Nitrogen cluster doping for high-mobility/conductivity graphene films with millimeter-sized domains

Towards the selective growth of two-dimensional ordered CxNy compounds via epitaxial substrate mediation

Two-dimensional (2D) ordered carbon-nitrogen binary compounds (CxNy) show great potential in many fields owing to their diverse structures and outstanding properties. However, the scalable and selective synthesis of 2D CxNy compounds remain a challenge due to the variable C/N stoichiometry induced coexistence of graphitic, pyridinic, and pyrrolic N species and the competitive growth of graphene. Here, this work systematically explored the mechanism of selective growth of a series of 2D ordered CxNy compounds, namely, the g-C3N4, C2N, C3N, and C5N, on various epitaxial substrates via first-principles calculations. By establishing the thermodynamic phase diagram, it is revealed that the individualized surface interaction and symmetry match between 2D CxNy compounds and substrates together enable the selective epitaxial growth of single crystal 2D CxNy compounds within distinct chemical potential windows of feedstock. The kinetics behaviors of the diffusion and attachment of the decomposed feedstock C/N atoms to the growing CxNy clusters further confirmed the feasibility of the substrate mediated selective growth of 2D CxNy compounds. Moreover, the optimal experimental conditions, including the temperature and partial pressure of feedstock, are suggested for the selective growth of targeted 2D CxNy compound on individual epitaxial substrates by carefully considering the chemical potential of carbon/nitrogen as the functional of experimental parameters based on the standard thermochemical tables. This work provides an insightful understanding on the mechanism of selective epitaxial growth of 2D ordered CxNy compounds for guiding the future experimental design.

Position-Induced Controllable Growth of Vertically Oriented Graphene Using Plasma-Enhanced Chemical Vapor Deposition

Because the morphology of vertically oriented graphene (VG) synthesized by the plasma-enhanced chemical vapor deposition process determines the application performance of VG, morphology control is always an important part of the research. A concise correspondence between plasma and the morphology of VG is the key to investigating the morphology control of VG, which is still under research. In this study, a simple but effective parameter, position, is used to grow VG, by which the continuous morphology evolution of VG is realized. As a result, the morphology of VGs varies from a porous structure to a "wall-like" structure, thus leading to a continuous change in its hydrophobicity and thermal emissivity. An ultrahigh emissivity of 0.999 with superhydrophobicity is obtained among these VGs, showing great potential in the area of the black body and infrared thermometer. Finally, the states of active particles in plasma depending on the positions are diagnosed to investigate their relations with the morphology of VGs.

Engineering Interface Defects and Interdiffusion at the Degenerate Conductive In2O3/Al2O3 Interface for Stable Electrodes in a Saline Solution

A low-temperature Al₂O₃ deposition process provides a simplified method to form a conductive two-dimensional electron gas (2DEG) at the metal oxide/Al₂O₃ heterointerface. However, the impact of key factors of the interface defects and cation interdiffusion on the interface is still not well understood. Furthermore, there is still a blank space in terms of applications that go beyond the understanding of the interface's electrical conductivity. In this work, we carried out a systematic experimental study by oxygen plasma pretreatment and thermal annealing post-treatment to study the impact of interface defects and cation interdiffusion at the In₂O₃/Al₂O₃ interface on the electrical conductance, respectively. Combining the trends in electrical conductance with the structural characteristics, we found that building a sharp interface with a high concentration of interface defects provides a reliable approach to producing such a conductive interface. After applying this conductive interface as electrodes for fabricating a field-effect transistor (FET) device, we found that this interface electrode exhibited ultrastability in phosphate-buffered saline (PBS), a commonly used biological saline solution. This study provides new insights into the formation of conductive 2DEGs at metal oxide/Al₂O₃ interfaces and lays the foundation for further applications as electrodes in bioelectronic devices.

