Jiuxiang Dai

Ultrathin vdW heterostructure of WO_3-x/WS₂ is constructed in a facile one-step manner. The heterostructure demonstrates an ultrahigh photo-responsivity and detectivity with a remarkable high-speed switching due to its Type-II band alignment. This study presents a facile and highly controllable approach to synthesize ultrathin TMOs/TMDs vdW heterostructures with strong potential in high-performance photodetection applications.

Abstract

Ultrathin semiconducting van der Waals (vdW) heterostructures based on transition metal dichalcogenides (TMDs) play a critical role in developing next-generation electronic and optoelectronic devices. The replacement of one component of the heterostructure by transition metal oxides (TMOs) certainly brings in numerous benefits including long-term stability and novel functionalities. However, the single-step chemical-vapor deposition growth of TMOs/TMDs vdW heterostructures, as a highly desired approach for large-scale fabrication and practical implementation, is challenging due to contradictory growth atmospheres of TMOs and TMDs. Here, the single-step growth of an ultrathin WO_3–x/WS₂ vdW heterostructure based on the quantity-driven discrepant interaction between S and the precursor, in which S induces sulfidation to produce WS₂ in the S-rich phase and is changed to the reduction role to obtain sub-stoichiometric WO_3–x in the S-deficient phase is realized. Both WO_3–x and WS₂ exhibit semiconducting properties with a favorable type-II band alignment. A wide response across the entire visible spectrum with a large photo-responsivity of 4375 A W⁻¹, a detectivity of 5.47 × 10¹¹ Jones, and sub-ms switching kinetics at 405 nm is achieved without gating bias, which is significantly improved over other reported ultrathin vdW heterostructures. This study demonstrates the possibility of single-step-growing TMOs/TMDs vdW heterostructures and their strong potential in high-performance optoelectronic devices.

Z. Huo, Y. Wei, Y. Wang, Z. L. Wang, Q. Sun

In this review, integrated self-powered sensors based on 2D materials are introduced in detail. The applications of TENG/PENG self-powered sensors based on 2D materials in different research fields are discussed.

With the development of the Internet of Things, there is an increasing need for clean energy and large-scale sensory systems. Triboelectric/piezoelectric nanogenerators (TENGs/PENGs), have attracted considerable attention as a new type of power generation terminal, which can harvest surrounding energy and convert it into electrical energy. To improve the output performance of nanogenerators (NGs) and diversify related applications, 2D materials with high carrier mobility and excellent piezoelectric properties can be directly used or integrated as different types of self-powered sensors. In this review, the authors first introduce the excellent piezoelectric and optoelectronic properties of 2D materials, followed by the triboelectric series of 2D materials used in TENGs. The categories of integrated self-powered sensors based on 2D materials are then summarized according to their different structures and compositions. We also discuss in detail the recent applications of integrated self-powered sensors based on 2D materials from five aspects. Finally, the challenges and outlooks in the research field of self-powered sensors are featured. Given the continuous development of self-powered sensors based on 2D materials, they are considered to have significant potential for applications in biomedicine, environmental detection, human motion monitoring, energy harvesting, and smart wearable devices.

Nature Nanotechnology, Published online: 02 August 2022; doi:10.1038/s41565-022-01191-4

Layered 2D crystals are exemplar energy conversion materials, able to convert light, heat, or motion to electricity. These energy conversion processes can also occur synchronously, leading to complex energy interplay networks during energy harvesting/transducing. This synchronous multisource energy conversion needs to be studied and understood to enable the fabrication of next-generation energy harvesting and conversion systems.

Abstract

Layered 2D crystals have unique properties and rich chemical and electronic diversity, with over 6000 2D crystals known and, in principle, millions of different stacked hybrid 2D crystals accessible. This diversity provides unique combinations of properties that can profoundly affect the future of energy conversion and harvesting devices. Notably, this includes catalysts, photovoltaics, superconductors, solar-fuel generators, and piezoelectric devices that will receive broad commercial uptake in the near future. However, the unique properties of layered 2D crystals are not limited to individual applications and they can achieve exceptional performance in multiple energy conversion applications synchronously. This synchronous multisource energy conversion (SMEC) has yet to be fully realized but offers a real game-changer in how devices will be produced and utilized in the future. This perspective highlights the energy interplay in materials and its impact on energy conversion, how SMEC devices can be realized, particularly through layered 2D crystals, and provides a vision of the future of effective environmental energy harvesting devices with layered 2D crystals.

