Jiuxiang Dai

The WS₂-ReS₂ van der Waals heterostructure is formed by the manually vertical stack. Anisotropic and isotropic materials are introduced in the heterojunction system to manipulate in-plane symmetry, offering a new way to control their properties. The study confirms the formation of effective heterojunctions and shows how the anisotropy and asymmetry of these heterostructures can be adjusted, leading to potential applications in electronics and photonics.

Abstract

Van der Waals (vdW) heterostructures are composed of atomically thin layers assembled through weak (vdW) force, which have opened a new era for integrating materials with distinct properties and specific applications. However, few studies have focused on whether and how anisotropic materials affect heterostructure system. The study introduces anisotropic and isotropic materials in a heterojunction system to change the in-plane symmetry, offering a new degree of freedom for modulating its properties. The sample is fabricated by manually stacking ReS₂ and WS₂ flakes prepared by mechanical exfoliation. Raman spectra and photoluminescence measurements confirm the formation of an effective heterojunction, indicating interlayer coupling of the system. The anisotropy and asymmetry of the WS₂-ReS₂ heterostructure system can be adjusted by the introduction of isotropic WS₂ and anisotropic ReS₂, which can be proved by the change of the polarized Raman pattern. In the transient absorption measurement, the transient absorption spectra of WS₂-ReS₂ heterostructure are red-shifted compared to those of WS₂ monolayer, and the charge transfer is observed in the heterostructure. These results show the potential of anisotropic 2D materials in anisotropy modulation of heterostructures, which may promote future electronic or photonic application.

Heterogeneity of defect density and edge structure on single-layer graphene nanoribbon (sGNR) obtained by a chemical unzipping method is investigated by silver nanowire-based tip-enhanced Raman spectroscopy (TERS) below 10 nm resolution.

Abstract

Graphene nanoribbons (GNRs), a quasi-one-dimensional form of graphene, have gained tremendous attention due to their potential for next-generation nanoelectronic devices. The chemical unzipping of carbon nanotubes is one of the attractive fabrication methods to obtain single-layered GNRs (sGNRs) with simple and large-scale production.  The authors recently found that unzipping from double-walled carbon nanotubes (DWNTs), rather than single- or multi-walled, results in high-yield production of crystalline sGNRs. However, details of the resultant GNR structure, as well as the reaction mechanism, are not fully understood due to the necessity of nanoscale spectroscopy. In this regard, silver nanowire-based tip-enhanced Raman spectroscopy (TERS) is applied for single GNR analysis and investigated ribbon-to-ribbon heterogeneity in terms of defect density and edge structure generated through the unzipping process.  The authors found that sGNRs originated from the inner walls of DWNTs showed lower defect densities than those from the outer walls. Furthermore, TERS spectra of sGNRs exhibit a large variety in graphitic Raman parameters, indicating a large variation in edge structures. This work at the single GNR level reveals, for the first time, ribbon-to-ribbon heterogeneity that can never be observed by diffraction-limited techniques and provides deeper insights into unzipped GNR structure as well as the DWNT unzipping reaction mechanism.

A new sacrificial layer, Sr₄Al₂O₇, is proposed to fabricate freestanding oxide membranes with high water dissolution rate, which is ≈10 times higher than that of Sr₃Al₂O₆. The high-dissolution-rate of Sr₄Al₂O₇ is most likely attributed to the more discrete Al-O networks and higher concentration of water-soluble Sr-O species in its crystal structure.

