Perspective of graphene-based electronic devices: Graphene synthesis and diverse applications

Graphene, the first two-dimensional material combining spectacular physical and chemical properties, displays great potential in electronic devices. After years of development, graphene, along with a series of derivatives, was widely synthesized, and graphene-based electronic devices (G-EDs) ranging from graphene-based field-effect transistors (GFETs) to specific sensors surged, opening the way of the graphene industry. The ultrahigh electron mobility can significantly improve the performance of electronics including various transistors and detectors, providing a performance advantage. The chemical durability, thermal stability, and high mechanical strength make it suitable for harsh working environment, and the combination of diversified superior properties (high transparency, flexibility, thermal conductivity, regulable electronic properties, etc.) enables it to be used for wider applications, exhibiting additional advantages.^1–3 For instance, compared to the currently used indium-tin-oxide (ITO) which is brittle and chemically unstable, graphene-based transparent electrodes show superiority in reducing the cost, high stability, flexibility, and outstanding mechanical property. The high surface area and conductivity make graphene an ideal material for sensing. By tailoring the morphology and structure of graphene, ultrasensitive sensors have been reported for different stimulations, such as NO₂ (detection limit: 95.2 ppt), NH₃ (detection limit: 59.9 ppb),⁴ and pressure (123 aF Pa⁻¹ mm⁻²).⁵ In particular, G-EDs can also be used for biosensors, such as the cancer cell,⁶ protein, and DNA.⁷ Moreover, due to the high mobility and carrier concentration, graphene-based photonic devices also have the potential for high-performance datacom and telecom applications.^1,8 Surprisingly, novel phenomena are continuously being discovered, such as unconventional superconductivity behavior in magic-angle graphene superlattices,⁹ robust microscale superlubricity,¹⁰ and Dirac cones in a quasicrystal.^11,12 However, all these are rooted on high-quality graphene with optimal properties in laboratory. As for mature products, especially high-performance devices, flaws or fluctuations need to be eliminated as much as possible. To date, scalable graphene films and flakes, including graphene oxides (GO) and reduced graphene oxides (RGO), are far from meeting the stringent requirement of uniformity and quality. As predicted by the company Mckinsey,¹³ the market for graphene semiconductors will reach $70 × 10⁹ in 2030. Controllable mass-production of high-quality graphene is urgently needed, which plays a decisive role in commercialization. Besides, purposive modification of graphene and an optimized processing technique of G-EDs are key issues for versatile devices. The cost and performance will deeply affect whether G-EDs can find its way to marketplace. These present challenges also mean unprecedented opportunities for both researchers and industries.

The production technologies and applications of graphene are growing rapidly, achieving abundant accomplishments. Graphene films, sheets, and corresponding applications become the vanguard of G-EDs commercialization (Fig. 1). Other graphenes (functionalized graphene, graphene based heterostructures, etc.) for different target devices are still at the stage of laboratory research. Moreover, novel phenomena are explored continuously, indicating more amazing applications.

