Reduced Graphene Oxide: fundamentals and applications

In this paper we report our recent studies on the fundamental physical/chemical properties of supported reduced Graphene Oxide (rGO) obtained either via standard thermal annealing or under extreme-UV (EUV) light exposure alongside with investigations on its possible technological applications. rGO has been studied by X-ray Photoelectron Spectroscopy (XPS), micro-Raman Spectroscopy (μRS), and Optical Microscopy. rGO reduction degree has been calibrated on the basis of its color contrast (CC) providing a handy tool to quantitatively determine the fraction of sp The original choice of using EUV instead of UV light to photo-reduce supported GO is not only advantageous in terms of reduction efficiency but it also allows to introduce the concept of EUV photolithography (today limited to the silicon technology only) for the processing of graphene-based materials. Here we demonstrate resistless sub-micrometer GO photo-patterning over large areas ( 10 mm 2 ) This result is a relevant upgrade for the graphene-based technology that can take advantage, in this way, from the entire know-how of the EUV-based technology in view of an eco-sustainable all-carbon technology

Use of Optical Contrast To Estimate the Degree of Reduction of Graphene Oxide

We report an optical contrast study of graphene oxide on 72 nm Al2O3/Si(100) and 300 nm SiO2/Si(100) as a function of its reduction degree. The reduction has been performed by means of ultrahigh vacuum thermal annealing from 25 °C (pristine graphene oxide) to 670 °C. In parallel to the optical contrast investigation, performed with optical microscopy, the graphene oxide films have been characterized with core level X-ray photoemission spectroscopy and micro- Raman spectroscopy. The optical contrast of graphene oxide (normalized to the one measured for pure graphene) on both substrates ranges from ∼0.4 to 1.0 for pristine and 670 °C annealed graphene oxide, respectively. Optical microscopy and X-ray photoemission spectroscopy data have been crosscorrelated, leading to calibration graphs that demonstrate that just by simply measuring the optical contrast of graphene oxide one can determine with very good approximation the fraction of sp2 hybridized carbon

Response to NO<inf>2</inf> and other gases of resistive chemically exfoliated MoS<inf>2</inf>-based gas sensors

We report on the fabrication, the morphological, structural, and chemical characterization, and the study of the electrical response to NO2 and other gases of resistive type gas sensors based on liquid chemically exfoliated (in N-methyl pyrrolidone, NMP) MoS2 flakes annealed in air either at 150 °C or at 250 °C. The active material has been analyzed by scanning electron microscopy (SEM), and micro Raman and X-ray core level photoemission spectroscopies. SEM shows that MoS2 exfoliated flakes are interconnected between electrodes of the sensing device to form percolation paths. Raman spectroscopy of the flakes before annealing demonstrates that the flakes are constituted by crystalline MoS2, while, annealing at 250 °C, does not introduce a detectable bulk contamination in the expected form of MoO3. The sensor obtained by thermal annealing in air at 150 °C exhibits a peculiar p-type response under exposure to NO2. In line with core level spectroscopy evidences, this behavior is potentially ascribed to nitrogen substitutional doping of S vacancies in the MoS2 surface (nitrogen atoms being likely provided by the intercalated NMP). Thermal annealing the MoS2 flakes in air at 250 °C irreversibly sets an n-type behavior of the gas sensing device, with a NO2 detection limit of 20 ppb. This behavior is assigned, in line with core level spectroscopy data, to a significant presence of S vacancies in the MoS2 annealed flakes and to the surface co-existence of MoO3 arising from the partial oxidation of the flakes surface. Both p- and n-type sensors have been demonstrated to be sensitive also to relative humidity. The n-type sensor shows good electrical response under H2 exposure

