Jiuxiang Dai

The van der Waals interfaces of graphene/MoS₂ heterostructures are tuned through controlled functionalization of the bottom graphene layer. Adjusting the size of the functional groups enables nm-scale modification of interlayer distances. Both the size and electronic properties of the functional groups are key to regulating charge transport, with size being particularly decisive.

Abstract

Surface chemistry and interface interactions profoundly influence the properties of two-dimensional (2D) materials and heterostructures. Therefore, developing methods to precisely control surfaces and interfaces is crucial for harnessing the properties and functions of 2D materials and heterostructures. Here, we developed a facile approach to tuning the interface distance and properties of graphene/MoS₂ heterostructures (G/MoS₂) by varying the functional groups attached to the surface of graphene bottom layer. We systematically investigated how different functionalized graphene bottom layers affect the interlayer distance, coupling between the interlayers, and optical properties of resulting G/MoS₂ heterostructures. Our findings indicate that both the size and electron-withdrawing/donating properties of functional groups are pivotal in regulating charge transport properties, with size playing a particularly decisive role. Our approach demonstrates an efficient and flexible pathway to regulate the interlayer spacing and charge transport, highlighting the potential of engineering interface chemistry in optimizing properties of van der Waals heterostructures.

Twisted hBN moiré superlattices offer a highly tunable quantum material platform, with large-area ferroelectric domain visualization possible via KPFM. The cumulative moiré potential enhances tunability, while 1D moiré pattern analysis enables high-resolution local strain analysis. Additionally, femtosecond pulse lasers can engineer the moiré potential by inducing optical phonon-driven atomic displacements.

Abstract

Moiré superlattice of twisted hexagonal boron nitride (hBN) has emerged as an advanced atomically thin van der Waals interfacial ferroelectricity platform. Nanoscale periodic ferroelectric moiré potentials in twisted hBN allow the hosting of remote Coulomb superlattice potentials to adjacent 2D materials. Therefore, the new strategies for engineering moiré length, angle, and potential strength are essential for developing programmable quantum materials. Here, it demonstrates the realization of twisted hBN-based moiré superlattice platforms and visualizes the moiré domains and ferroelectric properties using Kelvin probe force microscopy (KPFM). Also, the regular moiré superlattice in the large area is reported. It offers the possibility to reproduce uniform moiré structures with precise control piezo stage stacking and heat annealing. It demonstrates cumulative multi-ferroelectric polarization and multi-level domains with multiple angle mismatched interfaces. Additionally, it observes the quasi-1D anisotropic moiré domains and show the highest resolution analysis of the local built-in strain between adjacent hBN layers compared to the conventional methods. Furthermore, in-situ manipulation of moiré potential is demonstrated using femtosecond pulse laser, which results in the optical phonon-induced atomic displacement at the hBN moiré interfaces. The results pave the way to develop precisely programmable moiré superlattice platforms and investigate strongly correlated physics.

Giant linear dichroism absorption and second-harmonic generation (SHG) phenomena associated with Peierls distortion are demonstrated in NbOBr₂. In addition, NbOBr₂ exhibits single-domain ferroelectricity, with the polarization switchable by electric field. Temperature-dependent SHG reveals a Curie temperature of 502 K. NbOBr₂ holds promise for polarized nonlinear optical devices, as well as for nonvolatile information processing and storage devices.

Abstract

The Peierls distortion plays an essential role in governing the in-plane ferroelectricity and nonlinear optical characteristics of anisotropic niobium oxide dihalides, such as NbOCl₂ and NbOI₂. Despite its significance, experimental investigation into the structural, optical, and ferroelectric properties of NbOBr₂ has been lacking. Here, the successful fabrication of centimeter-sized, high-quality NbOBr₂ single crystals, enabling direct observation of Peierls distortion using aberration-corrected scanning transmission electron microscopy, is reported. Notably, the magnitude of Peierls distortion in NbOBr₂ falls between that observed in NbOCl₂ and NbOI₂. Furthermore, the existence of giant linear dichroism absorption and second-harmonic generation (SHG) phenomena associated with Peierls distortion is confirmed. Exfoliated NbOBr₂ flakes exhibit single-domain ferroelectricity, with the polarization switchable by an electric field. Temperature-dependent SHG measurements reveal a Curie temperature of ≈502 K for ferroelectric NbOBr₂. This work demonstrates that NbOBr₂ holds promise for polarized nonlinear optical devices, as well as for nonvolatile information processing and storage devices.

