•Accurate ε of MoS2 is obtained by both point-by-point method and Lorentz fitting.•Transition energies are extracted from Lorentz fitting and explained physically.•The evolution of optical properties with film thickness has been revealed.•Film thickness can be quantitatively controlled via sputtering time modulation.As a promising candidate for applications in future electronic and optoelectronic devices, MoS2 has been a research focus in recent years. Therefore, investigating its optical properties is of practical significance. Here we synthesized different MoS2 thin films with quantitatively controlled thickness and sizable thickness variation, which is vital to find out the thickness-dependent regularity. Afterwards, several characterization methods, including X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), Raman spectroscopy, photoluminescence (PL), optical absorption spectra, and spectroscopic ellipsometry (SE), were systematically performed to character the optical properties of as-grown samples. Accurate dielectric constants of MoS2 are obtained by fitting SE data using point-by-point method, and precise energies of interband transitions are directly extracted from the Lorentz dispersion model. We assign these energies to different interband electronic transitions between the valence bands and conduction bands in the Brillouin zone. In addition, the intrinsic physical mechanisms existing in observed phenomena are discussed in details. Results derived from this work are reliable and provide a better understanding of MoS2, which can be expected to help people fully employ its potential for wider applications.Download high-res image (139KB)Download full-size image

Atomically thin 2D-layered transition-metal dichalcogenides have been studied extensively in recent years because of their intriguing physical properties and promising applications in nanoelectronic devices. Among them, ReSe2 is an emerging material that exhibits a stable distorted 1T phase and strong in-plane anisotropy due to its reduced crystal symmetry. Here, the anisotropic nature of ReSe2 is revealed by Raman spectroscopy under linearly polarized excitations in which different vibration modes exhibit pronounced periodic variations in intensity. Utilizing high-quality ReSe2 nanosheets, top-gate ReSe2 field-effect transistors were built that show an excellent on/off current ratio exceeding 107 and a well-developed current saturation in the current–voltage characteristics at room temperature. Importantly, the successful synthesis of ReSe2 directly onto hexagonal boron nitride substrates has effectively improved the electron motility over 500 times and the hole mobility over 100 times at low temperatures. Strikingly, corroborating with our density-functional calculations, the ReSe2-based photodetectors exhibit a polarization-sensitive photoresponsivity due to the intrinsic linear dichroism originated from high in-plane optical anisotropy. With a back-gate voltage, the linear dichroism photodetection can be unambiguously tuned both in the electron and hole regime. The appealing physical properties demonstrated in this study clearly identify ReSe2 as a highly anisotropic 2D material for exotic electronic and optoelectronic applications.Keywords: ambipolar; anisotropy; linear dichroism photodetection; ReSe2

Vertically stacking two-dimensional (2D) materials can enable the design of novel electronic and optoelectronic devices and realize complex functionality. However, the fabrication of such artificial heterostructures on a wafer scale with an atomically sharp interface poses an unprecedented challenge. Here, we demonstrate a convenient and controllable approach for the production of wafer-scale 2D GaSe thin films by molecular beam epitaxy. In situ reflection high-energy electron diffraction oscillations and Raman spectroscopy reveal a layer-by-layer van der Waals epitaxial growth mode. Highly efficient photodetector arrays were fabricated, based on few-layer GaSe on Si. These photodiodes show steady rectifying characteristics and a high external quantum efficiency of 23.6%. The resultant photoresponse is super-fast and robust, with a response time of 60 μs. Importantly, the device shows no sign of degradation after 1 million cycles of operation. We also carried out numerical simulations to understand the underlying device working principles. Our study establishes a new approach to produce controllable, robust, and large-area 2D heterostructures and presents a crucial step for further practical applications.

Charge-trap memory with high-κ dielectric materials is considered to be a promising candidate for next-generation memory devices. Ultrathin layered two-dimensional (2D) materials like graphene and MoS2 have been receiving much attention because of their fantastic physical properties and potential applications in electronic devices. Here, we report on a dual-gate charge-trap memory device composed of a few-layer MoS2 channel and a three-dimensional (3D) Al2O3/HfO2/Al2O3 charge-trap gate stack. Because of the extraordinary trapping ability of both electrons and holes in HfO2, the MoS2 memory device exhibits an unprecedented memory window exceeding 20 V. Importantly, with a back gate the window size can be effectively tuned from 15.6 to 21 V; the program/erase current ratio can reach up to 104, allowing for multibit information storage. Moreover, the device shows a high endurance of hundreds of cycles and a stable retention of ∼28% charge loss after 10 years, which is drastically lower than ever reported MoS2 flash memory. The combination of 2D materials with traditional high-κ charge-trap gate stacks opens up an exciting field of nonvolatile memory devices.Keywords: charge-trap memory; dual gate; memory characteristics; memory window; MoS2;

