Highly sensitive SPR response of Au/chitosan/graphene oxide nanostructured thin films toward Pb (II) ions

Optical sensors based on surface plasmon resonance (SPR) are utilized for detecting toxic heavy metals in solutions. To improve the sensitivity of SPR sensors, nanostructured thin films with active layers can be synthesized. In this study, the response to Pb (II) was measured and compared for SPR sensors incorporating gold–chitosan–graphene oxide (Au/CS/GO) nanostructured thin films and Au/CS films. The characterization of Au/CS/GO using FESEM analysis revealed a film composed of nanosheets with wrinkled, rough surfaces. The results from XRD analysis confirmed the successful incorporation of GO in the prepared films. Additionally, AFM analysis determined that the Au/CS/GO films had a root mean square (rms) roughness of 28.38 nm and were considerably rougher than the Au/CS films. Upon exposure to a 5 ppm Pb (II) ion solution, the Au/CS/GO films exhibited higher SPR sensitivity, as much as 1.11200 ppm−1, than Au/CS films, 0.77600 ppm−1. This enhancement of the SPR response was attributed to strong covalent bonding between CS and GO in these films. These results indicated that the Au/CS/GO films show potential for the detection of heavy metal pollution in environmental applications

Seedless and catalyst-free growth of zinc oxide nanostructures on graphene by thermal evaporation

Metal-oxide, namely zinc oxide (ZnO) nanostructures and thin films on graphene is interesting because these structures can offer additional functionality to graphene for realizing advanced electronic and optoelectronic applications. Graphene has a great potential for novel electronic devices because of its extraordinary electrical mobility exceeding 104 cm2/Vs and a thermal conductivity of 103 W/mK. Therefore, with the excellent electrical and thermal characteristics of graphene layers, the hybrid ZnO/graphene structure is expected to offer many sophisticated device applications such as sensing devices. In this study, the seed/catalyst-free growth of ZnO on single layer (SL) and multilayer (ML) graphene by thermal evaporation of Zn in the presence of oxygen (O2) gas was performed. The effects of substrate temperatures, substrate positions and graphene thicknesses on the morphological, structural, and optical properties were found to be very pronounced. The grown ZnO structures exhibit three different structures, i.e., nanoclusters, nanorods, and thin films at 600°C, 800°C, and 1,000°C, respectively. By setting the substrate to be inclined at 90°, the growth of ZnO nanostructures, namely nanoclusters and nanorods, on SL graphene was successfully realized at temperatures of 600°C and 800°C, respectively. However, no growth was achieved at 1,000°C due to the possible severe oxidation of graphene. For the growth on ML graphene at 600°C with an inclination angle of 90°, the grown structures show extremely thick and continuous cluster structures as compared to the growth with substrate’s inclination angle of 45°. Moreover, the base of nanorod structures grown at 800°C with an inclination angle of 90° also become thicker as compared to 45°, even though their densities and aspect ratios were almost unchanged. The morphologies of grown structures at 1,000°C with an inclination angle of 90° do not show significant difference with 45°. The intensity ratio of UV emission (IUV) and visible emission (IVIS) was changed, depending on the temperature. The structures grown at a low temperature of 600°C show the highest value of IUV/IVIS of 16.2, which is almost two times higher than the structures grown on SL graphene, indicating fewer structural defects. From the results obtained, the temperature below 800°C, substrate position inclined at 90° towards the gas flow, and ML graphene seems to be preferable parameters for the growth of ZnO structures by thermal evaporation because these factors can overcome the problem of graphene’s oxidation that takes place during the growth

Seed/catalyst-free growth of zinc oxide on graphene by thermal evaporation: effects of substrate inclination angles and graphene thicknesses

A seed/catalyst-free growth of ZnO on graphene by thermal evaporation of Zn in the presence of O-2 gas was further studied. The effects of substrate positions and graphene thicknesses on the morphological, structural, and optical properties were found to be very pronounced. By setting the substrate to be inclined at 90 degrees, the growth of ZnO nanostructures, namely, nanoclusters and nanorods, on single-layer (SL) graphene was successfully realized at temperatures of 600 degrees C and 800 degrees C, respectively. For the growth on multilayer (ML) graphene at 600 degrees C with an inclination angle of 90 degrees, the grown structures show extremely thick and continuous cluster structures as compared to the growth with substrate's inclination angle of 45 degrees. Moreover, the base of nanorod structures grown at 800 degrees C with an inclination angle of 90 degrees also become thicker as compared to 45 degrees, even though their densities and aspect ratios were almost unchanged. Photoluminescence (PL) spectra of the grown ZnO structures were composed of the UV emission (378-386 nm) and the visible emission (517-550 nm), and the intensity ratio of the former emission (I-UV) to the latter emission (I-VIS) changed, depending on the temperature. The structures grown at a low temperature of 600 degrees C show the highest value of I-UV/I-VIS of 16.2, which is almost two times higher than the structures grown on SL graphene, indicating fewer structural defects. The possible growth mechanism was proposed and described which considered both the nucleation and oxidation processes. From the results obtained, it can be concluded that temperature below 800 degrees C, substrate position inclined at 90 degrees towards the gas flow, and ML graphene seems to be preferable parameters for the growth of ZnO structures by thermal evaporation because these factors can be used to overcome the problem of graphene's oxidation that takes place during the growth

