Temperature dependence of the Breit–Wigner–Fano Raman line in single-wall carbon nanotube bundles

Broadband and Spectrally Selective Photothermal Conversion through Nanocluster Assembly of Disordered Plasmonic Metasurfaces

Plasmonic metasurfaces have been realized for efficient light absorption, thereby leading to photothermal conversion through nonradiative decay of plasmonic modes. However, current plasmonic metasurfaces suffer from inaccessible spectral ranges, costly and time-consuming nanolithographic top-down techniques for fabrication, and difficulty of scale-up. Here, we demonstrate a new type of disordered metasurface created by densely packing plasmonic nanoclusters of ultrasmall size on a planar optical cavity. The system either operates as a broadband absorber or offers a reconfigurable absorption band right across the visible region, resulting in continuous wavelength-tunable photothermal conversion. We further present a method to measure the temperature of plasmonic metasurfaces via surface-enhanced Raman spectroscopy (SERS), by incorporating single-walled carbon nanotubes (SWCNTs) as an SERS probe within the metasurfaces. Our disordered plasmonic system, generated by a bottom-up process, offers excellent performance and compatibility with efficient photothermal conversion. Moreover, it also provides a novel platform for various hot-electron and energy-harvesting functionalities.

High-quality, highly concentrated semiconducting single-wall carbon nanotubes for use in field effect transistors and biosensors

We developed a simple and scalable selective synthesis method of high-quality, highly concentrated semiconducting single-wall carbon nanotubes (s-SWCNTs) by in situ hydrogen etching. Samples containing ~93% s-SWCNTs were obtained in bulk. These s-SWCNTs with good structural integrity showed a high oxidation resistance temperature of ~800 °C. Thin-film transistors based on the s-SWCNTs demonstrated a high carrier mobility of 21.1 cm(2) V(-1) s(-1) at an on/off ratio of 1.1 × 10(4) and a high on/off ratio of 4.0 × 10(5) with a carrier mobility of 7.0 cm(2) V(-1) s(-1). A biosensor fabricated using the s-SWCNTs had a very low dopamine detection limit of 10(-18) mol/L at room temperature.

Investigation of temperature dependent multi-walled nanotube G and D doublet using pseudo-Voigt functions

A pseudo-Voigt (PV) function is used as a representation of the Stoke's phonon frequency distributions for a multi-walled nanotube (MWNT) composite G and D doublet. Variable peak assignments with the PV function have been shown to enhance the resolution of these bands commonly used for characterization of carbon nanotube (CNT) composites. The peak assignment study was applied to an in-situ temperature experiment where the addition of new sub-bands in the G and D doublet was determined to reduce the uncertainty of the Raman characteristics. Fitting the spectrum with five pseudo-Voigt bands was concluded to give the most consistent results, producing the lowest uncertainty values for G-peak position (νG) and D/G intensity ratio.

Direct observation of Born-Oppenheimer approximation breakdown in carbon nanotubes

Raman spectra and electrical conductance of individual, pristine, suspended, metallic single-walled carbon nanotubes are measured under applied gate potentials. The G(-) band is observed to downshift with small applied gate voltages, with the minima occurring at E(F) = +/-(1)/(2)E(phonon), contrary to adiabatic predictions. A subsequent upshift in the Raman frequency at higher gate voltages results in a "W"-shaped Raman shift profile that agrees well with a nonadiabatic phonon renormalization model. This behavior constitutes the first experimental confirmation of the theoretically predicted breakdown of the Born-Oppenheimer approximation in individual single-walled carbon nanotubes.

Unique laser-scanning optical microscope for low-temperature imaging and spectroscopy

Low-temperature optical characterization of single quantum nanostructures can reveal detailed information on structure-dependent properties of these materials. We describe the development of a unique laser-scanning optical microscope capable of low-temperature single molecule/particle imaging and spectroscopy. Making use of the magnification of a microscope objective, the laser- scanning scheme of the present microscope allows for high-repeatability imaging over large sample areas. The microscope is utilized to measure the low-temperature Raman scattering spectra of individual single-walled carbon nanotubes and single molecule fluorescence spectra of conjugated polymers. The developed low-temperature microscope can be applied to study a wide array of nanomaterials at a single particle level.

Voltage-induced dependence of Raman-active modes in single-wall carbon nanotube thin films

We report on electrical Raman measurements in transparent and conducting single-wall carbon nanotube (SWNT) thin films. Application of external voltage results in downshifts of the D and G modes and in reduction of their intensity. The intensities of the radial breathing modes increase with external electric field related to the application of the external voltage in metallic SWNTs, while decreasing in semiconducting SWNTs. A model explaining the phenomenon in terms of both direct and indirect (Joule heating) effects of the field is proposed. Our work rules out the elimination of large amounts of metallic SWNTs in thin film transistors using high field pulses. Our results support the existence of Kohn anomalies in the Raman-active optical branches of metallic graphitic materials.

Effects of Surfactant and Boron Doping on the BWF Feature in the Raman Spectrum of Single-Wall Carbon Nanotube Aqueous Dispersions

We examine the Breit-Wigner-Fano (BWF) line shape in the Raman spectra of carbon single-wall nanotubes (SWNTs) dispersed in aqueous suspensions. Bundling and electronic effects are studied by comparing undoped SWNTs (C-SWNTs) to boron-doped nanotubes (B-SWNTs) in a variety of different surfactant solutions. For SWNTs dispersed with nonionic surfactants that are less effective in debundling than ionic surfactants, the Raman spectra retain a large BWF feature. However, we demonstrate that even for SWNTs dispersed as isolated nanotubes by ionic surfactants the BWF feature may be present and that the intensity of the BWF is highly sensitive to the specific surfactant. In particular, surfactants with electron-donating groups tend to enhance the BWF feature. Also, modification of the SWNT electronic properties by boron doping leads to enhanced surfactant dispersion relative to undoped C-SWNTs and also to modification of the BWF feature. These observations are in agreement with reports demonstrating an enhancement of the BWF by bundling but also agree with reports that suggest electron donation can enhance the BWF feature even for isolated SWNTs. Importantly, these results serve to caution against using the lack or presence of a BWF feature as an independent measure of SWNT aggregation in surfactant dispersions.