Jiuxiang Dai

A design concept of carrier transport/relaxation bilayer amorphous-oxide-semiconductor TFTs is demonstrated to avoid the bucket effect inevitable in single-layer devices. In&Sn-rich InSnZnO/InSnZnO:Pr bilayer TFTs that combine high mobility (75.5 cm² V⁻¹ s⁻¹), robust normalized drain current (225 µA), and satisfactory NBIS/PBTS stability (voltage shift ∼ −1.64/0.76 V) are successfully developed.

Abstract

Amorphous oxide semiconductor thin-film transistors (AOS TFTs) are ever-increasingly utilized in displays. However, to bring high mobility and excellent stability together is a daunting challenge. Here, the carrier transport/relaxation bilayer stacked AOS TFTs are investigated to solve the mobility-stability conflict. The charge transport layer (CTL) is made of amorphous In-rich InSnZnO, which favors big average effective coordination number for all cations and more edge-shared structures for better charge transport. Praseodymium-doped InSnZnO is used as the charge relaxation layer (CRL), which substantially shortens the photoelectron lifetime as revealed by femtosecond transient absorption spectroscopy. The CTL and CRL with the thickness suitable for industrial production respectively afford minute potential barrier fluctuation for charge transport and fast relaxation for photo-generated carriers, resulting in transistors with an ultrahigh mobility (75.5 cm² V⁻¹ s⁻¹) and small negative-bias-illumination-stress/positive-bias-temperature-stress voltage shifts (−1.64/0.76 V). The design concept provides a promising route to address the mobility-stability conflict for high-end displays.

Logic-in-memory “processing with polar textures” at room temperature in HfO₂-ZrO₂ nanolaminates are demonstrated along with 14 Boolean logic operations. Utilizing local probe force microscopy, microscopic evidence of electric field switchable polar nanotexture is provided, which is used to design ultrafast (≈83 ns) nonvolatile multilevel memory with high on/off ratio (>10⁶), long-term durability (>4000 s), and giant tunnel electroresistance (10⁸%).

Abstract

Nontrivial topological polar textures in ferroelectric materials, including vortices, skyrmions, and others, have the potential to develop ultrafast, high-density, reliable multilevel memory storage and conceptually innovative processing units, even beyond the limit of binary storage of 180° aligned polar materials. However, the realization of switchable polar textures at room temperature in ferroelectric materials integrated directly into silicon using a straightforward large area fabrication technique and effectively utilizing it to design multilevel programable memory and processing units has not yet been demonstrated. Here, utilizing vector piezoresponse force and conductive atomic force microscopy, microscopic evidence of the electric field switchable polar nanotexture is provided at room temperature in HfO₂-ZrO₂ nanolaminates grown directly onto silicon using an atomic layer deposition technique. Additionally, a two-terminal Au/nanolaminates/Si ferroelectric tunnel junction is designed, which shows ultrafast (≈83 ns) nonvolatile multilevel current switching with high on/off ratio (>10⁶), long-term durability (>4000 s), and giant tunnel electroresistance (10⁸%). Furthermore, 14 Boolean logic operations are tested utilizing a single device as a proof-of-concept for reconfigurable logic-in-memory processing. The results offer a potential approach to “processing with polar textures” and addressing the challenges of developing high-performance multilevel in-memory processing technology by virtue of its fundamentally distinct mechanism of operation.

Heterosynaptic MoS₂ memtransistors are developed to enable tunable synaptic plasticity by applying paired voltage pulses at the drain and gate terminals serving as presynaptic neurons and neuromodulators, respectively, for energy-efficient neuromorphic electronics. Consequently, a SET voltage of 2.0 V and sub-1 fJ energy consumption are achieved, which are the best recorded values for three-terminal synaptic devices reported so far.

