Electronic Thesis/Dissertation

 

Synthesis and Characterization of 2H-MoTe2 for Electrical and Gas Sensing Properties with Applications to Chemical Gas Sensing Open Access

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Molybdenum ditelluride (MoTe2) is a member of two-dimensional (2D) transition metal dichalcogenides (TMDCs) family, which have two different stable structures: semi-metallic (2H) and metallic (1T’) phase. Theoretically, 2H-MoTe2 has direct bandgap [1] (~1.10 eV) for single and indirect bandgap [2, 3] of (~1 eV) for bulk layer respectively. Single crystals of MoTe2 have been grown by chemical vapor transport (CVT) method [4] using tellurium tetrachloride (TeCl4) and iodine (I2) as transport agent (TA). For simplicity, MoTe2 crystals grown by TeCl4 and I2 TA are referred to as MoTe2-Cl and MoTe2-I2 in this work respectively. The CVT grown MoTe2 crystals were characterized by optical microscopy, Raman spectroscopy, Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), X-Ray Diffraction (XRD) and Secondary Ion Mass Spectroscopy (SIMS). All electronic measurements of MoTe2 FETs were performed in ambient environment using probe station and custom-built gas sensing setup. Until now, a detail study of channel thickness in range of ~5nm – 70nm for MoTe2 devices is not found in literatures [5]. Thickness of channel plays an important role in electrical properties of semiconducting materials. In this thesis, electronic properties of field effect transistors (FETs) fabricated from exfoliated MoTe2 single crystals are investigated as a function of channel thickness. The conductivity type in FETs gradually changes from n-type for thick MoTe2 layers (>50nm) to ambipolar behavior for intermediate MoTe2 thickness (between 50 - 12 nm) to p-type for thin layer (<12 nm) [6]. Change in polarity of FET was not observed in MoTe2-I2 devices which showed only p-type conducting behavior >5nm channel thickness. Change in polarity of MoTe2-Cl devices as function of channel thickness was also verified by ammonia (NH3) sensing. This thesis presents Gas Sensing by MoTe2 flakes. We studied the effect of ultraviolet (UV) light on MoTe2 channel for gas sensing applications. An anomalous behavior of current collapse is observed in all devices in presence of UV light. This study explains the probable cause of current collapse in MoTe2 devices in presence of UV light. We further investigated the sensing properties of fabricated device by exposing oxidizing (NO2) and reducing (NH3) gases on MoTe2 surface. The study is conducted in compressed air and N2 as carrier gas. The behavior of target gas (NO2 and NH3) sensing in presence and absence of UV illumination is also studied to improve the sensing response.

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