Xi Wan

Electronic Properties of MoS2–WS2 Heterostructures Synthesized with Two-Step Lateral Epitaxial Strategy

Formation of heterojunctions of transition metal dichalcogenides (TMDs) stimulates wide interest in new device physics and technology by tuning optical and electronic properties of TMDs. TMDs heterojunctions are of scientific and technological interest for exploration of next generation flexible electronics. Herein, we report on a two-step epitaxial ambient-pressure CVD technique to construct in-plane MoS2-WS2 heterostructures. The technique has the potential to artificially control the shape and structure of heterostructures or even to be more potentially extendable to growth of TMD superlattice than that of one-step CVD technique. Moreover, the unique MX2 heterostructure with monolayer MoS2 core wrapped by multilayer WS2 is obtained by the technique, which is entirely different from MX2 heterostructures synthesized by existing one-step CVD technique. Transmission electron microscopy, Raman and photoluminescence mapping studies reveal that the obtained heterostructure nanosheets clearly exhibit the modulated structural and optical properties. Electrical transport studies demonstrate that the special MoS2 (monolayer)/WS2 (multilayer) heterojunctions serve as intrinsic lateral p-n diodes and unambiguously show the photovoltaic effect. On the basis of this special heterostructure, depletion-layer width and built-in potential, as well as the built-in electric field distribution, are obtained by KPFM measurement, which are the essential parameters for TMD optoelectronic devices. With further development in future studies, this growth approach is envisaged to bring about a new growth platform for two-dimensional atomic crystals and to create unprecedented architectures therefor.

Lateral Built‐In Potential of Monolayer MoS2–WS2 In‐Plane Heterostructures by a Shortcut Growth Strategy

Lateral WS2-MoS2 heterostructures are synthesized by a shortcut one-step growth recipe with low-cost and soluble salts. The 2D spatial distributions of the built-in potential and the related electric field of the lateral WS2-MoS2 heterostructure are quantitatively analyzed by scanning Kelvin probe force microscopy revealing the fundamental attributes of the lateral heterostructure devices.

Controllable modulation of the electronic properties of graphene and silicene by interface engineering and pressure

Using first-principles calculations, we show that the band gap and electron effective mass (EEM) of D-X/G/H-D, Si-X/S/H-Si and D-X/S/H-D can be modulated effectively by tuning the pressure (interlayer spacing) and stacking arrangement. The electron effective mass (EEM) is proportional to the band gap. The band gap of confined silicene is more sensitive to pressure than that of confined graphene. Moreover, a heterogeneous interface structure would be beneficial for effectively regulating the band gap and carrier effective masses of confined graphene and silicene. Using a confinement technique and pressure, the integrity of the honeycomb structure of graphene and silicene will be preserved so that the small effective masses and high mobility of graphene and silicene will be retained during compression. The tunable band gap and high carrier mobility of the sandwich structures are promising for building high-performance nanodevices.

Interface Engineering for CVD Graphene: Current Status and Progress

In the past decade, graphene and graphene-like 2D materials have drawn more and more attention in both academia and industry due to their fascinating properties. As an atomically thin 2D layered material, graphene has extremely high environmental susceptibility, that is, its properties are strongly affected by its surroundings. In this review, the current status and progress in graphene interface engineering are systematically discussed, including the interface between graphene (carbon sources) and an underlying growth substrate (catalyst), the interface between graphene and a supporting layer during a transfer process, as well as the interface between graphene and a modified substrate from the viewpoint of device applications. These key techniques involved in graphene synthesis, transfer, and device substrates can be further applied to other related 2D layered materials such as MoS2 . Moreover, by combining 2D crystals in one particular stack, 2D-based heterostructures with desired functionalities can be achieved, which opens up a new avenue for the future applications of 2D layered materials.

Quantitative Analysis of Scattering Mechanisms in Highly Crystalline CVD MoS2 through a Self‐Limited Growth Strategy by Interface Engineering

The electrical performance of highly crystalline monolayer MoS2 is remarkably enhanced by a self-limited growth strategy on octadecyltrimethoxysilane self-assembled monolayer modified SiO2 /Si substrates. The scattering mechanisms in low-κ dielectric, including the dominant charged impurities, acoustic deformation potentials, optical deformation potentials), Fröhlich interaction, and the remote interface phonon interaction in dielectrics, are quantitatively analyzed.

Enhanced optical Kerr nonlinearity of MoS 2 on silicon waveguides

A quasi-two-dimensional layer of MoS2 was placed on top of a silicon optical waveguide to form a MoS2–silicon hybrid structure. Chirped pulse self-phase modulation measurements were carried out to determine the optical Kerr nonlinearity of the structure. The observed increase in the spectral broadening of the optical pulses in the MoS2–silicon waveguide compared with the silicon waveguides indicated that the third-order nonlinear effect in MoS2 is about 2 orders of magnitude larger than that in silicon. The measurements show that MoS2 has an effective optical Kerr coefficient of about 1.1×10−16  m2/W. This work reveals the potential application of MoS2 to enhance the nonlinearity of hybrid silicon optical devices.

Influence of Annealing on Raman Spectrum of Graphene in Different Gaseous Environments

ABSTRACT Graphene grown by a coronene (C-graphene) source is transferred to an SiO2 surface, and its Raman spectra are investigated in annealing environments of O2, Ar, and N2. An irreversible doping effect is observed in all the annealing environments, which is attributed to the enhancement of substrate doping. Compared with the mechanically exfoliated graphene on SiO2, stronger remnant stress remains in the transferred C-graphene, and wrinkles prevail on the surface. It is found that the defect density increases only after O2 annealing, and the full width half maximum (FWHM) of the G and 2D bands in the Raman spectrum increases in all the annealing atmospheres. We suggest that the increase of FWHM is caused by the crystalline disorders.

Growth of Large-Scale, Large-Size, Few-Layered α-MoO3 on SiO2 and Its Photoresponse Mechanism

Layered α-MoO3 is a multifunctional material that has significant application in optoelectronic devices. In this study, we show the growth of large-scale, large-size, few-layered (FL) α-MoO3 nanosheet directly on technical substrates (SiO2 and Si) by physical vapor deposition. We suggest that the growth is self-limiting in the [010] direction because of the re-evaporation and high diffusion capacity of MoOx species at high temperature. As-prepared FL α-MoO3 is nonconductive and shows poor response to photoillumination with wavelength of 405 and 630 nm. Its work function is strongly altered by the substrate. Improvement of conductivity and photoresponse is observed after the FL device is annealed in vacuum. Line defects along the [001], [100], and [101] directions belonging to the generation of Os and Oa vacancy states appear, and the interfacial effect is suppressed. Scanning near-field optical microscope shows that the defects are absorption sites. Kelvin probe force microscope reveals decrease of apparent work function under illumination, which confirms that electrons are excited from defects states. Our findings show that intense studies on defect engineering are required to push forward the application of two-dimensional metal oxides.