Graphene based photo-detecting has received great attentions and the performance of such detector is stretching to both ends of high sensitivity and ultra-fast response. However, limited by the current photo-gating mechanism, the price for achieving ultra-high sensitivity is sacrificing the response time. Detecting weak signal within short response time is crucial especially in applications such as optical positioning, remote sensing, and biomedical imaging. In this work, we bridge the gap between ultra-fast response and ultra-high sensitivity by employing a graphene/SiO 2 /lightly-doped-Si architecture with revolutionary interfacial gating mechanism. Such device is capable to detect < 1 nW signal (with responsivity of ~1000 A W -1 ) and the spectral response extends from visible to near-infrared. More importantly, the photoresponse time of our device has been pushed to ~400 ns. The current device structure does not need complicated fabrication process and is fully compatible with the silicon technology. This work will not only open up a route to graphene-based high performance optoelectronic devices, but also have great potential in ultra-fast weak signal detection.

The electrical performance of two dimensional transitional metal dichalcogenides (TMDs) is strongly influenced by the amount of structural defects inside. In this work, we provide an optical spectroscopic characterization approach to correlate the amount of structural defects and the electrical performance of WSe 2 devices. Low temperature photoluminescence (PL) spectra of electron beam lithography (EBL) processed WSe 2 presents a clear defect-induced PL emission due to excitons bound to defects, which would strongly degrade the electrical performance. By adopting an e-beam-free transfer-electrode technique, we are able to prepare backgated WSe 2 device with limited amount of defects. A maximum hole-mobility of about 200 cm 2 /Vs was achieved due to reduced scattering sources, which is the highest reported value among its type.This work would not only provide a versatile and nondestructive method to monitor the defects in TMDs, but also a new route to approach the room temperature phonon-limited mobility in high performance TMDs devices.

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Position-sensitive-detectors (PSDs) based on lateral photoeffect has been widely used in diverse applications 1-9 , including optical engineering, aerospace and military fields. With increasing demand in long distance, low energy consumption, and weak signal sensing systems, the poor responsivity of conventional PSDs has become a bottleneck limiting its applications, e.g. silicon p-n or p-i-n junctions 2-5 , or other materials and architectures 6-10 . Herein, we present a high performance graphene based PSDs with revolutionary interfacial amplification mechanism. Signal amplification in the order of ~10 4 has been demonstrated by utilizing the ultrahigh mobility of graphene and long lifetime of photo-induced carriers at the interface of SiO 2 /Si. This would improve the detection limit of Si-based PSDs from W to nW level, without sacrificing the spatial resolution and response speed. Such interfacial amplification mechanism is compatible with current Si technology and can be easily extended to other sensing systems 11,12 .

We report on the manipulation of the fluorescence quenching of Rhodamine 6G (R6G) on graphene by defect engineering via hydrogen and Ar+ plasma treatments. The amount and nature of defects in graphene were estimated on the basis of the Raman intensity ratios I(D)/I(G) and I(D)/I(D′) of graphene. Results showed that the quenching factor (QF) gradually decreases from ∼40 to ∼4 and ∼12 for hydrogenated graphene (sp3 defects) and Ar+-plasma-treated graphene (vacancy-like defects), respectively, with different amounts of defects. Our results indicated that the fluorescence quenching efficiency of graphene is strongly dependent on the amount and nature of defects.

Vapor transport method has been successfully used to synthesize high quality VO2 thin films on SiO2/Si substrate using V2O5 as a precursor in an inert-gas environment. The morphological and structural evolutions of the intermediate phases during the nucleation and growth processes were investigated by SEM and Raman spectroscopy, respectively. The results showed that the conversion of V2O5 powder to VO2 thin films was dominated by a melting-evaporation-nucleation-growth mechanism. Further characterization results demonstrated that the high quality crystals of monoclinic VO2 thin films exhibit a sharp resistance change up to 4 orders of magnitude. In addition, the VO2 thin films exhibited good near-infrared response, high stability, and reproducibility under ambient conditions, which should be promising for sensitive near-infrared detection. Our work not only provided a simple and direct approach to synthesize high quality VO2 thin films with distinct phase transition properties but also demonstrated the possible infrared sensing application in the future.

CVD graphene is a promising candidate for optoelectronic applications due to its high quality and high yield. However, multi-layer domains could inevitably form at the nucleation centers during the growth. Here, we propose an optical imaging technique to precisely identify the multilayer domains and also the ratio of their coverage in large-scale CVD monolayer graphene. We have also shown that the stacking disorder in twisted bilayer graphene as well as the impurities on the graphene surface could be distinguished by optical imaging. Finally, we investigated the effects of bilayer domains on the optical and electrical properties of CVD graphene, and found that the carrier mobility of CVD graphene is seriously limited by scattering from bilayer domains. Our results could be useful for guiding future optoelectronic applications of large-scale CVD graphene.

We present a simple, but rapid and accurate approach to identify the layer number of graphene oxide (GO) by using its thermally enhanced optical contrast via vacuum heating. As expected, changes have been observed both in the thicknesses and chemical structures of the material upon the thermal treatment, which can be attributed to the reduction of the amount of intercalated water and oxygen content. This results in the increase of refractive index and absorption coefficient approaching the values for intrinsic graphene. Finally, we achieve an almost complete recovery of optical contrast of GO compared with the one of graphene. The method would be made suitable for the thickness identification of mass-produced GO since it can greatly facilitate sample evaluation and manipulation, and provide immediate feedback to improve synthesis and processing strategies.