Covalent Functionalization Of Graphene Research Articles

Band Gap Engineering in Two-Dimensional Materials by Functionalization: Methylation of Graphene and Graphene Bilayers.

Graphene is well-known for its unique combination of electrical and mechanical properties. However, its vanishing band gap limits the use of graphene in microelectronics. Covalent functionalization of graphene has been a common approach to address this critical issue and introduce a band gap. In this Article, we systematically analyze the functionalization of single-layer graphene (SLG) and bilayer graphene (BLG) with methyl (CH3) using periodic density functional theory (DFT) at the PBE+D3 level of theory. We also include a comparison of methylated single-layer and bilayer graphene, as well as a discussion of different methylation options (radicalic, cationic, and anionic). For SLG, methyl coverages ranging from 1/8 to 1/1, (i.e., the fully methylated analogue of graphane) are considered. We find that up to a coverage θ of 1/2, graphene readily accepts CH3, with neighbor CH3 groups preferring trans positions. Above θ = 1/2, the tendency to accept further CH3 weakens and the lattice constant increases. The band gap behaves less regularly, but overall it increases with increasing methyl coverage. Thus, methylated graphene shows potential for developing band gap-tuned microelectronics devices and may offer further functionalization options. To guide in the interpretation of methylation experiments, vibrational signatures of various species are characterized by normal-mode analysis (NMA), their vibrational density of states (VDOS), and infrared (IR) spectra, the latter two are obtained from ab initio molecular dynamics (AIMD) in combination with a velocity-velocity autocorrelation function (VVAF) approach.

Attomolar detection of hepatitis C virus core protein powered by molecular antenna-like effect in a graphene field-effect aptasensor

Biosensors based on graphene field-effect transistors have become a promising tool for detecting a broad range of analytes. However, their performance is substantially affected by the functionalization protocol. In this work, we use a controlled in-vacuum physical method for the covalent functionalization of graphene to construct ultrasensitive aptamer-based biosensors (aptasensors) able to detect hepatitis C virus core protein. These devices are highly specific and robust, achieving attomolar detection of the viral protein in human blood plasma. Such an improved sensitivity is rationalized by theoretical calculations showing that induced polarization at the graphene interface, caused by the proximity of covalently bound molecular probe, modulates the charge balance at the graphene/aptamer interface. This charge balance causes a net shift of the Dirac cone providing enhanced sensitivity for the attomolar detection of the target proteins. Such an unexpected effect paves the way for using this kind of graphene-based functionalized platforms for ultrasensitive and real-time diagnostics of different diseases.

Grafting Ink for Direct Writing: Solvation Activated Covalent Functionalization of Graphene.

Covalent functionalization of graphene (CFG) has shown attractive advantages in tuning the electronic, mechanical, optical, and thermal properties of graphene. However, facile, large‐scale, controllable, and highly efficient CFG remains challenging and often involves highly reactive and volatile compounds, requiring complex control of the reaction conditions. Here, a diazonium‐based grafting ink consisting of only two components, i.e., an aryl diazonium salt and the solvent dimethyl sulfoxide (DMSO) is presented. The efficient functionalization is attributed to the combination of the solvation of the diazonium cations by DMSO and n‐doping of graphene by DMSO, thereby promoting electron transfer (ET) from graphene to the diazonium cations, resulting in the generation of aryl radicals which subsequently react with the graphene. The grafting density of CFG is controlled by the reaction time and very high levels of functionalization, up to the failing of the Tuinstra–Koenig (T–K) relation, while the functionalization layer remains at monolayer height. The grafting ink, effective for days at room temperature, can be used at ambient conditions and renders the patterning CFG by direct writing as easy as writing on paper. In combination with thermal sample treatment, reversible functionalization is possible by subsequent writing/erasing cycles.

