Modern electronics don't just rely on the charge of electrons, but also use their intrinsic magnetic property called 'spin'. This is the exciting field of spintronics, promising faster, smaller, and more energy-efficient devices. To build these gadgets, scientists are hunting for unique materials called dilute magnetic semiconductors (DMSs). These are hybrid materials, combining the electrical properties of semiconductors, like silicon in computer chips, with the magnetic behaviour of metals, like iron.
Researchers at VIT-AP University recently explored a promising candidate by taking a semiconductor called tin disulfide (SnS2) and strategically adding small amounts of a rare earth metal, gadolinium (Gd), creating nanoparticles with intriguing properties.
Using a relatively simple and affordable 'hydrothermal' method – essentially cooking the ingredients under pressure in water, like a nanoscale pressure cooker – the scientists created tiny powders of pure SnS2 and SnS2 doped with varying, small percentages (1% to 7%) of gadolinium. They then put these nanopowders through a battery of tests to understand their structure, how they interact with light, and their magnetic characteristics. X-ray diffraction acted like a structural fingerprint check, confirming the nanoparticles had the expected hexagonal crystal arrangement, similar to layers stacked neatly.
Interestingly, adding gadolinium subtly changed the spacing between these layers, indicating that Gd atoms were indeed taking the place of some tin (Sn) atoms within the structure – a process called doping. Electron microscopes revealed the nanoparticles weren't just tiny specks, but often formed intricate flower-like structures, hundreds of nanometers across, though composed of much smaller crystal grains.
When the researchers shone light on the materials, they observed how the nanoparticles absorbed and reflected different colours using UV-Visible spectroscopy. This allowed them to measure the 'band gap' –the energy hurdle electrons must overcome to conduct electricity or emit light. Generally, adding more gadolinium increased this hurdle, a phenomenon known as blue shift, meaning more energy was needed. This change is partly due to the quantum confinement effect – when materials get incredibly small, their electronic properties change.
They also studied photoluminescence, which is the light emitted by the material after absorbing energy. The Gd-doped SnS2 glowed with a mix of blue, green, and even red light when energised. This light emission arises when excited electrons fall back from higher energy levels, sometimes taking detours through 'trap states' caused by imperfections or defects in the crystal structure, like missing atoms. The specific colours emitted suggest these materials could be tuned for applications like domestic lighting, potentially offering warm, yellowish light as indicated by colour analysis (CIE).
Perhaps the most crucial test was examining the magnetic behaviour using a vibrating sample magnetometer. Pure SnS2 is non-magnetic or diamagnetic. However, the sample with just 1% gadolinium doping exhibited weak ferromagnetism at room temperature, meaning it behaved like a very weak magnet. This was a key finding, as creating room-temperature magnetism in semiconductors is a major goal for spintronics. Interestingly, adding more gadolinium (3% or higher) made the material non-magnetic again.
The researchers suggest the weak magnetism in the 1% sample arises indirectly. The Gd doping likely creates vacancies – missing tin atoms – in the crystal structure. These vacancies, surrounded by sulfur atoms, can generate tiny magnetic moments. The gadolinium might help the tiny magnetic swirls caused by the missing atoms align, but only when the doping concentration is low; too much Gd seems to disrupt the effect.
The study builds upon previous work on DMSs by specifically investigating gadolinium in SnS2, a combination not previously explored for these properties, and successfully synthesizing it using an accessible method. The main limitation observed is that the desired ferromagnetic behaviour was only present at a very low doping level and was relatively weak.
The research demonstrates how carefully adding a dash of gadolinium to tin disulfide nanoparticles can significantly alter their properties, inducing weak magnetism and tuning their light emission. While challenges remain in strengthening the magnetic effect, these findings open doors for exploring Gd-doped SnS2 further. Success in developing robust DMS materials like these could lead to revolutionary spintronic devices, merging logic and memory functions, and potentially contribute to new, efficient lighting technologies. This illustrates how manipulating materials at the nanoscale can unlock exciting possibilities for the future.
This research article was written with the help of generative AI and edited by an editor at Research Matters.