Colloidal crystals have brought the promise of revolution to modern engineering. The authors report a surface tension gradient-driven self-assembly strategy for the ultrafast fabrication of large-area colloidal crystals. The colloidal crystal monolayers exceeding 1000 cm² can be fabricated within minutes. The nanoparticle transfer printing method is further proposed to convert the close-packed nanoparticle monolayers into high-resolution conformal micropatterns.

Abstract

Colloidal crystals have brought the promise of revolution to modern engineering, yet commonly used fabrication technologies are still limited by the small preparation area, time-consuming process, and dependence on sophisticated equipment. Here, a surface tension gradient-driven self-assembly strategy is proposed for the ultrafast fabrication of large-area colloidal crystals. The hydrogel loaded with sodium dodecyl sulfate is devised to construct a stable and continuous liquid-air interfacial tension gradient, and the resulting Marangoni effect can drive the micro-nano particles to instantaneously form (within several seconds) highly ordered colloidal crystals. Benefiting from the long range of surface tension gradients, the fabrication area of colloidal crystal films is demonstrated to exceed an astonishing 1000 cm² without compromising their quality, showing great potential in scale-up manufacture. Moreover, particles of a wide variety of sizes, materials, and functionalities can form close-packed self-assembly monolayers and be transferred to various substrates without damage, exhibiting great versatility. Inspired by ink microprinting, an ultrafast nanoparticle transfer printing method is further proposed to convert the close-packed nanoparticle monolayers into large-area conformal micropatterns with single-nanoparticle resolution. The great potential of nanoparticle micropatterns is demonstrated in flexible micro-electronics/skin electronics. This user-friendly, efficient self-assembly, and micropatterning strategy provide promising opportunities for academic and real industrial applications.

Erbium-doped WS₂ hybrids with down- and up-conversion photoluminescence are integrated on silicon and are highly beneficial to 980-nm detection with a responsivity of 39.8 mA W⁻¹ at a weak power of 4.4 µW and a high detectivity of 2.79 × 10¹⁰ Jones, thus supporting the significance of rare-earth doping as a robust strategy for boosting the characteristics of 2D optoelectronic devices.

Abstract

The integration of 2D nanomaterials with silicon is expected to enrich the applications of 2D functional nanomaterials and pave the way for next-generation, nanoscale optoelectronics with enhanced performances. Herein, a strategy for rare earth element doping is utilized for the synthesis of 2D WS₂:Er nanosheets to achieve up-conversion and down-conversion emissions ranging from visible to near-infrared regions. Moreover, the potential integration of the synthesized 2D nanosheets in silicon platforms is demonstrated by the realization of an infrared photodetector based on a WS₂:Er/Si heterojunction. These devices operate at room temperature and show a high photoresponsivity of ≈39.8 mA W⁻¹ (at 980 nm) and a detectivity of 2.79 × 10¹⁰ cm Hz^1/2 W⁻¹. Moreover, the dark current and noise power density are suppressed effectively by van der Waals-assisted p–n heterojunction. This work fundamentally contributes to establishing infrared detection by rare element doping of 2D materials in heterojunctions with Si, at the forefront of infrared 2DMs-based photonics.

In this study, Na as the n-type dopant is introduced in the polycrystalline Bi₂Te₃ material for enhancing the thermoelectric and mechanical properties. Benefitting from the synergistic optimization of electrical and thermal transport properties, a maximum ZT value of 1.03 is achieved at 303 K for the Bi₂Te₃-0.25 wt% sample, which is 70% higher than that of the pristine sample.

Abstract

The recent growing energy crisis draws considerable attention to high-performance thermoelectric materials. n-type bismuth telluride is still irreplaceable at near room temperature for commercial application, and therefore, is worthy of further investigation. In this work, nanostructured Bi₂Te₃ polycrystalline materials with highly enhanced thermoelectric properties are obtained by alkali metal Na solid solution. Na is chosen as the cation site dopant for n-type polycrystalline Bi₂Te₃. Na enters the Bi site, introducing holes in the Bi₂Te₃ matrix and rendering the electrical conductivity tunable from 300 to 1800 Scm^–1. The solid solution limit of Na in Bi₂Te₃ exceeds 0.3 wt%. Owing to the effective solid solution, the Fermi level of Bi₂Te₃ is properly regulated, leading to an improved Seebeck coefficient. In addition, the scattering of both charge carriers and phonons is modulated, which ensured a high-power factor and low lattice thermal conductivity. Benefitting from the synergistic optimization of both electrical and thermal transport properties, a maximum figure of merit (ZT) of 1.03 is achieved at 303 K when the doping content is 0.25 wt%, which is 70% higher than that of the pristine sample. This work disclosed an effective strategy for enhancing the performance of n-type bismuth telluride-based alloy materials.

