Studies on Physical Properties of Two Dimensional [2d] Mos2 Films: Synthesis, Characterizations and Application
Singh Gaur, Anand Prakash
Katiyar, Ram S. (Consejero)
Atomically thin two dimensional [2D] materials, also known as van der Waals solids, are emerging as important class of materials relevant to technological and fundamental research studies in condensed matter physics. Bulk as well as single layer of MoS2 is being studied very well and the demonstrated optical, electronics, catalytic and spintronic properties have put forward MoS2 as promising material for multifunctional applications. Henceforth, there is a thrust to develop state of the art synthesis of high quality MoS2 films over a large area to facilitate the scalable MoS2 based device fabrication. This dissertation has emphasis on the synthesis methods of MoS2 film on insulating substrates as well as their physical properties measurements. We adapted sulfurization of Molybdenum comprised films and chemical vapor deposition (CVD) method for the synthesis of the MoS2 films. The ultra thin MoS2 films obtained by sulfurization of Molybdenum contained films at three different temperatures namely 550 ºC, 750 ºC and 900 ºC exhibit distinguished microstructure. We obtained high quality well crystallined MoS2 films containing basal oriented planes by sulfurizing Mo films at higher temperature. On the contrary, lower temperature synthesis yield MoS2 films comprised of edge terminated vertically aligned (ETVA) MoS2 layers. From high resolution transmission electron microscopy (HRTEM) experiments, the average grain size of ETVA MoS2 layers were found ~5 nm and consist of 3-5 MoS2 layer. Further, we utilized CVD synthesis for the deposition of monolayer MoS2 films. We modified the conventional CVD deposition for 1L-MoS2 and developed to achieve the deposition of uniform 1L of MoS2 films covering a wide area ~100 mm2 of substrate. Additionally we were able to fabricate thickness dependent MoS2 films by modifying the CVD method. We used Raman spectroscopy to characterize the crystallinity and phase of the deposited MoS2 films. We analyzed the line shape of the A1g mode using the phonon confinement model and the calculated grain size. Whereas the calculated and observed grain size distribution in the HRTEM studies of ETVA-MoS2 film were found in good agreement. Since few and monolayer MoS2 have been proposed for energy efficient logic and memory device applications, hence for the sustainable application, it is critically significant to determine the thermal conductivity. We utilized the Raman probe and by performing the temperature and laser power depenendent measurements upon suspended few layer MoS2 film (FLMS), estimated the thermal conductivity ~52 W/m-K. Moreover, anisotropy of layered structure alters their physical properties dramatically from passive planes to edges including number of layers, hence, for MoS2 its critically important to understand the fact that how the surface energy alters at the basal as well as at the edges. Our wetting measurements via measuring the static contact angle (CA) for water droplet reviled that ETVA MoS2 films are hydrophilic while basal oriented FLMS MoS2 films are hydrophobic. The measure static CA values for ETVA and FLMS MoS2 films are found to be 23.8º and 91.6º respectively. We extended the wetting measurement for monolayer and bilayer MoS2 as well to understand the how the wettability changes with number of layers. The increase CA magnitude ~98º for 1L confirmed the hydrophobic nature of MoS2. We approximated the specific surface free energy for FLMS and 1L MoS2 by Neuman’s and Fowkes method respectively and the estimated values are found to be about 46.5 mJ/m2 and 48.3 mJ/m2 respectively. In the final part of the dissertation, we present a comparative field emission (FE) studies on ETVA, FLMS and 1L MoS2 films and our FE studies establish that ETVA MoS2 has lower turn-on field Eto (defined as required applied electric field to emit current density of 10 μA/cm2), ~4.5 V/μm and higher current density ~1 mA/cm2. However, FLMS and 1L MoS2 films the Eto magnitude further raised to 5.7 and 11 V/μm respectively, with one order decrease in emission current density. The current voltage curves are analyzed by Fowler-Nordheim (FN) theory and the robust emission behavior of particularly for ETVA MoS2 is attributed to the high value of geometrical field enhancement factor (β), found to be ~1064 and lower value of work function. We sketch a two step emission mechanism to explain the FE characteristics of FLMS and 1L MoS2 films. Our studies suggest that the further tailoring the microstructure of ultra thin ETVA MoS2 films would result in elegant FE properties.