Role of Oxygen in Ionic Liquid Gating on Two-Dimensional Cr2Ge2Te6: A Non-oxide Material

Effect of Surface Oxidation and Crystal Thickness on the Magnetic Properties and Magnetic Domain Structures of Cr2Ge2Te6

van der Waals (vdW) magnetic materials, such as Cr2Ge2Te6 (CGT), show promise for memory and logic applications. This is due to their broadly tunable magnetic properties and the presence of topological magnetic features such as skyrmionic bubbles. A systematic study of thickness and oxidation effects on magnetic domain structures is important for designing devices and vdW heterostructures for practical applications. Here, we investigate thickness effects on magnetic properties, magnetic domains, and bubbles in oxidation-controlled CGT crystals. We find that CGT exposed to ambient conditions for 5 days forms an oxide layer approximately 5 nm thick. This oxidation leads to a significant increase in the oxidation state of the Cr ions, indicating a change in local magnetic properties. This is supported by real-space magnetic texture imaging through Lorentz transmission electron microscopy. By comparing the thickness-dependent saturation field of oxidized and pristine crystals, we find that oxidation leads to a nonmagnetic surface layer that is thicker than the oxide layer alone. We also find that the stripe domain width and skyrmionic bubble size are strongly affected by the crystal thickness in pristine crystals. These findings underscore the impact of thickness and surface oxidation on the properties of CGT, such as saturation field and domain/skyrmionic bubble size, and suggest a pathway for manipulating magnetic properties through a controlled oxidation process.

Exchange Bias Between van der Waals Materials: Tilted Magnetic States and Field-Free Spin-Orbit-Torque Switching

Magnetic van der Waals heterostructures provide a unique platform to study magnetism and spintronics device concepts in the 2D limit. Here, studies of exchange bias from the van der Waals antiferromagnet CrSBr acting on the van der Waals ferromagnet Fe3 GeTe2 (FGT) are reported. The orientation of the exchange bias is along the in-plane easy axis of CrSBr, perpendicular to the out-of-plane anisotropy of the FGT, inducing a strongly tilted magnetic configuration in the FGT. Furthermore, the in-plane exchange bias provides sufficient symmetry breaking to allow deterministic spin-orbit torque switching of the FGT in CrSBr/FGT/Pt samples at zero applied magnetic field. A minimum thickness of the CrSBr of >10 nm is needed to provide a non-zero exchange bias at 30 K.

Formation of Highly Conductive Interfaces in Crystalline Ionic Liquid-Gated Unipolar MoTe2/h-BN Field-Effect Transistor

2H MoTe2 (molybdenum ditelluride) has generated significant interest because of its superconducting, nonvolatile memory, and semiconducting of new materials, and it has a large range of electrical properties. The combination of transition metal dichalcogenides (TMDCs) and two dimensional (2D) materials like hexagonal boron nitride (h-BN) in lateral heterostructures offers a unique platform for designing and engineering novel electronic devices. We report the fabrication of highly conductive interfaces in crystalline ionic liquid-gated (ILG) field-effect transistors (FETs) consisting of a few layers of MoTe2/h-BN heterojunctions. In our initial exploration of tellurium-based semiconducting TMDs, we directed our attention to MoTe2 crystals with thicknesses exceeding 12 nm. Our primary focus centered on investigating the transport characteristics and quantitatively assessing the surface interface heterostructure. Our transconductance (gm) measurements indicate that the very efficient carrier modulation with an ILG FET is two times larger than standard back gating, and it demonstrates unipolarity of the device. The ILG FET exhibited highly unipolar p-type behavior with a high on/off ratio, and it significantly increased the mobility in MoTe2/h-BN heterochannels, achieving improvement as one of the highest recorded mobility increments. Specifically, we observed hole and electron mobility values ranging from 345 cm2 V-1 s-1 to 285 cm2 V-1 s-1 at 80 K. We predict that our ability to observe the intrinsic, heterointerface conduction in the channels was due to a drastic reduction of the Schottky barriers, and electrostatic gating is suggested as a method for controlling the phase transitions in the few layers of TMDC FETs. Moreover, the simultaneous structural phase transitions throughout the sample, achieved through electrostatic doping control, presents new opportunities for developing phase change devices using atomically thin membranes.