Nature Communications, Published online: 09 November 2024; doi:10.1038/s41467-024-54114-3

The intrinsic out-of-plane piezoelectric response (|d ₃₃ ^eff| = 0.76 nm V⁻¹) of Janus Nb₃SeI₇ is experimentally demonstrated. After proving the termination atom plane-related properties, the stability of distinct termination planes are found to be independent of the termination plane due to similar defect formation energies for Se and I atoms and the oxidation kinetics are self-limiting. This self-limiting and polarization-insensitive oxidation broadens the applications of out-of-plane piezoelectricity and other intriguing physical properties.

Abstract

As a newly discovered Janus van der Waals (vdW) material, semiconducting Nb₃SeI₇ offers several notable advantages, including spontaneous out-of-plane polarization, facile exfoliation to the monolayer limit, and significant out-of-plane emission dipole in second harmonic generation. These properties make it a promising candidate for piezoelectric and piezophototronic applications in highly efficient energy conversion. However, Nb₃SeI₇ is prone to oxidation when exposed to oxygen, which can severely limit the exploration and utilization of these intriguing physical properties. Therefore, understanding the oxidation mechanism of pristine Nb₃SeI₇ and its correlation with electrical polarization—an area that remains largely unexplored—is highly significant. In this study, the out-of-plane piezoelectricity of Nb₃SeI₇ is experimentally demonstrated, with a piezoelectric coefficient (|d ₃₃ ^eff|) of 0.76 nm V⁻¹. Furthermore, by combining near ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS), Time-of-Flight secondary ion mass spectrometry (ToF-SIMS), and Density functional theory (DFT) calculations, it is revealed that the oxidation of Nb₃SeI₇ is self-limiting and independent of its electrical polarization, owing to the similar defect formation energies of Se and I atoms. This self-limiting and polarization-insensitive oxidation provides valuable insights into the stabilization mechanisms and expands the potential applications of out-of-plane piezoelectricity and other intriguing physical properties in Janus vdW Nb₃SeI₇.

Light: Science & Applications, Published online: 05 November 2024; doi:10.1038/s41377-024-01630-y

npj 2D Materials and Applications, Published online: 05 November 2024; doi:10.1038/s41699-024-00507-3

The vortex-state magnetic domains are reported in 2D nonlayered Fe₃O₄ nanosheets at room temperature. The Verwey phase transition temperature (T _V) is determined to be ≈110 K for the ≈3 nm thick nanosheet. The anisotropic magnetoresistance ratio decreases near T _V, yet both the spin Hall magnetoresistance ratio and spin mixing conductance (G _r) exhibit an increase at T _V. It is also found that the anomalous Hall effect ratio tends to approach zero as temperature is close to 112 K. The maximum G _r reaches ≈5 × 10¹⁵ Ω⁻¹m⁻², which is attributed to the clean and flat interface between Pt and 2D nanosheet. These findings indicate potential applications of 2D Fe₃O₄ nanosheets in high-power microspintronic storage devices.

Abstract

Realizing spin transport between heavy metal and two-dimensional (2D) magnetic materials at high Curie temperature (T _C) is crucial to advanced spintronic information storage technology. Here, environmentally stable 2D nonlayered Fe₃O₄ nanosheets are successfully synthesized using a reproducible process and found that they exhibit vortex magnetic domains at room temperature. A Verwey phase transition temperature (T _V) of ≈110 K is identified for ≈3 nm thick nanosheet through Raman characterization and spin Hall device measurement of the Pt/Fe₃O₄ bilayer. The anisotropic magnetoresistance ratio decreases near T _V, while both the spin Hall magnetoresistance ratio and spin mixing conductance (G_r ) increase at T _V. As the temperature approaches 112 K, the anomalous Hall effect ratio tends to become zero. The maximum G_r reaches ≈5 × 10¹⁵ Ω⁻¹m⁻² due to the clean and flat interface between Pt and 2D nanosheet. The observed spin transport behavior in Pt/Fe₃O₄ spin Hall devices indicates that 2D Fe₃O₄ nanosheets possess potential for high-power micro spintronic storage devices applications.

