Jiuxiang Dai

High-performance molybdenum disulfide (MoS₂) single-nanoribbon photodetectors are successfully demonstrated. The MoS₂ nanoribbons grown using a dual pulsed laser deposition-chemical vapor deposition approach exhibit single-layer edges with enhanced nonlinear optical response and photoluminescence emission. The photoresponsivity of the nano-photodetector device reaches a value of 8.72 × 10² A W⁻¹ at visible wavelengths.

Abstract

MoS₂ nanoribbons have attracted increased interest due to their properties, which can be tailored by tuning their dimensions. Herein, the growth of MoS₂ nanoribbons and triangular crystals formed by the reaction between films of MoOx (2<x<3) grown by pulsed laser deposition and NaF in a sulfur-rich environment is demonstrated. The nanoribbons can reach up to 10 µm in length, and feature single-layer edges, forming a monolayer–multilayer junction enabled by the lateral modulation in thickness. The single-layer edges show a pronounced second harmonic generation due to the symmetry breaking, in contrast to the centrosymmetric multilayer structure, which is unsusceptible to the second-order nonlinear process. A splitting of the Raman spectra is observed in MoS₂ nanoribbons arising from distinct contributions from the single–layer edges and multilayer core. Nanoscale imaging reveals a blue-shifted exciton emission of the monolayer edge compared to the isolated MoS₂ monolayers due to built-in local strain and disorder. We further report on an ultrasensitive photodetector made of a single MoS₂ nanoribbon with a responsivity of 8.72 × 10² A W⁻¹ at 532 nm, among the highest reported up-to-date for single-nanoribbon photodetectors. These findings can inspire the design of MoS₂ semiconductors with tunable geometries for efficient optoelectronic devices.

Nature Photonics, Published online: 29 May 2023; doi:10.1038/s41566-023-01217-w

Nature Photonics, Published online: 29 May 2023; doi:10.1038/s41566-023-01220-1

A simple sprinkling of Se nanopowders onto sliding Mo and W thin films leads to the tribochemical in operando formation of lubricious 2D selenides. Due to the thermal and vacuum stability of the Se nanopowders they can be used to replenish sliding components with solid lubricants, avoiding the long-lasting problem of transition metal dichalcogenidelubricity degradation caused by environmental molecules.

Abstract

Transition metal dichalcogenide (TMD) coatings have attracted enormous scientific and industrial interest due to their outstanding tribological behavior. The paradigmatic example is MoS₂, even though selenides and tellurides have demonstrated superior tribological properties. Here, an innovative in operando conversion of Se nanopowders into lubricious 2D selenides, by sprinkling them onto sliding metallic surfaces coated with Mo and W thin films, is described. Advanced material characterization confirms the tribochemical formation of a thin tribofilm containing selenides, reducing the coefficient of friction down to below 0.1 in ambient air, levels typically reached using fully formulated oils. Ab initio molecular dynamics simulations under tribological conditions reveal the atomistic mechanisms that result in the shear-induced synthesis of selenide monolayers from nanopowders. The use of Se nanopowder provides thermal stability and prevents outgassing in vacuum environments. Additionally, the high reactivity of the Se nanopowder with the transition metal coating in the conditions prevailing in the contact interface yields highly reproducible results, making it particularly suitable for the replenishment of sliding components with solid lubricants, avoiding the long-lasting problem of TMD-lubricity degradation caused by environmental molecules. The suggested straightforward approach demonstrates an unconventional and smart way to synthesize TMDs in operando and exploit their friction- and wear-reducing impact.

A mid-IR NLO tellurite, Cd₂Nb₂Te₄O₁₅, with the pseudo-Aurivillius-type perovskite layered structure, exhibits the largest SHG effect (31 × KDP) among all reported metal tellurites, a large band gap (3.75 eV), a wide optical transparency window (0.33–14.5 µm), superior birefringence (0.12@ 546 nm), and a high laser-induced damage threshold (23 × AgGaS₂).

Abstract

Oxides are emerging candidates for mid-infrared (mid-IR) nonlinear optical (NLO) materials. However, their intrinsically weak second harmonic generation (SHG) effects hinder their further development. A major design challenge is to increase the nonlinear coefficient while maintaining the broad mid-IR transmission and high laser-induced damage threshold (LIDT) of the oxides. In this study, it is reported on a polar NLO tellurite, Cd₂Nb₂Te₄O₁₅ (CNTO), featuring a pseudo-Aurivillius-type perovskite layered structure composed of three types of NLO active groups, including CdO₆ octahedra, NbO₆ octahedra, and TeO₄ seesaws. The uniform orientation of the distorted units induces a giant SHG response that is ≈31 times larger than that of KH₂PO₄, the largest value among all reported metal tellurites. Additionally, CNTO exhibits a large band gap (3.75 eV), a wide optical transparency window (0.33–14.5 µm), superior birefringence (0.12@ 546 nm), high LIDT (23 × AgGaS₂), and strong acid and alkali resistance, indicating its potential as a promising mid-IR NLO material.

