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Towards doctoral degree in Columbia University and affiliated with Mechanical Engineering and Physics Departments, my research is mainly focused on applying large strains to atomically-thin nanomaterials. Strain is expected to modify the evolution of phonon spectra by changing the effective masses, lattice constants and harmonicity of molecular potentials. In addition, it affects electronic band structures which results in bandgap shifts on semiconductors. I developed a novel platform and experimental techniques to apply large, reversible and nondestructive strains (several percent) on thin films and MEMS devices followed by optical measurements. These materials are but not limited to: semiconducting transitional metal dichalcogenides (TMDs) such as WS2, WSe2, MoS2, MoSe2 and any hetero structure combinations of them. Strain platforms are made out of flexible polymer substrates in order to adjust, probe and sense optical, mechanical and electronical properties which could be used for either tensile or compressive strains. Large mechanical strains and lack of inversion symmetry of any odd layer(s) of these materials leads us to a novel class of innovative piezoelectric devices with huge and unexpected electric responses.
Besides, I worked on quantum transport characteristics of nitrogen doped graphene. Chemical doping is a well-known technique to introduce carriers into semiconductors. Such dopants are sources of strong intervalley scattering in graphene. I studied the effects of these scattering centers on electronic transports in monolayer films produced on SiO2 and BN substrates. I probed mobility and charge carriers inferred from field effect measurements from cryogenic (2K) to room temperature. Data analysis revealed interesting physical properties such as anomalous inelastic and elastic scattering lengths and rates obtained from Weak Localization. I also observed giant temperature-dependency of resistivity and also interesting oscillation properties of conductivity which did not exist in pristine graphene Hall bars.
In addition, I experienced challenges with thin film growth by Chemical Vapor Deposition (CVD) method. I successfully grew MoSe2 (semiconductor TMD) in atmospheric pressure conditions. Having this experience, I could grow graphene with extra-large continuous film (~1mm) and large grain boundaries (~100um) which is not easily achievable. These high quality growths were used in my transport experiments.
(For more information: https://www.linkedin.com/in/ali-dadgar)