Boldt, Klaus
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Localized Co2+ Doping of CdSe/CdS Seeded Nanorods
Transition-metal-doped semiconductor nanocrystals have garnered interest in the field of spintronics because of their capability to couple an applied magnetic field to a photonic response, and vice versa. These structures are a synthetic challenge due to difficulties in controlling the distribution of dopants as well as introducing the impurity atoms into the host material in the first place. In this paper we present a route toward anisotropically Co2+-doped CdSe/CdS dot-in-rods, in which the dopants are localized at the nanorod tips, at a defined distance from the CdSe seed particle. The localized doping of seeded nanorods allows to gain a deeper understanding of the interaction of the dopant with an exciton through competitive photoluminescence quenching, highlighting the fact that the dopant locale is more important than dopant concentration. It also yields information about the kinetics that govern impurity diffusion, with an activation energy for Co2+ in nanocrystalline CdS of Ea = 61.11 kJ mol–1.
Control of Hot Carrier Relaxation in CsPbBr3 Nanocrystals Using Damping Ligands
In photon-conversion processes, rapid cooling of photo-induced hot carriers is a dominant energy loss channel. We herein report a 3-fold reduced hot carrier cooling rate in CsPbBr 3 nanocrystals capped with a cross-linked polysiloxane shell in comparison to single alkyl-chain oleylamine ligands. Relaxation of hot charge carriers depends on the carrier-phonon coupling (CPC) process as an important channel to dissipate energies in nanostructured perovskite materials. The CPC strengths in the two samples were measured through cryogenic photoluminescence spectroscopic measurements. The effect of organic ligands on the CPC in CsPbBr 3 nanocrystals is elucidated based on a damped oscillation model. This supplements the conventional polaron-based CPC model, by involving a damping effect on the CPC from the resistance of the ligands against nanocrystal lattice vibrations. The model also accounts for the observed linear temperature-dependence of the CPC strength. Our work enables predictions about the effect of the ligands on the performance of perovskite nanocrystals in future applications.
Sensibilization of p-NiO with ZnSe/CdS and CdS/ZnSe quantum dots for photoelectrochemical water reduction
Core/shell quantum dots (QDs) paired with semiconductor photocathodes for water reduction have rarely been implemented so far. We demonstrate the integration of ZnSe/CdS and CdS/ZnSe QDs with porous p-type NiO photocathodes for water reduction. The QDs demonstrate appreciable enhancement in water-reduction efficiency, as compared with the bare NiO. Despite their different structure, both QDs generate comparable photocurrent enhancement, yielding a 3.8- and 3.2-fold improvement for the ZnSe/CdS@NiO and CdS/ZnSe@NiO system, respectively. Unraveling the carrier kinetics at the interface of these hybrid photocathodes is therefore critical for the development of efficient photoelectrochemical (PEC) proton reduction. In addition to examining the carrier dynamics by the Mott–Schottky technique and electrochemical impedance spectroscopy (EIS), we performed theoretical modelling for the distribution density of the carriers with respect to electron and hole wave functions. The electrons are found to be delocalized through the whole shell and can directly actuate the PEC-related process in the ZnSe/CdS QDs. The holes as the more localized carriers in the core have to tunnel through the shell before injecting into the hole transport layer (NiO). Our results emphasize the role of interfacial effects in core/shell QDs-based multi-heterojunction photocathodes.
The Future of Colloidal Semiconductor Magic-Size Clusters
Atomically defined, zero-dimensional magic-size clusters play pivotal roles in the nucleation and growth of semiconductor nanocrystals. Thus, they provide new opportunities to understand and to control nucleation and growth reactions beyond classical nucleation theory and to employ these reactions in the colloidal synthesis of increasingly complex and anisotropic nanomaterials with atomic level monodispersity. Both challenges require reliable determination of the exact structure and size of these ultrasmall and metastable nanoclusters. In this Perspective, we review and discuss the current challenges in analytics of magic-size clusters, in elucidating their formation mechanism, and in using them as next-generation reagents in colloidal chemistry.
Analysis of Magic-Size Clusters in Crude Reaction Mixtures Using Multiwavelength Analytical Ultracentrifugation
Magic-size clusters are ultrasmall, transient species that appear early during semiconductor nanocrystal growth and have been assigned to stoichiometrically and geometrically well-defined structures. Yet, a direct analysis has proved difficult due to their limited stability and challenging isolation. Analytical ultracentrifugation is a promising tool to analyze these compounds, because it circumvents the need to purify and, thereby, destabilize the clusters, while yielding information on their sedimentation and diffusion behavior, size, shape, and composition. We employ a multiwavelength detector that allows the detection of whole UV/vis absorption spectra during centrifugation to separate and identify individual magic-size clusters in crude reaction mixtures sampled directly from the colloidal syntheses. Analytical ultracentrifugation thus constitutes an elegant, independent, high-resolution, and statistically significant method to show the stepwise growth of magic-size clusters, confirming recent growth models.
