Surface passivated and encapsulated ZnO atomic layers by high-κ ultrathin MgO layers.

Atomically transparent vertically aligned ZnO-based van der Waals materials have been developed by surface passivation and encapsulation with atomic layers of MgO using materials by design, and their physical properties have been investigated. The passivation and encapsulation led to a remarkable improvement in the optical and electronic properties. The valence-band offset ΔEv between MgO and ZnO, ZnO and MgO/ZnO, and ZnO and MgO/ZnO/MgO heterointerfaces is determined to be 0.37 ± 0.02, -0.05 ± 0.02, and -0.11 ± 0.02 eV, respectively, and the conduction-band offset ΔEc is deduced to be 0.97 ± 0.02, 0.46 ± 0.02, and 0.59 ± 0.02 eV, respectively, indicating straddling type-I in MgO and ZnO, and staggering type-II heterojunction band alignment in ZnO and the various heterostructures. The band-offsets and interfacial charge transfer are used to explain the origin of n-type conductivity in the superlattices. Enhanced optical absorption due to carrier confinement in the layers demonstrates that MgO is an excellent high-κ dielectric gate oxide for encapsulating ZnO-based optoelectronic devices.

Resonant surface plasmon-exciton interaction in hybrid MoSe2@Au nanostructures.

In this work we investigate the interaction between plasmonic and excitonic resonances in hybrid MoSe2@Au nanostructures. The latter were fabricated by combining chemical vapor deposition of MoSe2 atomic layers, Au disk processing by nanosphere lithography and a soft lift-off/transfer technique. The samples were characterized by scanning electron and atomic force microscopy. Their optical properties were investigated experimentally using optical absorption, Raman scattering and photoluminescence spectroscopy. The work is focused on a resonant situation where the surface plasmon resonance is tuned to the excitonic transition. In that case, the near-field interaction between the surface plasmons and the confined excitons leads to interference between the plasmonic and excitonic resonances that manifests in the optical spectra as a transparency dip. The plasmonic-excitonic interaction regime is determined using quantitative analysis of the optical extinction spectra based on an analytical model supported by numerical simulations. We found that the plasmonic-excitonic resonances do interfere thus leading to a typical Fano lineshape of the optical extinction. The near-field nature of the plasmonic-excitonic interaction is pointed out experimentally from the dependence of the optical absorption on the number of monolayer stacks on the Au nanodisks. The results presented in this work contribute to the development of new concepts in the field of hybrid plasmonics.

Conduction Mechanisms in CVD-Grown Monolayer MoS2 Transistors: From Variable-Range Hopping to Velocity Saturation.

We fabricate transistors from chemical vapor deposition-grown monolayer MoS2 crystals and demonstrate excellent current saturation at large drain voltages (Vd). The low-field characteristics of these devices indicate that the electron mobility is likely limited by scattering from charged impurities. The current-voltage characteristics exhibit variable range hopping at low Vd and evidence of velocity saturation at higher Vd. This work confirms the excellent potential of MoS2 as a possible channel-replacement material and highlights the role of multiple transport phenomena in governing its transistor action.