Volume 229, Issue 1-2 - Special Issue: Chemical Nanoscience Dedicated to Horst Weller on the occasion of his 60th birthday / Alf Mews

The present work describes the structural and optical characterization of PbSe and PbSe/CdSe colloidal quantum dots (CQDs), the latter being produced by cation exchange of Pb 2+ for Cd 2+ ions. The cation exchange occurs on preferred crystallographic facets and results in either non-concentric CdSe shells or Pb x Cd 1 –x Se alloyed-shell layers. The obtained heterostuctures are referred to as PbSe/CdSe “thin shell” CQDs. The parent PbSe CQDs are limited to relatively small diameters of 3–4 nm, with absorption edge between 1.0 and 1.3 eV. The steady-state and time-resolved photoluminescence spectra recorded at various temperatures reveal the following properties of the PbSe/CdSe CQDs: (1) the photoluminescence intensity of air-free CQDs is maintained upon their exposure to oxygen; (2) the band-edge exciton lifetime is extended by about a factor of two relative to the parent PbSe CQDs. The experimental results and the effective mass-based calculations suggest the formation of alloyed shells and highlight a pronounced effect of core displacement from the CQD center on the heterostructure optical properties.

The development of the research into the synthesis and optoelectronic applications of HgTe and related quantum dots (QDs) is reviewed, from the early days when it was felt that it might be a useful replacement for rare earths in telecom optical amplifiers, through to its more recent and broader appeal as an IR photodetector technology. Appropriately the early investigation of the material sprang from a contact with Prof. Horst Weller and his group at Hamburg University. Though the problem of Auger recombination meant that it was not so easy to make telecom amplifiers and lasers as many had hoped, it has proved to have an extraordinarily broad bandgap tuning range and just recently this has resulted in the demonstration of photodetectors in the mid- to long-IR region operating at up to 12 μm wavelength. This impressive flexibility has led to interest in HgTe QDs and optoelectronic devices based on them to be as strong as ever and growing as the prospects for commercialization improve with every narrowing in the gap between the performance of present day epitaxial devices and QD-based technology. In a further satisfying and well timed twist, Prof. Weller's 60 th year has seen preliminary reports of extended gain lifetime in `doped' HgTe QDs which may yet lead to a completing of the circle with the distinct possibility of electrically driven telecom wavelength lasers and amplifiers in the coming years. This review follows the development of the several synthetic methods to grow colloidal HgTe QDs, and their deployment in optoelectronic devices to the present.

The design of new protocols for the colloidal synthesis of complex nanocrystals (NCs) with advanced functionalities, comprising both hybrid and hollow structures, and the study of their fundamental properties is of paramount importance for the development of a new generation of nanostructured materials. The possibility of tailoring the dimensional regime of NCs, along with its composition and structure, represents a landmark achievement in the control of their unique physico-chemical properties. These properties, alongside with the ability to cheaply produce high quality NCs in fairly large amounts by wet-chemistry techniques, leads to their potential applicability from materials science to nanomedicine. Within this context, this review is focused on describing a successful framework for designing synthetic strategies for the production of advanced complex NCs, integrating the development of new synthetic methods with its structural characterization, monitoring of their properties, and study of its reactivity. As a result, it is expected to provide new routes to produce robust and easy-to-process NCs in a wide range of sizes, shapes and configurations that can be explored to achieve the combination of all degrees of control, aiming to produce a complete and diverse library of material combinations that will expand its applicability in a wide diversity of fields.

Since the discovery of metal chalcogenide complexes (MCCs) as capping ligands for colloidal nanocrystals (NCs) in 2009, the chemistry of inorganic ligands for NCs has provided a new paradigm for surface design of nanomaterials. Various inorganic anions including MCCs, metal-free chalcogenides, oxoanions/oxometallates, and halides/pseudohalides/halometallates have been employed to replace the original long-chain organic ligands on NCs. This ligand exchange can also be achieved through a two-step route using ligands stripping agents like HBF 4 . This review outlines recent advances in inorganically-capped colloidal NCs and details the ligand exchange process for NCs using MCCs and metal-free chalcogenides. The binding affinities of ligands to NC surface have been rationalized in terms of Pearson's hard and soft acids and bases (HSAB) principle. We also demonstrate that inorganic ligands broaden the functionality of NCs by tailoring their electro-optical properties or generating new inorganic phases through chemical reactions between nanomaterials and their surface ligands. Especially promising are the electronic, optoelectronic, and thermoelectric applications of solution-processed, inorganically-capped colloidal NCs, which substantially outperform their organically-capped couterparts.

