Jiuxiang Dai

Publication date: 5 February 2022

Source: Journal of Hazardous Materials, Volume 423, Part B

Author(s): Hsu-Sheng Tsai, You Wang, Chaoming Liu, Tianqi Wang, Mingxue Huo

An atomic chemical-solution strategy allows wafer-size NiO thin films to be grown with controllable thickness down to sub-nanometer scale (0.92 nm) for the first time. The sub-nanometric NiO thin film exhibits the highest reported room-temperature ferromagnetic behavior.

Abstract

Adding ferromagnetism into semiconductors attracts much attentions due to its potential usage of magnetic spins in novel devices, such as spin field-effect transistors. However, it remains challenging to stabilize their ferromagnetism above room temperature. Here we introduce an atomic chemical-solution strategy to grow wafer-size NiO thin films with controllable thickness down to sub-nanometer scale (0.92 nm) for the first time. Surface lattice defects break the magnetic symmetry of NiO and produce surface ferromagnetic behaviors. Our sub-nanometric NiO thin film exhibits the highest reported room-temperature ferromagnetic behavior with a saturation magnetization of 157 emu/cc and coercivity of 418 Oe. Attributed to wafer size, the easily-transferred NiO thin film is further verified in a magnetoresistance device. Our work provides a sub-nanometric platform to produce wafer-size ferromagnetic NiO thin films as atomic layer magnetic units in future transparent magnetoelectric devices.

Nanopores turn semi metallic graphene into semiconducting nanoporous graphene-2 or insulating nanoporous graphene-1. These two nanoporous graphenes with variable structures and band gaps are synthesized via surface-assisted reactions. Nanoporous graphene-1 with a twisted structure shows an insulating bandgap of 5.0 eV, and nanoporous graphene-2 has a semiconducting bandgap of 3.8 eV, opening new applications for nanoporous graphene.

Abstract

Tuning the bandgap of nanoporous graphene is desirable for applications such as the charge transport layer in organic-hybrid devices. The holy grail in the field is the ability to synthesize 2D nanoporous graphene with variable pore sizes, and hence tunable band gaps. Herein, the on-surface synthesis of nanoporous graphene with variable bandgaps is demonstrated. Two types of nanoporous graphene are synthesized via hierarchical CC coupling, and are verified by low-temperature scanning tunneling microscopy and non-contact atomic force microscopy. Nanoporous graphene-1 is non-planar, and nanoporous graphene-2 is a single-atom thick planar sheet. Scanning tunneling spectroscopy measurements reveal that nanoporous graphene-2 has a bandgap of 3.8 eV, while nanoporous graphene-1 has a larger bandgap of 5.0 eV. Corroborated by first-principles calculations, it is proposed that the large bandgap opening is governed by the confinement of π-electrons induced by pore generation and the non-planar structure. The finding shows that by introducing nanopores or a twisted structure, semi metallic graphene is converted into semiconducting nanoporous graphene-2 or insulating wide-bandgap nanoporous graphene-1.

Publication date: 5 February 2022

Source: Journal of Hazardous Materials, Volume 423, Part B

Author(s): Fuhar Dixit, Karl Zimmermann, Rahul Dutta, Niranjana Jaya Prakash, Benoit Barbeau, Madjid Mohseni, Balasubramanian Kandasubramanian

Dimensionality-driven anomalous phase transition of MoTe₂ is demonstrated. The thinner MoTe₂ has a higher 2H-to-T_d phase transition temperature with distinct temperature differences. Vertical and lateral phase-patterning is achieved by modulating the thickness via stacking and insertion of graphene. By using dimensionality-driven phase transition, seamless T_d contacts for 2H-MoTe₂ transistors are fabricated, leading to low contact resistance and high mobility.

Abstract

Phase 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 T_d-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-T_d 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 T_d 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.

Field-Effect-Transistors

Metal-semiconductor junction is an efficient structure to control the carrier concentration of channel semiconductors, benefiting to the regulation of carrier mobility. In article number 2102323, Lei Liao, Johnny C. Ho, Zai-xing Yang, and co-workers demonstrate that by simply constructing the metal-semiconductor junctions, the peak hole mobility of GaSb nanowire field-effect-transistor can be enhanced to the highest value of 3372 cm² V⁻¹ s⁻¹ in the atmosphere, showing three times than the un-deposited one.

