Gas sensing properties of graphene synthesized by chemical vapor deposition

A Comparative Review of Graphene and MXene-Based Composites towards Gas Sensing

Graphene and MXenes have emerged as promising materials for gas sensing applications due to their unique properties and superior performance. This review focuses on the fabrication techniques, applications, and sensing mechanisms of graphene and MXene-based composites in gas sensing. Gas sensors are crucial in various fields, including healthcare, environmental monitoring, and industrial safety, for detecting and monitoring gases such as hydrogen sulfide (H2S), nitrogen dioxide (NO2), and ammonia (NH3). Conventional metal oxides like tin oxide (SnO2) and zinc oxide (ZnO) have been widely used, but graphene and MXenes offer enhanced sensitivity, selectivity, and response times. Graphene-based sensors can detect low concentrations of gases like H2S and NH3, while functionalization can improve their gas-specific selectivity. MXenes, a new class of two-dimensional materials, exhibit high electrical conductivity and tunable surface chemistry, making them suitable for selective and sensitive detection of various gases, including VOCs and humidity. Other materials, such as metal-organic frameworks (MOFs) and conducting polymers, have also shown potential in gas sensing applications, which may be doped into graphene and MXene layers to improve the sensitivity of the sensors.

Building a cm2scale CVD graphene-based gas sensor: modelling the kinetic with a three-site adsorption/desorption Langmuir model

In this article, we aim to develop and study a highly sensitive and selective cm2scale graphene-based gas sensor. We present the technology used to fabricate sensors which integrate monolayer chemical vapour deposition graphene: photolithography and transfer of layers. Characterization techniques (optical microscopy, AFM, micro-Raman spectroscopy, transport electrical measurements) ensure a diagnosis of graphene ribbons and allow good reproducibility of technological processes. We present the results of gas characterizations after a 200 ppm NO2exposure. We propose a novel approach for the modelling of the sensor response with a three-site adsorption/desorption Langmuir model. This innovative way of modelling the sensor response should provide a better understanding of the sensor's kinetic and help to overcome the long response time observed with graphene gas sensors.

Green syntheses of graphene and its applications in internet of things (IoT)-a status review

Internet of Things (IoT) is a trending technological field that converts any physical object into a communicable smarter one by converging the physical world with the digital world. This innovative technology connects the device to the internet and provides a platform to collect real-time data, cloud storage, and analyze the collected data to trigger smart actions from a remote location via remote notifications, etc. Because of its wide-ranging applications, this technology can be integrated into almost all the industries. Another trending field with tremendous opportunities is Nanotechnology, which provides many benefits in several areas of life, and helps to improve many technological and industrial sectors. So, integration of IoT and Nanotechnology can bring about the very important field of Internet of Nanothings (IoNT), which can re-shape the communication industry. For that, data (collected from trillions of nanosensors, connected to billions of devices) would be the 'ultimate truth', which could be generated from highly efficient nanosensors, fabricated from various novel nanomaterials, one of which is graphene, the so-called 'wonder material' of the 21st century. Therefore, graphene-assisted IoT/IoNT platforms may revolutionize the communication technologies around the globe. In this article, a status review of the smart applications of graphene in the IoT sector is presented. Firstly, various green synthesis of graphene for sustainable development is elucidated, followed by its applications in various nanosensors, detectors, actuators, memory, and nano-communication devices. Also, the future market prospects are discussed to converge various emerging concepts like machine learning, fog/edge computing, artificial intelligence, big data, and blockchain, with the graphene-assisted IoT field to bring about the concept of 'all-round connectivity in every sphere possible'.

Numerical Investigation of Graphene as a Back Surface Field Layer on the Performance of Cadmium Telluride Solar Cell

This paper numerically explores the possibility of ultrathin layering and high efficiency of graphene as a back surface field (BSF) based on a CdTe solar cell by Personal computer one-dimensional (PC1D) simulation. CdTe solar cells have been characterized and studied by varying the carrier lifetime, doping concentration, thickness, and bandgap of the graphene layer. With simulation results, the highest short-circuit current (I_sc = 2.09 A), power conversion efficiency (η = 15%), and quantum efficiency (QE~85%) were achieved at a carrier lifetime of 1 × 10³ μs and a doping concentration of 1 × 10¹⁷ cm⁻³ of graphene as a BSF layer-based CdTe solar cell. The thickness of the graphene BSF layer (1 μm) was proven the ultrathin, optimal, and obtainable for the fabrication of high-performance CdTe solar cells, confirming the suitability of graphene material as a BSF. This simulation confirmed that a CdTe solar cell with the proposed graphene as the BSF layer might be highly efficient with optimized parameters for fabrication.

