Tuning the Inter-Nanoplatelet Distance and Coupling Strength by Thermally Induced Ligand Decomposition.

CdSe nanoplatelets (NPLs) are promising 2D semiconductors for optoelectronic applications, in which efficient charge transport properties are desirable. It is reported that thermal annealing constitutes an effective strategy to control the optical absorption and electrical properties of CdSe NPLs by tuning the inter-NPL distance. Combining optical absorption, transmission electron microscopy, and thermogravimetric analysis, it is revealed that the thermal decomposition of ligands (e.g., cadmium myristate) governs the inter-NPL distance and thus the inter-NPL electronic coupling strength. Employing ultrafast terahertz spectroscopy, it is shown that this enhanced electronic coupling increases both the free carrier generation efficiency and the short-range mobility in NPL solids. The results show a straightforward method of controlling the interfacial electronic coupling strength for developing functional optoelectronic devices through thermal treatments.

The Crucial Role of Solvation Forces in the Steric Stabilization of Nanoplatelets.

The steric stability of inorganic colloidal particles in an apolar solvent is usually described in terms of the balance between three contributions: the van der Waals attraction, the free energy of mixing, and the ligand compression. However, in the case of nanoparticles, the discrete nature of the ligand shell and the solvent has to be taken into account. Cadmium selenide nanoplatelets are a special case. They combine a weak van der Waals attraction and a large facet to particle size ratio. We use coarse grained molecular dynamics simulations of nanoplatelets in octane to demonstrate that solvation forces are strong enough to induce the formation of nanoplatelet stacks and by that have a crucial impact on the steric stability. In particular, we demonstrate that for sufficiently large nanoplatelets, solvation forces are proportional to the interacting facet area, and their strength is intrinsically tied to the softness of the ligand shell.

Anisotropic and hyperbranched InP nanocrystals via chemical transformation of in situ produced In2O3.

We synthesized indium phosphide (InP) nanoparticles of different shapes and sizes, utilizing triphenyl phosphite (TPOP) as the phosphorus source. We show that this reaction proceeds via the formation of in situ formed In2O3 nanoparticles followed by subsequent transformation with triphenyl TPOP acting as the phosphorus source. Our findings open up new synthetic possibilities utilizing a cost-effective, non-pyrophoric and non-toxic phosphorus precursor. The large surface area of hyperbranched InP NCs might be ideally suited for surface-driven processes such as catalysis and energy storage.

Nanocarriers Made of Proteins: Intracellular Visualization of a Smart Biodegradable Drug Delivery System.

This work analyzes the intracellular fate of protein-based nanocarriers along their endolysosomal pathway by means of correlative light and electron microscopy methods. To unambiguously identify the nanocarriers and their degradation remnants in the cellular environment, they are labeled with fluorescent, inorganic nanoplatelets. This allows tracking the nanocarriers on their intracellular pathway by means of electron microscopy imaging. From the present data, it is possible to identify different cell compartments in which the nanocarriers are processed. Finally, three different terminal routes for the intracellular destiny of the nanocarriers are presented. These findings are important to reveal the degradation process of protein nanocapsules and contribute to the understanding of the therapeutic success of an encapsulated drug.

Surface Properties of Colloidal Particles Affect Colloidal Self-Assembly in Evaporating Self-Lubricating Ternary Droplets.

In this work, we unravel the role of surface properties of colloidal particles on the formation of supraparticles (clusters of colloidal particles) in a colloidal Ouzo droplet. Self-lubricating colloidal Ouzo droplets are an efficient and simple approach to form supraparticles, overcoming the challenge of the coffee stain effect in situ. Supraparticles are an efficient route to high-performance materials in various fields, from catalysis to carriers for therapeutics. Yet, the role of the surface of colloidal particles in the formation of supraparticles using Ouzo droplets remains unknown. Therefore, we used silica particles as a model system and compared sterically stabilized versus electrostatically stabilized silica particles─positively and negatively charged. Additionally, we studied the effect of hydration. Hydrated negatively charged silica particles and sterically stabilized silica particles form supraparticles. Conversely, dehydrated negatively charged silica particles and positively charged amine-coated particles form flat film-like deposits. Notably, the assembly process is different for all the four types of particles. The surface modifications alter (a) the contact line motion of the Ouzo droplet and (b) the particle-oil and particle-substrate interactions. These alterations modify the particle accumulation at the various interfaces, which ultimately determines the shape of the final deposit. Thus, by modulating the surface properties of the colloidal particles, we can tune the shape of the final deposit, from a spheroidal supraparticle to a flat deposit. In the future, this approach can be used to tailor the supraparticles for applications such as optics and catalysis, where the shape affects the functionality.