Advances and future outlook in epoxy/graphene composites for anticorrosive applications

Coatings are the prevailing, practical and preferred way to safeguard metals from corrosion. Graphene based organic polymer coatings are extensively used as corrosion fortification aids in this era. This short review highlights the importance of diverse features of such engineered coatings. Graphene has engrossed momentous responsiveness in many industrial fields due to its superior anti-corrosion and barrier properties. The current review honestly debated the recent works and reports on epoxy/graphene coatings for metal protection from extensive corrosion and oxidation. The various methodologies to acquire stable-homogenous graphene/epoxy dispersion, covalent functionalization of graphene, doping of graphene with other nanofillers and their effect on corrosion protection of epoxy/graphene coatings along with detailed corrosion protection mechanism are discussed. This review also outlines the integration of potentially relevant multifunctionalities in epoxy/graphene coatings for self-healing, antifouling, self-cleaning and corrosion protection with suitable examples.

(Invited) Covalent Functionalization of Graphene and MoS2 : Towards Nanometer Scale Chemical Patterning

Atomically thin sheets of materials (2D materials) came to prominence in the early 2000s with the successful isolation of graphene. Since then, the interest in graphene has steadily increased thanks to its exceptional electronic, optical, mechanical and thermal properties. It also led to the isolation of other 2D materials such as black phosphorous, silicene, germanene and transition metal dichalcogenides (TMDs) such as MoS2. Due to their exceptional properties, these materials offer virtually endless opportunities for fundamental research as well as cutting edge applications.Most of these 2D materials can be stabilized by the covalent attachment of organic molecules. For instance, the chemisorption of organic units can promote the stabilization of dispersions of 2D materials in organic solvents improving their solution processability. Moreover, such chemisorption of organic molecules allows manipulation of their physicochemical properties by addition of desired functional groups in a tailor made fashion. However, controlling the spatial distribution of the covalently bound units remains a major challenge due to the highly reactive nature of the reagents used for the covalent modification process.In this talk, I will discuss the covalent modification of graphene, graphite and MoS2 using diazonium chemistry. The chemically activated decomposition of diazonium salts using mild conditions will be discussed in detail. The characterization of the modified materials using atomic force microscopy (AFM), scanning tunnelling microscopy (STM), and Raman spectroscopy will be presented. The micrometre-scale multicomponent patterning of surface-supported graphene achieved through a combination of e-beam lithography and diazonium chemistry will also be discussed. The impact of this functionalization on the surface potential will be evaluated through scanning probe microscopies. Lastly, I will also show how the chemical patterning can be extended to the lower end of the nanoscale by using physisorbed self-assembled monolayers of alkanes as sacrificial templates.

Covalent Functionalization of Graphene for the Enhancement of Thermophysical Properties in Nanofluids

AbstractCovalent functionalization of graphene nanoparticles by autoclave, reflux‐condensation, and microwave‐assisted methods was performed. Nanofluids prepared by acidic functionalized graphene exhibit a higher dispersion stability than raw graphene. The thermal conductivity of nanofluids obtained by the microwave‐assisted method was higher than with other functionalization methods, and a signifcant maximum thermal conductivity enhancement compared to deionized water was achieved. Microwave‐assisted thermochemical synthesis is a highly effective energy‐saving method over other conventional approaches.

Enhancing and Tuning the Nonlinear Optical Response and Wavelength-Agile Strong Optical Limiting Action of N-octylamine Modified Fluorographenes.

Fluorographene has been recently shown to be a suitable platform for synthesizing numerous graphene derivatives with desired properties. In that respect, N-octylamine-modified fluorographenes with variable degrees of functionalization are studied and their nonlinear optical properties are assessed using 4 ns pulses. A very strong enhancement of the nonlinear optical response and a very efficient optical limiting action are observed, being strongly dependent on the degree of functionalization of fluorographene. The observed enhanced response is attributed to the increasing number of defects because of the incorporation of N-heteroatoms in the graphitic network upon functionalization with N-octylamine. The present work paves the way for the controlled covalent functionalization of graphene enabling a scalable access to a wide portfolio of graphene derivatives with custom-tailored properties.

Covalent Functionalization of Graphene with PAMAM Dendrimer and Its Implications on Graphene’s Dispersion and Cytotoxicity

Functionalizing graphene with polymers is an important area of research, owing to the potential applications of graphene in biomedicine and nanotechnology. In this paper, we investigate the covalen...

