"Coupling of atoms is the basis of chemistry, yielding the beauty and richness of molecules. We utilize semiconductor nanocrystals as artificial atoms to form nanocrystal molecules that are structurally and electronically coupled. CdSe/CdS core/shell nanocrystals are linked to form dimers which are then fused via constrained oriented attachment. The possible nanocrystal facets in which such fusion takes place are analyzed with atomic resolution revealing the distribution of possible crystal fusion scenarios. Coherent coupling and wave-function hybridization are manifested by a redshift of the band gap, in agreement with quantum mechanical simulations. Single nanoparticle spectroscopy unravels the attributes of coupled nanocrystal dimers related to the unique combination of quantum mechanical tunneling and energy transfer mechanisms. This sets the stage for nanocrystal chemistry to yield a diverse selection of coupled nanocrystal molecules constructed from controlled core/shell nanocrystal building blocks. These are of direct relevance for numerous applications in displays, sensing, biological tagging and emerging quantum technologies."

"Structural transformations in molecules and solids have generally been studied in isolation, whereas intermediate systems have eluded characterization. We show that a pair of cadmium sulfide (CdS) cluster isomers provides an advantageous experimental platform to study isomerization in well-defined, atomically precise systems. The clusters coherently interconvert over an ~1–electron volt energy barrier with a 140–milli–electron volt shift in their excitonic energy gaps. There is a diffusionless, displacive reconfiguration of the inorganic core (solid-solid transformation) with first order (isomerization-like) transformation kinetics. Driven by a distortion of the ligand-binding motifs, the presence of hydroxyl species changes the surface energy via physisorption, which determines “phase” stability in this system. This reaction possesses essential characteristics of both solid-solid transformations and molecular isomerizations and bridges these disparate length scales."

"Semiconductor nanocrystals have been shown to have unique advantages over traditional organic photoinitiators for polymerization in solution. However, efficient photoinitiation with such nanoparticles in solvent-free and additive-free formulations so far has not been achieved. Herein, the ability to use semiconductor nanocrystals for efficient bulk polymerization as sole initiators is reported, operating under modern UV-blue-LED light sources found in 3D printers and other photocuring applications. Hybrid semiconductor–metal nanorods exhibit superior photoinitiation capability to their pristine semiconductor counterparts, attributed to the enhanced charge separation and oxygen consumption in such systems. Moreover, photoinitiation by semiconductor nanocrystals overcoated by inorganic ligands is reported, thus increasing the scope of possible applications and shedding light on the photoinitiation mechanism; in light of the results, two possible pathways are discussed – ligand-mediated and cation-coordinated oxidation. A demonstration of the unique attributes of the quantum photoinitiators is reported in their use for high-resolution two-photon printing of optically fluorescing microstructures, demonstrating a multi-functionality capability. The bulk polymerization demonstrated here can be advantageous over solvent based methods as it alleviates the need of post-polymerization drying and reduces waste and exposure to toxic solvents, as well as broadens the possible use of quantum photoinitiators for industrial and research uses."

"Colloidal quantum materials are nanocrystals containing hundreds to thousands of atoms that exhibit unique properties resulting from their small finite dimensions. The extraordinary flexibility in tuning their properties via composition, size- and dimensionality-related quantum confinement effects and surface engineering combined with their scalable bottom-up manufacturing has already led to their commercialization in different light-emitting applications, such as materials for displays and as fluorescent agents for imaging and sensing. Beyond light emission, harnessing absorbed light energy to perform useful chemical work is an important new avenue for diverse applications of the colloidal quantum materials. Here, we introduce the colloidal quantum materials and their virtues, focusing on the “all-in-one system” concept for semiconductor–metal hybrid nanoparticles acting as photocatalysts. Next, their emerging photocatalytic functionalities are highlighted, including their action as photocatalysts for solar-to-fuel conversion and as photoinitiators for photo-curing and biomedical applications, such as phototherapy, sterilization, and diagnostics."

