Exploring the Formation of Black Phosphorus Intercalation Compounds with Alkali Metals.

Black phosphorus intercalation compounds (BPICs) with alkali metals (namely: K and Na) have been synthesized in bulk by solid-state as well as vapor-phase reactions. By means of a combination of in situ X-ray diffraction, Raman spectroscopy, and DFT calculations the structural behavior of the BPICs at different intercalation stages has been demonstrated for the first time. Our results provide a glimpse into the very first steps of a new family of intercalation compounds, with a distinct behavior as compared to its graphite analogues (GICs), showing a remarkable structural complexity and a dynamic behavior.

Confined crystals of the smallest phase-change material.

The demand for high-density memory in tandem with limitations imposed by the minimum feature size of current storage devices has created a need for new materials that can store information in smaller volumes than currently possible. Successfully employed in commercial optical data storage products, phase-change materials, that can reversibly and rapidly change from an amorphous phase to a crystalline phase when subject to heating or cooling have been identified for the development of the next generation electronic memories. There are limitations to the miniaturization of these devices due to current synthesis and theoretical considerations that place a lower limit of 2 nm on the minimum bit size, below which the material does not transform in the structural phase. We show here that by using carbon nanotubes of less than 2 nm diameter as templates phase-change nanowires confined to their smallest conceivable scale are obtained. Contrary to previous experimental evidence and theoretical expectations, the nanowires are found to crystallize at this scale and display amorphous-to-crystalline phase changes, fulfilling an important prerequisite of a memory element. We show evidence for the smallest phase-change material, extending thus the size limit to explore phase-change memory devices at extreme scales.

Highly Luminescent TCNQ in Melamine.

Optical properties of molecules change drastically as a result of interactions with surrounding environments as observed in solutions, clusters, and aggregates. Here, we make 7,7,8,8-tetracyanoquinodimethane (TCNQ) highly luminescent by encapsulating it in crystalline melamine. Colored single crystals are synthesized by slow evaporation of aqueous tetrahydrofuran solutions of melamine and TCNQ. Single-crystal X-ray diffraction reveals the lattice structure of pure melamine, meaning that the color is of impurities. Both mass spectrometry and UV-vis spectroscopy combined with density-functional theory calculations elucidate that the impurity species are neutral TCNQ and its oxidation product, dicyano-p-toluoyl cyanide anion (DCTC-), whose concentrations in a melamine crystal can be controlled by adjusting the molar ratio between melamine and TCNQ in the precursor solution. Fluorescence excitation-emission wavelength mappings on the precursor solutions illustrate dominant emissions from DCTC- while the emission from TCNQ is quenched by the resonance energy transfer to DCTC-. On the contrary, TCNQ in crystalline melamine is a bright fluorophore whose emission wavelength centered at 450 nm with internal quantum yields as high as 19% and slow fluorescence lifetimes of about 2 ns. The method of encapsulating molecules into transparent melamine would make many other molecules fluorescent in solids.

Metamagnetism in a coordination polymer built of trimeric cobalt units and melamine.

A coordination polymer of linear trimeric cobalt units and melamine has been synthesized. The magnetic isotherms of violet coloured crystals as long as 400 µm show a field-induced transition in an external field of about 2 T at temperatures approximately below 2 K. It is addressed that by assuming the coexistent positive and negative exchange between the nearest-neighbour spins in the linear trimer, this metamagnetism can be interpreted as a transition from antiferromagnetic to ferromagnetic exchange within each trimeric spin cluster. Although weak inter-cluster or inter-chain exchange to yield a long-range magnetic order is another possible and often attributed origin of metamagnetism in low-dimensional spin systems, this study demonstrates the significance of the exchange flip within each cluster in clustered spin networks.

A Novel Molecular Assembly of a Cobalt-Sulfate Coordination Polymer and Melamine: A Manifestation of Magnetic Anisotropy.

A novel molecular assembly of a cobalt-sulfate coordination polymer and melamine is synthesized under acidic conditions. Bar-shaped pink monocrystals as long as 1 mm are found to align along magnetic field lines in the proximity of a strong magnet. Magnetometry shows no hysteresis at temperatures down to 2 K but instead magnetic anisotropy and antiferromagnetic coupling. X-ray diffraction on a single crystal reveals that the cobalt-sulfate chains are along the shortest lattice vector or the crystal's long axis. The crystal alignment along the magnetic flux can be attributed to single-ion anisotropy that results in longitudinal antiferromagnetic coupling along the chain. Both structurally and magnetically isotropic crystals of metal-organic hybrid materials can be highly useful as elemental components in magneto-optics.

