Angela R. Hight Walker

Thermal Conductivity of Monolayer Molybdenum Disulfide Obtained from Temperature-Dependent Raman Spectroscopy

Atomically thin molybdenum disulfide (MoS2) offers potential for advanced devices and an alternative to graphene due to its unique electronic and optical properties. The temperature-dependent Raman spectra of exfoliated, monolayer MoS2 in the range of 100-320 K are reported and analyzed. The linear temperature coefficients of the in-plane E2g 1 and the out-of-plane A1g modes for both suspended and substrate-supported monolayer MoS2 are measured. These data, when combined with the first-order coefficients from laser power-dependent studies, enable the thermal conductivity to be extracted. The resulting thermal conductivity κ = (34.5(4) W/mK at room temperature agrees well with the first principles lattice dynamics simulations. However, this value is significantly lower than that of graphene. The results from this work provide important input for the design of MoS2-based devices where thermal management is critical.

High-speed coherent Raman fingerprint imaging of biological tissues

An imaging platform based on broadband coherent anti-Stokes Raman scattering (BCARS) has been developed which provides an advantageous combination of speed, sensitivity and spectral breadth. The system utilizes a configuration of laser sources that probes the entire biologically-relevant Raman window (500 cm−1 to 3500 cm−1) with high resolution (< 10 cm−1). It strongly and efficiently stimulates Raman transitions within the typically weak “fingerprint” region using intrapulse 3-colour excitation, and utilizes the nonresonant background (NRB) to heterodyne amplify weak Raman signals. We demonstrate high-speed chemical imaging in two- and three-dimensional views of healthy murine liver and pancreas tissues and interfaces between xenograft brain tumours and the surrounding healthy brain matter.

Isolation of Specific Small‐Diameter Single‐Wall Carbon Nanotube Species via Aqueous Two‐Phase Extraction

Aqueous two-phase extraction is demonstrated to enable isolation of single semiconducting and metallic single-wall carbon nanotube species from a synthetic mixture. The separation is rapid and robust, with remarkable tunability via modification of the surfactant environment set for the separation.

Evolution of DNA sequences toward recognition of metallic armchair carbon nanotubes.

The armchair carbon nanotube is an ideal system to study fundamental physics in one-dimensional metals and potentially a superb material for applications such as electrical power transmission. Synthesis and purification efforts to date have failed to produce a homogeneous population of such a material. Here we report evolutionary strategies to find DNA sequences for the recognition and subsequent purification of (6,6) and (7,7) armchair species from synthetic mixtures. The new sequences were derived by single-point scanning mutation and sequence motif variation of previously identified ones for semiconducting tubes. Optical absorption spectroscopy of the purified armchair tubes revealed well-resolved first- and second-order electronic transitions accompanied by prominent sideband features that have neither been predicted nor observed previously. Resonance Raman spectroscopy showed a single Lorentzian peak for the in-plane carbon-carbon stretching mode (G band) of the armchair tubes, repudiating the common practice of using such a line shape to infer the absence of metallic species. Our work demonstrates the exquisite sensitivity of DNA to nanotube metallicity and makes the long-anticipated pure armchair tubes available as seeds for their mass amplification.

Analyzing Surfactant Structures on Length and Chirality Resolved (6,5) Single-Wall Carbon Nanotubes by Analytical Ultracentrifugation

The structure and density of the bound interfacial surfactant layer and associated hydration shell were investigated using analytical ultracentrifugation for length and chirality purified (6,5) single-wall carbon nanotubes (SWCNTs) in three different bile salt surfactant solutions. The differences in the chemical structures of the surfactants significantly affect the size and density of the bound surfactant layers. As probed by exchange of a common parent nanotube population into sodium deoxycholate, sodium cholate, or sodium taurodeoxycholate solutions, the anhydrous density of the nanotubes was least for the sodium taurodeoxycholate surfactant, and the absolute sedimentation velocities greatest for the sodium cholate and sodium taurodeoxycholate surfactants. These results suggest that the thickest interfacial layer is formed by the deoxycholate, and that the taurodeoxycholate packs more densely than either sodium cholate or deoxycholate. These structural differences correlate well to an observed 25% increase in fluorescence intensity relative to the cholate surfactant for deoxycholate and taurodeoxycholate dispersed SWCNTs displaying equivalent absorbance spectra. Separate sedimentation velocity experiments including the density modifying agent iodixanol were used to establish the buoyant density of the (6,5) SWCNT in each of the bile salt surfactants; from the difference in the buoyant and anhydrous densities, the largest hydrated diameter is observed for sodium deoxycholate. Understanding the effects of dispersant choice and the methodology for measurement of the interfacial density and hydrated diameter is critical for rationally advancing separation strategies and applications of nanotubes.

