Imaging layer number and stacking order through formulating Raman fingerprints obtained from hexagonal single crystals of few layer graphene.

Quantitative mapping of layer number and stacking order for CVD-grown graphene layers is realized by formulating Raman fingerprints obtained on two stepwise stacked graphene single-crystal domains with AB Bernal and turbostratic stacking (with ~30°interlayer rotation), respectively. The integrated peak area ratio of the G band to the Si band, A(G)/A(Si), is proven to be a good fingerprint for layer number determination, while the area ratio of the 2D and G bands, A(2D)/A(G), is shown to differentiate effectively between the two different stacking orders. The two fingerprints are well formulated and resolve, quantitatively, the layer number and stacking type of various graphene domains that used to rely on tedious transmission electron microscopy for structural analysis. The approach is also noticeable in easy discrimination of the turbostratic graphene region (~30° rotation), the structure of which resembles the well known high-mobility graphene R30/R2(±) fault pairs found on the vacuum-annealed C-face SiC and suggests an electron mobility reaching 14,700 cm(3) V(-1) s(-1). The methodology may shed light on monitoring and control of high-quality graphene growth, and thereby facilitate future mass production of potential high-speed graphene applications.

Correlating defect density with carrier mobility in large-scaled graphene films: Raman spectral signatures for the estimation of defect density.

We report a correlation between carrier mobility and defect density in large-scaled graphene films prepared by chemical vapor deposition (CVD). Raman spectroscopy is used for investigating the layer number and the crystal quality of graphene films, and the defect density is estimated by the intensity ratios of the D and G peaks. By carefully controlling the growth parameters, especially the H(2)/CH(4) ratios during growth, and employing H(2) during cooling, monolayer-dominant graphene films can be obtained with different D peak intensities in Raman spectra, which show good correspondence with their carrier mobility obtained by Hall measurements. Also, a progressive shift of neutrality points to a more negative gate voltage is observed with the increase in defect density. Both the connections of carrier mobility and the shift of neutrality points to a negative direction in relation to the defect density in graphene are observed for the first time in CVD-grown graphene films. With the best growth conditions, a cm-scaled graphene film with carrier mobility of ∼ 1350 cm(2) V(-1) s(-1) (p-type in air) can be obtained.

Polymer structure and solvent effects on the selective dispersion of single-walled carbon nanotubes.

Combinations of different aromatic polymers and organic solvents have been studied as dispersing agents for preparing single-walled carbon nanotubes solutions, using optical absorbance, photoluminescence-excitation mapping, computer modeling, and electron microscopic imaging to characterize the solutions. Both the polymer structure and solvent used strongly influence the dispersion of the nanotubes, leading in some cases to very high selectivity in terms of diameter and chiral angle. The highest selectivities are observed using toluene with the rigid polymers PFO-BT and PFO to suspend isolated nanotubes. The specific nanotube species selected are also dependent on the solvent used and can be adjusted by the use of THF or xylene. Where the structure has more flexible conformations, the polymers are shown to be less selective but show an enhanced overall solubilization of nanotube material. When chloroform is used as the solvent, there is a large increase in the overall solubilization, but the nanotubes are suspended as bundles rather than as isolated tubes which leads to a quenching of their photoluminescence.

Band gap engineering of chemical vapor deposited graphene by in situ BN doping.

Band gap opening and engineering is one of the high priority goals in the development of graphene electronics. Here, we report on the opening and scaling of band gap in BN doped graphene (BNG) films grown by low-pressure chemical vapor deposition method. High resolution transmission electron microscopy is employed to resolve the graphene and h-BN domain formation in great detail. X-ray photoelectron, micro-Raman, and UV-vis spectroscopy studies revealed a distinct structural and phase evolution in BNG films at low BN concentration. Synchrotron radiation based XAS-XES measurements concluded a gap opening in BNG films, which is also confirmed by field effect transistor measurements. For the first time, a significant band gap as high as 600 meV is observed for low BN concentrations and is attributed to the opening of the π-π* band gap of graphene due to isoelectronic BN doping. As-grown films exhibit structural evolution from homogeneously dispersed small BN clusters to large sized BN domains with embedded diminutive graphene domains. The evolution is described in terms of competitive growth among h-BN and graphene domains with increasing BN concentration. The present results pave way for the development of band gap engineered BN doped graphene-based devices.

Top laminated graphene electrode in a semitransparent polymer solar cell by simultaneous thermal annealing/releasing method.

In this article, we demonstrate a semitransparent inverted-type polymer solar cell using a top laminated graphene electrode without damaging the underlying organic photoactive layer. The lamination process involves the simultaneous thermal releasing deposition of the graphene top electrode during thermal annealing of the photoactive layer. The resulting semitransparent polymer solar cell exhibits a promising power conversion efficiency of approximately 76% of that of the standard opaque device using an Ag metal electrode. The asymmetric photovoltaic performances of the semitransparent solar cell while illuminated from two respective sides were further analyzed using optical simulation and photocarrier recombination measurement. The devices consisting of the top laminated transparent graphene electrode enable the feasible roll-to-roll manufacturing of low-cost semitransparent polymer solar cells and can be utilized in new applications such as power-generated windows or multijunction or bifacial photovoltaic devices.

Direct spectroscopic evidence of energy transfer from photo-excited semiconducting polymers to single-walled carbon nanotubes.

Single-walled carbon nanotubes have been efficiently dispersed in an organic solvent using the semiconducting polymers MEHPPV and PFO. It has been found that energy is transferred to the carbon nanotubes when the polymer is photo-excited across its minimum energy gap. This is shown using photoluminescence excitation mapping in the range of both the polymer's and the nanotube's optical absorbance. Possible mechanisms for the energy transfer are discussed.

Highly selective dispersion of single-walled carbon nanotubes using aromatic polymers.

Solubilizing and purifying carbon nanotubes remains one of the foremost technological hurdles in their investigation and application. We report a dramatic improvement in the preparation of single-walled carbon nanotube solutions based on the ability of specific aromatic polymers to efficiently disperse certain nanotube species with a high degree of selectivity. Evidence of this is provided by optical absorbance and photoluminescence excitation spectra, which show suspensions corresponding to up to approximately 60% relative concentration of a single species of isolated nanotubes with fluorescence quantum yields of up to 1.5%. Different polymers show the ability to discriminate between nanotube species in terms of either diameter or chiral angle. Modelling suggests that rigid-backbone polymers form ordered molecular structures surrounding the nanotubes with n-fold symmetry determined by the tube diameter.