Jiuxiang Dai

A Bottom-Electrode Contact

In article number 2102207, Jongin Cha, Jangyup Son, and Jongill Hong investigate the contact resistivity of graphene on Pt as a bottom-electrode in single-layer graphene field-effect transistors. The contact resistivity remains constant with increasing graphene-metal contact width, and it becomes lowest at the Dirac point. These two unique features confirm the presence of high density of states at the Dirac point of the graphene edges, which is attributed to low contact resistivity.

The promise of BaSnO₃ (BSO) band structure and scattering channels to relieve low room temperature mobility challenges of interfacial oxide two-dimensional electron gases has been blocked by structural and point defects. The authors overcome this with a new synthesis approach combining a BSO atomically flat pseudo-substrate layer with a band-aligned BSO/LaScO₃ oxide interface.

Abstract

The prospect of 2-dimensional electron gases (2DEGs) possessing high mobility at room temperature in wide-bandgap perovskite stannates is enticing for oxide electronics, particularly to realize transparent and high-electron mobility transistors. Nonetheless only a small number of studies to date report 2DEGs in BaSnO₃-based heterostructures. Here, 2DEG formation at the LaScO₃/BaSnO₃ (LSO/BSO) interface with a room-temperature mobility of 60 cm² V⁻¹ s⁻¹ at a carrier concentration of 1.7 × 10¹³ cm^–2 is reported. This is an order of magnitude higher mobility at room temperature than achieved in SrTiO₃-based 2DEGs. This is achieved by combining a thick BSO buffer layer with an ex situ high-temperature treatment, which not only reduces the dislocation density but also produces a SnO₂-terminated atomically flat surface, followed by the growth of an overlying BSO/LSO interface. Using weak beam dark-field transmission electron microscopy imaging and in-line electron holography technique, a reduction of the threading dislocation density is revealed, and direct evidence for the spatial confinement of a 2DEG at the BSO/LSO interface is provided. This work opens a new pathway to explore the exciting physics of stannate-based 2DEGs at application-relevant temperatures for oxide nanoelectronics.

Nature Electronics, Published online: 21 February 2022; doi:10.1038/s41928-022-00718-w

High-performance indium oxide transistors with dimensions smaller than advanced silicon technologies can be fabricated using an industry-compatible atomic layer deposition process.

CrSBr is a 2D semiconductor of interest for its magnetic properties. Temperature- and gate-dependent transport exhibit an extreme anisotropy (ca. 10² to 10⁵)—stronger than in any other 2D semiconductor—and the photocurrent shows clear signatures of 1D behavior. This work shows that CrSBr is a quasi-1D electronic system, a unique characteristic in the panorama of known 2D semiconductors.

Abstract

Electronic transport through exfoliated multilayers of CrSBr, a 2D semiconductor of interest because of its magnetic properties, is investigated. An extremely pronounced anisotropy manifesting itself in qualitative and quantitative differences of all quantities measured along the in-plane a and b crystallographic directions is found. In particular, a qualitatively different dependence of the conductivities σ _a and σ _b on temperature and gate voltage, accompanied by orders of magnitude differences in their values (σ _b /σ _a  ≈ 3 × 10² to 10⁵ at low temperature and negative gate voltage) are observed, together with a different behavior of the longitudinal magnetoresistance in the two directions and the complete absence of the Hall effect in transverse resistance measurements. These observations appear not to be compatible with a description in terms of conventional band transport of a 2D doped semiconductor. The observed phenomenology—and unambiguous signatures of a 1D van Hove singularity detected in energy-resolved photocurrent measurements—indicate that electronic transport through CrSBr multilayers is better interpreted by considering the system as formed by weakly and incoherently coupled 1D wires, than by conventional 2D band transport. It is concluded that CrSBr is the first 2D semiconductor to show distinctly quasi-1D electronic transport properties.

Publication date: April 2022

Source: Materials Today, Volume 54

Author(s): Xu Wang, Keyuan Ding, Mengchao Shi, Junhua Li, Bin Chen, Mengjiao Xia, Jie Liu, Yaonan Wang, Jixue Li, En Ma, Ze Zhang, He Tian, Feng Rao

Nature, Published online: 23 February 2022; doi:10.1038/d41586-022-00466-z

Two-dimensional materials have been restricted to systems in which strong chemical bonds hold atoms together in sheets. Now, 2D materials consisting of molecules linked by weak non-covalent bonds have been peeled from crystals.

MXene possesses unique characteristics that make it a promising material for numerous applications. This review highlights the most recent developments in MXene-enabled wearable and flexible electronics. The emerging prospects of MXene nanomaterials as a key frontier in wearable electronics are envisioned and the design challenges of these electronic systems are also discussed.

