Isotope Engineered Fluorinated Single and Bilayer Graphene: Insights into Fluorination Selectivity,Stability, and Defect Passivation

Tailoring the physicochemical properties of graphene through functionalization remains a major interest for next-generation technological applications. However, defect formation due to functionalization greatly endangers the intrinsic properties of graphene, which remains a serious concern. Despite numerous attempts to address this issue, a comprehensive analysis has not been conducted. This work reports a two-step fluorination process to stabilize the fluorinated graphene and obtain control over the fluorination-induced defects in graphene layers. The structural, electronic and isotope-mass-sensitive spectroscopic characterization unveils several not-yet-resolved facts, such as fluorination sites and C F bond stability in partially-fluorinated graphene (F-SLG). The stability of fluorine has been correlated to fluorine co-shared between two graphene layers in fluorinated-bilayer-graphene (F-BLG). The desorption energy of co-shared fluorine is an order of magnitude higher than the C F bond energy in F-SLG due to the electrostatic interaction and the inhibition of defluorination in the F-BLG. Additionally, F-BLG exhibits enhanced light–matter interaction, which has been utilized to design a proof-of-concept field-effect phototransistor that produces high photocurrent response at a time <200 µs. Thus, the study paves a new avenue for the in-depth understanding and practical utilization of fluorinated graphenic carbon.     

Dichlorocarbene‐Functionalized Fluorographene: Synthesis and Reaction Mechanism

Halogen functionalization of graphene is an important branch of graphene research as it provides opportunities to tailor the band gap and catalytic properties of graphene. Monovalent C–X bond obviates pitfalls of functionalization with atoms of groups 13, 15, and 16, which can introduce various poorly defined groups. Here, the preparation of functionalized graphene containing both fluorine and chlorine atoms is shown. The starting material, fluorographite, undergoes a reaction with dichlorocarbene to provide dichlorocarbene‐functionalized fluorographene (DCC‐FG). The material is characterized by X‐ray photoelectron spectroscopy, Raman spectroscopy, and high‐resolution transmission electron microscopy with X‐ray dispersive spectroscopy. It is found that the chlorine atoms in DCC‐FG are distributed homogeneously over the entire area of the fluorographene sheet. Further density functional theory calculations show that the mechanism of dichlorocarbene attack on fluorographene sheet is a two‐step process. Dichlorocarbene detaches fluorine atoms from fluorographene sheet and subsequently adds to the newly formed sp² carbons. Halogenated graphene consisting of two (or eventually three) types of halogen atoms is envisioned to find its way as new graphene materials with tailored properties.     

Recent Progress on Germanene and Functionalized Germanene: Preparation,Characterizations, Applications,and Challenges

A new family of single‐atom‐thick 2D germanium‐based materials with graphene‐like atomic arrangement, germanene and functionalized germanene, has attracted intensive attention due to their large bandgap and easily tailored electronic properties. Unlike carbon atoms in graphene, germanium atoms tend to adopt mixed sp²/sp³ hybridization in germanene, which makes it chemically active on the surface and allows its electronic states to be easily tuned by chemical functionalization. Impressive achievements in terms of the applications in energy storage and catalysis have been reported by using germanene and functionalized germanene. Herein, the fabrication of epitaxial germanene on different metallic substrates and its unique electronic properties are summarized. Then, the preparation strategies and the fundamental properties of hydrogen‐functionalized germanene (germanane or GeH) and other ligand‐terminated forms of germanene are presented. Finally, the progress of their applications in energy storage and catalysis, including both experimental results and theoretical predictions, is analyzed.     

