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Because of their advanced properties inherited from their constituent atomic layers, van der Waals heterostructures such as graphene/MoS 2 are promising candidates for many optical and electronic applications. However, because heat tends to be generated during the operation of nanodevices, thermal expansion is an important phenomenon to consider for the thermal stability of such heterostructures. In the present work, molecular dynamics simulations are used to investigate the thermal expansion coefficient of the graphene/MoS 2 heterostructure, and how the unavoidable misfit strain affects that coefficient is revealed. The misfit strain can tune the thermal expansion coefficient by a factor of six, and this effect is quite robust in the sense that it is insensitive to the size or direction of the heterostructure. Further analysis shows that the misfit strain offers an efficient means of engineering thermally induced ripples, this being the key mechanism for how the misfit strain affects the thermal expansion coefficient. These findings provide valuable information about the thermal stability of van der Waals heterostructures and offer help for practical applications of nanodevices based on such heterostructures.

Compared with two-dimensional (2D) graphene sheets, recently, three-dimensional (3D) structured and porous graphene has attracted much attention in gas and humidity sensing owing to its increased s...

To address the low gas sensitivity of pristine graphene (Gr), chemical modification of Gr has been proved as a promising route. However, the existing chemical functionalization method imposes the u...

The thermal conductivity of two-dimensional materials like graphene can efficiently be tuned by introducing holes, in which the density and distribution of the holes are the key parameters. Furthermore, the distribution of holes can induce a variation as high as 74% in the thermal conductivity for porous graphene with a given density of holes. Therefore, an existing challenge is to find the optimal distribution of holes that can minimize or maximize the thermal conductivity of porous graphene as the design space expands dramatically with increasing hole density. We therefore apply an inverse design methodology based on machine learning to reveal the relationship between hole distribution and thermal conductivity reduction in monolayer graphene. The methodology reveals that holes that are randomly distributed transverse to the direction of heat flow, but that exhibit some periodicity along the direction of heat flow, represent the optimal distribution to minimizing the thermal conductivity for porous graphene. Lattice dynamics calculations and wave packet simulations reveal that this spatial distribution effectively causes localization of the phonon modes in porous graphene, which reduces the thermal conductivity. Overall, this work demonstrates the power of machine learning-based design approaches to efficiently obtain new physical insights for scientific problems of interest.

A facile, one-step hydrothermal route was exploited to prepare SnO2-decorated reduced graphene oxide hydrogel (SnO2/RGOH) with three-dimensional (3D) porous structures for NO2 gas detection. Various material characterizations demonstrate the effective deoxygenation of graphene oxide and in situ growth of rutile SnO2 nanoparticles (NPs) on 3D RGOH. Compared with the pristine RGOH, the SnO2/RGOH displayed much lower limit of detection (LOD) and an order of magnitude higher sensitivity, revealing the distinct impact of SnO2 NPs in improving the NO2-sensing properties. An exceptional low theoretical LOD of 2.8 ppb was obtained at room temperature. The p-n heterojunction formed at the interface between RGOH and SnO2 facilitates the charge transfer, improving both the sensitivity in NO2 detection and the conductivity of hybrid material. Considering that existing SnO2/RGO-based NO2 sensors suffer from great vulnerability to humidity, here we employed integrated microheaters to effectively suppress the response to humidity, with nearly unimpaired response to NO2, which boosted the selectivity. Notably, a flexible NO2 sensor was constructed on a liquid crystal polymer substrate with endurance to mechanical deformation. This work indicates the feasibility of optimizing the gas-sensing performance of sensors by combining rational material hybridization, 3D structural engineering with temperature modulation.

