Jiuxiang Dai

The degree of valley polarization at room temperature is found to increase drastically from <5% in monolayer WS₂ to 40% in monolayer WTe_{2 x} S_{2(1- x )} due to enhanced spin-orbit coupling, and is further enhanced to 75% by electrostatically gating the 1T′-WTe₂/WTe_0.12S_1.88 heterostructures, where the use of 1T′-WTe₂ substantially lowers the Schottky barrier height and weakens the Fermi-level pinning.

Abstract

Monolayer ternary tellurides based on alloying different transition metal dichalcogenides (TMDs) can result in new two-dimensional (2D) materials ranging from semiconductors to metals and superconductors with tunable optical and electrical properties. Semiconducting WTe_{2 x} S_{2(1- x )} monolayer possesses two inequivalent valleys in the Brillouin zone, each valley coupling selectively with circularly polarized light (CPL). The degree of valley polarization (DVP) under the excitation of CPL represents the purity of valley polarized photoluminescence (PL), a critical parameter for opto-valleytronic applications. Here, new strategies to efficiently tailor the valley-polarized PL from semiconducting monolayer WTe_{2 x} S_{2(1- x )} at room temperature (RT) through alloying and back-gating are presented. The DVP at RT is found to increase drastically from < 5% in WS₂ to 40% in WTe_0.12S_1.88 by Te-alloying to enhance the spin-orbit coupling. Further enhancement and control of the DVP from 40% up to 75% is demonstrated by electrostatically doping the monolayer WTe_0.12S_1.88 via metallic 1T′-WTe₂ electrodes, where the use of 1T′-WTe₂ substantially lowers the Schottky barrier height (SBH) and weakens the Fermi-level pinning of the electrical contacts. The demonstration of drastically enhanced DVP and electrical tunability in the valley-polarized emission from 1T′-WTe₂/WTe_0.12S_1.88 heterostructures paves new pathways towards harnessing valley excitons in ultrathin valleytronic devices for RT applications.

Multiplexing is of great significance in biology, medicine, and microscopic imaging. Here, a new approach is introduced using the optical nonlinearity of lanthanide-doped upconversion nanoparticles (UCNPs) for super-resolved multiplexing microscopy. By applying a vortex beam, imaging resolution is enhanced and UCNPs are differentiated based on their distinctive nonlinearities. This enables a complementary dimension to spectral, temporal, and polarized dimensions for nanoscale multiplexing.

Abstract

Optical multiplexing for nanoscale object recognition is of great significance within the intricate domains of biology, medicine, anti-counterfeiting, and microscopic imaging. Traditionally, the multiplexing dimensions of nanoscopy are limited to emission intensity, color, lifetime, and polarization. Here, a novel dimension, optical nonlinearity, is proposed for super-resolved multiplexing microscopy. This optical nonlinearity is attributable to the energy transitions between multiple energy levels of the doped lanthanide ions in upconversion nanoparticles (UCNPs), resulting in unique optical fingerprints for UCNPs with different compositions. A vortex beam is applied to transport the optical nonlinearity onto the imaging point-spread function (PSF), creating a robust super-resolved multiplexing imaging strategy for differentiating UCNPs with distinctive optical nonlinearities. The composition information of the nanoparticles can be retrieved with variations of the corresponding PSF in the obtained image. Four channels multiplexing super-resolved imaging with a single scanning, applying emission color and nonlinearity of two orthogonal imaging dimensions with a spatial resolution higher than 150 nm (1/6.5λ), are demonstrated. This work provides a new and orthogonal dimension – optical nonlinearity – to existing multiplexing dimensions, which shows great potential in bioimaging, anti-counterfeiting, microarray assays, deep tissue multiplexing detection, and high-density data storage.

Instant-in-air liquid metal printing facilitates the deposition of ultrathin tin dioxide nanosheet which is used for fabricating ammonia sensors featuring high repeatability, high selectivity, large dynamic range with low detection limit, and insignificant memory effects. A proof of concept for flexible sensors is demonstrated, signifying the high potential of employing liquid metal printing for realizing wearable sensors.