Chemical functionalization of graphene and its applications

Functionalization and dispersion of graphene sheets are of crucial importance for their end applications. Chemical functionalization of graphene enables this material to be processed by solvent-assisted techniques, such as layer-by-layer assembly, spin-coating, and filtration. It also prevents the agglomeration of single layer graphene during reduction and maintains the inherent properties of graphene. Therefore, a detailed review on the advances of chemical functionalization of graphene is presented. Synthesis and characterization of graphene have also been reviewed in the current article. The functionalization of graphene can be performed by covalent and noncovalent modification techniques. In both cases, surface modification of graphene oxide followed by reduction has been carried out to obtain functionalized graphene. It has been found that both the covalent and noncovalent modification techniques are very effective in the preparation of processable graphene. However, the electrical conductivity of the functionalized graphene has been observed to decrease significantly compared to pure graphene. Moreover, the surface area of the functionalized graphene prepared by covalent and non-covalent techniques decreases significantly due to the destructive chemical oxidation of flake graphite followed by sonication, functionalization and chemical reduction. In order to overcome these problems, several studies have been reported on the preparation of functionalized graphene directly from graphite (one-step process). In all these cases, surface modification of graphene can prevent agglomeration and facilitates the formation of stable dispersions. Surface modified graphene can be used for the fabrication of polymer nanocomposites, super-capacitor devices, drug delivery system, solar cells, memory devices, transistor device, biosensor, etc.     

Room-temperature metastability of multilayer graphene oxide films

Graphene oxide potentially has multiple applications. The chemistry of graphene oxide and its response to external stimuli such as temperature and light are not well understood and only approximately controlled. This understanding is crucial to enable future applications of this material. Here, a combined experimental and density functional theory study shows that multilayer graphene oxide produced by oxidizing epitaxial graphene through the Hummers method is a metastable material whose structure and chemistry evolve at room temperature with a characteristic relaxation time of about one month. At the quasi-equilibrium, graphene oxide reaches a nearly stable reduced O/C ratio, and exhibits a structure deprived of epoxide groups and enriched in hydroxyl groups. Our calculations show that the structural and chemical changes are driven by the availability of hydrogen in the oxidized graphitic sheets, which favours the reduction of epoxide groups and the formation of water molecules.     

Hyperstage Graphite: Electrochemical Synthesis and Spontaneous Reactive Exfoliation

Covalent modification of the π‐electron basal planes of graphene enables the formation of new materials with enhanced functionality. An electrochemical method is reported for the formation of what is referred to as a Hyperstage‐1 graphite intercalation compound (GIC), which has a very large interlayer spacing d₀₀₁ > 15.3 Å and contains disordered interstitial molecules/ions. This material is highly activated and undergoes spontaneous exfoliation when reacted with diazonium ions to produce soluble graphenes with high functionalization densities of one pendant aromatic ring for every 12 graphene carbons. Critical to achieving high functionalization density is the Hyperstage‐1 GIC state, a weakening of the van der Waals coupling between adjacent graphene layers, and the ability of reactants to diffuse into the disordered intercalate phase between the layers. Graphene functionalization with 3,5‐dinitrophenyl groups provides for exceptional dispersibility (0.24 mg mL^?1) in N,N‐dimethylformamide and for conjugation with amines.     

Understanding Defect‐Stabilized Noncovalent Functionalization of Graphene

The noncovalent functionalization of graphene by small molecule aromatic adsorbates, phenanthrenequinone (PQ), is investigated systematically by combining electrochemical characterization, high‐resolution interfacial X‐ray scattering, and ab initio density functional theory calculations. The findings in this study reveal that while PQ deposited on pristine graphene is unstable to electrochemical cycling, the prior introduction of defects and oxygen functionality (hydroxyl and epoxide groups) to the basal plane by exposure to atomic radicals (i.e., oxygen plasma) effectively stabilizes its noncovalent functionalization by PQ adsorption. The structure of adsorbed PQ molecules resembles the graphene layer stacking and is further stabilized by hydrogen bonding with terminal hydroxyl groups that form at defect sites within the graphene basal plane. The stabilized PQ/graphene interface demonstrates persistent redox activity associated with proton‐coupled‐electron‐transfer reactions. The resultant PQ adsorbed structure is essentially independent of electrochemical potentials. These results highlight a facile approach to enhance functionalities of the otherwise chemically inert graphene using noncovalent interactions.     

