Jiuxiang Dai

Two-dimensional (2D) materials-based terahertz (THz) modulators are attracting increased attention because they play important roles in THz communication, sensing, and biomedical diagnostics. This review summarizes the recent progress of 2D materials for THz modulation with different operation principles and photonic structures. The challenges and prospects for 2D-materials-based THz modulators are also proposed to further push forward the development of THz modulation.

Abstract

Terahertz (THz) technology has attracted great attention in the past few decades for its unique applications in various fields, including spectroscopy, noninvasive detection, wireless communications, and imaging. In parallel to this, the practical, fast, and broadband modulation of THz waves is becoming indispensable. Two-dimensional (2D) materials exhibit unusual optical and electrical properties, which has prompted tremendous interest and significant advances in THz modulation. This review provides the recent progress in 2D materials-based THz modulators, outlining the operating principles, including all-optical, electro-optic, magneto-optic, and other exotic mechanisms. We focus on the recent advances in THz modulation by the designed photonic structures, such as heterostructure, metamaterial, capacitor, optical cavity, and waveguide integration. Lastly, we discussed the challenges and opportunities for 2D materials-based THz modulators and presented our prospects for the future development.

npj 2D Materials and Applications, Published online: 07 September 2021; doi:10.1038/s41699-021-00257-6

Atomic-scale imaging and spectroscopy of boron-doped diamond (BDD) film electrodes reveal localized dopant-induced surface graphitization. This work presents the first experimental demonstration of dopant effects on the surface morphology of doped diamond and explains the role of boron in the formation of density of states in BDD and the strong crystallographic orientation dependence of its electrochemical activity.

Abstract

Doped diamond electrodes have attracted significant attention for decades owing to their excellent physical and electrochemical properties. However, direct experimental observation of dopant effects on the diamond surface has not been available until now. Here, low-temperature scanning tunneling microscopy is utilized to investigate the atomic-scale morphology and electronic structures of (100)- and (111)-oriented boron-doped diamond (BDD) electrodes. Graphitized domains of a few nanometers are shown to manifest the effects of boron dopants on the BDD surface. Confirmed by first-principles calculations, local density of states measurements reveal that the electronic structure of these features is characterized by in-gap states induced by boron-related lattice deformation. The dopant-related graphitization is uniquely observed in BDD (111), which explains its electrochemical superiority over the (100) facet. These experimental observations provide atomic-scale information about the role of dopants in modulating the conductivity of diamond, as well as, possibly, other functional doped materials.

The microscopic mechanism for nucleation and growth of nano-sized bubbles within 2D flakes is thoroughly revealed by in situ transmission electron microscopy observations. The nucleation is initiated by spinodal decomposition of solid precursor, which then leads to irregular voids before reorganizing into spherical bubbles. Afterward, the bubbles grow and merge each other via formation of either metastable or unstable necks.

Abstract

The nucleation and growth of bubbles within a solid matrix is a ubiquitous phenomenon that affects many natural and synthetic processes. However, such a bubbling process is almost “invisible” to common characterization methods because it has an intrinsically multiphased nature and occurs on very short time/length scales. Using in situ transmission electron microscopy to explore the decomposition of a solid precursor that emits gaseous byproducts, the direct observation of a complete nanoscale bubbling process confined in ultrathin 2D flakes is presented here. This result suggests a three-step pathway for bubble formation in the confined environment: void formation via spinodal decomposition, bubble nucleation from the spherization of voids, and bubble growth by coalescence. Furthermore, the systematic kinetics analysis based on COMSOL simulations shows that bubble growth is actually achieved by developing metastable or unstable necks between neighboring bubbles before coalescing into one. This thorough understanding of the bubbling mechanism in a confined geometry has implications for refining modern nucleation theories and controlling bubble-related processes in the fabrication of advanced materials (i.e., topological porous materials).

Thinnest Light Disk

In article number 2103140, Hongwei Liu, Junpeng Lu, Yunfei Chen, Zhenhua Ni, and co-workers present the thinnest light disk with an encryption functionality based on WS₂ monolayers. The writing-in and reading-out of information are enabled by directly controlling the fluorescence contrast of the WS₂ monolayers. The writing speed can be up to ≈6.25 MB s⁻¹, while the data storage density can reach ≈62.5 MB cm⁻² within the thickness of <1 nm.

A fully bottom-up technique for fabricating a self-releasing ultrathin silicon wafer without sacrificing any of the substrate is presented, whereas conventional technologies waste large amounts of such material. A plasma-assisted epitaxially grown silicon seed layer with a self-organized nanogap is a key for the realization of the fully bottom-up process. The results represent a technological breakthrough in advanced silicon microelectronics and photovoltaics.