Spontaneous emergence of long-range shape symmetry.

Self-organization of matter is essential for natural pattern formation, chemical synthesis, as well as modern material science. Here we show that isovolumetric reactions of a single organometallic precursor allow symmetry breaking events from iron nuclei to the creation of different symmetric carbon structures: microspheres, nanotubes, and mirrored spiraling microcones. A mathematical model, based on mass conservation and chemical composition, quantitatively explains the shape growth. The genesis of such could have significant implications for material design.

Room temperature synthesis of a luminescent crystalline Cu-BTC coordination polymer and metal-organic framework.

Synthesis of crystalline materials is elemental in the field of coordination chemistry towards optical applications. In the present work, coordination between copper and benzene-1,3,5-tricarboxylic acid (BTC) is controlled by adjusting the pH scale of the reaction mixture at room temperature to synthesize two crystalline structures: metal-organic framework HKUST-1 and coordination polymer Cu(BTC)·3H2O. The post-synthesis transformation of HKUST-1 into Cu(BTC)·3H2O is further demonstrated. Single crystals of both structures are studied by multi-laser Raman and luminescence spectroscopy. It is found that both crystals exhibit photoluminescence in the range of 700-900 cm-1 within the optical gap of the bulk materials, which can be associated with crystallographic defects. This work gives impetus for the synthesis of large metal-organic crystals based on which optical properties can be studied in depth.

Crystal engineering with copper and melamine.

Coordination complexes and polymers are central in inorganic and materials chemistry as a variety of metal centers and coordination geometries lead to a diverse range of interesting properties. Here, size and structure control of gem-like quality monocrystals is demonstrated at room temperature. Using the same set of precursors, the copper-to-melamine molar ratio is adjusted to synthesize either a novel coordination complex of dinuclear copper and melamine (Cu2M1), or a barely-studied coordination polymer of zigzag copper-chlorine chains (Cu4M1). Crystals of the former are dark green and square with a size up to 350 µm across. The latter is light green, octagonal, and as large as 5 mm across. The magnetic properties of both crystals reflect the low-dimensional arrangements of copper. The magnetic susceptibility of Cu2M1 is modelled with a spin-1/2 dimer, and that of Cu4M1 with a spin-1/2 one-dimensional Ising chain. Controlled synthesis of such quality magnetic crystals is a prerequisite for various magnetic and magneto-optical applications.

Direct observation of Tomonaga-Luttinger-liquid state in carbon nanotubes at low temperatures.

The electronic transport properties of conventional three-dimensional metals are successfully described by Fermi-liquid theory. But when the dimensionality of such a system is reduced to one, the Fermi-liquid state becomes unstable to Coulomb interactions, and the conduction electrons should instead behave according to Tomonaga-Luttinger-liquid (TLL) theory. Such a state reveals itself through interaction-dependent anomalous exponents in the correlation functions, density of states and momentum distribution of the electrons. Metallic single-walled carbon nanotubes (SWNTs) are considered to be ideal one-dimensional systems for realizing TLL states. Indeed, the results of transport measurements on metal-SWNT and SWNT-SWNT junctions have been attributed to the effects of tunnelling into or between TLLs, although there remains some ambiguity in these interpretations. Direct observations of the electronic states in SWNTs are therefore needed to resolve these uncertainties. Here we report angle-integrated photoemission measurements of SWNTs. Our results reveal an oscillation in the pi-electron density of states owing to one-dimensional van Hove singularities, confirming the one-dimensional nature of the valence band. The spectral function and intensities at the Fermi level both exhibit power-law behaviour (with almost identical exponents) in good agreement with theoretical predictions for the TLL state in SWNTs.

Host-Guest Interactions in Metal-Organic Frameworks Doped with Acceptor Molecules as Revealed by Resonance Raman Spectroscopy.