Abstract

Freestanding perovskite oxide membranes have drawn great attention recently since they offer exceptional structural tunability and stacking ability, providing new opportunities in fundamental research and potential device applications in silicon-based semiconductor technology. Among different types of sacrificial layers, the (Ca, Sr, Ba)₃Al₂O₆ compounds are most widely used since they can be dissolved in water and prepare high-quality perovskite oxide membranes with clean and sharp surfaces and interfaces; However, the typical transfer process takes a long time (up to hours) in obtaining millimeter-size freestanding membranes, let alone realize wafer-scale samples with high yield. Here, a new member of the SrO-Al₂O₃ family, Sr₄Al₂O₇ is introduced, and its high dissolution rate, ≈10 times higher than that of Sr₃Al₂O₆ is demonstrated. The high-dissolution-rate of Sr₄Al₂O₇ is most likely related to the more discrete Al-O networks and higher concentration of water-soluble Sr-O species in this compound. This work significantly facilitates the preparation of freestanding membranes and sheds light on the integration of multifunctional perovskite oxides in practical electronic devices.

Here, the role of meniscus shape on the morphology and charge transport behavior of organic semiconductors is investigated using meniscus-guided coating. By varying the height of the meniscus and the substrate velocity, four different morphologies are obtained: I) stick-slip, II) unidirectional, III) spherulite, and IV) directional branching morphologies. Among these, type II showed the highest performance in field-effect transistors.

Abstract

Meniscus-guided coating (MGC) is a promising method that offers predictable fabrication of highly crystalline thin films. For the integration of molecular semiconductors into large-area electronic devices with high efficiency and reliability, homogeneous and highly ordered film morphologies are required. The solution processing of such defect-free film structures requires comprehensive understanding of the complex relationship between molecular crystallization, fluid dynamics, and meniscus shape. In this work, the role of the meniscus shape on fluid dynamics in the coating bead and the crystallization process of the low molecular weight semiconductor 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) during zone-casting is systematically investigated. Depending on meniscus shape and coating velocity, four morphological subregimes are found: stick-slip morphology, unidirectional homogenous crystal stripes, spherulitic morphology, and directional branched morphology; of which the second exhibits the highest crystallinity with a reduced trap density in the thin film, resulting in improved saturation and effective mobilities in field-effect transistors (FET). Numerical simulation of fluid dynamics explains the observed morphological trends, which are correlated with the electrical behavior of the devices. This work provides a fundamental basis for upscaling MGC methods for the application of functional thin films.

Abstract

Given its intriguing band structure and unique tunable bandgap, AB-stacked bilayer graphene has great potentials in the applications of high-end electronics, optoelectronics and semiconductors. The epitaxial growth of AB-stacked single-crystal bilayer graphene films requires a strict AB-stacked lattice, identical orientations and seamless stitching of bilayer graphene islands. However, the particles inevitably present on the metal surface that produced during high temperature growth would induce random orientations, twisted stacking islands, and uncontrollable multilayers, which is a great challenge to overcome. Here, we propose a heat-resisting-box assisted strategy to produce nearly pure AB-stacked bilayer graphene single-crystal films on Cu/Ni (111) foils. With our technique, the particles on the Cu/Ni (111) surface are effectively eliminated, which greatly minimizes the occurrence of randomly twisted islands and uncontrollable multilayers. The as-grown AB-stacked bilayer graphene films show > 99% alignment and > 99% AB stacking order. Our work provides a promising method towards the growth of pure AB-stacked bilayer graphene single crystals and would accelerate its device applications.

Nature Materials, Published online: 19 January 2024; doi:10.1038/s41563-023-01774-z

Surface passivation using intermediate phase transfer (IPT) enhances the performance of InAs colloidal quantum dot (CQD) photodetectors. IPT enables stable CQD ink in green solvents and facile control over the surface condition of the nanocrystals. The resultant photodiodes exhibit high gain (≈10) and fast response times (≈12/36 ns), resulting in a notable figure of merit (FOM).

