Christian E. Halbig

Transparent conductive graphene films are typically fabricated at processing temperatures above 500°C, making the applied processes unsuitable for flexible polymer substrates and limiting their integration into semiconductor device technologies. Here we demonstrate a method for fabricating such films at temperatures below 200°C. To this end, oxo‐graphene flakes are assembled into continuous films using the double self‐assembly technique. Subsequent hydroiodic acid (HI) vapor treatment at 40°C removes the oxygen‐containing groups and, combined with a mild 180°C annealing step on heat‐stable substrates, lowers the sheet resistance by more than a factor of five to about 3 kΩ/sq while preserving optical transparencies of up to 88% at 550 nm wavelength. The trade‐off between sheet resistance and transparency is set by the number of stacked layers, and on flexible polyethylene terephthalate (PET), where the annealing step is precluded, the films reach a sheet resistance of about 2.8 kΩ/sq after the HI step alone. These results demonstrate that the double self‐assembly of oxo‐functionalized graphene flakes, combined with mild HI‐vapor reduction and a 180°C annealing step, can deliver highly transparent and conductive graphene films while remaining compatible with semiconductor device technologies.

ZUSAMMENFASSUNG: Hexacyanotrimethylencyclopropan ( CN6CP ) ist in seiner neutralen Form ein außergewöhnlich starker organischer Elektronenakzeptor und findet breite Anwendung bei der Dotierung von Materialien, um Ladungstransferprozesse zu induzieren und elektrochemische Systeme anzupassen. Dennoch sind seine grundlegenden molekularen Eigenschaften bislang weitgehend unbekannt geblieben. Hier präsentieren wir die erste umfassende strukturelle Charakterisierung von CN6CP . Möglich wurde dies durch eine verbesserte Synthese bei niedrigen Temperaturen. Auch die erste Festkörperstruktur der neutralen Verbindung wurde zugänglich. Das resultierende Verfahren liefert isolierbares, kristallines CN6CP , das bei –30°C über Wochen stabil ist. Über alle Redoxzustände hinweg zeigen kombinierte IR/Raman‐, UV/Vis‐ und NMR‐Untersuchungen zusammen mit NICS‐Berechnungen eine vom Oxidationszustand abhängige Umverteilung der Elektronendichte. Die Daten zeigen, dass CN6CP einen σ‐aromatischen Cyclopropan‐Kern mit abgestimmter π‐Delokalisierung besitzt, die bei der Reduktion verstärkt wird, während die zusätzliche Ladung überwiegend auf dem exocyclischen Akzeptorgerüst lokalisiert ist. Zyklische Voltammetrie‐Experimente zeigen zwei reversible Ein‐Elektronen‐Prozesse und eine außergewöhnlich niedrige LUMO‐Energie von –5,85 eV, die die niedrigste ist, die für kleine organische Moleküle berichtet wurde, und deutlich unter den Werten von Referenzakzeptoren wie F ₄ TCNQ oder F ₆ TCNNQ liegt. Insgesamt bestätigen diese Ergebnisse, dass CN6CP ein strukturell einzigartiger, extrem starker Elektronenakzeptor ist, und liefern die molekulare Grundlage für seine Leistungsfähigkeit in der organischen Elektronik und in redoxaktiven Materialien.

Hexacyanotrimethylenecyclopropane ( CN6CP ) is an exceptionally strong organic electron acceptor in its neutral form, and widely applied for molecular doping to induce charge transfer processes and enable electrochemical systems. Yet, its fundamental molecular properties have remained largely unknown. Here, we show the first comprehensive structure‐analytical characterization of CN6CP , enabled by an improved, low‐temperature synthesis and the first solid‐state structure of the neutral compound. The resulting procedure affords isolable, crystalline CN6CP that is stable for weeks at –30°C and can be recrystallised. Across all redox states, combined IR/Raman, UV–Vis and NMR measurements, together with NICS calculations, reveal an oxidation‐state‐dependent redistribution of electron density. These data show that CN6CP possesses a σ‐aromatic cyclopropane core with tunable π‐delocalisation, which is enhanced upon reduction while the additional charge is predominantly localised on the exocyclic acceptor framework. Cyclic voltammetry experiments unveil two reversible one‐electron processes and an exceptionally low LUMO energy of –5.85 eV, which is the lowest reported for small organic molecules being significantly lower than those of benchmark acceptors such as F ₄ TCNQ or F ₆ TCNNQ. All together, these findings establish CN6CP as a structurally unique, extremely strong electron acceptor and provide the molecular basis underlying its performance in organic electronics and redox‐active materials.

Strong interest in graphene oxide (GO) over the past decades has resulted in significant advances toward numerous applications and progress in understanding its chemistry. A rather broad community of scientists is involved in GO research including specialists in chemistry, physics, life, and materials sciences. While this diversity is a strength, common characterization techniques, such as Raman spectroscopy, FTIR, XPS, and XRD are often misused and inconsistently interpreted. Errors in data processing and analysis often invalidate key conclusions made in research papers. In many cases, experimental data provided in different studies are difficult to compare due to the lack of standardized ways of interpretations. The purpose of this review is to clarify common misunderstandings and errors in GO characterization by the four above-mentioned methods and to provide useful recommendations for best practices in data acquisition, processing, and interpretation. It also aims to offer guidance for new researchers entering the field of GO. The review is based on the authors' extensive experience in the field. By promoting standardized approaches, this review seeks to improve data comparability, enhance reliability in GO research, and establish a solid foundation for analyzing the structure and reactivity of GO-based materials.

Functionalization of graphene derivatives is a common approach to tune material properties for use in various applications. Because of the low reactivity of the unsaturated carbon lattice of graphene, not only are few chemical approaches suitable for successful functionalization, such as those involving highly reactive in situ formed radical species or nitrene and carbene compounds, but also the degree of functionalization is usually limited, modifying only a few percent of the carbon atoms. Typically, uncontrolled side reactions such as homocoupling and oligomerization of newly introduced functional groups can occur instead of direct coupling to the carbon lattice. We want to turn this unwanted side reaction into an advantage and use intentionally formed covalent dendrimeric oligophenylene structures for secondary functionalization. We show that these oligomeric structures can be grown to specific thicknesses and used for further functionalization with bromomethyl groups at high density on the surface. This functionalization opens further avenues for subsequent nucleophilic substitution, as exemplified by the introduction of versatile azide, nitrile, and phosphonate groups. The results presented here are not only applicable to large oligophenylene structures, but also demonstrate that, in principle, single aryl moieties on graphene of any size and density can be successfully functionalized.

Oxo‐graphene nanoribbons (oxo‐GNRs) can be prepared by the oxidative unzipping of single‐walled carbon nanotubes. We present an orthogonal functionalization method for the functionalization of the rims and the π‐surface, respectively, what is only possible due to the high rim portion in oxo‐GNRs. In particular, X‐ray photoelectron spectroscopy could be used to follow the reaction and detect the marker atoms contained in the addends. We propose that the reported orthogonal functionalization strategy can also be applied on other oxo‐functionalized carbon materials, such as graphene quantum dots, or reduced graphene oxide flakes.

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.

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.