Thermal Mechanical Properties of Graphene Nano-Composites with Kevlar-Nomex Copolymer: A Comparison of the Physical and Chemical Interactions

This paper reports the preparation of Kevlar-Nomex copolymer nano-composites with exfoliated pristine and functionalized graphene sheets (Grs). The graphene oxide (GrO) platelets were amidized by the reaction of amine-terminated aramid (Ar) with the functional groups present on the GrO surface to prepare the nano-composites films with different loadings of GrO. Chemical changes involved during the oxidation and subsequent amidation were monitored by Raman, FTIR and XP spectroscopic analyses. Morphology of the composite films was studied by atomic force and scanning electron microscopies. Viscoelastic properties of the hybrid films were studied for their glass transition temperature (Tg) and storage modulus by dynamical mechanical thermal analysis (DMTA). A higher shift in glass transition temperature was obtained by chemically binding the aramid copolymer chains on the functionalized Gr sheets. The increase in tensile strength and modulus at various loadings of GrO are compared with the composites using pristine Gr. The effect of interfacial interactions between the matrix chains and the reinforcement on the properties of these hybrids have been explained.

Preparation and Test of NH3 Gas Sensor Based on Single-Layer Graphene Film

The ammonia sensing properties of single-layer graphene synthesized by chemical vapor deposition (CVD) were studied. The Au interdigitated electrode (IDE) was prepared by microelectromechanical systems (MEMS) technology, and then, the single-layer graphene was transferred to the IDE by wet transfer technology. Raman spectroscopy was used to monitor the quality of graphene films transferred to SiO₂/Si substrates. Moreover, the theory of graphene’s adsorption of gases is explained. The results show that gas sensing characteristics such as response/recovery time and response are related to the target gas, gas concentration, test temperature, and so on. In the stability test, the difference between the maximum resistance and the minimum resistance of the device is 1 ohm without ammonia, the change is less than 1% of its initial resistance, and the repeatability is up to 98.58%. Therefore, the sensor prepared with high quality single-layer graphene has good repeatability and stability for ammonia detection.

Fabrication of LaFeO3 and rGO-LaFeO3 microspheres based gas sensors for detection of NO2 and CO

In the present report, gas sensing devices based on LaFeO₃ and rGO-LaFeO₃ were fabricated by a photolithography technique. The X-ray diffraction, Raman spectra and FT-IR results confirm the formation of a perovskite phase and composite. XPS and TEM give the chemical compositions for both products. The higher roughness, greater surface area (62.1 m² g⁻¹), larger pore size (16.4 nm) and lower band gap (1.94 eV) of rGO-LaFeO₃ make it a suitable candidate to obtain high sensitivity. The gas sensing performance of the devices was investigated for various concentrations of NO₂ and CO gases at temperatures of 200 and 250 °C. It was observed that the rGO-LaFeO₃ based device exhibited a high relative response (183.4%) for a 3 ppm concentration of NO₂ at a 250 °C operating temperature. This higher response is attributed to the large surface area, greater surface roughness, and numerous active sites of rGO-LaFeO₃. The gas sensing properties investigated show that rGO-LaFeO₃ is an excellent candidate for an NO₂ sensor.

In the present report, gas sensing devices based on LaFeO₃ and rGO-LaFeO₃ were fabricated by a photolithography technique.

Bionanosensor based on N-doped graphene quantum dots coupled with CoOOH nanosheets and their application for in vivo analysis of ascorbic acid

Herein, we employ 3D nitrogen-doped porous graphene frameworks (NPG) as raw material to prepare emissive nitrogen doped graphene quantum dots (r-NGQDs) via chemical oxidation method. The as-prepared fluorescent r-NGQDs was integrated with CoOOH nanosheets to construct a sensing platform for in vivo ascorbic acid (AA) analysis. Initially, the fluorescence emission intensity of r-NGQDs was quenched by CoOOH nanosheets based on the inner filter effect (IFE). Then the quenched intensity of r-NGQDs and CoOOH nanosheets system was enlightened by addition of AA, since AA could consume CoOOH nanosheets through redox reaction, leading to the release of r-NGQDs and fluorescence restoration. Moreover, the restored fluorescence intensity of r-NGQDs is highly dependent on the concentration of AA which endows them as a quantitative analysis of AA with a limit of detection (LOD) reach up to1.85 μM (n = 3) in aqueous solution. Finally, the as constructed bionanosensor was further employed for in vivo analysis of AA in living rat brain microdialysate with basal value up to 9.4 ± 1.4 μM (n = 3).

