Absence of single critical dose for the amorphization of quartz under ion irradiation.

In this work, we first simulated the amorphization of crystalline quartz under 50 keV [Formula: see text]Na ion irradiation with classical molecular dynamics (MD). We then used binary collision approximation algorithms to simulate the Rutherford backscattering spectrometry in channeling conditions (RBS-C) from these irradiated MD cells, and compared the RBS-C spectra with experiments. The simulated RBS-C results show an agreement with experiments in the evolution of amorphization as a function of dose, showing what appears to be (by this measure) full amorphization at about 2.2 eV⋅[Formula: see text]. We also applied other analysis methods, such as angular structure factor, Wigner-Seitz, coordination analysis and topological analysis, to analyze the structural evolution of the irradiated MD cells. The results show that the atomic-level structure of the sample keeps evolving after the RBS signal has saturated, until the dose of about 5 eV⋅[Formula: see text]. The continued evolution of the [Formula: see text] structure makes the definition of what is, on the atomic level, an amorphized quartz ambiguous.

Dual origin of defect magnetism in graphene and its reversible switching by molecular doping.

Control of magnetism by applied voltage is desirable for spintronics applications. Finding a suitable material remains an elusive goal, with only a few candidates found so far. Graphene is one of them and attracts interest because of its weak spin-orbit interaction, the ability to control electronic properties by the electric field effect and the possibility to introduce paramagnetic centres such as vacancies and adatoms. Here we show that the magnetism of adatoms in graphene is itinerant and can be controlled by doping, so that magnetic moments are switched on and off. The much-discussed vacancy magnetism is found to have a dual origin, with two approximately equal contributions; one from itinerant magnetism and the other from dangling bonds. Our work suggests that graphene's spin transport can be controlled by the field effect, similar to its electronic and optical properties, and that spin diffusion can be significantly enhanced above a certain carrier density.

Cutting and controlled modification of graphene with ion beams.

Using atomistic computer simulations, we study how ion irradiation can be used to alter the morphology of a graphene monolayer, by introducing defects of specific type, and to cut graphene sheets. Based on the results of our analytical potential molecular dynamics simulations, a kinetic Monte Carlo code is developed for modeling morphological changes in a graphene monolayer under irradiation at macroscopic time scales. Impacts of He, Ne, Ar, Kr, Xe, and Ga ions with kinetic energies ranging from tens of eV to 10 MeV and angles of incidence between 0° and 88° are studied. Our results provide microscopic insights into the response of graphene to ion irradiation and can directly be used for the optimization of graphene cutting and patterning with focused ion beams.

Self-starting stretched-pulse fiber laser mode locked and stabilized with slow and fast semiconductor saturable absorbers.

A self-starting stretched-pulse mode-locked erbium-doped fiber laser that uses fast and slow semiconductor saturable absorbers is described. By using two absorbers, we obtained reliable operation at a fundamental repetition rate with 250 pJ of pulse energy without multiple-pulse breakup. External chirp compensation was used to compress the highly chirped pulses to durations of 135 fs.

Atomic layer deposition of oxide thin films with metal alkoxides as oxygen sources

A chemical approach to atomic layer deposition (ALD) of oxide thin films is reported here. Instead of using water or other compounds for an oxygen source, oxygen is obtained from a metal alkoxide, which serves as both an oxygen and a metal source when it reacts with another metal compound such as a metal chloride or a metal alkyl. These reactions generally enable deposition of oxides of many metals. With this approach, an alumina film has been deposited on silicon without creating an interfacial silicon oxide layer that otherwise forms easily. This finding adds to the other benefits of the ALD method, especially the atomic-level thickness control and excellent uniformity, and takes a major step toward the scientifically challenging and technologically important task of replacing silica as the gate dielectric in the future generations of metal oxide semiconductor field effect transistors.