Organic Reactions in Covalent Functionalization of Graphene

Organic Reactions in Covalent Functionalization of Graphene

Impact of covalent functionalization by diazonium chemistry on the electronic properties of graphene on SiC.

Plenty of strategies focused on covalent interaction have been developed to functionalize graphene's surface in order to employ it in a wide range of applications. Among them, the use of radical species including nitrene, carbene and aryl diazonium salts is regarded as a promising strategy to establish the covalent functionalization of graphene. In this work, we highlight the effect of diazonium chemistry on the electronic properties of graphene on SiC. On the basis of X-ray and synchrotron-based photoemission experiments, we were able to prove that 3,4,5-trimethoxybenzenediazonium (TMeOD) units, reduced and chemisorbed onto graphene using electrochemistry, preserve the electronic structure of the Dirac cone, through inducing a slightly additional n-type doping of graphene, as revealed by a downshift of the Dirac cone probed by angle-resolved photoemission experiments.

Chemical Tuning of Specific Capacitance in Functionalized Fluorographene.

Owing to its high surface area and excellent conductivity, graphene is considered an efficient electrode material for supercapacitors. However, its restacking in electrolytes hampers its broader utilization in this field. Covalent graphene functionalization is a promising strategy for providing more efficient electrode materials. The chemistry of fluorographene is particularly attractive as it allows scalable chemical production of useful graphene derivatives. Nevertheless, the influence of chemical composition on the capacitance of graphene derivatives is a largely unexplored field in nanomaterials science, limiting further development of efficient graphene-based electrode materials. In the present study, we obtained well-defined graphene derivatives differing in chemical composition but with similar morphologies by controlling the reaction time of 5-aminoisophthalic acid with fluorographene. The gravimetric specific capacitance ranged from 271 to 391 F g–1 (in 1 M Na2SO4), with the maximum value achieved by a delicate balance between the amount of covalently grafted functional groups and density of the sp2 carbon network governing the conductivity of the material. Molecular dynamics simulations showed that covalent grafting of functional groups with charged and ionophilic/hydrophilic character significantly enhanced the ionic concentration and hydration due to favorable electrostatic interactions among the charged centers and ions/water molecules. Therefore, conductive and hydrophilic graphitic surfaces are important features of graphene-based supercapacitor electrode materials. These findings provide important insights into the role of chemical composition on capacitance and pave the way toward designing more efficient graphene-based supercapacitor electrode materials.

(Invited) Covalent Functionalization of Graphene and MoS2: Chemical Versus Electrochemical Routes

Atomically thin sheets of materials, the so-called two-dimensional (2D) materials arrived on the scene in early 2000s with the successful isolation of graphene as freestanding monolayer films. Graphene, a single atom thick sheet of sp2-hybridized carbon bonded in a honeycomb lattice, has exceptional electronic, optical, mechanical and thermal properties that outperform those of most of the existing materials. The research on graphene further fuelled emergence of related 2D materials such as black phosphorous, silicene, germanene and transition metal dichalcogenides (TMDs) such as MoS2. Due to their exotic properties, these materials offer virtually endless opportunities for fundamental research as well as cutting edge applications. Most of these 2D materials however exist as single layers only when supported by another solid surface or when stabilized by physisorbed or chemisorbed organic molecules. Given their layered nature, most 2D materials are extremely difficult to disperse in typical solvents. Chemisorption of organic molecules onto their basal plane allows dispersion of these materials in (organic) solvents thereby improving their solution processability. Such dispersions can be used in composite materials and as functional inks. Moreover, covalent functionalization allows modification of the intrinsic electrical, electronic and optical properties of these 2D materials. In this talk, I will discuss covalent modification graphene, graphite and MoS2 using diazonium chemistry. Two different routes for reductive decomposition of diazonium salts namely, chemical and electrochemical, will be discussed in detail. Special focus is on sub-nanometer characterization of modified materials using scanning tunneling and atomic force microscopy (STM and AFM). Initial results towards patterned covalent functionalization using physisorbed self-assembled templates will also be presented besides the discussion on the use of such surfaces as seed layers for atomic layer deposition (ALD).

