The eukaryotic exosome is a multi-subunit complex typically composed of a catalytically inactive core and the Rrp44 protein, which contains 3’ to 5’ exo- and endo-ribonuclease activities. RNA substrates have been shown to be recruited through the core to reach Rrp44's exoribonuclease (EXO) site. Using single particle electron microscopy and biochemical analysis, we provide visual evidence that two distinct substrate recruitment pathways exist. In the through-core route, channeling of the single stranded substrates from the core to Rrp44 induces a characteristic conformational change in Rrp44. In the alternative direct-access route, this conformational change does not take place and the RNA substrate is visualized to avoid the core and enter Rrp44's EXO site directly. Our results provide mechanistic explanations for several RNA processing scenarios by the eukaryotic exosome and indicate substrate specific modes of degradation by this complex.

Owing to a large diamagnetism, graphene flakes can respond and be aligned to magnetic field like a ferromagnetic material. Aligned graphene flakes exhibit emergent properties approaching single-layer graphene. Anisotropic optical properties also give rise to a magnetic writing board using graphene suspension and a bar magnet as a pen. This simple alignment technique opens up enormous applications of graphene.

The eukaryotic multi-subunit RNA exosome complex plays crucial roles in 3′-to-5′ RNA processing and decay. Rrp6 and Ski7 are the major cofactors for the nuclear and cytoplasmic exosomes, respectively. In the cytoplasm, Ski7 helps the exosome to target mRNAs for degradation and turnover via a through-core pathway. However, the interaction between Ski7 and the exosome complex has remained unclear. The transaction of RNA substrates within the exosome is also elusive. In this work, we used single-particle cryo-electron microscopy to solve the structures of the Ski7-exosome complex in RNA-free and RNA-bound forms at resolutions of 4.2 Å and 5.8 Å, respectively. These structures reveal that the N-terminal domain of Ski7 adopts a structural arrangement and interacts with the exosome in a similar fashion to the C-terminal domain of nuclear Rrp6. Further structural analysis of exosomes with RNA substrates harboring 3′ overhangs of different length suggests a switch mechanism of RNA-induced exosome activation in the through-core pathway of RNA processing.

Monitoring and controlling the neutral and charged excitons (trions) in two-dimensional (2D) materials are essential for the development of high-performance devices. However, nanoscale control is challenging because of diffraction-limited spatial resolution of conventional far-field techniques. Here, we extend the classical tip-enhanced photoluminescence based on tip-substrate nanocavity to quantum regime and demonstrate controlled nano-optical imaging, namely, tip-enhanced quantum plasmonics. In addition to improving the spatial resolution, we use the scanning probe to control the optoelectronic response of monolayer WS2 by varying the neutral/charged exciton ratio via charge tunneling in Au-Ag picocavity. We observe trion “hot spots” generated by varying the picometer-scale probe-sample distance and show the effects of weak and strong coupling, which depend on the spatial location. Our experimental results are in agreement with simulations and open an unprecedented view of a new range of quantum plasmonic phenomena with 2D materials that will help to design new quantum optoelectronic devices.

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The monolithic integration of electronics and photonics has attracted enormous attention due to its potential applications. A major challenge to this integration is the identification of suitable materials that can emit and absorb light at the same wavelength. In this paper we utilize unique excitonic transitions in WS 2 monolayers and show that WS 2 exhibits a perfect overlap between its absorption and photoluminescence spectra. By coupling WS 2 to Ag nanowires, we then show that WS 2 monolayers are able to excite and absorb surface plasmons of Ag nanowires at the same wavelength of exciton photoluminescence. This resonant absorption by WS 2 is distinguished from that of the ohmic propagation loss of silver nanowires, resulting in a short propagation length of surface plasmons. Our demonstration of resonant optical generation and detection of surface plasmons enables nanoscale optical communication and paves the way for on-chip electronic−photonic integrated circuits.

Planar alignment of disc-like nanomaterials is required to transfer their superior anisotropic properties from microscopic individual structures to macroscopic collective assemblies. However, such alignment by electrical or magnetic field is challenging due to their additional degrees of orientational freedom compared to that of rod-like nanostructures. Here, the realization of planar alignment of suspended graphene sheets using a rotating magnetic field produced by a pair of small NdFeB magnets and subsequent demonstration of high optical anisotropy and potential novel device applications is reported. Compared to partially aligned sheets with a static magnetic field, planar aligned graphene suspensions exhibit a near-perfect order parameter, much higher birefringence and anisotropic absorption/transmission. A unique feature of discotic nanomaterial assemblies is that the observed order parameter and optical property can vary from isotropic to partial and complete alignment depending on the experimental configuration. By immobilizing and patterning aligned graphene in a UV-curable polymer resin, we further demonstrated an all-graphene permanent display, which exhibits wide-angle, high dark-bright contrast in either transmission or reflection mode without any polarizing optics. The ability to control and pattern graphene orientation in all three dimensions opens up new exploration and broad device applications of graphene.

This paper studies graphene levitation and orientation control using a magnetic field. The torques in all three spatial directions induced by diamagnetic forces are used to predict stable conditions for different shapes of millimeter-sized graphite plates. We find that graphite plates, in regular polygon shapes with an even number of sides, will be levitated in a stable manner above four interleaved permanent magnets. In addition, the orientation of micrometer-sized graphene flakes near a permanent magnet is studied in both air and liquid environments. Using these analyses, we are able to simulate optical transmission and reflection on a writing board and thereby reveal potential applications using this technology for display screens. Understanding the control of graphene flake orientation will lead to the discovery of future applications using graphene flakes.