Jiuxiang Dai

The experimental evidence of the nearly independent single topological flat band (FB) generated by the 2D Nb breathing-kagome layer in Nb₃TeCl₇ with identical on-site energy of Nb sites is reported. The possibility of modulating the bandwidth, energy position, and topology of the FB by altering the on-site energy over Nb sites in the 2D breathing-kagome frustrated geometry is disclosed.

Abstract

Flat bands (FBs) can appear in two-dimensional (2D) geometrically frustrated systems caused by quantum destructive interference (QDI). However, the scarcity of pure 2D frustrated crystal structures in natural materials makes FBs hard to be identified, let alone modulate FBs relating to electronic properties. Here, the experimental evidence of the complete electronic QDI induced FB contributed by the 2D breathing-kagome layers of Nb atoms in Nb₃TeCl₇ (NTC) is reported. An identical chemical state and 2D localization characteristics of the Nb breathing-kagome layers are experimentally confirmed, based on which NTC is demonstrated to be a superior concrete candidate for the breathing-kagome tight-binding model. Furthermore, it theoretically establishes the tunable roles of the on-site energy over Nb sites on bandwidth, energy position, and topology of FBs in NTC. This work opens an aveanue to manipulate FB characteristics in these 4d transition-metal-based breathing-kagome materials.

Sulfur vacancy engineering is a vital strategy to activate the hydrogen evolution activity of the MoS₂ basal plane. Unlike traditional costly post-treatment methods, this work demonstrates a novel salt-assisted chemical vapor deposition method for synthesizing vacancy-rich 2H-MoS₂ electrocatalysts with exceptional catalytic activity. The generation of such defects is closely related to ion adsorption in the growth process.

Abstract

Emerging non-noble metal 2D catalysts, such as molybdenum disulfide (MoS₂), hold great promise in hydrogen evolution reactions. The sulfur vacancy is recognized as a key defect type that can activate the inert basal plane to improve the catalytic performance. Unfortunately, the method of introducing sulfur vacancies is limited and requires costly post-treatment processes. Here, a novel salt-assisted chemical vapor deposition (CVD) method is demonstrated for synthesizing ultrahigh-density vacancy-rich 2H-MoS₂, with a controllable sulfur vacancy density of up to 3.35 × 10¹⁴ cm⁻². This approach involves a pre-sprayed potassium chloridepromoter on the growth substrate. The generation of such defects is closely related to ion adsorption in the growth process, the unstable MoS₂-K-H₂O triggers the formation of sulfur vacancies during the subsequent transfer process, and it is more controllable and nondestructive when compared to traditional post-treatment methods. The vacancy-rich monolayer MoS₂ exhibits exceptional catalytic activity based on the microcell measurements, with an overpotential of ≈158.8 mV (100 mA cm⁻²) and a Tafel slope of 54.3 mV dec⁻¹ in 0.5 m H₂SO₄ electrolyte. These results indicate a promising opportunity for modulating sulfur vacancy defects in MoS₂ using salt-assisted CVD growth. This approach represents a significant leap toward achieving better control over the catalytic performances of 2D materials.

A high-speed phototransistor is designed, which has a type III junction between p-MoTe₂ channel and n-SnS₂ top layer. The photodetecting device operates with a basis of negative photoresponse, which originates from the recombination of photoexcited electrons within the n-SnS₂ layer and accumulated holes in the p-MoTe₂ channel.

Abstract

Extensive study on 2D van der Waals (vdW) heterojunctions has primarily focused on PN diodes for fast-switching photodetection, while achieving the same from 2D channel phototransistors is rare despite their other advantages. Here, a high-speed phototransistor featuring a type III junction between p-MoTe₂ channel and n-SnS₂ top layer is designed. The photodetecting device operates with a basis of negative photoresponse (NPR), which originates from the recombination of photoexcited electrons in n-SnS₂ and accumulated holes in the p-MoTe₂ channel. For the NPR to occur, high-energy photons capable of exciting SnS₂ (band gap ≈2.2 eV) are found to be effective because lower-energy photons simply penetrate the SnS₂ top layer only to excite MoTe₂, leading to normal positive photoresponse (PPR) which is known to be slow due to the photogating effects. The NPR transistor showcases 0.5 ms fast photoresponses and a high responsivity over 5000 A W⁻¹. More essentially, such carrier recombination mechanism is clarified with three experimental evidences. The phototransistor is finally modified with Au contact on n-SnS₂, to be a more practical device displaying voltage output. Three different photo-logic states under blue, near infrared (NIR), and blue-NIR mixed photons are demonstrated using the voltage signals.

