Origin of Oxygen in Graphene Oxide Revealed by 17O and 18O Isotopic Labeling.

Wet-chemical oxidation of graphite in a mixture of sulfuric acid with a strong oxidizer, such as potassium permanganate, leads to the formation of graphene oxide with hydroxyl and epoxide groups as the major functional groups. Nevertheless, the reaction mechanism remains unclear and the source of oxygen is a subject of debate. It could theoretically originate from the oxidizer, water, or sulfuric acid. In this study, we employed 18O and 17O labeled reagents to experimentally elucidate the reaction mechanism and, thus, determine the origin of oxo-functional groups. Our findings reveal the multifaceted roles of sulfuric acid, acting as a dispersion medium, a dehydrating agent for potassium permanganate, and an intercalant. Additionally, it significantly acts as a source of oxygen next to manganese oxides. Through 17O solid-state magic-angle spinning (MAS) NMR experiments, we exclude water as a direct reaction partner during oxygenation. With labeling experiments, we conclude on mechanistic insights, which may be exploited for the synthesis of novel graphene derivatives.

Evidence for Trans-Oligoene Chain Formation in Graphene Induced by Iodine.

Functionalization of pristine graphene by hydrogen and fluorine is well studied, resulting in graphane and fluorographene structures. In contrast, functionalization of pristine graphene with iodine has not been reported. Here, the functionalization of graphene with iodine using photochemical activation is presented, which is thermally reversible at 400 °C. Additional dispersive dominant Raman modes that are probed by resonance Raman spectroscopy are observed. Additionally, iodinated graphene is probed by Kelvin probe force microscopy and by transport measurements showing p-doping surpassing non-covalent iodine doping by charge transfer-complex formation. The emergent Raman modes combined with strong p-doping indicate that iodine functionalization is distinct from simple iodine doping. A reaction mechanism based on these findings is proposed, identifying the large size of iodine atoms as the probable cause governing regiochemically controlled addition due to steric hinderance of reactive sites. The modification of the electronic structure is explained by the confinement of 1D trans-oligoene chains between sp3 -defects. These results demonstrate the uniqueness of iodine reactivity toward graphene and the modification of the electronic structure of iodinated graphene, highlighting its dependence on the spatial arrangement of substituents.

Influence of Lattice Defects on Trans-Oligoene Substructure Formation in Graphene.

The photochemical reaction of iodine and graphene induces strong new Raman modes due to the formation of trans-oligoene substructures in graphene domains. This unique reactivity was demonstrated before on defect-free graphene, however leaving the influence of e. g. carbon vacancies, unexplored. Here, we investigate the photochemical reaction applied on graphene with varying average distances of lattice defects and statistically analyze the characteristic Raman modes which develop with the iodination reaction. We show that the iodination reaction does not lead to Raman-active defects and thus, the newly formed trans-oligoene substructures do not contribute to the D-mode of graphene. A statistical analysis reveals the correlation between the average distance of lattice defects and the intensity of the v1-mode. For defective graphene with average defect distances below ~1 nm no new Raman modes evolve, which is the lower limit of the substructure size probed at 532 nm and explains why this observation was not possible before using common graphene oxide as graphene source.

Dimethylene-Cyclopropanide Units as Building Blocks for Fluorescence Dyes.

Many organic dyes are fluorescent in solution. In the solid state, however, quenching processes often dominate, hampering material science applications such as light filters, light-emitting devices, or coding tags. We show that the dimethylene-cyclopropanide scaffold can be used to form two structurally different types of chromophores, which feature fluorescence quantum yields up to 0.65 in dimethyl sulfoxide and 0.53 in solids. The increased fluorescence in the solid state for compounds bearing malonate substituents instead of dicyanomethid ones is rationalized by the induced twist between the planes of the cyclopropanide core and a pyridine ligand.

Managing Excess Lead Iodide with Functionalized Oxo-Graphene Nanosheets for Stable Perovskite Solar Cells.

Stability issues could prevent lead halide perovskite solar cells (PSCs) from commercialization despite it having a comparable power conversion efficiency (PCE) to silicon solar cells. Overcoming drawbacks affecting their long-term stability is gaining incremental importance. Excess lead iodide (PbI2 ) causes perovskite degradation, although it aids in crystal growth and defect passivation. Herein, we synthesized functionalized oxo-graphene nanosheets (Dec-oxoG NSs) to effectively manage the excess PbI2 . Dec-oxoG NSs provide anchoring sites to bind the excess PbI2 and passivate perovskite grain boundaries, thereby reducing charge recombination loss and significantly boosting the extraction of free electrons. The inclusion of Dec-oxoG NSs leads to a PCE of 23.7 % in inverted (p-i-n) PSCs. The devices retain 93.8 % of their initial efficiency after 1,000 hours of tracking at maximum power points under continuous one-sun illumination and exhibit high stability under thermal and ambient conditions.

