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.

We have fabricated and characterized suspended single-layer MoS2 devices to investigate the substrate effect on the electrical properties of MoS2. The MoS2 devices were fabricated on Si/SiO2 first by using e-beam lithography and were suspended by etching away half of the SiO2 layer with buffered oxide etchant and drying them with critical point dryer. Compared with SiO2 substrate-supported devices, the suspended devices show 2-10 times of mobility and on/off ratio improvement. While measuring the electronic properties, we observed that the suspended devices were annealed by joule heating and showed the performance improvement, whereas the supported devices did not. Our observations reveal that MoS2 devices are substrate-sensitive in their electrical properties and that proper substrates and cleaning is necessary for the optimal device performance.

The stability of hafnium indium zinc oxide thin film transistors under negative bias stress with simultaneous exposure to white light was evaluated. Two different inverted staggered bottom gate devices, each with a silicon oxide and a silicon nitride passivation, were compared. The latter exhibits higher field effect mobility but inferior subthreshold swing, and undergoes more severe shifts in threshold voltage (VT) during negative bias illumination stress. The time evolution of VT fits the stretched exponential equation, which implies that hydrogen incorporation during the nitride growth has generated bulk defects within the semiconductor and/or at the semiconductor/gate dielectric interface.

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.

Electrical characteristics of self-aligned coplanar amorphous-indium-gallium-zinc-oxide (a-IGZO) thin-film transistors (TFTs) were investigated for different annealing temperatures. The field-effect mobility of the a-IGZO TFTs increased with the annealing temperature, in particular, for a small channel length. The effective channel length (Leff) of the a-IGZO TFTs was extracted using the transmission line method. The decrease in Leff significantly depended on the annealing temperature, due to the hydrogen diffusion into the a-IGZO channel region. The intrinsic mobility calculated from the channel resistance of the a-IGZO TFTs was in good agreement with the mobility corrected by Leff.