2D Layered Graphene Oxide Films Integrated with Micro-Ring Resonators for Enhanced Nonlinear Optics.

We report enhanced nonlinear optics in complementary metal oxide\nsemiconductor compatible photonic platforms through the use of layered two\ndimensional (2D) graphene oxide (GO) films. We integrate GO films with silicon\non insulator nanowires (SOI), high index doped silica glass (Hydex) and silicon\nnitride (SiN) waveguides and ring resonators, to demonstrate an enhanced\noptical nonlinearity including Kerr nonlinearity and four wave mixing (FWM).\nThe GO films are integrated using a large area, transfer free, layer by layer\nmethod while the film placement and size are controlled by photolithography. In\nSOI nanowires we observe a dramatic enhancement in both the Kerr nonlinearity\nand nonlinear figure of merit (FOM) due to the highly nonlinear GO films. Self\nphase modulation (SPM) measurements show significant spectral broadening\nenhancement for SOI nanowires coated with patterned films of GO. The dependence\nof GO Kerr nonlinearity on layer number and pulse energy shows trends of the\nlayered GO films from 2D to quasi bulk like behavior. The nonlinear parameter\nof GO coated SOI nanowires is increased 16 fold, with the nonlinear FOM\nincreasing over 20 times to a FOM greater than 5. We also observe an improved\nFWM efficiency in SiN waveguides integrated with 2D layered GO films. FWM\nmeasurements for samples with different numbers of GO layers and at different\npump powers are performed, achieving up to 7.3 dB conversion efficiency (CE)\nenhancement for a uniformly coated device with 1 layer of GO and 9.1 dB for a\npatterned device with 5 layers of GO. These results reveal the strong potential\nof GO films to improve the nonlinear optics of silicon, Hydex and SiN photonic\ndevices.\n

We report enhanced nonlinear optics in nanowires, waveguides, and ring resonators by introducing layered two-dimensional (2D) graphene oxide (GO) films through experimental demonstration. The GO films are integrated on silicon-on-insulator nanowires (SOI), high index doped silica glass, and silicon nitride (SiN) waveguides and microring resonators (MRRs), to demonstrate an improved optical nonlinearity including Kerr nonlinearity and four-wave mixing (FWM). By using a large-area, transfer-free, layer-by-layer GO coating method with photolithography and lift-off processes, we integrate GO films on these complementary metal-oxide-semiconductor (CMOS)-compatible devices. For SOI nanowires, significant spectral broadening of optical pulses in GO-coated SOI nanowires induced by self-phase modulation (SPM) is observed, achieving a high spectral broadening factor of 4.34 for a device with a patterned film including 10 layers of GO. A significant enhancement in the nonlinear figure of merit (FOM) for silicon nanowires by a factor of 20 is also achieved, resulting in a FOM > 5. For Hydex and SiN waveguides, enhanced FWM in the GO-coated waveguides is achieved, where conversion efficiency (CE) enhancements of up to 6.9 dB and 9.1 dB relative to the uncoated waveguides. For MRRs, an increase of up to ~10.3 dB in the FWM CE is achieved due to the resonant enhancement effect. These results reveal the strong potential of GO films to improve the nonlinear optics of nanowires, waveguides, and ring resonators.

We demonstrate enhanced four-wave mixing (FWM) in doped silica waveguides integrated with graphene oxide (GO) layers. Owing to strong mode overlap between the integrated waveguides and GO films that have a high Kerr nonlinearity and low loss, the FWM efficiency of the hybrid integrated waveguides is significantly improved. We perform FWM measurements for different pump powers, wavelength detuning, GO coating lengths, and number of GO layers. Our experimental results show good agreement with theory, achieving up to ∼9.5-dB enhancement in the FWM conversion efficiency for a 1.5-cm-long waveguide integrated with 2 layers of GO. We show theoretically that for different waveguide geometries an enhancement in FWM efficiency of ∼20 dB can be obtained in the doped silica waveguides and more than 30 dB in silicon nanowires and slot waveguides. This demonstrates the effectiveness of introducing GO films into integrated photonic devices in order to enhance the performance of nonlinear optical processes.

Ti:Sapphire laser irradiation of graphene oxide film in order to tune its structural, chemical and electrical properties: Patterning and characterizations

In this paper, the shear modulus of uniform graphene oxide (GO) film is extracted using a quartz crystal resonator (QCR). The GO film is prepared by drop-casting and vacuum filtration methods. The tunable mechanical parameters of GO film with various humidity (from 10% to 90% relative humidity) have been fully investigated. The interlayer shear modulus (∼500 MPa) of GO film by vacuum filtration shows a strengthening with increasing humidity, which is ascribed to the strong hydrogen bonding networks made up of oxygen-containing functional groups of GO film and intercalated water molecules. The interlayer shear modulus (∼27 MPa) of GO film by drop-casting displays a degraded strength. These obtained properties provide good guidance to serve the practical realization of the GO-based project.

The reduction of graphene oxide (GO) thin films deposited on substrates is crucial to achieve a technologically useful supported graphene material. However, the well-known thermal reduction process cannot be used with thermally unstable substrates (e.g., plastics and paper), in addition photo-reduction methods are expensive and only capable of reducing the external surface. Therefore, solid-state chemical reduction techniques could become a convenient approach for the full thickness reduction of the GO layers supported on thermally unstable substrates. Here, a novel experimental procedure for quantitative reduction of GO films on paper by a green and low-cost chemical reductant (L-ascorbic acid, L-aa) is proposed. The possibility to have an effective mass transport of the reductant inside the swelled GO solid (gel-phase) deposit was ensured by spraying a reductant solution on the GO film and allowing it to reflux in a closed microenvironment at 50 °C. The GO conversion degree to reduced graphene oxide (r-GO) was evaluated by Fourier transform infrared spectroscopy (FT-IR) in attenuated total reflectance (ATR) mode and X-ray Diffraction (XRD). In addition, morphology and wettability of GO deposits, before and after reduction, were confirmed by digital USB microscopy, scanning electron microscopy (SEM), and contact angle measurements. According to these structural characterizations, the proposed method allows a bulky reduction of the coating but leaves to a GO layer at the interface, that is essential for a good coating-substrate adhesion and this special characteristic is useful for industrial exploitation of the material.

