The quantum Hall (QH) effect in two-dimensional electrons and holes in high quality graphene samples is studied in strong magnetic fields up to 45 T. QH plateaus at filling factors nu = 0, +/-1, +/-4 are discovered at magnetic fields B > 20 T, indicating the lifting of the fourfold degeneracy of the previously observed QH states at nu = +/-4(absolute value(n) + 1/2), where n is the Landau-level index. In particular, the presence of the nu = 0, +/-1 QH plateaus indicates that the Landau level at the charge neutral Dirac point splits into four sublevels, lifting sublattice and spin degeneracy. The QH effect at nu = +/-4 is investigated in a tilted magnetic field and can be attributed to lifting of the spin degeneracy of the n = 1 Landau level.

Molecular electronics is often limited by the poorly defined nature of the contact between the molecules and the metal surface. We describe a method to wire molecules into gaps in single-walled carbon nanotubes (SWNTs). Precise oxidative cutting of a SWNT produces carboxylic acid-terminated electrodes separated by gaps of

We present an experimental investigation on the scaling of resistance in individual single-walled carbon nanotube devices with channel lengths that vary 4 orders of magnitude on the same sample. The electron mean free path is obtained from the linear scaling of resistance with length at various temperatures. The low temperature mean free path is determined by impurity scattering, while at high temperature, the mean free path decreases with increasing temperature, indicating that it is limited by electron-phonon scattering. An unusually long mean free path at room temperature has been experimentally confirmed. Exponentially increasing resistance with length at extremely long length scales suggests anomalous localization effects.

We report a simple but powerful method for engineering multiwalled carbon nanotubes (MWNTs) by using manipulation by an atomic-force microscope. The successive shell-by-shell extraction process of ultralong MWNTs allows the exposure of the innermost single-walled carbon nanotubes (SWNTs), which have diameters as small as Ϸ0.4 nm. The inner-shell extraction process changes the electrical characteristics of the MWNTs. Whereas the outer hollowed-out nanotubes show either metallic or semiconducting character, the innermost SWNTs of small diameter exhibit predominantly metallic transport properties.atomic-force microscope ͉ double-walled carbon nanotube ͉ extraction ͉ single-walled carbon nanotube ͉ resonance Raman spectroscopy M ultiwalled carbon nanotubes (MWNTs) (1) exhibit high ductility with extremely high tensile strength (2). These properties have enabled a variety of mechanical manipulations of these structures (2-6), as well as possible device applications. Engineering the MWNT shell structure further permits controlled modification of the electrical properties (7,8). Moreover, MWNTs are composed of strong, single-walled carbon nanotube (SWNT) shells coupled only by weak van der Waals interactions (9) associated with the nonlocal nature of the -dispersion forces (10). This situation results in ultralow friction between shell layers (9, 11) and leads to the possibility of nanoscale engineering of MWNT structures by displacing the shells with respect to one another. Such striking manipulations have indeed been realized in earlier investigations (11,12) in which a group of inner shells was extended by a few micrometers (8) in a transmission electron microscope chamber after electrical vaporization of MWNT end caps. Potential applications of these MWNT structures, such as nanobearings (11) and nanooscillators (13, 14), have been suggested. In the present work, we introduce a simple and highly flexible approach based on standard atomic-force microscope (AFM) techniques that yields extensions over distances of several hundreds of micrometers. This method achieves controlled shell-by-shell extraction down to the innermost SWNT without chemical (15, 16) or electrical (11, 12) modification of the outer shells. MethodsThe essence of the extraction method of inner shells from MWNTs can be understood from Fig. 1A. The image shows the result of moving an AFM tip along the substrate perpendicular to the axis of a long MWNT at a rate of 0.5-0.8 cm͞s. The height profile along the resulting structure (Fig. 1B) shows that the nanotube diameter drops in a stepwise fashion, corresponding to the successive extraction of the inner shells. We understand the nanotube extraction process as follows. As the length of MWNT segment displaced by the AFM tip increases, the total force required to overcome the friction of moving the MWNT across the substrate increases. The outer shell of the MWNT is subject to rupture when the force applied by the AFM tip exceeds its critical tensile strength (Ϸ10-100 GPa) (2). As the motion of the AFM tip...