We have applied the perturbation theory for calculating the piezoelectric potential distribution in a nanowire (NW) as pushed by a lateral force at the tip. The analytical solution given under the first-order approximation produces a result that is within 6% from the full numerically calculated result using the finite element method. The calculation shows that the piezoelectric potential in the NW almost does not depend on the z-coordinate along the NW unless very close to the two ends, meaning that the NW can be approximately taken as a "parallel plated capacitor". This is entirely consistent to the model established for nanopiezotronics, in which the potential drop across the nanowire serves as the gate voltage for the piezoelectric field effect transistor. The maximum potential at the surface of the NW is directly proportional to the lateral displacement of the NW and inversely proportional to the cube of its length-to-diameter aspect ratio. The magnitude of piezoelectric potential for a NW of diameter 50 nm and length 600 nm is ∼0.3 V. This voltage is much larger than the thermal voltage (∼25 mV) and is high enough to drive the metal−semiconductor Schottky diode at the interface between atomic force microscope tip and the ZnO NW, as assumed in our original mechanism for the nanogenerators.
Ceria nanoparticles are one of the key abrasive materials for chemical-mechanical planarization of advanced integrated circuits. However, ceria nanoparticles synthesized by existing techniques are irregularly faceted, and they scratch the silicon wafers and increase defect concentrations. We developed an approach for large-scale synthesis of single-crystal ceria nanospheres that can reduce the polishing defects by 80% and increase the silica removal rate by 50%, facilitating precise and reliable mass-manufacturing of chips for nanoelectronics. We doped the ceria system with titanium, using flame temperatures that facilitate crystallization of the ceria yet retain the titania in a molten state. In conjunction with molecular dynamics simulation, we show that under these conditions, the inner ceria core evolves in a single-crystal spherical shape without faceting, because throughout the crystallization it is completely encapsulated by a molten 1- to 2-nanometer shell of titania that, in liquid state, minimizes the surface energy. The principle demonstrated here could be applied to other oxide systems.
The mechanical properties of miniaturized materials depend strongly on their structure, which can be altered by wet chemistry methods common in microsystems postprocessing.In a comprehensive and systematic study, we examine the dissolution of silicon when immersed in various hydrofluoric acid (HF)-based chemistries.Specifically, the frequency of mechanical resonance f R of microcantilever beams is used as a vehicle to examine the corrosion of polycrystalline silicon (polySi).A decrease in f R that occurs as a function of immersion time in HF was measured for microcantilevers as well as "comb drives" in contact with a noble metal (gold).Time-dependent variation was also observed in the modulus and hardness measured during indentation testing, sometimes with pronounced difference for specimens contacted to gold.Secondary sources of influence, such as in-plane-oriented residual strain (which remained unchanged), through-thickness-oriented residual strain gradient (increased away from the substrate), and electrical resistance (greatly increased) are examined, but were found not to significantly contribute to the decrease in fR of the microcantilevers.Morphological characterization identified attack on the surface along with grain delineation for the polySi, with the formation of a nanoscale porous layer at the near surface.The damage to the microcantilevers can be modeled by approximating the beams as a laminated composite structure.Such analysis suggests that damage, induced as the result of galvanic corrosion, results from the decreased stiffness of the near surface porous silicon (PS) layer as well as a change in the effective thickness of the beams.Last, corrosion damage is compared between eight representative HF-based chemistries.The measurements here suggest that the fabrication and postprocessing of microsystems components are important, because they can greatly influence the material properties, design, performance, lifetime, tribology, manufacture, and required operating environment of microscale and nanoscale devices.