Abstract

The fabrication of ultrathin silicon wafers at low cost is crucial for advancing silicon electronics toward stretchability and flexibility. However, conventional fabrication techniques are inefficient because they sacrifice a large amount of substrate material. Thus, advanced silicon electronics that have been realized in laboratories cannot move forward to commercialization. Here, a fully bottom-up technique for producing a self-releasing ultrathin silicon wafer without sacrificing any of the substrate is presented. The key to this approach is a self-organized nanogap on the substrate fabricated by plasma-assisted epitaxial growth (plasma-epi) and subsequent hydrogen annealing. The wafer thickness can be independently controlled during the bulk growth after the formation of plasma-epi seed layer. In addition, semiconductor devices are realized using the ultrathin silicon wafer. Given the high scalability of plasma-epi and its compatibility with conventional semiconductor process, the proposed bottom-up wafer fabrication process will open a new route to developing advanced silicon electronics.

Chemical vapor deposition of an ionic liquid allows the formation of ionogel thin films and microdrops in high resolution. The non-volatile ionic liquid is formed on a surface by reaction of two non-volatile precursors. A broad range of applications will benefit from this new “IL-CVD” approach, which enables the fabrication of microdevices.

Abstract

Film deposition and high-resolution patterning of ionic liquids (ILs) remain a challenge, despite a broad range of applications that would benefit from this type of processing. Here, we demonstrate for the first time the chemical vapor deposition (CVD) of ILs. The IL-CVD method is based on the formation of a non-volatile IL through the reaction of two vaporized precursors. Ionogel micropatterns can be easily obtained via the combination of IL-CVD and standard photolithography, and the resulting microdrop arrays can be used as microreactors. The IL-CVD approach will facilitate leveraging the properties of ILs in a range of applications and microfabricated devices.

The room-temperature ferromagnetism of monolayer Cr₃Te₄, which is not an inherently layered material, is reported. Molecular beam epitaxy allows the successful growth of 2D Cr₃Te₄ in the monolayer form on graphite at low substrate temperatures, e.g., ≈100 °C, which is compatible with Si complementary metal–oxide–semiconductor technology.

Abstract

The realization of long-range magnetic ordering in 2D systems can potentially revolutionize next-generation information technology. Here, the successful fabrication of crystalline Cr₃Te₄ monolayers with room temperature (RT) ferromagnetism is reported. Using molecular beam epitaxy, the growth of 2D Cr₃Te₄ films with monolayer thickness is demonstrated at low substrate temperatures (≈100 °C), compatible with Si complementary metal oxide semiconductor technology. X-ray magnetic circular dichroism measurements reveal a Curie temperature (T _c) of v344 K for the Cr₃Te₄ monolayer with an out-of-plane magnetic easy axis, which decreases to v240 K for the thicker film (≈7 nm) with an in-plane easy axis. The enhancement of ferromagnetic coupling and the magnetic anisotropy transition is ascribed to interfacial effects, in particular the orbital overlap at the monolayer Cr₃Te₄/graphite interface, supported by density-functional theory calculations. This work sheds light on the low-temperature scalable growth of 2D nonlayered materials with RT ferromagnetism for new magnetic and spintronic devices.

A junction field-effect transistor (JFET) photodetector based on PdSe₂/MoS₂ is demonstrated with high responsivity (6 × 10² A W⁻¹) and high detectivity (10¹¹ Jones), which is realized by effective dual-gate modulation. The JFET photodetector provides a new approach to realize photodetectors with high responsivity and detectivity.

Abstract

2D materials have shown great promise for next-generation high-performance photodetectors. However, the performance of photodetectors based on 2D materials is generally limited by the tradeoff between photoresponsivity and photodetectivity. Here, a novel junction field-effect transistor (JFET) photodetector consisting of a PdSe₂ gate and MoS₂ channel is constructed to realize high responsivity and high detectivity through effective modulation of top junction gate and back gate. The JFET exhibits high carrier mobility of 213 cm² V⁻¹ s⁻¹. What is more, the high responsivity of 6 × 10² A W⁻¹, as well as the high detectivity of 10¹¹ Jones, are achieved simultaneously through the dual-gate modulation. The high performance is attributed to the modulation of the depletion region by the dual-gate, which can effectively suppress the dark current and enhance the photocurrent, thereby realizing high detectivity and responsivity. The JFET photodetector provides a new approach to realize photodetectors with high responsivity and detectivity.

Air-stable large-area 2D-In _{x Ga 1−x} alloys with tunable composition and no evidence of phase segregation are realized by confinement heteroepitaxy. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.

Abstract

Chemically stable quantum-confined 2D metals are of interest in next-generation nanoscale quantum devices. Bottom-up design and synthesis of such metals could enable the creation of materials with tailored, on-demand, electronic and optical properties for applications that utilize tunable plasmonic coupling, optical nonlinearity, epsilon-near-zero behavior, or wavelength-specific light trapping. In this work, it is demonstrated that the electronic, superconducting, and optical properties of air-stable 2D metals can be controllably tuned by the formation of alloys. Environmentally robust large-area 2D-In _x Ga_{1− x} alloys are synthesized byConfinement Heteroepitaxy (CHet). Near-complete solid solubility is achieved with no evidence of phase segregation, and the composition is tunable over the full range of x by changing the relative elemental composition of the precursor. The optical and electronic properties directly correlate with alloy composition, wherein the dielectric function, band structure, superconductivity, and charge transfer from the metal to graphene are all controlled by the indium/gallium ratio in the 2D metal layer.