Abstract

Wearable electronics offer incredible benefits in mobile healthcare monitoring, sensing, portable energy harvesting and storage, human-machine interactions, etc., due to the evolution of rigid electronics structure to flexible and stretchable devices. Lately, transition metal carbides and nitrides (MXenes) are highly regarded as a group of thriving two-dimensional nanomaterials and extraordinary building blocks for emerging flexible electronics platforms because of their excellent electrical conductivity, enriched surface functionalities, and large surface area. This article reviews the most recent developments in MXene-enabled flexible electronics for wearable electronics. Several MXene-enabled electronic devices designed on a nanometric scale are highlighted by drawing attention to widely developed nonstructural attributes, including 3D configured devices, textile and planer substrates, bioinspired structures, and printed materials. Furthermore, the unique progress of these nanodevices is highlighted by representative applications in healthcare, energy, electromagnetic interference (EMI) shielding, and humanoid control of machines. The emerging prospects of MXene nanomaterials as a key frontier in next-generation wearable electronics are envisioned and the design challenges of these electronic systems are also discussed, followed by proposed solutions.

Emerging two-dimensional (2D) materials have been studied as fascinating bioelectronics due to their ultrathin structures and excellent physicochemical properties. This review summarizes the structural optimization of them toward bioelectronics and biosensors, which encompasses neural interface simulation, biomolecular/biomarker detection, and skin sensors. Current challenges and future perspectives of utilizing emerging 2D materials and their composites for bioelectronics and biosensors are highlighted.

Abstract

Bioelectronics are powerful tools for monitoring and stimulating biological and biochemical processes, with applications ranging from neural interface simulation to biosensing. The increasing demand for bioelectronics has greatly promoted the development of new nanomaterials as detection platforms. Recently, owing to their ultrathin structures and excellent physicochemical properties, emerging two-dimensional (2D) materials have become one of the most researched areas in the fields of bioelectronics and biosensors. In this timely review, the physicochemical structures of the most representative emerging 2D materials and the design of their nanostructures for engineering high-performance bioelectronic and biosensing devices are presented. We focus on the structural optimization of emerging 2D material-based composites to achieve better regulation for enhancing the performance of bioelectronics. Subsequently, the recent developments of emerging 2D materials in bioelectronics, such as neural interface simulation, biomolecular/biomarker detection, and skin sensors are discussed thoroughly. Finally, we provide conclusive views on the current challenges and future perspectives on utilizing emerging 2D materials and their composites for bioelectronics and biosensors. This review will offer important guidance in designing and applying emerging 2D materials in bioelectronics, thus further promoting their prospects in a wide biomedical field.

npj 2D Materials and Applications, Published online: 23 February 2022; doi:10.1038/s41699-022-00285-w

Large-scale epitaxy of two-dimensional van der Waals room-temperature ferromagnet Fe₅GeTe₂

Author(s): Donghai Li, Hangyong Shan, Christoph Rupprecht, Heiko Knopf, Kenji Watanabe, Takashi Taniguchi, Ying Qin, Sefaattin Tongay, Matthias Nuß, Sven Schröder, Falk Eilenberger, Sven Höfling, Christian Schneider, and Tobias Brixner

Excitons in atomically thin transition-metal dichalcogenides (TMDs) have been established as an attractive platform to explore polaritonic physics, owing to their enormous binding energies and giant oscillator strength. Basic spectral features of exciton polaritons in TMD microcavities, thus far, we...

[Phys. Rev. Lett. 128, 087401] Published Wed Feb 23, 2022

Nature Communications, Published online: 24 February 2022; doi:10.1038/s41467-022-28625-w

Borophene exhibits attractive electronic and optical properties, but its instability has so far limited its applications. Here, the authors report the synthesis of a liquid-state borophene analogue showing a good thermal stability up to 350 °C and an electrically-controlled optical switching behaviour.

Nature Nanotechnology, Published online: 24 February 2022; doi:10.1038/s41565-022-01072-w

Marginal twisting of 2D semiconductor crystals enables the emergence of room temperature interfacial ferroelectricity.

Inspired by a world entering a quantum age, a human-in-the-loop framework is presented, which addresses key challenges and opportunities to overcome in the manufacturing of quantum materials.

Abstract

The quantum age is just around the corner. As quantum systems become more stable, robust, and mainstream, tackling the challenge of high-throughput manufacturing will require further developments in materials synthesis, characterization, assembly, and diagnostics. As the building blocks of future technologies scale down to atomic and molecular scales, a paradigm shift in manufacturing will begin to take shape. Inspired by a quantum manufacturing world that elevates the Materials Genome Initiative to the next level, a “human-in-the-loop” framework for high-throughput manufacturing, which addresses key opportunities and challenges to be overcome, is outlined.