Abstract

Heterosynaptic neuromodulation is a key enabler for energy-efficient and high-level biological neural processing. However, such manifold synaptic modulation cannot be emulated using conventional memristors and synaptic transistors. Thus, reported herein is a three-terminal heterosynaptic memtransistor using an intentional-defect-generated molybdenum disulfide channel. Particularly, the defect-mediated space-charge-limited conduction in the ultrathin channel results in memristive switching characteristics between the source and drain terminals, which are further modulated using a gate terminal according to the gate-tuned filling of trap states. The device acts as an artificial synapse controlled by sub-femtojoule impulses from both the source and gate terminals, consuming lower energy than its biological counterpart. In particular, electrostatic gate modulation, corresponding to biological neuromodulation, additionally regulates the dynamic range and tuning rate of the synaptic weight, independent of the programming (source) impulses. Notably, this heterosynaptic modulation not only improves the learning accuracy and efficiency but also reduces energy consumption in the pattern recognition. Thus, the study presents a new route leading toward the realization of highly networked and energy-efficient neuromorphic electronics.

An Fe-assisted epitaxial strategy to synthesize 4 in. length transition-metal dichalcogenides (TMDCs) single crystals on c-plane sapphire is designed, ultrahigh mobility and remarkable on/off current ratio are discovered due to the ultralow contact resistance. The introduction of Fe decreases the formation energy of parallel steps on sapphire surfaces and contributes to the edge-nucleation of unidirectional alignment of TMDCs domains.

Abstract

Epitaxial growth and controllable doping of wafer-scale atomically thin semiconductor single crystals are two central tasks to tackle the scaling challenge of transistors. Despite considerable efforts are devoted, addressing such crucial issues simultaneously under 2D confinement is yet to be realized. Here, an ingenious strategy to synthesize record-breaking 4 in. length Fe-doped transition-metal dichalcogenides (TMDCs) single crystals on industry-compatible c-plane sapphire without special miscut angle is designed. Atomically thin transistors with high electron mobility (≈146 cm² V⁻¹ s⁻¹) and remarkable on/off current ratio (≈10⁹) are fabricated based on 4 in. length Fe-MoS₂ single crystals, due to the ultralow contact resistance (≈489 Ω µm). In-depth characterizations and theoretical calculations reveal that the introduction of Fe significantly decreases the formation energy of parallel steps on sapphire surfaces and contributes to the edge-nucleation of unidirectional alignment TMDCs domains (>99%). This work represents a substantial leap in terms of bridging synthesis and doping of wafer-scale 2D semiconductor single crystals, which should promote the further device downscaling and extension of Moore's law.

2D non-van der Waals materials offer exciting, and unexplored properties for energy storage and other technologically important fields.

Abstract

The development of advanced electrode materials for the next generation of electrochemical energy storage (EES) solutions has attracted profound research attention as a key enabling technology toward decarbonization and electrification of transportation. Since the discovery of graphene's remarkable properties, 2D nanomaterials, derivatives, and heterostructures thereof, have emerged as some of the most promising electrode components in batteries and supercapacitors owing to their unique and tunable physical, chemical, and electronic properties, commonly not observed in their 3D counterparts. This review particularly focuses on recent advances in EES technologies related to 2D crystals originating from non-layered 3D solids (non-van der Waals; nvdW) and their hallmark features pertaining to this field of application. Emphasis is given to the methods and challenges in top-down and bottom-up strategies toward nvdW 2D sheets and their influence on the materials’ features, such as charge transport properties, functionalization, or adsorption dynamics. The exciting advances in nvdW 2D-based electrode materials of different compositions and mechanisms of operation in EES are discussed. Finally, the opportunities and challenges of nvdW 2D systems are highlighted not only in electrochemical energy storage but also in other applications, including spintronics, magnetism, and catalysis.

Defect-free van der Waals contacts have been achieved by utilizing topological Bi₂Se₃ as the electrodes. Such clean and atomically sharp contacts avoid the consumption of photogenerated carriers at the interface, enabling a markedly boosted sensitivity as compared to counterpart devices with directly deposited metal electrodes.

