Synergistic Combination of Reductive Covalent Functionalization and Atomic Layer Deposition-Towards Spatially Defined Graphene-Organic-Inorganic Heterostructures.

Three-dimensionally (3D) well-ordered and highly integrated graphene hybrid architectures are considered to be next-generation multifunctional graphene materials but still remain elusive. Here, we report the first realization of unprecedented 3D-patterned graphene nano-ensembles composed of a graphene monolayer, a tailor-made structured organophenyl layer, and three metal oxide films, providing the first example of such a hybrid nano-architecture. These spatially resolved and hierarchically structured quinary hybrids are generated via a two-dimensional (2D)-functionalization-mediated atomic layer deposition growth process, involving an initial lateral molecular programming of the graphene lattice via lithography-assisted 2D functionalization and a subsequent stepwise molecular assembly in these regions in the z-direction. Our breakthrough lays the foundation for the construction of emerging 3D-patterned graphene heterostructures.

Spatially Resolved Janus Patterning of Graphene by Direct Laser Writing.

Covalently patterned Janus-functionalized graphene featuring a spatially defined asymmetric bifacial addend binding motif remains a challenging synthetic target. Here, a facile and universal laser writing approach for a one-step covalent Janus patterning of graphene is reported, leading to the formation of up to now elusive graphene architectures, solely consisting of antaratopically functionalized superlattices. The structurally defined covalent functionalization procedure is based on laser-triggered concurrent photolysis of two different photosensitizers situated on both sides of the graphene plane, generating radicals and subsequent addend binding in the laser-irradiated areas only. Careful structure analysis was performed by Raman spectroscopy and Kelvin probe force microscopy. In terms of the advantages of our newly established concept, including a simple/easy-to-operate patterning procedure, arbitrary pattern availability, and a high degree of addend binding, an easy access to tailor-designed Janus-functionalized graphene devices with spatially resolved functional entities can be envisaged.

Molecular Stacking on Graphene.

The sequential vertical polyfunctionalization of 2D addend-patterned graphene is still elusive. Here, we report a practical realization of this goal via a "molecular building blocks" approach, which is based on a combination of a lithography-assisted reductive functionalization approach and a post-functionalization step to sequentially and controllably link the molecular building blocks ethylpyridine, cis-dichlorobis(2,2'-bipyridyl)ruthenium, and triphenylphosphine (4-methylbenzenethiol, respectively) on selected lattice regions of a graphene matrix. The assembled 2D hetero-architectures are unambiguously characterized by various spectroscopic and microscopic measurements, revealing the stepwise stacking of the molecular building blocks on the graphene surface. Our method overcomes the current limitation of a one-layer-only binding to the graphene surface and opens the door for a vertical growth in the z-direction.

Carbon Nano-onions: Potassium Intercalation and Reductive Covalent Functionalization.

Herein we report the synthesis of covalently functionalized carbon nano-onions (CNOs) via a reductive approach using unprecedented alkali-metal CNO intercalation compounds. For the first time, an in situ Raman study of the controlled intercalation process with potassium has been carried out revealing a Fano resonance in highly doped CNOs. The intercalation was further confirmed by electron energy loss spectroscopy and X-ray diffraction. Moreover, the experimental results have been rationalized with DFT calculations. Covalently functionalized CNO derivatives were synthesized by using phenyl iodide and n-hexyl iodide as electrophiles in model nucleophilic substitution reactions. The functionalized CNOs were exhaustively characterized by statistical Raman spectroscopy, thermogravimetric analysis coupled with gas chromatography and mass spectrometry, dynamic light scattering, UV-vis, and ATR-FTIR spectroscopies. This work provides important insights into the understanding of the basic principles of reductive CNOs functionalization and will pave the way for the use of CNOs in a wide range of potential applications, such as energy storage, photovoltaics, or molecular electronics.

Covalent 2D Patterning, Local Electronic Structure and Polarization Switching of Graphene at the Nanometer Level.