In this work, boron doped Al₂O₃ films are deposited from two precursors, phenylboronic acid (PBA) and trimethylaluminum (TMA). AlCl₃ is used as an alternative aluminum precursor in reference experiments. The film growth process is well controllable. In addition, the dielectric constant and leakage property of the boron doped Al₂O₃ films are also investigated. The boron doped Al₂O₃ can be used as a low-k spacer material.

Abstract

This paper presents preparation of boron-doped Al₂O₃ thin films by atomic layer deposition (ALD) using phenylboronic acid (PBA) and trimethylaluminum (TMA) as precursors. Deposition temperatures of 160–300 °C are studied, giving a maximum growth per cycle (GPC) of 0.77 Å at 200 °C. Field emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM) are used to study the surface morphology and roughness of the films. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), Time-of-flight elastic recoil detection analysis (ToF-ERDA), and X-ray photoelectron spectroscopy (XPS) are used to study the composition of the films. An annealing process is carried out at 450 °C for 1 h to investigate its effect on the elemental composition and electrical properties of the boron-doped Al₂O₃ thin films. The boron-doped Al₂O₃ 70 nm thick film deposited at 200 °C has a boron content of 3.7 at.% with low leakage current density (10⁻⁹ to 10⁻⁶ A cm⁻²) when the film thickness is 70 nm. The dielectric constant of this boron doped Al₂O₃ film is 5.18.

Cubic PbS nanosheets with preferential (200) facets are synthesized and introduced into formamidinium lead triiodide (FAPbI₃) films. Similar interplanar spacings ensure the formation of epitaxial PbS nanosheet-FAPbI₃ heterostructure with better crystallization and lower defect density. Chemical bonding and compression strain at the interfaces between PbS (200) and FAPbI₃ (200) help achieve solar cells with higher efficiency and better stability simultaneously.

Abstract

Nanomaterials such as quantum dots and 2D materials have been widely used to improve the performance of perovskite solar cells due to their favorable optical properties, conductivity, and stability. Nevertheless, the interfacial crystal structures between perovskites and nanomaterials have always been ignored while large mismatches can result in a significant number of defects within solar cells. In this work, cubic PbS nanosheets with (200) preferred crystal planes are synthesized through anisotropy growth. Based on the similar crystal structure between cubic PbS (200) and cubic-phase formamidinium lead triiodide (α-FAPbI₃) (200), a nanoepitaxial PbS nanosheets-FAPbI₃ heterostructure with low defect density is observed. Attribute to the epitaxial growth, PbS nanosheets-FAPbI₃ hybrid polycrystalline films show decreased defects and better crystallization. Optimized perovskite solar cells perform both improved efficiency and stability, retaining 90% of initial photovoltaic conversion efficiency after being stored at 20 °C and 20% RH for 2500 h. Notably, the significantly improved stability is ascribed to the interfacial compression strain and chemical bonding between (200) planes of PbS nanosheets and α-FAPbI₃ (200). This study provides insight into high-performance perovskite solar cells achieved by manipulating nanomaterial surfaces.

Here, the basic principles and measurement methods for piezoelectric coefficients of ferroelectric polymers are introduced. Approaches to improve piezoelectric property are discussed from the structure manipulation of the amorphous phase, the crystalline phase, and the crystalline–amorphous interface. Especially, the crucial role of the oriented amorphous fraction in enhancing piezoelectricity of poly(vinylidene fluoride)-based polymers is discussed.

Abstract

Poly(vinylidene fluoride) (PVDF)-based polymers demonstrate great potential for applications in flexible and wearable electronics but show low piezoelectric coefficients (e.g., −d ₃₃ < 30 pC N⁻¹). The effective improvement for the piezoelectricity of PVDF is achieved by manipulating its semicrystalline structures. However, there is still a debate about which component is the primary contributor to piezoelectricity. Therefore, current methods to improve the piezoelectricity of PVDF can be classified into modulations of the amorphous phase, the crystalline region, and the crystalline–amorphous interface. Here, the basic principles and measurements of piezoelectric coefficients for soft polymers are first discussed. Then, three different categories of structural modulations are reviewed. In each category, the physical understanding and strategies to improve the piezoelectric performance of PVDF are discussed. In particular, the crucial role of the oriented amorphous fraction at the crystalline–amorphous interface in determining the piezoelectricity of PVDF is emphasized. At last, the future development of high performance piezoelectric polymers is outlooked.