Cocatalyst modification engineering has emerged as a promising technology to ameliorate the solar utilization efficiency and redox activity of g-C₃N₄. Herein, this review emphasizes the loading of 2D configuration cocatalyst onto g-C₃N₄ nanosheet toward effective photocatalytic water splitting, H₂O₂ production, N₂ fixation, CO₂ reduction and environmental purification. Future recommendations and prospects are provided to advance this rapidly progressing field.

Abstract

Sparked by natural photosynthesis, solar photocatalysis using metal-free graphitic carbon nitride (g-C₃N₄) with appealing electronic structure has turned up as the most captivating technique to the quest for sustainable energy generation and pollution-free environment. Nonetheless, low-dimensional g-C₃N₄ is thwarted from sluggish kinetics and rapid recombination of photogenerated carriers upon light irradiation. Among multifarious modification strategies, engineering 2D cocatalysts is anticipated to accelerate redox kinetics, augment active sites and ameliorate electron–hole separation of 2D g-C₃N₄ for boosted activity thanks to its face-to-face contact surface. It is of timely and technological significance to review the 2D/2D interfaces with state-of-the-art 2D cocatalysts, spanning from carbon-containing to phosphorus-containing, metal dichalcogenide, and other cocatalysts. Fundamental principles for each photocatalytic application will be introduced. Thereafter, the recent advances of 2D/2D cocatalyst-mediated g-C₃N₄ systems will be critically evaluated based on their interfacial engineering, emerging roles, and impacts toward stability and catalytic efficiency. Importantly, mechanistic insights into the charge dynamics and structure–performance relationship will be deciphered. Last, noteworthy research directions are prospected to deliver insightful ideas for future development of g-C₃N₄. Overall, this review is anticipated to serve as a scaffold and cornerstone in designing dimensionality-dependent 2D cocatalyst-assisted g-C₃N₄ toward renewable energy and ecologically green environment.

Nanodots of correlated oxides are expected to yield emergent physical phenomena but remain rarely explored to date. Here, SrRuO₃/SrTiO₃ quantum structures are fabricated by patterning as-grown SrRuO₃ thin-film into nanodots with sizes down to 15 nm, showing dot-size-dependent magnetic behaviors. Atomically resolved scanning transmission electron microscopy unveils a unique oxygen octahedral distortion mechanism mediating anisotropic exchange interactions in SrRuO₃ nanodots.

Abstract

Artificial perovskite oxide nanostructures possess intriguing magnetic properties due to their tailorable electron–electron interactions, which are extremely sensitive to the oxygen coordination environment. To date, perovskite oxide nanodots with sizes below 50 nm have rarely been reported. Furthermore, the oxygen octahedral distortion and its relation to magnetic properties in perovskite oxide nanodots remain unexplored thus far. Here, the magnetic anisotropy in patterned SrRuO₃ (SRO) nanodots as small as 30 nm are studied. The constituent elements, in particular oxygen ions, are directly visualized via performing atomic resolution electron microscopy and spectroscopy. It is observed that the magnetic anisotropy and RuO₆ octahedra distortion in SRO nanodots are both nanodot size-dependent but remain unchanged in the first 3-unit-cell interfacial SRO monolayers regardless of the dots’ size. Combined with first principle calculations, a unique structural mechanism behind the nanodots’ size-dependent magnetic anisotropy in SRO nanodots is unraveled, suggesting that the competition between lattice anisotropy and oxygen octahedral rotation mediates anisotropic exchange interactions in SRO nanodots. These findings demonstrate a new avenue toward tuning magnetic properties of correlated perovskite oxides and imply that patterned nanodots could be a promising playground for engineering emergent functional behaviors.

An ultrathin layer of coronene is introduced for the heteroepitaxial nanostructuring of molecular semiconductor films. It is shown that nanostructured films exhibit enhanced light absorption and emission, and greater electron mobilities, compared to their amorphous counterparts. This approach can be readily integrated into existing diode manufacturing routines to realize large-area flexible optoelectronic devices, including organic light-emitting diodes and organic solar cells.