A very facile and efficient protocol for the covalent patterning and properties tuning of graphene is reported. Highly reactive fluorine radicals were added to confined regions of graphene directed by laser writing on graphene coated with 1-fluoro-3,3-dimethylbenziodoxole. This process allows for the realization of exquisite patterns on graphene with resolutions down to 200 nm. The degree of functionalization, ranging from the unfunctionalized graphene to extremely high functionalized graphene, can be precisely tuned by controlling the laser irradiation time. Subsequent substitution of the initially patterned fluorine atoms afforded an unprecedented graphene nanostructure bearing thiophene groups. This substitution led to a complete switch of both the electronic structure and the polarization within the patterned graphene regions. This approach paves the way towards the precise modulation of the structure and properties of nanostructured graphene.

Interface Amorphization of Two-Dimensional Black Phosphorus upon Treatment with Diazonium Salts.

Two-dimensional (2D) black phosphorus (BP) represents one of the most appealing 2D materials due to its electronic, optical, and chemical properties. Many strategies have been pursued to face its environmental instability, covalent functionalization being one of the most promising. However, the extremely low functionalization degrees and the limitations in proving the nature of the covalent functionalization still represent challenges in many of these sheet architectures reported to date. Here we shine light on the structural evolution of 2D-BP upon the addition of electrophilic diazonium salts. We demonstrated the absence of covalent functionalization in both the neutral and the reductive routes, observing in the latter case an unexpected interface conversion of BP to red phosphorus (RP), as characterized by Raman, 31 P-MAS NMR, and X-ray photoelectron spectroscopies (XPS). Furthermore, thermogravimetric analysis coupled to gas chromatography and mass spectrometry (TG-GC-MS), as well as electron paramagnetic resonance (EPR) gave insights into the potential underlying radical mechanism, suggesting a Sandmeyer-like reaction.

Covalently Doped Graphene Superlattices: Spatially Resolved Supratopic- and Janus-Binding.

Rational design and fabrication of graphene nanoarchitectures with multifunctionality and multidimensionality remains quite a challenge. Here, we present a synthetic sequence, based on the combination of two advanced patterned-functionalization principles, namely, laser-writing and poly(methyl methacrylate) (PMMA)-assisted lithographic processes, leading to unprecedented covalently doped graphene superlattices. Spatially resolved supratopic- and Janus-binding were periodically weaved on the graphene sheet, leading to four different types of zones with distinct chemical doping and structural properties. Notably, this is also the first realization of patterned Janus graphene. The elaborate chemical doping with micrometer resolution is unequivocally evidenced by scanning Raman spectroscopy (SRS) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS). The design of the pattern as well as the degree of chemical doping on both opposite sides of graphene can be easily manipulated, rendering exciting potential for graphene nanosystems.

Direct Laser Writing on Graphene with Unprecedented Efficiency of Covalent Two-Dimensional Functionalization.

Laser writing as a simple and straightforward method for covalent 2D patterning of graphene remains challenging. Here, we report a facile and efficient approach for a laser-induced 2D patterning of graphene utilizing silver trifluoroacetate, providing an unprecedented high degree of functionalization. The use of laser-triggered photolysis of silver trifluoroacetate to generate trifluoromethyl radicals, confined only to the laser-irradiated region, leads to the selective reaction of graphene, thereby completing direct laser writing on graphene toward a spatially resolved 2D-patterned architecture. This highly 2D-functionalized graphene is completely reversible. Furthermore, a more complex patterned graphene hybrid architecture was constructed, taking advantage of the simultaneously produced/patterned silver nanoparticles during the laser-writing process. Considering the simplicity of this approach and its ability to provide high degrees of functionalization, the prerequisite of 2D patterning of other 2D materials based on this method is provided.

Understanding the Electron-Doping Mechanism in Potassium-Intercalated Single-Walled Carbon Nanotubes.

