Jiuxiang Dai

A series of layered oxyselenides Bi₂LnO₄Cu₂Se₂ (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) are synthesized by a time-saving method. The origin of the excellent thermoelectric performance of Bi₂LnO₄Cu₂Se₂ is thoroughly investigated. A maximum ZT value of ≈0.27 at 923K is achieved in Bi₂DyO₄Cu₂Se₂, which proves to be a potential thermoelectric system for further investigation.

Abstract

Layered oxyselenides have been widely investigated as promising thermoelectric materials due to their unique merits such as super-lattice structural features and intrinsic complexity, which contributes to low thermal conductivity and easily controllable electrical properties. Newly developed Bi₂LnO₄Cu₂Se₂ (Ln stands for lanthanide) oxyselenides are found to be potential thermoelectric systems since they have excellent electrical conductivity over 10³ S cm⁻¹. In this work, unique energy and time-saving method combined self-propagating high-temperature synthesis (SHS) with spark plasma sintering (SPS) is adopted to successfully prepare a highly pure Bi₂LnO₄Cu₂Se₂ instead of a traditional solid-state reaction. To explore the most suitable lanthanide for Bi₂LnO₄Cu₂Se₂, thermoelectric performance in a wide temperature range (300 to 923 K) of Bi₂LnO₄Cu₂Se₂ (Ln = Nd, Sm, Eu, Gd, Tb, Dy, Ho, and Er) is deeply evaluated and studied. Ultimately, with a relatively high electrical conductivity, moderate Seebeck coefficient, and extremely low thermal conductivity, a maximum ZT value of ≈0.27 at 923K is achieved in Bi₂DyO₄Cu₂Se₂, which is 4 times larger than that of the ever-reported Bi₂YO₄Cu₂Se₂ and proves a potential thermoelectric system for further investigation. This work may provide some enlightenment and broaden the horizon in finding new thermoelectric materials, especially for complex layered compounds.

Nature Communications, Published online: 26 January 2022; doi:10.1038/s41467-022-28116-y

Nature Communications, Published online: 26 January 2022; doi:10.1038/s41467-022-28187-x

Publication date: January–February 2022

Source: Materials Today, Volume 52

Author(s): Siyan Dong, Xiang Zhang, S. Shiva. P. Nathamgari, Andrey Krayev, Xu Zhang, Jin Wook Hwang, Pulickel M. Ajayan, Horacio D. Espinosa

Nature, Published online: 26 January 2022; doi:10.1038/s41586-021-04238-z

Power-detector circuits are realized based on MoS₂ channel transistors fabricated on flexible polyimide substrates. The work compares the DC and radio-frequency performance of multilayer and monolayer wafer-scale grown MoS₂ for power detection and shows that both materials allow the fabrication of circuits that work at GHz frequencies with excellent dynamic range and responsivity.

Abstract

The design, fabrication, and characterization of wafer-scale, zero-bias power detectors based on 2D MoS₂ field-effect transistors (FETs) are demonstrated. The MoS₂ FETs are fabricated using a wafer-scale process on 8 μm-thick polyimide film, which, in principle, serves as a flexible substrate. The performances of two chemical vapor deposition MoS₂ sheets, grown with different processes and showing different thicknesses, are analyzed and compared from the single device fabrication and characterization steps to the circuit level. The power-detector prototypes exploit the nonlinearity of the transistors above the cut-off frequency of the devices. The proposed detectors are designed employing a transistor model based on measurement results. The fabricated circuits operate in the Ku-band between 12 and 18 GHz, with a demonstrated voltage responsivity of 45 V W⁻¹ at 18 GHz in the case of monolayer MoS₂ and 104 V W⁻¹ at 16 GHz in the case of multilayer MoS₂, both achieved without applied DC bias. They are the best-performing power detectors fabricated on flexible substrate reported to date. The measured dynamic range exceeds 30 dB, outperforming other semiconductor technologies like silicon complementary metal–oxide–semiconductor circuits and GaAs Schottky diodes.

Van der Waals heterostructures have great potential in next-generation optoelectronic devices. Here we present a synergistic-strategy of configuration-design and thickness-modulation to fabricate high-performance photodiodes based on monolayer MoS₂/multilayer WSe₂/graphene. The synergistic-engineered photodetector exhibits high external quantum efficiency (61%) and ultrafast photoresponse (4.1 μs) in self-powered mode, which advances the prospects of photovoltaic applications.

Abstract

Van der Waals (vdW) heterostructures based on two-dimensional transition-metal dichalcogenides have provided unprecedented opportunities for photovoltaic detectors owing to their strong light-matter interaction and ultrafast interfacial charge transfer. Despite continued advancement, insufficient control of photocarrier behaviors still limits the external quantum efficiency (EQE) and operation speed of such detectors. Here, we propose a synergistic strategy of contact-configuration design and thickness-modulation to construct high-performance vdW photodiodes based on the typical type II heterostructure (MoS₂/WSe₂). Through integrating three contact architectures into one device to exclude other factors, we solid the superiority of designed 1L-MoS₂/WSe₂/graphene heterostructures incorporating efficient photocarrier collection and gate modulation. Together with leveraging the layer-number-dependent properties of WSe₂, we observe the critical thickness of WSe₂ (11 layers) for the highest EQE, which verifies the thickness-dependent competition between photocarrier generation, dissociation, and collection. Finally, we demonstrate the synergistic-engineered vdW heterostructure can trigger record-high EQE (61%) and manifest ultrafast photoresponse (4.1 μs) at the atomically thin limit (8 nm). The proposed strategy enables architecture-design and thickness-engineering to unlock the potential to realize high-performance optoelectronic devices.