Influence of the coffee-ring effect and size of flakes of graphene oxide films on their electrochemical reduction.

Electrodes for electrochemical reduction of graphene oxide (GO) are coated with thin films using drop-casting and evaporation-assisted self-assembly. The influence of loading, the size of the flakes of GO, and the macroscopic coffee-ring effect occurring during drying are investigated. The effective transfer of protons and electrons in the electrochemical reduction of GO is decisive.

Aggregation-induced emission leading to two distinct emissive species in the solid-state structure of high-dipole organic chromophores.

The concept of aggregation-induced emission represents a means to rationalise photoluminescence of usually nonfluorescent excimers in solid-state materials. In this publication, we study the photophysical properties of selected diaminodicyanoquinone (DADQ) derivatives in the solid state using a combined approach of experiment and theory. DADQs are a class of high-dipole organic chromophores promising for applications in non-linear optics and light-harvesting devices. Among the compounds investigated, we find both aggregation-induced emission and aggregation-caused quenching effects rationalised by calculated energy transfer rates. Analysis of fluorescence spectra and lifetime measurements provide the interesting result that (at least) two emissive species seem to contribute to the photophysical properties of DADQs. The main emission peak is notably broadened in the long-wavelength limit and exhibits a blue-shifted shoulder. We employ high-level quantum-chemical methods to validate a molecular approach to a solid-state problem and show that the complex emission features of DADQs can be attributed to a combination of H-type aggregates, monomers, and crystal structure defects.

Room-Temperature Transport Properties of Graphene with Defects Derived from Oxo-Graphene.

In recent years, graphene oxide has been considered as a soluble precursor of graphene for electronic applications. However, the performance lags behind that of graphene due to lattice defects. Here, the relation between the density of defects in the range of 0.2 % and 1.5 % and the transport properties is quantitatively studied. Therefore, the related flakes of monolayers of graphene were prepared from oxo-functionalized graphene (oxo-G). The morphologic structure of oxo-G was imaged by atomic force microscopy (AFM) and scanning tunneling microscopy (STM). Field-effect mobility values were determined to range between 0.3 cm2 V-1 s-1 and 33.2 cm2 V-1 s-1 , which were inversely proportional to the density of defects. These results provide the first quantitative description of the density of defects and transport properties, which plays an important role for potential applications.

Between Aromatic and Quinoid Structure: A Symmetrical UV to Vis/NIR Benzothiadiazole Redox Switch.

Reversibly switching the light absorption of organic molecules by redox processes is of interest for applications in sensors, light harvesting, smart materials, and medical diagnostics. This work presents a symmetrical benzothiadiazole (BTD) derivative with a high fluorescence quantum yield in solution and in the crystalline state and shows by spectroelectrochemical analysis that reversible switching of UV absorption in the neutral state, to broadband Vis/NIR absorption in the 1st oxidized state, to sharp band Vis absorption in the 2nd oxidized state, is possible. For the one-electron oxidized species, formation of a delocalized radical is confirmed by electron paramagnetic resonance spectroelectrochemistry. Furthermore, our results reveal an increasing quinoidal distortion upon the 1st and 2nd oxidation, which can be used as the leitmotif for the development of BTD based redox switches.

Evidence for Electron Transfer between Graphene and Non-Covalently Bound π-Systems.

Hybridizing graphene and molecules possess a high potential for developing materials for new applications. However, new methods to characterize such hybrids must be developed. Herein, the wet-chemical non-covalent functionalization of graphene with cationic π-systems is presented and the interaction between graphene and the molecules is characterized in detail. A series of tricationic benzimidazolium salts with various steric demand and counterions was synthesized, characterized and used for the fabrication of graphene hybrids. Subsequently, the doping effects were studied. The molecules are adsorbed onto graphene and studied by Raman spectroscopy, XPS as well as ToF-SIMS. The charged π-systems show a p-doping effect on the underlying graphene. Consequently, the tricationic molecules are reduced through a partial electron transfer process from graphene, a process which is accompanied by the loss of counterions. DFT calculations support this hypothesis and the strong p-doping could be confirmed in fabricated monolayer graphene/hybrid FET devices. The results are the basis to develop sensor applications, which are based on analyte/molecule interactions and effects on doping.