Versatile and Tunable Electrical Properties of Doped Nonoxidized Graphene Using Alkali Metal Chlorides

With the rapid development of wearable and flexible electronics, graphene has been intensively studied for the transparent, hole transport electrode layer (HTL) of field-effect transistors, light-emitting diodes, and organic photovoltaic (OPV) cells. To modulate the sheet resistance and the work function of graphene as a HTL, the surface doping is versatile while retaining high transparency. In this work, we used a chemical doping method to control the charge carrier density, band gap, and work function of graphene with minimizing the damage of the carbon network, for which metal chlorides (NaCl, KCl, and AuCl₃) were used as chemical dopants. The high-quality graphene flakes were synthesized with large lateral sizes of more than 5 μm using ternary graphite intercalation compounds. Interestingly, the AuCl₃-doped graphene flake film with a film thickness of about 20 nm showed the lowest reported sheet resistance of ∼249 Ω/sq with ∼75% transmittance. Furthermore, it could control the work function from 4.32 to 5.1 eV. The interfacial dipole complexes of metal cations with a low work function and the reactive radicals such as -OH were discussed to explain this result. For the practical application, an OPV device using the AuCl₃-doped graphene flake film as the HTL was fabricated and it demonstrated enhanced power conversion efficiency while maintaining high optical transparency in visible light.

A uniform stable P-type graphene doping method with a gold etching process

Graphene is one of the materials with the most potential for post-silicon electronics because of its outstanding electrical, optical, and mechanical properties. However, the lack of a uniform stable doping method extremely limits the various possible applications of graphene. Here, we developed a uniform and stable graphene efficient p-doping method. Through etching a thin gold film on graphene with a KI/I₂ solution, iodine complexes are produced as the dopant absorbing on the graphene surface, and induce extra holes in graphene. Utilizing this method, the graphene film can be effectively doped to p-type without producing undesirable defects, and the roughness of the graphene surface can still be maintained at an ultra-low nanoscale (RMS roughness ∼0.739 nm). The doping effectiveness can be clearly verified by the changes in the Raman spectrum, and the Dirac point shift of the graphene-based transistor, and the reduction of sheet resistance (∼27.2%). Furthermore, the substantially coincident transfer curves after 45 days reveal the long-term stable doping effects. Therefore, this doping method can exploit a way for various graphene-based applications, such as phototransistors, sensors, and organic thin-film transistors.

Graphene Decorated with Silver Nanoparticles as a Low-Temperature Methane Gas Sensor

This paper is devoted to an investigation on the methane sensing properties of graphene (G), decorated with silver nanoparticles (AgNPs), under ambient conditions. To do so, we first present an effective modification in the standard manner of decorating graphene by AgNPs. From structural analysis of the product (AgNPs/G), it is concluded that graphene is indeed decorated by AgNPs of a mean size 29.3 nm, free of aggregation, with a uniform distribution. The so-produced material is then used, as a resistivity-based sensor, to examine its response to the presence of methane gas. Our measurements are performed at relatively low temperatures, for various silver-to-graphene mass ratios (SGMRs) and methane concentrations. To account for the effects of humidity, we have made the measurements, at room temperature, for different levels of humidity. Our results demonstrate that an increase in the SGMR enhances the response of AgNPs/G to methane with an optimum value of SGMR ≅ 12%. It is also illustrated that for methane concentrations less than 2000 ppm, the maximal response increases linearly and rapidly, even at room temperature. Moreover, we demonstrate that AgNPs/G is of low limit of detection, highly stable, selective, reversible, repeatable, and sensor-to-sensor reproducible, for methane sensing. The results thus promise a low-cost and simple-to-fabricate methane sensing device.