Biomass-Derived Carbon Materials in Heterogeneous Catalysis: A Step towards Sustainable Future

Biomass-derived carbons are emerging materials with a wide range of catalytic properties, such as large surface area and porosity, which make them ideal candidates to be used as heterogeneous catalysts and catalytic supports. Their unique physical and chemical properties, such as their tunable surface, chemical inertness, and hydrophobicity, along with being environmentally friendly and cost effective, give them an edge over other catalysts. The biomass-derived carbon materials are compatible with a wide range of reactions including organic transformations, electrocatalytic reactions, and photocatalytic reactions. This review discusses the uses of materials produced from biomass in the realm of heterogeneous catalysis, highlighting the different types of carbon materials derived from biomass that are potential catalysts, and the importance and unique properties of heterogeneous catalysts with different preparation methods are summarized. Furthermore, this review article presents the relevant work carried out in recent years where unique biomass-derived materials are used as heterogeneous catalysts and their contribution to the field of catalysis. The challenges and potential prospects of heterogeneous catalysis are also discussed.

Coexistence of Urbach-Tail-Like Localized States and Metallic Conduction Channels in Nitrogen-Doped 3D Curved Graphene

Nitrogen (N) doping is one of the most effective approaches to tailor the chemical and physical properties of graphene. By the interplay between N dopants and 3D curvature of graphene lattices, N-doped 3D graphene displays superior performance in electrocatalysis and solar-energy harvesting for energy and environmental applications. However, the electrical transport properties and the electronic states, which are the key factors to understand the origins of the N-doping effect in 3D graphene, are still missing. The electronic properties of N-doped 3D graphene are systematically investigated by an electric-double-layer transistor method. It is demonstrated that Urbach-tail-like localized states are located around the neutral point of N-doped 3D graphene with the background metallic transport channels. The dual nature of electronic states, generated by the synergistic effect of N dopants and 3D curvature of graphene, can be the electronic origin of the high electrocatalysis, enhanced molecular adsorption, and light absorption of N-doped 3D graphene.

Structural Design and Fabrication of Multifunctional Nanocarbon Materials for Extreme Environmental Applications

Extreme environments represent numerous harsh environmental conditions, such as temperature, pressure, corrosion, and radiation. The tolerance of applications in extreme environments exemplifies significant challenges to both materials and their structures. Given the superior mechanical strength, electrical conductivity, thermal stability, and chemical stability of nanocarbon materials, such as carbon nanotubes (CNTs) and graphene, they are widely investigated as base materials for extreme environmental applications and have shown numerous breakthroughs in the fields of wide-temperature structural-material construction, low-temperature energy storage, underwater sensing, and electronics operated at high temperatures. Here, the critical aspects of structural design and fabrication of nanocarbon materials for extreme environments are reviewed, including a description of the underlying mechanism supporting the performance of nanocarbon materials against extreme environments, the principles of structural design of nanocarbon materials for the optimization of extreme environmental performances, and the fabrication processes developed for the realization of specific extreme environmental applications. Finally, perspectives on how CNTs and graphene can further contribute to the development of extreme environmental applications are presented.

Gate-Tunable Resonance State and Screening Effects for Proton-Like Atomic Charge in Graphene

The ability to create a robust and well-defined artificial atomic charge in graphene and understand its carrier-dependent electronic properties represents an important goal toward the development of graphene-based quantum devices. Herein, we devise a new pathway toward the atomically precise embodiment of point charges into a graphene lattice by posterior (N) ion implantation into a back-gated graphene device. The N dopant behaves as an in-plane proton-like charge manifested by formation of the characteristic resonance state in the conduction band. Scanning tunneling spectroscopy measurements at varied charge carrier densities reveal a giant energetic renormalization of the resonance state up to 220 meV with respect to the Dirac point, accompanied by the observation of gate-tunable long-range screening effects close to individual N dopants. Joint density functional theory and tight-binding calculations with modified perturbation potential corroborate experimental findings and highlight the short-range character of N-induced perturbation.