Particle Size Determines the Shape of Supraparticles in Self-Lubricating Ternary Droplets.

Supraparticles are large clusters of much smaller colloidal particles. Controlling the shape and anisotropy of supraparticles can enhance their functionality, enabling applications in fields such as optics, magnetics, and medicine. The evaporation of self-lubricating colloidal ouzo droplets is an easy and efficient strategy to create supraparticles, overcoming the problem of the "coffee-stain effect" during drop evaporation. Yet, the parameters that control the shape of the supraparticles formed in such evaporating droplets are not fully understood. Here, we show that the size of the colloidal particles determines the shape of the supraparticle. We compared the shape of the supraparticles made of seven different sizes of spherical silica particles, namely from 20 to 1000 nm, and of the mixtures of small and large colloidal particles at different mixing ratios. Specifically, our in situ measurements revealed that the supraparticle formation proceeds via the formation of a flexible shell of colloidal particles at the rapidly moving interfaces of the evaporating droplet. The time tc0 when the shell ceases to shrink and loses its flexibility is closely related to the size of particles. A lower tc0, as observed for smaller colloidal particles, leads to a flat pancake-like supraparticle, in contrast to a more curved American football-like supraparticle from larger colloidal particles. Furthermore, using a mixture of large and small colloidal particles, we obtained supraparticles that display a spatial variation in particle distribution, with small colloids forming the outer surface of the supraparticle. Our findings provide a guideline for controlling the supraparticle shape and the spatial distribution of the colloidal particles in supraparticles by simply self-lubricating ternary drops filled with colloidal particles.

Unraveling the Growth Mechanism of Magic-Sized Semiconductor Nanocrystals.

Magic-sized clusters (MSCs) of semiconductor are typically defined as specific molecular-scale arrangements of atoms that exhibit enhanced stability. They often grow in discrete jumps, creating a series of crystallites, without the appearance of intermediate sizes. However, despite their long history, the mechanism behind their special stability and growth remains poorly understood. It is particularly difficult to explain experiments that have shown discrete evolution of MSCs to larger sizes well beyond the "cluster" regime and into the size range of colloidal quantum dots. Here, we study the growth of MSCs, including these larger magic-sized CdSe nanocrystals, to unravel the underlying growth mechanism. We first introduce a synthetic protocol that yields a series of nine magic-sized nanocrystals of increasing size. By investigating these crystallites, we obtain important clues about the mechanism. We then develop a microscopic model that uses classical nucleation theory to determine kinetic barriers and simulate the growth. We show that magic-sized nanocrystals are consistent with a series of zinc-blende crystallites that grow layer by layer under surface-reaction-limited conditions. They have a tetrahedral shape, which is preserved when a monolayer is added to any of its four identical facets, leading to a series of discrete nanocrystals with special stability. Our analysis also identifies strong similarities with the growth of semiconductor nanoplatelets, which we then exploit to further increase the size range of our magic-sized nanocrystals. Although we focus here on CdSe, these results reveal a fundamental growth mechanism that can provide a different approach to nearly monodisperse nanocrystals.

Plasmonic and Semiconductor Nanoparticles Interfere with Stereolithographic 3D Printing.

Two-photon polymerization stereolithographic three-dimensional (3D) printing is used for manufacturing a variety of structures ranging from microdevices to refractive optics. Incorporation of nanoparticles in 3D printing offers huge potential to create even more functional nanocomposite structures. However, this is difficult to achieve since the agglomeration of the nanoparticles can occur. Agglomeration not only leads to an uneven distribution of nanoparticles in the photoresin but also induces scattering of the excitation beam and altered absorption profiles due to interparticle coupling. Thus, it is crucial to ensure that the nanoparticles do not agglomerate during any stage of the process. To achieve noninteracting and well-dispersed nanoparticles on the 3D printing process, first, the stabilization of nanoparticles in the 3D printing resin is indispensable. We achieve this by functionalizing the nanoparticles with surface-bound ligands that are chemically similar to the photoresin that allows increased nanoparticle loadings without inducing agglomeration. By systematically studying the effect of different nanomaterials (Au nanoparticles, Ag nanoparticles, and CdSe/CdZnS nanoplatelets) in the resin on the 3D printing process, we observe that both, material-specific (absorption profiles) and unspecific (radical quenching at nanoparticle surfaces) pathways co-exist by which the photopolymerization procedure is altered. This can be exploited to increase the printing resolution leading to a reduction of the minimum feature size.