Colloidal quantum dots (QDs) bear a huge potential in different application fields due to their unique properties. Unfortunately, their limited stability against environmental influences and the limited processability of the colloidal solutions restricts, among other factors, their widespread implementation. Incorporating the QDs into an inorganic matrix allows overcoming of these barriers. In this manuscript, we demonstrate the reproducible incorporation of high quality CdTe QDs capped with thioglycolic acid into NaCl and KCl salt crystals. In comparison to previous reports, here we avoided the production of powder-like, weakly emitting mixed crystals and/or a loss of large amounts of QDs due to aggregation. The optimum conditions for this process were found by the variation of the amount of additional free-stabilizer and the pH of the saturated salt solutions. Both variables exhibit high influence on the colloidal stability of the QDs, ensuring the electrostatic forces on their surface to be high enough to prevent agglomeration. Incorporation in NaCl as host matrix is accompanied with pronounced etching of the QDs, causing a blue shift of the photoluminescence (PL)-spectra upon incorporation while KCl preserves the original optical properties of the QDs. Under optimized conditions, even the matrix-induced red shift of the emission upon incorporation can be overcome, resulting in a higher predictability of the mixed crystal preparation.

The presence of halogen compounds in the synthesis of semiconductor nanocrystals (NCs) produced by hot injection drastically influence their capping ligand sphere and/or chemical reactivity, allowing for the growth of hybrid systems, including composites combining NCs decorating carbon sp 2 surfaces. As a result, the presence of halide anions on the surface of some NCs has proven to have a positive impact on the optical and electrical properties. In this work the effects that halogen co-solvents (including alkylchlorides, bromides, and iodides) induce in the synthesis of rod-like Wurtzite CdSe NCs generated by hot-injection will be reviewed. The proposed mechanism of the reaction as well as a detailed characterization of the capping ligand sphere by Nuclear Magnetic Resonance (NMR) and X-Ray photoelectron Spectroscopy (XPS) will be discussed. Correlated cyclic voltammetry (CV) and XPS studies have been performed to address the effect of the halide anions on the NCs surface. Finally an example of the effect of the addition of chlorine-containing co-solvents on the synthesis of PbS rock-salt NCs will be shown.

Colloidal lead sulfide is a versatile material with great opportunities to tune the bandgap by electronic confinement and to adapt the optical and electrical properties to the target application. We present a new and simple synthetic route to control size and shape of PbS nanoparticles. Increasing concentrations of explicitly added acetic acid are used to tune the shape of PbS nanoparticles from quasi-spherical particles via octahedrons to six-armed stars. The presence of acetate changes the intrinsic surface energies of the different crystal facets and enables the growth along the 〈100〉 direction. Furthermore, the presence of 1,2-dichloroethane alters the reaction kinetics, which results in smaller nanoparticles with a narrower size distribution.

In order to assess the anisotropy-related uncertainties of relatively determined photoluminescence quantum yields ( Φ PL ) of molecular emitters and luminescent nanomaterials, we compared Φ PL values measured without and with polarizers using magic angle conditions and studied systematically the dependence of the detected emission intensity on the polarizer settings for samples of varying anisotropy. This includes a dispersion of a spherical quantum dot (QD) with an ideally isotropic emission, a solution of a common small organic dye in a fluid solvent as well as dispersions of elongated quantum dot rods (QDR) with an anisotropic luminescence and a small organic dye in a rigid polymeric matrix, as ideally anisotropic emitter. Our results show that for instruments lacking polarizers, anisotropy-related measurement uncertainties of relative photoluminescence quantum yields can amount to more than 40%, with the size of these systematic errors depending on the difference in emission anisotropy between the sample and the standard.

Recent experimental data on charge carrier transport in quantum dot solids is analyzed in light of the question whether band-like transport is the most plausible mechanism to account for the observed behavior. Upon reviewing some physical fundamentals of temperature-activated hopping, small polaron hopping and band-like transport, it is established that a combined approach of different experimental methods is required to achieve a satisfactory degree of unambiguousness to address this question. In doing so, at least one example is identified for which a high degree of supporting evidence exists that transport is in fact diffusive. It is highlighted that temperature-dependent Hall Effect measurements play an important role in this respect.

Quantum confinement effects are observed in transport measurements of Cu 2 S nanocrystal based devices. Two nanocrystals sizes are studied, 3 nm being in the quantum-confinement regime, and 14 nm, lacking confinement. The effect of ligand length on the charge transport mechanism is studied via conductance temperature dependence measurements. While in the 14 nm nanocrystal based devices unique non-monotonic temperature dependence is observed, the 3 nm based devices show only thermally activated transport for all ligands. The difference is attributed to a cross-over from inter-particle hopping to intra-particle dominated transport as the ligand length increases. In the 3 nm devices the effect of ligand length on the charge-hopping activation energy is also discussed.

Colloidal semiconductor nanocrystals with tunable optical properties are promising materials for light harvesting in solar cells. So far, in particular cadmium and lead chalcogenide nanocrystals were intensively studied in this respect, and the device performance has made rapid progress in recent years. In contrast, less research efforts were undertaken to develop solar cells based on Cd- and Pb-free nanoparticles as absorber material. In the present work, we report on Schottky solar cells with the absorber layer made of colloidal copper indium disulfide nanocrystals. Absorber films with up to ∼ 500 nm thickness were realized by a solution-based layer-by-layer deposition technique. The device performance was systematically studied dependent on the absorber layer thickness. Decreasing photocurrent densities with increasing thickness revealed charge transport to be a limiting factor for the device performance.