Graphene Infrared Radiation Management Targeting Photothermal Conversion for Electric-Energy-Free Crude Oil Collection

Graphene has been widely used as a solar absorber for its broad-band absorption. However, targeting a higher photothermal efficiency, the intrinsic infrared radiation loss of graphene requires to be further reduced. Herein, band structure engineering is performed to modulate graphene infrared radiation. Nitrogen-doped vertical graphene is grown on quartz foam (NVGQF) by the plasma-enhanced chemical vapor deposition method. Under the premise of keeping high solar absorption (250-2500 nm), graphitic nitrogen doping effectively modulates the infrared emissivity (2.5-25 μm) of NVGQF from 0.96 to 0.68, reducing the radiation loss by ∼31%. Based on the excellent photothermal properties of NVGQF, a temperature-gradient-driven crude oil collecting raft is designed, where the crude oil flows along the collecting path driven by the viscosity gradient without any external electric energy input. Compared with a nondoped vertical graphene quartz foam raft, the NVGQF raft with a superior photothermal efficiency shows a significantly enhanced crude oil collecting efficiency by three times. The advances in this work suggest broad radiation-managed application platforms for graphene materials, such as seawater desalination and personal or building thermal management.

High-Quality N-Doped Graphene with Controllable Nitrogen Bonding Configurations Derived from Ionic Liquids

Controllable nitrogen doping is an effective way to regulate the electronic properties of graphene and further to facilitate its wider application. However, the synthesis of high-quality nitrogen-doped graphene (NG) with a controllable nitrogen configuration still faces considerable challenges. In this work, we present for the first time a simple method for the one-step synthesis of NG with ionic liquids (ILs) as precursors, which avoids the defects introduced by secondary doping and simplifies the process. Using 1-Ethyl-3-methylimidazolium dicyanamide (EMIM-dca) as the precursor, we obtained a high-quality NG with few defects (I_D /I_G is 0.83), nitrogen content (4.11 at%), and graphite-N proportion of 92% at a growth temperature of 1000 °C and field effect transistors (FETs) fabricated on SiO₂ /Si substrates using the NG exhibited typical n-type semiconductor behavior in air. Our findings bring more inspiration for the controllable growth of high-quality graphitic N-doped graphene, thereby promoting its application possibilities in numerous fields.

Ultra-Broadband Strong Electromagnetic Interference Shielding with Ferromagnetic Graphene Quartz Fabric

Flexible electromagnetic interference (EMI) shielding materials with ultrahigh shielding effectiveness (SE) are highly desirable for high-speed electronic devices to attenuate radiated emissions. For hindering interference of their internal or external EMI fields, however, a metallic enclosure suffers from relatively low SE, band-limited anti-EMI responses, poor corrosion resistance, and non-adaptability to the complex geometry of a given circuit. Here, a broadband, strong EMI shielding response fabric is demonstrated based on a highly structured ferromagnetic graphene quartz fiber (FGQF) via a modulation-doped chemical vapor deposition (CVD) growth process. The precise control of the graphitic N-doping configuration endows graphene coatings on specifically designable quartz fabric weave with both high conductivity (3906 S cm^-1 ) and high magnetic responsiveness (a saturation magnetization of ≈0.14 emu g^-1 under 300 K), thus attaining synergistic effect of EMI shielding and electromagnetic wave (EMW) absorption for broadband anti-EMI technology. The large-scale durable FGQF exhibits extraordinary EMI SE of ≈107 dB over a broadband frequency (1-18 GHz), by configuring ≈20 nm-thick graphene coatings on a millimeter-thick quartz fabric. This work enables the potential for development of an industrial-scale, flexible, lightweight, durable, and ultra-broadband strong shielding material in advanced applications of flexible anti-electronic reconnaissance, antiradiation, and stealthy technologies.

Spin-Resolved Visible Optical Spectra and Electronic Characteristics of Defect-Mediated Hexagonal Boron Nitride Monolayer

Defect-mediated hexagonal boron nitride (hBN) supercells display visible optical spectra and electronic characteristics. The defects in the hBN supercells included atomic vacancy, antisite, antisite vacancy, and the substitution of a foreign atom for boron or nitrogen. The hBN supercells with VB, CB, and NB-VN were characterized by a high electron density of states across the Fermi level, which indicated high conductive electronic characteristics. The hBNs with defects including atomic vacancy, antisite at atomic vacancy, and substitution of a foreign atom for boron or nitride exhibited distinct spin-resolved optical and electronic characteristics, while defects of boron and nitrogen antisite did not display the spin-resolved optical characteristics. The hBNs with positively charged defects exhibited dominant optical and electronic characteristics in the longer spectral region. Acknowledgment: This work at HU is supported by ARO W911NF-15-1-0535, NSF HRD-1137747, and NASA NNX15AQ03A.

