A. Nicholas G. Parra-Vasquez

Spontaneous high-concentration dispersions and liquid crystals of graphene

Graphene combines unique electronic properties and surprising quantum effects with outstanding thermal and mechanical properties. Many potential applications, including electronics and nanocomposites, require that graphene be dispersed and processed in a fluid phase. Here, we show that graphite spontaneously exfoliates into single-layer graphene in chlorosulphonic acid, and dissolves at isotropic concentrations as high as approximately 2 mg ml(-1), which is an order of magnitude higher than previously reported values. This occurs without the need for covalent functionalization, surfactant stabilization, or sonication, which can compromise the properties of graphene or reduce flake size. We also report spontaneous formation of liquid-crystalline phases at high concentrations ( approximately 20-30 mg ml(-1)). Transparent, conducting films are produced from these dispersions at 1,000 Omega square(-1) and approximately 80% transparency. High-concentration solutions, both isotropic and liquid crystalline, could be particularly useful for making flexible electronics as well as multifunctional fibres.

Spontaneous dissolution of ultralong single- and multiwalled carbon nanotubes.

We report that chlorosulfonic acid is a true solvent for a wide range of carbon nanotubes (CNTs), including single-walled (SWNTs), double-walled (DWNTs), multiwalled carbon nanotubes (MWNTs), and CNTs hundreds of micrometers long. The CNTs dissolve as individuals at low concentrations, as determined by cryo-TEM (cryogenic transmission electron microscopy), and form liquid-crystalline phases at high concentrations. The mechanism of dissolution is electrostatic stabilization through reversible protonation of the CNT side walls, as previously established for SWNTs. CNTs with highly defective side walls do not protonate sufficiently and, hence, do not dissolve. The dissolution and liquid-crystallinity of ultralong CNTs are critical advances in the liquid-phase processing of macroscopic CNT-based materials, such as fibers and films.

Role of Surfactants and Salt in Aqueous Two-Phase Separation of Carbon Nanotubes toward Simple Chirality Isolation

Aqueous two-phase extraction has recently been demonstrated as a new method to separate single-wall carbon nanotubes (SWCNTs). In this work, we determined that the mechanism of separation is driven by the hydrophobicity of the surfactant, or combination of surfactants, at the SWCNT surface. This knowledge allowed us to develop a simple approach for obtaining highly enriched single-chirality suspensions in only 1 or 2 steps. These results were obtained by strategically combining multiple surfactants with different diameter-dependent binding affinities for SWCNTs and salts that readjust the surfactant structure within the mixed micelle surrounding the SWCNTs. The procedure is successfully applied to SWCNTs from different sources (CoMoCAT and HiPco) with various diameter distributions (from 0.53 to 1.2 nm). Each separation step is characterized by optical absorption, resonant Raman, and photoluminescence excitation spectroscopies. By determining the SWCNT sorting mechanism, we were able to develop a new set of parameters that separated another chirality.

Modeling the phase behavior of polydisperse rigid rods with attractive interactions with applications to single-walled carbon nanotubes in superacids

The phase behavior of rodlike molecules with polydisperse length and solvent-mediated attraction and repulsion is described by an extension of the Onsager theory for rigid rods. A phenomenological square-well potential is used to model these long-range interactions, and the model is used to compute phase separation and length fractionation as a function of well depth and rod concentration. The model closely captures experimental data points for isotropic/liquid crystalline phase coexistence of single-walled carbon nanotubes (SWCNTs) in superacids. The model also predicts that the isotropic-biphasic boundary approaches zero as the acid strength diminishes, with the possibility of coexistence of isotropic and liquid crystalline phases at very low concentrations; this counterintuitive prediction is confirmed experimentally. Experimental deviations from classical theories for rodlike liquid crystals are explained in terms of polydispersity and the balance between short-range repulsion and long-range attractions. The predictions of the model also hold practical importance for applications of SWCNT/superacid solutions, particularly in the processing of fibers and films from liquid crystalline SWCNT/superacid mixtures.

