While there has been increasing studies of MoS2 and other two-dimensional (2D) semiconducting dichalcogenides on hard conventional substrates, experimental or analytical studies on flexible substrates has been very limited so far, even though these 2D crystals are understood to have greater prospects for flexible smart systems. In this article, we report detailed studies of MoS2 transistors on industrial plastic sheets. Transistor characteristics afford more than 100x improvement in the ON/OFF current ratio and 4x enhancement in mobility compared to previous flexible MoS2 devices. Mechanical studies reveal robust electronic properties down to a bending radius of 1 mm which is comparable to previous reports for flexible graphene transistors. Experimental investigation identifies that crack formation in the dielectric is the responsible failure mechanism demonstrating that the mechanical properties of the dielectric layer is critical for realizing flexible electronics that can accommodate high strain. Our uniaxial tensile tests have revealed that atomic-layer-deposited HfO2 and Al2O3 films have very similar crack onset strain. However, crack propagation is slower in HfO2 dielectric compared to Al2O3 dielectric, suggesting a subcritical fracture mechanism in the thin oxide films. Rigorous mechanics modeling provides guidance for achieving flexible MoS2 transistors that are reliable at sub-mm bending radius.

Negative differential resistance and effect of defects and deformations in MoS2 armchair nanoribbon metaloxide-semiconductor field effect transistor

Due to the drastically different intralayer versus interlayer bonding strengths, the mechanical, thermal, and electrical properties of two-dimensional (2D) materials are highly anisotropic between the in-plane and out-of-plane directions. The structural anisotropy may also play a role in chemical reactions, such as oxidation, reduction, and etching. Here, the composition, structure, and electrical properties of mechanically exfoliated WSe 2 nanosheets on SiO 2 /Si substrates were studied as a function of the extent of thermal oxidation. A major component of the oxidation, as indicated from optical and Raman data, starts from the nano-sheet edges and propagates laterally towards the center. Partial oxidation also occurs in certain areas at the surface of the flakes, which are shown to be highly conductive by microwave impedance microscopy. Using secondary ion mass spectroscopy, we also observed extensive oxidation at the WSe 2 −SiO 2 interface. The combination of multiple microcopy methods can thus provide vital information on the spatial evolution of chemical reactions on 2D materials and the nanoscale electrical properties of the reaction products.Keywords: Tungsten diselenide, 2D materials, thermal oxidation, microwave impedance microscopy, secondary ion mass spectroscopy 2 Layered van der Waals (vdW) materials, in which the electronic properties are inherently anisotropic, have been studied for a relatively long time [1], and have also attracted renewed interest in recent years [1,2]. This family of materials include elemental atomic sheets (e.g.,, and phosphorene [6,7]) and transition metal dichalcogenides [8] (TMDCs, e.g., MoS 2 and WSe 2 ), among others, presenting a wide array of candidates for research and applications. Given the extensive knowledge obtained on the solidstate chemistry of conventional Group IV elemental semiconductors and III-V compounds, and the potential use of vdW materials in nanoelectronics, it is anticipated that much effort will be devoted to investigate the spatial and temporal evolution of various chemical reactions [9], such as solution or vapor-based synthesis, reduction and oxidation, wet and dry etching, in 2D materials. Specifically, the understanding of these processes from atomic to mesoscopic length scales will provide important insight on their performance at the device level.The oxidation process of TMDCs is of particular interest due to its strong influence on the characteristics of devices that are not hermetically sealed and could potentially be oxidized in ambient environments. In addition, the fully oxidized products, e.g., WO 3 , are semiconducting metal oxides with a band gap in the range of 2.5 -3.7 eV, which may find applications in many areas [10][11][12]. To date, the oxidation of TMDCs has been studied at elevated temperatures [13], under intense laser illumination [14], and upon exposure to oxygen plasma [15] or ozone [16]. In contrast to conventional semiconductors such as silicon, the TMDCs show larger oxidative reactivity at the edges and surfac...

By acoustically irradiating pristine, white, electrically insulating h- BN in aqueous environment we were able to invert its material properties. The resulting dark, electrically conductive h- BN (referred to as partially oxidized h- BN or PO- hBN ) shows a significant decrease in optical transmission (>60%) and bandgap (from 5.46 eV to 3.97 eV). Besides employing a wide variety of analytical techniques (optical and electrical measurements, Raman spectroscopy, SEM imaging, EDS, X-Ray diffraction, XPS and TOF-SIMS) to study the material properties of pristine and irradiated h- BN , our investigation suggests the basic mechanism leading to the dramatic changes following the acoustic treatment. We find that the degree of inversion arises from the degree of h- BN surface or edge oxidation which heavily depends on the acoustic energy density provided to the pristine h- BN platelets during the solution-based process. This provides a facile avenue for the realization of materials with tuned physical and chemical properties that depart from the intrinsic behavior of pristine h- BN .

Transparent and solution‐processable nanoscale polyimide (NPI) films less than 100 nm thick and their applications as flexible gate dielectrics for 2D‐materials‐based transistor devices are reported. Stable electrical performances of NPI dielectric under high tensile strains up to 10% are demonstrated by in situ bending experiments. A welcome benefit of the NPI nanoscale thickness is that the optical transparency is improved over 84% across the visible spectrum compared to conventional thick polyimide, indicating suitability for transparent electronics, such as displays and sensors. Prototypical 2D active materials, molybdenum disulfide (MoS2), and graphene using NPI gate dielectric show outstanding thin‐film transistors (TFTs) properties comparable to performances of similar devices using atomic layer deposition (ALD) gate dielectrics. For instance, MoS2 FETs with NPI dielectric affords maximum field‐effect mobility of 30 cm2 V−1 s−1 and ON/OFF current ratio >107. Graphene FETs (GFETs), fabricated with NPI dielectric, also show DC and radio frequency (RF) performances comparable to similar devices with high‐κ dielectrics, such as maximum carrier mobility of ≈5170 cm2 V−1 s−1. An extrinsic cutoff frequency ≈6.5 GHz is achieved, which reveals that NPI is also a suitable dielectric for flexible RF TFTs for wireless communication systems.