Nature, Published online: 03 August 2022; doi:10.1038/s41586-022-04933-5

npj 2D Materials and Applications, Published online: 01 August 2022; doi:10.1038/s41699-022-00327-3

Nature, Published online: 01 August 2022; doi:10.1038/s41586-022-05134-w

Publication date: September 2022

Source: Materials Today, Volume 58

Author(s): Aravind Puthirath Balan, Anand B. Puthirath, Soumyabrata Roy, Gelu Costin, Eliezer Fernando Oliveira, M.A.S.R. Saadi, Vishnu Sreepal, Rico Friedrich, Peter Serles, Abhijit Biswas, Sathvik Ajay Iyengar, Nithya Chakingal, Sohini Bhattacharyya, Sreehari K. Saju, Samuel Castro Pardo, Lucas M. Sassi, Tobin Filleter, Arkady Krasheninnikov, Douglas S Galvao, Robert Vajtai

A new platform is developed for assembling freestanding oxide thin films with different materials and orientations into artificial stacks of heterointerfaces. The heterointerfaces can be tailored by controlling the stacking sequences, as well as the twist angle between the constituent layers with atomically sharp interfaces, leading to distinct moiré patterns.

Abstract

The integration of dissimilar materials in heterostructures has long been a cornerstone of modern materials science—seminal examples are 2D materials and van der Waals heterostructures. Recently, new methods have been developed that enable the realization of ultrathin freestanding oxide films approaching the 2D limit. Oxides offer new degrees of freedom, due to the strong electronic interactions, especially the 3d orbital electrons, which give rise to rich exotic phases. Inspired by this progress, a new platform for assembling freestanding oxide thin films with different materials and orientations into artificial stacks with heterointerfaces is developed. It is shown that the oxide stacks can be tailored by controlling the stacking sequences, as well as the twist angle between the constituent layers with atomically sharp interfaces, leading to distinct moiré patterns in the transmission electron microscopy images of the full stacks. Stacking and twisting is recognized as a key degree of structural freedom in 2D materials but, until now, has never been realized for oxide materials. This approach opens unexplored avenues for fabricating artificial 3D oxide stacking heterostructures with freestanding membranes across a broad range of complex oxide crystal structures with functionalities not available in conventional 2D materials.

Nonlayered molybdenum phosphide (MoP) with well-defined 2D morphology and tunable crystallinity can be prepared from layered molybdenum dichalcogenides via surface-confined atomic substitution. In contrast to the well-documented edge-dominated electrocatalytic performance of the MoS₂ precusor, the entire basal plane of MoP demonstrates satisfying electrocatalytic hydrogen evolution reaction performance owing to its coordination-unsaturated surface atoms with abundant dangling bonds.

Abstract

The emerging nonlayered 2D materials (NL2DMs) are sparking immense interest due to their fascinating physicochemical properties and enhanced performance in many applications. NL2DMs are particularly favored in catalytic applications owing to the extremely large surface area and low-coordinated surface atoms. However, the synthesis of NL2DMs is complex because their crystals are held together by strong isotropic covalent bonds. Here, nonlayered molybdenum phosphide (MoP) with well-defined 2D morphology is synthesized from layered molybdenum dichalcogenides via surface-confined atomic substitution. During the synthesis, the molybdenum dichalcogenide nanosheet functions as the host matrix where each layer of Mo maintains their hexagonal arrangement and forms isotropic covalent bonds with P that substitutes S, resulting in the conversion from layered van der Waals material to a covalently bonded NL2DM. The MoP nanosheets converted from few-layer MoS₂ are single crystalline, while those converted from monolayers are amorphous. The converted MoP demonstrates metallic charge transport and desirable performance in the electrocatalytic hydrogen evolution reaction (HER). More importantly, in contrast to MoS₂, which shows edge-dominated HER performance, the edge and basal plane of MoP deliver similar HER performance, which is correlated with theoretical calculations. This work provides a new synthetic strategy for high-quality nonlayered materials with well-defined 2D morphology for future exploration.