Activation-Induced Surface Modulation of Biowaste-Derived Hierarchical Porous Carbon for Supercapacitors

Wheat straw-derived carbon from the Wheatbelt region in Western Australia was subjected to chemical activation in an electrolyte containing either acid or base treatment. The findings showed an increase in electron/hole mobility towards the interfaces due to the presence of different surface functional groups such as C-SO_x -C and S=C in the carbon framework for acid activation. Likewise, the galvanostatic capacitance measured at a current density of 2 mA cm^-2 in a three-electrode configuration for acid-activated wheat straw exhibited 162 F g^-1 , while that for base-activated wheat straw exhibited 106 F g^-1 . An increase of 34.5 % more capacitance was achieved for acid-treated wheat straw. This improvement is attributed to the synergistic effects between surface functional groups and electrolyte ions, as well as the electronic structure of the porous electrode.

Electrochemically Exfoliated Chlorine-Doped Graphene for Flexible All-Solid-State Micro-Supercapacitors with High Volumetric Energy Density

Graphene-constructed micro-supercapacitors (MSCs) have received considerable attention recently, as part of the prospective wearable and portable electronics, owing to their distinctive merits of well-tunable power output, robust mechanical flexibility, and long cyclability. In the current work, the focus is on the fabrication of high-quality and solution-processible chlorine-doped graphene (Cl-G) nanosheets through a handy yet eco-friendly electrochemical exfoliation process. The Cl-G is characteristic of the large lateral size of ≈10 µm, abundant nanopores with sizes of as small as 2 nm, as well as numerous steps from the rugged surface. Arising from the rich chemical functionalities and structure defects, the all-solid-state MSC built by using Cl-G via a facile mask-assisted method delivers a large reversible capacity and ultrasteady charge/discharge performance, with the capacitance being maintained at 98.1% even after 250 000 cycles. The Cl-G-MSC with EMIMBF₄ /PVDF-HFP as the electrolyte displays a large volumetric capacitance up to 160 F cm^-3 at the scan rate of 5 mV s^-1 and high volumetric energy density of 97.9 mW h cm^-3 at the power density of 3.4 W cm^-3 . The device can also output a high voltage up to 3.5 V and robust capability with 94.8% of capacitance retention upon 10 000 cycles.

Ultrasensitive Pressure Sensor Sponge Using Liquid Metal Modulated Nitrogen-Doped Graphene Nanosheets

Wearable pressure sensors are crucial for real-time monitoring of human activities and biomimetic robot status. Here, the ultrasensitive pressure sensor sponge is prepared by a facile method, realizing ultrasensitive pressure sensing for wearable health monitoring. Since the liquid metal in the sponge-skeleton structure under pressure is conducive to adjust the contact area with nitrogen-doped graphene nanosheets and thus facilitates the charge transfer at the interface, such sensors exhibit a fast response and recovery speed with the response/recovery time 0.41/0.12 s and a comprehensive response range with a sensitivity of up to 476 KPa^-1. Notably, the liquid metal-based spongy pressure sensor can accurately monitor the human body's pulse, the pressure on the skin, throat swallowing, and other activities in real time, demonstrating a broad application prospect. Those results provide a convenient and low-cost way to fabricate easily perceptible pressure sensors, expanded the application potential of liquid metal-based composites for future electronic skin development.