A Reduced GO-Graphene Hybrid Gas Sensor for Ultra-Low Concentration Ammonia Detection

A hybrid structure gas sensor of reduced graphene oxide (RGO) decorated graphene (RGO-Gr) is designed for ultra-low concentration ammonia detection. The resistance value of the RGO-Gr hybrid is the indicator of the ammonia concentration and controlled by effective charge transport from RGO to graphene after ammonia molecule adsorption. In this hybrid material, RGO is the adsorbing layer to catch ammonia molecules and graphene is the conductive layer to effectively enhance charge/electron transport. Compared to a RGO gas sensor, the signal-to-noise ratio (SNR) of the RGO-Gr is increased from 22 to 1008. Meanwhile, the response of the RGO-Gr gas sensor is better than that of either a pristine graphene or RGO gas sensor. It is found that the RGO reduction time is related to the content of functional groups that directly reflect on the gas sensing properties of the sensor. The RGO-Gr gas sensor with 10 min reduction time has the best gas sensing properties in this type of sensor. The highest sensitivity is 2.88% towards 0.5 ppm, and the ammonia gas detection limit is calculated to be 36 ppb.

Fe3O4-decorated graphene assembled porous carbon nanocomposite for ammonia sensing: study using an optical fiber Fabry-Perot interferometer

A porous graphene-coated optical fiber Fabry-Perot interferometer (G-FPI) and Fe3O4-graphene nanocomposite coated Fabry-Perot interferometer (FG-FPI) have been investigated and compared for the detection of ammonia gas at room temperature. The sensor probes were subjected to ammonia concentrations varying from 1.5 ppm to 150 ppm. An increased sensitivity was observed for FG-FPI (36 pm ppm-1) when compared with that of G-FPI (25 pm ppm-1). The observed sensor detection limits for FG-FPI and G-FPI were around 7 and 10 ppb, respectively. The sensing mechanism was based on the change in refractive index/dielectric constant of the material; which changed the conductivity of coated material in presence of NH3. It was observed that the modified refractive index induced a wavelength shift in the FPI. The highly porous structure of graphene and the uniform dispersion of Fe3O4 nanoparticles into this framework effectively facilitated the target gas diffusion and hence improved the sensing performance. The sensing was correlated to the oxygen vacancies on the Fe3O4 surfaces and the depletion region manipulations with the ammonia interactions along with Schottky-type electron conductivity via the conducting graphene assembled porous carbon framework. The mathematical evaluation of the phenomenon also justified the excellent repeatability and reversibility of this sensitive, room temperature sensor.

Chemiresistive Graphene Sensors for Ammonia Detection

The primary objective of this work is to demonstrate a novel sensor system as a convenient vehicle for scaled-up repeatability and the kinetic analysis of a pixelated testbed. This work presents a sensor system capable of measuring hundreds of functionalized graphene sensors in a rapid and convenient fashion. The sensor system makes use of a novel array architecture requiring only one sensor per pixel and no selector transistor. The sensor system is employed specifically for the evaluation of Co(tpfpp)ClO₄ functionalization of graphene sensors for the detection of ammonia as an extension of previous work. Co(tpfpp)ClO₄ treated graphene sensors were found to provide 4-fold increased ammonia sensitivity over pristine graphene sensors. Sensors were also found to exhibit excellent selectivity over interfering compounds such as water and common organic solvents. The ability to monitor a large sensor array with 160 pixels provides insights into performance variations and reproducibility-critical factors in the development of practical sensor systems. All sensors exhibit the same linearly related responses with variations in response exhibiting Gaussian distributions, a key finding for variation modeling and quality engineering purposes. The mean correlation coefficient between sensor responses was found to be 0.999 indicating highly consistent sensor responses and excellent reproducibility of Co(tpfpp)ClO₄ functionalization. A detailed kinetic model is developed to describe sensor response profiles. The model consists of two adsorption mechanisms-one reversible and one irreversible-and is shown capable of fitting experimental data with a mean percent error of 0.01%.

Chemical sensing with 2D materials

During the last decade, two-dimensional materials (2DMs) have attracted great attention due to their unique chemical and physical properties, which make them appealing platforms for diverse applications in sensing of gas, metal ions as well as relevant chemical entities.

