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Human beings perceive the world through the senses of sight, hearing, smell, taste, touch, space, and balance. The first five senses are prerequisites for people to live. The sensing organs upload information to the nervous systems, including the brain, for interpreting the surrounding environment. Then, the brain sends commands to muscles reflexively to react to stimuli, including light, gas, chemicals, sound, and pressure. MXene, as an emerging two-dimensional material, has been intensively adopted in the applications of various sensors and actuators. In this review, we update the sensors to mimic five primary senses and actuators for stimulating muscles, which employ MXene-based film, membrane, and composite with other functional materials. First, a brief introduction is delivered for the structure, properties, and synthesis methods of MXenes. Then, we feed the readers the recent reports on the MXene-derived image sensors as artificial retinas, gas sensors, chemical biosensors, acoustic devices, and tactile sensors for electronic skin. Besides, the actuators of MXene-based composite are introduced. Eventually, future opportunities are given to MXene research based on the requirements of artificial intelligence and humanoid robot, which may induce prospects in accompanying healthcare and biomedical engineering applications.

Large-area and high-quality graphene grown on metal by chemical vapor deposition (CVD) has great potential applications in electronics and photo electronics. After more than 10 years of development, great progress has been made in the growth of graphene, and the role of various factors in the growth of graphene has been gradually clear. In this review, the role of hydrogen and oxygen in the formation and elimination of carbon-based impurities and silicon oxide particles and in the formation, observation, and control of grain boundary, point defects, and wrinkles is introduced. The content focuses on the specific experimental methods, results, and mechanism. Finally, the challenges of hydrogen and oxygen in the study of graphene growth on metal are introduced. This review can increase the theoretical knowledge and experimental design ability related to hydrogen and oxygen in CVD graphene grown on metal.

The Internet of Things era has promoted enormous research on sensors, communications, data fusion, and actuators. Among them, sensors are a prerequisite for acquiring the environmental information for delivering to an artificial data center to make decisions. The MXene-based sensors have aroused tremendous interest because of their extraordinary performances. In this review, the electrical, electronic, and optical properties of MXenes are first introduced. Next, the MXene-based sensors are discussed according to the sensing mechanisms such as electronic, electrochemical, and optical methods. Initially, biosensors are introduced based on chemiresistors and field-effect transistors. Besides, the wearable pressure sensor is demonstrated with piezoresistive devices. Third, the electrochemical methods include amperometry and electrochemiluminescence as examples. In addition, the optical approaches refer to surface plasmonic resonance and fluorescence resonance energy transfer. Moreover, the prospects are delivered of multimodal data fusion toward complicated human-like senses. Eventually, future opportunities for MXene research are conveyed in the new material discovery, structure design, and proof-of-concept devices.

Wafer-scale single-crystal graphene with strong antioxidation is fundamentally important for their applications in electronics and optoelectronics. Although significant progress has been achieved in the chemical vapor deposition (CVD) growth of graphene, the production of wafer-scale graphene with high crystalline and excellent oxidation resistance still remains a challenge. Here, we report the epitaxial growth of 6-inch single-crystal graphene on twinned Pt (111) films with in-plane rotation of 60°(T-Pt) by ambient-pressure CVD. Our results show that the CVD-grown graphene on T-Pt exhibits fast growth rate and ultrahigh stability under the high-temperature air condition (>500 °C). The density functional theory (DFT) calculations reveal that the twinned Pt(111) surface does not change the preferential orientation of graphene nucleation, leading to highly aligned graphene domains on the T-Pt substrate. Moreover, the edge growth of graphene cannot be limited by the Pt twin boundaries (TBs), which is responsible for the fast growth of graphene single crystals. This work provides a reliable route to produce wafer-size single-crystal graphene monolayers with excellent oxidation resistance and clarifies the oriented growth mechanism of graphene domains on twinned Pt substrate.

AB-stacked bilayer graphene (BLG) has wide application prospects in the field of optoelectronic devices. Chemical vapor deposition (CVD) is an important method used to prepare BLG, but no definite stacking orientation is observed between the bilayers. Besides AB stacking, a number of disordered stacking orientations that could seriously affect the performance of BLG also exist. In this paper, we report a method to control the stacking orientation of reverse-transferred CVD-BLG via high pressure environment. We found that the orientation of BLG could be changed from weak-coupling stacking to AB stacking by increasing the temperature of the solvent in the autoclave. MoS2 grains grown on graphene confirmed the stacking orientation variation of BLG. Reductions in the distance between layers in BLG under a high-pressure environment were considered to explain the observed orientation change. Reverse transfer allows the easy modification of the stacking orientation of BLG. The proposed high-pressure assistant method introduces a new route to prepare AB-stacked BLG and could be used to adjust the stacking direction of other two-dimensional materials.

