Ph.D. Thesis Defense


                                     Ph.D. Thesis Defense 

NAME OF THE CANDIDATE    :  Mr. Rahul Tripathi. 

DEGREE                                    :  Ph.D 

TITLE OF THE THESIS             :  Synergetic Effect of Electrostatic Gating and Interfacial States in Molecular Switching                                                                Operation in Molybdenum Disulfide Based Thin Hetero-Interfaces 

 SUPERVISOR                           :  Prof. Abha Misra 

DATE & TIME                           :  Wednesday, 7th April 2021, @ 04:00 PM. 

VENUE                                      :  Online (Microsoft team) Link




Two-dimensional (2D) materials have stimulated intensive research due to their intriguing physical properties and excellent electronic applications. Recently, van der Waals (vdW) hetero-interfaces — a combination of diverse 2D materials have drawn attention for electrically controllable carrier confinement to enable superior electrostatic control. The charge-confined region in hetero-interfaces further extends its application toward non-volatile molecular memory devices.

This talk encompasses molecular response study of atomically thin heterostructures combining molybdenum disulfide (MoS2), graphene, and hexagonal boron nitride (h-BN). The defect induced interfacial states are created in an atomically thin two-dimensional MoS2 channel by underlying a narrow pattern of a graphene layer in a field-effect transistor. The presence of interfacial states in the channel leads to a conductance fluctuation. Its magnitude is modulated nearly three-order of magnitude at room temperature using the nitrogen dioxide gas molecules in the subthreshold region. The study provides a systematic experimental approach to establish a correlation between modulated conductance fluctuation and the molecular concentration up to parts-per-billion. First-principles density functional theory further explains the role of unique interfacial configuration on conductance fluctuation. The study determines a novel approach to induce charge-state for the modulation of carrier concentration and exploits the role of defect induced interfacial states in atomically thin interfaces for the molecular interaction.

So far, sensing molecular interaction characteristics have been exploited extensively to reach a detection limit to a few parts-per-billion (ppb) of molecules. Far less attention is given to the evolution of persistent current state due to molecular exposure. The metastable resistance state in the MoS2-graphene heterostructure based field-effect transistor due to external perturbation of molecules is further tuned to attain a near relaxation free current state at a much lower molecular concentration of 10 ppb. The process is co-controlled both by molecular as well as external charge density. The switching operation in a persistent current state facilitates non-volatile memory features for molecular memory operation. An ultrafast switching operation in milli-second order was achieved at room temperature for the fastest recovery obtained so far in any molecular sensor.

Furthermore, the electrical gating constriction is adapted for the real-time molecular switching interaction at room temperature, after examining the deterministic role of interfacial states and persistent current state present in hetero-interfaces of the MoS2 based vdW materials. We further investigated transport properties of a multilayer MoS2/h-BN heterojunction via a tunable electrostatic barrier using artificially designed different local gates width. A systematic transport characteristic revealed that the charge transfer switching (CTS) is a bias dependent conductance phenomenon which is highly dependent on the local gate width and bias in the channel due to the gating constriction with an ON-OFF ratio of ~103. Furthermore, the CTS can be precisely controlled upon molecular interaction through electrotuneable gated constriction. Interestingly, the large-conductance change (102) due to the 100 ppb level of gas concentration leads to a complete switching off the channel, thus acting as a molecular switch. The first-principle calculations explain the mechanism of molecular CTS in the device by the Fermi level shift with the nitrogen dioxide molecules adsorbed in MoS2. The precise tunability of CTS has not been observed earlier in any atomically thin 2D materials. The molecular interaction study in vdW materials contribute to the research of various other types of heterostructure for precise interaction and can further be applied for mesoscopic transport phenomena for molecular memory, switching operation at room temperature.




Date(s) - 07/04/2021
4:00 pm - 5:30 pm

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