Property by design is one appealing idea in material synthesis but hard to achieve in practice. A recent successful example is the demonstration of van der Waals (vdW) heterostructures, 1-3 in which atomic layers are stacked on each other and different ingredients can be combined beyond symmetry and lattice matching. This concept, usually described as a nanoscale Lego blocks, allows to build sophisticated structures layer by layer. However, this concept has been so far limited in two dimensional (2D) materials. Here we show a class of new material where different layers are coaxially (instead of planarly) stacked. As the structure is in one dimensional (1D) form, we name it "1D vdW heterostructures". We demonstrate a 5 nm diameter nanotube consisting of three different materials: an inner conductive carbon nanotube (CNT), a middle insulating hexagonal boron nitride nanotube

No abstract

No abstract

The dynamic viscoelastic properties of broad-distribution polystyrene and blends of narrowdistribution polystyrenes have been measured in the molten state. A comparison of the frequency dependences of the storage shear modulus G', loss modulus G", and relaxation spectrum H(7) of the blends and the broaddistribution polymer with those of the narrow-distribution polymers reveals that the effect of molecular weight distribution is very marked in the terminal and rubbery zones. The log G' us. log w curve for a binary blend composed of two components having very different molecular weights manifests the "two-step" rubbery plateau, suggesting that the blend has two sets of relaxation times associated with different kinds of entanglement couplings. The zero-shear viscosity qo of the blends is almost equal to that of a narrow-distribution polymer having the same weight-average molecular weight M,, but the elasticity coefficient AG for the blends is proportional to Mw4.3, as against M7.5 for the narrow-distribution polystyrene. The steady-state compliance .leo of the blends is proportional to w2-2 when wp, the weight fraction of the high molecular weight component, is close to unity. The entanglement compliance J,NO is also approximately proportional to W Z -~, and J,NO is lower than J, O by a factor of about 3. The existence of three kinds of entanglement couplings has been pointed out, in which molecular chains of highhigh, high-low, and low-low molecular weight components are involved.any investigations of the effect of molecular weight

Synthesis of single-walled carbon nanotubes (SWNTs) with well-defined atomic arrangements has been widely recognized in the past few decades as the biggest challenge in the SWNT community, and has become a bottleneck for the application of SWNTs in nano-electronics. Here, we report a selective synthesis of (12, 6) SWNTs with an enrichment of 50%-70% by chemical vapor deposition (CVD) using sputtered Co-W as a catalyst. This is achieved under much milder reduction and growth conditions than those in the previous report using transition-metal molecule clusters as catalyst precursors (Nature, 2014, 510, 522). Meanwhile, in-plane transmission electron microscopy unambiguously identified an intermediate structure of Co6W6C, which is strongly associated with selective growth. However, most of the W atoms disappear after a 5 min CVD growth, which implies that anchoring W may be important in this puzzling Co-W system.