Metal-organic frameworks (MOFs) represent a class of porous materials whose properties can be altered by doping with redox-active molecules. Despite advanced properties such as enhanced electrical conduction that doped MOFs exhibit, understanding physical mechanisms remains challenging because of their heterogeneous nature hindering experimental observations of host-guest interactions. Here, we show a study of charge transfer between Mn-MOF-74 and electron acceptors, 7,7,8,8-tetracyanoquinodimethane (TCNQ) and XeF2, employing selective enhancement of Raman scattering of different moieties under various optical-resonance conditions. We identify Raman modes of molecular components and elucidate that TCNQ gets oxidized into dicyano-p-toluoyl cyanide (DCTC-) while XeF2 fluorinates the MOF upon infiltration. The framework's linker in both cases acts as an electron donor as deduced from blue shifts of the C-O stretching mode accompanied by the emergence of a quinone-like mode. This work demonstrates a generally applicable methodology for investigating charge transfer in various donor-acceptor systems by means of resonance Raman spectroscopy.

Exchange coupling in a frustrated trimetric molecular magnet reversed by a 1D nano-confinement.

Single-molecule magnets exhibit magnetic ordering due to exchange coupling between localized spin components that makes them primary candidates as nanometric spintronic elements. Here we manipulate exchange interactions within a single-molecule magnet by nanometric structural confinement, exemplified with single-wall carbon nanotubes that encapsulate trimetric nickel(ii) acetylacetonate hosting three frustrated spins. It is revealed from bulk and Ni 3d orbital magnetic susceptibility measurements that the carbon tubular confinement allows a unique one-dimensional arrangement of the trimer in which the nearest-neighbour exchange is reversed from ferromagnetic to antiferromagnetic, resulting in quenched frustration as well as the Pauli paramagnetism is enhanced. The exchange reversal and enhanced spin delocalisation demonstrate the means of mechanically and electrically manipulating molecular magnetism at the nanoscale for nano-mechatronics and spintronics.

Near-field infrared microscopy of nanometer-sized nickel clusters inside single-walled carbon nanotubes.

Nickel nanoclusters grown inside single-walled carbon nanotubes (SWCNT) were studied by infrared scattering-type scanning near-field optical microscopy (s-SNOM). The metal clusters give high local contrast enhancement in near-field phase maps caused by the excitation of free charge carriers. The experimental results are supported by calculations using the finite dipole model, approximating the clusters with elliptical nanoparticles. Compared to magnetic force microscopy, s-SNOM appears much more sensitive to detect metal clusters inside carbon nanotubes. We estimate that these clusters contain fewer than ≈700 Ni atoms.

Metal-Organic Framework Co-MOF-74-Based Host-Guest Composites for Resistive Gas Sensing.

Increasing demands in the field of sensing, especially for gas detection applications, require new approaches to chemical sensors. Metal-organic frameworks (MOFs) can play a decisive role owing to their outstanding performances regarding gas selectivity and sensitivity. The tetrathiafulvalene (TTF)-infiltrated MOF, Co-MOF-74, has been prepared following the host-guest concept and evaluated in resistive gas sensing. The Co-MOF-74-TTF crystal morphology has been characterized via X-ray diffraction and scanning electron microscopy, while the successful incorporation of TTF into the MOF has been validated via X-ray photoemission spectroscopy, thermogravimetric analysis, UV/vis, infrared (IR), and Raman investigations. We demonstrate a reduced yet ample uptake of CO2 in the pores of the new material by IR imaging and adsorption isotherms. The nanocomposite Co-MOF-74-TTF exhibits an increased electrical conductivity in comparison to Co-MOF-74 which can be influenced by gas adsorption from a surrounding atmosphere. This effect could be used for gas sensing.

Doping of metal-organic frameworks towards resistive sensing.

Coordination polymerization leads to various metal-organic frameworks (MOFs) with unique physical properties and chemical functionalities. One of the challenges towards their applications as porous materials is to make MOFs optimally conductive to be used as electronic components. Here, it is demonstrated that Co-MOF-74, a honeycomb nano-framework with one-dimensionally arranged cobalt atoms, advances its physical properties by accommodating tetracyanochinodimethan (TCNQ), an acceptor molecule. Strong intermolecular charge transfer reduces the optical band gap down to 1.5 eV of divalent TCNQ and enhances the electrical conduction, which allows the MOF to be utilized for resistive gas- and photo-sensing. The results provide insight into the electronic interactions in doped MOFs and pave the way for their electronic applications.

