Magnetism at the Interface of Magnetic Oxide and Nonmagnetic Semiconductor Quantum Dots

An effective strategy for the development of multiferroic composite nanostructures with enhanced magnetoelectric coupling performance: a perovskite-spinel approach

An energy efficient move toward the regulation of magnetization vector solely with E-field by developing multiferroic (MF) magnetoelectric (ME) nanostructures' have opened up vast avenues for novel low power consumption memories and magnetoelectric devices. The present study delineates the development of multiferroic nanocomposites (MF NCs) with perovskite BiFeO3 and spinel NiFe2O4. A simple Pechini one-pot method is employed for the preparation of nanocomposites with different molar concentrations and the modified structural, magnetic, dielectric, and magnetoelectric performance of NCs is carried out at room temperature. The purity in the crystalline phase of BiFeO3 and NiFe2O4 in the NCs are corroborated by XRD and FTIR spectroscopy, which is further substantiated, while EDAX analysis pointed out the elemental composition. The SEM images provide an evidence for the closely compacted and agglomerated grains with some voids in the prepared sample. A well saturated magnetic hysteresis (M-H) loop reveals long range ferromagnetic ordering and the remanence value ascertains the multidomain structure. The dielectric performance and impedance analysis imparts greater knowledge of the dependence of grain and grain boundaries on the resistive and conductive behavior of the composites. The frequency dependence of electric modulus and impedance exhibits a non-Debye type of relaxation process. The P-E hysteresis loops clearly show the lossy behavior of the composites with an increase in the NiFe2O4 concentration. The ME coupling coefficient of the composites evaluated using the dynamic magnetic field method demonstrates the effective coupling interaction between ferroelectric (FE) perovskite BiFeO3 and ferromagnetic (FM) spinel NiFe2O4. Thus, the findings disclose a greater potential for innovative applications in spintronics and information technology.

Crystal Facet Engineering of CoPt Quantum Dots for Diverse Colloidal Heterostructures

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.

Corroboration and efficacy of Magneto-Fluorescent (NiZnFe/CdS) Nanostructures Prepared using Differently Processed Core

The selected and controlled preparation of core@shell nanostructures, which unite the multiple functions of ferromagnetic Ni-Zn ferrite core and CdS shell in a single material with tuneable fluorescence and magnetic properties, have been proposed by the seed mediated aqueous growth process. The shell particle thickness and core of nanostructures were precisely tuned. Current work exhibits the comparative study of core@shell multifunctional nanostructures where core being annealed at two different temperatures. The core@shell nanostructure formation was confirmed by complementary structural, elemental, optical, magnetic and IR measurements. Optical and magnetic characterizations were performed to study elaborative effects of different structural combinations of core@shell nanostructures to achieve best configuration with high-luminescence and magnetic outcomes. The interface of magnetic/nonmagnetic NiZnFe₂O₄/CdS nanostructures was inspected. Unexpectedly, in some of the core@shell nanostructures presence of substantial exchange-bias was observed in spite of the non-magnetic nature of CdS QDs which is clearly an “optically-active” and “magnetically-inactive” material. Presence of “exchange-bias” was confirmed by the change in “magnetic-anisotropy” as well as shift in susceptibility derivative. Finally, successful formulation of stable and efficient core@shell nanostructures achieved, which shows no exchange-bias and shift. Current findings suggest that these magneto-fluorescent nanostructures can be used in spintronics; and drug delivery-diagnosis-imaging applications in nanomedicine field.

Magnetic performance and defect characterization studies of core-shell architectured MgFe2O4@BaTiO3 multiferroic nanostructures

Multiferroics that permit manipulation of the magnetization vector exclusively by electric fields have spawned extensive interest for memory and logic device applications. In line with this understanding, we herein report the encapsulation of non-ferroelectric magnesium ferrite (MgFe2O4) nanoparticles in a ferroelectric shell of BaTiO3 to produce a system with engineered dielectric, magnetic, magneto-electric and ferroelectric properties. The interface effect on the strain transfer was observed to strongly influence the magneto-electric coupling and the electric and magnetic properties of the system. The model polyhedral image of MgFe2O4@BaTiO3 has helped to get an insight into the core-shell structure. The multiferroicity induced by the excellent coupling between the ferroelectric and magnetostrictive phases at the core-shell interface unlocks wide prospects for device downscaling and information storage applications. The influence of magnetostrictive stress on the magneto-electric coupling effects and domain dynamics was further studied using transmission electron microscopy (TEM) and atomic force microscopy images. Interestingly, the realization of a superparamagnetic multiferroic system has been a breakthrough and facilitates ultra high density magnetic data storage technologies. Evidence for spontaneous polarization and the ferroelectric trait exhibited by the multiferroic samples was revealed from the P-E hysteresis loop. The investigation of defect evolution in the system was carried out using positron annihilation lifetime spectroscopy (PALS) and coincidence Doppler broadening spectroscopy (CDBS) of annihilation radiation and the studies revealed thermal diffusion of positrons into the interfacial regions within the core-shell structure and the "formation and pick-off annihilation of orthopositronium atoms". It is concluded that interface engineering is a strong means for manipulation of the magnetic, dielectric and magneto-electric properties in multiferroic heterostructures for high density electrical energy and magnetic data storage.

Frontier challenges in doping quantum dots: synthesis and characterization

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.

We discuss state-of-the-art doping strategies for colloidal quantum dots, their principle, advantages and challenges in implementing the strategies.

Hyaluronate dots for highly efficient photodynamic therapy

Nanoscale particles, such as quantum dots and carbon dots, are important materials for use in sensing and treating irregular biological events, but their versatility for biomedical applications are usually limited by their undesirable properties such toxicity and non-degradability. Here, we report biofunctional hyaluronic acid (HA) dots containing biodegradable/biocompatible HA. HA dots were prepared by conjugating multiple HA molecules to C₆₀ (used as a base dot) without any hydrothermal treatment. The hydroxyl groups of HA completely linked to all π-π carbon bonds in C₆₀. The chemically synthesized HA dots (Mn=16.1kDa) were 2nm in diameter, soluble in aqueous solution, and possessed multiple functional (carboxyl) groups. The HA dots were biofunctional, enabling highly efficient binding to CD44 receptors overexpressed on in vitro/in vivo tumors. With light illumination, we demonstrated that the HA dots bearing a photosensitizing model drug (chlorin e6: Ce6) resulted in a significant enhancement in in vitro/in vivo tumor cell ablation. We believe that this approach offers a new strategy to create biopolymer dots.