Anand P. S. Gaur

Surface energy engineering for tunable wettability through controlled synthesis of MoS2.

MoS2 is an important member of the transition metal dichalcogenides that is emerging as a potential 2D atomically thin layered material for low power electronic and optoelectronic applications. However, for MoS2 a critical fundamental question of significant importance is how the surface energy and hence the wettability is altered at the nanoscale in particular, the role of crystallinity and orientation. This work reports on the synthesis of large area MoS2 thin films on insulating substrates (SiO2/Si and Al2O3) with different surface morphology via vapor phase deposition by varying the growth temperatures. The samples were examined using transmission electron microscopy and Raman spectroscopy. From contact angle measurements, it is possible to correlate the wettability with crystallinity at the nanoscale. The specific surface energy for few layers MoS2 is estimated to be about 46.5 mJ/m(2). Moreover a layer thickness-dependent wettability study suggests that the lower the thickness is, the higher the contact angle will be. Our results shed light on the MoS2-water interaction that is important for the development of devices based on MoS2 coated surfaces for microfluidic applications.

Temperature dependent Raman scattering studies of three dimensional topological insulators Bi2Se3

We investigate the temperature dependent (83 K≤T≤523 K) frequency shift of 2Ag1 and 1Eg2 phonon modes in the three dimensional topological insulator Bi2Se3, using Raman spectroscopy. The high quality single crystals of Bi2Se3 were grown using a modified Bridgman technique and characterized by Laue diffraction and high resolution transmission electron microscopy. A significant broadening in the line shape and red-shift in the frequencies were observed with increase in temperature. Polarized Raman scattering measurement shows a strong polarization effect of Ag1 and Ag2 phonon modes which confirms the good quality single crystals of Bi2Se3. Temperature co-efficient for A1g1, Eg2, and A1g2 modes was estimated to be −1.44 × 10−2, −1.94 × 10−2, and −1.95 × 10−2 cm−1∕K, respectively. Our results shed light on anharmonic properties of Bi2Se3.

Absence of magnetism in Cr-doped In2O3: a case study of phase separation versus phase formation

Bulk ceramic Cr-doped In2O3 have been prepared using a solid-state reaction method at two different annealing temperatures. Room-temperature ferromagnetic behaviour is observed for all the compositions prepared at relatively low temperature. But from the Raman study the ferromagnetic oxide phase of CrO2 was identified. On the other hand, samples prepared at relatively high temperature show no detectable secondary phases as confirmed by x-ray diffraction and Raman study, but also no ferromagnetism. With increase in Cr content the paramagnetism state is merely enhanced. This is a strong indication that reports of ferromagnetism in Cr : In2O3 are due to a second phase, probably CrO2.

Studies on chemical charge doping related optical properties in monolayer WS2

Thermal stability of quasi particles, i.e., exciton and trion, and a strong particle-particle interaction significantly tune the optical properties of atomically thin two dimensional (2D) metal dichalcogenides. The present work addresses the effect of inherent defects upon optical properties of chemical vapor deposition grown 1 L-WS2 and proposes the use of chemical transfer doping as a reversible and simple method for identification of the type of excess charge in the system. Photoluminescence (PL) studies in pristine 1 L-WS2 show that an additional band at ∼0.06 eV below trion (X±) PL band was evolved (at low temperature) which was associated to the bound exciton with charged/neutral defect. Using 7,7,8,8-Tetracyanoquinodimethane and 2,2-bis1,3-dithiolylidene as p and n-type dopants, respectively, we determined that the inherent defects/metal vacancies, which could be due to the presence of Tungsten metal deficiency, contributed in p-type nature of the pristine 1 L-WS2. Doping of 2D transition metal dic...

Spin-Polarized Tunneling through Chemical Vapor Deposited Multilayer Molybdenum Disulfide

The two-dimensional (2D) semiconductor molybdenum disulfide (MoS2) has attracted widespread attention for its extraordinary electrical-, optical-, spin-, and valley-related properties. Here, we report on spin-polarized tunneling through chemical vapor deposited multilayer MoS2 (∼7 nm) at room temperature in a vertically fabricated spin-valve device. A tunnel magnetoresistance (TMR) of 0.5-2% has been observed, corresponding to spin polarization of 5-10% in the measured temperature range of 300-75 K. First-principles calculations for ideal junctions result in a TMR up to 8% and a spin polarization of 26%. The detailed measurements at different temperature, bias voltages, and density functional theory calculations provide information about spin transport mechanisms in vertical multilayer MoS2 spin-valve devices. These findings form a platform for exploring spin functionalities in 2D semiconductors and understanding the basic phenomena that control their performance.

Cold cathode emission studies on topographically modified few layer and single layer MoS2 films

Nanostructured materials, such as carbon nanotubes, are excellent cold cathode emitters. Here, we report comparative field emission (FE) studies on topographically tailored few layer MoS2films consisting of ⟨0001⟩ plane perpendicular (⊥) to c-axis (i.e., edge terminated vertically aligned) along with planar few layer and monolayer (1L) MoS2films. FE measurements exhibited 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, for edge terminated vertically aligned (ETVA) MoS2films. However, Eto magnitude for planar few layer and 1L MoS2films increased further to 5.7 and 11 V/μm, respectively, with one order decrease in emission current density. The observed differences in emission behavior, particularly for ETVA MoS2 is attributed to the high value of geometrical field enhancement factor (β), found to be ∼1064, resulting from the large confinement of localized electric field at edge exposed nanograins. Emission behavior of planar few layers and 1L MoS2films are explained under a two step emission mechanism. Our studies suggest that with further tailoring the microstructure of ultra thin ETVA MoS2films would result in elegant FE properties.