Single-walled carbon nanotubes (SWCNTs) can be doped with potassium, similar to graphite, leading to intercalation compounds. These binary systems exhibit a clear metallic character. However, the entire picture of how electron doping (e-doping) modifies the SWCNTs' vibrational spectra as a function of their diameter, chirality, and metallicity is still elusive. Herein, we present a detailed study of the intercalation and solid state reduction of metallic and semiconducting enriched HiPco SWCNTs. We performed a combined experimental and theoretical study of the evolution of their Raman response with potassium exposure, focusing specifically on their radial breathing mode (RBM). We found the charge donated from the potassium atoms occupies antibonding π orbitals of the SWCNTs, weakening their C-C bonds, and reducing the RBM frequency. This RBM downshift with increasing doping level is quasi-linear with a steplike behavior when the Fermi level crosses a van Hove singularity for semiconducting species. Moreover, this weakening of the C-C bonds is greater with decreasing curvature, or increasing diameter. Overall, this suggests the RBM downshift with e-doping is proportional to both the SWCNT's integrated density of states (DOS) ϱ(ε) and diameter d. We have provided a precise and complete description of the complex electron doping mechanism in SWCNTs up to a charge density of -18 me/C, far beyond that achievable by standard gate voltage studies, not being the highest doping possible, but high enough to track the effects of doping in SWCNTs based on their excitation energy, diameter, band gap energy, chiral angle, and metallicity. This work is highly relevant to tuning the electronic properties of SWCNTs for applications in nanoelectronics, plasmonics, and thermoelectricity.

Covalent Inter-Synthetic-Carbon-Allotrope Hybrids.

Inter-synthetic-carbon-allotrope (SCA) architectures are constructed by hybridizing different types of carbon allotropes such as 0D-fullerene, 1D-carbon nanotubes (CNTs), and 2D-graphene. Also hybridizations of different fullerene families (e.g., empty fullerene, heterofullerenes, and endohedral fullerenes as well as their isomers) are considered as inter-SCA architectures and represent an emerging class of nanofunctional materials. Such highly integrated hybrids are quite promising, since they hold the potential of combining unique properties of each building block. For instance, hybridization of 2D-graphene, a synthetic carbon allotrope with outstanding chemical/physical properties, with 0D-fullerenes leads to materials with both outstanding solid state properties (e.g., electron mobility, flexibility, transparency, and mechanical stability) and distinct molecular properties (spatially resolved and tailor-made exohedral cage functionalization and endohedral guest encapsulation). Therefore, such integrated hybrids have potential as multifunctional nanocarbon materials applicable, for example, in energy storage, electronic devices, solar cells, or advanced sensors. Recognizing these significant advantages, a series of methods and techniques has been developed to synthesize such integrated hybrids. Based on in-depth understanding of fullerene chemistry, interfullerene hybrids where multiple fullerenes are linked together have been synthesized and fully characterized. However, due to the difficulty in functionalizing graphene or CNTs as a result of their poor dispersibility and weak reactivity the corresponding hybrids including these macromolecular forms are less explored. Furthermore, few approaches toward such systems reported so far inevitably involve some drawbacks such as low degree of addition and low production ability. This Account presents the concepts and strategies of our studies on the construction of inter-SCA hybrids. We first emphasize on our efficient "reductive functionalization route" as a versatile strategy for graphene/CNT functionalization. In sharp contrast to previous approaches, our strategy enables unprecedented functionalization of graphene/CNT without damaging their structures. As a consequence, the door for cross-dimensional architectures via hybridizing graphene/CNT with fullerenes has been opened. We will then summarize the diverse inter-SCA hybrids that we recently synthesized, ranging from interfullerene hybrids to those of cross-dimensional graphene/CNT-(endo)fullerenes hybrids as well as CNTs networks. For interfullerene hybrids, different types of fullerenes including empty fullerenes, heterofullerenes, and endohedral fullerenes have been employed. Finally, the prospects on the future challenges on inter-SCA hybrids are envisioned. This Account will provide fundamental insight into construction of inter-SCA hybrids and stimulate further efforts toward research on this emerging topic.