Nature, Published online: 15 September 2021; doi:10.1038/s41586-021-03815-6

Nature, Published online: 15 September 2021; doi:10.1038/d41586-021-02433-6

While MoS₂ monolayers degrade when left in ambient air, MoS₂ bilayers and thicker-layers are observed to be resistant to degradation for time periods up to two years. This effect is attributed to the Forster Resonance Energy Transfer mechanism, whereby the indirect band gap of bilayers and thicker-layers inhibits reactive-oxygen oxidation of the layers.

Abstract

It is reported that chemical vapor deposition (CVD) grown bilayer (BL) MoS₂ films are significantly more structurally stable in ambient air than CVD-grown monolayer (ML) MoS₂ films that have been reported to structurally degrade in ambient air. The authors present atomic force microscopy (AFM) images of preheated and as-grown ML and multilayer MoS₂ films after exposure to ambient air for periods of up to 2 years. The AFM images show that, in ambient air, preheated and as-grown BL and thicker-layer MoS₂ films do not exhibit the growth of dendrites that is characteristic of ML degradation. Dendrites are observed to stop at the ML-BL boundary. Raman spectra of BLs exposed for up to 2 years are similar to those reported for as-grown BLs. The greater stability of BLs and thicker layers are attributed to their indirect band gaps that suppress Förster resonance energy transfer processes that have been proposed to be responsible for ML degradation. The results show that BL and thicker-layer transition metal dichalcogenides with indirect band gaps may be structurally stable in air and useful for ambient-air applications.

The SeSe bonds are folded when the Ag⁺-ion vacancies are ordered and become stretched when these vacancies are disordered. Such a stretch/fold of the SeSe bonds enables a fast and large deformation occurring during the phase transition. A new ordered high-temperature phase of α ′-Ag₂Se acts as a buffer to flexibly accommodate the stress caused by the phase transformation.

Abstract

In most semiconducting metal chalcogenides, a large deformation is usually accompanied by a phase transformation, while the deformation mechanism remains largely unexplored. Herein, a phase-transformation-induced deformation in Ag₂Se is investigated by in situ transmission electron microscopy, and a new ordered high-temperature phase (named as α ′-Ag₂Se) is identified. The SeSe bonds are folded when the Ag⁺-ion vacancies are ordered and become stretched when these vacancies are disordered. Such a stretch/fold of the SeSe bonds enables a fast and large deformation occurring during the phase transition. Meanwhile, the different SeSe bonding states in α-, α ′-, β-Ag₂Se phases lead to the formation of a large number of nanoslabs and the high concentration of dislocations at the interface, which flexibly accommodate the strain caused by the phase transformation. This study reveals the atomic mechanism of the deformation in Ag₂Se inorganic semiconductors during the phase transition, which also provides inspiration for understanding the phase transition process in other functional materials.

Author(s): Timo Frauhammer, Hongyan Chen, Timofey Balashov, Gabriel Derenbach, Svetlana Klyatskaya, Eufemio Moreno-Pineda, Mario Ruben, and Wulf Wulfhekel

Rare-earth based single-molecule magnets are promising candidates for magnetic information storage including qubits as their large magnetic moments are carried by localized 4f electrons. This shielding from the environment in turn hampers a direct electronic access to the magnetic moment. Here, we p...

[Phys. Rev. Lett. 127, 123201] Published Mon Sep 13, 2021

Growth of ultra-long straight grain boundaries isotropic liquid surface is ideal for synthesizing 2D materials with ultra-long straight twin boundaries. The above figure illustrates that when a graphene island grows on a liquid copper surface, ultra-long straight twin boundaries are formed as expected.

Abstract

Although grain boundaries (GBs) in two-dimensional (2D) materials have been extensively observed and characterized, their formation mechanism still remains unexplained. Here a general model has reported to elucidate the mechanism of formation of GBs during 2D materials growth. Based on our model, a general method is put forward to synthesize twinned 2D materials on a liquid substrate. Using graphene growth on liquid Cu surface as an example, the growth of twinned graphene has been demonstrated successfully, in which all the GBs are ultra-long straight twin boundaries. Furthermore, well-defined twin boundaries (TBs) are found in graphene that can be selectively etched by hydrogen gas due to the preferential adsorption of hydrogen atoms at high-energy twins. This study thus reveals the formation mechanism of GBs in 2D materials during growth and paves the way to grow various 2D nanostructures with controlled GBs.