Nonclassical Mechanism of Metal-Enhanced Photoluminescence of Quantum Dots.

Metal-enhanced photoluminescence is able to provide a robust signal even from a single emitter and is promising in applications in biosensors and optoelectronic devices. However, its realization with semiconductor nanocrystals (e.g., quantum dots, QDs) is not always straightforward due to the hidden and not fully described interactions between plasmonic nanoparticles and an emitter. Here, we demonstrate nonclassical enhancement (i.e., not a conventional electromagnetic mechanism) of the QD photoluminescence at nonplasmonic conditions and correlate it with the charge exchange processes in the system, particularly with high efficiency of the hot-hole generation in gold nanoparticles and the possibility of their transfer to QDs. The hole injection returns a QD from a charged nonemitting state caused by hole trapping by surface and/or interfacial traps into an uncharged emitting state, which leads to an increased photoluminescence intensity. These results open new insights into metal-enhanced photoluminescence, showing the importance of the QD surface states in this process.

Role of the Crystal Structure in Cation Exchange Reactions Involving Colloidal Cu2Se Nanocrystals.

Stoichiometric Cu2Se nanocrystals were synthesized in either cubic or hexagonal (metastable) crystal structures and used as the host material in cation exchange reactions with Pb2+ ions. Even if the final product of the exchange, in both cases, was rock-salt PbSe nanocrystals, we show here that the crystal structure of the starting nanocrystals has a strong influence on the exchange pathway. The exposure of cubic Cu2Se nanocrystals to Pb2+ cations led to the initial formation of PbSe unselectively on the overall surface of the host nanocrystals, generating Cu2Se@PbSe core@shell nanoheterostructures. The formation of such intermediates was attributed to the low diffusivity of Pb2+ ions inside the host lattice and to the absence of preferred entry points in cubic Cu2Se. On the other hand, in hexagonal Cu2Se nanocrystals, the entrance of Pb2+ ions generated PbSe stripes "sandwiched" in between hexagonal Cu2Se domains. These peculiar heterostructures formed as a consequence of the preferential diffusion of Pb2+ ions through specific (a, b) planes of the hexagonal Cu2Se structure, which are characterized by almost empty octahedral sites. Our findings suggest that the morphology of the nanoheterostructures, formed upon partial cation exchange reactions, is intimately connected not only to the NC host material, but also to its crystal structure.

Probing Absolute Electronic Energy Levels in Hg-Doped CdTe Semiconductor Nanocrystals by Electrochemistry and Density Functional Theory.

The absolute electronic energy levels in Hg-doped CdTe semiconductor nanocrystals (CdHgTe NCs) with varying sizes/volumes and Hg contents are determined by using cyclic voltammetry (CV) measurements and density functional theory (DFT) -based calculations. The electrochemical measurements demonstrate several distinct characteristic features in the form of oxidation and reduction peaks in the voltammograms, where the peak positions are dependent on the volume of CdHgTe NCs as well as on their composition. The estimated absolute electronic energy levels for three different volumes, namely 22, 119 and 187 nm(3) with 2.7±0.3 % of Hg content, show strong volume dependence. The volume-dependent shift in the characteristic reduction and oxidation peak potential scan can be attributed to the alteration in the energetic band positions owing to the quantum confinement effect. Moreover, the composition (Cd/Hg=98.3/1.7 and 97.0/3.0) -dependent alteration in the electronic energy levels of CdHgTe NCs for two different samples with similar volumes (ca. 124±5 nm(3) ) are shown. Thus obtained electronic energy level values of CdHgTe NCs as a function of volume and composition demonstrate good congruence with the corresponding absorption and emission spectral data, as well as with DFT-based calculations. DFT calculations reveal that incorporation of Hg into CdTe NCs mostly affects the energy levels of conduction band edge, whereas the valence band edge remains almost unaltered.

Cyclic voltammetry as a sensitive method for in situ probing of chemical transformations in quantum dots.

The application of electrochemical methods for the characterization of colloidal quantum dots (QDs) attracts considerable attention as these methods may allow for monitoring of some crucial parameters, such as energetic levels of conduction and valence bands as well as surface traps and ligands under real conditions of colloidal solution. In the present work we extend the applications of cyclic voltammetry (CV) to in situ monitoring of degradation processes of water-soluble CdTe QDs. This degradation occurs under lowering of pH to the values around 5, i.e. under conditions relevant to bioimaging applications of these QDs, and is accompanied by pronounced changes of their photoluminescence. Observed correlations between characteristic features of CV diagrams and the fluorescence spectra allowed us to propose mechanisms responsible for evolution of the photoluminescence properties as well as degradation pathway of CdTe QDs at low pH.