Spatially Resolved Bottom-Side Fluorination of Graphene by Two-Dimensional Substrate Patterning.

Patterned functionalization can, on the one hand, open the band gap of graphene and, on the other hand, program demanding designs on graphene. The functionalization technique is essential for graphene-based nanoarchitectures. A new and highly efficient method was applied to obtain patterned functionalization on graphene by mild fluorination with spatially arranged AgF arrays on the structured substrate. Scanning Raman spectroscopy (SRS) and scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM-EDS) were used to characterize the functionalized materials. For the first time, chemical patterning on the bottom side of graphene was realized. The chemical nature of the patterned functionalization was determined to be the ditopic scenario with fluorine atoms occupying the bottom side and moieties, such as oxygen-containing groups or hydrogen atoms, binding on the top side, which provides information about the mechanism of the fluorination process. Our strategy can be conceptually extended to pattern other functionalities by using other reactants. Bottom-side patterned functionalization enables utilization of the top side of a material, thereby opening up the possibilities for applications in graphene-based devices.

Covalent 2D-Engineering of Graphene by Spatially Resolved Laser Writing/Reading/Erasing.

We report a facile and efficient method for the covalent 2D-patterning of monolayer graphene via laser irradiation. We utilized the photo-cleavage of dibenzoylperoxide (DBPO) and optimized the subsequent radical additions to non-activated graphene up to that level where controlled covalent 2D-patterning of graphene initiated by spatially resolved laser writing is possible. The covalent 2D-functionalization of graphene, which is monitored by scanning Raman microscopy (SRM) is completely reversible. This new concept enables write/read/erase control over the covalent chemical information stored on the graphene surface.

Quantifying the Covalent Functionalization of Black Phosphorus.

A straightforward quantification method to consistently determine the overall functionalization degree of covalently modified two-dimensional (2D) black phosphorus (BP) by Raman spectroscopy has been carried out. Indeed, the successful reductive methylation of the BP lattice using sodium intercalation compounds and exhibiting different functionalization degrees has been demonstrated by 31 P-magic angle spinning (MAS) NMR spectroscopy. Furthermore, the correlation of 31 P-MAS NMR spectroscopy and statistical Raman spectroscopy (SRS) revealed the first method to determine the functionalization degree of BP solely by evaluating the intensities of distinct peaks in the Raman spectra of the covalently modified material, in a similar way to the widely employed ID /IG ratio of graphene research.

Highly Efficient and Reversible Covalent Patterning of Graphene: 2D-Management of Chemical Information.

Patterned graphene-functionalization with a tunable degree of functionalization can tailor the properties of graphene. Here, we present a new reductive functionalization approach combined with lithography rendering patterned graphene-functionalization easily accessible. Two types of covalent patterning of graphene were prepared and their structures were unambiguously characterized by statistical Raman spectroscopy together with scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM-EDS). The reversible defunctionalization processes, as revealed by temperature-dependent Raman spectroscopy, enable the possibility to accurately modulate the degree of functionalization by annealing. This allows for the management of chemical information through complete write/store/erase cycles. Based on our strategy, controllable and efficient patterning graphene-functionalization is no longer a challenge and facilitates the development of graphene-based devices.

Controlling the Degree of Functionalization: In-Depth Quantification and Side-Product Analysis of Diazonium Chemistry on SWCNTs.