Abstract

Liquid metal-based printing techniques are emerging as an exemplary platform for harvesting non-layered 2D materials with a thickness down to a few nanometres, leading to an ultra-large surface-area-to-volume ratio that is ideal for sensing applications. In this work, the synthesis of 2D tin dioxide (SnO₂) by exfoliating the surface oxide of molten tin is reported which highlights the enhanced sensing capability of the obtained materials to ammonia (NH₃) gas is reported. It is demonstrated that amperometric gas sensors based on liquid metal-derived 2D SnO₂ nanosheets can achieve excellent NH₃ sensing performance at low temperature (150 °C) with and without UV light assistance. Detection over a wide range of NH₃ concentrations (5–500 ppm) is observed, revealing a limit of detection at the parts per billion (ppb) level. The 2D SnO₂ nanosheets also feature excellent cross-interference performance toward different organic and inorganic gas species, showcasing a high selectivity. Further, ab initio DFT calculations reveal the NH₃ adsorption mechanism is dominated by chemisorption with a charge transfer into 2D SnO₂ nanosheets. In addition, a proof of concept for prototype flexible ammonia sensors is demonstrated by depositing 2D SnO₂ on a polyimide substrate, signifying the high potential of employing liquid metal printed SnO₂ for realizing wearable gas sensors.

The exact molecular reaction pathway and crystallization mechanisms of LiNbO₃ nanoparticles under solvothermal conditions are derived. A nonclassical nucleation scheme is demonstrated after the identification of new octanuclear complexes. Upon heating, a frustrated aggregation-mediated crystallization process is evidenced leading to nanocrystals of adjustable size due to the variable ligand binding interactions at the surface of crystalline intermediates.

Abstract

The exact molecular reaction pathway and crystallization mechanisms of LiNbO₃ nanoparticles under solvothermal conditions are derived through extensive time- and temperature-resolved experiments allowing to track all the transient molecular and solid species. Starting with a simple mixing of Li/Nb ethoxides, water addition is used to promote condensation after ligand exchange with different co-solvents including alcohols and glycols of variable carbon-chain length. A nonclassical nucleation scheme is first demonstrated after the identification of new octanuclear complexes with a {Li₄Nb₄O₁₀} core whose solvophobic interactions mediate their aggregation, thus, resulting in a colloidal gel at room-temperature. Upon heating, a more or less frustrated aggregation-mediated crystallization process is then evidenced leading to LiNbO₃ nanocrystals of adjustable mean size between 20 and 100 nm. Such a fine control can be attributed to the variable Nb−OR (R = alkoxy/glycoxy ligand) binding interactions at the surface of crystalline intermediates. Demonstration of such a nonclassical nucleation process and crystallization mechanism for LiNbO₃ not only sheds light on the entire growth process of multifunctional nanomaterials with non-perovskite crystalline structures, but also opens new avenues for the identification of novel bimetallic oxoclusters involved in the formation of several mixed oxides from the aqueous alkoxide route.

Nature Electronics, Published online: 16 November 2023; doi:10.1038/s41928-023-01083-y

Growing materials remotely

Nature Electronics, Published online: 16 November 2023; doi:10.1038/s41928-023-01085-w

Ferroelectricity in zero dimensions

Nature Electronics, Published online: 16 November 2023; doi:10.1038/s41928-023-01051-6

A silicon photonics modulator design approach is proposed, in which the inductive networks and termination resistors are designed in conjunction with the optical phase shifter. A complementary metal–oxide–semiconductor (CMOS) silicon photonics transmitter developed with this approach achieved 112 gigabaud transmission with an energy efficiency better than 1 pJ per bit.

