Engineering Fluorescent Gold Nanoclusters Using Xanthate-Functionalized Hydrophilic Polymers: Toward Enhanced Monodispersity and Stability.

We introduce xanthate-functionalized poly(cyclic imino ethers)s (PCIEs), specifically poly(2-ethyl-2-oxazoline) and poly(2-ethyl-2-oxazine) given their stealth characteristics, as an attractive alternative to conventional thiol-based ligands for the synthesis of highly monodisperse and fluorescent gold nanoclusters (AuNCs). The xanthate in the PCIEs interacts with Au ions, acting as a well-controlled template for the direct formation of PCIE-AuNCs. This method yields red-emitting AuNCs with a narrow emission peak (λem = 645 nm), good quantum yield (4.3-4.8%), long fluorescence decay time (∼722-844 ns), and unprecedented product yield (>98%). The PCIE-AuNCs exhibit long-term colloidal stability, biocompatibility, and antifouling properties, enabling a prolonged blood circulation, lower nonspecific accumulation in major organs, and better renal clearance when compared with AuNCs without polymer coating. The advances made here in the synthesis of metal nanoclusters using xanthate-functionalized PCIEs could propel the production of highly monodisperse, biocompatible, and renally clearable nanoprobes in large-scale for different theranostic applications.

Quantification of Material Gradients in Core/Shell Nanocrystals Using EXAFS Spectroscopy.

Core/shell nanocrystals with a graded interface between core and shell exhibit improved optoelectronic properties compared with particles with an abrupt, sharp interface. Material gradients mitigate interfacial defects and define the shape of the confinement potential. So far, few works exist that allow to quantify the width of the gradient. In this study, ZnSe/CdS nanocrystals with graded shells made at different temperatures are characterized using extended X-ray absorption fine structure (EXAFS) and Raman spectroscopies. The average coordination number of the probed element with respect to the two possible counterions is fit to a simple, geometric model. It is shown that at the lower temperature limit for shell growth (260 °C), substantial interfacial alloying can be attributed mainly to cation migration. At higher temperature (290 °C), strain minimization leads to atomic ordering of the metal ions and an anomalously low degree of phase mixing.

Spectroscopic Evidence for the Contribution of Holes to the Bleach of Cd-Chalcogenide Quantum Dots.

In transient absorption (TA) measurements on Cd-chalcogenide quantum dots (QDs), the presence of a band-edge (BE) bleach signal is commonly attributed entirely to conduction-band electrons in the 1S(e) state, neglecting contributions from BE holes. While this has been the accepted view for more than 20 years, and has often been used to distinguish electron and hole kinetics, the reason for the absence of a hole contribution to the BE-bleach has remained unclear. Here, we show with three independent experiments that holes do in fact have a significant impact on the BE-bleach of well-passivated Cd-chalcogenide QD samples. Transient absorption experiments on high photoluminescence quantum yield CdSe/CdS/ZnS core-shell-shell QDs clearly show an increase of the band-edge bleach as holes cool down to the band edge. The relative contribution of electron-to-hole bleach is 2:1, as predicted by theory. The same measurements on core-only CdSe QDs with a lower quantum yield do not show a contribution of holes to the band-edge bleach. We assign the lack of hole bleach to the presence of ultrafast hole trapping in samples with insufficient passivation of the QD surface. In addition, we show measurements of optical gain in core-shell-shell QD solutions, providing clear evidence of a significant hole contribution to the BE transient absorption signal. Finally, we present spectroelectrochemical measurements on CdTe QDs films, showing the presence of a BE-bleach for both electron and hole injections. The presence of a contribution of holes to the bleach in passivated Cd-chalcogenides QDs bears important implications for quantitative studies on optical gain as well as for TA determinations of carrier dynamics.

Tuning Single Quantum Dot Emission with a Micromirror.