Intercalation Strategy in 2D Materials for Electronics and Optoelectronics

Intercalation is an effective approach to tune the physical and chemical properties of 2D materials due to their abundant van der Waals gaps that can host high-density intercalated guest matters. This approach has been widely employed to modulate the optical, electrical, and photoelectrical properties of 2D materials for their applications in electronic and optoelectronic devices. Thus it is necessary to review the recent progress of the intercalation strategy in 2D materials and their applications in devices. Herein, various intercalation strategies and the novel properties of the intercalated 2D materials as well as their applications in electronics and optoelectronics are summarized. In the end, the development tendency of this promising approach for 2D materials is also outlined.

Tunable Tunneling Magnetoresistance in van der Waals Magnetic Tunnel Junctions with 1-CrTe2 Electrodes

Two-dimensional (2D) van der Waals (vdW) heterostructures have opened new avenues for spintronic applications with novel properties. Here, by density functional theory calculations, we investigated the spin-dependent transport in vdW magnetic tunnel junctions (MTJs) composed of 1T-CrTe₂ ferromagnetic electrodes. Meanwhile, graphene and h-BN are employed as tunnel barriers. It has been found that the tunneling magnetoresistance (TMR) effects of two types of vdW MTJs present analogous trends: thicknesses of barriers have a great influence on the TMR ratios, which reach up to the maximum when barriers increase to five monolayers. However, despite the similarity, the graphene-barrier junction is more promising for optimization. Through observing the energy-resolved transmission spectra of vdW MTJs, we noticed that TMR ratios of graphene-barrier junctions are tunable and could be enhanced through tuning the position of Fermi energy. Therefore, we successfully realized the TMR optimization by substitutional doping. When substituting one carbon atom with one boron atom in the graphene barrier, TMR ratios are drastically improved, and a TMR ratio as high as 6962% could be obtained in the doped seven-monolayer-barrier junction. Our results pave the way for vdW MTJ applications in spintronics.

Solution-gated transistors of two-dimensional materials for chemical and biological sensors: status and challenges

Two-dimensional (2D) materials have been the focus of materials research for many years due to their unique fascinating properties and large specific surface area (SSA). They are very sensitive to the analytes (ions, glucose, DNA, protein, etc.), resulting in their wide-spread development in the field of sensing. New 2D materials, as the basis of applications, are constantly being fabricated and comprehensively studied. In a variety of sensing applications, the solution-gated transistor (SGT) is a promising biochemical sensing platform because it can work at low voltage in different electrolytes, which is ideal for monitoring body fluids in wearable electronics, e-skin, or implantable devices. However, there are still some key challenges, such as device stability and reproducibility, that must be faced in order to pave the way for the development of cost-effective, flexible, and transparent SGTs with 2D materials. In this review, the device preparation, device physics, and the latest application prospects of 2D materials-based SGTs are systematically presented. Besides, a bold perspective is also provided for the future development of these devices.

Designing Multiple Crystallization in Superlattice-like Phase-Change Materials for Multilevel Phase-Change Memory

A multilevel phase-change memory device was successfully designed, which was fabricated using a Ge₄₀Te₆₀/Cr superlattice-like (SLL) structure. In the SLL films, a two-step phase change process is observed at elevated temperatures, which reveals the crystallization of Ge₄₀Te₆₀ (GT) and an interface-dominated formation of Cr₂Ge₂Te₆ (CrGT). The bonding of Cr-Te and Ge-Ge is accompanied by the breaking of a Ge-Te bond, which is mainly in the Ge-rich GeTe_4-nGe_n units. The formation of CrGT is related to the breaking apart of the edge-sharing octahedron in GT and Cr replacement at Ge sites. The crystalline GT acts as the crystallization precursors in the formation of the CrGT phase. The stable reversible two-step phase change can guarantee the reliability of the multilevel storage. The present work may shed light on the possible mechanism of the CrGT phase transition-based interfacial dynamic process. The designed multiple crystallization system demonstrates a potential for multilevel storage.

