Jiuxiang Dai

This review introduces novel synthetic strategies to produce large-area single crystal thin films of various materials and discusses the technological challenges of dealing with the isostructured crystals (3D materials such as metals, III-V semiconductors, oxide perovskites, halide perovskites, organic semiconductors) and anisotropic crystals (2D materials such as graphene, hBN, and metal chalcogenides).

Abstract

Wafer-scale growth of single crystal thin films of metals, semiconductors, and insulators is crucial for manufacturing high-performance electronic and optical devices, but still challenging from both scientific and industrial perspectives. Recently, unconventional advanced synthetic approaches have been attempted and have made remarkable progress in diversifying the species of producible single crystal thin films. This review introduces several new synthetic approaches to produce large-area single crystal thin films of various materials according to the concepts and principles.

Nature Communications, Published online: 20 June 2022; doi:10.1038/s41467-022-31174-x

The preparation of quantum silicon nanowires, materials with potential application in high-performance nanodevices, is challenging. Here, the authors synthesize vertically aligned sub-5 nm silicon nanowires via a vapor phase silicon etching process; the resulting material features unusual lattice reduction and significant phonon and electronic confinement effects.

Abstract

Broken-gap (type-III) two-dimensional (2D) van der Waals heterostructures (vdWHs) offer an ideal platform for interband tunneling devices due to their broken-gap band offset and sharp band edge. Here, we demonstrate an efficient control of energy band alignment in a typical type-III vdWH, which is composed of vertically-stacked molybdenum telluride (MoTe₂) and tin diselenide (SnSe₂), via both electrostatic and optical modulation. By a single electrostatic gating with hexagonal boron nitride (h-BN) as the dielectric, a variety of electrical transport characteristics including forward rectifying, Zener tunneling, and backward rectifying are realized on the same heterojunction at low gate voltages of ±1 V. In particular, the heterostructure can function as an Esaki tunnel diode with a room-temperature negative differential resistance. This great tunability originates from the atomically-flat and inert surface of h-BN that significantly suppresses the interfacial trap scattering and strain effects. Upon the illumination of an 885 nm laser, the band alignment of heterojunction can be further tuned to facilitate the direct tunneling of photogenerated charge carriers, which leads to a high photocurrent on/off ratio of > 10⁵ and a competitive photodetectivity of 1.03 × 10¹² Jones at zero bias. Moreover, the open-circuit voltage of irradiated heterojunction can be switched from positive to negative at opposite gate voltages, revealing a transition from accumulation mode to depletion mode. Our findings not only promise a simple strategy to tailor the bands of type-III vdWHs but also provide an in-depth understanding of interlayer tunneling for future low-power electronic and optoelectronic applications.

npj 2D Materials and Applications, Published online: 21 June 2022; doi:10.1038/s41699-022-00318-4

Low-energy Se ion implantation in MoS₂ monolayers

Light: Science & Applications, Published online: 20 June 2022; doi:10.1038/s41377-022-00877-7

The image depicts a schematic illustration of a van der Waals heterostructure used to electrically activate quantum emitters in hexagonal boron nitride.

Abstract

Metastable materials offer a broad and novel platform for the development of next-generation science and technology. Phase engineering including synthesis of materials with unconventional phases and phase transition of metastable materials has been explored in layered materials but has not tackled their anisotropy issue yet. The high anisotropy in layered materials further adds the cost of orientation screening of materials. Herein, we report the effect of Ag doping on facilitating the formation of metastable π-cubic phase SnS during the solvothermal synthesis process. On this basis, we construct cubic-to-orthorhombic (CTO) samples and elucidate the intrinsic mechanisms of its nearly isotropic thermoelectric properties by characterizing the texturing information and analyzing the valence charge density calculated by density functional theory (DFT). This work demonstrates a convenient approach to synthesize layered materials with isotropic electrical and thermal transport behaviors through a precursor of metastable phase.

Nature Materials, Published online: 21 June 2022; doi:10.1038/s41563-022-01315-0

Publisher Correction: Unconventional excitonic states with phonon sidebands in layered silicon diphosphide

Different optical functions were achieved by regulating the structures of the aggregation-induced emission luminogens (AIEgens)–sub-1 nm nanowire (SNW) co-assemblies. The helical and parallel co-assembly of AIEgens and SNWs lead to circularly polarized luminescence and linearly polarized luminescence activity, respectively.

Abstract

Sub-1 nm nanowires (SNWs) combine the properties of inorganic materials and polymers. They can be highly oriented through assembly, and can also be easily processed. Meanwhile, aggregation-induced emission luminogens (AIEgens) show high potential for optical applications, but they are usually hard to process. The combination of SNWs and AIEgens can enrich both of their applications. In this study, we report that the fluorescence emission intensity of the AIEgens–SNW dispersion is dramatically enhanced due to the flexibility of SNWs. Furthermore, we fabricate two kinds of functional films with circularly polarized luminescence (CPL) and linearly polarized luminescence (LPL) activities. The construction of CPL materials didn't require any chiral chemicals. The construction of LPL materials didn't require an additional stretching process. As a result, we endowed common achiral AIEgens with a high dissymmetry factor of 0.033 and a polarization ratio of 0.44, respectively.

