Jiuxiang Dai

With the uniquely-designed one-pot method and space-confined process, the controlled growth of a series of 2D-3D perovskite lateral heterostructures is achieved. Based on the tunable dual-band optical response characteristics, a wavelength-tunable light communication system based on the lateral heterostructure is realized. This work provides a convenient and reliable approach for the direct growth of mixed-dimensional halide perovskite heterostructures, further demonstrating their potential in high-performance detecting and dual-band sensing fields.

Abstract

Lateral heterostructures based on halide perovskites exhibit great potential in the advancement of next-generation optoelectronic devices. Among them, mixed dimensional perovskite heterostructures, particularly 2D-3D ones, offer promising opportunities for semiconductor integration and device miniaturization by combining the advantages of 2D and 3D perovskites. However, the controllable and rapid growth of 2D-3D halide perovskite lateral heterostructures has not yet been achieved. This study presents an efficient strategy that integrates one-pot method and space-confined process to enable liquid-phase lateral growth of a series of 2D Ruddlesden-Popper (RP) perovskites on the sides of 3D perovskites. The photodetectors (PDs) based on (BA)₂MA_n-1Pb_nBr_3n+1-MAPbBr₃ (n = 1, 2, 3) lateral heterostructures demonstrate outstanding optoelectronic performance, featuring an on/off ratio of up to 1.4 × 10⁴, a high responsivity of 4.4 A W⁻¹ and a detectivity of 3.9 × 10¹³ Jones at 425 nm, 3 V bias. In addition, by combining the tunable dual-band photoresponse characteristic with the dual-beam irradiation modes, a wavelength-tunable light communication system based on the lateral heterostructure PDs is realized. This work provides a convenient and reliable approach for the direct growth of mixed-dimensional halide perovskite heterostructures, further demonstrating their potential in high-performance detecting and dual-band sensing fields.

An Iron-assisted floating solid catalyst chemical vapor deposition (IA-FSCCVD) method is developed for the controlled growth of carbon nanotube films. Using the IA-FSCCVD platform, carbon nanotube films with high quality (average I_G/I_D ≈166), a small diameter deviation (mean diameter of 1.6 nm), and a high content of single-walled carbon nanotubes (up to 97%) are prepared.

Abstract

Single-walled carbon nanotube (SWNT) films, with exciting electronic properties are increasingly important for next-generation technologies. Here, an Iron-assisted floating solid catalyst chemical vapor deposition (IA-FSCCVD) method is developed for the controlled growth of high-quality and high-purity SWNT films. Titanium carbide nanoparticles with a high melting point are used as the solid catalysts, which provide a stable template for SWNT growth through the perpendicular growth mode. Trace amounts of iron are introduced to increase the efficiency of SWNT growth. Gas chromatography measurements and density functional theory show that the gas-phase iron acts as a pre-cracking assistance for the carbon source, promoting the growth of SWNTs. Carbon nanotube films with a high quality (average I_G/I_D = 166) are successfully prepared, a small diameter deviation (mean diameter of 1.6 nm), and a high content of SWNTs (97%) using the IA-FSCCVD platform. This work provides a powerful way to prepare the carbon nanotube aggregates with a controlled structure.

This work reports the construction of a compact system that functions as both a transfer stage for deterministic transfer processes and a mask aligner for contact photolithography. The prototype instrument provides a feasible solution for performing both high-quality transfer and photolithography processes on one single tool in-house.

A home-built compact system that functions as both a transfer stage for deterministic transfer processes and a mask aligner for contact photolithography, is constructed. The precision translation sample stage and optical microscope are shared between the two modes. In the transfer mode, assisted by either an adhesive or a heating element, the setup has been used to deterministically transfer freestanding semiconductors, 2D materials, and van der Waals electrodes. When configured in photolithography mode, the proposed instrument has been employed to fabricate various microscale patterns and devices, with minimum feature sizes of 1–2 μm achieved. The prototype instrument provides a feasible solution for performing high-quality deterministic transfer and photolithography processes on one single tool in-house.

Nature Electronics, Published online: 23 October 2024; doi:10.1038/s41928-024-01244-7

Combining time-resolved optical spectroscopy and surface acoustic waves provide a powerful tool for investigating charge carrier dynamics in 2D materials. When interacting with the 2D semiconductor WSe₂, dynamic acoustic Poole–Frenkel activation of traps enhances the optical emission efficiency. The sub-nanosecond modulation of the optical signal serves as a sensitive local probe of defects and charge carrier trapping sites.