We report a small-angle ultra-narrowband mid-infrared tunable absorber that uses graphene and dielectric metamaterials. The absorption bandwidth of the absorber at the graphene Fermi level of 0.2 eV is 0.055 nm, and the absorption peaks can be tuned from 5.14803 to 5.1411 μm by changing the graphene Fermi level. Furthermore, the resonance absorption only occurs in the angle range of several degrees. The simulation field distributions show the magnetic resonance and Fabry–Pérot resonance at the resonance absorption peak. The one-dimensional photonic crystals (1DPCs) in this absorber act as a Bragg mirror to efficiently reflect the incidence light. The simulation results also show that the bandwidth can be further narrowed by increasing the resonance cavity length. As a tunable mid-infrared thermal source, this absorber can possess both high temporal coherence and near-collimated angle characteristics, thus providing it with potential applications.

Heteroatom-doping has been proved as an effective method to modulate the electronic, physical, and chemical properties of graphene (Gr). Developing a new strategy of heteroatom-doping for high-perf...

A biomimetic CoPe@CdS/rGO hybrid that self-assembles via the integration of a molecular cobalt catalyst and CdS nano-semiconductor on reduced graphene oxide was constructed for boosting photocatalytic H2 production. Photoinduced electron transfer from CdS/rGO to the molecular catalyst occurs and a long-lived charge-separation state forms for high H2 production.

Flexible humidity sensors enabling the real-time monitoring of the humidity level in the environment or around the human body have increasing application prospects in electronic skin and personal healthcare. Tremendous efforts have been devoted to improve the performance of humidity sensors, but little effort has been made to investigate the effect of the substrate. This work reveals the vital role of the sensor substrate in determining the humidity sensing performance by performing a comparative study. A porous, hydrophobic and flexible liquid crystal polymer (LCP) substrate is employed to speed up the response and recovery processes and boost the sensitivity simultaneously. Graphene oxide-based sensors fabricated on the LCP substrate exhibit 8.1 and 3.2 times higher response and recovery speeds, respectively, and 1.37 times higher sensitivity than corresponding sensors generated on a smooth SiO2 substrate. Similar performance improvement is observed when reduced graphene oxide and carbon nanocoils are deployed as the humidity sensing materials. These distinct structure-related properties enable the fabrication of humidity sensors to monitor various human activities, including fast and normal respiration, skin hydration, finger movement, etc. Furthermore, a positive correlation between the sensitivity and the oxygen content in carbon materials is revealed.

Gas sensor technology is widely utilized in various areas ranging from home security, environment and air pollution, to industrial production. It also hold great promise in non-invasive exhaled breath detection and an essential device in future internet of things. The past decade has witnessed giant advance in both fundamental research and industrial development of gas sensors, yet current efforts are being explored to achieve better selectivity, higher sensitivity and lower power consumption. The sensing layer in gas sensors have attracted dominant attention in the past research. In addition to the conventional metal oxide semiconductors, emerging nanocomposites and graphene-like two-dimensional materials also have drawn considerable research interest. This inspires us to organize this comprehensive 2020 gas sensing materials roadmap to discuss the current status, state-of-the-art progress, and present and future challenges in various materials that is potentially useful for gas sensors.

The successful discovery and isolation of graphene in 2004, and the subsequent synthesis of layered semiconductors and heterostructures beyond graphene have led to the exploding field of two-dimensional (2D) materials that explore their growth, new atomic-scale physics, and potential device applications. This review aims to provide an overview of theoretical, computational, and machine learning methods and tools at multiple length and time scales, and discuss how they can be utilized to assist/guide the design and synthesis of 2D materials beyond graphene. We focus on three methods at different length and time scales as follows: (i) nanoscale atomistic simulations including density functional theory (DFT) calculations and molecular dynamics simulations employing empirical and reactive interatomic potentials; (ii) mesoscale methods such as phase-field method; and (iii) macroscale continuum approaches by coupling thermal and chemical transport equations. We discuss how machine learning can be combined with computation and experiments to understand the correlations between structures and properties of 2D materials, and to guide the discovery of new 2D materials. We will also provide an outlook for the applications of computational approaches to 2D materials synthesis and growth in general.

