Two-Dimensional Wedge-Shaped Magnetic EuS: Insight into the Substrate Step-Guided Epitaxial Synthesis on Sapphire

Abstract

Rare earth chalcogenides (RECs) with novel luminescence and magnetic properties offer fascinating opportunities for fundamental research and applications. However, controllable synthesis of RECs down to the two-dimensional (2D) limit still has a great challenge. Herein, 2D wedge-shaped ferromagnetic EuS single crystals are successfully synthesized via a facile molten-salt-assisted chemical vapor deposition method on sapphire. Based on the theoretical simulations and experimental measurements, the mechanisms of aligned growth and wedge-shaped growth are systematically proposed. The wedge-shaped growth is driven by a dual-interaction mechanism, where the coupling between EuS and the substrate steps impedes the lateral growth, and the strong bonding of nonlayered EuS itself facilitates the vertical growth. Through temperature-dependent Raman and photoluminescence characterization, the nanoflakes show a large Raman temperature coefficient of −0.030 cm–1 K–1 and uncommon increasing band gap with temperature. More importantly, by low-temperature magnetic force microscopy characterization, thickness variation of the magnetic signal is revealed within one sample, indicating the great potential of the wedge-shaped nanoflake to serve as a platform for highly efficient investigation of thickness-dependent magnetic properties. This work sheds new light on 2D RECs and will offer a deep understanding of 2D wedge-shaped materials.

Growth of 2D Cr2O3–CrN Mosaic Heterostructures with Tunable Room‐Temperature Ferromagnetism

“…In MFM, to ensure the phase shift stemming from the interaction between the out-of-plane component of the magnetic field originating from the sample and magnetized tip, the tip is lifted to a constant height above the surface [9,15,16,22,[33][34][35] to eliminate the influence of surface morphology as illustrated in Figure 4a,b. [36] For nonmagnetic samples, no phase shift will occur, while ferromagnetic samples will generate a negative (positive) phase shift induced by the attractive (repulsive) force between the tip and the samples. The shifting angle is related to the strength of the force and reflects the M s value of the sample.…”

Judicious Selection of Precursors with Suitable Chemical Valence State for Controlled Growth of Transition Metal Chalcogenides

“…[66][67][68][69][70][71][72] 2D non-layered EuS, is an emerging TMCs material, which presents distinctive magnetic and optical properties and exhibits great potential in spintronic devices. [73] However, the challenge in their controllable synthesis limits their experimental investigations of monocrystalline 2D EuS nanoflakes. We also demonstrate that the valence state of precursors (EuBr 3 and EuBr 2 ) can play an important role in controlling the growth of EuS nanoflakes.…”

“…Figure 6d,e show Raman and PL spectra for EuS nanoflakes grown by EuBr 2 , respectively, which is consistent with a previous report. [ 73 ] In Raman spectrum, a fundamental vibrational mode at ≈237 cm −1 and an overtone sequence at ≈494 cm −1 can be observed, which only contains longitudinal optical phonon modes. In PL spectrum, a peak is located at 743 nm, corresponding to a band gap of ≈1.67 eV.…”

Synthesis Methods and Property Control of Two-Dimensional Magnetic Materials

“…Lately, Zhang et al successfully synthesized 2D wedge-shaped ferromagnetic EuS single crystals on sapphire substrates using sodium chloride-assisted CVD. [57] They employed sulfur powder and EuBr2 particles as precursors, mixing them with NaCl powder and utilizing a pre-treated stepped sapphire substrate for material epitaxial growth, as illustrated in the growth setup diagram shown in Fig. 4(a).…”