Xingxing Zhang

A precision, laser‐assisted, humidity‐controlled, layer‐by‐layer thinning method in 2D MoTe₂ films is presented. Field effect transistors fabricated from thinned layers exhibit an order of magnitude increase in on/off current, enhanced field‐effect mobility, and the fastest photoresponse for (visible) MoTe₂ photodetectors reported to date. Localized band gap engineering is also performed, with sub‐200 nm spatial resolution, via the creation of lateral homojunctions.

Abstract

A highly effective laser thinning method is demonstrated to accurately control the thickness of MoTe₂ layers. By utilizing the humidity present in the ambient atmosphere, multilayered MoTe₂ films can be uniformly thinned all the way down to monolayer with layer‐by‐layer precision using an ultralow laser power density of 0.2 mW µm⁻². Localized bandgap engineering is also performed in MoTe₂, by creating regions with different bandgaps on the same film, enabling the formation of lateral homojunctions with sub‐200 nm spatial resolution. Field‐effect transistors fabricated from these thinned layers exhibit significantly improved electrical properties with an order of magnitude increase in on/off current ratios, along with enhancements in on‐current and field‐effect mobility values. Thinned devices also exhibit the fastest photoresponse (45 µs) for an MoTe₂‐based visible photodetector reported to date, along with a high photoresponsivity. A highly sensitive monolayer MoTe₂ photodetector is also reported. These results demonstrate the efficiency of the presented thinning approach in producing high‐quality MoTe₂ films for electronic and optoelectronic applications.

Scalable high performance radio frequency electronics based on large domain bilayer MoS₂

Scalable high performance radio frequency electronics based on large domain bilayer MoS₂, Published online: 14 November 2018; doi:10.1038/s41467-018-07135-8

Large area two-dimensional materials show promise for applications in DC and RF flexible electronics. Here, the authors report RF transistors based on chemical vapor deposited bilayer MoS2 with 23 GHz extrinsic maximum oscillation frequency, and gigahertz mixers on flexible polyimide substrates.

An antipolar orthorhombic van der Waals structure is found to be responsible for the enhanced ferroelectric transition temperature in ultrathin SnTe films. Isostructural to GeS, the several‐layer SnTe films can maintain their spontaneous polarization above 400 K. This discovery directly bridges the experiments of SnTe and predictions on the robust ferroelectricity in GeS‐type monochalcogenide monolayers.

Abstract

2D SnTe films with a thickness of as little as 2 atomic layers (ALs) have recently been shown to be ferroelectric with in‐plane polarization. Remarkably, they exhibit transition temperatures (T_c ) much higher than that of bulk SnTe. Here, combining molecular beam epitaxy, variable temperature scanning tunneling microscopy, and ab initio calculations, the underlying mechanism of the T_c enhancement is unveiled, which relies on the formation of γ‐SnTe, a van der Waals orthorhombic phase with antipolar inter‐layer coupling in few‐AL thick SnTe films. In this phase, 4n − 2 AL (n = 1, 2, 3…) thick films are found to possess finite in‐plane polarization (space group Pmn2₁), while 4n AL thick films have zero total polarization (space group Pnma). Above 8 AL, the γ‐SnTe phase becomes metastable, and can convert irreversibly to the bulk rock salt phase as the temperature is increased. This finding unambiguously bridges experiments on ultrathin SnTe films with predictions of robust ferroelectricity in GeS‐type monochalcogenide monolayers. The observed high transition temperature, together with the strong spin‐orbit coupling and van der Waals structure, underlines the potential of atomically thin γ‐SnTe films for the development of novel spontaneous polarization‐based devices.

2D materials with phase transitions, such as charge density wave and magnetic and dipole orderings, are an important subfamily of 2D materials. Strong charge–spin–lattice couplings in the materials enable vast potentials for new‐concept and new‐structure devices. Recent experimental progress on the synthesis and device demonstration based 2D phase‐transition materials, such as 1T‐TaS₂, CrI₃, and Cr₂Ge₂Te₆ monolayers, is reviewed.

Abstract

Layered materials with phase transitions, such as charge density wave (CDW) and magnetic and dipole ordering, have potential to be exfoliated into monolayers and few‐layers and then become a large and important subfamily of two‐dimensional (2D) materials. Benefitting from enriched physical properties from the collective interactions, long‐range ordering, and related phase transitions, as well as the atomic thickness yet having nondangling bonds on the surface, 2D phase‐transition materials have vast potential for use in new‐concept and functional devices. Here, potential 2D phase‐transition materials with CDWs and magnetic and dipole ordering, including transition metal dichalcogenides, transition metal halides, metal thio/selenophosphates, chromium silicon/germanium tellurides, and more, are introduced. The structures and experimental phase‐transition properties are summarized for the bulk materials and some of the obtained monolayers. In addition, recent experimental progress on the synthesis and measurement of monolayers, such as 1T‐TaS₂, CrI₃, and Cr₂Ge₂Te₆, is reviewed.

A photoresist‐free p–n junction and inverter in the MoTe₂ homostructure are achieved by spatially controlling the photodoping region in the MoTe₂/BN heterostructure. The MoTe₂ diode demonstrates an ideality factor of ≈1.13 with a current on/off ratio of ≈1.7 × 10⁴, and the gain of the inverter reaches ≈98, illustrating its great potential in the application of 2D logic electronics.