A facile, cost-effective and two-step strategy is developed to fabricate a nanoporous graphene (Gr) thin film for enhanced gas sensing at room temperature. Microwave plasma enhanced chemical vapor deposition (MPCVD) is first exploited to synthesize 3D Gr spherical structures, which are subsequently fragmented by ultrasonic treatment in liquid. Deposition of the fragmented 3D Gr structures on a substrate leads to the formation of a nanoporous Gr thin film, which is exploited as a conductance channel to detect CO2 and NH3 with impressive performance, including high sensitivity, linearity, low limit of detection (LOD) and reversibility. Compared with traditional gas sensors based on planar Gr sheets, this nanoporous Gr thin film displays remarkable 4.2 times higher response to CO2 and 10.4 times larger response to NH3, demonstrating the advantage of nanoporous Gr in improving the sensitivity. CO2 with a wide concentration range from 20 to 1000 ppm is detected with good linearity and repeatability. Furthermore, NH3 with the low concentration of 2 ppm is detected with a good response of 19.2%. Note that ultrafast and complete recovery is achieved at room temperature, bypassing the requirement of external stimulus to facilitate the recovery. In addition, long-term stability and immunity to high humidity are achieved. This work underscores the efficacy of a nanoporous Gr thin film structure in improving gas sensing performance of Gr materials.

Two-dimensional (2D) layered crystals are prone to bending and folding owing to their ultra-low bending stiffness. Folds are traditionally viewed as defects that degrade the material performance. Here, we demonstrate that folds and cohesive forces in 2D layered crystals like graphene and MoS2 can be exploited to collect and clean up interlayer impurities, wherein multiple separated impurities agglomerate into a single, large cluster. We combine classical molecular dynamics simulations and an analytical model to elucidate the competing roles of membrane bending and impurity-membrane cohesive energies in the self-cleaning process. Our findings shed light on the mechanisms by which the forces that are present in 2D layered crystals can positively impact, through the possibility of intrinsic cleaning and defect engineering, the synthesis of van der Waals homo- and heterostructures with improved reliability and functionalities.

The monitoring of heavy metal ions (HMIs) in rice phloem sap is of great significance for studying the migration and accumulation of HMIs in rice plants and, ultimately, for human health. However, achieving sensitive and selective electroanalysis of HMIs in such complex real samples is a huge challenge due to severe interference. In this work, a sensitive anti-interference sensing interface for the analysis of Pb2+ in rice phloem sap is fabricated based on glutathione-functionalized gold/multi-walled carbon nanotubes (MWCNTs-GSH-Au-GSH). Combining the selective capturing abilities of glutathione and the electrocatalytic abilities of small sized gold nanoparticles (mean diameter: 3.1 nm) toward Pb2+, the sensitivity of MWCNTs-GSH-Au-GSH is as high as 1122.8 μA μM−1 cm−2 without interference from other small ions, sucrose, ATP, amino acids, plant hormones, and so on. The limit of detection (0.01 μM) satisfactorily meets the requirements for sensing trace Pb2+. Importantly, a MWCNTs-GSH-Au-GSH electrode exhibited excellent stability over a test period of 6 months. In addition, the developed sensor shows high reproducibility, which is checked using ten different MWCNTs-GSH-Au-GSH electrodes. Based on the above advantages, the MWCNTs-GSH-Au-GSH electrode can be suitably applied to the detection of Pb2+ in rice phloem sap. The mechanism behind the high selectivity and sensitivity of the MWCNTs-GSH-Au-GSH electrode toward Pb2+ is clearly investigated via X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) studies.

