Developing New Materials for Spintronic, Photovoltaic, and Optoelectronic Applications

Abstract

In 1965, Gordon Moore predicted that the number of components on an integrated circuit (IC) would double every year for the foreseeable future, later revised to doubling every two years. Moore’s Law has ushered in a technological revolution, leading to ubiquitous products such as laptops and smartphones. However, Moore’s Law has recently begun to slow down. As such, it is crucial to search for solutions to this problem and spin-electronic (spintronic) materials are a promising candidate, taking advantage of the charge and spin of the electron to process and store information. A class of M1-xM’xPn2Se4 (M = Fe, Mn; M’ = Sn, Cu, Zn; Pn = Sb, Bi) materials show great potential for spintronics, as the lattice provides for separate and precise electronic and magnetic tunability. Here, Mn1-xZnxSb2Se4 (x = 0-0.15) was synthesized, a p-type antiferromagnetic semiconductor. Increasing the Zn content was found to enhance the antiferromagnetic coupling and effective magnetic moment. Throughout the composition range, the effective moment was larger than the theoretical moment, so a substitution mechanism of Zn replacing Mn at the M1 and M2 sites was proposed, generating a ferrimagnetic lattice. Research on FeSb2-xSnxSe4 has revealed that a p-type to n-type transition at low-temperature (Lifshitz transition) can be induced by doping Sb3+ with Sn2+. In the literature, a Lifshitz transition has been observed to generate interesting properties such as colossal magnetoresistance and superconductivity. With FeSb2-xSnxSe4 as the model system, inducing a Lifshitz transition in p-type Sb2Se3 by Sn doping (Sb2-xSnxSe3, x = 0-0.2) could be intriguing. Sb2Se3 and Sb2Se3: Sn have shown promise in near-infrared (NIR) photodetectors, thermoelectrics, optoelectronics, and photovoltaics. In this work, Ultraviolet (UV)-visible (Vis)-NIR spectroscopy on Sb2-xSnxSe3 revealed an absorption edge at 1.4 eV, with a decreasing optical bandgap from 1.17 eV to 1.07 eV for increasing Sn. The tunable bandgap of the material around ~1.1 eV makes it an ideal candidate as a photovoltaic absorber layer, as it can absorb wavelengths in the UV, Vis, and NIR. With increasing Sn content, the amount of low-energy band tailing also increased. This could be evidence of impurity sub-bands. In the future, photoluminescence and low-T electrical transport would help elucidate further information on the doping effects and if a Lifshitz transition is induced upon large enough Sn. Rising greenhouse gas levels is linked to an increase in global temperature and an increase in extreme weather events. As such, it is essential to curb fossil fuel consumption and turn to clean, renewable energy sources such as wind, geothermal, and solar. Within solar, it is vital to choose cheap, abundant, and non-toxic elements. In this work, the face-centered cubic Fm-3m Cu8Se4 structure with 8/8 tetrahedral sites filled was used as the model lattice, and Cu2Zn2xTi(3-2x)/2Se4 (x = 1.5, 1, 0.5, 0) and Cu2Mn3Se4 were synthesized. Single crystals of all five compounds were grown and unique 2x2x2 or 3x3x3 supercell structures were revealed. Lowering the x value was found to increase the level of tetrahedral vacancies and increased the bandgap from 2.24 eV-2.75 eV. Cu2Zn3Se4 was revealed to be a promising UV or Vis (violet to green) photodetector material, exhibiting p-type conduction and a large electrical/thermal conductivity. Cu2Mn3Se4 holds promise both as an optical material, as well as the potential of exhibiting antiferromagnetic order; a bulk single-crystal synthesis route could be implemented to explore coupled magneto-optic effects.