Phase-Selective Synthesis of Ultrathin FeTe Nanoplates by Controllable Fe/Te Atom Ratio in the Growth Atmosphere

Reaction-driven formation of anisotropic strains in FeTeSe nanosheets boosts low-concentration nitrate reduction to ammonia

FeM (M = Se, Te) chalcogenides have been well studied as promising magnets and superconductors, yet their potential as electrocatalysts is often considered limited due to anion dissolution and oxidation during electrochemical reactions. Here, we show that by using two-dimensional (2D) FeTeSe nanosheets, these conventionally perceived limitations can be leveraged to enable the reaction-driven in-situ generation of anisotropic in-plane tensile and out-of-plane compressive strains during the alkaline low-concentration nitrate reduction reaction (NO3-RR). The reconstructed catalyst demonstrates enhanced performance, yielding ammonia with a near-unity Faradaic efficiency and a high yield rate of 42.14 ± 2.06 mg h-1 mgcat-1. A series of operando synchrotron-based X-ray measurements and ex-situ characterizations, alongside theoretical calculations, reveal that strain formation is ascribed to chalcogen vacancies created by partial Se/Te leaching, which facilitate the adsorption and dissociation of OH-/NO3- from the electrolyte, resulting in an O(H)-doped strained lattice. Combined electrochemical and computational investigations suggest that the superior catalytic performance arises from the synergistic contributions from the exposed strained Fe sites and surface hydroxyl groups. These findings highlight the potential of 2D transition metal chalcogenides for in-situ structural engineering during electrochemical reactions to enhance catalytic activity for NO3-RR and beyond.

Ultrahigh Exchange Bias Field/Coercive Field Ratio in In Situ Formed Two-Dimensional Magnetic Te-Cr2O3/Cr5Te6 Heterostructures

The exchange bias (EB) effect is a fundamental magnetic phenomenon, in which the exchange bias field/coercive field ratio (|HEB/HC|) can improve the stability of spintronic devices. Two-dimensional (2D) magnetic heterostructures have the potential to construct low-power and high-density spintronic devices, while their typically air unstable and |HEB/HC| lesser, limiting the possibility of applications. Here, 2D Cr5Te6 nanosheets have been systematically synthesized with an in situ formed ≈2 nm-thick Te doped Cr2O3 layer (Te-Cr2O3) on the upper surface by chemical vapor deposition (CVD) method. The strong and air stable EB effect, achieving a |HEB/HC| of up to 80% under an ultralow cooling field of 0.01 T, which is greater than that of the reported 2D magnetic heterostructures. Meanwhile, the uniformity of thickness and chemical composition of the Te-Cr2O3 layer can be controlled by the growth conditions which are highly correlated with the EB effect of 2D Te-Cr2O3/Cr5Te6 heterostructures. First-principles calculations show that the Te-Cr2O3 can provide uncompensated spins in the Cr2O3, thus forming strong spin pinning effect. The systematical investigation of the EB effect in 2D Te-Cr2O3/Cr5Te6 heterostructures with high |HEB/HC| will open up exciting opportunities in low-power and high-stability 2D spintronic devices.

Single-crystalline High-κ GdOCl dielectric for two-dimensional field-effect transistors

Two-dimensional (2D) dielectrics, integrated with high-mobility semiconductors, show great promise to overcome the scaling limits in miniaturized integrated circuits. However, the 2D dielectrics explored to date still face the challenges of low crystallinity, diminished dielectric constant, and the lack of effective synthesis methods. Here, we report the controllable synthesis of ultra-thin gadolinium oxychloride (GdOCl) nanosheets via a chloride hydrate-assisted chemical vapor deposition (CVD) method. The resultant GdOCl nanosheets display good dielectric properties, including a high dielectric constant (high-κ) of 15.3, robust breakdown field strengths (Ebd) exceeding 9.9 MV/cm, and minimal gate leakage currents of approximately 10-6 A/cm2. The top-gated GdOCl/MoS2 field-effect transistors (FETs) exhibit commendable switch characteristics, a negligible hysteresis of ~5 mV and a subthreshold swing down to 67.9 mV dec-1. The GdOCl/MoS2 FETs can also be employed to construct functional logic gates. Our study underscores the significant potential of the 2D GdOCl dielectric for innovative high-speed operated nanoelectronic devices.