Nature Communications, Published online: 29 May 2023; doi:10.1038/s41467-023-38856-0

The amino-As based synthesis of InAs@ZnSe core@shell nanocrystals with shell thickness up to seven monolayers is reported. The InAs@ZnSe nanocrystals exhibit photoluminescence quantum yields up to 70%, a record value for such system, thanks to an InZnSe interlayer featuring a structure similar to that of In₂ZnSe₄, which dampens the strain between the InAs core and the ZnSe shell.

Abstract

InAs-based nanocrystals can enable restriction of hazardous substances (RoHS) compliant optoelectronic devices, but their photoluminescence efficiency needs improvement. We report an optimized synthesis of InAs@ZnSe core@shell nanocrystals allowing to tune the ZnSe shell thickness up to seven mono-layers (ML) and to boost the emission, reaching a quantum yield of ≈70% at ≈900 nm. It is demonstrated that a high quantum yield can be attained when the shell thickness is at least ≈3ML. Notably, the photoluminescence lifetimeshows only a minor variation as a function of shell thickness, whereas the Auger recombination time (a limiting aspect in technological applications when fast) slows down from 11 to 38 ps when increasing the shell thickness from 1.5 to 7MLs. Chemical and structural analyses evidence that InAs@ZnSe nanocrystals do not exhibit any strain at the core-shell interface, likely due to the formation of an InZnSe interlayer. This is supported by atomistic modeling, which indicates the interlayer as being composed of In, Zn, Se and cation vacancies, alike to the In₂ZnSe₄ crystal structure. The simulations reveal an electronic structure consistent with that of type-I heterostructures, in which localized trap states can be passivated by a thick shell (>3ML) and excitons are confined in the core.

Biomanufacturing of high-purity rare earth (RE) materials is achieved by establishing a microbial synthesis system. RE bioproducts can be extracellularly collected from RE tailings by microbial in situ synthesis. A series of RE-biosorbent columns with high affinity are developed by immobilizing specifically designed proteins with an agarose matrix. Moreover, the lanthanide-immobilized methanol dehydrogenase is biofabricated for high-value utilization of RE.

Abstract

Rare earth materials play an irreplaceable role in biomedical and high technology fields. However, typical mining and extraction approaches to rare earth elements (REEs) often lead to severe environmental problems and resource wastage due to the involvement of hazardous chemicals. Although biomining shows elegant alternatives, there are still grand challenges to sustainably isolate and recover REEs in nature because of insufficient metal-extracting microbes and RE-scavenging macromolecular tools. To obtain high-performance rare earth materials directly from rare earth ore, a new generation of biological synthesis strategies needs to be developed for the efficient preparation of REEs. The microbial synthesis system established here has achieved active biomanufacturing of high-purity rare earth products. Further, through employing robust affinity columns bioconjugated with structurally engineered proteins, outstanding separation of Eu/Lu and Dy/La is acquired with the purity of 99.9% (Eu), 97.1% (La), and 92.7% (Dy). More importantly, in situ one-pot synthesis of lanthanide-dependent methanol dehydrogenase is well harnessed and exclusively adsorbs La, Ce, Pr, and Nd in RE tailing for advanced biocatalysis, indicating high value-added application. Therefore, this novel biosynthetic platform provides an insightful roadmap to expand the scope of chassis engineering in terms of biofoundry and to manufacture valuable bioproducts related to REEs.

Exotic functionalities of transition metal oxides are appealing to multifunctional devices if integrated with low-dimensional materials on Si-based platforms. In this study, heterostructures of MoS₂ and magnetic Sr-doped LaMnO₃ are achieved on Si using a freestanding form of Sr-doped LaMnO₃. The multifunctionality of the heterostructures is demonstrated based on three applications, including field-effect transistors, photodiodes, and magnetoresponsive heterostructure devices.