CNRs and CNBs have been a research hotspot owing to their aesthetic structures and potential applications. This review extensively surveys advancements made in the field of material syntheses and molecular structures, especially over the last two years. In addition, current applications such as organic field-effect transistors, carbon nanomaterials, and supramolecular chemistry are summarized. For the first time, organic field-effect transistors on CNRs and CNBs are discussed in detail. This review also presents future challenges and research directions for CNRs and CNBs.

Abstract

Carbon nanorings (CNRs) and carbon nanobelts (CNBs) of various sizes and innovative molecular architectures have ushered improvement in research in recent years. This serves as a prerequisite for further advances and applications, such as organic semiconductors and bottom-up synthesis of single-walled carbon nanotubes. Nevertheless, challenges such as high strain energy, excessive reactivity, end-product stability, low-scale yields, and side reactions inhibit material synthesis and applications. In this review, CNRs, CNBs, and other heteroatom-doped molecular belts are discussed based on their molecular structural characteristics, with a special focus on their developments in the past two years. Furthermore, current applications, research obstacles, and future developments related to CNRs and CNBs are discussed. For the first time, studies and advancements in organic field-effect transistors (OFETs) have been summarized in detail.

Abstract

Broadband photodetectors with self-driven functions have attracted intensive scientific interest due to their low energy consumption and high optical gain. However, high-performance broadband self-driven photodetectors are still a significant challenge due to the complex fabrication processes, environmental toxicity, high production costs of traditional 3D semiconductor materials and sharply raised contact resistance, severe interfacial recombination of 2D materials and 2D/3D mixed dimension heterojunction. Here, 1D p-Te/2D n-Bi₂Te₃ heterojunctions are constructed by the simple and low-cost hydrothermal method. 1D p-Te/2D n-Bi₂Te₃ devices are applied in photoelectrochemical (PEC) photodetectors, with their high performance attributed to the good interfacial contacts reducing interface recombination. The device demonstrated a broad wavelength range (365–850 nm) with an/_ph//_dark as high as 377.45. The R_iD*, and external quantum efficiency (EQE) values of the device were as high as 12.07 mA/W, 5.87 × 10¹⁰ Jones, and 41.05%, respectively, which were significantly better than the performance of the prepared Bi₂Te₃ and Te devices. A comparison of the freshly fabricated device and the device after 30 days showed that 1D p-Te/2D n-Bi₂Te₃ had excellent stability with only 18.08% decay of photocurrent. It is anticipated that this work will provide new emerging material for future design and preparation of a high-performance self-driven broadband photodetector.

Nature Communications, Published online: 29 July 2023; doi:10.1038/s41467-023-40341-7

Data security of internet is increasingly more demanding in the current era, yet the traditional electronic approach is limited in speed and efficiency. Here, the authors proposed a dual-layer optical encryption fluorescent polymer waveguide chip based on optical pulse-code modulation to mitigate the limitations.

This article presents an approach for broadening the spectral range of complementary metal–oxide–semiconductor (CMOS) image sensors. Surface nanostructuring is employed to achieve a device that is highly sensitive from ultraviolet (UV) to visible wavelengths. The same technique can be implemented in many modern image sensors to replace inefficient antireflective coatings and allow imaging of the challenging UV spectrum.

Abstract

Even though the recent progress made in complementary metal–oxide–semiconductor (CMOS) image sensors (CIS) has enabled numerous applications affecting our daily lives, the technology still relies on conventional methods such as antireflective coatings and ion-implanted back-surface field to reduce optical and electrical losses resulting in limited device performance. In this work, these methods are replaced with nanostructured surfaces and atomic layer deposited surface passivation. The results show that such surface nanoengineering applied to a commercial backside illuminated CIS significantly extends its spectral range and enhances its photosensitivity as demonstrated by >90% quantum efficiency in the 300–700 nm wavelength range. The surface nanoengineering also reduces the dark current by a factor of three. While the photoresponse uniformity of the sensor is seen to be slightly better, possible scattering from the nanostructures can lead to increased optical crosstalk between the pixels. The results demonstrate the vast potential of surface nanoengineering in improving the performance of CIS for a wide range of applications.

Author(s): Binghao Guo, Wangqian Miao, Victor Huang, Alexander C. Lygo, Xi Dai, and Susanne Stemmer

We report a topological phase transition in quantum-confined cadmium arsenide (Cd3As2) thin films under an in-plane Zeeman field when the Fermi level is tuned into the topological gap via an electric field. Symmetry considerations in this case predict the appearance of a two-dimensional Weyl semimet…

[Phys. Rev. Lett. 131, 046601] Published Thu Jul 27, 2023

Nature Nanotechnology, Published online: 27 July 2023; doi:10.1038/s41565-023-01457-5

The development of flexible thermoelectrics is limited by the low power factor and brittleness of materials. Here the authors present strategy to turn Bi2Te3-based single crystals into flexible films with staggered-layer structure while maintaining superior thermoelectric performance.