Abstract

Solution-processed low-bandgap semiconductors are crucial to next-generation infrared (IR) detection for various applications, such as autonomous driving, virtual reality, recognitions, and quantum communications. In particular, III–V group colloidal quantum dots (CQDs) are interesting as nontoxic bandgap-tunable materials and suitable for IR absorbers; however, the device performance is still lower than that of Pb-based devices. Herein, a universal surface-passivation method of InAs CQDs enabled by intermediate phase transfer (IPT), a preliminary process that exchanges native ligands with aromatic ligands on the CQD surface is presented. IPT yields highly stable CQD ink. In particular, desirable surface ligands with various reactivities can be obtained by dispersing them in green solvents. Furthermore, CQD near-infrared (NIR) photodetectors are demonstrated using solution processes. Careful surface ligand control via IPT is revealed that enables the modulation of surface-mediated photomultiplication, resulting in a notable gain control up to ≈10 with a fast rise/fall response time (≈12/36 ns). Considering the figure of merit (FOM), EQE versus response time (or −3 dB bandwidth), the optimal CQD photodiode yields one of the highest FOMs among all previously reported solution-processed nontoxic semiconductors comprising organics, perovskites, and CQDs in the NIR wavelength range.

The synergistic effect of thermal activation and plasma enabled low-temperature synthesis (150–200 °C) of various mesoporous metal oxide (V₂O₅, V₆O₁₃, TiO₂, WO₃, Nb₂O₅, and MoO₃), suitable for flexible polymeric substrates. As a proof of concept, the direct synthesis of mesoporous V₂O₅ is demonstrated on an indium-tin oxide-coated polyimide film and its application as electrode materials.

Abstract

Mesoporous metal oxides exhibit excellent physicochemical properties and are widely used in various fields, including energy storage/conversion, catalysis, and sensors. Although several soft-template approaches are reported, high-temperature calcination for both metal oxide formation and template removal is necessary, which limits direct synthesis on a plastic substrate for flexible devices. Here, a universal synthetic approach that combines thermal activation and oxygen plasma to synthesize diverse mesoporous metal oxides (V₂O₅, V₆O₁₃, TiO₂, Nb₂O₅, WO_3, and MoO₃) at low temperatures (150–200 °C), which can be applicable to a flexible polymeric substrate is introduced. As a demonstration, a flexible micro-supercapacitor is fabricated by directly synthesizing mesoporous V₂O₅ on an indium-tin oxide-coated colorless polyimide film. The energy storage performance is well maintained under severe bending conditions.

Scanning tunneling microscopy (STM) supported by density functional theory (DFT) calculations are used to probe and characterize the multiferroic order in monolayer NiI₂. Its type-II multiferroic order arises from the combination of a magnetic spin spiral order and a strong spin-orbit coupling. These results on magnetoelectric effects at the atomic scale suggest new ways to engineer multiferroic orders in van der Waals materials and their heterostructures.

Abstract

Progress in layered van der Waals materials has resulted in the discovery of ferromagnetic and ferroelectric materials down to the monolayer limit. Recently, evidence of the first purely 2D multiferroic material was reported in monolayer NiI₂. However, probing multiferroicity with scattering-based and optical bulk techniques is challenging on 2D materials, and experiments on the atomic scale are needed to fully characterize the multiferroic order at the monolayer limit. Here, scanning tunneling microscopy (STM) supported by density functional theory (DFT) calculations is used to probe and characterize the multiferroic order in monolayer NiI₂. It is demonstrated that the type-II multiferroic order displayed by NiI₂, arising from the combination of a magnetic spin spiral order and a strong spin-orbit coupling, allows probing the multiferroic order in the STM experiments. Moreover, the magnetoelectric coupling of NiI₂ is directly probed by external electric field manipulation of the multiferroic domains. The findings establish a novel point of view to analyze magnetoelectric effects at the microscopic level, paving the way toward engineering new multiferroic orders in van der Waals materials and their heterostructures.

Light: Science & Applications, Published online: 22 January 2024; doi:10.1038/s41377-023-01330-z

Orthogonal UCL is realized in crystals with a simple structure simply by introducing modulator Tm³⁺ ions to control the photon transition processes between different energy levels of activator Er³⁺ ions. The obtained crystals emit red and green luminescence under excitation of 980 nm and 808 nm lasers, respectively, and are shown to be well suited for advanced optical encryption.