The photoluminescence of single quantum dots fluctuates between bright (on) and dark (off) states, also termed fluorescence intermittency or blinking. This blinking limits the performance of quantum dot-based devices such as light-emitting diodes and solar cells. However, the origins of the blinking remain unresolved. Here, we use a movable gold micromirror to determine both the quantum yield of the bright state and the orientation of the excited state dipole of single quantum dots. We observe that the quantum yield of the bright state is close to unity for these single QDs. Furthermore, we also study the effect of a micromirror on blinking, and then evaluate excitation efficiency, biexciton quantum yield, and detection efficiency. The mirror does not modify the off-time statistics, but it does change the density of optical states available to the quantum dot and hence the on times. The duration of the on times can be lengthened due to an increase in the radiative recombination rate.

The Role of Dopant Ions on Charge Injection and Transport in Electrochemically Doped Quantum Dot Films.

Control over the charge density is very important for implementation of colloidal semiconductor nanocrystals into various optoelectronic applications. A promising approach to dope nanocrystal assemblies is charge injection by electrochemistry, in which the charge compensating electrolyte ions can be regarded as external dopant ions. To gain insight into the doping mechanism and the role of the external dopant ions, we investigate charge injection in ZnO nanocrystal assemblies for a large series of charge compensating electrolyte ions with spectroelectrochemical and electrochemical transistor measurements. We show that charge injection is limited by the diffusion of cations in the nanocrystal films as their diffusion coefficient are found to be ∼7 orders of magnitude lower than those of electrons. We further show that the rate of charge injection depends strongly on the cation size and cation concentration. Strikingly, the onset of electron injection varies up to 0.4 V, depending on the size of the electrolyte cation. For the small ions Li+ and Na+ the onset is at significantly less negative potentials. For larger ions (K+, quaternary ammonium ions) the onset is always at the same, more negative potential, suggesting that intercalation may take place for Li+ and Na+. Finally, we show that the nature of the charge compensating cation does not affect the source-drain electronic conductivity and mobility, indicating that shallow donor levels from intercalating ions fully hybridize with the quantum confined energy levels and that the reorganization energy due to intercalating ions does not strongly affect electron transport in these nanocrystal assemblies.

Finding and Fixing Traps in II-VI and III-V Colloidal Quantum Dots: The Importance of Z-Type Ligand Passivation.

Energy levels in the band gap arising from surface states can dominate the optical and electronic properties of semiconductor nanocrystal quantum dots (QDs). Recent theoretical work has predicted that such trap states in II-VI and III-V QDs arise only from two-coordinated anions on the QD surface, offering the hypothesis that Lewis acid (Z-type) ligands should be able to completely passivate these anionic trap states. In this work, we provide experimental support for this hypothesis by demonstrating that Z-type ligation is the primary cause of PL QY increase when passivating undercoordinated CdTe QDs with various metal salts. Optimized treatments with InCl3 or CdCl2 afford a near-unity (>90%) photoluminescence quantum yield (PL QY), whereas other metal halogen or carboxylate salts provide a smaller increase in PL QY as a result of weaker binding or steric repulsion. The addition of non-Lewis acidic ligands (amines, alkylammonium chlorides) systematically gives a much smaller but non-negligible increase in the PL QY. We discuss possible reasons for this result, which points toward a more complex and dynamic QD surface. Finally we show that Z-type metal halide ligand treatments also lead to a strong increase in the PL QY of CdSe, CdS, and InP QDs and can increase the efficiency of sintered CdTe solar cells. These results show that surface anions are the dominant source of trap states in II-VI and III-V QDs and that passivation with Lewis acidic Z-type ligands is a general strategy to fix those traps. Our work also provides a method to tune the PL QY of QD samples from nearly zero up to near-unity values, without the need to grow epitaxial shells.

Colloidal Stability of Apolar Nanoparticles: Role of Ligand Length.

Inorganic nanoparticle cores are often coated with organic ligands to render them dispersible in apolar solvents. However, the effect of the ligand shell on the colloidal stability of the overall hybrid particle is not fully understood. In particular, it is not known how the length of an apolar alkyl ligand chain affects the stability of a nanoparticle dispersion against agglomeration. Here, small-angle X-ray scattering and molecular dynamics simulations have been used to study the interactions between gold nanoparticles and between cadmium selenide nanoparticles passivated by alkanethiol ligands with 12-18 carbons in the solvent decane. We find that increasing the ligand length increases colloidal stability in the core-dominated regime but decreases it in the ligand-dominated regime. This unexpected inversion is connected to the transition from ligand-dominated to core-dominated agglomeration when the core diameter increases at constant ligand length. Our results provide a microscopic picture of the forces that determine the colloidal stability of apolar nanoparticles and explain why classical colloid theory fails.

