Hollow to bamboolike internal structure transition observed in carbon nanotube films

The transition of the internal structure in microwave chemical-vapor- deposited carbon nanotubes is investigated using scanning electron microscopy and high-resolution transmission electron microscopy. By controlling the thickness of the iron catalyst layer, a sequence of carbon nanotube films was obtained with diameters ranging from a few nanometers to over 100 nm. Experiments have established that by continuous reduction of the Fe layer thickness to <1 nm, single- and double-wall carbon nanotube films can be produced, whereas for an Fe film thickness <1 nm, multiwall carbon nanotube films can be synthesized. It was also found that for an Fe thickness ≥5 nm, interlayers (i.e., bamboolike or periodically compartmentalized nanotubes) were formed, while for an iron thickness <2 nm the tubes were primarily hollow. For an intermediate Fe thickness the internal structure of the carbon nanotubes was a mixture of hollow and bamboolike. A growth model which considers bulk and surface diffusions of carbon into andor onto the Fe catalyst surface is proposed to describe this transition and the internal periodic structure

Vibrational signature of broken chemical order in a GeS2 glass: a molecular dynamics simulation

Using density functional molecular dynamics simulations, we analyze the broken chemical order in a GeS$_2$ glass and its impact on the dynamical properties of the glass through the in-depth study of the vibrational eigenvectors. We find homopolar bonds and the frequencies of the corresponding modes are in agreement with experimental data. Localized S-S modes and 3-fold coordinated sulfur atoms are found to be at the origin of specific Raman peaks whose origin was not previously clear. Through the ring size statistics we find, during the glass formation, a conversion of 3-membered rings into larger units but also into 2-membered rings whose vibrational signature is in agreement with experiments.Comment: 11 pages, 8 figures; to appear in Phys. Rev.

Interaction of quasilocal harmonic modes and boson peak in glasses

The direct proportionality relation between the boson peak maximum in glasses, $\omega_b$, and the Ioffe-Regel crossover frequency for phonons, $\omega_d$, is established. For several investigated materials $\omega_b = (1.5\pm 0.1)\omega_d$. At the frequency $\omega_d$ the mean free path of the phonons $l$ becomes equal to their wavelength because of strong resonant scattering on quasilocal harmonic oscillators. Above this frequency phonons cease to exist. We prove that the established correlation between $\omega_b$ and $\omega_d$ holds in the general case and is a direct consequence of bilinear coupling of quasilocal oscillators with the strain field.Comment: RevTex, 4 pages, 1 figur

PHONONS OF THE METAL/AMORPHOUS SILICON INTERFACE STUDIED BY INTERFERENCE ENHANCED RAMAN SCATTERING

The interference enhanced Raman scattering (IERS) configuration is used to study the initial interfacial interactions of thin films of Pd or Pt on hydrogenated amorphous silicon (a-Si:H). Sharp spectral features are observed in the Raman spectrum of as-deposited Pd on a-Si:H which are-attributed to crystalline Pd2Si. In contrast, for as-deposited Pt on a-Si:H, broad spectral features are observed which are attributed to an intermixed Pt-Si phase

STRUCTURAL AND ELECTRICAL PROPERTIES OF NOBLE METAL-HYDROGENATED AMORPHOUS SILICON INTERFACES

Interfaces between hydrogenated amorphous Si and noble metals of Pd, Pt, and Au are probed by interference enhanced Raman spectroscopy and Schottky electrical measurements. Vacuum annealing of freshly prepared Pd, Pt, and Au Schottky diodes to ~200°C changes the structure of interfaces by forming crystalline Pd2Si, both crystalline Pt2Si and PtSi which grow simultaneously, and an intermixed Au-Si phase which lacks long-ranged order. Such structural changes are accompanied by an improvement in diode ideality factors and modifications of barrier heights. With the growth of silicides or intermixed phases at the interfaces, stable and almost ideal Schottky barriers can be formed