The Impact of Graphene on the Fabrication of Thin Film Solar Cells: Current Status and Future Prospects

Commercial solar cells have a power conversion efficiency (PCE) in the range of 10–22% with different light absorbers. Graphene, with demonstrated unique structural, physical, and electrical properties, is expected to bring the positive effects on the development of thin film solar cells. Investigations have been carried out to understand whether graphene can be used as a front and back contacts and active interfacial layer in solar cell fabrication. In this review, the current progress of this research is analyzed, starting from the graphene and graphene-based Schottky diode. Also, the discussion was focused on the progress of graphene-incorporated thin film solar cells that were fabricated with different light absorbers, in particular, the synthesis, fabrication, and characterization of devices. The effect of doping and layer thickness of graphene on PCE was also included. Currently, the PCE of graphene-incorporated bulk-heterojunction devices have enhanced in the range of 0.5–3%. However, device durability and cost-effectiveness are also the challenging factors for commercial production of graphene-incorporated solar cells. In addition to the application of graphene, graphene oxides have been also used in perovskite solar cells. The current needs and likely future investigations for graphene-incorporated solar cells are also discussed.

Human-Like Sensing and Reflexes of Graphene-Based Films

Humans have numerous senses, wherein vision, hearing, smell, taste, and touch are considered as the five conventionally acknowledged senses. Triggered by light, sound, or other physical stimulations, the sensory organs of human body are excited, leading to the transformation of the afferent energy into neural activity. Also converting other signals into electronical signals, graphene-based film shows its inherent advantages in responding to the tiny stimulations. In this review, the human-like senses and reflexes of graphene-based films are presented. The review starts with the brief discussions about the preparation and optimization of graphene-based film, as where as its new progress in synthesis method, transfer operation, film-formation technologies and optimization techniques. Various human-like senses of graphene-based film and their recent advancements are then summarized, including light-sensitive devices, acoustic devices, gas sensors, biomolecules and wearable devices. Similar to the reflex action of humans, graphene-based film also exhibits reflex when under thermal radiation and light actuation. Finally, the current challenges associated with human-like applications are discussed to help guide the future research on graphene films. At last, the future opportunities lie in the new applicable human-like senses and the integration of multiple senses that can raise a revolution in bionic devices.

High-Performance Hybrid Electronic Devices from Layered PtSe2 Films Grown at Low Temperature

Layered two-dimensional (2D) materials display great potential for a range of applications, particularly in electronics. We report the large-scale synthesis of thin films of platinum diselenide (PtSe₂), a thus far scarcely investigated transition metal dichalcogenide. Importantly, the synthesis by thermally assisted conversion is performed at 400 °C, representing a breakthrough for the direct integration of this material with silicon (Si) technology. Besides the thorough characterization of this 2D material, we demonstrate its promise for applications in high-performance gas sensing with extremely short response and recovery times observed due to the 2D nature of the films. Furthermore, we realized vertically stacked heterostructures of PtSe₂ on Si which act as both photodiodes and photovoltaic cells. Thus, this study establishes PtSe₂ as a potential candidate for next-generation sensors and (opto-)electronic devices, using fabrication protocols compatible with established Si technologies.

Spectroscopic Investigation of Plasma-Fluorinated Monolayer Graphene and Application for Gas Sensing

Large-area monolayer fluorinated graphene (FG) is synthesized by a controllable SF6 plasma treatment. The functional groups of FG are elucidated by various spectroscopies, including Raman spectroscopy, X-ray photoemission spectroscopy (XPS), and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Raman results suggest that the defects are introduced into the monolayer graphene during the fluorination process. The fluorine content can be varied by the plasma treatment and can reach the maximum (∼24.6 atom % F) under 20 s of plasma treatment as examined by XPS measurement. The angle-dependent NEXAFS results reveal that the fluorine atoms interact with the graphene matrix to form covalent C-F bonds, which are perpendicular to the basal plane of FG. FG is applied as a gas-sensing material and owns much better performance for ammonia detection compared to pristine graphene. On the basis of our density functional theory simulation results, the fast response/recovery behavior and high sensitivity of the FG gas sensor are attributed to enhanced physical absorption due to the C-F covalent bonds on the surface of FG.

Flexible humidity sensors composed of graphite-like carbon micro-pinecone arrays

We have prepared flexible graphite-like carbon humidity sensors at room temperature in air.