A one-step, facile hydrothermal approach was developed to synthesize gold nanoparticles (Au NPs) modified reduced graphene oxide hydrogel (Au-RGOH) with three-dimensional (3D) porous microstructures for sensitive NO2 and NH3 monitoring. It was found that the Au-RGOH displayed an order of magnitude higher sensitivity and much lower limit of detections (LODs) than the unmodified RGOH counterpart, demonstrating the remarkable role of Au NPs in boosting the responsivity toward NO2 and NH3. To address the disturbance of humidity on gas sensing, an integrated microheater platform was exploited to quench the response to humidity without impairing the response to target gas, enhancing the resistance to humidity. The mechanisms of Au NPs decoration effect and temperature-assisted screening effect were elucidated by molecular dynamics and density functional theory simulations for the first time. It revealed that Au NPs were active adsorption sites for NO2 molecules, and the strong and weak dependence of the adsorption of H2O and NO2 molecules on temperature, respectively, providing theoretical evidences for experimental results. Benefiting from the chemical sensitization effect of Au NPs, 3D porous structure, and localized temperature modulation, the Au-RGOH gas sensor exhibited high sensitivity, low LODs (1.16 NO2 and 55.3 ppb NH3), excellent selectivity, reversibility, fast recovery speed, etc. This study offers a facile and practical solution to boost the gas sensing performance by engineering the surface, structure and operation temperature of transducing materials, as well as understand the sensing behaviors via theoretical simulations.

Van der Waals heterostructures inherit many novel electronic and optical properties from their constituent atomic layers. Mechanical stability is key for realizing high-performance nanodevices based on van der Waals heterostructures. However, buckling instability is a critical mechanical issue for heterostructures associated with its two-dimensional nature. Using molecular dynamics simulations of graphene/MoS2heterostructures, we demonstrate the relationship between buckling instability and the misfit strain that arises inevitably in such heterostructures. The impact of misfit strain on buckling depends on its magnitude: (1) A negative misfit strain causes a pre-compression of the graphene layer, which in turn initiates and accelerates buckling in this layer and reduces the buckling stability in the heterostructure as a whole. (2) A small positive misfit strain enhances the buckling stability of the graphene/MoS2heterostructure by pre-stretching and hence decelerating the buckling of the graphene layer (where heterostructure buckling is initiated). (3) In the case of a large positive misfit strain, the graphene layer is pre-stretched while the MoS2layer is significantly pre-compressed, so that heterostructure buckling is initiated by the MoS2layer. Consequently, the buckling stability of the graphene/MoS2heterostructure is reduced by increasing the large positive misfit strain. These findings are valuable for understanding the mechanical properties of van der Waals heterostructures.

The interaction area and defect sites play important roles in determining the gas sensing properties of graphene. In this work, steam etching is exploited to fragment large graphene sheets on 3D graphene hydrogel (GH) to small pieces with increased defect sites. The obtained steamed graphene hydrogel (S-GH) displays three and ten times higher responses to NO2 and NH3, respectively, compared with its unmodified GH counterpart before steaming treatment. Furthermore, the S-GH-based NO2 sensor exhibits a low limit of detection (LOD) of 57 ppb, which is much lower than that of a GH-based NO2 sensor (178 ppb). This work demonstrates a facile hydrothermal steaming method to enhance the performance of graphene-based gas sensor by combining 3D structural design and defect modulation of graphene materials.

Graphene has been expected to revolutionize the electronics, but suffered from its zero bandgap that will lead to an unacceptable leakage current in graphene-based transistors. Although a bandgap engineered graphene structure may have a bandgap, such bandgap is sensitively dependent on the geometry of the structure. Finding new two-dimensional (2D) semiconducting carbon allotropes is therefore extremely important for the new emerging all-carbon electronics. Based on first principles calculations, here we predict a new 2D semiconducting carbon allotrope, referred to as twin graphene. The allotrope has an intrinsic direct bandgap, very close to that of silicon and tunable by in-plane strain, indicating its great potential in nanoelectronics. The stability of twin graphene is carefully verified by both the first principles calculations and the molecular dynamics simulations. We determine also the mechanical properties of twin graphene, including the in-plane stiffness, Poisson’s ratio, shear stiffness and bending stiffness. Its excellent electronic and mechanical properties suggest that twin graphene may have a significant impact on many disciplines and fields related to nanotechnology.