Bio-catalytic nanoparticle shaping for preparing mesoscopic assemblies of semiconductor quantum dots and organic molecules.

We report a unique bio-catalytic nanoparticle shaping (BNS) method for preparing a variety of mesoscopic particles by a facile process. For example, the BNS method affords mesoscopic QD assembly dispersions. Large-size sedimentations (>1 µm) of QDs are first formed using oligo-L-lysine linkers. These then undergo controlled enzymatic cleavage of the linkers using trypsin, which surprisingly leads to mesoscopic particles about 84 nm in size with a narrow size distribution. A detailed mechanism of the BNS method is investigated using tetrakis(4-carboxyphenyl)porphyrin (TCPP), instead of QDs, as a probe molecule. Interestingly, the BNS method can also be applied to other combinations of enzymes and enzymatically degradable linkers, such as hyaluronidase with hyaluronan. As a potential application, the mesoscopic particles of QDs and oligo-lysine exhibit their ability to act as a drug delivery carrier originating from the features of both QDs and oligo-lysine. The BNS method demonstrates the universality and versatility of preparing mesoscopic particles and opens new doors for studying QD assemblies and molecular-based mesoscopic particles.

Synthesis of Size-Tunable Indium Nitride Nanocrystals.

Indium nitride (InN) is an air stable III-V semiconductor with a small band gap of 0.7 eV and as such is of interest as a key material in near-infrared (NIR) LEDs, photodetectors, and multijunction solar cells. Conventionally, InN has been synthesized through physical deposition techniques which involve extreme temperatures and harsh conditions and yield nonluminescent materials. Here we report a new low temperature, solution-based synthetic route to colloidal InN nanocrystals that produces phase-pure wurtzite InN with tunable photoluminescence. The luminescence is attributed to the presence of InN surface states.

Evaluation of a lanthanide nanoparticle-based contrast agent for microcomputed tomography of porous channels in subchondral bone.

Osteoarthritis (OA) is a chronic joint disease that causes disability and pain. The osteochondral interface is a gradient tissue region that plays a significant role in maintaining joint health. It has been shown that during OA, increased neoangiogenesis creates porous channels at the osteochondral interface allowing the transport of molecules related to OA. Importantly, the connection between these porous channels and the early stages of OA development is still not fully understood. Microcomputed tomography (microCT) offers the ability to image the porous channels at the osteochondral interface, however, a contrast agent is necessary to delineate the different X-ray attenuations of the tissues. In this study BaYbF5 -SiO2 nanoparticles are synthesized and optimized as a microCT contrast agent to obtain an appropriate contrast attenuation for subsequent segmentation of structures of interest, that is, porous channels, and mouse subchondral bone. For this purpose, BaYbF5 nanoparticles were synthesized and coated with a biocompatible silica shell (SiO2 ). The optimized BaYbF5 -SiO2 27 nm nanoparticles exhibited the highest average microCT attenuation among the biocompatible nanoparticles tested. The BaYbF5 -SiO2 27 nm nanoparticles increased the mean X-ray attenuation of structures of interest, for example, porous channel models and mouse subchondral bone. The BaYbF5 -SiO2 contrast attenuation was steady after diffusion into mouse subchondral bone. In this study, we obtained for the first time, the average microCT attenuation of the BaYbF5 -SiO2 nanoparticles into porous channel models and mouse subchondral bone. In conclusion, BaYbF5 -SiO2 nanoparticles are a potential contrast agent for imaging porous channels at the osteochondral interface using microCT.

Tuning the Photoluminescence Anisotropy of Semiconductor Nanocrystals.