Superconducting Two-Dimensional FeSe Grown on the Fe-Enriched Interface

Two-dimensional (2D) tetragonal FeSe has sparked extensive research interest owing to its tunable superconductivity, providing valuable insights into the design of high-temperature superconductors. Currently, the intricate Fe-Se phase diagram poses a challenge to the controlled synthesis of superconducting 2D FeSe in a pure tetragonal phase. Here, we exploit the ion-exchange property of fluorophlogopite mica to devise a straightforward approach for the phase-controlled synthesis of tetragonal FeSe on an Fe-enriched mica surface within a molten salt environment. This method successfully produces highly crystalline FeSe in a pure tetragonal phase with adjustable thickness. We investigated the surface composition of the postgrowth mica substrate using various microscopic and spectroscopic characterizations to highlight the importance of the Fe-enriched growth interface in the phase-selective synthesis of 2D tetragonal FeSe. The obtained 2D FeSe exhibited 2D superconductivity, comparable to that of FeSe mechanically exfoliated from bulk crystals, confirming the high quality of our samples. Beyond tetragonal FeSe, 2D antiferromagnetic FeTe and superconducting FeSxSeyTe1-x-y have been phase-selectively synthesized via this approach. Our study elucidates the significance of the growth interface on the phase-selective synthesis of 2D materials and presents potential opportunities for the phase-controlled synthesis of 2D multiphase materials via the rational design of the growth interface.

Recent Progress on Phase Engineering of Nanomaterials

As a key structural parameter, phase depicts the arrangement of atoms in materials. Normally, a nanomaterial exists in its thermodynamically stable crystal phase. With the development of nanotechnology, nanomaterials with unconventional crystal phases, which rarely exist in their bulk counterparts, or amorphous phase have been prepared using carefully controlled reaction conditions. Together these methods are beginning to enable phase engineering of nanomaterials (PEN), i.e., the synthesis of nanomaterials with unconventional phases and the transformation between different phases, to obtain desired properties and functions. This Review summarizes the research progress in the field of PEN. First, we present representative strategies for the direct synthesis of unconventional phases and modulation of phase transformation in diverse kinds of nanomaterials. We cover the synthesis of nanomaterials ranging from metal nanostructures such as Au, Ag, Cu, Pd, and Ru, and their alloys; metal oxides, borides, and carbides; to transition metal dichalcogenides (TMDs) and 2D layered materials. We review synthesis and growth methods ranging from wet-chemical reduction and seed-mediated epitaxial growth to chemical vapor deposition (CVD), high pressure phase transformation, and electron and ion-beam irradiation. After that, we summarize the significant influence of phase on the various properties of unconventional-phase nanomaterials. We also discuss the potential applications of the developed unconventional-phase nanomaterials in different areas including catalysis, electrochemical energy storage (batteries and supercapacitors), solar cells, optoelectronics, and sensing. Finally, we discuss existing challenges and future research directions in PEN.

The Controllable Synthesis of High-Quality Two-Dimensional Iron Sulfide with Specific Phases

2D Fe-chalcogenides have drawn significant attention due to their unique structural phases and distinct properties in exploring magnetism and superconductivity. However, it remains a significant challenge to synthesize 2D Fe-chalcogenides with specific phases in a controllable manner since Fe-chalcogenides have multiple phases. Herein, a molecular sieve-assisted strategy is reported for synthesizing ultrathin 2D iron sulfide on substrates via the chemical vapor deposition method. Using a molecular sieve and tuning growth temperatures to control the partial pressures of precursor concentrations, hexagonal FeS, tetragonal FeS, and non-stoichiometric Fe₇ S₈ nanoflakes can be precisely synthesized. The 2D h-FeS, t-FeS, and Fe₇ S₈ have high conductivities of 5.4 × 10⁵ S m^-1 , 5.8 × 10⁵ S m^-1 , and 1.9 × 10⁶ S m^-1 . 2D tetragonal FeS shows a superconducting transition at 4 K. The spin reorientation at ≈30 K on the non-stoichiometric Fe₇ S₈ nanoflakes with ferrimagnetism up to room temperature has also been observed. The controllable synthesis of various phases of 2D iron sulfide may provide a route for synthesizing other 2D compounds with various phases.