This study reports the growth of aligned monolayer molybdenum disulfide (MoS2) ribbons on a sapphire substrate via NaOH-assisted chemical vapor deposition. The length of MoS2 ribbon is up to 400 μm. The MoS2 ribbon has excellent single crystal properties, a carrier mobility of {150 cm2 V−1 s−1, and a optical response of 103 mA W−1 at a wavelength of 550 nm. The growth model of MoS2 ribbons was given. NaOH reacts with MoO3 to form sodium molybdate droplets, which increase the fluidity of the molybdenum source on the substrate, realizing the vapor–liquid–solid growth of MoS2 on sapphire. The monolayer MoS2 ribbons have two kinds of arrangements on the sapphire substrate, one is the oriented growth affected by the interlayer van der Waals force and the lattice, and the other is the aligned growth constrained by the sapphire step. Our results promote the basic research and device applications of MoS2, and introduce a new way of synthesizing other one dimensional (1D) and 2D nanostructures.

Inherent sulfur vacancies in molybdenum disulfide (MoS2) films grown via chemical vapor deposition have restricted the practical application of MoS2 in optoelectronic devices. The repair of sulfur vacancies is important for improving the quality of MoS2 and enhancing the performance of devices based on MoS2 films. Here we report a route that uses hydrogen (H2) etching to detect the repair effect of sulfur vacancies. The domains with and without sulfur annealing were etched via H2 under the same conditions, and the changes of surface microstructures and properties were compared. It was found that the tolerance of MoS2 to H2 etching was enhanced after sulfur annealing. Results of the photoluminescence spectrum further proved that H2 etching could effectively detect the repair effect of sulfur vacancies. As a practical application, the optimum condition of sulfur annealing was obtained through H2 etching. Our work promotes the practical application of MoS2 in the field of new-generation electronics and optoelectronics.

Growing graphene on metal thin films, especially Cu or CuNi thin films, has become an efficient way to obtain large single-crystal graphene. Most studies focus on the epitaxial growth of graphene, but the effects of defects, such as wrinkles and point defects, on graphene stability are rarely explored. In this work, we mainly focus on the distribution of wrinkles and point defects to study the stability of graphene growth on CuNi thin films. It is found that graphene grown on a CuNi thin film shows the same peripheral etching phenomena other than central etching holes or wrinkle stripes whether it is cooled or not. Raman and TEM studies show that the graphene has high quality. We believe that graphene grown on a CuNi thin film has few wrinkles and point defects. Thus, the results show that graphene grown on a CuNi thin film is more stable than graphene grown on a Cu foil in a high-temperature hydrogen and air environment.

So far, it is still difficult to construct composites with a gradient distribution of graphene for decreasing the reflection and increasing the absorption of electromagnetic energy. Here, we introduce an electrochemical method to efficiently prepare a graphene/polyurethane composite with a gradient graphene distribution. And the composite shows obvious anisotropic reflection of electromagnetic waves, with low reflection loss (-30 dB) and high absorption (99.5%) in the whole X-band when electromagnetic waves are incident to the surface that has low graphene content. More importantly, the electrochemical method could be extended to the preparation of functional materials with similar structures based on the electrophoresis of charged nanoparticles.

Three-dimensional (3D) graphene with a high specific surface area and excellent electrical conductivity holds extraordinary potential for molecular gas sensing. Gas molecules adsorbed onto graphene serve as electron donors, leading to an increase in conductivity. However, several challenges remain for 3D graphene-based gas sensors, such as slow response and long recovery time. Therefore, research interest remains in the promotion of the sensitivity of molecular gas detection. In this study, we fabricate oxygen plasma-treated 3D graphene for the high-performance gas sensing of formaldehyde. We synthesize large-area, high-quality, 3D graphene over Ni foam by chemical vapor deposition and obtain freestanding 3D graphene foam after Ni etching. We compare three types of strategies—non-treatment, oxygen plasma, and etching in HNO3 solution—for the posttreatment of 3D graphene. Eventually, the strategy for oxygen plasma-treated 3D graphene exceeds expectations, which may highlight the general gas sensing based on chemiresistors.