Multifarious methods from mechanical exfoliation to chemical vapor deposition (CVD) have been developed to synthesize graphene with different properties. Exfoliation of graphite is the major top-down routes towards mass-production of graphene flakes.¹⁴ The key point is to overcome the interaction force in graphite layers to produce few layer sheets by physical and chemical methods.^15,16 Mechanical techniques including ball milling, sonication, and shear exfoliation were developed to produce graphene flakes from graphite. Alternatively, oxidation exfoliation of graphite was widely used to produce large-scale few-layer GO which was subsequently reduced to RGO. These graphene flakes are easy processing into a variety of devices. It is facile to print or coat these products onto target substrates for flexible electronics, exhibiting high applicability in roll to roll processing.¹⁷ Due to the simplicity and high productivity, mass production of graphene sheets and RGO has been realized for several years. The capacities are increasing as the technology improves. Recently, NanoXplore is beginning to study the largest graphene production plants with productivity up to 10 000 metric ton/year. In addition, many green ways were developed to reduce the consumption of chemical reagents and waters, for instance, electrolytic oxidation and reduction, microwave, and photo reduction. The exfoliated graphenes are promising for devise applications, such as sensors,⁷ transparent conductive films,^18–20 and photo-controlled electrical switches.²¹ CVD exhibits remarkable advantages in the controllable production of large-area high-quality graphene films which are more suitable for high-performance G-EDs. In laboratory, graphene crystals as large as meter-length were prepared on industrial copper foils,²² paving the way toward the industrial production of graphene crystals with mobilities over 10 000 cm² V⁻¹ s⁻¹. Besides, direct growth on insulators and near room-temperature growth also offer new opportunities for bulk production.^23,24 The evolution of the production technique is closely related with the deeper understanding of the growth mechanism. The crucial factors including oxygen effects, hydrogen, engineering of substrates, temperature, time, and pressure have been demonstrated to control the size, morphology, layer number, quality, and growth rate of graphene, which will further accelerate the development of graphene production. Benefiting from the self-limited growth of graphene on copper, roll-to-roll and batch-to-batch processes were developed to achieve large-area uniform graphene films.²⁵ By 2017, an annual production capability up to 10 00 000 m² was realized by several companies and the wafer size of graphene on SiO₂/Si or polyethylene terephthalate was as large as 100 × 100 cm².¹⁶ The mobilities of as-fabricated GFETs exceeded 2000 cm² V⁻¹ s⁻¹. These graphene films can be used in touch screens, transparent electrodes, and sensors. With rapidly falling costs and gradually optimized processes, it is expected that many low-end G-EDs will mature in the immediate future.

As an atomic-thick material, the property of graphene deeply depends on its nanoscale features. The distribution of the spin and charge density states can be altered by quantum confinement, heteroatoms, functional groups, and molecules, as well as strain, offering opportunities to reform the property of graphene. Thus, graphene with intriguing properties can be achieved by designing its morphology and structure, motivating the development of various derivatives. Limited by the zero bandgap structure, the engineering of bandgap is one of the most critical points for G-EDs. Graphene derivatives like graphene nanoribbons (GNRs), heteroatoms (N, S, B) doped, functionalized, and bilayer graphene were explored to improve the on/off current ratios of GFETs and modulate the electronic structure of graphene. The diversified approaches would promote the development of G-EDs in logical circuits. Particularly, remarkable progress has been made in the investigation of GNRs. GFETs prepared with sub-10-nm GNRs showed high on–off current ratios of 10⁴.²⁶ Moreover, high-yield GNRs (250 000 cm⁻²) were prepared by a rapid-heating plasma CVD,²⁷ which is accessible for large-scale production. Excitingly, atomic-level defined GNRs were synthesized by the on-surface bottom-up approach with designed halogenated aromatic precursors, arousing explosive interests. The monomers can be converted into GNRs with defined width and edge-type through dehalogenation and coupling reactions, which is affected by the structure of precursors.^28,29 Benefiting from the precise control at the atomic level, breakthroughs were made in engineering the topological quantum phases and topological band of GNRs, which is significant for quantum technology.^30,31 The synthesis strategy not only enables us to atomically design the structure of GNRs according to our requirements but also provides a route for the atomic-controllable synthesis of other graphene derivatives and graphene-like materials with unprecedented properties. Besides, graphene can be modified for different sensors by introducing heteroatoms, functional groups, DNAs, or proteins.⁷ The interaction between anchored groups and target molecules will cause variations in electrical characteristics, resulting in response signals. The high mobility of graphene can significantly enhance the signal-to-noise ratio, leading to an explosion of interest in graphene-based sensors. Compared to chemical inert pristine graphene, RGO with oxygen-containing groups, which is easy to decorate, is a more suitable platform to functionalize for desired devices. Additionally, with better hydrophilicity, films produced by RGO can be used for biosensors. Eventually, graphene-based heterostructures, consisting of graphene and other materials, can combine the advantages of different materials.³² For instance, transition metal dichalcogenides with strong spin–orbit coupling can match well with graphene, improving the spin dependent features.³³ 