A new efficient post-ionization strategy for MALDI-MS imaging is presented: Simple RF-Kr lamps were pulsed and synchronized with a MALDI laser using custom-made electronics. Initial photoionization of dopant vapor in the fine-vacuum ion-funnel source yielded >100× signal boosts of numerous lipid classes from mammal tissue sections. Deuterated components indicated complex gas-phase ionization pathways generating fragment-less [M+H]⁺/[M−H]⁻ species.

Abstract

Matrix-assisted laser desorption/ionization mass spectrometry imaging (MALDI-MSI) is a rapidly growing method in the life sciences. However, for many analyte classes, its sensitivity is limited due to poor ionization efficiencies. To mitigate this problem, we here introduce a novel post-ionization scheme based on single-photon induced chemical ionization using pulsed RF-Kr lamps. The fine-vacuum conditions of a dual ion-funnel ion source effectively thermalize the evolving MALDI plume and enable ample gas-phase reactions. Injected chemical dopants crucially support fragment-less ionization to [M+H]⁺/[M−H]⁻ species. Based on this interplay, numerous glycerophospho-, sphingo-, and further lipids, registered from mammalian tissue sections, were boosted by up to three orders of magnitude, similar to results obtained with laser-based post-ionization (MALDI-2). Experiments with deuterated matrix and dopant, however, indicated complex chemical ionization pathways different from MALDI-2.

A pass-transistor logic configuration based on pseudo-NMOS is realized on a 4-inch high-quality monolayer MoS₂ wafer. Such preliminary integration efforts exhibit a promising future for 2D semiconductors in integrated circuit application.

Abstract

2D semiconductors, such as molybdenum disulfide (MoS₂), have attracted tremendous attention in constructing advanced monolithic integrated circuits (ICs) for future flexible and energy-efficient electronics. However, the development of large-scale ICs based on 2D materials is still in its early stage, mainly due to the non-uniformity of the individual devices and little investigation of device and circuit-level optimization. Herein, a 4-inch high-quality monolayer MoS₂ film is successfully synthesized, which is then used to fabricate top-gated (TG) MoS₂ field-effect transistors with wafer-scale uniformity. Some basic circuits such as static random access memory and ring oscillators are examined. A pass-transistor logic configuration based on pseudo-NMOS is then employed to design more complex MoS₂ logic circuits, which are successfully fabricated with proper logic functions tested. These preliminary integration efforts show the promising potential of wafer-scale 2D semiconductors for application in complex ICs.

Abstract

Vertically stacked transition metal dichalcogenide (TMD) heterostructures provide an opportunity to explore optoelectronic properties within the two-dimensional limit. In such structures, spatially indirect interlayer excitons (IXs) can be generated in adjacent layers because of strong Coulomb interactions. However, due to the complexity of the multilayered heterostructure (HS), the capture and study of the IXs in trilayer type-II HSs have so far remained elusive. Here, we present the observation of the IXs in trilayer type-II staggered band alignment of MoS₂/MoSe₂/WSe₂ van der Waals (vdW) HSs by photoluminescence (PL) spectroscopy. The central energy of IX is 1.33 eV, and the energy difference between the extracted double peaks is 23 meV. We confirmed the origin of IX through PL properties and calculations by the density functional theory, we also studied the dependence of the IX emission peak on laser power and temperature. Furthermore, the polarization-resolved PL spectra of HS were also investigated, and the maximum polarizability of the emission peak of WSe₂ reached 11.40% at 6 K. Our findings offer opportunities for the study of new physical properties of excitons in TMD HSs and therefore are valuable for exploring the potential applications of TMDs in optoelectronic devices.

Nature, Published online: 22 June 2022; doi:10.1038/s41586-022-04768-0

Developments, challenges and opportunities in using two-dimensional materials for the next generation of non-volatile spin-based memory technologies are reviewed, and possible disruptive improvements are discussed.

Nature, Published online: 22 June 2022; doi:10.1038/s41586-022-04837-4

An organic bipolar junction transistor composed of highly crystalline rubrene thin films has a device architecture that could be used in organic electronics with greatly improved high-frequency performance

Nature, Published online: 22 June 2022; doi:10.1038/d41586-022-01635-w

A transistor fabricated from the crystalline phase of an organic semiconductor material could provide a path to improved switching speeds — rivalling those of devices built from inorganic materials such as silicon.

npj 2D Materials and Applications, Published online: 23 June 2022; doi:10.1038/s41699-022-00317-5

Exploration of sub-bandgap states in 2D halide perovskite single-crystal photodetector

Nature Materials, Published online: 23 June 2022; doi:10.1038/s41563-022-01301-6

A general method by controlling reaction kinetics is proposed to synthesize 67 kinds of two-dimensional crystal with custom-made phases and compositions, in particular, Fe- and Cr-based (layered and non-layered) chalcogenides and phosphorous chalcogenides, which show interesting ferromagnetism and superconductivity properties.