We perform molecular dynamics simulations to investigate the Poisson's ratio of graphene oxide. We find that the Poisson's ratio can be effectively tuned by increasing the degree of oxidation of graphene oxide. More specifically, the Poisson's ratio decreases linearly from positive to negative with increasing oxidation, turning negative at room temperature for a degree of oxidation of 0.27, and reaching a value of -0.567 for fully oxidized graphene. The oxidation dependence of the Poisson's ratio is attributed to the tension-induced suppression of the ripples resulting from the oxidation, whose amplitude increases with increasing oxidation. Finally, we also demonstrate the temperature dependence of the Poisson's ratio in the graphene oxide.

Misfit strain is inevitable in various heterostructures like the graphene/MoS 2 van der Waals heterostructure. Although the misfit strain effect on electronic and other physical properties have been well studied, it is still unclear how will the misfit strain affect the performance of the nanomechanical resonator based on the graphene/MoS 2 heterostructure. By performing molecular dynamics simulations, we disclose a misfit strain-induced decoupling phenomenon between the graphene layer and the MoS 2 layer during the resonant oscillation of the heterostructure. A direct relationship between the misfit strain and the decoupling mechanism is successfully established through the retraction force analysis. We further suggest to use the graphene/MoS 2 /graphene sandwich heterostructure for the nanomechanical resonator application, which is able to prevent the misfit strain-related decoupling phenomenon. These results provide valuable information for the future application of the graphene/MoS 2 heterostructure in the nanomechanical resonator field.

Stimulated by the success of graphene and diamond, a variety of carbon allotropes have been discovered in recent years in either two-dimensional or three-dimensional configurations. Although these emerging carbon allotropes share some common features, they have certain different and novel mechanical or physical properties. In this review, we present a comparative survey of some of the major properties of fifteen newly discovered carbon allotropes. By comparing their structural topology, we propose a general route for designing most carbon allotropes from two mother structures, namely, graphene and diamond. Furthermore, we discuss several future prospects as well as current challenges in designing new carbon allotropes.

Graphene oxide (GO) and nanoscale zero-valent iron (nZVI), two of the most widely used nanomaterials, may come into contact with each other after their application in environmental remediation. To assess the role of nZVI in the environmental behavior of GO, the removal of GO by nZVI as a function of physicochemical properties of nZVI (i.e., initial concentration of nZVI, nZVI nanoparticle size and Fe dissolution) and solution chemistry (i.e., pH, ionic strength and cation type) is conducted by using batch experiments. The results indicate that nZVI can promote the aggregation of GO depending on the particle size and concentration of nZVI at pH

Using non-equilibrium molecular dynamics simulations, we investigate the thermal transport in van der Waals heterostructures consisting of alternating multilayer graphene and multilayer MoS2. It is found that the thermal conductance at graphene/MoS2 (G/M) interface is much lower than that at graphene/graphene (G/G) and MoS2/MoS2 (M/M) interfaces. This low interfacial thermal conductance is attributed to the low friction at G/M interface, which significantly reduces the contribution of shearing modes to the thermal conductance. It is also found that there is no thermal rectification at the G/M interface as the thermal conductance is independent on the heat flux direction. Moreover, the interfacial thermal conductance can be effectively tuned by cross-plane strain. More specifically, a 5% tensile strain is able to reduce the interfacial thermal conductance by 70%; while a 5% compressive strain is able to increase the thermal conductance by 150%. Unexpectedly, the G/M interfacial thermal conductance is found to increase with increasing the defect density near the interface, which is in strong contrast to the in-plane thermal conductivity. This unexpected increase in thermal conductance can be explained by the enhanced phonon coupling at the G/M interface arising from the enhanced interface friction caused by the defects.