Nature Nanotechnology, Published online: 27 July 2023; doi:10.1038/s41565-023-01460-w

Using metal–organic chemical vapour deposition, high-crystallinity MoS2 monolayers are grown directly on polymers and thin glass substrates at about 150 °C, thus avoiding any transfer process, preserving the electronic properties of MoS2.

This study reports an ultrawide-bandgap 2D oxide semiconductor, AsSbO₃, with a highly anisotropic structure. The photodetectors fabricated based on the corresponding nanosheets exhibit excellent selectivity and photoresponse properties for the solar-blind UV (200–280 nm) band. Finally, a simple machine vision system is simulated by combining the device performance with a convolutional neural network.

Abstract

Facing the future development trend of miniaturization and intelligence of electronic devices, solar-blind photodetectors based on ultrawide-bandgap 2D semiconductors have the advantages of low dark current, and high signal-to-noise ratio, as well as the features of micro-nanometer miniaturization and multi-functionalization of 2D material devices, which have potential applications in the photoelectric sensor part of high-performance machine vision systems. This study reports a 2D oxide semiconductor, AsSbO₃, with an ultrawide bandgap (4.997 eV for monolayer and 4.4 eV for multilayer) to be used to fabricate highly selective solar-blind UV photodetectors, of which the dark current as low as 100 fA and rejection ratio of UV-C and UV-A reaches 7.6 × 10³. Under 239 nm incident light, the responsivity is 105 mA W⁻¹ and the detectivity is 7.58 × 10¹² Jones. Owing to the remarkable anisotropic crystal structure, AsSbO₃ also shows significant linear dichroism and nonlinear optical properties. Finally, a simple machine vision system is simulated by combining the real-time imaging function in solar-blind UV with a convolutional neural network. This study enriches the material system of ultrawide-bandgap 2D semiconductors and provides insight into the future development of high-performance solar-blind UV optoelectronic devices.

New photodetectors based on low-dimensional materials and reports on their integration on silicon complementary metal-oxide semiconductor chips are presented. Methods for creating and testing high-resolution, multi-spectral, bio-inspired, and smart imaging sensors based on low-dimensional materials are described. Major technical barriers against the widespread use of low-dimensional materials in image sensors are discussed and some promising solutions are presented.

Abstract

The last two decades have witnessed a dramatic increase in research on low-dimensional material with exceptional optoelectronic properties. While low-dimensional materials offer exciting new opportunities for imaging, their integration in practical applications has been slow. In fact, most existing reports are based on single-pixel devices that cannot rival the quantity and quality of information provided by massively parallelized mega-pixel imagers based on complementary metal-oxide semiconductor (CMOS) readout electronics. The first goal of this review is to present new opportunities in producing high-resolution cameras using these new materials. New photodetection methods and materials in the field are presented, and the challenges involved in their integration on CMOS chips for making high-resolution cameras are discussed. Practical approaches are then presented to address these challenges and methods to integrate low-dimensional material on CMOS. It is also shown that such integrations could be used for ultra-low noise and massively parallel testing of new material and devices. The second goal of this review is to present the colossal untapped potential of low-dimensional material in enabling the next-generation of low-cost and high-performance cameras. It is proposed that low-dimensional materials have the natural ability to create excellent bio-inspired artificial imaging systems with unique features such as in-pixel computing, multi-band imaging, and curved retinas.

Optical phonons are known to become confined when the size of the host crystal approaches atomic limits. This confinement offers a unique yet underexplored pathway toward the modification and design of optical phonons in tailored atomic-scale devices. This study sheds light on the criteria for, and ramifications of phonon confinement, paving the way for designer phonon applications.

Abstract

Polar dielectrics are key materials of interest for infrared (IR) nanophotonic applications due to their ability to host phonon-polaritons that allow for low-loss, subdiffractional control of light. The properties of phonon-polaritons are limited by the characteristics of optical phonons, which are nominally fixed for most “bulk” materials. Superlattices composed of alternating atomically thin materials offer control over crystal anisotropy through changes in composition, optical phonon confinement, and the emergence of new modes. In particular, the modified optical phonons in superlattices offer the potential for so-called crystalline hybrids whose IR properties cannot be described as a simple mixture of the bulk constituents. To date, however, studies have primarily focused on identifying the presence of new or modified optical phonon modes rather than assessing their impact on the IR response. This study focuses on assessing the impact of confined optical phonon modes on the hybrid IR dielectric function in superlattices of GaSb and AlSb. Using a combination of first principles theory, Raman, FTIR, and spectroscopic ellipsometry, the hybrid dielectric function is found to track the confinement of optical phonons, leading to optical phonon spectral shifts of up to 20 cm⁻¹. These results provide an alternative pathway toward designer IR optical materials.