Nature Electronics, Published online: 06 November 2024; doi:10.1038/s41928-024-01265-2

Nature Materials, Published online: 06 November 2024; doi:10.1038/s41563-024-02043-3

Nature, Published online: 06 November 2024; doi:10.1038/s41586-024-08156-8

Nature Reviews Methods Primers, Published online: 07 November 2024; doi:10.1038/s43586-024-00367-7

A novel laser bidirectional graphene printing process integrating synthesis, transfer, and patterning of graphene is proposed for efficient preparation of graphene films, which can obtain two graphene films in one-step laser irradiation. The obtained graphene films are applied to Joule heaters and flexible pressure sensors, indicating that the proposed process has great potential in the preparation of high-performance graphene electronics.

Abstract

Graphene has tremendous potential in future electronics due to its superior force, electrical, and thermal properties. However, the development of graphene devices is limited by its complex, high-cost, and low-efficiency preparation process. This study proposes a novel laser bidirectional graphene printing (LBGP) process for the large-scale preparation of patterned graphene films. In LBGP, a sandwich sample composed of a thermoplastic elastomer (TPE) substrate, carbon precursor powder, and a glass cover is irradiated by a nanosecond pulsed laser. The laser photothermal effect converts the carbon precursor into graphene, with partial graphene sheets deposited directly on the TPE substrate and the remaining transferred to the glass cover via a laser-induced plasma plume. This method simultaneously prepares two face-to-face graphene films in a single laser irradiation, integrating synthesis, transfer, and patterning. The resulting graphene patterns demonstrate good performance in flexible pressure sensing and Joule heating, showcasing high sensitivity (7.7 kPa⁻¹), fast response (37 ms), and good cycling stability (2000 cycles) for sensors, and high heating rate (1 °C s⁻¹) and long-term stability (3000 s) for heaters. It is believed that the simple, low-cost, and efficient LBGP process can promote the development of graphene electronics and laser manufacturing processes.

Manganese-doped shells revolutionize InP quantum dots, enhancing their optical properties through improved core-shell interface engineering. This novel approach results in increased crystallinity, reduced defects, and stronger quantum confinement. The optimized quantum dots exhibit higher photoluminescence quantum yield in the green emission region, paving the way for high-performance, environmentally friendly optoelectronic materials.

Abstract

Indium phosphide (InP) quantum dots (QDs) offer a promising alternative to (Restriction of Hazardous Substances) restricted cadmium-based QDs, however, their performance is limited by surface defects and weak quantum confinement. This study introduces a novel approach to enhance the optical properties of InP QDs through manganese (Mn) doping into the zinc selenide and zinc sulfide shell. This aims to expand the bandgap of the shells and adjust its lattice constant to better match the InP core. A comprehensive investigation of the effect of Mn-doping concentration reveals that optimal properties are developed at a 10% feed ratio, resulting in improved crystallinity, reduced interfacial defects, and enhanced quantum confinement. X-ray diffraction and transmission electron microscopy confirm the structural improvements and spectroscopic analyses demonstrate remarkable enhancement of optical properties. Notably, the photoluminescence quantum yield reaches 83% in the green emission region (λ ≈535 nm), a significant improvement over undoped QDs. Time-resolved photoluminescence measurements indicate extended carrier lifetimes, supporting the effect of defect reduction. This strategy not only addresses the long-standing challenges of InP QDs but also opens new avenues for designing high-performance, environmentally friendly nanomaterials for various optoelectronic applications, including displays, lighting, and photovoltaics.

Experimentally, gate voltage and strain is introduced to achieve a tunable linear dichroism property in GeSe, witnessing a maximum enhancement of 44%. The explanation of the underlying mechanism is grounded in rigorous theoretical calculations. Moreover, two promising application avenues are showcased for GeSe-based flexible photoelectric sensors in the realm of wearable electronics.