Stable Luminescence from Individual Carbon Nanotubes in Acidic, Basic, and Biological Environments

Aqueous surfactant suspensions of single walled carbon nanotubes (SWNTs) are very sensitive to environmental conditions. For example, the photoluminescence of semiconducting SWNTs varies significantly with concentration, pH, or salinity. In most cases, these factors restrict the range of applicability of SWNT suspensions. Here, we report a simple strategy to obtain stable and highly luminescent individualized SWNTs at pH values ranging from 1 to 11, as well as in highly saline buffers. This strategy relies on combining SWNTs previously suspended in sodium dodecylbenzene sulfonate (SDBS) with biocompatible poly(vinyl pyrrolidone) (PVP), which can be polymerized in situ to entrap the SWNT-SDBS micelles. We present a model that accounts for the photoluminescence stability of these suspensions based on PVP morphological changes at different pH values. Moreover, we demonstrate the effectiveness of these highly stable suspensions by imaging individual luminescent SWNTs on the surface of live human embryonic kidney cells (HEK cells).

Alignment Dynamics of Single-Walled Carbon Nanotubes in Pulsed Ultrahigh Magnetic Fields

We have measured the dynamic alignment properties of single-walled carbon nanotube (SWNT) suspensions in pulsed high magnetic fields through linear dichroism spectroscopy. Millisecond-duration pulsed high magnetic fields up to 56 T as well as microsecond-duration pulsed ultrahigh magnetic fields up to 166 T were used. Because of their anisotropic magnetic properties, SWNTs align in an applied magnetic field, and because of their anisotropic optical properties, aligned SWNTs show linear dichroism. The characteristics of their overall alignment depend on several factors, including the viscosity and temperature of the suspending solvent, the degree of anisotropy of nanotube magnetic susceptibilities, the nanotube length distribution, the degree of nanotube bundling, and the strength and duration of the applied magnetic field. To explain our data, we have developed a theoretical model based on the Smoluchowski equation for rigid rods that accurately reproduces the salient features of the experimental data.

Metrological Investigation of the (6,5) Carbon Nanotube Absorption Cross Section

Using single-nanotube absorption microscopy, we measured the absorption cross section of (6,5) carbon nanotubes at their second-order optical transition. We obtained a value of 3.2 × 10(-17) cm(2)/C atom with a precision of 15% and an accuracy below 20%. This constitutes the first metrological investigation of the absorption cross section of chirality-identified nanotubes. Correlative absorption-luminescence microscopies performed on long nanotubes reveal a direct manifestation of exciton diffusion in the nanotube.

Cryogenic-temperature electron microscopy direct imaging of carbon nanotubes and graphene solutions in superacids.

Cryogenic electron microscopy (cryo‐EM) is a powerful tool for imaging liquid and semiliquid systems. While cryogenic transmission electron microscopy (cryo‐TEM) is a standard technique in many fields, cryogenic scanning electron microscopy (cryo‐SEM) is still not that widely used and is far less developed. The vast majority of systems under investigation by cryo‐EM involve either water or organic components. In this paper, we introduce the use of novel cryo‐TEM and cryo‐SEM specimen preparation and imaging methodologies, suitable for highly acidic and very reactive systems. Both preserve the native nanostructure in the system, while not harming the expensive equipment or the user. We present examples of direct imaging of single‐walled, multiwalled carbon nanotubes and graphene, dissolved in chlorosulfonic acid and oleum. Moreover, we demonstrate the ability of these new cryo‐TEM and cryo‐SEM methodologies to follow phase transitions in carbon nanotube (CNT)/superacid systems, starting from dilute solutions up to the concentrated nematic liquid‐crystalline CNT phases, used as the ‘dope’ for all‐carbon‐fibre spinning. Originally developed for direct imaging of CNTs and graphene dissolution and self‐assembly in superacids, these methodologies can be implemented for a variety of highly acidic systems, paving a way for a new field of nonaqueous cryogenic electron microscopy.