Carbon nanotubes (CNTs) have attracted great attentions in the field of electronics, sensors, healthcare, and energy conversion. Such emerging applications have driven the carbon nanotube research in a rapid fashion. Indeed, the structure control over CNTs has inspired an intensive research vortex due to the high promises in electronic and optical device applications. Here, this in-depth review is anticipated to provide insights into the controllable synthesis and applications of high-quality CNTs. First, the general synthesis and post-purification of CNTs are briefly discussed. Then, the state-of-the-art electronic device applications are discussed, including field-effect transistors, gas sensors, DNA biosensors, and pressure gauges. Besides, the optical sensors are delivered based on the photoluminescence. In addition, energy applications of CNTs are discussed such as thermoelectric energy generators. Eventually, future opportunities are proposed for the Internet of Things (IoT) oriented sensors, data processing, and artificial intelligence.

Monolayer transition metal dichalcogenides (TMDCs) with high crystalline quality are important channel materials for next-generation electronics. Researches on TMDCs have been accelerated by the development of chemical vapor deposition (CVD). However, antiparallel domains and twin grain boundaries (GBs) usually form in CVD synthesis due to the special threefold symmetry of TMDCs lattices. The existence of GBs severely reduces the electrical and photoelectrical properties of TMDCs, thus restricting their practical applications. Herein, the epitaxial growth of single crystal MoS

Contact resistance (R C ) is of great importance for radio frequency (RF) applications of graphene, especially graphene field effect transistors (FETs) with short channel. FETs and transmission line model test structures based on chemical vapor deposition grown graphene are fabricated. The effects of employing traditional lithography solvent (Acetone) and strong solvents for photo resist, such as N, N-Dimethylacetamide (ZDMAC) and N-Methyl pyrrolidone (NMP), are systematically investigated. It was found that ZDMAC and NMP have more proficiency than acetone to remove the photo-resist residues and contaminations attached on graphene surface, enabling clean surface of graphene. However, strong solvents are found to destroy the lattice structure of graphene channel and induce defects in graphene lattice. Clean surface contributes to a significant reduction in the R C between graphene channel and metal electrode, and the defects introduced on graphene surface underneath metal electrodes also contribute the reduction of R C . But defects and deformation of lattice will increase the resistance in graphene channel and lead to the compromise of device performance. To address this problem, a mix wet-chemical approach employing both acetone and ZDMAC was developed in our study to realize a 19.07% reduction of R C , without an unacceptable mass production of defects.

Recently, graphene has led to unprecedented progress in device performance at the atom limit. A high performance of field-effect transistors requires a low graphene–metal contact resistance. However, the chemical doping methods used to tailor or improve the properties of graphene are sensitive to ambient conditions. Here, we fabricate a single-layer perfluorinated polymeric sulfonic acid (PFSA), also known as Nafion, between the graphene and the substrate as a p-type dopant. The PFSA doping method, without inducing any additional structural defects, reduces the contact resistance of graphene by ∼28.8%, which has a significant impact on practical applications. This reduction can be maintained for at least 67 days due to the extreme stability of PFSA. Effective, uniform and stable, the PFSA doping method provides an efficient way to reduce the contact resistance of graphene applications.

Recently, graphene has provided unprecedent progress in device performance at the atom limit. High performance of field-effect transistors (FETs) requires a low graphene-metal contact resistance. However, the chemical doping methods used to tailor or improve the properties of graphene are sensitive to ambient conditions. Here, we fabricate a single layer perfluorinated polymeric sulfonic acid (PFSA), also known as Nafion, between graphene and the substrate as a p-type dopant. The PFSA doping method, without inducing any additional structural defects, reduces the contact resistance of graphene by ~28.8%, which has a significant impact on practical applications. This reduction can keep at least 67 days due to the extreme stability of PFSA. Effective, uniform and stable, the PFSA doping method provides an efficient way to reduce the contact resistance of graphene applications.

We have studied the field-effect carrier transport of graphene on 4H silicon carbide substrate. In order to extract the electrical parameters, the top-gated field effect transistor has been fabricated. By fitting the measured results with Kim’s model, the field effect carrier mobility (µ) and the metal/graphene contact resistance (Rc) and the residual carrier concentration (n0) are derived to be 3382cm2/Vs, 2250Ω▪µm and 2.18×1013cm-2, respectively. It is noted that the large contact resistance did not affect the high field effect carrier mobility of our device. The high carrier mobility suggests that the SiC epitaxial graphene may be quite suitable for the future high speed electronic applications.