As a new two-dimensional (2D) material, phosphorene has drawn growing attention owing to its novel electronic properties, such as layer-dependent direct bandgaps and high carrier mobility. Herein we investigate the in-plane and cross-plane thermal conductivities of single- and multi-layer phosphorene, focusing on geometrical (sample size, orientation and layer number) and strain (compression and tension) effects. A strong anisotropy is found in the in-plane thermal conductivity with its value along the zigzag direction being much higher than that along the armchair direction. Interestingly, the in-plane thermal conductivity of multi-layer phosphorene is insensitive to the layer number, which is in strong contrast to that of graphene where the interlayer interactions strongly influence the thermal transport. Surprisingly, tensile strain leads to an anomalous increase in the in-plane thermal conductivity of phosphorene, in particular in the armchair direction. Both the in-plane and cross-plane thermal conductivities can be modulated by external strain; however, the strain modulation along the cross-plane direction is more effective and thus more tunable than that along the in-plane direction. Our findings here are of great importance for the thermal management in phosphorene-based nanoelectronic devices and for thermoelectric applications of phosphorene.

The performance of chemical sensors can be optimized not only by material structural design, but also via functionalization of sensing materials. For the first time, we develop a high performance, cost-effective NO2 and CO2 sensor by exploiting a 3D chemically functionalized reduced graphene oxide hydrogel (FRGOH). The self-assembly and chemical modification of the 3D FRGOH are realized using hydroquinone molecules in a simple, one-step hydrothermal synthesis process. Compared with an unmodified RGOH counterpart, the chemically derived FRGOH sensor not only displays twofold higher sensitivity in both NO2 and CO2 sensing, but also exhibits significantly faster recovery and a lower limit of detection (LOD). Thus, a theoretical LOD of 57 ppb NO2 is obtained and a low concentration of 200 ppb NO2 is detected experimentally by the FRGOH sensor. Importantly, an integrated microheater is employed not only to significantly improve the selectivity of NO2 sensing, but also to accelerate the response and recovery. This work sheds light on improving the performance of a graphene-based gas sensor by simultaneous chemical functionalization, 3D structural design and temperature modulation.

Inspired by macroscale self-assembly using the higher order resonant modes of Chladni plates, we use classical molecular dynamics to investigate the self-assembly of water molecules using graphene nanoresonators. We find that water molecules can assemble into water chains and that the location of the assembled water chain can be controlled through the resonant frequency. More specifically, water molecules assemble at the location of maximum amplitude if the resonant frequency is lower than a critical value. Otherwise, the assembly occurs near the nodes of the resonator provided the resonant frequency is higher than the critical value. We provide an analytic formula for the critical resonant frequency based on the interaction between water molecules and graphene. Furthermore, we demonstrate that the water chains assembled by the graphene nanoresonators have some universal properties including a stable value for the number of hydrogen bonds.

Silver nanoparticles (NPs) and 6-Mercaptohexanoic acid (MHA) co-modified reduced graphene oxide (RGO-Ag-MHA) is rationally designed for ultrasensitive and highly reversible NO2 sensing at room temperature (RT). Due to the enhanced adsorption of NO2 on the abundant active sites provided by Ag NPs and MHA, both the NO2 adsorption and charge transfer processes are promoted. Consequently, the RGO-Ag-MHA exhibits orders of magnitude higher sensitivity and much lower limit of detection (LOD) toward NO2 than the unmodified RGO. These experimental observations are supported by theoretical simulations, as density functional theory simulation reveals that the binding energy of NO2 on Ag NPs and MHA is 2.03 and 10.9 times higher than that on RGO, respectively. Furthermore, molecular dynamics simulation indicates the adsorbed NO2 density on RGO-Ag-MHA is twofold higher than that on RGO. The RGO-Ag-MHA NO2 sensor displays very competitive performance, including ultrahigh sensitivity (53 ppm-1), exceptional low theoretical LOD (3.5 ppb), excellent selectivity, linearity and repeatability. This work not only unveils the role of surface co-modification in dictating the chemical sensing performance of graphene, but also provides a feasible route to fabricate high-performance, RT NO2 sensors for various applications.