Isolation of >1 nm Diameter Single-Wall Carbon Nanotube Species Using Aqueous Two-Phase Extraction

In this contribution we demonstrate the effective separation of single-wall carbon nanotube (SWCNT) species with diameters larger than 1 nm through multistage aqueous two-phase extraction (ATPE), including isolation at the near-monochiral species level up to at least the diameter range of SWCNTs synthesized by electric arc synthesis (1.3-1.6 nm). We also demonstrate that refined species are readily obtained from both the metallic and semiconducting subpopulations of SWCNTs and that this methodology is effective for multiple SWCNT raw materials. Using these data, we report an empirical function for the necessary surfactant concentrations in the ATPE method for separating different SWCNTs into either the lower or upper phase as a function of SWCNT diameter. This empirical correlation enables predictive separation design and identifies a subset of SWCNTs that behave unusually as compared to other species. These results not only dramatically increase the range of SWCNT diameters to which species selective separation can be achieved but also demonstrate that aqueous two-phase separations can be designed across experimentally accessible ranges of surfactant concentrations to controllably separate SWCNT populations of very small (∼0.62 nm) to very large diameters (>1.7 nm). Together, the results reported here indicate that total separation of all SWCNT species is likely feasible by the ATPE method, especially given future development of multistage automated extraction techniques.

Influence of Metal–MoS2 Interface on MoS2 Transistor Performance: Comparison of Ag and Ti Contacts

In this work, we compare the electrical characteristics of MoS2 field-effect transistors (FETs) with Ag source/drain contacts with those with Ti and demonstrate that the metal-MoS2 interface is crucial to the device performance. MoS2 FETs with Ag contacts show more than 60 times higher ON-state current than those with Ti contacts. In order to better understand the mechanism of the better performance with Ag contacts, 5 nm Au/5 nm Ag (contact layer) or 5 nm Au/5 nm Ti film was deposited onto MoS2 monolayers and few layers, and the topography of metal films was characterized using scanning electron microscopy and atomic force microscopy. The surface morphology shows that, while there exist pinholes in Au/Ti film on MoS2, Au/Ag forms a smoother and denser film. Raman spectroscopy was carried out to investigate the metal-MoS2 interface. The Raman spectra from MoS2 covered with Au/Ag or Au/Ti film reveal that Ag or Ti is in direct contact with MoS2. Our findings show that the smoother and denser Au/Ag contacts lead to higher carrier transport efficiency.

Targeted Therapeutic Nanotubes Influence the Viscoelasticity of Cancer Cells to Overcome Drug Resistance

Resistance to chemotherapy is the primary cause of treatment failure in over 90% of cancer patients in the clinic. Research in nanotechnology-based therapeutic alternatives has helped provide innovative and promising strategies to overcome multidrug resistance (MDR). By targeting CD44-overexpressing MDR cancer cells, we have developed in a single-step a self-assembled, self-targetable, therapeutic semiconducting single-walled carbon nanotube (sSWCNT) drug delivery system that can deliver chemotherapeutic agents to both drug-sensitive OVCAR8 and resistant OVCAR8/ADR cancer cells. The novel nanoformula with a cholanic acid-derivatized hyaluronic acid (CAHA) biopolymer wrapped around a sSWCNT and loaded with doxorubicin (DOX), CAHA-sSWCNT-DOX, is much more effective in killing drug-resistant cancer cells compared to the free DOX and phospholipid PEG (PL-PEG)-modified sSWCNT formula, PEG-sSWCNT-DOX. The CAHA-sSWCNT-DOX affects the viscoelastic property more than free DOX and PL-PEG-sSWCNT-DOX, which in turn allows more drug molecules to be internalized. Intravenous injection of CAHA-sSWCNT-DOX (12 mg/kg DOX equivalent) followed by 808 nm laser irradiation (1 W/cm2, 90 s) led to complete tumor eradication in a subcutaneous OVCAR8/ADR drug-resistant xenograft model, while free DOX alone failed to delay tumor growth. Our newly developed CAHA-sSWCNT-DOX nanoformula, which delivers therapeutics and acts as a sensitizer to influence drug uptake and induce apoptosis with minimal resistance factor, provides a novel effective means of counteracting the phenomenon of multidrug resistance.

Carbon Nanotubes: Measuring Dispersion and Length

Advanced technological uses of single-walled carbon nanotubes (SWCNTs) rely on the production of single length and chirality populations that are currently only available through liquid-phase post processing. The foundation of all of these processing steps is the attainment of individualized nanotube dispersions in solution. An understanding of the colloidal properties of the dispersed SWCNTs can then be used to design appropriate conditions for separations. In many instances nanotube size, particularly length, is especially active in determining the properties achievable in a given population, and, thus, there is a critical need for measurement technologies for both length distribution and effective separation techniques. In this Progress Report, the current state of the art for measuring dispersion and length populations, including separations, is documented, and examples are used to demonstrate the desirability of addressing these parameters.

Multicomponent Chemical Imaging of Pharmaceutical Solid Dosage Forms with Broadband CARS Microscopy

We compare a coherent Raman imaging modality, broadband coherent anti-Stokes Raman scattering (BCARS) microscopy, with spontaneous Raman microscopy for quantitative and qualitative assessment of multicomponent pharmaceuticals. Indomethacin was used as a model active pharmaceutical ingredient (API) and was analyzed in a tabulated solid dosage form, embedded within commonly used excipients. In comparison with wide-field spontaneous Raman chemical imaging, BCARS acquired images 10× faster, at higher spatiochemical resolution and with spectra of much higher SNR, eliminating the need for multivariate methods to identify chemical components. The significant increase in spatiochemical resolution allowed identification of an unanticipated API phase that was missed by the spontaneous wide-field method and bulk Raman spectroscopy. We confirmed the presence of the unanticipated API phase using confocal spontaneous Raman, which provided spatiochemical resolution similar to BCARS but at 100× slower acquisition times.