With the widespread utilization of indium-phosphide-based high-electron-mobility transistors (InP HEMTs) in the millimeter-wave (mmW) band, the distributed and high-frequency parasitic coupling behavior of the device is particularly prominent. We present an InP HEMT extrinsic parasitic equivalent circuit, in which the conductance between the device electrodes and a new gate–drain mutual inductance term L mgd are taken into account for the high-frequency magnetic field coupling between device electrodes. Based on the suggested parasitic equivalent circuit, through HFSS and advanced design system (ADS) co-simulation, the equivalent circuit parameters are directly extracted in the multi-step system. The HFSS simulation prediction, measurement data, and modeled frequency response are compared with each other to verify the feasibility of the extraction method and the accuracy of the equivalent circuit. The proposed model demonstrates the distributed and radio-frequency behavior of the device and solves the problem that the equivalent circuit parameters of the conventional InP HEMTs device are limited by the device model and inaccurate at high frequencies when being extracted.

In this paper, we studied the growth of graphene on an untreated Cu substrate and further studied the effect of carbon-based impurities on the nucleation of graphene in different growth environments. It is found that the impurities on the surface of the Cu substrate easily lead to damage of the graphene, and the impurities do not always promote nucleation as previously reported, but inhibit nucleation in a high etching environment. Finally, based on experimental results, a model of nucleation and growth of graphene around impurities is presented.

In recent years, researchers have explored the interlayer stacking structure of the bilayer (BL) transition metal dichalcogenides (TMDCs) and found that it has unique impact on the electrical, optical, and vibrational properties of TMDCs. Unfortunately, the size of BL TMDCs synthesized by the pre-existing methods is not large. In addition, there is not any effective mean to control the number of layers as well as the stack structure. This is the major limiting factor for further exploration of the properties of BL TMDCs. We report a new method for the rapid and controllable growth of AB-stacked BL MoS2. Firstly, the mixed solution of Na2MoO4 and NaOH was spun on the sapphire substrate and then sulfurated. The BL MoS2 of AB-stacked can be obtained by changing the time of sulfur introduction. The size of the BL MoS2 is related to the parameters of sulfur. The maximum size of BL MoS2 grown by this method can reach 288 μm, and the average size can reach about 200 μm, which is larger than the size reported in most articles. In addition, the growth rate of BL MoS2 can be clearly increased by the spin-coating growth method and the fastest growth rate is up to 144 μm min−1, which is far higher than other published reports at present. Subsequently, a photodetector based on BL MoS2 was prepared. The response time of rising edge is about 0.32 ms, and that of falling edge is about 3.2 ms. The detector prepared by us shows excellent photoelectric detection performance. Therefore, the excellent performance of BL MoS2 makes it have a broad application prospect in the field of high-performance electronics and optoelectronics.

Monolayer transition metal dichalcogenides (TMDCs) with high crystalline quality are important channel materials for next-generation electronics. Researches on TMDCs have been accelerated by the development of chemical vapor deposition (CVD). However, antiparallel domains and twin grain boundaries (GBs) usually form in CVD synthesis due to the special threefold symmetry of TMDCs lattices. The existence of GBs severely reduces the electrical and photoelectrical properties of TMDCs, thus restricting their practical applications. Herein, the epitaxial growth of single crystal MoS2 (SC-MoS2 ) monolayer is reported on Au (111) film across a two-inch c-plane sapphire wafer by CVD. The MoS2 domains obtained on Au (111) film exhibit unidirectional alignment with zigzag edges parallel to the direction of Au (111). Experimental results indicated that the unidirectional growth of MoS2 domains on Au (111) is a temperature-guided epitaxial growth mode. The high growth temperature provides enough energy for the rotation of the MoS2 seeds to find the most favorable orientation on Au (111) to achieve a unidirectional ratio of over 99%. Moreover, the unidirectional MoS2 domains seamlessly stitched into single crystal monolayer without GBs formation. The progress achieved in this work will promote the practical applications of TMDCs in microelectronics.

A top-gated graphene FET with an ultralow 1/f noise level of 1.8 × 10–12 μm2Hz1– (f = 10 Hz) has been fabricated. The noise has the least value at Dirac point, it then increases fast when the current deviates from that at Dirac point, the noise slightly decreases at large current. The phenomenon can be understood by the carrier-number-fluctuation induced low frequency noise, which caused by the trapping-detrapping processes of the carriers. Further analysis suggests that the effect trap density depends on the location of Fermi level in graphene channel. The study has provided guidance for suppressing the 1/f noise in graphene-based applications.