Semiconductor nanocrystals are promising optoelectronic materials. Understanding their anisotropic photoluminescence is fundamental for developing quantum-dot-based devices such as light-emitting diodes, solar cells, and polarized single-photon sources. In this study, we experimentally and theoretically investigate the photoluminescence anisotropy of CdSe semiconductor nanocrystals with various shapes, including plates, rods, and spheres, with either wurtzite or zincblende structures. We use defocused wide-field microscopy to visualize the emission dipole orientation and find that spheres, rods, and plates exhibit the optical properties of 2D, 1D, and 2D emission dipoles, respectively. We rationalize the seemingly counterintuitive observation that despite having similar aspect ratios (width/length), rods and long nanoplatelets exhibit different defocused emission patterns by considering valence band structures calculated using multiband effective mass theory and the dielectric effect. The principles are extended to provide general relationships that can be used to tune the emission dipole orientation for different materials, crystalline structures, and shapes.

Ultrafast imaging of terahertz electric waveforms using quantum dots.

Microscopic electric fields govern the majority of elementary excitations in condensed matter and drive electronics at frequencies approaching the Terahertz (THz) regime. However, only few imaging schemes are able to resolve sub-wavelength fields in the THz range, such as scanning-probe techniques, electro-optic sampling, and ultrafast electron microscopy. Still, intrinsic constraints on sample geometry, acquisition speed and field strength limit their applicability. Here, we harness the quantum-confined Stark-effect to encode ultrafast electric near-fields into colloidal quantum dot luminescence. Our approach, termed Quantum-probe Field Microscopy (QFIM), combines far-field imaging of visible photons with phase-resolved sampling of electric waveforms. By capturing ultrafast movies, we spatio-temporally resolve a Terahertz resonance inside a bowtie antenna and unveil the propagation of a Terahertz waveguide excitation deeply in the sub-wavelength regime. The demonstrated QFIM approach is compatible with strong-field excitation and sub-micrometer resolution-introducing a direct route towards ultrafast field imaging of complex nanodevices in-operando.

Developing Lattice Matched ZnMgSe Shells on InZnP Quantum Dots for Phosphor Applications.

Indium phosphide quantum dots (QDs) have drawn attention as alternatives to cadmium- and lead-based QDs that are currently used as phosphors in lamps and displays. The main drawbacks of InP QDs are, in general, a lower photoluminescence quantum yield (PLQY), a decreased color purity, and poor chemical stability. In this research, we attempted to increase the PLQY and stability of indium phosphide QDs by developing lattice matched InP/MgSe core-shell nanoheterostructures. The choice of MgSe comes from the fact that, in theory, it has a near-perfect lattice match with InP, provided MgSe is grown in the zinc blende crystal structure, which can be achieved by alloying with zinc. To retain lattice matching, we used Zn in both the core and shell and we fabricated InZnP/Zn x Mg1-x Se core/shell QDs. To identify the most suitable conditions for the shell growth, we first developed a synthesis route to Zn x Mg1-x Se nanocrystals (NCs) wherein Mg is effectively incorporated. Our optimized procedure was employed for the successful growth of Zn x Mg1-x Se shells around In(Zn)P QDs. The corresponding core/shell systems exhibit PLQYs higher than those of the starting In(Zn)P QDs and, more importantly, a higher color purity upon increasing the Mg content. The results are discussed in the context of a reduced density of interface states upon using better lattice matched Zn x Mg1-x Se shells.

When Like Destabilizes Like: Inverted Solvent Effects in Apolar Nanoparticle Dispersions.

We report on the colloidal stability of nanoparticles with alkanethiol shells in apolar solvents. Small-angle X-ray scattering and molecular dynamics simulations were used to characterize the interaction between nanoparticles in linear alkane solvents ranging from hexane to hexadecane, including 4 nm gold cores with hexadecanethiol shells and 6 nm cadmium selenide cores with octadecanethiol shells. We find that the agglomeration is enthalpically driven and that, contrary to what one would expect from classical colloid theory, the temperature at which the particles agglomerate increases with increasing solvent chain length. We demonstrate that the inverted trend correlates with the temperatures at which the ligands order in the different solvents and show that the inversion is due to a combination of enthalpic and entropic effects that enhance the stability of the ordered ligand state as the solvent length increases. We also explain why cyclohexane is a better solvent than hexadecane despite the two having very similar solvation parameters.

