Search

Surface contamination by polymer residues has long been a critical problem in probing graphene's intrinsic properties and in using graphene for unique applications in surface chemistry, biotechnology, and ultrahigh speed electronics. Poly(methyl methacrylate) (PMMA) is a macromolecule commonly used for graphene transfer and device processing, leaving a thin layer of residue to be empirically cleaned by annealing. Here we report on a systematic study of PMMA decomposition on graphene and of its impact on graphene's intrinsic properties using transmission electron microscopy (TEM) in combination with Raman spectroscopy. TEM images revealed that the physisorbed PMMA proceeds in two steps of weight loss in annealing and cannot be removed entirely at a graphene susceptible temperature before breaking. Raman analysis shows a remarkable blue-shift of the 2D mode after annealing, implying an anneal-induced band structure modulation in graphene with defects. Calculations using density functional theory show that local rehybridization of carbons from sp(2) to sp(3) on graphene defects may occur in the random scission of polymer chains and account for the blue-shift of the Raman 2D mode., (© 2011 American Chemical Society)

The usefulness of graphene for electronics has been limited because it does not have an energy bandgap. Although graphene nanoribbons have non-zero bandgaps, lithographic fabrication methods introduce defects that decouple the bandgap from electronic properties, compromising performance. Here we report direct measurements of a large intrinsic energy bandgap of approximately 50 meV in nanoribbons (width, approximately 100 nm) fabricated by high-temperature hydrogen-annealing of unzipped carbon nanotubes. The thermal energy required to promote a charge to the conduction band (the activation energy) is measured to be seven times greater than in lithographically defined nanoribbons, and is close to the width of the voltage range over which differential conductance is zero (the transport gap). This similarity suggests that the activation energy is in fact the intrinsic energy bandgap. High-resolution transmission electron and Raman microscopy, in combination with an absence of hopping conductance and stochastic charging effects, suggest a low defect density.

Unique low-dimensional SiO(2)-based nanomaterials can be encapsulated and synthesized inside the nanometer-scale one-dimensional internal spaces of carbon nanotubes (CNTs). In this study, various single-walled CNTs (SWNTs) and double-walled CNTs (DWNTs) having different diameters are used as containers for cubic octameric H(8)Si(8)O(12) molecules. High-resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FT-IR) spectroscopy, and Raman spectroscopy observations revealed that, depending on the diameter of the CNTs, two types of structures are formed inside the SWNTs and DWNTs: In the case of those CNTs having inner diameters ranging from 1.2 to 1.4 nm, a new ordered self-assembled structure composed of H(8)Si(4n)O(8n-4) molecules was formed through the transformation of H(8)Si(8)O(12); however, in the case of CNTs having inner diameters larger than 1.7 nm, a disordered structure was formed. This behavior may indicate that strong interactions occur between the CNTs and the encapsulated H(8)Si(4n)O(8n-4) molecules.

Chiral indices (n, m) of metallic and semiconducting single-walled carbon nanotubes (SWNTs) selectively separated via the density-gradient ultracentrifugation process were individually assigned by using an aberration-corrected transmission electron microscope (TEM) operated at 80 kV. Our statistical analysis revealed that armchair (n, n) and chiral (n, n-3) SWNTs with large chiral angles (>20 degrees) are dominant metallic nanotubes in the separated samples, whereas such a noticeable preference of particular indices was not observed for semiconducting nanotubes. Some significant discrepancies were found between the TEM and spectroscopic results on the major chiral indices and the metal/semiconductor ratios in these SWNTs.

Catalyst-free inner growth of single-wall carbon nanotubes has been directly realized and monitored by means of in situ high-resolution transmission electron microscopy, with particular attention paid to the evolution of the cap shape. The cap of a carbon nanotube is surprisingly found to be kept closed during the growing/shrinking process, and the cap shape evolves inhomogeneously with a few particular sites growing faster during the growth, while the cap of a carbon nanotube keeps a round shape during the shrinkage process. The closed cap should be specific for noncatalytic growth of carbon nanotubes. We infer, from the results above, the possible atomistic mechanism and how the carbon network can accommodate or release the carbon atoms during the growth/shrinkage of carbon nanotubes.

The imaging properties and observation of the sterically regulated translational motion of discrete tungsten polyoxometalate Linqvist ions (i.e., [W(6)O(19)](2-)) within carbon nanotubes of specific internal diameter are reported. The translational motion of the nonspheroidal anion within the nanotube capillary is found to be impeded by its near-perfect accommodation to the internal van der Waals surface of the nanotube wall. Rotational motion of the anion about one remaining degree of freedom permits translational motion of the anion along the nanotube followed by locking in at sterically favorable positions in a mechanism similar to a molecular ratchet. This steric locking permits the successful direct imaging of the constituent octahedral cation template of individual [W(6)O(19)](2-) anions by high resolution transmission electron microscopy thereby permitting meterological measurements to be performed directly on the anion. Direct imaging of pairs of equatorial W(2) atoms within the anion reveal steric relaxation of the anion contained within the nanotube capillary relative to the bulk anion structure.

The smallest carbon nanotubes with the chiral index (3,3), (4,3), or (5,1) were unambiguously identified for the first time. They were grown inside single-wall carbon nanotubes with the diameter of 1.0-1.2 nm, and the chiral indices were experimentally assigned beyond a doubt by an aberration-corrected high-resolution transmission electron microscopy (HR-TEM). In contrast to a theoretical prediction, the (3,3) nanotube is rather unstable and extremely sensitive to the electron beam and, therefore, may not survive alone without the protection of outer nanotube. The cap structure of (3,3) nanotube is also well-explained by a half-dome of C20 fullerene, which consists of six pentagons only.

The growth of double-walled carbon nanotubes from peapods was studied. The transformation was monitored by the decrease of fullerene Raman lines, the growth of inner tube Raman lines, and the development of X-ray diffraction patterns. A visual check of the growth process by HRTEM provided additional information. From the difference in time constants for the bleaching of fullerene Raman lines and for the growth of nanotube Raman lines, the existence of an intermediate phase was concluded that was eventually observed in X-ray diffraction and HRTEM. Time constants for the growth of large diameter inner tubes were up to a factor two larger than for small diameter inner tubes. The results fully support the fullerene coalescence growth model triggered by Stone-Wales transformations.

Atomic chains and crystal of iodine were successfully generated in a controlled manner inside single-walled carbon nanotubes (SWNTs). The structure is strongly dependent on the diameter of SWNTs; the single, double, and triple helical structures became quite stable when the diameter of SWNTs matches the certain size. More than three chains of iodine are not very stable, and they often crystallize inside the carbon nanotube when the diameter is larger than 1.45 nm. The crystallization or phase transition in a confined nanospace is thus directly observed, and there is indeed a critical size of the hollow nanospace for the stable formation of the atomic chains of iodine.

Local modifications to the electronic structure during the bending deformation of carbon nanotubes are probed by in situ electron energy-loss spectroscopy. Reversible changes in the carbon K(1s) absorption near-edge fine structures are detected at the inner bending side for a multiwalled carbon nanotube and attributed to the curvature-induced electronic structure alteration of the graphite layers. More intensive changes in the carbon K edge across an abrupt kink of a bundle of single-walled carbon nanotubes are observed over a region of 4-5 nm around the kink. These results clearly demonstrate the unique deformation behavior of the carbon nanotube, namely, a high elastic deformability and variable electronic properties associated with bending. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]