2D metal halide layered perovskites
2D metal halide perovskites can be formed by replacing the A-site cations in ABX3 perovskites with long-chain organic spacer cations. Depending on the bonding motif of spacer cations they can be categorized into two types: Ruddlesden-Popper (RP) phase (spacer cation is monodentate type) and (ii) Dion-Jacobson (DJ) phase (spacer cation is bidentate type). Recently, these 2D perovskites are emerged as an alternative to 3D perovskite nanocrystals since tuning the layer thickness provides bandgap tunability as well as environmental stability. Due to these properties, these materials are considered as potential candidates for next-generation optoelectronic devices (LEDs, Solar cells, etc.).
Source: Nano Lett., 2016, 16, 7001–7007 and Joule, 2019, 3, 794–806
Semiconductor Quantum Dots
Quantum dots (QDs) are semiconducting nanocrystals where the excitons are confined in all three spatial dimensions (i.e. they are zero-dimensional material as compared to the bulk semiconductor). Moreover, QDs are made up of a few numbers of atoms (~200-1000) and an organic layer (surface ligand) with a typical size (2-10 nm) comparable with its bulk exciton Bohr radius. These semiconductor QDs possess unique size-dependent optical and magnetic properties that arise due to the quantum confinement effect. These exciting properties of semiconductor QDs make them excellent materials for photovoltaic, light-emitting devices, spintronics, and biomedical applications.
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Source: Science, 373,eaaz8541(2021). DOI: 10.1126/science.aaz8541
Diluted Magnetic Semiconductor Quantum Dots (DMSQDs)
The un-doped semiconductor QDs has some severe weaknesses, for example, diamagnetic, lack of thermal stability, self-diffusion, self-absorbance, and low quantum yield. The intentional impurities can be used to change their properties in a required and controllable manner to overwhelm these weaknesses. Paramagnetic Transition Metal (TM) ions doped â…¡-â…¥ semiconductor QDs attracted much attention to the researchers in the last two decades. These TM ions doped semiconductor QDs pronounced as diluted magnetic semiconductor QDs (DMSQDs) and have various technological applications, especially in the field of spintronics, solar cell, and magneto-optics. Doping of TMs ions in â…¡-â…¥ semiconductor results in orbital exchange interaction between the d-levels of the dopant and the sp-levels of the host and which can produce a variety of optical and magnetic phenomena.
Synthesis of DMSQDs: Single-source cluster approach
I have been using a single-source cluster approach for the synthesis of II-VI semiconductor quantum dots (QDs) and diluted magnetic semiconductor (DMS) QDs, which has been developed by Prof. Strouse group (FSU, USA).
In this method, we have been using inorganic clusters with formula Li4[M10X4(SPh)16] where (M= Cd or Zn and X= S or Se) as a single-source precursor in dodecylamine (DDA) or HDA at 80ËšC under N2 environment. Further, after the dissolution of the clusters, the doping precursor has been added to exchange the metal ions into the clusters. The growth of QDs has been achieved at ~220ËšC using different synthesis times, depending on the required size of the QDs (see the figure).
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From this method, nucleation at relatively low temperatures can be achieved, and the reaction can be scaled to large quantities (1–50 g/L). Moreover, this new method provides greater synthesis control, slow thermodynamic growth at low temperature and monodisperse QDs with less than 5% size disparity. Using this method, several DMSQDs such as Co: CdSe, Cu: CdSe, Mn: CdSe, Mn: ZnSe, Cr: CdSe, Cr: ZnSe, Fe: CdSe, Fe: ZnSe, can be synthesized.
Nanoparticles/Graphene composite
In 2D carbon materials, graphene has found a perfect supporting material for semiconductor nanocrystals and metal oxide nanoparticles because of its large surface area, light-weight, less toxicity, and hydrophilic nature. The excellent and unique properties of 2D graphene come out from its tightly packed carbon atoms that form an sp2-hybrid network in a honeycomb lattice. In transition metal NPs-graphene hybrid, the NPs are attached with the surface of the graphene sheet through strong covalent bonding which further avoids evaporation and migration of NPs. Moreover, graphene has unsaturated pz orbital and zero bandgaps, these properties are useful for the electronic interaction with 3d orbital of transition metal NPs. Furthermore, the resulting hybrid material may hold unique properties of graphene such as long spin coherence lengths and times because of limited fine interactions and small spin-orbit coupling. Thus, graphene is a promising material to alternate the electronic band structure of MNPs efficiently and can promote room temperature ferromagnetic interactions.
Synthesis of nanoparticle/graphene composite: Solvothermal Method
Nanocomposite can be synthesized by a simple solvothermal synthesis method. In a typical synthesis protocol, 80 ml ethylene glycol, 15 ml ethylene-diamine, 6 gm sodium acetate, 200 mg graphite oxide and 50 mg as-synthesized nanoparticles have been sonicated in a beaker for 3 hrs. Further, the dispersed solution has been transferred into a 100 ml autoclave and heated at a temperature of 200ËšC for 12 hrs. Finally, the reaction mixture is allowed to cool at ambient temperature, purified by ethanol several times, and the obtained product has been dried in a vacuum furnace at 60ËšC.