A novel sulfide electrode system allows sulfur-vacancy self-healing in 2D MoS₂. Ultrathin CuS electrode heals defects in the MoS₂ channel spontaneously upon mild thermal annealing. The self-healed CuS/MoS₂ transistors and phototransistors show impressive device performance.

Abstract

Contact engineering for monolayered transition metal dichalcogenides (TMDCs) is considered to be of fundamental challenge for realizing high-performance TMDCs-based (opto) electronic devices. Here, an innovative concept is established for a device configuration with metallic copper monosulfide (CuS) electrodes that induces sulfur vacancy healing in the monolayer molybdenum disulfide (MoS₂) channel. Excess sulfur adatoms from the metallic CuS electrodes are donated to heal sulfur vacancy defects in MoS₂ that surprisingly improve the overall performance of its devices. The electrode-induced self-healing mechanism is demonstrated and analyzed systematically using various spectroscopic analyses, density functional theory (DFT) calculations, and electrical measurements. Without any passivation layers, the self-healed MoS₂ (photo)transistor with the CuS contact electrodes show outstanding room temperature field effect mobility of 97.6 cm² (Vs)⁻¹, On/Off ratio > 10⁸, low subthreshold swing of 120 mV per decade, high photoresponsivity of 1 × 10⁴ A W⁻¹, and detectivity of 10¹³ jones, which are the best among back-gated transistors that employ 1L MoS₂. Using ultrathin and flexible 2D CuS and MoS₂, mechanically flexible photosensor is also demonstrated, which shows excellent durability under mechanical strain. These findings demonstrate a promising strategy in TMDCs or other 2D material for the development of high performance and functional devices including self-healable sulfide electrodes.

Nature Nanotechnology, Published online: 02 September 2021; doi:10.1038/s41565-021-00963-8

Air-stable fullerene-based single-molecule magnet Tb₂@C₈₀(CH₂Ph) is functionalized with pyrene groups. Self-assembled monolayers of the derivative on graphite and graphene demonstrate magnetic hysteresis up to 28 K.

Abstract

The chemical functionalization of fullerene single molecule magnet Tb₂@C₈₀(CH₂Ph) enables the facile preparation of robust monolayers on graphene and highly oriented pyrolytic graphite from solution without impairing their magnetic properties. Monolayers of endohedral fullerene functionalized with pyrene exhibit magnetic bistability up to a temperature of 28 K. The use of pyrene terminated linker molecules opens the way to devise integration of spin carrying units encapsulated by fullerene cages on graphitic substrates, be it single-molecule magnets or qubit candidates.

Nature, Published online: 01 September 2021; doi:10.1038/s41586-021-03926-0

Nature, Published online: 01 September 2021; doi:10.1038/s41586-021-03938-w

Mn-rich MnSb₂Te₄ is shown to exhibit ferromagnetic hysteresis with out-of-plane anisotropy and Curie temperature of 45–50 K. It is a topological insulator with Dirac point close to the Fermi level and a magnetic gap that closes at the Curie temperature. These favorable properties are caused by excess Mn that substitutes Sb within the septuple layers.

Abstract

Ferromagnetic topological insulators exhibit the quantum anomalous Hall effect, which is potentially useful for high-precision metrology, edge channel spintronics, and topological qubits.  The stable 2+ state of Mn enables intrinsic magnetic topological insulators. MnBi₂Te₄ is, however, antiferromagnetic with 25 K Néel temperature and is strongly n-doped. In this work, p-type MnSb₂Te₄, previously considered topologically trivial, is shown to be a ferromagnetic topological insulator for a few percent Mn excess. i) Ferromagnetic hysteresis with record Curie temperature of 45–50 K, ii) out-of-plane magnetic anisotropy, iii) a 2D Dirac cone with the Dirac point close to the Fermi level, iv) out-of-plane spin polarization as revealed by photoelectron spectroscopy, and v) a magnetically induced bandgap closing at the Curie temperature, demonstrated by scanning tunneling spectroscopy (STS), are shown. Moreover, a critical exponent of the magnetization β ≈ 1 is found, indicating the vicinity of a quantum critical point. Ab initio calculations reveal that Mn–Sb site exchange provides the ferromagnetic interlayer coupling and the slight excess of Mn nearly doubles the Curie temperature. Remaining deviations from the ferromagnetic order open the inverted bulk bandgap and render MnSb₂Te₄ a robust topological insulator and new benchmark for magnetic topological insulators.