Thickness-Dependent Topological Hall Effect in 2D Cr5 Si3 Nanosheets with Noncollinear Magnetic Phase

Antiferromagnets with noncollinear spin order are expected to exhibit unconventional electromagnetic response, such as spin Hall effects, chiral abnormal, quantum Hall effect, and topological Hall effect. Here, 2D thickness-controlled and high-quality Cr5 Si3 nanosheets that are compatible with the complementary metal-oxide-semiconductor technology are synthesized by chemical vapor deposition method. The angular dependence of electromagnetic transport properties of Cr5 Si3 nanosheets is investigated using a physical property measurement system, and an obvious topological Hall effect (THE) appears at a large tilted magnetic field, which results from the noncollinear magnetic structure of the Cr5 Si3 nanosheet. The Cr5 Si3 nanosheets exhibit distinct thickness-dependent perpendicular magnetic anisotropy (PMA), and the THE only emerges in the specific thickness range with moderate PMA. This work provides opportunities for exploring fundamental spin-related physical mechanisms of noncollinear antiferromagnet in ultrathin limit.

Two-Dimensional Room-Temperature Magnetic Nonstoichiometric Fe7Se8 Nanocrystals: Controllable Synthesis and Magnetic Behavior

Two-dimensional (2D) magnetic materials have attracted significant attention for promising applications in energy-saving logic and robust memory devices. However, most 2D magnets discovered so far typically feature drawbacks for practical applications due to low critical temperatures. Herein, we synthesize ultrathin room-temperature (RT) magnetic Fe₇Se₈ nanoflakes via the space-confined chemical vapor deposition method. It is found that the appropriate supply and control of Se concentration in the reaction chamber is crucial for synthesizing high-quality nonstoichiometric Fe₇Se₈ nanoflakes. Cryogenic electrical and magnetic characterizations reveal the emergence of spin reorientation at ∼130 K and the survival of long-range magnetic ordering up to room temperature. The RT magnetic domain structures with different thicknesses are also uncovered by magnetic force microscopy. Moreover, theoretical calculations confirm the spin configuration and metallic band structure. The outstanding characteristics exhibited by Fe₇Se₈ nanoflakes, including RT magnetism, spin reorientation property, and good electrical conductivity, make them a potential candidate for RT spintronics.

Phase-Tunable Synthesis and Etching-Free Transfer of Two-Dimensional Magnetic FeTe

Two-dimensional (2D) Fe-chalcogenides (e.g., FeS, FeSe, and FeTe, etc.) have sparked extensive interest due to their rich phase diagrams including superconductivity, magnetism, and topological state, as well as versatile applications in electronic devices and energy related fields. However, the phase-tunable synthesis and green transfer of such fascinating materials still remain challenging. Herein, we develop a temperature-mediated chemical vapor deposition (CVD) approach to grow ultrathin nonlayered hexagonal and layered tetragonal FeTe nanosheets on mica substrates, with their thicknesses down to ∼2.3 and ∼4.0 nm, respectively. Interestingly, we have observed exciting ferromagnetism with the Curie temperature approaching ∼300 K and high conductivity (∼1.96 × 10⁵ S m^-1) in 2D hexagonal FeTe. More significantly, we have designed a swift, high-efficiency, and etching-free method for the transfer of 2D FeTe nanosheets onto arbitrary substrates, and such a transfer strategy enables the cyclic utilization of growth substrates. These results should propel the further development of phase-tunable synthesis and green transfer of 2D Fe-chalcogenides, as well as their potential applications in spintronic devices.