Jiuxiang Dai

Nature Reviews Methods Primers, Published online: 14 July 2022; doi:10.1038/s43586-022-00133-7

Nature Nanotechnology, Published online: 14 July 2022; doi:10.1038/s41565-022-01184-3

Light: Science & Applications, Published online: 13 July 2022; doi:10.1038/s41377-022-00871-z

Author(s): Md. S. Hossain, M. K. Ma, K. A. Villegas-Rosales, Y. J. Chung, L. N. Pfeiffer, K. W. West, K. W. Baldwin, and M. Shayegan

Researchers have made electrons crystallize into an anisotropic structure, which could lead to new insights into quantum many-body systems.

[Phys. Rev. Lett. 129, 036601] Published Wed Jul 13, 2022

Robust room-temperature ferroelectric modulated resonant tunneling and negative differential resistance (NDR) behaviors under external voltage in perovskite oxide BaTiO₃/SrRuO₃/BaTiO₃ quantum-well structures are reported. The NDR ratio can be modified by ≈3 orders of magnitude, depending on the polarization states of the ferroelectric barrier, and a large OFF/ON resistance ratio up to 23 000 is obtained.

Abstract

Resonant tunneling is a quantum-mechanical effect in which electron transport is controlled by the discrete energy levels within a quantum-well (QW) structure. A ferroelectric resonant tunneling diode (RTD) exploits the switchable electric polarization state of the QW barrier to tune the device resistance. Here, the discovery of robust room-temperature ferroelectric-modulated resonant tunneling and negative differential resistance (NDR) behaviors in all-perovskite-oxide BaTiO₃/SrRuO₃/BaTiO₃ QW structures is reported. The resonant current amplitude and voltage are tunable by the switchable polarization of the BaTiO₃ ferroelectric with the NDR ratio modulated by ≈3 orders of magnitude and an OFF/ON resistance ratio exceeding a factor of 2 × 10⁴. The observed NDR effect is explained an energy bandgap between Ru-t_2g and Ru-e_g orbitals driven by electron–electron correlations, as follows from density functional theory calculations. This study paves the way for ferroelectric-based quantum-tunneling devices in future oxide electronics.

In this article, time-resolved momentum-microscopy is highlighted as a powerful new technique for characterizing photoexcited two-dimensional materials in momentum-space. The recent scientific progress that is made thanks to this technique is surveyed. The future directions in which it will be impactful are outlined: the dynamics of photoexcitations, and the control of the band-structure by Floquet band engineering.

Abstract

Van der Waals (vdW) materials at their 2D limit are diverse, flexible, and unique laboratories to study fundamental quantum phenomena and their future applications. Their novel properties rely on their pronounced Coulomb interactions, variety of crystal symmetries and spin-physics, and the ease of incorporation of different vdW materials to form sophisticated heterostructures. In particular, the excited state properties of many 2D semiconductors and semi-metals are relevant for their technological applications, particularly those that can be induced by light. In this paper, the recent advances made in studying out-of-equilibrium, light-induced, phenomena in these materials are reviewed using powerful, surface-sensitive, time-resolved photoemission-based techniques, with a particular emphasis on the emerging multi-dimensional photoemission spectroscopy technique of time-resolved momentum microscopy. The advances this technique has enabled in studying the nature and dynamics of occupied excited states in these materials are discussed. Then, the future research directions opened by these scientific and instrumental advancements are projected for studying the physics of 2D materials and the opportunities to engineer their band-structure and band-topology by laser fields.

2D Si₉C₁₅ monolayer is fabricated on Ru (0001) and Rh(111) substrates via the reaction between Si and graphene under high temperatures. The as-grown Si₉C₁₅ layer is micrometer-scale, high quality, and single crystalline with a bandgap of ≈1.9 eV. Combined measurements and density functional calculations confirm the buckled honeycomb structure. The novel 2D material also shows good air stability.