3D Assembly of All-Inorganic Colloidal Nanocrystals into Gels and Aerogels.

We report an efficient approach to assemble a variety of electrostatically stabilized all-inorganic semiconductor nanocrystals (NCs) by their linking with appropriate ions into multibranched gel networks. These all-inorganic non-ordered 3D assemblies benefit from strong interparticle coupling, which facilitates charge transport between the NCs with diverse morphologies, compositions, sizes, and functional capping ligands. Moreover, the resulting dry gels (aerogels) are highly porous monolithic structures, which preserve the quantum confinement of their building blocks. The inorganic semiconductor aerogel made of 4.5 nm CdSe colloidal NCs capped with I(-) ions and bridged with Cd(2+) ions had a large surface area of 146 m(2) g(-1) .

Simultaneous Identification of Spectral Properties and Sizes of Multiple Particles in Solution with Subnanometer Resolution.

We report an unsurpassed solution characterization technique based on analytical ultracentrifugation, which demonstrates exceptional potential for resolving particle sizes in solution with sub-nm resolution. We achieve this improvement in resolution by simultaneously measuring UV/Vis spectra while hydrodynamically separating individual components in the mixture. By equipping an analytical ultracentrifuge with a novel multi-wavelength detector, we are adding a new spectral discovery dimension to traditional hydrodynamic characterization, and amplify the information obtained by orders of magnitude. We demonstrate the power of this technique by characterizing unpurified CdTe nanoparticle samples, avoiding tedious and often impossible purification and fractionation of nanoparticles into apparently monodisperse fractions. With this approach, we have for the first time identified the pure spectral properties and band-gap positions of discrete species present in the CdTe mixture.

Cu Vacancies Boost Cation Exchange Reactions in Copper Selenide Nanocrystals.

We have investigated cation exchange reactions in copper selenide nanocrystals using two different divalent ions as guest cations (Zn(2+) and Cd(2+)) and comparing the reactivity of close to stoichiometric (that is, Cu2Se) nanocrystals with that of nonstoichiometric (Cu(2-x)Se) nanocrystals, to gain insights into the mechanism of cation exchange at the nanoscale. We have found that the presence of a large density of copper vacancies significantly accelerated the exchange process at room temperature and corroborated vacancy diffusion as one of the main drivers in these reactions. Partially exchanged samples exhibited Janus-like heterostructures made of immiscible domains sharing epitaxial interfaces. No alloy or core-shell structures were observed. The role of phosphines, like tri-n-octylphosphine, in these reactions, is multifaceted: besides acting as selective solvating ligands for Cu(+) ions exiting the nanoparticles during exchange, they also enable anion diffusion, by extracting an appreciable amount of selenium to the solution phase, which may further promote the exchange process. In reactions run at a higher temperature (150 °C), copper vacancies were quickly eliminated from the nanocrystals and major differences in Cu stoichiometries, as well as in reactivities, between the initial Cu2Se and Cu(2-x)Se samples were rapidly smoothed out. These experiments indicate that cation exchange, under the specific conditions of this work, is more efficient at room temperature than at higher temperature.

Tetrazoles: Unique Capping Ligands and Precursors for Nanostructured Materials.

Capping agents play an important role in the colloidal synthesis of nanomaterials because they control the nucleation and growth of particles, as well as their chemical and colloidal stability. During recent years tetrazole derivatives have proven to be advanced capping ligands for the stabilization of semiconductor and metal nanoparticles. Tetrazole-capped nanoparticles can be prepared by solution-phase or solventless single precursor approaches using metal derivatives of tetrazoles. The solventless thermolysis of metal tetrazolates can produce both individual semiconductor nanocrystals and nanostructured metal monolithic foams displaying low densities and high surface areas. Alternatively, highly porous nanoparticle 3D assemblies are achieved through the controllable aggregation of tetrazole-capped particles in solutions. This approach allows for the preparation of non-ordered hybrid structures consisting of different building blocks, such as mixed semiconductor and metal nanoparticle-based (aero)gels with tunable compositions. Another unique property of tetrazoles is their complete thermal decomposition, forming only gaseous products, which is employed in the fabrication of organic-free semiconductor films from tetrazole-capped nanoparticles. After deposition and subsequent thermal treatment these films exhibit significantly improved electrical transport. The synthetic availability and advances in the functionalization of tetrazoles necessitate further design and study of tetrazole-capped nanoparticles for various applications.

Single-mode lasing from colloidal water-soluble CdSe/CdS quantum dot-in-rods.