Graphene (Gr) has been recognized as a promising candidate for room temperature (RT), high-performance gas sensing due to its unique structure and electronic properties. However, the poor selectivity of pristine Gr hinders its practical application in NO2 detection. SnS2 displays a strong physical affinity to NO2, while the ultrahigh resistance and low sensitivity at RT limit its utility. Here, 3D structured SnS2/reduced graphene oxide (RGO) heterojunction, a novel gas responsive material merging the merits of RGO and SnS2, is synthesized through a facile hydrothermal route. The 3D SnS2/RGO displays impressive NO2 sensing performance, including high sensitivity (6.1 ppm−1), excellent selectivity, low theoretical limit of detection (8.7 ppb), linearity and reversibility. The SnS2/RGO heterojunction exhibits 22.6 times higher response to NO2 than the pristine RGO, indicating the remarkable role of surface decoration in enhancing the gas sensing performance of RGO. The high sensitivity originates from the formation of heterojunction and 3D porous structures, which promote NO2 adsorption, diffusion and charge transfer by providing alternative adsorption sites and charge transport paths. Notably, a liquid crystal polymer substrate is employed to fabricate a flexible NO2 sensor with endurance to mechanical deformation. The elevated temperature improves the response and recovery kinetics.

Stretchable strain sensors have promising applications in health monitoring and human motion detection. However, only a few of the strain sensors reported to date have exhibited a multiscale strain range and a high gauge factor simultaneously. As such, most strain sensors cannot be used in applications that require both high sensitivity and a multiscale strain range. In this work, we develop a wearable multifunctional strain sensor using graphene and a new ionic conductor as the sensing material and Ecoflex as the encapsulant. In the ionic conductor, KCl and glycerol are used as the electrolyte and solvent, respectively. This deformable ionic conductor connects cracked graphene sheets electronically, enabling the strain sensor to be stretched to 300% of its original length with a moderate gauge factor of 25.2. The sensor can respond to various mechanical deformations including stretching, bending, and pressing. When attached to human body, the sensor can monitor large-scale strains (>50%) for joint movement and small-scale strains (

Superhydrophobic reduced graphene oxide (RGO) with unique 3D hierarchical structures is synthesized by exploiting one-step spark plasma sintering (SPS) within 60 s for high-performance NO2 detection. The effective removal of oxygenated groups and generation of 3D hierarchical structures in SPS render the RGO superhydrophobic. The superhydrophobicity makes the fabricated RGO sensor exceptionally immune to high relative humidity (RH). Specifically, the RGO sensor exhibits a response degradation less than 5.5% to 1 ppm NO2 when the RH increases from 0% to 70%. Importantly, an integrated microheater array is employed to remarkably activate the RGO-based NO2 sensor, boosting the sensitivity. Consequently, the NO2 sensor displays a high sensitivity (25.5 ppm-1) and an extremely low limit of detection (9.1 ppb). The boosted NO2 sensing performance is attributed to superhydrophobicity, 3D hierarchical structures with high specific surface area (850 m2/g), abundant defect sites and thermal activation with microheaters. NRF (Natl Research Foundation, S’pore)

Monolayer molybdenum disulfide (MoS2), a new two-dimensional material beyond graphene, has attracted tremendous attention recently. Its applications in nanoelectronic and thermoelectric devices usually require manipulating the thermal transport properties. Using nonequilibrium molecular dynamics simulations, we investigated the effects of lattice defects and mechanical strain on the thermal conductivity of MoS2. We found that the thermal conductivity of monolayer MoS2 can be effectively tuned by introducing even a small amount of lattice defects. For example, a 0.5% concentration of mono-Mo vacancies is able to reduce the thermal conductivity by about 60%. Remarkably, the thermal conductivity of the defected sample can further be tuned by mechanical strain. For example, a 12% tensile strain is able to reduce the thermal conductivity by another 60%. We also found that the tensile strain exerts almost the same impact on the thermal conductivity of both pristine and defective MoS2, which signifies that there...

