Author Correction: Impact of pore anisotropy on the thermal conductivity of porous Si nanowires.

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Impact of pore anisotropy on the thermal conductivity of porous Si nanowires.

Porous materials display enhanced scattering mechanisms that greatly influence their transport properties. Metal-assisted chemical etching (MACE) enables fabrication of porous silicon nanowires starting from a doped Si wafer by using a metal template that catalyzes the etching process. Here, we report on the low thermal conductivity (κ) of individual porous Si nanowires (NWs) prepared from MACE, with values as low as 0.87 W·m-1·K-1 for 90 nm diameter wires with 35-40% porosity. Despite the strong suppression of long mean free path phonons in porous materials, we find a linear correlation of κ with the NW diameter. We ascribe this dependence to the anisotropic porous structure that arises during chemical etching and modifies the phonon percolation pathway in the center and outer regions of the nanowire. The inner microstructure of the NWs is visualized by means of electron tomography. In addition, we have used molecular dynamics simulations to provide guidance for how a porosity gradient influences phonon transport along the axis of the NW. Our findings are important towards the rational design of porous materials with tailored thermal and electronic properties for improved thermoelectric devices.

A nanomechanical mass sensor with yoctogram resolution.

Nanomechanical resonators have been used to weigh cells, biomolecules and gas molecules, and to study basic phenomena in surface science, such as phase transitions and diffusion. These experiments all rely on the ability of nanomechanical mass sensors to resolve small masses. Here, we report mass sensing experiments with a resolution of 1.7 yg (1 yg = 10(-24) g), which corresponds to the mass of one proton. The resonator is a carbon nanotube of length ∼150 nm that vibrates at a frequency of almost 2 GHz. This unprecedented level of sensitivity allows us to detect adsorption events of naphthalene molecules (C(10)H(8)), and to measure the binding energy of a xenon atom on the nanotube surface. These ultrasensitive nanotube resonators could have applications in mass spectrometry, magnetometry and surface science.

Site-Dependent Coordination Bonding in Self-Assembled Metal-Organic Networks.

The combination of organic linkers with metal atoms on top of inorganic substrates offers promising perspectives for functional electronic and magnetic nanoscale devices. Typically, coordination bonds between electron-rich end groups and transition-metal atoms lead to the self-assembly of metal-organic nanostructures, whose shape and electronic and magnetic properties crucially depend on the type of ligand. Here, we report on the site-selective bonding properties of Co atoms to the dichotomic dicyanoazobenzene molecule with its carbonitrile and diazo N-based moieties as possible ligands. Using low-temperature scanning tunneling microscopy (STM) and spectroscopy measurements, we resolve the formation of self-assembled metal-organic motifs. Cobalt atoms exhibit a clear spectroscopic fingerprint dependent on the different coordination site, which is further used to map their position, otherwise not clearly visible in the topographic STM images. Density functional theory corroborates the observed bonding patterns and evidences their coordinative nature.

The BN-pair impurity in carbon nanotubes and the possibility for disorder-induced frustration of gap formation.

We study the BN-pair impurity complex inside a metallic and a semiconducting single-walled carbon nanotube host. For the single impurity in the semiconducting tube, we find that no electron or hole bound states can be sustained because the distance between the B and the N is less than the effective Fermi-Teller radius for that system. If the BN pairs are incorporated at stoichiometric concentrations (BC(10)N nanotubes), achievable for example with a borabenzene-pyridine adduct C(10)H(10)BN precursor, the metallic tube becomes semiconducting for an ordered arrangement of the impurities, but the introduction of disorder restores a finite density of states at the Fermi level. Thus, in the mechanism presented here, disorder effectively restores the symmetry of the nanotube, returning the nanotube to its original metallic character.

Ferrodistortive instability at the (001) surface of half-metallic manganites.

We present the structure of the fully relaxed (001) surface of the half-metallic manganite La0.7Sr0.3MnO3, calculated using density functional theory. Two relevant ferroelastic order parameters are identified and characterized. The known tilting of the oxygen octahedra, which is present in the bulk phase, decreases towards the surface. A ferrodistortive Mn off-centering, triggered by the surface and not reported before, decays monotonically into the bulk. This distortion affects neither the half-metallicity nor the zero-temperature magnetization, but does change the effective spin-spin interactions, and thus the temperature dependence of the magnetic properties.

Electron transport via local polarons at interface atoms.

Electronic transport is profoundly modified in the presence of strong electron-vibration coupling. We show that in certain situations, the electron flow takes place only when vibrations are excited. By controlling the segregation of boron in semiconducting Si(111)-square root 3 x square root 3 R 30 degrees surfaces, we create a type of adatom with a dangling-bond state that is electronically decoupled from any other electronic state. However, probing this state with scanning tunnelling microscopy at 5 K yields high currents. These findings are rationalized by ab-initio calculations that show the formation of a local polaron in the transport process.

Spontaneous formation of triptycene supramolecules on surfaces.

In the limit of weak molecular interaction with an inorganic surface, noncovalent interactions between molecules dominate the nucleation and thin-film growth. Here, we report on the formation of three-dimensional triptycene clusters with a particularly stable structure. Once formed at the early stage of molecular adsorption, the clusters are stable for all temperatures until desorption. Furthermore, the clusters diffuse and nucleate as individual entities, therefore constituting building blocks for the later thin-film formation. High resolution scanning tunneling microscopy images indicate that the cluster is stabilized by C-H-pi interactions. The formation of such molecular structures at a surface is possible because the three-dimensional structure of the triptycene molecule leads to a very weak and mobile adsorption state. These results show that it is possible to investigate complex pathways in the formation of three-dimensional supramolecules at surfaces using a scanning tunneling microscope.

Metallic and semimetallic silicon 100 nanowires.

Silicon nanowires grown along the 100 direction with a bulk Si core are studied with density-functional calculations. Two surface reconstructions prevail after exploration of a large fraction of the phase space of nanowire reconstructions. Despite their energetical equivalence, one of the reconstructions is found to be strongly metallic while the other one is semimetallic. This electronic-structure behavior is dictated by the particular surface states of each reconstruction. These results imply that doping is not required in order to obtain good conducting Si nanowires.