Abstract

Monolayer Si _x C _y constitutes an important family of 2D materials that is predicted to feature a honeycomb structure and appreciable bandgaps. However, due to its binary chemical nature and the lack of bulk polymorphs with a layered structure, the fabrication of such materials has so far been challenging. Here, the synthesis of atomic monolayer Si₉C₁₅ on Ru (0001) and Rh(111) substrates is reported. A combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), and density functional theory (DFT) calculations is used to infer that the 2D lattice of Si₉C₁₅ is a buckled honeycomb structure. Monolayer Si₉C₁₅ shows semiconducting behavior with a bandgap of ≈1.9 eV. Remarkably, the Si₉C₁₅ lattice remains intact after exposure to ambient conditions, indicating good air stability. The present work expands the 2D-materials library and provides a promising platform for future studies in nanoelectronics and nanophotonics.

Nature Communications, Published online: 12 July 2022; doi:10.1038/s41467-022-31612-w

Flexible fully-carbon-integrated transistors are realized based on mixed-dimensional van der Waals (vdW) heterojunctions. This work combines the advantages of vdW engineering and carbon-based materials, achieves expected electrical and mechanical performance, and studies the charge transfer mechanism involved. Either the strategy for all-vdW transistor realization or the charge transfer mechanism provides general approach to improve device performance.

Abstract

Flexible electronics draw intense interest because of their promising potential for emerging applications, which, however, encounter challenging obstacles of material self-limiting fabrication, trade-off mechanical flexibility, and associated moderate electrical performance. Here, wafer-level flexible fully-carbon-integrated transistors via mixed-dimensional van der Waals (vdW) engineering is realized. Remarkable performance includes subthreshold swing of 51.8 mV dec⁻¹ breaking thermionic limit, outstanding field-effect mobility as high as 313.8 cm² V⁻¹ s⁻¹, and sub-1 V operating voltage. The charge transfer modulation of graphene oxide on carbon nanotube in the vdW-integrated transistors is designed to enhance channel conductance, which is simultaneously confirmed by theoretical calculations and electrical characterizations. Besides, the transistors maintain stable electrical performance after bending under an ultra-small radius of 250 µm. Exponential-sensitivity temperature sensors and binary-logic inverters are further realized to demonstrate the feasibility of the devices as the building blocks of all-vdW electronics. These results indicate that either the strategy of all-vdW transistor realization or the charge transfer provides general approach to improve device performance and further advance flexible electronic technologies.

Taking advantage of the ferroelectric-semiconducting coupling property of α-In₂Se₃ and the light-induced ferroelectric flipping effect, photodetectors with sensitivity approaching twenty photons and photoelectric memories with excellent endurance of more than 10⁶ cycles are achieved on the ferroelectric semiconductor phototransistors with a CMOS-compatible device architecture and operation voltage.

Abstract

Low-light-level photodetections are highly desired in the fields of astronomy and quantum information. However, the existing techniques suffer from high operation voltages and complexity of fabrication, which reduces its compatibility with complementary metal oxide semiconductors (CMOS) based read-out circuit and prevent the use of imaging. Here, a low-light-level phototransistor that employs a photo-induced ferroelectric reversal mechanism in a ferroelectric semiconductor channel: α-In₂Se₃ is demonstrated. It shows a record-low noise-equivalent power of 7.9 × 10⁻²² W Hz^−1/2, a record-high specific detectivity of 6.34 × 10¹⁷ Jones, and sensitivity approaching 20 photons in a photon-counting mode, and fast time response of 260 µs/50 ns in the rise/decay period. It also works as an optoelectronic memory with an on/off ratio of 2.9 × 10⁵, retention of longer than 10 years, and endurance of more than 10⁶ cycles. Due to its high performance, simple architecture, and small operation voltage, the phototransistor provides a feasible platform for new-generation low-light-level image sensors.

The recent progress of pentagonal 2D transition metal dichalcogenides, including preparation, defect engineering, physical and chemical properties, as well as their functionalities for various applications are summarized. The promising functional applications in electronics, optoelectronics, catalysts, and sensors are highlighted. A forward-looking outlook of pentagonal 2D transition metal dichalcogenides is also provided.

