Journal: Nano letters
Metal halides perovskites, such as hybrid organic-inorganic CH3NH3PbI3, are newcomer optoelectronic materials that have attracted enormous attention as solution-deposited absorbing layers in solar cells with power conversion efficiencies reaching 20%. Herein we demonstrate a new avenue for halide perovskties by designing perovskite-based quantum dot materials. We have synthesized monodisperse, colloidal nanocubes (4-15 nm edge lengths) of fully inorganic cesium lead halide perovskites (CsPbX3, X=Cl, Br, I or mixed halide systems Cl/Br and Br/I) using inexpensive commercial precursors. Through compositional modulations and quantum size-effects, the bandgap energies and emission spectra are readily tunable over the entire visible spectral region of 410-700 nm. The photoluminescence of CsPbX3 nanocrystals is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, high quantum yields of up to 90% and radiative lifetimes in the range of 4-29 ns. The compelling combination of enhanced optical properties and chemical robustness makes CsPbX3 nanocrystals appealing for optoelectronic applications, particularly for blue and green spectral regions (410-530 nm), where typical metal chalcogenide-based quantum dots suffer from photodegradation.
The electrostatic confinement of massless charge carriers is hampered by Klein tunneling. Circumventing this problem in graphene mainly relies on carving out nanostructures or applying electric displacement fields to open a band gap in bilayer graphene. So far, these approaches suffer from edge disorder or insufficiently controlled localization of electrons. Here we realize an alternative strategy in monolayer graphene, by combining a homogeneous magnetic field and electrostatic confinement. Using the tip of a scanning tunneling microscope, we induce a confining potential in the Landau gaps of bulk graphene without the need for physical edges. Gating the localized states towards the Fermi energy leads to regular charging sequences with more than 40 Coulomb peaks exhibiting typical addition energies of 7-20meV. Orbital splittings of 4-10meV and a valley splitting of about 3meV for the first orbital state can be deduced. These experimental observations are quantitatively reproduced by tight binding calculations, which include the interactions of the graphene with the aligned hexagonal boron nitride substrate. The demonstrated confinement approach appears suitable to create quantum dots with well-defined wave function properties beyond the reach of traditional techniques.
Direct imaging becomes important when the knowledge at few/single molecule level is requested and where the diffraction doesn’t allow to get structural and functional information. Here we report on the direct imaging of double strand (ds) λ-DNA in the A conformation, obtained by combining a novel sample preparation method based on super hydrophobic DNA molecules self aggregation process with Transmission Electron Microscopy (TEM). The experimental breakthrough is the production of robust and highly ordered paired DNA nanofibers that allowed its direct TEM imaging and the double helix structure revealing.
Bacterial microcompartments (BMCs) are proteinaceous organelles widespread among bacterial phyla. They compartmentalize metabolic enzymes within a selectively permeable shell, and play important roles in CO2 fixation, pathogenesis and microbial ecology. Here we combine X-ray crystallography and high-speed atomic force microscopy to characterize, at molecular resolution, the structure and dynamics of BMC shell facet assembly. Our results show that pre-formed hexamers assemble into uniformly oriented shell layers, a single hexamer thick. We also observe the dynamic process of shell facet assembly. Shell hexamers can dissociate from and incorporate into assembled sheets, indicating a flexible intermolecular interaction. Furthermore, we demonstrate that the self-assembly and dynamics of shell proteins are governed by specific contacts at the interfaces of shell proteins. Our study provides novel insights into the formation, interactions and dynamics of BMC shell facets, which are essential for the design and engineering of self-assembled biological nanoreactors and scaffolds based on BMC architectures.
Carbon nanofibers, CNFs, due to their superior strength, conductivity, flexibility and durability have great potential as a material resource, but still have limited use due to the cost intensive complexities of their synthesis. Herein, we report the high-yield and scale-able electrolytic conversion of atmospheric CO2 dissolved in molten carbonates into CNFs. It is demonstrated that the conversion of CO2 → CCNF + O2 can be driven by efficient solar, as well as conventional, energy at inexpensive steel or nickel electrodes. The structure is tuned by controlling the electrolysis conditions, such as the addition of trace transition metals to act as CNF nucleation sites, the addition of zinc as an initiator and the control of current density. A less expensive source of CNFs will facilitate its adoption as a societal resource, and using carbon dioxide as a reactant to generate a value added product such as CNFs provides impetus to consume this greenhouse gas to mitigate climate change.
