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Epitaxial growth and magnetic properties of kagome metal FeSn/elemental ferromagnet heterostructures

Binary kagome compounds TmXn (T = Mn, Fe, Co; X = Sn, Ge; m:n = 3:1, 3:2, 1:1) have garnered recent interest owing to the presence of both topological band crossings and flatbands arising from the geometry of the metal-site kagome lattice. To exploit these electronic features for potential applications in spintronics, the growth of high-quality heterostructures is required. Here, we report the synthesis of Fe/FeSn and Co/FeSn bilayers on Al2O3 substrates using molecular beam epitaxy to realize heterointerfaces between elemental ferromagnetic metals and antiferromagnetic kagome metals. Structural characterization using high-resolution x-ray diffraction, reflection high-energy electron diffraction, and electron microscopy reveals that the FeSn films are flat and epitaxial. Rutherford backscattering spectroscopy was used to confirm the stoichiometric window where the FeSn phase is stabilized, while transport and magnetometry measurements were conducted to verify metallicity and magnetic ordering in the films. Exchange bias was observed, confirming the presence of antiferromagnetic order in the FeSn layers, paving the way for future studies of magnetism in kagome heterostructures and potential integration of these materials into devices.
Publications

Anomalous Dimensionality‐Driven Phase Transition of MoTe2 in Van der Waals Heterostructure

AbstractPhase transition in nanomaterials is distinct from that in 3D bulk materials owing to the dominant contribution of surface energy. Among nanomaterials, 2D materials have shown unique phase transition behaviors due to their larger surface‐to‐volume ratio, high crystallinity, and lack of dangling bonds in atomically thin layers. Here, the anomalous dimensionality‐driven phase transition of molybdenum ditelluride (MoTe2) encapsulated by hexagonal boron nitride (hBN) is reported. After encapsulation annealing, single‐crystal 2H‐MoTe2 transformed into polycrystalline Td‐MoTe2 with tilt‐angle grain boundaries of 60°‐glide‐reflection and 120°‐twofold rotation. In contrast to conventional nanomaterials, the hBN‐encapsulated MoTe2 exhibit a deterministic dependence of the phase transition on the number of layers, in which the thinner MoTe2 has a higher 2H‐to‐Td phase transition temperature. In addition, the vertical and lateral phase transitions of the stacked MoTe2 with different crystalline orientations can be controlled by inserted graphene layers and the thickness of the heterostructure. Finally, it is shown that seamless Td contacts for 2H‐MoTe2 transistors can be fabricated by using the dimensionality‐driven phase transition. The work provides insight into the phase transition of 2D materials and van der Waals heterostructures and illustrates a novel method for the fabrication of multi‐phase 2D electronics.
Publications

Fluorinated Graphene Contacts and Passivation Layer for MoS2 Field Effect Transistors

AbstractRealizing a future of 2D semiconductor‐based devices requires new approaches to channel passivation and nondestructive contact engineering. Here, a facile one‐step technique is shown that simultaneously utilizes monolayer fluorinated graphene (FG) as the passivation layer and contact buffer layer to 2D semiconductor transistors. Monolayer graphene is transferred onto the MoS2, followed by fluorination by XeF2 gas exposure. Metal electrodes for source and drain are fabricated on top of FG‐covered MoS2 regions. The MoS2 transistor is perfectly passivated by insulating FG layer and, in the contacts, FG layer also acts as an efficient charge injection layer, leading to the formation of Ohmic contacts and high carrier mobility of up to 64 cm2 V−1 s−1 at room temperature. This work shows a novel strategy for simultaneous fabrication of passivation layer and low‐resistance contacts by using ultrathin functionalized graphene, which has applications for high performance 2D semiconductor integrated electronics.
Publications

Nonlinear electrohydrodynamic ion transport in graphene nanopores

Pressure-modulated ion transport through single graphene nanopores reveals a nonlinear electrohydrodynamic coupling phenomenon.
Publications

Ultra-resilient multi-layer fluorinated diamond like carbon hydrophobic surfaces

AbstractSeventy percent of global electricity is generated by steam-cycle power plants. A hydrophobic condenser surface within these plants could boost overall cycle efficiency by 2%. In 2022, this enhancement equates to an additional electrical power generation of 1000 TWh annually, or 83% of the global solar electricity production. Furthermore, this efficiency increase reduces CO2 emissions by 460 million tons /year with a decreased use of 2 trillion gallons of cooling water per year. However, the main challenge with hydrophobic surfaces is their poor durability. Here, we show that solid microscale-thick fluorinated diamond-like carbon (F-DLC) possesses mechanical and thermal properties that ensure durability in moist, abrasive, and thermally harsh conditions. The F-DLC coating achieves this without relying on atmospheric interactions, infused lubricants, self-healing strategies, or sacrificial surface designs. Through tailored substrate adhesion and multilayer deposition, we develop a pinhole-free F-DLC coating with low surface energy and comparable Young’s modulus to metals. In a three-year steam condensation experiment, the F-DLC coating maintains hydrophobicity, resulting in sustained and improved dropwise condensation on multiple metallic substrates. Our findings provide a promising solution to hydrophobic material fragility and can enhance the sustainability of renewable and non-renewable energy sources.

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