Electronic, Structural, and Magnetic Upgrading of Coal-Based Products through Laser Annealing

Most coal-to-product routes require complex thermal treatment to carbonize the raw materials. However, the lack of unified comparison of products made from different kinds of coals downplays the role of initial coal chemistry in high-temperature reactions. Here, we used a CO₂ laser to investigate the roles that aromatic content and maturity play in the structural evolution and doping of coals during annealing. Results show that a bituminous coal (DECS 19) with aromatic content and maturity in between higher rank, more mature anthracite (DECS 21) and lower rank, lower maturity lignite (DECS 25) leads to more graphite-like structure observed from the highest 2D peak on the Raman spectrum and conductivity (sheet resistance ∼30 ohm sq^-1) after lasing. When nitrogen dopants are incorporated with saturated urea dopants into coals through laser ablation, nitrogen preferentially incorporates at the edge sites of graphitic grains. Furthermore, oxide nanoparticles can be incorporated into the graphitic backbone of coal to modify their electronic and magnetic properties through laser annealing. Leveraging tunable magnetic behavior, we demonstrate a soft actuator using both conductive and magnetic coal-Fe/Co oxide. Through laser annealing, we propose a paradigm to understand and control coal chemistry toward flexible and tunable doping and magnetism.

ZIF-Derived Carbon Nanoframes as a Polysulfide Anchor and Conversion Mediator for High-Performance Lithium-Sulfur Cells

Improving the redox kinetics of sulfur species, while suppressing the "shuttle effects" to achieve stable cycling under high sulfur loading is an inevitable problem for lithium-sulfur (Li-S) cells to commercialization. Herein, the three-dimensional Zn, Co, and N codoped carbon nanoframe (3DZCN-C) was successfully synthesized by calcining precursor which protected by mesoporous SiO₂ and was used as cathode host for the first time to improve the performance of Li-S cells. Combining the merits of strong lithium polysulfides (LiPSs) anchoring and accelerating the conversion kinetics of sulfur species, 3DZCN-C effectively inhibit the shuttling of LiPSs and achieves excellent cyclability with capacity fading rate of 0.03% per cycle over 1000 cycles. Furthermore, the Li-S pouch cell has been assembled and has been shown to operate reliably with high energy density (>300 Wh kg^-1) even under a high sulfur loading of 10 mg cm^-2. This work provides a simple and effective way for the promotion and commercial application of Li-S cells.

N-doped reduced graphene oxide-PEDOT nanocomposites for implementation of a flexible wideband antenna for wearable wireless communication applications

We report a flexible and highly efficient wideband slot antenna based on a highly conductive composite of poly(3,4-ethylenedioxythiophene) (PEDOT) and N-doped reduced graphene oxide (N-doped rGO) for wearable applications. The high conductivity of this hybrid material with low sheet resistance of 0.56 Ω/square, substantial thickness of 55μm, and excellent mechanical resilience (<5.5% resistance change after 1000 bending cycles) confirmed this composite to be a suitable antenna conductor. The antenna achieved an estimated conduction efficiency close to 80% over a bandwidth from 3 to 8 GHz. Moreover, the successful operation of a realized antenna prototype has been demonstrated in free space and as part of a wearable camera system. The read range of the system was measured to be 271.2 m, which is 23 m longer than that of the original monopole antennas provided by the supplier. The synergistic effects between the dual conjugated structures of N-doped rGO and PEDOT in a single composite with fine distribution and interfacial interactions are critical to the demonstrated material performance. The N-doped rGO sheet reinforces the mechanical stability whereas the PEDOT functions as additive and/or binder, leading to an improved electrical and mechanical performance compared to that of the graphene and PEDOT alone. This high-performing nanocomposite material meets requirements for antenna design and opens the door for diverse future non-metallic flexible electronic device developments.

New Growth Frontier: Superclean Graphene

The last 10 years have witnessed significant progress in chemical vapor deposition (CVD) growth of graphene films. However, major hurdles remain in achieving the excellent quality and scalability of CVD graphene needed for industrial production and applications. Early efforts were mainly focused on increasing the single-crystalline domain size, large-area uniformity, growth rate, and controllability of layer thickness and on decreasing the defect concentrations. An important recent advance was the discovery of the inevitable contamination phenomenon of CVD graphene film during high-temperature growth processes and the superclean growth technique, which is closely related to the surface defects and to the peeling-off and transfer quality. Superclean graphene represents a new frontier in CVD graphene research. In this Perspective, we aim to provide comprehensive understanding of the intrinsic growth contamination and the experimental solution of making superclean graphene and to provide an outlook for future commercial production of high-quality CVD graphene films.