Halide perovskite) single crystals are promising for electro-optical and photovoltaic devices. The challenging size- and shape-controlled single-crystal synthesis is demonstrated by separating and optimizing the crystal nucleation-growth processe . The cuboid single-crystals with controlled size show optimal photoluminescence and minimal defects. This method offers a route to synthesize high-quality perovskite single crystals with size- and shape-control for devices.

Abstract

Halide perovskites are ideal for next-generation optical devices and photovoltaics. Although perovskite single-crystals show reproducible optoelectronic properties, significant variations in the crystal size, anisotropy, density, defects, photoluminescence (PL), and carrier lifetime affect the sample properties and device performances. Homogenous size and shape FA/MAPbBr₃ single microcrystals (MCs) with controlled edge lengths, crystal densities, PL lifetimes, and PL intensities are prepared by thermodynamically controlling and kinetically separating the crystal nucleation-growth processes using optimum N-cyclohexyl-2-pyrrolidone (CHP) concentration. The crystal growth kinetics at different CHP concentrations and temperatures are estimated spectroscopically by measuring the concentration of Pb (II). High-density cubic MCs with a homogenous size distribution, high PL intensities, and long PL lifetimes are obtained within minutes at high temperatures by the controlled addition of the pyrrolidone derivative. Conversely, the crystal size nonlinearly increases with time at low temperatures. The isotropically grown high-density single crystals at controlled nucleation-growth rates at 190 °C with 20% CHP show the highest PL intensity and the longest PL lifetimes. This method offers thermodynamic and kinetic control of perovskite single-crystal growth with shape control.

A universal in situ TEM cyclic loading–unloading tensile acceleration test was constructed to study the planar cyclic performance and deformation behavior of three-layer MoS₂ NSs including the elastic deformation, fatigue crack initiation, propagation and fracture. The experimental cyclic deformation fracture strength of the MoS₂ NS was approximately 3.13 GPa, which is far lower than its planar intrinsic strength (>3.97 GPa). The natural overlaying affix fragments or wrinkle folds in the NSs have proved to be effective in enhancing their planar cyclic performance and deformation behavior. The effect of the angle between the natural overlays and external force on the fatigue crack initiation and propagation behavior of the NS is as follows: the effects gradually change from deceleration or termination to acceleration as the angle was increased from 0° to 90°.

Abstract

As a typical two-dimensional (2D) transition metal dichalcogenides (TMDCs) material with nonzero band gap, MoS₂ has a wide range of potential applications as building blocks in the field of nanoelectronics. The stability and reliability of the corresponding nanoelectronic devices depend critically on the mechanical performance and cyclic reliability of 2D MoS₂. Although an in situ technique has been used to analyze the mechanical properties of 2D materials, the cyclic mechanical behavior, that is, fatigue, remains a major challenge in the practical application of the devices. This study was aimed at analyzing the planar cyclic performance and deformation behavior of three-layer MoS₂ nanosheets (NSs) using an in situ transmission electron microscopy (TEM) variable-amplitude uniaxial low-frequency and cyclic loading–unloading tensile acceleration test. We also elucidated the strengthening effect of the natural overlaying affix fragments (other external NSs) or wrinkle folds (internal folds from the NS itself) on cycling performances and service life of MoS₂ NSs by delaying the whole process of fatigue crack initiation, propagation, and fracture. The results have been confirmed by molecular dynamics (MDs) simulations. The overlaying enhancement effect effectively ensures the long-term reliability and stability of nanoelectronic devices made of few-layer 2D materials.

Nature, Published online: 26 July 2023; doi:10.1038/s41586-023-06452-3

Thermodynamic evidence of fractional Chern insulator in moiré MoTe₂

Nature Electronics, Published online: 26 July 2023; doi:10.1038/s41928-023-01005-y

Transistors pile up

Nature Electronics, Published online: 26 July 2023; doi:10.1038/s41928-023-01008-9

A ferroelectric capacitor that scales

Nature Electronics, Published online: 26 July 2023; doi:10.1038/s41928-023-01006-x

The next generation of gate-all-around transistors

Nature Electronics, Published online: 26 July 2023; doi:10.1038/s41928-023-01009-8

A subdural chip with 65,000 channels

Nature Electronics, Published online: 26 July 2023; doi:10.1038/s41928-023-01007-w

Superconducting radiofrequency transistors

Nature Electronics, Published online: 26 July 2023; doi:10.1038/s41928-023-00986-0

Carbon nanotube transistors with high performance and integration density can be created using a full-contact structure to scale the nanotube–electrode contact length.