Template Stripping of Perovskite Thin Films for Dry Interfacing and Surface Structuring.

Combining excellent optoelectronic properties with the benefits of solution processability, metal-halide perovskites are promising materials for photovoltaic and light-emitting technologies. To facilitate the integration of perovskite thin films into optoelectronic devices, a need exists for simple and efficient fabrication methods. Here, we present a template-stripping technique to produce ultraflat and flexible perovskite thin films. We apply a one-step spin-coating procedure to produce high-quality CH3NH3PbBr3 perovskite thin films on top of ultraflat silicon templates. These films can be mechanically cleaved from the template using a polymer adhesive to reveal the ultraflat perovskite surfaces. We demonstrate how the flatness and flexibility of the template-stripped films enable new processing strategies based on dry interfacing, by interfacing them with plasmonic hole arrays. Moreover, we show that by using prepatterned silicon templates, structured perovskite surfaces can be produced with feature sizes down to a micrometer.

Kinetic Control over Self-Assembly of Semiconductor Nanoplatelets.

Semiconductor nanoplatelets exhibit spectrally pure, directional fluorescence. To make polarized light emission accessible and the charge transport effective, nanoplatelets have to be collectively oriented in the solid state. We discovered that the collective nanoplatelets orientation in monolayers can be controlled kinetically by exploiting the solvent evaporation rate in self-assembly at liquid interfaces. Our method avoids insulating additives such as surfactants, making it ideally suited for optoelectronics. The monolayer films with controlled nanoplatelets orientation (edge-up or face-down) exhibit long-range ordering of transition dipole moments and macroscopically polarized light emission. Furthermore, we unveil that the substantial in-plane electronic coupling between nanoplatelets enables charge transport through a single nanoplatelets monolayer, with an efficiency that strongly depends on the orientation of the nanoplatelets. The ability to kinetically control the assembly of nanoplatelets into ordered monolayers with tunable optical and electronic properties paves the way for new applications in optoelectronic devices.

Water-dispersed semiconductor nanoplatelets with high fluorescence brightness, chemical and colloidal stability.

Quasi-two dimensional semiconductor nanoplatelets (NPLs) exhibit high spectral brightness and large absorption cross sections, making them promising for various applications including bioimaging. However, the synthesis of NPLs takes place in organic solvents, therefore they require phase transfer in order to use them in aqueous environments. The phase transfer of NPLs has so far been challenging with few examples in literature. This is likely due to the facile agglomeration of materials with plate-like geometries during the coating procedure. Here we demonstrate how to overcome agglomeration and transfer NPLs, individually coated with amphiphilic polymer chains, to aqueous phase. Upon one and two-photon excitation the water transferred NPLs exhibit more than two fold higher fluorescent brightness relative to commercially available quantum dots. Additionally, the polymer coating increase the stability of nanoparticles in physiological conditions (pH 4.5-7.4, [NaCl] 5.8-11.7 g L-1, and in human serum). Our experiments with NPL labeled RAW264.7 cells demonstrate the capabilities of NPLs as next generation ultra-bright fluorescent labels for bioimaging.

Identifying reactive organo-selenium precursors in the synthesis of CdSe nanoplatelets.

In the synthesis of CdSe nanoplatelets, the selenium-to-selenide reduction pathway is unknown. We study solvent-free growth of CdSe nanoplatelets and identify bis(acyl) selenides as key reactive intermediates. Based on our findings, we prepare a series of bis(acyl) selenides that provide useful precursors with tailored reactivity for liquid-phase syntheses of nanoplatelets.

Photocatalytic Water-Splitting Enhancement by Sub-Bandgap Photon Harvesting.