Abstract

Correlated oxides and related heterostructures are intriguing for developing future multifunctional devices by exploiting their exotic properties, but their integration with other materials, especially on Si-based platforms, is challenging. Here, van der Waals heterostructures of La_0.7Sr_0.3MnO₃ (LSMO) , a correlated manganite perovskite, and MoS₂ are demonstrated on Si substrates with multiple functions. To overcome the problems due to the incompatible growth process, technologies involving freestanding LSMO membranes and van der Waals force-mediated transfer are used to fabricate the LSMO-MoS₂ heterostructures. The LSMO-MoS₂ heterostructures exhibit a gate-tunable rectifying behavior, based on which metal-semiconductor field-effect transistors (MESFETs) with on-off ratios of over 10⁴ can be achieved. The LSMO-MoS₂ heterostructures can function as photodiodes displaying considerable open-circuit voltages and photocurrents. In addition, the colossal magnetoresistance of LSMO endows the LSMO-MoS₂ heterostructures with an electrically tunable magnetoresponse at room temperature. This work not only proves the applicability of the LSMO-MoS₂ heterostructure devices on Si-based platform but also demonstrates a paradigm to create multifunctional heterostructures from materials with disparate properties.

Scalable synthesis of 2D magnetic materials and heterostructures exhibiting a ferromagnetic order above room temperature is a crucial step toward the development of all-2D spintronic devices. Here the bottom-up synthesis of epitaxial Fe_{5− x} GeTe₂ (FGT)/graphene van der Waals heterostructures exhibiting a sharp interface between FGT and graphene, and ferromagnetism persisting well above 300 K with a perpendicular magnetic anisotropy, is demonstrated.

Abstract

Van der Waals (vdW) heterostructures combining layered ferromagnets and other 2D crystals are promising building blocks for the realization of ultracompact devices with integrated magnetic, electronic, and optical functionalities. Their implementation in various technologies depends strongly on the development of a bottom-up scalable synthesis approach allowing for realizing highly uniform heterostructures with well-defined interfaces between different 2D-layered materials. It is also required that each material component of the heterostructure remains functional, which ideally includes ferromagnetic order above room temperature for 2D ferromagnets. Here, it is demonstrated that the large-area growth of Fe_{5− x} GeTe₂/graphene heterostructures is achieved by vdW epitaxy of Fe_{5− x} GeTe₂ on epitaxial graphene. Structural characterization confirms the realization of a continuous vdW heterostructure film with a sharp interface between Fe_{5− x} GeTe₂ and graphene. Magnetic and transport studies reveal that the ferromagnetic order persists well above 300 K with a perpendicular magnetic anisotropy. In addition, epitaxial graphene on SiC(0001) continues to exhibit a high electronic quality. These results represent an important advance beyond nonscalable flake exfoliation and stacking methods, thus marking a crucial step toward the implementation of ferromagnetic 2D materials in practical applications.

Long exposure to the air triggers the formation of a stable ferromagnetic phase of Cr₂Te₃ (T _C2 ≈160 K) in the parent van der Waals ferromagnetic Cr₂Ge₂Te₆ (T _C1  69 K) causing the enhancement of the magnetic anisotropy in the time-elapsed crystal. Air-stable layered magnets having multiple ordering may have applications in low-dimensional nanoelectronics and spintronic devices.

Abstract

Manipulation of long-range order in 2D van der Waals (vdW) magnetic materials (e.g., CrI₃, CrSiTe₃ ,etc.), exfoliated in few-atomic layer, can be achieved via application of electric field, mechanical-constraint, interface engineering, or even by chemical substitution/doping. Usually, active surface oxidation due to the exposure in the ambient condition and hydrolysis in the presence of water/moisture causes degradation in magnetic nanosheets that, in turn, affects the nanoelectronic /spintronic device performance. Counterintuitively, the current study reveals that exposure to the air at ambient atmosphere results in advent of a stable nonlayered secondary ferromagnetic phase in the form of Cr₂Te₃ (T _C2 ≈160 K) in the parent vdW magnetic semiconductor Cr₂Ge₂Te₆ (T _C1 ≈69 K). The coexistence of the two ferromagnetic phases in the time elapsed bulk crystal is confirmed through systematic investigation of crystal structure along with detailed dc/ac magnetic susceptibility, specific heat, and magneto-transport measurement. To capture the concurrence of the two ferromagnetic phases in a single material, Ginzburg-Landau theory with two independent order parameters (as magnetization) with a coupling term can be introduced. In contrast to the rather common poor environmental stability of the vdW magnets, the results open possibilities of finding air-stable novel materials having multiple magnetic phases.