CdSe/CdS core-shell nanocrystals with controlled CdS shell thickness and CdSe core size were synthesized for several different values of these two parameters. The particles in aqueous dispersion were in situ decorated with Ni nanoparticles and evaluated for photocatalytic hydrogen generation capacity. The highest H 2 production quantum yield at 457 nm illumination was 4.7%, while at 530 nm it rose to 9.1%. We have found that ensuring equal absorption at illumination wavelength the H 2 formation rate increases with the shell thickness, but decreases with the increasing core size. Interestingly, the same trend was observed for the photoluminesce quantum yield and lifetime for samples without Ni decoration, suggesting that they reflect the balance between charge transfer and avoidance of recombination sites which is crucial for photocatalysis with semiconductor nanocrystals. Moreover, the core-shell nanocrystals constitute a very convenient model system in which the charge carrier dynamics in photocatalytic applications can be studied. In comparison with well-known CdSe/CdS dot-in-rod structures they are less efficient at blue illumination, but they allow for significant extension of the usable absorption range.

In this article, we are reporting the catalytic activity of gold nanoparticles (AuNPs) dispersed in a novel eco-friendly Deep Eutectic Solvent (DES; choline chloride and urea) at room temperature. A low energy sputter deposition method is employed to synthesize AuNPs on the DES surface. This is a simple, economical and clean method for producing monodisperse gold nanoparticles by a self-limited growth mechanism. Detailed Small Angle X-ray Scattering (SAXS), UV–Vis and Transmission Electron Microscopy (TEM) investigations show the formation of spherical AuNPs of 5 nm in diameter. Moreover, these particles have a strong tendency to self-assemble at the DES surface as well as into the bulk. This is a new addition to a novel generation of gold based catalysts. The catalytic activity of Au nanoparticles in DES is revealed quantitatively by analyzing the apparent conversion reaction rate constant k app of para-nitrophenol to para-aminophenol in the presence of sodium borohydride. Catalytic activity reported in the present article is much faster ( k app  = 0.34 s – 1 ) than the rate reported earlier ( k app  = 0.030 min – 1 ) by Chirea et al. for gold nanowire networks [Langmuir, 2011, 27, 3906]. In agreement with the literature, we explain the higher catalytic activity in terms of a larger surface area of AuNPs in a non-self-assembled state.

Spheric and anisotropic gold nanoparticles (GNPs) such as rods, stars or nanoprism prepared using hexadecyltrimethyl ammonium bromide (CTAB) as the stabilising agent have received a great deal of interest in the last years. The literature procedures exploited lead to GNPs in aqueous solution. We herein describe a fast, efficient, and cheap method to transfer particles of different shapes from water into toluene solution via ligand exchange (CTAB to dodecanethiol), which was mediated by acetone as a cosolvent. Absorption spectra and TEM-pictures before and after the transfer revealed that the particles survived the transfer intact and without change in shape.

β -NaYF 4 :Yb,Er upconversion nanocrystals are synthesized by a hot-injection method employing purified α -NaYF 4 :Yb,Er particles with a size of approximately 2–3 nm as single-source precursor. The method allows to prepare either spherical nanoparticles with different sizes or nanorods by simply controlling the addition rate of the solution of α -phase particles and performing the synthesis either in the absence or in the presence of seed particles. The luminescence properties of the different particles are compared also including core-shell particles.

The integration of different types of materials in a single hybrid system allows the combination of multiple functionalities, which can even be used in conjunction with each other. This strategy has been exploited in nanoscale systems for the creation of so-called smart nanomaterials. Within this category, the combination of inorganic nanoparticles with stimuli-responsive microgels is of very high interest because of the wide variety of potential applications. We present here a short overview of this type of materials in which the nano- and micro-scales get nicely integrated, with a great potential to expand the range of technological applications. We focus mainly on the integration of metal nanoparticles, either by themselves or in combination with semiconductor and magnetic nanoparticles. Various examples of the synergic properties that can be obtained are described, as well as the possibility to extract useful information when optical tweezers are used to manipulate single particles. We expect that this review will stimulate additional research in this field.

In the present short review, loosely packed two-dimensional arrays of hydrogel colloids with quasi-hexagonal order, which are fabricated by self-assembly on solid substrates, are discussed. For this purpose the most prominent hydrogel microgel, composed of poly( N -isopropylacrylamide), is considered. Its extraordinary properties dictate not only the formation of loosely packed 2D colloidal crystals but also pave the road to various applications ranging from optical devices to cell culture substrates.

We report on the growth of carbon nanowalls and few-layer graphene sheets on glass substrates coated with different transparent conductive oxides or gold. The growth is accomplished by a capacitively-coupled radio-frequency plasma-enhanced chemical vapor deposition setup with a gas mixture of C 2 H 2 , H 2 , and Ar or He. This system allows the synthesis of thin vertical carbon nanosheets without catalyst at mild reaction conditions due to the low plasma density, which is preferable for sensitive substrate materials. In fact, the electrical properties of the transparent conductive indium tin oxide and fluorine-doped tin oxide stay intact upon carbon growth. Carbon nanowalls and few-layer graphene sheets on transparent conductive oxides are promising candidates for solar-cell applications while these nanostructures on gold coated glass can act as catalyst support material.