Role of Buffer Layer and Building Unit in the Monolayer CrI3 Growth: A First-Principles Perspective

Perfect spin-filtering effect in molecular junctions based on half-metallic penta-hexa-graphene nanoribbons

The design and control of spintronic devices is a research hotspot in the field of electronics, and pure carbon-based materials provide new opportunities for the construction of electronic devices with excellent performance. Using density functional theory in combination with nonequilibrium Green's functions method, we design spin filter devices based on Penta-hexa-graphene (PHG) nanoribbons-a carbon nanomaterial in which the intrinsic magnetic moments combines with edge effects leading to a half-metallic property. Spin-resolved electronic transport studies show that such carbon-based devices can achieve nearly 100% spin filtering effect at low bias voltages. Such SEF can resist the influence of hydrogen passivation at different positions, but hardly survive under a hydrogen-rich environment. Our analysis show that the perfect SEF transport properties are caused by the magnetic and electronic properties of PHG nanoribbons, especially the magnetic moments on the quasi-sp³carbons. These interesting results indicate that PHG nanomaterials have very prominent application prospects in future spintronic devices.

Ambient-Stable Two-Dimensional CrI3 via Organic-Inorganic Encapsulation

Two-dimensional transitional metal halides have recently attracted significant attention due to their thickness-dependent and electrostatically tunable magnetic properties. However, this class of materials is highly reactive chemically, which leads to irreversible degradation and catastrophic dissolution within seconds in ambient conditions, severely limiting subsequent characterization, processing, and applications. Here, we impart long-term ambient stability to the prototypical transition metal halide CrI₃ by assembling a noncovalent organic buffer layer, perylenetetracarboxylic dianhydride (PTCDA), which templates subsequent atomic layer deposition (ALD) of alumina. X-ray photoelectron spectroscopy demonstrates the necessity of the noncovalent organic buffer layer since the CrI₃ undergoes deleterious surface reactions with the ALD precursors in the absence of PTCDA. This organic-inorganic encapsulation scheme preserves the long-range magnetic ordering in CrI₃ down to the monolayer limit as confirmed by magneto-optical Kerr effect measurements. Furthermore, we demonstrate field-effect transistors, photodetectors, and optothermal measurements of CrI₃ thermal conductivity in ambient conditions.

Switchable electronic and enhanced magnetic properties of CrI3 edges

Owing to its novel electronic and magnetic properties, two-dimensional CrI₃ has great potential in the application of spintronic devices. However, as an inevitable line defect, the properties of the edges of CrI₃ remain elusive. Here, via first-principles calculations with spin-orbit coupling, we investigated the thermodynamic stabilities, electronic and magnetic properties of thirteen CrI₃ edges with different structures. We showed that zigzag edges are more stable than armchair edges, and a CrI₃ nanoribbon can be either metallic or insulating depending on its chemical growth conditions. The edge stability and associated electronic properties can be understood in terms of the octahedron ligand field and electron counting model. In most cases, both the magnetic moment and Curie temperature can be enhanced by edges, which are in startle contrast to the surfaces of three-dimensional ferromagnetic materials, where a magnetic dead layer is often observed.

Epitaxial growth of large-grain-size ferromagnetic monolayer CrI3 for valley Zeeman splitting enhancement

Two-dimensional (2D) magnetic CrI₃ has received considerable research attention because of its intrinsic features, including insulation, Ising ferromagnetism, and stacking-order-dependent magnetism, as well as potential in spintronic applications. However, the current strategy for the production of ambient-unstable CrI₃ thin layer is limited to mechanical exfoliation, which normally suffers from uncontrollable layer thickness, small size, and low yet unpredictable yield. Here, via a confined vapor epitaxy (CVE) method, we demonstrate the mass production of flower-like CrI₃ monolayers on mica. Interestingly, we discovered the crucial role of K ions on the mica surface in determining the morphology of monolayer CrI₃, reacting with precursors to form a KI_x buffer layer. Meanwhile, the transport agent affects the thickness and size of the as-grown CrI₃. Moreover, the Curie temperature of CrI₃ is greatly affected by the interaction between CrI₃ and the substrate. The monolayer CrI₃ on mica could act as a magnetic substrate for valley Zeeman splitting enhancement of WSe₂. We reckon our work represents a major advancement in the mass production of monolayer 2D CrI₃ and anticipate that our growth strategy may be extended to other transition metal halides.