Correction: Protic additives determine the pathway of CdSe nanocrystal growth.

Correction for 'Protic additives determine the pathway of CdSe nanocrystal growth' by Klaus Boldt et al., Nanoscale, 2018, 10, 18238-18248.

Electrochemical Modulation of the Photophysics of Surface-Localized Trap States in Core/Shell/(Shell) Quantum Dot Films.

In this work, we systematically study the spectroelectrochemical response of CdSe quantum dots (QDs), CdSe/CdS core/shell QDs with varying CdS shell thicknesses, and CdSe/CdS/ZnS core/shell/shell QDs in order to elucidate the influence of localized surface trap states on the optoelectronic properties. By correlating the differential absorbance and the photoluminescence upon electrochemically raising the Fermi level, we reveal that trap states near the conduction band (CB) edge give rise to nonradiative recombination pathways regardless of the CdS shell thickness, evidenced by quenching of the photoluminescence before the CB edge is populated with electrons. This points in the direction of shallow trap states localized on the CdS shell surface that give rise to nonradiative recombination pathways. We suggest that these shallow trap states reduce the quantum yield because of enhanced hole trapping when the Fermi level is raised electrochemically. We show that these shallow trap states are removed when additional wide band gap ZnS shells are grown around the CdSe/CdS core/shell QDs.

Engineering the Band Alignment in QD Heterojunction Films via Ligand Exchange.

Colloidal quantum dots (QDs) allow great flexibility in the design of optoelectronic devices, thanks to their size-dependent optical and electronic properties and the possibility to fabricate thin films with solution-based processing. In particular, in QD-based heterojunctions, the band gap of both components can be controlled by varying the size of the QDs. However, control over the band alignment between the two materials is required to tune the dynamics of carrier transfer across a heterostructure. We demonstrate that ligand exchange strategies can be used to control the band alignment of PbSe and CdSe QDs in a mixed QD solid, shifting it from a type-I to a type-II alignment. The change in alignment is observed in both spectroelectrochemical and transient absorption measurements, leading to a change in the energy of the conduction band edges in the two materials and in the direction of electron transfer upon photoexcitation. Our work demonstrates the possibility to tune the band offset of QD heterostructures via control of the chemical species passivating the QD surface, allowing full control over the energetics of the heterostructure without requiring changes in the QD composition.

Protic additives determine the pathway of CdSe nanocrystal growth.

The formation of semiconductor nanocrystals by hot-injection synthesis follows complex reaction mechanisms that are not yet fully understood. In particular the occurrence of intermediate species indicated by sharp, stationary spectral lines poses an important deviation from the predictions of classical nucleation theory. We show that trace amounts of water and other protic additives strongly impact the structure of these reaction intermediates, forming either coordination polymers under dry conditions or small clusters in the presence of moisture. These intermediates bind monomer during the initial nucleation phase. The structure of the intermediate determines the monomer release rate, either continuously or in a rapid dissolution event, and hence controls the reaction kinetics. From this we propose a kinetic model that allows us to predict secondary nucleation events. By directing the type of intermediate formed, protic additives provide a lever to manipulate this equilibrium and control nanocrystal synthesis in a rational fashion.

Two Mechanisms Determine Quantum Dot Blinking.

Many potential applications of quantum dots (QDs) can only be realized once the luminescence from single nanocrystals (NCs) is understood. These applications include the development of quantum logic devices, single-photon sources, long-life LEDs, and single-molecule biolabels. At the single-nanocrystal level, random fluctuations in the QD photoluminescence occur, a phenomenon termed blinking. There are two competing models to explain this blinking: Auger recombination and surface trap induced recombination. Here we use lifetime scaling on core-shell chalcogenide NCs to demonstrate that both types of blinking occur in the same QDs. We prove that Auger-blinking can yield single-exponential on/off times in contrast to earlier work. The surface passivation strategy determines which blinking mechanism dominates. This study summarizes earlier studies on blinking mechanisms and provides some clues that stable single QDs can be engineered for optoelectronic applications.