Thermal Oxidation of WSe₂ Nanosheets Adhered on SiO₂/Si Substrates

Abstract

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...

Environmental Instability and Degradation of Single‐ and Few‐Layer WTe2 Nanosheets in Ambient Conditions

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Local characterization of mobile charge carriers by two electrical AFM modes: multi-harmonic EFM versus sMIM

“…While the conductivity, permittivity, carrier type and concentration can be extracted quantitatively combining the FEA with spectroscopy study (i.e. C −V, R−V and C−z curves) in sMIM mode [11,46,52,53].…”

Lithographically patterned metallic conduction in single-layer MoS2 via plasma processing

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