Jiuxiang Dai

Abstract

Two-dimensional (2D) van der Waals (vdW) magnetic materials with strong in-plane anisotropy can make possible novel applications such as optospintronics and strain sensors. In this work, the strong in-plane optical anisotropy in 2D vdW antiferromagnet VOCl has been systematically investigated. The optical brightness and absorption coefficient exhibit evident periodic variation with the change of incident polarization, unveiling the strong in-plane anisotropic optical absorption. The Raman intensity in this material shows obvious dependence on the polarization angle of incident laser, demonstrating that the phonon properties possess strong in-plane anisotropy. Besides, we have also realized in-situ visualization of in-plane optical reflection anisotropy in this material. Moreover, the strong second harmonic generation (SHG) signal can only be detected when the incident polarization is along specific in-plane crystal orientations, illustrating the presence of strong in-plane nonlinear optical anisotropy. These findings will benefit the applications of VOCl in the field of polarization-dependent electronics and spintronics.

Nature, Published online: 04 January 2023; doi:10.1038/d41586-022-04491-w

Ferroelectricity has been found in a superconducting compound. Strong coupling between these two properties enables ferroelectric control of the superconductivity, which could prove useful for quantum devices.

Nature, Published online: 04 January 2023; doi:10.1038/s41586-022-05401-w

A two-dimensional crystalline polymer of C60, termed graphullerene, is synthesized by chemical vapour transport, and mechanically exfoliated to produce molecularly thin flakes with clean interfaces for potential optoelectronic applications.

Nature, Published online: 04 January 2023; doi:10.1038/s41586-022-05521-3

The authors show a hysteretic behaviour of superconductivity as a function of electric field in bilayer Td-MoTe2, representing observations of coupled ferroelectricity and superconductivity.

Nature, Published online: 04 January 2023; doi:10.1038/s41586-022-05393-7

A van der Waals crystal, niobium oxide dichloride, with vanishing interlayer electronic coupling and considerable monolayer-like excitonic behaviour in the bulk, as well as strong and scalable second-order optical nonlinearity, is discovered, which enables a high-performance quantum light source.

Nature Communications, Published online: 04 January 2023; doi:10.1038/s41467-022-35702-7

All-inorganic nanocrystals are of great importance for a variety of electronic applications. Here, the authors use metal salts to remove organic ligands to obtain passivated nanocrystals with improved fluorescence yield for direct optical patterning.

Correlative electrical and chemical information on a WS₂ layer grown on a sapphire substrate. The different surface terminations (in blue and pink in the figure) lead to regions with different conductivity characterized by sulfur (lower current, in blue) or oxygen (higher current, in pink).

Abstract

Atomically thin, 2D semiconductors, such as transition metal dichalcogenides, complement silicon in ultra-scaled nano-electronic devices. However, the semiconductor and its interfaces become increasingly more difficult to characterize chemically and electrically. Conventional methodologies, including scanning probe microscopies, fail to capture insight into the chemical and electronic nature of the semiconductor, albeit vital to understand its impact on the semiconductor performance. Therefore, this work presents a unique and universal in situ approach combining time-of-flight secondary ion mass spectrometry and atomic force microscopy to map chemical differences between regions of different electrical conductivity in epitaxially deposited tungsten disulfide (WS₂) on sapphire substrates. Surprisingly, WS₂ regions of lower electrical conductivity possess a larger amount of sulfur compared to regions with higher conductivity, for which oxygen is also detected. Such difference in chemical composition likely roots from the non-homogeneously terminated sapphire starting surface, altering the WS₂ nucleation behavior and associated defect formation between neighboring sapphire terraces. These resulting sapphire terrace-dependent doping effects in the WS₂ hamper its electrical conductivity. Thus, accurate chemical assignment at a sub-micrometer lateral resolution of atomically thin 2D semiconductors is vital to achieve a more detailed understanding on how the growth behavior affects the electrical properties.

This work establishes the first computational framework capable of capturing 2D van der Waals (vdW) magnets with high probability for experimental demonstration. Historical validation of predictions demonstrates its remarkable capacity for accelerating experimental discovery of 2D vdW magnets. Introduction of the theory of mutual information is the key to the resounding success of this framework.

Abstract

2D van der Waals (vdW) magnets have opened intriguing prospects for next-generation spintronic nanodevices. Machine learning techniques and density functional theory calculations enable the discovery of 2D vdW magnets to be accelerated; however, current computational frameworks based on these state-of-the-art approaches cannot offer probability analysis on whether a 2D vdW magnet can be experimentally demonstrated. Herein, a new framework can be established to overcome this challenge. Via the framework, 2D vdW magnets with high probability for experimental demonstration are captured from materials science literature. The key to the successful establishment is the introduction of the theory of mutual information. Historical validation of predictions substantiates the high reliability of the framework. For example, half of the 30 2D vdW magnets discovered in the literature published prior to 2017 have been experimentally demonstrated in the subsequent years. This framework has the potential to become a revolutionary force for progressing experimental discovery of 2D vdW magnets.