Seed/catalyst-free vertical growth of high-density electrodeposited zinc oxide nanostructures on a single-layer graphene

We report the seed/catalyst-free vertical growth of high-density electrodeposited ZnO nanostructures on a single-layer graphene. The absence of hexamethylenetetramine (HMTA) and heat has resulted in the formation of nanoflake-like ZnO structure. The results show that HMTA and heat are needed to promote the formation of hexagonal ZnO nanostructures. The applied current density plays important role in inducing the growth of ZnO on graphene as well as in controlling the shape, size, and density of ZnO nanostructures. High density of vertically aligned ZnO nanorods comparable to other methods was obtained. The quality of the ZnO nanostructures also depended strongly on the applied current density. The growth mechanism was proposed. According to the growth timing chart, the growth seems to involve two stages which are the formation of ZnO nucleation and the enhancement of the vertical growth of nanorods. ZnO/graphene hybrid structure provides several potential applications in electronics and optoelectronics such as photovoltaic devices, sensing devices, optical devices, and photodetectors

Seed/catalyst-free growth of zinc oxide nanostructures on multilayer graphene by thermal evaporation

We report the seed/catalyst-free growth of ZnO on multilayer graphene by thermal evaporation of Zn in the presence of O2 gas. The effects of substrate temperatures were studied. The changes of morphologies were very significant where the grown ZnO structures show three different structures, i.e., nanoclusters, nanorods, and thin films at 600°C, 800°C, and 1,000°C, respectively. High-density vertically aligned ZnO nanorods comparable to other methods were obtained. A growth mechanism was proposed based on the obtained results. The ZnO/graphene hybrid structure provides several potential applications in electronics and optoelectronics

Sensitivity Enhancement of Pb(II) Ion Detection in Rivers Using SPR-Based Ag Metallic Layer Coated with Chitosan–Graphene Oxide Nanocomposite

The detection of Pb(II) ions in a river using the surface plasmon resonance (SPR)-based silver (Ag) thin film technique was successfully developed. Chitosan–graphene oxide (CS-GO) was coated on top of the Ag thin film surface and acted as the active sensing layer for Pb(II) ion detection. CS-GO was synthesized and characterized, and the physicochemical properties of this material were studied prior to integration with the SPR. In X-ray photoelectron spectroscopy (XPS), the appearance of the C=O, C–O, and O–H functional groups at 531.2 eV and 532.5 eV, respectively, confirms the success of CS-GO nanocomposite synthesis. A higher surface roughness of 31.04 nm was observed under atomic force microscopy (AFM) analysis for Ag/CS-GO thin film. The enhancement in thin film roughness indicates that more adsorption sites are available for Pb(II) ion binding. The SPR performance shows a good sensor sensitivity for Ag/CS-GO with 1.38° ppm−1 ranging from 0.01 to 5.00 ppm of standard Pb(II) solutions. At lower concentrations, a better detection accuracy was shown by SPR using Ag/CS-GO thin film compared to Ag/CS thin film. The SPR performance using Ag/CS-GO thin film was further evaluated with real water samples collected from rivers. The results are in agreement with those of standard Pb(II) ion solution, which were obtained at incidence angles of 80.00° and 81.11° for local and foreign rivers, respectively

Fabry–Pérot resonances and a crossover to the quantum Hall regime in ballistic graphene quantum point contacts

Abstract We report on the observation of quantum transport and interference in a graphene device that is attached with a pair of split gates to form an electrostatically-defined quantum point contact (QPC). In the low magnetic field regime, the resistance exhibited Fabry–Pérot (FP) resonances due to np’n(pn’p) cavities formed by the top gate. In the quantum Hall (QH) regime with a high magnetic field, the edge states governed the phenomena, presenting a unique condition where the edge channels of electrons and holes along a p–n junction acted as a solid-state analogue of a monochromatic light beam. We observed a crossover from the FP to QH regimes in ballistic graphene QPC under a magnetic field with varying temperatures. In particular, the collapse of the QH effect was elucidated as the magnetic field was decreased. Our high-mobility graphene device enabled observation of such quantum coherence effects up to several tens of kelvins. The presented device could serve as one of the key elements in future electronic quantum optic devices

Effect of gap width on electron transport through quantum point contact in hBN/graphene/hBN in the quantum hall regime

This study investigates quantized electron transport in high-mobility quantum point contact (QPC) devices in hBN/graphene/hBN in the quantum Hall regime. This study primarily focuses on the effect of the gap width of split gates on edge-channel manipulations, which defines the QPC structure and its electrostatic potential distribution. The quantized conductance is governed by the dynamics of edge channels passing through or backscattered at the QPC, which is controlled by both the top-gate and back-gate biases. The effects of the split-gate gap width and the filling in the QPC on the edge-channel manipulations are experimentally verified. The experimental results are consistent with the theoretical predictions of open/closed configurations of the edge channels around QPC with different gate gap widths