Abstract

2D transition‐metal dichalcogenide (TMD)‐based electronic devices have been extensively explored toward the post‐Moore era. Huge efforts have been devoted to modulating the doping profile of TMDs to achieve 2D p–n junctions and inverters, the fundamental units in logic circuits. Here, photoinduced nonvolatile and programmable electron doping in MoTe₂ based on a heterostructure of MoTe₂ and hexagonal boron nitride (BN) is reported. The electron transport property in the MoTe₂ device can be precisely controlled by modulating the magnitude of the photodoping gate exerted on BN. Through tuning the polarity of the photodoping gate exerted on BN under illumination, such a doping effect in MoTe₂ can be programmed with excellent repeatability and is retained for over 14 d in the absence of an external perturbation. By spatially controlling the photodoping region in MoTe₂, a photoresist‐free p–n junction and inverter in the MoTe₂ homostructure are achieved. The MoTe₂ diode exhibits a near‐unity ideality factor of ≈1.13 with a rectification ratio of ≈1.7 × 10⁴. Moreover, the gain of the MoTe₂ inverter reaches ≈98, which is among the highest values for 2D‐material‐based homoinverters. These findings promise photodoping as an effective method to achieve 2D‐TMDs‐based nonvolatile and programmable complementary electronic devices.

Contact resistance between the channel and electrodes in MoS₂ devices is significantly reduced using a low work function metal (Ag) and graphene as an interfacial layer between the MoS₂ and the Ag because the Schottky barrier height is lowered at the contacts. Using graphene/Ag contacts instead of Ti/Au improves the field‐effect mobility, on/off current ratio, and photoresponsivity of the devices.

Abstract

2D transition metal dichalcogenides (TMDCs) have emerged as promising candidates for post‐silicon nanoelectronics owing to their unique and outstanding semiconducting properties. However, contact engineering for these materials to create high‐performance devices while adapting for large‐area fabrication is still in its nascent stages. In this study, graphene/Ag contacts are introduced into MoS₂ devices, for which a graphene film synthesized by chemical vapor deposition (CVD) is inserted between a CVD‐grown MoS₂ film and a Ag electrode as an interfacial layer. The MoS₂ field‐effect transistors with graphene/Ag contacts show improved electrical and photoelectrical properties, achieving a field‐effect mobility of 35 cm² V⁻¹ s⁻¹, an on/off current ratio of 4 × 10⁸, and a photoresponsivity of 2160 A W⁻¹, compared to those of devices with conventional Ti/Au contacts. These improvements are attributed to the low work function of Ag and the tunability of graphene Fermi level; the n‐doping of Ag in graphene decreases its Fermi level, thereby reducing the Schottky barrier height and contact resistance between the MoS₂ and electrodes. This demonstration of contact interface engineering with CVD‐grown MoS₂ and graphene is a key step toward the practical application of atomically thin TMDC‐based devices with low‐resistance contacts for high‐performance large‐area electronics and optoelectronics.

While it has been demonstrated that large scale liquid exfoliation of graphene is possible using high-shear exfoliation, it has not yet been shown to be applicable to a broader range of layered materials. In addition, it would be useful to determine whether the mechanisms reported for shear exfoliation of graphene also apply to other 2D materials. In this work we show that previous models describing high-shear exfoliation of graphene apply to MoS 2 and WS 2 . However, we find the minimum shear rate required to exfoliate MoS 2 and WS 2 to be ~3  ×  10 4 s −1 , somewhat higher than the value for graphene. We also demonstrate the scalability of shear exfoliation of WS 2 . By measuring and then optimising the scaling parameters, shear exfoliation of WS 2 is shown to be capable of reaching concentrations of 1.82 g l −1 in 6 h and demonstrating a maximum production rate of 0.95 g h −1 .

Monolayers and multilayers of semiconducting transition metal dichalcogenides (TMDCs) offer an ideal platform to explore valley-selective physics with promising applications in valleytronics and information processing. Here we manipulate the energetic degeneracy of the ##IMG## [http://ej.iop.org/images/2053-1583/6/1/015010/tdmaae7e5ieqn001.gif] and ##IMG## [http://ej.iop.org/images/2053-1583/6/1/015010/tdmaae7e5ieqn002.gif] valleys in few-layer TMDCs. We perform high-field magneto-reflectance spectroscopy on WSe 2 , MoSe 2 , and MoTe 2 crystals of thickness from monolayer to the bulk limit under magnetic fields up to ##IMG## [http://ej.iop.org/images/2053-1583/6/1/015010/tdmaae7e5ieqn003.gif] applied perpendicular to the sample plane. Because of a strong spin-layer locking, the ground state A excitons exhibit a monolayer-like valley Zeeman splitting with a negative g

Strong light-matter interactions in layered transition metal dichalcogenides (TMDs) open up vivid possibilities for novel excitonic quasiparticle-based devices. The optical properties of TMDs are dominated mostly by the tightly bound excitons and more complex quasiparticles, the biexcitons. Instead of physically exfoliated monolayers, the solvent-mediated chemical exfoliation of these 2D crystals is a cost-effective, large-scale production method suitable for substantial practical implications. Here, we explore the ultrafast excitonic phenomena in layered WS 2 (mono-to-quad) dispersion using broadband (350–750 nm) femtosecond pump-probe spectroscopy at room temperature (300 K) which are inaccessible to the steady-state absorption or emission spectroscopy. The transient absorption spectra (TAS) suggest that the mono-to-quad layered dispersion of WS 2 has similar spectral features as monolayer WS 2 in terms of saturation absorptions (SA) and excited s...