The novel thermal properties of the Janus MoSSe/WSSe heterostructure are manipulated using thermo-mechanical coupling.

Abstract

2D Janus transition metal dichalcogenide (TMD) semiconductor materials have attracted great interest for their potential applications. Because of the increased requirement for thermal management in 2D devices with single-atom thickness, a fundamental understanding of interfacial thermal conduction (ITC) has emerging significance. In this work, the ITC of in-plane heterostructures constructed using MoSSe and WSSe is reported. In addition to the interface connected normally by MoSSe and WSSe with the same type of chalcogen atoms are on the same side of left and right sections, inversional interface by rotation of 180° of WSSe is also considered, in which S atoms are on the opposite side of the left and right sections. Interestingly, the ITC in the normally connected heterostructure is found to be almost twice as much as that in the inversely connected heterostructure. The unusually large change in ITC is attributed to the bending curvature and additional discontinuity in the inversely connected heterostructure. Euler–Bernoulli beam model gives further insight into the origin of such interface bending. The findings offer the very first insight into the phonon transport in Janus heterostructures, and benefit thermal management of 2D devices based on Janus monolayers.

The evolution of multiple valence bands, band convergence and density-of-states distortion, strongly enhances the effective mass of thermoelectrics, leading to an ultrahigh weighted mobility. The lattice thermal conductivity is significantly reduced through introducing all-scale defects via alloying CdSe. Consequently, an ultrahigh ZT of 2.3 at 673 K and a ZT _ave of 1.46 at 303–773 K are obtained in CdSe-alloyed GeTe.

Abstract

Thermoelectric materials can achieve the direct conversion between electricity and heat, which has drawn extensive attention in recent decades. Understanding the chemical nature of band structure and microstructure is essential to boost the thermoelectric performance of given materials. Herein, CdSe alloying promotes the evolution of multiple valence bands in GeTe, resulting in the contemporaneous appearance of band convergence and density of state distortion, which benefits the sharply enhanced effective mass from 2.3 m ₀ to 5.0 m ₀. The carrier mobility and effective mass are well optimized via CdSe alloying, contributing to the ultrahigh weighted mobility of ≈199 cm² V⁻¹ s⁻¹ at 300 K in CdSe-alloyed GeTe. Accordingly, a superior power factor of ≈41 µW cm⁻¹ K⁻² is attained at 673 K. Meanwhile, the nanoprecipitates, strain, and mass field fluctuations introduced by CdSe alloying result in a significantly decreased lattice thermal conductivity. A highest figure of merit (ZT) of ≈2.3 at 673 K and the ultrahigh ZT _ave of ≈1.46 at 303–773 K are achieved in CdSe-alloyed GeTe. This work illustrates that the charge and phonon transport properties of GeTe can be simultaneously optimized through integrating band engineering and all-scale defects incorporation via CdSe alloying.

The progress in employing MXenes in energy harvesting applications is summarized. Different energy sources, including solar, ultrasound, electrokinetic, salinity-gradient, piezoelectric, triboelectric, thermoelectric, and humidity energy, are discussed separately. The future challenges and promising directions of MXene research for energy harvesting are presented.

Abstract

Energy harvesting modules play an increasingly important role in the development of autonomous self-powered microelectronic devices. MXenes (i.e., 2D transition metal carbide/nitride) have recently emerged as promising candidates for energy applications due to their excellent electronic conductivity, large specific surface area, and tunable properties. Herein, a perspective on using MXenes to harvest energy from various sources in the environment is presented. First, the characteristics of MXenes that facilitate energy capturing are systematically introduced and the preparation strategies of MXenes and their derived nanostructures tailored toward such applications are summarized. Subsequently, the harvesting mechanism of different energy sources (e.g., solar energy, thermoelectric energy, triboelectric energy, piezoelectric energy, salinity-gradient energy, electrokinetic energy, ultrasound energy, and humidity energy) are discussed. Then, the recent progress of MXene-based nanostructures in energy harvesting, as well as their applications, is introduced. Finally, opinions on the existing challenges and future directions of MXene-based nanostructure for energy harvesting are presented.

npj 2D Materials and Applications, Published online: 21 January 2022; doi:10.1038/s41699-021-00284-3

Low-cost, high-volume, and high-quality 2D materials play an important role in the applications of flexible devices. At this stage of their development, the key issues concern how to make further improvements to high-performance and scalable-production. Different properties and synthesis techniques of 2D materials are discussed in detail in this review and important directions for future advancements are outlined.