Abstract

With the rapid development of two-dimensional semiconductor technology, the inevitable chemical disorder at a typical metal–semiconductor interface has become an increasingly serious problem that degrades the performance of 2D semiconductor optoelectronic devices. Herein, defect-free van der Waals contacts have been achieved by utilizing topological Bi₂Se₃ as the electrodes. Such clean and atomically sharp contacts avoid the consumption of photogenerated carriers at the interface, enabling a markedly boosted sensitivity as compared to counterpart devices with directly deposited metal electrodes. Typically, the device with 2D WSe₂ channel realizes a high responsivity of 20.5 A W⁻¹, an excellent detectivity of 2.18 × 10¹² Jones, and a fast rise/decay time of 41.66/38.81 ms. Furthermore, high-resolution visible-light imaging capability of the WSe₂ device is demonstrated, indicating its promising application prospect in future optoelectronic systems. More inspiringly, the topological electrodes are universally applicable to other 2D semiconductor channels, including WS₂ and InSe, suggesting its broad applicability. These results open fascinating opportunities for the development of high-performance electronics and optoelectronics.

The surface reconstruction of Lattice oxygen oxidation mechanism (LOM)-based metal oxides in alkaline medium was unveiled to be triggered by a spontaneous chemical reaction, instead of an electrochemical reaction. During the chemical process, the activated lattice oxygen atoms were attacked by adsorbed water molecules firstly, leading to the formation of hydroxide ions (OH⁻). Then, the metal-site soluble atoms leached from the oxygen-deficient surface.

Abstract

A fundamental understanding of surface reconstruction process is pivotal to developing highly efficient lattice oxygen oxidation mechanism (LOM) based electrocatalysts. Traditionally, the surface reconstruction in LOM based metal oxides is believed as an irreversible oxygen redox behavior, due to the much slower rate of OH⁻ refilling than that of oxygen vacancy formation. Here, we found that the surface reconstruction in LOM based metal oxides is a spontaneous chemical reaction process, instead of an electrochemical reaction process. During the chemical process, the lattice oxygen atoms were attacked by adsorbed water molecules, leading to the formation of hydroxide ions (OH⁻). Subsequently, the metal-site soluble atoms leached from the oxygen-deficient surface. This work also suggests that the enhancement of surface hydrophilicity could accelerate the surface reconstruction process. Hence, such a finding could add a new layer for the understanding of surface reconstruction mechanism.

A molecular sieve-assisted strategy is used to modify the phase of 2D iron sulfide on mica by chemical vapor deposition (CVD) method. By controlling the growth of S concentration and temperature and precursor, non-stoichiometric Fe₇S₈ nanoflakes, non-layered 2D hexagonal FeS nanoflakes, and layered tetragonal FeS nanoflakes can be precisely grown with high quality.

Abstract

2D Fe-chalcogenides have drawn significant attention due to their unique structural phases and distinct properties in exploring magnetism and superconductivity. However, it remains a significant challenge to synthesize 2D Fe-chalcogenides with specific phases in a controllable manner since Fe-chalcogenides have multiple phases. Herein, a molecular sieve-assisted strategy is reported for synthesizing ultrathin 2D iron sulfide on substrates via the chemical vapor deposition method. Using a molecular sieve and tuning growth temperatures to control the partial pressures of precursor concentrations, hexagonal FeS, tetragonal FeS, and non-stoichiometric Fe₇S₈ nanoflakes can be precisely synthesized. The 2D h-FeS, t-FeS, and Fe₇S₈ have high conductivities of 5.4 × 10⁵ S m⁻¹, 5.8 × 10⁵ S m⁻¹, and 1.9 × 10⁶ S m⁻¹. 2D tetragonal FeS shows a superconducting transition at 4 K. The spin reorientation at ≈30 K on the non-stoichiometric Fe₇S₈ nanoflakes with ferrimagnetism up to room temperature has also been observed. The controllable synthesis of various phases of 2D iron sulfide may provide a route for synthesizing other 2D compounds with various phases.

Wafer-scale single crystal h-BN layers with controllable thickness from monolayer to tens of nanometers were successfully grown on dielectric sapphire substrates by a facile catalyst-free submicron-spacing vapor deposition method using a boron film as the solid source. The epitaxial h-BN layer exhibits extremely high crystalline quality, paving the way for future two-dimensional semiconductor-based device applications.

