RESUMEN
Fe2+ doping in II-VI semiconductors, due to the absence of energetically accessible multiple spin state configurations, has not given rise to interesting spintronic applications. In this work, we demonstrate for the first time that the interaction of homogeneously doped Fe2+ ions with the host CdS nanocrystal with no clustering is different for the two spin states and produces two magnetically inequivalent excitonic states upon optical perturbation. We combine ultrafast transient absorption spectroscopy and density functional theoretical analysis within the ground and excited states to demonstrate the presence of the magneto-optical Stark effect (MOSE). The energy gap between the spin states arising due to MOSE does not decay within the time frame of observation, unlike optical and electrical Stark shifts. This demonstration provides a stepping-stone for spin-dependent applications.
RESUMEN
Digestive ripening of bimetallic magnetic nanocrystals from uniform microsheets to spherical nanocrystals was observed in FeCoS2 nanocrystals leading to the formation of monodisperse nanocrystals. Earlier examples of digestive ripening are associated with the transformation of polydisperse particles to monodisperse particles deriving energetic stabilization from the monodispersity. However, it is interesting to note that in the current case, not only did we observe a transformation from uniform sheets to spherical particles but we also observed it in the absence of thiol, the most commonly used digestive ripening agent. We have then studied the effect of ligands such as oleic acid and oleylamine responsible for this ripening process. Long chain acids were found to be majorly responsible for digestive ripening while the amines assist in the formation of microsheets. A plausible mechanism has then been proposed.
RESUMEN
Precise control of crystal orientation, and specifically the exposed surface, is critical for the engineering of heterostructures. Here, using CoPt as a model system, we explore the energetics to expose suitable facets to promote the required heterostructure formation. Different heterostructures are grown ranging from core/shell structure, diffused interface, dumbbell structured dimers, and embedded island structures wherein these hybrids are fabricated via micro/macrolevel facet-selective growth. The reaction conditions used to achieve such diversity starting from the same seed offer insights into the growth mechanisms of these heterostructures. Such a microscopic understanding of surface chemistry paves the way for the design of new heterostructures with exciting properties.
RESUMEN
Dual doping is a powerful way to tailor the properties of semiconductor quantum dots (QDs) arising out of host-dopant and dopant-dopant interactions. Nevertheless, it has seldom been explored due to a variety of thermodynamic challenges, such as the differential bonding strength and diffusion constant within the host matrix that integrates with the host in dissimilar ways. This work discusses the challenges involved in administering them within the constraints of one host under similar conditions of temperature, time, and chemical parameters such as solubility and reactivity using CoPt-doped CdS QDs as a model system. In addition, the various forces in play, such as Kirkendall diffusion, solid- and liquid-state diffusion, hard acid soft base interaction with the host, and the effect of lattice strain due to lattice mismatch, are studied to understand the feasibility of the core to doped transformation. These findings suggest a potential approach for manipulating the properties of semiconductors by dual doping engineering.
RESUMEN
Impurity doping in semiconductor quantum dots (QDs) has numerous prospects in implementing and altering their properties and technologies. Herein, we review the state-of-the-art doping techniques arising from colloidal synthesis methods. We first discuss the advantages and challenges involved in doping; we then discuss various doping techniques, including clustering of dopants as well as expulsion out of the lattice due to self-purification. Some of these techniques have been shown to open up a new generation of robust doped semiconductor quantum dots with cluster-free doping which will be suitable for various spin-based solid-state device technologies and overcome the longstanding challenges of controlled impurity doping. Further, we discuss inhibitors such as defects, clustering and interfaces, followed by current open questions. These include pathways to obtain uniform doping in the required radial position with unprecedented control over the dopant concentration and the size of the QDs.
RESUMEN
Semiconducting materials uniformly doped with optical or magnetic impurities have been useful in a number of potential applications. However, clustering or phase separation during synthesis has made this job challenging. Recently the "inside out" diffusion doping was proposed to be successful in obtaining large sized quantum dots (QDs) uniformly doped with a dilute percentage of dopant atoms. Herein, we demonstrate the use of basic physical chemistry of diffusion to control the size and concentration of the dopants within the QDs for a given transition metal ion. We have studied three parameters; the bond strength of the core molecules and the diffusion coefficient of the diffusing metal ion are found to be important while the ease of cation exchange was not highly influential in the control of size and concentration of the single domain dilute magnetic semiconductor quantum dots (DMSQDs) with diverse dopant ions M2+ (Fe2+, Ni2+, Co2+, Mn2+). Steady state optical emission spectra reveal that the dopants are incorporated inside the semiconducting CdS and the emission can be tuned during shell growth. We have shown that this method enables control over doping percentage and the QDs show a superior ferromagnetic response at room temperature as compared to previously reported systems.