Core-shell CdSe/CdS nanocrystals are a very promising material for light emitting applications. Their solution-phase synthesis is based on surface-stabilizing ligands that make them soluble in organic solvents, like toluene or chloroform. However, solubility of these materials in water provides many advantages, such as additional process routes and easier handling. So far, solubilization of CdSe/CdS nanocrystals in water that avoids detrimental effects on the luminescent properties poses a major challenge. This work demonstrates how core-shell CdSe/CdS quantum dot-in-rods can be transferred into water using a ligand exchange method employing mercaptopropionic acid (MPA). Key to maintaining the light-emitting properties is an enlarged CdS rod diameter, which prevents potential surface defects formed during the ligand exchange from affecting the photophysics of the dot-in-rods. Films made from water-soluble dot-in-rods show amplified spontaneous emission (ASE) with a similar threshold (130 µJ/cm(2)) as the pristine material (115 µJ/cm(2)). To demonstrate feasibility for lasing applications, self-assembled microlasers are fabricated via the "coffee-ring effect" that display single-mode operation and a very low threshold of ∼10 µJ/cm(2). The performance of these microlasers is enhanced by the small size of MPA ligands, enabling a high packing density of the dot-in-rods.

Morphology-Dependent Electrochemical Properties of CuS Hierarchical Superstructures.

Hierarchical superstructures formed by self-assembled nanoparticles exhibit interesting electrochemical properties that can potentially be exploited in Li-ion batteries (LIBs) as possible electrode materials. In this work, we tested two different morphologies of CuS superstructures for electrodes, namely, tubular dandelion-like and ball-like assemblies, both of which are composed of similar small covellite nanoparticles. These two CuS morphologies are characterized by their markedly different electrochemical performances, suggesting that their complex structures/morphologies influence the electrochemical properties. At 1.12 A g(-1), the cells made with CuS tubular structures delivered about 420 mAh g(-1), and at 0.56 A g(-1), the capacity was as high as about 500 mAh g(-1) with good capacity retention. Their ease of preparation and processing, together with good electrochemical performance, make CuS tubular dandelion-like clusters attractive for developing low-cost LIBs based on conversion reactions.

Colloidal semiconductor nanocrystals: the aqueous approach.

This article summarizes the main achievements and challenges in the field of the aqueous synthesis of semiconductor quantum dots in colloidal solutions. Developments in the last two decades demonstrate the great potential of this approach to synthesize nanocrystalline materials with superior properties such as strong photoluminescence, long time stability and compatibility with biological media, and the variability in assembling and self-assembling into larger structures or on surfaces. Being relatively straightforward, the aqueous approach provides some advantages such as versatility, scalability, environmental friendliness and cost effectiveness, leading in summary to very attractive application perspectives.

Facile and Scalable Colloidal Synthesis of Transition Metal Dichalcogenide Nanoparticles with High-Performance Hydrogen Production.

Transition metal dichalcogenides (TMDs) have garnered significant attention as efficient electrocatalysts for the hydrogen evolution reaction (HER) due to their high activity, stability, and cost-effectiveness. However, the development of a convenient and economical approach for large-scale HER applications remains a persistent challenge. In this study, we present the successful synthesis of TMD nanoparticles (including MoS2, RuS2, ReS2, MoSe2, RuSe2, and ReSe2) using a general colloidal method at room temperature. Notably, the ReSe2 nanoparticles synthesized in this study exhibit superior HER performance compared with previously reported nanostructured TMDs. Importantly, the synthesis of these TMD nanoparticles can readily be scaled up to gram quantities while preserving their exceptional HER performance. These findings highlight the potential of colloidal synthesis as a versatile and scalable approach for producing TMD nanomaterials with outstanding electrocatalytic properties for water splitting.

Emissive ZnO@Zn3 P2 nanocrystals: synthesis, optical, and optoelectrochemical properties.

ZnO@Zn3 P2 quantum dots (QDs) are synthesized, with emission from yellow to red. Photoelectrochemical investigations reveal that the current and voltage of the QD-derivatized electrodes show a response upon illumination. A photocurrent of ca. 8 nA cm(-2) for a monolayer of ZnO@Zn3 P2 QDs deposited on indium tin oxide (ITO) electrode is recorded.

Colloidal nanocrystals embedded in macrocrystals: robustness, photostability, and color purity.

The incorporation of colloidal quantum dots (QDs) into ionic crystals of various salts (NaCl, KCl, KBr, etc.) is demonstrated. The resulting mixed crystals of various shapes and beautiful colors preserve the strong luminescence of the incorporated QDs. Moreover, the ionic salts appear to be very tight matrices, ensuring the protection of the QDs from the environment and as a result providing them with extraordinary high photo- and chemical stability. A prototype of a white light-emitting diode (WLED) with a color conversion layer consisting of this kind of mixed crystals is demonstrated. These materials may also find applications in nonlinear optics and as luminescence standards.

