30 research outputs found
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 (111)/SiO2 substrates are reported. The results are
significantly different from those known for graphite. Spectra were collected
using 514 nm radiation on films containing from n=1 to 20 graphene layers, as
determined by atomic force microscopy. Both the 1st and 2nd order Raman spectra
show unique signatures of the number of layers in the film. The nGL film analog
of the Raman G-band in graphite exhibits a Lorentzian lineshape whose center
frequency shifts linearly relative to graphite as ~1/n (for n=1 G-band
frequency ~1588 cm-1). Three weak bands, identified with disorder-induced 1st
order scattering, are observed at ~ 1350, 1450 and 1500 cm-1. The 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 2nd order bands are also observed (~2450, ~2700 and 3248 cm-1). They
are analogs to those observed in graphite. However, the ~2700 cm-1 band
exhibits an interesting and dramatic change of shape with n. Interestingly, for
n<5 this 2nd order band is more intense than the G-band.Comment: 10 pages, 7 fig'
A microchip optomechanical accelerometer
The monitoring of accelerations is essential for a variety of applications
ranging from inertial navigation to consumer electronics. The basic operation
principle of an accelerometer is to measure the displacement of a flexibly
mounted test mass; sensitive displacement measurement can be realized using
capacitive, piezo-electric, tunnel-current, or optical methods. While optical
readout provides superior displacement resolution and resilience to
electromagnetic interference, current optical accelerometers either do not
allow for chip-scale integration or require bulky test masses. Here we
demonstrate an optomechanical accelerometer that employs ultra-sensitive
all-optical displacement read-out using a planar photonic crystal cavity
monolithically integrated with a nano-tethered test mass of high mechanical
Q-factor. This device architecture allows for full on-chip integration and
achieves a broadband acceleration resolution of 10 \mu g/rt-Hz, a bandwidth
greater than 20 kHz, and a dynamic range of 50 dB with sub-milliwatt optical
power requirements. Moreover, the nano-gram test masses used here allow for
optomechanical back-action in the form of cooling or the optical spring effect,
setting the stage for a new class of motional sensors.Comment: 16 pages, 9 figure
MEMS Technologies for Energy Harvesting
The objective of this chapter is to introduce the technology of Microelectromechanical Systems, MEMS, and their application to emerging energy harvesting devices. The chapter begins with a general introduction to the most common MEMS fabrication processes. This is followed with a survey of design mechanisms implemented in MEMS energy harvesters to provide nonlinear mechanical actuations. Mechanisms to produce bistable potential will be studied, such as introducing fixed magnets, buckling of beams or using slightly slanted clamped-clamped beams. Other nonlinear mechanisms are studied such as impact energy transfer, or the design of nonlinear springs. Finally, due to their importance in the field of MEMS and their application to energy harvesters, an introduction to actuation using piezoelectric materials is given. Examples of energy harvesters found in the literature using this actuation principle are also presented
Polymers and biopolymers at interfaces
This review updates recent progress in the understanding of the behaviour of polymers at surfaces and interfaces, highlighting examples in the areas of wetting, dewetting, crystallization, and 'smart' materials. Recent developments in analysis tools have yielded a large increase in the study of biological systems, and some of these will also be discussed, focussing on areas where surfaces are important. These areas include molecular binding events and protein adsorption as well as the mapping of the surfaces of cells. Important techniques commonly used for the analysis of surfaces and interfaces are discussed separately to aid the understanding of their application
Performance of micromachined quartz gravimetric sensors upon electrochemical adsorption of monolayers
This paper presents the results of electrochemical calibration experiments performed on micromachined quartz gravimetric sensors. The absolute mass sensitivity of bulk acoustic quartz crystal microbalance (QCM) can be improved into the sub-10-12 g range upon miniaturization of the resonator thickness and area. Using plasma etching, we have fabricated miniaturized QCMs with thicknesses of ~29 µ^m and diameters of 500 µ^m with f0=58 MHz. Resonators with 60 nm thick Ti/Pt top electrodes were used to study the electrochemically induced oxide layer formation on the metal surface, the adsorption of hydrogen, and underpotential deposition (UPD) of Cu on Pt electrodes. The performance of microQCM is compared with the performance of a frequency matched overtone mode of a commercial 5 MHz resonator. Micromachined QCMs showed expected sensitivity improvement to UPD of Cu, however an unexpected hundred fold enhancement to oxygen and ~28 times enhancement in the sensitivity to hydrogen adsorption was observed which may be due to the roughness/porosity of the electrodes. © 2008 IEEE
Characterization of dielectric properties of polycrystalline aluminum nitride for high temperature wireless sensor nodes
An aluminium nitride (AlN) passive resonance circuit intended for thermallymatched high temperature wireless sensor nodes (WSN) was manufactured using thick-lmtechnology. Characterization was done for temperatures up to 900C in both a hot-chuck forfrequencies below 5 MHz, and using wireless readings of resonating circuits at 15 MHz, 59 MHz,and 116 MHz. The substrate for the circuits was sintered polycrystalline AlN. Using a simpliedmodel for the resonators where the main contribution of the frequency-shift was considered tocome from a shift of the dielectric constant for these frequencies, the temperature dependency ofthe dielectric constant for AlN was found to decrease with increasing frequency up to 15 MHz.With an observed frequency shift of 0.04% at 15 MHz, and up to 0.56% at 59 MHz over atemperature range of 900C, AlN looks as a promising material for integration of resonancecircuits directly on the substrate