Effect of surface morphology on friction of graphene on various substrates.

The friction of graphene on various substrates, such as SiO2, h-BN, bulk-like graphene, and mica, was investigated to characterize the adhesion level between graphene and the underlying surface. The friction of graphene on SiO2 decreased with increasing thickness and converged around the penta-layers due to incomplete contact between the two surfaces. However, the friction of graphene on an atomically flat substrate, such as h-BN or bulk-like graphene, was low and comparable to that of bulk-like graphene. In contrast, the friction of graphene folded onto bulk-like graphene was indistinguishable from that of mono-layer graphene on SiO2 despite the ultra-smoothness of bulk-like graphene. The characterization of the graphene's roughness before and after folding showed that the corrugation of graphene induced by SiO2 morphology was preserved even after it was folded onto an atomically flat substrate. In addition, graphene deposited on mica, when folded, preserved the same corrugation level as before the folding event. Our friction measurements revealed that graphene, once exfoliated from the bulk crystal, tends to maintain its corrugation level even after it is folded onto an atomically flat substrate and that ultra-flatness in both graphene and the substrate is required to achieve the intimate contact necessary for strong adhesion.

Improvement in the breakdown properties of electrowetting using polyelectrolyte ionic solution.

We report the improvement in the breakdown properties of electrowetting using a mixture of poly(acrylic acid) (PAA) polyelectrolyte and a surfactant (Tween 80, TW80). Onset of breakdown was initially determined via visual observation and further verified by investigating impedance phase shift. Breakdown characteristics of the large-molecule ionic solution were compared with those of conventional electrolytes (Na(2)SO(4)) that produce small molecules. Experiments with various conductivities and hydrophobic coatings on a thin silicon nitride dielectric layer (∼500 Å) showed that the breakdown voltage of the PAA-TW80 system was at least two times higher than that of the Na(2)SO(4)-TW80 system. Our results demonstrate that defects in the dielectric and hydrophobic layers are less vulnerable to larger ionic molecules.

Piezoelectric touch-sensitive flexible hybrid energy harvesting nanoarchitectures.

In this work, we report a flexible hybrid nanoarchitecture that can be utilized as both an energy harvester and a touch sensor on a single platform without any cross-talk problems. Based on the electron transport and piezoelectric properties of a zinc oxide (ZnO) nanostructured thin film, a hybrid cell was designed and the total thickness was below 500 nm on a plastic substrate. Piezoelectric touch signals were demonstrated under independent and simultaneous operations with respect to photo-induced charges. Different levels of piezoelectric output signals from different magnitudes of touching pressures suggest new user-interface functions from our hybrid cell. From a signal controller, the decoupled performance of a hybrid cell as an energy harvester and a touch sensor was confirmed. Our hybrid approach does not require additional assembly processes for such multiplex systems of an energy harvester and a touch sensor since we utilize the coupled material properties of ZnO and output signal processing. Furthermore, the hybrid cell can provide a multi-type energy harvester by both solar and mechanical touching energies.