Nature Electronics, Published online: 16 November 2023; doi:10.1038/s41928-023-01072-1

Ring oscillator circuits that operate at gigahertz frequencies and are based on monolayer molybdenum disulfide can be created with the help of a design–technology co-optimization approach.

The intercoupled ferroelectricity of 𝛼-In₂Se₃ is utilized to introduce non-volatile electrostatic doping in ambipolar WSe₂. The fabricated device presents stable p–n to n–p switching, superior rectification ≈10⁶, and low leakage current ≈10⁻¹² A. Furthermore, the switchable shortcircuit current is utilized to demonstrate self-powered, non-volatile memory based on photovoltaic reading.

Abstract

It is impossible to imagine modern electronic circuitry without a p–n junction—an essential building block for transistors, rectifiers, amplifiers, photovoltaics, etc. Conventional fabrication processes (ion implantation or chemical diffusion) result in an immutable potential configuration depriving reconfigurability. In contrast, the superior electrostatic tunability, dangling bonds- and reconstruction-free interfaces are some of the key features of 2D based heterostructures, making them promising candidates for cutting-edge optoelectronic and memory applications. Herein, the intercoupled 2D ferroelectricity of 𝛼-In₂Se₃ is utilized to introduce micron-scale, non-volatile electrostatic doping in ambipolar WSe₂, enabling reconfigurable p–n junction. The actuation mechanism is based on the strong polarization field along the edge topology of In₂Se₃. The fabricated device presents stable p–n to n–p switching, a superior rectification ratio of ≈10⁶, and a low leakage current of ≈10⁻¹² A. Furthermore, the switchable short-circuit current response is utilized to demonstrate a novel self-powered, non-volatile memory based on photovoltaic reading. The ferroelectric non-volatility coupled with the ability to control the device operation using optical and electrical signals paves the way for ultrathin energy-efficient, multi-level optoelectronic and in-memory logic devices.

High concentration of Er³⁺/Yb³⁺ co-doped WS₂ monolayer is creatively prepared by in-situ chemical vapor deposition technique. The performances of the devices (photodetectors and field-effect transistors) are significantly enhanced after rare-earth (RE) co-doping, which can be attributed to the stronger light absorption and higher the transitions of electronic states, providing an excellent strategy for practical application in optoelectronics.

Abstract

Chemical doping is a significant means to modulate bandgap structures and optoelectronic properties of transition metal dichalcogenides (TMDCs). Herein, an Er³⁺/Yb³⁺ co-doped WS₂ monolayer with ultrahigh and tunable concentrations is successfully fabricated by in-situ chemical vapor deposition (CVD) technique. The morphologies, thicknesses, components, and structures of the samples are systemically characterized by optical microscope, atomic force microscopy, Raman, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscope with energy dispersive spectrometer, and high-resolution transmission electron microscopy, respectively. Photoluminescent peaks are enhanced significantly with red shifts, and the absorption is broadened to near-infrared, implying a shrinked bandgap after RE co-doping, which is consistent to the calculation results by density functional theory (DFT). The Er³⁺/Yb³⁺ co-doped WS₂ device demonstrates high carrier mobility, photocurrent, photoresponsivity, external quantum efficiency, and specific detectivity, which are approximately two orders of magnitudes compared with those of the pristine WS₂ device. The values of photoresponsivity and specific detectivity approach 4.8 × 10⁴ A W⁻¹ and 5.5 × 10¹⁴ Jones, respectively, at 20 V bias and 1.77 mW cm⁻² luminescence, which may refresh the records as has been reported. The excellent performances of the WS₂ photodetector prove the effectiveness of Er³⁺/Yb³⁺ co-doping for practical application in optoelectronics.

The overlooked process of partial melting is exploited to achieve continuous optical levels in a single film of the phase-change material Ge₂Sb₂Te₅. The stability of the optical switch is improved by introducing a multi-layer structure containing materials Ge₂Sb₂Te₅ and GeTe, with four discrete and reversible optical levels. The discussion is supported by nanosecond laser switching and multi-physics modeling.