Mechanism of Functionalization of the Surfaces of Detonation Nanodiamonds: Mass-Spectrometric Investigation

The comparison of the formation processes of volatile products at heating mixtures of detonation nanodiamonds with perfluorochemical alcohol and nanodiamonds subjected to directed chemical modification with perfluorochemical alcohol was made using the thermodesorption mass-spectrometry. A change of the destruction mechanism and increase of the thermostability of perfluorochemical alcohol by ~ 100°C as a result of a directional chemical modification was revealed, the most possible models of functionalization through hydrogen and covalent bonds of the hydroxyl group of fluorine radicals with carboxyl groups of surfaces of nanodiamond particles are suggested, the degrees of the functionalization and energy of the desorption activation of the products of the destruction functionalized molecules are assessed.     

Simultaneous N-intercalation and N-doping of epitaxial graphene on 6H-SiC(0001) through thermal reactions with ammonia

Surface functionalization of epitaxial graphene overlayers on 6H-SiC(0001) has been attempted through thermal reactions in NH₃. X-ray photoelectron spectroscopy and micro-region low energy electron diffraction results show that a significant amount of N is present at the NH₃-treated graphene surface, which results in strong band bending at the SiC surface as well as decoupling of the graphene overlayers from the substrate. The majority of the surface N species can be removed by annealing in vacuum up to 850 °C, weakening the surface band bending and resuming the strong coupling of graphene with the SiC surface. The desorbed N atoms can be attributed to the intercalated species between graphene and SiC. Low temperature scanning tunneling spectroscopy and density functional theory simulations confirm the presence of N dopants in the graphene lattice, which are in the form of graphitic substitution and can be stable above 850 °C. This is the first report of simultaneous N intercalation and N doping of epitaxial graphene overlayers on SiC, and it may be employed to alter the surface physical and chemical properties of epitaxial graphene overlayers.      

Zwitterionic Conjugated Surfactant Functionalization of Graphene with pH‐Independent Dispersibility: An Efficient Electron Mediator for the Oxygen Evolution Reaction in Acidic Media

The functionalization of graphene has been extensively used as an effective route for modulating the surface property of graphene, and enhancing the dispersion stability of graphene in aqueous solutions via functionalization has been widely investigated to expand its use for various applications across a range of fields. Herein, an effective approach is described for enhancing the dispersibility of graphene in aqueous solutions at different pH levels via non‐covalent zwitterion functionalization. The results show that a surfactant with electron‐deficient carbon atoms in its backbone structure and large π–π interactive area enables strong interactions with graphene, and the zwitterionic side terminal groups of the molecule support the dispersibility of graphene in various pH conditions. Experimental and computational studies confirm that perylene diimide amino N‐oxide (PDI–NO) allows efficient functionalization and pH‐independent dispersion of graphene enabled by hydration repulsion effects induced by PDI–NO. The PDI–NO functionalized graphene is successfully used in the oxygen evolution reaction as an electron mediator for boosting the electrocatalytic activity of a Ru‐based polyoxometalate catalyst in an acidic medium. The proposed strategy is expected to bring significant advances in producing highly dispersible graphene in aqueous medium with pH‐independent stability, thus broadening the application range of graphene.     

Molecular Self‐Assembly on Graphene

The formation of ordered arrays of molecules via self‐assembly is a rapid, scalable route towards the realization of nanoscale architectures with tailored properties. In recent years, graphene has emerged as an appealing substrate for molecular self‐assembly in two dimensions. Here, the first five years of progress in supramolecular organization on graphene are reviewed. The self‐assembly process can vary depending on the type of graphene employed: epitaxial graphene, grown in situ on a metal surface, and non‐epitaxial graphene, transferred onto an arbitrary substrate, can have different effects on the final structure. On epitaxial graphene, the process is sensitive to the interaction between the graphene and the substrate on which it is grown. In the case of graphene that strongly interacts with its substrate, such as graphene/Ru(0001), the inhomogeneous adsorption landscape of the graphene moiré superlattice provides a unique opportunity for guiding molecular organization, since molecules experience spatially constrained diffusion and adsorption. On weaker‐interacting epitaxial graphene films, and on non‐epitaxial graphene transferred onto a host substrate, self‐assembly leads to films similar to those obtained on graphite surfaces. The efficacy of a graphene layer for facilitating planar adsorption of aromatic molecules has been repeatedly demonstrated, indicating that it can be used to direct molecular adsorption, and therefore carrier transport, in a certain orientation, and suggesting that the use of transferred graphene may allow for predictible molecular self‐assembly on a wide range of surfaces.     