Large Tunneling Magnetoresistance in VSe2/MoS2 Magnetic Tunnel Junction

Two-dimensional (2D) van der Waals (vdW) materials provide the possibility of realizing heterostructures with coveted properties. Here, we report a theoretical investigation of the vdW magnetic tunnel junction (MTJ) based on VSe₂/MoS₂ heterojunction, where the VSe₂ monolayer acts as a ferromagnet with room-temperature ferromagnetism. We propose the concept of spin-orbit torque (SOT) vdW MTJ with reliable reading and efficient writing operations. The nonequilibrium study reveals a large tunneling magnetoresistance of 846% at 300 K, identifying significantly its parallel and antiparallel states. Thanks to the strong spin Hall conductivity of MoS₂, SOT is promising for the magnetization switching of VSe₂ free layer. Quantum-well states come into being and resonances appear in MTJ, suggesting that the voltage control can adjust transport properties effectively. The SOT vdW MTJ based on VSe₂/MoS₂ provides desirable performance and experimental feasibility, offering new opportunities for 2D spintronics.

Charge-governed phase manipulation of few-layer tellurium

Few-layer tellurium is an emerging quasi-one-dimensional layered material. The striking feature of Te is its presence as various few-layer allotropes (α-δ). Although these allotropes offer substantially different physical properties, only the α phase has been synthesized in neutral few-layers as it is so far the most stable few-layer form. Herein, we show that hole or electron doping could maintain a certain Te phase. The β, α, γ and δ phases appear as the most stable forms of Te bilayer, in sequence, with bandgap variations over 1 eV. In Te trilayer, a novel metallic chiral α + δ phase emerges, leading to the appearance of chirality. Transitions among these phases, understood at the wavefunction level, are accompanied by the emergence or elimination of inversion centers (α-β, α-γ, α-α + δ), structural anisotropy (α-γ, γ-δ) and chirality (α-α + δ), which could result in substantial changes in optical and other properties. In light of these findings, our work opens a new avenue for stabilizing different allotropes of layered materials; this is crucial for using their outstanding properties. This study also suggests the possibility of building mono-elemental electronic and optoelectronic heterostructures or devices, which are attractive for future applications in electronics.

Ultrasensitive negative photoresponse in 2D Cr2Ge2Te6 photodetector with light-induced carrier trapping

Cr₂Ge₂Te₆, a layered ferromagnetic semiconductor, has triggered extensive research interest due to its fantastic ferromagnetism and semiconducting characteristics as well as potential applications in next-generation spintronic and nanoelectronic devices. On the basis of its ferromagnetism, combined with rich electronic and optical properties, Cr₂Ge₂Te₆ could be a promising candidate for optoelectronics including magnetophotonics and photodetectors. However, there are no relevant studies addressing this to date. In this work, we comprehensively investigated the photoresponse characteristics of few-layer Cr₂Ge₂Te₆-based detectors. An uncommon negative photoconductivity (NPC) and correlated mechanism are explored with the Cr₂Ge₂Te₆ photodetector. It is found that, both NPC and positive photoconductivity (PPC) may exist in an individual Cr₂Ge₂Te₆ device, which are adjustable by control of the incident light intensity. More significantly, the NPC behavior enables ultrasensitive photoresponses of the Cr₂Ge₂Te₆ photodetectors, where the detection of a weak light with an incident power intensity as low as 0.04 pW and a high responsivity of 340 AW^-1 is achieved. This extraordinary performance demonstrates that the two-dimensional (2D) Cr₂Ge₂Te₆ holds great promise for applications in ultraweak light detection.