Graphene becomes an appealing platform that can be designed for desired G-EDs, which will give full play to its potential. The high carrier concentration and mobility lay the foundation of high-performance, and the outstanding flexibility, transparency, and chemical stability bring additional competitiveness to the currently used materials. Therefore, one direction is to develop advanced G-EDs with incomparable performances, such as the flexible graphene terahertz detector,³⁴ mechanical terahertz modulator,³⁵ graphene radio frequency transistor with a high f_max of 200 GHz,³⁶ and functionalized graphene photodetectors with a linear dynamic range of 44 dB.³⁷ For another direction, a variety of graphene based transparent electrodes, touch screens, ultrasensitive sensors, and flexible and wearable devices were developed to handle the shortages of existing materials (for instance, the brittleness and high-cost of ITO). These devices can make current electronics thinner, lighter, more adaptable, deformable, and stable, achieving technological innovations. Achievements in laboratory will lead to killer applications, promoting the commercialization of graphene.

With the development of graphene research, compelling phenomena are excavated unceasingly. Recently, Yankowitz et al. realized in tuning the band-structure of graphene moiré superlattices with hydrostatic pressure.³⁸ This result provides a way to engineer the electronic characters of graphene. Cao et al. demonstrated the unconventional superconductivity and insulator behavior in magic-angle graphene superlattices,^9,39 arousing worldwide attention. This is significant to unveil the mystery of superconductivity. Terahertz harmonics up to 10⁷ can be generated in single layer graphene efficiently by the hot Dirac fermions, indicating the potential for ultrahigh-speed electronic devices.⁴⁰ The booming breakthroughs are based on the unique 2D structure and the tunable electronic features of graphene. As graphene is sensitive to miscellaneous stimulations (deformation, light, interlayer interaction, etc.), more amazing discovery can be expected in the near future with increasing controllability of graphene at the atomic level. The magic of graphene is far from over, offering infinite possibilities.

In the development of G-EDs, there are also challenges as well as opportunities.

The atomic-thick structure, which endows graphene fascinating properties, makes it difficult to produce large-scale graphene with a well-defined structure at the same time. High-performance G-EDs have strict requirements for the quality and uniformity of graphene. Atom-scale features (Fig. 2), such as defects, disorders, rotations, impurities, grain boundaries, and anchored groups, which currently exist in mass-produced graphene films, will lead to the alteration of properties. Therefore, the controllable synthesis of different graphene materials in bulk is the most challenging issue.

In the case of graphene films produced by CVD, the control of layer numbers, orientations of graphene crystals, etching effects, and substrate engineering are the basic issues in graphene growth. For the graphene film produced by Graphenea, the grain sizes (less than 10 μm) are far behind those of graphene crystals synthesized in the laboratory, indicating a great part of the room for improvements. The seamless stitching of orientated graphene domains shows apparent advantages in the fast growth of a large-area crystal that has less boundaries and defects, which is helpful to improve the quality of the whole film as well as the performance of G-EDs. Further exploration on the growth mechanism will contribute to the control of graphene structures. To date, mass-production of other graphene products like heteroatom-doped graphene and bilayer graphene has not been realized. But the gradually mature production technology of the monolayer graphene film will provide much reference. The other extensively concerned challenge originates from the transfer technology of graphene which is easy to cause damages (breakages, wrinkles, impurities, etc.) and lead to the degradradation of G-EDs. The deterioration of graphene derives from several stages: separation of graphene and the catalyst, the contact between graphene and the target substrate, and the removal of supporting layers. A supporting layer is usually needed to protect graphene during the transfer. The surface energy of the polymer layer and target substrate, which is closely related to the interaction between them and graphene, is crucial to avoid cracks and wrinkles. Despite the progress achieved, an effective, noninvasive, large-scale, transfer approach is urgently required. Direct growth technology on arbitrary substrates is an alternative way to avoid this issue, but the quality of graphene synthesized by this method is far from that using metal catalysts. Strategies, including novel growth mechanisms and system modifications, should be developed to achieve transfer-free and high-quality graphene synchronously.