Chirality-dependent growth of single-wall carbon nanotubes as revealed inside nano-test tubes.

Growth dynamics of single-wall carbon nanotubes (SWCNTs) have been studied with nickelocene as a precursor encapsulated in the interior of template SWCNTs. By means of multi-laser Raman spectroscopy, growth curves of nine different SWCNTs, (8,8), (12,3), (13,1), (9,6), (10,4), (11,2), (11,1), (9,3) and (9,2), have been determined upon in situ annealing at various temperatures. The data reveal that the nanotubes grow through fast and slow reaction pathways with high and low activation energies, respectively. While the activation energy of the slow growth is independent of the nanotube's chiral vector, that of the fast growth exhibits a monotonic increase as the tube diameter reduces from ∼1.1 down to 0.8 nm and no dependency on the chiral angle, which can be attributed to the size-dependent properties of catalyst clusters. The chirality dependent catalytic growth properties exploited in this study provide the basis for a large-scale synthesis of single-chiral vector SWCNTs.

Doping of single-walled carbon nanotubes controlled via chemical transformation of encapsulated nickelocene.

Controlled doping of carbon nanotubes is elemental for their electronic applications. Here we report an approach to tune the polarity and degree of doping of single-walled carbon nanotubes via filling with nickelocene followed by encapsulated reactions. Using Raman, photoemission spectroscopy and transmission electron microscopy, we show that nickelocene molecules transform into nickel carbides, nickel and inner carbon nanotubes with reaction temperatures as low as 250 °C. The doping efficiency is determined for each chemical component. Synchronous charge transfer among the molecular components allows bipolar doping of the carbon nanotubes to be achieved in a broad range of ±0.0012 e(-) per carbon.

Nickel clusters embedded in carbon nanotubes as high performance magnets.

Ensembles of fcc nickel nanowires have been synthesized with defined mean sizes in the interior of single-wall carbon nanotubes. The method allows the intrinsic nature of single-domain magnets to emerge with large coercivity as their size becomes as small as the exchange length of nickel. By means of X-ray magnetic circular dichroism we probe electronic interactions at nickel-carbon interfaces where nickel exhibit no hysteresis and size-dependent spin magnetic moment. A manifestation of the interacting two subsystems on a bulk scale is traced in the nanotube's magnetoresistance as explained within the framework of weak localization.

Microscopic insight into the bilateral formation of carbon spirals from a symmetric iron core.

Mirrored carbon-spirals have been produced from pressured ferrocene via the bilateral extrusion of the spiral pairs from an iron core. A parametric plot of the surface geometry displays the fractal growth of the conical helix made with the logarithmic spiral. Electron microscopy studies show the core is a crystalline cementite which grows and transforms its shape from spherical to biconical as it extrudes two spiralling carbon arms. In a cross section along the arms we observe graphitic flakes arranged in a herringbone structure, normal to which defects propagate. Local-wave-pattern analysis reveals nanoscale defect patterns of two-fold symmetry around the core. The data suggest that the bilateral growth originates from a globular cementite crystal with molten surfaces and the nano-defects shape emerging hexagonal carbon into a fractal structure. Understanding and knowledge obtained provide a basis for the controlled production of advanced carbon materials with designed geometries.

Hybrid carbon nanotube networks as efficient hole extraction layers for organic photovoltaics.

Transparent, highly percolated networks of regioregular poly(3-hexylthiophene) (rr-P3HT)-wrapped semiconducting single-walled carbon nanotubes (s-SWNTs) are deposited, and the charge transfer processes of these nanohybrids are studied using spectroscopic and electrical measurements. The data disclose hole doping of s-SWNTs by the polymer, challenging the prevalent electron-doping hypothesis. Through controlled fabrication, high- to low-density nanohybrid networks are achieved, with low-density hybrid carbon nanotube networks tested as hole transport layers (HTLs) for bulk heterojunction (BHJ) organic photovoltaics (OPV). OPVs incorporating these rr-P3HT/s-SWNT networks as the HTL demonstrate the best large area (70 mm(2)) carbon nanotube incorporated organic solar cells to date with a power conversion efficiency of 7.6%. This signifies the strong capability of nanohybrids as an efficient hole extraction layer, and we believe that dense nanohybrid networks have the potential to replace expensive and material scarce inorganic transparent electrodes in large area electronics toward the realization of low-cost flexible electronics.