Abstract

Optical encryption shows great potential in meeting the growing demand for advanced anti-counterfeiting in the information age. The development of upconversion luminescence (UCL) materials capable of emitting different colors of light in response to different external stimuli holds great promise in this field. However, the effective realization of multicolor UCL materials usually requires complex structural designs. In this work, orthogonal UCL is achieved in crystals with a simple structure simply by introducing modulator Tm³⁺ ions to control the photon transition processes between different energy levels of activator Er³⁺ ions. The obtained crystals emit red and green UCL when excited by 980 nm and 808 nm lasers, respectively. The orthogonal excitation-emission properties of crystals are shown to be very suitable for high-level optical encryption, which is important for information security and anti-counterfeiting. This work provides an effective strategy for obtaining orthogonal UCL in simple structural materials, which will encourage researchers to further explore novel orthogonal UCL materials and their applications, and has important implications for the development of the frontier photonic upconversion fields.

A novel strategy is employed to achieve a giant negative photoconductance (NPC) effect in a graphene/AgBiP₂Se₆/graphene van der Waals vertical heterostructure device by applying a high drain-source voltage bias. Remarkably, this device demonstrates an exceptionally high negative responsivity (R) of 4.9 × 10⁵ A W⁻¹, surpassing the previous records for NPC photodetectors.

Abstract

The positive photoconductive (PPC) effect is a well-established primary detection mechanism employed by photodetectors. In contrast, the negative photoconductive (NPC) effect is not extensively investigated thus far, and research on the NPC effect is still in its early stage. Herein, a quaternary van der Waals material, AgBiP₂Se₆ atomic layers, is discovered to achieve a giant NPC effect. Through experimental observations in a Graphene/AgBiP₂Se₆/ Graphene-based vertical photodetector, an irreversible conversion is identified from common PPC photoresponse to atypical NPC photoresponse. Notably, this device demonstrates an exceptionally high negative responsivity (R) of 4.9 × 10⁵ A W⁻¹, surpassing the previous records for NPC photodetectors. Additionally, it exhibits remarkable optoelectronic performances, including an external quantum efficiency of 1.3 × 10⁸% and a detectivity (D) of 3.60 × 10¹² Jones. The exceptionally high NPC photoresponse observed in this device can be attributed to the swift suppression of photogenerated free carriers at robust recombination centers situated at significant depths, induced by the elevated drain-source voltage bias. The remarkably high NPC photoresponse also positions AgBiP₂Se₆ as a promising 2D material for multifunctional optoelectronic devices and an excellent platform for systematic exploration of the NPC effect.

Highlights

• Atomically substitutional engineering in binary transition metal dichalcogenides (TMDs) enables the facile production of ternary or quaternary TMDs with tunable (opto)electronic properties spanning the entire compositional spectrum.

• A comprehensive overview is provided on multinary TMDs, including Janus-type structures, aiming to elaborate on their theoretical foundations, synthetic strategies, tailored properties, and superior applications.

• The challenges and opportunities faced in accelerating the exploitation of multinary TMDs as highly promising nanomaterials are discussed.

BiFeO₃ lead-free ceramics exhibit a disappearance of piezoelectricity in BiFeO₃ upon heating to ≈300–450 °C, and this temperature increases with increasing measuring frequency. Remarkably, the piezoelectricity recovers upon cooling. These behaviors are attributed to its significant electrical leakage but outstanding thermal resistance of its poled state to very high temperature approaching its Curie point of 830 °C.

Abstract

BiFeO₃ is a ferroelectric with a Curie temperature of 830 °C, however, its piezoelectric performance at high temperature remains unclear. The current work reveals a disappearance/recovery of piezoelectricity in BiFeO₃ at elevated temperature and upon cooling. In particular, that temperature is strongly frequency-dependent and ranges from 280 to 430 °C between 0.5 and 140 Hz, respectively. Meanwhile, in situ and ex situ X-ray diffraction and piezoresponse microscope analysis demonstrate thermally-resistant domain texture to temperatures as high as 750 °C. This demonstrates that the piezoelectricity of BiFeO₃ is strongly influenced by its resistance/conductance variations due to charge carrier motion limiting the operational frequency. The investigation enhances the understanding of BiFeO₃ complex piezoelectric behavior at various temperatures, offering insights into its potential applications.