Ultrasensitive gas detection of large-area boron-doped graphene

Heteroatom doping is an efficient way to modify the chemical and electronic properties of graphene. In particular, boron doping is expected to induce a p-type (boron)-conducting behavior to pristine (nondoped) graphene, which could lead to diverse applications. However, the experimental progress on atomic scale visualization and sensing properties of large-area boron-doped graphene (BG) sheets is still very scarce. This work describes the controlled growth of centimeter size, high-crystallinity BG sheets. Scanning tunneling microscopy and spectroscopy are used to visualize the atomic structure and the local density of states around boron dopants. It is confirmed that BG behaves as a p-type conductor and a unique croissant-like feature is frequently observed within the BG lattice, which is caused by the presence of boron-carbon trimers embedded within the hexagonal lattice. More interestingly, it is demonstrated for the first time that BG exhibits unique sensing capabilities when detecting toxic gases, such as NO2 and NH3, being able to detect extremely low concentrations (e.g., parts per trillion, parts per billion). This work envisions that other attractive applications could now be explored based on as-synthesized BG.

Thiourea-treated graphene aerogel as a highly selective gas sensor for sensing of trace level of ammonia

As a result of this study, a new and simple method was proposed for the fabrication of an ultra sensitive, robust and reversible ammonia gas sensor. The sensing mechanism was based upon the change in electrical resistance of a graphene aerogel as a result of sensor exposing to ammonia. Three-dimensional graphene hydrogel was first synthesized via hydrothermal method in the absence or presence of various amounts of thiourea. The obtained material was heated to obtain aerogel and then it was used as ammonia gas sensor. The materials obtained were characterized using different techniques such as Fourier transform infra red spectroscopy (FT-IR), thermal gravimetric analysis (TGA), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The thiourea-treated graphene aerogel was more porous (389 m(2) g(-1)) and thermally unstable and exhibited higher sensitivity, shorter response time and better selectivity toward ammonia gas, compared to the aerogel produced in the absence of thiourea. Thiourea amount, involved in the hydrogel synthesis step, was found to be highly effective factor in the sensing properties of finally obtained aerogel. The sensor response time to ammonia was short (100 s) and completely reversible (recovery time of about 500 s) in ambient temperature. The sensor response to ammonia was linear between 0.02 and 85 ppm and its detection limit was found to be 10 ppb (3S/N).

Multifunctional graphene sensors for magnetic and hydrogen detection

Multifunctional graphene magnetic/hydrogen sensors are constructed for the first time through a simple microfabrication process. The as-fabricated graphene sensor may act as excellent Hall magnetic detector, demonstrating small linearity error within 2% and high magnetic resolution up to 7 mG/Hz(0.5). Meanwhile the same graphene sensor is also demonstrated as high-performance hydrogen sensor with high gas response, excellent linearity, and great repeatability and selectivity. In particular, the graphene sensor exhibits high hydrogen response up to 32.5% when exposed to 1000 ppm hydrogen, outperforming most graphene-based hydrogen sensors. In addition the hydrogen-sensing mechanism of Pd-decorated graphene is systematically explored through investigating its transfer characteristics during gas detection. Our work demonstrates that graphene is a terrific material for multifunctional sensing, which may in principle reduce the complexity of manufacturing process, lower the number of sensors required in the sensing systems, and potentially derive new and more powerful functions.

Environmental applications of graphene-based nanomaterials

A critical assessment of recent developments in environmental applications of graphene and graphene-based materials.

A backside fluorine-functionalized graphene layer for ammonia detection

Sensing properties of the back-side fluorinated graphene to ammonia gas are significantly enhanced in comparison with pristine graphene. The NH₃–graphene interaction model based on the concentration dependence measurements is supported by quantum-chemical calculations.

The CO gas sensing properties of direct-patternable SnO2 films containing graphene or Ag nanoparticles

The CO gas sensing properties of direct-patternable SnO₂ composite thin films were more effectively improved by the incorporation of Ag nanoparticles than graphene.

Monatomic chemical-vapor-deposited graphene membranes bridge a half-millimeter-scale gap

One of the main concerns in nanotechnology is the utilization of nanomaterials in macroscopic applications without losing their extreme properties. In an effort to bridge the gap between the nano- and macroscales, we propose a clever fabrication method, the inverted floating method (IFM), for preparing freestanding chemical-vapor-deposited (CVD) graphene membranes. These freestanding membranes were then successfully suspended over a gap a half-millimeter in diameter. To understand the working principle of IFM, high-speed photography and white light interferometry were used to characterize and analyze the deformation behaviors of the freestanding graphene membranes in contact with a liquid during fabrication. Some nanoscale configurations in the macroscopic graphene membranes were able to be characterized by simple optical microscopy. The proposed IFM is a powerful approach to investigating the macroscopic structures of CVD graphene and enables the exploitation of freestanding CVD graphene for device applications.

