Large-deformation and high-strength amorphous porous carbon nanospheres.

Carbon is one of the most important materials extensively used in industry and our daily life. Crystalline carbon materials such as carbon nanotubes and graphene possess ultrahigh strength and toughness. In contrast, amorphous carbon is known to be very brittle and can sustain little compressive deformation. Inspired by biological shells and honeycomb-like cellular structures in nature, we introduce a class of hybrid structural designs and demonstrate that amorphous porous carbon nanospheres with a thin outer shell can simultaneously achieve high strength and sustain large deformation. The amorphous carbon nanospheres were synthesized via a low-cost, scalable and structure-controllable ultrasonic spray pyrolysis approach using energetic carbon precursors. In situ compression experiments on individual nanospheres show that the amorphous carbon nanospheres with an optimized structure can sustain beyond 50% compressive strain. Both experiments and finite element analyses reveal that the buckling deformation of the outer spherical shell dominates the improvement of strength while the collapse of inner nanoscale pores driven by twisting, rotation, buckling and bending of pore walls contributes to the large deformation.

LEEM investigations of surfaces using a beam of energetic self-ions.

This article reviews recent research using a low-energy electron microscope, built by Tromp at IBM, and equipped with an accelerator that permits in situ irradiation with a beam of self-ions. The available ion energies of 20 eV to 5 keV span the range from epitaxial growth by a hyperthermal beam to sputtering at the level of approximately 10 atoms per incident ion. The design criteria and instrument calibration are described. The research described is surface science that requires a vacuum maintained below 10(-10) Torr, with all components contained in the same vacuum. Two general categories of applications are sketched. Experiments that accurately measure important physical quantities include surface mass diffusion over an extended temperature range; determining the critical chemical potential at which island nucleation occurs; observation and explanation of the universal evolution by which adatom and advacancy islands both grow and shrink by beam-driven processes; and the study of sublimation (regarded as negative ion beam intensity). Experiments described here with other goals include beam-assisted synthesis first of large pans and mesas for isolating surface experiments (e.g., nucleation) from the surrounding crystal, and second of Fourier waves on steps, for studies of diffusive relaxation. Operation of exotic structures including Bardeen-Herring sources and Frank growth spirals deformed by crystal anisotropy are also described.

LEEM investigations of ion beam effects on clean metal surfaces: quantitative studies of the driven steady state.

The technique of low-energy electron microscopy (LEEM) pioneered by Bauer has been adapted here to the investigation of ion beam processes on crystal surfaces by the incorporation of an intense and tunable source of selectable, energetic ions into a LEEM designed by Tromp et al. In this paper we explain principles that constrain the design of this tandem instrument, to permit observation of surfaces during irradiation. We also describe experiments that probe the driven steady state of surfaces subject to the perturbation of a uniform and constant flux of self-ions. The emphasis is on the example of Pt(-) ions irradiating the Pt(111) surface. We explore a regime of linear response at elevated temperature in which the driven nucleation and universal driven growth of surface islands, and the driven cycling of Bardeen-Herring sources and other surface clocks, may be understood in a fully quantitative manner.