The interface of liquid metals is used as natural filtering for doping and inducing asymmetry in harvested two-dimensional (2D) metal oxides. 2D HZO-doped SnO ferroelectric sheets are produced based on the different migration tendencies of Hf, Zr, and Sn metals within the bulk competing for the selective enrichment of the liquid metal interface.

Abstract

The emergence of ferroelectricity in two-dimensional (2D) metal oxides is a topic of significant technological interest; however, many 2D metal oxides lack intrinsic ferroelectric properties. Therefore, introducing asymmetry provides access to a broader range of 2D materials within the ferroelectric family. Here, the generation of asymmetry in 2D SnO by doping the material with Hf_0.5Zr_0.5O₂ (HZO) is demonstrated. A liquid metal process as a doping strategy for the preparation of 2D HZO-doped SnO with robust ferroelectric characteristics is implemented. This technology takes advantage of the selective interface enrichment of molten Sn with HZO crystallites. Molecular dynamics simulations indicate a strong tendency of Hf and Zr atoms to migrate toward the surface of liquid metal and embed themselves within the growing oxide layer in the form of HZO. Thus, the liquid metal-based harvesting/doping technique is a feasible approach devised for producing novel 2D metal oxides with induced ferroelectric properties, represents a significant development for the prospects of random-access memories.

Abstract

Spintronic devices are driving new paradigms of bio-inspired, energy efficient computation like neuromorphic stochastic computing and in-memory computing. They have also emerged as key candidates for non-volatile memories for embedded systems as well as alternatives to persistent memories. To meet the growing demands from such diverse applications, there is need for innovation in materials and device designs which can be scaled and adapted according to the application. Two-dimensional (2D) magnetic materials address challenges facing bulk magnet systems by offering scalability while maintaining device integrity and allowing efficient control of magnetism. In this review, we highlight the progress made in experimental studies on 2D magnetic materials towards their integration into spintronic devices. We provide an account of the various relevant material discoveries, demonstrations of current and voltage-based control of magnetism and reported device systems, while also discussing the challenges and opportunities towards integration of 2D magnetic materials in commercial spintronic devices.

Liquid-phase exfoliation is a powerful methodology to produce 2D materials. The use of rhodamine B base and PBA-BODIPY dispersants has allowed generating few-layer graphene and hexagonal boron nitride (hBN) through a sonication-assisted exfoliation. The exfoliated graphene and hBN with a high loading of dispersants exhibit good dispersibility and low defect content. Moreover, atomistic simulations are exploited to understand the interaction between the nanosheets and the two dispersant molecules.

Abstract

Liquid-phase exfoliation (LPE) in aqueous solutions provides a simple, scalable, and green approach to produce 2D materials. By combining atomistic simulations with exfoliation experiments, the interaction between a surfactant and a 2D layer at the molecular scale can be better understood. In this work, two different dyes, corresponding to rhodamine B base (Rbb) and to a phenylboronic acid BODIPY (PBA-BODIPY) derivative, are employed as dispersants to exfoliate graphene and hexagonal boron nitride (hBN) through sonication-assisted LPE. The exfoliated 2D sheets, mostly as few-layers, exhibit good quality and high loading of dyes. Using molecular dynamics (MD) simulations, the binding free energies are calculated and the arrangement of both dyes on the layers are predicted. It has been found that the dyes show a higher affinity toward hBN than graphene, which is consistent with the higher yields of exfoliated hBN. Furthermore, it is demonstrated that the adsorption behavior of Rbb molecules on graphene and hBN is quite different compared to PBA-BODIPY.