A few-layer covalent network of fullerenes.

The two natural allotropes of carbon, diamond and graphite, are extended networks of sp3-hybridized and sp2-hybridized atoms, respectively1. By mixing different hybridizations and geometries of carbon, one could conceptually construct countless synthetic allotropes. Here we introduce graphullerene, a two-dimensional crystalline polymer of C60 that bridges the gulf between molecular and extended carbon materials. Its constituent fullerene subunits arrange hexagonally in a covalently interconnected molecular sheet. We report charge-neutral, purely carbon-based macroscopic crystals that are large enough to be mechanically exfoliated to produce molecularly thin flakes with clean interfaces-a critical requirement for the creation of heterostructures and optoelectronic devices2. The synthesis entails growing single crystals of layered polymeric (Mg4C60)∞ by chemical vapour transport and subsequently removing the magnesium with dilute acid. We explore the thermal conductivity of this material and find it to be much higher than that of molecular C60, which is a consequence of the in-plane covalent bonding. Furthermore, imaging few-layer graphullerene flakes using transmission electron microscopy and near-field nano-photoluminescence spectroscopy reveals the existence of moiré-like superlattices3. More broadly, the synthesis of extended carbon structures by polymerization of molecular precursors charts a clear path to the systematic design of materials for the construction of two-dimensional heterostructures with tunable optoelectronic properties.

Indefinite and bidirectional near-infrared nanocrystal photoswitching.

Materials whose luminescence can be switched by optical stimulation drive technologies ranging from superresolution imaging1-4, nanophotonics5, and optical data storage6,7, to targeted pharmacology, optogenetics, and chemical reactivity8. These photoswitchable probes, including organic fluorophores and proteins, can be prone to photodegradation and often operate in the ultraviolet or visible spectral regions. Colloidal inorganic nanoparticles6,9 can offer improved stability, but the ability to switch emission bidirectionally, particularly with near-infrared (NIR) light, has not, to our knowledge, been reported in such systems. Here, we present two-way, NIR photoswitching of avalanching nanoparticles (ANPs), showing full optical control of upconverted emission using phototriggers in the NIR-I and NIR-II spectral regions useful for subsurface imaging. Employing single-step photodarkening10-13 and photobrightening12,14-16, we demonstrate indefinite photoswitching of individual nanoparticles (more than 1,000 cycles over 7 h) in ambient or aqueous conditions without measurable photodegradation. Critical steps of the photoswitching mechanism are elucidated by modelling and by measuring the photon avalanche properties of single ANPs in both bright and dark states. Unlimited, reversible photoswitching of ANPs enables indefinitely rewritable two-dimensional and three-dimensional multilevel optical patterning of ANPs, as well as optical nanoscopy with sub-Å localization superresolution that allows us to distinguish individual ANPs within tightly packed clusters.

Electronic Level Structure of Novel Guanine Octuplex DNA Single Molecules.

Throughout the past few decades, guanine quadruplex DNA structures have attracted much interest both from a fundamental material science perspective and from a technologically oriented perspective. Novel guanine octuplex DNA, formed from coiled quadruplex DNA, was recently discovered as a stable and rigid DNA-based nanostructure. A detailed electronic structure study of this new nanomaterial, performed by scanning tunneling spectroscopy on a subsingle-molecule level at cryogenic temperature, is presented herein. The electronic levels and lower energy gap of guanine octuplex DNA compared to quadruplex DNA dictate higher transverse conductivity through guanine octads than through guanine tetrads.

Electronic Level Structure of Silver-Intercalated Cytosine Nanowires.

Metal-mediated base-paired DNA has long been investigated for basic scientific pursuit and for nanoelectronics purposes. Particularly attractive in these domains is the Ag+-intercalated polycytosine DNA duplex. Extensive studies of this molecule have led to our current understanding of its self-assembly properties, high thermodynamic and structural stability, and high longitudinal conductivity. However, a high-resolution morphological characterization of long Ag+-intercalated polycytosine DNA has hitherto not been carried out. Furthermore, the electronic level structure of this molecule has not been studied before. Here we present a scanning tunneling microscopy and spectroscopy study of this intriguing nanowire. Its temperature-independent morphological and electronic properties suggest substantial stability, while its emergent electronic levels and energy gap provide the basis for its high conductivity.