Identification of Semiconductive Patches in Thermally Processed Monolayer Oxo-Functionalized Graphene.

The thermal decomposition of graphene oxide (GO) is a complex process at the atomic level and not fully understood. Here, a subclass of GO, oxo-functionalized graphene (oxo-G), was used to study its thermal disproportionation. We present the impact of annealing on the electronic properties of a monolayer oxo-G flake and correlated the chemical composition and topography corrugation by two-probe transport measurements, XPS, TEM, FTIR and STM. Surprisingly, we found that oxo-G, processed at 300 °C, displays C-C sp3 -patches and possibly C-O-C bonds, next to graphene domains and holes. It is striking that those C-O-C/C-C sp3 -separated sp2 -patches a few nanometers in diameter possess semiconducting properties with a band gap of about 0.4 eV. We propose that sp3 -patches confine conjugated sp2 -C atoms, which leads to the local semiconductor properties. Accordingly, graphene with sp3 -C in double layer areas is a potential class of semiconductors and a potential target for future chemical modifications.

Brodie's or Hummers' Method: Oxidation Conditions Determine the Structure of Graphene Oxide.

Synthesis and studies of graphite oxide started more than 150 years ago and turned into a boom by the measurements of the outstanding physical properties of graphene. A series of preparation protocols emanated trying to optimize the synthesis of graphene oxide in order to obtain a less defective material, as source for graphene. However, over-oxidation of the carbon framework hampered establishing structure-property relationships. Here, the fact that two different synthetic methods for graphene oxide preparation lead to very similar types of graphene oxide with a preserved graphene lattice is demonstrated. Either sodium chlorate in nitric acid (similar to Brodie's method) or potassium permanganate in sulfuric acid (similar to Hummers' method) treatment are possible; however, reaction conditions must be controlled. With a preserved carbon lattice analytical differences between the samples relate to the altered on-plane functionality. Consequently, terming preparation protocols "according to Brodie's/Hummers' method" is not sufficient.

Molecular Lipid Films on Microengineering Materials.

In this study, we have systematically investigated the formation of molecular phospholipid films on a variety of solid substrates fabricated from typical surface engineering materials and the fluidic properties of the lipid membranes formed on these substrates. The surface materials comprise of borosilicate glass, mica, SiO2, Al (native oxide), Al2O3, TiO2, ITO, SiC, Au, Teflon AF, SU-8, and graphene. We deposited the lipid films from small unilamellar vesicles (SUVs) by means of an open-space microfluidic device, observed the formation and development of the films by laser scanning confocal microscopy, and evaluated the mode and degree of coverage, fluidity, and integrity. In addition to previously established mechanisms of lipid membrane-surface interaction upon bulk addition of SUVs on solid supports, we observed nontrivial lipid adhesion phenomena, including reverse rolling of spreading bilayers, spontaneous nucleation and growth of multilamellar vesicles, and the formation of intact circular patches of double lipid bilayer membranes. Our findings allow for accurate prediction of membrane-surface interactions in microfabricated devices and experimental environments where model membranes are used as functional biomimetic coatings.

Diaminodicyanoquinones: Fluorescent Dyes with High Dipole Moments and Electron-Acceptor Properties.

Fluorescent dyes are applied in various fields of research, including solar cells and light-emitting devices, and as reporters for assays and bioimaging studies. Fluorescent dyes with an added high dipole moment pave the way to nonlinear optics and polarity sensitivity. Redox activity makes it possible to switch the molecule's photophysical properties. Diaminodicyanoquinone derivatives possess high dipole moments, yet only low fluorescence quantum yields, and have therefore been neglected as fluorescent dyes. Here we investigate the fluorescence properties of diaminodicyanoquinones using a combined theoretical and experimental approach and derive molecules with a fluorescence quantum yield exceeding 90 %. The diaminodicyanoquinone core moiety provides chemical versatility and can be integrated into novel molecular architectures with unique photophysical features.

Selective Functionalization of Graphene at Defect-Activated Sites by Arylazocarboxylic tert-Butyl Esters.