Abstract

Organized nano- and microstructures of molecular semiconductors display interesting optical and photonic properties, and enhanced charge carrier mobilities, as compared to disordered thin films. However, known directed-growth and self-organization strategies cannot create structured molecular heterojunctions and cannot be practically incorporated into existing device fabrication routines to create large-area optoelectronic devices. Here, an ultrathin (<2 nm) seed layer of the compound coronene creates 1D nanostructures of an electron-transporting molecule (IFD) is shown, which possesses an intrinsic proclivity to form disordered thin films in the absence of the seed layer. It is revealed that nanostructured IFD films exhibit enhanced light absorption and emission, and greater electron mobilities, as compared to amorphous counterparts. This seed layer strategy creates uniform IFD nanowires over large areas of up to 18 mm² at low processing temperatures. Notably, the coronene seed layer creates IFD nanowires when applied over either oxide surfaces or predeposited organic layers, meaning that this structuring approach can be integrated into diode manufacturing routines to realize large-area flexible optoelectronic devices. Flexible organic light-emitting diodes and fullerene-free organic solar cells containing IFD nanowires in the photoactive layer to demonstrate that molecular nanostructures can lead to robust, large-area device arrays on flexible substrates being fabricated.

The screw pyramid MoS₂ with abundant edge active sites and eliminated interlayer potential barrier can facilitate the formation of micro eddy current and efficiently utilize magnetic heating to boost electrocatalytic activity under alternating magnetic field. It provides a new strategy for further improvement of electrocatalytic performance and advanced catalyst technology.

Abstract

Eddy current is a magnetic field effect generated in alternating magnetic field (AMF), which could trigger continuous local heating, reducing the energy consumption without impairing the life of the catalyst or reactor. Unfortunately, the investigation of eddy current effect on transition metal disulfides (TMDs) electrocatalysis is still in its infancy, and its actual electrocatalytic applications has been impeded by the multilayered structure of traditional layered TMDs. Typically, the step pyramid MoS₂, with a layer-by-layer stacking structure just like the silicon steel plate in the transformer, showing an inevitable interlayer potential barrier will suppress the generation of eddy current and cause low efficiency of magnetic heating. In this work, the designed screw pyramid MoS₂ can facilitate the formation of micro eddy current and maximize utilization of magnetic heating to boost electrocatalytic activity, benefiting from its eliminated interlayer potential barrier. This work provides a new thinking for the design of field-assisted electrocatalytic reactions and development of the advanced catalyst technology.

This work is about fundamental Raman spectroscopy and transistor applications of a wide-bandgap and air-stable 2D β-type zirconium nitride chloride (β-ZrNCl). Systematical investigation of layer-dependent Raman scattering of the 2D β-ZrNCl from monolayer to bulk reveals a blueshift of its out-of-plane A _1g peak at ≈189 cm^-1. The back gate field-effect transistor shows a high on/off ratio, suggesting the potential for electronic applications.

Abstract

In recent years, 2D layered semiconductors have received much attention for their potential in next-generation electronics and optoelectronics. Wide-bandgap 2D semiconductors are especially important in the blue and ultraviolet wavelength region, although there are very few 2D materials in this region. Here, monolayer β-type zirconium nitride chloride (β-ZrNCl) is isolated for the first time, which is an air-stable layered material with a bandgap of ≈3.0 eV in bulk. Systematical investigation of layer-dependent Raman scattering of ZrNCl from monolayer, bilayer, to bulk reveals a blueshift of its out-of-plane A _1g peak at ≈189 cm^–1. Importantly, this A _1g peak is absent in the monolayer, suggesting that it is a fingerprint to quickly identify the monolayer and for the thickness determination of 2D ZrNCl. The back gate field-effect transistor based on few-layer ZrNCl shows a high on/off ratio of 10⁸. These results suggest the potential of 2D β-ZrNCl for electronic applications.

Heterostructure of nanolayered MoSe₂ and n-GaN is formed to elucidate broadband photodetection from ultraviolet to near-infrared spectrum. Kelvin probe force microscopy investigation is performed at the interface of MoSe₂ and n-GaN to understand the band-alignment and, hence, the current transport mechanism.

Abstract

Heterojunction photodiodes comprising of layered metal chalcogenides and wide-bandgap semiconductors are a promising candidate for broadband photodetection. In this work, nanolayered Molybdenum diselenide (MoSe₂)/Gallium Nitride (GaN) based photodetector has been demonstrated from ultraviolet to near-infrared range (300–1000 nm). The performance is investigated by conducting Kelvin probe force microscopy to measure the conduction band offset at the interface of the heterojunction. It is used to analyze the energy-band diagrams to understand the current transport mechanism. The device exhibits a high photoconductive gain of 1.8 × 10⁴, responsivity of 5580 A W⁻¹, detectivity of 1.9 × 10¹¹ Jones and low noise equivalent power of 10 fW Hz^−1/2 at 365 nm. Additionally, finite difference time domain simulations are performed for the MoSe₂/n-GaN heterostructure to authenticate the broadband photodetection spectrum. Therefore, the outcomes of this study validate the suitability of MoSe₂/n-GaN heterojunction devices for high responsivity and detectivity applications.