For graphene sheets produced by liquid exfoliation and reduction of GO, the main features are lateral dimensions, layer numbers, ratio of C/O, and C-C sp², which are closely related to the property of products as well as the G-EDs. Influenced by the different production technologies and complicated composition of graphene flakes, the controllability is a major challenge. As reported by Kauling et al., the graphene flakes on the market are intermingled.¹⁴ They found that many graphene flakes have lateral size below 5 µm, graphene content lower than 50%, and layer numbers over 3. However, only large-size (>10 µm) flakes with fewer layers can exhibit the outstanding electronic, optical, thermal properties of ideal graphene, which are expected by the scientists. It is of great significant to control the graphene quality or grade the products according to their features. The publication of the International Organization for Standardization (ISO) graphene standard (ISO/TS 80004-13: 2017) will benefit to standardize the production and application of graphene flakes, laying the foundation for the beneficial development of the graphene industry. Currently, it is hard to produce large-size graphene flakes efficiently. Because during the exfoliation of highly ordered pyrolytic graphite, the rate of in-plane oxidation is much higher than that of cross-planar splitting,⁴¹ leading to small-size graphene sheets. A similar challenge also exists in mechanical exfoliation. It is essential to achieve balance between the flake size and productivity. At present, GO sheets with lateral size up to 33 µm have been achieved using thermally or chemically expanded graphite, offering avenues toward large-size graphene sheets.⁴² The other challenge lies in the synthesis of monolayer graphene sheets which are favorable for electrodes due to the strong interaction between adjacent graphene layers. To realize the mass production of the graphene sheet with high carbon content, novel methods based on CVD were developed by modifying the equipment and process, such as the bubbling CVD using molten copper,⁴³ microwave oven “snowing” process.⁴⁴ These techniques break the limitation of low yield of CVD, giving other approaches to produce large-scale high-quality graphene sheets. In fact, as the growth mechanism of graphene becomes clear, innovation in process design plays an important role in developing more effective approaches.

Apart from graphene films and flakes, other derivatives synthesized by functionalization should also be controlled when constructing desired G-EDs. The modification extent, defects in graphene, and the performance of G-EDs are closely related. Because, modification not only endows or enhances the specialty but also brings about the deterioration of conductivity due to the break of the initial π-conjugated structure. Consequently, controllable decoration of graphene is the other grand challenge after production. Macroscopical control of the types and contents of function groups is adequate for graphene used in medium- and low-grade products. But controllable synthesis and modification of graphene at the nanoscale are demanded in sophisticated nanodevices (such as DNA sequencing⁴⁵ and devices fabricated from structure-defined GNRs), which requires further investigations. In order to further improve the reliability and accuracy, it is essential to precisely control the types, locations, and contents of the functional groups on these derivatives. The development of the on-surface bottom-up approach for atomically defined GNRs offers a direction to control the microstructure of graphene.

The control of the structure and property of graphene is bound up with characterization techniques. Conventional characterization means in laboratory, such as Raman spectroscopy and scanning tunneling microscopy (STM), are not applicable to large-batch graphene films. Novel techniques, which should be non-destructive, high accuracy, and speed, are urgently required to rapidly evaluate the properties (uniformity, conductivity, continuity, etc.) of large-area graphene, providing a basis for the quality control of graphene. Terahertz time-domain spectroscopy without physical contact has shown the potential for the effective characterization of the conductivity of graphene.⁴⁶ Merely, the microstructure of graphene, such as edges, vacancies, and disorders, is mainly revealed by equipment like STM and the high-resolution transmission electron microscope. It will be of extraordinary significance to map or estimate the defects, boundaries, and other features efficiently. Feasible methods have been demonstrated such as optical microscopy and Raman spectroscopy,^47,48 though further development is required. The evolution of characterizing technology indicates great opportunities in both scientific research and business.

High-quality graphene films synthesized by CVD should be the most desirable material for G-EDs. However, compared to cost-effective graphene sheets, RGO, and GO, CVD-grown graphene films were limited by the high cost resulting from the consumption of the substrate and energy. After the global growth of graphene during the past years, process optimization and equipment improvement become more important to further reduce the cost. The reuse of metal by the optimized transfer technique and cold-wall CVD would be beneficial to handle this problem. As reported by Banszerus et al., the Hall bar devices fabricated with graphene grown on recovered copper exhibited mobilities as high as 3 50 000 cm² V⁻¹ s⁻¹,⁴⁹ comparable to exfoliated graphene. Besides, the breakthrough in the low-temperature and transfer-free growth technique, such as PECVD at temperatures below 500 °C,⁵⁰ could greatly promote the mass-production of graphene and integrated G-EDs. At last, to maximize resource utilization, the combination with related industries (feedstocks, application products, and byproducts) will further perfect the industrial production of graphene.