The work function evolution of a graphene monolayer under two-dimensional metal electrodes was studied by combining in situ metal deposition and ultraviolet photoelectron spectroscopy under an ultra-high vacuum system. The process of the charge transfer at the graphene–metal interfaces was investigated. The transfer of electrons from metal to graphene and then from graphene to metal, with deposition of the metal, was observed. The work function of the graphene–metal contact shifted, and was finally stopped at the theoretical work function of the metal when the metal turned from a two-dimensional film into a bulk material. Meanwhile, the energy barrier of the metal–graphene interface could be tailored by altering the metal thickness. This research makes it possible to use the graphene device in future applications.

Reduced graphene oxide nano-bridges and tunable electrical transport properties of reduced graphene oxide based transistors are achieved by using tip-based nanolithography. The polarity dependence of the reduction is revealed with a threshold reduction bias of −6 V on the nano-scale tip. The best carrier mobilities up to now for holes and for electrons in reduced-graphene-oxide-based nano-scale transistors are about 5.6 cm2 V−1 s−1 and 3.2 cm2 V−1 s−1 at room temperature. Moreover, the tunable output and transport properties have been realized for the first time by the controlled nano-tip electrochemical reaction with different reduction nano-tip biases. These tunable electrical properties of graphene based transistors will be extended to develop various novel optoelectronic and microelectronic applications. It opens a possible way to mass production of a graphene oxide based device, representing a significant step forward for electrical applications.

The dependence of the absorbance of few-layer graphene oxide on reduction level is shown to originate from different inter- and intra-band transitions using infrared–visible spectroscopy. In addition, the band-gap of reduced graphene oxide is tunable from 2 to 0.02 eV depending on its reduction level. These results indicate that reduced graphene oxide possesses great potential as a candidate for photodetection in the mid-infrared range by controlling its band-gap. This study not only gives further insight into the absorption mechanism of graphene oxide reduced to different levels, but also reveals a way to tune and measure the band-gap of graphene-based materials using a simple, economical, and nondestructive approach. This approach should be readily adapted for use in photodetection applications.

In this work, the advances in the resistance switching characteristics of stoichiometric ZrO2 thin films were studied. The Al/ZrO2/Al structure exhibits reliable and reproducible switching behaviours. The thickness dependence and electrode size effect was demonstrated and understood in terms of a combined model of conductive filament/carriers trapping. Analyses of current–voltage characteristics were performed and it is suggested that the resistive switching characteristics of the ZrO2 film are governed by both the electrode/interface effect and the formation of conductive multi-filaments.

Reduced graphene oxide nano-bridges and tunable electrical transport properties of reduced graphene oxide based transistors are achieved by using tip-based nanolithography. The polarity dependence of the reduction is revealed with a threshold reduction bias of −6 V on the nano-scale tip. The best carrier mobilities up to now for holes and for electrons in reduced-graphene-oxide-based nano-scale transistors are about 5.6 cm2 V−1 s−1 and 3.2 cm2 V−1 s−1 at room temperature. Moreover, the tunable output and transport properties have been realized for the first time by the controlled nano-tip electrochemical reaction with different reduction nano-tip biases. These tunable electrical properties of graphene based transistors will be extended to develop various novel optoelectronic and microelectronic applications. It opens a possible way to mass production of a graphene oxide based device, representing a significant step forward for electrical applications.

The work function evolution of a graphene monolayer under two-dimensional metal electrodes was studied by combining in situ metal deposition and ultraviolet photoelectron spectroscopy under an ultra-high vacuum system. The process of the charge transfer at the graphene–metal interfaces was investigated. The transfer of electrons from metal to graphene and then from graphene to metal, with deposition of the metal, was observed. The work function of the graphene–metal contact shifted, and was finally stopped at the theoretical work function of the metal when the metal turned from a two-dimensional film into a bulk material. Meanwhile, the energy barrier of the metal–graphene interface could be tailored by altering the metal thickness. This research makes it possible to use the graphene device in future applications.