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.

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 ($l_\phi$) and spin-orbit length ($l_{so}$) by a factor of $\sim$ 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

We probe electro-mechanical properties of InAs nanowire (diameter ∼100 nm) resonators where the suspended nanowire (NW) is also the active channel of a field effect transistor (FET). We observe and explain the non-monotonic dispersion of the resonant frequency with DC gate voltage). 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 V DC g . We also experimentally probe and quantitatively explain the hysteretic non-linear properties, as a function of V DC g , of the resonator using the Duffing equation.

Microwave microscopy enables three-dimensional characterization of atomically thin semiconductor structures with nanometer precision.

We measure the thermal conductivity (κ) of individual InAs nanowires (NWs), and find that it is 3 orders of magnitude smaller than the bulk value in the temperature range of 10 to 50 K. We argue that the low κ arises from the strong localization of phonons in the random superlattice of twindefects oriented perpendicular to the axis of the NW. We observe significant electronic contribution arising from the surface accumulation layer which gives rise to tunability of κ with the application of electrostatic gate and magnetic field. Our devices and measurements of κ at different carrier concentrations and magnetic field without introducing structural defects, offer a means to study new aspects of nanoscale thermal transport.Thermal transport measurements on semiconducting nanowires (NWs) have attracted a lot of attention in the last few years. Measurements of thermal transport in nanostructures are important as they provide a platform to test the existing descriptions of phonons in confined structures and across complex interfaces 1 , and have the potential to result in technological applications as thermoelectric systems 1-3 . Different materials are benchmarked using the thermoelectric figure of merit ZT = S 2 T ρκ , where S is the Seebeck coefficient, ρ is the electrical resistivity, κ is the thermal conductivity and T is the absolute temperature. ZT can be increased by appropriate engineering of nanostructures. One of the ways to increase ZT is by reducing κ without degrading its electrical conductivity and Seebeck coefficient 4 . As a result, an ideal thermoelectric material is a glass for phonons and ordered for electronic transport. Recently, several theoretical models as well as experimental studies have been carried out in different semiconductor NWs like Si, Ge, Bi 2 Te 3 etc.5-9 . It is found that for Si NWs, the value of κ is reduced by two orders of magnitude 5,8,9 compared to bulk values by tuning the roughness of the surface. III-V semiconductors are also known to be good thermoelectric materials, and theoretical studies suggest that InSb and InAs NWs are good candidates for better ZT 10 . InAs NWs have been studied extensively to probe their charge and spin-transport 11-13 . An aspect of InAs NWs that makes studying their thermal transport, hitherto little explored 14,15 , of interest is the ability to tune the density of twin defects and polytypes along its length by varying growth parameters [16][17][18][19][20] . Exploiting this control over crystal structure can help synthesize defect-engineered NWs, whose lattice has aperiodic array of twins along its length that modify phonon behavior, without significantly compromising their electrical properties 20 . Such NWs satisfy the key criteria for a good thermoelectric material -localization of phonons without localizing electrons.In this work we explore the κ of suspended InAs NW field effect transistors (FETs); the NWs have high density of aperiodic twins and polytypes perpendicular to their axis [16][17][18]20 . The random nature of defects suppress...

We study nanoelectromechanical resonators fabricated from suspended flakes of NbSe 2 ͑thickness ϳ 30-50 nm͒ to probe charge density wave ͑CDW͒ physics at nanoscale. Variation in elastic and electronic properties accompanying the CDW phase transition ͑around 30 K͒ are investigated simultaneously using the devices as self-sensing heterodyne mixers. Elastic modulus is observed to change by 10%, an amount significantly larger than what had been reported earlier in the case of bulk crystals. We also study the modulation of conductance by electric field effect, and examine its relation to the order parameter and the CDW energy gap at the Fermi surface.Charge density waves ͑CDWs͒ ͑Ref. 1͒ result from the coupling between electrons and phonons, which distorts the electron distribution inside the crystal and the lattice also undergoes a periodic deformation. The experimental techniques used so far to study charge density waves include neutron scattering, 2 electrical transport measurements, 3 vibrating reed technique, 4 and angle-resolved photoemission spectroscopy ͑ARPES͒ ͑Ref. 5͒ among others. The CDW modifies both the elastic and electrical transport properties of the system. Many questions regarding the microscopic origin of CDW in transition-metal dichalcogenides ͑TMD͒, 6 which are quasi-two-dimensional materials, have still not been resolved. In the last few years, insights have emerged about the formation of CDW in TMD ͑issues such as Dirac fermion excitations 7 and feasibility of Peierls instability 8 ͒. In this paper, we will focus on the CDW transition in 2H-NbSe 2 ͑a TMD͒. This is a layered material, where each layer comprises of two sheets of Se atoms with a layer of Nb atoms in between ͓see inset of Fig. 1͑a͔͒. 9 NbSe 2 has elicited a lot of interest for its unique properties and is known to support an incommensurate CDW at low temperatures ͑below 35 K͒. It also undergoes a superconducting transition at 7.2 K.With technological advances in small scale device fabrication, CDW in mesoscale and nanoscale systems 10,11 have become the subject of experimental and theoretical studies. The isolation of atomically thin NbSe 2 layers and transport measurements 12 on them has been reported. Our study has been conducted on nanoelectromechanical resonators made from suspended flakes of NbSe 2 ͑thicknessϳ 30-50 nm, i.e., 45-80 monolayers͒ in doubly clamped geometry. Resonators based on nanoelectromechanical systems ͑NEMSs͒ have been realized earlier in carbon nanotubes, 13 nanowires, 14,15 and graphene, 16 to name a few materials. Using them to investigate the CDW transition in NbSe 2 , we demonstrate here for the first time that such resonators can also act as highly sensitive probes for concomitant structural and electronic phase transitions at nanoscale.To fabricate our devices, we deposit NbSe 2 flakes on a silicon wafer ͑coated with 300 nm of insulating SiO 2 ͒ by mechanical exfoliation. 12 Standard electron-beam lithography techniques, followed by sputtering Cr and Au, are used to establish metallic contacts to thes...

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 K 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 300K and 30K. 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 lowering of temperature, we find that the positively dispersing electromechanical modes evolve to negatively dispersing ones. We quantitatively explain this crossover and discuss optimal electromechanical properties that are desirable for temperature compensated sensors.

We present a simple fabrication technique for lateral nanowire wrap-gate devices with high capacitive coupling and field-effect mobility. Our process uses e-beam lithography with a single resist-spinning step, and does not require chemical etching. We measure, in the temperature range 1.5-250 K, a subthreshold slope of 5-54 mV/decade and mobility of 2800-2500 cm 2 /V s -significantly larger than previously reported lateral wrap-gate devices. At depletion, the barrier height due to the gated region is proportional to applied wrapgate voltage.