Raman spectroscopy of colloidal semiconductor nanocrystals
Raman spectroscopy is a powerful method that gives insight into the atomic structure and composition of nanomaterials, but also allows to draw conclusions about their electronic properties. It is based on the inelastic scattering of light, which is able to excite phonons in the material. In the field of semiconductor nanocrystals, Raman spectroscopy has been employed to make significant contributions to the analysis of lattice distortion, interfaces, phase mixing, and defect formation. Yet, there is no clear consensus on how the electronic and crystal structure of the material interacts with the incident light to yield the observed spectra. This review gives a brief overview over the method. It then reviews the most important findings, current developments, and discusses the efforts to formulate a consistent model that allows to establish the method as a tool for structural analysis.
Rapid synthesis of vertically aligned α-MoO3 nanostructures on substrates
We report a new procedure for large scale, reproducible and fast synthesis of polycrystalline, dense, vertically aligned α-MoO3 nanostructures on conducting (FTO) and non-conducting substrates (Si/SiO2) by using a simple, low-cost hydrothermal technique. The synthesis method consists of two steps, firstly formation of a thermally evaporated Cr/MoO3 seed layer, and secondly growth of the nanostructures in a highly acidic precursor solution. In this report, we document a growth process of vertically aligned α-MoO3 nanostructures with varying growth parameters, such as pH and precursor concentration influencing the resulting structure. Vertically aligned MoO3 nanostructures are valuable for different applications such as electrode material for organic and dye-sensitized solar cells, as a photocatalyst, and in Li-ion batteries, display devices and memory devices due to their high surface area.
Chemoselective Surface Trap-Mediated Metal Growth on Semiconductor Nanocrystals
We present a highly chemoselective deposition of precious metals on semiconductor nanoheterostructures with a strong preference for cadmium and zinc telluride over the lighter chalcogenides. The selectivity is explained by p-type surface traps on the tellurides, compared to n-type defects of the homologous sulfides and selenides, and can be turned off by passivating the particle surface. The results give insight into the nature and role of surface defects for semiconductor nanocrystals. The fast formation of many, small metal seeds leads to aggregation of the particles into star-shaped or branched superstructures, leaving the rest of the semiconductor surface exposed. It provides a preparative route toward complex, yet well-defined semiconductor-metal hybrid structures with potential application in photocatalysis.
Oxygen vacancies in oxidized and reduced vertically aligned α-MoO3 nanoblades
Functionalized materials are highly desired for technological advancements spanning physics, chemistry, materials science, and biology due to their unique electronic properties. One such example is molybdenum trioxide (MoO3), a metal oxide with multiple oxidation states. Manipulating these oxidation states can alter the electronic properties, for instance, defects and electrical conductivity, by several orders of magnitude. In this work, oxygen vacancy-mediated intrinsic defects in vertically aligned a-MoO3 crystals are systematically tuned via thermal treatment under different reducing and oxidizing atmospheres. The positions and the concentration of the oxygen vacancies and restitution of the oxygen ions have been experimentally demonstrated via a range of techniques including electron paramagnetic resonance, X-ray diffraction, and high-resolution electron microscopy. The calculated concentration of the oxygen vacancies in the a-MoO3-x via EPR measurements is in the range of x = 0.004–0.049. The mechanism of the formation of oxygen vacancies in the a-MoO3-x crystal is understood via color center formation and polaron migration models. These oxygen vacancies show no influence on the optical band gap. However, they significantly impact the electrical conductivity on the order of 102 Sm-1 by altering the MoO3 properties from semi-insulating to conducting.
Kinetically Driven Cadmium Chalcogenide Nanorod Growth Fed by Local Cluster Aggregates
The process of symmetry breaking that leads to the formation of anisotropic, colloidal semiconductor nanocrystals is an important issue for understanding the physical mechanism of nucleation and growth. One-dimensional growth of nanorods is assumed to occur under reaction-limited control at a high concentration of monomer, which preferentially reacts at the crystal facet that has the highest energy and least passivation by surface ligands. In this study, it is shown that instead of assuming a homogeneous distribution of monomer in the reaction solution, the seeded growth of CdS nanorods on CdSe particles is driven by the formation of one-dimensional reaction intermediates that act as local monomer reservoirs. These result in heterogeneously distributed hotspots of the nucleating species that guarantees fast deposition and one-dimensional growth of the nanorod exclusively in one direction. Thus, performing anisotropic particle growth reactions under conditions that favor formation of transient, metastable intermediates lead to particles with a higher aspect ratio and better mechanistic reaction control.