"DNA nanostructures provide a powerful platform for the programmable assembly of nanomaterials. Here, this approach is extended to semiconductor nanorods that possess interesting electrical properties and could be utilized for the bottom‐up fabrication of nanoelectronic building blocks. The assembly scheme is based on an efficient DNA functionalization of the nanorods. A complete coverage of the rod surface with DNA ensures a high colloidal stability while maintaining the rod size and shape. It furthermore supports the assembly of the nanorods at defined docking positions of a DNA origami platform with binding efficiencies of up to 90 % as well as the formation of nanorod dimers with defined relative orientations. By incorporating orthogonal binding sites for gold nanoparticles, defined metal‐semiconductor heterostructures can be fabricated. Subsequent application of a seeded growth procedure onto the gold nanoparticles (AuNPs) allows for to establish a direct metal‐semiconductor interface as a crucial basis for the integration of semiconductors in self‐assembled nanoelectronic devices."

We investigate the electronic properties of individual n-type (Cu) doped and p-type (Ag) doped InAs colloidal nanocrystals (NCs) in the 2–8 nm size range from their charge transfers toward a highly oriented pyrolytic graphite (HOPG) substrate, using ultrahigh vacuum Kelvin probe force microscopy (KPFM) with elementary charge sensitivity at 300 K. The NC active dopant concentration is measured as N_D = 8 × 10²⁰ cm^–3 and N_A > 5 × 10²⁰ cm^–3 for n- and p-type doping, respectively. The electrostatic equilibrium between the NC and the HOPG reference substrate is investigated and reveals an enhancement of the Fermi-level mismatch between the NCs and the HOPG substrate at reduced NC sizes, both for n- and p-type doping. It also shows, for n-type doped NCs with smallest sizes (∼2 nm), the existence of a full depletion regime, in which smallest NCs contain single ionized dopants. Results are compared with self-consistent tight-binding calculations of the electronic structure of InAs NCs, including hydrogenoid impurities and the presence of a host substrate, in the case of n-type doped NCs. The observed enhancement of the NC–HOPG Fermi-level mismatch can be understood by considering a size-dependent electrostatic contribution attributed to dipolar effects at the NC–ligand interface. The estimated surface dipole density equals a few Debye/nm² and is increased at smallest NC sizes, which follows the enhancement of ligand densities at small NC sizes previously reported for metallic or semiconducting NCs. The results put forward the role played by the NC–ligand interface electrostatics for electronic applications.

"Coupled colloidal quantum dot molecules composed of two fused CdSe/CdS core/shell sphere monomers were recently presented. Upon fusion, the potential energy landscape changes into two quantum dots separated by a pretuned potential barrier with energetics dictated by the conduction and valence band offsets of the core/shell semiconductors and the width controlled by the shell thickness and the fusion reaction conditions. In close proximity of the two nanocrystals, orbital hybridization occurs, forming bonding and antibonding states in analogy to the hydrogen molecule. In this study, we examine theoretically the electronic and optical signatures of such a quantum dot dimer compared to its monomer core/shell building-blocks. We examine the effects of different core sizes, barrier widths, different band offsets, and neck sizes at the interface of the fused facets on the system wave-functions and energetics. Due to the higher effective mass of the hole and the large valence band offset, the hole still essentially resides in either of the cores, breaking the symmetry of the potential for the electron as well. We found that the dimer signature is well expressed in a red shift of the band gap both in absorption and emission, in slower radiative lifetimes and in an absorption cross section which is significantly enhanced relative to the monomers at energies above the shell absorption onset, while remains essentially at the same level near the band-edge. This study provides essential guidance to predesign of coupled quantum dot molecules with specific attributes which can be utilized for various new opto-electronic applications."

"Colloidal semiconductor quantum dots (QDs) are promising for solar energy harvesting, particularly the excess energy of hot and/or high energy excitons before they relax, which would allow efficiencies beyond the Shockley-Queisser limit. Going beyond QDs, nanostructured materials such as nanorods (NRs) are attracting significant attention, as their geometrical differences with QDs imply different electronic properties, and their one-dimensional character makes them ideal building blocks for hybrid nanostructures [1,2]. Experimental observation of the charge relaxation pathways in such nanostructured systems is challenging, as high resolution in time (to resolve ultrafast processes) and in excitation energy (to distinguish the relaxation pathways of different excitons) are simultaneously required, which are incompatible with regular transient absorption (TA). Both requirements are met by two-dimensional electronic spectroscopy (2DES), which can be thought as an extension of TA employing two phase-locked pump pulses. Acquiring the data as a function of the delay t ₁ between them and using Fourier transform spectroscopy yields a correlation map between signal excitation and signal emission energies for each waiting time t ₂ [3]. As it provides resolution in the excitation energy, 2DES is ideal to study systems with spectrally congested electronic transitions and was applied to study many nanostructured systems."