Effect of micro-scale surface texture with different densities on wear resistance of graphene oxide (GO) films was studied using a revolving ball-on-flat tribometer in humid environments. The micro-scale surface texture was produced by depositing about 363 nm thick GO films onto silicon substrates pre-textured with the patterns of dimples. The goal of the surface patterning was to improve the wear resistance of the GO films so that to extend lubricating life. The experimental results demonstrated excellent ability of the textured films to improve wear resistance of the silicon substrate. Furthermore, the GO films with an appropriate dimple area density (4%) were effective in reducing the friction coefficient (0.024) and exhibiting outstanding wear resistance owing to the entrapment of wear particles in the dimples. Finally, a simple friction mechanism model was drawn to explain the frictional properties of the different textured area density GO films.

We experimentally demonstrate integrated waveguide and micro-ring resonator polarizers incorporating two-dimensional layered graphene oxide films, with a high polarization dependent loss of 53.8 dB and a high polarization extinction ratio of 8.3 dB, respectively.

Enhanced Kerr optical nonlinearities of waveguides integrated with layered graphene oxide (GO) films is experimentally demonstrated. Up to ~9.5-dB enhancement of four-wave mixing conversion efficiency is achieved for a waveguide with 2 layers of GO.

Tailoring mechanical and electrical properties of graphene oxide film for structural dielectric capacitors

A simple approach to fabricating smart actuators with large directional bending actuation and fast response has yet to be developed. Herein, a self-standing actuator is fabricated through layer-by-layer assembly of a graphene oxide (GO) film on a flexible commercial cellulose acetate (CA) polymer membrane. The GO/CA actuator responds to acetone vapor and the bending angle depends on the strip length and the thickness of GO film. Bending greater than 720° is achieved within 19 s in acetone vapor at a fixed strip length of 30 mm and a GO film thickness of 29 μm, and recovery within 38 s in air. A bending mechanism of GO/CA bilayer actuator is proposed based on the swelling effect of the CA polymer membrane in alcohol vapor. The GO film plays an important role as the barrier layer to avoid over-swelling of the CA polymer, induced by the organic vapor, and to create tension in the recovery process. The new driven system is used in example applications for designing various devices to demonstrate the dynamic process.

The proton conductivity of graphene oxide (GO) and Nafion films was studied depending on the humidity and voltage on electrodes. The electric properties of the films were similar, but the mobility of positive charges in Nafion was approximately two orders of magnitude higher than in GO. In GO films, the negative ion current with a positive voltage bias was up to ~10% of the proton current, while in Nafion films it was almost absent (<1%). The sensors based on GO and Nafion films were most effective at humidity (RH) in the range 20–80%.

Thin graphene oxide (GO) film layers are being widely used as sensing layers in different types of electrical and optical sensor devices. GO layers are particularly popular because of their tuned interface reflectivity. The stability of GO layers is fundamental for sensor device reliability, particularly in complex aqueous environments such as wastewater. In this work, the stability of GO layers in layer-by-layer (LbL) films of polyethyleneimine (PEI) and GO was investigated. The results led to the following conclusions: PEI/GO films grow linearly with the number of bilayers as long as the adsorption time is kept constant; the adsorption kinetics of a GO layer follow the behavior of the adsorption of polyelectrolytes; and the interaction associated with the growth of these films is of the ionic type since the desorption activation energy has a value of 119 ± 17 kJ/mol. Therefore, it is possible to conclude that PEI/GO films are suitable for application in optical fiber sensor devices; most importantly, an optical fiber-based interrogation setup can easily be adapted to investigate in situ desorption via a thermally stimulated process. In addition, it is possible to draw inferences about film stability in solution in a fast, reliable way when compared with the traditional ones.

Conductivity of graphene oxide films: Dependence from solvents and photoreduction

Introduction. An incomplete list of graphene properties includes high electric conductivity, thermal conductivity, strength, large surface area, high light transmittance. Graphene is a very promising material from the point of view of its application in micro- and nanoelectronics. In addition, graphene advantage is a possibility of its obtaining by various ways. It allows creating materials with desired physicochemical properties by using appropriate technological methods. Objective. The investigation of a thermal reduction temperature influence on physicochemical properties of graphene oxide (GO) films. Materials and methods. In the present work, GO films are obtained on a slide surface by its immersing and removing from a graphene oxide water dispersion (dip coating). Obtained samples are studied by methods of scanning electron microscopy, Raman spectroscopy, and elemental CHN analysis. A sheet resistance is measured by a four-point probes method. Results. A content difference of elements (C, H, N) in studied samples, and both graphene structure defectiveness and sheet resistance decrease, are found to be proportional to a reduction temperature increase. A GO films thickness decrease during a heat treatment is also observed, which is presumably associated with a functional GO groups loss while thermal reduction. Conclusion. Research results demonstrate a possibility of a carbon films with desired physicochemical properties obtaining from a reduced graphene oxide (RGO), which can be used in thin-film technologies. Presented materials can also be useful in issues related to GO and RGO obtaining and applying.