Improving the magnetoelectric performance of Metglas/PZT laminates by annealing in a magnetic field.

A comprehensive investigation of magnetostriction optimization in Metglas 2605SA1 ribbons is performed to enhance magnetoelectric performance. We explore a range of annealing conditions to relieve remnant stress and align the magnetic domains in the Metglas, while minimizing unwanted crystallization. The magnetostriction coefficient, magnetoelectric coefficient, and magnetic domain alignment are correlated to optimize magnetoelectric performance. We report on direct magnetostriction observed by in-plane Doppler vibrometer and domain imagining using scanning electron microscopy with polarization analysis for a range of annealing conditions. We find that annealing in an oxygen-free environment at 400 °C for 30 min yields an optimal magnetoelectric coefficient, magnetostriction and magnetostriction coefficient. The optimized ribbons had a magnetostriction of 50.6 ± 0.2 µm m-1 and magnetoelectric coefficient of 79.3 ± 1.5 µm m-1 mT-1. The optimized Metglas 2605SA1 ribbons and PZT-5A (d31 mode) sensor achieves a magnetic noise floor of approximately 600 pT Hz-1/2 at 100 Hz and a magnetoelectric coefficient of 6.1 ± 0.03 MV m-1 T-1.

Intrinsic doping and gate hysteresis in graphene field effect devices fabricated on SiO2 substrates.

We have studied the intrinsic doping level and gate hysteresis of graphene-based field effect transistors (FETs) fabricated over Si/SiO(2) substrates. It was found that the high p-doping level of graphene in some as-prepared devices can be reversed by vacuum degassing at room temperature or above depending on the degree of hydrophobicity and/or hydration of the underlying SiO(2) substrate. Charge neutrality point (CNP) hysteresis, consisting of the shift of the charge neutrality point (or Dirac peak) upon reversal of the gate voltage sweep direction, was also greatly reduced upon vacuum degassing. However, another type of hysteresis, consisting of the change in the transconductance upon reversal of the gate voltage sweep direction, persists even after long-term vacuum annealing at 200 °C, when SiO(2) surface-bound water is expected to be desorbed. We propose a mechanism for this transconductance hysteresis that involves water-related defects, formed during the hydration of the near-surface silanol groups in the bulk SiO(2), that can act as electron traps.

Raman scattering from high-frequency phonons in supported n-graphene layer films.

Results of room-temperature Raman scattering studies of ultrathin graphitic films supported on Si (100)/SiO2 substrates are reported. The results are significantly different from those known for graphite. Spectra were collected using 514.5 nm radiation on films containing from n = 1 to 20 graphene layers, as determined by atomic force microscopy. Both the first- and second-order Raman spectra show unique signatures of the number of layers in the film. The nGL film analogue of the Raman G-band in graphite exhibits a Lorentzian line shape whose center frequency shifts linearly relative to graphite as approximately 1/n (for n = 1 omegaG approximately 1587 cm-1). Three weak bands, identified with disorder-induced first-order scattering, are observed at approximately 1350, 1450, and 1500 cm-1. The approximately 1500 cm-1 band is weak but relatively sharp and exhibits an interesting n-dependence. In general, the intensity of these D-bands decreases dramatically with increasing n. Three second-order bands are also observed (approximately 2450, approximately 2700, and 3248 cm-1). They are analogues to those observed in graphite. However, the approximately 2700 cm-1 band exhibits an interesting and dramatic change of shape with n. Interestingly, for n < 5 this second-order band is more intense than the G-band.

Force-deflection spectroscopy: a new method to determine the Young's modulus of nanofilaments.

We demonstrate the determination of Young's modulus of nanowires or nanotubes via a new approach, that is, force-deflection spectroscopy (FDS). An atomic force microscope is used to measure force versus deflection (F-D) curves of nanofilaments that bridge a trench patterned in a Si substrate. The FD data are then fit to the Euler-Bernoulli equation to determine Young's modulus. Our approach provides a generic platform from which to study the mechanical and piezoelectric properties of a variety of materials at the nanoscale level. Young's modulus measurements on ZnS (wurtzite) nanowires are presented to demonstrate this technique. We find that the Young's modulus for rectangular cross section ZnS nanobelts is 52 +/- 7.0 GPa, about 30% smaller than that reported for the bulk.