Abstract

The optical properties of phase-change materials (PCMs) can be tuned to multiple levels by controlling the transition between their amorphous and crystalline phases. In multi-material PCM structures, the number of discrete reflectance levels can be increased according to the number of PCM layers. However, the effect of increasing the number of layers on quenching and reversibility has not been thoroughly studied. In this work, the phase-change physics and thermal conditions required for reversible switching of single and multi-material PCM switches are discussed based on thermo-optical phase-change models and laser switching experiments. By using nanosecond laser pulses, 16 different reflectance levels in Ge₂Sb₂Te₅ are demonstrated via amorphization. Furthermore, a multi-material switch based on Ge₂Sb₂Te₅ and GeTe with four discrete reflectance levels is experimentally proven with a reversible multi-level response. The results and design principles presented herein will impact active photonics applications that rely on dynamic multi-level operation, such as optical computing, beam steering, and next-generation display technologies.

This study addresses a crucial need for effective thermal management in electronics by integrating diamonds with semiconductors. The successful transfer of AlGaN/GaN/3C-SiC layers to a large diamond substrate, followed by GaN high electron mobility transistors (HEMT) fabrication, demonstrates a breakthrough. The 3C-SiC/diamond interface remains intact even after annealing at 1100 °C, enabling high-quality GaN crystal growth. GaN HEMTs on diamond exhibit superior performance and thermal resistance reduction compared with those on Si and SiC, suggesting a promising future for enhanced electronic systems.

Abstract

Thermal management is critical in contemporary electronic systems, and integrating diamond with semiconductors offers the most promising solution to improve heat dissipation. However, developing a technique that can fully exploit the high thermal conductivity of diamond, withstand high-temperature annealing processes, and enable mass production is a significant challenge. In this study, the successful transfer of AlGaN/GaN/3C-SiC layers grown on Si to a large-size diamond substrate is demonstrated, followed by the fabrication of GaN high electron mobility transistors (HEMTs) on the diamond. Notably, no exfoliation of 3C-SiC/diamond bonding interfaces is observed even after annealing at 1100 °C, which is essential for high-quality GaN crystal growth on the diamond. The thermal boundary conductance of the 3C-SiC-diamond interface reaches ≈55 MW m⁻² K⁻¹, which is efficient for device cooling. GaN HEMTs fabricated on the diamond substrate exhibit the highest maximum drain current and the lowest surface temperature compared to those on Si and SiC substrates. Furthermore, the device thermal resistance of GaN HEMTs on the diamond substrate is significantly reduced compared to those on SiC substrates. These results indicate that the GaN/3C-SiC on diamond technique has the potential to revolutionize the development of power and radio-frequency electronics with improved thermal management capabilities.

Author(s): Matteo Zanfrognini, Alexandre Plaud, Ingrid Stenger, Frédéric Fossard, Lorenzo Sponza, Léonard Schué, Fulvio Paleari, Elisa Molinari, Daniele Varsano, Ludger Wirtz, François Ducastelle, Annick Loiseau, and Julien Barjon

Despite its simple crystal structure, layered boron nitride features a surprisingly complex variety of phonon-assisted luminescence peaks. We present a combined experimental and theoretical study on ultraviolet-light emission in hexagonal and rhombohedral bulk boron nitride crystals. Emission spectr…

[Phys. Rev. Lett. 131, 206902] Published Wed Nov 15, 2023

Nature, Published online: 15 November 2023; doi:10.1038/s41586-023-06663-8

Scanning tunnelling microscopy imaging of the correlated phases of magic-angle twisted trilayer graphene shows marked signatures of interaction-driven spatial symmetry breaking.

Reducing the thickness of indium oxide, enabled by atomic layer deposition (ALD) process, can tune its material properties to achieve high performance devices beyond the capabilities of conventional oxide semiconductors. In this work, the history leading to the re-emergence of indium oxide, its fundamental material properties, growth techniques with a focus on ALD, and state-of-the-art indium oxide device research are reviewed.