A novel 19×19$\sqrt {19} \times \sqrt {19} $ charge order is observed in few-layer thick 1T-TaTe₂ films grown by molecular beam epitaxy. The photoemission and scanning probe measurements demonstrate that monolayer 1T-TaTe₂ exhibits various metastable charge density wave orders, including the 19×19$\sqrt {19} \times \sqrt {19} $ superstructure, which can be selectively stabilized by postgrowth annealing. In 1T-TaTe₂ films thicker than a monolayer, only the 19×19$\sqrt {19} \times \sqrt {19} $ order persists.

Abstract

The spontaneous formation of electronic orders is a crucial element for understanding complex quantum states and engineering heterostructures in 2D materials. A novel 19$\sqrt {19} $ ×19$\sqrt {19} $ charge order in few-layer-thick 1T-TaTe₂ transition metal dichalcogenide films grown by molecular beam epitaxy, which has not been realized, is report. The photoemission and scanning probe measurements demonstrate that monolayer 1T-TaTe₂ exhibits a variety of metastable charge density wave orders, including the 19$\sqrt {19} $ × 19$\sqrt {19} $ superstructure, which can be selectively stabilized by controlling the post-growth annealing temperature. Moreover, it is found that only the 19$\sqrt {19} $ × 19$\sqrt {19} $ order persists in 1T-TaTe₂ films thicker than a monolayer, up to 8 layers. The findings identify the previously unrealized novel electronic order in a much-studied transition metal dichalcogenide and provide a viable route to control it within the epitaxial growth process.

A step-climbing epitaxy growth strategy is proposed, in which epilayers grow across the stepped surfaces and form continuous films. Based on the step-climbing epitaxy, wafer-scale uniform 2D Bi₂O₂Se films with controllable thickness can be obtained. One-unit-cell films exhibit a high room-temperature Hall mobility of 180 cm² V⁻¹ s⁻¹, which exceeds that of silicon and other 2D semiconductors with comparable thickness.

Abstract

Heteroepitaxy with large lattice mismatch remains a great challenge for high-quality epifilm growth. Although great efforts have been devoted to epifilm growth with an in-plane lattice mismatch, the epitaxy of 2D layered crystals on stepped substrates with a giant out-of-plane lattice mismatch is seldom reported. Here, taking the molecular-beam epitaxy of 2D semiconducting Bi₂O₂Se on 3D SrTiO₃ substrates as an example, a step-climbing epitaxy growth strategy is proposed, in which the n-th (n = 1, 2, 3…) epilayer climbs the step with height difference from out-of-plane lattice mismatch and continues to grow the n+1-th epilayer. Step-climbing epitaxy can spontaneously relax and release the strain from the out-of-plane lattice mismatch, which ensures the high quality of large-area epitaxial films. Wafer-scale uniform 2D Bi₂O₂Se single-crystal films with controllable thickness can be obtained via step-climbing epitaxy. Most notably, one-unit-cell Bi₂O₂Se films (1.2 nm thick) exhibit a high Hall mobility of 180 cm² V⁻¹ s⁻¹ at room temperature, which exceeds that of silicon and other 2D semiconductors with comparable thickness. As an out-of-plane lattice mismatch is generally present in the epitaxy of layered materials, the step-climbing epitaxy strategy expands the existing epitaxial growth theory and provides guidance toward the high-quality synthesis of layered materials.

Monolayers of Janus SeMoS with asymmetric top (Se) and bottom (S) chalcogen atomic planes with respect to the central transition metal (Mo) atoms are synthesized using a one-pot chemical vapor deposition process and its high optical quality is demonstrated by low-temperature magneto optical spectroscopy.

Abstract

One-pot chemical vapor deposition (CVD) growth of large-area Janus SeMoS monolayers is reported, with the asymmetric top (Se) and bottom (S) chalcogen atomic planes with respect to the central transition metal (Mo) atoms. The formation of these 2D semiconductor monolayers takes place upon the thermodynamic-equilibrium-driven exchange of the bottom Se atoms of the initially grown MoSe₂ single crystals on gold foils with S atoms. The growth process is characterized by complementary experimental techniques including Raman and X-ray photoelectron spectroscopy, transmission electron microscopy, and the growth mechanisms are rationalized by first principle calculations. The remarkably high optical quality of the synthesized Janus monolayers is demonstrated by optical and magneto-optical measurements which reveal the strong exciton–phonon coupling and enable an exciton g-factor of −3.3.