Temperature Dependence of the STM Morphology and Electronic Level Structure of Silver-Containing DNA.

Understanding the effect of external conditions, temperature in particular, on novel nanomaterials is of great significance. The powerful ability of scanning tunneling microscopy (STM) to characterize topography and electronic levels on a single molecule scale is utilized herein to characterize individual silver-containing poly(dG)-poly(dC) DNA molecules, at different temperatures. These measurements indicate that the molecule is a truly hybrid metal-organic nanomaterial with electronic states originating from both the DNA and the embedded silver. The temperature dependence of this density of states (DOS) leads to the temperature dependent STM apparent height of the molecule-a phenomenon that has not been observed before for other complex nanostructures.

Formation of Novel Octuplex DNA Molecules from Guanine Quadruplexes.

Guanine quadruplex (G4)-DNA structures have sparked the interest of many scientists due to their important biological roles and their potential use in molecular nanoelectronics and nanotechnology. The high guanine content in G4-DNA endows it with mechanical stability, robustness, and improved charge transport properties-attractive attributes for a molecular nanowire. The self-driven formation of a novel G4-DNA-based nanostructure, coined guanine octuplex (G8)-DNA, is reported herein. Atomic force microscopy and scanning tunneling microscopy characterization of this molecule reveal its organized coiled-coil structure, which is found to be stable under different temperatures and surrounding conditions. G8-DNA exhibits enhanced stiffness, mechanical and thermodynamic stability when compared to its parent G4-DNA. These, along with its high guanine content, make G8-DNA a compelling new molecule, and a highly prospective candidate for molecular nanoelectronics.

Scanning Tunneling Microscopy and Spectroscopy of Novel Silver-Containing DNA Molecules.

The quest for a suitable molecule to pave the way to molecular nanoelectronics has been met with obstacles for over a decade. Candidate molecules such as carbon nanotubes lack the appealing trait of self-assembly, while DNA seems to lack the desirable feature of conductivity. Silver-containing poly(dG)-poly(dC) DNA (E-DNA) molecules have recently been reported as promising candidates for molecular electronics, owing to the selectivity of their metallization, their thin and uniform structure, their resistance to deformation, and their maximum possible high conductivity. Ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) of E-DNA presents an elaborate high-resolution morphology characterization of these unique molecules, along with a detailed depiction of their electronic level structure. The energy levels found for E-DNA indicate a novel truly hybrid metal-molecule structure, potentially more conductive than other DNA-based alternatives.

Advances in Synthesis and Measurement of Charge Transport in DNA-Based Derivatives.

Charge transport through molecular structures is interesting both scientifically and technologically. To date, DNA is the only type of polymer that transports significant currents over distances of more than a few nanometers in individual molecules. For molecular electronics, DNA derivatives are by far more promising than native DNA due to their improved charge-transport properties. Here, the synthesis of several unique DNA derivatives along with electrical characterization and theoretical models is surveyed. The derivatives include double stranded poly(G)-poly(C) DNA molecules, four stranded G4-DNA, metal-DNA hybrid molecular wires, and other DNA molecules that are modified either at the bases or at the backbone. The electrical characteristics of these nanostructures, studied experimentally by electrostatic force microscopy, conductive atomic force microscopy, and scanning tunneling microscopy and spectroscopy, are reviewed.

DNA-Metalization: Synthesis and Properties of Novel Silver-Containing DNA Molecules (Adv. Mater. 24/2016).

D. Porath, A. Kotlyar, and co-workers transform DNA to a conducting material by metalization through coating or chemical modifications, as described on page 4839. Specific and reversible metalization of poly(dG)-poly(dC) DNA by migration of atoms from silver nanoparticles to the DNA is demonstrated. As the transformation occurs gradually, novel, truly hybrid molecular structures are obtained, paving the way to their usage as nanowires in programmable molecular electronic devices and circuits.

Synthesis and Properties of Novel Silver-Containing DNA Molecules.

Migration of silver atoms from silver nano-particles selectively to a double-stranded poly(dG)-poly(dC) polymer leads to metallization of the DNA. As a result the DNA molecules become shorter and thicker (higher), as evident from the atomic force microscopy imaging analysis. The metalized molecules can be detected by transmission and scanning electron microscopy in contrast to the initial non-metalized ones.