Graphene oxide for gas detection under standard humidity conditions

Graphene oxide (GO) synthesis is the easiest way to functionalize graphene, preserving the high graphene surface to volume ratio. Therefore, GO is a promising candidate for gas sensing applications. In this paper, an easy-to-fabricate and high sensitivity GO-based gas sensor is proposed. The device is fabricated by drop-casting a solution of GO flakes dispersed in water on a prepatterned Si3N4 substrate with 30 mu m spaced Pt electrodes. The sensing material has been studied using scanning electron microscopy and x-ray photoelectron spectroscopy. The large lateral dimensions of the flakes (tens of microns) allow single GO flake to bridge adjacent electrodes. The high quality of the synthesized flakes results in the gas sensor high sensitivity to and low detection limit (20 ppb) of NO2. The gas sensor response to NO2 has been studied in various relative humidity environments and it is demonstrated not to be affected by the presence of water vapor. Finally, the gas sensor responses to acetone, toluene, ethanol, and ammonia are reported

Graphene Oxide as a Practical Solution to High Sensitivity Gas Sensing

Graphene and its related materials have attracted much interest in sensing applications because of their optimized ratio between active surface and bulk volume. In particular, several forms of oxidized graphene have been studied to optimize the sensing efficiency, sometimes moving away from practical solutions to boost performance. In this paper, we propose a practical, high-sensitivity, and easy to fabricate gas sensor based on high quality graphene oxide (GO), and we give the rationale to the high performance of the device. The device is fabricated by drop-casting water-dispersed single-layer GO flakes on standard 30 μm spaced interdigitated Pt electrodes. The exceptional size of the GO flakes (27 μm mean size and ∼500 μm maximum size) allows single GO flake to bridge adjacent electrodes. A typical p-type response is observed by testing the device in both reducing and oxidizing environments. The specific response to NO₂ is studied by varying the operating temperature and the gas concentration. Sensing activity is demonstrated to be mainly mediated by the oxygen functional groups. A 20 ppb detection limit is measured. Besides illustrating a simple and efficient approach to gas sensing, this work is an example of the versatility of graphene oxide, accomplishing tasks that are complementary to graphene

Large Area Extreme-UV Lithography of Graphene Oxide via Spatially Resolved Photoreduction

The ability to pattern graphene over large areas with nanometer resolution is the current request for nanodevice fabrication at the industrial scale. Existing methods do not match high throughput with nanometer resolution. We propose a high-throughput resistless extreme-UV (EUV) photolithographic approach operating with sub-micrometer resolution on large area (∼10 mm²) graphene oxide (GO) films via spatially resolved photoreduction. The efficiency of EUV photoreduction is tested with 46.9 nm coherent light produced by a table top capillary discharge plasma source. Irradiated samples are studied by X-ray photoemission spectroscopy (XPS) and micro-Raman Spectroscopy (μRS). XPS data show that 200 mJ/cm² EUV dose produces, onto pristine GO, a 6% increase of sp² carbon bonds and a 20% decrease of C–O bonds. μRS data demonstrate a photoreduction efficiency 2 orders of magnitude higher than the one reported in the literature for UV-assisted photoreduction. GO patterning is obtained modulating the EUV dose with a Lloyd’s interferometer. The lithographic features consist of GO stripes with modulated reduction degree. Such modulation is investigated and demonstrated by μRS on patterns with 2 μm periodicity

Use of Optical Contrast To Estimate the Degree of Reduction of Graphene Oxide

We report an optical contrast study of graphene oxide on 72 nm Al₂O₃/Si­(100) and 300 nm SiO₂/Si­(100) as a function of its reduction degree. The reduction has been performed by means of ultrahigh vacuum thermal annealing from 25 °C (pristine graphene oxide) to 670 °C. In parallel to the optical contrast investigation, performed with optical microscopy, the graphene oxide films have been characterized with core level X-ray photoemission spectroscopy and micro-Raman spectroscopy. The optical contrast of graphene oxide (normalized to the one measured for pure graphene) on both substrates ranges from ∼0.4 to 1.0 for pristine and 670 °C annealed graphene oxide, respectively. Optical microscopy and X-ray photoemission spectroscopy data have been cross-correlated, leading to calibration graphs that demonstrate that just by simply measuring the optical contrast of graphene oxide one can determine with very good approximation the fraction of sp² hybridized carbon