"Quasi‐1D colloidal semiconductor nanorods (NRs) are at the forefront of nanoparticle (NP) research owing to their intriguing size‐dependent and shape‐dependent optical and electronic properties. The past decade has witnessed significant advances in both fundamental understanding of the growth mechanisms and applications of these stimulating materials. Herein, the state‐of‐the‐art of colloidal semiconductor NRs is reviewed, with special emphasis on heavy‐metal‐free materials. The main growth mechanisms of heavy‐metal‐free colloidal semiconductor NRs are first elaborated, including anisotropic‐controlled growth, oriented attachment, solution–liquid–solid method, and cation exchange. Then, structural engineering and properties of semiconductor NRs are discussed, with a comprehensive overview of core/shell structures, alloying, and doping, as well as semiconductor–metal hybrid nanostructures, followed by highlighted practical applications in terms of photocatalysis, photodetectors, solar cells, and biomedicine. Finally, challenges and future opportunities in this fascinating research area are proposed."

"The majority of developed and approved anticancer nanomedicines have been designed to exploit the dogma of the enhanced permeability and retention (EPR) effect, which is based on the leakiness of the tumor’s blood vessels accompanied by impeded lymphatic drainage. However, the EPR effect has been under scrutiny recently because of its variable manifestation across tumor types and animal species and its poor translation to human cancer therapy. To facilitate the EPR effect, systemically injected NPs should overcome the obstacle of rapid recognition and elimination by the mononuclear phagocyte system (MPS). We hypothesized that circulating monocytes, major cells of the MPS that infiltrate the tumor, may serve as an alternative method for achieving increased tumor accumulation of NPs, independent of the EPR effect. We describe here the accumulation of liposomal quantum dots (LipQDs) designed for active delivery via monocytes, in comparison to LipQDs designed for passive delivery (via the EPR effect), following IV administration in a mammary carcinoma model. Hydrophilic QDs were synthesized and entrapped in functionalized liposomes, conferring passive (“stealth” NPs; PEGylated, neutral charge) and active (monocyte-mediated delivery; positively charged) properties by differing in their lipid composition, membrane PEGylation, and charge (positively, negatively, and neutrally charged). The various physicochemical parameters affecting the entrapment yield and optical stability were examined in vitro and in vivo. Biodistribution in the blood, various organs, and in the tumor was determined by the fluorescence intensity and Cd analyses. Following the treatment of animals (intact and mammary-carcinoma-bearing mice) with disparate formulations of LipQDs (differing by their lipid composition, neutrally and positively charged surfaces, and hydrophilic membrane), we demonstrate comparable tumor uptake of QDs delivered by the passive and the active routes (mainly by Ly-6C^hi monocytes). Our findings suggest that entrapping QDs in nanosized liposomal formulations, prepared by a new facile method, imparts superior structural and optical stability and a suitable biodistribution profile leading to increased tumor uptake of fluorescently stable QDs."

"We report a solution for the challenge of having luminescence and metal conductivity from the same material. The fabrication of a hybrid metal–conductive luminescent film that manifests this dual property is described: the conductivity arising from a continuous gold thin film structure and luminescence originating from the embedded fluorescent emitters (nanoparticles of silica-coated CdSe/CdS quantum dots (QD/SiO₂ NPs)). The embedding of the QD/SiO₂ NPs is performed via a self-templating gold electroless process. The presence of the insulating silica layer on the QDs avoids quenching and enables luminescence, while still allowing plasmonic coupling of the QDs, as observed by luminescence lifetime analysis and by surface-enhanced Raman scattering. The potential applications of this special dual functionality are demonstrated by its used as a temperature probe: Passing current (heating the gold thin film) affects the emission intensity and induces a spectral red-shift of the QD/SiO₂ NPs. All properties of this metal–conductive luminescent film required the special embedding architecture and are not observed with simple adsorption of QD/SiO₂ NPs on a continuous Au film."