Abstract

The direct detection of light polarization poses a crucial challenge in the field of optoelectronics and photonics. Herein, the tunable linear dichroism (LD) in GeSe-based polarized photodetectors is presented through electronic and structural asymmetry modulation, and demonstrate their application prospects in wearable electronics. An improvement in the dichroic ratio up to 34% is achieved under a gate voltage of 20 V, and the improvement reaches 44% by applying a tensile strain along the zigzag direction. Theoretical calculations reveal that the gate regulation of barrier height between GeSe and Au electrodes is responsible for the electrical-tunable LD, while the anisotropic optical absorption in response to strains leads to the strain-tunable LD. Moreover, flexible GeSe transistors are developed for wearable applications including motion sensors and glucose monitors. This study offers viable approaches for modulating the optical anisotropy of low-dimensional materials and emphasizes the versatility of van der Waals materials for practical applications in wearable electronic devices.

Nature Communications, Published online: 02 November 2024; doi:10.1038/s41467-024-53907-w

A novel two-step method that facilitates the heterogeneous integration of high-quality BaTiO₃ (BTO) capacitors onto silicon substrates is presented, resulting in robust ferroelectric, electromechanical, and endurance characteristics. By employing a templated layer of BTO, larger-area heterogeneous integration of BTO on silicon is achieved. This advancement paves the way for incorporating epitaxial complex oxides with various functionalities onto different inorganic substrates.

Abstract

Integrating epitaxial BaTiO₃ (BTO) with Si is essential for leveraging its ferroelectric, piezoelectric, and nonlinear optical properties in microelectronics. Recently, heterogeneous integration approaches that involve growth of BTO on ideal substrates followed by transfer to a desired substrate show promise of achieving excellent device-quality films. However, beyond simple demonstrations of the existence of ferroelectricity, robust devices with high endurance are not yet demonstrated on Si using the latter approach. Here, using a novel two-step approach to synthesize epitaxial BTO using pulsed laser deposition on water-soluble Sr₃Al₂O₆ (on SrTiO₃ substrates), successful integration of high-quality BTO capacitors on Si is demonstrated, with remanent polarisation P_r = 7 µC cm⁻², coercive field E_c = 150 kV cm⁻¹, ferroelectric and electromechanical endurance of > 10⁶ cycles. The study further addresses the challenge of cracking and disintegration of thicker films by first transferring a large area (5 mm x 5 mm) of the templated layer of BTO (≈30 nm thick) on the desired substrate, followed by the growth of high-quality BTO on this substrate, as revealed by high-resolution X-ray diffraction (HRXRD) and high-resolution scanning transmission electron microscopy (HRSTEM) measurements. These templated Si substrates offer a versatile platform for integrating any epitaxial complex oxides with diverse functionalities onto any inorganic substrate.

In this work, the syntheses of two photo-sensitive and thermoresponsive hydrogel systems, a photo-stiffening and a photo-softening hydrogel, are reported. Their potential for fabricating soft materials with patterned mechanical properties is then demonstrated by fabricating an actuator whose higher-order bending properties can be switched on with light, and by encoding mechanical properties for digital information encryption and storage.

Abstract

Although different chemistries for the spatio-temporal localization of molecules and gradients of chemical signals within soft materials are now available, the achievement of spatio-temporal patterns of mechanical properties in such materials and their characterization remain considerable challenges. This study presents the syntheses of two novel photo-sensitive and thermoresponsive hydrogel systems, a photo-stiffening and a photo-softening hydrogel. Their potential for fabricating soft materials with patterned mechanical properties is then demonstrated by fabricating an actuator whose higher-order bending properties can be switched on with light, and by encoding mechanical properties for digital information encryption and storage. Microindentation and a custom-made data analysis software are essential for the characterization of all the materials. From a general perspective, this work opens a route to the fabrication of soft materials with patterned mechanical properties, addressing an important emerging challenge in soft materials science with applications in soft robotics and information encryption and storage.

A novel synthesis route of achieving 5-armchair graphene nanoribbons with high purity and long length via confined polymerization of non-halogen precursors inside single-walled carbon nanotubes is proposed, breaking the strong dependence on metal substrates and halogen-containing precursors in current on-surface synthesis of such graphene nanoribbons with a quasi-metallic gap promising in electronics and optoelectronics.