Nature Materials, Published online: 23 June 2022; doi:10.1038/s41563-022-01291-5

A competitive-chemical-reaction-based growth mechanism by controlling the kinetic parameters can easily realize the growth of transition metal chalcogenides and transition metal phosphorous chalcogenides with different compositions and phases.

Nature Communications, Published online: 23 June 2022; doi:10.1038/s41467-022-31148-z

Internet of things (IoT) sensors can collect, store and communicate large volumes of information, which require effective security measures. Here, the authors report the realization of low-power edge sensors based on photosensitive and programmable 2D memtransistors, integrating sensing, storage and encryption functionalities.

Quantum Dots

In article 2103907, Kuan Eng Johnson Goh and co-workers report electrostatically defined quantum dots in bilayer WS₂ grown by chemical vapor deposition and capped by HfO₂ dielectric using atomic layer deposition. This marks a key milestone in scalable approaches toward 2D-semiconductor-based quantum devices, which had hitherto only been demonstrated with micrometer-sized exfoliated flakes.

Hexagonal boron nitride (h-BN) has attracted great interest motivated by fascinating properties in the fields of quantum optics, electronics, and optoelectronics. The most recent discoveries of structural, optical, and electrical properties of h-BN and advancements in emerging photonic and electronic applications are reviewed.

Abstract

Hexagonal boron nitride (h-BN), an insulating 2D layered material, has recently attracted tremendous interest motivated by the extraordinary properties it shows across the fields of optoelectronics, quantum optics, and electronics, being exotic material platforms for various applications. At an early stage of h-BN research, it is explored as an ideal substrate and insulating layers for other 2D materials due to its atomically flat surface that is free of dangling bonds and charged impurities, and its high thermal conductivity. Recent discoveries of structural and optical properties of h-BN have expanded potential applications into emerging electronics and photonics fields. h-BN shows a very efficient deep-ultraviolet band-edge emission despite its indirect-bandgap nature, as well as stable room-temperature single-photon emission over a wide wavelength range, showing a great potential for next-generation photonics. In addition, h-BN is extensively being adopted as active media for low-energy electronics, including nonvolatile resistive switching memory, radio-frequency devices, and low-dielectric-constant materials for next-generation electronics.

An ultralow power consumption force field-effect transistor with large modulation by the FE in 2D MoS₂ is demonstrated. A new principle for mechanical modulation can realize ultrahigh electromechanical resolution (nano-Newton) and sensitivity (GF >4801), and this process only consumes atto-Joule energy and the leakage power approaches zero, indicating the direction for the next generation of low-power electronics.

Abstract

In addition to electrical, optical, and magnetic fields, mechanical forces have demonstrated a strong ability to modulate semiconductor devices. With the rapid development of piezotronics and flexotronics, force regulation has been widely used in field-effect transistors (FETs), human–machine interfaces, light-emitting diodes (LEDs), solar cells, etc. Here, a large mechanical modulation of electronic properties by nano-Newton force in semiconductor materials with a large Young's modulus-based force FET is reported. More importantly, this FET has ultralow switching energy dissipation (7 aJ per decuple current gain) and nearly zero leakage power; these values are even better than those of electronic FETs. This finding paves the way for practical applications of nanoforce modulation devices at high power efficiency.

The synthesis of a Nb, Re dual-doped monolayer WSe₂ with the phase modulation is reported. The as-synthesized Nb, Re-WSe₂ demonstrates an ultrasensitive molecular sensing performance with a record low concentration of 5 × 10^–15 m, which is superior to that of most state-of-the-art non-noble metal substrates, and exhibits excellent environmental stability (≈6 months) and selective detection.

Abstract

Surface-enhanced Raman scattering (SERS) is a sensitive, fast, and nondestructive technology to detect trace amounts of molecules. The development of ultrasensitive and environmentally stable noble-metal-free SERS substrates is crucial for practical applications but still very challenging. In this contribution, an in situ substitutional doping strategy to synthesize Re-doped WSe₂ (Re-WSe₂) with different doping levels is reported. By increasing the Re content to ≈50 at%, the Re-WSe₂ alloy inherits the 1T″ phase of the ReSe₂ lattice. Furthermore, Nb atoms are doped into the 1T″ Re-WSe₂ alloy to further modulate its electronic structure. The as synthesized 1T″ Nb, Re-WSe₂ demonstrates a femtomolar-level molecular sensing capability with a detectable concentration of 5 × 10^–15 m and the corresponding enhancement factor is 2.0 × 10⁹, which is superior to that of most non-noble-metal SERS substrates and comparable or even superior to that of noble-metal substrates to the best of the authors’ knowledge. More importantly, the as-synthesized 1T″ Nb, Re-WSe₂ exhibits excellent air-stability over a long term (≈6 months) and selective detection capability in the mixed molecular solution, which are essential for their practical applications. The work provides a new strategy for the rational design of noble-metal-free SERS substrates to achieve ultrasensitive molecular sensing.