Graphene and MoS2 are two well-known quasi two-dimensional materials. This review is a comparative survey of the complementary lattice dynamical and mechanical properties of graphene and MoS2. This comparison facilitates the study of graphene/MoS2 heterostructures, which is expected to mitigate the negative properties of each individual constituent., Frontiers of Physics, published, Focus Review

A new method of combining microwave plasma enhanced chemical vapor deposition (MPCVD) with ultrasonic fragmentation is exploited to prepare nanoporous graphene (Gr) thin film for detection of hazardous gases with high performance, including high sensitivity, linearity, low limit of detection (LOD) and reversibility. Compared with gas sensors based on traditional planar Gr sheets, this nanoporous Gr thin film displays 3.6 times higher response to 1 ppm NO 2 , 10.4 times larger response to NH 3 and 4.2 times higher response to CO2, demonstrating the improved sensitivity of nanoporous Gr thin film. The boosted sensitivity is attributed to nanoporous Gr structures, large amounts of defects and oxygenated functional groups. Note that ultrafast signal recovery is achieved at room temperature, bypassing the requirement of external stimulus to facilitate the recovery. Furthermore, the response is linear with gas concentration.

One-step, hydrothermal synthesized 3D sulfonated reduced graphene oxide hydrogel (S-RGOH) is employed to fabricate a chemical sensor with high sensitivity, good selectivity, fast response and reversibility toward several gases. Compared with unmodified RGOH counterparts, the 3D NaHSO 3 functionalized S-RGOH sensor exhibits 61.3 and 58.9 times higher responses to NO 2 and NH 3 respectively. A low limit of detection (LOD) of 4.1 ppb and 1.48 ppm for NO 2 and NH 3 , respectively, has been achieved. The sensor exhibits fast and complete recovery at room temperature. Importantly, for the first time, the characteristics of a linear fitted response-temperature relationship are exploited to discriminate many different gases. An integrated microheater is deployed to modulate substrate temperature rapidly with low power consumption.

The emerging 2D materials such as graphene (Gr) has attracted widespread attention in chemical sensing due to its unique structural and electronic properties such as high conductivity, large surface areas and high sensitivity to electrical perturbations from gas molecules. This book chapter discusses and summarizes recent advancement of Gr-based gas sensors from basic principles to applications. The performance of Gr-based gas sensors can be optimized from several aspects, such as chemical composition, structures and defects, which are discussed systematically in this chapter. The technical barriers that limit their practical application and an outlook of Gr in next-generation gas sensors is presented.

Three-dimensional, superhydrophobic, reduced graphene oxide (RGO) with unique hierarchical structures is synthesized by spark plasma sintering (SPS) in one step for highly selective NO2 detection. Because the oxygenated functional groups in graphene oxide (GO) can be effectively removed to a minimal content (8.8%) by SPS within just 60 s, the formed 3D RGO exhibits superhydrophobicity that endows the fabricated RGO sensor with exceptional immunity to high relative humidity (RH). Specifically, the RGO sensor exhibits a response degradation less than 5.5% to 1 ppm NO2 in a wide temperature range from 25 to 140 °C when the RH increases from 0% to 70%. In addition, an integrated microheater array is employed to remarkably activate the RGO-based NO2 sensor, boosting the sensitivity. The RGO sensor demonstrates the practical capability to detect 50 ppb NO2 and exhibits an extremely low theoretical limit of detection of 9.1 ppb. The good tolerance to environmental variations such as humidity and temperature makes this sensor suitable for reliable application in the Internet of Things (IoT) under ambient conditions. The high NO2 sensing performance is attributed to the unique 3D hierarchical structures with a high specific surface area (850 m2 g−1), a superhydrophobic surface, abundant defect sites and thermal activation. NRF (Natl Research Foundation, S’pore)