Abstract

Armchair graphene nanoribbons (AGNRs) known as semiconductors are holding promise for nanoelectronics applications and sparking increased research interest. Currently, synthesis of 5-AGNRs with a quasi-metallic gap has been achieved using perylene and its halogen-containing derivatives as precursors via on-surface synthesis on a metal substrate. However, challenges in controlling the polymerization and orientation between precursor molecules have led to side reactions and the formation of by-products, posing a significant issue in purity. Here a precision synthesis of confined 5-AGNRs using molecular-designed precursors without halogens is proposed to address these challenges. Perylene and its dimer quaterrylene are utilized for filling into single-walled carbon nanotubes (SWCNTs), following a precursor-driven transition into 5-AGNRs by heat-induced polymerization and cyclodehydrogenation. SWCNTs restrict the alignment of confined quaterrylene enabling their polymerization with a head-to-tail arrangement, which results in the formation of pure 5-AGNRs with three times higher yield than that of perylene, as the free rotation capability of perylene molecules inside SWCNTs lead to the formation of 5-AGNRs concomitant with by-products. This work provides a templated route for synthesizing desired GNRs based on molecular-designed precursors and confined polymerization, bringing advantages for their applications in electronics and optoelectronics.

A series of hollow TMDCs@C fibers are synthesized via a sacrificial template method and the confined growth strategy. The complex permittivity can be adjusted by tuning the content of CdS templates. Multidimensional interfacial design of the composites can enhance interfacial polarization and establish conductive networks simultaneously, thereby improving microwave absorption performance.

Abstract

Interface design has enormous potential for the enhancement of interfacial polarization and microwave absorption properties. However, the construction of interfaces is always limited in components of a single dimension. Developing systematic strategies to customize multidimensional interfaces and fully utilize advantages of low-dimensional materials remains challenging. Two-dimensional transition metal dichalcogenides (TMDCs) have garnered significant attention owing to their distinctive electrical conductivity and exceptional interfacial effects. In this study, a series of hollow TMDCs@C fibers are synthesized via sacrificial template of CdS and confined growth of TMDCs embedded in the fibers. The complex permittivity of the hollow TMDCs@C fibers can be adjusted by tuning the content of CdS templates. Importantly, the multidimensional interfaces of the fibers contribute to elevating the microwave absorption performance. Among the hollow TMDCs@C fibers, the minimum reflection loss (RL_min) of the hollow MoS₂@C fibers can reach −52.0 dB at the thickness of 2.5 mm, with a broad effective absorption bandwidth of 4.56 GHz at 2.0 mm. This work establishes an alternative approach for constructing multidimensional coupling interfaces and optimizing TMDCs as microwave absorption materials.

Here, results on the tailored growth of various 2D semiconductors’ monolayers of transition metal dichalcogenides are presented using chemical vapor deposition techniques. Basic electronic, photonic and optoelectronic properties of the grown quantum materials are studied by high-resolution microscopy and spectroscopy techniques and are analyzed for possible device applications.

Abstract

Here, results on the tailored growth of monolayers (MLs) of transition metal dichalcogenides (TMDs) are presented using chemical vapor deposition (CVD) techniques. To enable reproducible growth, the flow of chalcogen precursors is controlled by Knudsen cells providing an advantage in comparison to the commonly used open crucible techniques. It is demonstrated that TMD MLs can be grown by CVD on large scale with structural, and therefore electronic, photonic and optoelectronic properties similar to TMD MLs are obtained by exfoliating bulk crystals. It is shown that besides the growth of the “standard” TMD MLs also the growth of MLs that are not available by the exfoliation is possible including examples like lateral TMD₁–TMD₂ ML heterostructures and Janus TMDs. Moreover, the CVD technique enables the growth of TMD MLs on various 3D substrates on large scale and with high quality. The intrinsic properties of the grown MLs are analyzed by complementary microscopy and spectroscopy techniques down to the nanoscale with a particular focus on the influence of structural defects. Their functional properties are studied in devices including field-effect transistors, photodetectors, wave guides and excitonic diodes. Finally, an outlook of the developed methodology in both applied and fundamental research is given.