Abstract

Amorphous oxide semiconductor transistors have been a mature technology in display panels for upward of a decade, and have recently been considered as promising back-end-of-line compatible channel materials for monolithic 3D applications. However, achieving high-mobility amorphous semiconductor materials with comparable performance to traditional crystalline semiconductors has been a long-standing problem. Recently it has been found that greatly reducing the thickness of indium oxide, enabled by an atomic layer deposition (ALD) process, can tune its material properties to achieve high mobility, high drive current, high on/off ratio, and enhancement-mode operation at the same time, beyond the capabilities of conventional oxide semiconductor materials. In this work, the history leading to the re-emergence of indium oxide, its fundamental material properties, growth techniques with a focus on ALD, state-of-the-art indium oxide device research, and the bias stability of the devices are reviewed.

The unique structural features of d-H,M-Nb₂O₅ promote swift lithium-ion transport, facillitating fast charging, while also maintaining excellent capacity retention and an extended cyclying lifespan.

Abstract

The development of low-cost, high-power lithium-ion batteries requires durable anode materials that can store and release lithium quickly. Here a mixed phase of H-Nb₂O₅ and M-Nb₂O₅ (denoted as d-H,M-Nb₂O₅) that demonstrates excellent performance as an anode material for lithium storage is reported. Experimental and computational analyses reveal several salient features of d-H,M-Nb₂O₅. First, the edge-sharing arrangement between the mixed niobium oxygen polyhedral block structures helps alleviate volume expansion during cycling, thereby enhancing stability and reversibility. Second, the mixed-phase structure facilitates a continuous pathway for lithium ion adsorption These characteristics allow for sequential transport of lithium ions, enabling fast charging. As a result, the d-H,M-Nb₂O₅ electrode material exhibits a high capacity of 142 mAh g⁻¹ at an ultra-fast charging rate of 100 C, while maintaining 85% of its initial capacity after 5000 cycles. Moreover, its practical feasibility is established by demonstrating full-cell performance at a 5 C discharge/charge rate (13 mA cm⁻²), maintaining 79% of its capacity over 1000 cycles.

Nature Electronics, Published online: 13 November 2023; doi:10.1038/s41928-023-01064-1

An in-memory computing chip for vector–matrix multiplication and discrete signal processing applications can be fabricated using floating-gate field-effect transistors based on monolayer molybdenum disulfide.

A 2D float gate transistor with steep threshold memory switching behavior is developed by modifying its semiconductor channel in to depletion mode. The device displays potential in emulating complex hetero-modulated neuron functions, including integrate-and-fire and sigmoid type non-linear activation, which are applied to realize visual recognition tasks in biomimetic manner.

Abstract

Hetero-modulated neural activation is vital for adaptive information processing and learning that occurs in brain. To implement brain-inspired adaptive processing, previously various neurotransistors oriented for synaptic functions are extensively explored, however, the emulation of nonlinear neural activation and hetero-modulated behaviors are not possible due to the lack of threshold switching behavior in a conventional transistor structure. Here, a 2D van der Waals float gate transistor (FGT) that exhibits steep threshold switching behavior, and the emulation of hetero-modulated neuron functions (integrate-and-fire, sigmoid type activation) for adaptive sensory processing, are reported. Unlike conventional FGTs, the threshold switching behavior stems from impact ionization in channel and the coupled charge injection to float gate. When a threshold is met, a sub-30 mV dec⁻¹ increase of transistor conductance by more than four orders is triggered with a typical switch time of approximately milliseconds. Essentially, by feeding light sensing signal as the modulation input, it is demonstrated that two typical tasks that rely on adaptive neural activation, including collision avoidance and adaptive visual perception, can be realized. These results may shed light on the emulation of rich hetero-modulating behaviors in biological neurons and the realization of biomimetic neuromorphic processing at low hardware cost.