Graphene to Graphane: A Theoretical Study

Graphane is a two-dimensional system consisting of a single layer of fully saturated (sp$^3$ hybridization) carbon atoms. In an ideal graphane structure C-H bonds exhibit an alternating pattern (up and down with relation to the plane defined by the carbon atoms). In this work we have investigated using \textit{ab initio} and reactive molecular dynamics simulations the role of H frustration (breaking the H atoms up and down alternating pattern) in graphane-like structures. Our results show that significant percentage of uncorrelated H frustrated domains are formed in the early stages of the hydrogenation process leading to membrane shrinkage and extensive membrane corrugations. These results also suggest that large domains of perfect graphane-like structures are unlikely to be formed, H frustrated domains are always present.Comment: 15 pages, 6 figure

Defect-Free Carbon Nanotube Coils

Carbon nanotubes are promising building blocks for various nanoelectronic components. A highly desirable geometry for such applications is a coil. However, coiled nanotube structures reported so far were inherently defective or had no free ends accessible for contacting. Here we demonstrate the spontaneous self-coiling of single-wall carbon nanotubes into defect-free coils of up to more than 70 turns with identical diameter and chirality, and free ends. We characterize the structure, formation mechanism, and electrical properties of these coils by different microscopies, molecular dynamics simulations, Raman spectroscopy, and electrical and magnetic measurements. The coils are highly conductive, as expected for defect-free carbon nanotubes, but adjacent nanotube segments in the coil are more highly coupled than in regular bundles of single-wall carbon nanotubes, owing to their perfect crystal momentum matching, which enables tunneling between the turns. Although this behavior does not yet enable the performance of these nanotube coils as inductive devices, it does point a clear path for their realization. Hence, this study represents a major step toward the production of many different nanotube coil devices, including inductors, electromagnets, transformers, and dynamos

Origin of anomalously long interatomic distances in suspended gold chains

The discovery of long bonds in gold atom chains has represented a challenge for physical interpretation. In fact, interatomic distances frequently attain 3.0-3.6 A values and, distances as large as 5.0 A may be seldom observed. Here, we studied gold chains by transmission electron microscopy and performed theoretical calculations using cluster ab initio density functional formalism. We show that the insertion of two carbon atoms is required to account for the longest bonds, while distances above 3 A may be due to a mixture of clean and one C atom contaminated bonds.Comment: 4 pages, 4 Postscript figures, to be published in Physical Review Letter

Origin Of Anomalously Long Interatomic Distances In Suspended Gold Chains.

The discovery of long bonds in gold atom chains has represented a challenge for physical interpretation. In fact, interatomic distances frequently attain 3.0-3.6 A values, and distances as large as 5.0 A may be occasionally observed. Here we studied gold chains by transmission electron microscopy and performed theoretical calculations using cluster ab initio density functional formalism. We show that the insertion of two carbon atoms is required to account for the longest bonds, while distances above 3 A may be due to a mixture of clean and one C atom contaminated bonds.8807610

Electric-field gradients at the Zr sites in Zr/sub 3/Fe : measured using perturbed-angular-correlation spectroscopy and calculated using band theory

We have measured the electric-field-gradient (EFG) parameters Vzz and η and their temperature dependences at the two Zr sites in the intermetallic compound Zr₃Fe using perturbed-angular-correlation spectroscopy and the probe ¹⁸¹Hf→¹⁸¹Ta. At temperatures below the peritectic transformation at ≈1158 K, at each Zr site, we observed well-defined EFG’s, characterized by sharp spectral lines. A high-frequency, very asymmetric nuclear electric-quadrupole interaction characterizes the first Zr site, which represents approximately one-third of the probes. A low-frequency, nearly axially symmetric nuclear electric-quadrupole interaction characterizes the second Zr site, which represents approximately two-thirds of the probes. Near and above the peritectic transformation, the results show the effects of the decomposition of Zr₃Fe into Zr and Zr₂Fe and subsequent melting. We have compared the values of Vzz and h measured at laboratory temperature to those calculated using the first-principles, self-consistent real-space linear muffin-tin atomic sphere approximation (RS-LMTOASA) band-theory method. Overall the magnitudes of Vzz and η calculated using the RS-LMTO-ASA method agree reasonably well with the experimental values