Controlled alignment of supermoir\'e lattice in double-aligned graphene heterostructures

The supermoir\'e lattice, built by stacking two moir\'e patterns, provides a platform for creating flat mini-bands and studying electron correlations. An ultimate challenge in assembling a graphene supermoir\'e lattice is in the deterministic control of its rotational alignment, which is made highly aleatory due to the random nature of the edge chirality and crystal symmetry of each component layer. Employing the so-called golden rule of three, here we present an experimental strategy to overcome this challenge and realize the controlled alignment of double-aligned hBN/graphene/hBN supermoir\'e lattice, where graphene is precisely aligned with both top hBN and bottom hBN. Remarkably, we find that the crystallographic edge of neighboring graphite can be used to better guide the stacking alignment, as demonstrated by the controlled production of 20 moir\'e samples with an accuracy better than 0.2 degree. Finally, we extend our technique to other strongly correlated electron systems, such as low-angle twisted bilayer graphene and ABC-stacked trilayer graphene, providing a strategy for flat-band engineering in these moir\'e materials.Comment: 20 pages, 4 figure

Nanotechnology

In this paper, we propose a modified wavevector (WV) model that takes account of the N-process relaxation time and second-order three-phonon process to predict the length dependence of the thermal conductivity of single-wall carbon nanotubes (SWNTs). The model is validated by length-dependent thermal conductivities of individual SWNTs measured using the four-pad 3 omega method. The fitted Gruneisen parameter is close to 2. for SWNTs. These results indicate that the effect of the second-order three-phonon process cannot be neglected at room temperature. Both the experimental and theoretical results prove that the thermal conductivity increases with length of SWNTs over the range of 0.5-7 mu m.In this paper, we propose a modified wavevector (WV) model that takes account of the N-process relaxation time and second-order three-phonon process to predict the length dependence of the thermal conductivity of single-wall carbon nanotubes (SWNTs). The model is validated by length-dependent thermal conductivities of individual SWNTs measured using the four-pad 3 omega method. The fitted Gruneisen parameter is close to 2. for SWNTs. These results indicate that the effect of the second-order three-phonon process cannot be neglected at room temperature. Both the experimental and theoretical results prove that the thermal conductivity increases with length of SWNTs over the range of 0.5-7 mu m

Nonreciprocal light propagation induced by a subwavelength spinning cylinder

Nonreciprocal optical devices have broad applications in light manipulations for communications and sensing. Non-magnetic mechanisms of optical nonreciprocity are highly desired for high-frequency on-chip applications. Here, we investigate the nonreciprocal properties of light propagation in a dielectric waveguide induced by a subwavelength spinning cylinder. We find that the chiral modes of the cylinder can give rise to unidirectional coupling with the waveguide via the transverse spin-orbit interaction, leading to different transmissions for guided wave propagating in opposite directions and thus optical isolation. We reveal the dependence of the nonreciprocal properties on various system parameters including mode order, spinning speed, and coupling distance. The results show that higher-order chiral modes and larger spinning speed generally give rise to stronger nonreciprocity, and there exists an optimal cylinder-waveguide coupling distance where the optical isolation reaches the maximum. Our work contributes to the understanding of nonreciprocity in subwavelength moving structures and can find applications in integrated photonic circuits, topological photonics, and novel metasurfaces.Comment: 14 pages, 6 figure

Appl. Phys. Lett.

The thermal conductivity of single-wall carbon nanotubes (SWCNTs) is predicted to increase with length, but this has never been proved experimentally because of limitations in previous measurement methods. Here, the authors report the measurement of the length-dependent thermal conductivities of individual SWCNTs on a Si substrate using a four-pad 3 omega method. An increase in thermal conductivity with length was observed at room temperature, which is consistent with a theoretical prediction that considers higher order three-phonon processes. When SWCNTs are longer than the phonon mean path, they showed dissipative thermal transport. The observed increase of thermal conductivity with length makes SWCNTs ideal for thermal management.(c) 2007 American Institute of Physics.The thermal conductivity of single-wall carbon nanotubes (SWCNTs) is predicted to increase with length, but this has never been proved experimentally because of limitations in previous measurement methods. Here, the authors report the measurement of the length-dependent thermal conductivities of individual SWCNTs on a Si substrate using a four-pad 3 omega method. An increase in thermal conductivity with length was observed at room temperature, which is consistent with a theoretical prediction that considers higher order three-phonon processes. When SWCNTs are longer than the phonon mean path, they showed dissipative thermal transport. The observed increase of thermal conductivity with length makes SWCNTs ideal for thermal management.(c) 2007 American Institute of Physics

Remarkably enhanced thermal transport based on a flexible horizontally-aligned carbon nanotube array film

It has been more than a decade since the thermal conductivity of vertically aligned carbon nanotube (VACNT) arrays was reported possible to exceed that of the best thermal greases or phase change materials by an order of magnitude. Despite tremendous prospects as a thermal interface material (TIM), results were discouraging for practical applications. The primary reason is the large thermal contact resistance between the CNT tips and the heat sink. Here we report a simultaneous sevenfold increase in in-plane thermal conductivity and a fourfold reduction in the thermal contact resistance at the flexible CNT-SiO2 coated heat sink interface by coupling the CNTs with orderly physical overlapping along the horizontal direction through an engineering approach (shear pressing). The removal of empty space rapidly increases the density of transport channels, and the replacement of the fine CNT tips with their cylindrical surface insures intimate contact at CNT-SiO2 interface. Our results suggest horizontally aligned CNT arrays exhibit remarkably enhanced in-plane thermal conductivity and reduced out-of-plane thermal conductivity and thermal contact resistance. This novel structure makes CNT film promising for applications in chip-level heat dissipation. Besides TIM, it also provides for a solution to anisotropic heat spreader which is significant for eliminating hot spots.Published versio

Broadband MoS₂ Square Nanotube-Based Photodetectors

Although the research on layered MoS2 photodetectors has made great progress, their poor light absorption ability and complex preparation process hinder their further commercial application. In the present work, we report the growth of MoS2 square nanotubes with high purity via a facile hydrothermal method for the first time. Microstructure characterization demonstrates that the cavity structure of the nanotubes can bring about a light trapping effect, thus obtaining a strong photoelectric performance. The as-constructed MoS2 square nanotube photodetector with a paper substrate displays a broadband response with a detection range of 375 to 915 nm. It exhibits excellent performance with a high responsivity of 2.33 mA/W under 915 nm light irradiation, which is comparable to the best ones ever reported for polycrystalline MoS2 photodetectors