The development of versatile functionalization concepts for graphene is currently in the focus of research. Upon oxo-functionalization of graphite, the full surface of graphene becomes accessible for C-C bond formation to introduce out-of-plane functionality. Herein, we present the arylation of graphene with arylazocarboxylic tert-butyl esters, which generates aryl radicals after activation with an acid. Surprisingly, the degree of functionalization is related to the concentration of lattice vacancy defects in the graphene material. Consequently, graphene materials that are free from lattice defects are not reactive. The reaction can be applied to graphene dispersed in solvents and leads to bitopic functionalization as well as monotopic functionalization when the graphene is deposited on surfaces. As the arylazocarboxylic tert-butyl ester moiety can be attached to various molecules, the presented method paves the way to functional graphene derivatives, with the density of defects determining the degree of functionalization.

γ-Iron Phase Stabilized at Room Temperature by Thermally Processed Graphene Oxide.

Stabilizing nanoparticles on surfaces, such as graphene, is a growing field of research. Thereby, iron particle stabilization on carbon materials is attractive and finds applications in charge-storage devices, catalysis, and others. In this work, we describe the discovery of iron nanoparticles with the face-centered cubic structure that was postulated not to exist at ambient conditions. In bulk, the γ-iron phase is formed only above 917 °C, and transforms back to the thermodynamically favored α-phase upon cooling. Here, with X-ray diffraction and Mössbauer spectroscopy we unambiguously demonstrate the unexpected room-temperature stability of the γ-phase of iron in the form of the austenitic nanoparticles with low carbon content from 0.60% through 0.93%. The nanoparticles have controllable diameter range from 30 nm through 200 nm. They are stabilized by a layer of Fe/C solid solution on the surface, serving as the buffer controlling carbon content in the core, and by a few-layer graphene as an outermost shell.

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.

Focused electron beam based direct-write fabrication of graphene and amorphous carbon from oxo-functionalized graphene on silicon dioxide.

Controlled patterning of graphene is an important task towards device fabrication and thus is the focus of current research activities. Graphene oxide (GO) is a solution-processible precursor of graphene. It can be patterned by thermal processing. However, thermal processing of GO leads to decomposition and CO2 formation. Alternatively, focused electron beam induced processing (FEBIP) techniques can be used to pattern graphene with high spatial resolution. Based on this approach, we explore FEBIP of GO deposited on SiO2. Using oxo-functionalized graphene (oxo-G) with an in-plane lattice defect density of 1% we are able to image the electron beam-induced effects by scanning Raman microscopy for the first time. Depending on electron energy (2-30 keV) and doses (50-800 mC m-2) either reduction of GO or formation of permanent lattice defects occurs. This result reflects a step towards controlled FEBIP processing of oxo-G.

Thermal Disproportionation of Oxo-Functionalized Graphene.

Graphene production by wet chemistry is an ongoing scientific challenge. Controlled oxidation of graphite introduces oxo functional groups; this material can be processed and converted back to graphene by reductive defunctionalization. Although thermal processing yields conductive carbon, a ruptured and undefined carbon lattice is produced as a consequence of CO2 formation. This thermal process is not understood, but it is believed that graphene is not accessible. Here, we thermally process oxo-functionalized graphene (oxo-G) with a low (4-6 %) and high degree of functionalization (50-60 %) and find on the basis of Raman spectroscopy and transmission electron microscopy performed at atomic resolution (HRTEM) that thermal processing leads predominantly to an intact carbon framework with a density of lattice defects as low as 0.8 %. We attribute this finding to reorganization effects of oxo groups. This finding holds out the prospect of thermal graphene formation from oxo-G derivatives.

Graphene Oxide: A One- versus Two-Component Material.

The structure of graphene oxide (GO) is a matter of discussion. While established GO models are based on functional groups attached to the carbon framework, another frequently used model claims that GO consists of two components, a slightly oxidized graphene core and highly oxidized molecular species, oxidative debris (OD), adsorbed on it. Those adsorbents are claimed to be the origin for optical properties of GO. Here, we examine this model by preparing GO with a low degree of functionalization, combining it with OD and studying the optical properties of both components and their combination in an artificial two-component system. The analyses of absorption and emission spectra as well as lifetime measurements reveal that properties of the combined system are distinctly different from those of GO. That confirms structural models of GO as a separate oxygenated hexagonal carbon framework with optical properties governed by its internal structure rather than the presence of OD. Understanding the structure of GO allows further reliable interpretation of its optical and electronic properties and enables controlled processing of GO.