Sajal Dhara

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.

Direct Observation of Metal–Insulator Transition in Single-Crystalline Germanium Telluride Nanowire Memory Devices Prior to Amorphization

Structural defects and their dynamics play an important role in controlling the behavior of phase-change materials (PCM) used in low-power nonvolatile memory devices. However, not much is known about the influence of disorder on the electronic properties of crystalline PCM prior to a structural phase-change. Here, we show that the application of voltage pulses to single-crystalline GeTe nanowire memory devices introduces structural disorder in the form of dislocations and antiphase boundaries (APB). The dynamic evolution and pile-up of APBs increases disorder at a local region of the nanowire, which electronically transforms it from a metal to a dirty metal to an insulator, while still retaining single-crystalline long-range order. We also observe that close to this metal-insulator transition, precise control over the applied voltage is required to create an insulating state; otherwise the system ends up in a more disordered amorphous phase suggesting the role of electronic instabilities during the structural phase-change.

Magnetotransport properties of individual InAs nanowires

We probe the magnetotransport properties of individual InAs nanowires in a field effect transistor geometry. In the low magnetic field regime we observe magnetoresistance that is well described by the weak localization (WL) description in diffusive conductors. The weak localization correction is modified to weak anti-localization (WAL) as the gate voltage is increased. We show that the gate voltage can be used to tune the phase coherence length (

Tuning mechanical modes and influence of charge screening in nanowire resonators

l_\phi

Tunable thermal conductivity in defect engineered nanowires at low temperatures

) and spin-orbit length (

Carrier transport in high mobility InAs nanowire junctionless transistors.

l_{so}

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

) by a factor of

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

\sim

Quantum-Confined Stark Effect of Individual Defects in a van der Waals Heterostructure

2. In the high field and low temperature regime we observe the mobility of devices can be modified significantly as a function of magnetic field. We argue that the role of skipping orbits and the nature of surface scattering is essential in understanding high field magnetotransport in nanowires.