Udo Mundloch

Noncovalent Functionalization of Black Phosphorus

Black phosphorus (BP) was functionalized with organic moieties on the basis of liquid exfoliation. The treatment of BP with electron-withdrawing 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) led to electron transfer from BP to the organic dopant. On the other hand, the noncovalent interaction of BP with a perylene diimide was mainly due to van der Waals interactions but also led to considerable stabilization of the BP flakes against oxygen degradation.

Mapping charge transport by electroluminescence in chirality-selected carbon nanotube networks.

We demonstrate random network single-walled carbon nanotube (SWNT) field-effect transistors (FETs) in bottom contact/top gate geometry with only five different semiconducting nanotube species that were selected by dispersion with poly(9,9-dioctylfluorene) in toluene. These FETs are highly ambipolar with balanced hole and electron mobilities and emit near-infrared light with narrow peak widths (<40 meV) and good efficiency. We spatially resolve the electroluminescence from the channel region during a gate voltage sweep and can thus trace charge transport paths through the SWNT thin film. A shift of emission intensity to large diameter nanotubes and gate-voltage-dependent photoluminescence quenching of the different nanotube species indicates excitation transfer within the network and preferential charge accumulation on small band gap nanotubes. Apart from applications as near-infrared emitters with selectable emission wavelengths and narrow line widths, these devices will help to understand and model charge transport in realistic carbon nanotube networks.

Fundamental Insights into the Degradation and Stabilization of Thin Layer Black Phosphorus

Herein, we have developed a systematic study on the oxidation and passivation of mechanically exfoliated black phosphorus (BP). We analyzed the strong anisotropic behavior of BP by scanning Raman microscopy providing an accurate method for monitoring the oxidation of BP via statistical Raman spectroscopy. Furthermore, different factors influencing the environmental instability of the BP, i.e., thickness, lateral dimensions or visible light illumination, have been investigated in detail. Finally, we discovered that the degradation of few-layer BP flakes of <10 nm can be suppressed for months by using ionic liquids, paving the way for the development of BP-based technologies.

Solvent-driven electron trapping and mass transport in reduced graphites to access perfect graphene.

Herein, we report on a significant discovery, namely, the quantitative discharging of reduced graphite forms, such as graphite intercalation compounds, graphenide dispersions and graphenides deposited on surfaces with the simple solvent benzonitrile. Because of its comparatively low reduction potential, benzonitrile is reduced during this process to the radical anion, which exhibits a red colour and serves as a reporter molecule for the quantitative determination of negative charges on the carbon sheets. Moreover, this discovery reveals a very fundamental physical–chemical phenomenon, namely a quantitative solvent reduction induced and electrostatically driven mass transport of K+ ions from the graphite intercalation compounds into the liquid. The simple treatment of dispersed graphenides suspended on silica substrates with benzonitrile leads to the clean conversion to graphene. This unprecedented procedure represents a rather mild, scalable and inexpensive method for graphene production surpassing previous wet-chemical approaches.

Basic Insights into Tunable Graphene Hydrogenation

The hydrogenation and deuteration of graphite with potassium intercalation compounds as starting materials were investigated in depth. Characterization of the reaction products (hydrogenated and deuterated graphene) was carried out by thermogravimetric analysis coupled with mass spectrometry (TG-MS) and statistical Raman spectroscopy (SRS) and microscopy (SRM). The results reveal that the choice of the hydrogen/deuterium source, the nature of the graphite (used as starting material), the potassium concentration in the intercalation compound, and the choice of the solvent have a great impact on the reaction outcome. Furthermore, it was possible to prove that both mono- and few-layer hydrogenated/deuterated graphene can be produced.

Region-Selective Self-Assembly of Functionalized Carbon Allotropes from Solution

Approaches for the selective self-assembly of functionalized carbon allotropes from solution are developed and validated for 0D-fullerenes, 1D-carbon nanotubes and 2D-graphene. By choosing the right molecular interaction of self-assembled monolayers (serving the surface) with the functionalization features of carbon materials, which provide the solubility but also serve the driving force for assembly, we demonstrate a region-selective and self-terminating assembly of the materials. Active layers of the carbon allotropes can be selectively deposited in the channel region of thin-film transistor (TFT) devices by this approach. As an example for a 0D system, molecules of C60 functionalized octadecylphosphonic acids are used to realize self-assembled monolayer field-effect transistors (SAMFETs) based on a selective molecular exchange reaction of stearic acid in the channel region. For noncovalently functionalized single-walled carbon nanotubes (SWCNTs) and graphene oxide (GO) flakes, the electrostatic Coulomb interactions between the functional groups of the carbon allotropes and the charged head groups of a SAM dielectric layer are utilized to implement the selective deposition.