Abstract

The direct growth of wafer-scale single crystal two-dimensional (2D) hexagonal boron nitride (h-BN) layer with a controllable thickness is highly desirable for 2D-material-based device applications. Here, for the first time, a facile submicron-spacing vapor deposition (SSVD) method is reported to achieve 2-inch single crystal h-BN layers with controllable thickness from monolayer to tens of nanometers on the dielectric sapphire substrates using a boron film as the solid source. In the SSVD growth, the boron film is fully covered by the same-sized sapphire substrate with a submicron spacing, leading to an efficient vapor diffusion transport. The epitaxial h-BN layer exhibits extremely high crystalline quality, as demonstrated by both a sharp Raman E _2g vibration mode (12 cm⁻¹) and a narrow X-ray rocking curve (0.10°). Furthermore, a deep ultraviolet photodetector and a ZrS₂/h-BN heterostructure fabricated from the h-BN layer demonstrate its fascinating properties and potential applications. This facile method to synthesize wafer-scale single crystal h-BN layers with controllable thickness paves the way to future 2D semiconductor-based electronics and optoelectronics.

Supramolecular assemblies shifting from 2D to 3D revealed by scanning tunneling microscopy/spectroscopy (STM/STS) provide insights into the design of supramolecular architectures with increasing complexity and desired functionality. This review summarizes STM/STS studies of several molecular systems with an emphasis on the unique characteristics and electronic properties of multilayered assembled structures at the liquid–solid interface.

Abstract

Exploring supramolecular architectures at surfaces plays an increasingly important role in contemporary science, especially for molecular electronics. A paradigm of research interest in this context is shifting from 2D to 3D that is expanding from monolayer, bilayers, to multilayers. Taking advantage of its high-resolution insight into monolayers and a few layers, scanning tunneling microscopy/spectroscopy (STM/STS) turns out a powerful tool for analyzing such thin films on a solid surface. This review summarizes the representative efforts of STM/STS studies of layered supramolecular assemblies and their unique electronic properties, especially at the liquid–solid interface. The superiority of the 3D molecular networks at surfaces is elucidated and an outlook on the challenges that still lie ahead is provided. This review not only highlights the profound progress in 3D supramolecular assemblies but also provides researchers with unusual concepts to design surface supramolecular structures with increasing complexity and desired functionality.

Nature Physics, Published online: 16 March 2023; doi:10.1038/s41567-023-01947-2

Nature Materials, Published online: 16 March 2023; doi:10.1038/s41563-023-01494-4

Nature Materials, Published online: 16 March 2023; doi:10.1038/s41563-023-01498-0

BaMnO₃ rods, synthesized via a low-temperature low-cost method, are characterized by the 2H-perovskite structure. Despite their crystalline nature, the rods are coated in an amorphous surface layer. The rods display electrocatalytic activity for the oxygen reduction reaction, originating from the presence of reduced Mn states within this layer.

Abstract

In relation to perovskites, tweaking the oxidation state of the B-site cation is fundamental to controlling the catalytic activity of these materials, thus necessitating a complete characterization of surface oxidation states. Herein, using a combination of atomic-scale imaging and spectroscopic techniques, structure-property correlation in barium manganese oxide (BaMnO₃) is established for the oxygen reduction reaction (ORR). Electron energy loss spectroscopy (EELS) on the synthesized BaMnO₃ find the rods to contain an amorphous surface layer with reduced Mn³⁺ states compared to Mn⁴⁺ states in the bulk. Consequently, the BaMnO₃ rods show electrocatalytic activity for the ORR, which originates from the presence of Mn³⁺ at the rod surface. Furthermore, heating of the samples in air at 300 and 800 °C results in a decrease in the number of Mn³⁺ states, and thus lowering of the ORR activity. This study represents a step-stone study in understanding the mechanism of ORR activity and its association to the Mn³⁺ state at the perovskite's surface, opening up possibilities for further surface engineering and tuning catalytic properties.

Nature, Published online: 15 March 2023; doi:10.1038/s41586-023-05800-7

Nature, Published online: 15 March 2023; doi:10.1038/s41586-023-05724-2