Graphene epitaxy by chemical vapor deposition on SiC

We demonstrate the growth of high quality graphene layers by chemical vapor deposition (CVD) on insulating and conductive SiC substrates. This method provides key advantages over the well-developed epitaxial graphene growth by Si sublimation that has been known for decades. (1) CVD growth is much less sensitive to SiC surface defects resulting in high electron mobilities of ～1800 cm(2)/(V s) and enables the controlled synthesis of a determined number of graphene layers with a defined doping level. The high quality of graphene is evidenced by a unique combination of angle-resolved photoemission spectroscopy, Raman spectroscopy, transport measurements, scanning tunneling microscopy and ellipsometry. Our measurements indicate that CVD grown graphene is under less compressive strain than its epitaxial counterpart and confirms the existence of an electronic energy band gap. These features are essential for future applications of graphene electronics based on wafer scale graphene growth.     

Preparation of non-covalently functionalized graphene using 9-anthracene carboxylic acid

A simple way of achieving stable aqueous dispersion of graphene by non-covalent functionalization using 9-anthracene carboxylic acid has been successfully accomplished. Unlike in chemically reduced graphene, the C-sp(2) network of the graphene remains undistorted and therefore of superior quality. The non-covalent functionalization facilitates the exfoliation of graphite layers in a polarity controlled combination of media. A detailed exfoliation mechanism is proposed based on the controlled experiment and is supported by the data from UV-vis spectroscopy, transmission electron microscopy, and x-ray diffraction studies. Formation of monolayer graphene has been confirmed from Raman spectroscopy. The graphene based ultracapacitor shows a high value of specific capacitance (148 F g(-1)).     

Orientation‐Dependent Strain Relaxation and Chemical Functionalization of Graphene on a Cu(111) Foil

Epitaxial graphene grown on single crystal Cu(111) foils by chemical vapor deposition is found to be free of wrinkles and under biaxial compressive strain. The compressive strain in the epitaxial regions (0.25–0.40%) is higher than regions where the graphene is not epitaxial with the underlying surface (0.20–0.25%). This orientation‐dependent strain relaxation is through the loss of local adhesion and the generation of graphene wrinkles. Density functional theory calculations suggest a large frictional force between the epitaxial graphene and the Cu(111) substrate, and this is therefore an energy barrier to the formation of wrinkles in the graphene. Enhanced chemical reactivity is found in epitaxial graphene on Cu(111) foils as compared to graphene on polycrystalline Cu foils for certain chemical reactions. A higher compressive strain possibly favors lowering the formation energy and/or the energy gap between the initial and transition states, either of which can lead to an increase in chemical reactivity.     

Real-time microscopy of graphene growth on epitaxial metal films: role of template thickness and strain

Epitaxial transition metal films have recently been introduced as substrates for the scalable synthesis of transferable graphene. Here, real-time microscopy is used to study graphene growth on epitaxial Ru films on sapphire. At high temperatures, high-quality graphene grows in macroscopic (>100 μm) domains to full surface coverage. Graphene nucleation and growth characteristics on thin (100 nm) Ru films are consistent with a pure surface chemical vapor deposition process, without detectable contributions from C segregation. Experiments on thicker (1 μm) films show a systematic suppression of the C uptake into the metal to levels substantially below those expected from bulk C solubility data, consistent with a strain-induced reduction of the C solubility due to gas bubbles acting as stressors in the epitaxial Ru films. The results identify two powerful approaches--i) limiting the template thickness and ii) tuning the interstitial C solubility via strain--for controlling graphene growth on metals with high C solubility, such as Ru, Pt, Rh, Co, and Ni.     