Abstract

The charge carrier dynamics are investigated by surface acoustic waves (SAWs) inside a WSe₂ monolayer on LiNbO₃ by scanning acousto-optoelectric spectroscopy. A strong enhancement of the PL emission intensity is observed almost over the entire area of the flake. This enhancement increases with increasing amplitude of the wave and is especially strong at or in the vicinity to defects. The latter is attributed to the SAW-driven Poole–Frenkel activation of trapped charge carriers bound to trapping sites at these defects. In addition, the PL intensity exhibit clear periodic modulations at the SAW's frequency f _SAW and at 2 f _SAW. These modulations are clear and unambiguous fingerprints of spatio-temporal carrier dynamics driven by the SAW. These occur on sub-nanosecond timescales which are found in good agreement with calculated exciton dissociation times. Mapping and analyzing both effects, this study shows that scanning acousto-electric spectroscopy provides a highly sensitive and local contact-free probe which uncovers distinct local features not resolved by conventional quasi-static photoluminescence techniques. The method is ideally suited to study carrier transport in 2D and other types of nanoscale materials and to reveal dynamic exciton modulation, and carrier localization and activation dynamics in the technologically important megahertz to gigahertz frequency range.

A robust plasma-assisted (RPA) synthesis strategy is introduced with a specially designed tube for a uniform plasma atmosphere. This enables broader growth parameter variations while preserving Janus MoSSe's morphology. Enhancements in photoluminescence (PL) are achieved through bis(trifluoromethane) sulfonimide (TFSI) treatment, resulting in a 70-fold PL intensity increase and a quantum yield (QY) of 31.2%.

Abstract

Janus transition metal dichalcogenides (TMDs) are a novel class of 2D materials with unique mirror asymmetry. Plasma-assisted synthesis at room temperature is favored for producing Janus TMDs due to its energy efficiency and prevention of alloying. However, current methods require stringent control over growth conditions, risking defects or unintended materials. A robust plasma-assisted (RPA) synthesis strategy is introduced, incorporating a built-in tube with a suitable inner diameter into the plasma-assisted system. This innovation creates a mild, uniform plasma atmosphere, allowing for broader variations in growth parameters without significantly affecting Janus MoSSe's morphology and characteristics. This approach simplifies the synthesis process and enhances the success rate of Janus TMD production. Additionally, methods are explored to enhance the photoluminescence (PL) of Janus MoSSe. Releasing MoSSe from the growth substrate and annealing it removes strain and unintentional doping, improving PL performance. MoSSe on hexagonal boron nitride (h-BN) flakes after annealing shows a 32-fold increase in PL intensity. Bis(trifluoromethane) sulfonimide (TFSI) treatment of MoSSe results in a remarkable 70-fold increase in PL intensity, a 2.5-fold extension in exciton lifetime, and quantum yield (QY) reaching up to ≈31.2%. These findings provide critical insights for optimizing the luminescence properties of 2D Janus materials, advancing Janus optoelectronics.

In this paper, it is observed that polarization retention and stability of ferroelectric Hf_0.5Zr_0.5O₂ (HZO) are significantly dependent on humidity. Elimination of absorbed water shows a significant effect in improving polarization stability and suppressing imprinting characteristics. This work provides a novel understanding of the relationship between surface electrochemistry and ferroelectricity in HfO₂-based ferroelectric materials.

Abstract

The discovery of nanoscale ferroelectricity in hafnia (HfO₂) has paved the way for next generation high-density, non-volatile devices. Although the surface conditions of nanoscale HfO₂ present one of the fundamental mechanism origins, the impact of external environment on HfO₂ ferroelectricity remains unknown. In this study, the deleterious effect of ambient moisture is examined on the stability of ferroelectricity using Hf_0.5Zr_0.5O₂ (HZO) films as a model system. It is found that the development of an intrinsic electric field due to the adsorption of atmospheric water molecules onto the film's surface significantly impairs the properties of domain retention and polarization stability. Nonetheless, vacuum heating efficiently counteracts the adverse effects of water adsorption, which restores the symmetric electrical characteristics and polarization stability. This work furnishes a novel perspective on previous extensive studies, demonstrating significant impact of surface water on HfO₂-based ferroelectrics, and establishes the design paradigm for the future evolution of HfO₂-based multifunctional electronic devices.

Nature Materials, Published online: 29 October 2024; doi:10.1038/s41563-024-02013-9

Out-of-plane magnetic field (B)-induced giant electric hysteretic responses to back-gate voltages in monolayer (ML) MoS₂ field-effect transistors (FETs) below 20 Kelvin are observed and attributed to asymmetric lattice expansion with increasing |B| in ML-MoS₂ FETs on rigid SiO₂/Si substrates, leading to mirror symmetry breaking in ML-MoS₂ and the emergence of a tunable out-of-plane ferroelectric-like polar order.

