Graphene oxide for enhanced optical nonlinear performance in CMOS compatible integrated devices

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.

Layered 2D graphene oxide (GO) films are integrated with micro-ring resonators (MRRs) to experimentally demonstrate enhanced nonlinear optics. Both uniformly coated (1-5 layers) and patterned (10-50 layers) GO films are integrated on complementary-metal-oxide-semiconductor (CMOS)-compatible doped silica MRRs using a large-area, transfer-free, layer-by-layer GO coating method with precise control of the film thickness. The patterned devices further employ photolithography and lift-off processes to enable precise control of the film placement and coating length. Four-wave-mixing (FWM) measurements for different pump powers and resonant wavelengths show a significant improvement in efficiency of ≈7.6 dB for a uniformly coated device with 1 GO layer and ≈10.3 dB for a patterned device with 50 GO layers. The measurements agree well with theory, with the enhancement in FWM efficiency resulting from the high Kerr nonlinearity and low loss of the GO films combined with the strong light-matter interaction within the MRRs. The dependence of GO's third-order nonlinearity on layer number and pump power is also extracted from the FWM measurements, revealing interesting physical insights about the evolution of the GO films from 2D monolayers to quasi bulk-like behavior. These results confirm the high nonlinear optical performance of integrated photonic resonators incorporated with 2D layered GO films.

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.

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.

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%.

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.

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.

Many environmental technologies rely on containment by engineered barriers that inhibit the release or transport of toxicants. Graphene is a new, atomically thin, two-dimensional sheet material, whose aspect ratio, chemical resistance, flexibility, and impermeability make it a promising candidate for inclusion in a next generation of engineered barriers. Here we show that ultrathin graphene oxide (GO) films can serve as effective barriers for both liquid and vapor permeants. First, GO deposition on porous substrates is shown to block convective flow at much lower mass loadings than other carbon nanomaterials, and can achieve hydraulic conductivities of 5 × 10(-12) cm/s or lower. Second we show that ultrathin GO films of only 20-nm thickness coated on polyethylene films reduce their vapor permeability by 90% using elemental mercury as a model vapor toxicant. The barrier performance of GO in this thin-film configuration is much better than the Nielsen model limit, which describes ideal behavior of flake-like fillers uniformly imbedded in a polymer. The Hg barrier performance of GO films is found to be sensitive to residual water in the films, which is consistent with molecular dynamics (MD) simulations that show lateral diffusion of Hg atoms in graphene interlayer spaces that have been expanded by hydration.

The strong, broad and tunable fluorescence emission of graphene oxide (GO) has shown the exciting optical applications in many areas, such as fluorescence imaging in living cell, high sensitive detection of heavy metal ions, and the fabrication of optoelectronic devices. However, the intrinsic heterogeneous fluorescence intensity resulting from the variability in the power density of excitation laser and the non-uniform thickness of GO film, hinders its further applications in the micropatterning, information storage and display technology, which requires homogeneous fluorescence emission. In contrast to the fluorescence intensity, the fluorescence lifetime of GO is determined by the intrinsic nature of chromophores, rather than the film thickness or excitation power density. Here we report that the fluorescence lifetime is homogeneous for GO film, which eliminates the anisotropic optical properties of GO film. By reducing the GO film through the irradiation from a 405 nm continuous-wave laser at a certain power density on a home-built scanning confocal microscope, we find that the lifetime can be precisely modulated by controlling the duration of laser irradiation. It is determined that the lifetime gradually decreases with the increase of duration. As reported in the previous researches, the GO fluorescence originates from the graphene-like confined sp2 clusters and sp3 domains consisting of oxygen-containing functional groups, where the lifetime of sp3 domain is about 1.4 ns, and that of sp2 domain is 0.14 ns. During the photoreduction, the long-lived sp3 domains will decrease or convert into short-lived sp2 domains, resulting in the decrease of lifetime. Hence, by controlling the reduction degree or the ratio of the two domains, the lifetime of GO film can be determined. More importantly, the lifetime distributions of the reduction areas are very narrow, leading to a relatively homogenous background. The precise manipulation of lifetime can be used to fabricate micropatterns with high contrast. Combining with laser direct writing with features of maskless, facile processing ability and high spatial resolution, many versatile micropatterns, such as quick response code, barcode, graphic, alphabet, and numbers can be readily created based on the modulation of fluorescence lifetime. By using three optimized durations of laser irradiation, three distributions with narrow widths are obtained. Based on this processing, the micropatterns with three colors are determined, which indicates that the multimode optical recording can be created on the GO film based on the modulation of fluorescence lifetime. Furthermore, the multilayer micropatterns are also created. The robust and versatile micropatterns with film-thickness and excitation-power-independent features show their promising applications in electronics, photonics, display technology and information storage.

The electrical conductivity of graphene oxide (GO) and reduced graphene oxide (RGO) films with poly(allylamine hydrochloride) (PAH) supporting layers is investigated. Graphene-PAH hybrid films were produced in a two-step procedure that consisted of vacuum filtration for GO (or RGO) dispersion to fabricate the graphene thin films on quartz substrates, followed by the deposition of PAH onto the graphene films via solution casting. Highly selective deposition of the PAH layer on the graphene sheets was confirmed through the detection of the fluorescence signals of hybridized Cy3-DNA onto the PAH-coated graphene surfaces. In this case, electrostatic interaction plays an important role in the selective deposition process. Interestingly, it was found that the electrical conductivity of RGO films was significantly enhanced by 120% after PAH treatment, whereas that of the GO films was reduced by 98% of its initial conductivity. This finding was interpreted in terms of the molecular structure and oxygen functionalities of GO and RGO films combined with the ionic conduction characteristics of hydrated PAH on the RGO film.