No abstract

Polaritons in two-dimensional (2D) materials have shown their unique capabilities to concentrate light into deep subwavelength scales. Precise control of the excitation and propagation of 2D polaritons has remained a central challenge for future on-chip nanophotonic devices and circuits. To solve this issue, we exploit Cherenkov radiation, a classic physical phenomenon that occurs when a charged particle moves at a velocity greater than the phase velocity of light in that medium, in low-dimensional material heterostructures. Here, we report an experimental observation of Cherenkov phonon polariton wakes emitted by superluminal one-dimensional plasmon polaritons in a silver nanowire and hexagonal boron nitride heterostructure using near-field infrared nanoscopy. The observed Cherenkov radiation direction and radiation rate exhibit large tunability through varying the excitation frequency. Such tunable Cherenkov phonon polaritons provide opportunities for novel deep subwavelength-scale manipulation of light and nanoscale control of energy flow in low-dimensional material heterostructures.

We herein report on the guest-responsive hierarchical self-assembly of dissymmetric cage DC-1 with intrinsic dipole along its C 3 -symmetric axis. DC-1 molecules self-assemble into supramolecular columns with the molecular dipoles aligned along the columnar axis. Mediated by different host-guest interactions of ethyl acetate (EtOAc) and chloroform (CHCl 3 ), the columns are arranged in an anti-parallel and parallel fashion, respectively, leading to a switch of centrosymmetric and non-centrosymmetric superstructures. The symmetry of the molecular packing of DC-1 molecules of the non-centrosymmetric crystalline phase is therefore broken, producing a supramolecular ferroelectric with second-harmonic generation and piezoelectric responses. We demonstrate that cages could serve as promising building blocks for the discovery of supramolecular materials with emergent functions and properties, including but not limited to, organic ferroelectrics and non-linear optics.

Graphene nanoribbons (GNRs) with widths of a few nanometers are promising candidates for future nanoelectronic applications due to their structurally tunable bandgaps, ultrahigh carrier mobilities, and exceptional stability. However, the direct growth of micrometer‐long GNRs on insulating substrates, which is essential for the fabrication of nanoelectronic devices, remains an immense challenge. Here, the epitaxial growth of GNRs on an insulating hexagonal boron nitride (h‐BN) substrate through nanoparticle‐catalyzed chemical vapor deposition is reported. Ultranarrow GNRs with lengths of up to 10 µm are synthesized. Remarkably, the as‐grown GNRs are crystallographically aligned with the h‐BN substrate, forming 1D moiré superlattices. Scanning tunneling microscopy reveals an average width of 2 nm and a typical bandgap of ≈1 eV for similar GNRs grown on conducting graphite substrates. Fully atomistic computational simulations support the experimental results and reveal a competition between the formation of GNRs and carbon nanotubes during the nucleation stage, and van der Waals sliding of the GNRs on the h‐BN substrate throughout the growth stage. This study provides a scalable, single‐step method for growing micrometer‐long narrow GNRs on insulating substrates, thus opening a route to explore the performance of high‐quality GNR devices and the fundamental physics of 1D moiré superlattices.

No abstract

Much of the richness and variety of physics today are based on coupling phenomena where multiple interacting systems hybridize into new ones with completely distinct attributes. Recent development in building...

Polaritons in low‐dimensional materials have shown their unique capabilities to concentrate light at the deep subwavelength scale. They behave quite differently from those in conventional metals. On the one hand, the strongly confined polaritons exhibit a large tunability in wavelength following different scaling behaviors in different systems. On the other hand, they show novel reflection behaviors when they encounter topographic boundaries or electronic discontinuities. Here, recent progress on the scaling and reflection behaviors of strongly confined polaritons in three representative low‐dimensional material systems is reviewed. It is shown that the polariton wavelength changes sensitively with carrier density, thickness, and the number of conducting channels for graphene, hexagonal boron nitride, and carbon nanotubes. Also, it is demonstrated how polaritons reflect in low‐dimensional materials when encountering edges, corrugations, domain walls, strain regions, etc.

Graphene nanoribbons (GNRs) and carbon nanotubes (CNTs), two representative one-dimensional (1D) graphitic materials, have attracted tremendous research interests due to their promising applications for future high-performance nanoelectronics. Although various methods have been developed for fabrication of GNRs or CNTs, a unified method allowing controllable synthesis of both of them, as well as their heterojunctions, which could largely benefit their nano-electronic applications, is still lacking. Here, we report on a generic growth of 1D carbon using nanoparticles catalyzed chemical vapor deposition (CVD) on atomically flat hexagonal boron nitride (h-BN) substrates. Relative ratio of the yielded GNRs and CNTs is able to be arbitrarily tuned by varying the growth temperature or feeding gas pressures. The tunability of the generic growth is quantitatively explained by a competing nucleation theory: nucleation into either GNRs or CNTs by the catalysts is determined by the free energy of their formation, which is controlled by the growth conditions. Under the guidance of the theory, we further realized growth of GNR/CNT intramolecular junctions through changing H2 partial pressure during a single growth process. Our study provides not only a universal and controllable method for growing 1D carbon nanostructures, but also a deep understanding of their growth mechanism, which would largely benefit future carbon-based electronics and optoelectronics.