In an attempt to replace the position of the current mature material, G-EDs need to stand out in both cost and performance. The cost, properties of graphene, and practicable G-EDs are closely connected and interacted (Fig. 3). Accordingly, a balance between these three factors is required for each application. The development of different-level products also benefits to improve the cost-effectiveness of graphene. With the maturity of production technologies and equipment, the reducing cost of high-quality graphene films will accelerate the commercialization of high-performance G-EDs.

As yet, there is a gap between the performances we expected and the reported results of G-EDs. First, the deterioration of G-EDs also depends on the controllable synthesis and modification of graphene, which has been discussed earlier. Second, the work function, high sheet resistance, and contact resistance of graphene, which are critical to charge transport, are the predominant questions to improve the efficiency of most G-EDs. To date, various strategies have been used to overcome these advantages without sacrificing the transparency and conductivity, for instance, doping, surface treatments, and edge contacts.^51,52 More universal strategies are urgently needed. Sustainable efforts are still required to understand and govern the complex interaction, which is also important for the application of other 2D materials. Nonetheless, with outstanding and mechanical strength, graphene has an outstanding chance of being applied in flexible electronics. Additionally, the interaction between graphene and the substrate is also important. Compared with SiO₂/Si, hexagonal boron nitride (h-BN), another 2D material with an atomically smooth surface and less dangling bonds, showed better lattice matching with graphene, which can improve the performance of G-EDs fabricated on h-BN.⁵³ But the synthesis of large-area h-BN becomes another challenge. The good news is that the wafer-scale single-crystal hexagonal boron nitride film has been achieved recently,⁵⁴ which will boost the development of G-EDs based on the graphene/h-BN heterostructure. Furthermore, multifarious concrete issues remain to be solved for specific devices. For instance, the sensing behavior of GFETs in physiological solutions is limited by the Debye screening effect which is caused by the influence of mobile ions.⁷ It requires continuous improvement to optimize the structure and fabrication of G-EDs. We should overcome the drawbacks of G-EDs without sacrificing their advantages. Modifications of substrates, graphene, and the interaction between them are basic outlines to surmount these obstacles. Combination between graphene and other materials (such as TMDs, organic semiconductors, etc.) also provides practicable approaches.

The industrialization of a newly arisen material generally needs a long-term process. Great efforts have been directed at developing the production approach and related applications. The cooperation of research institutes and companies has contributed enormous advances to the graphene industry. With the decreasing cost and increasing productivity, products based on graphene films, such as touch screens and transparent flexible devices, will exhibit high competitiveness to existing products, becoming the pioneer of commercialized G-EDs in less than 10 years. The development of superior G-EDs will give full play of advantages of graphene, such as high-performance electronics, ultrasensitive sensors designed to work under extreme conditions, and smart devices which can react to the fluctuations of the environment from multiple aspects (temperature, gas molecules, pressure, and light). These advanced electronics, high-cost but possessing unparalleled features, should first find their place in strategic sectors like aerospace, space technology, and defence.

The development of G-EDs faces the opportunity and challenge simultaneously. Continuous optimization of the production and processing techniques is demanded to improve the performance, reliability, and reproducibility. The growing accessible applications will contribute to the emergence of killer products. The evolution of the graphene industry will expedite the birth of advanced characterization and processing techniques at the same time, providing the basis for the commercialization of other 2D materials. It is anticipated that graphene can fulfil its promise in electronics in the near future.

We are grateful for the financial support from the National Natural Science Foundation of China (Nos. 61390502 and 51521003), the Strategic Priority Research Program of the Chinese Academy of Sciences (Nos. XDB 12030100 and XDB30000000), Self-Planned Task of State Key Laboratory of Robotics and System (HIT) (No. SKLRS201607B), and Newton Mobility Grant through Royal Society and NFSC (Grant No. IE161019).