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...

We develop networks of international stock market indices using information and correlation based measures. We use 83 stock market indices of a diversity of countries, as well as their single day lagged values, to probe the correlation and the flow of information from one stock index to another taking into account different operating hours. Additionally, we apply the formalism of partial correlations to build the dependency network of the data, and calculate the partial Transfer Entropy to quantify the indirect influence that indices have on one another. We find that Transfer Entropy is an effective way to quantify the flow of information between indices, and that a high degree of information flow between indices lagged by one day coincides to same day correlation between them.