Superclean Growth of Graphene Using a Cold-Wall Chemical Vapor Deposition Approach

Chemical vapor deposition (CVD) has become a promising approach for the industrial production of graphene films with appealing controllability and uniformity. However, in the conventional hot-wall CVD system, CVD-derived graphene films suffer from surface contamination originating from the gas-phase reaction during the high-temperature growth. Shown here is that the cold-wall CVD system is capable of suppressing the gas-phase reaction, and achieves the superclean growth of graphene films in a controllable manner. The as-received superclean graphene film, exhibiting improved optical and electrical properties, was proven to be an ideal candidate material used as transparent electrodes and substrate for epitaxial growth. This study provides a new promising choice for industrial production of high-quality graphene films, and the finding about the engineering of the gas-phase reaction, which is usually overlooked, will be instructive for future research on CVD growth of graphene.

Superhigh Uniform Magnetic Cr Substitution in a 2D Mo2 C Superconductor for a Macroscopic-Scale Kondo Effect

Substitutional doping provides an effective strategy to tailor the properties of 2D materials, but it remains an open challenge to achieve tunable uniform doping, especially at high doping level. Here, uniform lattice substitution of a 2D Mo₂ C superconductor by magnetic Cr atoms with controlled concentration up to ≈46.9 at% by chemical vapor deposition and a specifically designed Cu/Cr/Mo trilayer growth substrate is reported. The concentration of Cr atoms can be easily tuned by simply changing the thickness of the Cr layer, and the samples retain the original structure of 2D Mo₂ C even at a very high Cr concentration. The controlled uniform Cr doping enables the tuning of the competition of the 2D superconductor and the Kondo effect across the whole sample. Transport measurements show that with increasing Cr concentration, the superconductivity of the 2D Cr-doped Mo₂ C crystals disappears along with the emergence of the Kondo effect, and the Kondo temperature increases monotonously. Using scanning tunneling microscopy/spectroscopy, the mechanism of the doping level effect on the interplay and evolution between superconductivity and the Kondo effect is revealed. This work paves a new way for the synthesis of 2D materials with widely tunable doping levels, and provides new understandings on the interplay between superconductivity and magnetism in the 2D limit.

Chemical vapor deposition of layered two-dimensional MoSi2N4 materials

Identifying two-dimensional layered materials in the monolayer limit has led to discoveries of numerous new phenomena and unusual properties. We introduced elemental silicon during chemical vapor deposition growth of nonlayered molybdenum nitride to passivate its surface, which enabled the growth of centimeter-scale monolayer films of MoSi2N4. This monolayer was built up by septuple atomic layers of N-Si-N-Mo-N-Si-N, which can be viewed as a MoN2 layer sandwiched between two Si-N bilayers. This material exhibited semiconducting behavior (bandgap ~1.94 electron volts), high strength (~66 gigapascals), and excellent ambient stability. Density functional theory calculations predict a large family of such monolayer structured two-dimensional layered materials, including semiconductors, metals, and magnetic half-metals.

Recent Advancement in Bio-precursor derived graphene quantum dots: Synthesis, Characterization and Toxicological Perspective

Graphene quantum dots (GQDs), impressive materials with enormous future potential, are reviewed from their inception, including different precursors. Considering the increasing burden of industrial and ecological bio-waste, there is an urgency to develop techniques which will convert biowaste into active moieties of interest. Amongst the various materials explored, we selectively highlight the use of potential carbon containing bioprecursors (e.g. plant-based, amino acids, carbohydrates), and industrial waste and its conversion into GQDs with negligible use of chemicals. This review focuses on the effects of different processing parameters that affect the properties of GQDs, including the surface functionalization, paradigmatic characterization, toxicity and biocompatibility issues of bioprecursor derived GQDs. This review also examines current challenges and s the ongoing exploration of potential bioprecursors for ecofriendly GQD synthesis for future applications. This review sheds further light on the electronic and optical properties of GQDs along with the effects of doping on the same. This review may aid in future design approaches and applications of GQDs in the biomedical and materials design fields.