Formation of oxygen protective layer on monolayer MoS2 via low energy electron irradiation

Monolayer molybdenum disulfide (MoS2) semiconductors are the new generation of two-dimensional materials that possess several advantages compared to graphene due to their tunable bandgap and high electron mobility. Several approaches have been used to modify their physical properties for optical device applications. Here, we report a facile and non-destructive surface modification method for monolayer MoS2 via electron irradiation at a low, 5 kV accelerating voltage. After electron irradiation, the results of Raman and photoluminescence spectroscopy confirmed that the structure remains unchanged. However, when the modified surface was illuminated with a 532 nm laser for a prolonged period, the PL intensity was quenched as a result of oxygen desorption. Interestingly, the PL intensity can be recovered when left in ambient conditions for 10 h. The analysis of the PL spectrum revealed a decrease of trion, which is consistent with the readsorbed O2 molecules on the surface that deplete electrons and lead to PL recovery. We attribute this effect to the enhancement of the n-type character of monolayer MoS2 after electron irradiation. The sensitive nature of the modified surface to oxygen suggests that this approach may be used as a tool for the fabrication of MoS2 oxygen sensors.

Sub-9 nm high-performance and low-power transistors based on an in-plane NbSe2/MoSe2/NbSe2 heterojunction

Due to the ability to reduce the gate length of field-effect transistors (FETs) down to sub-10 nm without obviously affecting the performance of the device, the utilization of two-dimensional (2D) semiconductor materials as channel materials for FETs is of great interest. However, in-plane 2D/2D heterojunction FETs have received less attention in previous studies than vertical van der Waals heterojunction devices. Based on the above reasons, this study has investigated the transport properties of an in-plane NbSe2/MoSe2/NbSe2 heterojunction FET with different gate lengths by using ab initio quantum transport simulation. The results reveal that a gate length of sub-9 nm gives the device a low subthreshold swing down to 62 mV dec-1 and a high on-state current up to 1040 μA μm-1. Most importantly, the on-state current, delay time, and power dissipation of the FET with the optimized channel length can nearly meet or even exceed the high-performance and low-power requirements of the International Technology Roadmap for Semiconductors. The findings for this FET can provide the design and development guidance for other in-plane heterojunction electrical devices in the post-Moore era.

Fermi Level Pinning Dependent 2D Semiconductor Devices: Challenges and Prospects

Motivated by the high expectation for efficient electrostatic modulation of charge transport at very low voltages, atomically thin 2D materials with a range of bandgaps are investigated extensively for use in future semiconductor devices. However, researchers face formidable challenges in 2D device processing mainly originated from the out-of-plane van der Waals (vdW) structure of ultrathin 2D materials. As major challenges, untunable Schottky barrier height and the corresponding strong Fermi level pinning (FLP) at metal interfaces are observed unexpectedly with 2D vdW materials, giving rise to unmodulated semiconductor polarity, high contact resistance, and lowered device mobility. Here, FLP observed from recently developed 2D semiconductor devices is addressed differently from those observed from conventional semiconductor devices. It is understood that the observed FLP is attributed to inefficient doping into 2D materials, vdW gap present at the metal interface, and hybridized compounds formed under contacting metals. To provide readers with practical guidelines for the design of 2D devices, the impact of FLP occurring in 2D semiconductor devices is further reviewed by exploring various origins responsible for the FLP, effects of FLP on 2D device performances, and methods for improving metallic contact to 2D materials.

Photoluminescence of monolayer MoS2 modulated by water/O2/laser irradiation

The low photoluminescence (PL) quantum yields of transition metal dichalcogenide monolayers have been a limiting factor for their optoelectronic applications. Various and even inconsistent mechanisms have been proposed to modulate their PL efficiencies. Herein, we use PL/Raman microspectroscopy and the corresponding in situ mapping, atomic force microscopy, and field-effect transistor (FET) characterization to investigate the changes in the structural and optical properties of monolayer MoS₂. Relatively low power density (<4.08 × 10⁵ W cm^-2) of laser irradiation in ambient air can cause a slight PL suppression effect on monolayer MoS₂, whereas relatively high power density (∼1.02 × 10⁶ W cm^-2) of laser irradiation brings significant PL enhancement. Experiments under different atmospheres reveal that the laser-irradiation-induced enhancement only occurs in the atmosphere containing O₂ and is more remarkable in pure O₂. In addition, physically adsorbed water can also induce PL enhancement of monolayer MoS₂. FET devices suggest that the adsorbed water produces a p-doping effect on MoS₂, and the laser irradiation in ambient air generates an n-doping effect, and both types of doping can enhance the PL intensity. The island-shaped defects caused by laser irradiation can be stabilized by oxygen atoms and act as trapping centers for excited trions or electrons, thus reducing the non-radiative recombination ratio and enhancing the PL intensity. The physically adsorbed water works in a similar way. A low power density of laser irradiation can sweep away the originally adsorbed H₂O on the surface, thus reducing the PL.