Nature Communications, Published online: 03 January 2023; doi:10.1038/s41467-022-35651-1

Deep level transient spectroscopy (DLTS) is an established characterization technique used to study electrically active defects in 3D semiconductors. Here, the authors show that DLTS can also be applied to monolayer semiconductors, enabling in-situ characterization of the energy states of different defects and their interactions.

Nature Communications, Published online: 03 January 2023; doi:10.1038/s41467-022-35731-2

Heteroepitaxy on colloidal nanocrystals often yields defective heterostructures due to intricate reaction pathways. Here, the authors decode the surface chemistry at the molecular level to realise defect-free interfaces with atomic uniformity.

Nature Communications, Published online: 03 January 2023; doi:10.1038/s41467-022-35385-0

Self-assembled nanocomposites present opportunities for a range of phase morphologies and desirable properties. Here authors present tuneable self-assembled nanostructures in the SnO2:NiO system; the double-percolated system expands the design of self-assembled oxides for practical applications, e.g. in optoelectronics.

Nature Communications, Published online: 03 January 2023; doi:10.1038/s41467-022-34699-3

Despite their vast potential, the practical deployment of MXenes has been hampered by their tendency to be oxidized. Here, the authors show that simply vibrating MXene films in just a minute can remove the oxide layer formed and restore their electrochemical performance close to its original state.

The fundamentals of 2D transition metal dichalcogenides (TMDs), their synthesis, their advantages in photocatalysis, and the strategies for boosting their photocatalytic performance are summarized in this review. Currently problems and their solutions are presented.

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs), a rising star in the post-graphene era, are fundamentally and technologically intriguing for photocatalysis. Their extraordinary electronic, optical, and chemical properties endow them as promising materials for effectively harvesting light and catalyzing the redox reaction in photocatalysis. Here, we present a tutorial-style review of the field of 2D TMDs for photocatalysis to educate researchers (especially the new-comers), which begins with a brief introduction of the fundamentals of 2D TMDs and photocatalysis along with the synthesis of this type of material, then look deeply into the merits of 2D TMDs as co-catalysts and active photocatalysts, followed by an overview of the challenges and corresponding strategies of 2D TMDs for photocatalysis, and finally look ahead this topic.

A broadband and polarimetric photodetector with reconfigurable operation mode based on anisotropic As_0.4P_0.6-MoTe₂ heterostructure is developed, where photogating and photovoltaic modes are operated and switchable, yielding the gate-modulated responsivity up to 14.7 A W⁻¹ at 1550 nm. A novel optical reversal and gate-tuanble dichroism ratio (from unity to ≈10) can also enable the reconfigurable detection mode from polarization-independence to polarization-susceptibility.

Abstract

The sensitive detection of light polarization besides the intensity and wavelength, can provide a new degree of freedom for more and clearer information of imaging targets in night, fog, and smoke environment. However, the conventional filter-integrated polarimetric photodetectors suffer from the complicated fabrication process and limited spectral range. Herein, broadband and polarization-sensitive photodetectors are achieved with reconfigurable operation mode, utilizing the linear dichroism and narrow band gap of 2D As_0.4P_0.6 with in-plane anisotropic structure. In As_0.4P_0.6-MoTe₂ heterojunction device, both photo-gating and photovoltaic modes are operated and switchable, contributing to high responsivity (1590 A W⁻¹ at 405 nm and 14.7 A W⁻¹ at 1550 nm) and ultrafast speed (25 µs) in the wide spectral band (405–1550 nm). Interestingly, an optical reversal is observed on both linear dichroism and polarimetric photocurrent due to the wavelength-dependent polarization reverse nature of the As_0.4P_0.6 flakes. The dichroism ratio of photocurrent can be modulated from unity to ≈10 by varying the gate voltage, enabling the reconfigurable detection mode from polarization-independence to polarization-susceptibility. This study demonstrates a new prototype device comprising low symmetric van der Waals heterostructure, possessing the gate-tunability on both photo-gain and dichroism ratio, toward high performance, reconfigurable, broadband, and polarization-resolved photodetection and imaging applications.

A new nonmetallic catalyst (V₂O₅) is found for graphene growth, which has a new growth mechanism because of the phase-change properties of V₂O₅. With V₂O₅ as the interface layer, in situ growth of graphene on a dielectric substance at 400 °C is successfully realized. Furthermore, based on this, wafer-scale graphene Schottky junction photodetector array is achieved with good performance consistency.