Enhanced Adsorption Affinity of Anionic Perylene-Based Surfactants towards Smaller-Diameter SWCNTs

We present evidence from multiple characterization methods, such as emission spectroscopy, zeta potential, and analytical ultracentrifugation, to shed light on the adsorption behavior of synthesized perylene surfactants on single-walled carbon nanotubes (SWCNTs). On comparing dispersions of smaller-diameter SWCNTs prepared by using cobalt-molybdenum catalysis (CoMoCAT) with the larger-diameter SWCNTs prepared by high-pressure carbon monoxide decomposition (HiPco), we find that the CoMoCAT-perylene surfactant dispersions are characterized by more negative zeta potentials, and higher anhydrous specific volumes (the latter determined from the sedimentation coefficients by analytical ultracentrifugation), which indicates an increased packing density of the perylene surfactants on nanotubes of smaller diameter. This conclusion is further supported by the subsequent replacement of the perylene derivatives from the nanotube sidewall by sodium dodecyl benzene sulfonate (SDBS), which first occurs on the larger-diameter nanotubes. The enhanced adsorption affinity of the perylene surfactants towards smaller-diameter SWCNTs can be understood in terms of a change in the supramolecular arrangement of the perylene derivatives on the scaffold of the SWCNTs. These findings represent a significant step forward in understanding the noncovalent interaction of π-surfactants with carbon nanotubes, which will enable the design of novel surfactants with enhanced selectivity for certain nanotube species.

Density Gradient Ultracentrifugation on Carbon Nanotubes According to Structural Integrity as a Foundation for an Absolute Purity Evaluation

fundamental aspects is the precise analysis of as-produced SWCNTs, as a widely applicable, readily and quickly available standard protocol for the determination of the absolute nanotube purity (with regard to structurally perfect SWCNTs in the bulk sample) is still lacking so that evidence from multiple spectroscopic and microscopic techniques usually needs to be cumulated in time-consuming studies. In 2003 the group of Haddon and coworkers started to follow the approach of determining relative nanotube purities by the aid of solution phase absorption spectroscopy. [13] The methodology relies on calculating the resonant ratio (RR) in the absorption spectra of nanotubes which they defined as the ratio of the integrated resonant SWCNT peaks arising from the excitonic interband transitions [14–16] [A(S)] to the total integrated area of the same region including the non-resonant background [A(T)]. In a variety of subsequent investigations, they were able to establish the technique as quick and readily available relative purity evaluation of bulk samples. [17] However, a bottleneck is formed, as no absolute purity can be determined due to the lack of a perfectly pure SWCNT sample that can be used as a reference system. Herein, we present that such a perfect SWCNT reference can be obtained from the as-produced bulk material by the aid of density gradient ultracentrifugation (DGU). In the past five years, DGU has evolved to a highly potent and versatile nanotube sorting technique where SWCNT samples can be separated according to diameter, electronic properties, or even down to single chiralities or helicities. [18–24] In principle, surfactant-dispersed SWCNTs are separated in a centrifugal field according to their buoyant densities by the aid of a density gradient medium. As the buoyant density is dependent on the surfactant adsorption, the fractionation process can be fine-tuned by using different combinations of surfactants. Additional to the “classical” DGU nanotube sorting approach, evidence has been provided that the buoyant density of covalently functionalized SWCNTs is significantly higher compared to the non-functionalized counterparts with differences in the range of 100 kg m 3 in the case of sodium-cholate-dispersed hydroxyphenylated SWCNTs. [25] We now applied this technique to separate the perfect, defect-free, nonfunctionalized SWCNTs of a bulk HiPco nanotube sample [26] from the defect material and amorphous carbon in DGU separation according to structural integrity. This enabled us to provide a reference spectrum for the determination of the absolute nanotube purity following Haddon’s approach of solution phase absorption spectroscopy. As the nanotube absorption spectra are also a fingerprint of the aggregation state, [27] we furthermore revealed that the resonant ratio in the absorption spectra is independent on the degree of SWCNT debundling by comparing the RR in different solvent systems (N,N’-dimethylformamide, N-cyclohexyl-2-pyrrolidone and 2 wt % sodium deoxycholate (SDC) aqueous surfactant solution), where it is known from literature that nanotube individualization strongly varies. [28–32] Accordingly, we provide an

Effect of TCNQ Layer Cover on Oxidation Dynamics of Black Phosphorus

The puckered surface of black phosphorus represents an ideal substrate for an unconventional arrangement of physisorbed species and the resulting specific two-dimensional chemistry of this system. This opens the way to investigate the chemical and physical properties of locally confined areas of black phosphorus without the necessity for further physical downscaling of the material. We have evaporated TCNQ on top of black phosphorus under over-saturation non-equilibrium conditions in vacuum. The evolution of linear density and height of droplets formed through oxidation during exposure to air was studied time-dependently by scanning-force microscopy. Our study suggests that the TCNQ molecules spontaneously arrange in a thin layer of the order of a few nm height, which, however, is fragmented with a periodicity of about 100 nm. It is shown that within the confined space separating the layer fragments the chemical dynamics of the oxidation process is remarkably different than on a bare black phosphorus surface.