"Zinc chalcogenides nanostructures are promising newly‐studied materials for photocatalytic reactions such as water reduction for hydrogen generation and photoinitiation of radical polymerization processes. Herein, we introduced a straightforward synthesis of thin ZnS shell on ZnSe nanorods and characterized the resulting core/shell ZnSe/ZnS nanorods by electron microscopy techniques and X‐ray photoelectron spectroscopy. Then, the photocatalytic activities of the nanorods were studied by gas chromatography for hydrogen generation and by Fourier transform infrared spectroscopy for the photoinitiation capability. While such core/shell systems may limit charge transfer to the solution due to the type‐I band alignment, we find that the photochemical stability and photocatalytic activity of this system is significantly enhanced over pristine ZnSe nanorods. ZnSe/ZnS nanorods with Ni(NO₃)₂ co‐catalyst are used for hydrogen generation and the effects of the nanorods’ surface coating, pH conditions and type of hole scavenger are examined. The best performances were observed for nanorods after ligand stripping with Meerwein′s reagent using ascorbic acid as a hole acceptor at pH 4.5. All three parameters were found to affect the photocatalytic activity. Yet, the surface coating and pH dependence differ from previous reports on cadmium chalcogenides photocatalysts. The origin of their differences is discussed and attributed to the band alignment of the systems and the nature of the co‐catalyst. The stable heavy‐metal free ZnSe/ZnS nanorods system presented here holds promise for various photocatalytic applications."

"Silica aerogel is a solid matrix, capable of preserving the unique properties of nanoparticles incorporated in it; forming a macroscopic structure that benefits from nano-metric properties.

We developed a simple method for preparation of doped silica aerogels. Dopants are implemented as silica coated nanoparticles suspended in alcohol or alcohol:water mixture. This suspension replaces a portion of the pure solvent in the one-step base catalyzed silica aerogel recipe.

Aerogels doped with, Au nano-spheres, Ag nano-platelets, and CdSe/CdS core/shell quantum dots, were prepared. These aerogels exhibit homogeneous mono-dispersion of the nanoparticles, and possess the nanoparticles' unique optical properties. Additionally, doping does not affect the gelation process or the silica matrix.

We envision that using this methodology, many other silica coated nanoparticles can be implemented as dopants. Furthermore, complex systems of several co-dopants can be obtained in a reproducible manner. Such aerogels can be tailor made for a vast range of applications."

"Semiconductor heterostructure nanocrystals, especially with core/shell architectures, are important for numerous applications.  Here we show that by decreasing the shell growth rate the morphology of ZnS shells on ZnSe quantum rods can be tuned from flat to islands-like, which decreases the interfacial strain energy. Further reduced growth speed, approaching the thermodynamic limit, leads to coherent shell growth forming unique helical-shell morphology.  This reveals a template-free mechanism for induced chirality at the nanoscale. The helical morphology minimizes the sum of the strain and surface energy and maintains band gap emission due to its coherent core/shell interface without traps, unlike the other morphologies. Reaching the thermodynamic controlled growth regime for colloidal semiconductor core/shell nanocrystals thus offers morphologies with clear impact on their applicative potential."

"The Electrical Functionality of an Array of Semiconductor Nanocrystals (NCs) depends critically on the free carriers that may arise from impurity or surface doping. Herein, we used InAs NC thin films as a model system to address the relative contributions of these doping mechanisms by comparative analysis of as-synthesized and Cu-doped NC-based field effect transistor (FET) characteristics. By applying FET simulation methods used in conventional semiconductor FETs, we elucidate surface and impurity-doping contributions to the overall performance of InAs NC-based FETs. As-synthesized InAs NC-based FETs show n-type characteristics assigned to the contribution of the surface electron accumulation layer that can be considered as an actual electron-donating doping level with specific doping density and is energetically located just below the conduction band. The Cu-doped InAs NC FETs show enhanced n-type conduction as expected from the Cu impurity location as an interstitial n-dopant in InAs NCs. The simulated curves reveal the additional contribution from electrons within an impurity sub-band close to the conduction band onset of the InAs NCs. The work therefore demonstrates the utility of the bulk FET simulation methodology also to NC-based FETs. It provides guidelines for control of doping of NC arrays separately from surface contributions and impurity doping in colloidal semiconductor NCs toward their future utilization as building blocks in bottom-up prepared optoelectronic devices."