Abstract

2D materials with common hexagonal crystal structures, such as graphene, hexagonal boron nitride, and transition metal dichalcogenides have attracted great interest due to their novel physical and chemical properties. Pentagonal transition metal dichalcogenides (TMDs) exhibit distinct optical, electrical, and chemical properties, with valuable functionalities for various applications. This review highlights some of the most important developments in this field, with emphasis on their functionalities for neuromorphic computing, transistors, photodetection, catalysts, etc. Strategies for modifying their physical and chemical properties as well as device performance including defect engineering and interface engineering are presented. Finally, a forward-looking outlook of pentagonal 2D materials is discussed.

In this article, Ghulam Dastgeer, and co-workers report an atomically thin vdW stacking of “MoTe₂/GeSe/MoTe₂”, exhibiting excellent output characteristics with a considerably high current gain. The hexagonal surface of MoTe₂ offers an ideal plate-form of π–π stacking for supporter molecules composed of a “pyrene ring” loaded with “lysine-biotin” to detect the “streptavidin” biomolecules as analytes.

Abstract

Bipolar junction transistors (BJTs), the basic building blocks of integrated circuits, are deployed to control switching applications and logic operations. However, as the thickness of a conventional BJT device approaches a few atoms, its performance decreases substantially. The stacking of atomically thin 2D semiconductor materials is advantageous for manufacturing atomically thin BJT devices owing to the high carrier density of electrons and holes. Here, an atomically thin n–p–n BJT device composed of heavily doped molybdenum ditelluride (n-MoTe₂) and germanium selenide (p-GeSe) sheets stacked over each other by van der Waals interactions is reported. In a common-emitter configuration, MoTe₂/GeSe/MoTe₂ BJT devices exhibit a considerably high current gain (β  =  I _c /I _b = 29.3) at V _be = 2.5 V. The MoTe₂/GeSe/MoTe₂ BJT device is employed to detect streptavidin biomolecules as analytes within <10 s. The real-time response of the functionalized BJT device is examined at various concentrations of streptavidin biomolecules ranging from 250 to 5 pm. Such vdW BJT devices can trigger the development of state-of-the-art electronic devices that can be used as biosensors to detect the various kinds of target DNA and proteins like spike protein of Covid-19.

In-plane heterostructure light-emitting diode is realized using various combinations of chemical vapor deposition-grown transition metal dichalcogenide monolayers. The sharp and strained heterostructure makes it possible to capture the electroluminescence (EL) fixed along the heterojunction interfaces, which results in circularly polarized EL at room temperature. These results pave the way to expand the potential of monolayer in-plane heterostructures for use in quantum optoelectronic devices.

Abstract

Atomically thin transition metal dichalcogenides (TMDCs) are attractive materials for future optoelectronic applications because of their excellent electrical, optical, and quantum (spin-valley) properties. In particular, in-plane heterostructures based on TMDC monolayers provide opportunities to directly modulate band structures and lattice strains by the spatial distribution of constituent elements, leading to efficient control of their carrier transport and recombination. However, it is still challenging to create light-emitting devices using such in-plane heterostructures because of the technical difficulties associated with sample/device fabrication. This study demonstrated interfacial electroluminescence (EL) in diverse TMDC monolayer in-plane heterostructures. Various combinations of large-area, single-crystalline in-plane heterostructures with sharp interfaces are grown by chemical vapor deposition, followed by the adoption of electrolyte-based light-emitting devices to observe EL. The fine heterostructures enabled the capture of the linear-shaped EL fixed along the junction interfaces. Significantly, the WS₂/WSe₂ in-plane heterostructures exhibited circularly polarized EL with polarizability of 10% at room temperature. This can be explained by the interfacial strain-mediated electronic structure evolution, in which the combination of electric fields and strain-induced valley drifts realizes selective EL from the K/K’ valley. These findings pave the way for expanding the potential of monolayer in-plane heterostructures for use in functional optoelectronic devices.

This work exploits the bidirectionality of interlayer charge transfer in graphene-transition metal dichalcogenide heterostructures and demonstrates a reverse charge transfer mechanism to detect sub-bandgap NIR photons. This novel mechanism allows fast and repeatable detection of low energy photons (<E _g) with a photoresponsivity as high as ≈3000 A W⁻¹ close to the communication wavelength.

