Shamashis Sengupta

Probing thermal expansion of graphene and modal dispersion at low-temperature using graphene nanoelectromechanical systems resonators.

We use suspended graphene electromechanical resonators to study the variation of resonant frequency as a function of temperature. Measuring the change in frequency resulting from a change in tension, from 300 to 30 K, allows us to extract information about the thermal expansion of monolayer graphene as a function of temperature, which is critical for strain engineering applications. We find that thermal expansion of graphene is negative for all temperatures between 300 and 30 K. We also study the dispersion, the variation of resonant frequency with DC gate voltage, of the electromechanical modes and find considerable tunability of resonant frequency, desirable for applications like mass sensing and RF signal processing at room temperature. With a lowering of temperature, we find that the positively dispersing electromechanical modes evolve into negatively dispersing ones. We quantitatively explain this crossover and discuss optimal electromechanical properties that are desirable for temperature-compensated sensors.

Field-effect modulation of conductance in VO2 nanobeam transistors with HfO2 as the gate dielectric

We study field-effect transistors realized from VO2 nanobeams with HfO2 as the gate dielectric. When heated up from low to high temperatures, VO2 undergoes an insulator-to-metal transition. We observe a change in conductance (~ 6 percent) of our devices induced by gate voltage when the system is in the insulating phase. The response is reversible and hysteretic, and the area of hysteresis loop becomes larger as the rate of gate sweep is slowed down. A phase lag exists between the response of the conductance and the gate voltage. This indicates the existence of a memory of the system and we discuss its possible origins.

Dense Electron System from Gate-Controlled Surface Metal–Insulator Transition

Two-dimensional electron systems offer enormous opportunities for science discoveries and technological innovations. Here we report a dense electron system on the surface of single-crystal vanadium dioxide nanobeam via electrolyte gating. The overall conductance of the nanobeam increases by nearly 100 times at a gate voltage of 3 V. A series of experiments were carried out which rule out electrochemical reaction, impurity doping, and oxygen vacancy diffusion as the dominant mechanism for the conductance modulation. A surface insulator-to-metal transition is electrostatically triggered, thereby collapsing the bandgap and unleashing an extremely high density of free electrons from the original valence band within a depth self-limited by the energetics of the system. The dense surface electron system can be reversibly tuned by the gating electric field, which provides direct evidence of the electron correlation driving mechanism of the phase transition in VO(2). It also offers a new material platform for implementing Mott transistor and novel sensors and investigating low-dimensional correlated electron behavior.

Tuning mechanical modes and influence of charge screening in nanowire resonators

We probe electromechanical properties of InAs nanowire (diameter

Tunable thermal conductivity in defect engineered nanowires at low temperatures

\ensuremath{\sim}100\text{ }\text{nm}

Facile fabrication of lateral nanowire wrap-gate devices with improved performance

) resonators where the suspended nanowire is also the active channel of a field-effect transistor. We observe and explain the nonmonotonic dispersion of the resonant frequency with dc gate voltage

Coupling between quantum Hall state and electromechanics in suspended graphene resonator

({V}_{g}^{\text{dc}})

Electromechanical resonators as probes of the charge density wave transition at the nanoscale in NbSe2

. The effect of electronic screening on the properties of the resonator can be seen in the amplitude. We observe the mixing of mechanical modes with

Dynamically tracking the strain across the metal-insulator transition in VO2 measured using electromechanical resonators.

{V}_{g}^{\text{dc}}

Wide bandwidth nanowire electromechanics on insulating substrates at room temperature.

. We also experimentally probe and quantitatively explain the hysteretic nonlinear properties, as a function of