Wurtzite CuInS2: solution based one pot direct synthesis and its doping studies with non-magnetic Ga3+ and magnetic Fe3+ ions

Abstract

Reactions of air stable copper-thiourea precursors [Cu(tu)₃]Cl and [Cu₂(tu)₆]SO₄·H₂O with indium(III) acetate (In(OAc)₃·xH₂O) and indium(III) sulfate (In₂(SO₄)₃·xH₂O) under refluxing in ethyleneglycol for 1–3.5 h yielded metastable, wurtzite (WZ) and zincblende (ZB) forms of CuInS₂ (CIS). While the yields of CIS from the reactions using In₂(SO₄)₃ were quite high, reactions involving In(OAc)₃ were sluggish producing low yields. A flower like morphology has been observed in the FE-SEM image of the WZ-CIS with EDX analysis yielding Cu : In : S ratio as 1.05 : 0.95 : 2.00. The SAED pattern of WZ-CIS recorded from the HR-TEM images could very well be indexed in hexagonal symmetry. The room temperature Raman spectrum also confirmed the formation of crystalline CIS. Solid WZ-CIS samples showed a band gap of 1.40 eV as revealed by UV-Visible diffuse reflectance spectroscopic analysis. Doping of non-magnetic Ga³⁺-ion and magnetic Fe³⁺-ion for the In³⁺-ion in the WZ-CIS has been examined by reacting the sulfate salts of gallium and iron with [Cu(tu)₃]Cl and In₂(SO₄)₃. FE-SEM-EDX and TEM-EDX analyses confirmed the presence of gallium and iron in CIS samples. 3.5 at% gallium doped CIS sample showed WZ to be the major phase with few reflections appearing due to the chalcopyrite phase in both the PXRD and TEM-SAED patterns. On doping CIS with 20.8 at% of iron, the hexagonal symmetry of the CIS changed to either cubic (zincblende) or tetragonal (chalcopyrite) depending on the experimental conditions. The tetragonal symmetry of the iron doped CIS has also been verified from TEM-SAED patterns. Introduction of intermediate states in the bandgap of CIS has been observed on doping with iron in the UV-visible diffuse reflectance spectrum with the estimated band gap of 1.05 eV. From magnetization measurements at room temperature, Fe³⁺-doped CIS showed paramagnetic behavior with χ_g of 5.89 × 10⁻⁶ emu/g. Also, they showed transitions between the defect levels resulting in intense photoluminescence (PL) emission at 750 nm on excitation with λ = 500 nm. The electron paramagnetic resonance (EPR) spectrum confirmed the presence of Fe³⁺ ions in the CIS lattice exhibiting a broad signal at g = 1.90. The versatility of using this rapid and scalable synthetic approach has been extended to produce orthorhombic AgInS₂.