Abstract

The use of metal foil catalysts in the chemical vapor deposition of graphene films makes graphene transfer an ineluctable part of graphene device fabrication, which greatly limits industrialization. Here, an oxide phase-change material (V₂O₅) is found to have the same catalytic effect on graphene growth as conventional metals. A uniform large-area graphene film can be obtained on a 10 nm V₂O₅ film. Density functional theory is used to quantitatively analyze the catalytic effect of V₂O₅. Due to the high resistance property of V₂O₅ at room temperature, the obtained graphene can be directly used in devices with V₂O₅ as an intercalation layer. A wafer-scale graphene-V₂O₅-Si (GVS) Schottky photodetector array is successfully fabricated. When illuminated by a 792 nm laser, the responsivity of the photodetector can reach 266 mA W⁻¹ at 0 V bias and 420 mA W⁻¹ at 2 V. The transfer-free device fabrication process enables high feasibility for industrialization.

npj 2D Materials and Applications, Published online: 02 January 2023; doi:10.1038/s41699-022-00363-z

Friction hysteretic behavior of supported atomically thin nanofilms

An edge-dependent dopant distribution effect (i.e., sulfur-zigzag edge terminated domains host a higher density of transition metal dopants) is demonstrated using examples of hexagonal Fe- and V-doped WS₂ monolayers. This work highlights the important role of edge termination in tuning the spatial distribution of dopants, which constitutes a novel knob for creating in-plane hetero-/multi-junctions that locally display intriguing physicochemical properties.

Abstract

The ability to control the density and spatial distribution of substitutional dopants in semiconductors is crucial for achieving desired physicochemical properties. Substitutional doping with adjustable doping levels has been previously demonstrated in 2D transition metal dichalcogenides (TMDs); however, the spatial control of dopant distribution remains an open field. In this work, edge termination is demonstrated as an important characteristic of 2D TMD monocrystals that affects the distribution of substitutional dopants. Particularly, in chemical vapor deposition (CVD)-grown monolayer WS₂, it is found that a higher density of transition metal dopants is always incorporated in sulfur-terminated domains when compared to tungsten-terminated domains. Two representative examples demonstrate this spatial distribution control, including hexagonal iron- and vanadium-doped WS₂ monolayers. Density functional theory (DFT) calculations are further performed, indicating that the edge-dependent dopant distribution is due to a strong binding of tungsten atoms at tungsten-zigzag edges, resulting in the formation of open sites at sulfur-zigzag edges that enable preferential dopant incorporation. Based on these results, it is envisioned that edge termination in crystalline TMD monolayers can be utilized as a novel and effective knob for engineering the spatial distribution of substitutional dopants, leading to in-plane hetero-/multi-junctions that display fascinating electronic, optoelectronic, and magnetic properties.

The physics and materials science of electrical contact resistance in 2D materials-based nanoelectronics, interface configurations, charge injection mechanisms, and numerical modeling of electrical contacts, as well as the most pressing issues that need to be resolved in the field of research and development, are discussed in this article.

Abstract

Current electrical contact models are occasionally insufficient at the nanoscale owing to the wide variations in outcomes between 2D mono and multi-layered and bulk materials that result from their distinctive electrostatics and geometries. Contrarily, devices based on 2D semiconductors present a significant challenge due to the requirement for electrical contact with resistances close to the quantum limit. The next generation of low-power devices is already hindered by the lack of high-quality and low-contact-resistance contacts on 2D materials. The physics and materials science of electrical contact resistance in 2D materials-based nanoelectronics, interface configurations, charge injection mechanisms, and numerical modeling of electrical contacts, as well as the most pressing issues that need to be resolved in the field of research and development, will all be covered in this review.

A pulsed physical vapor deposition method is designed for burst nucleation-assisted fabrication of ultrathin tellurium nanowires with sub-10 nm thickness, which displays excellent hole mobility and on/off ratio in a field effect transistor.

Abstract

Physical vapor deposition (PVD) methods have been widely employed for high-quality crystal growth and thin-film deposition in semiconductor electronics. However, the fabrication of emerging low dimensional nanostructures is hitherto challenging in conventional PVD systems due to their large thermal mass and near-continuous operation which hinder flexible control of the nucleation and growth events. Herein, a pulsed PVD method is reported that features finely controllable temperature and heating time (down to milliseconds), which enables programming of the vapor supersaturation and decoupling of nucleation and growth events. Take tellurium as an example, the pulsed PVD allows transient source vaporization (≈1000 °C, 30 ms) for burst nucleation, followed by relatively low-temperature volatilization (≈600 °C, 5 min) for steady-state growth with well-suppressed random nucleation. As a result, uniform and high-density tellurium nanowires are obtained at the ultrathin thickness of sub-10 nm and length >10 µm, which is in sharp contrast to the randomly formed nanostructures in conventional PVD. When used in the field-effect transistor, the thin tellurium nanowires display a high on-off ratio of >10⁴ and hole mobility of ≈40 cm² V⁻¹ s⁻¹, showing the potential for high-performance electronics. Pulsed PVD therefore enables to flexibly program and finely tailor the nucleation and growth events during vapor phase deposition, which are otherwise impossible in conventional PVD.