Hong-Zhang Geng

Fermi Level Engineering of Single-Walled Carbon Nanotubes by AuCl3 Doping

We investigated the modulation of optical properties of single-walled carbon nanotubes (SWCNTs) by AuCl 3 doping. The van Hove singularity transitions (E 11 (S), E 22 (S), E 11 (M)) in absorption spectroscopy disappeared gradually with an increasing doping concentration and a new peak appeared at a high doping concentration. The work function was downshifted up to 0.42 eV by a strong charge transfer from the SWCNTs to AuCl 3 by a high level of p-doping. We propose that this large work function shift forces the Fermi level of the SWCNTs to be located deep in the valence band, i.e., highly degenerate, creating empty van Hove singularity states, and hence the work function shift invokes a new asymmetric transition in the absorption spectroscopy from a deeper level to newly generated empty states.

Doping and de-doping of carbon nanotube transparent conducting films by dispersant and chemical treatment

Single-walled carbon nanotubes (SWCNTs) dispersed with Nafion in a solvent mixture containing de-ionized water and 1-propanol (bisolvent) were sprayed on a poly(ethylene terephthalate) substrate to fabricate flexible transparent conducting films (TCFs). Different SWCNT-to-Nafion ratios were used to optimize the film performance of transparence and sheet resistance. The TCFs were then immersed in nitric acid. These steps resulted in p-type doping due to the presence of Nafion in the SWCNT network and de-doping (removal of doping effect) by the acid treatment. X-Ray photoelectron and Raman spectroscopy confirmed that the de-doping effect occurred with the partial removal of Nafion from the nanotube surface by the nitric acid treatment, which improved the film conductivity by a factor of ∼4 with negligible change in transmittance.

DEPENDENCE OF MATERIAL QUALITY ON PERFORMANCE OF FLEXIBLE TRANSPARENT CONDUCTING FILMS WITH SINGLE-WALLED CARBON NANOTUBES

Several single-walled carbon nanotubes (SWCNTs) prepared by different methods have been used to investigate the material dependence on the optimal film performance of flexible transparent conducting films. The nanotubes were dispersed in water with sodium dodecyl sulfate by sonication. These SWCNT solutions were then sprayed onto the Poly(ethylene terephthalate) substrate by a spray coater to form the film. Several factors such as purity, diameter, defects, metallicity, and degree of dispersion were evaluated individually to examine how they affect the film performance. We found that the metallicity of SWCNTs and the degree of dispersion are the most crucial factors in determining the film performance. We also proposed a material quality factor to estimate the material quality of SWCNTs as a figure of merit for the film performance.

Carbon nanotube/polyurethane films with high transparency, low sheet resistance and strong adhesion for antistatic application

Antistatic technology has been applied in all aspects of modern life, including the manufacture of electronic products, aerospace systems, daily necessities and so on. The main purpose of this study is to obtain, using the Mayer rod-coating method, a flexible antistatic film with high transmittance, low sheet resistance and strong adhesion. With the help of the dispersant, single-walled carbon nanotubes (SWCNTs) were dispersed in water to form an homogeneous dispersion. The SWCNT dispersion was coated onto a poly(ethylene terephthalate) (PET) film with use of a rod to produce a uniform film. The antistatic films obtained had special characteristics, such as high transparency, low sheet resistance and excellent resistance to water and heat. Sheet resistance varied between 102–105 Ω sq−1 by controlling the content of water-based polyurethane (WPU), changing the thickness of the films and the post-treatment. The best film had a sheet resistance of 423 Ω sq−1 with 82.7% transmittance. In particular, the addition of the WPU binder greatly improved the adhesion between the CNTs and the PET film. Scanning electron microscopy, energy dispersive X-ray spectroscopy and Scotch™ tape method were conducted to determine the microstructure, cleanliness, and adhesion of the film, respectively.

Fabrication and evaluation of adhesion enhanced flexible carbon nanotube transparent conducting films

Single-walled carbon nanotubes (SWCNTs) were dispersed in water with the help of a combination of surfactants to achieve a high concentration SWCNT ink. Transparent conducting films (TCFs) were fabricated through a rod-coating method using the SWCNT ink. The addition of binders (polyacrylic acid or carboxymethyl cellulose) greatly enhanced the adhesion of SWCNT films to substrates and the cohesion between CNTs, which produced a uniform film of SWCNTs by preventing damage during the post-treatment process. The thickness of SWCNT films is controlled by the amount of SWCNTs in the solution and the diameter of the wire used. To test the film adhesion, Scotch™ tape was used to detach some loosely bound SWCNTs. Then the SWCNT films were further post-treated with nitric acid to improve the conductivity. The addition of polyacrylic acid to the SWCNT dispersion improved the film adhesion obviously without decreasing its electrical conductivity. This rod-coating method demonstrates great potential for the scalable fabrication of flexible SWCNT-TCFs.

