behavior. This correlation between nanoscale structure and function demands interrogation of both atomic structure and photophysics at the level of single nanostructures to be fully understood. Herein, by conducting detailed analyses of fine atomic structure, chemical composition, and time-resolved single-photon photoluminescence data for the same individual nanocrystals, we reveal inhomogeneity in the quantum yields of single nonblinking "giant" CdSe/CdS core/shell quantum dots (g-QDs). We find that each g-QD possesses distinctive single exciton and biexciton quantum yields that result mainly from variations in the degree of charging, rather than from volume or structure inhomogeneity. We further establish that there is a very limited nonemissive "dark" fraction (<2%) among the studied g-QDs and present direct evidence that the g-QD core must lack inorganic passivation for the g-QD to be "dark". Therefore, in contrast to conventional QDs, ensemble photoluminescence quantum yield is principally defined by charging processes rather than the existence of dark g-QDs.

Intraband Luminescence from HgSe/CdS Core/Shell Quantum Dots

HgSe/CdS core/shell CQD are synthesized, and the changes in the optical absorption and luminescence are investigated. While HgSe quantum dots are naturally n-doped after synthesis, both as colloidal solutions and as films, the HgSe/CdS core/shell dots in solution lose the n-doping, as seen from the optical absorption in solution. However, n-doping is regained in films, and the intraband luminescence of the films of HgSe/CdS is greater than that of the cores. The shell also vastly improves the stability of the quantum dots films against sintering at 200 °C. After annealing at that temperature, the HgSe/CdS films retain a narrow intraband emission and sustain a higher laser power leading to brighter emission at 5 μm.

Insights into the Mechanistic Role of Diphenylphosphine Selenide, Diphenylphosphine, and Primary Amines in the Formation of CdSe Monomers

The formation mechanism of CdSe monomers from the reaction of cadmium oleate (Cd(OA)2) and SePPh2H in the presence of HPPh2 and RNH2 was studied systematically at the M06//B3LYP/6-31++G(d,p),SDD level in 1-octadecene solution. Herein, SePPh2H, HPPh2, and RNH2 act as hydrogen/proton donors with a decreased capacity, leading to the release of oleic acid (RCOOH). The longer the radius of the coordinated atom is, the larger the size of the cyclic transition state is, which lowers the activation strain and the Gibbs free energy of activation for the release of RCOOH. From the resulting RCOOCdSe-PPh2, for the formation of Ph2P-CdSe-PPh2 (G), SePPh2H acts as a catalyst, in which the turnover frequency determining transition state (TDTS) is characteristic of the Se-P bond cleavage. For the formation of RHN-CdSe-PPh2 (H), SePPh2H also serves as a catalyst, in which the TDTS is representative of the N-H bond cleavage. For the formation of Ph2PSe-CdSe-NHR (I), HPPh2 behaves as a catalyst, in which the TDTS is typical of the Se-P and N-H bond cleavage. The rate constants increase as kI < kH < kG, which is in good agreement with our previous experimental observations reported. The present study brings insight into the use of additives such as HPPh2 and RNH2 to synthesize colloidal quantum dots.

Acetate ligands determine the crystal structure of CdSe nanoplatelets – a density functional theory study

Density functional theory calculations show that acetate ligands play a crucial role in stabilising cadmium selenide nanoplatelets in the zinc blende crystal structure.

Stable and Low-Threshold Optical Gain in CdSe/CdS Quantum Dots: An All-Colloidal Frequency Up-Converted Laser

An all-solution processed and all-colloidal laser is demonstrated using tailored CdSe/CdS core/shell quantum dots, which exhibit highly stable and low-threshold optical gain owing to substantially suppressed non-radiative Auger recombination.