We present an in-depth qualitative and quantitative analysis of a reaction between 4-iodobenzenediazonium tetrafluoroborate and single-walled carbon nanotubes (SWCNTs) via thermogravimetric analysis coupled with mass spectrometry (TG-MS) or a gas chromatography and mass spectrometry (TG-GC-MS) as well as Raman spectroscopy. We propose a method for precise determination of the degree of functionalization and quantification of physisorbed aromates, detaching around their boiling point, alongside covalently bonded ones (cleavage over 200 °C). While the presence of some side products like phenol- or biphenyl species could be excluded, residual surfactant and minor amounts of benzene could be identified. A concentration-dependent experiment shows that the degree of functionalization increases with the logarithm of the concentration of applied diazonium salt, which can be exploited to precisely adjust the amount of aryl addends on the nanotube sidewall, up to 1 moiety per 100 carbon atoms.

Exohedral Addition Chemistry of the Fullerenide Anions C60 2- and C60 ⋠.

A systematic screening study of the exohedral reactivity of the reduced fullerenes (fullerenides) C60 2- and C60 ⋅- is reported. These doubly and singly negatively charged carbon cages were prepared by two-fold reduction of C60 with potassium, leading to K2 C60 , or by in situ monoreduction with the radical anion of benzonitrile PhCN⋅- , respectively. Several series of electrophiles, including geminal and distant dihalides, benzyl bromides, and diazonium compounds, were employed as addition partners. In general, the investigated bromides proved to be the most suitable reaction partners. A series of fullerene adducts and cycloadducts involving either 1,2- or 1,4-addition patterns, depending on the precise architecture and the steric demand of the addends, were synthesized and fully characterized. Some of the reaction products are unprecedented and inaccessible forms of neutral C60 . The fullerenide chemistry presented here closely resembles related reactions of graphenides and carbon nanotubides, which are the most powerful methods for the functionalization of these macromolecular forms of synthetic carbon allotropes (SCAs). Activation of C60 by negative charging represents a little explored concept of fullerene chemistry, providing both new insights of fullerene reactivity itself and new types of exohedral derivatives.

A Straightforward Approach to Multifunctional Graphene.

Graphene has been covalently functionalized through a one-pot reductive pathway using graphite intercalation compounds (GICs), in particular KC8 , with three different orthogonally protected derivatives of 4-aminobenzylamine. This novel multifunctional platform exhibits excellent bulk functionalization homogeneity (Hbulk ) and degree of addition while preserving the chemical functionalities of the organic addends through different protecting groups, namely: tert-butyloxycarbonyl (Boc), benzyloxycarbonyl (Cbz) and phthalimide (Pht). We have employed (temperature-dependent) statistical Raman spectroscopy (SRS), X-ray photoelectron spectroscopy (XPS), magic angle spinning solid state 13 C NMR (MAS-NMR), and a characterization tool consisting of thermogravimetric analysis coupled with gas chromatography and mass spectrometry (TG-GC-MS) to unambiguously demonstrate the covalent binding and the chemical nature of the different molecular linkers. This work paves the way for the development of smart graphene-based materials of great interest in biomedicine or electronics, to name a few, and will serve as a guide in the design of new 2D multifunctional materials.

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.

Covalent Inter-Carbon-Allotrope Architectures Consisting of the Endohedral Fullerene Sc3 N@C80 and Single-Walled Carbon Nanotubes.

Using a reductive sidewall functionalization concept, we prepared for the first time a covalent inter-carbon-allotrope hybrid consisting of single-walled carbon nanotubes (SWCNTs) and the endohedral fullerene Sc3 N@C80 . The new compound type was characterized through a variety of techniques including absorption spectroscopy, Raman spectroscopy, TG-MS, TG-GC-MS, and MALDI-TOF MS. HRTEM investigations were carried out to visualize this highly integrated architecture.

Lattice Opening upon Bulk Reductive Covalent Functionalization of Black Phosphorus.

The chemical bulk reductive covalent functionalization of thin-layer black phosphorus (BP) using BP intercalation compounds has been developed. Through effective reductive activation, covalent functionalization of the charged BP by reaction with organic alkyl halides is achieved. Functionalization was extensively demonstrated by means of several spectroscopic techniques and DFT calculations; the products showed higher functionalization degrees than those obtained by neutral routes.