Atomic‐Scale Oxygen‐Mediated Etching of 2D MoS₂ and MoTe₂

Abstract

In contrast, MoTe 2 has been reported to be one of the most reactive TMDs. [9] However, not much is known about the atomic-level processes leading to the drastically different behavior of these materials.Aberration corrected (scanning) transmission electron microscopy (STEM) provides access to the exact atomic structure of materials with a sub-second time resolution. However, high energy electrons used for imaging can also cause structural changes, as has been already demonstrated for both MoS 2 and MoTe 2 . In MoS 2 , continuous electron exposure leads quickly to the formation of sulfur vacancies [10] through a combined effect of electronic excitations and knock-on damage, [11] which first agglomerate into vacancy lines before pores with molybdenum-rich edges appear. [12] In contrast, presumably due to suppressed knock-on damage resulting from the larger mass of Te compared to S, vacancy formation in MoTe 2 is considerably slower allowing dynamic phase transitions to take place without removal of atoms. [13] Nevertheless, structural changes are inevitable during prolonged imaging of both materials. Therefore, in order to study oxidation-related structural changes, they must be separated from pure electronirradiation-induced effects. This requires an instrument with ultra high vacuum and means to introduce a controlled low-pressure atmosphere around the sample during imaging. [14] Such experiments have already revealed that chemical etching in graphene takes place at a partial oxygen pressures of >3 × 10 −8 torr, [15] well below typical pressures of TEM instruments with side entry holders, leading to pore growth that starts at defective sites. [16] Pristine graphene areas remain unaffected. However, similar studies remain lacking for all other 2D materials.Here, we use the same strategy to compare the behavior of suspended 2D MoS 2 and MoTe 2 mono-layers under low-pressure (9 × 10 −10 − 4 × 10 −7 torr) oxygen atmospheres in situ while being imaged at atomic resolution via STEM. Under electron irradiation, O 2 molecules can split into atomic oxygen, accelerating the chemical effects up to an experimentally accessible time scale. In our experiments, structural damage in MoS 2 shows no dependency on the oxygen partial pressure, displaying the well-known [10,12,17] electron-beam-related creation of vacancies and later pores with molybdenum-rich edge structures. In contrast, in MoTe 2 there is a marked difference in structural changes at different oxygen pressures. Specifically, in ultra high vacuum, damage in MoTe 2 is similar to that in MoS 2 apart from Oxidation is the main cause of degradation of many 2D materials, including transition metal dichalcogenides (TMDs), under ambient conditions. Some of the materials are more affected by oxidation than others. To elucidate the oxidationinduced degradation mechanisms in TMDs, the chemical effects in single layer MoS 2 and MoTe 2 are studied in situ in an electron microscope under controlled low-pressure oxygen environments at room temperature. MoTe 2 is found to be reac...

Exploring the Surface Oxidation and Environmental Instability of 2H‐/1T’‐MoTe2 Using Field Emission‐Based Scanning Probe Lithography

“…electron beam lithography (EBL) in combination with a subsequent dry etching procedure, such as SF 6 /O 2 /Ar-plasma exposure. [30][31][32] However, not only do residues of the used resist and other chemicals that are involved in the patterning process result in a reduced device performance, moreover can especially electron beam irradiation lead to significant changes inside the material, such as doping or the previously mentioned phase transitions. [26,27,[33][34][35][36][37] Apart from the conventional electron beam lithography-based nanopatterning, atomic force microscopy (AFM) is widely used to determine the actual number of atomic layers present in the investigated 2D nanosheet.…”

“…Nanostructuring of 2D materials like MoTe 2 is typically done using electron beam lithography (EBL) in combination with a subsequent dry etching procedure, such as SF 6 /O 2 /Ar‐plasma exposure. [ 30–32 ] However, not only do residues of the used resist and other chemicals that are involved in the patterning process result in a reduced device performance, moreover can especially electron beam irradiation lead to significant changes inside the material, such as doping or the previously mentioned phase transitions. [ 26,27,33–37 ]…”