Localized Excitons in NbSe2-MoSe2 Heterostructures

Neutral and charged excitons (trions) in atomically thin materials offer important capabilities for photonics, from ultrafast photodetectors to highly efficient light-emitting diodes and lasers. Recent studies of van der Waals (vdW) heterostructures comprised of dissimilar monolayer materials have uncovered a wealth of optical phenomena that are predominantly governed by interlayer interactions. Here, we examine the optical properties in NbSe₂-MoSe₂ vdW heterostructures, which provide an important model system to study metal-semiconductor interfaces, a common element in optoelectronics. Through low-temperature photoluminescence (PL) microscopy, we discover a sharp emission feature, L1, that is localized at the NbSe₂-capped regions of MoSe₂. L1 is observed at energies below the commonly studied MoSe₂ excitons and trions and exhibits temperature- and power-dependent PL consistent with exciton localization in a confining potential. This PL feature is robust, observed in a variety of samples fabricated with different stacking geometries and cleaning procedures. Using first-principles calculations, we reveal that the confinement potential required for exciton localization naturally arises from the in-plane band bending due to the changes in the electron affinity between pristine MoSe₂ and NbSe₂-MoSe₂ heterostructure. We discuss the implications of our studies for atomically thin optoelectronics devices with atomically sharp interfaces and tunable electronic structures.

Photochemically Induced Phase Change in Monolayer Molybdenum Disulfide

Monolayer transition metal dichalcogenide (TMDs) are promising candidates for two-dimensional (2D) ultrathin, flexible, low-power, and transparent electronics and optoelectronics. However, the performance of TMD-based devices is still limited by the relatively low carrier mobility and the large contact resistance between the semiconducting 2D channel material and the contact metal electrodes. Phase-engineering in monolayer TMDs showed great promise in enabling the fabrication of high-quality hetero-phase structures with controlled carrier mobilities and heterojunction materials with reduced contact resistance. However, to date, general methods to induce phase-change in monolayer TMDs either employ highly-hostile organometallic compounds, or have limited compatibility with large-scale, cost-effective device fabrication. In this paper, we report a new photochemical method to induce semiconductor to metallic phase transition in monolayer MoS₂ in a benign chemical environment, through a bench-top, cost-effective solution phase process that is compatible with large-scale device fabrication. It was demonstrated that photoelectrons produced by the band-gap absorption of monolayer MoS2 have enough chemical potential to activate the phase transition in the presence of an electron-donating solvent. This novel photochemical phase-transition mechanism advances our fundamental understanding of the phase transformation in 2D transition metal dichalcogenides (TMDs), and will open new revenues in the fabrication of atomically-thick metal-semiconductor heterostructures for improved carrier mobility and reduced contact resistance in TMD-based electronic and optoelectronic devices.

Harnessing Plasmon-Induced Hot Carriers at the Interfaces With Ferroelectrics

This article reviews the scientific understanding and progress of interfacing plasmonic particles with ferroelectrics in order to facilitate the absorption of low-energy photons and their conversion to chemical fuels. The fundamental principles of hot carrier generation and charge injection are described for semiconductors interfaced with metallic nanoparticles and immersed in aqueous solutions, forming a synergistic juncture between the growing fields of plasmonically-driven photochemistry and semiconductor photocatalysis. The underlying mechanistic advantages of a metal-ferroelectric vs. metal-nonferroelectric interface are presented with respect to achieving a more optimal and efficient control over the Schottky barrier height and charge separation. Notable recent examples of using ferroelectric-interfaced plasmonic particles have demonstrated their roles in yielding significantly enhanced photocurrents as well as in the photon-driven production of molecular hydrogen. Notably, plasmonically-driven photocatalysis has been shown to occur for photon wavelengths in the infrared range, which is at lower energies than typically possible for conventional semiconductor photocatalysts. Recent results thus demonstrate that integrated ferroelectric-plasmonic systems represent a potentially transformative concept for use in the field of solar energy conversion.

Functionalized MXenes as ideal electrodes for Janus MoSSe

Combining functionalized MXenes and mirror asymmetric MoSSe can form ideal electrical contacts.

Modified MXene: promising electrode materials for constructing Ohmic contacts with MoS2for electronic device applications

Promising electrode materials for constructing Ohmic contact with MoS₂for electronic device application.