Abstract

Binary van der Waals heterostructures of graphene (Gr) and transition metal dichalcogenide (TMDC) have evolved as a promising candidate for photodetection with very high responsivity due to the separation of photo-excited electron–hole pairs across the interface. The spectral range of optoelectronic response in such hybrids has so far been limited by the optical bandgap of the light absorbing TMDC layer. Here, the bidirectionality of interlayer charge transfer is utilized for detecting sub-band gap photons in Gr-TMDC heterostructures. A Gr/MoSe₂ heterostructure sequentially driven by visible and near infra-red (NIR) photons is employed, to demonstrate that NIR induced back transfer of charge allows fast and repeatable detection of the low energy photons (less than the optical band gap of the TMDC layer). This mechanism provides photoresponsivity as high as ≈3000 A W⁻¹ close to the communication wavelength. The experiment provides a new strategy for achieving highly efficient photodetection over a broad range of energies beyond the spectral bandgap with the 2D semiconductor family.

Abstract

Vertical van der Waals (vdW) heterostructures composed of two-dimensional (2D) layered materials have recently attracted substantial interests due to their unique properties. However, the direct synthesis of moiré superlattice remains a great challenge due to the difficulties in heterogeneous nucleation on smooth vdW surfaces. Here, we report a controllable chemical vapor deposition growth of complete monolayer WS₂ on highly ordered pyrolytic graphite (HOPG) substrates through the plasma pretreatment. The results show that the morphologies of the grown WS₂ have a strong dependence on the plasma parameters, including gas composition, source power, and treatment time. It is found that the surface C-C bonds are broken in the plasma pretreated HOPG, and the formed small clusters can act as the nucleation sites for the subsequent growth of WS₂. Moreover, the height of clusters dominates the growth mode of WS₂ islands. A transition from a 2D mode to three-dimensional (3D) growth mode occurs when the height is higher than the interlayer spacing of the heterostructure. Besides, diverse moiré superlattices with different twist angles for WS₂/HOPG heterostructures are observed, and the formation mechanism is further analyzed by first-principles calculations.

Abstract

Amorphous materials are one kind of nonequilibrium materials and have become one of the most active research fields. Compared with crystalline solids, the theory of amorphous materials is still in infancy because their characteristic of atomic arrangement is more like liquid and has no long-range periodicity. Recently, as the representative of amorphous materials, amorphous molybdenum sulfide (a-MoS_x) with unique physical and chemical properties has been studied extensively. However, considerable debate surrounds the structure—property relationships of a-MoS_x owing to its diverse Mo-S motifs. Herein, we summarize recent discoveries and research results regarding a-MoS_x, whose structural characteristics, synthetic strategies, formation criteria, and comprehensive applications are discussed in detail. Finally, this review is ended with our personal insights and critical outlooks over the development of a-MoS_x.

In this review, the recent progress of the oxide etching mechanisms of nanostructured metals/metallic oxides revealed by an in situ liquid cell TEM technique are systematically summarized. The challenges and opportunities for utilizing this technique to understand the oxide etching mechanisms are also discussed, aiming to offer guidance for the rational design and engineering of nanomaterials for high-end advanced devices.

Abstract

Oxidative etching in nanostructured metals and metallic oxides plays a fundamental role in numerous areas of chemical synthesis and materials processing for the semiconductor industry. An in-depth understanding of the oxidative etching mechanisms is of great significance to design various fascinating nanomaterials for practical application. In situ liquid cell transmission electron microscopy (TEM) has the merits of high temporal and spatial resolutions in real-time, and thus it can provide solid evidence directly for the dynamic evaluation of oxidative etching in nanostructured materials that occur in solution. Herein, the recent progress of oxidative etching in nanostructured metals and metallic oxides is overviewed. First, the advancements in liquid cells designs are briefly introduced for in situ TEM observation. Subsequently, in situ liquid cell TEM/STEM advances for the oxide etching mechanisms in different surface chemistry surroundings are systematically described. In addition, both the galvanic replacement and electrochemical etching reactions are also discussed. Finally, the challenges and opportunities in utilizing the in situ liquid cell TEM technique to visualize a specific dynamic oxidative etching process are proposed. This review will provide deep insights into dynamic changes in oxidative etching processes of nanostructured materials and assist in formulating design rules for developing high-end advanced devices.