Perfect spin-filtering in p-aminophenol functionalized zigzag graphene nanoribbons: The role of sp3 hybridized nitrogen

Covalent functionalization of graphene is recently developed from the formation of sp3 hybridized carbon atom (sp3-C) to the sp3 hybridized nitrogen (sp3-N) at the anchoring site. Here, we investigated the electronic structures and transport properties of the zigzag graphene nanoribbons functionalized by covalently bonding of p-aminophenol (p-AP) molecule. First principles results demonstrate that the formed sp3-N plays a vital role in determining the electronic structure and transport properties of the system, resulting in a halfmetallic characteristic with a perfect spin-filtering behavior (100%). Interestingly, the performance of the spin-filtering is find to be insensitive to the sub-structures of the molecule. Our findings reveal the importance of sp3-N and suggest a new mechanism for realizing high-performance spin-filtering devices with functionalized graphene.

Covalently functionalized graphene as a supercapacitor electrode material

The need for efficient, lightweight, flexible, stable and eco-friendly portable electric power delivery has placed research on supercapacitor electrode materials at the forefront of endeavors to provide future energy storage. Graphene-based materials and composites have been shown to be promising materials toward this goal. In this review, we describe recent progress in the development of electrode materials by the covalent functionalization of graphene. Covalent graphene derivatives are an emerging and potentially large class of very efficient materials, whose structural and physicochemical properties can be tailored according to the specific needs of capacitive charge storage. They also open doors to new composites and hybrids, which may further advance the field-encoding properties and other features as yet unexplored.

Modular Covalent Graphene Functionalization with C60 and the Endohedral Fullerene Sc3N@C80: A Facile Entry to Synthetic‐Carbon‐Allotrope Hybrids

AbstractTwo novel graphene‐fullerene hybrid structures, containing C60 and endohedral Sc3N@C80 bound to graphene, instead of the formerly used graphene oxide, were efficiently synthesized via a reductive activation/exfoliation approach starting from pristine graphite. The structures of these multifunctional hybrid systems were unambiguously characterized by statistical Raman spectroscopy, TG‐MS, TG‐GC‐MS, and LD‐TOF mass spectroscopy, confirming the covalent bonding of the respective C60/Sc3N@C80 moieties to the pristine graphene. Furthermore, assisted by temperature‐dependent Raman spectroscopy studies the corresponding defunctionalization processes were also investigated. Finally, the formation of a carbon allotrope hybrid material on the basis of C60/Sc3N@C80 moieties coupled to graphene could be visualized by HRTEM.

Modular Covalent Graphene Functionalization with C60 and the Endohedral Fullerene Sc3 N@C80 : A Facile Entry to Synthetic-Carbon-Allotrope Hybrids.

Two novel graphene-fullerene hybrid structures, containing C60 and endohedral Sc3 N@C80 bound to graphene, instead of the formerly used graphene oxide, were efficiently synthesized via a reductive activation/exfoliation approach starting from pristine graphite. The structures of these multifunctional hybrid systems were unambiguously characterized by statistical Raman spectroscopy, TG-MS, TG-GC-MS, and LD-TOF mass spectroscopy, confirming the covalent bonding of the respective C60 /Sc3 N@C80 moieties to the pristine graphene. Furthermore, assisted by temperature-dependent Raman spectroscopy studies the corresponding defunctionalization processes were also investigated. Finally, the formation of a carbon allotrope hybrid material on the basis of C60 /Sc3 N@C80 moieties coupled to graphene could be visualized by HRTEM.

