Imaging Spatial Distribution of Photogenerated Carriers in Monolayer MoS2 with Kelvin Probe Force Microscopy.

The spatial distribution of photogenerated carriers in atomically thin MoS2 flakes is investigated by measuring surface potential changes under light illumination using Kelvin probe force microscopy (KPFM). It is demonstrated that the vertical redistribution of photogenerated carriers, which is responsible for photocurrent generation in MoS2 photodetectors, can be imaged as surface potential changes with KPFM. The polarity of surface potential changes points to the trapping of photogenerated holes at the interface between MoS2 and the substrate as a major mechanism for the photoresponse in monolayer MoS2. The temporal response of the surface potential changes is compatible with the time constant of MoS2 photodetectors. The spatial inhomogeneity in the surface potential changes at the low light intensity that is related to the defect distribution in MoS2 is also investigated.

Carbon Nanotubes as Etching Masks for the Formation of Polymer Nanostructures.

We investigate the interaction of carbon nanotubes (CNTs) embedded in a polymer matrix [poly(methyl methacrylate) (PMMA)] with Ar plasma, which results in the formation of PMMA nanostructures, as CNTs act as an etching mask. Because of the large differences in the Ar ion sputtering yields between CNTs and PMMA, PMMA lines with the width comparable to that of CNTs and as high as 20 nm (for single-walled CNTs) or 80 nm (for multiwalled CNTs) can be obtained after repeated exposure of CNT/PMMA films to Ar plasma. We also follow the etching process by investigating changes in the IV characteristics and Raman spectra of CNTs after each exposure to Ar plasma, which shows progressive defect generations in CNTs while they maintain structural integrity long enough to act as the etching mask for PMMA underneath. We demonstrate that the PMMA nanostructure patterns can be transferred to a different polymer substrate using nanoimprinting.

Active control of all-fibre graphene devices with electrical gating.

Active manipulation of light in optical fibres has been extensively studied with great interest because of its compatibility with diverse fibre-optic systems. While graphene exhibits a strong electro-optic effect originating from its gapless Dirac-fermionic band structure, electric control of all-fibre graphene devices remains still highly challenging. Here we report electrically manipulable in-line graphene devices by integrating graphene-based field effect transistors on a side-polished fibre. Ion liquid used in the present work critically acts both as an efficient gating medium with wide electrochemical windows and transparent over-cladding facilitating light-matter interaction. Combined study of unique features in gate-variable electrical transport and optical transition at monolayer and randomly stacked multilayer graphene reveals that the device exhibits significant optical transmission change (>90%) with high efficiency-loss figure of merit. This subsequently modifies nonlinear saturable absorption characteristics of the device, enabling electrically tunable fibre laser at various operational regimes. The proposed device will open promising way for actively controlled optoelectronic and nonlinear photonic devices in all-fibre platform with greatly enhanced graphene-light interaction.