Extremely high response of electrostatically exfoliated few layer graphene to ammonia adsorption

Gas Sensors Based on Chemically Reduced Holey Graphene Oxide Thin Films

The nanosheet stacking phenomenon in graphene thin films significantly deteriorates their gas-sensing performance. This nanosheet stacking issue should be solved and reduced to enhance the gas detection sensitivity. In this study, we report a novel ammonia (NH₃) gas sensor based on holey graphene thin films. The precursors, holey graphene oxide (HGO) nanosheets, were prepared by etching graphene under UV irradiation with Fenton reagent (Fe²⁺/Fe³⁺/H₂O₂). Holey graphene was prepared by the reduction of HGO (rHGO) with pyrrole. Holey graphene thin-film gas sensors were prepared by depositing rHGO suspensions onto the electrodes. The resulting sensing devices show excellent response, sensitivity, and selectivity to NH₃. The resistance change is 2.81% when the NH₃ level is as low as 1 ppm, whereas the resistance change is 11.32% when the NH₃ level is increased to 50 ppm. Furthermore, the rHGO thin-film gas sensor could be quickly restored to their initial states without the stimulation with an IR lamp. In addition, the devices showed excellent repeatability. The resulting rHGO thin-film gas sensor has a great potential for applications in numerous sensing fields because of its low cost, low energy consumption, and outstanding sensing performance.

Improved NO2 Gas Sensing Properties of Graphene Oxide Reduced by Two-beam-laser Interference

We report on the fabrication of a NO₂ gas sensor from room-temperature reduction of graphene oxide(GO) via two-beam-laser interference (TBLI). The method of TBLI gives the distribution of periodic dissociation energies for oxygen functional groups, which are capable to reduce the graphene oxide to hierarchical graphene nanostructures, which holds great promise for gaseous molecular adsorption. The fabricated reduced graphene oxide(RGO) sensor enhanced sensing response in NO₂ and accelerated response/recovery rates. It is seen that, for 20 ppm NO₂, the response (R_a/R_g) of the sensor based on RGO hierarchical nanostructures is 1.27, which is higher than that of GO (1.06) and thermal reduced RGO (1.04). The response time and recovery time of the sensor based on laser reduced RGO are 10 s and 7 s, which are much shorter than those of GO (34 s and 45 s), indicating that the sensing performances for NO₂ sensor at room temperature have been enhanced by introduction of nanostructures. This mask-free and large-area approach to the production of hierarchical graphene micro-nanostructures, could lead to the implementation of future graphene-based sensors.

Flexible Transparent Electrode of Hybrid Ag-Nanowire/Reduced-Graphene-Oxide Thin Film on PET Substrate Prepared Using H2/Ar Low-Damage Plasma

We employ H₂/Ar low-damage plasma treatment (H₂/Ar-LDPT) to reduce graphene oxide (GO) coating on a polymer substrate-polyethylene terephthalate (PET)-with the assistance of atomic hydrogen (H_α) at low temperature of 70 °C. Four-point probing and ultraviolet-visible (UV-Vis) spectroscopy demonstrate that the conductivity and transmittance can be controlled by varying the H₂/Ar flow rate, treatment time, and radio-frequency (RF) power. Optical emission spectroscopy reveals that the H_α intensity depends on these processing parameters, which influence the removal of oxidative functional groups (confirmed via X-ray photoelectron spectroscopy) to yield reduced GO (rGO). To further improve the conductivity while maintaining high transmittance, we introduce silver nanowires (AgNWs) between rGO and a PET substrate to obtain a hybrid rGO/AgNWs/PET with a sheet resistance of ~100 Ω/sq and 81% transmittance. In addition, the hybrid rGO/AgNWs thin film also shows high flexibility and durability and is suitable for flexible and wearable electronics applications.

Nanostructured Materials for Room-Temperature Gas Sensors

Sensor technology has an important effect on many aspects in our society, and has gained much progress, propelled by the development of nanoscience and nanotechnology. Current research efforts are directed toward developing high-performance gas sensors with low operating temperature at low fabrication costs. A gas sensor working at room temperature is very appealing as it provides very low power consumption and does not require a heater for high-temperature operation, and hence simplifies the fabrication of sensor devices and reduces the operating cost. Nanostructured materials are at the core of the development of any room-temperature sensing platform. The most important advances with regard to fundamental research, sensing mechanisms, and application of nanostructured materials for room-temperature conductometric sensor devices are reviewed here. Particular emphasis is given to the relation between the nanostructure and sensor properties in an attempt to address structure-property correlations. Finally, some future research perspectives and new challenges that the field of room-temperature sensors will have to address are also discussed.

Molecular adsorption on graphene

Current studies addressing the engineering of charge carrier concentration and the electronic band gap in epitaxial graphene using molecular adsorbates are reviewed. The focus here is on interactions between the graphene surface and the adsorbed molecules, including small gas molecules (H(2)O, H(2), O(2), CO, NO(2), NO, and NH(3)), aromatic, and non-aromatic molecules (F4-TCNQ, PTCDA, TPA, Na-NH(2), An-CH(3), An-Br, Poly (ethylene imine) (PEI), and diazonium salts), and various biomolecules such as peptides, DNA fragments, and other derivatives. This is followed by a discussion on graphene-based gas sensor concepts. In reviewing the studies of the effects of molecular adsorption on graphene, it is evident that the strong manipulation of graphene's electronic structure, including p- and n-doping, is not only possible with molecular adsorbates, but that this approach appears to be superior compared to these exploiting edge effects, local defects, or strain. However, graphene-based gas sensors, albeit feasible because huge adsorbate-induced variations in the relative conductivity are possible, generally suffer from the lack of chemical selectivity.

Tunable properties of few-layer graphene-N-methylpyrrolidone hybrid structures

A few-layer graphene-based hybrid material with high thermal and chemical stability and reproducible and tunable electronic properties was fabricated by intercalation of N-methylpyrrolidone into a few-layer graphene combined with heat treatment. Depending on the process temperature, the obtained material could be produced with the following properties: a broad range of resistivity values (six to seven orders of magnitude) in combination with a high carrier mobility, a tunable band-gap (from 0 up to 3-4 eV) and sp² or sp³ hybridization of carbon atoms. The extremely strong step-like temperature dependence (within 10 °C) of its properties observed in the vicinity of two temperatures, 90 and 200 °C, seems to be important for various applications. The hybrid material opens viable routes to progress in the design of three-dimensional nanostructures.