Linear and Helical Cesium Iodide Atomic Chains in Ultranarrow Single-Walled Carbon Nanotubes: Impact on Optical Properties.

One-dimensional (1D) atomic chains of CsI were previously reported in double-walled carbon nanotubes with ∼0.8 nm inner diameter. Here, we demonstrate that, while 1D CsI chains form within narrow ∼0.73 nm diameter single-walled carbon nanotubes (SWCNTs), wider SWCNT tubules (∼0.8-1.1 nm) promote the formation of helical chains of CsI 2 × 1 atoms in cross-section. These CsI helices create complementary oval distortions in encapsulating SWCNTs with highly strained helices formed from strained Cs2I2 parallelogram units in narrow tubes to lower strain Cs2I2 units in wider tubes. The observed structural changes and charge distribution were analyzed by density-functional theory and Bader analysis. CsI chains also produce conformation-selective changes to the electronic structure and optical properties of the encapsulating tubules. The observed defects are an interesting variation from defects commonly observed in alkali halides as these are normally associated with the Schottky and Frenkel type. The energetics of CsI 2 × 1 helix formation in SWCNTs suggests how these could be controllably formed.

Electronic Structure Control of Sub-nanometer 1D SnTe via Nanostructuring within Single-Walled Carbon Nanotubes.

Nanostructuring, e. g., reduction of dimensionality in materials, offers a viable route toward regulation of materials electronic and hence functional properties. Here, we present the extreme case of nanostructuring, exploiting the capillarity of single-walled carbon nanotubes (SWCNTs) for the synthesis of the smallest possible SnTe nanowires with cross sections as thin as a single atom column. We demonstrate that by choosing the appropriate diameter of a template SWCNT, we can manipulate the structure of the quasi-one-dimensional (1D) SnTe to design electronic behavior. From first principles, we predict the structural re-formations that SnTe undergoes in varying encapsulations and confront the prediction with TEM imagery. To further illustrate the control of physical properties by nanostructuring, we study the evolution of transport properties in a homologous series of models of synthesized and isolated SnTe nanowires varying only in morphology and atomic layer thickness. This extreme scaling is predicted to significantly enhance thermoelectric performance of SnTe, offering a prospect for further experimental studies and future applications.

Single-Atom Scale Structural Selectivity in Te Nanowires Encapsulated Inside Ultranarrow, Single-Walled Carbon Nanotubes.

Extreme nanowires (ENs) represent the ultimate class of crystals: They are the smallest possible periodic materials. With atom-wide motifs repeated in one dimension (1D), they offer a privileged perspective into the physics and chemistry of low-dimensional systems. Single-walled carbon nanotubes (SWCNTs) provide ideal environments for the creation of such materials. Here we present a comprehensive study of Te ENs encapsulated inside ultranarrow SWCNTs with diameters between 0.7 nm and 1.1 nm. We combine state-of-the-art imaging techniques and 1D-adapted ab initio structure prediction to treat both confinement and periodicity effects. The studied Te ENs adopt a variety of structures, exhibiting a true 1D realization of a Peierls structural distortion and transition from metallic to insulating behavior as a function of encapsulating diameter. We analyze the mechanical stability of the encapsulated ENs and show that nanoconfinement is not only a useful means to produce ENs but also may actually be necessary, in some cases, to prevent them from disintegrating. The ability to control functional properties of these ENs with confinement has numerous applications in future device technologies, and we anticipate that our study will set the basic paradigm to be adopted in the characterization and understanding of such systems.

A DFT study of halogen atoms adsorbed on graphene layers.

In this work, ab initio density functional theory calculations were performed in order to study the structural and electronic properties of halogens (X = fluorine, chlorine, bromine or iodine) that were deposited on both sides of graphene single layers (X-graphene). The adsorption of these atoms on only one side of the layer with hydrogen atoms adsorbed on the other was also considered (H,X-graphene). The results indicate that the F-C bond in the F-graphene system causes an sp(2) to sp(3) transition of the carbon orbitals, and similar effects seem to occur in the H,X-graphene systems. For the other cases, two configurations are found: bonded (B) and non-bonded (NB). For the B configuration, the structural arrangement of the atoms was similar to F-graphene and H-graphene (graphane), although the electronic structures present some differences. In the NB configuration, the interaction between the adsorbed atoms and the graphene layer seems to be essentially of the van der Waals type. In these cases, the original shape of the graphene layer presents only small deviations from the pristine form and the adsorbed atoms reach equilibrium far from the sheet. The F-graphene structure has a direct bandgap of approximately 3.16 eV at the Γ point, which is a value that is close to the value of 3.50 eV that was found for graphane. The Cl-graphene (B configuration), H,F-graphene and H,Cl-graphene systems have smaller bandgap values. All of the other systems present metallic behaviours. Energy calculations indicate the possible stability of these X-graphene layers, although some considerations about the possibility of spontaneous formation have to be taken into account.

Adsorption of monovalent metal atoms on graphene: a theoretical approach.

This work investigates, using first-principles calculations, electronic and structural properties of hydrogen, lithium, sodium, potassium and rubidium that are adsorbed, in a regular pattern, on a graphene surface. The results for H-graphene (graphane) and Li-graphene were compared with previous calculations. The present results do not support previous claims that the Li-C bond in such a layer would result in an sp(2) to an sp(3) transition of carbon orbitals, being more compatible with some ionic character for the covalent bond and with lithium acting as an electron acceptor in a bridging environment. Calculations were also performed for the Na, K, and Rb-graphene systems, resulting in a similar electronic behaviour but with a more pronounced ionic character than for Li-graphene. Energy calculations indicate the possible stability of such ad-graphene layers, with only the Li-graphene being possible to be spontaneously obtained.