Abstract

2D materials are now at the forefront of state-of-the-art nanotechnologies due to their fascinating properties and unique structures. As expected, low-cost, high-volume, and high-quality 2D materials play an important role in the applications of flexible devices. Although considerable progress has been achieved in the integration of a series of novel 2D materials beyond graphene into flexible devices, a lot remains to be known. At this stage of their development, the key issues concern how to make further improvements to high-performance and scalable-production. Herein, recent progress in the quest to improve the current state of the art for 2D materials beyond graphene is reviewed. Namely, the properties and synthesis techniques of 2D materials are first introduced. Then, both the advantages and challenges of these 2D materials for flexible devices are also highlighted. Finally, important directions for future advancements toward efficient, low-cost, and stable flexible devices are outlined.

The growth of adlayer graphene spirals and onions with novel spiral arms by chemical vapor deposition (CVD) is confirmed. These CVD-prepared graphene spirals and onions exhibit more than one arm (with two, three, four, and five arms) together with clockwise and anticlockwise direction of rotation of each arm, which can be reproduced by using the kinetic Monte Carlo simulations.

Abstract

The morphology of as-grown graphene in chemical vapor deposition (CVD) experiments is sensitive to the reaction environment. Understanding the mechanism of formation of different graphene morphologies is essential to achieve controlled graphene CVD growth. Here the growth and formation mechanism of adlayer graphene spirals are reported. An adlayer graphene spiral is formed by fast propagation of the tips of spiral arms along the edge of the first graphene layer. The driving force to form spirals is the limited availability of carbon diffusing from the Cu surface through the edge of the first graphene layer. In addition, it is found that graphene onions are formed by overlapping graphene spirals with clockwise and anticlockwise arms. Based on these features, a kinetic Monte Carlo (kMC) method is demonstrated using which all the observed graphene spiral structures are successfully reproduced at the atomic level. This study thus unravels the hither-to unresolved mechanism of graphene onion growth and paves the way to the controllable growth of few-layer graphene by increasing the carbon supply at the edge of the first layer graphene.

Nature Nanotechnology, Published online: 20 January 2022; doi:10.1038/s41565-021-01052-6

Nature Materials, Published online: 20 January 2022; doi:10.1038/s41563-021-01174-1

As a fast-emerging platform, van der Waals heterojunctions have exhibited exotic carrier dynamics in the quantum limit, including charge and energy transfer. Based on the recent experimental and theoretical progress, this review summarizes the state-of-art understanding, followed by the representative applications in optoelectronic devices. We also summarize the remaining challenges and opportunities for future development in this field.

Abstract

Van der Waals heterojunctions are fast-emerging quantum structures fabricated by the controlled stacking of two-dimensional (2D) materials. Owing to the atomically thin thickness, their carrier properties are not only determined by the host material itself, but also defined by the interlayer interactions, including dielectric environment, charge trapping centers, and stacking angles. The abundant constituents without the limitation of lattice constant matching enable fascinating electrical, optical, and magnetic properties in van der Waals heterojunctions toward next-generation devices in photonics, optoelectronics, and information sciences. This review focuses on the charge and energy transfer processes and their dynamics in transition metal dichalcogenides (TMDCs), a family of quantum materials with strong excitonic effects and unique valley properties, and other related 2D materials such as graphene and hexagonal-boron nitride. In the first part, we summarize the ultrafast charge transfer processes in van der Waals heterojunctions, including its experimental evidence and theoretical understanding, the interlayer excitons at the TMDC interfaces, and the hot carrier injection at the graphene/TMDCs interface. In the second part, the energy transfer, including both Förster and Dexter types, are reviewed from both experimental and theoretical perspectives. Finally, we highlight the typical charge and energy transfer applications in photodetectors and summarize the challenges and opportunities for future development in this field.

Ultra-steep-slope MoS₂ resistive-gate field-effect transistors (RG-FETs) by integrating atomic-scale resistive filamentary with conventional MoS₂ transistors, demonstrating an ultra-low SS below 1 mV dec⁻¹ at room temperature are reported. Ultra-high gain ≈2000 is demonstrated in the fabricated inverter.

Abstract

The fundamental Boltzmann limitation dictates the ultimate limit of subthreshold swing (SS) to be 60 mV dec⁻¹, which prevents the continued scaling of supply voltage. With atomically thin body, 2D semiconductors provide new possibilities for advanced low-power electronics. Herein, ultra-steep-slope MoS₂ resistive-gate field-effect transistors (RG-FETs) by integrating atomic-scale-resistive filamentary with conventional MoS₂ transistors, demonstrating an ultra-low SS below 1 mV dec⁻¹ at room temperature are reported. The abrupt resistance transition of the nanoscale-resistive filamentary ensures dramatic change in gate potential, and switches the device on and off, leading to ultra-steep SS. Simultaneously, RG-FETs demonstrate a high on/off ratio of 2.76 × 10⁷ with superior reproducibility and reliability. With the ultra-steep SS, the RG-FETs can be readily employed to construct logic inverter with an ultra-high gain ≈2000, indicating exciting potential for future low-power electronics and monolithic integration.