Quantum Dots Facilitate 3D Two-Photon Laser Lithography.

In the past two decades, direct laser writing (DLW) technologies have seen tremendous growth. However, strategies that enhance the printing resolution and the development of printing material with assorted functionalities are still sparser than expected. Herein, a cost-effective method to tackle this bottleneck is presented. Semiconductor quantum dots (QDs) are selected to carry out this task, most importantly via surface chemistry modification to enable their copolymerization with themonomers, resulting in transparent composites. The evaluations indicate that the QDs show great colloidal stability and their photoluminescent properties are well-preserved. This allows further exploration of the printing characteristics of such composite material. It is shown that in the presence of the QDs, the material provides a much lower polymerization threshold with faster linewidth growth, indicating that the QDs form a synergetic relationship with the monomer and the photoinitiator, widening the dynamic range of the material and thus increasing the writing efficiency for broader fields of applications. Lowering the polymerization threshold reduces the minimum achievable feature size by ≈32%, which is well-matched with STED-based (i.e., stimulated-emission depletion microscopy) methods in writing 3D structures. The study further elucidates the mechanism of the synergetic behavior, further guiding the future development of functional materials for DLW-related printing technologies.

Incoherent photon conversion in selectively infiltrated hollow-core photonic crystal fibers for single photon generation in the near infrared.

At present, there exist a number of on-demand single photon sources with high emission rates and stability even at room temperature. However, their emission wavelength is restricted to specific transitions in single quantum emitters. Single photon generation in the near infrared, possibly within the telecom band, though most urgently needed, is particularly crucial. In this paper, we suggest an experimental method to convert visible single photons from a defect center in diamond to the near infrared. The conversion relies on efficient absorption by colloidal quantum dots and subsequent Stokes-shifted emission. The desired target wavelength can be chosen almost arbitrarily by selecting quantum dots with a suitable emission spectrum. A hollow core photonic crystal fiber selectively filled with a solution of quantum dots was used to achieve at the same time a single photon absorption probability of near unity and a very high re-collection efficiency of Stokes-shifted fluorescence (theoretically estimated to be 26%). A total conversion efficiency of light of 0.1% is achieved. Experimental strategies to significantly enhance this number are presented.

Surface plasmon enhanced energy transfer between donor and acceptor CdTe nanocrystal quantum dot monolayers.

Surface plasmon enhanced Förster resonant energy transfer (FRET) between CdTe nanocrystal quantum dots (QDs) has been observed in a multilayer acceptor QD-gold nanoparticle-donor QD sandwich structure. Compared to a donor-acceptor QD bilayer structure without gold nanoparticles, the FRET rate is enhanced by a factor of 80 and the Förster radius increases by 103%. Furthermore, a strong impact of the donor QD properties on the surface plasmon mediated FRET is reported.

3D assembly of semiconductor and metal nanocrystals: hybrid CdTe/Au structures with controlled content.

A 3D metal ion assisted assembly of nanoparticles has been developed. The approach relies on the efficient complexation of cadmium ions and 5-mercaptomethyltetrazole employed as the stabilizer of both colloidal CdTe and Au nanoparticles. It enables in a facile way the formation of hybrid metal-semiconductor 3D structures with controllable and tunable composition in aqueous media. By means of critical point drying, these assemblies form highly porous aerogels. The hybrid architectures obtained are characterized by electron microscopy, nitrogen adsorption, and optical spectroscopy methods.

One-phase synthesis of gold nanoparticles with varied solubility.

We developed a straightforward synthesis of gold nanoparticles with diameters in the range 2.1-7.0 nm which display solubility in both aqueous and nonpolar (toluene, chloroform) media. This versatile solubility of the nanoparticles is achieved by the use of a thiolated PEG capping agent. Their plasmon resonance band is virtually unaltered in different media.

Influence of annealing on composition and optical properties of CdTe nanoparticle layer-by-layer films.

CdTe nanoparticle-polymer composite films were deposited conformally using a layer-by-layer (LbL) process onto planar or ZnO nanorod-coated substrates. Films were annealed between 150-450 degrees C. Under air this led to oxidation of the nanoparticles while under vacuum their composition was retained. Annealing at 450 degrees C led to complete removal of the polymer with a loss of quantum confinement as shown by UV-vis spectroscopy. Annealing at 350 degrees C gave partial removal of the polymer and retained quantum confinement. Such annealed nanoparticle composite systems may have application in photovoltaics.