Abstract

In semiconducting monolayer transition metal dichalcogenides (ML-TMDs), broken inversion symmetry and strong spin-orbit coupling result in spin-valley lock-in effects so that the valley degeneracy may be lifted by external magnetic fields, potentially leading to real-space structural transformation. Here, magnetic field (B)-induced giant electric hysteretic responses to back-gate voltages are reported in ML-MoS₂ field-effect transistors (FETs) on SiO₂/Si at temperatures < 20 K. The observed hysteresis increases with |B| up to 12 T and is tunable by varying the temperature. Raman spectroscopic and scanning tunneling microscopic studies reveal significant lattice expansion with increasing |B| at 4.2 K, and this lattice expansion becomes asymmetric in ML-MoS₂ FETs on rigid SiO₂/Si substrates, leading to out-of-plane mirror symmetry breaking and the emergence of a tunable out-of-plane ferroelectric-like polar order. This broken symmetry-induced polarization in ML-MoS₂ shows typical ferroelectric butterfly hysteresis in piezo-response force microscopy, adding ML-MoS₂ to the single-layer material family that exhibits out-of-plane polar order-induced ferroelectricity, which is promising for such technological applications as cryo-temperature ultracompact non-volatile memories, memtransistors, and ultrasensitive magnetic field sensors. Moreover, the polar effect induced by asymmetric lattice expansion may be further generalized to other ML-TMDs and achieved by nanoscale strain engineering of the substrate without magnetic fields.

Publication date: December 2024

Source: Materials Today, Volume 81

Author(s): Bixuan Li, Lei Zheng, Yongji Gong, Peng Kang

Nature, Published online: 30 October 2024; doi:10.1038/s41586-024-08118-0

Nature, Published online: 30 October 2024; doi:10.1038/s41586-024-08116-2

Synthesis, processing, and designing approaches of photochemically controllable and mechanically robust smart spring actuators are presented here. By combining azobenzene into a semicrystalline liquid crystal elastomer matrix (AC-LCE), several unprecedented properties can be realized within a single material system, including enhanced actuation stress, twist processability, and large-stroke latch-like underwater actuation.

Abstract

As actuated devices become smaller and more complex, there is a need for smart materials and structures that directly function as complete mechanical units without an external power supply. The strategy uses light-powered, twisted, and coiled azobenzene-functionalized semicrystalline liquid crystal elastomer (AC-LCE) springs. This twisting and coiling, which has previously been used for only thermally, electrochemically, or absorption-powered muscles, maximizes uniaxial and radial actuation. The specially designed photochemical muscles can undergo about 60% tensile stroke and provide 15 kJ m⁻³ of work capacity in response to light, thus providing about three times and two times higher performance, respectively, than previous azobenzene actuators. Since this actuation is photochemical, driven by ultraviolet (UV) light and reversed by visible light, isothermal actuation can occur in a range of environmental conditions, including underwater. In addition, photoisomerization of the AC-LCEs enables unique latch-like actuation, eliminating the need for continuous energy application to maintain the stroke. Also, as the light-powered muscles processed to be either homochiral or heterochiral, the direction of actuation can be reversed. The presented approach highlights the novel capabilities of photochemical actuator materials that can be manipulated in untethered, isothermal, and wet environmental conditions, thus suggesting various potential applications, including underwater soft robotics.

This study uncovers the oxidation mechanisms of NbOI₂ and its heterostructures, demonstrating tunable oxidation behavior through in situ experiments and first-principles calculations. Charge transfer at hetero-interfaces drives continuous oxidation in NbOI₂/TMDs and suppresses it in NbOI₂/graphene. By linking energy band structures to oxidation behavior, this work offers insights and strategies for enhancing the chemical stability of 2D air-sensitive materials.

Abstract

2D transition metal halide oxides (TMHOs) have attracted much interest due to their intriguing ferroelectrics and excellent nonlinear optics, however, their susceptibility to oxidation makes their basic research and practical application a challenge. Therefore, it is crucial to understand the oxidation mechanism and explore effective strategies for protection. Here, taking van der Waals (vdWs) ferroelectric NbOI₂ as an example, oxidation mechanisms and tuning the oxidation behaviors of NbOI₂ and its heterostructures by a variety of in situ experiments and first-principles calculations are discovered. The ambient-pressure X-ray photoelectron spectra reveal a self-limiting oxidation in isolated NbOI₂, driven by spontaneously formed iodine vacancies that react with oxygen molecules due to their lower formation and adsorption energies. For the heterostructures with a lower Fermi level (such as WSe₂) than transition state V_I@NbOI₂ (NbOI₂ rich in I-vacancies), the charge transfer from NbOI₂ to WSe₂ drives the continuous and complete oxidation of NbOI₂ flakes. Moreover, the heterostructures with a higher Fermi level (such as graphene) than V_I@NbOI₂ can weaken the oxidation of NbOI₂. By linking the energy band structures to oxidation behavior, the work offers a new oxidation mechanism of 2D air-sensitive materials and a crucial strategy for improving their chemical stability.