Y-junction carbon nanocoils: synthesis by chemical vapor deposition and formation mechanism

Y-junction carbon nanocoils (Y-CNCs) were synthesized by thermal chemical vapor deposition using Ni catalyst prepared by spray-coating method. According to the emerging morphologies of Y-CNCs, several growth models were advanced to elucidate their formation mechanisms. Regarding the Y-CNCs without metal catalyst in the Y-junctions, fusing of contiguous CNCs and a tip-growth mechanism are considered to be responsible for their formation. However, as for the Y-CNCs with catalyst presence in the Y-junctions, the formation can be ascribed to nanoscale soldering/welding and bottom-growth mechanism. It is found that increasing spray-coating time for catalyst preparation generates agglomerated larger nanoparticles strongly adhering to the substrate, resulting in bottom-growth of CNCs and appearance of the metal catalyst in the Y-junctions. In the contrary case, CNCs catalyzed by isolated smaller nanoparticles develop Y-junctions with an absence of metal catalyst by virtue of weaker adhesion of catalyst with the substrate and tip-growth of CNCs.

Three-dimensional architecture of carbon nanotube-anchored polymer nanofiber composite

Carbon nanotube (CNT)-anchored polymer nanofiber mats were designed by in situ spraying of carbon nanotubes while simultaneously electrospinning polymer nanofibers. The improved conductivity of the composite mat, high catalytic current (3400 mA/cm2/mg Pt) and long term stability of the composite mat was attributed to the efficient formation of CNT bridges between nanofibers.

Effect of Different Concentrations of Nitric Acid on the Conductivity of Single-walled Carbon Nanotube Transparent Films

Single-walled carbon nanotubes were dispersed in deionized water with sodium dodecyl benzene sulfonate as surfactant. The solutions were sprayed on UV and plasma treated polyethylene terephthalate to achieve transparent conductive films with excellent adhesion. The carbon nanotube films were further treated with different concentrations of nitric acid to improve conductivity. SWCNTs and films were characterized by thermo gravimetric analysis, field-emitting scanning electron microscopy, UV-VIS spectrophotometer, four-point probe method, and Raman spectroscopy. The results demonstrated that the conductivity of carbon nanotube films with high transparency was improved to a greater degree with higher concentration of nitric acid due to effectively removing residual surfactants. The low sheet resistance films of ~100 Ω/sq @ 80T% have widely applications in touch screen, flat panel displays, organic light emitting diode, and etc.

Recent progresses in carbon nanotube-based flexible transparent conducting film

Flexible transparent conducting films (TCFs) were fabricated on a PET substrate by various methods using carbon nanotubes dispersed in organic or water-based solution. Thin multi-walled carbon nanotubes, double-walled carbon nanotubes, and single-walled carbon nanotubes were used to compare the performance for TCFs. Optimal design rules for types of nanotubes, surfactants, the degree of dispersion, and film preparation methods were discussed. The TCFs were characterized by scanning electron microscopy, TGA, Raman, optical absorption spectra, and sheet resistance. The dispersion of CNTs in water and in bisolvent has been tried. A simple acid treatment on the TCF film increased the conductivity by about four times. Doping and functionalization techniques will be further introduced to improve the conductivity of the film.

Transparent Conducting Films by Using Carbon Nanotubes

Summary of thematerial parameters forSWCNTs: (a) diameters,(b) burning temperature andpurity, (c) The ratio of theintensity of D-band to G-bandand G -band to G-band of theSWCNTs at excitationenergies of 514 and 633nm where n and p are the n -type (electrons) and p -type (holes) carrier concentrations,respectively, and μ n and μ p are the respective electron and hole mobility. Themobility is dominated by a succession of random scattering from collisions withlattice atoms, impurity atoms, and other scattering centers. The intrinsic carrierconcentration decreases exponentially with band gap, n i = n o exp( − E g / 2 k B T) ,where k B and T are the Boltzmann constant and temperature of the system, respec-tively. The p -type and n -type nanotube carrier concentrationsare, p = n i exp [ (E i − E f )/k B T ], and n = n i exp [ (E f − E i )/k B T ], where the intrinsic Fermi level E i is frequently used as a reference level when the extrinsic semiconductors are dis-cussed with a Fermi level of