Upconversion is a photon-management process especially suited to water-splitting cells that exploit wide-bandgap photocatalysts. Currently, such catalysts cannot utilize 95% of the available solar photons. We demonstrate here that the energy-conversion yield for a standard photocatalytic water-splitting device can be enhanced under solar irradiance by using a low-power upconversion system that recovers part of the unutilized incident sub-bandgap photons. The upconverter is based on a sensitized triplet-triplet annihilation mechanism (sTTA-UC) obtained in a dye-doped elastomer and boosted by a fluorescent nanocrystal/polymer composite that allows for broadband light harvesting. The complementary and tailored optical properties of these materials enable efficient upconversion at subsolar irradiance, allowing the realization of the first prototype water-splitting cell assisted by solid-state upconversion. In our proof-of concept device the increase of the performance is 3.5%, which grows to 6.3% if concentrated sunlight (10 sun) is used. Our experiments show how the sTTA-UC materials can be successfully implemented in technologically relevant devices while matching the strict requirements of clean-energy production.

Ripening of Semiconductor Nanoplatelets.

Ostwald ripening describes how the size distribution of colloidal particles evolves with time due to thermodynamic driving forces. Typically, small particles shrink and provide material to larger particles, which leads to size defocusing. Semiconductor nanoplatelets, thin quasi-two-dimensional (2D) particles with thicknesses of only a few atomic layers but larger lateral dimensions, offer a unique system to investigate this phenomenon. Experiments show that the distribution of nanoplatelet thicknesses does not defocus during ripening, but instead jumps sequentially from m to (m + 1) monolayers, allowing precise thickness control. We investigate how this counterintuitive process occurs in CdSe nanoplatelets. We develop a microscopic model that treats the kinetics and thermodynamics of attachment and detachment of monomers as a function of their concentration. We then simulate the growth process from nucleation through ripening. For a given thickness, we observe Ostwald ripening in the lateral direction, but none perpendicular. Thicker populations arise instead from nuclei that capture material from thinner nanoplatelets as they dissolve laterally. Optical experiments that attempt to track the thickness and lateral extent of nanoplatelets during ripening appear consistent with these conclusions. Understanding such effects can lead to better synthetic control, enabling further exploration of quasi-2D nanomaterials.

High-temperature growth of thick-shell CdSe/CdS core/shell nanoplatelets.

To optimize the optical properties of semiconductor nanoplatelets, simple routes to add high-quality shells are needed. We demonstrate uniform growth of CdS shells on CdSe nanoplatelets at 300 °C, overcoming limitations of previous low-temperature syntheses. We obtain core/shell nanoplatelets with spectrally narrow (20 nm) and efficient emission for shells up to 4 nm thick.

Tuning and Locking the Localized Surface Plasmon Resonances of CuS (Covellite) Nanocrystals by an Amorphous CuPd x S Shell.

We demonstrate the stabilization of the localized surface plasmon resonance (LSPR) in a semiconductor-based core-shell heterostructure made of a plasmonic CuS core embedded in an amorphous-like alloyed CuPd x S shell. This heterostructure is prepared by reacting the as-synthesized CuS nanocrystals (NCs) with Pd2+ cations at room temperature in the presence of an electron donor (ascorbic acid). The reaction starts from the surface of the CuS NCs and proceeds toward the center, causing reorganization of the initial lattice and amorphization of the covellite structure. According to density functional calculations, Pd atoms are preferentially accommodated between the bilayer formed by the S-S covalent bonds, which are therefore broken, and this can be understood as the first step leading to amorphization of the particles upon insertion of the Pd2+ ions. The position and intensity in near-infrared LSPRs can be tuned by altering the thickness of the shell and are in agreement with the theoretical optical simulation based on the Mie-Gans theory and Drude model. Compared to the starting CuS NCs, the amorphous CuPd x S shell in the core-shell nanoparticles makes their plasmonic response less sensitive to a harsh oxidation environment (generated, for example, by the presence of I2).

An intrinsic growth instability in isotropic materials leads to quasi-two-dimensional nanoplatelets.

Colloidal nanoplatelets are atomically flat, quasi-two-dimensional sheets of semiconductor that can exhibit efficient, spectrally pure fluorescence. Despite intense interest in their properties, the mechanism behind their highly anisotropic shape and precise atomic-scale thickness remains unclear, and even counter-intuitive for commonly studied nanoplatelets that arise from isotropic crystal structures (such as zincblende CdSe and lead halide perovskites). Here we show that an intrinsic instability in growth kinetics can lead to such highly anisotropic shapes. By combining experimental results on the synthesis of CdSe nanoplatelets with theory predicting enhanced growth on narrow surface facets, we develop a model that explains nanoplatelet formation as well as observed dependencies on time and temperature. Based on standard concepts of volume, surface and edge energies, the resulting growth instability criterion can be directly applied to other crystalline materials. Thus, knowledge of this previously unknown mechanism for controlling shape at the nanoscale can lead to broader libraries of quasi-two-dimensional materials.

