Nishshanka N. Hewa-Kasakarage

Radiative recombination of spatially extended excitons in (ZnSe/CdS)/CdS heterostructured nanorods.

We report on organometallic synthesis of luminescent (ZnSe/CdS)/CdS semiconductor heterostructured nanorods (hetero-NRs) that produce an efficient spatial separation of carriers along the main axis of the structure (type II carrier localization). Nanorods were fabricated using a seeded-type approach by nucleating the growth of 20-100 nm CdS extensions at [000 +/- 1] facets of wurtzite ZnSe/CdS core/shell nanocrystals. The difference in growth rates of CdS in each of the two directions ensures that the position of ZnSe/CdS seeds in the final structure is offset from the center of hetero-NRs, resulting in a spatially asymmetric distribution of carrier wave functions along the heterostructure. Present work demonstrates a number of unique properties of (ZnSe/CdS)/CdS hetero-NRs, including enhanced magnitude of quantum confined Stark effect and subnanosecond switching of absorption energies that can find practical applications in electroabsorption switches and ultrasensitive charge detectors.

Ultrafast Carrier Dynamics in Type II ZnSe/CdS/ZnSe Nanobarbells

We employ femtosecond transient absorption spectroscopy to get an insight into ultrafast processes occurring at the interface of type II ZnSe/CdS heterostructured nanocrystals fabricated via colloidal routes and comprising a barbell-like arrangement of ZnSe tips and CdS nanorods. Our study shows that resonant excitation of ZnSe tips results in an unprecedently fast transfer of excited electrons into CdS domains of nanobarbells (<0.35 ps), whereas selective pumping of CdS components leads to a relatively slow injection of photoinduced holes into ZnSe tips (tau(h)= 95 ps). A qualitative thermodynamic description of observed electron processes within the classical limit of Marcus theory was used to identify a specific charge transfer regime associated with the ultrafast electron injection into CdS. Potential photocatalytic applications of the observed fast separation of carriers along the main axis of ZnSe/CdS barbells are discussed.

Micromachined piezoelectric microphones with in-plane directivity

Micromachined piezoelectric microphones with in-plane directivity are introduced. A beam rotates about center torsional pivots and is attached to piezoelectrically active end-springs. Rotation of the beam in response to sound pressure gradients produces spring deflections, which, in turn, produce an open-circuit voltage at the piezoelectric films. Prototypes are presented that contain a 20-μm-thick silicon beam and end-springs with 900-nm-thick chemical solution deposited lead zirconate titanate atop the surface of the end-springs. Acoustic directivity measurements are presented that confirm device functionality.

The effect of dielectric friction on the rate of charge separation in type II ZnSe/CdS semiconductor nanorods

The effect of dielectric friction on the rate of charge separation in type II ZnSe/CdS semiconductor nanorods has been investigated using picosecond transient absorption spectroscopy. The spatial separation of an excited electron-hole pair was estimated from the redshift in band edge absorption corresponding to the decrease in the exciton binding energy. The present study identifies a considerable effect of the solvent polarity on the rate of charge separation in semiconductor heterostructures, which should be taken into account when selecting nanorod caging media, such as solvents or polymer matrices.

Piezoelectric micromachined microphones with out-of-plane directivity

Piezoelectric microphones with out-of-plane directivity are introduced. Structures are comprised of circular diaphragms suspended on compliant circumferential springs and open to ambient at front and back sides. The springs contain thin piezoelectric films for integrated piezoelectric readout. Prototypes are presented in which diaphragm and springs are etched into a 10-μm-thick epitaxial Si layer with 800-nm-thick lead-zirconate-titanate films on the spring surface. Directivity and frequency response measurement confirm anticipated device functionality. A discussion of signal-to-noise ratio (SNR) merits of the approach is presented, concluding that up to 20-dB SNR improvements may be possible beyond what is achievable with present state-of-the-art commercial microphones.

On the minimum coupling required for maximum theoretical power capture from vibration energy harvesters

The minimum transducer coupling to enable maximum theoretical power capture from vibration energy harvesters is derived, leading to the simple conclusion that the product of the transducer coupling coefficient and resonance quality factor must be greater than two. Maximum theoretical power capture is experimentally demonstrated on a micromachined piezoelectric energy harvester comprised of a 20 μm thick epitaxial silicon cantilever with 800 nm thick lead-zirconate-titanate along the top surface and a bulk silicon mass at the tip. The coupling of these structures, although small (κ2=0.0033), is entirely sufficient to enable maximum theoretical power capture owing to light damping (Q=906).

Network modeling of multiple-port transducers across multiple modes of vibration

A network modeling procedure is presented that is capable of modeling transducers across a broad frequency regime with multiple coupling ports. The model is based on modal superposition, and a separate network is crafted for each vibration mode of the device. Modal velocity, rather than a particular physical velocity on the vibrating transducer, is chosen as the flow variable in each network. Multiple ports are modeled with the use of multiple transformers in series. A procedure for performing system identification to complete the network parameters is also presented, which can be performed experimentally, analytically, or through use of a finite element model in the design stage. Application of the procedure to a multiple port piezoelectric microphone is presented.

Experimental comparison of 3-3 and 3-1 mode piezoelectric microelectromechanical systems

A common architecture for piezoelectric MEMS sensors and actuators is a thin piezoelectric film patterned atop a much thicker passive bending structure (e.g., a silicon beam or plate). In a first common configuration, parallel plate electrodes reside above and below the piezoelectric film to realize a 3-1 mode device. In a second configuration, a top electrode is patterned in the form of an interdigitated transducer (IDT) to realize a parallel network of 3-3 mode cells. A theoretical comparison of figures-of-merit for each configuration has been presented by research teams in the past. Figures-of-merit include coupling coefficient, actuator strength, and signal to noise ratio for sensing applications. Less work has been performed directly comparing these configurations experimentally using micro-processed thin films. In this presentation, a micromachined accelerometer structure employing a set of multiple springs is used to experimentally compare the two configurations. Each silicon spring contains a 1mic...