Self-assembly of one-side-functionalized graphene nanosheets in bilayered superstructures for drug delivery

In this article, we describe the one-side functionalization of graphene nanosheets with hydrophilic catechol-bearing pyrrolidine rings. For this purpose, we used, for the first time, a solvothermal alternative of 1,3 dipolar cycloaddition of azomethine ylide. To achieve asymmetrical reaction, graphene nanosheets were initially and during reaction deposited on glass substrate. The result of one-side functionalization of graphene was the formation of amphiphilic few-layered graphene nanosheets. The modified side becomes hydrophilic due to the attachment of catechols, while the nonmodified side remains hydrophobic. In the literature, there are limited examples of functionalized graphene with different sides, the so-called Janus-type graphenes. These amphiphilic graphene nanosheets dispersed in water were self-organized in bilayer superstructures, with hydrophilic outer surface and hydrophobic internal space. The later can host hydrophobic molecules such as anticancer drugs and could be used in drug delivery systems. As an example, camptothecin, a drug practically insoluble in water, was used here to show that it can be transferred to water phase using graphene as transporter.     

石墨烯的制备研究进展

近年来, 石墨烯以其独特的结构和优异的性能, 在化学、物理和材料学界引起了广泛的研究兴趣. 人们已经在石墨烯的制备方面取得了积极的进展, 为石墨烯的基础研究和应用开发提供了原料保障. 本文大量引用近三年最新参考文献, 综述了石墨烯的制备方法: 物理方法(微机械剥离法、液相或气相直接剥离法)与化学法(化学气相沉积法、晶体外延生长法、氧化?还原法), 并详细介绍了石墨烯的各种修饰方法. 分析比较了各种方法的优缺点, 指出了石墨烯制备方法的发展趋势.     

Peptide-functionalized colloidal graphene via interdigited bilayer coating and fluorescence turn-on detection of enzyme

Synthesis of colloidal functional graphene is challenging because graphene is water-insoluble and its relatively inert surface made the functionalization a difficult task. Here we report interdigited bilayer type coating that provide both colloidal stability and functionalization option for graphene. Colloidal graphene oxide is first converted into interdigited bilayer coated graphene oxide and next they are transformed into colloidal graphene by hydrazine reduction. These coated graphenes can be further transformed into colloidal functional graphene using covalent conjugation chemistry. Functional graphene has been synthesized for optical detection of enzyme where a fluorescent dye is covalently linked through a peptide so that the dye fluorescence is quenched by graphene but switches on once enzymes cleave the peptide bond. The interdigited bilayer coating reported here is unique as it provides coating thickness <3 nm, offering optically responsive graphene-fluorophore substrate with high colloidal stability.     

The safer and scalable mechanochemical synthesis of edge-chlorinated and fluorinated few-layer graphenes

Halogen functionalization of the edges of the graphene sheets can improve processability, add new properties, and enhance its interactions with other materials. Through functionalization, improved materials can be realized. Typically, halogenated graphenes are produced from pure or reactive halogen sources. Current approaches present significant safety challenges. By generating reactive dichlorine monoxide (Cl₂O) in situ, a chlorinated graphene with the nominal composition C₁₇Cl₂OH can be realized safely and scalably. Chlorinated graphene can be used as a precursor for an array of functionalized materials by mechanically driven solid-state metathesis reactions. For example, nearly 75% of the chlorine in chlorinated graphene can be exchanged with fluorine by using the safer fluorine-containing compound ammonium fluoride (NH₄F) as a reagent. A material with the composition C₃₄Cl₃F(OH)₂ is realized. Preliminary work shows that F–graphene has oxygen reduction properties and Cl–graphene can improve existing zinc–air fuel cells. A scalable production of chlorinated and fluorinated graphenes and graphites will accelerate their adoption in fuel cells, batteries, polymer composites, and catalysts.     

The possibility of colossal magnetoresistance in heterostructures based on epitaxial graphene

The magnetoresistance of a heterostructure comprising parallel-connected epitaxial graphene on a metal substrate and graphene on a dielectric substrate has been studied in the framework of the Drude theory. The phenomenon of colossal magnetoresistance in this system is predicted on the basis of previous results for the conductivity of epitaxial graphene.