Abdel F. Isakovic

Synergistic toughening of epoxy with carbon nanotubes and graphene oxide for improved long-term performance

Epoxy based nanocomposites using graphene oxide (GO) sheets dispersed multi-walled carbon nanotubes (CNTs) as combination fillers were prepared using an in situ polymerization technique. A remarkable synergetic effect was observed between CNTs and GO sheets which improved the mechanical properties of the epoxy. It was confirmed by optical and field-emission scanning electron microscopy (FESEM) images that the dispersion of CNTs in epoxy matrix can be significantly improved by adding GO sheets. The overall mechanical properties of CNT–GO/epoxy composites were greatly enhanced with only adding 0.04 wt% (percent by weight) CNTs and 0.2 wt% GO sheets. Moreover, the fatigue and creep rupture lives of pure epoxy was also significantly increased by the addition of GO dispersed CNTs. Approximately a 950% improvement in fatigue life, and 400% improvement in creep rupture life were observed at the applied stress levels tested.

Diamond Amplified Photocathodes

High-average-current linear electron accelerators require photoinjectors capable of delivering tens to hundreds of mA average current, with peak currents of hundreds of amps. Standard photocathodes face significant challenges in meeting these requirements, and often have short operational lifetimes in an accelerator environment. We report on recent progress toward development of secondary emission amplifiers for photocathodes, which are intended to increase the achievable average current while protecting the cathode from the accelerator. The amplifier is a thin diamond wafer which converts energetic (few keV) primary electrons into hundreds of electron-hole pairs via secondary electron emission. The electrons drift through the diamond under an external bias and are emitted into vacuum via a hydrogen-terminated surface with negative electron affinity (NEA). Secondary emission gain of over 200 has been achieved. Two methods of patterning diamond, laser ablation and reactive-ion etching (RIE), are being developed to produce the required geometry. A variety of diagnostic techniques, including FTIR, SEM and AFM, have been used to characterize the diamonds.

Voltage dependent spin tunneling and spin relaxation in spin-leds

Spin diodes are potential building blocks of spin transistors, themselves units for future spintronics “microchips” for quantum information processing. Ferromagnet-semiconductor Schottky diodes are useful model devices that allow for understanding of basic physical and electronic processes in transport of spin-polarized electrons across the interface between a conventional ferromagnet (itself a natural reservoir of spins) and a spin hospitable semiconductor like galliumarsenide (GaAs), where spin-carrying electrons can be used for quantum information processing. This paper will introduce a model that explains experimentally observed voltage dependence of finite spin transfer efficiency, using Schottky tunneling contact, and drift-diffusion equations. In the same framework we present a rate-equation based explanation for voltage dependent spin relaxation of hot electrons, which has also been experimentally observed in spin light emitting diodes (spin-LEDs). Based on this model, we present device suggestions that are realizable within the modern semiconductor growth and nanoprocessing R&D sector.

The potential of electron-nuclear spin interactions for use in quantum information processing

Recent experiments have demonstrated that an ensemble of spin polarized electrons can transfer its spin polarization to the spin of the host atomic nuclei in a “spin-friendly” semiconductor like gallium arsenide (GaAs). In this paper, we analyze this process in terms of its efficiency for using natural nuclear spin in a solid state quantum computer. Among the appealing features of the proposed design is the potential to mate it to existing memory technology. We propose a realizable and scalable model of such a device based on growth-implanted quantum dots in a III-V semiconductor matrix and analyze the feasibility of transferring spin polarized information to such a system, keeping it stored in the system and processing it.