The metal-graphene contact resistance has been identified to be a key bottleneck for achieving high performance of graphene transistors. It is crucial to understand the electrical properties of graphene and the carrier transport mechanism under the contact metal. Here, we have developed a new method of characterizing the electrical properties of graphene under the metal contact. It was found that the electrical properties of graphene under the metal can be tuned via the back-gate voltage and display ambipolar behavior. A quantum tunneling model for graphene-metal physical contact has been proposed. The probability of electric field-tunable tunneling has been derived from the results of measurements for the first time. The model predicts that even for physical contact the contact resistance can be much lower than 100 Ω μm when graphene is more heavily doped and the interfacial layer is eliminated. This study paves the way to achieving ultralow graphene-metal contact resistance in graphene devices for terahertz applications.

We investigated the influence of the flake thickness for molybdenum disulfide (MoS2) field effect transistors on the effect of a 150 keV high-energy proton beam applied on these devices. The electrical characteristics of the devices with channel thicknesses ranging from monolayer to bulk were measured before and after proton irradiation with a proton fluence of 5 × 1014 cm−2. The subthreshold swing (SS), threshold voltage shift and electron mobility were extracted with the Y-function method after proton irradiation and significant degradation were observed. It is found that, with the increase of layer thickness, mobility degradation and threshold voltage shift both eased, but the SS degradation was insensitive to the MoS2 flake thickness increase. We also demonstrate that the threshold voltage shift is dominated by oxide charges; however, the mobility and SS degradations are mainly affected by the interface traps. Our study will enhance the understanding of the influence of high-energy particles on MoS2-based nano-electronic devices. By increasing the MoS2 flake thickness to a certain extent, one can hopefully find a balance between effectively resisting V T H shift and achieving high mobility and small SS degradation.

To understand the hysteresis phenomenon in the transfer characteristic curve of graphene field effect transistor is crucial to develop new graphene based sensors and memory devices. We investigated the hysteresis through a charging and discharging procedure and found that there exist two time constants with different timescales in the trapping mechanism. A fast trapping at the interface with a time constant of 0.68 s and a slow trapping in the bulk oxide with a time constant of 23.85 s were presented in our work. By identifying a linear relation between the interface trap densities and the gate voltage, we also found that the release of trapped charges was decided by the magnitude decrease of gate voltage instead of the voltage’s polarity switch. Based on the findings, a bi-resistive state memory prototype with a shorter charging time was realized at last.

Thick graphene film can be widely used in surface protection, heat dissipation, heating devices and other fields. Here we present a study on the growth and characterization of large area continuous and patterned graphite-like films prepared on Ni foil through CVD. Effects of parameters on the material growth are studied experimentally and theoretically in detail. Thickness of the graphite-like film in this work can reach 500 nm and can be regulated by growth process. Characterization results show that the graphite-like film is formed by free stacking of large area graphene continuous films and possesses strong mechanical strength, chemical stability, good electrical conductivity and flexibility. The graphite-like film shows potential for application in metal surface protection, flexible conduction, heating and electromagnetic shielding.

This study investigates the growth promotion of vertically oriented graphene in plasma-enhanced chemical vapor deposition through Ar plasma treatment. Combined with various substrate treatments, including hydrofluoric acid etching and oxidation after Ar plasma treatment, Ar plasma pretreatment promotes vertical growth through the microcavity on the rough substrate surface and the active growth sites. The microcavity affects the strain distribution and defects of as-deposited planar films, which benefit the transition of 2D deposition to 3D vertical growth. A growth model on the effect of Ar plasma pretreatment is proposed.

It is normally assumed that the sheet resistances under and outside the metal contact are identical when deriving specific contact resistivity of graphene from transmission line model. We considered the contact end resistance and obtained the sheet resistance under contact of 670 Ω/□, which is much different from that outside the contact of 1840 Ω/□. Considering the difference, the value of specific contact resistivity is determined to be 3.3 × 10 −6 Ω cm 2 , which is three times as large as the unmodified value. This indicates that the difference between the sheet resistances under and outside the contact affects the derived specific contact resistivity of graphene significantly.

The transfer of chemical vapor deposition (CVD)-grown graphene is the prerequisite for many applications. Herein, we introduce a simple and eco-friendly electrochemical technique to transfer graphene using agarose gel as the solid electrolyte and a mechanical support layer for graphene sheets. This transfer technique will be a perfect candidate for the industrial applications of graphene.

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.