Publication date: June 2023

Source: Materials Today, Volume 66

Author(s): Jinfeng Dong, Ady Suwardi, Xian Yi Tan, Ning Jia, Kivanc Saglik, Rong Ji, Xizu Wang, Qiang Zhu, Jianwei Xu, Qingyu Yan

Nature Reviews Chemistry, Published online: 26 May 2023; doi:10.1038/s41570-023-00508-8

Nature Electronics, Published online: 26 May 2023; doi:10.1038/s41928-023-00972-6

Nature Electronics, Published online: 26 May 2023; doi:10.1038/s41928-023-00974-4

Nature Electronics, Published online: 26 May 2023; doi:10.1038/s41928-023-00969-1

Atomic scale epitaxy enables realization of artificial lattice structures unattainable in nature. Particularly, artificial 4d and 5d perovskite oxide superlattices, in which the finite spin–orbit coupling gives rise to novel functionalities, frequently exhibiting correlated quantum phenomena with practical controllability. The review summarizes the versatile correlated quantum functionalities of ruthenate and iridate based oxide superlattices in terms of the growth, underlying physics, and promising applications.

Abstract

Unexpected, yet useful functionalities emerge when two or more materials merge coherently. Artificial oxide superlattices realize atomic and crystal structures that are not available in nature, thus providing controllable correlated quantum phenomena. This review focuses on 4d and 5d perovskite oxide superlattices, in which the spin–orbit coupling plays a significant role compared with conventional 3d oxide superlattices. Modulations in crystal structures with octahedral distortion, phonon engineering, electronic structures, spin orderings, and dimensionality control are discussed for 4d oxide superlattices. Atomic and magnetic structures, J _eff = 1/2 pseudospin and charge fluctuations, and the integration of topology and correlation are discussed for 5d oxide superlattices. This review provides insights into how correlated quantum phenomena arise from the deliberate design of superlattice structures that give birth to novel functionalities.

2D materials provide a rich platform to study novel physical phenomena arising from quantum confinement. This article presents a simple and generic method of kinetic in situ single-layer synthesis, which enables the exfoliation of sub-millimeter flakes of air-sensitive 2D materials directly in vacuum. The method does not require the usage of a glovebox or other specialized equipment, making it well-suited for surface science techniques.

Abstract

2D materials provide a rich platform to study novel physical phenomena arising from quantum confinement of charge carriers. Many of these phenomena are discovered by surface sensitive techniques, such as photoemission spectroscopy, that work in ultra-high vacuum (UHV). Success in experimental studies of 2D materials, however, inherently relies on producing adsorbate-free, large-area, high-quality samples. The method that yields 2D materials of highest quality is mechanical exfoliation from bulk-grown samples. However, as this technique is traditionally performed in a dedicated environment, the transfer of samples into vacuum requires surface cleaning that might diminish the quality of the samples. In this article, a simple method for in situ exfoliation directly in UHV is reported, which yields large-area, single-layered films. Multiple metallic and semiconducting transition metal dichalcogenides are exfoliated in situ onto Au, Ag, and Ge. The exfoliated flakes are found to be of sub-millimeter size with excellent crystallinity and purity, as supported by angle-resolved photoemission spectroscopy, atomic force microscopy, and low-energy electron diffraction. The approach is well-suited for air-sensitive 2D materials, enabling the study of a new suite of electronic properties. In addition, the exfoliation of surface alloys and the possibility of controlling the substrate-2D material twist angle is demonstrated.

The SnSe₂ single crystal consists of a large-period polytype with an 18R low-symmetry structure. This work in situ applies pressure to the specimen and observes pressure-derived dynamic phase transitions from 18R to 4H, and finally to 2H-SnSe₂. This results in superior and near isotropic plasticity along the direction parallel and perpendicular to the cleavage plane.

Abstract

Plastic/ductile inorganic van der Waals (vdW) thermoelectric semiconductors offer transformative advantages for high-performance flexible thermoelectric devices, which can displace the self-charge system of wearable electronics. However, the chemical origin of their plasticity remains unclear. Here, it is reported that the exceptionally large plastic strain of the bulk SnSe₂ crystal results from its polytype conversion under an external force. The SnSe₂ single crystal consists of a large-period polytype with 18R low-symmetry structure rather than the trigonal and hexagonal-phase that are frequently observed in the polycrystalline specimen. In situ applied pressure to the specimen drives a phase transition from low to high-symmetry, that is, from 18R to 4H, and finally to 2H-SnSe₂. First principle calculations corroborate that the dynamic phase transition is a pressure-activated process and only 15 MPa pressure erases their energy gaps, consistent with experimentally measured strain–stress curves. This dynamic phase transition results in superior and near isotropic plasticity along the direction parallel and perpendicular to the cleavage plane.