Defect induced magnetism in green synthesized Cadmium Sulfide nanoparticles for spintronics applications

Highlights

• Introduction of green synthesis of CdS by simple chemical method using widely available leaf extract.

• Room temperature ferromagnetism in CdS and CdS- leaf extract composite without any substitution/doping.

• Vacancy induced and ligand- induced magnetism in CdS- leaf extract composite.

• Wasp-waisted behaviour of hysteresis loop at low temperature.

• Enhanced electrical conductivity in the composite nanoparticle on light illumination.

Abstract

Introducing room – temperature ferromagnetism, in wide bandgap semiconductors has gained significant interest in the field of diluted magnetic semiconductors. Here, the Cadmium Sulfide and CdS – C. odorata leaf extract composites, fabricated through green synthesis routes, show room – temperature ferromagnetism and improved electrical conductivity. Here, the origin of magnetism is attributed to the trapped electrons in the F – center, which is emanated from the sulfur vacancies. Besides, the composite has ligand – induced magnetism due to the electron transfer at the interface between the CdS and the leaf extract. Enhanced conductivity observed in the composite nanoparticle is due to the loss of a few electrons from the F – center to the conduction band. The leaf extract modified CdS samples exhibit unconventional ferromagnetism which makes them ideal for spintronics applications without any magnetic impurity doping. The improved electrical conductivity exhibited by the samples forecast their multi – utility potential to be exploited for electrical and photovoltaic applications in addition to their DMS and spintronic applications.

Introduction

Group II – VI semiconductors have attracted the researchers owing to their wide optoelectronic applications and ease of synthesis. CdS are one of the renowned semiconductors in the group II – VI and are the best candidate for solar cells as a window material [1]. During the late 70 s, researchers realized that a small percentage of magnetic impurity could induce large magnetic effects in the host material, especially in group II – VI semiconductors, without deteriorating its inherent optical and electrical transport properties. These magnetic semiconductor materials can provide control on the conduction of carriers via quantum spin state (up, down spin) along with the control of charge carriers (n, p – type) in electronic devices, leading to the evolution of a new era of spintronic devices. The origin of ferromagnetism in these materials, with the increase in the concentration of magnetic impurities, predominantly transition metals, left some fundamental aspects unclear. Due to the wide range of functional properties, magnetically doped oxides have achieved considerable appreciation in the field of diluted magnetic semiconductors (DMS). In addition to that, the researchers observed room – temperature ferromagnetism in metal oxides without injecting any magnetic impurity. Here, the magnetism is vacancy (cationic or anionic) mediated. Rather than oxides, chalcogenides are also widely researched because of its potential as a DMS material when doped with magnetic impurities, like Co, Fe, Ni, Mn, etc. Special attention is given to CdS – based DMSs due to their facile synthesis as well as abundance and lower cost of the precursor materials.

Development of novel synthesis and fabrication techniques for semiconductor nanomaterial is a promising task because of their size and chemical environment – dependent properties. Semiconductor nanoparticles are the most widely sought – after nanosystems because of their wide demand in the field of cost – effective energy harvesters. Synthesizing particles in the confinement size regime through chemical routes needs intense attention and care. During the synthesis, the particles are free to grow in any direction. These free particles interact with the neighbouring particle and lead to agglomeration, which in turn increases the particle size. This type of growth can be controlled by using surfactants or capping agents (organic/inorganic) by ceasing agglomeration [2], which can ensure uniform particle size and shape. Some of the researchers have used polymer layer like PVA/PVP as a capping agent [3] which encloses the nanoparticle but does not react with its surface which in turn results in the formation of larger particles. Also, the morphological anisotropy in nanoparticles positively influences the physical properties of the material and hence received researchers’ significant attention. Most of the published works based on the morphology [4], [5] and size control [6] followed complicated synthesis procedures with the use of toxic precursors.

To avoid toxic by – products, researchers, especially in the field of nanobiotechnology, use environmentally friendly methods for the synthesis of nanomaterials such as Ag, Au, Pd, and Pt, as it is an essential aspect for the industrial and medical applications [7]. Eco – friendly biological methods have two parts [8]; synthesis based on microorganisms [9] and plant tissues [10], [11]. Among the green synthesis techniques, plant extract – based preparation is quite common since it helps in avoiding the formation of hazardous by – products. Bokka Durga et al. studied the antibacterial and antifungal activity against different test organisms using CdS nanoparticles, synthesized using Artabotrys hexapetalus leaf extract [12], and Reishi mushroom aqueous extract [13]. K S Prasad et al. [14] studied the DNA damage activity of water – soluble CdS nanoparticles prepared using the leaf extract of Asparagus racemosus. In addition to that, the leaf extract of Aloe vera [15] and Banana peel [16] extracts are used as capping agents for the green synthesis of CdS.

Here, the leaf extract of Chromolaena odorata is used as the medium for the growth of CdS nanoparticles. C. odorata is a medicinal plant which is originated in America and later introduced into Asian countries [17]. C. odorata is a rapidly growing herb that is available in India throughout the year. The leaves are renowned for the treatment of soft tissue wounds, skin infections and burn wounds [18] and are being explored for other potential medicinal applications. Reports on the magnetic behavior of CdS as well as doped CdS showed only very weak magnetism even in the presence of magnetic impurities (a detailed report is tabulated in the supplementary data). A new methodology for enhancing magnetism in CdS with defect engineering (without any magnetic dopants) is the main objective of the work. Here, a slightly modified synthesis protocol of boiling leaf extract technique was employed for the growth of CdS nanoparticles [19]. The optical, electrical, and magnetic properties of CdS – C. odorata composite (CdS – B) were evaluated and compared with the pure CdS synthesized in the absence of leaf extract (CdS – A).