Wrinkled reduced graphene oxide nanosheets for highly sensitive and easy recoverable NH3 gas detector

Highly wrinkled reduced graphene oxide nanosheets were prepared by chemical exfoliation of ball-milled graphite powder. This material showed high sensitivity and simple recovery ability for NH₃ gas detection.

Reduced graphene oxide and graphene composite materials for improved gas sensing at low temperature

Reduced graphene oxide (rGO) was investigated as a material for use in chemiresistive gas sensors. The carbon nanomaterial was transferred onto a silicon wafer with interdigital gold electrodes. Spin coating turned out to be the most reliable transfer technique, resulting in consistent rGO layers of reproducible quality. Fast changes in the electrical resistance at a low operating temperature of 85 °C could be detected for the gases NO₂, CH₄ and H₂. Especially upon adsorption of NO₂ the high signal changes allowed a minimum detection of 0.3 ppm (S/N = 3). To overcome the poor selectivity, rGO was chemically functionalized with octadecylamine, or modified by doping with metal nanoparticles such as Pd and Pt, and also metal oxides such as MnO₂, and TiO₂. The different response patterns for six different materials allowed the discrimination of all of the test gases by pattern recognition based on principal component analysis.

Graphene/poly(methyl methacrylate) chemiresistor sensor for formaldehyde odor sensing

It was demonstrated that a thin film of chemically exfoliated graphene flakes, blended with poly(methyl methacrylate), could be used as a chemiresistor sensor for the sensitive and selective determination of formaldehyde vapor. Formaldehyde adsorption on the sensing film led to an increase in the electrical resistance of the sensing film. It was demonstrated that direct interaction of formaldehyde with graphene flakes was responsible for the observed response. Graphene/polymer ratio was found to be important parameter in defining the dominant sensing mechanism and sensor performance. A reasonable relationship was found between formaldehyde concentration and electrical resistance response of the sensor. The amounts of graphene and polymer in the sensing composite were optimized by using three-level full factorial design in order to acquire higher sensitivity and selectivity. Final sensor response for formaldehyde concentration was linear between 0.05 and 5.0 ppm. The detection limit of this sensing device was calculated equal to 10 ppb.

How do the electrical properties of graphene change with its functionalization?

Functionalization of graphene is essential to interface it with other moieties to expand the scope of its electrical/electronic applications. However, chemical functionalization and/or molecular interactions on graphene sensitively modulate its electrical properties. To evaluate and take advantage of the properties of functionalized graphene, it is important to understand how its electrical attributes (such as carrier scattering, carrier concentration, charge polarity, quantum-capacitance enhanced doping, energy levels, transport mechanisms, and orbital hybridization of energy-bands) are influenced by a change in carbon's structural conformation, hybridization state, chemical potential, local energy levels, and dopant/interface coupling induced via functionalization or molecular interactions. Here, a detailed and integrated model describes factors influencing these electrical characteristics of functionalized graphene (covalent bonds, adsorption, π-π bonds, and lattice incorporation). The electrical properties are governed via three mechanisms: (a) conversion of carbon's hybridized state, (b) dipole interactions enhanced via quantum capacitance, and (c) orbital hybridization with an interfacing molecule. A few graphenic materials are also identified where further studies are essential to understand the effect of their functionalization.

Enhancement of hydrogen gas sensing of nanocrystalline nickel oxide by pulsed-laser irradiation

This paper reports the effect of post-laser irradiation on the gas-sensing behavior of nickel oxide (NiO) thin films. Nanocrystalline NiO semiconductor thin films were fabricated by a sol-gel method on a nonalkaline glass substrate. The NiO samples were irradiated with a pulsed 532-nm wavelength, using a Nd:YVO(4) laser beam. The effect of laser irradiation on the microstructure, electrical conductivity, and gas-sensing properties was investigated as a function of laser power levels. It was found that the crystallinity and surface morphology were modified by the pulsed-laser irradiation. Hydrogen gas sensors were fabricated using both as-deposited and laser-irradiated NiO films. It was observed that the performance of gas-sensing characteristics could be changed by the change of laser power levels. By optimizing the magnitude of the laser power, the gas-sensing property of NiO thin film was improved, compared to that of as-deposited NiO films. At the optimal laser irradiation conditions, a high response of NiO sensors to hydrogen molecule exposure of as little as 2.5% of the lower explosion threshold of hydrogen gas (40,000 ppm) was observed at 175 °C.