Silkworm silk is gaining significant attention from both the textile industry and research society because of its outstanding mechanical properties and lustrous appearance. The possibility of creating tougher silks attracts particular research interest. Carbon nanotubes and graphene are widely studied for their use as reinforcement. In this work, we report mechanically enhanced silk directly collected by feeding Bombyx mori larval silkworms with single-walled carbon nanotubes (SWNTs) and graphene. We found that parts of the fed carbon nanomaterials were incorporated into the as-spun silk fibers, whereas the others went into the excrement of silkworms. Spectroscopy study indicated that nanocarbon additions hindered the conformation transition of silk fibroin from random coil/α-helix to β-sheet, which may contribute to increased breaking elongation and toughness modules. We further investigated the pyrolysis of modified silk, and a highly developed graphitic structure with obviously enhanced electrical conductivity was obtained through the introduction SWNTs and graphene. The successful generation of these SWNT- or graphene- embedded silks by in vivo feeding is expected to open up possibilities for the large-scale production of high strength silk fibers.
Plasmonic lasers exploit strong electromagnetic field confinement at dimensions well below the diffraction limit. However, lasing from an electromagnetic hot spot supported by discrete, coupled metal nanoparticles (NPs) has not been explicitly demonstrated to date. We present a new design for a room-temperature nanolaser based on three-dimensional (3D) Au bowtie NPs supported by an organic gain material. The extreme field compression, and thus ultrasmall mode volume, within the bowtie gaps produced laser oscillations at the localized plasmon resonance gap mode of the 3D bowties. Transient absorption measurements confirmed ultrafast resonant energy transfer between photoexcited dye molecules and gap plasmons on the picosecond time scale. These plasmonic nanolasers are anticipated to be readily integrated into Si-based photonic devices, all-optical circuits, and nanoscale biosensors.
The direct manipulation of individual atoms in materials using scanning probe microscopy has been a seminal achievement of nanotechnology. Recent advances in imaging resolution and sample stability have made scanning transmission electron microscopy a promising alternative for single-atom manipulation of covalently bound materials. Pioneering experiments using an atomically focused electron beam have demonstrated the directed movement of silicon atoms over a handful of sites within the graphene lattice. Here, we achieve a much greater degree of control, allowing us to precisely move silicon impurities along an extended path, circulating a single hexagon, or back and forth between the two graphene sublattices. Even with manual operation, our manipulation rate is already comparable to the state-of-the-art in any atomically precise technique. We further explore the influence of electron energy on the manipulation rate, supported by improved theoretical modeling taking into account the vibrations of atoms near the impurities, and implement feedback to detect manipulation events in real time. In addition to atomic-level engineering of its structure and properties, graphene also provides an excellent platform for refining the accuracy of quantitative models and for the development of automated manipulation.
Structural DNA nanotechnology, and specifically scaffolded DNA origami, is rapidly developing as a versatile method for bottom-up fabrication of novel nanometer-scale materials and devices. However, lengths of conventional single-stranded scaffolds, e.g., 7,249-nucleotide circular genomic DNA from the M13mp18 phage, limit the scales of these uniquely addressable structures. Additionally, increasing DNA origami size generates the cost burden of increased staple-strand synthesis. We addressed this two-fold problem by developing the following methods: 1) production of the largest to-date biologically-derived single-stranded scaffold using a lambda/M13 hybrid virus to produce a 51,466-nucleotide DNA in a circular, single-stranded form and 2) inexpensive DNA synthesis via an inkjet-printing process on a chip embossed with functionalized micro-pillars made from cyclic olefin copolymer. We have experimentally demonstrated very efficient assembly of a 51-kilobasepair origami from the lambda/M13 hybrid scaffold folded by chip-derived staple strands. In addition, we have demonstrated two-dimensional, asymmetric origami sheets with controlled global curvature such that they land on a substrate in predictable orientations that have been verified by atomic force microscopy.
Semiconductor heterostructures form the cornerstone of many electronic and optoelectronic devices and are traditionally fabricated using epitaxial growth techniques. More recently, heterostructures have also been obtained by vertical stacking of two-dimensional crystals, such as graphene and related two-dimensional materials. These layered designer materials are held together by van der Waals forces and contain atomically sharp interfaces. Here, we report on a type-II van der Waals heterojunction made of molybdenum disulfide and tungsten diselenide monolayers. The junction is electrically tunable and under appropriate gate bias, an atomically thin diode is realized. Upon optical illumination, charge transfer occurs across the planar interface and the device exhibits a photovoltaic effect. Advances in large-scale production of two-dimensional crystals could thus lead to a new photovoltaic solar technology.