Nanoscale Transformations in Covellite (CuS) Nanocrystals in the Presence of Divalent Metal Cations in a Mild Reducing Environment.

We studied the structural and compositional transformations of colloidal covellite (CuS) nanocrystals (and of djurleite (Cu1.94S) nanocrystals as a control) when exposed to divalent cations, as Cd2+ and Hg2+, at room temperature in organic solvents. All the experiments were run in the absence of phosphines, which are a necessary ingredient for cation exchange reactions involving copper chalcogenides, as they strongly bind to the expelled Cu+ ions. Under these experimental conditions, no remarkable reactivity was indeed seen for both CuS and Cu1.94S nanocrystals. On the other hand, in the covellite structure 2/3 of sulfur atoms form covalent S-S bonds. This peculiarity suggests that the combined presence of electron donors and of foreign metal cations can trigger the entry of both electrons and cations in the covellite lattice, causing reorganization of the anion framework due to the rupture of the S-S bonds. In Cu1.94S, which lacks S-S bonds, this mechanism should not be accessible. This hypothesis was proven by the experimental evidence that adding ascorbic acid increased the fraction of metal ions incorporated in the covellite nanocrystals, while it had no noticeable effect on the Cu1.94S ones. Once inside the covellite particles, Cd2+ and Hg2+ cations engaged in exchange reactions, pushing the expelled Cu+ ions toward the not-yet exchanged regions in the same particles, or out to the solution, from where they could be recaptured by other covellite nanoparticles/domains. Because no good solvating agent for Cu ions was present in solution, they essentially remained in the nanocrystals.

Post-Synthesis Incorporation of 64Cu in CuS Nanocrystals to Radiolabel Photothermal Probes: A Feasible Approach for Clinics.

We report a simple method for the incorporation of Cu(I) or (64)Cu(I) radionuclides in covellite nanocrystals (CuS NCs). After the in situ reduction of Cu(II) or (64)Cu(II) ions by ascorbic acid, their incorporation in PEG-coated CuS NCs takes place at room temperature. In all the reaction steps, the stability of the NCs under physiological conditions was ensured. The copper incorporation reaction could also take place on CuS NCs bearing biotin molecules at their surface, with no detrimental effects on the specific binding affinity of the NCs toward streptavidin after incorporation. At low loading of Cu ions, the strong near-infrared (NIR) absorption band of the starting CuS NCs was essentially preserved, which allowed for efficient plasmonic photothermal therapy. The combined presence in the NCs of (64)Cu ions, well suitable for positron emission tomography, and of free carriers responsible for the NIR absorption, should enable their theranostic use as radiotracers and as photothermal probes in tumor ablation treatments. Moreover, the simplicity of the preparation scheme, which involves the use of radioactive species only as a last step, makes the protocol easily transferable to the clinical practice.

Wedge Waveguides and Resonators for Quantum Plasmonics.

Plasmonic structures can provide deep-subwavelength electromagnetic fields that are useful for enhancing light-matter interactions. However, because these localized modes are also dissipative, structures that offer the best compromise between field confinement and loss have been sought. Metallic wedge waveguides were initially identified as an ideal candidate but have been largely abandoned because to date their experimental performance has been limited. We combine state-of-the-art metallic wedges with integrated reflectors and precisely placed colloidal quantum dots (down to the single-emitter level) and demonstrate quantum-plasmonic waveguides and resonators with performance approaching theoretical limits. By exploiting a nearly 10-fold improvement in wedge-plasmon propagation (19 µm at a vacuum wavelength, λvac, of 630 nm), efficient reflectors (93%), and effective coupling (estimated to be >70%) to highly emissive (~90%) quantum dots, we obtain Ag plasmonic resonators at visible wavelengths with quality factors approaching 200 (3.3 nm line widths). As our structures offer modal volumes down to ~0.004λvac(3) in an exposed single-mode waveguide-resonator geometry, they provide advantages over both traditional photonic microcavities and localized-plasmonic resonators for enhancing light-matter interactions. Our results confirm the promise of wedges for creating plasmonic devices and for studying coherent quantum-plasmonic effects such as long-distance plasmon-mediated entanglement and strong plasmon-matter coupling.