Aziridine-functionalized graphene: Effect of aromaticity for aryl functional groups on enhanced power conversion efficiency of organic photovoltaic cells

We have demonstrated the covalent functionalization of graphene by nitrene chemistry, resulting in highly dispersed aryl azides functionalized graphene. Two different ortho-substituted aryl azides having different aromatic ring (π-conjugation) and ester/anhydride substituent have been synthesized. Further, on thermolysis they undergo [2+1] cycloaddition ensuing in the formation of aziridine adduct with sp2 carbons of graphene. Resulting aryl azide functionalized graphene sheets (f-graphene I & II) are soluble in most common organic (polar and non polar) solvents, facilitating the structural/property characterization and the device fabrication by solution processing. The aziridine linkage modification of the π conjugation and the strong electronic interaction between the f-graphenes and poly (3-hexylthiophene) (P3HT):Phenyl-C61-Butyric-Acid-Methyl Ester (PCBM) has been confirmed by various spectroscopic analysis. The implications of changing the aromaticity of the aryl azides attached to f-graphenes on the P3HT:PCBM bulk heterojunction (BHJ) photovoltaic device are discussed. The solution cast f-graphene I (5%) doped P3HT:PCBM BHJ showed the power conversion efficiency ɳ = 1.94( ± 0.06) %, while f-graphene II (5%) doped exhibited ɳ = 2.16( ± 0.07) % under AM 1.5 illumination (100 mW/cm2), which is found to be more than double the efficiency of the reference cell.

Oxo‐Functionalized Graphene: A Versatile Precursor for Alkylated Graphene Sheets by Reductive Functionalization

Controlled covalent functionalization of graphene remains a challenging task owing to the heterogeneous nature of materials. Functionalization approaches for graphene either lack in quantifying the degree of functionalization or they do not discriminate between covalent and non-covalent functionalization. Here, graphite is oxidized and exfoliated in a three-step procedure and subsequently reduced and functionalized by hexylation. Although Raman spectroscopy is powerful to determine the degree of in-plane lattice defects (θLD ) and functionalization (θFD ), the method fails at detecting introduced hexyl groups at a concentration of about 0.03 %, next to the pre-existing in-plane lattice defects of 0.7 %. However, sensitive thermogravimetric analysis coupled with gas chromatography and mass spectrometry (TGA-GC/MS) can prove the hexylation reaction. The efficiency of functionalization is comparable to reductive functionalization of pristine chemical vapor deposition (CVD)-graphene and bulk graphite.

Photoswitchable Monolayer and Bilayer Graphene Devices Enabled by In Situ Covalent Functionalization

AbstractA chemical approach is demonstrated for covalent functionalization of graphene devices with photochromic azobenzene moieties using diazonium chemistry. The approach utilizing in situ generated aryl diazonium cations enables multilayer deposition of photochromic species on graphene surfaces. It is demonstrated that the thickness of the resulting optically responsive films can be tuned from about 1 to over 20 nm by varying the functionalization time. Cis and trans forms of azobenzene can be achieved by illumination with UV and visible light, respectively, which enables reversible optically tunable change in the doping level of graphene. Interestingly, the bilayer graphene devices, while showing robust photoswitching, do not exhibit any considerable degradation of conductivity and charge carrier mobilities upon chemical functionalization, which is not the case for monolayer graphene devices. This work paves a way for multilayer functionalization of graphene devices with photochromic species and highlights bilayer graphene as a promising platform for high mobility devices with covalent functionalization.

2D Chemistry: Chemical Control of Graphene Derivatization.

Controllable synthesis of graphene derivatives with defined composition and properties represents the holy grail of graphene chemistry, especially in view of the low reactivity of graphene. Recent progress in fluorographene (FG) chemistry has opened up new routes for synthesizing a plethora of graphene derivatives with widely applicable properties, but they are often difficult to control. We explored nucleophilic substitution on FG combining density functional theory calculations with experiments to achieve accurate control over the functionalization process. In-depth analysis revealed the complexity of the reaction and identified basic rules for controlling the 2D chemistry. Their application, that is, choice of solvent and reaction time, enabled facile control over the reaction of FG with N-octylamine to form graphene derivatives with tailored content of the alkylamine functional group (2.5–7.5% N atomic content) and F atoms (31.5–3.5% F atomic content). This work substantially extends prospects for the controlled covalent functionalization of graphene.